Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τ_d) between laser and microwave pulses, with τ_d ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.     

四氰基乙烯-四甲基乙烯体系光诱导电子转移双势阱的从头算研 究

以两态模型为基础,用从头算方法,在DZP[所有原子带极化函数的Dunning(9s,5p)/(3s,2p)]基组水平上对四氰基乙烯与四甲基乙烯间的电子转移进行理论计算。通过孤立给体和受体的几何构型优化,计算了给体的电离能和受体的电子亲和能。计算表明,在光诱导电荷分离之后的返回电子转移处于高放热的Marcus反转区。通过碰撞配合物的结构优化和电荷分离处理,在线性反应坐标近似下得到四甲基乙烯-四氰基乙烯配合物电荷分离反应的双势阱,进而获得反应热,键重组能,以及跃迁能。     

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。     

Molecular energy and electron transfer assemblies made of self-organized lipid-porphyrin bilayer vesicles

Novel molecular energy and electron transfer assemblies in vesicular form, which are made of self-organized amphiphilic porphyrins bearing phospholipid-like substituents (lipid-porphyrins), have been photochemically characterized. Tetraphenylporphyrin (TPP) derivatives with four dialkylphosphocholine groups [free-base (1 a), Zn(2+) complex (1 b), and Fe(3+) complex (1 c)] are spontaneously associated in water to form spherical unilamellar vesicles with a diameter of 100-150 nm. Exciton calculations based on the bilayered sheet model of 1 b, which has a porphyrin packing similar to that seen in the triclinic unit cell of the Zn(2+)TPP crystals, reproduced the Soret band bathochromic shift appearing in the aqueous solution of 1 b well. The UV/Vis absorption spectrum of the 1 a/1 b hybrid vesicles (molar ratio: 1/1) showed no electronic interaction between the two porphyrin chromophores in the ground state, but efficient intermolecular singlet-singlet energy transfer took place from the excited 1 b donors to the 1 a acceptor within the vesicle. Near-field scanning optical microspectroscopy of the 1 a/1 b vesicles on a graphite surface also showed only free-base porphyrin fluorescence. The efficiency of the energy transfer was 0.81 and the rate constant was 3.1 x 10(9) s(-1). On the other hand, protoporphyrin IX bearing two alkylphosphocholine propionates (2) was incorporated into the 1 a or 1 c bilayer vesicles (ca. 100 nm phi, molar ratio: 1 a/2 or 1 c/2=10). The UV/Vis absorption spectrum showed that 2 was successfully anchored into the fluid alkylene region of the membrane without stacking. Photoirradiation (lambda(ex): 390 nm) of the 1 c/2 vesicles in the presence of triethanolamine led a vectorial electron transfer from the outer aqueous phase to the membrane center, which allowed reduction of the ferric ion of the Fe(3+)TPP platform.     

Regioselective photoaddition of alcohols to 1,2,3-butatriene derivatives via photoinduced electron transfer

Photochemical polar addition of alcohols to 1,1-diaryl-1,2,3-butatriene derivatives in the presence of a catalytic amount of 9,10-dicyanoanthracene regioselectively afforded 1,1-diaryl-4-alkoxy-1,2-butadienes as addition products in good yields via photoinduced electron transfer.     

Semiconductor photoinduced cycloreversion of the dimer of methyl 2-naphthoate

The dimer of methyl 2-naphthoate (1) has been found to undergo efficient cyclorever-sion to its monomer, methyl naphtha)ene-2-carboxylate (2) on illuminated ZnO and TiO2 but not on CdS. An electron transfer and cation radical chain mechanism is proposed. Quantum yields, solvent effect, the role of oxygen, and the quenching of the reaction were investigated, and were consistent with the proposed mechanism.     

Electron transfer reactivity of O2+O system in low-spin coupling: Ab Initio study at electron correlation level

The electron transfer reactivity of the O₂+O system in low-spin coupling is studied at the second-order unrestricted Møller–Plesset (full)/6-311+G* basis set level by using different transition state structures. The properties and stabilities of the encounter complexes are compared for the five selected coupling structures: two T type, collinear, parallel, and crossing. The activation barriers and the coupling matrix elements are also calculated. The results indicate that the structures of the encounter complexes directly affect the electron transfer mechanism and rate. These encounter complexes are structurally unstable, the contact distances between the acceptor O₂ and the donor O are generally large, the interaction is weak, and the structures are floppy. The electronic transmission factor for the reacting system, O₂+O, is less than unity; thus, the electron transfer reaction is nonadiabatic in nature. Analysis of the dependence of relevant kinetic parameters on various influencing factors has shown that the effect of the solvent medium on the coupling matrix element is small but that on the electron transfer rate is very large. Among the five selected transition state structures, the electron transfer is more likely to take place via T₁-type and P-type structures. In the low-spin coupling the favorable electronic states for two reacting species are ¹∑(O₂) and X²Π_g(O) instead of X³∑(O₂) and X²π_g(O), which are favorable for the high-spin (quartet state) coupling mechanism. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 989–998, 1999     

Photoinduced electron transfer in ion pairs of cation-anion polymethine dyes

Photoinduced electron transfer in ion pairs of cation-anion polymethine dyes was studied by flash photolysis. The formation of radicals, which are the products of photoinduced transfer of an electron from an anion to a cation in the ion pairs, was observed during photoexcitation of a number of cation-anion dyes in nonpolar and some weakly polar solvents (in particular, in toluene and chloroform). Photoinduced electron transfer is also observed during triplet sensitization of ion pairs of the cation-anion dyes. The redox potentials of the cations and anions constituting the dyes were measured; the radical yields were compared with the free energies of photoinduced electron transfer. Photoinduced electron transfer in the systems under study was compared with similar process in cyanineborate ion pairs.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 878–884, May, 1995.The authors thank I. Ya. Levitin for help in measuring redox potentials.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-4217).     

