A study on fluorescence behavior of pyrene at the interface of polymer Langmuir-Blodgett films

This study describes the fluorescence behavior of the pyrene chromophore, which was incorporated into polymer Langmuir-Blodgett (LB) films, from the viewpoint of molecular motion and molecular orientations in the LB film matrix. 1-Pyrenylmethyl acrylate (PyMA) was copolymerized with a long alkylacrylamide [N-dodecylacrylamide (DDA)] and a short, branched alkylacrylamide [tert-pentylacrylamide (tPA)]. Copolymers (p(DDA/PyMA) and p(tPA/PyMA)) formed stable monolayers on the water surface, and monolayers were transferred onto solid substrates, yielding polymer LB films. Photophysical properties of the pyrene chromophore placed in polymer LB films were studied by UV-vis spectroscopy, steady-state fluorescence spectroscopy, and time-resolved fluorescence spectroscopy. Molecular motion of the pyrene molecule was highly restricted in both LB films, so that pyrene fluorescence lifetimes were longer than those in tetrahydrofuran (THF) solution and excimer emission was from a partially overlapped conformation. Excimer intensity in LB films is smaller than that in THF solution and cast films. Moreover, capping layers on LB films drastically affected emission behavior of the pyrene chromophore in the p(tPA/PyMA) LB film, although emission behavior of the pyrene chromophore in p(DDA/PyMA) was not influenced by capping layers. This can be explained by the fact that the pyrene chromophore is exposed to the surface in the p(tPA/PyMA) LB monolayer, whereas it is surrounded by long alkyl chains in p(DDA/PyMA).