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.