Singlet quenching of chlorophyll A by nitroaromatics. Effect of solvents on the electron transfer from excited singlet of chlorophyll A to nitroaromatic acceptors

Abstract

Optical absorption and singlet emission properties of chlorophyll a in the presence of diverse nitroaromatic acceptors of varying ring size and electron affinity have been investigated. Chl a forms 1:1 intermolecular complexes with the acceptors and the nature of interaction between the donor Chl a and the acceptor has its origin in charge transfer. The K_sv values are found to vary linearly with the electrochemical reduction potentials of the acceptors. The oxidation potential of the excited singlet state of Chl a, Chl a^∗/Chl a⁺ is found to be 1.28 V vs. saturated calomel electrode. The rates of bimolecular quenching (k_q) increase linearly with the change in free-energy for the excited-state electron transfer reaction in the weakly exergonic region, indicating that the electron transfer reaction is predominantly controlled by the activation energy. In the moderate exergonic region, the k_q values increase exponentially, with ΔG₂₃ values without the exhibition of inverted region even upto a ΔG₂₃ value of - 100 kJ · mol^-1. This has been ascribed to the presence of low-lying electronically excited state in the ion-pair. The ΔG₂₃ values have been used to distinguish the various acceptors based on their ability to form ionic or covalent exciplexes with Chl a. Ten solvents of varying polarizability and viscosity have been employed to study the influence of the rates of excited state electron transfer in Chl a-tetranitrofluorenone system. The solvent reorganization energies are found to over-estimate the magnitudes of free-energy change for radical-pair formation, ΔG_rip. It is shown that specific solvent effects, viz. H-bonding and coordinating ability, contribute to the k_q values. It is found that solvents can interact specifically with Chl a exhibiting maximum efficiency of quenching of the singlet emission of Chl a by the acceptor.