Spatial Control of Curing Kinetics in Thiol-Ene-Systems through Antagonistic Photoreactions

Spatial Control of Curing Kinetics in Thiol-Ene-Systems through Antagonistic Photoreactions | 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 Article Spatial Control of Curing Kinetics in Thiol-Ene-Systems through Antagonistic Photoreactions Thomas Griesser, Rita Johanna Höller, Dmitry Sivun, Stefanie Monika Müller, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5521813/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract The use of two wavelengths to activate different photoreactions in a resin system has recently attracted much attention in the scientific community. Here, wavelength orthogonal photochemistry was used to spatially control the curing kinetics of the thiol-ene photopolymerization reaction. In the investigated resin system, radical curing is activated by a type II photoinitiator at 450 nm, while light at 365 nm is used to photorelease a base, resulting in an inhibition of the curing reaction. The antagonistic nature of these photoreactions is demonstrated in laser writing and grey scale patterning experiments. The controlled inhibition and retardation of the thiol-ene curing reaction in a spatial manner paves the way for numerous applications in 3D printing or sub-µm photolithography. Physical sciences/Materials science/Soft materials/Polymers Physical sciences/Chemistry/Photochemistry antagonistic photoreactions thiol-ene dual-wavelength photochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Full Text Light as a trigger for network formation has been widely used over the past decades due to its powerful advantages over other stimuli. [1] Using photochemical initiation, spatial and temporal control is readily achievable, explaining the increasing interest in photosensitive reactive systems. [2,3] Additional control over light-triggered reactions is provided through the concept of wavelength-selective activation, in which the responses exhibited by different chromophores can be influenced by the wavelength applied. As highlighted by various authors, [4,5] the integration of multiple wavelength-responsive reactions within a single material is key to unlocking the full potential of light-triggered reactions in photochemistry. In a recent article, I. Irshadeen and coworkers defined three types of two-color interactions – namely, synergistic, orthogonal, and antagonistic systems. Synergistic means that simultaneous illumination with at least two different wavelengths is required to activate the photoreaction. When photochemical chromophores behave orthogonally, they lead to different reactions that do not compete with each other. In antagonistic interactions, one wavelength initiates a reaction while the other wavelength quenches the reaction initiated by the first wavelength. [6,7] The principle of antagonistic photochemistry has already been applied to Stimulated Emission Depletion (STED)-inspired lithography, allowing photopolymerization to be performed with single-digit nanometer spatial resolution. [8,9] In addition, the group of Scott et al. reported on the use of antagonistic photochemistry to spatially suppress (meth)acrylate polymerization, enabling volumetric stereolithography. [10] However, to the best of our knowledge, this type of strategy has not been applied to precisely control reaction kinetics of radical mediated thiol-ene photopolymerization. Thiol-ene chemistry has been the subject of extensive research for several years, which can be explained by the excellent biocompatibility and toughness of these networks compared to cured (meth)acrylates. [11] Whether Scott's concept of selective termination of photopolymerization reactions by radical recombination can be applied to thiol-based chemistry is more than questionable due to the complex chain transfer reactions in such systems. [10] In previous work, Bowman et al. showed that amines exert a retarding effect on the radical-mediated thiol-ene reaction. [12] Generally, thiol-ene reactions are deemed click-reactions, with high yields among other qualities. [13] In order to reach sufficient inhibition of the thiol-ene curing reaction, basic amines need to be introduced to the resin system. Those amines lead to deprotonation of the thiol, resulting in thiolate anions. Subsequently, a metastable disulfide anion species is formed as a result of interaction between thiyl radicals and thiolate anions. Consequently, the thiol-ene coupling reaction is inhibited due to the removal of the reacting thiyl radicals from the reaction. This effect occurs provided that the pK a of the thiol is lower than the pK a of the conjugated acid