有机半导体材料中的电荷转移

在介绍有机半导体材料电荷转移基本理论的基础上,对利用电荷转移研究有机半导体材料的导电、光电导和发光过程的现状进行评述,认为电荷转移是有机半导体材料研究的关键问题,开展其研究不仅有助于弄清一些新现象、新效应的物理起因,还可望找到预测有机半导体材料相关性能的有效手段.     

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D^⊕, A^?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D^⊕, A^?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene^⊕, NO₂] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.     

Charge transfer in duplex DNA containing mismatch

A theory for charge transfer between the electrode and the donor/acceptor molecule coupled through a DNA bridge in solution is developed. We explore the crossover between the coherent tunneling and the incoherent sequential transfer regimes by varying the electrode potential and discuss the effects of single-base mismatches in DNA duplex in both regimes. In the former regime a single-base mismatch in DNA duplex causes a reduction in the charge transfer rate simply by decreasing the electron tunneling matrix element, however, in the latter regime the effects are rather complicated.     

The role of vibronic interactions on intramolecular and intermolecular electron transfer in π-conjugated oligomers

The importance of electron-vibrational coupling for intermolecular and intramolecular electron-transfer processes is discussed on the basis of first-principles correlated quantum-mechanical calculations and of a dynamic vibronic approach. The methodology is illustrated for examples selected from some of our recent work. In all instances, the theoretical results are thoroughly compared to experimental data.     

Ethylene polymerization catalyzed by nickel-based complex combined with various modified EBAOs

A series of novel aluminoxanes, ethyl-isobutylalulminoxanes (EBAO) modified with n-butaneboronic acid, phenylboronic acid and 4-fluorobenzeneboronic acid are synthesized. The ethylene polymerization activity of [diacetyl-bis(2,6-diisopropylanil)] NiBr₂(1)/different aluminoxanes were investigated. It is found that boronic acid greatly affects the catalytic activity of modified EBAO. Bulky substituted boronic site of EBAO improved ethylene polymerization activity of nickel complex.     

不对称沉积合成PdS-CdSe@CdS-Au一维纳米异质结构及其光解水制氢性能

提高光催化剂在光照射下产生的电子/孔穴分离效率是一个关键的科学问题之一,目前也是一个很大的挑战. 最近,在纳米尺度, 通过材料设计, 在窄带半导体上沉积助催化剂(比如引进双助催化剂)形成异质结构, 能够建立内建电场, 从而使电子和空穴快速分离和传输, 显示出很好的可见光量子效率. 对于异质结构, 纳米结构半导体如硫化镉具有表面积大、规整形貌、电子和空穴迁移路径短等优势. 用纳米半导体硫化镉制备异质结构光催化剂已有很多报道, 大多数研究集中于单一助催化剂来提高光催化活性, 对于纳米结构的设计制备研究较少; 对于稳定性研究, 侧重于利用超薄碳膜包敷策略来提高光催化的稳定性. 因此, 复杂纳米异质结构的精准合成和稳定性仍是个不小的挑战. 我们研究组发展了一种催化剂制备方法, 可选择性地将Au纳米颗粒和PdS纳米颗粒分别沉积于一维硫化镉纳米棒的两端, 并将所制备的催化剂应用于可见光光催化分解水制氢反应中.本文报道了一种高选择性沉积助催化剂的新方法, 制备了PdS-CdSe@CdS-Au一维纳米异质结构. 首先用高温分解法和种子法制备了核壳结构的CdSe@CdS纳米棒, 预先沉积纳米金在纳米棒的一端, 然后PdS通过阳离子交换法高度选择性地沉积到纳米棒的另一端, 形成火柴棒纳米结构. HRTEM结果显示Au和Pd分别高选择性地沉积在纳米棒顶的两端, 助催化剂和纳米棒之间有一个清晰的界面, 非外延生长. 紫外-可见吸收光谱显示, Au和PdS与CdSe@CdS纳米棒之间有很强的电子耦合效应, 相应的荧光光谱也显示, 顶端的助催化剂使CdSe@CdS发生强的荧光淬灭效应. 将PdS-CdSe@CdS-Au一维纳米异质结构用于光催化分解水制氢, 发现5 h内产氢达到1100 μmol, 是相应Au-CdSe@CdS催化剂产氢速率的2个数量级.同时考察了它的光催化稳定性, 发现双助催化剂形成的火柴棒型纳米结构稳定性大大提高, 经过4 h光照仍能保持很好的形貌.通过对照实验考察了PdS-CdSe@CdS-Au一维纳米异质结构的形成机理. 一端金纳米颗粒的形成主要是由于顶端曲率的Gibbs-Thompson效应和纳米棒顶端组成分布不对称的缘故, 而PdS的顶端高选择性沉积是在阳离子交换过程中两端化学性质发生变化等原因造成的. 最后提出了光催化性能提高机理, 主要是由于电子和空穴在一维纳米棒上快速向相反方向分离和传输, 既大大提高了光催化制氢效率, 也大大提高了光催化稳定性.     

