Fluorescence detection study of molecular triplet states in chlorophyll and related compounds

Abstract

The lowest triplet state T ₁ of chlorophylls-a and -b, pheophytin-a, protochlorophyll and tetracene are investigated by a fluorescence detection method consisting of the measurement of fluorescence fading (FF) curves. Triplet state lifetimes of photosynthetic pigments in glassy solutions at 77 K, determined from the FF curves, are found to be dependent on the wavelength of fluorescence monitoring within the inhomogeneously broadened emission band. In the case of protochlorophyll, the double-exponential character of the FF curves is explained by suggesting the existence of two nearly degenerate triplet states below the S ₁ state. Estimates for the yield of S ₁ → T ₁ interconversion are obtained. From the dependence of the T ₁ population on excitation intensity it is estimated that the relative yield of the decay of the higher triplet state T_n to the ground state is about 0·2. From the FF curves measured at 4·2 K the lifetimes of the T ₁ sublevels are calculated, and compared with magnetic resonance and phosphorescence data. Some evidence for the contribution of the spin-lattice relaxation rates to the observed decay rates is found. In the case of tetracene the possibility of distinguishing sharp pseudo-lines and Shpolskii multiplet lines by investigating FF curves is demonstrated. The theoretical part, deals in particular with the influence of T-T absorption, excitation intensity and the anisotropy of excitation on the triplet state population kinetics.