Modification of Aspen Wood Ethanol Lignin via Azo Coupling; Promising Polymers from Renewable Plant Biomass

Modification of Aspen Wood Ethanol Lignin via Azo Coupling; Promising Polymers from Renewable Plant Biomass | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Modification of Aspen Wood Ethanol Lignin via Azo Coupling; Promising Polymers from Renewable Plant Biomass Viktor A. Golubkov, Valentina S. Borovkova, Maxim A. Lutoshkin, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4235328/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 20 Aug, 2024 Read the published version in Wood Science and Technology → Version 1 posted 8 You are reading this latest preprint version Abstract Plant biomass, in particular forestry wastes, is and promising renewable feedstock for deep chemical processing. Organosolv methods allow the use of underutilized lignin. The synthesis of modified polymers by azo coupling with the use of aspen ( Populus tremula ) ethanol lignin and its sulfated modification is studied. The success of the synthesis has been proven and the features of the structure and properties of the synthesized samples were studied by the physicochemical techniques, including Fourier-transform infrared and nuclear magnetic resonance spectroscopy, gel permeation chromatography and thermogravimetric analysis. It is shown that the new azopolymers have the ability to photoisomerize, which opens up prospects for their high-tech applications. The modified lignins are proven to be bioactive antioxidants. ethanol lignin aspen wood azo coupling photoisomerization antioxidant activity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 Introduction Plant biomass is the most intensively studied and promising renewable feedstock for a sustainable global economy (Ragauskas et al. 2006 ). The transformation of plant biomass polysaccharides into high-demand chemicals has long been a major focus of biorefinery. The history of chemical processing of lignocellulosic biomass has shown that, among its main components, lignin is the most difficult to utilize (Abu-Omar et al. 2021 , Tarabanko 2021 ). Lignin is a complex heteropolymer, usually highly branched. Lignin monomers are phenylpropane units – syringyl, guaiacyl, and hydroxyphenyl. During the processing of lignocellulosic biomass, lignin significantly changes, condenses, loses its reactivity, and becomes a waste (lignosulfonates, Kraft lignin). Such lignins are very cheap, but unwanted, since there has been a lack of efficient techniques for their valorization (Kuznetsov et al. 2018 ). The advantages of native lignins over technical lignins have led to the modern concept expressed in two words: «Lignin first» (Renders et al. 2017 , Tarabanko 2021 ). This means that the processing of lignocellulosic biomass should begin with the conversion of lignin into high-demand products. A way of the efficient and environmentally friendly processing of native lignin is the use of organosolv methods, i.e., the removal of significant amounts of lignin from the initial plant biomass with organic solvents at elevated temperatures and pressures (Kuznetsov et al. 2018 ). One of the most attractive solvents is ethanol, a cheap and readily available aliphatic alcohol, which is produced from the carbohydrates. The properties of the isolated lignin differ from the native one and depend on organosolv fractionation time (Tao et al. 2016 ), temperature (Meyer et al. 2020 ), type of lignocellulosic biomass (Xu et al. 2020 ). The ethanol lignin production technique was developed specially for extraction from hardwood (Pan et al. 2006 ). Organosolv lignins as well as ethanol lignin can be used in high-value-added applications as extracted and isolated: adhesives, membranes, carbon fibers, among others (Rabelo et al. 2023 ). The aromatic units of lignin are suitable for further functionalization (Eraghi Kazzaz et al. 2019 ) and, with properly chosen substituents, which ensure the occurrence of new properties, high value-added products can be obtained. One of the most interesting reactions for the modification of lignin is azo coupling. First, this reaction is well known in organic chemistry. In addition, for lignin itself, it became a starting point in determining its structure (Karlivan V.P. 1959, Karlivan V.P. 1960). Secondly, the obtained azo derivatives of lignin are interesting for a wide application range. They exhibit a high efficiency in pyrocondensates processing as inhibitors of polymerization (Hai et al. 2011 ). Due to their ability to supramolecular assembly (Ago et al. 2017 ), the azo derivatives can serve as precursors for the production of anodes from nitrogen-doped carbon nanospheres (Zhao et al. 2016 ). Water solubility of lignin can be improved by adding hydrophilic substituents, for example, the diazobenzenesulfonic acid (Borovkova et al. 2023 ). An azo-coupling-modified lignin is most commonly used in coloring and reflecting coatings (Frolova et al. 2020 , Pandian et al. 2020 ). Lignin is currently considered to be a promising ingredient of sunscreens for use in cosmetics industry (Qian et al. 2017 , Gordobil et al. 2020 ). In addition, it exhibits the high antioxidant activity (Barapatre et al. 2016 ), as well as the antitumor, antiviral, and antimicrobial activities, which opens up new prospects for pharmacology and biomedicine (Spiridon et al. 2018 ). It must be taken into account, however, that there is evidence of the pronounced cytotoxic effect of lignin (Barapatre et al. 2016 , Gordobil et al. 2020 ). At the same time, various azo dyes demonstrate bioaffinity and attract attention of researchers in the field of biomedicine (Alsantali et al. 2022 ). Although the azo derivatives of lignin have not yet evoked much interest of the biomedical community, they may prove to be effective drugs in therapy and components of personal care products. The combination of the optical and photosensitive properties of these derivatives and their stimuli-response properties to can find high-tech applications, e.g., in biosensors, drug delivery systems (Wu et al. 2013 ), and nanomaterials (Urban 2009 ). In this study, we discuss the use of ethanol lignin from hardwood raw materials in the production of azo and sulfated-azo modified materials. A source of native lignin was the aspen wood ( Populus tremula) , a forestry waste attracts attention as a raw material for deep chemical processing (Borovkova et al. 2022 ). The aim of this study was to develop methods for modifying aspen ethanol lignin by azo coupling with photosensitive and antioxidant properties. 2 Materials and Methods The initial sample for the synthesis and physicochemical study of the azo and sulfated derivatives of ethanol lignin (EL) was prepared from wood of Populus tremula aspen by the original technique (Kuznetsov et al. 2018 ). Aspen wood sawdust collected near Krasnoyarsk (Russia) was crushed on a BP-2 vibrating mill and a fraction of less than 0.5 mm was selected. Composition of materials: 46.3% – cellulose; 20.4% – lignin; 24.1% – hemicellulose; 5.2% - extractives; 0.5 – ash (Sharypov et al. 2016 ). Ethanol lignin was isolated by treatment of 60 wt. % ethanol-water mixture at 190°C during 3 h. The solution was separated by filtration and ethanol lignin was recovered with ice water at 4°C. The obtained ethanol lignin was modified using the well-known method for obtaining water-soluble sulfated ethanol lignin (SEL) proposed in (Malyar et al. 2022 ). Per 1 g air-dry EL 25 mL of 1,4-dioxane, 5 g of sulfamic acid and 3 g of urea were taken and placed into three neck flask. The prepared mixture was heated to 90°C under constant stirring for 3.0 h. After the solvent was decanted and the residue was dissolved in 25 mL of water, neutralized with aqueous ammonia and purified by dialysis. 2.1 Synthesis of the Azo Derivatives 2.1.1 Synthesis of azo lignin using p-nitroaniline To obtain the diazonium salt based on p-nitroaniline (4-nitrobenzenediazonium): water − 1.125 ml, concentrated HCl − 1.125 ml and 0.5 g of p-nitroaniline were mixed and cooled to 0°C in an ice bath and added with a solution of 0.35 g of NaNO 2 in 1 mL of water cooled to 0°C. In a separate beaker, 0.9 g of lignin and 2 ml of the 9% NaOH solution were mixed and cooled to 0°С. The alkaline solution of lignin was gradually added with a diazonium salt solution under stirring at ~ 0°C. The reaction mixture was incubated in an ice bath for 0.5 h. The EL and SEL samples modified with p-nitroaniline are hereinafter referred to as EL- azo -NO 2 and SEL- azo -NO 2 , respectively. The water-insoluble EL- azo -NO 2 precipitate was then filtered with a Büchner funnel and dried in air. The water soluble SEL- azo -NO 2 samples were subjected to dialysis in an MF-503-46 MFPI dialysis bag (US) with a pore size of 3.5 kDa against water for 10 h with changing water every hour. After dialysis, the solution was evaporated to dryness on a rotary evaporator under vacuum until a water-soluble solid residue was obtained. 2.1.2 Synthesis of azo lignin with sulfanilic acid To obtain the diazonium salt based on sulfanilic acid (diazobenzenesulfonic acid): sulfanilic acid − 1 g, 2 М NaOH − 2.5 ml and 0.4 g of NaNO 2 in 5 mL of water were placed in a glass beaker (100 ml). The solution was cooled to 0°C in an ice bath and added with 10 ml of 2 M HCl cooled to 0°С. In a separate beaker, 0.9 g of EL (SEL) and 2 ml of the 9% NaOH solution were mixed and cooled to 0°С. The alkaline solution of lignin was gradually added with the diazonium salt solution under stirring at a temperature of ~ 0°C. The reaction mixture was incubated for 0.5 h in an ice bath. To purify the product was subjected to dialysis and dried same as described above. The EL and SEL samples modified with sulfanilic acid are hereinafter referred to as EL- azo -SO 3 H and SEL- azo -SO 3 H, respectively. 2.2 Fourier-Transform Infrared (FTIR) Spectroscopy The Tensor 27 spectrometer was used for record FTIR spectra in the wavelength range of 4000–400 cm –1 with a resolution of 4 cm − 1 , the number of scans was 32. Specimens for the FTIR study were prepared in the form of tablets in a potassium bromide matrix. The substance concentration in the tablets was constant and amounted to 4 mg per 1000 mg of KBr. 2.3 Nuclear Magnetic Resonance (NMR) Spectroscopy NMR data were collected on a Bruker Avance III 600 spectrometer system at 295 K. Samples of 5–10 mg of lignin were placed into a 5 mm NMR tube and dissolved in 0.5 ml of DMSO-d 6 . The two-dimensional multiplicity edited 1 H- 13 C heteronuclear single quantum correlation (HSQC) spectra were recorded with four scans of 2048 data points, 256 increments and relaxation delay of 2.5 s. All spectra were acquired and processed using Top Spin 2 software supplied with the spectrometer. 