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Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.

This paper describes a series of three-ring phenylene-ethynylenes (PEs) in which specific, non-covalent arene-arene interactions control conformation in the solid-state. As determined by single crystal X-ray structures, edge-face interactions between benzyl ester side chains and conjugated main chains are observed. In contrast, perfluorobenzyl ester side chains interact cofacially with main chains, resulting in $60 torsional angles between neighboring aryl rings in crystalline PEs. Absorbance and fluorescence spectra of films of these compounds reflect these conformational effects, with the spectra of perfluorobenzyl-substituted compounds shifting hypsochromically from solution-to solid-state. In a demonstration of how balancing non-covalent interactions can open the way to new responsive materials, a main chain twisted derivative with octyloxy substituents displayed significant piezochromic behavior.

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