Ultrafast dynamics of dithienylethenes differently linked to the surface of TiO(2) nanoparticles

The photoinduced dynamics of a dithienylethene chromophore coupled to the surface of TiO(2) by either a tripodal linker or a carboxyl group was investigated with ultrafast transient absorption spectroscopy. The absence of electron transfer from the photoexcited tripodal dithienylethene chromophore demonstrates that the tripod efficiently uncouples the electronic systems of dithienylethene and TiO(2). Contrary to this situation, photoinduced electron transfer can compete with ultrafast intramolecular relaxation in the COOH-dithienylethene/TiO(2) coupled system. An electron transfer rate of 1.1?×?10(12)?s(-1) can be extracted, which is considerably slower than the intramolecular relaxation rate of the dithienylethene (3.7?×?10(12)?s(-1)). Consequently, the electron transfer reaction exhibits a low efficiency.     

Direct observation of radical intermediates during electron transfer between DNA and a ternary copper complex

The photoinduced electron transfer (PET) reaction within a ternary copper complex [Cu(phen)(Htrp)]⁺ (Htrp: l-tryptophanato; phen: 1,10-phenanthroline) (1) and in presence of DNA has been studied in homogeneous buffer medium and in reverse micelles. An intramolecular electron transfer occurs within the photoexcited complex (1) from tryptophan to phen. The copper complex can displace ethidium bromide from DNA backbone and on photoexcitation can oxidize DNA in a deoxygenated environment due to intermolecular electron transfer, although the intramolecular electron transfer is thermodynamically favorable. A prominent magnetic field effect (MFE) has been found even in homogeneous aqueous medium for the triplet born radicals both in case of intra and intermolecular electron transfer reactions. In case of intramolecular electron transfer the observation of MFE is similar to that of linked donor-acceptor system. However the observation of MFE for the intermolecular electron transfer between non-covalently bound complex-DNA systems is rather rare. Some non-covalent weak interaction, e.g. hydrophobic interaction between the phen ligand and DNA base pairs and electrostatic force of attraction between [Cu(phen)(Htrp)]⁺ complex and DNA may lead to partial intercalation of the copper complex within DNA that is responsible for such a rare observation.     

Photoinduced intramolecular electron transfer reactions in fullerene—phenothiazine linked compounds: effects of magnetic field and spacer chain length

Spectroscopic and electrochemical properties of two fullerene(C₆₀)-phenothiazine(PH) linked compounds with different spacer chain length have been compared in benzonitrile (polar solvent) and in benzene (non-polar solvent). Transient absorption and fluorescence spectra indicated that photoinduced intramolecular electron transfer occurred in benzonitrile, but not in benzene. The results are due to solvent effect on energy levels of the photogenerated biradical. The driving forces for the electron transfer were determined by measuring the redox potentials of the C₆₀ and PH moieties. Thermodynamic parameters for the electron transfer processes were evaluated and compared. In benzonitrile, the lifetime of the photo-generated biradical was very long, in spite of being around the top region in Marcus theory. The decay rate of the biradicals was retarded in the presence of magnetic fields. The decay rate constant decreased quickly with increasing the magnetic field and became constant above about 0.2 T. The magnetic field effects verified that the triplet biradical was generated by the intramolecular electron transfer from PH to the triplet excited state of C₆₀. The long lifetime is most probably ascribed to the spin multiplicities of the biradical.     

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     

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.     

The influence of electronic and structural factors on the photoinduced electron transfer in sterically strained <Emphasis Type="Italic">meso</Emphasis>-nitrophenyl-substituted porphyrins

The electronic and structural factors affecting the efficiency of the photoinduced electron transfer in meso-nitrophenyl-substituted octaethylporphyrins are theoretically analyzed by semiempirical methods of quantum chemistry. It is shown that the experimental differences between the rate constants of electron transfer associated with the change in the position of the nitro group in the meso-phenyl ring (ortho, meta, or para positions) are determined by such factors as torsional vibrations of the phenyl ring around the single C₁-C_m bond, electronic properties of the phenyl group, rotations of the nitro group around the single C-N bond, and out-ofplane deformations of the porphyrin macrocycle. It is ascertained that the matrix elements of electronic interactions and the energies of excited charge-transfer states depend substantially both on the position of the nitro group in the meso-phenyl ring and on intramolecular vibrations of the phenyl and the nitro groups. In nonpolar media, the fluorescence quenching in the compounds under study occurs mainly due to the admixture of chargetransfer excitations to the lowest S ₁ state of porphyrin. Upon increasing polarity, the main channel of deactivation of excited singlet states is direct photoinduced electron transfer either through space (the ortho position) or through a bond involving the participation of orbitals of the phenyl spacer (the meta and para positions).     