2.4 Gel Permeation Chromatography (GPC) The molecular weight characteristics of lignin samples were determined by the GPC technique using an Agilent 1260 Infinity II Multi-Detector GPC/SEC System with triple detection: refractometer, viscometer, and light scattering. The eluent flow rate was 1 mL/min and the injected sample volume was 100 µl. Before the analysis, the water-soluble samples were dissolved in water (1.5 mg/mL) and the remaining samples, due to their insolubility in water, were dissolved in THF (1.5 mg/mL) and filtered through a 0.45-µm Millipore PTFE membrane filter. For aqueous solutions separation was performed on two combined PL Aquagel-OH Mixed-M columns (7.5x300 mm) using the mixture 0.2M NaNO 3 + 0.01M NaHPO 4 as a mobile phase. For organic solutions PlGel Mixed-E column (7.5x300 mm) using tetrahydrofuran (THF) stabilized with 250 ppm butylated hydroxytoluene as a mobile phase was used. Calibration was carried out using polydisperse standards of polyethylene glycol and polystyrene. 2.5 Thermal Analysis The TGA study was carried out on a NETZSCH TG 209 F1 thermobalance. The thermal decomposition of the samples was analyzed in nitrogen in the temperature range from 25 to 700°C in the dynamic temperature regime (10°C/min) using cylindrical corundum crucibles. The protective and blow out gas flow rate was 20 mL/min. 2.6 Spectrophotometry Analysis The UV-vis spectra were measured on Shimadzu UV-Vis-NIR 3600 plus scanning spectrophotometer (Japan) at a 1-nm spectral gap in a 1-cm quartz cuvette. The samples were dissolved in dimethyl sulfoxide (DMSO). The cuvettes were irradiated by LED assemblies with wavelengths of 360 and 450 nm and a luminous flux specific power of 50 mW ∙ cm –2 . Photoreactor walls were darkened to prevent additional irradiation of the sample caused by reflection. 2.7 Study of the Antioxidant Activity Based on the data on the absorption capacity of 1,1-diphenyl-2-picrylhydrazyl (DPPH), which served as a reference free radical compound (Rumpf et al. 2023 ), the antioxidant activity of lignins was determined by the somewhat modified method from (Lu et al. 2012 , Alzagameem et al. 2018 ). Before the UV measurements, a DPPH solution in ethanol (0.2 mmol/L) was prepared. The SEL- azo samples were dissolved in ethanol (Alzagameem et al. 2018 ) in a concentration series of 0.05, 0.1, 0.2, 0.5, 2, and 5 mg/mL. The SEL- azo solutions (1 mL) were thoroughly mixed with 2 mL of the freshly prepared DPPH solution and 2 mL of ethanol. The mixtures were well-stirred and incubated at room temperature in the dark for 30 min. After that, the absorbance was measured at 517 nm (Alzagameem et al. 2018 ) on a SPEKOL-1300 spectrophotometer (Analytik Jena AG, Germany) against a blank. In this study, vitamin C (Vc) was used as a positive control. The experiments were repeated for three times and the values obtained were averaged. The DPPH radical scavenging ability was calculated as $$\text{D}\text{P}\text{P}\text{H} \text{R}\text{a}\text{d}\text{i}\text{c}\text{a}\text{l} \text{S}\text{c}\text{a}\text{v}\text{e}\text{n}\text{g}\text{i}\text{n}\text{g} \text{A}\text{b}\text{i}\text{l}\text{i}\text{t}\text{y} \left(\text{%}\right)=\left(1-\frac{{\text{A}}_{\text{S}}-{\text{A}}_{\text{B}}}{{\text{A}}_{\text{C}}}\right)\times 100\text{%},$$ 1 where A C is the absorbance of the DPPH solution without a sample, A S is the absorbance of the test sample mixed with the DPPH solution, and A B is the absorbance of the sample without the DPPH solution. 3 Results and Discussion Aspen ethanol lignin was obtained with yield 12.5 mass.%, i.e. half of native lignin was isolated. Further, the sulfated and azo-coupling derivatives were obtained. The initial ethanol lignin is insoluble in water, but soluble in organic solvents (THF, DMSO, ethanol, etc.). Sulfation introduces ionic groups, which make the SEL polymer water-soluble (Malyar et al. 2022 ), but reduce its solubility in organic solvents. Via azo coupling, new functional groups are introduced into the polymer. The general scheme of the reaction is shown in Fig. 1 . A diazonium salts react with lignin in an alkaline medium to form azo derivatives. The substitution of aromatic H proceeds in the para position to phenolic hydroxyl. Thus, guaiacyl and hydroxyphenyl lignin units can be modified, syringyl units cannot. Also, phenylpropane units having esterified phenolic hydroxyl groups do not react (Gogotov and Luzhanskaya 2005 ). The latter fact is interesting because with the sulfation method used, phenolic hydroxyls are usually not sulfated (Levdansky et al. 2022 , Malyar et al. 2022 ). The 4-nitrobenzenediazonium-modified EL- azo -NO 2 and SEL- azo -NO 2 compounds contain nitro groups; the EL- azo -NO 2 derivative is water-insoluble. The coupling with diazobenzenesulfonic acid (EL- azo -SO 3 H and SEL- azo -SO 3 H) improves the solubility in the water, which is maintained in the pH range of 2‒12. 3.1 FTIR Study of the Polymers The chemical changes of ethanol lignin were studied by FTIR spectroscopy (see the spectra in Fig. 2 ). The spectra of all the samples contain a set of characteristic absorption bands near 1594, 1423, 1328, 1214 and 1122 cm ‒1 (L in Fig. 2 ). These wavenumbers are typical of the guaiacylsyringyl-type lignin, i.e., lignin from hard wood (Levdansky et al. 2019 ). However, the intensity of these absorption bands strongly changes from one sample to another. Their intensity is maximum for initial ethanol lignin and significantly decreases for the SEL and azo derivative samples. Another characteristic difference of the modified samples from the initial one is a drastic change in the shape of the absorption band between 3100 and 3700 cm –1 , which corresponds to OH groups. A decrease in the intensity of the signal of C‒H vibrations in methyl and methylene at 2930 and 2840 cm –1 characteristic of organosolv lignins (Yáñez-S et al. 2014 , Michelin et al. 2018 ) is related to a decrease in the content of the aliphatic part at the sulfation and azo coupling modification of ethanol lignin. The p-nitroaniline-modified EL- azo -NO 2 and SEL- azo -NO 2 samples are characterized by the intense absorption bands with maxima at 1518 and 1345 cm ‒1 , which belong to the NO 2 in nitrobenzene group (Sundaraganesan et al. 2007 ). The medium-intensity absorption bands at 700, 750, and 855 cm –1 are also attributed to the NO 2 vibrations (Khaikin et al. 2015 ). In the spectra of the EL- azo -SO 3 H and SEL- azo -SO 3 H samples with the grafted azobenzensulfonic acid groups, there are bands at 1030, 1006 and 835 cm ‒1 characteristic of sulfonic groups (Wang et al. 2002 ). There is an absorption band with the maximum at ~ 1190 cm ‒1 corresponding to C = S stretching vibrations. In addition, absorption bands associated with the presence of sulfate and sulfonic groups is located in the region of 660 − 500 cm − 1 . 3.2 Nuclear Magnetic Resonance Study of Azo Lignins The structural features of the modification of aspen ethanol lignin by the azo coupling reaction were studied by the example of EL and EL- azo -NO 2 using 2D HSQC NMR. The comparison of this pair of the samples is most expedient, since there are no changes, except for the azo coupling. For example, the water-soluble samples are likely to be fractionated during the dialysis purification. The HSQC spectra of initial ethanol lignin include characteristic correlation peaks of phenylpropane units, β-aryl ethers, and pinoresinol and phenylcoumarane lignin fragments (Figs. 3 a, 4 a) (Li and Gellerstedt 2008 , Kuznetsov et al. 2018 , Levdansky et al. 2019 ). As compared with the case of the EL sample, in the aliphatic region of the EL- azo -NO 2 spectra (Fig. 3 b), one can see an abrupt drop in the intensity of the cross peaks corresponding to the structures of β-aryl ethers, phenylcoumarane fragments, and even methoxyl groups. This is obviously due to a change in their concentration in the sample, rather than the chemical transformations and fractionation. During the reaction, 4-nitrobenzenediazonium species are added to guaiacyl and hydroxyphenyl units, while the contents of other ethanol lignin structural elements decrease. The most intense peaks in the aromatic structure region (δ 1 Н/δ 13 С 6.2–7.3/103–123 ppm) correspond to guaiacyl (G) and syringyl (S) structural units characteristic of hardwood lignins (Li and Gellerstedt 2008 , Kuznetsov et al. 2018 , Levdansky et al. 2019 ). After the modification the signal of the S structures is preserved in the spectrum, while the G structures completely disappear. In the aromatic region, the most characteristic is the appearance of cross peaks of nitrobenzene structures (δ 1 Н/ δ 13 С 8.0-8.5/122–127) in the EL- azo -NO 2 spectrum. The C atom signal positions (2, 3, 5, and 6) correspond to the simulation data using nmrdb.org (Banfi and Patiny 2008 ). At the same time, C atoms in positions 3 and 5 of the guaiacile structure of lignin, which has an N = N bond in position 2, are responsible for the peaks in the region of δ 1 Н/ δ 13 С 7.6–7.9/127–137. 3.3 Gel Permeation Chromatography The chemical modification of ethanol lignin affects the distribution of molecular weights of polymer molecules, which is reflected in its properties and potential applications. The characteristics of the molecular weights and their differential distributions were studied by GPC in an aqueous medium for the water-soluble samples and in THF for the water-insoluble samples. According to the data given in Table 1 , the azo coupling modification increases the number average ( M n ) and weight average ( M w ) molecular weights. Table 1 Molecular weight characteristics of lignin samples Sample M n (g/mol) M w (g/mol) PDI EL 1080 2420 2.24 SEL 2470 4020 1.62 EL- azo -NO 2 1190 3250 2.74 SEL- azo -NO 2 2570 4180 1.63 EL- azo -SO 3 H 3630 6350 1.75 SEL- azo -SO 3 H 5440 9220 1.70 The comparison of the EL and EL- azo -NO 2 samples analyzed in the THF medium revealed a shift in the molecular weight distribution towards higher molecular weights due to the azo coupling. The other modifications (SEL, SEL- azo -NO 2 , EL- azo -SO 3 H, and SEL- azo -SO 3 H) are water-soluble and their molecular weight distributions are strongly shifted towards larger values. Their M n and M w parameters exceed the values for the initial EL sample by a factor of 2‒4. One can estimate the molecular weight growth by comparing the molar weights of the model lignin structure ‒ coniferyl alcohol (180 g/mol) ‒ and its conjugate with diazosulfobenzene (366 g/mol). An increase in the molecular weight after the reaction should not exceed a twofold growth even in the simplest lignin model, when each monomer phenylpropane unit is modified. The