Generation of Phenyl Alky and Diphenyl Selenide Radical Cations: a Laser Flash Photolysis Study

LYDIA M. BOUCHET; ALICIA B. PEÑÉÑORY; JUAN E. ARGÜELLO

Mendoza

Conferencia; 21st IAPS Conference, Inter-American Photochemical Society Conference; 2011

Inter-American Photochemical Society

Organic selenium compounds have been suggested to have antioxidant properties due to the rather low one-electron reduction potentials of the corresponding radical cations. Some mechanistic aspects and synthetic potentials of photoinduced electron transfer (PET) activation of organoselenium substrates have been explored by Pandey et al. These interesting studies have been useful in initiating various synthetic reactions. However, direct evidence of the mediation of selenide radical cations or of other electrophilic selenium and carbocations species is further needed. In the present communication the radical cations of PhSePh (1), PhSeMe (2) and PhSeCHPh2 (3) have been produced by PET in acetonitrile solution, using dicyano naphthalene (DCN), N-methylquinolinium (NMQ+) and 2,4,6-triphenyl thiapyrylium (STPP+) cations, and Chloranil (Ch) as photosensitizers. The fluorescence of DCN and the low-lying triplet state of STPP+ and Ch are quenched by selenides at diffusion rate constants, in agreement with the high exergonic value of those of GET calculated by the Gibbs free energy equation. The nature of the PET step was confirmed by the observation of the semireduced form of the sensitizer, namely, DCN and Ch radical anions and the NMQ radical by the laser flash photolysis technique. Thus the PhSeR radical cations were observed for the first time by transient absorption spectroscopy. PhSePh.+ presents three maximum absorption bands in agreement with pulsed radiolysis experiments. The monomeric form of PhSeMe.+ and PhSeCHPh2.+ shows a maximum at 500 nm; however, another band at 640 nm was observed for the former, attributed to a -dimmer between the selenide radical cation an a neutral molecule, similar to its periodic neighbor PhSeMe.+. Cosensitizers such as toluene or biphenyl were necessary to avoid back-electron transfer for NMQ+ and DCN, respectively. Finally, the life times of the selenide radical cations depend on the sensitizer employed and the concentration of molecular oxygen. Different reactions channels will be discussed in order to explain these observations.