Mechanochromic luminogens with hypsochromically shifted emission switching property: recent advances and perspectives

Abstract

Mechanoresponsive luminescent (MRL) materials are a class of luminescent materials that have the ability to show changes in their emission when subjected to various mechanical stimuli. A wide range of MRL materials have been reported for application in advanced opto-electronic technologies. In most of the MRL materials, mechanical stimuli induced emission switching behaviour arises either due to the change in their aggregation states or molecular orientation or disruption of intermolecular interactions. A majority of mechanochromic luminogens display red-shifted emission when subjected to mechanical stress. On the other hand, mechanochromic materials showing blue shift in emission on applying mechanical stimuli are rare. The basic mechanism of shifting of emission wavelength to the shorter wavelength region in response to mechanical stimuli is unclear due to the scarcity of mechanochromic luminogens capable of exhibiting stimuli responsive blue shifted emissions. The main goal of this review is to provide a critical assessment of recently developed mechanochromic luminogens that hypsochromically shift their emission in response to mechanical stimuli. In this review, we have emphasized on various factors such as grinding induced transformation of intermolecular charge transfer to intramolecular charge transfer, crumbling of a crystal to microcrystalline states, grinding induced conformational transformation of a molecule to a more twisted structure, etc. that contribute to the hypsochromic emission shifting in response to mechanical stimuli. This review provides insight into various mechanisms that influence the hypsochromic emission switching mechanochromic behaviour and opens possibilities for developing new materials with the capability of producing distinct emission in response to various mechanical stimuli.