title: Rotationally resolved C2 symmetric conformers of bis-(4-hydroxyphenyl)methane: prototypical examples of excitonic coupling in the S1 and S2 electronic states.
abstract: 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.
authors.affiliation: Biophysics Group, Optical Technology Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8441, USA.
authors.forename: Shin Grace
authors.initials: SG
authors.lastname: Chou
authors.forename: Chirantha P
authors.initials: CP
authors.lastname: Rodrigo
authors.forename: Christian W
authors.initials: CW
authors.lastname: Müller
authors.forename: Kevin O
authors.initials: KO
authors.lastname: Douglass
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
authors.forename: David F
authors.initials: DF
authors.lastname: Plusquellic
mesh_terms: D001559:Benzhydryl Compounds / Q000737:chemistry*; D002627:Chemistry, Physical; D007700:Kinetics; D016018:Least-Squares Analysis; D008697:Methane / Q000031:analogs & derivatives / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D012399:Rotation; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*; D013237:Stereoisomerism; D013816:Thermodynamics; D014732:Vibration
chemical_list: D001559:Benzhydryl Compounds; C010129:diphenylmethane; D008697:Methane