E.P.R. studies of the lowest excited triplet states of aromatic carbonyl molecules in mixed organic crystals

Abstract

Detailed single-crystal E.P.R. studies of the lowest excited triplet states (T ₁) of a series of aromatic carbonyl molecules, such as acetophenone, substituted acetophenones and substituted benzaldehydes, were made using 1,4-dibromobenzene and durene as hosts. Signs and magnitudes of the zero field splittings (ZFS), directions of the principal axes of the ZF tensor, and the orientations of the guest molecules with respect to the host molecules were determined for a series of aromatic carbonyl molecules. The results obtained are consistent with the ³ππ* assignment of the T ₁ states of the systems studied here. Comparisons of the observed ZFS and those estimated from the Hayashi-Nagakura analysis were made for several systems. Hyperfine interactions and spin distributions were investigated in the case of 4-fluoroacetophenone. The large spin density at the 4 position (ρ₄ = 0·28) was discussed in terms of the ³ππ* assignment and compared with those of the related triplet state molecules and radical anions.