Rotationally resolved studies of S-0 and the exciton coupled S-1/S-2 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.