Tunable emission of static excimer in a pyrene-modified polyamidoamine dendrimer aggregate through positive solvatochromism

Significant aggregation is observed in pyrene-modified zero- and first-generation polyamidoamine (PAMAM) dendrimers above their critical aggregation concentration (CAC, >10(-6) M). The pyrene units are attached to the dendrimer skeleton through imine bonds, which play a pivotal role in enhancing the aggregation propensity of the PAMAM dendrimers. Scanning electron microscopy studies suggest that pyrene-modified PAMAM dendrimers aggregate into doughnut-shaped assemblies. As a result of aggregation, the pyrene chromophores are pre-arranged in a face-to-face geometry in the ground state, and readily generate pyrene "static excimer" on photoexcitation. The static pyrene excimer emits with an unprecedented quantum yield of 0.62 ± 0.01 in dichloromethane, and also exhibits remarkable positive solvatochromism from 498 to 638 nm, which leads to the highest bathochromic shift for pyrene excimer emission in solution reported so far. Lippert-Mataga analysis of the system suggests that general and specific solvent effects play a crucial role in the positive solvatochromism exhibited by the system. Luminescence quenching studies on both monomer and aggregate systems were carried out in the presence of various metal ions, and the results imply that pyrene-modified PAMAM dendrimer can be utilized for selective detection of Hg ions in the presence of a wide variety of transition, alkali, and alkaline earth metal ions. This report presents the first dendrimer-based chromophoric system exhibiting positive solvatochromism over a range of 140 nm, and shows that pyrene-modified PAMAM dendrimers can be effectively utilized to generate wavelength-tunable emitting systems displaying bluish green, greenish yellow, and orange-red colors at room temperatures.