Triplet species, ultraviolet spectra

TMB (42) was first generated by Roth el al. by photochemical decarbonyla-tion of the ketone 44 in a low-temperature matrix. This preparation was intensely colored, with a main transition at 490 nm and several subsidiary absorptions. Earlier ti-CI quantum chemical computations had predicted ultraviolet-visible (UV-vis) is transitions for the singlet and triplet states of TMB, and the bands observed by the Roth group were in better agreement with the predictions for the triplet. The preparation also showed a narrow ESR spectrum interpreted by the authors as that of a triplet species with D = 0.0042 cm and E = 0.0009 cm, which gave a linear Curie plot. The authors assumed that the carriers of the UV-vis and ESR spectra were the same species, namely, triplet TMB. They concluded that TMB is a ground-state triplet, contrary to the disjoint theory and to the computational results described above. 

Spectra.—Ultraviolet Spectra, The transient, extra absorption band which appeared in the 400—600 nm region (maximum absorption at ca. 475 nm) in the u.v. spectrum of thiobenzophenone as a result of the flash photolysis of this thione has been tentatively assigned to short-lived, triplet thio-benzophenone. " A similar species, generated in the same manner from Michler s thione, was found to exhibit absorption in the 470—520 nm region. " The influence of substituents (R) on the u.v. absorptions of 3-alkyl- and 3-aryl-6-methylthiouracils (368) has been studied, and, based on a comparison with the u.v. spectra of alkyl-, aryl-, and benzoyl-thioureas, the two bands at 280 nm and 240 nm (the intensities of which... 

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