Knowledge Bank

Diphenylmethane is a simple bichromophore in which two phenyl rings are connected only by a methylene group, giving considerable flexibility in the ring torsion coordinates as well as substantial electronic coupling between the rings. According to DFT calculations, the minimum energy structure has C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ symmetry with a ring torsion angle near 60. There are two such minima connected by a C structure 176 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ higher in energy. The rotationally-resolved fluorescence excitation spectrum of the S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ origin indicated a 70% a-type, 30% c-type transition moment, suggestive of an excitonic state in which the zero-order transition moment is rotated significantly from that of toluene. CIS underestimates the a-type character of the band, while time-dependent DFT overestimates it. The ground state experimental rotational constants are consistent with those predicted by DFT. A Franck-Condon progession in the torsional coordinate was observed in the experimental jet-cooled resonant two-photon ionization and dispersed fluorescence spectra. This progression was well-fit using a harmonic Franck-Condon analysis indicating a 3 change in the ring torsion angle upon excitation to S$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$. The S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ origin was assigned to a vibration 123 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ above the S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ origin. The dispersed fluorescence spectrum from this transition showed a great deal of activity in low frequency vibrations which were not present in the excitation spectrum, indicating that a vibronic band of the S$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ state may be resonant with the S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ origin. To test the effect of asymmetry on the excitonic coupling, the spectroscopy of 4-methyldiphenylmethane has also been studied. Addition of a methyl group to one chromophore completely localizes the electronic excitation, demonstrated by the fact that the two electronic origins are very near those of toluene and {\it para}-xylene. Dispersed fluorescence from the S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ origin showed only S$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-S$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ origin-like emission broadened by IVR, indicative of extremely rapid electronic energy transfer.