Design for multi-step mechanochromic luminescence property by enhancement of environmental sensitivity in a solid-state emissive boron complex

Abstract

We have recently reported multi-state emission of fused boron ketoiminate (FBKI) complexes and their mechanochromic luminescence behaviors. Based on this structure, we introduced the thienyl unit for improving molecular planarity and facilitating intermolecular electronic conjugation in order to enhance stimuli responsive luminescent chromism in the solid state. It was found that the synthesized complex (FBKI-thio) can form two types of crystal polymorphs and their mechanochromic luminescence behaviors were studied. The green-emissive polymorph (crystal A) shows a typical peak shift triggered by grinding the crystalline powder sample, meanwhile the orange-emissive one (crystal B) exhibits multi-step luminescent chromism. By tapping, two new peaks appeared in the shorter wavelength region in the photoluminescence spectrum, and the intensities of these emission bands were enhanced by additional grinding. From the lifetime measurements, it was revealed that new energy levels should be generated after the mechanical treatments. By applying weak mechanical forces, collapse of the regular structures could be partially induced, and subsequently the amorphous states should be realized by the following strong forces. It is proposed that FBKI-thio is able to sensitively detect these morphological changes in the solid state and present a different luminescent color because of enhanced planarity and electronic conjugation. We also mention that the applied physical forces can be evaluated from the changes in intensity ratios between these emission bands.