Coupling of charge-transfer transitions to low-wavenumber phonons in quasi-one-dimensional radical ion salts: resonance Raman study of tetraethylammonium-DDQ

Low-wavenumber resonance Raman spectra of the quasi-1-D anion radical salt tetratethylammoniumdichlorodicyano-p-benzoquinone are reported. By investigating the resonance behavior of the observed bands when the excitation wavelength is tuned from resonance with a localized electronic transition at 530 nm to the charge-transfer transition at 761 nm, three phonon modes at 148, 173, and 194 cm-1 coupled with the radical electrons through the modulation of their transfer integral are unambiguously identified. The Raman excitation profiles for these modes, measured in the 650–860 nm excitation wavelength range, were analyzed according to a Peierls–Hubbard dimer Hamiltonian using a time correlator approach. By combining the analysis of the excitation profiles with that of previously reported magnetic susceptibility data, it is possible to obtain values for the transfer integral, for the on-site electron correlation and for the electron–phonon coupling constants.