Difference Bands in Time-Resolved Femtosecond Stimulated Raman Spectra of Photoexcited Intermolecular Electron...

The time-resolved femtosecond stimulated Raman spectra (FSRS) of a charge transfer (CT) excited noncovalent complex tetracyanoethylene:1-chloronaphthalene (TCNE:ClN) in dichloromethane (DCM) is reported with 40 fs time resolution. In the frequency domain, five FSRS peaks are observed with frequencies of 534, 858, 1069, 1392, and 1926 cm^-1. The most intense peaks at 534 and 1392 cm^-1 correspond to fundamentals while the features at 858, 1069, and 1926 cm^-1 are attributed to a difference frequency, an overtone and a combination frequency of the fundamentals, respectively. The frequency of the 1392 cm^-1 fundamental corresponding to the central C═C stretch of TCNE^•- is red-shifted from the frequency of the steady state radical due to the close proximity and electron affinity of the countercation. The observation of a FSRS band at a difference frequency is analyzed. This analysis lends evidence for alternative nonlinear pathways of inverse Raman gain scattering (IRGS) or vertical-FSRS (VFSRS) which may contribute to the time-evolving FSRS spectrum on-resonance. Impulsive stimulated Raman measurements of the complex show coherent oscillations of the stimulated emission with frequencies of 153, 278, and 534 cm^-1. The 278 cm^-1 mode corresponds to Cl bending of the dichloromethane solvent. The center frequency of the 278 cm^-1 mode is modulated by a frequency of ∼30 cm^-1 which is attributed to the effect of librational motion of the dichloromethane solvent as it reorganizes around the nascent contact ion pair. The 153 ± 15 cm^-1 mode corresponds to an out-of-plane bending motion of TCNE. This motion modulates the intermolecular separation of the contact ion pair and thereby the overlap of the frontier orbitals which is crucial for rapid charge recombination in 5.9 ± 0.2 ps. High time-frequency resolution vibrational spectra provide unique molecular details regarding charge localization and recombination.