Rotationally Resolved C2 Symmetric Conformers of Bis-(4-hydroxyphenyl)methane: Textbook Examples of Excitonic Coupling in the S1 and S2 Electronic States
Shin G. Chou, Chirantha P. Rodrigo, Christian Muller, Kevin O. Douglass, Timothy Zwier, David F. Plusquellic
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.