Steady state and time resolved spectroscopic studies of the photoinduced electron-transfer reactions between electron donor xylenol and acceptor tetracyanoquinodimethane in acetonitrile at room temperature

Abstract

The present study was undertaken both by steady state and time correlated single photon counting techniques to reveal the nature of the electron transfer (ET) process within excited (or ground) xylenol donor and ground state (excited) well-known acceptor tetracyanoquinodimethane (TCNQ) in highly polar solvent acetonitrile (ACN) at 300 K. Observations of poor overlap between the emission of the donor and the electronic absorption spectra of the acceptor and the high negative value of Δ G, the driving energy (Gibbs free energy) when either donor or acceptor chromophore is excited seemingly indicate that the observed fluorescence quenching of the donor in presence of TCNQ is mainly due to highly exothermic electron transfer reactions and no significant energy transfer process is operative here. It is suggested that such highly exothermic ET reaction, which has been proposed to be of outersphere type, occurs at a long distance (⩾7 A ̊ ) between the fluorescer (donor) and the quencher (acceptor). Observations of slow electron transfer rate, k ET (∼10 7 s −1) and − ΔG > λ (nuclear reorganization energy parameter) suggest that the reaction might occur in the Marcus inverted region. It has been proposed that in an ACN fluid solution the transient geminate ion-pair complex formed by ET reaction is of loose structure and due to the rapid dissociation of this complex stable anionic species TCNQ - ion is formed in the excited state. This anion is observed to emit at nearly 432 nm region. It is suggested that in the Marcus inverted region as the reaction becomes more exothermic, the necessary reorganization energy λ also increases.