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 n-pi* state with a small contribution from the pi-pi* state. The T1 states of 2, 3, and 4 are considered to be of mixed character between the pyridine B3(1) (n-pi*) and A3(1)(pi-pi*) states, though the interaction is smaller than that of unsubstituted pyridine because of raising the pi-pi* 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.