Electron acceptors of the fluorene series. Part 5. Intramolecular charge transfer in nitro-substituted 9-(aminomethylene)fluorenes
作者:Igor F. Perepichka、Anatolii F. Popov、Tatyana V. Orekhova、Martin R. Bryce、Alexander N. Vdovichenko、Andrei S. Batsanov、Leonid M. Goldenberg、Judith A. K. Howard、Nikolai I. Sokolov、Joanne L. Megson
DOI:10.1039/p29960002453
日期:——
Strong intramolecular charge transfer (ICT) in fluorenes 2 and 3 from a donor amino group to acceptor fluorene moiety leads to exceptionally easy rotation around the C(9)(α) bond that has been registered by 1H NMR spectroscopy; single crystal X-ray analysis of 2i confirms the changes in the bond numbers. Cyclic voltammetry (CV) of compounds 2 and 3 shows two closely spaced reversible single-electron reduction waves (in the range of –0.4 V to –1.16 V, E1red½– E2red½⩽ 0.16 V) resulting in radical anions and dianions, respectively, and a third reduction wave [E3red½≈–(1.31 – 1.53) V] resulting in radical trianions. E3red½ shows very little dependence on the structure of compounds 2 and 3, whereas E1red7½ and E2red½ correlate well with σp– constants of substituents in the fluorene ring. At +0.71 V to + 1.55 V a single-electron oxidation wave yielding radical cations of the compounds 2 and 3 is observed in CV. Parameters ρcv– for reduction (E1red½ and E2red½) and oxidation (Eox½) of compounds 2 and 3 are in the region of 0.12 V to 0.20 V. The influence of the structure and solvent effects on the ICT energies have been studied by UV–VIS spectroscopy. It has been found that the ICT energies also correlate well with σp– constants in the fluorene ring; however, in contrast to CV investigations which demonstrated close ρcv– values for both series of the compounds [ρ1red–(2)= 0.195 ± 0.005 V ≈ρ1red–(3) 0.175 ± 0.005 V, in acetonitrile], the sensitivity parameter ρICT–for 2 is approximately twice that for 3 (–0.175 ± 0.008 eV and –0.085 ± 0.008 eV, respectively, in acetonitrile). A quantitative estimation of solvent effects on the ICT energies using the four-parameter Koppel-Palm equation shows that only polarity and basicity of the solvent are statistically relevant in all the cases. Spectroelectrochemical studies on the compound 2i show the disappearance of the ICT band in the visible spectra when the negative potential yielding the radical anion and/or the dianion is applied. An increased electrophotographic sensitivity of poly-N-(2,3-epoxypropyl)carbazole (PEPC) films sensitized by compound 2d has been found in the ICT region; this phenomenon can be used for elaboration of photothermoplastic films with selective regions of the photosensitivity.
在荧光苊(fluorenes) 2 和 3 中,强内分子电荷转移(ICT)发生在供体氨基团和受体荧光苊部分之间,导致围绕 C(9)(α) 键的旋转异常容易,这一点通过 1H NMR 谱学得到了验证;对 2i 的单晶 X 射线分析证实了键数的变化。化合物 2 和 3 的循环伏安法(CV)显示出两个紧密相邻的可逆单电子还原波(在 -0.4 V 至 -1.16 V 范围内,E1red½– E2red½⩽ 0.16 V),分别导致自由基阴离子和二阴离子的形成,以及一个第三个还原波 [E3red½≈–(1.31 – 1.53) V],导致自由基三阴离子的形成。E3red½ 对化合物 2 和 3 的结构几乎没有依赖性,而 E1red½ 和 E2red½ 与荧光苊环中取代基的 σp– 常数有很好的相关性。在 +0.71 V 到 +1.55 V 的范围内,观察到产生化合物 2 和 3 自由基阳离子的单电子氧化波。化合物 2 和 3 的还原(E1red½ 和 E2red½)与氧化(Eox½)参数 ρcv– 在 0.12 V 至 0.20 V 的范围内。通过 UV–VIS 光谱研究了结构和溶剂效应对 ICT 能量的影响。发现 ICT 能量与荧光苊环中的 σp– 常数也有很好的相关性;然而,与 CV 研究结果相比,后者显示两组化合物具有接近的 ρcv– 值 [ρ1red–(2)= 0.195 ± 0.005 V ≈ ρ1red–(3) 0.175 ± 0.005 V,在乙腈中],而 2 的灵敏度参数 ρICT– 约为 3 的两倍(在乙腈中分别为 -0.175 ± 0.008 eV 和 -0.085 ± 0.008 eV)。用四参数 Koppel-Palm 方程对溶剂效应对 ICT 能量的定量估计表明,只有溶剂的极性和碱性在所有情况下是统计上相关的。对化合物 2i 的光谱电化学研究表明,当施加负电位以产生自由基阴离子和/或二阴离子时,ICT 带在可见光谱中消失。发现用化合物 2d 感光的聚-N-(2,3-环氧丙基)喹喔烯(PEPC)薄膜在 ICT 区域具有提高的电光敏感性;这一现象可以用于开发具有选择性光敏区域的光热塑性薄膜。