On the Lowest Triplet State of Substituted Benzenes. III. Optically Detected Magnetic Resonance Studies on the 3π π State of 1,2,4,5-Tetrabromobenzene and 1,2,4,5-Tetrachlorobenzene

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Abstract
The optically detected magnetic resonance (ODMR) in zero field and the phosphorescence microwave double resonance (PMDR) spectra of 1,2,4,5‐tetrabromobenzene and 1,2,4,5‐tetrachlorobenzene have been obtained monitoring the low temperature (1.3°K) phosphorescence from both trap emission in neat single crystals and emission from a durene host. From the PMDR the major vibronic features of the spectra are assigned. The orbital symmetries in the excited triplet state, the principal and secondary contributions from the individual triplet spin sublevels to the electronic and vibronic origins in phosphorescence, the spin sublevel populations, and lifetimes are determined. In addition the spin Hamiltonian parameters, including the zero‐field splittings and Br and Cl excited state nuclear quadrupole coupling constants, are presented and interpreted. Finally, the relationship between spin sublevel activity in phosphorescence, Frank‐Condon maxima in emission, and molecular distortions are discussed.