Department of Chemistry, University of North Carolina at Charlotte, Charlotte, NC 28223-0001, USA.
Molecules. 2021 Dec 22;27(1):35. doi: 10.3390/molecules27010035.
The study of excited-state energy diffusion has had an important impact in the development and optimization of organic electronics. For instance, optimizing excited-state energy migration in the photoactive layer in an organic solar cell device has been shown to yield efficient solar energy conversion. Despite the crucial role that energy migration plays in molecular electronic device physics, there is still a great deal to be explored to establish how molecular orientation impacts energy diffusion mechanisms. In this work, we have synthesized a new library of solution-processable, Zn (alkoxycarbonyl)phenylporphyrins containing butyl (ZnTCBPP), hexyl (ZnTCHPP), 2-ethylhexyl (ZnTCEHPP), and octyl (ZnTCOPP) alkoxycarbonyl groups. We establish that, by varying the length of the peripheral alkyl chains on the metalloporphyrin macrocycle, preferential orientation and molecular self-assembly is observed in solution-processed thin films. The resultant arrangement of molecules consequently affects the electronic and photophysical characteristics of the metalloporphyrin thin films. The various molecular arrangements in the porphyrin thin films and their resultant impact were determined using UV-Vis absorption spectroscopy, steady-state and time-resolved fluorescence emission lifetimes, and X-ray diffraction in thin films. The films were doped with C quencher molecules and the change in fluorescence was measured to derive a relative quenching efficiency. Using emission decay, relative quenching efficiency, and dopant volume fraction as input, insights on exciton diffusion coefficient and exciton diffusion lengths were obtained from a Monte Carlo simulation. The octyl derivative (ZnTCOPP) showed the strongest relative fluorescence quenching and, therefore, the highest exciton diffusion coefficient (5.29 × 10 cm s) and longest exciton diffusion length (~81 nm). The octyl derivative also showed the strongest out-of-plane stacking among the metalloporphyrins studied. This work demonstrates how molecular self-assembly can be used to modulate and direct exciton diffusion in solution-processable metalloporphyrin thin films engineered for optoelectronic and photonic applications.
激发态能量扩散的研究对有机电子学的发展和优化产生了重要影响。例如，优化有机太阳能电池器件中光活性层中的激发态能量迁移已被证明可以有效地转换太阳能。尽管能量迁移在分子电子器件物理学中起着至关重要的作用，但仍有许多需要探索的地方，以确定分子取向如何影响能量扩散机制。在这项工作中，我们合成了一系列新的可溶液加工的锌（烷氧基羰基）苯基卟啉，其中包含丁基（ZnTCBPP）、己基（ZnTCHPP）、2-乙基己基（ZnTCEHPP）和辛基（ZnTCOPP）烷氧基羰基基团。我们发现，通过改变金属卟啉大环上外围烷基链的长度，可以在溶液加工的薄膜中观察到优先取向和分子自组装。分子的这种排列方式继而影响金属卟啉薄膜的电子和光物理特性。通过紫外-可见吸收光谱、稳态和时间分辨荧光发射寿命以及薄膜中的 X 射线衍射来确定卟啉薄膜中的各种分子排列及其对薄膜的影响。将这些薄膜掺杂 C 猝灭剂分子，并测量荧光的变化，以得出相对猝灭效率。通过发射衰减、相对猝灭效率和掺杂剂体积分数作为输入，从蒙特卡罗模拟中获得了激子扩散系数和激子扩散长度的信息。辛基衍生物（ZnTCOPP）显示出最强的相对荧光猝灭，因此具有最高的激子扩散系数（5.29×10 cm s）和最长的激子扩散长度（~81nm）。在研究的金属卟啉中，辛基衍生物也表现出最强的面外堆积。这项工作展示了如何通过分子自组装来调节和控制用于光电和光子应用的可溶液加工金属卟啉薄膜中的激子扩散。
