Rotationally resolved studies of S0 and the exciton coupled S1/S2 origin regions of diphenylmethane and the d(12) isotopologue.

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.