Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-4 hydroxy)-diphenylmethane (b4HBM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM1 has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. The rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S1) levels, the transition dipole moment (TDM) orientations are perpendicular to the C2 symmetry axes and consist of 41(2):59(2) % and 34(2):66(2) % a:c hybrid-type character. The S1 levels are therefore delocalized states of B symmetry and represent the anti-symmetric combinations of the zero-order locally excited states of the p-cresol-like chromophores. The TDM polarizations of bands located at ≈ 132 cm-1 above the S1 origins are exclusively b-type and identify them as the upper exciton S2 origin levels of A symmetry. These TDM orientations and the relative band strengths from the vibronic study have been analyzed within a dipole-dipole coupling model in terms of the localized TDM orientations, μloc, on the two chromophores. The out-of-the-ring plane angles of μloc are both near 20º and are similar to results for DPM2. The in-plane angles are, however, rotated by 14º and 18º relative to DPM and may explain the smaller than expected exciton splitting of these two conformers.
Citation: Journal of Physical Chemistry A
Pub Type: Journals
dipole-dipole coupling model, exciton coupling, excitonic interactions, rotationally resolved uv spectroscopy