PHOTOINDUCED ELECTRON-TRANSFER IN COVALENTLY LINKED DONOR-ACCEPTOR ASSEMBLIES IN LIQUID-CRYSTALS - TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin-cyclohexylene-quinone, oriented in liquid crystals (LCs), is monitored by time-domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 2 1 0 < T < 320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130 < T < 160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet- and probably singlet-initiated ET routes can concurrently be detected, and the free energy of the charge-separated state is estimated from the spectral dynamics.