Croconaine Rotaxane Dye with 984 nm Absorption: Wavelength-Selective Photothermal Heating.

Croconaines are an emerging class of near-infrared dyes that are useful for various sensing, photothermal, optoelectronic, and photoacoustic applications. Previous work encapsulated a dumbbell-shaped croconaine dye whose structure contains two thiophene flanking units inside a tetralactam macrocycle and produced a croconaine rotaxane 1 with a narrow 824 nm absorption band. Herein, a new rotaxane 2 is reported that encapsulates a croconaine dye with two thienothiophene flanking units. The new croconaine rotaxane 2 exhibits a narrow 984 nm absorption band that is distinct from the 824 nm absorption of rotaxane 1. Photothermal heating experiments showed that an 830 nm diode laser selectively heats a solution containing rotaxane 1, with no heating of a solution containing rotaxane 2. Conversely, a 980 nm diode laser selectively heats a solution containing rotaxane 2, with no heating of a solution containing rotaxane 1. The new croconaine rotaxane 2 shows no fatigue after four cycles of laser heating and cooling.