Effect of a bridge unit on the mechanochromism of phenothiazine-based Schiff bases

Abstract

Two phenothiazine derivatives bridged with Schiff base groups were prepared, and investigations were conducted to study their mechanofluorochromic (MFC) behavior. When a mechanical force was applied, the compound PPD (N,N′-bis(10-ethyl-phenothiazin-2-ylmethylene)-p-phenylenediamine) changed its color from yellow to orange, exhibiting a significant MFC behavior. The reversible MFC behavior of PPD could be accomplished via grinding and fuming with CHCl₃. Spectral analysis indicated that PPD molecules were packed in an H-aggregate form in the as-synthesized crystal, whilst it had transformed to an amorphous state under external grinding. The X-ray analysis revealed that the transition from the crystalline structure to the amorphous state would result in the color change. Moreover, PED (N,N′-bis(10-ethyl-phenothiazin-2-ylmethylene)-ethylenediamine) with two isolated chromophores did not exhibit an MFC behavior, while no phase transition from crystal to the amorphous solid was found under grinding.