吩噻嗪衍生物的分子内电荷转移激发态的特性

合成了一系列的N-10位取代的吩噻嗪给体受体化合物,这些受体包括苯、苯甲醚、吡啶、萘、苯乙酮和苯乙腈.研究了不同极性溶剂中这些化合物的分子内的光诱导电荷转移现象.稳态荧光的溶剂化效应和较大的Stokes位移清楚表明,仅仅后四种吩噻嗪衍生物的激发态存在着分子内的电荷转移,而苯和苯甲醚取代吩噻嗪因为受体部分氧化电势低,所以不具有这种特性.修正过的Lippert-Mataga公式被用来分析Stokes位移值,从而获得激发态偶极矩.较大的激发态偶极矩说明在激发态时这四种给体受体化合物体系内发生了完全的电子转移.氧化还原电势的数据表明基态时这四种衍生物给体受体部分的作用比较弱.分析荧光光谱获得的结果说明伴随着电荷转移这四种衍生物的激发态构型变化比较小,给体和受体间的扭转角在电荷转移后的激发态与在基态时相似.     

Push–pull hyperbranched molecules. A theoretical study

The electronic properties of the ground state, unrelaxed and relaxed first excited states of push–pull hyperbranched molecules bearing amino and nitro terminal groups have been studied at BB1K/cc‐pvdz//HF/6‐31g(d), TD‐BB1K/cc‐pvdz//HF/6‐31g(d) and TD‐BB1K/cc‐pvdz//CIS/6‐31g(d) levels of theory, respectively. It was demonstrated that dendritic architecture of push–pull molecules favours the charge transfer in the excited state compared to linear molecules. The possibility of adopting a plane conformation is an important condition for the charge transfer in an excited state. According to the calculations 1:1 ratio of donor and acceptor groups is another important precondition for the manifestation of strong charge separation in the excited state. In case of excess of nitro groups over the amino, some of the excitations participating in the S0 → S1 transition favour the charge transfer in the excited state in the opposite directions, thus decreasing the charge separation. Copyright © 2008 John Wiley & Sons, Ltd.     

Red Emitting Coumarin—Azo Dyes : Synthesis,Characterization, Linear and Non-linear Optical Properties-Experimental and Computational Approach

The coumarin molecules with 7-(N,N-diethylamino) substitution and aryl azo (Ar–N=N-) at 3-position were synthesized, by reacting diazonium salt of substituted amines and 7-(N, N-diethylamino)-4-hydroxy coumarin under basic conditions. They were found to be fluorescent despite the presence of azo group. The azo group rotation was blocked by complexing with -BF₂, so as to get a red shift in absorption. The azo molecules show charge transfer, whereas BF₂-complexes do not. The dipole moment ratios between the ground and excited states calculated suggest highly polar excited state and an intra-molecular charge transfer at the excited state in the case of azo dyes. The NLO properties were calculated by solvatochromic method and computationally. Second order hyperpolarizability was found to be 46 to 1083 times more than urea. DFT and TDTDF calculations were performed to understand the electronic properties of the molecules at the ground as well as excited states.     

Hydroquinone–Benzonitrile System: Intramolecular Charge-Transfer and Computational Studies

A novel intramolecular donor–acceptor system of hydroquinone–benzonitrile was synthesized. Its photo-induced intramolecular charge-transfer (ICT) transition was confirmed by (1) shift of its emission maximum with increasing solvent polarity, (2) high dipole moment for the ICT excited state calculated from the Lippert equation, and (3) its HOMO and LUMO. According to the extent of separation between HOMO and LUMO, it is suggested that substituent position (ortho, meta, or para) in the donor–acceptor biphenyls is not a key point for the photo-induced intramolecular charge transfer and the donor with two alkoxy or hydroxy groups has more photo-induced charge transfer transitions than the one with one alkoxy or hydroxy group. In other words, the hydroquinone–benzonitrile system displays more photo-induced charge transfer transitions than 4COB (4-cyano-4′-butyloxybiphenyl).     