含蒽醌生色团的水溶性聚合物的制备及其光致电子转移

采用1-氨基蒽醌经酰氯化,合成含双键的单体,再与丙烯酰胺单体共聚,随后对共聚物进行了Hofmann反应,制得含伯胺侧基和蒽醌生色团的水溶性聚合物．利用傅立叶红外光谱、核磁共振氢谱及紫外,可见光等测试手段对所合成的聚合物的结构进行了表征,并探讨了含蒽醌生色团的水溶性聚合物的光致电子转移性能．     

A novel photoinduced electron transfer reaction of N-alkylphenothiazines in carbon tetrachloride

U.V. irradiation of N-alkylphenothiazines (1a—c) in carbon tetrachloride produced 3-(phenothiazin-N'-yl-carbonyl)-N′-alkylphenothiazine (2a—b) and/or 3-(N′-alkylphenothiazin-3′-yl-carbonyl)-N-alkylphenothiazine (3b—c) together with the radical cation carbon tetrachloride salt 1~( ·)CCl_4~(-·)(4a—c), presumably via a photoinduced electron transfer mechanism.     

Tuning the photoinduced electron transfer in near-infrared heptamethine cyanine dyes

An efficient photoinduced electron transfer (PET) system in near-infrared region was described. The PET in heptamethine cyanine dyes was tuned by changing the electron-donating ability of the substituent at the central position of the polymehine chain. 4-Aminophenylthio-substitution led to an efficient PET and the lowest fluorescence quantum yield. The acetylation, protonation or transition metal cation coordination of the amino group could recover fluorescence greatly via suppressing the PET.     

Outer-sphere electron transfer in aqueous solutions of lithium hexacyanoferrates

Electron transfer between the hexacyanoferrate anions in the presence of the Li⁺ cations in aqueous solutions was studied by spectrophotometry. The results obtained were compared with the data for the K⁺- and Me₄N⁺-containing systems. The proposed explanation of the observed cationic catalysis effect of the thermal electron transfer in homogeneous and heterogeneous systems is based on the analysis of electron-transfer distances (viz., contact distances between the anions) in the activated complexes.     

Binding of 3O2 and 1O2 to dyes used in photodynamic therapy in gas phase and aqueous media

Density functional theory (DFT) was employed at the B3LYP/6‐31+G* level to study complexes of ¹O₂ and ³O₂ with the dye molecules proflavine, methylene blue, and acridine orange, which are useful in photodynamic therapy. It was found that the most stable complex between ¹O₂ and proflavine are formed when ¹O₂ is located above the central ring, while the most stable complex between ¹O₂ and methylene blue is formed when ¹O₂ is located above the molecular plane, but not above any of the rings, near the sulfur atom. ¹O₂ can make a stable complex with acridine orange, as it is located above the outer ring of the dye. The binding energies of the complexes of ¹O₂ with all three dyes are enhanced considerably in going from gas phase to aqueous media. The complexes of ³O₂ with the dyes will be unstable in all cases, while those of ¹O₂ with the same will be quite stable and will not be dissociated due to thermal fluctuations at room temperature. In the complexes of ¹O₂ and ³O₂ with the dyes, charge transfer occurs from the dyes to the O₂ moiety, the amount of charge transfer being much more to ¹O₂ than to ³O₂ in each case. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Effect of different metal ions in a new kind of functionalized self-assembled films of trinary complexes on its photoinduced electron transfer property

Three new kinds of self-assembled films were first fabricated through a new strategy. These films have Sn4 , Zr4 and Ce4 as bridging central ions, 3, 4, 9, 10-perylene tetracarboxylic acid (PTA) as functional molecules and oxalic acid as connecting ligand. All films were characterized by contact angle, UV spectra, X-ray photoelectron spectra (XPS) and cyclic voltammetry. The effects of light intensity, bias voltage, and electron donor on photocurrents of these films were also studied.     

Novel naphthalene-based fluorescent chemosensors for Cu2+ and Fe3+ in aqueous media

Three novel compounds bearing 2,7-dihydroxylnaphthalene capable of detecting Cu²⁺ or Fe³⁺ have been synthesised based on photoinduced electron transfer. The ability of these compounds for complex transition metal ions has been studied, and complex stoichiometry for Cu²⁺ and Fe³⁺ complex has been determined in the Tris–HCl (0.01 M DMSO/H₂O (v/v) 1:1, buffer, pH 7.4) solution system by fluorescence titration experiments. These chemosensors form a 1:1 complex with Cu²⁺ or Fe³⁺ and show a fluorescent quenching with a binding constant of (4.46 ± 0.29) × 10³ and (8.04 ± 0.26) × 10⁴, respectively.     

Intermolecular and intracomplex photoinduced electron transfer from planar and nonplanar metalloporphyrins to p-quinones

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP(2-)=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP(2-)=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO(-) of [Al(TPP)](+) and [Al(DPP)](+) was replaced by anthraquinone-2-carboxylate (AqCOO(-)) to afford the electron donor-acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO(-) to the Al(3+) ion of [Al(TPP)](+) and the existence of π-π interactions between AqCOO(-) and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO(-) moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)](+) and [Al(DPP)](+) to the AqCOO(-) moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.     

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.