ROTATIONAL MOTIONS OF PROBES AND LABELS IN POLYETHYLACRYLATE NETWORKSSTUDIED BY TIME-RESOLVED FLUORESCENCE ANISOTROPY

Time-dependent decays of fluorescence anisotropy (r) of probes and lab els, were measured by the single-photon counting technique on polyethy lacrylate networks of different cross-linking degrees. In the case of labels in the knots (phenylene groups) or in the elastic chains (anthr anyl side groups), r (t) can be fitted with a biexponential function p lus a limiting anitropy (r(infinity)) which was ascribed to a fraction of chromophores immobilized in the nanosecond time scale, having r = r(0), the initial anisotropy. The limiting anisotropy decreases upon i ncreasing temperature in the range 10-40 degrees C and decreasing the network cross-linking degree. Anthracene probes behave like ellipsoids with rotational times equal to about 2 and 20 ns, in networks with th e lowest degree of cross-linking.