of the amine. [12] Inspired by this study, in the current work the concept was advanced by using a photobase generator to enable the spatial release of an amine base. To realize the antagonistic control of the reaction kinetics in the used thiol-ene resin, wavelength orthogonality of the initiation and quenching reaction is required ( Figure 1 ). Camphorquinone (CQ) was chosen as the radical photoinitiator (PI) because of its particularly low absorption at 365 nm and its ability to initiate polymerization in the 450 nm region. Due to its unique UV-Vis absorption characteristics ( Figure 2 b ), CQ is often used in dual wavelength reactive systems. [14] Recently, Hu et al. demonstrated rapid continuous 3D printing of acrylates using CQ as a PI with blue light while UV-activated butyl nitrite inhibits the radical chain growth locally. [15] CQ is a type II initiator that abstracts hydrogen from donor molecules (e.g., amines or thiols) during irradiation via a bimolecular mechanism. [16] Thiol monomers based on esters of mercaptopropionic acid, as the monomer used in the system at hand, show a pK a value in the range of 10. [17] Considering the pK a of aliphatic thiols as well as the UV absorption characteristics of CQ, 2-(2-nitrophenyl)-propyloxycarbonyl-1,1,3,3-tetramethylguanidine (NPPOC-TMG) was chosen as a strong latent amine base. It was synthesized according to a literature protocol and provides a pK a of 13.6 in its unprotected state. [18,19] The wavelength dependent response of the CQ and NPPOC-TMG was investigated in a stochiometric mixture of the monomers pentaerythritol-tetrakis(3-mercaptopropionat) PETMP and triallyl-triazine-2,4,6(1H,3H,5H)-trione (TATO, see Figure 2 a ). Importantly, TATO shows hardly any homopolymerization and, due to the high electron density of the carbon double bond, is not susceptible to thiol anions making it inert to base catalyzed thiol-Michael reactions. [20–22] Figure 3 a depicts the wavelength dependent curing kinetics of the resin as obtained by FTIR spectroscopy. In the first step, visible light irradiation at 450 nm initiates the radical curing process, which can be followed by the decrease of the C=C double bond (3080 cm -1 ) and thiol (2570 cm -1 ) absorption bands (the related FTIR spectra are provided in Figure S3 and Figure S4 ). Under these conditions, the photobase generator remains unaffected. As soon as the formulation is additionally irradiated with UV light at 365 nm (to unblock NPPOC-TMG and to release TMG as strongly basic amine), the curing speed of the thiol-ene reaction decreases significantly to almost zero. This effect is due to the aforementioned consumption of the reacting thiyl radicals through the formation of a metastable disulfide radical anion species. [12] The formation of TMG can be monitored by the decrease of the absorption band of the C-N bond at 1530 cm -1 , which is broken during the photocleavage. [23,24] The data clearly demonstrate both the wavelength orthogonal reaction response of the CQ and NPPOC-TMG as well as their antagonistic behavior in the PETMP-TATO system. Furthermore, Figure 3 b shows that base-induced retardation of the reaction can be triggered at any stage of the curing process. It is worth noting that the introduction of these amines via cleavage of the photobase generator inevitably initiates the inhibition process, whether before or during activation of the radical photoinitiator. In addition to curing kinetics, the influence of the photogenerated base on network formation was investigated by photorheology. Figure 2 c shows the loss and storage moduli over time, with the gel point defined as the intersection of the two moduli. While the gel point is reached after an illumination dose of 140,4 Jcm -2 at 450 nm, the resins pre-exposed to light of 365 nm show no gelation, confirming the inhibitory effect of the formed amine on network formation. The major advantage of this antagonistic approach is that the reactivity of the thiol-ene formulation can be tuned in a spatially resolved manner. To demonstrate this, a thin resin layer on a glass substrate was illuminated with 365 nm (7.84 Jcm -2 ) through a cherry-shaped photomask. After removing the photomask, the irradiation was switched to 450 nm, illuminating the entire layer with 146.40 Jcm -2 . The solvent treatment showed that the area not previously covered by the photomask was completely soluble, indicating that no polymer network had formed in this area. Figure 3 c shows the cured layer after washing with isopropanol.Going a step beyond low-resolution mask lithography, the antagonistic nature of the two photoreactions has also been demonstrated in laser writing