low-molecular-weight part of the water-soluble samples was removed by dialysis (Kuznetsov et al. 2020 ), which strongly shifted the distribution towards higher molecular weights. An unexpected decrease in the observed molecular weight, or rather the size of the molecules, is observed when SEL is modified via azo coupling with p-nitroaniline. Obviously, this is not due to depolymerization, but due to the compaction of the molecule due to the introduction of a more hydrophobic fragment and a decrease in the hydrate shell size. 3.4 Thermal Analysis The thermogravimetry (TG) and differential thermogravimetry (DTG) curves of the samples of ethanol lignin and its azo derivatives are shown in Fig. 6 . The weight loss in the initial sample at 700°C attained 74.1%. The DTG curve of the initial lignin sample contains a broad peak between 200 and 500°C, which is typical of the thermal decomposition of aspen ethanol lignin (Fetisova et al. 2019 ). Further weight loss is due to graphitization (Ma et al. 2016 ). The modified samples exhibit a significantly higher thermal stability. The EL- azo -NO 2 and SEL- azo -NO 2 weight losses at 700°С were 43.8 and 38.4%, respectively. The thermograms of the EL- azo -SO 3 H and SEL- azo -SO 3 H samples also show that their stability exceeds that of the EL, EL- azo -NO 2 , and SEL- azo -NO 2 samples. Their weight losses were 37.0 and 31.7%. The azo coupling modification leads to the formation of thermostable condensed structures during pyrolysis. The weight loss in the EL- azo -NO 2 and SEL- azo -NO 2 samples at ~ 260°С is related to the decomposition of a nitro compounds (Simeonov et al. 1990 ); as the temperature further increases, the azo compounds and aromatic matrix of lignin decompose, but, in general, the modified ethanol lignin weight loss at 350°C noticeably weakens. The thermal event at 345°C can be attributed to the decomposition of sulfate group in the SEL- azo -NO 2 and SEL- azo -SO 3 H samples (Malyar et al. 2022 ). The decomposition of sulfonic groups in diazobenzenesulfonic acid occurs at temperatures of 440‒470°C. The results of the thermal analysis show that the azo derivatives of ethanol lignin are promising for use in the production of nitrogen-doped carbon materials. They exhibit a fairly high thermal stability; the weight loss at 700°C is reduced by a factor of more than 2 as compared with the value for the initial ethanol lignin. 3.5 Photoisomerization Study The cis-trans photoisomerization reaction represents a change in the configuration of a molecule during the transition from the stable ground state to the excited state after absorption of a photon with a certain wavelength (Fig. 7 ). Azobenzene is a well-studied chromophore which demonstrates photoisomerization activity; its derivatives attract attention as photofunctional materials for use in biochemistry and materials science (Bandara and Burdette 2012 ). Since the synthesized azo lignins contain the N = N conjugated double bonds, they can can from two isomers upon photoisomerization. The spectra, as well as the optical changes in the investigated samples at different excitation wavelengths are shown in Fig. 8 . Although the initial spectra are rather poorly distinguishable from each other, an analysis of their spectral differences (ΔA-λ plots in Fig. 8 ) gives an idea about the different optical changes under the light excitation. Figure 9 presents the dependence between the absorbance (ΔA) and exposure time by the example of EL- azo -SO 3 H. It can be seen that the absorption maximum is an invariant for both excitation wavelengths and independent of the irradiation time, while the ΔA value depends only on the excitation time. All this shows that, under irradiation by light with wavelengths of 360 and 450 nm, individual isomerization products are formed. The characteristic variation region at 300‒360 nm is typical of the π‒π* transitions in the azo compounds (Mirković et al. 2017 , Lađarević et al. 2019 ). First, for all the investigated samples, the spectral maxima for the cis and trans forms are noticeably different. This indicates a fundamental redistribution of shapes of the molecular orbitals for the cis- and trans-forms of azo derivates ethanol lignins and, consequently, their different geometric structures. The second point to note is the nonuniformity of the spectral changes: for the two investigated samples (SEL- azo -SO 3 H and SEL- azo -NO 2 ), the differences in the absorbance under appropriate irradiation are no more than hundredths. Obviously, this is due to the fact that the sulfate groups initially incorporated into the lignin structure reduces the content of inherent phenylpropane units, reducing the number of available reaction sites. In another hand sulfate groups create steric obstacles for the attachment of the N = N chromophore group. In the other two azo lignins (EL- azo -NO 2 and EL- azo -SO 3 H), on the contrary, the optical changes are several tenths. The difference between the extinctions of the cis- and trans-forms is most likely due to the kinetic difficulties (the transition kinetics are much faster for the last two samples) and the fact that the EL- azo -NO 2 and EL- azo -SO 3 H samples undergo the most dramatic transformation of the electronic structure of molecular orbitals. All the synthesized azo derivatives can photoisomerize, but, depending on the properties of lignin and the nature of the azo component, the activity and depth of the cis‒trans‒cis transitions change. 3.6 Study of the Antioxidant Activity The free radical scavenging assay of 1,1-diphenyl-2-picrylhydrazyl (DPPH) is based on the redox reaction of DPPH with an antioxidant, which results in a decrease in the color intensity in proportion to the antioxidant concentration (Alzagameem et al. 2018 ). According to the literature data (Alzagameem et al. 2018 , Du et al. 2022 ), lignins can exhibit the antioxidant properties, and, as a rule, their inhibitory effect on DPPH increases with the lignin concentration. The complex structure of lignin, which includes aromatic rings with hydroxyl and methoxyl functional groups, is responsible for the antioxidant potential. This depends, first of all, on the termination of oxidation reactions due to hydrogen donation and single electron transfer reactions. The ability of the samples and vitamin C solutions of different concentrations to absorb the DPPH free radicals is illustrated in Fig. 10 . According to the data obtained, the SEL- azo -NO 2 sample exhibits a higher ability to inhibit free radicals than the SEL- azo -SO 3 H sample. The maximum antioxidant activity of SEL- azo -NO 2 was achieved at 5 mg/mL and amounted to 64.6%, while the maximum value for SEL and SEL- azo -SO 3 H at the same concentration was merely 6.2 and 3.3%, respectively. Such a strong difference is explained by several factors. Previously, it has been repeatedly reported (Alzagameem et al. 2018 , Du et al. 2022 ) on the inverse relationship between the molecular weight characteristics of lignins and their antioxidant activity. In particular, the SEL and sample SEL- azo -NO 2 has a lower weight average molecular weight (4.0-4.2 kDa) than SEL- azo -SO 3 H (9.2 kDa). In addition, according to the molecular weight distributions in these samples (Fig. 5 ), the SEL- azo -NO 2 sample is a mixture of fractions with different molecular weights, in which phenolic hydroxyls may be present in greater quantities, causing increased antioxidant activity. In addition, the increase in the antioxidant activity of the phenolic part of lignin in the SEL-azo-NO 2 sample may be caused by the influence of the nitro group, which has a negative mesomeric effect. It is likely that the shift in electron density along the p-bond system increases the mobility of hydrogen in the phenolic hydroxyl, which causes inhibition of DPPH. In the case of the sulfonic group, which also has a negative mesomeric effect, a similar effect is not observed due to the replacement of hydrogen in the sulfonic group with sodium, which greatly reduces the acceptor properties of the substituent. 4 Conclusions A method for modifying Populus tremula aspen ethanol lignin via azo coupling with diazonium salts based on p-nitroaniline and sulfanilic acid was developed. These reactions were studied also for sulfated ethanol lignin. The novel synthesized polymers were examined by FTIR and NMR spectroscopy, which confirmed their successful functionalization. The p-nitroaniline modification does not make ethanol lignin water-soluble. The introduction of azobenzenegroups with a sulfonic acid group improves the water solubility of the polymer, as does sulfation. It was established that the modification via the azo coupling reaction increases the lignin molecular weights. The study of the photosensitive properties showed that the synthesized azo derivatives exhibit the photoisomerization ability, but it depends on the properties of lignin and the nature of the azo component. The modified lignins, especially p-nitroaniline based, are proven to be antioxidants. Declarations Funding This study was carried out in part within the State assignment of the Ministry of Science and Higher Education of the Russian Federation for the Institute of Chemistry and Chemical Technology, Siberian Branch of the Russian Academy of Sciences, project no. FWES-2021-0012. Acknowledgements This study was carried out using the equipment of the Krasnoyarsk Regional Centre for Collective Use, Krasnoyarsk Scientific Center, Siberian Branch of the Russian Academy of Sciences. CRediT authorship contribution statement Viktor A. Golubkov: Conceptualization, Visualization, Validation, Writing - Original Draft; Valentina S. Borovkova: Methodology, Investigation, Writing - Original Draft; Maxim A. Lutoshkin: Investigation, Writing - Original Draft; Nikolay A. Zos’ko: Resources, Software; Natalya Yu. Vasilieva: Methodology, Writing - Review & Editing, Supervision. Yuriy N. Malyar: Writing - Review & Editing, Supervision, Conceptualization, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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Supplementary Files Graphicalabstract.png Cite Share Download PDF Status: Published Journal Publication published 20 Aug, 2024 Read the published version in Wood Science and Technology → Version 1 posted Editorial decision: Revision requested 08 Jul, 2024 Reviews received at journal 17 May, 2024 Reviewers agreed at journal 11 Apr, 2024 Reviewers agreed at journal 11 Apr, 2024 Reviewers invited by journal 11 Apr, 2024 Editor assigned by journal 11 Apr, 2024 Submission checks completed at journal 09 Apr, 2024 First submitted to journal 08 Apr, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Vasilieva","email":"","orcid":"","institution":"Siberian Federal University","correspondingAuthor":false,"prefix":"","firstName":"Natalya","middleName":"Yu.","lastName":"Vasilieva","suffix":""},{"id":290205171,"identity":"f2733615-0822-4055-8902-10edf271ea85","order_by":5,"name":"Yuriy N. Malyar","email":"","orcid":"","institution":"Institute of Chemistry and Chemical Technology","correspondingAuthor":false,"prefix":"","firstName":"Yuriy","middleName":"N.","lastName":"Malyar","suffix":""}],"badges":[],"createdAt":"2024-04-08 09:30:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4235328/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4235328/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00226-024-01590-x","type":"published","date":"2024-08-20T15:57:48+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":54557463,"identity":"dc497be7-7617-4d8b-8ff3-89387c488805","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":114722,"visible":true,"origin":"","legend":"\u003cp\u003eThe general scheme of the azo coupling reaction phenylpropane units, the building blocks of lignin\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/8b21e54d0f5d09e2f4bad433.png"},{"id":54557464,"identity":"5a1417ae-921e-44dd-ab9d-cf0187dd1458","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":385904,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of ethanol lignin and its derivatives\u003c/p\u003e","description":"","filename":"Fig2FTIRaspenazolignin.