Time-resolved fluorescence studies of the interactions between the thermoresponsive polymer host, poly(N-isopropylacrylamide), and a hydrophobic guest, pyrene

The time-resolved fluorescence anisotropy behaviour of pyrene solubilized ( 10(-6) M) in ultra-dilute (10(-3) wt%) aqueous solutions of the thermorespo nsive polymer poly(N-isoplopylacrylamide), PNIPAM, shows unusual characteri stics. Rather than decay to zero las expected of a freely rotating species in a homogeneous fluid environment) the anisotropy of the emission from the probe attains a minimum but finite value, some 40 ns after excitation of t he pyrene. Thereafter, over a timescale of hundreds of nanoseconds, the ani sotropy increases: the fluorescence from the solubized pyrene guest becomes more polarized with time! These deviations from "expectation" are signific ant both in terms of the aqueous solution behaviour of the polymer and basi c photophysics. The anisotropy data confirm that, above the lower critical solution tempera ture (LCST) of PNIPAM, the polymer host lin the form of single, "collapsed" macromolecules) is capable of solubilizing hydrophobic guests. The complex ity of the time dependence of anisotropy of fluorescence from pyrene, above the LCST of the system, reflects the heterogeneous nature of the medium in which the probe is dispersed. At one extreme, we have pyrene molecules loc ated within the bulk aqueous phase. At the other extreme, pyrene species ar e occluded within the hydrophobic interiors of the collapsed PNIPAM globule s. The initial rapid loss in emission anisotropy reflects the contribution from shorter lived and rapidly tumbling solutes dispersed largely in the aq ueous phase. The subsequent enhancement in the anisotropy at longer times, results from the ever-increasing importance of fluorescence from the longer lived, slowly rotating probes solubilized within the highly viscous cores of the globular conformation of the macromolecule. Below the polymer's LCST, most of the pyrene guest is released to the bulk aqueous phase but a proportion of the organic guests remains associated wit h the (otherwise) open coils of the polymer. In photophysical terms, the data presented constitute the first example of a study in which the motion of a fluorescent guest, mediated by the influen ce of its interactions with a synthetic polymer, appears to become more res tricted as the time of sampling of its emission, following excitation, incr eases. (C) 2000 Elsevier Science Ltd. All rights reserved.