Excited singlet (S1) state interactions of 2,2′- and 4,4′-biphenyldiols with chloroalkanes: Photoinduced dissociative electron transfer

Abstract

Interactions of the excited singlet (S1) state of 2,2′- and 4,4′-biphenyldiols with a number of chloroalkanes (CA) have been investigated in acetonitrile solutions using fluorescence quenching measurements. For any particular diol–CA pair, the bimolecular quenching constants kq obtained from steady-state and time-resolved measurements are found to be the same, indicating the dynamic nature of the interaction. The kq values for different diol–CA pairs are seen to increase as the oxidation potential of the diol becomes less positive or the reduction potential of the CA becomes less negative, indicating the electron transfer (ET) type of interaction for the observed quenching. Following Marcus’ outer-sphere ET theory, the correlation of the observed kq values with the free-energy changes for such reactions (ΔG0) results in the recovery of an unusually higher intramolecular reorganization energy (λin), indicating that the ET in the systems studied might not be of outer sphere in nature. Since the CAs are prone to undergo C–Cl bond cleavage following their reduction, a dissociative ET (DET) mechanism has been proposed for the observed fluorescence quenching. The evidence for the DET mechanism has been obtained by characterizing and estimating the Cl− ions in the photolyzed diol–CA solutions. Following a suitable theory for concerted DET reactions, it is seen that the observed kq values correlate well with the free-energy changes (ΔGDET0) for such reactions. It is seen that the reorganization energy recovered from such correlation accounts well for the C–Cl bond dissociation energy of the CAs, supporting a concerted DET mechanism in these systems.