Far-Red- to NIR-Emitting Adamantyl-Functionalized Squaraine Dye: J-Aggregation, Dissociation, and Cell Imaging

Squaraine dyes AM-1′ and AM-1 bearing adamantyl termini were synthesized and spectroscopically characterized. AM-1′ exhibited intense absorption and sharp emission in the far-red to near-infrared (NIR) spectral region. The degenerate two-photon absorption (2PA) spectrum of AM-1′ was obtained over a broad spectral range with a maximum cross section of ca. 360 GM. In aqueous solutions, AM-1′ displayed a strong tendency for J-aggregation and the reference AM-1 exhibited entirely different aggregation behavior. We conclude that both the terminal pendant groups and substituents adjacent to the squaraine core play important roles in determining the aggregation behaviors and optical properties of the resultant squaraine dyes. Further an AM-1′/β-cyclodextrin complex dramatically suppressed squaraine π–π stacking and dissociated the J-aggregate. Fluorescence microscopy imaging of HCT 116 cells incubated with the complex was accomplished, suggesting the potential application of this complex in fluorescence bioimaging.