PHOTOCHEMISTRY OF DICYANOPYRIDINES

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 mu s and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (tau approximate to 10-12 mu s, lambda(max)approximate to 360-380 nm) was observed, suggesting the formation of a protonated excited state. Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (tau approximate to 40-80 mu s, air sensitive, lambda(max)approximate to 360-380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stem-Volmer analysis gave electron transfer rate constants in the range (1-8)X10(-8)M(-1)s(-1). In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (tau approximate to 30-200 mu s, lambda(max)approximate to 370-385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.