Complexation of Fluorescent Tetraphenylthiophene-Derived Ammonium Chloride to Poly(N‑isopropylacrylamide) with Sulfonate Terminal: Aggregation-Induced Emission, Critical Micelle Concentration, and Lower Critical Solution Temperature Chih-Min Yang, Yi-Wei Lai, Shiao-Wei Kuo, and Jin-Long Hong* Department of Materials and Optoelectric Science, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424 * S Supporting Information ABSTRACT: Amphiphilic polymers with hydrophilic poly(N- isopropylacylamide) (PNIPAM) shell connecting hydrophobic tetraphenylthiophene (TP) core, which has the novel aggregation- induced emission (AIE) property, by ionic bonds were prepared to explore the AIE-operative emission responses toward critical micelle concentration (CMC) and lower critical solution temperature (LCST). To exercise the idea, ammonium-function- alized TP2NH 3 + and sulfonate-terminated PNIPAM were separately prepared and mixed in different molar ratios to yield three amphiphilic TP-PNIPAMn complexes for the evaluations of CMC and LCST by fluorescence responses. The nonemissive dilute aqueous solutions of TP-PNIPAMn became fluorescent when increasing concentrations above CMC. Heating micelles solution to temperatures above LCSTs causes further enhancement on the emission intensity. The fluorescence responses are explained by the extent of aggregation in the micelles and in the globules formed at room temperature and at high temperatures, respectively. 1. INTRODUCTION Polymer micelles 1 responsive to external stimuli have received considerable research interest due to their potential applications on drug delivery 2 and catalysis. 3 Among them, amphiphilic block copolymer is one attractive system and much effort 4 has been directed toward engineering “smart” micelle from the hydrophilic/hydrophobic polymers featuring multiple-stimuli. In water, the amphiphilic block copolymers are aggregated to form different molecular assemblies by the repelling and coordinating action between the hydrophilic and hydrophobic parts to the surrounding environment. At low concentrations, copolymers in aqueous solutions exist as individual molecules; their self-assembly starts when copolymer concentration reaches the specific value of critical micelle concentration (CMC). Previous study on the amphiphilic poly(ethylene glycol)-b-poly(L-lactide) (PEG-PLLA), 5 polystyrene-b-poly- (ethylene oxide), 6 and polycarbonate-b-poly(N-isopropyacryla- mide) (PC-PNIPAM) 7 has suggested that CMC depends strongly on the chemical nature and the ratio between the hydrophilic and the hydrophobic blocks. With the higher PLLA/PEG ratio, the corresponding PEG-PLLA 5 tends to form micelles with a lower CMC. In addition to CMC, the thermally induced conformational change of PNIPAM also stimulated much research interest. 8-13 Under semidilute conditions, PNIPAM in water forms a clear solution that rapidly clears, becomes cloudy upon heating to temperatures above 32 °C, the lower critical solution temperature (LCST). A possible model relating to the corresponding coil-to-globule collapse of PNIPAM in water 14-16 has been proposed and the temperature-driven single-chain conformational transformation and the subsequent macroscopic phase separation reflect rather subtle changes in polymer/water hydrogen-bond (H-bond) interactions, primar- ily the release of water molecules from a polymer hydrophilic layer into the bulk water. Minor changes in the chemical composition of PNIPAM were anticipated to have significant influences on the phase diagram of PNIPAM in water. 16 Conventional organic luminogens enjoy the high fluores- cence in the dilute solutions but suffer from the detrimental aggregation-caused quenching (ACQ) in the concentrated solution and solid states. When dispersed in liquid media or fabricated into solid film, the fluorescence of conventional luminogens is often weakened or even quenched, which greatly limits their real-world application. In 2001, Tang’s group discovered that one particular silole molecule (1-methyl- 1,2,3,4,5-pentaphenylsilole) emits strongly in the concentrated solution or the solid state even though it is nonemissive in the dilute solution. 17 This interesting phenomenon was designated as “aggregation-induced emission” (AIE) later on to emphasize the phenomenon that the originally nonemissive solution of silole can be tuned to emit strongly when the corresponding aggregates formed after introduction of the water nonsolvent. Received: September 20, 2012 Revised: October 16, 2012 Published: October 17, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 15725 dx.doi.org/10.1021/la303783n | Langmuir 2012, 28, 15725-15735