Substituent and State-Specific Solvent Influences on Novel NIR-Emitting ESIPT-Active Push–Pull Fluorescent Dyes Exhibiting Large Stokes Shifts

The most attractive features of the ESIPT-cyanine dyes are their potential in molecular imaging, due to their unique photophysical properties, excellent biocompatibility, and high cell penetration ability. This study utilizes the TD-CAM-B3LYP quantum chemical method to investigate the photophysical properties of a series of near-infrared emitting dyes. These dyes are constructed around a central phenol ring that is functionalized with a strongly electron-withdrawing cyanine group (–CH = CH–R₂⁺) located ortho to the hydroxyl function, and feature tunable substituents (R₁) at the para position. The S₀ → S ₁ absorption wavelengths for L1–L4 range approximately from 323 nm ( L1 ) to 527 nm ( L4 ) in water and from 329 nm (L1) to 520 nm ( L4 ) in DCM. Similarly, the emission wavelengths corresponding to the S₁–E (S₁–K) forms range from ~ 406 (~ 465) nm for L1 up to ~ 865 (~ 781) nm for L4 in water and 407 (468) nm to 809 (769) nm in DCM. The calculated Stokes shifts at S ₁ -K state for L1-L4 in water and DCM are 141.0 and 138.1 nm for L1 , 297.3 and 257.8 nm for L2 , 249.2 and 243.4 nm for L3 , and 254.6 and 249.1 nm for L4 , respectively. Aligned with recent advances in ESIPT–cyanine dyes for fluorescence imaging, particularly in targeting acidic organelles such as lysosomes, the push–pull electron L2–L4 systems specifically L4 characterized by their large Stokes shifts that reduce self-reabsorption for high-contrast imaging, are anticipated to offer comparable benefits.