Photochemical Properties of Supramolecular Dyad with Pyrenylethenylquinoline as a Photochrome

Spectral and photochemical properties of a bichromophoric dyad with two photochromes, 2-[(2-(pyren-1-yl)ethenyl]quinoline (PEQ) bridged by ten methylene groups have been investigated. Stationary spectroscopy does not show substantial interaction between the PEQ groups in the dyad, but the appearance of another, more long-living component in the f luorescence decay kinetics indicates possible excimer formation. The formation of the chemically active excimer is also confirmed by the intramolecular [2+2] photocycloaddition reaction to give tetrasubstituted cyclobutane with two vicinal pyrene substituents. This reaction competes with trans–cis photoisomerization of the dyad. It has been proposed that the π-stacking interaction of two PEQ groups with a long system of π-bonds plays an important role in the excimer formation followed by the photocycloaddition reaction.     

Covalently linked dimer of chlorophyll-a derivative with an amide bond and its folded conformer

A chlorophyll-a derivative homo-dimer covalently linked with a flexible methylene-amino-methylene group at the 3-position was benzoylated to give the dyad bearing a CH₂N(COPh)CH₂ linker. The synthetic dyad with the relatively rigid spacer showed red-shifted visible absorption bands and was oxidized more readily, in comparison with the corresponding monomer. The optical and electrochemical properties of the dimer are ascribable to the partial π-stacking of chlorin moieties in the benzoylated dyad, mimicking the special dimeric species in reaction centers of photochemical systems in natural phototrophs.     

Photochemical properties of the naphthol-styrylquinoline dyad in neutral and ionic forms

The photochemical properties of the naphthol-styrylquinoline dyad 2-(E)-{4-[4-(3-hydroxynaphthalen-2-yloxy)butoxy]styryl}quinoline (SQ4Np) have been investigated. It has been found that the excited-state acidity of the styrylquinoline (SQ) moiety is reduced by six orders of magnitude and that of the naphthol (Np) moiety increases by four orders of magnitude. As part of the dyad in the neutral and protonated forms, the SQ moiety retains a high photoisomerization quantum yield characteristic of model styrylquinoline. Deprotonation of the Np moiety of the dyad reduces the SQ photoisomerization quantum yields, presumably because of the formation of intramolecular complexes (exciplexes) or energy transfer to the Np anion.     

A High‐Energy Charge‐Separated State of 1.70 eV from a High‐Potential Donor–Acceptor Dyad: A Catalyst for Energy‐Demanding Photochemical Reactions

A high potential donor–acceptor dyad composed of zinc porphyrin bearing three meso‐pentafluorophenyl substituents covalently linked to C₆₀, as a novel dyad capable of generating charge‐separated states of high energy (potential) has been developed. The calculated energy of the charge‐separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin–fullerene dyad. Intramolecular photoinduced electron transfer leading to charge‐separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto‐ to nanosecond transient absorption techniques. The high energy stored in the form of charge‐separated states along with its persistence of about 50–60 ns makes this dyad a potential electron‐transporting catalyst to carry out energy‐demanding photochemical reactions. This type of high‐energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light‐to‐fuel products.     

Design and synthesis of a self-assembled photochemical dyad based on selective imidazole recognition

The unique recognition properties of phenanthroline-strapped zinc porphyrin 1, which displays extremely high affinity for N-unsubstituted imidazoles, has been used as the driving force for the assembly of a photochemical dyad involving a zinc(II) porphyrin as energy donor and a free base porphyrin as energy acceptor. The synthesis of the imidazole-substituted porphyrin is described together with the assembly of the dyad. (1)H NMR titrations confirm the formation of a 1/1 complex between 1 and 6, as well as insertion of the imidazole of the acceptor within the phenanthroline strap of the donor. Preliminary fluorescence quenching measurements show that efficient energy transfer occurs between the self-assembled components.     

The solution photochemistry of substituted cycloheptatrienes

The solution photochemistry of several ethoxycarbonyl- and ethoxycarbonyl-methoxy-substituted cycloheptatrienes has been investigated. The results have been used to analyse the directional specificity of ring-closure and 1,7-hydrogen migration reactions. This specificity is interpreted in terms of polarisation within the S₁ state which undergoes either (a) allowed photochemical reactions to give the observed products directly or (b) isomerisation to a trans, cis, cis-cyclohepatriene which then reacts via allowed ground-state processes. In at least two instances the directional specificity of the ring-closure reaction is wavelength dependent and this is attributed to the differing reactivities of the S₁ and S₂ states.     

