Photochemical Dimerization Research Articles

Direct observation of 4‐nitrophenyl disulfide produced from p‐nitrothiophenol in air by Raman spectroscopy

AbstractTaking the fingerprint characteristics of Raman spectroscopy, the dimerization of p‐nitrothiophenol (PNTP) to 4‐nitrophenyl disulfide in air was investigated, which could be accelerated by the external laser illumination. The dimerization was qualitatively confirmed by both mass spectroscopy experimentally and by Raman spectrum simulated calculations theoretically and quantitatively analyzed by Raman spectroscopy in a simple and quick manner. On the basis of the inductively coupled plasma optical mass spectrometry analysis, the dimerization was possibly ignited by the trace metal residues as catalyst in the PNTP. This study clearly shows that it is very important to strictly and preciously control the experimental conditions of Raman measurement and other laser‐based spectroscopies to avoid any wrong conclusion during the characterization or identification. Different to the plasmon‐induced photochemical reactions, we reported the photochemical reaction of PNTP to 4‐nitrophenyl disulfide, which is free of the plasmoic effect from the metal nanostrucutres. On the other hand, the photochemical dimerization reaction may provide a new and important guideline for the formation of thiols to disulfides.

Asymmetric Synthesis Involving Reversible Photodimerization of a Prochiral Flavonoid Followed by Crystallization

Asymmetric synthesis involving photochemical dimerization of a prochiral flavonoid derivative in solution without any chiral source was achieved. Irradiation of ethyl 6‐bromochromonecarboxylate in solution efficiently gave a C2‐chiral anti‐head‐to‐head dimer in excellent chemical yield with good quantum efficiency (Φ365 = 0.15). X‐ray crystallographic analysis revealed that the dimer crystallized as a conglomerate of C2 space group. The crystalline dimer precipitated upon irradiation of the monomer in solution, and indirect racemization of the dimer through a reversible photoreaction in solution and selective crystallization simultaneously occurred to give the C2‐chiral dimer in optically active form with up to 80 % ee. Optically active photoproducts could be obtained by simply irradiating achiral materials in solution without an external chiral source.

Versatile Bioorthogonal Hydrogel Platform by Catalyst-Free Visible Light Initiated Photodimerization of Anthracene

Recent developments in photochemistry have introduced new methods to prepare hydrogels initiated by nonharmful light which is essential for encapsulation of cells and bioactive components. However, bioorthogonal photoclick reactions generally requires two components for cross-linking and, in many cases, the formation of a reactive intermediate that may cross-react with nucleophiles in biological media. Here we report the utilization of a visible light triggered dimerization of electron-rich anthracene for polymer cross-linking to form bulk hydrogels and microgels. Incorporation of gelatin within the hydrogel enhanced cell attachment and viability after 7 days of culture and spatiotemporal conjugation of a bioactive component using photochemical dimerization of anthracene was demonstrated. This work therefore introduces a simple yet powerful tool for light modulated bioorthogonal polymer cross-linking, which can be utilized in various bioengineering applications.

One-Dimensional Coordination Polymers of Bis(3-pyridyl-acrylamido)ethane: Influence of Anions and Metal Ions on Their Solid State [2 + 2] Photochemical Polymerization and Dimerization Reactions

The coordination polymers (CPs) of bis(3-pyridyl-acrylamido)ethane (L) with nitrate, acetate, and perchlorate salts of Cu(II), Cd(II), and Zn(II) were synthesized, and their crystal structures have been analyzed with respect to the influence of anions, hydrogen bonding, and their solid state [2 + 2] photochemical reactivity. Four out of the six CPs (1–4) containing NO3– and AcO– ions were found to form one-dimensional chains and exhibit solid state [2 + 2] reactivity. Two CPs (5–6) containing ClO4– ions were found to form ML2 discrete complexes which are photostable. The CPs 1 and 4 were found to form single [2 + 2] dimers, while 2 and 3 resulted in the formation of polymers via solid state [2 + 2] polymerization upon irradiation. The photoproducts are characterized by 1H NMR, electrospray ionization mass spectrometry, and matrix-assisted laser desorption/ionization–time of flight (MALDI-TOF) spectra. The dimer was charecterized by single crystal X-ray structure determination. The MALDI-TOF analyses of po...

