Tunable mechanochromic luminescence of benzofuran-fused pyrazine: effects of alkyl chain length and branching pattern

Abstract

A series of bis(benzofuro)[2,3-b:2′,3′-e]pyrazines (BBFPz) with various alkyl chains of different length have been synthesized, and the effects of chain length on their mechanochromic luminescent properties were investigated. Some derivatives gave two polymorphs depending on the recrystallization conditions. Although the luminescent properties of the derivatives were almost the same in the solution state, distinct luminescence depending on their alkyl chains was observed in the crystalline state. The obtained crystals could be categorized into four packing motifs (Type A–D) with unique mechanochromic luminescent properties. Type D crystals with linear alkyl groups did not show mechanochromic luminescent characters. The luminescent color of Linear-Me slightly and reversibly changed upon grinding. Linear-Et (Form 1) and Linear-sBu, which belong to Type B and C, respectively, exhibited mechanochromic luminescent properties with rapid self-recovery character. The other crystals showed mechanochromic luminescent properties with relatively slow self-recovery rates. The present work demonstrates a quite limited example of covering both on/off mechanochromic luminescence (MCL) and self-recovery characters with a specific luminophore skeleton by changing the alkyl chains.