Time-resolved EPR studies on the photochemical hydrogen abstraction reactions and the excited triplet states of 4-substituted pyridines

Photochemical hydrogen abstraction reactions and the lowest excited triplet states taking part in the initial process have been investigated for several pyridine derivatives by using the time-resolved EPR method. The emissive CIDEP spectrum obtained from the laser photolysis of 4-acetylpyridine (1) in 2-propanol was assigned to the corresponding ketyl radical, proving initial hydrogen abstraction by the carbonyl group. In the cases of 4-cyano-(2) and 4-methoxycarbonyl-(3) pyridines, and 4-pyridinecarboxamide (4), enhanced absorptive CIDEP spectra due to the corresponding 1-hydropyridinyl radicals were observed, suggesting the preferential population to the lowest sublevel in the intersystem crossing. The triplet EPR and phosphorescence spectra observed at 77 K indicate that T₁ of 1 is mainly the carbonyl nπ* state with a small contribution from the ππ* state. The T₁ states of 2, 3, and 4 are considered to be of mixed character between the pyridine ³B₁(nπ*) and ³A₁(ππ*) states, though the interaction is smaller than that of unsubstituted pyridine because of raising of the ππ* state by electron-withdrawing groups. It is proposed that the T₁ states of 2-4 have smaller deviation from the planar conformation than the T₁ state of unsubstituted pyridine.