Synthesis,photophysical properties,and photochemical activity of the water-soluble dyad based on fullerene С<Subscript>60</Subscript> and chlorin <Emphasis Type="Italic">e</Emphasis><Subscript>6</Subscript> derivatives

Chlorin e₆ derivative and water-soluble dyad resulting from covalent bonding of polyanionic fullerene С₆₀ derivative to chlorin e₆ derivative were synthesized and studied for spectral properties and photochemical activity. A considerable change in the absorption spectra and pronounced fluorescence quenching for the chlorin moiety included in the dyad were identified. The singlet excited state of chlorin is quenched via electron transfer from the excited chlorin to the fullerene core. A comparison of the photochemical activities of the test compounds in aqueous solutions showed a tenfold increase in the photochemical activity of the chlorin–fullerene dyad compared with free chlorin per absorbed light quantum.     

Synthesis of seven and higher membered nitrogen containing heterocycles using photochemical irradiation

Photochemical reactions have been applied for the synthesis of complex targets in many examples recently. In many cases, these processes provide access to unique modes of reactivity or offer unrivaled increases in molecular complexity. The key-features of photochemical reactions include increased selectivity, conversion, and yield and are beneficial for industrial and “green” processes. Despite these advantages, however, photochemical reactions in chemical production or R and D processes are rare. Most technical processes are limited to commodity chemicals and have been developed decades ago. The application of photochemical reactions for the synthesis of fine chemicals, natural products, and pharmaceutically active compounds, has become very popular. Photochemical reactions are used for organic synthesis and this review article highlighted the syntheses of heterocycles. Photochemistry is particularly fascinating and afforded an exotic charm due to its unconventional nature. In this review, I have given a clear idea of applicability of photochemical irradiations for the synthesis of a number of seven and higher membered N-heterocycles.     

(E)-2,5-二甲基-3-呋喃乙叉(异丙叉)-丁二酸酐的光发色和光消色反应过程

淡黄色的(E)-2,5-二甲基-3-呋喃乙叉(异丙叉)-丁二酸酐,1,在紫外光作用下同旋闭环生成深红色的闭环体,7,7,a-二氢苯骈呋喃-5,6-二羧酸酐,2,在Ar⁺激光(515nm)作用下,2同旋开环成1,由于在2的分子中无1-位氢,因此不发生与光消色反应相竞争的1,5氢迁移反应。用Fischer的方法测定了光化学反应1→2的光化学转化率,发现转化率是100%,证明光发色反应是定量的。对于用Ar⁺激光引起的光消色反应进行稳态的光化学分析处理,结果揭示在2的激发态和1的基态之间的某一个能级处有一个过渡态X,X分别以22.43ms^-1和1.11ms^-1的速率常数转变成化合物2和1,提出一个一般性的反应能量座标以解释俘精酸酐的光致色变过程,化合物1的光发色和光消色反应遵循两条不同的反应路线。     

Photoablation experiments of thin polysilane-films prepared by the Langmuir-Blodgett-Kuhn technique

We give a brief survey of our recent research on polysilane Langmuir-Blodgett-Kuhn films. Surface-plasmon experiments, IR and flourescence-lifetime measurements have been performed to study the complex photophysical and photochemical processes in these materials following UV irradiation. Quenching effects of the polysilane backbone and their influence on the ablation-kinetics are discussed.     

Synthesis and photochemical properties of porphyrie-quinone compounds

The results of studies on the synthesis of porphyrin-quinone compounds and investigation of their photochemical properties are summarized. Effects of various factors (the redox potential, the distance between donor and acceptor moieties, their spatial orientation, the free energy of the reaction, and solvents) on the photoinduced electron transfer in these model systems are discussed. The dyad and triad model systems have been compared. The possibility of using these systems for modeling the primary steps of photosynthesis is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1. pp. 9–24, January, 1996.     

Photoinduced Electron Transfer in a Distyryl BODIPY–Fullerene Dyad

A novel distyryl BODIPY–fullerene dyad is prepared. Upon excitation at the distyryl BODIPY moiety, the dyad undergoes photoinduced electron transfer to give a charge‐separated state with lifetimes of 476 ps and 730 ps in polar (benzonitrile) and nonpolar (toluene) solvents, respectively. Transient absorption measurements show the formation of the triplet excited state of distyryl BODIPY in the dyad, which is populated from charge‐recombination processes in both solvents.     

C60富勒烯-哌啶硫代荒酸酯稠合体的合成与三线态特征

C₆₀富勒烯与2-(哌啶-硫代荒酸酯)-1,3-丁二烯通过Diels-Alder环加成反应得到C₆₀富勒烯-哌啶硫代荒酸酯稠合体,运用现代波谱技术等确定了产物结构;用半经验方法PM3和AM1计算预测环加成反应性和C₆₀富勒烯-哌啶硫代荒酸酯稠合体的性能.激光光解时间分辨技术初步探究了单加成的C₆₀富勒烯-哌啶硫代荒酸酯稠合体(C₆₀-PX)三线态特征以及与四-(2-噻吩基)-四硫富瓦烯(TT-TTF)分子间的光诱导电子转移反应.     

