Time-Resolved and Steady-State Fluorescence Studies of Hydrophobically Modified Water-Soluble Polymers J. Seixas de Melo,* ,² Telma Costa, ² Maria da G. Miguel, ² Bjo 1 rn Lindman, ²,‡ and Karin Schille ´ n ‡ Chemistry Department, UniVersity of Coimbra, 3004-535 Coimbra, Portugal, and Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden ReceiVed: March 10, 2003; In Final Form: September 11, 2003 A comprehensive study of the photophysical behavior of poly(acrylic acids) randomly labeled with pyrene using steady-state and time-resolved fluorescence spectroscopy is presented. The influence of external factors, such as different solvents and pH in the aqueous solution, on the polymer photophysics has been investigated. These factors induce major changes in the polymer conformation, which are reflected in the fluorescence experiments. The random introduction of the hydrophobic pyrene groups along the macromolecule favors the coexistence of static (preformed dimers) and dynamic (excimers) monomer quenching phenomena together with a fraction of isolated monomers (not able to form excimer), as revealed by time-resolved and steady- state fluorescence data. In dioxane and methanol solvents, the global analysis of the fluorescence time profiles shows a rise time of ∼21-32 ns followed by a decay of ∼90-138 ns with an additional longer decay component with a low preexponential factor. This is consistent with the fact that in organic solvents such as methanol and dioxane, which are considered to be good solvents, the fraction of preassociated and isolated chromophores is highly reduced and excimer formation is essentially due to a dynamic mechanism. A kinetic scheme involving two types of monomers (M A and M B ) and one excimer (E) is proposed. From the fluorescence decays it was possible to extract quantitatively the percentage of ground-state preformed dimers along with the percentage of isolated chromophores at room temperature. In addition, it is shown that the fraction of associated ground-state chromophores that can be excited is always larger than that of the isolated chromophores. The rate constants for excimer formation (k a ), dissociation (k d ), and deactivation (k E ) were determined considering the absence and presence of preformed dimers. Additional photophysical and spectroscopic data consisting of wavelength shifts, peak-to-valley ratios and differences (obtained both from absorption and from excitation spectra collected at the monomer and excimer emission region), and the vibronic I 1 /I 3 ratio in the pyrene monomer emission were found to be pH-dependent for the polymers in aqueous solution. Hence, by combining the results from steady-state fluorescence measurements with time-resolved fluorescence data, information is provided on how the chain conformation of the labeled PAA polymers changes depending on the solvent (water at different pH values or organic solvents). In water, the conformation changes from compact at low pH to an open polymer coil at high pH, whereas the polymers are in an extended state when in dioxane and methanol. Introduction The use of fluorescent probes to study conformational changes in polymers, polymer-surfactant interactions, micelle formation, the aggregation number of surfactants, and so forth has been widely reported in the literature. See, for example, refs 1 and 2 and references therein for a recent review. The study of copolymers, which incorporate chromophores of interest in polymer photophysics, has the major advantage of varying the level of grafted chromophore (intramolecular chromophore concentration), allowing the possibility to obtain photophysical parameters in ideal situations of “zero intramo- lecular concentration” and remove the concentration dependence on rate constants for excimer formation. This is so because the pyrene concentration, needed to promote intermolecular excimer formation, is usually found to be 10 -3 M 3 , whereas with intramolecular excimer formation the effective pyrene concen- tration can be lower than 10 -6 M. In polymer photophysics, the implications of the above are obvious, consisting of the absence of an inter-polymeric association (use of low polymer concentration), and at the same time, because the pyrene absorbing probe is also found in low concentrations, situations such as reabsorption, 3,4 which may have a strong influence on the shape of the monomer band, 5 can be easily avoided. Water-soluble polymers covalently labeled with fluorescent hydrophobic dyes are self-organizing and a major focus of interest in polymer photophysics. Among the most studied cases within this group of polymers are the polyelectrolytes, poly- (acrylic acid) (PAA), 6-9 and the cellulose-containing 10-12 polymers with aromatic fluorophores (e.g., pyrene or naphtha- lene) randomly attached. By employing photophysically active groups as hydrophobes in hydrophobically modified polymers, a direct molecular-level study of the association and, conse- quently, molecular conformation can be made. Associating polymers have important applications for surface modification, structuring and, in particular, rheology control. Hydrophobically modified water-soluble polymers, developed * Corresponding author. E-mail: sseixas@ci.uc.pt. Fax: 351 239 827703. ² University of Coimbra. ‡ Lund University. 12605 J. Phys. Chem. B 2003, 107, 12605-12621 10.1021/jp0346054 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003