pubs.acs.org/Macromolecules Published on Web 10/26/2009 r 2009 American Chemical Society 282 Macromolecules 2010, 43, 282–291 DOI: 10.1021/ma9019619 Synthesis, Nanostructure, Functionality, and Application of Polyfluorene-block-poly(N-isopropylacrylamide)s Yanqing Tian,* ,†,§ Ching-Yi Chen, † Hin-Lap Yip, † Wen-Chung Wu, † Wen-Chang Chen, ‡ and Alex K.-Y. Jen* ,† † Department of Materials Science and Engineering, Box 352120, University of Washington, Seattle, Washington 98195-2120, and ‡ Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106. § Current address: Center for Ecogenomics, The Biodesign Institute, Arizona State University, Tempe, AZ 85287-5001. Received September 2, 2009; Revised Manuscript Received October 6, 2009 ABSTRACT: A series of amphiphilic rod-coil diblock copolymers with a polyfluorene (PF) as a hydro- phobic and light-emitting rod and a poly(N-isopropylacrylamide) (PNIPAAm) as a hydrophilic coil were prepared. Their micellar nanostructures and fluorescence properties were stimulated using solvents, e.g., water and tetrahydrofuran (THF)/toluene (1:10 by volume), which were characterized using dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and fluores- cence spectroscopy. A typical block copolymer P2 (M n =14 400, M w /M n =1.80, the PF weight fraction is 9.4%) in water exhibited micelles with the PNIPAAm block as the corona, whereas the polymer showed inverse micelles in mixed organic solvents of THF/toluene with PF as the corona. Such different nano- structures resulted in their different fluorescence properties. The fluorescence spectrum of P2 did not indicate aggregations of the PF chains in the THF/toluene mixture. Blue emission with a high quantum yield (0.80) was observed. In contrast, strong aggregations of the PF chains were observed in the aqueous solution, resulting in low quantum efficient (0.26) fluorescence. These results clearly demonstrated that the function- ality of the P2 could be tuned through the solvent stimuli. The micelles of P2 were further used as nanocarriers to load a water-insoluble tetrakis(mesityl)porphyrin (H 2 (Me 3 )TPP) into its micellar aqueous solution. Efficient fluorescence resonance energy transfer (FRET) between the PF (donor) and H 2 (Me 3 )TPP (acceptor) (as high as 98% quenching of donor emission) was observed in their micellar solutions. Moreover, FRET contributes significantly to the efficiency of singlet oxygen generation from H 2 (Me 3 )TPP molecules (with an efficiency factor of 2 as compared to non-FRET micelles). This study provided a feasible way to enable the application of water-insoluble porphyrin in aqueous solution and enhance its singlet oxygen generation efficiency. Introduction Rod-coil block copolymers with conjugated polymers/oligo- mers as the rod portion are considered as new nanostructured functional materials because they exhibit interesting morpholo- gies of honeycomb, cylindrical, and nanowire structures as well as electrical and optical properties. 1 Some recent studies in this field have shifted their gravities to create one-, two-, or three-dimen- sional confinement of the functional moieties with desirable microphase-separated structures with nanometer periodicity to effectively affect the optical or electronic properties. 1a,1g,1h Thus, the synthesis and investigation of new rod-coil block copolymers with well-ordered nanostructures are necessary and important. Furthermore, it is quite challenging to understand the modula- tion of their morphologies through a stimulus such as tempera- ture, solvent, and pH value and the relationship between the functionality and nanostructure. Polyfluorene (PF) 2,3 is a conju- gated polymer, widely studied as a blue emitter and biosensor. PF-containing block copolymers 1d,1i-1k,4,5 have been reported and were investigated as new block copolymers exhibiting fibril- lar and spherical morphologies 1k and stable blue-light-emitting materials in the organic light-emitting diodes. 1d,1l More recently, a few coil-rod-coil triblock copolymers 5 with the PF as the hydrophobic segment and a poly(N-isopropylacrylamide) (PNIPAAm) block as the temperature-stimulated hydrophilic/ hydrophobic chain were synthesized, and the thermal responsive morphologies around the lower critical solution temperature (LCST) 6 of the PNIPAAm polymers were investigated. On the other hand, porphyrin and its derivatives are a class of typical photosensitizers and widely used as drugs for photo- dynamic therapy (PDT) to treat keratosis, esophageal tumors, lung cancer, and bladder carcinoma. 7 Under light irradiation, the photosensitizer is excited to reach its singlet state, and then intersystem crossing of the photosensitizer to its triplet state occurs. In the presence of molecular oxygen, the photosensitizer further transfers its triplet state energy to nearby oxygen mole- cules to form reactive oxygen species, such as the singlet oxygen ( 1 O 2 ) or free radicals, and in the meantime, the photosensitizer returns to its ground state and the generated highly active 1 O 2 will kill the tumors around it. PDT efficacy is strongly related with its photoactivity and singlet oxygen generation efficiency. However, as with other photosensitizing agents, many porphyrins have poor solubility in water and tend to aggregate in aqueous solutions, resulting in a loss of photochemical activity in bio- logical environment. 8 To alleviate the above problems, one can encapsulate porphyrins into micelles or vesicles generated by *To whom all correspondence should be addressed: Tel þ1-480-965- 9601, Fax þ1-480-727-6588, e-mail yanqing.tian@asu.edu (Y.T.); Tel þ1-206-543-2626, Fax 206-543-3100, e-mail ajen@u.washington.edu (A.K.-Y.J.).