Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4/I, A-8010 Graz, Austria.
J Phys Chem A. 2013 Feb 21;117(7):1439-48. doi: 10.1021/jp3104358. Epub 2013 Feb 11.
The properties of p-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV-vis spectra of HMPD(2+••) were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet-doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH(2))(3) to dimethylated CH(2)CMe(2)CH(2) in OMPD(2+••) causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD(2+••) and OMPD(2+••), in which the positive charges are localized separately in each PD(+•) ring, in the extended conformation, exchange of the electron ("hole hopping") between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD(+•) and OMPD(+•). This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established.
对苯二胺（PD）基体系的性质在很大程度上取决于分子拓扑结构。单桥联 PD 类似物 HMPD 和 OMPD 中 PD 环通过柔性连接键相连，其自由基阳离子和二阳离子具有特殊的电子性质。HMPD(2+••)的 EPR 和 UV-vis 谱表现出异常的温度敏感性，其电子状态从室温下的伸展构象（双峰态）转变为干冰温度下的π堆积构象（单态）。在 OMPD(2+••)中，将单桥从（CH(2))(3)变为二甲化的 CH(2)CMe(2)CH(2)，由于桥接甲基的空间位阻，低温下π堆积构象的存在显著减少。与 HMPD(2+••)和 OMPD(2+••)不同，在 HMPD(2+••)和 OMPD(2+••)中，正电荷分别定域在每个 PD(+•)环上，在伸展构象中，两个 PD 单元之间的电子（“空穴跳跃”）发生交换（在 EPR 实验的时间尺度上很快）。该过程随着温度的降低以可逆的方式减缓，因此在低温下形成了未配对电子自旋密度主要位于一个 PD 核心上的自由基阳离子。因此，可以在构象变化、电子离域和自旋态之间建立微妙的平衡。
