Solvent dependence of the photophysical properties of 2-chlorothioxanthone, the principal photoproduct of chlorprothixene

2‐chlorothioxanthone (CTX) is used as photoinitiator for the reticulation of synthetic resins and for the preparation of pharmaceuticals. It was previously determined that CTX is the primary photoproduct of z‐chlorprothixene (CPTX) when irradiated at 313 nm and is formed in an autocatalyzed reaction through an energy‐transfer mechanism (Piñero et al. [2009] Photochem. Photobiol., 85 , 895–900). In this work, the photophysical properties of CTX were measured in acetonitrile/water solutions to determine if their magnitude can affect the side effects of CPTX. The results show that CTX has higher absorption coefficients in the visible region (400–420 nm) and higher triplet quantum yields than its parent compound. Similar to TX, both properties strongly depend on the solvent polarity/hydroxylicity. The quantum yield of the triplet intermediate is very close to the value of the phenothiazine triplets. The phenothiazines are the most phototoxic antidepressants. Therefore, given the appropriate microenvironment, the photosensitization side effects of CPTX can be intensified on the production of CTX.     

Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory

The E and Z geometric isomers of a stable silene (tBu₂MeSi)(tBuMe₂Si)Si=CH(1‐Ad) ( 1 ) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z ? 1E isomerization are: E_a=24.4 kcal mol^?1, ΔH^≠=23.7 kcal mol^?1, ΔS^≠=?13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86‐D3BJ/def2‐TZVP(‐f)//BP86‐D3BJ/def2‐TZVP(‐f)) activation parameters, the Z?E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration–rotation–migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.     

Photochromic fulgides: Transformation of the non‐photochromic (Z)‐isomer of a fulgide into a highly photochromic (E)‐isomer via structural modification involving enhanced conjugation

The non‐photochromic fulgide (1‐Z) has been successfully converted into the highly photochromic ( 3‐Z ) analogue. A dicyanomethylene group was introduced at the 5‐position of 1‐Z in order to enhance the latter's conjugation properties that would facilitate the photochemical Z→E isomerization process. The irradiation of the product 3‐Z with a UV light at λ_max 350 nm formed a bluish green solution which absorbed at λ_max 620 nm, corresponding to the ring‐closed product 4. The latter was also formed from the reference dicyanomethylene product 3‐E synthesized from 1‐E. The irradiation of 4 at λ_max 532 nm produced the reversion to the original pale yellow color of 3‐E.     

Photoresponsive Formation of an Intermolecular Minimal G‐Quadruplex Motif

The ability of three different bifunctional azobenzene linkers to enable the photoreversible formation of a defined intermolecular two‐tetrad G‐quadruplex upon UV/Vis irradiation was investigated. Circular dichroism and NMR spectroscopic data showed the formation of G‐quadruplexes with K⁺ ions at room temperature in all three cases with the corresponding azobenzene linker in an E conformation. However, only the para–para‐substituted azobenzene derivative enables photoswitching between a nonpolymorphic, stacked, tetramolecular G‐quadruplex and an unstructured state after E–Z isomerization.     

Mesomorphism,polymerization, and chirality induction in α‐cyanostilbene‐functionalized diacetylene‐assembled films: Photo‐triggered Z/E isomerization

Engineering of molecular stacking arrangement via environmental stimuli is of particular interest in stimuli‐responsive self‐assembling architectures. A novel dual photo‐functionalized diacetylene ((Z)‐CNBE‐DA) molecule was synthesized, in which photo‐responsive cyanostilbene moieties exhibited interesting Z‐E isomerization upon UV light irradiation and could be utilized to modulate mesomorphism, molecular stacking arrangement and resulting polymerization behavior. Rod‐like (Z)‐CNBE‐DA could self‐assemble into well‐defined lamellar structures and the helical polydiacetylene (PDA) chains could be formed upon irradiation with circularly polarized ultraviolet light (CPUL). However, the bent‐shaped (E)‐CNBE‐DA molecules only self‐assembled into irregular loose packing, inhibiting the formation of ordered helical PDA chains upon CPUL irradiation. In this work, we established the links between chemical structures, molecular packing engineering and photophysical properties, which would be of great fundamental value for the rational design of smart soft materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2458–2466     

2‐(4′‐Pyridyl‐N‐oxide)‐Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl‐Extended Calix[4]pyrrole Receptor

