This study investigated the photophysical properties and acid-base equilibria of lumichrome (Lch) in acetonitrile and dimethyl sulfoxide. The methodology involved adding a base, such as tetrabutylammonium acetate and phosphate, to induce deprotonation. The resulting species were characterized using spectrophotometric and time-resolved fluorescence measurements, with structural assignments confirmed using methylated Lch derivatives.The addition of bases to a solution of Lch in acetonitrile or dimethyl sulfoxide led to ground-state deprotonation and the simultaneous formation of two monoanionic species: the alloxazinic and isoalloxazinic anions. The ground-state acidity constants (pKa) were ranged from approximately 3.9-4.3, while the excited-state values (pKa*) were between 4.0 and 5.0. Time-resolved fluorescence revealed distinct lifetimes for each species, with a time of 0.26-0.6 ns for the neutral Lch, 2.2-3.6 ns for the alloxazinic anion, and a significantly and consistently longer time of 7.4-8.0 ns for the isoalloxazinic anion. The measured pKa values of Lch in these organic solvents were significantly lower than those in water, highlighting the critical influence of the solvent environment on Lch acid-base behavior and photophysics.A clear mechanistic distinction is established between the effect of bases and acetic acid on Lch, as strong bases generate stable ground-state anions, whereas acetic acid acts solely as a catalyst for an excited-state double proton transfer (ESDPT) that produces a transient excited-state isoalloxazine tautomer.
Prukała et al. (Thu,) studied this question.
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