experiments. In this approach, two lasers were used – a curing laser with an emission maximum at 488 nm ( Figure S5 ) and a monochromatic 355 nm laser for inhibition and. Both lasers were focused on the same spot but activated independently. Figure 4 a shows successful writing with a minimum feature size of 1.4 µm (line width) using only the curing laser. As a control, a similar writing experiment was performed with 355 nm, but as expected, no gelation occurred. Figure 4 b shows the result of laser writing with the curing laser (488 nm) always on (displayed as blue lines in figure) and the inhibition laser (355 nm, pink line in figure) turned on for small and arbitrary time periods. Simultaneous illumination with both lasers stopped curing completely, in a manner similar to STED lithography [25] , resulting in gaps in the written lines, confirming the antagonistic behavior of both photoreactions even at the microscale. In addition to spatial control, this approach also enables precise adjustment of the reaction kinetics by the amount of photogenerated base. Since the introduction of the base into the resin is triggered photochemically, the intensity of the inhibition can be varied by changing the illumination dose. This concept was applied to gray scale experiments as shown in Figure 4 c . In this approach, 3 rectangles (4 mm x 10 mm) were inscribed with 365 nm with increasing dose (288 mJcm -2 to 1440 mJcm -2 ) within defined resin layer and then exposed to 460 nm with a dose of 1050 Jcm -2 over the complete area. Due to the differences in the inhibition, i.e. reaction kinetics, a structure with different heights was obtained. In summary, an antagonistically behaving photoactivated system within a thiol-ene network was successfully demonstrated. A dual wavelength approach was realized to locally inhibit the thiol-ene curing reaction. While curing was initiated at 450 nm, the reaction can be stopped at 365 nm. The photochemically induced inhibition on the macroscopic scale was shown by contact lithography, while laser writing experiments demonstrated the antagonistic character of the resin on the micrometer scale. In addition to spatial control, this approach also offers the possibility of tuning the kinetics of the thiol-ene photoreaction, which was demonstrated in gray-scale experiments. This approach holds great potential for advanced nanolithography using laser setups similar to those used in STED-inspired lithography or for volumetric stereolithography approaches for thiol-ene based resins. Declarations Acknowledgements Part of the research was carried out within the COMET-Module project” Repairtecture” (project-no.: 904927) at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Federal Ministry of Labour and Economy. Funding was provided by the Austrian Government and the State Governments of Styria and Upper Austria. We would like to thank the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Austrian Research Promotion Agency (FFG) for funding the “3DFit4Wear” project as part of the Production of the Future program line (project no. 891254). Supporting Information The authors have cited additional references within the Supporting Information. [26] References Y. Yagci, S. Jockusch, N. J. Turro, Macromolecules 2010 , 43 , 6245–6260. E. Blasco, M. Wegener, C. Barner-Kowollik, Adv. Mater. 2017 , 29 , DOI 10.1002/adma.201604005. S. Aubert, M. Bezagu, A. C. Spivey, S. Arseniyadis, Nat. Rev. Chem. 2019 , 3 , 706–722. P. Lu, D. Ahn, R. Yunis, L. Delafresnaye, N. Corrigan, C. Boyer, C. Barner-Kowollik, Z. A. Page, Matter 2021 , 4 , 2172–2229. X. Zhang, W. Xi, S. Huang, K. Long, C. N. Bowman, Macromolecules 2017 , 50 , 5652–5660. I. M. Irshadeen, S. L. Walden, M. Wegener, V. X. Truong, H. Frisch, J. P. Blinco, C. Barner-Kowollik, J. Am. Chem. Soc. 2021 , 143 , 21113–21126. J. Hobich, E. Blasco, M. Wegener, H. Mutlu, C. Barner-Kowollik, Macromol. Chem. Phys. 2023 , 224 , 1–10. J. Fischer, G. Von Freymann, M. Wegener, Adv. Mater. 2010 , 22 , 3578–3582. P. Müller, R. Müller, L. Hammer, C. Barner-Kowollik, M. Wegener, E. Blasco, Chem. Mater. 2019 , 1966–1972. M. P. De Beer, H. L. Van Der Laan, M. A. Cole, R. J. Whelan, M. A. Burns, T. F. Scott, Sci. Adv. 2019 , 5 , 1–8. C. E. Hoyle, T. Y. Lee, T. Roper, J. Polym. Sci. Part A Polym. Chem. 2004 , 42 , 5301–5338. D. M. Love, K. Kim, J. T. Goodrich, B. D. Fairbanks, B. T. Worrell, M. P. Stoykovich, C. B. Musgrave, C. N. Bowman, J. Org. Chem. 2018 , 83 , 2912–2919. C. E. Hoyle, C. N. Bowman, Angew. Chemie - Int. Ed. 2010 , 49 , 1540–1573. T. F. Scott, B. A. Kowalski, A. C. Sullivan, C. N. Bowman, R. R. McLeod, Science (80-. ). 2009 , 324 , 913–917. M. Hu, H. Cheng, Y. Feng, 3D Print. Addit. Manuf. 