Mani Tomoyasu, Grills David C
Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
Chemistry Division, Brookhaven National Laboratory , Upton, New York 11973-5000, United States.
J Phys Chem B. 2017 Aug 3;121(30):7327-7335. doi: 10.1021/acs.jpcb.7b02559. Epub 2017 Jul 21.
Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges. While dimerization reactions of radical cations are well investigated, studies on radical anions are still scarce. Upon dimerization of radical anions with neutral counterparts, an electron is considered to delocalize over the two molecules. Here, by using time-resolved infrared (TRIR) detection coupled with pulse radiolysis, we show that radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB) undergo such dimerization reactions, with an electron equally delocalized over the two molecules. We have recently demonstrated that nitrile ν(C≡N) vibrations respond to the degree of electron localization of nitrile-substituted anions: we can quantify the changes in the electronic charges from the neutral to the anion states in the nitriles by monitoring the ν(C≡N) IR shifts. In the first part of this article, we show that the sensitivity of the ν(C≡N) IR shifts does not depend on solvent polarity. In the second part, we describe how probing the shifts of the nitrile IR vibrational band unambiguously confirms the formation of dimer radical anions, with K = 3 × 10 M. IR findings are corroborated by electronic absorption spectroscopy and electronic structure calculations. We find that the presence of a hexyl chain and the formation of π-π interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. The present study provides clear evidence of spatial delocalization of electrons over two molecular fragments.
电荷离域是控制共轭分子中电荷传输的因素之一。它被认为在包括有机太阳能电池和有机电子学在内的广泛分子技术性能中发挥着重要作用。二聚反应非常适合作为研究电荷分子间空间离域的模型。虽然对自由基阳离子的二聚反应已有充分研究，但对自由基阴离子的研究仍然很少。当自由基阴离子与中性分子二聚时，一个电子被认为会在两个分子间离域。在此，通过使用与脉冲辐解相结合的时间分辨红外（TRIR）检测，我们表明4-正己基-4'-氰基联苯（6CB）的自由基阴离子会发生这种二聚反应，一个电子在两个分子间平均离域。我们最近证明，腈的ν(C≡N)振动对腈取代阴离子的电子离域程度有响应：我们可以通过监测ν(C≡N)红外位移来量化腈中从中性态到阴离子态的电子电荷变化。在本文的第一部分，我们表明ν(C≡N)红外位移的灵敏度不依赖于溶剂极性。在第二部分，我们描述了如何通过探测腈红外振动带的位移明确证实二聚自由基阴离子的形成，其平衡常数K = 3×10 M。红外研究结果得到了电子吸收光谱和电子结构计算结果的佐证。我们发现己基链的存在以及π-π相互作用的形成对于6CB自由基阴离子与中性6CB的二聚反应都至关重要。本研究为电子在两个分子片段间的空间离域提供了明确证据。
