Photophysical Investigation of Green Vitamin C Adducts as Synthetic pH Indicators: Solvatochromism, Halochromism, and DFT Calculations Integrating Optical Band Gap and Electronic Transition Analysis

Photophysical Investigation of Green Vitamin C Adducts as Synthetic pH Indicators: Solvatochromism, Halochromism, and DFT Calculations Integrating Optical Band Gap and Electronic Transition Analysis | 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 Photophysical Investigation of Green Vitamin C Adducts as Synthetic pH Indicators: Solvatochromism, Halochromism, and DFT Calculations Integrating Optical Band Gap and Electronic Transition Analysis Srood Rashid This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8164734/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Apr, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract The chemistry of vitamin C adducts was revisited to investigate their pH sensitivity, photophysical properties, optical behavior, and assess their potential as eco-friendly pH indicators. In this study, a series of ascorbic acid adducts were synthesized through a straightforward, high-yield method. Their acid–base responsiveness is driven by a reversible protonation–deprotonation process between the hydrazineyl and azo groups, causing observable color changes. Analysis of substituent effects demonstrated that strong electron-withdrawing groups like nitro (–NO₂) significantly impact the electronic structure, particularly when located at the ortho or para positions of the aromatic ring. Solvatochromic studies revealed that the ortho -nitro adduct 3(b) and para -nitro adduct 3(d) exhibited positive solvatochromism, with their absorption wavelengths increasing in more polar solvents. Notably, 3(b) displayed a consistent yellow color with λₘₐₓ around 396 nm across solvents, indicating its structural stability and solvent-independent optical behavior, making it a robust and versatile pH indicator. Halochromic studies in aqueous media revealed pronounced bathochromic shifts for both 3(b) and 3(d) under alkaline conditions. The 3(b) exhibited a color change from yellow to dark purple (λₘₐₓ=534 nm), while the 3(d) transitioned from colorless (λₘₐₓ=361 nm) to dark rosewood (λₘₐₓ= 472 nm). Spectrographic analysis determined that 3(b) had a pKa of 8.99 and 3(d) a pKa of 10.68. The band gap analyses conducted under both alkaline and acidic conditions provide detailed insights into the optical properties and electronic transitions of the most promising adducts, 3(b) and 3(d) . The optical band gap energies obtained from UV–Vis spectroscopy, E g(optical) and E g(Tauc) , showed strong agreement with the electrochemically measured band gap, E g(electronic) . These findings were further corroborated by computational results from DFT (B3LYP/6-311 + G(d,p)), where the theoretically calculated band gaps, E g(DFT) , validated the experimentally derived values and provided additional information through frontier molecular orbital (FMO) analysis, indicating the presence of an efficient push–pull electronic system throughout the adduct structures. Correlation of DFT outputs with Tauc-plot analyses clarified the nature of the electronic transitions, including π→π*, n→π*, and intramolecular charge-transfer (ICT) processes, and determined whether these transitions were direct or indirect, and whether they were symmetry-allowed or forbidden. Biochemical assays further supported the proposed neutral proton-transfer mechanisms operating under both acidic and basic conditions for the promising adducts. Thermal analysis divulged thermal stability up to 210°C. Acid-based titration tests showed that 3(b) and 3(d) produced sharp and accurate endpoints comparable to the standard methyl red indicator, while 3(c) did not exhibit a reliable color transition. Given their green origin, tunable structures, and visual responsiveness, these ascorbic pH indicators hold promising candidates for sustainable applications in food coloration, cosmetics, and medical diagnostics. Biological sciences/Biochemistry Physical sciences/Chemistry pH sensor Ascorbic acid Synthetic dyes Acid-base indicators Computational chemistry Optical properties Band gap Full Text Additional Declarations No competing interests reported. Supplementary Files ESIPaperDrSrood.pdf Cite Share Download PDF Status: Published Journal Publication published 11 Apr, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 27 Jan, 2026 Reviews received at journal 21 Jan, 2026 Reviewers agreed at journal 24 Dec, 2025 Reviews received at journal 23 Dec, 2025 Reviewers agreed at journal 14 Dec, 2025 Reviewers invited by journal 11 Dec, 2025 Editor assigned by journal 11 Dec, 2025 Editor invited by journal 10 Dec, 2025 Submission checks completed at journal 24 Nov, 2025 First submitted to journal 24 Nov, 2025 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. 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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-8164734","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":560222498,"identity":"c30b0c2f-6a5c-412d-9f91-423ce331a437","order_by":0,"name":"Srood 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Analysis","fulltext":[],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":true,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":true,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"pH sensor, Ascorbic acid, Synthetic dyes, Acid-base indicators, Computational chemistry, Optical properties, Band gap","lastPublishedDoi":"10.21203/rs.3.rs-8164734/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8164734/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe chemistry of vitamin C adducts was revisited to investigate their pH sensitivity, photophysical properties, optical behavior, and assess their potential as eco-friendly pH indicators. In this study, a series