Two novel rhodamine-based molecules with different mechanochromic and photochromic properties in solid state

Abstract

Two novel molecules (AnRhB-1 and AnRhB-2) were synthesized using Rhodamine B and anthracene as building units via Schiff base chemistry with different spacers. This small structural variation led to a dramatic difference in mechanochromic and photochromic properties in the solid state. They both showed a force-induced color change. However, AnRhB-1 not only displayed two color changes due to the ring-opening isomerization of the rhodamine derivatives, but also exhibited rarely reported photochromic properties. The photochromic behaviour of AnRhB-1 resulted from its large free volume in the amorphous solid state and the relatively high stability of its zwitterionic species in its ring-open form. In contrast, AnRhB-2 gave a tricolored switch upon force action but no photochromic behavior is observed. The three-color variations in this molecule originated from the transformation of the different physical packing modes of anthracene and force-triggered chemical reaction of Rhodamine B lactam. The Schiff base group for the molecular design of mechanochromic and photochromic materials can be extended to other systems, which may open a new strategy to provide a broad perspective for the development of multi-stimuli responsive materials.