We report the synthesis of two 2‐(4′‐pyridyl‐N‐oxide)‐substituted hemithioindigos (HTIs). We probed their photoisomerization by using UV/Vis and ¹H NMR spectroscopy techniques. Light irradiation at λ=450 nm provoked the isomerization of the HTI Z isomer to the E counterpart to a large extent (≈80 % at the photostationary state). ¹H NMR titration experiments revealed the formation of thermodynamically and kinetically stable 1:1 inclusion complexes of the (Z)‐HTI isomers with a super aryl‐extended host (association constant>10⁴ m ^?1). Photoirradiation at λ=450 nm of the inclusion complexes induced the isomerization of the bound HTI N‐oxide to afford the (E)‐HTI?calix[4]pyrrole complex. We determined accurate association constant values for the 1:1 inclusion complexes of the (Z)‐ and (E)‐HTI isomers by using isothermal titration calorimetry experiments. The results showed that the stability constants of the (E)‐HTI complexes were 2.2–2.8‐fold lower than those of the (Z)‐HTI counterparts, which explains the lack of light‐induced release of the former to the bulk solution.     

Patterned retarders prepared by photoisomerization and photopolymerization of liquid crystalline films

Isomerizable diacrylates derived from cinnamic acid are designed, synthesized and mixed with liquid crystalline diacrylates with the aim of making films with alternating birefringent and isotropic domains by applying the E–Z isomerization process at room temperature. The effects of the structure of the isomerizable‐mesogenic group on the isotropization efficacy, the efficiency of the E–Z isomerization reaction, and film formation are discussed. Compounds derived from cyclohexyl cinnamate are proved to be good candidates that meet a whole set of parameters related to processing and application. These compounds exhibit a low nematic‐to‐isotropic transition temperature. In addition, they show no yellowing upon irradiation, unlike similar compounds derived from phenyl cinnamate. To elucidate the origin of isotropization of the film by irradiation, the pure Z‐isomer is prepared by photolysis of the E‐isomer and subsequent chromatographic separation of both isomers. Analysis of reference samples containing the pure isomers reveals that the decrease in transition temperature can be attributed exclusively to the E–Z photoisomerization process. Finally, thin films with alternating birefringent and isotropic parts of 100×100 µm² are obtained by using a combination of photoisomerization in air and photopolymerization in a nitrogen atmosphere, which is referred to as photo‐patterning.     

Understanding the electronic reorganization in the thermal isomerization reaction of trans‐3,4‐dimethylcyclobutene. Origins of outward Pseudodiradical {2n + 2π} torquoselectivity

The thermal isomerization reaction of trans‐3,4‐dimethylcyclobutene (1,2,3,4‐DMC) to produce the isomer (2E, 4E)‐hexadiene have been studied using density functional theory at the B3LYP/6‐31+G level. For this reaction, two different channels of the conrotatory torquoselectivity allowing the formation of the two isomeric (E,E) and (Z,Z) have been characterized. The isomer (Z,Z) occurs through the inward conrotatory mechanism, whereas the isomer (E,E) occurs through the outward conrotatory mechanism. The outward conrotatory is favored by 11.3 kcal/mol with respect to inward conrotatory one. This behavior is consistent with the thermodynamic properties: enthalpy, free energy, and entropy calculated in both reaction pathways. The topological analysis of the electron localization function at the outward conrotatory transition state explicated the electronic reorganization through a pseudodiradical {2n + 2π} process and not a pericyclic reorganization. © 2012 Wiley Periodicals, Inc.     

Does one‐photon photocyclization of trans‐diarylethylenes involve adiabatic trans‐to‐cis photoisomerization? potential energy surface calculations for 1‐styrylnaphthalene

Potential energy surface (PES) for 1‐styrylnaphthalene was calculated by PM3 method for the S₀ state and PM3‐CI(2x2) method with configuration interaction for the S₁ state. Scanning PES along both isomerization and cyclization reaction coordinates enabled to reveal the minimum energy path (MEP) with low barriers on the S₁ PES from E‐isomer to dihydrocyclophotoproduct (DHP). This is consistent with formation of the photocyclization product in one‐photon process during irradiation of E‐isomer. Additionally, the MEP was found to bypass the coordinate region of Z‐isomer, i.e. one‐photon E‐isomer‐to‐DHP photocyclization does not demand participation of the excited Z‐isomer. Therefore, adiabatic trans‐to‐cis isomerization is likely not an intermediate stage on the E‐isomer photocyclization pathway, and experimentally observed one‐photon formation of the DHP from the E‐isomer is likely not an evidence for adiabatic trans‐to‐cis photoisomerization, as it is usually assumed. According to the results obtained, two photochemical reactions of E‐isomer, photoisomerization to Z‐isomer and photocyclization to DHP, are not consecutive but parallel reactions with branching at perpendicular conformer on the S₁ PES. © 2012 Wiley Periodicals, Inc.     

