Odd-Even Effect Controls Twist-Elasticity of an Organic Fluorophore in Cocrystals Prepared Using Mechanochemistry

Abstract

The odd-even effect is a fascinating phenomenon observed in various systems where properties of a series of compounds exhibit alternating patterns depending on whether a specific parameter (often the number of repeating units or a specific structural feature) is odd or even. This effect is significant in solid-state chemistry as it influences diverse physical properties of materials, viz. melting point, solubility, elastic modulus, intrinsic dissolution rate, thermal expansion, etc. However, the odd-even effect on the photophysical behaviour of organic chromophores during co-crystalization is not explored in the literature. Herein, as a proof-of-concept, we co-crystallized a novel organic chromophore (PDAN-1) with a series of aliphatic dicarboxylic acids and showed that fluorescence emission shows an odd-even alteration on their emission maximum similar to other physical parameters. Our in-depth crystal structure analysis reveals that variation of the dicarboxylic acids affects the twist-elasticity of PDAN-1 and thus results in a change of crystal packing, thereby, the odd-even effect in solid-state fluorescence. Moreover, nanomechanical analysis and melting point measurements compliment our odd-even effect on the cocrystal.