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.     

DFT and TD-DFT study on structure and properties of organic dye sensitizer TA-St-CA

The geometry, electronic structure, polarizability and hyperpolarizability of organic dye sensitizer TA-St-CA, which contains a π-conjugated oligo-phenylenevinylene unit with an electron donor–acceptor moiety, was studied using density functional theory (DFT), and the electronic absorption spectrum was investigated via time-dependent DFT (TD-DFT) with several hybrid functionals. The calculated geometry indicates that the strong conjugated effects are formed in the dye. The TD-DFT results show that the hybrid functional PBE1PBE and MPW1PW91 are more suitable than B3LYP for calculating electronic absorption spectra. The features of electronic absorption spectra were assigned on account of the qualitative agreement between the experiment and the calculations. The absorption bands in visible and near-UV region are related to photoinduced electron transfer processes, and the diphenylaniline group is major chromophore that contributed to the sensitization, and the interfacial electron transfer are electron injection processes from the excited dyes to the semiconductor conduction band. Compared with the similar dye D5, the good performance of TA-St-CA in dye-sensitized solar cells may be resulted from the higher energy level of the lowest unoccupied molecular orbital and the larger oscillator strengths for the most excited states with intramolecular electron transfer character.     

识别过渡金属离子的1,8-萘二甲酰亚胺多胺衍生物荧光传感器的研究

设计合成了用以检测过渡金属离子的荧光化学敏感器体系,它们是由1,8-萘二酰亚胺为荧光团,多胺衍生物为金属离子受体组成．在室温下对其光物理性质的研究中发现,在没有加入过渡金属离子时,由于体系内存在有效的光诱导电子转移过程使得荧光团的荧光被淬灭．加入过渡金属离子后,金属离子受体中的氮原子和过渡金属离子之间的配位作用阻断了光诱导电子转移过程,体系的荧光增强．不同的金属离子受体表现出了和过渡金属离子不同的配位识别能力,并且通过荧光的变化传递出受体-金属离子作用的信息．     

Intramolecular Fluorescence Quenching of Crowned 7-Aminocoumarins as Potential Fluorescent Chemosensors

The effects of the nature of solvent, temperature and complex formation with alkali and alkaline-earth metal cations, as well as protonation, on the efficiency and the kinetics of fluorescence of 3-azacrowned 7-diethylaminocoumarins have been studied. For the crown-ethers under investigation, the ratio of a dipole moment to the radius of Onsager cavity delta micro/rho is a constant value, and a macrocycle does not affect delta micro, and rho. The fluorescence of coumarin 1 in acetonitrile is quenched by an electron donor, triethylamine, with the Stern-Volmer constant being equal to (0.474+/-0.009) M(-1). The decrease in coumarin 1 fluorescence quantum yield upon the introduction of N-alkylazacrown moiety into position 3 is caused by an intramolecular photoinduced electron transfer from the nitrogen atom of macroheterocycle to the coumarin moiety, where the excitation is localized. The fluorescence quenching has an activation energy 2.32+/-0.05 kcal/mol in various hydrocarbons, and does not depend on the solvent viscosity. The fluorescence kinetics of free crowned coumarins in methanol is not monoexponential because of the existence of macrocycle conformers, or because of the hydrogen bond complex formation between the solvent and the nitrogen atom of macrocycle, in which the efficiency of intramolecular electron transfer is different. Upon complex formation with alkali and alkaline-earth metal cations and upon protonation, the fluorescence quantum yield increases and fluorescence decay becomes monoexponential.     

含喹啉基团的杯芳烃衍生物的合成及其Zn2+和Cu2+配合物光谱性质的研究

杯[4]芳烃下缘引入荧光基团,合成了一种具有荧光特性的新型杯[4]芳烃衍生物(25,27-二2-甲基喹啉杯[4]芳烃,简称MQBC)。通过红外光谱、元素分析、核磁共振氢谱、质谱等波谱分析确定其结构。研究了MQBC的紫外光谱和荧光光谱,并对其Zn2 和Cu2 配合物的荧光性质进行了分析。MQBC与Zn2 和Cu2 作用主要源于其杯式腔体下缘的氧原子提供孤对电子参与配位,紫外光谱实验发现MQBC与Zn2 作用后,226 nm处的吸收强度减弱,315 nm处吸收强度增强,同时测定了结合常数(K=2 064 L.mol-1)与结合比(n=1),MQBC有用于对微量Zn2 检测的前景。荧光光谱实验发现MQBC存在分子内光诱导电子转移过程(PET),导致其荧光较弱;当其与Zn2 和Cu2 生成配合物后,光诱导电子转移受阻,荧光增强。文章初步探讨了光诱导电子受阻作用机理,研究了Zn2 和Cu2 浓度对荧光强度的影响。     

Probing the metal-insulator phase transition in the (DMEDO-EBDT)2PF6 single crystal by optical measurements

The temperature and polarization dependence of the optical reflectivity spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT)(2)PF(6), have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (E(g)?>?0.1?eV) and transfer of the spectral weight accompanied by the metal-insulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase.     

