Effect of Substitution Pattern of tert‐Butyl Groups in a Bisbenzofuropyrazine Core π‐System on Optical Properties: Unique Mechanochromic Fluorescence Behavior

Abstract

AbstractBisbenzofuro[2,3‐b : 2′,3′‐e]pyrazine (BBFPy) and its 2,8‐ and 3,9‐di(tert‐butyl)‐substituted derivatives (2 a and 2 b) were synthesized by palladium‐catalyzed intramolecular direct arylation as the key step. The effect of introduction of two tert‐butyl groups to the BBFPy core at the different positions on the fluorescent properties was then investigated. The tert‐butyl groups not only enhance the tractability with increasing solubility, but also induce different crystal‐packing patterns as anticipated. The compounds 2 a and 2 b show relatively strong blue fluorescence in both the solution and solid state. The 2,8‐isomer 2 a specifically exhibits a unique mechanochromism in the solid state; 2 a exhibits two crystalline forms, both of which show green fluorescence by grinding, while different self‐recovering fluorescence patterns between them are observed by standing. The intriguing mechanochromism is rationally interpreted in terms of molecular arrangements determined by X‐ray crystallography.