Bright near-infrared (NIR) dyes are the cornerstones of high-resolution imaging of scattering media and will transform biology, diagnosis, and surgery. Given that NIR fluorescence quantum yields are intrinsically low as dictated by the energy gap law, improving their molar absorptivity is the only alternative strategy to enhance the brightness of NIR fluorophores. Unfortunately, molar absorptivity still remains a gifted rather than an engineerable photophysical property throughout the long course of synthetic dye chemistry. Through an extensive review of the photophysical, structural, and environmental factors governing the molar absorptivity, we envisioned that the site-specific installation of optimized steric groups along the longitudinal axis can significantly enhance the molar absorptivity, along with stability, i.e., the quantum confinement effect. With an iterative structure-property relationship study of a total of 46 cyanine dyes, both the viability and broad scope of this strategy were established. Unprecedented superabsorbing cyanine dyes (ε > 4 × 105 cm-1 M-1) are commonplace in this library, and three Cy7/Cy9 dyes even exhibit molar absorptivity higher than 5 × 105 cm-1 M-1. NIR dyes as bright as fluorescein/rhodamine have been made available. This is a milestone in synthetic dye chemistry and paves the way for high-resolution NIR confocal imaging of intricate biological ultrastructures, e.g., hierarchical nephron structures, and more demanding bioimaging applications.
Zhang et al. (Sat,) studied this question.