POLARIZED ABSORPTION AND PHOSPHORESCENCE SPECTRA AND MAGNETIC CIRCULAR-DICHROISM OF DITHIOIMIDES - ASSIGNMENT OF THE LOWER (1)N-PI-ASTERISKAND (3)N-PI-ASTERISK STATES

Electronic absorption, magnetic circular dichroism, and (polarized) ph osphorescence and phosphorescence excitation spectra are reported for dithiosuccinimide, dithioglutarimide, and dithiocamphorimide and their N-methyl derivatives. The theory of Engelbrecht et al. (Spectrochim. Acta 1975, 31A, 507) for magnetic circular dichroism (MCD) in singlet- triplet transitions is extended to show that not only the shape but al so the magnitude of the MCD is of help in characterizing such transiti ons. The weak shoulder at similar to 500 nm in the absorption spectra of the dithioimides is, using C-2v symmetry labels, identified as the S-0 --> B-3(1)(n pi)) transition. Its polarization is almost exclusiv ely (similar to 99%) parallel to the line C(S)-C(S) in the dithioimide chromophore, Presumably the responsible tripler sublevel derives its radiative character from spin-orbit coupling to the third excited sing let state (S-3), B-1(2)(pi pi) The radiation from the other active su blevel is polarized along the C-2 axis of the chromophore. From the ab sorption spectra and the excitation polarization data, which were obta ined via the photoselection method, the lowest excited singlet state i s assigned as B-1(1)(n pi), S-2 as (1)A(2)(n pi*). The experimental f indings are consistent with C-2v symmetry for dithiosuccinimide, and C -s symmetry for the chromophore in N-methyldithioglutarimide and N-met hyldithiocamphorimide. The data indicate that at 77 K the phosphoresce nt state, B-3(1)(n pi), is not appreciably out-of-plane distorted wit h respect to the ground state.