【JACS】共轭拓展偶氮噻吩分子：同步实现可逆光致异构与光调控电荷传输
文章标题：Coupling Photochromism and Charge Transport in π-Extended Arylazo Oligothiophenes and Oligothienoacenes
通讯作者：Massimo Baroncini, Francesca Di Maria
文章概要
在这项发表于《美国化学会志》（JACS）的前沿研究中，科研团队成功解决了一个长期困扰光电分子材料领域的难题：如何在单一分子架构中同时兼顾高效的光致变色（光控开关）性能与优异的电荷传输能力。研究人员系统设计并合成了两类新型的π共轭芳基偶氮寡聚噻吩和寡聚噻吩并酰苯材料，通过巧妙的晶体工程调控骨架排布，在固体薄膜状态下实现了高度稳定的光致异构化及材料形貌的可控重构。基于该材料制备的单载流子器件成功证明，在不同波长光照下，其体电子迁移率可展现出高达约4倍的电荷传输调制能力，为开发下一代智能光电开关、多功能光敏异构化器件及有机半导体架构提供了全新的构筑范式。
引言
光致变色化合物因其在光照下能够发生可逆的结构与电子态转变，在数据存储、机械驱动、分子马达以及精准光药理学等领域展现出巨大的应用潜力。在众多的光控开关分子中，偶氮苯及其衍生物由于合成简便、结构多变且异构化效率高，一直是学术界和工业界关注的焦点。然而，如果想要将这类分子引入到有机光电子器件中，通常需要延长分子的π共轭长度以增强其吸光特性并赋予其半导体性质。遗憾的是，传统的共轭体系延长策略往往伴随着难以调和的“双刃剑”效应：一方面，共轭度的提升会显著降低亚稳态顺式（Z）异构体的热稳定性，使其极易自发回复；另一方面，大面积的共轭平面在固体状态下倾向于形成致密的π-π堆积，这种极高的晶格限域效应会彻底锁死分子的自由度，导致材料在固体薄膜中失去光致变色活性。为了攻克这一瓶颈，本研究将具有优异半导体特性的寡聚噻吩骨架与偶氮变色基团直接相连，并提出了相应的分子修饰策略，实现了光变色与电荷传输的完美耦合。
Chart 1. Structures of the Investigated Compounds
主要实验及结论
研究团队首先合成了以α键相连的寡聚噻吩和外环熔融的寡聚噻吩并酰苯两类同系物。在溶液态下，随着噻吩单元数量的增加，由于材料光吸收能力的系统性提升，其紫外-可见吸收光谱表现出显著的红移特征。然而，伴随而来的共轭扩展使得顺式异构体的半衰期骤减。为了解决这一溶液态的双稳态退化问题，研究人员提出了芳环氟化调控策略。通过在芳环上引入强吸电子的氟原子，增强了分子内部硫原子的孤对电子与芳环之间的孤对电子-π相互作用，从而使得氟化衍生物顺式异构体的半衰期从原本的几分钟大幅延长至数小时，显著改善了溶液态下的光控双稳态特性。
Scheme 1. Synthetic Scheme Illustrating the Preparation of the Compounds Investigated in This Work
随后，研究的重心转移到了最具挑战性的固体薄膜活性调控上。正如前文所述，紧密的固体堆积会抑制分子的光异构化运动。对此，团队引入了大位阻的具有形状保持特性的三苯甲基基团。这种刚性的星状基团能够有效增加分子的自由体积并降低晶体堆积密度。在薄膜吸光实验中，未修饰的噻吩骨架由于堆积过紧完全失去了光控响应，而引入了三苯甲基修饰的特殊薄膜则展现出高度可逆的光谱调制行为。这一薄膜在可见光照射下可以高效地向顺式异构体转变，而在紫外光照射下又可以几乎定量地回复到反式（E）状态，连续循环十次以上仍表现出极其优异的抗疲劳特性。
Figure 1. UV–vis absorption spectra in air equilibrated CH2Cl2 of the E isomers of (a) arylazo oligothiophenes, and (b) arylazo oligothienoacenes.
Figure 2. Reconstructed UV–vis absorption spectra in air equilibrated CH2Cl2 and DFT-optimized ground-state geometries of the Z isomers of: (a, c) phenylazo oligothiophenes, (b, d) arylazo oligothienoacenes. The Z-4 spectrum could not be reconstructed due to fast thermal Z → E back-isomerization. Ground-state geometries overlays were generated by maximizing the overlap between heavy atoms of the phenyl–N═N–thienyl fragment. Color legend: Z-1, black; Z-2 and Z-2a, cyan; Z-3 and Z-3a, green; Z-4 and Z-4a, red. The intramolecular S(n)···π interaction is indicated in orange.