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/29220f68973d3d1edd4eacdb.png"},{"id":54557466,"identity":"d042ad3d-ae0d-4698-b250-b125aed36195","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":129752,"visible":true,"origin":"","legend":"\u003cp\u003eAliphatic region in the 2D HSQC NMR spectra of (а) the EL and (b) EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples.\u003c/p\u003e","description":"","filename":"Fig3NMRaliph.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/d565a3e3c206955ee6340df3.jpg"},{"id":54557472,"identity":"6be6d3d1-1015-4100-9957-ffe35c20f73e","added_by":"auto","created_at":"2024-04-12 08:56:03","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":91597,"visible":true,"origin":"","legend":"\u003cp\u003eAromatic region in the 2D HSQC NMR spectra of (а) the EL and (b) EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples.\u003c/p\u003e","description":"","filename":"Fig4NMRaromat.emf.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/8ac013d80c339d0bc21dce19.jpg"},{"id":54557467,"identity":"7d19ef1d-3bd0-4b5b-8600-52649dc6adaf","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":191731,"visible":true,"origin":"","legend":"\u003cp\u003eMolecular weight distribution curves of lignin samples.\u003c/p\u003e","description":"","filename":"Fig5GPC.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/98b1218212ec38bc5d46ac87.png"},{"id":54557992,"identity":"7ac9edd1-ce1e-44c3-a2a3-593ae5b2282d","added_by":"auto","created_at":"2024-04-12 09:04:02","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":377760,"visible":true,"origin":"","legend":"\u003cp\u003eThermogravimetry (TG) and differential thermogravimetry (DTG) curves of ethanol lignin and its azo derivatives.\u003c/p\u003e","description":"","filename":"Fig6TGDTGaspenazolignin.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/4a51478cb06289e004ef4914.png"},{"id":54557469,"identity":"26b0b565-9c48-48ea-8a7e-c8c95a78ee48","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":220547,"visible":true,"origin":"","legend":"\u003cp\u003ePhotosensitive behavior of the azo derivatives of ethanol lignin.\u003c/p\u003e","description":"","filename":"Fig7schem.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/1baf81577f06b9dcafb2554c.png"},{"id":54557470,"identity":"d167b71c-14c7-46d1-b50b-443572a2cc88","added_by":"auto","created_at":"2024-04-12 08:56:02","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":353167,"visible":true,"origin":"","legend":"\u003cp\u003eUV-vis absorption spectra (in DMSO) and ΔA-λcurves. The black curve corresponds to the mixture of isomers; the red curve – to the excitation at 360 nm; and the blue curve – the excitation at 450 nm.\u003c/p\u003e","description":"","filename":"Fig8UvVis.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/0d25ec846f24ce65d90f18e4.png"},{"id":54557994,"identity":"855b9495-1122-4544-9a86-b37536284d63","added_by":"auto","created_at":"2024-04-12 09:04:03","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":89306,"visible":true,"origin":"","legend":"\u003cp\u003eΔA-wavelength curves for the EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH sample excited at 360 and 450 nm for exposure times of 1, 3, 7, and 10 min.\u003c/p\u003e","description":"","filename":"Fig9dAdt.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/fab96ef2303395f646a3a9d0.png"},{"id":54557474,"identity":"60c42375-249e-4732-8a02-bed66251c846","added_by":"auto","created_at":"2024-04-12 08:56:03","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":439842,"visible":true,"origin":"","legend":"\u003cp\u003eDPPH radical scavenging activity: SEL and SEL-\u003cem\u003eazo\u003c/em\u003e obtained using p-nitroaniline and sulfanilic acid and Vc as a positive control at different concentrations.\u003c/p\u003e","description":"","filename":"Fig10AOA.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/2f2952f49d1e1057d87a3dde.png"},{"id":63300323,"identity":"40d27c7b-f591-4975-b15d-615072d88965","added_by":"auto","created_at":"2024-08-26 16:13:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2695650,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/96b95c3c-e421-4178-9cea-06e8a39bd782.pdf"},{"id":54557991,"identity":"3d03b620-555f-4fa8-8392-2597e226c5aa","added_by":"auto","created_at":"2024-04-12 09:04:02","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":467483,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.png","url":"https://assets-eu.researchsquare.com/files/rs-4235328/v1/0822c5d5cdd4ed6945ab9383.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Modification of Aspen Wood Ethanol Lignin via Azo Coupling; Promising Polymers from Renewable Plant Biomass","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003ePlant biomass is the most intensively studied and promising renewable feedstock for a sustainable global economy (Ragauskas et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The transformation of plant biomass polysaccharides into high-demand chemicals has long been a major focus of biorefinery. The history of chemical processing of lignocellulosic biomass has shown that, among its main components, lignin is the most difficult to utilize (Abu-Omar et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Tarabanko \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eLignin is a complex heteropolymer, usually highly branched. Lignin monomers are phenylpropane units \u0026ndash; syringyl, guaiacyl, and hydroxyphenyl. During the processing of lignocellulosic biomass, lignin significantly changes, condenses, loses its reactivity, and becomes a waste (lignosulfonates, Kraft lignin). Such lignins are very cheap, but unwanted, since there has been a lack of efficient techniques for their valorization (Kuznetsov et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The advantages of native lignins over technical lignins have led to the modern concept expressed in two words: \u0026laquo;Lignin first\u0026raquo; (Renders et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Tarabanko \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). This means that the processing of lignocellulosic biomass should begin with the conversion of lignin into high-demand products.\u003c/p\u003e \u003cp\u003eA way of the efficient and environmentally friendly processing of native lignin is the use of organosolv methods, i.e., the removal of significant amounts of lignin from the initial plant biomass with organic solvents at elevated temperatures and pressures (Kuznetsov et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). One of the most attractive solvents is ethanol, a cheap and readily available aliphatic alcohol, which is produced from the carbohydrates. The properties of the isolated lignin differ from the native one and depend on organosolv fractionation time (Tao et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), temperature (Meyer et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), type of lignocellulosic biomass (Xu et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The ethanol lignin production technique was developed specially for extraction from hardwood (Pan et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Organosolv lignins as well as ethanol lignin can be used in high-value-added applications as extracted and isolated: adhesives, membranes, carbon fibers, among others (Rabelo et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe aromatic units of lignin are suitable for further functionalization (Eraghi Kazzaz et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) and, with properly chosen substituents, which ensure the occurrence of new properties, high value-added products can be obtained. One of the most interesting reactions for the modification of lignin is azo coupling. First, this reaction is well known in organic chemistry. In addition, for lignin itself, it became a starting point in determining its structure (Karlivan V.P. 1959, Karlivan V.P. 1960).