International School for Optoelectronic Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China.
Int J Mol Sci. 2024 Nov 18;25(22):12365. doi: 10.3390/ijms252212365.
Organic molecular fluorophores have been extensively utilized for biological imaging in the visible and the first near-infrared windows. However, their applications in the second near-infrared (NIR-II) window remain constrained, primarily due to the insufficient fluorescence brightness. Herein, we employ a theoretical protocol combining the thermal vibration correlation function with the time-dependent density functional theory method to investigate the mechanism of the planar-twisted strategy for developing fluorophores with balanced NIR-II emission and fluorescence brightness. Based on a planar donor-acceptor-donor molecular skeleton, various ortho-positioned alkyl side chains with steric hindrances are tactfully incorporated into the backbone to construct a series of twisted fluorophores. Photophysical characterizations of the studied fluorophores demonstrate that the emission spectra located in the NIR-II region exhibited a hypsochromic shift with the structural distortion. Notably, conformational twisting significantly accelerated the radiative decay rate while simultaneously suppressing the nonradiative decay rate, resulting in an improved fluorescence quantum efficiency (FQE). This enhancement can be mainly attributed to both the enlarged adiabatic excitation energy and reduced nonadiabatic electronic coupling between the first excited state and the ground state. Compared with the planar fluorophore, the twisted structures possessed a more than fivefold increase in FQE. In particular, the optimal twisted fluorophore BBTD-4 demonstrated a desirable fluorescence brightness (16.59 M cm) on the premise of typical NIR-II emission (980 nm), making it a promising candidate for NIR-II fluorescence imaging in biomedical applications. The findings in this study elucidate the available experimental observations on the analogues, highlighting a feasible approach to modulating the photophysical performances of NIR-II chromophores for developing more highly efficient fluorophores toward optical imaging applications.
有机分子荧光团已被广泛应用于可见光和近红外第一窗口的生物成像。然而，它们在近红外二区（NIR-II）窗口的应用仍然受到限制，主要是由于荧光亮度不足。在此，我们采用理论协议，结合热振动相关函数和含时密度泛函理论方法，研究了发展具有平衡近红外二区发射和荧光亮度的荧光团的平面扭曲策略的机制。基于平面给体-受体-给体分子骨架，巧妙地在骨架中引入各种具有空间位阻的邻位烷基侧链，构建了一系列扭曲荧光团。所研究的荧光团的光物理特性表明，位于近红外二区的发射光谱随着结构扭曲呈现出蓝移。值得注意的是，构象扭曲显著加速了辐射衰减速率，同时抑制了非辐射衰减速率，从而提高了荧光量子效率（FQE）。这种增强主要归因于第一激发态和基态之间的绝热激发能增大和非绝热电子耦合减小。与平面荧光团相比，扭曲结构的 FQE 提高了五倍以上。特别是最优扭曲荧光团 BBTD-4 在具有典型近红外二区发射（980nm）的前提下表现出良好的荧光亮度（16.59Mcm），使其成为生物医学应用中近红外二区荧光成像的有前途的候选物。本研究阐明了对类似物的现有实验观察结果，突出了一种调节近红外二区发色团光物理性能的可行方法，以开发更高效的荧光团用于光学成像应用。
