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–0.5, 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 over2–3 porphyrin units for all the oligomers, except the 12-porphyrinnanotube, in which the spin is spread over about 4–6 porphyrins.These experimental studies of linear and cyclic cations give a consistentpicture, which is supported by DFT calculations and multiparabolicmodeling with a reorganization energy of 1400–2000 cm–1 and coupling of 2000 cm–1 for charge transferbetween neighboring sites, placing the system in the Robin–Dayclass III.
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机译:已经研究了π共轭卟啉阵列家族的自由基阳离子:N = 1-6卟啉的线性链,6-卟啉纳米环和12-卟啉纳米管。自由基阳离子是通过化学和电化学氧化在溶液中生成的,并通过vis–NIR–IR和EPR光谱仪进行探测。阳离子在〜1000 nm和2000-5000 nm处显示出很强的NIR谱带,随着低聚物长度的增加,这些谱带移至更长的波长。近红外和红外光谱分析表明,极化子在线性低聚物中的2-3卟啉单元上离域。某些IR振动带在氧化时会强烈增强,并且当激活的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EPR谱具有高斯线形,线宽与N -0.5 sup>成比例,这表明在室温下,自旋跃迁在超精细耦合的时间尺度上迅速遍及整个链(约100 ns)。通过电子核双共振(ENDOR)直接测量超精细偶联在冷冻溶液中(80 K)表明旋转分布除12-卟啉外,所有低聚物均含2-3个卟啉单元纳米管,其中自旋分布在大约4-6卟啉上。这些对线性和环状阳离子的实验研究给出了一致的结果图片,DFT计算和多抛物线支持建模,其重组能量为1400–2000 cm –1 sup>,耦合为2000 cm –1 sup>用于电荷转移相邻站点之间,将系统置于Robin-Day第三类。
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