2024 , 11 , 476–484. A. A. Pérez-Mondragón, C. E. Cuevas-Suárez, J. A. González-López, N. Trejo-Carbajal, A. M. Herrera-González, J. Photochem. Photobiol. A Chem. 2020 , 403 , DOI 10.1016/j.jphotochem.2020.112844. C. F. Bernasconi, R. B. Killion, J. Am. Chem. Soc. 1988 , 110 , 7506–7512. S. J. Angyal, W. K. Warburton, J. Chem. Soc. 1951 , 2492–2494. X. Zhang, W. Xi, G. Gao, X. Wang, J. W. Stansbury, C. N. Bowman, ACS Macro Lett. 2018 , 7 , 852–857. P. Shen, S. Z. Moghaddam, Q. Huang, A. E. Daugaard, Mater. Today Commun. 2019 , 21 , 100657. H. Lu, J. A. Carioscia, J. W. Stansbury, C. N. Bowman, Dent. Mater. 2005 , 21 , 1129–1136. K. Ganabady, N. C. Negrini, J. C. Scherba, B. M. Nitschke, M. R. Alexander, K. H. Vining, M. A. Grunlan, D. J. Mooney, A. D. Celiz, ACS Appl. Mater. Interfaces 2023 , 15 , 50908–50915. G. Socrates, Infrared and Raman Characteristic Group Frequencies , John Wiley & Sons, Ltd, Chichester, 2001 . F. S. Parker, Applications of Infrared Spectroscopy in Biochemistry, Biology, and Medicine , Springer New York, NY, New York, 1971 . R. Wollhofen, J. Katzmann, C. Hrelescu, J. Jacak, T. A. Klar, Opt. Express 2013 , 21 , 10831. X. Zhang, W. Xi, G. Gao, X. Wang, J. W. Stansbury, C. N. Bowman, ACS Macro Lett. 2018 , 7 , 852–857. Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformationManuscript.docx Supporting Information Cite Share Download PDF Status: Published Journal Publication published 26 Sep, 2025 Read the published version in Nature Communications → Version 1 posted 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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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5521813","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":392375578,"identity":"ff52d7b9-e0a5-4605-9753-3418f1cf3ecc","order_by":0,"name":"Thomas Griesser","email":"data:image/png;base64,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","orcid":"","institution":"Montanuniversität Leoben","correspondingAuthor":true,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Griesser","suffix":""},{"id":392375579,"identity":"eb57e8f7-9d1c-47a9-b415-a32009d45266","order_by":1,"name":"Rita Johanna Höller","email":"","orcid":"","institution":"Montanuniversität Leoben","correspondingAuthor":false,"prefix":"","firstName":"Rita","middleName":"Johanna","lastName":"Höller","suffix":""},{"id":392375580,"identity":"ea408af6-ae86-4982-b2d7-af2f16362270","order_by":2,"name":"Dmitry Sivun","email":"","orcid":"https://orcid.org/0000-0002-5531-1354","institution":"University of Applied Sciences Upper Austria","correspondingAuthor":false,"prefix":"","firstName":"Dmitry","middleName":"","lastName":"Sivun","suffix":""},{"id":392375581,"identity":"d60c43de-7f9f-4bf2-8f23-06985d8bc2e9","order_by":3,"name":"Stefanie Monika Müller","email":"","orcid":"","institution":"Montanuniversität Leoben","correspondingAuthor":false,"prefix":"","firstName":"Stefanie","middleName":"Monika","lastName":"Müller","suffix":""},{"id":392375582,"identity":"282d6cef-f6f8-4a0b-b024-8273290a9afc","order_by":4,"name":"Jaroslaw Jacak","email":"","orcid":"https://orcid.org/0000-0002-4989-1276","institution":"University of Applied Sicences Upper Austria","correspondingAuthor":false,"prefix":"","firstName":"Jaroslaw","middleName":"","lastName":"Jacak","suffix":""},{"id":392375583,"identity":"1bca9467-b0c4-44ed-a9ba-eb804ad30b9c","order_by":5,"name":"Sandra Schlögl","email":"","orcid":"https://orcid.org/0000-0002-2840-9700","institution":"Polymer Competence Center Leoben GmbH","correspondingAuthor":false,"prefix":"","firstName":"Sandra","middleName":"","lastName":"Schlögl","suffix":""}],"badges":[],"createdAt":"2024-11-25 16:12:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5521813/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5521813/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-63407-0","type":"published","date":"2025-09-26T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":71950776,"identity":"dfec239d-6e64-41e2-8e5c-165e05b67ea0","added_by":"auto","created_at":"2024-12-20 04:24:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":159211,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the curing and inhibition mechanism in the presented thiol-ene resin system\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/bdc84d26c6bead25b905424a.png"},{"id":71951219,"identity":"aa93a7da-5653-41f7-a2eb-6a626ecf9aa6","added_by":"auto","created_at":"2024-12-20 04:32:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":78043,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Resin formulation with photoinitiator and photobase generator. (b) Absorption spectra of CQ and NPPOC-TMG versus the emission spectra of the light sources used. (c) Comparison of the photorheological behavior of the resin with and without prior photobase release.