Triplet (T1) state and radical cation resonance Raman spectroscopy of N,N,N′,N′-tetramethyl- and N,N,N′,N′-tetraethyl-p-phenylenediamines

The time-resolved resonance Raman spectra of the first triplet state and the cw resonance Raman spectra of the radical cation are reported for various isotopic derivatives of the N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and of the N,N,N′,N′-tetraethyl-p-phenylenediamine (TEPD). Vibrational assignments and structural implications are discussed. The radical cation spectra are consistent with a semiquinoidal conformation of the ionized species. They are characterized by a significant frequency increase of the in-phase ν(N–ring) mode with respect to the ground state spectra (Δν≥120 cm−1) and by the high resonance enhancement of various bands assigned to vibrations of the N(alkyl)2 groups, resulting from couplings of these vibrations with modes of the π chromophore. The triplet spectra display almost only bands due to vibrations of the N(alkyl)2 groups, implying that similar couplings take place in the T1 state. Two signals characterize the N–ring–N framework: one is assigned to the Wilson ring mode 9a (CH bending ); the second one, a strong signal around 1500 cm−1, corresponds to a vibration involving a large contribution of the N–ring stretching motion and possibly also of the ring mode 8a.