The Electronic Structure of the Electron Donor-Acceptor Complex in Its Lowest Excited Singlet State. II

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Abstract
The electronic spectra of some 1,2,4,5-tetracyanobenzene complexes in the lowest excited singlet state at 77 K were obtained by laser photolysis method. The observed spectra can be well reproduced with the superposition of the bands similar to those of acceptor anion as well as donor cation, independent of donors. Comparing the spectra with those of ionic radical salts, it is concluded that the electronic structure of these complexes is quite polar and the fluorescent state is a contact ion-pair. The electronic structures of excited electron donor-acceptor systems have been discussed, on the basis of the interaction between donor cation and acceptor anion. The chargetransfer degree in the excited complexes was found to increase in the following order; 1,2,4,5-tetracyanobenzenehexamethylbenzene complex in the phosphorescent state<hydrocarbon-amine heteroexcimers<the present complexes in the fluorescent state.