Time-Resolved EPR Studies on the Photochemical Hydrogen Abstraction Reactions and the Excited Triplet States of 4-Substituted Pyridines

Article abstract of DOI:10.1021/j100155a051

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 n?^* state with small contribution from the ??_* state.The T₁ states of 2,3, and 4 are considered to be of mixed character between the pyridine ³B₁(n?^*) and ³A₁(??^*)states, though the interaction is smaller than that of unsubstituted pyridine because of raising of the ??^* 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.