Resonance effects of diabatic surface crossing within the torsional spectrum of 9‐(N‐carbazolyl) anthracene observed by supersonic jet fluorescence spectroscopy

Torsional Wave-Packet Dynamics in 2-Fluorobiphenyl Investigated by State-Selective Ionization-Detected Impulsive Stimulated Raman Spectroscopy

We report the creation and observation of vibrational wave packets pertinent to torsional motion in a biphenyl derivative in its electronic ground-state manifold. Adiabatically cooled molecular samples of 2-fluorobiphenyl were irradiated by intense nonresonant ultrashort laser pulses to drive impulsive stimulated Raman excitation of torsional motion. Spectral change due to the nonadiabatic vibrational excitation is probed in a state-selective manner using resonance-enhanced two-photon ionization through the S₁ ← S₀ electronic transition. The coherent nature of the excitation was exemplified by adopting irradiation with a pair of pump pulses: observed signals for excited torsional levels exhibit oscillatory variations against the mutual delay between the pump pulses due to wave-packet interference. By taking the Fourier transform of the time course of the signals, energy intervals among torsional levels with v = 0-3 were determined and utilized to calibrate a density functional theory (DFT)-calculated torsional potential-energy function. Time variation of populations in the excited torsional levels was assessed experimentally by measuring integrated intensities of the corresponding transitions while scanning the delay. Early time enhancement of the population (up to ∼2 ps) and gradual degradation of coherence (within ∼20 ps) appears. To explain the observed distinctive features, we developed a four-dimensional (4D) dynamical calculation in which one-dimensional (1D) quantum-mechanical propagation of the torsional motion was followed by solving the time-dependent Schrödinger equation, whereas three-dimensional (3D) molecular rotation was tracked by classical trajectory calculations. This hybrid approach enabled us to reproduce experimental results at a reasonable computational cost and provided a deeper insight into rotational effects on vibrational wave-packet dynamics.

A supramolecular approach for designing emissive solid-state carbazole arrays

Reactions of 5-(9H-carbazol-9-yl)-isophthalate and metal salts provide laminar metal organic frameworks (MOFs) wherein interlayer interactions of the pendant carbazoles engender extended aromatic stacks or non-stacked structures with enhanced solid-state emission.

Effect of ring torsion on intramolecular vibrational redistribution dynamics of 1,1'-binaphthyl and 2,2'-binaphthyl

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1'-binaphthyl (1,1'-BN) and 2,2'-BN. The dispersed fluorescence spectra of 1,1'-BN in the origin region of S(1)-S(0) were well resolved, which presented 25-27 cm(-1) gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1'-BN is similar to that of naphthalene. In contrast, the spectra of 2,2'-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm(-1) for 1,1'-BN and 658 cm(-1) for 2,2'-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1'-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2'-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into "strong" and "weak" torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.

Fluorescence and REMPI Spectroscopy of Jet-Cooled Isolated 2-Phenylindene in the S1 State

We investigated the spectroscopy of the first excited singlet electronic state S1 of 2-phenylindene using both fluorescence excitation spectroscopy and resonantly enhanced multiphoton ionization spectroscopy. Moreover, we investigated the dynamics of the S1 state by determining state-selective fluorescence lifetimes up to an excess energy of approximately 3400 cm(-1). Ab initio calculations were performed on the torsional potential energy curve and the equilibrium and transition state geometries and normal-mode frequencies of the first excited singlet state S1 on the CIS level of theory. Numerous vibronic transitions were assigned, especially those involving the torsional normal mode. The torsional potentials of the ground and first excited electronic states were simulated by matching the observed and calculated torsional frequency spacings in a least-squares fitting procedure. The simulated S1 potential showed very good agreement with the ab initio potential calculated on the CIS/6-31G(d,p) level of theory. TDDFT energy corrections improved the match with the simulated S(1) torsional potential. The latter calculation yielded a torsional barrier of V2 = 6708 cm(-1), and the simulation a barrier of V2 = 6245 cm(-1). Ground-state normal-mode frequencies were calculated on the B3LYP/6-31G(d,p) level of theory, which were used to interpret the infrared spectrum, the FDS spectrum of the transition and hot bands of the FES spectrum. The fluorescence intensities of the nu49 overtone progression could reasonably be reproduced by considering the geometry changes upon electronic excitation predicted by the ab initio calculations. On the basis of the torsional potential calculations, it could be ruled out that the uniform excess energy dependence of the fluorescence lifetimes is linked to the torsional barrier in the excited state. The rotational band contour simulation of the transition yielded rotational constants in close agreement to the ab initio values for both electronic states. Rotational coherence signals were obtained by polarization-analyzed, time-resolved measurements of the fluorescence decay of the transition. The simulation of these signals yielded corroborating evidence as to the quality of the ab initio calculated rotational constants of both states. The origin of the anomalous intensity discrepancy between the fluorescence excitation spectrum and the REMPI spectrum is discussed.