Photochemical Dimerization Research Articles

Photochemical Dimerization of Indones: A DFT Study

In this study, DFT calculations were performed in order to identify the reaction products of the photochemical dimerization of indones. Calculations allowed us to assume that all tested reactions occurred in the first excited singlet state. The irradiation of 2-phenyl-5-nitroindone gave the main product, the <i>anti-cis</i> head-to-tail dimer, while the other two compounds found in the experiments were the <i>syn-cis</i> head-to-head dimer and the <i>anti-cis</i> head-to-tail dimer. The irradiation of 2-phenylindone gave the main product, <i>syn-cis</i>-head-to-tail dimer, while the minor products were <i>anti-cis</i>-head-to-tail and <i>anti-cis</i>-head-to-head dimer. The photochemical dimerization of 2-methyl-3-phenylindone gave the main product, <i>anti-cis</i>-head-to-tail dimer, while the minor component of the mixture was <i>syn-cis</i>-head- to-head dimer.

The Photochemical Dimerization of 4-Nitro-2-Phenylindone. A DFT Study

A DFT study of the photochemical dimerization on 4-nitro-2-phenylindone has been performed. The reaction was performed by Marussia Bakunin in 1911 but the reaction products were not identified. Calculations allow us to know that the reaction occurred through the first excited singlet state. The nature of the first excited singlet state allowed the formation of both head-to-head and head-to-tail dimers. Finally, the main product was the syn-trans head-to-tail dimer, while the other two compounds found by Bakunin in lower yields were the syn-trans head-to-head dimer and the syn-cis head-to-tail dimer. The results described by Bakunin can not be interpreted assuming the reaction occurred through the formation of the excited triplet state.

Counting the truxillines-11 or more, the question is now.

Truxillines are a group of tropane alkaloids present in coca leaves that are formed by photochemical dimerization of cinnamoylcocaine(s). Proportion of different truxilline forms present in cocaine serves as its geographical, manufacture, and storage "fingerprint"; thus, the quantitative determination of truxilline content represents one of the powerful methods of analysis and characterization of cocaine samples. Contrary to the statements repeatedly presented in the literature, namely, that there exist exactly 11 truxillines and that every single truxilline is diastereomer of any other, here we show that,in fact, a total of 15 truxillines exist, which can be divided in two structurally isomeric groups-five mutually diastereomeric truxillates and 10 mutually diastereomeric truxinates.

Cucurbit[8]uril Mediated Supramolecular and Photocrosslinked Interpenetrating Network Hydrogel Matrices for 3D-Bioprinting.

Printing of biologically functional constructs is significant for applications in tissue engineering and regenerative medicine. Designing bioinks remains remarkably challenging due to the multifaceted requirements in terms of the physical, chemical, and biochemical properties of the three-dimensional matrix, such as cytocompatibility, printability, and shape fidelity. In order to promote matrix and materials stiffness, while not sacrificing stress relaxation mechanisms which support cell spreading, migration, and differentiation, this work reports an interpenetrating network (IPN) bioink design. The approach makes use of a chemically defined network, combining physical and chemical crosslinking units with a tunable composition and network density, as well as spatiotemporal control over post-assembly material stiffening. To this end, star-shaped poly(ethylene glycol)s functionalized with Phe-Gly-Gly tripeptide or photoactive stilbazolium are synthesized, and used to prepare three-dimensional networks with cucurbit[8]uril (CB[8]) through supramolecular host-guest complexation. The hydrogel obtained shows fast relaxation and thus supports the proliferation and differentiation of cells. Upon irradiation, the mechanical properties of the hydrogel can be rapidly adapted via selective photochemical dimerization of stilbazolium within CB[8], leading to IPNs with increased form stability while retaining the dynamic nature of the hydrogels. This modular approach opens new design opportunities for extrudable and cell-friendly dynamic biomaterials for applications in 3D-bioprinting.

Dynamic accumulation of cyclobutane pyrimidine dimers and its response to changes in DNA conformation.

Photochemical dimerization of adjacent pyrimidines is fundamental to the creation of mutagenic hotspots caused by ultraviolet light. Distribution of the resulting lesions (cyclobutane pyrimidine dimers, CPDs) is already known to be highly variable in cells, and in vitro models have implicated DNA conformation as a major basis for this observation. Past efforts have primarily focused on mechanisms that influence CPD formation and have rarely considered contributions of CPD reversion. However, reversion is competitive under the standard conditions of 254 nm irradiation as illustrated in this report based on the dynamic response of CPDs to changes in DNA conformation. A periodic profile of CPDs was recreated in DNA held in a bent conformation by λ repressor. After linearization of this DNA, the CPD profile relaxed to its characteristic uniform distribution over a similar time of irradiation to that required to generate the initial profile. Similarly, when a T tract was released from a bent conformation, its CPD profile converted under further irradiation to that consistent with a linear T tract. This interconversion of CPDs indicates thatboth its formation and reversion exert control on CPD populations long before photo-steady-state conditions are achieved and suggests that the dominant sites of CPDs will evolve as DNA conformation changes in response to natural cellular processes.