\u003c/p\u003e \u003cp\u003eSecondly, the obtained azo derivatives of lignin are interesting for a wide application range. They exhibit a high efficiency in pyrocondensates processing as inhibitors of polymerization (Hai et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Due to their ability to supramolecular assembly (Ago et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), the azo derivatives can serve as precursors for the production of anodes from nitrogen-doped carbon nanospheres (Zhao et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Water solubility of lignin can be improved by adding hydrophilic substituents, for example, the diazobenzenesulfonic acid (Borovkova et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). An azo-coupling-modified lignin is most commonly used in coloring and reflecting coatings (Frolova et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Pandian et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eLignin is currently considered to be a promising ingredient of sunscreens for use in cosmetics industry (Qian et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e, Gordobil et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In addition, it exhibits the high antioxidant activity (Barapatre et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), as well as the antitumor, antiviral, and antimicrobial activities, which opens up new prospects for pharmacology and biomedicine (Spiridon et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). It must be taken into account, however, that there is evidence of the pronounced cytotoxic effect of lignin (Barapatre et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2016\u003c/span\u003e, Gordobil et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). At the same time, various azo dyes demonstrate bioaffinity and attract attention of researchers in the field of biomedicine (Alsantali et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Although the azo derivatives of lignin have not yet evoked much interest of the biomedical community, they may prove to be effective drugs in therapy and components of personal care products. The combination of the optical and photosensitive properties of these derivatives and their stimuli-response properties to can find high-tech applications, e.g., in biosensors, drug delivery systems (Wu et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), and nanomaterials (Urban \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this study, we discuss the use of ethanol lignin from hardwood raw materials in the production of azo and sulfated-azo modified materials. A source of native lignin was the aspen wood (\u003cem\u003ePopulus tremula)\u003c/em\u003e, a forestry waste attracts attention as a raw material for deep chemical processing (Borovkova et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The aim of this study was to develop methods for modifying aspen ethanol lignin by azo coupling with photosensitive and antioxidant properties.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cp\u003eThe initial sample for the synthesis and physicochemical study of the azo and sulfated derivatives of ethanol lignin (EL) was prepared from wood of \u003cem\u003ePopulus tremula\u003c/em\u003e aspen by the original technique (Kuznetsov et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Aspen wood sawdust collected near Krasnoyarsk (Russia) was crushed on a BP-2 vibrating mill and a fraction of less than 0.5 mm was selected. Composition of materials: 46.3% \u0026ndash; cellulose; 20.4% \u0026ndash; lignin; 24.1% \u0026ndash; hemicellulose; 5.2% - extractives; 0.5 \u0026ndash; ash (Sharypov et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Ethanol lignin was isolated by treatment of 60 wt. % ethanol-water mixture at 190\u0026deg;C during 3 h. The solution was separated by filtration and ethanol lignin was recovered with ice water at 4\u0026deg;C.\u003c/p\u003e \u003cp\u003eThe obtained ethanol lignin was modified using the well-known method for obtaining water-soluble sulfated ethanol lignin (SEL) proposed in (Malyar et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Per 1 g air-dry EL 25 mL of 1,4-dioxane, 5 g of sulfamic acid and 3 g of urea were taken and placed into three neck flask. The prepared mixture was heated to 90\u0026deg;C under constant stirring for 3.0 h. After the solvent was decanted and the residue was dissolved in 25 mL of water, neutralized with aqueous ammonia and purified by dialysis.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Synthesis of the Azo Derivatives\u003c/h2\u003e \u003cdiv id=\"Sec4\" class=\"Section3\"\u003e \u003ch2\u003e2.1.1 Synthesis of azo lignin using p-nitroaniline\u003c/h2\u003e \u003cp\u003eTo obtain the diazonium salt based on p-nitroaniline (4-nitrobenzenediazonium): water \u0026minus;\u0026thinsp;1.125 ml, concentrated HCl \u0026minus;\u0026thinsp;1.125 ml and 0.5 g of p-nitroaniline were mixed and cooled to 0\u0026deg;C in an ice bath and added with a solution of 0.35 g of NaNO\u003csub\u003e2\u003c/sub\u003e in 1 mL of water cooled to 0\u0026deg;C. In a separate beaker, 0.9 g of lignin and 2 ml of the 9% NaOH solution were mixed and cooled to 0\u0026deg;С.\u003c/p\u003e \u003cp\u003eThe alkaline solution of lignin was gradually added with a diazonium salt solution under stirring at ~\u0026thinsp;0\u0026deg;C. The reaction mixture was incubated in an ice bath for 0.5 h. The EL and SEL samples modified with p-nitroaniline are hereinafter referred to as EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e, respectively. The water-insoluble EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e precipitate was then filtered with a B\u0026uuml;chner funnel and dried in air. The water soluble SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples were subjected to dialysis in an MF-503-46 MFPI dialysis bag (US) with a pore size of 3.5 kDa against water for 10 h with changing water every hour. After dialysis, the solution was evaporated to dryness on a rotary evaporator under vacuum until a water-soluble solid residue was obtained.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.1.2 Synthesis of azo lignin with sulfanilic acid\u003c/h2\u003e \u003cp\u003eTo obtain the diazonium salt based on sulfanilic acid (diazobenzenesulfonic acid): sulfanilic acid \u0026minus;\u0026thinsp;1 g, 2 М NaOH \u0026minus;\u0026thinsp;2.5 ml and 0.4 g of NaNO\u003csub\u003e2\u003c/sub\u003e in 5 mL of water were placed in a glass beaker (100 ml). The solution was cooled to 0\u0026deg;C in an ice bath and added with 10 ml of 2 M HCl cooled to 0\u0026deg;С. In a separate beaker, 0.9 g of EL (SEL) and 2 ml of the 9% NaOH solution were mixed and cooled to 0\u0026deg;С.\u003c/p\u003e \u003cp\u003eThe alkaline solution of lignin was gradually added with the diazonium salt solution under stirring at a temperature of ~\u0026thinsp;0\u0026deg;C. The reaction mixture was incubated for 0.5 h in an ice bath. To purify the product was subjected to dialysis and dried same as described above. The EL and SEL samples modified with sulfanilic acid are hereinafter referred to as EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH, respectively.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Fourier-Transform Infrared (FTIR) Spectroscopy\u003c/h2\u003e \u003cp\u003eThe Tensor 27 spectrometer was used for record FTIR spectra in the wavelength range of 4000\u0026ndash;400 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e with a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the number of scans was 32. Specimens for the FTIR study were prepared in the form of tablets in a potassium bromide matrix. The substance concentration in the tablets was constant and amounted to 4 mg per 1000 mg of KBr.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Nuclear Magnetic Resonance (NMR) Spectroscopy\u003c/h2\u003e \u003cp\u003eNMR data were collected on a Bruker Avance III 600 spectrometer system at 295 K. Samples of 5\u0026ndash;10 mg of lignin were placed into a 5 mm NMR tube and dissolved in 0.5 ml of DMSO-d\u003csub\u003e6\u003c/sub\u003e. The two-dimensional multiplicity edited \u003csup\u003e1\u003c/sup\u003eH-\u003csup\u003e13\u003c/sup\u003eC heteronuclear single quantum correlation (HSQC) spectra were recorded with four scans of 2048 data points, 256 increments and relaxation delay of 2.5 s. All spectra were acquired and processed using Top Spin 2 software supplied with the spectrometer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Gel Permeation Chromatography (GPC)\u003c/h2\u003e \u003cp\u003eThe molecular weight characteristics of lignin samples were determined by the GPC technique using an Agilent 1260 Infinity II Multi-Detector GPC/SEC System with triple detection: refractometer, viscometer, and light scattering. The eluent flow rate was 1 mL/min and the injected sample volume was 100 \u0026micro;l. Before the analysis, the water-soluble samples were dissolved in water (1.5 mg/mL) and the remaining samples, due to their insolubility in water, were dissolved in THF (1.5 mg/mL) and filtered through a 0.45-\u0026micro;m Millipore PTFE membrane filter. For aqueous solutions separation was performed on two combined PL Aquagel-OH Mixed-M columns (7.5x300 mm) using the mixture 0.2M NaNO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0.01M NaHPO\u003csub\u003e4\u003c/sub\u003e as a mobile phase. For organic solutions PlGel Mixed-E column (7.5x300 mm) using tetrahydrofuran (THF) stabilized with 250 ppm butylated hydroxytoluene as a mobile phase was used. Calibration was carried out using polydisperse standards of polyethylene glycol and polystyrene.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Thermal Analysis\u003c/h2\u003e \u003cp\u003eThe TGA study was carried out on a NETZSCH TG 209 F1 thermobalance. The thermal decomposition of the samples was analyzed in nitrogen in the temperature range from 25 to 700\u0026deg;C in the dynamic temperature regime (10\u0026deg;C/min) using cylindrical corundum crucibles. The protective and blow out gas flow rate was 20 mL/min.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Spectrophotometry Analysis\u003c/h2\u003e \u003cp\u003eThe UV-vis spectra were measured on Shimadzu UV-Vis-NIR 3600 plus scanning spectrophotometer (Japan) at a 1-nm spectral gap in a 1-cm quartz cuvette. The samples were dissolved in dimethyl sulfoxide (DMSO). The cuvettes were irradiated by LED assemblies with wavelengths of 360 and 450 nm and a luminous flux specific power of 50 mW ∙ cm\u003csup\u003e\u0026ndash;2\u003c/sup\u003e. Photoreactor walls were darkened to prevent additional irradiation of the sample caused by reflection.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Study of the Antioxidant Activity\u003c/h2\u003e \u003cp\u003eBased on the data on the absorption capacity of 1,1-diphenyl-2-picrylhydrazyl (DPPH), which served as a reference free radical compound (Rumpf et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), the antioxidant activity of lignins was determined by the somewhat modified method from (Lu et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2012\u003c/span\u003e, Alzagameem et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Before the UV measurements, a DPPH solution in ethanol (0.2 mmol/L) was prepared. The SEL-\u003cem\u003eazo\u003c/em\u003e samples were dissolved in ethanol (Alzagameem et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) in a concentration series of 0.05, 0.1, 0.2, 0.5, 2, and 5 mg/mL. The SEL-\u003cem\u003eazo\u003c/em\u003e solutions (1 mL) were thoroughly mixed with 2 mL of the freshly prepared DPPH solution and 2 mL of ethanol. The mixtures were well-stirred and incubated at room temperature in the dark for 30 min. After that, the absorbance was measured at 517 nm (Alzagameem et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) on a SPEKOL-1300 spectrophotometer (Analytik Jena AG, Germany) against a blank. In this study, vitamin C (Vc) was used as a positive control. The experiments were repeated for three times and the values obtained were averaged.