Rotationally Resolved C 2 Symmetric Conformers of Bis-(4-hydroxyphenyl)methane: Prototypical Examples of Excitonic Coupling in the S1 and S2 Electronic States

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-09, Vol.115 (34), p.9643-9652
• Main Authors: Chou, Shin Grace, Rodrigo, Chirantha P, Müller, Christian W, Douglass, Kevin O, Zwier, Timothy S, Plusquellic, David F
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp200804t

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 (S1) 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 S1 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 S1 origins are exclusively b-type and identify them as the upper exciton S2 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, μloc, on the two chromophores. The out-of-the-ring plane angles of μloc 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.