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The solid-state microstructure of semiconducting polymers is known to influence properties relevant for their function in optoelectronic devices. While several strategies exist in the literature for controlling desired morphological organization, preaggregation in solution via polymer chain end-functionalization remains relatively unexplored. In this work, we synthesized two poly(3-hexylthiophene) (P3HT) derivatives with different end-groups by using click chemistry. End-groups chosen for this study were derivatives of sulforhodamine 101 dye and phenyl-C 61 -butyric acid methyl ester (PC 60 BM). The chemical functionality of the end-group is found to influence the aggregation properties as a function of solvent quality. End-group-induced preaggregation ultimately impacts the optical properties and molecular orientation in the solid state. These results suggest future applications of this structural modification strategy to modulate device characteristics. ■ INTRODUCTIONMuch of the interest in organic semiconductors arises from the possibility of manufacturing optoelectronic devices via solution deposition methods. 1 In order to realize the advantages of solution-processing, it is necessary to understand how to control and ultimately optimize the resulting thin film device morphology. 2 For example, thin film morphology control is essential in applications such as organic photovoltaics and field effect transistors due to the impact of interchain contacts and large-scale phase separation on charge carrier mobility and photophysical processes. 3 One method of influencing the organization of conjugated polymers in the solid state is through preaggregation of the chains in solution and understanding polymer−solvent interactions is thus critical in this respect. 4 P3HT is a well-studied conjugated polymer. 5 Synthetic methods for achieving well-defined, regioregular P3HT have been developed, such as the Kumada coupling living polymerization (GRIM method), which provide control over molecular weight, dispersity, and regioregularity. 6 This method can be extended to provide control of the end-group. 7 P3HT is readily dissolved in organic solvents, such as chloroform and chlorobenzene, and is known to form different thin film morphologies depending upon solution-processing conditions. Such morphologies include for example well-defined "whiskers", formed by cooling P3HT solutions in cyclohexanone, 8 and long rectangular parallelepipeds, formed when a large volume fraction of hexane nucleates aggregate growth in solution via favorable π−π interactions. 9 P3HT is also known to form a fibrillar structure when deposited from solutions of odichlorobenzene where the size of the nanofiber increases with increasing volume fraction of nonsolvent hexane. 10 Similar nanowire morphologies are observed with the addition of small amounts of poor solvents, such as acetonitrile; however, round nanometer-sized aggregates are seen upon addition of larger amounts of acetonitrile. 11 Further work has shown higher boiling point solvents or p...

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