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

Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-$d12$ have been obtained in $S0$⁠, $S1$⁠, and $S2$ electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. The $S0$ and $S1$ 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-$d12$ are perpendicular to the $C2$ symmetry axes with 66(2)%:34(2)% $a:c$ hybrid-type character, establishing the lower exciton $S1$ 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 $S2$ origin bands at $S1+123cm−1$ and $S1+116cm−1$⁠, respectively, display $b$-type $Q$-branch transitions and lack the central $a$-type $Q$-branch features that characterize the $S1$ origins, indicating TDM orientations parallel to the $C2(b)$ 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. Müller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 114301 (2008)] where the $S2$ origins were found to be largely perturbed through vibronic interactions with the $S1$ 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.