Ultrafast excited states dynamics of metal ion complexes of the carotenoid astaxanthin

Abstract

Carotenoids are natural compounds with multiple functions in biological systems. They exhibit rich photophysics that can be modified by changes in properties of their local environments. Here we present time-resolved spectroscopy data from complexes resulting from interaction of the carotenoid astaxanthin with Zn2+ and Cu2+ ions. The astaxanthin-metal complexes exhibit significant alteration of their excited-state dynamics, resulting in enhancement of the charge transfer character of the lowest excited state, S1/ICT. The S1/ICT lifetime of pure astaxanthin, 4.4 ps, is shortened to 3.9 ps and 2.6 ps for astaxanthin-Zn and astaxanthin-Cu complex, respectively. The lifetime shortening is attributed to prolongation of effective conjugation caused by interaction of metal ions with keto oxygen at the terminal rings of astaxanthin. The significant shortening of the S1/ICT lifetime of astaxanthin-Cu complex also results in observation of the S* signal, which is likely due to a hot ground state generated by rapid depopulation of excited states.