四氰基乙烯—四甲基乙烯体系光诱导电子转移的溶剂效应及电荷分离激发态研究

用从头算方法,在HF/6-31 G^**和CASSCF（8,8）/6-31G^*基组水平上对四氰基乙烯与四甲基乙烯间电子转移的溶剂效应及电荷分离激发态进行了理论计算与研究。通过对给、受体各种几何构型的优化,计算了孤立给、受体之间的电荷分离反应热。在假定碰撞络合物形成过程中给、受体内部结构不发生变化的前提下,通过优化给、受体中心间距的方法,找出了络合物的稳定构型。计算了水溶剂及二氯甲烷溶剂中两种稳定构型络合物的电荷分离激发态,计算结果表明光激发可以直接导致体系的电荷分离。     

A theoretical study of photoinduced electron transfer between tetracyanoethylene and arene

 Ab initio calculations have been performed to investigate the state transition in photoinduced electron transfer reactions between tetracyanoethylene and biphenyl as well as naphthalene. Face-to-face conformations of electron donor–acceptor (EDA) complexes were selected for this purpose. The geometries of the EDA complexes were determined by using the isolated optimized geometries of the donor and the acceptor to search for the maximum stabilization energy along the center-to-center distance. The correction of interaction energies for basis set superposition error was considered by using counterpoise methods. The ground and excited states of the EDA complexes were optimized with complete-active-space self-consistent-field calculations. The theoretical study of the ground state and excited states of the EDA complex in this work reveals that the S₁ and S₂ states of the EDA complexes are charge–transfer (CT) excited states, and CT absorption which corresponds to the S₀→S₁ and S₀→S₂ transitions arise from π−π^* excitation. On the basis of an Onsager model, CT absorption in dichloromethane was investigated by considering the solvent reorganization energy. Detailed discussions on the excited state and on the CT absorptions were made. Received: 30 April 2001 / Accepted: 18 October 2001 / Published online: 9 January 2002     

9,10-二氰基蒽和杜烯间光诱导电子转移的电荷分离态和电子耦合

采用从头算方法,讨论了9,10-二氰基蒽(DCA)和杜烯(DUR)间光诱导电子转移反应的态-态跃迁.考虑基组重叠误差(BSSE)对相互作用能的校正,用MP₂方法优化得到重叠式[DCA…DUR]配合物的稳定构型.用单激发组态相互作用(CIS)方法讨论了[DCA…DUR]配合物的光诱导电荷分离和电荷复合过程.根据广义Mulliken-Hush(GMH)模型,计算了电荷复合过程的电子耦合矩阵元.结果表明,[DCA…DUR]配合物的S₀→S₁和S₀→S₂跃迁产生了两个强的局域激发态,S₀→S₃跃迁直接导致电荷分离态,小的振子强度预测该电荷转移(CT)跃迁是一弱跃迁,电荷分离态S₃衰变到低局域激发态或基态的电荷复合是可能的.     

Ab initio study of phosphorescent emitters based on rare-earth complexes with organic ligands for organic electroluminescent devices

An ab initio approach is developed for calculation of low-lying excited states in Ln(3+) complexes with organic ligands. The energies of the ground and excited states are calculated using the XMCQDPT2/CASSCF approximation; the 4f electrons of the Ln(3+) ion are included in the core, and the effects of the core electrons are described by scalar quasirelativistic 4f-in-core pseudopotentials. The geometries of the complexes in the ground and triplet excited states are fully optimized at the CASSCF level, and the resulting excited states have been found to be localized on one of the ligands. The efficiency of ligand-to-lanthanide energy transfer is assessed based on the relative energies of the triplet excited states localized on the organic ligands with respect to the receiving and emitting levels of the Ln(3+) ion. It is shown that ligand relaxation in the excited state should be properly taken into account in order to adequately describe energy transfer in the complexes. It is demonstrated that the efficiency of antenna ligands for lanthanide complexes used as phosphorescent emitters in organic light-emitting devices can be reasonably predicted using the procedure suggested in this work. Hence, the best antenna ligands can be selected in silico based on theoretical calculations of ligand-localized excited energy levels.     

