Red-shifting squaraine dyes in the near-infrared region using perimidine heterocycles: synthesis and optical properties

Squaraine dyes are an important class of near-infrared (NIR) probes due to their strong absorption, high molar absorptivity, and structural tunability. However, conventional squaraine dyes employ weak donor units, which suffer from an absorbance maximum in the low 700 nm region. Herein, we report the design, synthesis, and comprehensive characterization of a series of new perimidine-based squaraine dyes that utilize a stronger donor heterocycle to extend optical properties beyond 800 nm. The perimidine donor units were synthesized using a green methodology with high yields. The heterocycles were condensed with squaric acid to afford the squaraine dyes in moderate yields. Optical studies showed strong NIR absorbance, high molar extinction coefficients, and superior photothermal stability. All dyes exhibited low fluorescence. Computational analysis revealed that the facile <i>E</i>-<i>Z</i> photoisomerization and intramolecular quenching pathways led to low quantum yields. The <i>Z</i>-isomer was identified as the global energy minimum.