Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600 036, India.
Chemistry. 2010 Mar 22;16(12):3699-706. doi: 10.1002/chem.200902391.
Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrile-water mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene-modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15-20 vol % of water, and 2) aggregate formation in anthracene-modified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre-arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face-to-face, angular, and T-shaped geometry. The formation of different types of anthracene excimers was confirmed by steady-state and time-resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground-state aggregation. Most excitingly, increased pi conjugation in the molecular framework of anthracene-substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature.
低代数的聚酰胺-胺（PAMAM）树枝状大分子被蒽基外围修饰，在酸性和碱性 pH 值的乙腈-水混合物中研究了蒽发色团的激基缔合物发射。荧光光谱实验结果表明：1）在存在 15-20 体积%的水的情况下，蒽修饰的 PAMAM 树枝状大分子在乙腈中聚集的倾向是相当大的；2）在蒽修饰的 PAMAM 树枝状大分子中形成的聚集导致在基态下形成独特的形态，其中蒽单元预先排列以在光激发时形成稳定的激基缔合物。在该体系中产生了三种类型的蒽激基缔合物，具有面对面、角状和 T 形几何形状。通过稳态和时间分辨荧光光谱实验证实了不同类型的蒽激基缔合物的形成。实验结果进一步表明，通过改变基态聚集的倾向，有可能改变连接到 PAMAM 树枝状大分子上的蒽单元形成的激基缔合物的类型。最令人兴奋的是，蒽取代的 PAMAM 树枝状大分子的分子框架中增加的 pi 共轭导致室温下从蒽中产生强烈且独特的激基缔合物发射。
