Rotationally resolved C2 symmetric conformers of bis-(4-hydroxyphenyl)methane: prototypical examples of excitonic coupling in the S1 and S2 electronic states.

Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM [Rodrigo, C. P.; Müller, C. W.; Pillsbury, N. R.; James, W. H., III; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2011, 134, 164312] has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. In the present study, the rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S(1)) levels, the transition dipole moment (TDM) orientations are perpendicular to the C(2) symmetry axes and consist of 41(2):59(2) and 34(2):66(2)% a:c hybrid-type character. The S(1) levels are therefore delocalized states of B symmetry and represent the antisymmetric 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 S(1) origins are exclusively b-type and identify them as the upper exciton S(2) origin levels of A symmetry. The 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, μ(loc), on the two chromophores. The out-of-the-ring plane angles of μ(loc) are both near 20° and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K. O.; Müller, C. W.; Zwier, T. S.; Plusquellic, D. F. J. Chem. Phys. 2008, 129, 224305]. The in-plane angles are, however, rotated by 14 and 18° relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.