Knowledge Bank

Phosphorescence-microwave double-resonance (PMDR) techniques have been used to measure the zero-field splittings in the lowest triplet state of benzaldehyde and several para-substituted benzaldehydes in a variety of hosts. The spin splittings are found to be strongly correlated to the usual criteria for distinguishing between n, $\Pi \ast$ and $\Pi ,\Pi \ast$ triplet states. Namely, the more vibronic activity evident in the phosphorescence and the longer the phosphorescence lifetimes of the individual spin states, the smaller the overall spin splitting. The nature of the triplet state will be discussed in terms of the results.