Optical properties of trans-stilbene using semiempirical and time-dependent density functional theory: A comparative study

Abstract

The present study focuses on the low-lying valence electronically excited states of trans-stilbene. A comparison is made between semiempirical CIS-ZINDO/S and ab initio time-dependent density functional theory (TD-DFT) results obtained in conjunction with a cc-pVDZ basis set and using three different functionals, namely SVWN, B3LYP, and B3P86. Geometry optimization with the B3LYP functional and the cc-pVDZ basis set provides as most stable form an almost planar structure. With the exception of the TD-SVWN approach, and despite the limitation of the cc-pVDZ basis set, these methods provide overall a rather satisfactory insight into the main valence transitions identified experimentally from one-photon and two-photon absorption measurements. In particular, our CIS-ZINDO/S and TD-DFT results for excitation energies and symmetry assignments of lines are in rather close agreement with a recent CASPT2 study. On the other hand, only qualitative agreement can be claimed for the oscillator strengths and Line composition, in particular at the highest excitation energies, where strong configurational mixing between excited states occurs. As a by-product of the present study, our TD-DFT results for the energy excitation threshold, lambda (max), indirectly confirms the planar nature of trans-stilbene in the vacuum, even at room temperature. (C) 2000 John Wiley & Sons, Inc.