Conformationally Induced Changes in the Electronic Structures of Some Flexible Benzenes. A Molecular Orbital Model

Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-d(12) have been obtained in S(0), S(1), and S(2) electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. The S(0) and S(1) states of both isotopologues have been well fit to asymmetric rotor Hamiltonians that include only Watson distortion parameters. The transition dipole moment (TDM) orientations of DPM and DPM-d(12) are perpendicular to the C(2) symmetry axes with 66(2)%:34(2)% a:c hybrid-type character, establishing the lower exciton S(1) origin as a completely delocalized, antisymmetric combination of the zero-order locally excited states of the toluene-like chromophores. In contrast, the rotational structures of the S(2) origin bands at S(1)+123 cm(-1) and S(1)+116 cm(-1), respectively, display b-type Q-branch transitions and lack the central a-type Q-branch features that characterize the S(1) origins, indicating TDM orientations parallel to the C(2)(b) symmetry axes as anticipated for the upper exciton levels. However, rotational fits were not possible in line with expectations from previous work [N. R. Pillsbury, J. A. Stearns, C. W. Muller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 114301 (2008)] where the S(2) origins were found to be largely perturbed through vibronic interactions with the S(1) symmetric, antisymmetric torsional, and butterfly levels in close proximity. Predictions from a dipole-dipole coupling model and ab initio theories are shown to be in fair agreement with the observed TDM orientations and exciton splitting. The need to include out-of-ring-plane dipole coupling terms indicates that in-plane models are not sufficient to fully account for the excitonic interactions in this bichromophore.

Section: Dipole Model Predictions Of the Exciton Splittingsupporting

confidence: 59%

Rotationally resolved studies of S and the exciton coupled S1/S2 origin regions of diphenylmethane and the d12 isotopologue

Stearns

Pillsbury

Douglass

et al. 2008

“…17 Theoretical studies using the ab initio CIS (6-31G*) method revealed that the effect is due to different degrees of perturbations of the unoccupied benzene * molecular orbitals by the side chain. 18 The perturbations produce a mixing of the low-lying * orbitals which are responsible for optical transitions to the S 1 and S 2 states. Further, single rovibronic linewidth measurements of the origin bands in the S 1 ←S 0 fluorescence excitation spectra of two conformers of n-propylbenzene show that the homogeneous bandwidth of the gauche conformer ͑65 MHz͒ is larger than the trans conformer ͑50 MHz͒.…”

Section: Resultsmentioning

confidence: 99%

Conformational stability of allylbenzene: A combined study by dispersed fluorescence spectroscopy and quantum chemistry calculation

Panja

Chakraborty

2003

Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic π electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the Cα–Cβ bond by 120° from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers.

“…It is worth noting in passing that the fluorescence quantum yield of S 1 toluene has been observed to be temperature dependent, 29 and that the S 1 and S 2 excited states are believed to be mixed as a consequence of toluene adopting a quinoidal structure in S 1 . 30 Because the observed dynamics are occurring out of S 1 ͑v =0͒, IVR cannot be responsible, and this suggests that either intersystem crossing or internal conversion may be occurring, with the spectrum of the resulting T 1 or S 0 state contributing to the observed result, see later. However, even when overlapped 1 ps laser pulses are used to excite and ionize the sample, the ambient and jetcooled photoelectron spectra of S 1 ͑v =0͒ look noticeably different, although the congestion is significantly less pronounced, see Fig.…”

Section: Temperature Dependencementioning

confidence: 97%

Photoelectron spectroscopy of S1 toluene: II. Intramolecular dynamics of selected vibrational levels in S1 toluene studied by nanosecond and picosecond time-resolved photoelectron spectroscopies

Whiteside

King

Davies

et al. 2005

We present results which suggest that the photophysics of S(1) toluene is significantly more complicated than that of the related molecules p-fluorotoluene or p-difluorobenzene. We have measured a range of photoelectron spectra for a number of S(1) internal energies, on different time scales and at different temperatures, in an attempt to unravel the competing processes, but the final conclusion remains outstanding.