Resonance Raman Intensity Analysis of the Carbazole/Tetracyanoethylene Charge-Transfer Complex:  Mode-Specific Reorganization Energies for a Hole-Transport Molecule

Abstract

A resonance Raman intensity analysis is presented for the carbazole/tetracyanoethylene donor-acceptor chargetransfer 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.

Understanding the Resonance Raman Scattering of Donor−Acceptor Complexes using Long-Range Corrected DFT

“…First, the figure derived from experimental data has constructive interference present for modes involving ∆ k n values that are opposite in sign for the CT1 and CT2 states. This disagrees with the analysis presented in ref 36, where having ∆ k n values with opposite signs for the CT1 and CT2 states was thought to be the only factor causing destructive interference. Because the transition dipole moments are parallel (directed along the positive z-axis), their components have the same sign and cannot result in destructive interference effects.…”

“…The lowenergy portion of the optical absorbance spectra for these complexes include excitations where an electron is transferred from the donor (carbazole or HMB) to the acceptor (TCNE) molecule. [36][37][38] The complex HMB/TCNE has a single CT state on its absorbance spectrum, while carbazole/TCNE has two energetically close CT states that overlap in one band on the absorbance spectrum. For the carbazole/TCNE complex, contributions from two CT states result in interference effects, which have been observed in previous studies that focused on experimentally derived fits to resonance Raman spectra.…”

“…Absorbance spectra for carbazole/TCNE with fits compared to the experimental absorbance spectrum (black line, ref36). In the figures, the total fit is the blue line, the fit for the CT1 state (at lower energy) is the red line, and the fit for the CT2 state (at higher energy) is the green line.…”

Probing the excited state nature of coordination complexes with blended organic and inorganic chromophores using vibrational spectroscopy

“…This is because the resonance enhancement of band intensity is related to a number of factors including ΔQ (Figure 1) and the curvature of the normal mode of vibration (ω). [48] Such measurements are uncommon in metal polypyridyl systems [53][54][55][56][57] for a number of reasons: firstly the large number of normal modes that exist in many polypyridyl systems can make unambiguous determination of structural changes challenging; secondly the advent of ultrafast lasers means that many spectroscopic techniques can actually time-resolve evolution from the FC state and this can be as informative as a resonance excitation profile study. [58,59] Over the last few years accurate modelling of the resonance excitation profiles and resonance Raman spectra has been pioneered by the Guthmuller group.…”

Understanding excited-state structure in metal polypyridyl complexes using resonance Raman excitation profiles, time-resolved resonance Raman spectroscopy and density functional theory

“…A number of these are reported for common solvents such as dichloromethane [33,34], chloroform [35], methanol [34] and DMSO [36].…”