Geometrical Photoisomerization Research Articles

Light, Switch, Action! The Influence of Geometrical Photoisomerization in an Adaptive Self-Assembled System.

The ubiquitous ability of natural dynamic nanostructures to adapt to environmental changes is a highly desirable property for chemical systems, particularly in the development of complex matter, molecular machines, and life-like materials. Designing such systems is challenging due to the generation of complex mixtures with responses that are difficult to predict, characterize, and diversify. Here, we navigate between self-assembled architectures using light by operating an intrinsic photoswitchable building block that governs the state of the system. When complementary units are present, the photoswitch determines the predominant architecture, reversibly adapting between the cage and macrocycles, including (otherwise inaccessible) higher-energy assemblies. Our study showcases this concept with seven different transformations, offering an unprecedented degree of control, diversification, and adaptation by self-selecting complementary units. These findings could enable applications of on-demand dissipative macrocycles based on dynamic bonds. We also envision different transient nanostructures, e.g., reticular and polymeric materials, being explored by fine-tuning the nature of the complementary unit.

Photoisomerization of 1,2-Di(4-pyridyl)ethylene According to NMR and UV Spectroscopy Data and Density Functional Modeling.

Photoisomerization of 1,2-di(4-pyridyl)ethylene under UV irradiation was studied by NMR and UV spectroscopy in solutions of acetone, acetonitrile and chloroform. It has been established that in the studied solutions under irradiation, geometric photoisomerization occurs: the trans-isomer transforms into the cis-isomer and then cyclization occurs. DFT modeling of the listed processes, states and UV absorption spectra of the sample in an acetonitrile solution was performed.

The photochemistry and photophysics of all-trans-1,4-diindanylidenyl-2-butene, a rigid analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene

all-trans-1,4-Diindanylidenyl-2-butene (ttt-stiff-5-DPH), a torsionally constrained analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH), was synthesized and studied in order to evaluate the role of phenyl-vinyl torsional motions in the photophysical and photochemical responses of the DPH chromophore. Spectroscopic and photoisomerization measurements reveal that the behavior of the rigid DPH analogue is very similar to that of the parent DPH. This similarity is obtained despite the fact that the alkyl substitution from the five-membered rings selectively lowers the energy of the 1 (1)B(u)* state, leading to inversion of the order of the 1 (1)B(u)* and 2 (1)A(g)* energy levels in hydrocarbon solvents. In stiff-5-DPH, as in DPH, an increase in solvent polarity enhances terminal over central bond photoisomerization. Analyses of fluorescence and photoisomerization quantum yields show that, as in DPH, the torsional relaxation channel on the singlet excited state manifold is inefficient, falling far short of accounting for all radiationless decay. Significant (approximately 50 and 80% of all singlet decay in Bz and AN, respectively), photochemically unproductive, radiationless decay channels exist in both molecules. Competing one bond photoisomerizations give the two major photoproducts: tct-stiff-5-DPH and ctt-stiff-5-DPH. They were isolated in pure form and were spectroscopically characterized. Biacetyl-sensitization was used to study the behavior of the stiff-5-DPH triplet state. As in the parent DPH, stiff-5-DPH triplets undergo relatively efficient concentration dependent geometric photoisomerization.

Conformational equilibria in EE-1,3-di-(3′-thienylethenyl)benzene: One-way adiabatic interconversion of rotamers in S1 and their excited state properties

A refined procedure of the kinetic fluorescence analysis (KFA) (accounting for the lack of isoemissive points in the fluorescence spectra) has been applied to study the rotamerism of the EE isomer of 1,3-di-(3′-thienylethenyl)benzene [(3TE)2B]. The effect of the excitation wavelength and temperature on the fluorescence spectra, quantum yields and decay profiles allowed the excited state properties (radiative and reactive) of the two most abundant conformers (rotamers), implying the simultaneous, restricted rotation around the quasi-single bonds of the ethenic bridges with both the central ring and the 3′-thienyl groups, to be obtained. At high temperature (above 320K), another activated process of the lowest excited singlet state, implying the interconversion of the short-lived rotamer into the longer-lived one, is operative. The EE→EZ photoisomerization was also investigated as a function of temperature.The present results lead to a complete and quantitative description of the conformational equilibrium including the radiative and radiationless relaxation rate parameters of the lowest excited states of the two rotamers. A simultaneous singlet and triplet mechanism for the geometrical photoisomerization, with an involvement of the former above room temperature only, seems to reasonably account for the experimental results. The role played by the rotamers in the photoreactive behaviour of (3TE)2B is also discussed.