Effectiveness of charge separation from the long-lived second excited state of donors

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S₂ is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A₁–A₂ can be close to 100% in molecular systems with lifetimes of the S₂ state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A₁ and triads D–A₁–A₂ are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.     

超快非均质电子转移的优化控制研究(Ⅰ)

本文用三能级的单振动模型,模拟了从二萘嵌苯到TiO2超快电子转移的动力学过程,发现在弱电子转移耦合下,电子在激光场的作用下激发至分子激发态转移然后到半导体导带,在强电子转移耦合下,电子直接由基态转移到半导体导带。在优化控制的理论模拟中以电子激发态的振动基态为目标态,考虑了不同注入位置条件下的电子转移的动力学过程,研究了优化激光场在给定时间内实现目标态的过程,由于从分子激发态到半导体的超快电子转移,只有当分子激发态能级与半导体导带底能级简并时,才能实现较高的目标态产生率。     

Photoinduced energy and electron transfer in oligo(<Emphasis Type="Italic">p</Emphasis>-phenylene vinylene)-fullerene dyads

The intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene)–fulleropyrrolidine dyad with four phenyl rings (OPV4-C₆₀) has been investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state. In solution, photoexcitation of the OPV4 moiety of OPV4-C₆₀ results in an ultrafast (<190 fs) singlet energy transfer reaction, creating the fullerene singlet excited state. In polar solvents, the ultrafast energy transfer is followed in the picosecond time domain by an intramolecular electron transfer. In accordance with Marcus theory, the rates for forward and backward intramolecular electron transfer in OPV4-C₆₀ are influenced by the polarity of the solvent. In the solid state the photophysics of OPV4-C₆₀ is dramatically different. In thin films, the forward electron transfer proceeds within 500 fs, irrespective of which chromophore is photoexcited. The increased rate for charge separation in the solid state is attributed to a more favorable orientation of the donor and acceptor that results in an intermolecular electron transfer. In the films, energy and electron transfer processes compete at the earliest moments after photoexcitation. In the solid state, the photogenerated electrons and holes have long lifetimes as a result of migration of these charges to thermodynamically more favorable sites in the film. PACS 78.47.1+p; 34.30.+h; 33.50.-j     

Photoinduced Intramolecular Electron Transfer and Electronic Coupling Interactions in π- Expanded Imidazole Derivatives

An intramolecular excited charge transfer (CT) analysis of imidazole derivatives has been made. The determined electronic transition dipole moments has been used to estimate the electronic coupling interactions between the excited charge transfer singlet state (¹CT) and the ground state (S₀) or the locally excited state (¹LE). The properties of excited ¹CT state imidazole derivatives have been exploited by the significant contribution of the electronic coupling interactions. The excited state intramolecular proton transfer (ESIPT) analysis has also been discussed.     

An Intramolecular Charge Transfer Fluorescent Probe: Synthesis,Structure and Selective Fluorescent Sensing of Cu<Superscript>+2</Superscript>

A series of substituted imidazoles have been synthesized in very good yield under solvent free condition by grinding 1,2-diketone, aromatic aldehyde and ammonium acetate in the presence of molecular iodine as the catalyst. The short reaction time, good yield and easy workup make this protocol practically and economically attractive and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazoles (dpip and dptip) have been studied using emission spectroscopy and it was detected that the two distinct ground state rotamers are responsible for the normal and the tautomer emissions. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the imidazole derivatives were performed and discussed. DFT analysis about HOMO, HOMO-1, LUMO and LUMO + 1 were carried out and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.     

Fluorescence Properties of Donor–Acceptor-Substituted Pyrazoloquinolines

The photophysical properties of three newly synthesized pyrazoloquinolines, composed of N,N-dimethylaniline as donor subunit and various substituted forms of the acceptor pyrazoloquinoline (DPPQ), were investigated by absorption as well as by stationary and time resolved fluorescence spectroscopy. These compounds show generally highly efficient emission in nonpolar and medium polar solvents; the dipole moment of the emitting state increases and the quantum yield decreases with solvent polarity. These results are explained by state reversion in polar solvents: At low polarities emission originates from a state localized on the DPPQ moiety, whereas in the high-polarity regime the next excited state of charge transfer character, in which an electron is promoted from the amino nitrogen lone pair into an excited orbital of the DPPQ moiety, becomes the fluorescent state. This view is corroborated by semiempirical calculations including the solvent reaction field, low-temperature fluorescence measurements, and the observation of effects of protonation on the spectroscopic and photophysical properties.     

