Conformational effects on excitonic interactions in a prototypical H-bonded bichromophore: bis(2-hydroxyphenyl)methane.

Laser-induced fluorescence, single-vibronic level fluorescence (SVLF), UV hole burning, and fluorescence dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane in order to characterize the ground-state 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...pi H-bonded conformer with C(2) symmetry. High-resolution ultraviolet spectra ( approximately 50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment (TDM) direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S(2) origin is made to a set of transitions approximately 400 cm(-1) above S(1). In conformer B, the TDM direction firmly establishes C(2) symmetry for the conformer in its S(1) state and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S(2) state has not been clearly identified in the spectrum. Based on CIS calculations, the S(2) state is postulated to be several times weaker than S(1), 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 TDMs that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the TDM surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.