Polarizabilities and Hyperpolarizabilities of Dendritic Systems
Polarizabilities and Hyperpolarizabilities of Dendritic Systems
复制标题
树突系统的极化率和超极化率
DOI：
10.1142/9789812795373_0001
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
K. Yamaguchi
中科院分区：
文献类型：
--
作者：
M. Nakano;K. Yamaguchi
最近，一类新的聚合物体系，即树枝状大分子，由于其高度的结构可控性和一些有趣的性质，如高捕光性能和药物释放性能，引起了人们的极大关注。树枝状大分子的特征是大量末端基团起源于一个焦点(核心)，在每个重复单元处至少有一个分支(线性腿区域)。这种吸引人的体系结构预计将与系统的各种化学和物理功能密切相关。在这篇综述中，我们从独特的结构(树枝状结构)与光学响应性质之间的关系的角度，对这类体系的光学响应性质(如极化率和超极化率)进行了理论研究。作为一个例子，我们考虑了Cayley树型树状大分子，它们具有分形的天线结构，并表现出高的捕光性能。由于激子迁移被认为是这些树枝状大分子高捕光性能的关键，我们首先考虑了激子模型，即由两态单体组成的具有分形天线结构的分子聚集体模型。我们将数值Liouville方法应用于激子迁移动力学，研究了多步激子态和激子态之间的弛豫过程对激子极化率(α)和第二超极化率(γ)的影响。结果表明，激子对α和γ的主要空间贡献集中在线型支柱区，反映了树枝状结构。结构的空间尺寸差异也显著影响了分子间相互作用效应对α的尺寸依赖性。其次，用分子轨道方法研究了由苯乙炔单元组成的Cayley树型树状大分子所涉及的树状部分(低聚物)的纵向α和γ值以及苯乙炔树状大分子的γ值。借助于(超)极化率密度分析，这种具有分形天线结构的树枝状大分子类似于树枝状聚集态，提供了电子对(超)极化率的空间局域贡献，并被预测有可能通过改变代数的大小和线性支柱区之间的连接形式来控制其(超)极化率的大小和符号。这些特性有望为新型(可控)非线性光学材料的分子设计提供有利条件。
Recently, a new class of polymeric systems, i.e., dendrimers, has attracted much attention because of their high controllability of architecture and several interesting properties, e.g., high light-harvesting and drug delivery properties. "Dendrimers" are characterized by a large number of terminal groups originating in a focal point (core) with at least one branch (linear-leg region) at each repeat unit. Such attractive architecture is expected to have a close relation to various chemical and physicsl functionalities of systems. In this review, we focus on the theoretical studies on optical response properties such as polarizabilities and hyperpolarizabilities for these systems from the view point of the relation among the unique architecture (dendritic structure) and the optical response properties. As an example, we consider Cayley-tree-type dendrimers, which are known to have fractal antenna architectures and to exhibit high light-harvesting properties. Since exciton migration is presumed to be essential for the high light-harvesting properties for these dendrimers, we firstly consider exciton models, i.e., molecular aggregate models with fractal antenna structures composed of two-state monomers. We apply the numerical Liouville approach to the exciton migration dynamics and investigate the influence of the multi-step exciton states and relaxation processes among exciton states on the polarizabilities (α) and second hyperpolarizabilities (γ). It is found that the dominant spatial contributions of excitons to α nd γ are localized in linear-leg regions and reflect the dendritic structures. The difference among the spatial dimensions of structures is also found to remarkably affect the size dependency of intermolecular-interaction effects on α. Second, the longitudinal α and γ values of dendron parts (oligomers) involved in Cayley-tree-type dendrimers composed of phenylene vinylene units and the γ values of a phenylacetylene dendrimer are examined using the molecular orbital (MO) method. With the aid of (hyper)poarizability density analyses, such dendrimers with fractal antenna structures are found to provide spatially localized contributions of electrons to (hyper)polarizabilities similarly to the dendritic aggregate case and are predicted to have the possibility of controlling the magnitude and sign of their (hyper)polarizabilities by changing the size of generations and the connection forms between linear-leg regions. These features are expected to be advantage to the molecular design of novel (controllable) nonlinear optical materials.