Control of the Electronic Structure of Organic Conductors as Exemplified by [NMP][TCNQ] Charge-Transfer Complexes

Through replacement of cations with neutral molecules of similar shape and polarizability in a highly conducting “metal-like” change-transfer organic conductor, the deliberate control of the electronic structure from a quarter to a half-filled band is possible. This goal has been achieved with the N-methylphenazinium (NMP) salt of the tetracyanoquinodimethane (TCNQ) anion by partial replacement of the cation with phenazine. A detailed study of the optical, electrical, and magnetic properties of these conducting molecular solids has lead to the evolution of a broad understanding of the physics of one-dimensional organic conductors and a reinterpretation of the mechanism of electron transport in such solids. Phenomena such as switching from a low coulomb repulsion two-chain conductor to a high coulomb repulsion one-chain conductor as well as soliton-assisted electron transport are observed.     

Photoinduced electron transfer across linearly fused oligo-norbornyl structures

The rates of photoinduced electron transfer (ET) reactions across two oligo-norbornyl spacer groups (S), that is, structure 1 fused by two norbornadiene (NBD) units and structure 2 fused by three NBD units, are examined. Substituted naphthalene acted as an electron donor (D), whilst ethylene-1,2-dicarboxylate as an electron acceptor (A). ET rates were measured by fluorescence quenching experiments on these D-S-A dyads, and the results were correlated with reaction free energies according to the Marcus relationship. It was found that naphthalene with phenyl substituents showed relatively slower ET rates. The conformational flexibility of phenyl substituents may cause a hindrance on the electronic coupling between D and A. Another salient feature was the abnormally high quenching rates observed in nonpolar solvents such as cyclohexane, the results of which may be ascribed to a competing energy transfer process.     

光催化全解水助催化剂的设计与构建

能源和环境危机是当今社会面临的两大关键课题,利用太阳光驱动化学反应、将太阳能转化为化学能是解决上述问题的重要措施。通过光催化分解水是直接利用太阳能生产氢燃料的有效策略。光催化水分解过程可以分为三个基元步骤:光吸收、电荷分离与迁移、以及表面氧化还原反应。助催化剂可有效提高电荷分离效率、提供反应活性位点并抑制催化剂光腐蚀的发生,进而提高水分解效率。助催化剂也可以通过活化水分子以提高表面氧化还原动力学,进而提升整体光催化反应的太阳能转换效率。本文综述了助催化剂在光催化反应中的重要作用以及目前常用的助催化剂类型,详细说明了在光催化全解水过程中双助催化剂体系的构建及作用机理,并根据限制全解水的关键因素提出了新型助催化剂的设计策略。     

Characterization of infrared vibrational activity in specific totally symmetric bridging modes of mixed-valence systems near the localized-to-delocalized transition

