Kim Heechan, Kim Younghun, Lee Dongwhan
Department of Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, 08826 Seoul, Korea.
Acc Chem Res. 2024 Jan 2;57(1):140-152. doi: 10.1021/acs.accounts.3c00605. Epub 2023 Dec 21.
ConspectusSingle-benzene fluorophores (SBFs) are small molecules that produce visible light by using only one benzene ring as the sole aromatic core. This Account centers around the chemistry of a new class of SBF that we accidentally discovered but rationally developed and refined afterward. In a failed experiment that took an unintended reaction pathway, we encountered the bright green fluorescence of -diacetylphenylenediamine (). Despite its uninspiring look, reminiscent of textbook examples of simple benzene derivatives, this molecule had neither been synthesized nor isolated before. This discovery led to our studies on the larger DAPA family, including isomeric and . Remarkably, is the lightest red fluorophore, with a molecular weight of only 192. While - and are emissive, rapidly undergoes internal conversion, facilitated by sequential proton transfer reactions in the excited state.Leveraging the synthetic utility of the amine group, we carried out straightforward single-step modifications to create a full-color SBF library from as the common precursor. During the course of the investigation, we made another fortuitous discovery. With increasing acidity of the N-H group, the excited-state intramolecular proton transfer reaction is promoted, opening up additional pathways for emission to occur at even longer wavelengths. Tipping the balance between the two excited-state tautomers enabled the first example of a single-benzene white-light emitter. We demonstrated the practical utility of these molecules in white light-emitting devices and live cell imaging.According to the particle-in-a-box model, it is difficult to expect a molecule with only one small aromatic ring to produce long-wavelength emission. SBFs rise to this challenge by exploiting electron donor-acceptor pairs around the benzene core, which lowers the energy of light absorption. However, this answers only half of the question. Where do the exceptionally large spectral shifts in the light emission of SBFs originate from? Chemists have long been curious about the molecular mechanisms underlying the dramatic spectral shifts observed in SBFs. Prevailing paradigms invoke the charge transfer (CT) between electron donor and acceptor groups in the excited state. However, without a large π-skeleton for effective charge separation, how could benzene support a CT-type excited state? Our experimental and theoretical studies have revealed that large excited-state antiaromaticity (ESAA) of the benzene core itself is responsible for this remarkable phenomenon. The core matters, not the periphery. With appropriate molecular design, large and extended π-conjugation is no longer a prerequisite for long-wavelength light emission.
综述
单苯荧光团（SBFs）是一类小分子，仅以一个苯环作为唯一的芳香核来产生可见光。本综述围绕我们意外发现但随后经过合理开发和优化的一类新型SBF的化学性质展开。在一个走了意外反应路径的失败实验中，我们遇到了二乙酰苯二胺（ ）的亮绿色荧光。尽管它看起来平淡无奇，让人联想到简单苯衍生物的教科书示例，但这个分子此前从未被合成或分离出来过。这一发现促使我们对更大的DAPA家族进行研究，包括异构体 和 。值得注意的是， 是最轻的红色荧光团，分子量仅为192。虽然 和 有发射，但 由于激发态中的连续质子转移反应而迅速发生内转换。
利用胺基的合成效用，我们以 作为共同前体进行了直接的单步修饰，创建了一个全色SBF库。在研究过程中，我们又有了一个意外发现。随着N - H基团酸度的增加，激发态分子内质子转移反应得到促进，为在更长波长处发生发射开辟了额外的途径。打破两种激发态互变异构体之间的平衡，实现了单苯白光发射体的首个实例。我们展示了这些分子在白光发光器件和活细胞成像中的实际应用。
根据箱中粒子模型，很难期望一个只有一个小芳香环的分子产生长波长发射。SBFs通过利用苯核周围的电子供体 - 受体对来应对这一挑战，这降低了光吸收的能量。然而，这只回答了问题的一半。SBFs发光中异常大的光谱位移源自何处？长期以来，化学家们一直对SBFs中观察到的巨大光谱位移背后的分子机制感到好奇。流行的范式援引激发态中电子供体和受体基团之间的电荷转移（CT）。然而，没有用于有效电荷分离的大π骨架时，苯如何支持CT型激发态？我们的实验和理论研究表明，苯核本身的大激发态反芳香性（ESAA）是这一显著现象的原因。核心起作用，而非外围。通过适当的分子设计，大的和扩展的π共轭不再是长波长发光的先决条件。
