Fluorescence-detected triplet kinetics study of the specifically solvated chlorophyll a and protochlorophyll in frozen solutions

Abstract

The triplet lifetimes (τ T ) of the title compounds have been measured in a number of solvent hosts by using an indirect triplet state detection through fluorescence fading kinetics at 77 K. Three types of complexes with solvent molecules, i.e. with a mono-(L 1-type) and bisligated (L 2-type) central magnesium atom and the latter with hydrogen bridges (L 2H-type), have average τ T values of 2.5, 1.4 and 0.8 ms and 10, 4.5 and 2.5 ms for chlorophyll a (Chl a) and protochlorophyll (PChl), respectively. Hydrogen-bonded L 1H-type complexes of chl a occurring in tert-alcohols and perfluoroaniline have the same τ T as L 1-type solvates. The solvent deuteration isotopic effects show that in L 2H-complexes (e.g. in methanol) triplet quenching arises partially due to electronic energy transfer to the vibrational over tones of OH groups attached via Mg atom to the central part of Chl a macrocycle. The dependence of fluorescence band-shapes and maxima on the excitation wavelength indicates the presence of several types of pigment-solvent complexes in most matrices. Minor amounts (∼ 1 %) of L 2- and L 2 H-type solvates on the background of dominating monoligated species could be selectively excited with a 457.9 nm Ar + laser line.