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

Optical absorption and singlet emission properties of chlorophyll (Chl) a in the presence of diverse nitroaromatic acceptors of varying ring size and electron affinity were studied. 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 varied 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 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 $\Delta G_{23}$ values without the exhibition of inverted region even up to a $\Delta G_{23}$ value of $-100 kJ \cdot mol^{-1}$. This has been ascribed to the presence of a low-lying electronically excited state in the ion pair. The $\Delta G_{23}$ values were 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 were used to study the effect of the rates of excited state electron transfer in the Chl a-tetranitrofluorenone system. The solvent reorganization energies are found to over-estimate the magnitudes of free energy change for radical-pair formation, $\Delta G_{rip}$. It is shown that specific solvent effects, viz. H-bonding and coordinating ability, contributed to the $k_q$ values, and the solvents can interact specifically with Chl a, exhibiting maximum efficiency of quenching of the singlet emission of Chl a by the acceptor