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/164344148fb6d3492e69ca3e.png"},{"id":71950781,"identity":"e9ad62fa-87bf-4a6a-9b21-6309d84f4b68","added_by":"auto","created_at":"2024-12-20 04:24:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":142357,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Wavelength dependent behavior of the different resin components, the color of the background corresponds to the wavelength used (blue - 450 nm, pink - 365 nm). (b) Stopping curing at different times during the curing process. (c) Schematic masking experiment setup and cured masked layer after washing with solvent.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/797b83bd5cd3abc0187a0f15.png"},{"id":71950778,"identity":"2f978b75-b8c0-4b69-8296-744214d99f66","added_by":"auto","created_at":"2024-12-20 04:24:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":197289,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Constant illumination with curing laser while switching on and off the inhibition laser. Colored lines correspond to the illuminating lasers (blue – 488 nm, pink – 365 nm). (b) Laser writing with 488 and 355 respectively. (c) Greyscale experiment – height profile of the obtained structure and depth of the formed valleys over illumination dose\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/c08cce4e08811c00b3806bb8.png"},{"id":92305107,"identity":"bb128335-e1e3-44c9-9f76-05078bcad70f","added_by":"auto","created_at":"2025-09-27 07:09:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1020766,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/bc5f4cc3-858b-4aa9-80f0-3c9bd41ed539.pdf"},{"id":71950777,"identity":"47d175b6-b25f-4def-89f6-28aa9d284b05","added_by":"auto","created_at":"2024-12-20 04:24:55","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2030384,"visible":true,"origin":"","legend":"Supporting Information","description":"","filename":"SupportingInformationManuscript.docx","url":"https://assets-eu.researchsquare.com/files/rs-5521813/v1/2d8708b9a167fbc1beaec3a6.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Spatial Control of Curing Kinetics in Thiol-Ene-Systems through\r\nAntagonistic Photoreactions","fulltext":[{"header":"Full Text","content":"\u003cp\u003eLight as a trigger for network formation has been widely used over the past decades due to its powerful advantages over other stimuli.\u003csup\u003e[1]\u003c/sup\u003e Using photochemical initiation, spatial and temporal control is readily achievable, explaining the increasing interest in photosensitive reactive systems.\u003csup\u003e[2,3]\u003c/sup\u003e Additional control over light-triggered reactions is provided through the concept of wavelength-selective activation, in which the responses exhibited by different chromophores can be influenced by the wavelength applied. As highlighted by various authors,\u003csup\u003e[4,5]\u003c/sup\u003e the integration of multiple wavelength-responsive reactions within a single material is key to unlocking the full potential of light-triggered reactions in photochemistry. In a recent article, I. Irshadeen and coworkers defined three types of two-color interactions \u0026ndash; namely, synergistic, orthogonal, and antagonistic systems. Synergistic means that simultaneous illumination with at least two different wavelengths is required to activate the photoreaction. When photochemical chromophores behave orthogonally, they lead to different reactions that do not compete with each other. In antagonistic interactions, one wavelength initiates a reaction while the other wavelength quenches the reaction initiated by the first wavelength.\u003csup\u003e[6,7]\u003c/sup\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe principle of antagonistic photochemistry has already been applied to Stimulated Emission Depletion (STED)-inspired lithography, allowing photopolymerization to be performed with single-digit nanometer spatial resolution.\u003csup\u003e[8,9]\u003c/sup\u003e In addition, the group of Scott et al. reported on the use of antagonistic photochemistry to spatially suppress (meth)acrylate polymerization, enabling volumetric stereolithography.\u003csup\u003e[10]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eHowever, to the best of our knowledge, this type of strategy has not been applied to precisely control reaction kinetics of radical mediated thiol-ene photopolymerization.