Morphology Control via Manipulation of Chirality

Conjugated organic semiconductors have proven to be a credible optoelectronic device platform, revolutionizing the display industry, with other foreseen promising applications to photovoltaics and bioelectronics. Compared to their inorganic counterparts, their remarkable mechanical properties and vast chemical diversity make them incredibly versatile. Additionally, amongst molecular options are chiral materials which can selectively manipulate circularly-polarized light or dictate spin transport, holding potential for optical tuning and innovations in data storage, advanced sensors, and immersive 3D displays (<i>Adv. Photonics Res. <b>2021</b>, 2 (4), 2000136</i>). For such applications, having crystalline chiral materials is important to make the most of these chiral properties. To unlock such possibilities, here we study the influence of molecular chirality on the crystallization of conjugated organic thin films.<br/><br/>We focus on 2,2-bis-(diphenylphosphino)-1,1-naphthalene (BINAP), an axially chiral molecule, and how chiral ratios influence crystallization. Thermally evaporated and subsequently annealed films of the racemic mixture (rac-BINAP) exhibit micron-scale platelet crystals with smooth molecular terraces consistent with predictions based on their thermal properties (<i>J. Phys. Chem. C <b>2020</b>, 124 (49), 27213-27221</i>), and we found that the preparation of such films is possible on various substrates (e.g., glass, quartz, transparent conducting oxides, silicon, etc.). In contrast, enantiopure BINAPs (R- and S-BINAP) form spherulitic crystals with comparatively rougher surfaces. Systems with varying R- and S-BINAP ratios exhibit platelet-like crystallization morphology and large-area coverage with S-BINAP loadings of 35% to 80%, showing a higher tolerance to an excess of S-BINAP. Notably, identical crystallization behavior was observed from films regardless of whether they were produced via co-evaporation (simultaneous evaporation of R- and S-BINAP) or in a layered system with two discrete S-BINAP and R-BINAP layers, implying considerable molecular mobility upon annealing. The tolerance of lattice mismatch in epitaxially grown systems is also investigated by co-depositing varying ratios of R- and S-BINAP on crystalline template layers of BINAP, grown with the same or different ratios as the epitaxial layer.<br/><br/>Overall, this work highlights the distinct crystallization behaviors of chiral BINAPs and provides insights into controlling crystal morphology in chiral organic materials, an important step toward their applications in optoelectronic devices.