Effects of Self-Assembly on the Photogeneration of Radical Cations in Halogenated Triphenylamines

We investigate the effect of assembly on charge transfer, charge recombination, and the persistence of radical cations in halogen-substituted triphenylamine (TPA) dimers. A series of urea-tethered TPA derivatives 1 (X = H, Cl, Br, and I) are compared, which have one phenyl group modified at the para position with a halogen. Ureas direct the assembly of these derivatives while halogen substituents influence the packing of the TPA units. These modifications affect the generation and persistence of TPA radical cations as monitored by electron paramagnetic resonance (EPR) spectroscopy. The formation and degradation pathways of the radical cations in solution and gas phase were probed by ion-mobility spectrometry mass spectrometry. In contrast, supramolecular assembly enhanced the stability of these materials as well as the persistence of their photogenerated radical cations, which appear to undergo charge recombination without degradation. Greater quantities of these radical cations are observed for the bromo and non-halogenated derivatives (1Br, 1H). Time-dependent density functional theory (TD-DFT) calculations on single molecules and hydrogen-bonded dimers suggest the stability of TPA radical cations largely depends on initial photoinduced charge separation and electronic coupling between assembled TPA dimers. The latter was found to be about 7 times stronger in 1I than in 1Br dimers, which may explain faster charge recombination and shorter lifetimes of 1I radicals. Transient absorption (TA) spectroscopy and TD-DFT were able to identify the charged species for 1Br along with the kinetic traces and measured lifetime of ∼80 ns. Fluorescence quenching studies are consistent with initial charge separation and subsequent charge transfer event between nearby TPAs. Future exploration will focus on the mobility and application of these TPA assemblies as hole transport materials.

本研究探究了组装过程对卤代三苯胺（triphenylamine，TPA）二聚体中电荷转移、电荷复合及自由基阳离子存续性的影响。本研究对比了一系列脲桥联三苯胺衍生物1（X=H、Cl、Br、I），这类衍生物的一个苯基在对位被卤素原子修饰。脲基可引导这些衍生物的组装过程，而卤素取代基则会影响三苯胺单元的堆积方式。上述修饰会影响三苯胺自由基阳离子的生成与存续性，该过程可通过电子顺磁共振波谱（electron paramagnetic resonance，EPR）进行监测。研究人员通过离子迁移谱质谱法（ion-mobility spectrometry mass spectrometry）探究了自由基阳离子在溶液与气相中的形成与降解路径。与之相对，超分子组装可提升这类材料的稳定性，以及其光生自由基阳离子的存续性；这类自由基阳离子似乎可在不发生降解的情况下进行电荷复合。溴代与非卤代衍生物（1Br、1H）的自由基阳离子含量更高。针对单分子与氢键二聚体的含时密度泛函理论（time-dependent density functional theory，TD-DFT）计算结果表明，三苯胺自由基阳离子的稳定性主要取决于初始光诱导电荷分离过程，以及组装态三苯胺二聚体之间的电子耦合作用。研究发现，后者（电子耦合作用）在1I二聚体中的强度约为1Br二聚体的7倍，这或许可以解释1I自由基的电荷复合速率更快、寿命更短的现象。瞬态吸收光谱（transient absorption spectroscopy，TA）与含时密度泛函理论可识别1Br的带电物种，并得到其动力学轨迹与约80纳秒的实测寿命。荧光淬灭实验结果与邻近三苯胺单元间的初始电荷分离及后续电荷转移过程相符。未来的研究将聚焦于这类三苯胺组装体作为空穴传输材料的迁移率与应用前景。