The triplet-triplet absorption spectra of electron donor-acceptor complexes

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Abstract
Triplet-triplet (T-T) absorption spectra were measured in the wave-number region between 5000 and 30 000 cm-1 for the 1,2,4,5-tetracyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. From the analysis of the observed T-T absorption spectra, with the aid of theoretical considerations based on the interaction among zero-order charge-transfer and locally excited triplet states, the assignment was made and the nature of the lower triplet states of the complexes was clarified. We conclude that the lowest zero-order charge-transfer triplet state and the lowest zero-order locally excited triplet states are close to each other for the complexes, and the charge-transfer character of the lowest state changes from 81 to 15 per cent by changing the electron donor from hexamethylbenzene to toluene.