关于氧化锌/双吡啶高氯酸盐非均相光诱导电子转移过程的研究

用ESR方法对9种不同双吡啶高氯酸盐与氧化锌组成的非均相体系的光诱导电子转移过程进行了系统的研究。ZnO-PQ⁺⁺-胶束-苯的分散系,经350nm~420nm的紫外光照后,PQ⁺⁺被还原成PQ⁺。具有大共轭结构的PQ⁺的ESR信号较强。胶束的存在也可使ESR信号增强。当体系充氧时,PQ⁺信号消失,并有超氧阴离子基O₂^·和羟基·OH被检出。     

Early-stage dynamics of light-matter interaction leading to the insulator-to-metal transition in a charge ordered organic crystal

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator α-(BEDT-TTF)2I3 were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.     

Tuning Photoinduced Intramolecular Electron Transfer by Electron Accepting and Donating Substituents in Oxazolones

The solvatochromic and spectral properties of oxazolone derivatives in various solvents were reported. Fluorescence spectra clearly showed positive and negative solvatochromism depending on substituents. The solvatochromic plots and quantum chemical computations at DFT-B3LYP/6-31?+?G(d,p) level were used to assess dipole moment changes between the ground and the first excited singlet-states. The electron accepting nitro substituent at the para-position increased the π-electron mobility, however, the 3,5-dinitro substituent decreased the π-electron mobility as a result of inverse accumulation of the electronic density as compared with that of its ground state. Experimental and computational studies proved that the photoinduced intramolecular electron transfer (PIET) is responsible for the observed solvatochromic effects. We demonstrate that PIET can be finely tailored by the position of the electron accepting and donating substituents in the phenyl ring of the oxazolone derivatives. We propose that the photoactive CPO derivatives are new molecular class of conjugated push-pull structures using azlactone moiety as the π-conjugated linker and may find applications in photovoltaic cells and light emitting diodes.     

Fluorescence quenching of 9-cyanoanthracene in presence of zinc tetraphenylporphyrin in a polar liquid medium

Steady-state and time-resolved techniques are used to study photoinduced electron and/or excitational energy transfer processes involved within a novel donor (zinc tetraphenylporphyrin)-acceptor (9-cyanoanthracene) system in a polar liquid medium (acetonitrile) at the ambient temperature (300 K). After photoexcitation of 9-cyanoanthracene, its fluorescence emission as well as lifetime are found to be quenched in presence of zinc tetraphenylporphyrin. The fluorescence quenching is ascribed to be due to the combined effect of electron transfer from zinc tetraphenylporphyrin to 9-cyanoanthracene and energy transfer (radiative as well as non-radiative) from 9-cyanoanthracene to zinc tetraphenylporphyrin. The highly exergonic values of Gibbs free energy change for both forward electron transfer reaction (−1.15 eV) and charge recombination reaction (−1.94 eV) indicate the possibilities of occurrences of these two processes in the Marcus inverted region. The fluorescence quenching rate due to photoinduced electron transfer reaction is found to be close to the diffusion-controlled limit within the present donor-acceptor system upon excitation of the acceptor molecules.     

