Quantum chemical calculations by INDO/S method of stilbene photoisomers

Abstract

At a twisting of phenyl rings around of single C-C bond in trans-stilbene take place a short-wavelength shift the first (pi) (pi) * transition with decreasing of intensity and mixing (pi) * and (sigma) * MO due to modification of geometry. The planar form of trans-stilbene has a high symmetry and dipole moment in this form is equal to zero both in the ground, and excited states; in nonplanar conformation the molecule obtains some dipole moment. The planar confirmation of cis- stilbene is impossible ((theta) equals 0 degree(s)), as distance between the nearest atoms of hydrogen is equal 0,3 Angstrom. The rotation of rings only around of the double C equals C bond not yield results, which are in good agreement with the experimental absorption spectra. The good agreement of our calculations with the experimental absorption spectra are observed only at rotational angles (theta) equal from 10 to 45 degree(s) together with rotation of phenyl rings around of single C-C bond, (sigma) from 30 to 90 degree(s). The existence of cis-form is possible at (theta) equals 0 degree(s), but at the considerable twisting of phenyl rings rather each other (from 60 up to 90 degree(s)). It is possible to conclude from our calculations that the best emitting properties has molecule of trans-stilbene in planar geometry, at a twisting of phenyl rings around of single C-C bond, the spectral properties of trans-stilbene is similar to cis- conformations. `