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.