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Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: bis-(4-hydroxyphenyl)methane.

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV-UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S(0)-S(1) origins at 35,184 and 35,209 cm(-1), respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and T), ring-flapping (R and R), and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C(2) symmetric uu and dd conformers. The S(0)-S(2) origins are assigned to bands located 132 cm(-1) above the S(0)-S(1) origins of both conformers. DFL spectra from the S(2) origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S(2) origin occurs dominantly to S(1) R(1) and S(1) R(1)β(1) levels, which are located -15 and +31 cm(-1) from it. Unusual vibronic activity in the ring-breathing (ν(1)) and ring-deformation (ν(6a)) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S(2) origin is strongly mixed (60%/40%) with S(1) 6a(1) and 1(1) levels. The direct extension of this model to the T/T and R/R inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S(1)/S(2) mixing compared to the ring modes.