Chemistry Research Laboratory, Department of Chemistry, University of Oxford , Oxford OX1 3TA, United Kingdom.
Centre for Advanced Electron Spin Resonance, Department of Chemistry, University of Oxford , Oxford OX1 3QR, United Kingdom.
J Am Chem Soc. 2017 Aug 2;139(30):10461-10471. doi: 10.1021/jacs.7b05386. Epub 2017 Jul 24.
The radical cations of a family of π-conjugated porphyrin arrays have been investigated: linear chains of N = 1-6 porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The radical cations were generated in solution by chemical and electrochemical oxidation, and probed by vis-NIR-IR and EPR spectroscopies. The cations exhibit strong NIR bands at ∼1000 nm and 2000-5000 nm, which shift to longer wavelength with increasing oligomer length. Analysis of the NIR and IR spectra indicates that the polaron is delocalized over 2-3 porphyrin units in the linear oligomers. Some of the IR vibrational bands are strongly intensified on oxidation, and Fano-type antiresonances are observed when activated vibrations overlap with electronic transitions. The solution-phase EPR spectra of the radical cations have Gaussian lineshapes with linewidths proportional to N, demonstrating that at room temperature the spin hops rapidly over the whole chain on the time scale of the hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine couplings through electron-nuclear double resonance (ENDOR) in frozen solution (80 K) indicates distribution of the spin over 2-3 porphyrin units for all the oligomers, except the 12-porphyrin nanotube, in which the spin is spread over about 4-6 porphyrins. These experimental studies of linear and cyclic cations give a consistent picture, which is supported by DFT calculations and multiparabolic modeling with a reorganization energy of 1400-2000 cm and coupling of 2000 cm for charge transfer between neighboring sites, placing the system in the Robin-Day class III.
一系列π共轭卟啉阵列的自由基阳离子已经被研究：1-6 个卟啉的线性链、6 个卟啉的纳米环和 12 个卟啉的纳米管。自由基阳离子通过化学和电化学氧化在溶液中产生，并通过可见近红外-红外和电子顺磁共振光谱进行探测。阳离子在 ∼1000nm 和 2000-5000nm 处表现出强的近红外带，随着寡聚物长度的增加，这些带向长波长移动。近红外和红外光谱的分析表明，在线性寡聚物中，极化子在 2-3 个卟啉单元上离域。一些红外振动带在氧化时被强烈增强，当激活振动与电子跃迁重叠时，会观察到 Fano 型反共振。自由基阳离子的溶液相电子顺磁共振谱具有与 N 成正比的高斯线宽，这表明在室温下，自旋在超精细耦合（约 100ns）的时间尺度上快速跳跃过整个链。通过电子-核双共振（ENDOR）在冻结溶液（80K）中对超精细耦合的直接测量表明，对于所有的寡聚物，除了 12 个卟啉纳米管之外，自旋在 2-3 个卟啉单元上分布，在 12 个卟啉纳米管中，自旋在大约 4-6 个卟啉上分布。这些对线性和环状阳离子的实验研究给出了一致的图像，该图像得到了 DFT 计算和多抛物线模型的支持，其中重组能为 1400-2000cm，相邻位点之间的电荷转移耦合为 2000cm，这将系统置于 Robin-Day 类 III 中。
