The hydrophobic associations of three hydrophobically modified alkali swellable emulsion polymers (HASE) were studied by fluorescence. The chromophore pyrene was used as the hydrophobic pendant. The level of association was characterized qualitatively and quantitatively by monitoring the fluorescence behavior of the pyrene monomer and excimer. Excimer formation occurs mostly by diffusion in an organic solvent (tetrahydrofuran, THF) where the pyrene moieties are not associated. In aqueous solutions where pyrene is insoluble, excimer formation occurs mostly via direct excitation of ground-state pyrene aggregates. The level of association of these pyrene-labeled HASEs was determined quantitatively by analyzing the monomer and excimer fluorescence decays using a blob model. It was found that 58% of the hydrophobic pendants were associated in aqueous solutions, but that only 5% of them were associated in THF.

The S 1 -S 0 limiting anisotropy of a widely used fluorophore, rhodamine 101, is determined with unprecedented accuracy. From time-resolved and steady-state fluorescence measurements in several solvents, it is shown that the limiting anisotropy of rhodamine 101 is for all practical purposes equal to the theoretical one-photon fundamental anisotropy value of 2/5, both in rigid and in fluid media. This fact, along with the favorable chemical and photophysical properties of rhodamine 101, point to its use as a standard for fluorescence polarization measurements. It is also shown that if the excitation pulse can be considered a delta impulse with respect to the time scale of the anisotropy decay (but not necessarily to the time scale of the intensity decay), then no deconvolution procedure is needed for anisotropy decay analysis. IntroductionRhodamine 101 (also known as rhodamine 640), Chart 1, and derivatives retaining the same fluorophore are widely used as fluorescent probes. Rhodamine 101 was designed so that no rotation of the amino groups is possible, 1,2 in the hope of achieving unit fluorescence quantum yield, but this was not fulfilled. 3 Indeed, its fluorescence quantum yield, although high, is similar to that of rhodamine 6G [3][4][5] and is in fact distinctly lower than 1.0, 4-6 which is the value initially reported. 1,2,7 In room temperature acidified ethanol, where the dye is present in the cationic form, it has a fluorescence quantum yield of about 0.9 6 and a single exponential fluorescence decay with a lifetime of 4.3 ns. 6 The cation and the zwitterion share a common fluorophore, whereas the lactone form has quite different photophysical properties. 8 A precise determination of the S 1 -S 0 limiting anisotropy (i.e., the highest measured S 1 -S 0 anisotropy for a given molecule) of several xanthene dyes, including the rhodamines B, 6G and 101, and fluorescein, r 0 ) 0.373 ( 0.002, was reported by Johansson, 9 on the basis of both steady-state and time-resolved measurements. This begs the question as to why the limiting anisotropy is lower than the fundamental anisotropy, whose onephoton theoretical value is 0.4 for parallel absorption and emission transition moments.The fact that the limiting anisotropy seldom attains the theoretical value of 2 / 5 has been subject of considerable attention over the years; see, e.g., the classic work of Feofilov 10 for an early view, and Valeur's book 11 for a recent discussion. Possible reasons for the discrepancy fall in four categories: (i) instrumental effects (effect of wide angle collection and/or of polarizer misalignment, 12,13 etc.); (ii) matrix-dependent effects (depolarization by light scattering, depolarization by stress-induced optical activity in solid glasses, depolarization by residual rotational motion); (iii) intermolecular effects (depolarization by radiative and/or nonradiative energy migration); (iv) intramolecular effects (mixed polarization bands, significantly different geometries for the Franck-Condon and emissive states implying noncoinci...

Reversible addition-fragmentation chain transfer (RAFT) polymerization is a versatile process to obtain polymers with controlled architecture and molecular weight from a large variety of monomers. This appears to be a very attractive method for the synthesis of fluorescent polymers for different applications. However, fluorescent polymers obtained by RAFT are known to suffer strong fluorescence quenching. Here, we report the quenching of coumarin 343 (C343) fluorescence by four different chain transfer agents (CTA) used in RAFT polymerization: carboxymethyl dithiobenzoate (CMDB), tert-butyl dithiobenzoate (tBDB), menthonyl dithiobenzoate (MDB), and bis(3-methylbutyl)-2-(thiobenzoylthio)succinate (MBTS). The quenching ability of these CTAs was compared with that of a macroCTA (poly(DcA)-MBTS) obtained from the RAFT polymerization of N-decylacrylamide (DcA) using MBTS as chain transfer agent. The results of stationary and time-resolved fluorescence measurements of C343 with the different CTA and macroCTA can be interpreted with a single kinetic model, which considers the reversible formation of a C343-CTA exciplex and both static and dynamic quenching of C343. The photophysical properties of tert-butyl mercaptan (tBM), resulting from the aminolysis of the tBDB dithioester, were also studied using the same experimental conditions, confirming that the thiol group does not quench the C343 fluorescence. This was further tested by converting poly(DcA)-MBTS to the corresponding thiol-ended chain, eliminating the fluorescence quenching of coumarin previously observed.