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.