Theoretical study on second hyperpolarizabilities of phenylacetylene dendrimer: toward an understanding of structure-property relation in NLO responses of fractal antenna dendrimers.
Theoretical study on second hyperpolarizabilities of phenylacetylene dendrimer: toward an understanding of structure-property relation in NLO responses of fractal antenna dendrimers.
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DOI：
10.1021/ja0115969
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发表时间：
2002-07
影响因子：
15
通讯作者：
M. Nakano;H. Fujita;M. Takahata;K. Yamaguchi
中科院分区：
文献类型：
--
作者：
M. Nakano;H. Fujita;M. Takahata;K. Yamaguchi
本文用半经验分子轨道方法（INDO/S方法）研究了苯乙炔树枝状大分子的第二超极化率（γ）与分子构型的关系。的取向平均γ的大尺寸的苯乙炔树枝状聚合物，这是由24个单元的苯乙炔，并被称为D25，被发现是约50倍大的二苯基乙炔单体。相反，发现D25的γ（s）值是由24个苯乙炔单元组成的对位取代苯乙炔低聚物（L25）的γ（s）值的约6倍。为了研究D25和L25的伽马结构-性质关系，我们使用伽马密度分析来研究电子对伽马值的空间贡献。目前的分析表明，电子对伽马的D25的主要贡献是本地化的线性腿区域平行于所施加的电场和贡献也很好地分割在元连接点（苯环），而γ密度的空间分布的L25是扩展到整个区域的链，和占主导地位的贡献源于两端区域。发现对于D25，内部区域中对γ的贡献的幅度比外部区域中的贡献的幅度更大。我们进一步发现，在内部线性腿区域的D25的贡献的幅度略大于那些相同大小的孤立的线性腿分子。这表明，通过元分支点的略微剩余的π共轭仍然增强了对线性腿部区域中的伽马的贡献。这些功能的空间贡献的伽马D25被发现起源于分形结构，其中π共轭长度的线性腿区域增加去从外围到核心。最后，分形天线树枝状聚合物有望成为有前途的新型非线性光学（NLO）物质与可控的幅度和空间贡献的三阶非线性光学性能。
This contribution explores the relation between molecular second hyperpolarizabilities (gamma) and molecular architecture in phenylacetylene dendrimers using the semiempirical molecular orbital method, that is, INDO/S method. The orientationally averaged gamma of a large-size phenylacetylene dendrimer, which is composed of 24 units of phenylacetylenes and is referred to as D25, is found to be about 50 times as large as that of the diphenylacetylene monomer. In contrast, the gamma(s)() value of D25 is found to be about 6 times as small as that of the para-substituted phenylacetylene oligomer (L25) composed of 24 units of phenylacetylenes. To investigate the structure-property relation in gamma for D25 and L25, we examine the spatial contributions of electrons to gamma values using gamma density analysis. The present analysis reveals that the dominant contributions of electrons to gamma of D25 are localized in the linear-leg regions parallel to the applied electric field and the contributions are also well segmented at the meta-connected points (benzene rings), while the spatial distribution of the gamma density of L25 is extended over the whole region of the chain, and the dominant contribution stems from the both-end regions. It is found for D25 that the magnitude of contributions to gamma in the internal region is more enhanced than that in the outer region. We further found that the magnitudes of contributions in internal linear-leg regions of D25 are somewhat larger than those of the same-size isolated linear-leg molecules. This suggests that the slightly remaining pi-conjugations via the meta-branching points still enhance the contributions to gamma localized in the linear-leg regions. These features of spatial contributions to gamma of D25 are found to originate in the fractal architecture, in which pi-conjugation lengths in the linear-leg region increase on going from the periphery to the core. Finally, fractal antenna dendrimers are expected to be promising novel nonlinear optical (NLO) substances with a controllability of the magnitude and spatial contribution of the third-order NLO properties.