Toward combined photobiological–photochemical formation of kerosene-type biofuels: which small 1,3-diene photodimerizes most efficiently?

A transition from fossil- to bio-based hydrocarbon fuels is required to reduce greenhouse gas emissions; yet, traditional biomass cultivation for biofuel production competes with food production and impacts negatively on biodiversity. Recently, we reported a proof-of-principle study of a two-step photobiological–photochemical approach to kerosene biofuels in which a volatile hydrocarbon (isoprene) is produced by photosynthetic cyanobacteria, followed by its photochemical dimerization into C10 hydrocarbons. Both steps can utilize solar irradiation. Here, we report the triplet state (T1)-sensitized photodimerization of a broader set of small 1,3-dienes to identify which structural features lead to rapid photodimerization. Neat 1,3-cyclohexadiene gave the highest yield (93%) after 24 h of irradiation at 365 nm, followed by isoprene (66%). The long triplet lifetime of 1,3-cyclohexadiene, which is two orders of magnitude longer than those of acyclic dienes, is key to its high photoreactivity and stem from its planar T1 state structure. In contrast, while isoprene is conformationally flexible, it has both photochemical and photobiological advantages, as it is the most reactive among the volatile 1,3-dienes and it can be produced by cyanobacteria. Finally, we explored the influence of solvent viscosity, diene concentration, and triplet sensitizer loading on the photodimerization, with a focus on conditions that are amenable when the dienes are produced photobiologically. Our findings should be useful for the further development of the two-step photobiological–photochemical approach to kerosene biofuels.Graphical abstract

Topochemical, Single-Crystal-to-Single-Crystal [2+2] Photocycloadditions Driven by Chalcogen-Bonding Interactions.

The face‐to‐face association of (E)‐1,2‐di(4‐pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single‐crystal‐to‐single‐crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho‐substituted bis(selenocyanato)benzene derivatives 1–3, prepared from ortho‐diboronic acid bis(pinacol) ester precursors and SeO2 and malononitrile in 75–90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1–3 (3.22–3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52–3.54 Å in the chalcogen‐bonded adducts with bpen, a distance (<4 Å) well adapted to the face‐to‐face association of the bpen molecules into the reactive position toward photochemical dimerization.

Topochemical, Single‐Crystal‐to‐Single‐Crystal [2+2] Photocycloadditions Driven by Chalcogen‐Bonding Interactions

The face-to-face association of (E)-1,2-di(4-pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single-crystal-to-single-crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho-substituted bis(selenocyanato)benzene derivatives 1–3, prepared from ortho-diboronic acid bis(pinacol) ester precursors and SeO2 and malononitrile in 75–90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1–3 (3.22–3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52–3.54 Å in the chalcogen-bonded adducts with bpen, a distance (<4 Å) well adapted to the face-to-face association of the bpen molecules into the reactive position toward photochemical dimerization.

Synthesis, Purification, and Rotational Spectroscopy of 1-Cyanocyclobutene (C5H5N)

The rotational spectrum of 1-cyanocyclobutene from 130 to 360 GHz has been observed, assigned, and least-squares fit for the ground state and the two lowest-energy vibrationally excited states. Synthesis by UV photochemical dimerization of acrylonitrile and subsequent base-catalyzed dehydrocyanation affords a highly pure sample, yielding several thousand observable rotational transitions for this small organic nitrile. Over 2500 a-type, R-branch transitions of the ground state have been least-squares fit to low error with partial-octic A- and S-reduced Hamiltonians, providing precise determinations of the corresponding spectroscopic constants. In both reductions, computed spectroscopic constants are in close agreement with their experimentally determined counterparts. Two vibrationally excited states (ν27 and ν17) form a Coriolis-coupled dyad, displaying many a-type and b-type local resonances and related nominal interstate transitions. Somewhat unexpectedly, despite the very small permanent b-axis dipole moment, a number of b-type transitions could be observed for the ν17 state; this is explained in terms of state mixing by the Coriolis perturbations. Over 2200 transitions for each of these states have been least-squares fit to a low-error, two-state, partial-octic, A-reduced Hamiltonian with nine Coriolis-coupling terms (Ga , GaJ, GaK, GaJJ, Fbc , FbcK, Gb , GbJ, and Fac). The availability of so many observed rotational transitions, including resonant transitions and nominal interstate transitions, enables a very accurate and precise determination of the energy difference (ΔE27,17 = 14.0588093 (43) cm-1) between ν27 and ν17. The spectroscopic constants presented herein provide a starting point for future astronomical searches for 1-cyanocyclobutene.