Effect of oxidation state and coordination with Hg(II) on the electronic spectra of an anthraquinone–rhodamine sensor

ABSTRACT

In this study, a computational examination of the electronic transitions and through-space energy transfer processes lends insight into the experimental electronic spectra of a redox-sensitive rhodamine–anthraquinone dyad. Electronic transitions were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT) based on models optimised from single-crystal X-ray diffraction (XRD) ion positions. DFT calculations were performed on gas-phase models using the Vienna Ab Initio Software Package (VASP) with the functional developed by Perdew, Burke, and Ernzerhof (PBE) on a basis set of plane waves. Using the DFT results, select transitions were evaluated based on a dipole–dipole coupling mechanism to find the Förster resonance energy transfer coupling, the square of which is approximately proportional to the rate of energy transfer between the donor and the acceptor. Electronic transitions during the relaxation of charge carriers are also investigated using nonadiabatic molecular dynamics. In order to investigate the transitions contributing to key peaks in the experimental absorbance spectra, TDDFT calculations were performed in Gaussian 09 with the B3LYP functional on the sensor in solution phase, which is simulated using a polarisable continuum model (PCM). The computed electron transfer process from the excited rhodamine to the quinone correlates better with the experimental data than does the computed Förster resonance energy transfer (FRET) process. This computed electron transfer process is faster than the radiative lifetime of the fluorescent state, which collectively suggests that the charge transfer process quenches fluorescence. These results support the observation that fluorescence is more prominent in the oxidised sensor than in the reduced sensor.     

Synthesis and fluorescence properties of carbazole and fluorene-based compounds

Three donor-acceptor (D-A) conjugated compounds consisted of carbazole and fluorene (Scheme 1) were synthesized via Wittig-Horner, Ullmann or Sonogashira reaction. Their photoluminescence properties were investigated in solution and solid state, respectively. Luminescence in various solvents revealed the intramolecular charge transfer (ICT) process exists in these D-A molecules. Correlation between the Stokes’ shifts and solvent polarity parameters indicated that moderate large dipole moments exist in the excited states.     

Intramolecular charge transfer state and unusual fluorescence from an upper excited singlet of a nonplanar derivative of p-cyano-N,N-dimethylaniline

The TICT (twisted internal charge transfer state) fluorescence of 4-cyano-2,6,N,N-tetramethylaniline (CTMA) was found in the vapour phase. The dipole moments of excited Franck-Condon and Franck-Condon twisted ground states were estimated from the solvent shifts of absorption and fluorescence. The near-equality of excited Franck-Condon and relaxed TICT state dipole moments is discussed. The fluorescence from an upper excited singlet state is found and its mechanism is explained.     

Effect of positional substitution of amino group on excited state dipole moments of quinoline

The effect of positional substitution of amino group on the ground and excited state dipole moments of quinoline ring has been investigated using solvatochromic shift methods. The excited state dipole moments of 5aminoquinoline (5AQ) and 3aminoquinoline (3AQ) have been estimated from the spectral data in different non-polar, polar aprotic and polar protic solvents using Bakhshiev and Kawski-Chamma-Viallet equations. It has been observed that both grounds as well as excited state dipole moments for 5AQ are higher than those for 3AQ by approximately a factor of two. Higher values of the excited state dipole moments for both 3AQ and 5AQ as compared to corresponding ground state values have been attributed to intramolecular charge transfer processes. The role of specific solute-solvent interaction on excited state dipole moment in addition to the general solvent effects has been discussed.     

