The photophysical properties of betanin in aqueous and alcoholic solutions were determined at room temperature using ultrafast UV-vis-NIR transient absorption spectroscopy (λ_exc = 535 nm). Its S₁ → S_n (n > 1) absorption bands appear with maxima at about λ ∼ 450 and 1220 nm. The short betanin S₁ state lifetime (6.4 ps in water) is mainly determined by the efficient S₁ → S₀ radiationless relaxation, probably requiring a strong change in geometry, since the S₁ lifetime grows to 27 ps in the more viscous ethylene glycol. The fluorescence quantum yield is very low (Φ_f ∼ 0.0007 in water), therefore this deactivation path is of minor importance. Other processes, such as S₁ → T₁ intersystem crossing or photoproduct formation, are virtually absent, since full S₀ ← S₁ ground state recovery is observed within tens of picoseconds after photoexcitation. The observed fast light-to-heat conversion in the absence of triplet excited state formation supports the idea that betanin is a photoprotector in vivo.

Triphenylphosphine (Ph₃P) activated by various electrophiles (e.g., alkyl diazocarboxylates) represents an effective mediator of esterification and other nucleophilic substitution reactions. We report herein an aza-reagent-free procedure using flavin catalyst (3-methyl riboflavin tetraacetate), triphenylphosphine, and visible light (448 nm), which allows effective esterification of aromatic and aliphatic carboxylic acids with alcohols. Mechanistic study confirmed that photoinduced electron transfer from triphenylphosphine to excited flavin with the formation of Ph₃P˙⁺ is a crucial step in the catalytic cycle. This allows reactive alkoxyphosphonium species to be generated by reaction of an alcohol with Ph₃P˙⁺ followed by single-electron oxidation. Unexpected stereoselectivity control by the solvent was observed, allowing switching from inversion to retention of configuration during esterification of (S)- or (R)-1-phenylethanol; for example with phenylacetic acid, the ratio shifting from 10 : 90 (retention : inversion) in trifluoromethylbenzene to 99.9 : 0.1 in acetonitrile. Our method uses nitrobenzene to regenerate the flavin photocatalyst. This new approach to flavin re-oxidation has also been successfully proved in benzyl alcohol oxidation, which is a “standard” process among flavin-mediated photooxidations.

Fast radiationless S₁ → S₀ transition in photo-excited betaxanthins is due to conical intersection seam between S₁ and S₀ surfaces.