Katan Claudine, Tretiak Sergei, Werts Martinus H V, Bain Angus J, Marsh Richard J, Leonczek Nicholas, Nicolaou Nicholas, Badaeva Ekaterina, Mongin Olivier, Blanchard-Desce Mireille
Synthèse et ElectroSynthèse Organiques (CNRS, UMR 6510), Université de Rennes 1, Campus de Beaulieu, Bât 10A Case 1003, F-35042 Rennes Cedex, France.
J Phys Chem B. 2007 Aug 16;111(32):9468-83. doi: 10.1021/jp071069x. Epub 2007 Jul 20.
A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related pi-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.
为了评估支化和电荷对称性对吸收、光致发光和双光子吸收（TPA）性质的综合作用，对一系列模型化合物进行了实验和理论相结合的研究。本研究的主要问题是考察与偶极发色团的支化相比，四极发色团的支化如何导致不同的结果。因此，使用三个用供体端基对称取代的结构相关的π共轭四极发色团以及一个由三个四极支链通过一个共同的供体部分组装而成的支链结构作为模型化合物。使用紫外-可见光谱研究了它们的光物理性质，并通过在500-1000nm范围内使用飞秒脉冲的双光子激发荧光实验确定了TPA光谱。实验研究辅以理论计算。所应用的理论方法基于含时密度泛函理论、弗伦克尔激子模型以及根据相关电子态的自然跃迁轨道进行的分析。理论表明，在光激发时，所有四极发色团中都发生了从末端供体基团到分子中部的对称分子内电荷转移。相反，通过中心给电子三苯胺部分的支化破坏了支链的四极对称性。因此，所有弗兰克-康登激发态在激发时都具有显著的不对称多维电荷转移特征。激发态下支链发色团随后的振动弛豫导致激发和荧光局域在单个支链上。与之前观察到的偶极发色团通过相同的共同给电子部分支化时的情况相反，我们发现支链化合物的最大TPA响应相对于其四极支链的加和贡献仅略有增强。相反，观察到光谱形状有实质性的改变。这归因于支化引起的支链间电子耦合和不对称性的微妙相互作用。
