The Problematic Charge-Transfer Triplet State of Electron-Donor-Acceptor-Complexes

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Abstract
Iwata, Tanaka and Nagakura, and Hayashi, Iwata and Nagakura have observed in ethylether-isopentane solutions (1 : 1) at 77 °K, in the case of the complexes of tetracyanobenzene (TCNB) as the electron acceptor with the electron donors mesitylene (Me), durene (Du) and hexamethyl-benzene (HMB), an unstructured long-lived, long-wave emission band by exciting the complexes in the charge transfer (CT) band. They interpreted the 11, lw band as a CT-phosphorescence of a t t charge transfer triplet excited state | D + A" | . This would be of considerable theoretical interest and significance also for an interpretation of photochemical processes. In the present communication, we report results which lead us to a different interpretation of the 11, lw emission band under discussion. The emission spectra of the complexes of tetracyanobenzene (TCNB) with the electron donors toluene (To), p-xylene (Xy), mesitylene (Me), durene (Du) and hexamethylbenzene (HMB) were studied in glassy solutions in propylether at 100 °K. In the case of TCNB-To, -Xy, -Me and -Du, when the complexes were irradiated in the CT-band so that the complex components were not excited, we observed either the structured phosporescence band of TCNB which originates from a locally excited triplet state of complexed TCNB or, at large donor concentrations, the unstructured long-lived, long-wave emission band within the green spectral range. In the case of TCNB-HMB, only the 11, lw emission may be observed. The 11, lw unstructured emission superimposed on the structured TCNB phosphorescence may also be observed when TCNB, singly soluted in glassy solvents or crystallized, is irradiated in the 31,600 cm -1 (St S0) peak. Obviously, the 11, lw emission band may not be characteristic of the complexes of TCNB. Simultaneously with the 11, lw emission, a new molecular species X can be identified by a new absorption spectrum after irradiating either TCNB itself within the S0 —St 31,600 cm -1 peak or the TCNB-complexes within the CT-absorption band. X is formed by a reaction of the triplet ex-cited TCNB. The excitation in the long-wave absorption peak of X at about 21,000 — 22,000 cm -1 gives rise to an emission with a maximum in the same spectral range as the 11, lw emission band. Accordingly, we interpret the 11, lw emission as a delayed emission of X due to an energy transfer from the triplet excited TCNB.

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