Electronic Structure of Long Polyenes: States and Transition Dipoles
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ELECTRONIC STRUCTURE OF LONG POLYENES: STATES AND TRANSITION DIPOLES BRYAN E. KOHLER Chemistry Department, University of California, Riverside, CA 92521, U.S.A.
ABSTRACT Electronic excitation energies and transition dipoles measured for symmetrically substituted linear polyenes (R-(CH=CH)N-R) with chain lengths n ranging from 2 to 8 are well fit by the functional form A+B/N. Extrapolation to the long chain limit correlates the band gap transition of polyacetylene with the polyene I IAg to Il Bu transition. The amount by which the highly correlated polyene 2l A state is lower than the II B state increases with increasing chain length, extrapolating to roughly half of the I IBu excitation energy. The 1IA to I Bu transition dipole magnitude per chain repeat unit attains a maximum value of 3.0 Debye at an effective conjugation length 5 double bonds. Careful studies on linear polyenes provides insight into the electronic structure of polyacetylene. 1. INTRODUCTION Our understanding of polyene electronic structure has advanced significantly during the last decade [1]. The discovery that the lowest energy excited singlet state for linear polyenes is not the 1lBu state derived by promoting a single electron from the highest energy occupied molecular orbital (HOMO to the chemist, top of the valence band to the physicist) to the lowest energy unoccupied molecular orbital (LUMO to the chemist, bottom of the conduction band to the physicist) but, rather, the 21A state that can roughly be described as a double excitation from the HOMO to the LUMO is especially important. This unexpected ordering of excited states calls attention to the unusual importance of electron correlation in these molecules and raises questions about theoretical models for linear polyenes, including polyacetylene, that neglect explicit consideration of electron-electron interactions [1-3]. This contribution reviews two aspects of linear polyene electronic structure that are of special interest with respect to conjugated polymers. These are the chain length dependence of I A g to 21 A and I IBu excitation energies and transition dipole magnitudes. A detailed discussion of how the polyene 2 'A state isomerization behavior and excitation energy extrapolated to longer chain length is consistent with the transient absorption spectra and thermal isomerization of polyacetylene has been published elsewhere [4].
Mat. Res. Soc. Symp. Proc. Vol. 173. @1990 Materials Research Society
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2l A and I BU EXCITATION ENERGIES Excitation energies of finite polyenes with the general structure R-(CH=CH)N-R' depend
on the substituents R and R' and on the microscopic local environment (solvent). However, for N>3 the 2 1A state lies below the
IBu state for all substituents and all solvent environments that have so far been investigated [1,5]. The fact that the 21 A state is electronically distinct from the 11Bu state, that is, that it is not the I IBu state in a geometry distorted with respect to the ground state equilibrium geometry, deserves special emphasis. In li
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