\u003c/p\u003e\n\u003cp\u003eThiol-ene chemistry has been the subject of extensive research for several years, which can be explained by the excellent biocompatibility and toughness of these networks compared to cured (meth)acrylates.\u003csup\u003e[11]\u003c/sup\u003e Whether Scott\u0026apos;s concept of selective termination of photopolymerization reactions by radical recombination can be applied to thiol-based chemistry is more than questionable due to the complex chain transfer reactions in such systems.\u003csup\u003e[10]\u003c/sup\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn previous work, Bowman et al. showed that amines exert a retarding effect on the radical-mediated thiol-ene reaction.\u003csup\u003e[12]\u003c/sup\u003e Generally, thiol-ene reactions are deemed click-reactions, with high yields among other qualities.\u003csup\u003e[13]\u003c/sup\u003e In order to reach sufficient inhibition of the thiol-ene curing reaction, basic amines need to be introduced to the resin system. Those amines lead to deprotonation of the thiol, resulting in thiolate anions. Subsequently, a metastable disulfide anion species is formed as a result of interaction between thiyl radicals and thiolate anions. Consequently, the thiol-ene coupling reaction is inhibited due to the removal of the reacting thiyl radicals from the reaction. This effect occurs provided that the pK\u003csub\u003ea\u003c/sub\u003e of the thiol is lower than the pK\u003csub\u003ea\u003c/sub\u003e of the conjugated acid of the amine.\u003csup\u003e[12]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eInspired by this study, in the current work the concept was advanced by using a photobase generator to enable the spatial release of an amine base. To realize the antagonistic control of the reaction kinetics in the used thiol-ene resin, wavelength orthogonality of the initiation and quenching reaction is required (\u003cstrong\u003eFigure 1\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eCamphorquinone (CQ) was chosen as the radical photoinitiator (PI) because of its particularly low absorption at 365\u0026nbsp;nm and its ability to initiate polymerization in the 450\u0026nbsp;nm region. Due to its unique UV-Vis absorption characteristics (\u003cstrong\u003eFigure 2\u003c/strong\u003e\u003cstrong\u003eb\u003c/strong\u003e), CQ is often used in dual wavelength reactive systems.\u003csup\u003e[14]\u003c/sup\u003e Recently, Hu et al. demonstrated rapid continuous 3D printing of acrylates using CQ as a PI with blue light while UV-activated butyl nitrite inhibits the radical chain growth locally.\u003csup\u003e[15]\u003c/sup\u003e CQ is a type II initiator that abstracts hydrogen from donor molecules (e.g., amines or thiols) during irradiation via a bimolecular mechanism.\u003csup\u003e[16]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThiol monomers based on esters of mercaptopropionic acid, as the monomer used in the system at hand, show a pK\u003csub\u003ea\u003c/sub\u003e value in the range of 10.\u003csup\u003e[17]\u003c/sup\u003e Considering the pK\u003csub\u003ea\u003c/sub\u003e of aliphatic thiols as well as the UV absorption characteristics of CQ, 2-(2-nitrophenyl)-propyloxycarbonyl-1,1,3,3-tetramethylguanidine (NPPOC-TMG) was chosen as a strong latent amine base. It was synthesized according to a literature protocol and provides a pK\u003csub\u003ea\u003c/sub\u003e of 13.6 in its unprotected state.\u003csup\u003e[18,19]\u003c/sup\u003e The wavelength dependent response of the CQ and NPPOC-TMG was investigated in a stochiometric mixture of the monomers pentaerythritol-tetrakis(3-mercaptopropionat) PETMP and triallyl-triazine-2,4,6(1H,3H,5H)-trione (TATO, see \u003cstrong\u003eFigure 2\u003c/strong\u003e\u003cstrong\u003ea\u003c/strong\u003e). Importantly, TATO shows hardly any homopolymerization and, due to the high electron density of the carbon double bond, is not susceptible to thiol anions making it inert to base catalyzed thiol-Michael reactions.\u003csup\u003e[20\u0026ndash;22]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 3\u003c/strong\u003e\u003cstrong\u003ea\u003c/strong\u003e depicts the wavelength dependent curing kinetics of the resin as obtained by FTIR spectroscopy. In the first step, visible light irradiation at 450 nm initiates the radical curing process, which can be followed by the decrease of the C=C double bond (3080 cm\u003csup\u003e-1\u003c/sup\u003e) and thiol (2570 cm\u003csup\u003e-1\u003c/sup\u003e) absorption bands (the related FTIR spectra are provided in \u003cstrong\u003eFigure S3\u003c/strong\u003e and \u003cstrong\u003eFigure\u0026nbsp;S4\u003c/strong\u003e). Under these conditions, the photobase generator remains unaffected. As soon as the formulation is additionally irradiated with UV light at 365 nm (to unblock NPPOC-TMG and to release TMG as strongly basic amine), the curing speed of the thiol-ene reaction decreases significantly to almost zero. This effect is due to the aforementioned consumption of the reacting thiyl radicals through the formation of a metastable disulfide radical anion species.