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. Müller , 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.