Understanding the mechano-fluorochromic enhancement in a cyanoethylene-functionalized pyrene-based luminogen: An experimental and theoretical study

Abstract

Pyrene-based luminogens have attracted wide attention due to their promising applications in organic electronics, chemical sensing, and bioimaging. In this study, a cyanoethylene-functionalized pyrene-based luminogen CNPyCH₃ was designed and synthesized. Experimental results show that CNPyCH₃ exhibits distinct photophysical properties in dilute solution and various solid states. The mechano-fluorochromic (MFC) property is more attractive and interesting, as a red shift with enhanced emission was observed upon mechanical stimulation. The intriguing MFC enhancement behavior was further investigated by in-depth theoretical calculations combining Monte Carlo simulations, quantum mechanics/molecular mechanics calculations, and time-dependent density functional theory. It was found that the calculated emission wavelengths for both pristine and ground CNPyCH₃ are in good agreement with the experimental values, and that the ground CNPyCH₃ has a greater oscillator strength. From the perspective of intermolecular interactions, the weakened π-π interactions induced by the half-switched “face-to-edge” packing modes are beneficial for achieving brighter emission upon grinding. In terms of intramolecular structures, the ground CNPyCH₃ has a more planar conformation around the cyanoethylene moiety in the excited-state equilibrium geometry than the pristine CNPyCH₃, resulting in the grinding-induced red-shifted emission. Our study provides a unique insight into the MFC enhancement behavior of the pyrene-based luminogens.