Engineering the Amorphous-to-Crystalline Transformation, Fluorescence Enhancement and Reversible Switching of a Cyclobutane Derivative inside a Polymer Film

Abstract

The critical characteristic of molecular materials is the weak intermolecular interactions that determine their self-assembly and impact upon their photophysical attributes. Precise control of molecular self-assembly at the nano/microscale can be used to realize desired materials properties, paving the way for the development of innovative functional materials. This study is focused on the controlled assembly of a small molecule (cyclobutane derivative, CB) based material to obtain amorphous and crystalline phases, followed by a detailed investigation of their optical characteristics. Simple drop-casting and re-precipitation techniques are used to fabricate the amorphous and nano/microcrystalline phases of CB respectively. More significantly, a polymer-assisted, highly reversible amorphous-to-crystalline transformation of CB is demonstrated by solvent fuming techniques. During the transformation from the amorphous to the crystalline phase, an increase in fluorescence quantum yield and a blue shift in the emission wavelength were observed. CB exhibited a high quantum yield of ~70% and enhanced fluorescence emission, ~450 times greater in the solid state compared to its solution state. Crystallographic analysis and computational modelling provided insights into the significance of molecular self-assembly and the plausible mechanism underlying the fluorescence emission enhancement in the solid state. This work illustrates a simple and effective method to control molecular self-assembly within the polymer matrix, that can lead to tailored optical properties, offering insights into the design of smart soft materials.