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.