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 w ith 45 and 25 mol % in pyrene content (PPy(45)MLGln (DP 360) and PPy(25)MLG ln (DP 360)) were prepared, and their side-chain chromophore orientation wa s 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 p yrene chromophores are oriented with respect to the main chain and with res pect to one another owing to the rigid side-chain amide linkage and the bul kiness of the pyrene chromophores. In film, PPyMLGln (DP 700) showed CD sig nals 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. Molecu lar mechanics calculations gave two regulated conformations with theoretica l CD in accord with those observed with PPyMLGln (DP 700) in solution and i n film, respectively. In DMAc, these polymers gave excimer emission whose i ntensity decreased with decreasing DP and decreasing pyrene content. Dimini shed 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 intera ction.