Constructing NIR-II Fluorophores with Enhanced Fluorescence Quantum Efficiency via Asymmetric D-A-D' Molecular Architectures.

Organic donor-acceptor-donor (D-A-D) fluorophores with emission in the second near-infrared (NIR-II) window have attracted growing attention for bioimaging applications. While incorporating strong electron donor and acceptor units into the molecular framework enables long-wavelength emission, it inevitably leads to a decrease in the fluorescence quantum efficiency (FQE). Herein, we investigated the microscopic mechanism underlying luminescent efficiency enhancement in NIR-II chromophores featuring asymmetric D-A-D' molecular architectures. Based on a typical D-A-D scaffold, a series of compounds is designed through unilateral and bilateral heteroatom substitutions. It indicates that two independent intramolecular charge transfer transitions from different donors to the acceptor are responsible for the photophysical performances of the D-A-D' fluorophores, resulting in balanced spectral shift and FQE as compared to the D-A-D and D'-A-D' counterparts. Notably, the reduction of the nonradiative decay rate arising from the decreased nonadiabatic coupling and electron-vibration coupling caused by suppressed vibrational relaxations mainly contributes to the enhanced FQE, which exhibits a strong correlation with the adiabatic excitation energy of the compounds. With unilateral heteroatom substitution at the ortho-position, Fluo 2' achieves optimal NIR-II emission and FQE, making it a promising candidate for NIR-II imaging. These findings highlight the dual-pronged design strategy for optimizing the luminescent performances of the NIR-II fluorophores, which facilitates the development of highly efficient chromophores for biomedical imaging applications.