Advanced Membranes and Porous Materials (AMPM) Center, Physical Sciences and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia.
Functional Materials Design, Discovery and Development Research Group (FMD3), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia.
J Phys Chem B. 2021 Dec 9;125(48):13298-13308. doi: 10.1021/acs.jpcb.1c08889. Epub 2021 Nov 30.
Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10 to 2.48 ns at 2.0 × 10 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.
聚集诱导发光增强（AIEE）是一种最近在固态材料和有机发光体中得到利用的过程，它可以通过紧密的分子包装来解释。然而，溶液相 AIEE 及其形成机制尚未得到广泛探索。本工作研究了两种供体-受体-供体型苯并二氮杂卓基分子（金属有机框架中的有机构建块）在乙快和苯π共轭主链上带有羧酸基团的端基的 AIEE 现象。这是通过结合时间分辨电子和振动光谱以及时间依赖密度泛函理论（TD-DFT）计算来实现的。荧光上转换光谱和时间相关单光子计数法明确表明，分子内电荷转移驱动的聚集态发射增强。这表现为发射光谱的红移以及荧光寿命从 1.0×10时的 746 ps 增加到 2.0×10时的 2.48 ns。TD-DFT 计算表明，受限的分子内旋转机制是增强发射的原因。飞秒红外（IR）瞬态吸收结果直接揭示了聚集形成的结构动力学，这从乙炔单元的 C≡C 振动标记模式在光激发下的演变中可以明显看出。此外，IR 数据清楚地表明，聚集过程发生在 10 ps 的时间尺度内，这与荧光上转换结果一致。有趣的是，时间分辨结果和 DFT 计算清楚地表明，乙炔键和硫原子都是实现这种可控聚集诱导荧光增强的关键要求。这项工作的发现不仅表明荧光发色团的化学结构稍有变化就会对其光学行为产生巨大影响，而且还促使人们深入研究这种现象的机制起源。这可能会导致发现新的化学策略，旨在合成具有优异光收集应用光学性能的新型发色团。
