Dependence of mechanofluorochromic property at room temperature on alkyl chain structure for β-diketone boron complex and its polymer blend film

Abstract

A series of β-diketonate boron complexes (BF2-TPEn, n = 1, 2, 3, 4 and 5) with different alkyl chain lengths and tetraphenylethylene (TPE) group, were synthesized to investigate the effect of alkyl chain structure on their fluorescence properties. The results showed that all the as-prepared BF2-TPEn complexes emit fluorescence, but only the emission wavelength of BF2-TPEn (n = 1, 3 and 5) is changed from yellow to orange after grinding, which revealed that the mechanofluorochromic (MFC) property of complexes relies on the parity of the carbon atoms at the end of the molecules. Notably, it is also found that the fluorescence emission of the ground samples of BF2-TPE1 and BF2-TPE5 could recover spontaneously at room temperature without the need for solvent fumigation or other stimulates. This reversible MFC property at room temperature is distinctly different from the other MFC materials reported in previous literature. Additionally, the blend films of polycaprolactone (PCL) and complexes were prepared to investigate the effect of different alkyl chain length on fluorescence emission of BF2-TPEn existing in polymer. It is found that only the PCL film containing BF2-TPE5 showed MFC property at room temperature, which is expected to receive widespread applications in the field of optical devices and mechanical sensors. This work demonstrates that alkyl chain structure can tune the MFC properties of β-diketonate boron complexes, which provides good design ideas for the development of novel MFC materials.