Polymorphism-driven Distinct Nanomechanical, Optical, Photophysical, and Conducting Properties in a Benzothiophene-quinoline

Polymorphic forms of organic conjugated small molecules, with their unique molecular shapes, packing arrangements, and interaction patterns, provide an excellent opportunity to uncover how their microstructures influence their observable properties. Ethyl-2-(1-benzothiophene-2-yl)quinoline-4-carboxylate (BZQ) exists as dimorphs with distinct crystal habits - blocks (BZB) and needles (BZN). The crystal forms differ in their molecular arrangements - BZB has a slip-stacked column-like structure in contrast to a zig-zag crystal packing with limited pi-overlap in BZN. The BZB crystals characterized by extended pi-stacking along [100] demonstrated semiconductor behavior, whereas the BZN, with its zig-zag crystal packing and limited stacking characteristics, was reckoned as an insulator. Monotropically related crystal forms also differ in their nanomechanical properties, with BZB crystals being considerably softer than BZN crystals. This discrepancy in mechanical behavior can be attributed to the distinct molecular arrangements adopted by each crystal form, resulting in unique mechanisms to relieve the strain generated during nanoindentation experiments. Waveguiding experiments on the acicular crystals of BZN revealed the passive waveguiding properties. Excitation of these crystals using a 532 nm laser confirmed the propagation of elastically scattered photons (green) and the subsequent generation of inelastically scattered (orange) photons by the crystals. Further, the dimorphs display dissimilar photoluminescence properties; they are both blue-emissive, but BZN displays twice the quantum yield of BZB. The study underscores the integral role of polymorphism in modulating the mechanical, photophysical, and conducting properties of functional molecular materials. Importantly, our findings reveal the existence of light-emitting crystal polymorphs with varying electric conductivity, a relatively scarce phenomenon in the literature. Polymorphic forms of ethyl-2-(1-benzothiophene-2-yl)quinolone-4-carboxylate with distinct packing arrangements and interactions, vary in their nanomechanical response, photophysical, optical waveguiding, and conducting properties, highlight the significant role of polymorphism in modulating the microstructures in molecular materials and thereby their functional properties. DFT calculations on the single molecule and crystal dimorphs corroborate the experimental photophysical and conductive findings.image

Automated summary

Polymorphic forms of organic conjugated small molecules play an important role in modulating their mechanical, photophysical, and conducting properties. The crystal forms and molecular arrangements of dimorphs have an impact on their properties and exhibit differences in hardness, waveguiding properties, and photoluminescence.