Photochemical Reaction Mechanism of 2,2′-Pyridil in Alcoholic Media

Time-resolved transient absorption spectra of photo-irradiated 2,2′-pyridil were measured in PVA films. The broad bands at 552 and ∼455 nm, appearing just after photo-excitation of 2,2′-pyridil, are assigned to the skewed triplet species. The triplet species changes into 1-hydroxy-2-oxo-1,2-di(2-pyridyl)ethyl (pyridoil) radical via the abstraction of a hydrogen atom from the solvent. The radical produced by the photo-irradiation of 2,2′-pyridil is confirmed to be the pyridoil radical from the measurement of the polarized absorption spectrum, with the aid of the stretched polymer film technique and MO calculations. The time-resolved transient absorption spectra of 2,2′-pyridil irradiated by a xenon flash lamp in a 1:1 (v/v) mixed solvent of methanol and ethanol show that the pyridoil radical further abstracts a hydrogen atom from the solvent to afford cis-enediol (cis-1,2-di(2-pyridyl)-1,2-ethenediol) and that cis-enediol is converted into trans-enediol (trans-1,2-di(2-pyridyl)-1,2-ethenediol). 2,2′-Pyridil shows a structured Tn ← T1 absorption spectrum in a 1:1 (v/v) mixed solvent of methylcyclohexane and 2-methylbutane, indicating that 2,2′-pyridil takes a planar structure in the lowest triplet excited state in a nonpolar medium.