\u003csup\u003e[12]\u003c/sup\u003e The formation of TMG can be monitored by the decrease of the absorption band of the C-N bond at 1530 cm\u003csup\u003e-1\u003c/sup\u003e, which is broken during the photocleavage.\u003csup\u003e[23,24]\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe data clearly demonstrate both the wavelength orthogonal reaction response of the CQ and NPPOC-TMG as well as their antagonistic behavior in the PETMP-TATO system.\u003c/p\u003e\n\u003cp\u003eFurthermore, \u003cstrong\u003eFigure 3\u003c/strong\u003e\u003cstrong\u003eb\u003c/strong\u003e shows that base-induced retardation of the reaction can be triggered at any stage of the curing process. It is worth noting that the introduction of these amines via cleavage of the photobase generator inevitably initiates the inhibition process, whether before or during activation of the radical photoinitiator.\u003c/p\u003e\n\u003cp\u003eIn addition to curing kinetics, the influence of the photogenerated base on network formation was investigated by photorheology. \u003cstrong\u003eFigure 2\u003c/strong\u003e\u003cstrong\u003ec\u003c/strong\u003e shows the loss and storage moduli over time, with the gel point defined as the intersection of the two moduli. While the gel point is reached after an illumination dose of 140,4 Jcm\u003csup\u003e-2\u003c/sup\u003e at 450 nm, the resins pre-exposed to light of 365 nm show no gelation, confirming the inhibitory effect of the formed amine on network formation. The major advantage of this antagonistic approach is that the reactivity of the thiol-ene formulation can be tuned in a spatially resolved manner. To demonstrate this, a thin resin layer on a glass substrate was illuminated with 365 nm (7.84 Jcm\u003csup\u003e-2\u003c/sup\u003e) through\u0026nbsp;a cherry-shaped photomask. After removing the photomask, the irradiation was switched to 450\u0026nbsp;nm, illuminating the entire layer with 146.40\u0026nbsp;Jcm\u003csup\u003e-2\u003c/sup\u003e. The solvent treatment showed that the area not previously covered by the photomask was completely soluble, indicating that no polymer network had formed in this area.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eFigure 3\u003c/strong\u003e\u003cstrong\u003ec\u003c/strong\u003e shows the cured layer after washing with isopropanol.Going a step beyond low-resolution mask lithography, the antagonistic nature of the two photoreactions has also been demonstrated in laser writing experiments. In this approach, two lasers were used \u0026ndash; a curing laser with an emission maximum at 488 nm (\u003cstrong\u003eFigure S5\u003c/strong\u003e) and a monochromatic 355 nm laser for inhibition and. Both lasers were focused on the same spot but activated independently. \u003cstrong\u003eFigure 4\u003c/strong\u003e\u003cstrong\u003ea\u003c/strong\u003e shows successful writing with a minimum feature size of 1.4 \u0026micro;m (line width) using only the curing laser. As a control, a similar writing experiment was performed with 355 nm, but as expected, no gelation occurred. \u003cstrong\u003eFigure 4\u003c/strong\u003e\u003cstrong\u003eb\u003c/strong\u003e shows the result of laser writing with the curing laser (488 nm) always on (displayed as blue lines in figure) and the inhibition laser (355 nm, pink line in figure) turned on for small and arbitrary time periods. Simultaneous illumination with both lasers stopped curing completely, in a manner similar to STED lithography\u003csup\u003e[25]\u003c/sup\u003e, resulting in gaps in the written lines, confirming the antagonistic behavior of both photoreactions even at the microscale.\u003c/p\u003e\n\u003cp\u003eIn addition to spatial control, this approach also enables precise adjustment of the reaction kinetics by the amount of photogenerated base. Since the introduction of the base into the resin is triggered photochemically, the intensity of the inhibition can be varied by changing the illumination dose. This concept was applied to gray scale experiments as shown in \u003cstrong\u003eFigure 4\u003c/strong\u003e\u003cstrong\u003ec\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003eIn this approach, 3 rectangles (4 mm x 10 mm) were inscribed with 365 nm with increasing dose (288 mJcm\u003csup\u003e-2\u003c/sup\u003e to 1440 mJcm\u003csup\u003e-2\u003c/sup\u003e) within defined resin layer and then exposed to 460 nm with a dose of 1050 Jcm\u003csup\u003e-2\u003c/sup\u003e over the complete area. Due to the differences in the inhibition, i.e. reaction kinetics, a structure with different heights was obtained.