Modeling of the optical properties of cofacial chromophore pairs: Stilbenophane

Using electronic absorption and fluorescence spectroscopic techniques, as well as quantum chemical calculations, we have studied the electronic spectra of thia-bridged stilbenophane (TSP) with close cofacial contact of two trans-stilbene (t-SB) units. Compared to the t-SB monomer, the experimental consequences of the cofacial arrangement are (i) a splitting of the main absorption band with a weakly allowed emitting state, and (ii) a strongly red-shifted, unstructured emission spectrum with long fluorescence decay times. According to the theoretical investigations, the two t-SB units are strongly bent in the electronic ground state (SO), because of repulsive pi-pi overlap. In the first excited state (S I), the t-SB units become almost planar, because of attractive pi*-pi* overlap. As a consequence, the symmetry-forbidden S-0 <----> S-1 transition couples strongly to interchromophore breathing modes of low frequency (v(1) = 67 cm(-1), V-2 = 117 cm(-1)), yielding structureless spectra with large Stokes shifts. The features of the calculated spectra are in good agreement with the experimental data. The results indicate that strong intermolecular vibronic coupling is also responsible for "excimer-like" emission in organic molecular crystals of cofacially arranged molecules. Furthermore, the different geometries in the S-0 and S-1 states of TSP give evidence for the mechanism of [2+2]photodimerization of t-SB in solutions.