Self‐Assembled Architectures with Segregated Donor and Acceptor Units of a Dyad Based on a Monopyrrolo‐Annulated TTF–PTM Radical

An electron donor–acceptor dyad based on a polychlorotriphenylmethyl (PTM) radical subunit linked to a tetrathiafulvalene (TTF) unit through a π‐conjugated N‐phenyl–pyrrole–vinylene bridge has been synthesized and characterized. The intramolecular electron transfer process and magnetic properties of the radical dyad have been evaluated by cyclic voltammetry, UV/Vis spectroscopy, vibrational spectroscopy, and ESR spectroscopy in solution and in the solid state. The self‐assembling abilities of the radical dyad and of its protonated non‐radical analogue have been investigated by X‐ray crystallographic analysis, which revealed that the radical dyad produced a supramolecular architecture with segregated donor and acceptor units in which the TTF subunits were arranged in 1D herringbone‐type stacks. Analysis of the X‐ray data at different temperatures suggests that the two inequivalent molecules that form the asymmetric unit of the crystal of the radical dyad evolve into an opposite degree of electronic delocalization as the temperature decreases.     

Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad.

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (P(H2)) porphyrin. In the resulting dyads (P(Zn)-Sp(c) and P(H2)-Sp(c)), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp(o)) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn-Sp(o). The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Sp(o). The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1P(H2)-Sp(o) and P(H2)-1Sp(o) exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.     

A modified method for calculating quantum yields of photochemical reactions

The previously proposed approach to calculation of quantum yields of photochemical reactions has been improved. The new version uses a different procedure of accounting for the asymmetry (anharmonicity) of potential wells of the molecules involved in the photochemical conversion. The form of a doubleminimum potential energy surface has been considered in a more correct manner. It has been shown that the number of model parameters is reduced by two thirds compared with the model used previously, adequately reflecting the characteristics of structural transformations of polyatomic molecules in which a relatively small part of the total number of coordinates are transformed. The quantum yields of photochemical transformations of six dienes into their cyclic isomers have been calculated. Quantitative agreement of the calculated values with the experimental data has been achieved for all the reactions. It has been shown that the model parameters have good transferability in a series of related molecules, thereby rendering the simulation of photochemical processes by the method in question predictive.     

Fullerene–Coumarin Dyad as a Selective Metal Receptor: Synthesis,Photophysical Properties,Electrochemistry and Ion Binding Studies

A coumarin derivative with a malonate unit has been synthesized and used for the preparation of a fullerene–coumarin dyad through the Bingel cyclopropanation method. The newly synthesized dyad is soluble in organic solvents and has been fully characterized with traditional spectroscopic techniques. Electronic interactions between the two components of the dyad were probed with the aid of UV/Vis spectroscopy, fluorescence emission, and electrochemistry measurements. Our studies clearly show the presence of electronic interactions between C₆₀ and modified coumarin in the ground state; efficient electron‐transfer quenching of the singlet excited state of the coumarin moiety by the appended fullerene sphere was also observed. Time‐resolved fluorescence measurements revealed lifetimes for the coumarin–C₆₀ dyad at a maximum of 50 ps, while the quantum yield was reaching unity. Additionally, the redox potentials of the C₆₀–coumarin dyad were determined and the energetics of the electron‐transfer processes were evaluated. Finally, after alkaline treatment of C₆₀–coumarin, which resulted in the deprotection of carboxylate units, the dyad was tested as a metal receptor for divalent metal cations; ion competition studies and fluorescence experiments showed binding selectivity for lead ions.     

Photoreduction dehalogenation of polychlorobenzenes by ethanol

It has been demonstrated that in ethanolic solutions in the presence of oxygen the photochemical and photocatalytic consecutive dehalogenation of hexachlorobenzene to benzene can take place. The kinetic rules governing the processes have been established and the possible mechanisms discussed.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 30, No. 1, pp. 34–38, March–April, 1994.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N-DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ_F) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that ^1,3(n,π*) state(s) has(have) changed order relative to a lowest ¹(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as ³(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ_F and φ_T1 (φ_isc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.     

Spectral, luminescent, and photochemical properties of o-hydroxyazomethinespiropyrans in solid matrices

Spectral, luminescent, and photochemical properties of the novel bifunctional compounds BF1 and BF2 based on indoline spiropyrans and o-hydroxyazomethines have been studied in solid media. In PMMA films and frozen solutions, redistribution of photoexcitation relaxation channels in favor of photochemical processes leading to photochromic products in the azomethine moiety and luminescence is observed as compared with liquid solutions.