Theoretical study on excited-state molecular dynamics of conjugated polymers and nonvanishing first hyperpolarizabilities of centrosymmetric molecules
Date Issued
2008
Date
2008
Author(s)
Chang, Yao-Wen
Abstract
(1) QCFF/PI ( Quantum Chemistry Consistent Force Field with Pi Electron ), one of the earliest developed QM/MM force field consistent in both ground state and excited state, is used studying the excited-state dynamics of trans-polyacetylene. With applying quantum chemistry techniques, the excited-state molecular dynamics can perform efficiently and quantitatively simulate the behavior of excited molecules. Various static and dynamic effects such as soliton excitations, polaron excitations, exciton self-trapped and nonlinear breathers are observed in the simulation. The topological geometry of 1Bu exciton state of trans-polyacetylene and the dependence oflectron correlation are further investigated. We found the bound exciton or bound S+S− pair are the major mechanism for the photoexcited molecular dynamics of trans-polyacetylene.2) The centrosymmetric molecules are theoretically studied using VB-4CT model and the effect of broken symmetry on first hyperpolarizability is discussed. The stiffness of molecular geometry and the electric susceptibility to weak perturbation are related to the extent of charge transfer of the molecular ground-state. The charge transfer character is responsible to the softening of ground-state potential surface and instability of molecular wave function. If there exist an asymmetric perturbation, the conservation of symmetry will be lost and the molecular properties will be dramaticallyhanged because of the instability. The mechanism is applied to explain the exceptional high first hyperpolarizability of furan-containing [2.2]cyclophan-iene, which has centrosymmetric chromophores. A quantitative estimation is calculated by combining the simple VB-4CT model and quantum chemistry simulation.
Subjects
exciton
soliton
nonlinear optics
Type
thesis
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