POLARIZED TRIPLET-TRIPLET ABSORPTION SPECTRA IN 3 BENZENE DERIVATIVES

The triplet spectra of three amine derivatives of benzene - aniline (AN), N,N-dimethylaniline (DMA), and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) - are determined throughout the visible spectrum in rigid solutions at 77°K. The triplet spectrum of TMPD is extended into the region of the singlet absorption bands, revealing a remarkable similarity with the TMPD radical cation spectrum that extends from the visible bands down into the ultraviolet bands. Polarization measurements are made across the triplet bands for all three molecules. The results are consistent with a 3A1 → 3A1 assignment to the triplet bands in the visible for AN and DMA (C2v) and a 3B1u → 3Ag assignment for the visible triplet band in TMPD. The 3La nature of the lowest triplet is assumed. In the blue, the TMPD triplet absorption becomes weak and structureless, and the polarization becomes isotropic, revealing continuum or near-continuum transitions. The new structured TMPD triplet bands in the ultraviolet are speculated to be π-π transitions, presumably strongly polarized, indicating that transitions to bound states are being observed at energies above the ionization threshold.