Avalos Claudia E, Richert Sabine, Socie Etienne, Karthikeyan Ganesan, Casano Gilles, Stevanato Gabriele, Kubicki Dominik J, Moser Jacques E, Timmel Christiane R, Lelli Moreno, Rossini Aaron J, Ouari Olivier, Emsley Lyndon
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Centre for Advanced Electron Spin Resonance (CAESR), University of Oxford, South Parks Road, OX1 3QR Oxford, United Kingdom.
J Phys Chem A. 2020 Jul 23;124(29):6068-6075. doi: 10.1021/acs.jpca.0c03498. Epub 2020 Jul 9.
Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from -selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.
识别和表征能够产生具有大电子自旋极化的明确定义状态的系统，对于分子自旋电子学、高能物理和磁共振光谱学中的应用具有高度的研究价值。最近，连接在并五苯衍生物上的自由基取代基上电子自旋极化的产生，因其在分子电子学中的应用而备受关注。发色团光激发后，并五苯 - 自由基衍生物可通过增强的系间窜越迅速形成自旋极化的三重态激发态。在合适的条件下，由 - 选择性系间窜越速率产生的三重态自旋极化可转移到连接的稳定自由基上。这种自旋极化转移的效率取决于许多因素：局部磁场和电场、激发态能量、分子几何结构以及自旋 - 自旋耦合。在此，我们对连接芬兰三苯甲基、BDPA 或 TEMPO 自由基的三种并五苯衍生物进行了瞬态电子顺磁共振（EPR）测量，以探究自由基性质对自旋极化转移的影响。我们观察到并五苯激发三重态与三苯甲基自由基之间存在有效的极化转移，但对于 BDPA 和 TEMPO 衍生物则未观察到相同现象。还研究了并五苯 - 三苯甲基系统在不同玻璃态基质中的极化转移行为，发现其明显依赖于所使用的溶剂。借助飞秒和纳秒瞬态吸收测量对 EPR 结果进行了合理解释，从而获得了关于这三种并五苯衍生物激发态动力学的补充信息。值得注意的是，我们观察到并五苯 - 三苯甲基系统与连接 BDPA 和 TEMPO 自由基的并五苯系统之间在三重态形成的时间尺度上存在 2 个数量级的差异。
