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研究生:梁國淦
研究生(外文):Liang, Kuo Kan
論文名稱:光合作用天線複分子及發光共軛高分子中激子態之性質探討
論文名稱(外文):Aspects of Excitons in Photosynthesis Antenna Complexes and Light-Emitting Conjugated Polymers
指導教授:范文祥范文祥引用關係林聖賢林聖賢引用關係---
指導教授(外文):Fann, WunshainLin, Sheng-Hsien
學位類別:博士
校院名稱:國立臺灣大學
系所名稱:物理學系
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:1998
畢業學年度:86
語文別:英文
論文頁數:125
中文關鍵詞:分子激子光合作用天線複分子發光共軛高分子
外文關鍵詞:Molecular ExcitonPhotosynthesisAntenna ComplexLight-Emitting Conjugate Polymer
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共軛分子形成的分子聚積或分子晶體中的光物理性質主要決定於其中的中性的光激發態之
動態性質。自分子激子的概念出現以來,有許多理論工作都嘗試以分子激子模型來分析這
些中性激發態。
僅考慮系統中分子間的靜態交互作用,會得到同調的激子模型。將耦合系統的罕米耳吞量
對角化後所得之固有態是所謂的激子態。它們的波函數通常是廣布於整個系統之上。由這
些固有態的資料我們可以推得系統的吸收譜、螢光譜等等的光學性質。
由於在分子固體中電子與聲子的耦合很強,分子間和分子內之振動態皆提供了熱庫使得系
統很快地達到熱平衡,以至於系統的同調性很快地喪失。因此同調的激子模型並不足以描
述所有的激子過程。完全不同調的激子模型通常將激子視為古典的粒子,而研究其傳輸性
質。我們可以問:激子所攜帶的能量如何在系統中傳遞。
本文中我們探討完全同調和完全不同調的兩個極限下,以激子模型所研究的兩個個案。在
光合作用天線複分子系統中,我們用同調的激子模型來研究它的吸收光譜。我們特別強調
的重點在於了解天線複分子的環狀聚積方式對於它的吸收光譜性質有什麼影響。我們希望
從而了解天線複分子如何以極高的效率將吸收的光能傳遞到反應中心進行光合作用的化學
反應。
在發光共軛高分子系統中,激子可以復合發光。這是一種很受注目的新興光電材料。但是
在系統中有很多可能的缺陷會影響發光效率。要了解激子的發光效率如何受系統中的缺陷
影響,我們希望能對激子的傳輸性質加以研究,並且更定量地確定激子的自然生命期等有
用的物理量。因此我們採用完全不同調的激子模型。在此模型下,共軛高分子形同一個一
維的晶體,其中具有缺陷。我們修改以往對此類系統提出的模型,對傳統的理論重新檢視
,並試著了解這樣的模型是否真能忠實反映共軛高分子中的光物理過程。在此過程中,我
們也更了解了這樣的模型系統中各個參數所扮演的角色,並且對傳統理論中提出的幾種極
限情形下的漸近解,它們的適用範圍有了更深入的認識,可以提供日後分析類似系統行為
時的參考。
The photophysical properties of aggregates or molecular crystals of
conjugated molecules are deeply involved with the dynamical properties of
neutral excitation species inside. Since the introduction of the concept of
molecular exciton in molecular crystals, there had been a lot of efforts on
describing the behavior of these neutral excitations in a picture of Frenkel
type exciton.
Considering only the static (usually Coulombic) interaction between
molecules in the system, one comes to the coherent exciton picture.
Diagonalizing the Hamiltonian of this interacting molecular system, one
obtains the exciton levels and exciton wave functions. Very often the
continuous wave spectroscopic properties of the system, such as absorption,
circular dichroism, and fluorescence, can be explained well with these
exciton states.
Due to the significant electron-phonon interactions in such systems, the
coupling of molecular exciton states with their environment might be
important. Both intermolecular and intramolecular vibrational modes provide
a heatbath to which the excitations relax both their population and their
coherence. The coherent exciton model, therefore, is insufficient for
describing the dynamics of all kinds of excitonic processes. In the strong
electron-phonon coupling limit, hopping model of totally incoherent and
localized excitons, usually quoted as F\"{o}rster type energy transfer, is
often used to describe the energy transfer process. In between these two
limits, there are theories such as the generalized master equation approach
and the stochastic Liouville equation approach that deal with the coupled
coherent and incoherent motion of excitonic states.
In this thesis, two works in different limits were presented. The first one
is on the coherent exciton model of light-harvesting pigment-protein complex
in photosynthetic bacteria. According to the recently resolved X-ray
structure of these complexes, we constructed a model of circularly
aggregated two-state systems. Exciton levels and wavefunctions in terms of
molecular orbitals were solved. The arrangement of single-molecule
transition dipole moments were used as parameters to calculate the
transition dipole moment of the exciton states. Under point-dipole
approximation we can examine the implications of the detailed structure of
the aggregate on its spectroscopic properties. In the other work, the
dynamics of incoherent exciton on a one-dimensional lattice is studied. A
single polymer chain is viewed as a one-dimensional finite lattice with some
randomly allocated traps. The excitons are thought of as classical particles
moving on the lattice. They are assumed to be able to decay radiatively
while freely moving, and the fluorescence will be quenched when they are
captured by the traps. Pauli master equations of the motion of these
particles were solved in the discrete case. The general properties of the
eigenmodes of the equations of motion are discussed analytically, while the
dynamics are calculated numerically. It is shown that according to this
model, the measured nonexponential fluorescence decay dynamics can provide
information not only about the average properties of the system, such as the
concentration of defects and the diffusion coefficient, but also that of the
inhomogeneity in the system. Asymptotic forms of the dynamics of a similar
system from a classic work which were employed in many studies on the
exciton dynamics in polymers are compared with the numerical results, and
the applicability of the asymptotic forms is examined.
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