Mechanofluorochromic behaviors of β-iminoenolate boron complexes functionalized with carbazole

Abstract

Carbazole and tert-butylcarbazole functionalized β-iminoenolate boron complexes CB and TCB with different mechanofluorochromic (MFC) properties have been synthesized. It was found that the as-synthesized crystal of CB emitted orange light under UV illumination because of the appearance of the emission from the monomers and the excimers. After grinding for a while, the ground powder 1 of CB emitted bright yellow light since the ratio of the emission intensity from excimers/monomers decreased. On further grinding for a long time, the excimers disappeared and the obtained amorphous ground powder 2 emitted dark green light. However, TCB emitted sky blue light in the as-synthesized crystal because no excimer was formed due to the steric hindrance of tert-butyl. After grinding, the amorphous ground powder of TCB emitted bright green light derived from excimers. It should be noted that the fluorescence quantum yield of TCB in an amorphous solid state reached 0.53, which was the highest one for the β-iminoenolate boron complexes that have ever been reported. In addition, the emission changes of CB and TCB in different solid states were reversible upon repeating the treatment of mechanical grinding and fuming with CH₂Cl₂. Therefore, the obtained β-iminoenolate boron complexes might be used as sensors and memory chips on the basis of the solid fluorescence response to external mechanical forces and organic solvents.