Crown-containing butadienyl dyes. 5. Structure and photoisomerization of a butadienyl dye containing a 15-crown-5 fragment

The structure of the E,E isomer of a butadienyl dye of the benzothiazole series containing a 15-crown-5 fragment was established by X-ray diffraction analysis. The stacking motif of the dye molecules in the crystal structure is, apparently, attributable to intermolecular π-interactions between the large conjugated fragments. The possibility of [2+2] photocycloaddition occurring in the crystal is discussed. Reversible geometrical photoisomerization of the dye and its complex with Mg2+ in acetonitrile solution was studied by spectrophotometry and 1H NMR spectroscopy. Photoirradiation of the complex of the E,E isomer with Mg2+ leads predominantly to isomerization of the C=C bond adjacent to the benzothiazole fragment. Regioselective E,E→Z,E photoisomerization occurs via a singlet mechanism with a quantum yield of about 0.45. The quantum yield of reverse Z,E→E,E photoisomerization is approximately 0.52. The structures of different geometrical isomers of the dye complex with Mg2+ were calculated by the quantum-chemical density functional theory (DFT).

Cyclodextrin mediated enantio and diastereoselective geometric photoisomerization of diphenylcyclopropane and its derivatives

cis-Diphenylcyclopropanes upon direct excitation and triplet sensitization undergo geometric isomerization to the corresponding trans isomers. In solution the trans isomers are formed as a 1:1 enantiomeric or diasteromeric mixture. Upon inclusion within β-cyclodextrin the same molecules give the trans isomers enriched in one optical isomer. Enantiomeric excess and diasteromeric excess induced by the optically active host β-cyclodextrin although small is mechanistically significant.

Electron transfer mediated geometrical photoisomerization of α,β-unsaturated ketones in the presence of electron donors in solution

The phototransformation of α,β-unsaturated ketones ( I– IV) in the presence of electron donor triphenylamine in polar media has been investigated by using the chemically induced dynamic nuclear polarization (CIDNP) technique. For these compounds, it was demonstrated, that the electron transfer mechanism of isomerization results only in cis-isomers formation. Isomerization occurs in the triplet excited states of I– IV arising from back electron transfer in the triplet radical ion pairs of radical ions of ketone and triphenylamine. The set of isomers formed via this mechanism differs from phototransformation products observed after direct photolysis of these ketones in homogeneous and organized media.

The influence of electron donors and acceptors on the isomer distribution in the photolysis of β-ionone

The application of the CIDNP technique has shown that geometrical cis-trans photoisomerization of β-ionone and its isomers in the presence of pyrene in polar media occurs via radical ion pairs. Based on the combined analysis of CIDNP results and kinetic data on the formation and decay of isomers, it has been demonstrated that this channel exerts a significant influence on the isomer distribution in this system and on the reaction with other electron donors and acceptors.

Enantiodifferentiating photoisomerization of (Z)-cyclooctene sensitized by chiral C^2-symmetric phosphoramide

The optically active C2-symmetric phosphoramides (2) and (3) were employed as chiral sensitizers for the geometrical photoisomerization of (Z)-cyclooctene (1Z) in pentane at varying temperatures. At low temperature (–56˚C), the achieved enantiomeric excesses (e.e.) for (1Z) are 20 and 28%, respectively. Manuscript received: 14 March 2001.

Trapped Optically Active (E)-Cycloheptene Generated by Enantiodifferentiating Z−E Photoisomerization of Cycloheptene Sensitized by Chiral Aromatic Esters

Highly strained, optically active (E)-cycloheptene (1E) was prepared for the first time in the enantiodifferentiating geometrical photoisomerization of the (Z)-isomer (1Z) sensitized by chiral benzenetetracarboxylates at −40 to −80 °C. Low-temperature irradiations of 1Z in the presence of the chiral sensitizer, and subsequent stereospecific trapping of the optically active photoproduct, 1E, through a Diels−Alder reaction with 1,3-diphenylisobenzofuran or by oxidation with OsO4, afforded the cycloadduct or trans-1,2-cycloheptanediol, respectively. The enantiomeric excesses (ee's) of the two products were subsequently determined by chiral HPLC or GC. The ee of the product, which was used as a measure of the efficiency of chirality transfer in the excited state, was found to depend critically not only on the chiral sensitizer employed but also on the temperature and solvent employed. Thus, the ee of the product was doubled in an extreme case simply by changing the solvent from dichloromethane to hexane. Furt...

