Sterically Controlled Template-Assisted Macrocyclization of Hemisquaraine Rotaxanes: Synthesis, Characterization, and DFT Calculations

Abstract The encapsulation of squaraine dyes within a tetralactam macrocycle effectively mitigates their intrinsic limitations while maintaining favorable optical properties and improving both chemical and photochemical stability. However, the synthesis of such rotaxanes remains challenging due to the intricate balance of steric and electronic factors governing the macrocyclization. In this study, a series of rotaxanes derived from hemisquaraine dyes bearing primary and secondary amino groups was successfully synthesized using an oxocyclobutenolate template-assisted macrocyclization approach. Dyes with bulky tertiary amino groups, however, failed to undergo encapsulation. Upon encapsulation, the rotaxane systems in chloroform exhibited red-shifted absorption and emission bands, along with a substantial increase in fluorescence quantum yield. This enhancement is attributed to the restricted vibrational and rotational motions of the encapsulated dye. DFT calculations confirmed that all studied rotaxanes are thermodynamically viable, suggesting that the synthetic difficulties observed for tertiary amine derivatives arise from kinetic constraints. This hypothesis was supported by DFT calculations that simulated the macrocyclization process from the corresponding open-chain precursors. These calculations showed favorable energetics for hemisquaraine dyes that contain primary and secondary amino groups. In contrast, the significant steric effects of tertiary amino groups render the oxocyclobutenolate template-assisted macrocyclization energetically unfavorable.