Calculations of absorption and emission spectra: A study of cis-stilbene

Abstract

Absorption and emission spectra are calculated by modeling cis‐stilbene as a system of 12 displaced harmonic oscillators. We are able to obtain good agreement with the peaks of both the room temperature absorption and low temperature emission spectra using parameters from resonance Raman experiments by adjusting the position of the zero–zero transition energy (E_0,0) and slightly adjusting the displacements along the normal modes. The best fit value for E_0,0 is 29 000 cm⁻¹. Using the displacements along the 12 degrees of freedom, and a normal mode description based on semiempirical quantum force field calculations (QCFF/PI), we determine a ‘‘relaxed’’ excited state geometry which is twisted a maximum of approximately 37 deg along the ethylenic torsional coordinate relative to the relaxed ground state geometry. An extension of the spectral calculations is described which allows for one or more of the modes to be anharmonic and vibrationally unrelaxed. We apply this extension to show that cis‐stilbene emission can not be originating from a geometry with a 90 deg twist along the ethylenic coordinate. Comparison of our results with a recently obtained room temperature solution phase emission spectrum suggest that this emission originates from vibrationally unrelaxed molecules.