Laser induced fluorescence (LIF), single vibronic level fluorescence, UV holeburning, and fluorescence-dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane (2HDPM) in order to characterize the ground and first excited state vibronic spectroscopy of this model flexible bichromophore. These studies identified the presence of two conformational isomers. The FDIR spectra in the OH stretch region determine that conformer A is an OH O H-bonded conformer, while conformer B is a doubly OH H-bonded conformer with C2 symmetry. High resolution, ultraviolet spectra (~50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S2 origin is made to a set of transitions ~400 cm-1 above S1. In conformer B, the TDM direction firmly establishes C2 symmetry for the conformer in its S1 state, and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S2 state has not been clearly identified in the spectrum. Based on CIS calculations, the S2 state is postulated to be several times weaker than S1, making it difficult to identify, especially in the midst of overlap from vibronic bands due to conformer A. SVLF spectra show highly unusual vibronic intensity patterns, particularly in conformer B, which cannot be understood by simple harmonic Franck-Condon models, even in the presence of Duschinsky mixing. We postulate that these model flexible bichromophores have transition dipole moments that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the transition dipole moment surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.
Citation: Journal of Physical Chemistry A
Pub Type: Journals
flexible bichromophore, exciton splitting, moment, hydrogen bond, transition dipole vibronic coupling