Post-synthetic modification of near-infrared absorbing cyclodextrin-encapsulated cyanine dyes for controlling absorption wavelength, stability, and singlet oxygen generation

In this study, near-infrared absorbing rotaxane-type cyanine dyes exhibit high tolerance to various chemical reactions, which is attributed to the encapsulation effect of their cyclic molecules. As a result, rotaxane dyes can be post-modified on the host α-cyclodextrin or guest cyanine skeleton to adjust their solubility, absorption wavelength, stability, and singlet oxygen generation ability. The guest modification product obtained via the Heck reaction demonstrates a red shift of its absorption wavelength owing to the extended conjugation system. Moreover, the products of host modification through the methylation and benzylation of all cyclodextrin hydroxyl groups not only become lipophilic and show extended absorption, but also exhibit higher photooxidation tolerance, lower singlet oxygen generation rate, and increased singlet oxygen tolerance, indicating their potential applicability as highly durable dyes. Furthermore, the outstanding singlet oxygen tolerance of these dyes enables their use in long-life singlet oxygen generators, in which the total amount of singlet oxygen increases. This work demonstrates that an intrinsically unstable near-infrared cyanine dye can be used as a synthetic intermediate by stabilizing it via α-cyclodextrin encapsulation, allowing the post-modification of various properties of cyanine dyes toward the higher-order near-infrared-absorbing materials with complex functionalities and diverse utilities.