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Density Functional Studies of the Ground‐ and Excited‐State Potential‐Energy Curves of Stilbene cis–trans Isomerization
Author(s) -
Han WenGe,
Lovell Timothy,
Liu Tiqing,
Noodleman Louis
Publication year - 2002
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/1439-7641(20020215)3:2<167::aid-cphc167>3.0.co;2-g
Subject(s) - diradical , excited state , isomerization , potential energy surface , chemistry , singlet state , ground state , potential energy , photoisomerization , singlet fission , density functional theory , triplet state , molecular physics , atomic physics , computational chemistry , molecule , physics , biochemistry , organic chemistry , catalysis
Using spin‐unrestricted density functional theory (the VWN Becke–Perdew potential), including broken‐symmetry and spin‐projection methods, we have obtained the potential‐energy curves as a function of the central torsional angle of stilbene in the ground (S 0 ), the first excited triplet (T 1 ), the first excited singlet (S 1 ), and the doubly excited singlet (S 2 ) states. The thermal trans→cis isomerization of stilbene passes through a diradical broken‐symmetry electronic structure around the twisted conformation (90° central torsional angle) in the ground state. Our calculations support the proposed triplet mechanism for sensitized cis ⇌ trans photoisomerization and the nonadiabatic singlet mechanism proposed by Orlandi and Siebrand. 1 On the T 1 potential‐energy curve, the rotation of the C=C bond for both trans ‐ and cis ‐stilbene will lead stilbene to the twisted conformation, from which the twisted stilbene will decay to the ground‐state surface that is nearly isoenergetic with the T 1 surface and has diradical electronic structure in the twisted region. On the S 1 potential‐energy curve, the energy increases in the direction from trans ‐ to the twisted stilbene, and crosses with the neutral doubly excited S 2 potential‐energy curve, which has a minimum at the twisted structure and is lower in energy than the zwitterionic doubly excited state. The twisted stilbene around the energy minimum of the neutral doubly excited S 2 ‐state will decay onto the ground‐state surface from where the rotation of the C=C bond leads the twisted stilbene to either the trans or cis configuration and the isomerization of stilbene is then completed. Similar studies have also been performed on a stilbene derivative with a substituent group, NHCOCH 3 .