\u003c/p\u003e \u003cp\u003eThe DPPH radical scavenging ability was calculated as\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\text{D}\\text{P}\\text{P}\\text{H} \\text{R}\\text{a}\\text{d}\\text{i}\\text{c}\\text{a}\\text{l} \\text{S}\\text{c}\\text{a}\\text{v}\\text{e}\\text{n}\\text{g}\\text{i}\\text{n}\\text{g} \\text{A}\\text{b}\\text{i}\\text{l}\\text{i}\\text{t}\\text{y} \\left(\\text{%}\\right)=\\left(1-\\frac{{\\text{A}}_{\\text{S}}-{\\text{A}}_{\\text{B}}}{{\\text{A}}_{\\text{C}}}\\right)\\times 100\\text{%},$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere A\u003csub\u003eC\u003c/sub\u003e is the absorbance of the DPPH solution without a sample, A\u003csub\u003eS\u003c/sub\u003e is the absorbance of the test sample mixed with the DPPH solution, and A\u003csub\u003eB\u003c/sub\u003e is the absorbance of the sample without the DPPH solution.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cp\u003eAspen ethanol lignin was obtained with yield 12.5 mass.%, i.e. half of native lignin was isolated. Further, the sulfated and azo-coupling derivatives were obtained. The initial ethanol lignin is insoluble in water, but soluble in organic solvents (THF, DMSO, ethanol, etc.). Sulfation introduces ionic groups, which make the SEL polymer water-soluble (Malyar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e), but reduce its solubility in organic solvents.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eVia\u003c/em\u003e azo coupling, new functional groups are introduced into the polymer. The general scheme of the reaction is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. A diazonium salts react with lignin in an alkaline medium to form azo derivatives. The substitution of aromatic H proceeds in the \u003cem\u003epara\u003c/em\u003e position to phenolic hydroxyl. Thus, guaiacyl and hydroxyphenyl lignin units can be modified, syringyl units cannot. Also, phenylpropane units having esterified phenolic hydroxyl groups do not react (Gogotov and Luzhanskaya \u003cspan class=\"CitationRef\"\u003e2005\u003c/span\u003e). The latter fact is interesting because with the sulfation method used, phenolic hydroxyls are usually not sulfated (Levdansky et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e, Malyar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe 4-nitrobenzenediazonium-modified EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e compounds contain nitro groups; the EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e derivative is water-insoluble. The coupling with diazobenzenesulfonic acid (EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH) improves the solubility in the water, which is maintained in the pH range of 2‒12.\u003c/p\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1 FTIR Study of the Polymers\u003c/h2\u003e\n\u003cp\u003eThe chemical changes of ethanol lignin were studied by FTIR spectroscopy (see the spectra in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). The spectra of all the samples contain a set of characteristic absorption bands near 1594, 1423, 1328, 1214 and 1122 cm\u003csup\u003e‒1\u003c/sup\u003e (L in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). These wavenumbers are typical of the guaiacylsyringyl-type lignin, i.e., lignin from hard wood (Levdansky et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). However, the intensity of these absorption bands strongly changes from one sample to another. Their intensity is maximum for initial ethanol lignin and significantly decreases for the SEL and azo derivative samples. Another characteristic difference of the modified samples from the initial one is a drastic change in the shape of the absorption band between 3100 and 3700 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e, which corresponds to OH groups.\u003c/p\u003e\n\u003cp\u003eA decrease in the intensity of the signal of C‒H vibrations in methyl and methylene at 2930 and 2840 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e characteristic of organosolv lignins (Y\u0026aacute;\u0026ntilde;ez-S et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e, Michelin et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e) is related to a decrease in the content of the aliphatic part at the sulfation and azo coupling modification of ethanol lignin.\u003c/p\u003e\n\u003cp\u003eThe p-nitroaniline-modified EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples are characterized by the intense absorption bands with maxima at 1518 and 1345 cm\u003csup\u003e‒1\u003c/sup\u003e, which belong to the NO\u003csub\u003e2\u003c/sub\u003e in nitrobenzene group (Sundaraganesan et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e). The medium-intensity absorption bands at 700, 750, and 855 cm\u003csup\u003e\u0026ndash;1\u003c/sup\u003e are also attributed to the NO\u003csub\u003e2\u003c/sub\u003e vibrations (Khaikin et al. \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eIn the spectra of the EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH samples with the grafted azobenzensulfonic acid groups, there are bands at 1030, 1006 and 835 cm\u003csup\u003e‒1\u003c/sup\u003e characteristic of sulfonic groups (Wang et al. \u003cspan class=\"CitationRef\"\u003e2002\u003c/span\u003e). There is an absorption band with the maximum at ~\u0026thinsp;1190 cm\u003csup\u003e‒1\u003c/sup\u003e corresponding to C\u0026thinsp;=\u0026thinsp;S stretching vibrations. In addition, absorption bands associated with the presence of sulfate and sulfonic groups is located in the region of 660\u0026thinsp;\u0026minus;\u0026thinsp;500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2 Nuclear Magnetic Resonance Study of Azo Lignins\u003c/h2\u003e\n\u003cp\u003eThe structural features of the modification of aspen ethanol lignin by the azo coupling reaction were studied by the example of EL and EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e using 2D HSQC NMR. The comparison of this pair of the samples is most expedient, since there are no changes, except for the azo coupling. For example, the water-soluble samples are likely to be fractionated during the dialysis purification.\u003c/p\u003e\n\u003cp\u003eThe HSQC spectra of initial ethanol lignin include characteristic correlation peaks of phenylpropane units, \u0026beta;-aryl ethers, and pinoresinol and phenylcoumarane lignin fragments (Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea) (Li and Gellerstedt \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e, Kuznetsov et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e, Levdansky et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eAs compared with the case of the EL sample, in the aliphatic region of the EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e spectra (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb), one can see an abrupt drop in the intensity of the cross peaks corresponding to the structures of \u0026beta;-aryl ethers, phenylcoumarane fragments, and even methoxyl groups. This is obviously due to a change in their concentration in the sample, rather than the chemical transformations and fractionation. During the reaction, 4-nitrobenzenediazonium species are added to guaiacyl and hydroxyphenyl units, while the contents of other ethanol lignin structural elements decrease.\u003c/p\u003e\n\u003cp\u003eThe most intense peaks in the aromatic structure region (\u0026delta;\u003csup\u003e1\u003c/sup\u003eН/\u0026delta;\u003csup\u003e13\u003c/sup\u003eС 6.2\u0026ndash;7.3/103\u0026ndash;123 ppm) correspond to guaiacyl (G) and syringyl (S) structural units characteristic of hardwood lignins (Li and Gellerstedt \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e, Kuznetsov et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e, Levdansky et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). After the modification the signal of the S structures is preserved in the spectrum, while the G structures completely disappear.\u003c/p\u003e\n\u003cp\u003eIn the aromatic region, the most characteristic is the appearance of cross peaks of nitrobenzene structures (\u0026delta;\u003csup\u003e1\u003c/sup\u003eН/ \u0026delta;\u003csup\u003e13\u003c/sup\u003eС 8.0-8.5/122\u0026ndash;127) in the EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e spectrum. The C atom signal positions (2, 3, 5, and 6) correspond to the simulation data using nmrdb.org (Banfi and Patiny \u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e). At the same time, C atoms in positions 3 and 5 of the guaiacile structure of lignin, which has an N\u0026thinsp;=\u0026thinsp;N bond in position 2, are responsible for the peaks in the region of \u0026delta;\u003csup\u003e1\u003c/sup\u003eН/ \u0026delta;\u003csup\u003e13\u003c/sup\u003eС 7.6\u0026ndash;7.9/127\u0026ndash;137.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3 Gel Permeation Chromatography\u003c/h2\u003e\n\u003cp\u003eThe chemical modification of ethanol lignin affects the distribution of molecular weights of polymer molecules, which is reflected in its properties and potential applications. The characteristics of the molecular weights and their differential distributions were studied by GPC in an aqueous medium for the water-soluble samples and in THF for the water-insoluble samples. According to the data given in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, the azo coupling modification increases the number average (\u003cem\u003eM\u003c/em\u003e\u003csub\u003en\u003c/sub\u003e) and weight average (\u003cem\u003eM\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e) molecular weights.