8-羟基喹啉铝光电性质的Ab initio和DFT研究

利用abinitioHF和密度泛函理论B3LYP等方法,对金属有机配合物8-羟基喹啉铝(AlQ₃)进行几何结构优化,探索分子内部电子跃迁的机理.结果表明,电子从基态跃迁到低激发态时主要为π-π^*跃迁;电荷从含氧的苯酚环转移至含氮的吡啶环上,包括两环之间C→C转移和O→N转移,与金属离子关系不大.考虑到配体对发光性质的贡献,进一步设计了3种AlQ₃的衍生物.     

含氯不对称配体8-羟基喹啉铝配合物电子和光谱性质的TDDFT研究

以含时密度泛函理论(TDDFT) B3LYP方法通过电子结构计算研究了含氯不对称配体8-羟基喹啉铝配合物AlQ(ClQ)₂的吸收和发射光谱性质. 计算表明, 第一激发态与基态结构变化主要集中在单一含氯配体中, 与mer-AlQ₃相比, 前线占有轨道离域程度变大而未占据轨道则定域化程度提高. 最大吸收主要来自配体内电荷转移跃迁, 电荷从苯酚环和氯向含氮的吡啶环跃迁. 电子陷入态计算表明, 抽取电子能量比AlQ₃相应值略大, 与实验结果一致.     

胶束辅助的酞菁给体-碳纳米管受体超分子自组装及光诱导长寿命电荷分离态

利用十二烷基硫酸钠(SDS)阴离子胶束能够稳定分散单壁碳纳米管(SWCNT)和解聚富集四磺酸锌酞菁(ZnPcS₄)的能力, 组装了ZnPcS₄-SWCNT的电子给体-受体对来模拟光合作用的原初电子转移过程. 用稳态和时间分辨荧光法研究了相应的给体-受体分子间和分子内的光诱导电子转移速率, 用激光闪光光解技术检测了生成的电荷分离态. ZnPcS₄-SWCNT的电子给体-受体组装体在707 nm处出现了基态特征吸收峰, 但是复合体不产生荧光, 这主要归因于有效的分子内光诱导电子转移过程. 瞬态吸收光谱检测到相应的离子对, 动力学衰减结果表明, 电荷分离态的寿命长达42 μs. 这一长寿命电荷分离态的形成, 主要是因为ZnPcS₄是良电子给体(低氧化电位), SWCNT是好的电子受体, 使得三重态电子转移能够发生, 生成三重态电荷分离态.     

卟啉-富勒烯体系光激发电子转移的理论研究

在分子水平研究新型人工光俘获材料对于太阳能电池的发展具有重要意义。本文采用TD-DFT方法研究了卟啉-富勒烯(P-C60)体系的光诱导电子转移过程。该过程由三个过程组成:(1)光激发过程,P-C60由基态激发至卟啉局域激发(LE)态;(2)电荷分离(CS)过程形成卟啉至富勒烯的电荷转移(CT)态;(3)电荷重组(CR)过程,CT态返回到基态。我们通过分析分子轨道指认了LE态和CT,并获得了这两个激发态的结构。采用广义Mulliken-Hush(GMH)方法计算体系电荷分离和电荷重组过程的态态间电子耦合,和实验测量的电子转移速率获得定性一致的结果。本工作为分析、预测光诱导电荷转移过程提供了有效的手段。     

电子给受体复合物中电荷转移吸收光谱和溶剂效应

用MP2/6-31G^**方法研究了二氯甲烷溶剂分子与电子给体、受体以及电子给受体复合物间的相互作用,结果表明,二氯甲烷与电子受体和电子给受体复合物间有弱氢键相互作用.利用CIS/6-31++G^**方法研究了溶剂与溶质分子间形成氢键对激发态的影响.自然键电荷分析表明,电子给受体复合物的S₀→S₁跃迁导致一个电子从电子给体转移到受体.结合非平衡溶剂化处理和自洽反应场方法研究了溶剂分子与复合物间形成氢键时的电荷转移吸收光谱.计算表明氢键作用导致复合物的电荷转移吸收光谱蓝移.     

荧光素蒽醌甲酯分子内光诱导电子转移及电荷分离态的检测

合成了以荧光素为光敏剂的电子给体-受体二元化合物荧光素蒽醌甲酯(FL-AQ),用吸收光谱、荧光光谱、荧光寿命研究了该化合物在乙醇溶液中的光物理性质,并首次用纳秒级瞬态吸收光谱检测了此化合物分子内光诱导电子转移所形成的电荷分离态.在溶液中激发FL,电子可从FL有效地转移到AQ,其速率常数为3.95×10⁹s^-1,效率为95%.但由于电荷分离态寿命较短,瞬态吸收信号弱,若在此溶液中加入二氧化钛(TiO₂)纳米胶体,使FL-AQ吸附在胶体上,电荷分离态信号明显增强.480nm处FL^+·的寿命为11.1μs;560nm处AQ^-·的寿命为8.93μs.     

