共軛高分子激發態動力學模擬與中心對稱分子的非零一階超極化率之理論研究

Theoretical study on excited-state molecular dynamics of conjugated polymers and nonvanishing first hyperpolarizabilities of centrosymmetric molecules

張耀文(Yao-Wen Chang)

指導教授 ： 金必耀

國立臺灣大學/理學院/化學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.01455

摘要

（1）利用QCFF/PI (Quantum chemistry Consistence Force Field with PI electron) 來研究長鏈反式聚乙炔分子的激發態動力學。經由一些量子化學的技巧，執行長鏈分字的激發態動力學可以相當的有效率且定量化的模擬受激發分子的行為。許多不同的效應如孤子、激子、極化子與非線性的呼吸震動可在模擬中見到。進一步研究長鏈反式聚乙炔1Bu激子狀態的拓樸結構與其和電子間作用力的關連。我們發現束縛的激子狀態或束縛的孤子-反孤子狀態為長鏈反式聚乙炔之光激發態分子動力學的主要機制。
（2）利用VB-4CT模型作理論研究中心對稱分子並討論對稱性破壞對一階超極化率的效應。分子結構的堅硬度與對於弱微擾的電場感應與分子基態的電荷轉移程度有關。電荷轉移特性是造成基態位能面軟化與分子波函數不穩定的主因。如果給予一個非對稱的微擾，中心對稱守恆會因此失去且分子的性質會因為這不穩定而大幅改變。這機制被用來解釋擁有中心對稱發光團的 furan-containing [2,2]-cyclophandiene 具有異常高的一階超極化率。定量的估計經由結合VB-4CT與量子化學計算來求得。

關鍵字

激子 ； 孤立子 ； 非線性光學

並列摘要

(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 of
electron 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 dramatically
changed because of the instability. The mechanism is applied to explain the exceptional high first hyperpolarizability of furan-containing [2.2]cyclophan-
diene, which has centrosymmetric chromophores. A quantitative estimation is calculated by combining the simple VB-4CT model and quantum chemistry simulation.

並列關鍵字

exciton ； soliton ； nonlinear optics

參考文獻

neck, Nature, 397, 121 (1999).

[2] A. J. Heeger, Rev. Mod. Phys., 73, 681 (2001).

Schrieffer, Proc. Natl. Acad. Sci. USA, 77, 5626 (1980).

[4] W. Barford, Electronic and Optical Properties of Conjugated Polymers, Oxford Science Publications, 2005.

[5] S. Tretiak, A. Saxena, R. L. Martin and A. R. Bishop,