Structures and spectra of Na(NH3) n=1,2

Geometric structures and the energies for the ground and several excited electronic states of a sodium atom bound with one or two ammonia molecules are presented. All self consistent field (SCF) calculations are performed with extended basis sets. Geometry optimization and one electron properties have been performed within the SCF approximation. Excited states have been calculated with the multi-configuration SCF (MCSCF) technique. This system may be viewed as a precursor to solvation in a macroscopic system. The excited state calculations provide important information for spectroscopic studies of these complexes.     

High-efficiency energy transfer from carotenoids to a phthalocyanine in an artificial photosynthetic antenna

Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.     

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.     

Effects of chain length and Au spin-orbit coupling on 3(pi pi*) emission from bridging Cn2- units: theoretical characterization of spin-forbidden radiative transitions in metal-capped one-dimensional carbon chains [H3PAu(C[triple bond]C)nAuPH3

Density functional theory and CASSCF calculations have been used to optimize the geometries of binuclear gold(I) complexes [H(3)PAu(C[triple bond]C)(n)AuPH(3)] (n=1-6) in their ground states and selected lowest energy (3)(pi pi*) excited states. Vertical excitation energies obtained by time-dependent density functional calculations for the spin-forbidden singlet-triplet transitions have exponential-decay size dependence. The predicted singlet-triplet splitting limit of [H(3)PAu(C[triple bond]C)(proportional/variant)AuPH(3)] is about 8317 cm(-1). Calculated singlet-triplet transition energies are in reasonable agreement with available experimental observations. The effect of the heavy atom Au spin-orbit coupling on the (3)(pi pi*) emission of these metal-capped one-dimensional carbon allotropes has been investigated by MRCI calculations. The contribution of the spin- and dipole-allowed singlet excited state to the spin-orbit-coupling wave function of the (3)(pi pi*) excited state makes the low-lying acetylenic triplet excited states become sufficiently allowed so as to appear in both electronic absorption and emission.     

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.     

硅杂环戊二烯衍生物电子结构与光谱性质的理论研究

采用密度泛函理论(DFT)B3LYP/6-31G*方法, 对2,5位取代的硅杂环戊二烯(Silole)系列衍生物进行几何结构优化, 通过计算得到电离能、电子亲和势、空穴抽取能及电子抽取能等相关能量, 并使用TD-DFT方法研究其吸收光谱, 分析相关能量及光谱的变化规律. 采用单组态相互作用(CIS/6-31G*)方法优化得到它们的最低单重激发态(S1)结构, 在此基础上, 使用TD-DFT方法计算对应的发射光谱. 分析2,5位芳基取代硅杂环戊二烯衍生物(DADPS)激发态与基态的结构差异及原因, 研究前线分子轨道的分布情况, 并讨论发光特征及载流子传输性能. 研究结果表明, 激发态结构弛豫主要发生在Silole环和直接与2,5位芳基相连的部位; 前线轨道主要分布在Silole环和2,5位芳基上; 二吡咯取代物有望成为空穴传输材料, 二噻吩取代物和二呋喃取代物有望在发光器件中表现出较高的发光效率.     

簇合物Mo_2X_8~(4-)(X=Cl,Br)低能态电子光谱

应用间略微分重叠自洽场和组态作用(INDO/S-CI)方法,计算双核簇合物Mo_2X_8~(4-)(X=Cl,Br)基态~1A_(1g)至不同低能态的d电子自旋允许的跃迁能,得出Mo—Mo四重键均裂时所需的能量大小依次为键σ>π>>δ。对于Mo_2Cl_8~(4-)簇合物态~1A_(1g)→1~1A_(2u)(δ→δ~*)和~1A_(1g)→2~1E_g(π→δ~*)的电子跃迁能,理论值(15584和22587cm~(-1))与低温时测得光谱实验值(18083和22000cm~(-1))较为接近。此外,本文还提出了簇合物Mo_2X_8~(4-)的态相同而X不同时,电子跃迁能变化,主要决定于参与电子转移过程的分子轨道组成成份的看法。     

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     

Structure and properties of the low-lying electronic states of CeC(2) and CeC(2)(+)

