The 1-hydro-2,6-dimethyl-3,5-dicyanopyridinyl radical in solution: mechanisms of photochemical formation and kinetics of disappearance by self-combination

Abstract

The 1-hydro-2,6-dimethyl-3,5-dicyanopyridinyl radical (HDCP·) has been identified by electron spin resonance spectroscopy (ESR) in solution at room temperature as an intermediate during both the 280–300 nm photoreduction of 2,6-dimethyl-3,5-dicyanopyridine (DCP) in 2-propanol and the 270–300 nm photo-oxidation of the corresponding 1,4-dihydropyridine (H 2DCP) in oxygenated aqueous acetone. The hyperfine couplings and the g value of HDCP· are reported. The rate constant for the second-order self-combination of HDCP· as determined by time-resolved ESR is 2 k = (1.6 ± 0.4) × 10 9 M −1 s −1 in 2-propanol at 20 °C. In addition to the HDCP· radical dimer (yield, 24%), combination products of the (CH 3) 2ĊOH and the HDCP· radicals (total yield, 32%) have been identified among the products of the photoreduction of DCP in 2-propanol. Also reported are the quantum yields of DCP disappearance (0.07 and 0.15 at 280 nm and 300 nm respectively) and fluorescence quenching studies of aqueous solutions of DCP in the presence of 2-propanol. The various results are rationalized in terms of a mechanism proposed for the photoreduction of DCP in 2-propanol. It is suggested that the radical and product formation originates from the lowest excited 1(n-π *) state of DCP. The formation of the HDCP· radical by photo-oxidation of H 2DCP in oxygenated aqueous acetone is thought to proceed via singlet oxygen.