of ascorbic acid adducts were synthesized through a straightforward, high-yield method. Their acid\u0026ndash;base responsiveness is driven by a reversible protonation\u0026ndash;deprotonation process between the hydrazineyl and azo groups, causing observable color changes. Analysis of substituent effects demonstrated that strong electron-withdrawing groups like nitro (\u0026ndash;NO₂) significantly impact the electronic structure, particularly when located at the \u003cem\u003eortho\u003c/em\u003e or \u003cem\u003epara\u003c/em\u003e positions of the aromatic ring. Solvatochromic studies revealed that the \u003cem\u003eortho\u003c/em\u003e-nitro adduct \u003cb\u003e3(b)\u003c/b\u003e and \u003cem\u003epara\u003c/em\u003e-nitro adduct \u003cb\u003e3(d)\u003c/b\u003e exhibited positive solvatochromism, with their absorption wavelengths increasing in more polar solvents. Notably, \u003cb\u003e3(b)\u003c/b\u003e displayed a consistent yellow color with λₘₐₓ around 396 nm across solvents, indicating its structural stability and solvent-independent optical behavior, making it a robust and versatile pH indicator. Halochromic studies in aqueous media revealed pronounced bathochromic shifts for both \u003cb\u003e3(b)\u003c/b\u003e and \u003cb\u003e3(d)\u003c/b\u003e under alkaline conditions. The \u003cb\u003e3(b)\u003c/b\u003e exhibited a color change from yellow to dark purple (λₘₐₓ=534 nm), while the \u003cb\u003e3(d)\u003c/b\u003e transitioned from colorless (λₘₐₓ=361 nm) to dark rosewood (λₘₐₓ= 472 nm). Spectrographic analysis determined that \u003cb\u003e3(b)\u003c/b\u003e had a pKa of 8.99 and \u003cb\u003e3(d)\u003c/b\u003e a pKa of 10.68.\u003c/p\u003e \u003cp\u003eThe band gap analyses conducted under both alkaline and acidic conditions provide detailed insights into the optical properties and electronic transitions of the most promising adducts, \u003cb\u003e3(b)\u003c/b\u003e and \u003cb\u003e3(d)\u003c/b\u003e. The optical band gap energies obtained from UV\u0026ndash;Vis spectroscopy, \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg(optical)\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg(Tauc)\u003c/em\u003e\u003c/sub\u003e, showed strong agreement with the electrochemically measured band gap, \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg(electronic)\u003c/em\u003e\u003c/sub\u003e. These findings were further corroborated by computational results from DFT (B3LYP/6-311\u0026thinsp;+\u0026thinsp;G(d,p)), where the theoretically calculated band gaps, \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg(DFT)\u003c/em\u003e\u003c/sub\u003e, validated the experimentally derived values and provided additional information through frontier molecular orbital (FMO) analysis, indicating the presence of an efficient push\u0026ndash;pull electronic system throughout the adduct structures. Correlation of DFT outputs with Tauc-plot analyses clarified the nature of the electronic transitions, including π\u0026rarr;π*, n\u0026rarr;π*, and intramolecular charge-transfer (ICT) processes, and determined whether these transitions were direct or indirect, and whether they were symmetry-allowed or forbidden. Biochemical assays further supported the proposed neutral proton-transfer mechanisms operating under both acidic and basic conditions for the promising adducts. Thermal analysis divulged thermal stability up to 210\u0026deg;C. Acid-based titration tests showed that \u003cb\u003e3(b)\u003c/b\u003e and \u003cb\u003e3(d)\u003c/b\u003e produced sharp and accurate endpoints comparable to the standard methyl red indicator, while \u003cb\u003e3(c)\u003c/b\u003e did not exhibit a reliable color transition. Given their green origin, tunable structures, and visual responsiveness, these ascorbic pH indicators hold promising candidates for sustainable applications in food coloration, cosmetics, and medical diagnostics.\u003c/p\u003e","manuscriptTitle":"Photophysical Investigation of Green Vitamin C Adducts as Synthetic pH Indicators: Solvatochromism, Halochromism, and DFT Calculations Integrating Optical Band Gap and Electronic Transition Analysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-17 22:53:26","doi":"10.21203/rs.3.rs-8164734/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-27T05:42:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-21T16:00:58+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"33036783049974451920372667410751000133","date":"2025-12-24T17:45:22+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-23T13:42:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"134412798439813626803942843099373269970","date":"2025-12-14T11:20:36+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-12T03:08:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-12T03:02:27+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-12-10T17:40:55+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-24T21:28:41+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-24T21:23:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"476c0b7d-f7d6-46ff-9388-6b7b96579a1c","owner":[],"postedDate":"December 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":59633116,"name":"Biological sciences/Biochemistry"},{"id":59633117,"name":"Physical sciences/Chemistry"}],"tags":[],"updatedAt":"2026-04-13T15:59:44+00:00","versionOfRecord":{"articleIdentity":"rs-8164734","link":"https://doi.org/10.1038/s41598-026-45614-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-04-11 15:57:08","publishedOnDateReadable":"April 11th, 2026"},"versionCreatedAt":"2025-12-17 22:53:26","video":"","vorDoi":"10.1038/s41598-026-45614-x","vorDoiUrl":"https://doi.org/10.1038/s41598-026-45614-x","workflowStages":[]},"version":"v1","identity":"rs-8164734","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8164734","identity":"rs-8164734","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}