光诱导细胞色素b5还原反应及机理的光谱学研究

在生命体内有关电子转移的研究备受关注,其中蛋白质以及酶之间的电子转移研究已成为热点,目前有关光诱导血红素蛋白还原的详细机理尚不明确,电子转移也许是解释这一机理的有效途径。通过紫外-可见吸收光谱、稳态荧光光谱和圆二色等光谱系统地研究了体外近生理溶液环境中,不同的紫外定波长、pH、游离氨基酸、谷胱甘肽、咪唑对Cyt b₅光照还原的影响,以阐明Cyt b₅不能用传统提出的机制去解释的光还原机制。结果表明: 在近紫外区通过直接的光激发高铁细胞色素b₅ (Cyt b₅)可以使其被还原成亚铁Cyt b₅,经过分析发现了Cyt b₅被光还原的机制和促进条件。用280 nm单色光照射Cyt b₅溶液,发现随着光照时间的延长,Cyt b₅溶液紫外-可见光谱中Soret带由412 nm红移到421 nm,Q带556 nm处吸收峰逐渐增强。Cyt b₅被光照后发生了类似化学还原剂作用的还原反应,同时发现了不同的光照波长、不同的pH、氨基酸种类以及各种其他配体等存在下对Cyt b₅的还原程度影响不同。采用280 nm波长、在偏碱性条件下光照Cyt b₅时还原程度最强;溶液中加入谷胱甘肽和咪唑能通过提供电子和氢供体途径促进Cyt b₅的光照还原反应发生;溶液中游离Met的存在能以最大速率促进光照还原反应的发生。基于以上结果提出Cyt b₅光还原的机理是从卟啉环到三价铁的电子转移,通过280 nm激发形成卟啉π阳离子基团和亚铁。采用稳态荧光光谱和圆二色谱对Cyt b₅光照还原发生前后进行检测,发现光诱导蛋白发生还原后,Cyt b₅主链结构逐渐伸展,二级结构发生了一定改变,α-螺旋含量逐渐降低,β-折叠含量逐渐升高,但是整个Cyt b₅的二级结构仍以α-螺旋为主。该研究结果不仅对了解光对含血红素蛋白（酶）的结构和功能的影响有重要的理论和实践意义,而且对于生命体内的氧化还原反应和电子传递机制提供一定理论依据。     

Cyclic Voltammetry and Electron Paramagnetic Resonance Study of the Electrochemical Reduction of P-Nitrobenzyl Bromide in Aprotic Solvents

EPR has been used to investigate the radicals postulated as intermediates in the intramolecular electron transfer and dehalogenation of p-nitrobenzyl bromide (p-NBBr) in DMSO and DMFA at room temperature. The electrochemical behavior has been studied through the use of cyclic voltammetry. According to the postulated reaction mechanism, the one-electron reduction of p-nitrobenzyl bromide generates an anion radical which undergoes breakage of the C-Br bond followed by intramolecular electron transfer. The free radical obtained is finally transformed into p-nitrotoluene.     

采用非绝热动力学模拟探究溶剂效应对桨-轮形状的给体-受体复合物的激发态弛豫动力学的影响

充分理解给体-受体复合物的激发态动力学在实验和理论上都具有非常重要的意义. 本文首次结合电子结构计算和非绝热动力学模拟,探索了最近合成的一个桨-轮形状的给体-受体复合物的光致动力学过程,该复合物由三个氟硼二吡咯基团和一个六氧杂并三苯构成. 根据计算结果,得出结论,桨-轮形状的给体-受体复合物共轭物在激发时将被提升到氟硼二吡咯片段的局部激发状态,然后发生超快局部激发电子态到电荷转移电子态的激子转移. 与先前实验工作中提出的光致电子转移机理不同,这种激子转移过程伴随着从氟硼二吡咯基团到六氧杂并三苯基团上的空穴. 此外,还发现溶剂效应在该体系的光致动力学中起着重要作用. 具体而言,强极性的乙腈溶剂加速了空穴转移动力学,这可以归因于溶剂对电荷转移状态的显著影响,即局部激发和电荷转移激子之间的能隙大大减小,而与此同时非绝热耦合增加,这两个因素都可以促进空穴转移过程. 目前的工作不仅为桨-轮形状的给体-受体复合物的潜在光诱导机制提供了有价值的见解,而且有助于未来设计具有更好光电性能的新型给体-受体复合物.     

Magnetic field effect as a probe of distance-dependent electron transfer in systems undergoing free diffusion

A theoretical analysis is presented of the problem of how distance-dependent electron transfer in photoinduced forward electron transfer followed by geminate backward electron transfer in liquid solution is reflected in the viscosity dependence of the magnetic field effect (MFE) on the efficiency of free radical formation (φ_ce) in such reactions. The stochastic Liouville equation formalism is employed to model the reaction behaviour of distance-distributed, triplet-born radical pairs (RPs) undergoing free diffusion, distance- and spin-dependent backward electron transfer, coherent and incoherent spin evolution in the ps time domain. In comparison with real systems the spin situation is simplified by reducing it to a two state (S, T ₀) problem, yet it is parametrized in a way that allows sensible comparison of the results with those of recent experiments. It is predicted that the MFE on φ_ce exhibits characteristic minima in the MFE versus viscosity curves, and it is verified in detail that this feature is peculiar to the diffusional model with distance-dependent electron transfer, i.e. cannot be reproduced with the simpler (‘exponential’) RP model employing distance-independent rate constants. Thus, the MFE versus viscosity curves are established as a genuine fingerprint of distance-dependent electron transfer. The theoretical results compare favourably with recent experimental results obtained with Ru^III complex/methylviologen RPs.