Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-d^12 have been obtained in S0, Sd1 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) % ac-type character establishing the lower exciton S1 origin as completely delocalized and the anti-symmetric combination of the zero-order locally excited states. In contrast, similar b-type rotational fits of the upper excitonic S2 origin bands at S1 + 123 cm-1 and S1 + 116 cm 1, respectively, 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., in press.] where the S2 origins were found to be largely perturbed though vibronic interactions with the S1 symmetric (T), anti-symmetric torsional ( ) and butterfly (β) levels in close proximity. The symmetry and character of the mixed S1 |v(T), v( ), v(β)> levels previously assigned in DPM include |0,5,0>, |2,3,0>, |4,1,0> and |0,3,1> suggesting Fermi and/or Coriolis interactions as possible coupling mechanisms for such strong mixing between vibronic levels with Δν(T) and/or Δν( ) > 1. The observed TDM components and exciton splitting from these studies are shown to be in poor agreement with predictions from the dipole coupling model and TDDFT ab initio methods.
Citation: Journal of Chemical Physics
Pub Type: Journals
Exciton Interactions, Exciton Coupling, Dipole Coupling Model, Rotationally Resolved UV Spectroscopy.