Photochemical electron transfer reactions were studied in two different areas: (1) photooxidations of sulfides, and (2) development of new ET sensitizers.

In the first area, we studied the photooxidations of both 1,5-dithiocyclooctane (DTCO) and 5H, 7H-dibenzo[b,g][1,7] dithiocin (DTC) with three different sensitizers: methylene blue (MB), N-methylquinolinium tetrafluoroborate, and 2-methoxy-4,6-diphenyl pyrylium salts. The MB sensitized reactions exhibit typical characteristics of a singlet oxygen reaction including isotope effects for the formation of a hydroperoxysulfonium ylide and the ability of both DTCO and DTC to quench the time-resolved emission of singlet oxygen at 1270nm. The product compositions in quinolinium and pyrylium sensitized reaction are different and also different from the singlet oxygen products. These argue for different mechanisms for each of the sensitizers. The quinolinium and pyrylium salt sensitized reactions show all the characteristics of ET reactions including Stern-Volmer quenching with a diffusion controlled rate for both DTCO and DTC, and LFP observation of the reduced radical cation of the sensitizer in the presence of DTCO and DTC. We propose a new inner sphere ET mechanism for the pyrylium sensitizer photooxidation.

The second area is the development of a new type of ET sensitizers. Pyrylogens, which include both a pyrylium core and a pyridinium ring, were synthesized with different electron perturbing groups. The structural, photophysical and electrochemical properties of the pyrylogens were examined and compared with 2,4,6-triphenylpyrylium salts. The replacement of the phenyl ring with the electron deficient pyridinium ring causes the UV-Vis spectrum to change fundamentally. The singlet and triplet energies of these pyrylogens were measured. Preliminary results show that the pyrylogens can promote several ET reactions including dimerization of cyclohexadiene and the cycloreversion of cyclobutane. However, the ease of nucleophilic attack to open the pyrylium ring and the fast decomposition urges us to improve the stability of this type of new ET sensitizers.