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

Abstract

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.

Vibronic coupling in asymmetric bichromophores: Experimental investigation of diphenylmethane-d5

“…Electronic addresses: david.plusquellic@nist.gov and zwier@purdue.edu the delocalized nature of the S 1 and S 2 electronic states of both DPM-d 0 and DPM-d 12 . 5 Single vibronic level dispersed fluorescence spectra showed striking effects of vibronic coupling associated with the intermingled vibronic levels from the two states, which produces vibronic levels with mixed S 1 /S 2 character that is reflected in the emission. This experimental work has been complemented by the development of a multi-mode vibronic coupling model general enough to handle both symmetric and asymmetric bichromophores, and to allow for the inclusion of low-frequency inter-monomer modes.…”

Fundamentals of Electronic Spectroscopy

“…In the case (Stearns et al 2008). The principal axis system of structure (c) has the b axis along the C 2 symmetry axis, the c axis pointing out of the plane of the page, and the a axis parallel to the line connecting the centers of mass of the two chromophores.…”

“…In all these cases, the electronic spectrum can be qualitatively understood from the interaction of several localized excitations, which are referred to as excitons (Coffman and McClure 1958). Diphenylmethane, depicted in Figure 50, is a prototypical molecule with two nearly degenerate interacting aromatic chromophores (Stearns et al 2008). The π − π * excitations occur in the phenyl rings and interact with each other.…”

“…The electronic transitions to these two electronic states, also labeled S 1 and S 2 , were recently investigated by rotationally resolved laser-induced fluorescence spectroscopy in the unperturbed environment of a molecular jet (Stearns et al 2008). The origins of the S 1 ← S 0 and S 2 ← S 0 transitions in (doubly deuterated) diphenylmethane-d 1,2 are shown in Figure 51(a) and (b), respectively.…”

Binding water to a PEG-linked flexible bichromophore: IR spectra of diphenoxyethane-(H2O)n clusters, n = 2-4

“…zwier@purdue.edu While much of the focus of past work [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] has been on the vibronic coupling between the close-lying S 1 and S 2 states, a necessary prerequisite for such understanding is establishing the inherent ground-state conformational preferences of the molecule, which serve as the starting geometry for excitation onto the excited state surfaces. To that end, the observed vibronic and infrared (IR) transitions of DPOE have been previously assigned to the ttt and tgt conformers shown in Figure 1(a).…”