Synthesis and Structure of 3,3-Dimethylindoline Squaraine Rotaxanes

Squaraine rotaxanes are mechanically interlocked molecules comprised of a dumbbell shaped squaraine dye inside a tetralactam macrocycle. Previous squaraine rotaxanes have employed planar squaraine dyes with 4-aminophenyl, 2-aminothiophene, or N-amino units appended to the central C₄O₂ core. Here we describe two rotaxanes that encapsulate a 3,3-dimethylindoline squaraine inside a tetralactam with anthracene sidewalls. The rotaxanes were prepared by a templated clipping reaction and an X-ray crystal structure shows that the squaraine gem-dimethyl groups force a relatively wide separation between the macrocycle anthracene sidewalls. The decreased interaction between the encapsulated squaraine and the anthracene sidewalls leads to a smaller red shift of the squaraine absorption and emission bands. Solution-state studies show that the gem-dimethyl groups in 3,3-dimethylindoline squaraine dyes are large enough to prevent macrocycle threading or rotaxane unthreading. One of the new rotaxanes emits an orange light (560-650 nm), and there is a 10-fold enhancement in the squaraine fluorescence quantum yield upon encapsulation as a rotaxane. This orange-emitting dye completes the palette of known squaraine rotaxane fluorophores whose emission profiles span the color range from green to near-infrared.