EXPERIMENTAL INVESTIGATION OF EXCITED-STATE ELECTRON-TRANSFER REACTIONS BETWEEN SOME BICYCLIC MOLECULES AND TETRACYANOQUINODIMETHANE (TCNQ)

Investigations on photoinduced electron transfer (ET) reactions betwee n excited (ground) bicyclic electron donors 5,6,7,8-tetrahydro-2-napht hol (TH2N), 2-methoxy-5,6,7,8-tetrahydro naphthalene (2MTHN) and groun d stare (excited) acceptor tetracyanoquinodimethane (TCNQ) in fluid so lutions of different polarity at the ambient temperature (300 K) by el ectronic absorption, steady state fluorescence and time-resolved spect roscopic methods in the time domain of nanosecond order have been carr ied out. It is suggested that in highly polar solvent acetonitrile (AC N), a loosely-structured transient geminate ion-pair complex (GIP) in the excited singlet state (S-1) is formed due to the ET encounter betw een the present donor TH2N or 2MTHN and TCNQ and this GIP complex rapi dly dissociates into stable excited radical ions, as evidenced from st eady state spectra. In polar DMF solvents, TCNQ exhibits an electronic absorption band of its anion without the presence of donor molecules. Both steady state and time-resolved data indicate that ET reactions b etween the present donors and acceptor TCNQ are largely impeded in the less polar solvent tetrahydrofuran (THF). In the highly polar solvent ACN, ET reactions between the donors and acceptor TCNQ have been sugg ested to,be of adiabatic or intermediate between adiabatic and non-adi abatic types, from the observation of radical ion species in the elect ronic excited state. For some bicyclic donors and TCNQ acceptor system s, large negative Delta G, which is a measure of the gap between local ly excited and radical ion-pair states, shows reaction occurs in highl y exothermic regions. Further observations of -Delta G>lambda, nuclear reorganization energy parameters and the decrement of ET rate (k(ET)) With increasing exothermicity (more negative Delta G values) suggest the ET reaction for the bicyclic donor-TCNQ acceptor systems studied i n the present investigation might occur in the Marcus inverted region. The possibility of building up efficient photoconducting materials wi th the present donor acceptor systems is suggested.