Crystal Structure and Properties of a New 2, 2'‐Ortho(metaxylene) E‐Bridged Stilbenophane

The synthesis and characterization of novel E and Z‐2, 2'‐ortho(metaxylene)‐bridged stilbenophane 2 by reductive McMurry condensation are described and the X‐ray structure determination of the E‐isomer is reported. The structure analysis of 2 ‐E shows weak hydrogen bonding. The complexation of Na⁺ ions in 2, 2'‐ortho(metaxylene)‐Z‐bridged stilbenophane using sodium chloride and measurements of conductivity were performed. The formation constant logK was determined to 1.45.     

1‐Alkyl‐2‐{(O‐Thioalkyl)Phenylazo}Imidazole Complexes of PbII and Their Photochromic Property

Pb^II complexes of 1‐alkyl‐2‐{(o‐thioalkyl)phenylazo}imidazole (SRaaiNR'), [Pb(SRaaiNR')₂X₂] were characterized by spectroscopic studies. The single‐crystal X‐ray structure of [Pb(SEtaaiNEt)₂Cl₂] (SEtaaiNEt = 1‐ethyl‐2‐{(o‐thioalkyl)phenylazo}imidazole) proved imidazolyl‐N and –SEt coordination forming unusual puckered eight member chelate rings. UV light irradiation of the complexes in DMF solution shows E‐to‐Z (E and Z refer to trans and cis‐configuration about –N=N–, respectively) photoisomerization of the coordinated azoimidazole. The rate of isomerization follows the sequence: [Pb(SRaaiNR')₂Cl₂] < [Pb(SRaaiNR')₂Br₂] < [Pb(SRaaiNR')₂I₂]. Quantum yields (φ_E→Z) and the activation energy (E_a) of the isomerization of the complexes are lower than observed for the free ligand. This can be explained by considering the molecular assembly and the thus observed increase in mass and rotor volume of the complexes. DFT and TDDFT calculations of optimized geometry could explained the spectral properties and photochromic activity.     

Photoisomerization of a Maleonitrile‐Type Salen Schiff Base and Its Application in Fine‐Tuning Infinite Coordination Polymers

Strategically designed salen ligand 2,3‐bis[4‐(di‐p‐tolylamino)‐2‐hydroxybenzylideneamino]maleonitrile ( 1 ), which has pronounced excited‐state charge‐transfer properties, shows a previously unrecognized form of photoisomerization. On electronic excitation (denoted by an asterisk), 1Z *→ 1E isomerization takes place by rotation about the C2? C3 bond, which takes on single‐bond character due to the charge‐transfer reaction. The isomerization takes place nonadiabatically from the excited‐state ( 1Z ) to the ground‐state ( 1E ) potential‐energy surface in the singlet manifold; 1Z and 1E are neither thermally inconvertible at ambient temperature (25–30 °C), nor does photoinduced reverse 1E *→ 1Z (or 1Z *) isomerization occur. Isomers 1Z and 1E show very different coordination chemistry towards a Zn^II precursor. More prominent coordination chemistry is evidenced by a derivative of 1 bearing a carboxyl group, namely, N,N′‐dicyanoethenebis(salicylideneimine)dicarboxylic acid ( 2 ). Applying 2Z and its photoinduced isomer 2E as building blocks, we then demonstrate remarkable differences in morphology (sphere‐ and needlelike nanostructure, respectively) of their infinite coordination polymers with Zn^II.     

Polysubstituted 5‐Phenylazopyrimidines: Extremely Fast Non‐ionic Photochromic Oscillators

Photochromic systems with an ultrahigh rate of thermal relaxation are highly desirable for the development of new efficient photochromic oscillators. Based on DFT calculations, we designed a series of 5‐phenylazopyrimidines with strong push–pull character in silico and observed very low energy barriers for the thermal (Z)‐to‐(E) isomerization. The structure of the (Z)‐isomer of the slowest isomerizing derivative in the series was confirmed by NMR analysis with in situ irradiation at low temperature. The substituents can tune the lifetime of thermal back isomerization from hundreds of microseconds to several nanoseconds (8 orders of magnitude). The photoswitching parameters were extracted from transient absorption techniques and a dominant rotation mechanism of the (Z)‐to‐(E) thermal fading was proposed based on DFT calculations.     