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eMolecular weight characteristics of lignin samples\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSample\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eM\u003c/em\u003e\u003csub\u003en\u003c/sub\u003e\u0026nbsp;(g/mol)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eM\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e\u0026nbsp;(g/mol)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ePDI\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEL\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1080\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2420\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.24\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSEL\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2470\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4020\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.62\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1190\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3250\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.74\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2570\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4180\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.63\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3630\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e6350\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.75\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5440\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e9220\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.70\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eThe comparison of the EL and EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples analyzed in the THF medium revealed a shift in the molecular weight distribution towards higher molecular weights due to the azo coupling. The other modifications (SEL, SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e, EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH, and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH) are water-soluble and their molecular weight distributions are strongly shifted towards larger values. Their \u003cem\u003eM\u003c/em\u003e\u003csub\u003en\u003c/sub\u003e and \u003cem\u003eM\u003c/em\u003e\u003csub\u003ew\u003c/sub\u003e parameters exceed the values for the initial EL sample by a factor of 2‒4.\u003c/p\u003e\n\u003cp\u003eOne can estimate the molecular weight growth by comparing the molar weights of the model lignin structure ‒ coniferyl alcohol (180 g/mol) ‒ and its conjugate with diazosulfobenzene (366 g/mol). An increase in the molecular weight after the reaction should not exceed a twofold growth even in the simplest lignin model, when each monomer phenylpropane unit is modified. The low-molecular-weight part of the water-soluble samples was removed by dialysis (Kuznetsov et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e), which strongly shifted the distribution towards higher molecular weights.\u003c/p\u003e\n\u003cp\u003eAn unexpected decrease in the observed molecular weight, or rather the size of the molecules, is observed when SEL is modified \u003cem\u003evia\u003c/em\u003e azo coupling with p-nitroaniline. Obviously, this is not due to depolymerization, but due to the compaction of the molecule due to the introduction of a more hydrophobic fragment and a decrease in the hydrate shell size.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4 Thermal Analysis\u003c/h2\u003e\n\u003cp\u003eThe thermogravimetry (TG) and differential thermogravimetry (DTG) curves of the samples of ethanol lignin and its azo derivatives are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. The weight loss in the initial sample at 700\u0026deg;C attained 74.1%. The DTG curve of the initial lignin sample contains a broad peak between 200 and 500\u0026deg;C, which is typical of the thermal decomposition of aspen ethanol lignin (Fetisova et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e). Further weight loss is due to graphitization (Ma et al. \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eThe modified samples exhibit a significantly higher thermal stability. The EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e weight losses at 700\u0026deg;С were 43.8 and 38.4%, respectively. The thermograms of the EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH samples also show that their stability exceeds that of the EL, EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e, and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples. Their weight losses were 37.0 and 31.7%. The azo coupling modification leads to the formation of thermostable condensed structures during pyrolysis.\u003c/p\u003e\n\u003cp\u003eThe weight loss in the EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e samples at ~\u0026thinsp;260\u0026deg;С is related to the decomposition of a nitro compounds (Simeonov et al. \u003cspan class=\"CitationRef\"\u003e1990\u003c/span\u003e); as the temperature further increases, the azo compounds and aromatic matrix of lignin decompose, but, in general, the modified ethanol lignin weight loss at 350\u0026deg;C noticeably weakens.\u003c/p\u003e\n\u003cp\u003eThe thermal event at 345\u0026deg;C can be attributed to the decomposition of sulfate group in the SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH samples (Malyar et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The decomposition of sulfonic groups in diazobenzenesulfonic acid occurs at temperatures of 440‒470\u0026deg;C.\u003c/p\u003e\n\u003cp\u003eThe results of the thermal analysis show that the azo derivatives of ethanol lignin are promising for use in the production of nitrogen-doped carbon materials. They exhibit a fairly high thermal stability; the weight loss at 700\u0026deg;C is reduced by a factor of more than 2 as compared with the value for the initial ethanol lignin.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\n\u003ch2\u003e3.5 Photoisomerization Study\u003c/h2\u003e\n\u003cp\u003eThe cis-trans photoisomerization reaction represents a change in the configuration of a molecule during the transition from the stable ground state to the excited state after absorption of a photon with a certain wavelength (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). Azobenzene is a well-studied chromophore which demonstrates photoisomerization activity; its derivatives attract attention as photofunctional materials for use in biochemistry and materials science (Bandara and Burdette \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eSince the synthesized azo lignins contain the N\u0026thinsp;=\u0026thinsp;N conjugated double bonds, they can can from two isomers upon photoisomerization. The spectra, as well as the optical changes in the investigated samples at different excitation wavelengths are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. Although the initial spectra are rather poorly distinguishable from each other, an analysis of their spectral differences (\u0026Delta;A-\u0026lambda; plots in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e) gives an idea about the different optical changes under the light excitation.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e presents the dependence between the absorbance (\u0026Delta;A) and exposure time by the example of EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH. It can be seen that the absorption maximum is an invariant for both excitation wavelengths and independent of the irradiation time, while the \u0026Delta;A value depends only on the excitation time. All this shows that, under irradiation by light with wavelengths of 360 and 450 nm, individual isomerization products are formed. The characteristic variation region at 300‒360 nm is typical of the \u0026pi;‒\u0026pi;* transitions in the azo compounds (Mirković et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e, Lađarević et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eFirst, for all the investigated samples, the spectral maxima for the cis and trans forms are noticeably different. This indicates a fundamental redistribution of shapes of the molecular orbitals for the cis- and trans-forms of azo derivates ethanol lignins and, consequently, their different geometric structures.\u003c/p\u003e\n\u003cp\u003eThe second point to note is the nonuniformity of the spectral changes: for the two investigated samples (SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH and SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e), the differences in the absorbance under appropriate irradiation are no more than hundredths. Obviously, this is due to the fact that the sulfate groups initially incorporated into the lignin structure reduces the content of inherent phenylpropane units, reducing the number of available reaction sites.\u003c/p\u003e\n\u003cp\u003eIn another hand sulfate groups create steric obstacles for the attachment of the N\u0026thinsp;=\u0026thinsp;N chromophore group. In the other two azo lignins (EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH), on the contrary, the optical changes are several tenths. The difference between the extinctions of the cis- and trans-forms is most likely due to the kinetic difficulties (the transition kinetics are much faster for the last two samples) and the fact that the EL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e and EL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH samples undergo the most dramatic transformation of the electronic structure of molecular orbitals.\u003c/p\u003e\n\u003cp\u003eAll the synthesized azo derivatives can photoisomerize, but, depending on the properties of lignin and the nature of the azo component, the activity and depth of the cis‒trans‒cis transitions change.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n\u003ch2\u003e3.6 Study of the Antioxidant Activity\u003c/h2\u003e\n\u003cp\u003eThe free radical scavenging assay of 1,1-diphenyl-2-picrylhydrazyl (DPPH) is based on the redox reaction of DPPH with an antioxidant, which results in a decrease in the color intensity in proportion to the antioxidant concentration (Alzagameem et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eAccording to the literature data (Alzagameem et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e, Du et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e), lignins can exhibit the antioxidant properties, and, as a rule, their inhibitory effect on DPPH increases with the lignin concentration. The complex structure of lignin, which includes aromatic rings with hydroxyl and methoxyl functional groups, is responsible for the antioxidant potential. This depends, first of all, on the termination of oxidation reactions due to hydrogen donation and single electron transfer reactions. The ability of the samples and vitamin C solutions of different concentrations to absorb the DPPH free radicals is illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAccording to the data obtained, the SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e sample exhibits a higher ability to inhibit free radicals than the SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH sample. The maximum antioxidant activity of SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e was achieved at 5 mg/mL and amounted to 64.6%, while the maximum value for SEL and SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH at the same concentration was merely 6.2 and 3.3%, respectively. Such a strong difference is explained by several factors.