Resonance Raman intensity analysis of the carbazole/tetracyanoethylene charge-transfer complex: Mode-specific reorganization energies for a hole-transport molecule

A resonance Raman intensity analysis is presented for the carbazole/tetracyanoethylene donor-acceptor charge transfer complex in dichloromethane solution. The intent is to determine the nuclear reorganization contributions to the rates of charge hopping in carbazole polymers used as hole-transport agents in the xerographic (electrophotographic) process. Resonance Raman cross sections have been measured at seven excitation wavelengths spanning the broad visible charge-transfer absorption known to consist of two strongly overlapping charge-transfer electronic transitions. interference between the Raman amplitudes from these two transitions manifests itself in the Raman excitation profiles for a number of resonantly enhanced modes. Simultaneous modeling of the absorption spectrum and the Raman cross sections shows that explicit consideration of the signs as well as the magnitudes of the normal mode displacements is required to reproduce the experimental data. The signs of the mode displacements obtained from the modeling are shown to be consistent with qualitative predictions based on the nodal patterns of the donor molecular orbitals and the forms of the resonantly enhanced normal modes. Mode-specific reorganization energies for the carbazole donor and tetracyanoethylene acceptor are obtained from the analysis along with parameters describing the magnitude and time scale of the solvent contributions to the reorganization energy. Approximately 60% of the total reorganization energy of 5100 cm(-1) in each charge-transfer state is attributed to solvent and any other classically behaved low-frequency modes, with the remainder about equally divided between donor and acceptor modes. The partitioning of the reorganization energy among the carbazole modes is significantly different for the two electronic transitions.