Synthesis,characterization, and photoresponsive behavior of new azobenzene‐containing polyethers

Two new polyethers, bearing azobenzene moiety in the side chain, were synthesized in excellent yields by means of anionic polymerization of 4‐glycidyloxyazobenzene and 4‐cyano‐4′‐glycidyloxyazobenzene (leading to azo‐P1 and azo‐P2 polymers, respectively) with the system polyiminophosphazene base t‐Bu‐P₄/3,5‐di‐tert‐butylphenol as initiator. The polymers were characterized with respect to their molecular weights, structure, and calorimetric features. The polyether bearing cyanoazobenzene group in the side chain was found to exhibit nematic phase up to 200 °C. E–Z isomerization of both polymers in tetrahydrofuran solution, after irradiating with UV light at 364 nm for 15 min, was investigated by means of UV–visible absorption spectroscopy. In the case of glycidylic monomers as well as the resulting polymers, E–Z isomerization was also investigated by means of ¹H NMR, by direct irradiation in the NMR probe in deuterated 1,1,2,2‐tetrachloroethane solution. By this technique, in the case of 4‐cyano‐4′‐glycidyloxyazobenzene, it was found that irradiation led to a photostationary state corresponding to an amount of Z isomer equal to 25%. For azo‐P1 polymer, Z–E or “reverse” isomerization was investigated at 60, 70, or 80 °C directly in the NMR probe; as expected, the process followed a first‐order rate law. The kinetic constants as well as the activation energy for the process were determined in this temperature range. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5426–5436, 2009     

Applications de la Résonance Dynamique du Proton et du Carbone-13 à l'Etude d'Azines Hétérocycliques. Mécanismes de Formation et d'Isomérisation

A ¹H and ¹³C n.m.r. study of heterocyclic azines is presented and the influence of Z/E isomerism on the ¹³C chemical shifts is examined. The kinetic study of the formation and Z/E isomerization of these compounds has been carried out and mechanisms of these reactions are discussed.     

Light‐Mediated Total Synthesis of 17‐Deoxyprovidencin

An asymmetric synthesis of the diterpenoid 17‐deoxyprovidencin is described. Key steps include an aldol addition, a base‐catalyzed Wipf‐type furan formation, a Z‐selective ring‐closing metathesis for macrocyclization, a photochemical E/Z isomerization to a highly strained and conformationally restricted ring system, and the stereoselective formation of two epoxides on the ring.     

Contra‐Thermodynamic,Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two‐Dimensional Chemical Space

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z‐α‐substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp²)−B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E‐α‐substituted styrenyl BPins to generate the corresponding Z‐isomers with high fidelity. Preliminary validation of this contra‐thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z‐configured trisubstituted alkenes, as well as in a concise synthesis of the anti‐tumor agent Combretastatin A4.     

Synthesis of Citronellal by RhI‐Catalysed Asymmetric Isomerization of N,N‐Diethyl‐Substituted Geranyl‐ and Nerylamines or Geraniol and Nerol in the Presence of Chiral Diphosphino Ligands,under Homogeneous and Supported Conditions

For the asymmetric isomerization of geranyl‐ or neryldiethylamine ((E)‐ or (Z)‐ 1 , resp.) and allyl alcohols geraniol or nerol ((E)‐ or (Z)‐ 2 , resp.) to citronellal ( 4 ) in the presence of a [Rh^I(ligand)cycloocta‐1,5‐diene)]⁺ catalyst, the atropic ligands 5 – 11 are compared under homogeneous and polymer‐supported conditions with the non‐C₂‐symmetrical diphosphino ferrocene ligands 12 – 16 . The ^tBu‐josiphos ligand 13 or daniphos ligand 19 , available in both antipodal series, already catalyse the reaction of (E)‐ 1 at 20° (97% e.e.) and favourably compare with the binap ligand 5 (see Table 1). Silica‐gel‐ or polymer‐supported diphosphino ligands usually afford similar selectivity as compared to the corresponding ligands applied under homogeneous conditions, but are generally less reactive. In this context, a polymer‐supported ligand of interest is the polymer‐anchored binap (R)‐ 6 , in terms of reactivity, selectivity, and recoverability, with a turnover of more than 14400.     

Geometry‐Retentive C‐Alkenylation of Lithiated α‐Aminonitriles: Quaternary α‐Alkenyl Amino Acids and Hydantoins

α‐Amino nitriles tethered to alkenes through a urea linkage undergo intramolecular C‐alkenylation on treatment with base by attack of the lithionitrile derivatives on the N′‐alkenyl group. A geometry‐retentive alkene shift affords stereospecifically the E or Z isomer of the 5‐alkenyl‐4‐iminohydantoin products from the corresponding starting E ‐ or Z ‐N ′‐alkenyl urea, each of which may be formed from the same N ‐allyl precursor by stereodivergent alkene isomerization. The reaction, formally a nucleophilic substitution at an sp² carbon atom, allows the direct regioselective incorporation of mono‐, di‐, tri‐, and tetrasubstituted olefins at the α‐carbon of amino acid derivatives. The initially formed 5‐alkenyl iminohydantoins may be hydrolyzed and oxidatively deprotected to yield hydantoins and unsaturated α‐quaternary amino acids.