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有机树形分子激发态的光化学反应机理研究
黄静
Thesis Advisor兰峥岗
2015-05
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline化学工程
Keyword有机树形分子 分子内激发态的能量转移 非绝热动力学 Jahn-teller效应 荧光各向异性衰减
Abstract为了解决全球可再生能源的需求,光能的利用和转化显得尤为重要。因此,发展新型光伏材料用于光能的利用和转化,尤其是太阳能的捕获或传输面临着巨大挑战。作为光伏材料中具有广泛应用前景的有机树形分子近年来在光捕获体系,光发射设备,生物医药成像及非线性光谱设备等领域受到了广泛关注,这主要是由于有机树形分子具有高效的能量收集与捕获的特性;同时,在光学激发过程中分子内会发生有效的能量转移过程。一般来说,有机树形分子是由一个中心体和分叉单元构成,其中分叉单元的结构可分为等长与不等长两大类。本文主要针对这两类有机树形分子的激发态能量转移机理进行了研究。 对于体系一,我们采用不同的从头算电子结构方法SCS-ADC(2), TDHF, TDDFT 结合不同泛函(B3LYP, BH&HLYP, CAM-B3LYP),以及DFT/MRCI对含有不等长线性分叉单元的phenylene ethynylene(PE)树形分子的激发态性质进行了比较详细的研究。研究发现,光首先激发分子的边缘基团,能量从较短分支传向较长分支,并且传递方向是单向的,最终到达分子的中心体部位,完成能量的收集与捕获过程。理解这一过程对于指导实验合成新型分子用于光伏材料中的光捕获过程具有重要的意义。本体系采用基于单电子跃迁密度矩阵的理论方法理解分子激发态的电子特征,尤其是相互作用分支间的局域激发以及电荷转移激发。在此过程中,我们采用不同方法构建了分子较低激发态的势能面曲线,分析了激发态能量转移的物理机制。该计算结果在定性方面与实验结果一致。 对于体系二,研究的是从Jahn-Teller(JT)效应角度理解具有D3对称性的tris(4-ethynylphenyl)amine(TEPA)树形分子激发态的能量转移机理。该分子中心体为N元素,三个分叉单元具有相同的长度。光照激发后,最低的两个激发态具有简并的E对称性。两组e振动模式,即N元素附近二面角和金字塔角的扭曲振动,导致了JT扭曲和对称性的降低。通过在TDDFT方法下采用即时面跳跃的激发态动力学模拟发现,体系或者在S1态势能面的三个能量最小值点间来回窜越,或者在S1态和S2态间发生了非绝热跃迁。这些过程导致了分子不同分支间超快的能量转移过程以及跃迁偶极矩的重新定向,最终导致了超快的荧光各向异性衰减。计算结果表明,荧光各向异性衰减的时间尺度与实验结果是一致的,该能量转移机理能为理解更大的树形分子以及其他具有高对称性的过渡金属化合物的激发态动力学奠定了一定的基础。 总之,本文分别选取不等长和等长分支长度的两类最简单有机树形分子作为研究对象,采用我们自己编写的JADE动力学程序研究了分子激发态的能量转移动力学,并采用自己发展的跃迁密度方法对激发态的电子特征作了分析。通过动力学研究发现TEPA树形分子激发态的能量转移过程与著名的JT效应有关,同时我们也模拟了该体系的荧光各向异性衰减光谱,与实验报道一致。本文的研究为理解其他高对称性有机树形分子激发态的能量转移机理奠定了坚实基础。
Other AbstractTo address the global requirement of renewable energies, the utilization and conversion of solar energy seems to be very important. Thus, the development of novel photovoltaic materials conducting solar energy conversion represents a major challenging task. As a group of promising candidates of photovoltaic compounds, dendrimeric molecules have received wide research interests because of their possible applications in the areas of artificial light harvesting systems, light emitting devices, biological and medical imaging, as well as nonlinear optical devices. The interest in dendrimers for these applications stems from the possibility of very efficient energy transfer among the branches. In this work, we mainly select two kinds of organic dendrimers for studing the excited states energy transfer mechanisms. Firstly, the excited states of the phenylene ethynylene (PE) dendrimer are investigated comprehensively by various electronic-structure methods. Several computational methods, including SCS-ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM-B3LYP), and DFT/MRCI, are applied in systematic calculations. Through the study, we find that when the peripheral groups of the dendrimer are initially excited, the exciton is transferred along the branches from the margin to the center of the dendrimer in a unidirectional, multistep manner. Studying this energy transfer process can be very significant for synthesizing the novel photovoltaic materials. In this work, the theoretical approach based on the one electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge-transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low-lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited-state energy transfer mechanism of the dendrimers at the state-of-the-art electronic-structure theories. Secondly, we report an interesting view to understand the ultrafast excited-state energy transfer (EET) process in the D3-symmetric dendrimer tris(4-ethynylphenyl)amine (TEPA) from the perspective of the well-known E⊗e Jahn-Teller (JT) effect. Upon excitation to two lowest excited states (S1 and S2) with doubly degenerate E symmetry, two sets of e vibrational modes, dihedral angle twist and strong pyramidalization near the nitrogen core, lead to the JT distortion and symmetry lowering. Through the excited-state dynamics simulation with the on-the-fly surface hopping approach at the TDDFT level, we find that the system may either travel three equivalent minima of S1 state or undergo the nonadiabatic transitions between S1 and S2 states. These motions induce the ultrafast EET among different branches and the re-orientation of the transition dipole moments, finally leading to the ultrafast fluorescence anisotropy decay. This energy transfer mechanism can provide some new insights on the excited-state dynamics of large dendrimers with three equivalent branches and transition metal complexes with the C3 symmetry. In summary, two kinds of organic dendrimers were selected for studying their excited states energy transfer mechanisms, and furthermore, we employ our in-house program named “JADE” for investigating the energy transfer mechanism. Meanwhile, the transition density matrix was used to analyze the electronic excited states characters. Through the dynamics, we find the energy transfer process of TEPA dendrimer is related with the well-known JT effect. Additionally, we also simulated the spectrum of fluorescence anisotropy decay for the molecule, which is consistent with the experimental result. We believe this work will provide a solid basis for understanding the excited states energy transfer mechanisms of other dendrimers.
Department仿真模拟团队
Subject Area仿真模拟
Date Available2015-07-01
Subtype博士 ; 学位论文
Language中文
Document Type学位论文
Identifierhttp://ir.qibebt.ac.cn/handle/337004/8075
Collection蛋白质设计研究组
Affiliation中国科学院青岛生物能源与过程研究所
Recommended Citation
GB/T 7714
黄静. 有机树形分子激发态的光化学反应机理研究[D]. 北京. 中国科学院研究生院,2015.
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