A new squaraine rotaxane molecular shuttle exhibits high chemical stability and acts as a deep-red, fluorescent and colorimetric sensor for Cl À anion with reversible, ratiometric response. The molecular design encapsulates a dihydroxyl substituted squaraine dye inside an anthracene-containing tetralactam macrocycle and a "clicked capping" reaction was used to convert an appropriate pseudorotaxane precursor into a permanently interlocked rotaxane in high yield. Reversible binding of Cl À to the rotaxane in solution, or on the surface of prototype dipsticks, causes lateral displacement of the surrounding macrocycle away from the central squaraine station and a substantial 30-40 nm shift in the squaraine absorption/fluorescence maxima that can be easily detected by the naked eye. The collective attributes of intense absorption/emission and ratiometric response at deep-red wavelengths is a significant advance in optical Cl À sensor performance by an organic molecule.

Photooxygenation of permanently interlocked squaraine rotaxanes with anthracene-containing macrocycles produces the corresponding squaraine rotaxane endoperoxides (SREPs) quantitatively. SREPs are stored at low temperature, and upon warming, they undergo clean cycloreversion, releasing singlet oxygen and emitting light. The structural elucidation in 2010 assigned the structure as the SREP-int stereoisomer, with the endoperoxide unit directed inside the macrocycle cavity. New experimental and computational evidence reported here proves that the initial, kinetic photooxygenation product is the less stable SREP-ext stereoisomer with the endoperoxide unit directed outside the macrocycle. The photophysical properties and subsequent reactivity of mechanically strained SREP-ext depend on the size of the end groups of the encapsulated squaraine dye. If the end groups are sufficiently large to prevent dissociation of the interlocked components, the strained SREP-ext stereoisomer undergoes clean thermal cycloreversion. However, smaller squaraine end groups allow transient dissociation, resulting in a pseudorotaxane dissociation/association process that produces SREP-int as the thermodynamic stereoisomer that does not cyclorevert. The large difference in endoperoxide reactivity for the two SREP stereoisomers illustrates the power of the mechanical bond to induce cross-component steric strain and selective enhancement of a specific reaction pathway. The new insight enabled synthetic development of triptycene-containing squaraine rotaxanes with high fluorescence quantum yields and large Stokes shifts.

A squaraine rotaxane endoperoxide with a truncated squaraine chromophore undergoes a cycloreversion reaction and emits green light that can be transferred to red acceptor dyes. The results demonstrate that chemiluminescence emission for squaraine rotaxane endoperoxides comes from the excited squaraine inside the rotaxane.

Squaraine rotaxanes with macrocycles containing 1,3-adamantanedicarboxamide bridging units have altered chemical and luminescent properties due to a more loosely held rotaxane co-conformation.

Thiosquaraine dyes have sulfur atoms instead of oxygens on the central squaraine core, and they are powerful singlet oxygen photosensitizers. Stability studies show that they are susceptible to attack by nucleophiles. This problem was circumvented by preparing a mechanically interlocked thiosquaraine rotaxane. NMR studies of the rotaxane indicate an unusual dynamic molecular structure due to a nonsymmetrical coconformation. Upon irradiation with red light, the thiosquaraine rotaxane generates the same amount of singlet oxygen as the known photosensitizer methylene blue.