Deuterated Indocyanine Green (ICG) with Extended Aqueous Storage Shelf-Life: Chemical and Clinical Implications.

Indocyanine Green (ICG) is a clinically approved near‐infrared fluorescent dye that is used extensively for various imaging and diagnostic procedures. One drawback with ICG is its instability in water, which means that reconstituted clinical doses have to be used very shortly after preparation. Two deuterated versions of ICG were prepared with deuterium atoms on the heptamethine chain, and the spectral, physiochemical, and photostability properties were quantified. A notable mechanistic finding is that self‐aggregation of ICG in water strongly favors dye degradation by a photochemical oxidative dimerization reaction that gives a nonfluorescent product. Storage stability studies showed that replacement of C−H with C−D decreased the dimerization rate constant by a factor of 3.1, and it is likely that many medical and preclinical procedures will benefit from the longer shelf‐lives of these two deuterated ICG dyes. The discovery that ICG self‐aggregation promotes photoinduced electron transfer can be exploited as a new paradigm for next‐generation photodynamic therapies.

Phenylene-bridged bis(benzimidazolium) (BBIm2+): a dicationic organic photoredox catalyst.

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet–triplet absorption of the excited BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm2+ and a carboxylate anion.

Photochemically Induced Solid State Dimerisation of Resveratrol Analogues: A Greener Synthetic Process

The photochemical dimerisation of resveratrol analogues in the solid state to generate chiral phenyl substituted cyclobutanes is described. NMR spectroscopic and X-ray crystallographic methods have confirmed that the dimerisation leads to the head to tail orientation of the phenyl group substituents in the cyclobutane derivative. Interestingly, the parent compound, resveratrol, in the solid state, did not form a cyclobutane dimer, but the O-acetyl analogues gave the corresponding cyclobutane dimers in high yield, suggesting that the close packing of molecules together with the electron density through the conjugated double bond of the resveratrol structure are important determinants for photodimerisation to occur in the solid state.

Time-Dependent Differential and Integral Quantum Yields for Wavelength-Dependent [4+4] Photocycloadditions.

The [4+4] photocycloaddition of anthracene is one of most relevant photoreactions and is widely applied in materials science, as it allows to remote-control soft matter material properties by irradiation. However, highly energetic UV irradiation is commonly applied, which limits its application. Herein, the wavelength dependence of the photodimerization of anthracene is assessed for the first time, revealing that the reaction is induced just as effectively with mild visible light (410 nm). To fully establish [4+4] cycloadditions within defined chemical environments, a conceptual framework for the solution kinetics of the photo-dimerization up to long reaction times is established by developing a novel photoreaction rate law that is dependent on individual rate coefficients of the key reaction steps. These coefficients can be determined based on low conversion photochemical experiments. Both differential and integral quantum yields can subsequently be predicted that are strongly time-dependent, highlighting the need for a detailed reaction pathway analysis. The presented approach simplifies a complex photochemical scenario, making the photochemical anthracene dimerization, or potentially any other photochemical dimerization, amenable to a time-dependent understanding at the elementary reaction level.

Direct and Metal-Catalyzed Photochemical Dimerization of the Phthalide (Z)-Ligustilide Leading to Both [2 + 2] and [4 + 2] Cycloadducts: Application to Total Syntheses of Tokinolides A-C and Riligustilide.

Synthetically derived (Z)-ligustilide (1) has been subjected to photochemically-promoted dimerization processes under a range of conditions. By such means, varying distributions of the dimeric natural products tokinolides A-C (4, 3, and 6, respectively) and riligustilide (5) as well certain related (isomeric) compounds have been obtained. The structures of three of them have been confirmed by single-crystal X-ray analysis. The biosynthetic implications of the outcomes of this study are discussed.

A new class of flavonoids bearing macrocyclic polyethers by stereoselective photochemical cycloaddition reaction

A new class of flavonoids bearing cyclic polyethers involving a phenyl ring was conveniently provided by the intramolecular photochemical dimerization of 2-chromonecarboxylic esters. Irradiation of 2-chromonecarboxylate with a polyether tethered at both ends promoted intramolecular [2 + 2] cyclobutane formation leading to 14- to 27-membered cyclic polyethers. The efficiency depended on the substituted position of the phenyl ring, with ortho- and meta-substituted derivatives giving cycloadducts in good chemical yields and quantum efficiencies, whereas the para-derivatives were inert toward photolysis. X-ray crystallographic analysis revealed that the stereochemistry of the macrocyclic cycloadducts exhibited C2-symmetry.

