An electron spin resonance and CIDNP study of the structure and reactivity of some excited triplet biphenyl ketones

Abstract

The correlation of structure and reactivity of some excited triplet biphenyl ketones was approached by a combined esr and CIDNP study. The low temperature esr study confirmed the assignment of the triplet π,π* as the lowest triplet state in these biphenyl ketones. An analysis of the zero-field energies suggests that these π,π* triplets have a structure with both the biphenyl and the carbonyl groups being coplanar. Consequently a substantial unpaired spin density may result at the carbonyl oxygen atom which could lead to abstraction reactions. The CIDNP results are consistent with the triplet photoreduction mechanism. The polarization is mainly due to the cage products and the polarization magnitude is therefore dependent upon the light intensity