Characterization of the molecular environment of polymer Langmuir-Blodgett films using a pyrene fluorescent probe

The molecular environment of polymer Langmuir-Blodgett (LB) films with different alkyl chain length, poly(N-dodecyl acrylamide) (pDDA) and poly(tert-pentyl acrylamide) (ptPA), which are copolymerized with a small amount of 1-pyrenylmethyl acrylate (PyMA) comonomer as a luminescent probe, was investigated by means of the surface pressure (pi)-area (A) isotherms, UV-vis spectroscopy, and fluorescence spectroscopy. The pyrene ring orients perpendicular to the p(DDA/PyMA) layer in both the monolayer at the air/water interface and LB multilayers, while the pyrene moiety of p(tPA/PyMA) takes a parallel orientation to its layer plane. The pyrene residue is more exposed to its surroundings in p(tPA/PyMA) LB films and therefore sensitive to solvent polarity. The measurement of fluorescence anisotropy confirmed that the pyrene was densely packed in the LB films. These findings suggest that the molecular orientation of luminescent molecules can be controlled when the length of side chains is changed by only several angstroms in a polymer LB assembly system.