\u003c/p\u003e\n\u003cp\u003eIn summary, an antagonistically behaving photoactivated system within a thiol-ene network was successfully demonstrated. A dual wavelength approach was realized to locally inhibit the thiol-ene curing reaction. While curing was initiated at 450 nm, the reaction can be stopped at 365 nm. The photochemically induced inhibition on the macroscopic scale was shown by contact lithography, while laser writing experiments demonstrated the antagonistic character of the resin on the micrometer scale. In addition to spatial control, this approach also offers the possibility of tuning the kinetics of the thiol-ene photoreaction, which was demonstrated in gray-scale experiments. This approach holds great potential for advanced nanolithography using laser setups similar to those used in STED-inspired lithography or for volumetric stereolithography approaches for thiol-ene based resins.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePart of the research was carried out within the COMET-Module project\u0026rdquo; Repairtecture\u0026rdquo; (project-no.: 904927) at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Federal Ministry of Labour and Economy. Funding was provided by the Austrian Government and the State Governments of Styria and Upper Austria.\u003c/p\u003e\u003cp\u003eWe would like to thank the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Austrian Research Promotion Agency (FFG) for funding the \u0026ldquo;3DFit4Wear\u0026rdquo; project as part of the Production of the Future program line (project no. 891254).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have cited additional references within the Supporting Information.\u003csup\u003e[26]\u003c/sup\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eY. Yagci, S. Jockusch, N. J. Turro, \u003cem\u003eMacromolecules\u003c/em\u003e \u003cstrong\u003e2010\u003c/strong\u003e, \u003cem\u003e43\u003c/em\u003e, 6245\u0026ndash;6260.\u003c/li\u003e\n\u003cli\u003eE. Blasco, M. Wegener, C. 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Bowman, \u003cem\u003eACS Macro Lett.\u003c/em\u003e \u003cstrong\u003e2018\u003c/strong\u003e, \u003cem\u003e7\u003c/em\u003e, 852\u0026ndash;857.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"antagonistic photoreactions, thiol-ene, dual-wavelength, photochemistry","lastPublishedDoi":"10.21203/rs.3.rs-5521813/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5521813/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe use of two wavelengths to activate different photoreactions in a resin system has recently attracted much attention in the scientific community. Here, wavelength orthogonal photochemistry was used to spatially control the curing kinetics of the thiol-ene photopolymerization reaction. In the investigated resin system, radical curing is activated by a type II photoinitiator at 450 nm, while light at 365 nm is used to photorelease a base, resulting in an inhibition of the curing reaction. The antagonistic nature of these photoreactions is demonstrated in laser writing and grey scale patterning experiments. The controlled inhibition and retardation of the thiol-ene curing reaction in a spatial manner paves the way for numerous applications in 3D printing or sub-µm photolithography.\u003c/p\u003e","manuscriptTitle":"Spatial Control of Curing Kinetics in Thiol-Ene-Systems through\nAntagonistic Photoreactions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-20 04:24:50","doi":"10.21203/rs.3.rs-5521813/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4f39fd8f-33fc-48a7-9c91-480fce7bbe17","owner":[],"postedDate":"December 20th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":41825753,"name":"Physical sciences/Materials science/Soft materials/Polymers"},{"id":41825754,"name":"Physical sciences/Chemistry/Photochemistry"}],"tags":[],"updatedAt":"2025-09-27T07:09:15+00:00","versionOfRecord":{"articleIdentity":"rs-5521813","link":"https://doi.org/10.1038/s41467-025-63407-0","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-09-26 04:00:00","publishedOnDateReadable":"September 26th, 2025"},"versionCreatedAt":"2024-12-20 04:24:50","video":"","vorDoi":"10.1038/s41467-025-63407-0","vorDoiUrl":"https://doi.org/10.1038/s41467-025-63407-0","workflowStages":[]},"version":"v1","identity":"rs-5521813","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5521813","identity":"rs-5521813","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}