Chirantha P. Rodrigo, Christian Muller, Nathan Pillsbury, William H. James III,
, Timothy Zwier
The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) has been explored using fluorescence excitation (FE), dispersed fluorescence (DFL), UV-UV holeburning (UVHB), UV depletion (UVD), and fluorescence-dip infrared (FDIR) 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 S0-Sd1 origins at 35184 cmu1 and 35209 cmu1, 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-bar), ring-flapping (R and R-bar) and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C2 symmetric uu and dd conformers. The Sd0-S2 origins are assigned to bands located 132 cmu1 above the S0-Sd1 origins of both conformers. DFL spectra from the S2 origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the Sd2 origin occurs dominantly to S1 R-baru1 and S1 R-baru1 βu1 levels, which are located 15 and +31 cmu1 from it. Unusual vibronic activity in the ring-breathing (Ņ1) and ring deformation (Ņd6a) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multi-mode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton, R. L., Gouterman, M.; J. Chem. Phys. 1961, 35, 1059.] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the Sd2 origin is strongly mixed (60%/40%) with S^d1 and levels.
Journal of Chemical Physics
internal mixing, internal conversion, electronic energy transfer, vibronic coupling, bichromophore, vibronic coupling model