The presence or absence of infrared (IR) activity in totally symmetric bridging vibrations of ligand-bridged mixed-valence molecular systems has often been taken, on the basis of electronic (a)symmetry arguments, as markers for electronic localization (class II) or delocalization (class III) on the vibrational time scale. However, because IR absorption intensity in such nominally-forbidden modes may be anomalously enhanced due to vibronic coupling effects, especially for cases in the localized-to-delocalized (class II/III) transition regime, a direct and continuous correlation between the extent of (de)localization and IR activity is challenged. In order to experimentally illustrate such phenomena, here we provide a detailed comparison of relevant IR signatures across a series of structurally related, symmetric dinuclear complexes of the type [(bpy)₂(Cl)M-pz-M(Cl)(bpy)₂]ⁿ⁺ (bpy = 2,2′-bipyridine, pz = pyrazine, M = Ru^II,III or Os^II,III; n = 2, 3, or 4), in which the “mixed valency” (II,III; n = 3) has been characterized as largely localized for M = Ru and approaching the near-delocalized behavior for M = Os. To improve analytical accuracy and facilitate standardization in the comparison of results spanning the various isovalent and mixed-valent redox species, the IR spectroscopic features were probed via an in situ spectroelectrochemical (SEC) approach, in identical medium (solution) conditions. The parent monomers, i.e. [(bpy)₂(Cl)M-pz]ⁿ⁺ (M = Ru^II,III or Os^II,III; n = 1 or 2), which represent the extreme examples of structural and electronic symmetry breaking in the series, were also investigated and compared with the dimers. In all cases reported here, the spectroscopic assignments of vibrational modes were supported by isotopic modification (deuteration) of the bridging pyrazine ligand.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

电荷转移复合物的量子化学研究(1)列—1,3,5-三硝基苯与对位取代苯酚钾系

用CNDO/2法对1,3,5-三硝基苯与苯酚钾系列的电荷转移复合物作了研究以解释该系列稳定性的实验规律并探讨其稳定构型。在溶液中形成电荷转移复合物联系着不少生命过程,故颇受研究者重视。例如Wallwork利用X-光数据讨论了三硝基苯与一些芳烃生成的复合物的构型。仅能在溶液中存在的复合物则宜用量子化学法,例如严继民等用EHMO法计算苯与四氰乙烯生成的复合物以研究其构型。     

钨酸铋担载钯纳米粒子光催化甲苯及其衍生物到醛的氧化反应

甲苯及其衍生物的选择性氧化是化学工业中重要的一环.氧化产物醛、酮和酸类化合物是各种农药、染料、防腐剂、阻燃剂、香料、塑料的合成中间体.在传统的化工过程中,通常在苛刻的条件下,如高温、高压以及酸性溶剂中进行甲苯的选择性氧化.光催化有机合成作为一种"绿色"的合成方法受到越来越多的关注.我们发现钨酸铋作为可见光响应的光催化剂,可以利用氧气作为氧化剂,对甲苯及其衍生物进行催化氧化反应.通过调节水热法合成钨酸铋的酸碱条件,控制其成核生长过程,改变其形貌,发现在pH值为0.49时,得到花状钨酸铋粉末活性最高.X射线衍射、扫描电镜、紫外可见吸收光谱和比表面测定结果发现,花状钨酸铋粉末表现出最佳的甲苯氧化活性很可能是与它最大的比表面积有关系.为了进一步提高催化的活性,我们将助催化剂担载在钨酸铋粉末上,发现Pd的担载量为0.1 wt%时甲苯氧化反应活性最高.将邻、间、对位甲基取代以及对位甲氧基和氯取代的甲苯衍生物进行反应,发现均可高选择性地得到目标产物.在加入电子牺牲剂硝酸银和空穴牺牲剂草酸铵到反应体系后,发现反应被完全禁阻,说明在甲苯氧化反应过程中电子和空穴都起到了相应的作用.通过电化学测试发现,钯作为助催化剂担载在钨酸铋表面后,产生更强的氧化和还原信号,说明其在电子和空穴发生反应的过程中都起到了相应的促进作用.由此推测,在甲苯的氧化反应中,钨酸铋材料表面吸收可见光,产生光生电子和空穴,而钯的担载则促进了电子和空穴进行氧气还原和甲苯氧化的过程.