Secondary structure and side-chain chromophore orientation in poly(L-glutamines) having pyrene chromophores in the side chains

Poly(L-glutamines) with pyrene groups in the side chains (PPyMLGln) having different degrees of polymerization, i.e., DP 700, 360, and 70, and those with 45 and 25 mol % in pyrene content (PPy(45)MLGln (DP 360) and PPy(25)MLGln (DP 360)) were prepared, and their side-chain chromophore orientation was examined in solution and in film. In DMAc, these polymers, except PPy(25)MLGln, showed circular dichroism (CD) indicative of exciton coupling among the pyrene chromophores, suggesting that despite the long side chains the pyrene chromophores are oriented with respect to the main chain and with respect to one another owing to the rigid side-chain amide linkage and the bulkiness of the pyrene chromophores. In film, PPyMLGln (DP 700) showed CD signals remarkably different from those of the other polymers, suggesting that intermolecular interactions lead to a change in side-chain as well as main-chain conformation and thus to a change in chromophore orientation. Molecular mechanics calculations gave two regulated conformations with theoretical CD in accord with those observed with PPyMLGln (DP 700) in solution and in film, respectively. In DMAc, these polymers gave excimer emission whose intensity decreased with decreasing DP and decreasing pyrene content. Diminished excimer emission and rather strong exciton coupling found with PPy(45)MLGln suggest that many of its pyrene groups are situated in proximity and orientation, allowing exciton interaction but not allowing excimeric interaction.