Abstract
Molecular fluorophores emitting in the second near-infrared (NIR-II) window with good renal excretion ability are favorable for in vivo bio-imaging and clinical applications. So far, renally excretable fluorophores are still less studied. Understanding the influences of molecular structure on optical properties and renal excretion abilities are vital for fluorophore optimization. Herein, a series of shielding unit-donor-acceptor-donor-shielding unit (S-D-A-D-S) NIR-II molecular fluorophores are designed and synthesized with dialkoxy chains substituted benzene as the S unit. The anchoring positions of dialkoxy chains on benzene are tuned as meso-2,6, para-2,5, or ortho-3,4 to afford three fluorophores: BGM6P, BGP6P and BGO6P, respectively. Experimental and calculation results reveal that alkoxy side chains anchored closer to the conjugated backbone can provide better protection from water molecules and PEG chains, affording higher fluorescence quantum yield (QY) in aqueous solutions. Further, these side chains can enable good encapsulation of backbone, resulting in decreased binding with albumin and improved renal excretion. Thus, fluorophore BGM6P with meso-2,6-dialkoxy chains exhibits the highest quantum yield and fastest renal excretion. This work emphasizes the important roles of side chain patterns on optimizing NIR-II fluorophores with high brightness and renal excretion ability.
Highlights
To date, remarkable attention has been attracted for developing fluorophores for biological imaging in the second near-infrared window (1,000–1700 nm, NIR-II) due to the merits of deep photon penetration and low noise interference (Hong et al, 2017; Li and Pu, 2019; Lei and Zhang, 2021)
It is noteworthy that the substitution positions of dihexyloxy chains on S unit are elaborately engineered as meso-2,6, para-2,5 or ortho-3,4 position to yield three molecular fluorophores BGM6P, BGP6P and BGO6P, respectively
It provides a chance to scrutinize the influence of side chains substitution positions of S unit on optical performance and renal excretion behaviors of the S-D-A-D-S molecular fluorophores
Summary
Remarkable attention has been attracted for developing fluorophores for biological imaging in the second near-infrared window (1,000–1700 nm, NIR-II) due to the merits of deep photon penetration and low noise interference (Hong et al, 2017; Li and Pu, 2019; Lei and Zhang, 2021). Great challenges need to be addressed for clinical administration of most reported NIR-II molecular fluorophores due to their unfavorable metabolic. Molecular Design of NIR-II Fluorophores pathway with apparently accumulation in functional tissues and organs (Yang et al, 2017; Tian et al, 2019; Feng et al, 2021). Molecular fluorophores with rapid clearance from body but low retention in tissues and organs are preferred for preclinical studies and clinical translation (Li et al, 2020). Very few NIR-II fluorophores have been reported to be renally excreted, and it still lacks guidance for designing molecular fluorophores with simultaneously improved fluorescence performance and renal excretion efficiency (Huang et al, 2019; Yang et al, 2021). It is of great significance to study the relationship between molecular structure and optical properties as well as renal excretion ability of NIR-II fluorophores
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