Facilitating J-Aggregate Formation by Lowering Activation Energy to Achieve Ultrahigh Brightness for NIR-II Fluorescence Imaging and Photoacoustic Imaging.

Organic fluorophores operating in the second near-infrared window (NIR-II, 900-1700 nm) are plagued by low quantum yields (QYs) and diminished brightness in aqueous media. To address this challenge, we proposed a strategy of regulating the activation energy for aggregates transformation to achieve high-brightness NIR-II J-aggregates. Three aza-boron-dipyrromethene (aza-BODIPY) with heterocyclic substituents were synthesized. Among them compound ABS with substituents of dibenzothiophene exhibited the strongest J-aggregation propensity, as evidenced by aggregation kinetics studies. The self-assembly properties in water were modulated by grafting hydrophilic and hydrophobic chains onto the ABS core. This molecular engineering strategy promoted exciton delocalization and accelerated radiative decay (kr = 0.191 ns-1) for NIR-II J-aggregates of ABS-2 in water, achieving a record-high absolute quantum yield of 10.11%. Coupled with a molar extinction coefficient of 1.6 × 105 M-1·cm-1, the ABS-2 J-aggregates demonstrated ultrahigh brightness (B = 1.6 × 104 M-1·cm-1), surpassing most aqueous NIR-II emitters. Leveraging these photophysical merits, ABS-2 J-aggregates enabled NIR-II fluorescence imaging of blood vessels and also allowed NIR-II fluorescence and photoacoustic imaging of tumors in mice, showcasing its dual-modal diagnostic potential. We anticipate that the proposed strategy of regulating the activation energy for aggregates transformation may inspire further research for design of NIR-II fluorophores with high quantum yield and brightness, to improve the spatial resolution and tissue penetration depth of in vivo fluorescence imaging.