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

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.