ESR Spectra of the Charge-Transfer Triplet States of Some Molecular Complexes

ESR spectra were measured with various solutions containing 1,2,4,5-tetracyanobenzene (TCNB) and benzene or methyl-substituted benzenes. We succeeded in finding the ESR absorption lines due to the Δm = ± 2 and Δm = ± 1 transitions of the lowest triplet excited states of TCNB complexes and also in measuring their lifetimes. From the analysis of the ESR spectral data, we determined the fine-structure constants D and E for the TCNB–toluene complex in ether–toluene mixed solvent, and D* for the TCNB complexes with benzene, toluene, xylenes, mesitylene, durene, pentamethylbenzene, and hexamethylbenzene. From the theoretical consideration on the D* value, an equation representing its dependence upon the charge-transfer degree (x) in the lowest triplet excited state of a molecular complex was derived. The combination of the equation with the observed D* values gave the x values for the TCNB complexes, for example 95%, 72%, and 42% for TCNB complexes with hexamethylbenzene, durene, and mesitylene, respectively. Thus, excited triplet states of great charge-transfer character were found to exist.