Side-chain chromophore orientation and excitation energy transport in films prepared from poly[N⁵-(R)- and N⁵-(S)-1-(1-naphthyl)ethyl-L-glutamines]

The side-chain chromophore orientation and singlet excitation energy transport in films prepared from poly[N⁵-(R)- and N⁵-(S)-1-(1-naphthyl)ethyl-L-glutamines] (1 and 2) were examined by circular dichroic (CD) and fluorescence spectroscopy. A CD spectrum similar to that in solution was observed with 1, affording large CD signals suggestive of exciton coupling in the ¹B_b band of the naphthalene chromophore. 2, on the other hand, gave a CD spectrum much different from that in solution, suggesting that intermolecular interactions brought about by molecular packing give rise to significant changes in polymer conformation and thus in side-chain orientation. These films showed pseudo-second-order transitions at ca. 130 °C, which apparently relate to the onset of collective motion of the side chains. While the IR spectra initially show that the side-chain amide groups are extensively hydrogen-bonded, their changes upon annealing suggest that 2, unlike 1, undergoes considerable deformation of the α-helical main chain, causing marked changes in CD. Using the corresponding copolymers having 1.9 and 1.6 mol % of pyrene chromophore (3 and 4) and observing total quenching of naphthalene fluorescence by the pyrene chromophore in film, we confirmed efficient singlet excitation energy transport among the naphthalene chromophores. When the copolymers were dispersed in the corresponding homopolymers, we found that the film of 2 wt % of 4 in 2, particularly after annealing, gives a significantly higher efficiency of excitation energy-transport than that of 3 in 1, evidently reflecting better controlled chromophore orientation of 2 compared with that of 1.