环双(百草枯对苯撑)-双2-萘甲酸三缩四乙二醇酯二元超分子给受体体系光诱导电子转移研究

本文利用核磁氢谱、吸收光谱和荧光光谱证明了环双(百草枯对苯撑)(CBPQT)与双2-萘甲酸三缩四乙二醇(N-P₄-N)在乙腈溶液中能够形成1:1的二元超分子给受体体系.瞬态吸收光谱的研究表明该超分子体系中光诱导电子转移的速率k_CS>1.0×10⁸s^-1,电子回传的速率k_CR=1.26×10³s^-1,光诱导电子转移所生成电荷分离态的寿命长达794μs.     

二茂铁-酞菁-富勒烯超分子三元体系的构筑及光物理和电化学性质

采用富勒吡咯烷衍生物中的吡啶或咪唑基与二茂铁修饰的金属酞菁轴向配位构筑了二茂铁-酞菁-富勒烯超分子三元体系, 通过紫外-可见光谱滴定法测定了其配位稳定性(K_assoc约为8.58×10⁴ L/mol). 稳态和时间分辨荧光光谱研究结果表明, 在该超分子三元体系中发生了快速的光诱导电子转移(k_CS约为10⁹ s^-1), 并具有较高的电荷分离态量子产率(Ф_CS=0.88). 循环伏安法数据表明, 其电荷分离驱动力ΔG_CS为负值(-0.60 eV), 说明酞菁和富勒烯之间容易形成电荷分离态.     

供电子基团修饰对NNI-R系列分子光物理性质的影响

设计了一系列具有不同供电子基团的N-苯基-1,8-萘二甲酰亚胺衍生物(NNI-R),对它们在二氯甲烷和气相中的几何结构、电子结构以及室温磷光性能进行了研究.在二氯甲烷极性溶剂中,NNI-R系列分子的最低单重激发态(S₁)有2个异构体,分别表现为局域激发(LE)和电荷转移激发(CT).具有弱给电子体(R=OMe,OH)时的NNI-R分子,其S₁态为LE结构,给体和受体间二面角垂直,其总能量远低于CT结构,会抑制系间窜越(ISC)的发生,不会发生磷光现象.在气相下,NNI-R系列分子的S1态只有一种稳定的CT结构,该特征能显著抑制荧光发射,并有效促进系间窜越,使NNI-R系列分子的室温磷光发射成为一种可能.     

BPh-2(mqp)的电子结构和光谱性质的含时密度泛函理论研究

采用abinitioHF和DFTB3LYP方法,对配合物BPh₂(mqp)基态结构进行优化,分析了前线分子轨道特征和能级分布.用abinitioCIS方法优化体系激发态结构.用含时密度泛函理论(TD-DFT)对BPh₂(mqp)的电子光谱进行了研究.结果发现,该物质是配体发光配合物,其发光源于mqp配体内π^*→π的电子跃迁.这表明在mqp配体上进行修饰,可有效地影响配合物前线分子轨道分布,达到调整发光波段的目的.     

4-[2-(9,10-二氰基蒽)乙烯基] -N,N-二甲基苯胺的三荧光发射

从分子水平进行电子转移,电荷分离的研究是十分重要的,它不仅是自然界光合作用的基本过程,也是现代高新技术中的一个关键问题。近年来分子内含电子给体与电子受体的D-A化合物一直引起人们的极大兴趣。这些化合物能发生光致分子内电子转移,使其激发态分子的偶极矩远大于基态,它们的发射光谱对介质的粘度及极性十分敏感,随分子结构的变化而变化,展现出特有的光电性质,可利用作为非线性光学材料、光电转换材料以及荧光探针等。     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Computational characterization of low-lying states and intramolecular charge transfers in N-phenylpyrrole and the planar-rigidized fluorazene

Low-lying states and intramolecular charge transfers in N-phenylpyrrole (PP) and its planar-rigidized derivative fluorazene (FPP) have been investigated by ab initio methodologies. On the basis of calculations, properties of the excited states and plausible dual-fluorescence mechanisms have been elucidated. Present results show that S2 as a key state is involved in the consecutive photophysical processes. The S2 state is easily populated under excitation. In the polar MeCN solution, S2 can evolve to either a lower-energy locally excited state or a lower-energy solvated intramolecular charge-transfer state (S-ICT). The former emits a normal fluorescence back to the ground state, and the latter is exclusively responsible for the red-shifted fluorescence band. Calculations reveal that the emissive ICT states in both FPP and PP have similar geometric features, an elongated N-phenyl bond, a pyramidal carbon atom linking the pyrrole ring, and a quinonoid phenyl ring. The twisting of molecule around the N-phenyl bond is not necessary for the intramolecular charge transfer. Predicted absorption and emission spectra are in reasonable agreement with the experimental observations.     

Spectroscopic studies of bridge contributions to electronic coupling in a donor-bridge-acceptor biradical system

Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of Tp(Cum,Me)Zn(SQ-Ph-NN) (1), a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analogue of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong electronic coupling mediated by the p-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ(π)(SOMO) → NN-Ph (π*)(LUMO) D → A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph(8a) + SQ(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the phenylene bridge in promoting (1) electron transfer in D-Ph-A systems and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D → A interaction given by the magnitude of the electronic coupling matrix element, H(ab).     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Photoinduced energy and electron-transfer processes in porphyrin-perylene bisimide symmetric triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.     

