Photophysical Properties and Acid-Base Equilibria of Lumichrome Anions in Non-Aqueous Media

This study investigated the photophysical properties and acid-base equilibria of lumichrome (Lch) in acetonitrile and dimethyl sulfoxide. The methods involved adding a base, such as tetrabutylammonium acetate and phosphate, to induce deprotonation. Spectrophotometric and time-resolved fluorescence measurements were used to characterize the resulting species, with structural assignments confirmed using methylated Lch derivatives. The addition of bases to solution of Lch in acetonitrile or dimethyl sulfoxide led to ground-state deprotonation, simultaneously forming two monoanionic species: the alloxazinic and isoalloxazinic anions. The ground-state acidity constants (pKa) were found in the range of approximately 3.9–4.3, while the excited-state values (pKa*) were between 4.0–5.0. Time-resolved fluorescence revealed distinct lifetimes for each species: the neutral Lch (0.26–0.6 ns), the alloxazinic anion (2.2–3.6 ns), and the isoalloxazinic anion (7.4–8.0 ns), with the latter consistently exhibiting a significantly longer lifetime. The measured pKa values of Lch in these organic solvents are significantly lower than those in water, highlighting the critical influence of the solvent environment on Lch's acid-base behavior and photophysics. A clear mechanistic distinction is established between effect of bases and acetic acid on Lch: strong bases generate stable ground-state anions, whereas acetic acid acts solely as a catalyst for an excited-state double proton transfer (ESDPT), producing a transient excited-state isoalloxazine tautomer.本研究考察了光黄素(lumichrome, Lch)在乙腈(acetonitrile)与二甲基亚砜(dimethyl sulfoxide)中的光物理性质与酸碱平衡。实验方法为加入四丁基乙酸铵(tetrabutylammonium acetate)与磷酸盐等碱诱导去质子化；通过分光光度法与时间分辨荧光光谱对生成的物种进行表征，并利用甲基化光黄素衍生物确认结构归属。 向乙腈或二甲基亚砜中的光黄素溶液加入碱后，会发生基态去质子化，同时生成两种单阴离子物种：别咯嗪型阴离子与异咯嗪型阴离子。基态酸度常数(pKa)约为3.9~4.3，激发态酸度常数(pKa*)则处于4.0~5.0区间。时间分辨荧光光谱显示各物种具有截然不同的荧光寿命：中性光黄素为0.26~0.6 ns，别咯嗪型阴离子为2.2~3.6 ns，异咯嗪型阴离子为7.4~8.0 ns，后者的寿命显著更长。本研究测得的光黄素在上述有机溶剂中的pKa值远低于水溶液中的数值，凸显了溶剂环境对光黄素酸碱行为与光物理性质的关键影响。 本研究明确了碱与乙酸对光黄素的作用机制差异：强碱可生成稳定的基态阴离子，而乙酸仅作为激发态双质子转移(excited-state double proton transfer, ESDPT)的催化剂，生成瞬态激发态异咯嗪互变异构体。