Effects of protic and aprotic solvents on quenching mechanisms involving dimethyl-substituted donors and tetracyanoquinodimethane (TCNQ)

Steady-state and time-resolved spectroscopic techniques have been used to study photoinduced quenching reactions e.g. electron transfer (ET), energy transfer processes etc. between the electron donors 3,5-dimethylphenol (3,5-DMP) and 3,5-dimethylanisole (3,5-DMA) and the electron acceptor TCNQ in polar aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) at ambient temperature. In both solvents photoinduced ET reactions are found to be highly exothermic (Delta G degrees < -2 eV) and appear, since -Delta G degrees > lambda (where lambda is the nuclear reorganization energy parameter) and because the electron transfer rate k(ET) decreases with increasing exothermicity, to occur in the Marcus inverted region (MIR). However, relatively larger k(ET) values are observed in ACN than in EtOH. This has been explained in terms of the ordered structure of EtOH due to H-bonding. In ACN, the primary process responsible for quenching of the excited singlet (S-1) of the donors in the presence of TCNQ seems to be ET whereas, in EtOH, several other non-radiative processes can occur together with photoinduced ET. ACN would appear to be a better solvent in which to investigate the mechanism of the ET reactions. Reaction schemes showing the possible non-radiative deactivation routes within the donor-acceptor systems in both ACN and EtOH have been proposed.