Competing electron-transfer pathways in hydrocarbon frameworks: short-circuiting through-bond coupling by nonbonded contacts in rigid U-shaped norbornylogous systems containing a cavity-bound aromatic pendant group

This work explores electron transfer through nonbonded contacts in two U-shaped DBA molecules 1DBA and 2DBA by measuring electron-transfer rates in organic solvents of different polarities. These molecules have identical U-shaped norbornylogous frameworks, 12 bonds in length and with diphenyldimethoxynaphthalene (DPMN) donor and dicyanovinyl (DCV) acceptor groups fused at the ends. The U-shaped cavity of each molecule contains an aromatic pendant group of different electronic character, namely p-ethylphenyl, in 1DBA, and p-methoxyphenyl, in 2DBA. Electronic coupling matrix elements, Gibbs free energy, and reorganization energy were calculated from experimental photophysical data for these compounds, and the experimental results were compared with computational values. The magnitude of the electronic coupling for photoinduced charge separation, /V(CS)/, in 1DBA and 2DBA were found to be 147 and 274 cm(-1), respectively, and suggests that the origin of this difference lies in the electronic nature of the pendant aromatic group and charge separation occurs by tunneling through the pendant group, rather than through the bridge. 2DBA, but not 1DBA, displayed charge transfer (CT) fluorescence in nonpolar and weakly polar solvents, and this observation enabled the electronic coupling for charge recombination, /V(CR)/, in 2DBA to be made, the magnitude of which is approximately 500 cm(-1), significantly larger than that for charge separation. This difference is explained by changes in the geometry of the molecule in the relevant states; because of electrostatic effects, the donor and acceptor chromophores are about 1 A closer to the pendant group in the charge-separated state than in the locally excited state. Consequently the through-pendant-group electronic coupling is stronger in the charge-separated state--which controls the CT fluorescence process--than in the locally excited state--which controls the charge separation process. The magnitude of /V(CR)/ for 2DBA is almost 2 orders of magnitude greater than that in DMN-12-DCV, having the same length bridge as for the former molecule, but lacking a pendant group. This result unequivocally demonstrates the operation of the through-pendant-group mechanism of electron transfer in the pendant-containing U-shaped systems of the type 1DBA and 2DBA.     

黄芩素激发态分子内质予转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S1态吸收峰的缺失表明S1态是暗态. S1暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S1态是电荷转移态,然而S2态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无能垒的过程.     

Effect of the relaxation of high-frequency vibrational modes on the kinetics of charge separation and recombination

The processes of intramolecular electron transfer from the second excited electron state accompanied by superfast reverse transfer to the first excited state are studied. The kinetics of the populations of the first and second excited states, along with that the charge-separated states, is calculated within the generalized stochastic model, taking into account the reorganization of the medium and intramolecular high-frequency vibrations. It is shown that variations in the relaxation rate of the high-frequency vibrational modes can change the population of the quenching products by a factor of two to three. It is established that in the case of the weak exothermicity of the charge separation process, the population of the charge-separated states declines upon an increase in the vibrational relaxation rate, while the population of the first excited state increases; in the region of high exothermicity, these dependences change to ones that are opposite. To reveal the scales of these effects in real systems, the kinetics of the photo-induced processes in the zinc-porphyrin derivatives, including electron-acceptor imide groups covalently coupled with porphyrin rings, are calculated. It is shown that the results from calculating the kinetics of the population of the first and the second excited states agree well with the experimental data on the kinetics of the fluorescence of these states. The absolute values of the population of the charge-separated state and the first excited state are determined. The key role of the hot electron transitions that occur in parallel with the relaxation of the medium and intramolecular vibrations in the considered process is shown.     

Photophysics of untethered ZnTPP-fullerene complexes in solution

The spectroscopy and dynamic behavior of the self-assembled, Soret-excited zinc tetraphenylporphyrin (ZnTPP) plus fullerene (C(60)) model system in solution has been examined using steady state fluorescence quenching, nanosecond time-correlated single photon counting, picosecond fluorescence upconversion, and picosecond transient absorption methods. Evidence of ground state complexation is presented. Steady-state quenching of the S(2) and S(1) fluorescence of ZnTPP by C(60) reveals that the quenching processes only occur in the excited complexes, are ultrafast, and proceed at different rates in the two states. Only uncomplexed ZnTPP is observed by fluorescence lifetime methods; the locally excited complexes are either dark or, more likely, rapidly relax to products that do not radiate strongly. Both short-range (Dexter) energy transfer and electron transfer relaxation mechanisms are evaluated. Picosecond transient absorption data obtained from the subtle differences between the spectra of Soret-excited ZnTPP with and without a large excess of added C(60) reveal the formation, on a subpicosecond time scale, of relatively long-lived charge-separated species. Soret excitation of ZnTPP···C(60) does not produce a quantitative yield of species in the lower S(1) excited state.