Photochemistry of dicyanopyridines

Abstract 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 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient ( τ ≈ 10–12 μ s, λ max ≈ 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 ( τ ≈ 40–80 μ s, air sensitive, λ max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10 −8 M −1 s −1 . In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical ( τ ≈ 30–200 μ s, λ max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.