Improving the fluorescence brightness of NIR-II fluorophores via intramolecular covalent bond locking: a theoretical...

Fluorescence imaging in the second near-infrared (NIR-II) region, characterized by deep tissue penetration and high spatial resolution, has emerged as a promising modality for biomedical applications. However, the majority of NIR-II fluorophores suffer from insufficient brightness primarily attributed to the limited fluorescence quantum yields. Herein, the mechanism of fluorescence brightness enhancement through an intramolecular covalent bond locking strategy for donor-acceptor-donor NIR-II fluorophores is investigated. Compared with the unlocked TQ-1, fusing phenyl rings on the acceptor moiety induces bathochromic shifts in both the photoabsorption and photoemission spectra, while the modification to the donor unit results in a hypsochromic effect. Notably, incorporating intramolecular covalent bonds within the acceptor segment facilitates structural relaxation during the electronic transitions, which is mainly responsible for the reduction in luminescence efficiency. In contrast, by locking the terminal phenyl groups of the fluorophore, the adiabatic excitation energy is increased and the electron-vibration coupling as well as nonadiabatic electronic coupling is decreased, resulting in a significant reduction in the nonradiative decay rate. Consequently, TQ-4 achieves an optimal fluorescence quantum yield and brightness while maintaining NIR-II emission, demonstrating its potential as a high-performance NIR-II chromophore. This work highlights the feasibility of constructing efficient NIR-II fluorophores via intramolecular covalent bond locking, providing rational design principles for developing novel NIR-II fluorophores toward biomedical applications.