Twisted intramolecular charge transfer states: Rotationally resolved fluorescence excitation spectra of 4,4′-dimethylaminobenzonitrile in a molecular beam

Abstract

We report the observation at high resolution of seven vibronic bands that appear within ϳ200 cm −1 of the electronic origin in the S 1 -S 0 fluorescence excitation spectrum of 4,4Ј-dimethylaminobenzonitrile ͑DMABN͒ in a molecular beam. Surprisingly, each band is found to be split into two or more components by a ͑coordinated͒ methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality ͓͑Observed− Calculated͒ ഛ 30 MHz͔ fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized ͑ϳ1°͒ but otherwise ͑heavy-atom͒ planar molecule in its ground S 0 state, and that its electronically excited S 1 state has both a more pyramidalized ͑ϳ3°͒ and twisted ͑ϳ25°͒ dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S 1 ← S 0 electronic transition is accompanied by significant charge transfer from the nitrogen atom to the ‫ء‬ orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.

Photophysical properties of the intramolecular excited charge-transfer states of π-expanded styryl phenanthrimidazoles – effect of solvent polarity

“…These calculations suggest the presence of TICT state and highly Stokes shied uorescence in polar solvents and also support the experimental observations. 58,59 Further conrmation of the TICT phenomenon comes from the frontier molecular orbital (FMO) pictures. The nature of the molecular orbitals involved is illustrated by Fig.…”

High-resolution ultraviolet spectroscopy of p-fluorostyrene-water: Evidence for a σ-type hydrogen-bonded dimer

“…To interpret the observed spectrum and to deduce information on the relevant parameters such as the rotational constants in the ground and the excited states, respectively, and the transition moment ratio, computerassisted fitting techniques must be employed. This approach has been successfully used for the interpretation of high-resolution spectra with densely spaced peaks or with low signal-to-noise ratio, [11][12][13] and it has been applied even to large pliable molecules. They are based on downhill simplex method, simulation annealing, gradient method, etc., 7 and have been successfully applied to the analysis of the highly resolved spectra of isolated molecules and molecular clusters in the gas phase.…”

Ground State 14N Quadrupole Couplings in the Microwave Spectra of N,N′-Dimethylaniline and 4,4′-Dimethylaminobenzonitrile

“…In addition, the splittings of the nitrile group in DMABN should be similar to those of benzonitrile, which have been observed by Grabow and co-workers. 8 Reported here are studies of the microwave spectra of DMA (6-18 GHz) and DMABN (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) in the collision-free environment of a supersonic jet. Broadband experiments were performed to determine substitution structures of both molecules from the spectra of all 13 C-and 15 N-substituted isotopologues in natural abundance.…”