更为奇特的是，这种固体状态下的分子异构化还触发了微观层面的形貌重构。通过X射线粉末衍射（XRPD）和原子力显微镜（AFM）分析，科研人员清晰地观察到了材料在光照下发生的光致晶态向非晶态的转变。在原始状态下，由于三苯甲基的隔离，噻吩骨架在薄膜中形成了数十纳米的均匀结晶微区。当对其进行持续的可见光辐照时，分子的微观几何运动逐步扰乱了原本的晶格排布，导致X射线衍射峰强度持续下降，部分结晶区域逐渐淡化并发生横向展宽，表现为薄膜的逐渐变暗和无序化。这种形貌上的重构在室温暗处可以被冻结在动力学捕获的非晶态中，只有通过高温热退火处理，材料才能恢复最初的结晶度与双折射现象，表明其异构化速率与大尺度形貌扩散在时间尺度上是解耦的。
Figure 3. (a) Bar plot of the Z-isomer percentage at the photostationary state (PSS) and quantum yield for E→Z (Φ_E_→Z blue diamonds) and Z→E (Φ_Z_→E red diamonds) photoisomerization reactions (%) as a function of irradiation wavelength. (b) Semilogarithmic plot of the Z isomers half-life in the dark at 25 °C. Solvent: air equilibrated CH2Cl2.
Figure 4. (a) UV–vis absorption spectra of a spin-coated film of E-3aPh3 (thickness ≈ 60 nm, spin-coated from a 1 mg/mL solution at 1500 rpm): before (black trace), after exhaustive irradiation at 436 nm for 20 min (PSS436, red trace), and after subsequent exhaustive irradiation at 365 nm for 20 min (PSS365, blue trace). Spectra acquired at intermediate times during 436 nm irradiation are shown in light gray; inset: absorbance variation at 450 nm over repeated cycles of alternate irradiation at 436 nm (20 min, blue circles) and 365 nm (20 min, red circles); (b) UV–vis absorption spectra of a spin-coated film of E-3aPh3 (thickness ≈ 60 nm) after exhaustive irradiation at 436 nm for 20 min (PSS436, red trace) and after 6 h at 25 °C in the dark (green trace). Spectra acquired at intermediate times are shown in light gray; inset: absorbance variation at 450 nm. Light intensity at 365 and 436 nm: ≈20 mW·cm–2.
Figure 5. (a) XRPD patterns of a spin-coated film of E-3aPh3 (thickness ≈ 150 nm, spin-coated from a 3 mg/mL solution at 1500 rpm), before (I), after irradiation at 436 nm (≈20 mW·cm–2) for 2 h (II), and after subsequent thermal annealing at 120 °C for 20 min (III). Patterns acquired at intermediate times are shown in light gray. The pattern calculated from single-crystal data of E-3aPh3 is also shown (IV). (b) AFM morphology images showing the same area of a spin-coated film of E-3aPh3 (thickness ≈ 60 nm) before (I), and after irradiation at 436 nm (≈20 mW·cm–2) for 2 h (II). The white dotted circles highlight some of the clusters that become less visible and eventually disappear upon irradiation. Scale bar: 1 μm. (c) Polarized optical micrographs (POM) of a spin-coated film (thickness ≈ 150 nm) of E-3aPh3 before (I), after localized irradiation at 436 nm (dotted white circle) for 20 min (II), and after thermal annealing at 120 °C for 20 min (III). The white arrows represent the relative orientation of the polarizer and analyzer. Scale bar: 30 μm.
为了进一步揭示该一体化设计在光电器件中的实用价值，团队通过单晶结构分析深入探究了电荷传输的理论边界。尽管三苯甲基增大了分子间距，但得益于单元晶胞内不同构象异构体的交叉反平行排列，材料在晶体堆积方向上依然保留了可观的轨道重叠和较窄的晶体带隙。基于此，研究人员构筑了平面双电极器件与纵向单载流子电子传输器件。在平面器件中，交替的光照能够引发微弱但可逆的电流开关响应。由于平面器件中电极接触阻挡与大尺度通道形貌退化的干扰较为严重，为了探究其本质的体电荷传输性质，团队利用空间电荷限制电流（SCLC）方法对纵向电子器件进行了表征。在排除界面与形貌大范围变动干扰的纵向微型器件中，电流密度与电压平方表现出完美的二次方线性关系。实验结果清晰地表明，处于反式富集状态下的器件表现出较高的电子迁移率，而当通过可见光照射使其转换为顺式富集状态时，由于噻吩骨架平面的扭曲与连续共轭的受损，阻碍了电荷在分子间的跳跃传输，其整体的体电子迁移率降低了约4倍。这一数据确凿地证实了分子异构化状态能够直接且有效地对散装半导体材料的内在输运能力实施精密的光控调制。
Figure 6. Projection down the a-axis of single crystal structures for (a) E-3aPh3 and (b) E-3a. In (a), the different colored areas highlight the distinct portions of the crystal volume where the flat thiophene backbones or the bulky triphenyl branches are settled. Band structure and density-of-states plots of (c) E-3aPh3 and (d) E-3a crystals calculated at the HSE06/POB-TZVP-rev2 level of theory on geometries from single-crystal X-ray diffraction data.
