Characterization of triplet states of axially symmetric benzenes using the zeeman effect

Abstract

By means of low temperature spectroscopy of single crystals of sym-trichlorobenzene (TCB) and sym-tribromobenzene (TBB) we have determined that the principal Herzberg-Teller origins in the S_0 → T_1 transition correspond to out-of-plane vibrations. The assigned active modes for TCB are: 205 cm^(−1) (e"), 144 cm^(−1) (a"2) from phosphorescence; 119 cm^(−1) (e" or a"2) from absorption. For TBB the assigned modes are: 203 cm^(−1) (e") from phosphorescence; 166, 207 cm^(−1) (e" or a"2) from absorption. The singlet-triplet transitions are dominated by these vibronic origins, and the electronic origins are relatively weak and manifest crystal field induced interactions as evidenced by the Zeeman effect analysis. The appearance of these low frequency out-of-plane modes indicates that the halogen atom motion induced mixing of the pi and sigma type electronic states is dominating the transition mechanism.

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