Photochemical strengthening of silica aerogels modified with coumarin groups

Silica aerogels were silated with coumarin functionalities to allow for strengthening through the photochemical dimerization of the coumarin groups. Coumarin modified silica gels were prepared by either co-polymerizing tetramethoxysilane (TMOS) with small amounts of 7-(3-triethoxysilylpropoxy)coumarin (TEPC) or by surface modifying the silica gel with 7-(3-trimethoxysilylpropoxy)coumarin (TMPC). Aerogels, prepared by supercritical carbon dioxide drying, were irradiated with ultraviolet light (365nm) for varying lengths of time and the flexural strength of cylindrical samples was measured. Copolymerization of TEPC with TMOS resulted in aerogels that, while initially weaker than pure silica aerogels, showed a 41% increase in flexural strength with 20min exposure to UV. In contrast, the surface modified aerogels, while stronger than pure silica aerogels, showed no influence on flexural strength with exposure to UV.

Photochemical Synthesis of Nepetanudone.

Nepetanudone and nepetaparnone have been suspected of being the products of a photochemical dimerization of nepetapyrone. Both are natural products found in a variety of Nepeta species. The synthesis of (±)-nepetapyrone and subsequent photochemical experiments are described. (±)-Nepetanudone was produced upon irradiation of (±)-nepetapyrone, while (±)-nepetaparnone, a diastereomer of nepetanudone, was not observed.

A DFT Study of the Photochemical Dimerization of Methyl 3-(2-Furyl)acrylate and Allyl Urocanate

A DFT study of the photochemical dimerization of methyl 3-(2-furyl)acrylate is reported. The photochemical reaction gave a mixture of two dimers with high regioselectivity and good stereoselectivity. Calculations showed that benzophenone was able to act as a photosensitizer of the reaction. This compound populated the first excited triplet state of the substrate. The frontier orbitals interaction between LSOMO of the triplet state and HOMO of the ground state accounted for the observed high regioselectivity. Furthermore, the energy of all the possible triplet biradicals has been calculated, showing that the precursor of the main product was the triplet biradical with the lowest energy. The coupling of the atomic coefficients on the radical centres in the biradical intermediates allowed to justify the observed products. The same behavior was observed in the case of the photochemical dimerization of an urocanate ester and in the dimerization of liquid methyl cinnamate.

Host–guest accelerated photodimerisation of anthracene-labeled macromolecules in water

The rate enhancement of the [4 + 4]-photodimerisation of anthracene through a host–guest templation strategy in water is presented and applied to polymer ligation and cross-linking. The macrocycle cucurbit[8]uril (CB[8]) tethers together two anthracene moieties in a face-to-face π–π-stack arrangement, which results in significantly faster photochemical dimerisation (on the order of a few minutes) than in the absence of the host, minimises photochemical side reactions and works even under dilute conditions. Therefore, the host-accelerated photodimerisation was applied to macromolecules, shown here for anthracene end-group functionalised poly(ethylene glycol) (PEG) and side-chain functionalised hydroxyethyl cellulose (HEC) to induce polymer-ligation and network-formation (hydrogelation), respectively. In particular, only in the presence of CB[8] did polymer-ligation or chemical cross-linking through photodimerisation occur, whereas photochemical degradation reactions were exclusively observed in the absence of the CB[8] host.

Highly sensitive Raman spectroscopy using a gap mode plasmon under an attenuated total reflection geometry

We investigated a gap mode plasmon under an attenuated total reflection (ATR) configuration toward realization of near-field Raman spectroscopy with a single molecule sensitivity and spatial resolution. Additional enhancement in Raman scattering at a nanogap was obtained by a coupling of a propagating surface plasmon (PSP) of Ag films on a prism, and a gap mode between Ag films and Ag nanoparticles (AgNPs). Immobilization of AgNPs on Ag films through thiol-SAM slightly up-shifted the resonance angle of a PSP, which broadened the reflectivity dip owing to an increased out-coupling of a PSP. Raman enhancement factor at a nanogap increased with decreasing surface coverage of AgNPs, albeit the enhancement factor averaged over illuminated area in Ag films decreased, ensuring the largest enhancement factor in tip-enhanced Raman scattering. This is due to increased efficiency for a PSP excitation at lower coverage of AgNPs in consistent with that in theoretical evaluation using finite difference time domain calculations. A gap mode under an ATR configuration was applied to elucidate a plausible photochemical reaction of p-amino thiophenol (PATP) adsorbed on Ag films on a prism. Spectral changes in Raman scattering under laser illumination were observed for PATP with a deuterated amino group, but suppressed by a dimethyl amino group owing to steric hindrance, supporting the photochemical dimerization.

