Intramolecular Vibrational Energy Redistribution in Aromatic Molecules of Type C6H5X (X = H, D, F, Cl, CH3, CF3)

Investigation on the vibrational relaxation and ultrafast electronic dynamics of S1 state in 2,4-difluoroanisole

Intramolecular vibrational energy redistribution (IVR) has a profound impact on dynamic processes. We have studied two types of IVR processes, restricted and dissipative, and ultrafast dynamics of the S1 state of 2,4-difluoroanisole using time-resolved photoelectron spectroscopy and time-of-flight mass spectroscopy. The restricted IVR occurs in the intermediate regime of 219 cm-1 vibrational level, and the dissipative IVR occurs in the statistical regime of 1200 cm-1. The lifetimes of IVR processes are measured to be 90 and 11 ps, respectively, depending on the internal energies of the S1 state and differ by a factor of eight. Similar subsequent dynamics were observed at two vibrational levels in the S1 state. The population undergoes IVR following the initial excitation and subsequently leaks into a triplet state, accompanied by intersystem crossing within ∼400 ps followed by a slower nonradiative relaxation of the triplet state on the nanosecond time scale. Furthermore, the values of 3s and 3px Rydberg states of 2,4-difluoroanisole were experimentally determined to be 5.02 and 6.28 eV.

Effects of symmetry, methyl groups and serendipity on intramolecular vibrational energy dispersal

Intramolecular vibrational dispersal of vibrational energy is more efficient in the symmetrically-substituted p-xylene molecule than in p-fluorotoluene, p-chlorofluorobenzene or p-difluorobenzene.

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^-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.

Probing the origins of vibrational mode specificity in intramolecular dynamics through picosecond time-resolved photoelectron imaging studies

Enhanced torsion-vibration coupling associated with a selected vibrational mode is shown to accelerate intramolecular energy flow in p-fluorotoluene.

Structural Relaxation of Photoexcited Quaterthiophenes Probed with Vibrational Specificity

Ultrafast structural relaxation of photoexcited 2,2':5',2″:5″,2‴-quaterthiophene (4T) and 3,3‴-Dihexyl-2,2':5',2″:5″,2‴-quaterthiophene (DH4T) in solution were interrogated with femtosecond stimulated Raman spectroscopy (FSRS). Relaxation was observed through time-dependent evolution in frequencies and intensity ratios of out-of-phase (Z) and in-phase (Я) intraring C═C stretching features. Frequency shifts occurred on time scales of 0.4 and 0.86 ps, respectively, dominated by a blue shift in the Z mode (6.2 and 11.5 cm(-1) shifts for 4T and DH4T, respectively). Intensity ratios evolved on similar time scales due to correlated intensity decreases and increases of Z- and Я-mode features. Excited-state quantum-chemical calculations with bithiophene demonstrate that mode frequencies are coupled to the torsional dihedral, such that the spectral evolution observed reflects excited-state relaxation toward a planar conformation. This work demonstrates the power of ultrafast Raman spectroscopy for probing dynamics in photoexcited conjugated materials with structural detail given the parametric dependence of intraring vibrational modes on interring torsional dihedrals.