Time-resolved spectroscopy of the singlet excited state of betanin in aqueous and alcoholic solutions

The photophysical properties of betanin in aqueous and alcoholic solutions were determined at room temperature using ultrafast UV-vis-NIR transient absorption spectroscopy (lambda(exc) = 535 nm). Its S-1 -> S-n (n > 1) absorption bands appear with maxima at about lambda similar to 450 and 1220 nm. The short betanin S-1 state lifetime (6.4 ps in water) is mainly determined by the efficient S-1 -> S-0 radiationless relaxation, probably requiring a strong change in geometry, since the S-1 lifetime grows to 27 ps in the more viscous ethylene glycol. The fluorescence quantum yield is very low (Phi(f) similar to 0.0007 in water), therefore this deactivation path is of minor importance. Other processes, such as S-1 -> T-1 intersystem crossing or photoproduct formation, are virtually absent, since full S-0 <- S-1 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.