Evidence for Strong and Weak Phenyl-C61-Butyric Acid Methyl Ester Photodimer Populations in Organic Solar Cells

In polymer/fullerene organic solar cells, the photochemical dimerization of phenyl-C61-butyric acid methyl ester (PCBM) was reported to have either a beneficial or a detrimental effect on device performance and stability. In this work, we investigate the behavior of such dimers by measuring the temperature dependence of the kinetics of PCBM de-dimerization as a function of prior light intensity and duration. Our data reveal the presence of both “weakly” and “strongly” bound dimers, with higher light intensities preferentially generating the latter. DFT simulations corroborate our experimental findings and suggest a distribution of dimer binding energies, correlated with the orientation of the fullerene tail with respect to the dimer bonds on the cage. These results provide a framework to rationalize the double-edged effects of PCBM dimerization on the stability of organic solar cells.

Stereoselectivity and Regioselectivity of the Cycloaddition Dimerization of allyl 3-(2-pyridyl) acrylate and allyl 3-(2-pyrryl) acrylate: DFT Calculations

A theoretical study of the photochemical dimerization of allyl 3-(2-pyridyl) acrylate and allyl 3-(2-pyrryl) acrylate is reported. The reactions gave dimers with high regioselectivity and stereoselectivity through [2+2] cycloaddition mechanism. All calculations were computed by density functional theory method, B3LYP, in conjunction with the 6-31G(d) basis set. Conformation analysis, geometric parameters and IR spectrum for the target dimers were also studied. Conformation 2 of allyl 3-(2-pyridyl) acrylate and conformation 9 for allyl 3-(2-pyrryl) acrylate were found to be the most stable structures among the different conformations. Vibration frequencies and IR absorption intensities were calculated for the conformers 2 and 9 using the same computational method. In addition, thermodynamic parameters for the reactions of most stable conformations were analysed. The HOMO and LUMO molecular orbitals and the energy gap between them were estimated for the stable conformations.

Acid catalyzed aldol condensations of ketones with glyoxylic acid: A simple single-step synthesis of 4-oxo-2,5-heptdienedioic acids

The acid catalyzed condensations of ketones with glyoxylic acid at 80 °C, under neat conditions gives 4-oxo-2,5-heptdienedioic acids in 56–88% yield. The solid state photochemical dimerization of 4-oxo-2,5-heptdienedioic acid under UV light gives 2,7-dioxo-tricyclo[6.2.0.03,6]decane-4,5,9,10-tetracarboxylic acid in 90% yield.

In vivo photoinduced [4 + 2] dimerization of a neo-clerodane diterpene in Baccharis flabellata. ROS and RNS scavenging abilities

Secondary metabolites play a major role in the adaptation of plants to the environment. Furan neo-clerodane diterpenes are characteristic secondary metabolites in Baccharis flabellata Hook. & Arn. var. flabellata. One of the main compounds is the diene ent-15,16-epoxy-19-hydroxy-1,3,13(16),14-clerodatetraen-18-oic acid (DAC). In this work a new dimeric compound (DACD) has been isolated and identified by NMR and MS techniques. The presence of other minor dimers was also observed in the same plant methanolic extracts. Assuming that they may be the products of [4 + 2] condensation of two monomeric moieties, the formation of adducts by photochemical dimerization was checked by inducing the in vitro [4 + 2] cycloaddition of DAC. Moreover, the DAC and DACD accumulation rates in aerial parts of B. flabellata specimens were analyzed monthly during a complete phenological cycle. The accumulation of monomer depends on the plant phonological stage; meanwhile the dimer proportion arises in detriment of the monomer as the solar UV radiation increases. Since plants exposed to strong UV intensities produce radical species, the scavenger properties of these compounds toward reactive nitrogen species (RNS), and reactive oxygen species (ROS), were analyzed. Albeit DAC and DACD show significant superoxide radical scavenger activities, the monomer proved to be more effective than the dimer toward ROS, while DACD was an excellent RNS scavenger.

A Facile Synthesis of C2-Symmetric Macrocyclic Polyethers by Photodimerization of Covalently-linked Flavonoid Derivatives

C2-Symmetric cyclic polyethers were conveniently provided by the photochemical dimerization of covalently-linked 2-chromonecarboxylic esters. Irradiation of polyethers possessing 2-chromonecarboxylate groups at both ends efficiently promoted intramolecular [2+2] cyclobutane formation leading to 14- to 23-membered cyclic polyethers in good chemical yields and quantum efficiencies. X-ray crystallographic analysis revealed that the stereochemistry of the cyclobutane was the C2-symmetric structure.