Involvements of phenyldibenzophosphole and 9-phenylcarbazole in electron transfer reactions with photoexcited 9-cyanoanthracene

Abstract

9-Cyanoanthracene (9CNA), used as an electron acceptor, has been subjected to photoexcitation in the presence of both the electron donors, phenyldibenzophosphole (PDP) and 9-phenylcarbazole (9PC). Steady state and time resolved (fluorescence lifetime by time correlated single photon counting and laser flash photolysis) spectroscopic measurements were made both in non-polar n-heptane (NH) and polar aprotic acetonitrile (ACN) at the ambient temperature. The spectroscopic studies coupled with electrochemical measurements by cyclic voltammetry demonstrate the presence of photoinduced electron transfer (PET) processes with the present donor–acceptor systems in polar ACN. In non-polar NH, the nature of the mechanism involved in quenching of fluorescence of the acceptor 9CNA differs significantly. The donor 9PC, in such a non-polar environment, undergoes partial charge separation reactions with the excited singlet 9CNA and forms radiative exciplex, whereas formation of such exciplex was not apparent in case of the other donor PDP. It is presumed, both from steady state and time resolved fluorescence studies, that the exciplex, if forms from the interaction between PDP and the excited 9CNA may undergo very fast non-radiative decay possibly through intersystem crossing or some other channels. From the electrochemical measurements, steady-state fluorescence, transient absorption spectra and analysis of transient decays, measured in ACN, formation of radical ion-pair species both in the excited singlet and triplet states has been inferred. In ACN, triplet population of 9CNA from charge (ion-pair) recombination mechanism was apparent from transient absorption decay and supported by thermodynamical calculations but this phenomenon was not observed in NH due to lack of formation of triplet ion-pair in such medium. The observed results indicate that in non-polar medium NH, the radiative nature of the exciplex formed in the excited singlet state of 9CNA in the presence of 9PC changes to the non-radiative one when the other donor PDP containing the heavy atom “phosphorous” is used as a reacting species in place of 9PC.