Theoretical studies on the electronic and thermodynamic properties of several electronic states of CeC(2) and CeC(2)(+) have been carried out employing state-of-the-art single- and multireference techniques. The ground and the low-lying electronic states of these two species have been found to possess C(2v) triangular structures. A (3)B(2) state has been found to be the ground state of CeC(2) while for CeC(2)(+) (2)A(2) is the ground state. The computed electron ionization energy is in excellent agreement with experiment. The experimentally observed thermodynamic properties (dissociation and atomization energies) of reactions involving CeC(2) dissociation are corrected using the computed gas-phase properties of the molecule and the partition functions. The bent triplet and singlet state of CeC(2) exhibit large dipole moments (7.0-10.5 D) and it is consistent with the ionic character (through dative charge transfer) of the cluster in ground and excited states.     

Relativistic study on emission mechanism in palladium and platinum complexes

The present study investigates the spin-orbit coupling (SOC) effects in the radiative processes from the electronically excited states of bis[-2-(2-thienyl)-pyridine] platinum (Pt(thpy)2) and palladium (Pd(thpy)2). The transition probabilities among the low-lying spin-mixed states in these complexes are estimated using the discrete variable representation (DVR) method based on the assumption that the system obeys Fermi's golden rule. It is revealed that the low-lying excited singlets and triplets are strongly mixed with each other by SOC in Pt(thpy)2 and, as a result, a fast nonradiative transition occurs to the low-lying excited spin-mixed states. This is followed by the radiative transition from these low-lying spin-mixed states to the lowest spin-mixed state (the ground state); that is to say, a phosphorescence should be observed from these low-lying excited spin-mixed states in Pt(thpy)2. On the contrary, weak SOCs are obtained in Pd(thpy)2 and no phosphorescence at room temperature is expected to be observed in Pd(thpy)2. These results are in good agreement with the experimental reports.     

Excited state properties of 7-hydroxy-4-methylcoumarin in the gas phase and in solution. A theoretical study

TDDFT/B3LYP and RI-CC2 calculations with different basis sets have been performed for vertical and adiabatic excitations and emission properties of the lowest singlet states for the neutral (enol and keto), protonated and deprotonated forms of 7-hydroxy-4-methylcoumarin (7H4MC) in the gas phase and in solution. The effect of 7H4MC-solvent (water) interactions on the lowest excited and fluorescence states were computed using the Polarizable Continuum Method (PCM), 7H4MC-water clusters and a combination of both approaches. The calculations revealed that in aqueous solution the pi pi* energy is the lowest one for excitation and fluorescence transitions of all forms of 7H4MC studied. The calculated excitation and fluorescence energies in aqueous solution are in good agreement with experiment. It was found that, depending on the polarity of the medium, the solvent shifts vary, leading to a change in the character of the lowest excitation and fluorescence transition. The dipole-moment and electron-density changes of the excited states relative to the ground state correlate with the solvation effect on the singlet excited states and on transition energies, respectively. The calculations show that, in contrast to the ground state, the keto form has a lower energy in the pi pi* state as compared to enol, demonstrating from this point of view the energetic possibility of proton transfer from the enol to the keto form in the excited state.     

Time‐dependent Density Functional Theory Study on the Hydrogen‐bonded Dimers Formed by Gauche‐1PA and Trans‐1PA

Three hydrogen bonding complexes of the gauche‐1PA dimer (GG), trans‐1PA dimer (TT) and mixed dimer (GT) have been calculated for the geometry conformations and excited‐state energies. The electron distribution at the site of C‐O of H‐donor moiety in HOMO transfers to the direction of O‐H of H‐acceptor moiety in LUMO. The hydrogen bond between two 1PAs is the bridge of the intermolecular charge transfer. By the Zhao and Han's excited‐state hydrogen bonding dynamics rule, the first excited‐state hydrogen bonding change has been discussed without optimizing the excited‐state geometry conformations. According to the distinct difference between GT and GG (TT), we concluded that two gauche‐1PA monomers of one dimer are transformed at the same time to two trans‐1PA monomers.     

