CNRS, Chimie et Photonique Moléculaires (UMR 6510), 35042 Rennes, France.
J Phys Chem B. 2010 Mar 11;114(9):3152-69. doi: 10.1021/jp911445m.
The aim of the present work is to demonstrate that combined spatial tuning of fluorescence and two-photon absorption (TPA) properties of multipolar chromophores can be achieved by introduction of slight electronic chemical dissymmetry. In that perspective, two model series of structurally related chromophores have been designed and investigated. One is based on rod-like quadrupolar chromophores bearing either two identical or different electron-donating (D) end groups and the other on three-branched octupolar chromophores built from a trigonal donating moiety bearing identical or different acceptor (A) peripheral groups. The influence of the electronic dissymmetry is investigated by combined experimental and theoretical studies of the linear and nonlinear optical properties of dissymmetrical chromophores compared to their symmetrical counterparts. In both types of systems (i.e., quadrupoles and octupoles), experiments and theory reveal that excitation is essentially delocalized and that excitation involves synchronized charge redistribution (i.e., concerted intramolecular charge transfer) between the different D and A moieties within the multipolar structure. In contrast, the emission stems only from a particular dipolar subunit bearing the strongest D or A moiety due to fast excitation localization after excitation, prior to emission. Hence, control of emission characteristics (polarization and emission spectrum), can be achieved, in addition to localization, by controlled introduction of electronic dissymmetry (i.e., replacement of one of the D or A end-groups by a slightly stronger D' or A' unit). Interestingly, slight dissymmetrical functionalization of both quadrupolar and octupolar compounds does not lead to significant loss in TPA responses and can even be beneficial due to the spectral broadening and peak position tuning that it allows. This study thus reveals an original molecular engineering route allowing TPA enhancement in multipolar structures, due to concerted core-to-periphery or periphery-to-core intramolecular charge redistribution upon excitation, while providing for control of emission localization. Such a route could be extended to more intricate (dendritic) and multipolar (3D) systems.
本工作的目的是证明通过引入轻微的电子化学不对称性，可以实现多极发色团的荧光和双光子吸收（TPA）性质的联合空间调谐。为此，设计并研究了两个结构相关的发色团模型系列。一个基于带有两个相同或不同给电子（D）端基的棒状四极发色团，另一个基于由具有相同或不同受体（A）外围基团的三分支八极发色团构建的三分支八极发色团。通过比较不对称发色团与对称发色团的线性和非线性光学性质的实验和理论研究，研究了电子不对称性的影响。在这两种类型的系统（即四极体和八极体）中，实验和理论都表明，激发基本上是离域的，并且激发涉及多极结构内不同的 D 和 A 部分之间的同步电荷再分配（即协同分子内电荷转移）。相比之下，由于激发后，在发射之前，激发快速本地化，因此发射仅来自于带有最强 D 或 A 部分的特定偶极子部分。因此，除了局部化之外，还可以通过控制电子不对称性（即通过用稍微更强的 D'或 A'单元替换一个 D 或 A 端基）来实现对发射特性（偏振和发射光谱）的控制。有趣的是，轻微的不对称官能化对四极和八极化合物的 TPA 响应没有显著损失，甚至可能有益，因为它允许光谱展宽和峰位调谐。这项研究因此揭示了一种原始的分子工程途径，该途径允许在多极结构中增强 TPA，这是由于激发时协同的核心到外围或外围到核心的分子内电荷再分配，同时提供对发射局域化的控制。这种途径可以扩展到更复杂的（树枝状）和多极（3D）系统。
