Photodimerization induced Photomechanical behaviour in Akyldiammonium Salts of trans-Dichlorocinnamates: Exploration of [2+2] Reactions in as Synthesized Salts, Single Crystals and Organogels

Abstract

The development of light-responsive molecular crystals that convert light into mechanical energy is a burgeoning field with applications ranging from remote actuation to flexible electronics. This study explores the [2+2] photodimerization and subsequent photomechanical behaviour of a series of organic salts synthesized from alkyldiamines (H2N-(CH2)n-NH2, n= 2-4) Bn and trans-2,4/3,4-dichlorocinnamic acid (Am4, m =2-3). By employing chloro-substitution to direct crystal packing into photoreactive β-type structures, we investigated these reactions across three distinct states: as-synthesized salts, single crystals, and supramolecular organogels. Experimental results demonstrate that salts containing ethylenediamine (B2(A24)2) and propylenediamine (B3(A24)2) are photoactive across all states, while butylenediamine (B4(A24)2 & B3(A24)2) derivatives remain photoinactive. Notably, the [2+2] reaction in single crystals of B2(A24)2 and B3(A24)2 induced dramatic photomechanical effects, including macroscopic bending, surface peel, and the formation of surface cracks. The successful formation of cyclobutane dimers was confirmed via 1 H-NMR, MALDI-ToF mass spectra, and single-crystal X-ray diffraction.Furthermore, we report a rare instance of organogels derived from these simple organic salts undergoing [2+2] photodimerization while exhibiting significant electrical conductivity ((5.2 × 10 -3 S cm -1 ). Additionally, certain salt compositions demonstrated high efficiency for iodine absorption. This work highlights the versatility of organic diammonium salts as multifunctional materials for optomechanics, sensing, and conducting soft materials.