Department of Chemistry, The University of New Mexico , MSC03 2060, Albuquerque, New Mexico 87131-0001, United States.
J Am Chem Soc. 2013 Oct 2;135(39):14713-25. doi: 10.1021/ja405354x. Epub 2013 Sep 23.
A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp(Cum,Me)Zn(SQ-m-Ph-NN) (1-meta) a donor-bridge-acceptor (D-B-A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet-triplet splitting in this cross-conjugated D-B-A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D-B-A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D-B-A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D-A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D-B-A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ~ 0 cm(-1). We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D-B-A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated.
已对 Tp(Cum,Me)Zn(SQ-m-Ph-NN)（1-meta）施主-桥-受主（D-B-A）双自由基进行了变温电子顺磁共振波谱、电子吸收光谱和磁化率测量，该双自由基具有交叉共轭亚苯基（m-Ph）桥和单重态基态。为了理解 1-meta 的激发态电子结构，实验结果结合了详细的键合和激发态计算进行了解释。结果揭示了在这个交叉共轭 D-B-A 双自由基中对基态单重态三重态分裂有重要贡献的激发态，这有助于我们理解在交叉共轭分子中电子耦合，特别是量子干涉效应。与共轭异构体形成鲜明对比的是，共轭异构体是一个具有对亚苯基桥的 D-B-A 双自由基，在交叉共轭 D-B-A 双自由基基态中混入单个低能单重激发 D→A 型构型对基态磁交换相互作用的贡献可以忽略不计。相反，由 Ph-NN（HOMO）→Ph-NN（LUMO）单电子激发构型形成的激发态与 m-Ph 桥联 D-A 双自由基的基态混合。这导致双（动态）自旋极化机制成为 SQ 和 NN 自旋之间基态反铁磁交换耦合的主要贡献者。因此，主要的交换机制是通过具有亚苯基桥特征的双占据轨道的自旋极化来激活桥部分的机制。这种机制很重要，因为它增强了交叉共轭 D-B-A 分子中的电子和磁通信，在 1-meta 的情况下，在活性电子近似中，预计磁交换为 J≈0cm(-1)。我们假设，在所有交叉共轭 D-B-A 三联体中，都存在类似的超交换机制。我们的结果与交叉共轭分子作为桥或分子线元件时电子转移/输运的量子干涉效应进行了比较，并提出了一种可以调节电子耦合（因此电子转移/输运）的机制。