Excited State Properties of Fluorine-Substituted Hexabenzocoronene: A Quantum-Chemical Characterization

The first fluorine-substituted hexabenzocoronene has been synthesized and its electronic structure and optical properties have been reported [Q. Zhang, et al., Org. Lett.7 (2005) 5019]. In this letter, the electronic structure and excited state properties of the fluorine-substituted hexabenzocoronene are studied with quantum chemistry method as well as the transition and the charge difference densities. The transition densities show the orientations and strength of the dipole moments and the charge difference densities reveal the orientation and results of the intramolecular charge transfer. The calculated transition energies and oscillator strengths are consistent with the experimental data, and the theoretical results of transition and charge difference densities are valuable to understanding the excited state properties of the fluorine-substituted hexabenzocoronene.     

Photophysical properties of a series of 4‐aryl substituted 1,4‐dihydropyridines

In this article, a series of Hantzsch 1,4‐dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher Stokes’ shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation of the fluorescence maxima (ν_max) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation. A comparison between the experimental data and time‐dependent density functional theory‐polarizable continuum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics of these compounds. For these calculations, the S₀ and S₁ states were optimized using Becke, three‐parameter, Lee–Yang–Parr/6‐31 G* and Configuration Interaction Singles/6‐31 G*, respectively. The predicted absorption maxima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure. Copyright © 2012 John Wiley & Sons, Ltd.     

异质二聚体反应中心的电荷转移过程

在低温的状态下,类球红细菌内天然叶绿素的反应中心发生的电荷分离对激发波长有较弱的依赖性,而这种依赖可能来源于分子内电荷转移态。我们采用表现出更多的电荷转移态性质的突变叶绿素为反应中心,对这种电荷分离过程中的激发波长依赖和温度依赖的性质做了进一步的研究。研究发现:突变的异质二聚体存在的两种激发态(定域的激发态与分子内电荷转移态)是相互强耦合的,在突变的分子异质二聚体中形成的电荷转移态并不是电荷分离的有效途径。随着温度的降低,异质二聚体的活性降低,从而电荷分离产率下降。     

Excited State Properties of Fluorine-Substituted Hexabenzocoronene： A Quantum-Chemical Characterization

The first fluorine-substituted hexabenzocoronene has been synthesized and its electronic structure and optical properties have been reported [Q. Zhang, et al., Org. Lett. 7 （2005） 5019]. In this letter, the electronic structure and excited state properties of the fluorine-substituted hexabenzocoronene are studied with quantum chemistry method as well as the transition and the charge difference densities. The transition densities show the orientations and strength of the dipole moments and the charge difference densities reveal the orientation and results of the intramolecular charge transfer. The calculated transition energies and oscillator strengths are consistent with the experimental data, and the theoretical results of transition and charge difference densities are valuable to understanding the excited state properties of the fluorine-substituted hexabenzocoronene.     

Comparative charge transfer studies in nonmetallated and metallated porphyrin fullerene dyads

Single material organic solar cells become an interesting area of research to overcome the challenges with efficient charge separation efficiencies in conventional organic solar cells. In this article, we have synthesized nonmetallated and metallated porphyrin‐fullerene dyad materials (H2P‐C60 and ZnP‐C60, respectively) with simple structure, comprehensively studied their charge transfer mechanism, and established a proof of concept that nonmetallated porphyrin‐fullerene dyads are better candidates to be used in organic solar cells compared with metallated dyads. Absorption and electrochemical analysis revealed the ground state electronic interactions between donor‐acceptor moieties in both types of dyads. Driving force (?ΔG^o_ET) for intramolecular electron transfer process was calculated by first oxidation and reduction potentials of dyads. The excited state electronic interactions were characterized by time‐resolved fluorescence and pump‐probe transient absorption experiments. Strong fluorescence quenching of porphyrin along with reduced lifetimes in dyads due to deactivation of singlet excited states by photoinduced charge transfer process between porphyrin/Zn‐porphyrin core and fullerene in different polarity solvents was observed. Transient absorption spectroscopy was also applied to identify the transient spectral features, ie, cationic (H2P⁺/ZnP⁺) and anionic (C₆₀^?) radicals formed because of the charge separation in both types of dyads. Finally, organic solar cell device was also fabricated using the dyads. We obtained higher V_oc, J_sc, and fill factor in single material organic solar cell using H2P‐C60 compared to previous reports.