A semiempirical study for the ground and excited states of free-base and zinc porphyrin-fullerene dyads

The ground and excited states of a covalently linked porphyrin-fullerene dyad in both its free-base and zinc forms (D. Kuciauskas et al., J. Phys. Chem. 100 (1996) 15926) have been investigated by semiempirical methods. The excited-state properties are discussed by investigation of the character of the molecular orbitals. All frontier MOs are mainly localized on either the donor or the acceptor subunit. Thus, the absorption spectra of both systems are best described as the sum of the spectra of the single components. The experimentally observed spectra are well reproduced by the theoretical computations. Both molecules undergo efficient electron transfer in polar but not in apolar solvents. This experimental finding is explained theoretically by explicitly considering solvent effects. The tenth excited state in the gas phase is of charge-separated character where an electron is transferred from the porphyrin donor to the fullerene acceptor subunit. This state is stabilized in energy in polar solvents due to its large formal dipole moment. The stabilization energy for an apolar environment such as benzene is not sufficient to lower this state to become the first excited singlet state. Thus, no electron transfer is observed, in agreement with experiment. In a polar environment such as acetonitrile, the charge-separated state becomes the S, state and electron transfer takes place, as observed experimentally. The flexible single bond connecting both the donor and acceptor subunits allows free rotation by ca. +/- 30 degrees about the optimized ground-state conformation. For the charge-separated state this optimized geometry has a maximum dipole moment. The geometry of the charge-separated state thus does not change relatively to the ground-state conformation. The electron-donating properties of porphyrin are enhanced in the zinc derivative due to a reduced porphyrin HOMO-LUMO energy gap. This yields a lower energy for the charge-separated state compared to the free-base dyad.     

Hydrogen-bonding effects on the formation and lifetimes of charge-separated States in molecular triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d(6) metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ～750 ns in dichloromethane to ～3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ～50 to ～2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET.     

Spectroscopic constants and potential energy curves of yttrium carbide (YC)

The potential energy curves of the low-lying electronic states of yttrium carbide (YC) and its cation are calculated at the complete active space self-consistent field and the multireference single and double excitation configuration interaction (MRSDCI) levels of theory. Fifteen low-lying electronic states of YC with different spin and spatial symmetries were identified. The X (4)Sigma- state prevails as the ground state of YC, and a low-lying excited A (4)Pi state is found to be 1661 cm(-1) higher at the MRSDCI level. The computations of the authors support the assignment of the observed spectra to a B (4)Delta(Omega=72)<--A (4)Pi(Omega=52) transition with a reinterpretation that the A (4)Pi state is appreciably populated under the experimental conditions as it is less than 2000 cm(-1) of the X (4)Sigma- ground state, and the previously suggested (4)Pi ground state is reassigned to the first low-lying excited state of YC. The potential energy curves of YC+ confirm a previous prediction by Seivers et al. [J. Chem. Phys. 105, 6322 (1996)] that the ground state of YC+ is formed through a second pathway at higher energies. The calculated ionization energy of YC is 6.00 eV, while the adiabatic electron affinity is 0.95 eV at the MRSDCI level. The computed ionization energy of YC and dissociation energy of YC+ confirm the revised experimental estimates provided by Seivers et al. although direct experimental measurements yielded results with greater errors due to uncertainty in collisional cross sections for YC+ formation.     

Photochemical charge separation in closely positioned donor-boron dipyrrin-fullerene triads

A series of molecular triads, composed of closely positioned boron dipyrrin-fullerene units, covalently linked to either an electron donor (donor(1)-acceptor(1)-acceptor(2)-type triads) or an energy donor (antenna-donor(1)-acceptor(1)-type triads) was synthesized and photoinduced energy/electron transfer leading to stabilization of the charge-separated state was demonstrated by using femtosecond and nanosecond transient spectroscopic techniques. The structures of the newly synthesized triads were visualized by DFT calculations, whereas the energies of the excited states were determined from spectral and electrochemical studies. In the case of the antenna-donor(1)-acceptor(1)-type triads, excitation of the antenna moiety results in efficient energy transfer to the boron dipyrrin entity. The singlet-excited boron dipyrrin thus generated, undergoes subsequent energy and electron transfer to fullerene to produce a boron dipyrrin radical cation and a fullerene radical anion as charge-separated species. Stabilization of the charge-separated state in these molecular triads was observed to some extent.