Temperature effects on the UV-Vis electronic spectrum of trans-stilbene

Abstract

The ultraviolet (UV)-Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by coupling molecular dynamics (NM) simulations with the classical MM3 force field to ZINDO/S-CIS calculations of vertical excitation energies and transition dipole moments. The selection of a large number of structures along the MID trajectories enables a consistent treatment of temperature effects in the vacuum, whereas the ZINDO/S-CIS calculations permit a reliable treatment of electron correlation and relaxation, taking account of multistate interactions in the final state. Thermal motions are found to alter very differently the width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very strongly influence the appearance of the first valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) transition. At temperatures less than 400 K, these are found to yield a merely Gaussian and very pronounced thermal broadening of the related band (A), up to nearly 30 run, together with a minor blue shift of its maximum lambda (max). En contrast, a red shift by several nanometers occurs due to thermal motions for the remaining three valence bands. As can be expected, the broadening intensifies at higher temperatures, and for the A-band, becomes markedly asymmetric when T exceeds 400 K. The combination of MD(MM3) and ZINDO/S-CIS computations enables also consistent calculations of hot bands, which are forbidden by symmetry at 0 K. (C) 2001 John Wiley & Sons, Inc.