화학공학소재연구정보센터
Journal of Physical Chemistry A, Vol.115, No.34, 9643-9652, 2011
Rotationally Resolved C-2 Symmetric Conformers of Bis-(4-hydroxyphenyl)methane: Prototypical Examples of Excitonic Coupling in the S-1 and S-2 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.; Muller, 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 approximate to 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, mu(loc), on the two chromophores. The out plane angles of mu(loc) are both near 20 degrees and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K.O.; Muller, 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 degrees relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.