Rationally Designed Polymer Hosts of Fullerene

Abstract

The development of hosts for fullerenes is an active area of research with technological implications. [1][2][3][4][5][6][7][8] In this context, macrocycles, [9,10] dendrimers, [4,11] p-conjugated linear and cyclic oligomers, [12][13][14] molecular clips, [4,[15][16][17][18][19][20] and self-assembled small molecules [21] have been developed for their fullerene-encapsulation properties. A shift in this paradigm is possible by the design of polymer-based receptors. Progress in this direction, however, is restricted to the use of commercial materials, such as poly(vinyl pyrrolidone)s, that do not possess any molecular-recognition capacity towards fullerenes and therefore have limited potential as hosts. [22,23] Herein, we describe a general class of pristine C 60 hosts based upon a novel family of corannulene-encoded [24][25][26][27] polymers (Scheme 1) synthesized by a free-radical-polymerization technique. Each repeat unit of these polymers contains the necessary information to interact with fullerene C 60 through a "ball-and-socket"-type p-p stacking interactions (Scheme 1). [17,20,[28][29][30] Since the free-radical synthesis enables the preparation of different polymer architectures (linear and branched) of varying compositions (homopolymers, random copolymers, block copolymers), [31][32][33] the described new family of hosts can be readily transformed into materials of a desired chemical composition and architecture appropriate for use in a specific application. For example, block copolymers with an electron-donating block can give rise to materials for photovoltaic applications, [34,35] whereas the introduction of a watersoluble comonomer into the polymer structure can yield C 60rich water-soluble materials. Specifically, such water-soluble materials are constructed by preparing an amphiphilic block copolymer in which one block is water-soluble and the other block is composed of the host repeat units. This block copolymer is capable of dispersing hydrophobic C 60 molecules in water by forming a core-shell type of micellar structure in which the corannulene core hosts the water-insoluble C 60 molecules. Furthermore, the described system allows for tuning of the host-guest interaction in three different ways: a) by changing the linker length between the polymerizable site and the recognition unit, b) by altering the chemical nature of the solvent, and c) by varying the temperature. Moreover, powders as well as freestanding and transparent polymer films could be prepared from the C 60 /polymer composites. The modularity of the designed system, the ease of synthesis, the ability to fine-tune the host-guest interactions, and the processability due to the polymeric nature of the system distinguish the described new class of hosts from previous host classes and indicate broad applicability of the described approach to polymer-based fullerene-host materials. Such materials will be of significant value in the arena of photovoltaics and plastic electronics.Initially, we decided to prepare a homologous series of corann...

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