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.