Geometrical photoisomerization of (Z )-cyclooctene sensitized by aromatic phosphate, phosphonate, phosphinate, phosphine oxide and chiral phosphoryl esters

Aromatic phosphate, phosphonate, phosphinate and phosphine oxide have been used as effective sensitizers for the geometrical photoisomerization of (Z)-cyclooctene (1Z) to its highly strained (E)-isomer (1E). Photosensitizations with phosphate and phosphonate gave moderate photostationary-state E/Z ratios of 0.14–0.17. A comparative study of the fluorescence quenching experiments and kinetic analysis of the product yield demonstrate that the photosensitized isomerization proceeds through a mixed mechanism that involves both singlet and triplet excited states. Triphenylphosphine oxide is not fluorescent and gave a low photostationary-state E/Z ratio of 0.05. Using (–)-menthyl or (–)-bornyl phosphate, phosphonate and phosphinate as chiral sensitizers, the enantiodifferentiating photoisomerization of 1Z was performed to afford optically active 1E in enantiomeric excesses ⩽5%.

ChemInform Abstract: Direct and Sensitized Geometrical Photoisomerization of 1‐ Methylcyclooctene.

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Direct and sensitized geometrical photoisomerization of 1-methylcyclooctene

Effects of methylation at C(1) of cyclooctene on the ground-state structure, potential energy surfaces, and photochemical behaviour in the singlet and triplet manifolds have been investigated. Molecular mechanics calculations using the MM2 force field reveal that, as compared with the parent (E)-cyclooctene, methylation increases the strain of the (E)-isomer by 8.9 kJ mol–1, which is however released in part through the minimized steric repulsion between one of the ring methylenes and the introduced methyl by increasing the dihedral angle C(Me)–C(1)C(2)–C(3).

Emission control of a pyrene-thioindigo compound

Two distinct fluorescence modes of a thioindigo compound substituted with two pyrene groups on the 7 and 7′ positions were controlled by geometrical photoisomerization of the thioindigo skeleton.

Homogeneous acid catalysis of the trans—cis photoisomerization reaction of trans-3-(2-hydroxy- benzylidene)-4,5-dihydrofuran-2(3H)-one

The geometrical photoisomerization reaction of the title molecule ( trans-I) has been studied in methanol/water 90/10 wt.% mixtures containing sulphuric or hydrochloric acid. Quantum yields of the process, measured under acidic conditions in the range 0.095 N < [H +] <4.0 N, reach a maximum at pH ≈ 0. The effect of the counterion has also been investigated. On the basis of the fluorescence behaviour of trans-I as a function of pH, experimental quantum yields have been interpreted in the light of prototropic equilibria in the excited state. Mechanistic models are proposed which are able to fit results obtained in the investigated experimental conditions.

Silver(I) perturbation of (E)–(Z) photoisomerization of stilbene and azobenzene

The influence of AgClO4 on the photochemical behaviour of stilbene and azobenzene in acetonitrile and methanol has been compared with that of Nal, a well known perturber of the excited-state properties via the heavy-atom effect. Both the salts quench the fluorescence and the geometrical photoisomerization yields of stilbene, the effect being greater in CH3OH. l– has no effect on the photoisomerization of azobenzene whatever the solvent, while Ag+ increases the isomerization yield in CH3OH but has no effect in CH3CN. l– interacts with the singlet excited state of stilbene and l–via a charge-transfer complex which favours (owing to the heavy-atom effect) ISC to the isomerizable triplet state of the alkene. The lack of effect of l– on the azobenzene photoisomerization probably results from its exceedingly short lifetime. Ag+ forms a ground-state complex with stilbene and azobenzene in methanol solution, as is apparent from the variations of the absorption spectrum of the two aromatics. The light excitation of the complex causes (E)→(Z) isomerization of the bond chromophore.

