Rational Design of High Brightness NIR-II Organic Dyes with S-D-A-D-S Structure

ConspectusFluorescence bioimaging in the second near-infrared window (NIR-II, 1000–1700 nm) is a superior visualization tool with deeper penetration and higher spatial resolution than the traditional vis/NIR window (400–1000 nm). Developing desirable NIR-II agents with high brightness (high quantum yield (QY) and absorption coefficient), longer emission wavelengths, light stability, and good biocompatibility is a bottleneck that must be crossed in the process of clinical transformation. NIR-II organic agents attract more attention because of their good biocompatibility. Researchers are committed to develop small organic molecules with an adjustable narrow band gap to emit longer NIR-II wavelengths. However, the reduced band gap of molecular fluorophores generally triggered interactions between the conjugated skeleton and other molecules, leading to a decreased fluorescence QY, especially quenching in aqueous systems.Aiming to enhance NIR-II fluorophores' brightness in aqueous systems, the molecular engineering of a shielding and donor unit is introduced in NIR-II fluorophores, and these molecules are composed of a shielding unit, donor(s), acceptor, donor(s), shielding unit (S-D-A-D-S) structure. We found that the donor's steric hindrance brings about a molecular twist that can be tuned with electrostatic potential surfaces and the donor's hydrophobic effects, which can reduce water molecules approaching the excitation center. In addition to the protective effect of the electron shielding units, these changes can be weakened by the interactions between water and fluorophore molecules, which is beneficial to the stability of excited states for fluorophore brightness maintained in water-soluble environments. However, the molecular torsion will reduce intramolecular charge transfer (ICT), the weakened transition from the ground state to the excited state (S0–S1), leading to an undesired decrease in the absorption coefficient. Thus, it is necessary to develop molecular engineering to improve emission QY without sacrificing the absorption coefficient. Therefore, bright NIR-II fluorophores need to have a rational balanced donor unit, which can have decreased backbone distortion to strengthen ICT and better protect the conjugated backbone to reduce the water molecule effect, resulting in improvements to the QY and absorption coefficient simultaneously.In this Account, we will present a concise summary and analysis of the rational molecular design of S-D-A-D-S structural NIR-II fluorophores for tunable enhanced QY and absorption coefficients based on the shielding and donor engineering strategy. We expect that this Account will trigger more research interest to explore the inspiring performance of S-D-A-D-S structural NIR-II fluorophores and employ them in bioimaging to promote the process of clinical transformation.