Two-Step Mechanochromic Luminescence ofN,N′-Bis-Boc-3,3′-di(pyren-1-yl)-2,2′-biindole

Despite recent advances in the development of mechanochromic luminescent organic molecules, the mechano-responsive properties of intramolecularly stacked fluorophores are not well documented. Herein, the synthesis and emission properties of three N,N'-bis-Boc-3,3'-diaryl-2,2'-biindole (Boc=tert-butoxycarbonyl) derivatives as a new class of solid-state emissive fluorophores are described. For a 3,3'-dipyrenyl derivative in the crystalline state, partial intramolecular stacking of the dipyrenyl groups was observed, which results in a two-step mechanochromic luminescence from blue (λem =462 nm) to green (λem =516 nm) and yellow (λem =535 nm).