Rosenthal Joel, Hodgkiss Justin M, Young Elizabeth R, Nocera Daniel G
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA.
J Am Chem Soc. 2006 Aug 16;128(32):10474-83. doi: 10.1021/ja062430g.
A homologous set of porphyrin derivatives possessing an isocyclic five-membered ring appended with an amidinium functionality has been used to examine proton-coupled electron transfer (PCET) through well-characterized amidine-carboxylic acid interfaces. Conjugation between the porphyrin chromophore and the amidinium interface can be altered by selective reduction of the isocyclic ring of an amidinium-purpurin to produce an amidinium-chlorin. The highly conjugated amidinium-purpurin displays large spectral shifts in the visible region upon alteration of the amidinium/amidine protonation state; no such change is observed for the chlorin homologue. Analysis of the UV-vis absorption and emission profiles of the amidinium-purpurin upon deprotonation allows for the measurement of the porphyrinic-amidinium acidity constant for the ground state (pKa = 9.55 +/- 0.1 in CH3CN) and excited state (pKa)= 10.40 +/- 0.1 in CH3CN). The absorption spectrum of the purpurin also provides a convenient handle for determining the protonation state of assembled interfaces. In this way, the purpurin macrocycle provides a general tool for PCET studies because it can be used to determine the location of a proton within PCET interfaces formed from carboxylic acid electron acceptors including dinitrobenzenes (DNBs) and naphthalenediimide (NI), which have been used extensively in previous PCET studies. An amidine-carboxylic acid interface is observed for electron-rich acceptors, whereas the ionized amidinium-carboxylate interface is observed for electron-poor acceptors. The PCET kinetics for purpurin/chlorin associated to NI are consistent with an amidine-carboxylic acid interface, which is also verified spectrally.
一组具有异环五元环并带有脒基官能团的卟啉衍生物，已被用于通过特征明确的脒 - 羧酸界面来研究质子耦合电子转移（PCET）。通过选择性还原脒基 - 紫红素的异环来生成脒基 - 二氢卟吩，可改变卟啉发色团与脒基界面之间的共轭作用。高度共轭的脒基 - 紫红素在脒基/脒的质子化状态改变时，在可见光区域显示出较大的光谱位移；而二氢卟吩同系物则未观察到这种变化。对去质子化后的脒基 - 紫红素的紫外 - 可见吸收和发射光谱进行分析，可以测量基态（在乙腈中 pKa = 9.55 ± 0.1）和激发态（在乙腈中 pKa = 10.40 ± 0.1）的卟啉 - 脒基酸度常数。紫红素的吸收光谱也为确定组装界面的质子化状态提供了便利手段。通过这种方式，紫红素大环为 PCET 研究提供了一个通用工具，因为它可用于确定由羧酸电子受体形成的 PCET 界面内质子的位置，这些羧酸电子受体包括二硝基苯（DNB）和萘二酰亚胺（NI），它们在先前的 PCET 研究中已被广泛使用。对于富电子受体，观察到的是脒 - 羧酸界面，而对于贫电子受体，则观察到离子化的脒基 - 羧酸盐界面。与 NI 相关的紫红素/二氢卟吩的 PCET 动力学与脒 - 羧酸界面一致，这也通过光谱得到了验证。
