Photoinduced irreversible transformation from elastic to bent brittle crystals: importance of the reaction pathway in the irreversibility of the molecular movement

Abstract

A chalcone based molecular crystal, (E)-1-(2-thienyl)-3-(3,4,5-trimethoxyphenyl)prop-2-en-1-one, has been synthesised and analyzed with SXRD, PL-studies, ¹H-NMR, ¹³C-NMR, DSC, TGA, DTA, SEM, IR and PXRD. The compound crystallizes in an orthorhombic crystal system with the space group Pna2₁ and a short axis of 4.1184(2) Å. The single crystal exhibits three molecular properties, such as fluorescence (green light emission, λ = 537.21 nm), elastic bendability under mechanical force, and photoinduced bending under light (λ_max = 347 nm). The photoinduced bending is a consequence of [2 + 2] dimerization between the two parallelly-oriented molecules in the single crystal. Interestingly, after the dimerization, the photoluminescence properties and elastic bendability are extinct. The transparent emissive elastic single crystal transforms into an amorphous bent brittle solid with an arc morphology. The mechanism for this rare observation is analyzed by following the variable time light exposure PXRD pattern. During the [2 + 2] dimerization, the amorphous dimer phase randomly formed along the needle releases strain at a slow pace throughout the crystal, and the needle turns into a permanently bent solid.