Photochemical dimerization of a fluorinated dibenzylideneacetone in chloroform solution

(1E,4E)-1,5-Bis(2,6-difluorophenyl)penta-1,4-dien-3-one, C17H10F4O, (I), dimerizes under sunlight in chloroform solution to form the corresponding cyclobutane derivative, (2E,2'E)-1,1'-[2,4-bis(2,6-difluorophenyl)cyclobutane-1,3-diyl]bis[3-(2,6-difluorophenyl)prop-2-en-1-one], C34H20F8O2, (II). The crystal structure of (I) explains why no topochemical dimerization can occur in the solid state. In the solid, molecules of dimer (II) show the `truxillic acid'-type arrangement of crystallographic centres of inversion, with half a molecule per asymmetric unit and cell dimensions closely related to those of the monomer. Intermolecular interactions in both solids are dominated by C-H···O and C-H···F contacts and also comprise interactions with aromatic systems (C-H···π and π-π).

Matrix isopotential synchronous fluorescence spectrometry for the determination of gibberellic acid in watermelon after ultraviolet-irradiation

The determination of gibberellic acid in watermelon was performed by means of first derivative matrix isopotential synchronous fluorescence spectrometry in combination with photochemically induced fluorescence, without the need for tedious pre-separation. Its photochemical reactivity by effect of UV irradiation boosting the fluorescence is in agreement with the occurrence of photoaromatization and photochemical dimerization processes, with loss of carbon dioxide, and the probable formation of several fluorescent photoproducts. Watermelon extract presents a high native fluorescence, which drastically diminishes under UV irradiation treatment. The analyses were performed in 50% (v/v) methanol and pH 5 buffer solution, using 6 min as UV irradiation time in a flow photoreactor. The concentration of gibberellic acid in watermelon samples can be calculated by recording its total luminescence spectrum and applying the isopotential trajectory of the watermelon matrix that cuts the selected band of gibberellic acid. The unknown analytical signal of watermelon is eliminated in the matrix isopotential synchronous fluorescence spectrum obtained, by means of its first derivative. The calibration graph was linear between 40 and 200 ng mL−1 of gibberellic acid with a detection limit of 0.14 ng mL−1 and the obtained value of repeatability of the method was a standard deviation of 1.9%.

Synthesis of a Covalent Monolayer Sheet by Photochemical Anthracene Dimerization at the Air/Water Interface and its Mechanical Characterization by AFM Indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Direct Determination of Gibberellic Acid in Tomato and Fruit by Using Photochemically Induced Fluorescence

A simple, sensitive method for determining gibberellic acid based on photochemically induced fluorescence detection was developed to determine this plant growth regulator in a technical formulation, tomato, and fruit samples. The principle for the determination is the photochemical reactivity of the gibberellic acid, being consistent with the occurrence of photoaromatization and photochemical dimerization with loss of carbon dioxide, and with the likely formation of various fluorescent photoproducts. Six min of UV (mainly at 253.7 nm) irradiation in a solution containing 50% (v/v) methanol and buffer at pH 5 provided the best results. The calibration curve was linear over the concentration range 50-150 ng mL(-1), and the limit of detection was 1.7 ng mL(-1). The method is useful to determine gibberellic acid in samples with background fluorescence such as plum and tomato without the need for labor-intensive preparation as a result of UV irradiation suppressing the fluorescent background.

Accumulation of the cyclobutane thymine dimer in defined sequences of free and nucleosomal DNA

Photochemical cyclobutane dimerization of adjacent thymines generates the major lesion in DNA caused by exposure to sunlight. Not all nucleotide sequences and structures are equally susceptible to this reaction or its potential to create mutations. Photostationary levels of the cyclobutane thymine dimer have now been quantified in homogenous samples of DNA reconstituted into nucleosome core particles to examine the basis for previous observations that such structures could induce a periodicity in dimer yield when libraries of heterogeneous sequences were used. Initial rate studies did not reveal a similar periodicity when a homogenous core particle was analyzed, but this approach examined only formation of this photochemically reversible cyclobutane dimer. Photostationary levels result from competition between dimerization and reversion and, as described in this study, still express none of the periodicity within two alternative core particles that was evident in heterogeneous samples. Such periodicity likely arises from only a limited set of sequences and structural environments that are not present in the homogeneous and well-characterized assemblies available to date.

Photochemistry Is Back in the Undergraduate Organic Laboratory

A commercial four-foot display ultraviolet fluorescent tube works well both for the photodimerization of trans-cinnamic acid and for the photochemical dimerization of benzophenone to benzopinacol. These procedures were designed for the undergraduate organic chemistry laboratory course.

Concise Synthesis of Linderaspirone A and Bi-linderone.

