Sculpting Light with Chiral Organic Crystals

The commercialization of organic light-emitting diodes is attributed to the versatility and energy-efficient fabrication of organic semiconductors (OSCs). Among all the organic options, chiral materials can selectively control circularly polarized light and dictate spin transport, unlocking innovations for OSCs in data storage, advanced sensors, and immersive 3D displays (<i>Adv. Photonics Res. <b>2021,</b> 2 (4), 2000136</i>). As such, understanding the effects of molecular and solid-state chirality on the optical and electronic properties of chiral crystalline organic semiconductors is essential to fabricate state-of-the-art electronics.<br/>In this study, thin films of 2,2-bis-(diphenylphosphino)-1,1-naphthalene (BINAP), an axially chiral molecule, are fabricated by thermally evaporating varying proportions of its R and S enantiomer. The as-fabricated thin films show similar luminescent behavior regardless of their R and S enantiomeric ratio. After thermal annealing into crystalline thin films, the intensity of luminescence increases significantly. Interestingly, a red-shifted emission was observed for the crystalline racemic mixture (1:1 ratio of R- and S-BINAP, or rac-BINAP) upon crystallization, whereas the crystalline R- and S-BINAP show blue-shifted emission. The different luminescent properties may be attributed to the different molecular arrangements of the crystals. While rac-BINAP exhibits platelet crystals up to the size of hundreds of microns 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>), R- and S-BINAP crystallize as spherulites with relatively rougher surfaces. Both platelet and spherulitic crystals can be fabricated on various substrates (e.g., glass, quartz, transparent conducting oxides, silicon, etc.) with high surface coverage. Systems with R- and S-BINAP deposited in other ratios exhibit platelet-like crystals with large-area coverage, with a higher tolerance to an excess of S-BINAP from 35% to 80%. Notably, similar crystallization was observed for both co-evaporated and discrete layered systems of S-BINAP R-BINAP layers, implying considerable molecular mobility upon annealing. Furthermore, these films showed similar fluorescence properties as the rac-BINAP crystalline thin films. The optical properties of the BINAP thin films and their solutions are further explored by circular dichroism (CD) and UV-Vis spectroscopy.<br/><br/>As a result, this work highlights the distinct crystallization behavior of chiral BINAPs and provides insights into controlling crystal morphology in chiral organic materials and their optical properties, which are important for their use in optoelectronic devices.