Control of Polymer Solution Structure via Intra- and Intermolecular Aromatic Stacking Faysal Ilhan, Mark Gray, Keven Blanchette, and Vincent M. Rotello* Department of Chemistry, University of MassachusettssAmherst, Amherst, Massachusetts 01003 Received May 7, 1999; Revised Manuscript Received July 16, 1999 ABSTRACT: Anthracene-functionalized polymer 1 folds into a compact globular structure in nonpolar media. This folding arises from intramolecular aromatic-aromatic interactions of the anthracene side chains. Polymer 1 binds strongly to picric acid (2), through donor-acceptor electrostatic interactions between the electron-rich anthracene side chains and the electron-deficient picric acid guests. This binding stabilizes the thermally labile folded conformation of polymer 1, dramatically altering the temperature dependence of polymer unfolding. Control of the three-dimensional structure of confor- mationally flexible macromolecules is of fundamental importance to the fields of biology and material science. In biological chemistry, conformationally defined archi- tecture is critical to proper functioning of proteins 1 and polynucleic acid 2 systems.In materials science, 3 the ability to create specific polymer architectures provides a powerful tool for controlling both the structure and function of molecular materials and devices. 4 Noncovalent interactions between aromatic groups are responsible for a wide array of phenomena in fields of chemistry and biology. 5 These stacking interactions play a key role in recognition events of proteins 6 and contribute stabilization to structures ofbiopolymeric systems. 7 Application of these interactions to synthetic polymers allows the creation of higher order architec- ture required for devices and materials, 8 as well as the dynamic properties required for efficient utilization of these attributes. 9 To explore the application of this methodology to the creation of dynamically self-assembled materials, we have synthesized anthracene-functionalized polystyrene 1 (Scheme 1). 10 Polymer 1 adopts a folded globular architecture driven by intramolecular aromatic-aro- matic interactions. Additionally, polymer 1 strongly binds electron-deficient aromatic guests that are comple- mentary to the electron-rich anthracene moieties found in the core of the folded polymer. This binding reinforces the core of the polymer, imparting greater thermal stability to the folded structure. Results and Discussion The desired homogeneous dispersion of functionality in anthracene-functionalized polymer 1 was obtained via functionalization of 3,a 1:1 copolymer of styrene and chloromethylstyrene. 11 Reaction of polymer 3 with potassium phthalimide followed by hydrazine monohy- drate provided the amine-functionalized polymer 4. Coupling of 9-anthracenepropenoic acid fluoride 5 12 with amine-functionalized polymer 4 then provided quantita- tive conversion to polymer 1 (Scheme 1). Preliminary molecular dynamics calculations 13 of a model of polymer 1 predict a highly folded structure (Figure 1a), with multiple anthracene-anthracene and anthracene-phenylinteractions (Scheme 2).Further molecular dynamics studies predict that picric acid (2) efficiently intercalates 14 between the anthracenes in the interior of polymer 1, with concomitant swelling of the polymer globule (Figure 1b). This predicted intercalation is driven by the increased electrostatic attraction be- tween electron-deficient picric acid molecules and elec- tron-rich anthracene side chains relative to the weaker anthracene-anthracene interactions. Complexation between polymer 1 and picric acid (2) was established experimentally via fluorescence titra- tion in CHCl 3 . Addition of guest 2 strongly quenched the fluorescence of the anthracene side chains of poly- mer 1 (Figure 2). Multiple binding modes and stoichi- ometries are apparent from the titration curve for the polymer 1-guest 2 complex, preventing establishment of a binding constant.The binding curve in Figure 2, however,gradually leveled offwith increasing molar equivalencies of guest 2, a diagnostic feature of recogni- tion processes. 15 Additionally, we have performed variable tempera- ture fluorescence experiments on solutions of polymer 1 and the polymer 1‚2 complex in chloroform. Increasing temperatures resulted in greater quenching for both Scheme 1 6159 Macromolecules 1999, 32, 6159-6162 10.1021/ma990724z CCC: $18.00© 1999 American Chemical Society Published on Web 08/26/1999