On the assignment of the lowest triplet state of 1,4-dihalobenzenes. MODR spectrum of the 3B1u state of p -dichlorobenzene in a p -dibromobenzene host crystal

A careful examination of the orientational dependence of the microwave-optical double resonance (MODR) spectrum of the lowest (π,π*) triplet state of p -dichlorobenzene (DCB) in a host crystal of p -dibromobenzene at liquid helium temperatures yields accurate values of the principal components of the zero-field (ZF) splitting tensor (X = ∓ 0.0987, Y = ± 0.0780, and Z = ± 0.0207 cm−1) and the g tensor (gxx = 1.986, gyy = 2.001, and gzz = 2.002). Formally forbidden transitions involving simultaneous electron/nuclear spin flips are also observed at high microwave power levels, and these are attributed to a transferred hyperfine interaction between the guest triplet state and the bromine atoms of the host. When combined with the results of previous PMDR studies on this system, these experiments show conclusively that the ordering of the ZF levels of the lowest triplet state of DCB is τy > τz≫τx and that the orbital symmetry of this state is B1u. This is the state derived from the accepted assignment of the lowest triplet state of benzene itself.