Phosphorescence of the Charge-Transfer Triplet States of Some Molecular Complexes

Abstract

The phosphorescence spectra of some charge-transfer complexes were studied in glassy solution at 77°K. Long-lived emission spectra which were different from the phosphorescence spectra of the constituent molecules were observed for the 1,2,4,5-tetracyanobenzene complexes with mesitylene, durene, and hexamethylbenzene. The maxima of the long-lived emission spectra were found to shift to lower frequencies as the ionization potential of the donor was decreased; for example, maxima appear at 19.8, 19.5, and 18.3 (× 103 cm−1) in the complexes with mesitylene, durene, and hexamethylbenzene, respectively. This shift is parallel to that observed for charge-transfer fluorescence spectra which exhibit a mirror-image relationship with the corresponding absorption spectra. Similar long-lived emission spectra were also observed for complexes containing pyromellitic dianhydride, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors. A combination of theory and experiment indicates that the long-lived emission is attributable to a transition from a triplet state of charge-transfer character. This is the first observation of a ``charge-transfer triplet state.'' The lifetime of the phosphorescence due to the charge-transfer triplet state was measured. The results indicate that the observed lifetime decreases with increasing donor ability for a series of complexes with a fixed electron acceptor. The mechanism of singlet—triplet mixing in the charge-transfer complexes is discussed.