INVESTIGATION OF EXCITED-STATE ELECTRON-TRANSFER REACTIONS - A SEARCHFOR OTHER ASSOCIATIVE PROCESSES

The present investigations were carried out to reveal the nature of th e photoinduced electron-transfer (ET) process within the electron dono rs 1,2,3,4-tetrahydroquinoline (THQ) and 1-methyl-1,2,3,4-tetrahydroqu inoline (THMe), and widely used acceptor tetracyanoquinodimethane (TCN Q) in the highly polar solvent acetonitrile (ACN) at 300 K. Observatio ns of considerable overlapping between the emission spectrum of the do nor molecules studied in the present investigation and the electronic absorption spectrum of the acceptor TCNQ, coupled to a high negative v alue of DELTAG [the energy gap between the locally excited (LE) and ra dical ion pair (RIP) states] when one of the chromophores is excited, indicate the possibility of concurrent occurrence of the two processes , e.g. energy and electron transfer. Surprisingly even when the donor chromophore is photoexcited, no spectral manifestation of energy trans fer was observed, though both steady state and time resolved (in the t ime domain of nanosecond order) spectroscopic measurements strongly su ggest the occurrence of a highly exothermic ET reaction within the pre sent donor-acceptor systems. Furthermore such ET reactions have been s uggested to occur between donor and acceptor separated by a large dist ance (approximately 7 angstrom), and quenching of fluorescence emissio n of donor molecules is caused primarily due to outer sphere ET reacti ons with the acceptor. Measured electron transfer rates (k(ET)) were f ound to be of much lower value (approximately 10(7) s-1). It is demons trated that loose structure of the transient geminate ion pair complex is formed due to the encounter between excited acceptor (or donor) an d unexcited donor (or acceptor), and due to this structural property, a stable anionic species (TCNQ- ion) is produced due to the rapid diss ociation (probably in the picosecond time domain) of this excited comp lex. It is hinted that synthesis of biochromophoric systems in which t he present donor and acceptor chromophore would be linked by a polymet hylene type (sigma-type) spacer might be useful in building good photo conducting materials.