\u003c/p\u003e\n\u003cp\u003ePreviously, it has been repeatedly reported (Alzagameem et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e, Du et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e) on the inverse relationship between the molecular weight characteristics of lignins and their antioxidant activity. In particular, the SEL and sample SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e has a lower weight average molecular weight (4.0-4.2 kDa) than SEL-\u003cem\u003eazo\u003c/em\u003e-SO\u003csub\u003e3\u003c/sub\u003eH (9.2 kDa). In addition, according to the molecular weight distributions in these samples (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e), the SEL-\u003cem\u003eazo\u003c/em\u003e-NO\u003csub\u003e2\u003c/sub\u003e sample is a mixture of fractions with different molecular weights, in which phenolic hydroxyls may be present in greater quantities, causing increased antioxidant activity.\u003c/p\u003e\n\u003cp\u003eIn addition, the increase in the antioxidant activity of the phenolic part of lignin in the SEL-azo-NO\u003csub\u003e2\u003c/sub\u003e sample may be caused by the influence of the nitro group, which has a negative mesomeric effect. It is likely that the shift in electron density along the p-bond system increases the mobility of hydrogen in the phenolic hydroxyl, which causes inhibition of DPPH. In the case of the sulfonic group, which also has a negative mesomeric effect, a similar effect is not observed due to the replacement of hydrogen in the sulfonic group with sodium, which greatly reduces the acceptor properties of the substituent.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eA method for modifying \u003cem\u003ePopulus tremula\u003c/em\u003e aspen ethanol lignin via azo coupling with diazonium salts based on p-nitroaniline and sulfanilic acid was developed. These reactions were studied also for sulfated ethanol lignin. The novel synthesized polymers were examined by FTIR and NMR spectroscopy, which confirmed their successful functionalization. The p-nitroaniline modification does not make ethanol lignin water-soluble. The introduction of azobenzenegroups with a sulfonic acid group improves the water solubility of the polymer, as does sulfation. It was established that the modification via the azo coupling reaction increases the lignin molecular weights. The study of the photosensitive properties showed that the synthesized azo derivatives exhibit the photoisomerization ability, but it depends on the properties of lignin and the nature of the azo component. The modified lignins, especially p-nitroaniline based, are proven to be antioxidants.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThis study was carried out in part within the State assignment of the Ministry of Science and Higher Education of the Russian Federation for the Institute of Chemistry and Chemical Technology, Siberian Branch of the Russian Academy of Sciences, project no. FWES-2021-0012.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThis study was carried out using the equipment of the Krasnoyarsk Regional Centre for Collective Use, Krasnoyarsk Scientific Center, Siberian Branch of the Russian Academy of Sciences.\u003c/p\u003e\n\u003ch2\u003eCRediT authorship contribution statement\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eViktor A. Golubkov:\u0026nbsp;\u003c/strong\u003eConceptualization,\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eVisualization, Validation, Writing - Original Draft;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eValentina S. Borovkova:\u0026nbsp;\u003c/strong\u003eMethodology, Investigation, Writing - Original Draft;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaxim A. Lutoshkin:\u0026nbsp;\u003c/strong\u003eInvestigation, Writing - Original Draft;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNikolay A. Zos\u0026rsquo;ko:\u0026nbsp;\u003c/strong\u003eResources, Software;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNatalya Yu. Vasilieva:\u0026nbsp;\u003c/strong\u003eMethodology, Writing - Review \u0026amp; Editing,\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eSupervision.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYuriy N. Malyar:\u0026nbsp;\u003c/strong\u003eWriting - Review \u0026amp; Editing,\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eSupervision, Conceptualization, Project administration, Funding acquisition.\u003c/p\u003e\n\u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAbu-Omar MM, Barta K, Beckham GT, Luterbacher JS, Ralph J, Rinaldi R, Rom\u0026aacute;n-Leshkov Y, Samec JS, Sels BF and Wang F (2021) Guidelines for performing lignin-first biorefining Energy \u0026amp; Environmental Science 14:262-292. https://doi.org/10.1039/D0EE02870C\u003c/li\u003e\n\u003cli\u003eAgo M, Tardy BL, Wang L, Guo J, Khakalo A and Rojas OJ (2017) Supramolecular assemblies of lignin into nano-and microparticles MRS Bulletin 42:371-378. https://doi.org/10.1557/mrs.2017.88\u003c/li\u003e\n\u003cli\u003eAlsantali RI, Raja QA, Alzahrani AYA, Sadiq A, Naeem N, Mughal EU, Al-Rooqi MM, El Guesmi N, Moussa Z and Ahmed SA (2022) Miscellaneous azo dyes: a comprehensive review on recent advancements in biological and industrial applications Dyes and Pigments 199:110050. https://doi.org/10.1016/j.dyepig.2021.110050\u003c/li\u003e\n\u003cli\u003eAlzagameem A, Khaldi-Hansen BE, Buchner D, Larkins M, Kamm B, Witzleben S and Schulze M (2018) Lignocellulosic Biomass as Source for Lignin-Based Environmentally Benign Antioxidants Molecules 23:2664. https://doi.org/10.3390/molecules23102664\u003c/li\u003e\n\u003cli\u003eBandara HM and Burdette SC (2012) Photoisomerization in different classes of azobenzene Chem Soc Rev 41:1809-1825. https://doi.org/10.1039/c1cs15179g\u003c/li\u003e\n\u003cli\u003eBanfi D and Patiny L (2008) www.nmrdb.org: Resurrecting and Processing NMR Spectra On-line Chimia 62:280-280. https://doi.org/10.2533/chimia.2008.280\u003c/li\u003e\n\u003cli\u003eBarapatre A, Meena AS, Mekala S, Das A and Jha H (2016) In vitro evaluation of antioxidant and cytotoxic activities of lignin fractions extracted from Acacia nilotica Int J Biol Macromol 86:443-453. https://doi.org/10.1016/j.ijbiomac.2016.01.109\u003c/li\u003e\n\u003cli\u003eBorovkova VS, Malyar YN, Vasilieva NY, Skripnikov AM, Ionin VA, Sychev VV, Golubkov VA and Taran OP (2023) New Azo Derivatives of Ethanol Lignin: Synthesis, Structure, and Photosensitive Properties Materials (Basel) 16:1525. https://doi.org/10.3390/ma16041525\u003c/li\u003e\n\u003cli\u003eBorovkova VS, Malyar YN, Sudakova IG, Chudina AI, Zimonin DV, Skripnikov AM, Miroshnikova AV, Ionin VA, Kazachenko AS, Sychev VV, Ponomarev IS and Issaoui N (2022) Composition and Structure of Aspen (Populus tremula) Hemicelluloses Obtained by Oxidative Delignification Polymers (Basel) 14:4521. https://doi.org/10.3390/polym14214521\u003c/li\u003e\n\u003cli\u003eDu B, Li W, Bai Y, Pan Z, Wang Q, Wang X, Lv G, Ding H and Zhou J (2022) Effect of CO2 Concentration on Improving Yield and Antioxidant Activity of Lignin from Corn Cobs BioEnergy Research 16:954-966. https://doi.org/10.1007/s12155-022-10490-6\u003c/li\u003e\n\u003cli\u003eEraghi Kazzaz A, Hosseinpour Feizi Z and Fatehi P (2019) Grafting strategies for hydroxy groups of lignin for producing materials Green Chemistry 21:5714-5752. https://doi.org/10.1039/c9gc02598g\u003c/li\u003e\n\u003cli\u003eFetisova OY, Mikova NM and Chesnokov NV (2019) A Kinetic Study of the Thermal Degradation of Fir and Aspen Ethanol Lignins Kinetics and Catalysis 60:273-280. https://doi.org/10.1134/s0023158419030054\u003c/li\u003e\n\u003cli\u003eFrolova TS, Cheshkova AV and Loginova VA (2020) Preparation of Dyed Azolignins on Linen Cottonin Modified by Enzymes Izvestiya Vysshikh Uchebnykh Zavedenii Khimiya Khimicheskaya Tekhnologiya 63:64-70. https://doi.org/10.6060/ivkkt.20206302.5970\u003c/li\u003e\n\u003cli\u003eGogotov AF and Luzhanskaya IM (2005) Azoderivatives of lignin. 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Organosolv methods allow the use of underutilized lignin. The synthesis of modified polymers by azo coupling with the use of aspen (\u003cem\u003ePopulus tremula\u003c/em\u003e) ethanol lignin and its sulfated modification is studied. The success of the synthesis has been proven and the features of the structure and properties of the synthesized samples were studied by the physicochemical techniques, including Fourier-transform infrared and nuclear magnetic resonance spectroscopy, gel permeation chromatography and thermogravimetric analysis. It is shown that the new azopolymers have the ability to photoisomerize, which opens up prospects for their high-tech applications. The modified lignins are proven to be bioactive antioxidants.\u003c/p\u003e","manuscriptTitle":"Modification of Aspen Wood Ethanol Lignin via Azo Coupling; Promising Polymers from Renewable Plant Biomass","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-12 08:55:55","doi":"10.21203/rs.3.rs-4235328/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-07-08T10:15:14+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-05-17T12:51:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"125b2b9a-264a-492d-bfd1-c068ff63e38e","date":"2024-04-11T16:28:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"cc0e1b98-eb44-40df-ad23-3342b7f5b91d_SNPRID","date":"2024-04-11T15:45:02+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-04-11T15:37:29+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-04-11T15:15:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-04-09T07:27:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Wood Science and Technology","date":"2024-04-08T09:26:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"wood-science-and-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wsat","sideBox":"Learn more about [Wood Science and Technology](http://link.springer.com/journal/226)","snPcode":"226","submissionUrl":"https://submission.nature.com/new-submission/226/3","title":"Wood Science and Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b9643b3a-cd0e-4eac-a200-ef0b11eb4a95","owner":[],"postedDate":"April 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-08-26T16:04:02+00:00","versionOfRecord":{"articleIdentity":"rs-4235328","link":"https://doi.org/10.1007/s00226-024-01590-x","journal":{"identity":"wood-science-and-technology","isVorOnly":false,"title":"Wood Science and Technology"},"publishedOn":"2024-08-20 15:57:48","publishedOnDateReadable":"August 20th, 2024"},"versionCreatedAt":"2024-04-12 08:55:55","video":"","vorDoi":"10.1007/s00226-024-01590-x","vorDoiUrl":"https://doi.org/10.1007/s00226-024-01590-x","workflowStages":[]},"version":"v1","identity":"rs-4235328","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4235328","identity":"rs-4235328","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}