Effect of the binding of bilirubin to either the first class or the second class of binding sites of the human serum albumin molecule on its photochemical reaction.

The kinetics of the photochemical changes of bilirubin were studied at a constant concentration of bilirubin bound either to the first class or to the second class of binding sites of the human serum albumin molecule. The more the bilirubin binds to the first class of binding sites in the human serum albumin molecule, the more readily geometric photoequilibrium to give (ZE)-bilirubin takes place. The more the bilirubin binds to the second class of binding sites or allosterically transformed binding sites induced by added SDS, the more readily structural photoisomerization, i.e. the formation of (EZ)-cyclobilirubin, takes place. When the serum bilirubin concentration is at low, safe, values bilirubin binds exclusively to the first class of binding sites and serves as an antioxidant [Onishi, Yamakawa & Ogawa (1971) Perinatology 1, 373-379]; at these concentrations human serum albumin protects bilirubin from irreversible photodegradation by only allowing readily reversible geometric photoisomerization. As the serum bilirubin concentration increases to high, and potentially dangerous, values, bilirubin binds to the second class of binding sites, and under these conditions human serum albumin seems to promote the photocyclization of bilirubin. During irradiation human serum albumin seems to act by retaining low, useful, concentrations of bilirubin while facilitating irreversible photoisomerization of excess bilirubin.

Bilirubin photoconversion induced by monochromatic laser radiation. Comparison between aerobic and anaerobic experiments in vitro.

Structural and geometric photoisomerization of bilirubin bound to human serum albumin was investigated. Solutions were irradiated with monochromatic light emitted by an Ar ion laser, the 457.9, 488.0 and 514.5 nm wavelengths being selected. Photoproducts were separated and analysed by h.p.l.c. Visible-absorption spectra of pure ZZ-bilirubin, ZE-bilirubin and lumirubin in the eluent were registered in the 350-550 nm region by collecting single fractions by h.p.l.c. Wavelength-dependence of bilirubin photoconversion was studied within photoequilibrium and up to a large decrement of the total concentration. Experiments were performed in aerobic and anaerobic conditions in order to assess the contribution of the photo-oxidation to the overall process. The presence of O2 was found to increase the rate of bilirubin degradation and unexpectedly to favour lumirubin production. The ability of 514.5 nm irradiation to induce bilirubin cyclization was definitively confirmed.

Wavelength dependence of the geometric and structural photoisomerization of bilirubin bound to human serum albumin.

The two quantitatively important photoisomers in bilirubin metabolism during phototherapy are (ZE)-bilirubin and (EZ)-cyclobilirubin. We describe in vitro studies on the wavelength dependence for the geometric (delta 4Z, delta 15Z----delta 4Z, delta 15E) and structural (endovinyl cyclization) photoisomerization of bilirubin bound to human serum albumin by a high performance liquid chromatography method. For the geometric photoisomerization from (ZZ)-bilirubin to (ZE)-bilirubin, the most effective wavelength in vitro was 410 nm. For the structural photoisomerization, green light at 510 nm is the most efficient for causing cyclization of (ZZ)-bilirubin to (EZ)-cyclobilirubin via (EZ)-bilirubin and this may depend on a larger cross-section of (EZ)-bilirubin than (ZZ)-bilirubin in this spectral region and/or on a larger quantum yield for cyclization than geometric photoisomerization.

On the role of singlet oxygen in the self-sensitized photo-oxygenation of bilirubin and its pyrromethenone models

4-Ethyl-3-methyl-5-(4-ethyl-3,5-dimethylpyrrolyl-2-methylidene)-3-pyrrolin-2-one ( 1), 3,4- diethyl-5-(4-ethyl-3,5-dimethylpyrrolyl-2-methylidene)-3-pyrrolin-2-one ( 2) and xanthobilirubinic acid (XBR) undergo self-sensitized photo-oxygenation, as does bilirubin. The reaction products of 1 consist of methylethylmaleimide, kryptopyrrole aldehyde and its photo-oxygenation product and oxygenated dipyrroles. The same products are formed from 2, except xeronimide is found instead of methylethyl-maleimide. Kinetic studies indicate that singlet oxygen is not necessarily involved in the photo-oxygenation reactions. Geometric photoisomerization ( Z →2 E) of 1 and 2, and XBR proceeds at an even faster rate than photosensitized oxygenation.