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-dimet hylphenol (3,5-DMP) and 3,5-dimethylanisole (3,5-DMA) and the electron acceptor TCNQ in polar aprotic acetonitrile (ACN) and polar protic et hanol (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 nucle ar reorganization energy parameter) and because the electron transfer rate k(ET) decreases with increasing exothermicity, to occur in the Ma rcus inverted region (MIR). However, relatively larger k(ET) values ar e observed in ACN than in EtOH. This has been explained in terms of th e ordered structure of EtOH due to H-bonding. In ACN, the primary proc ess responsible for quenching of the excited singlet (S-1) of the dono rs in the presence of TCNQ seems to be ET whereas, in EtOH, several ot her non-radiative processes can occur together with photoinduced ET. A CN would appear to be a better solvent in which to investigate the mec hanism of the ET reactions. Reaction schemes showing the possible non- radiative deactivation routes within the donor-acceptor systems in bot h ACN and EtOH have been proposed.