Photogenic: The synthesis of linderaspirone A has been accomplished in only three steps by a Darzens cyclopentenedione synthesis and dioxygen-assisted photochemical dimerization. Moreover, the thermal isomerization of linderaspirone A into bi-linderone has been discovered, which may give clues to the biosynthetic pathway for bi-linderone.

The interplay between hydrogen bonding and π-π stacking interactions in the crystal packing of N1-thyminyl derivatives, and implications for the photo-chemical [2π + 2π]-cycloaddition of thyminyl compounds

The solid-state photo-chemical dimerisation of thyminyl derivatives occurs when two thyminyl units are aligned in such a way that the olefinic moieties are separated by a distance of less than 4.2 Å. When irradiated with >270 nm UV, the thyminyl olefinic groups undergo [2π + 2π]-cycloaddition to form a dimeric cyclobutane derivative. However, the design and execution of [2π + 2π]-cycloaddition reactions can be challenging due to the requirement to produce molecular crystals with the necessary olefinic alignment. In this investigation, the crystallographic and solid-state photo-chemical reactions of six N1-thyminyl derivatives are studied. Only one derivative, thyminyl propanamide (), was found to undergo [2π + 2π]-cycloaddition in the crystalline state. As such, quantum chemical methods were employed to study the photo-chemical transition states of the derivatives, as well as the strengths of typical intermolecular interactions that were observed in their crystal structures (such as π-π stacking between the thyminyl rings, Watson and Crick style hydrogen bonding and hydrogen bonding between functional groups of N1 substituents). These results were used to rationalise the solid-state photo-reactivity of more complex bis-thyminyl monomers.

Pre-organization of diarylideneacetonyl crownophanes in single crystals to photochemical transformations

Specific features of crystal packings and pre-organization of diarylideneacetonyl crownophane molecules to solid-phase photochemical transformations were studied on the basis of X-ray diffraction data using methods of simulation of molecular crystal packings. The tendency of the most part of the synthesized macrocyclic E,E-isomers to the formation of homochiral crystals (P212121) was revealed, while (23E,26E)-11,12,14,15-tetrahydro-8H-dinaphtho-[2,1-k:1′,2′-r][1,4,7,10]tetraoxacyclononadecine-23,26-dien-25(9H)-one is prone to polymorphism. A phenomenon of solid-phase stereospecific photochemical dimerization of molecules according to the syn-head-to-tail type without crystal destruction (single crystal—single crystal transformation) was found for one of the modifications of this crownophane.

Interfacial assembly of cinnamoyl-terminated bolaamphiphiles through the air/water interface: headgroup-dependent assembly, supramolecular nanotube and photochemical sewing

A series of cinnamoyl-terminated bolaamphiphiles were synthesized and their assemblies at the air/water interface were investigated. It was found that the assembly behaviour depended on the substituted groups on the cinnamoyl unit. The bolaamphiphile with 4-hydroxycinnamoyl head groups (HCDA) was found to assemble into a supramolecular nanotube, while the others formed only layer-structured films. Moreover, the nanotube formed from HCDA showed supramolecular chirality due to the symmetry breaking. Both the layered films and the nanotubes showed photochemical dimerization upon UV irradiation, which were studied from the UV-Vis, FT-IR spectral and MALDI-TOF MS analysis. Interestingly, such dimerization behavior of the cinnamoyl group could be used to stabilize the nanotube of HCDAvia photochemical sewing. During such a process both the supramolecular chirality and the tubular shapes were kept. Remarkably, such a photochemical sewed chiral nanotube could further induce the chirality of an achiral porphyrin derivative assembled on it, and produced the induced chirality without using any chiral molecules.

Formation of different photodimers of isoquinolinone by irradiation of solid molecular compounds

[4 + 4] Photodimerization of isoquinolin-3(2H)-one (isoquinolinone) (1) may yield 12 different isomers depending on the relative geometry of the two monomers prior to the reaction. Irradiation of the solid neat compound shows that of all possible dimers only the dimer between the two hetero-rings having an inversion center is produced. On the other hand, exposure to UV light of solid molecular compounds composed of isoquinolinone as the guest molecule and different host molecules shows to produce few other isomers of the dimer. The use of different host molecules affects the packing of the molecules in the crystal. As a result, the relative geometries between two monomer molecules are varied enabling photochemical dimerization to yield different isomers. When 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) is used as the host molecule, two polymorphic forms of the molecular compound are obtained. Irradiation of single-crystal of the two polymorphs yields a mixture of isomers of the dimer. When the host molecule is cyclohexanol-1,2,4,5-tetracarboxylic acid (II) three isomers are expected, and when the host molecule is 1,3-benzenediol (III) a single isomer is formed which is identical to that obtained from the neat isoquinolinone.