Resonance Raman intensity analysis of a dicyanovinyl-azaadamantane: Mode-specific reorganization energies for charge-transfer and locally-excited states

A resonance Raman intensity analysis is performed on the intramolecular charge-transfer molecule 1-aza-adamantane-4-ylidenemalononitrile in acetonitrile solution. We explore the extent to which changes in molecular structure upon charge transfer can be obtained from resonance Raman intensity analysis, and extend the analysis method for charge-transfer excitation to take into account the possible influence of nearby locally excited states. Absolute scattering cross sections are measured at five excitation wavelengths spanning both the charge-transfer band at 324 nm and the lowest locally excited band at 231 nm, and the absorption spectra and resonance Raman intensities are modeled self-consistently to obtain the mode-specific reorganization energies accompanying electronic excitation to both states. Interference effects between the two states are considered but are found to be of minimal importance for this particular charge-transfer molecule. The reorganization parameters in terms of dimensionless normal coordinates are converted to actual bond length and bond angle changes by making use of a previously developed ground-state normal mode analysis and by comparing with electronic structure calculations on models for the donor and acceptor ends to reduce the indeterminacy in the signs of the dimensionless displacements. The geometry changes upon excitation to the LE state are dominated by lengthening of the ethylenic C=C bond, while for CT excitation the distortions are distributed over the donor, acceptor, and adamantane bridge, with a smaller C=C bond length change.