Photophysical properties of betaxanthins: miraxanthin V – insight into the excited-state deactivation mechanism from experiment and computations

Abstract

Miraxanthin V is a betaxanthin dye occurring in Caryophyllales plants. This work describes its photophysical properties in aqueous and alcoholic solutions. After excitation at λ_exc = 480 nm (2.6 eV), transient absorption spectrum of the excited S₁ state was observed (S₁ → S_n absorption band at λ_probe = 394 nm). The S₁ state population decays with two time constants (4.2 ps and 24.2 ps in water) corresponding to the photoexcited miraxanthin V in its two major stereoisomeric forms. The S₁ state decay is mainly radiationless, since the fluorescence quantum yield is low (Φ_F = 0.003 in water, the smallest value among all of the studied betaxanthins). Strong correlations were obtained between the solvent viscosity and the S₁ state lifetime in linear alcohols, as well as in methanolic solutions at low temperatures. These correlations suggest that intramolecular rotation or conformational relaxation precedes the S₁ state decay. Full recovery of the electronic ground state S₀ was observed after excitations at λ_exc = 480 nm and 298 nm (4.2 eV), without forming the triplet T₁ state or any photoproducts. Experimental results are complemented by the ab initio S₀ state calculations using MP2 and DFT methods, and also by the excited-state calculations at the ADC(2) and TD-DFT levels of theory. The ab initio ADC(2) results give insight into the mechanism of the excited-state deactivation through the conical intersection region. We conclude that most probably the excited-state deactivation process is initiated by the rotation about the CN double bond, present in the chain linking the electron-donor catechol to the electron-acceptor dihydropyridine group, and it is followed by collective geometry changes involving a large rotation about the CC double bond. This mechanism seems to be universal for different betaxanthin dyes present in plants, and plays a photoprotective role.