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

Authors:

• Monika Wendel,
• Stanislaw Nizinski,
• Dorota Tuwalska,
• Karolina Starzak,
• Dominika Szot,
• Dorota Prukała,
• Marek Sikorski,
• Slawomir Wybraniec,
• Gotard Tadeusz Burdziński

Abstract

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.