Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg , Am Hubland, D-97074 Würzburg, Germany.
Institute of Physical and Theoretical Chemistry, Graz University of Technology , Stremayrgasse 9, A-8010 Graz, Austria.
J Am Chem Soc. 2017 May 3;139(17):6200-6209. doi: 10.1021/jacs.7b01650. Epub 2017 Apr 20.
To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.
为了探讨供体取代基是否可以用于加速连接到中央苯桥的对位或间位的氧化还原位点之间的空穴转移（HT），我们研究了基于三芳胺氧化还原中心的三个系列混合价化合物，这些化合物通过炔烃间隔基连接到苯桥的对位和间位。桥的电子密度通过具有不同供电子或吸电子特性的取代基进行调节。通过分析光学光谱和 DFT 计算，我们表明对于间位系列之一，HT 性质与桥取代基无关，而对于对位系列，供体取代基可以强烈降低固有势垒。相比之下，温度依赖的 ESR 测量表明，对于对位情况中的强供体取代基，表观势垒和 HT 速率都会急剧增加。这是由于前所未有的取代基依赖性 HT 机制从过渡态理论控制的机制转变为溶剂动力学控制的机制。对于具有缓慢纵向弛豫（PhNO，oDCB）的溶剂，介电弛豫过程会通过介电弛豫过程对固有势垒产生额外贡献。将供体取代基连接到分子轨道系数较大的桥的对位位置会加速间位共轭化合物的 HT 速率，就像对位系列一样。这种效应表明，如果选择了适当的取代基和取代模式，那么对位-间位范式不再成立，从而大大拓宽了间位拓扑结构在光电应用中的适用性。
