Zhou Jiawang, Folster Carlton P, Surampudi Sravan K, Jimenez Daniel, Klausen Rebekka S, Bragg Arthur E
Department of Chemistry, Johns Hopkins University, 3400 N. Charles St, Baltimore, MD 21218, USA.
Dalton Trans. 2017 Jul 11;46(27):8716-8726. doi: 10.1039/c7dt00384f.
The flexibility of σ-conjugated silanes presents new opportunities for manipulating charge generation, transport, and non-linear optical properties of materials. Recently we synthesized a series of acceptor-donor-acceptor (ADA) compounds in which a methylated oligosilane core (D) is flanked by electron-deficient cyanovinyl-substituted arenes (A). Based on a detailed characterization of the photophysics of ADA and donor-acceptor (DA) architectures using both steady state and ultrafast spectroscopic measurements we illustrate that asymmetric charge separation occurs directly following light absorption. Lippert analysis of solvatochromic emission indicates large changes in dipole moments on excitation consistent with the formation of dipolar emissive states. Time resolved absorption measurements reveal common excited-state relaxation behavior across molecular structures: spectral dynamics associated with the relaxation of nascent excited states occur on a common timescale for all structures within the same solvent environment, whereas charge recombination via excited-state decay consistently follows a common energy gap law. Ultrafast time-resolved Raman measurements reveal that reduction of the cyanovinyl moieties is instantaneous with excitation, with only minor shifts in vibrational features over the course of excited-state relaxation. We conclude that excited-state symmetry breaking that gives rise to asymmetric intramolecular charge transfer (ICT) is associated with the conformation of the central Si chain. In contrast, ultrafast solvent reorganization or solvent-controlled intramolecular dynamics only serve to stabilize nascent dipolar excited states, rather than induce charge separation from an initial quadrupolar state.
σ-共轭硅烷的灵活性为调控材料的电荷产生、传输及非线性光学性质带来了新机遇。最近我们合成了一系列给体-受体-给体（ADA）化合物，其中甲基化的低聚硅烷核心（D）两侧是缺电子的氰基乙烯基取代芳烃（A）。基于使用稳态和超快光谱测量对ADA和给体-受体（DA）结构的光物理性质进行的详细表征，我们表明光吸收后直接发生不对称电荷分离。对溶剂化显色发射的Lippert分析表明，激发时偶极矩发生了很大变化，这与偶极发射态的形成一致。时间分辨吸收测量揭示了不同分子结构间常见的激发态弛豫行为：在相同溶剂环境中，所有结构的新生激发态弛豫相关的光谱动力学都发生在同一时间尺度上，而通过激发态衰变的电荷复合始终遵循共同的能隙定律。超快时间分辨拉曼测量表明，氰基乙烯基部分的还原在激发时是即时的，在激发态弛豫过程中振动特征仅有微小变化。我们得出结论，导致不对称分子内电荷转移（ICT）的激发态对称性破缺与中心硅链的构象有关。相比之下，超快溶剂重组或溶剂控制的分子内动力学仅起到稳定新生偶极激发态的作用，而非诱导从初始四极态进行电荷分离。
