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Difference Bands in Time-Resolved Femtosecond Stimulated Raman Spectra of Photoexcited Intermolecular Electron Transfer from Chloronaphthalene to Tetracyanoethylene
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<sup>–1</sup>. The most
intense peaks at 534 and 1392 cm<sup>–1</sup> correspond to
fundamentals while the features at 858, 1069, and 1926 cm<sup>–1</sup> are attributed to a difference frequency, an overtone and a combination
frequency of the fundamentals, respectively. The frequency of the
1392 cm<sup>–1</sup> fundamental corresponding to the central
CC stretch of TCNE<sup>•–</sup> 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<sup>–1</sup>. The
278 cm<sup>–1</sup> mode corresponds to Cl bending of the dichloromethane
solvent. The center frequency of the 278 cm<sup>–1</sup> mode
is modulated by a frequency of ∼30 cm<sup>–1</sup> 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<sup>–1</sup> 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