Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy

Abstract

Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz(2)) and trimer (Bz(3)) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000-3100 cm(-1) region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz(2), a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C(6)H(6) and d = C(6)D(6), and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140-170 ps] is ~2.5 times shorter than that of the Top site [h(top)d(stem), 370-400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz(3) also exhibited the stepwise VER process, though each step is faster than Bz(2).

Low‐energy structures of benzene clusters with a novel accurate potential surface

“…Other attempts to obtain a complete PES for the benzene dimer rely on the DFT‐based symmetry‐adapted perturbation theory (i.e., the SAPT(DFT) approach) with triple‐ ζ basis set, which allows for relatively accurate results at a low computational cost. These and more accurate CCSD(T) calculations, at the complete basis set (CBS) limit, indicate that the T‐shape conformer of the benzene dimer (with a center‐to‐center distance of 4.96 Å) is more stable than the displaced‐stacked structure; this is in agreement with experimental results . However, since both conformers are almost isoergic, it has been observed that the energetic order is quite dependent on the accuracy of the method employed in the calculation; a comprehensive review on benzene dimer calculations involving different methods is given in a recent article by Xantheas and coworkers, where they also present a detailed study about the displaced‐stacked conformer at a high level of theory.…”

Intramolecular vibrational redistribution: from high-resolution spectra to real-time dynamics

“…Due to the high sensitivity of photoionization detection, the method successfully operates in conjunction with a cold molecular beam, and so it can also be used for studying IVR in small clusters. Such a possibility was recently demonstrated in paper [248] by the example of benzene dimers and trimers. …”

The Role of Torsional Dynamics on Hole and Exciton Stabilization in π‐Stacked Assemblies: Design of Rigid Torsionomers of a Cofacial Bifluorene

“…These findings underscore the diverse requirements for charge and energy delocalization across p-stacked assemblies.Charge and energy dynamics across p-stacked assemblies is of critical importance in biological systems as well in the performance of functional materials. [1][2][3] Va rious p-stacked dimers capable of excimer formation and charge-resonance stabilization based upon benzene, [4][5][6][7][8] fluorene, [9][10][11][12] pyrene, [13,14] and other aromatic donors [15][16][17][18] have served as model systems to gain fundamental insight into key factors controlling charge/energy stabilization. These studies have established that the extent of the cationic charge (that is,hole) and exciton stabilization/delocalization is dependent on the orbital overlap between the chromophores,a nd is optimal when the p-stacked assembly adopts ap erfect sandwich-like arrangement, where orbital overlap and electronic coupling are at their maxima.…”