Photodimerization Process Research Articles

4-Aminothiophenol Photodimerization Without Plasmons.

The photodimerization of 4‐aminothiophenol (PATP) into 4,4′‐dimercaptobenzene (DMAB) has been extensively utilized as a paradigm reaction to probe the role of surface plasmons in nanoparticle‐mediated light‐driven processes. Here I report the first observation of the PATP‐to‐DMAB photoreaction in the absence of any plasmonic mediators. The reaction was observed to occur with different kinetics either for PATP adsorbed on non‐plasmonic nanoparticles (TiO2, ZnO, SiO2) or deposited as macroscopic droplets. Confocal microRaman spectroscopy enabled to investigate the reaction progress in different plasmon‐free contexts, either aerobic or anaerobic, suggesting a new interpretation of the photodimerization process, based on direct laser‐induced activation of singlet oxygen species. These results provide new insights in light‐driven redox processes, elucidating the role of sample morphology, light and oxygen.

4‐Aminothiophenol Photodimerization Without Plasmons**

AbstractThe photodimerization of 4‐aminothiophenol (PATP) into 4,4′‐dimercaptobenzene (DMAB) has been extensively utilized as a paradigm reaction to probe the role of surface plasmons in nanoparticle‐mediated light‐driven processes. Here I report the first observation of the PATP‐to‐DMAB photoreaction in the absence of any plasmonic mediators. The reaction was observed to occur with different kinetics either for PATP adsorbed on non‐plasmonic nanoparticles (TiO2, ZnO, SiO2) or deposited as macroscopic droplets. Confocal microRaman spectroscopy enabled to investigate the reaction progress in different plasmon‐free contexts, either aerobic or anaerobic, suggesting a new interpretation of the photodimerization process, based on direct laser‐induced activation of singlet oxygen species. These results provide new insights in light‐driven redox processes, elucidating the role of sample morphology, light and oxygen.

Development of photoresponsive coumarin-modified ethylene-co-vinyl alcohol copolymers with antifouling behavior

In this work, the modification of ethylene-co-vinyl alcohol (EVOH) copolymer with a coumarin derivative, 7-(hydroxyethoxy)-4-methyl coumarin (HEOMC), has been performed through the incorporation of different reactive groups, p-nitrophenyl chloroformate, succinic anhydride and phthalic anhydride as different linkers between coumarin and EVOH with the aim to obtain photo-responsive materials with improved protein absorption resistance. After purification, the modification degree was calculated by nuclear magnetic resonance and ultraviolet-visible spectroscopy. The thermal properties revealed the crystallinity loss and higher maximum degradation temperatures when EVOH is modified with coumarin derivative. Copolymers were successfully photo-crosslinked through the photo-responsive behavior of coumarin moieties. The micro-hardness test shown an increase of the mechanical performance due to the coumarin presence. The water contact angle measurements revealed that while coumarin-modified EVOH copolymers present a higher hydrophilic behavior than EVOH, the photo-dimerization process mainly leads to slightly deviations on the surface wettability of neat EVOH. Finally, the absorption of bovine serum albumin onto the surface of the coumarin-modified materials revealed enhanced antifouling properties, particularly after the photo-crosslinked process. Therefore, the introduction of coumarin through different chain extenders leads to functional materials with tunable photo-responsive performance and protein absorption resistance.

The photodimerization of Schiff bases: Synthesis and crystal structures of benzophenone azines and their weak C[sbnd]H···π interactions

Four benzophenone azines were synthesized through a photodimerization process. The crystals are constructed through weak CH⋯π packing interactions, with additional π···π stabilization in two of the crystals. The four intermediates of the photodimerization process were also crystallized and their crystal structures are discussed. The packing of three of these intermediates involves weak CH⋯O interactions. The crystal structure of di-isoniazid azine, an expected by-product of the photodimerization process, is also described.

Multiple Electronic and Structural Factors Control Cyclobutane Pyrimidine Dimer and 6-4 Thymine-Thymine Photodimerization in a DNA Duplex.

The T-T photodimerization paths leading to the formation of cyclobutane pyrimidine dimer (CPD) and 6-4 pyrimidine pyrimidone (64-PP), the two main DNA photolesions, have been resolved for a T-T step in a DNA duplex by two complementary state-of-the-art quantum mechanical approaches: QM(CASPT2//CASSCF)/MM and TD-DFT/PCM. Based on the analysis of several different representative structures, we define a new-ensemble of cooperating geometrical and electronic factors (besides the distance between the reacting bonds) ruling T-T photodimerization in DNA. CPD is formed by a barrierless path on an exciton state delocalized over the two bases. Large interbase stacking and shift values, together with a small pseudorotation phase angle for T at the 3'-end, favor this reaction. The oxetane intermediate, leading to a 64-PP adduct, is formed on a singlet T→T charge-transfer state and is favored by a large interbase angle and slide values. A small energy barrier (<0.3 eV) is associated to this path, likely contributing to the smaller quantum yield observed for this process. Eventually, a clear directionality is always shown by the electronic excitation characterizing the singlet photoactive state driving the photodimerization process: an exciton that is more localized on T3 and a 5'-T→3'-T charge transfer for CPD and oxetane formation, respectively, thus calling for specific electronic constraints.

The Study of Photoreversibility, Photopolimerization and Photocleavage Properties of Polyurethane Coumarin by Means of Electronic Spectrometry and AFM Methods

The spectroscopic and photochemical properties of polyurethane coumarin (PUC) in dimethyl sulf-oxide (DMSO), thetra hydro furan (THF), dimethyl formamide (DMF) and film state were investigated at room temperature under one photon excitation. The results show that under irradiation of l ] 310 nm photodimerization process are increased and under UV irradiation with l[ 260 nm, photocleavage of polymer have been evidenced, too.

Photophysical and Photochemical Properties of Polyurethane Coumarin Studied by Means of Electronic Spectra

The spectroscopic and photochemical properties of polyurethane coumarine (PUC) in dimethyl sulf-oxide (DMSO), thetra hydro furan (THF), dimethyl formamide (DMF) and film state were investigated at room temperature under one photon excitation. Photodimer products were formed via UV irradiation. Syn head-to-head and syn head-to-tail photodimers were formed under one-photon (broadband UV) irradiation in polar solvents. The spectral investigation of the photochemical products of PUC revealed efficient fluorescence emission in the spectral region 300�400 nm, attributed to the syn-head-to-tail dimers of PUC. The results show that under irradiation of l ] 310 nm photodimerization process are increased and under UV irradiation with l [ 260 nm, photocleavage degree have been evidenced, too. Fluorescence spectra analysis of PUC in polar and film-based solvents reveals another very important result. The 375nm fluorescence wavelength of the PUC (exciting beam 310 nm) is about 30 times more intense than the same maximum in THF and in DMF. This result indicates a new method to confirm that PUC film photodimerisation occurs especially at its surface.

Silica-nanocomposites of photo-crosslinkable poly(urethane)s based on poly(ε-caprolactone) and coumarin

Photo-reversible nanocomposites of polyurethanes (PUs) based on coumarin (CD) and poly(ε-caprolactone) (PCL), with different molecular weights (PCL530, PCL2000) were developed. High molecular weight PUs were loaded with three different amounts of fumed silica nanoparticles (SiNPs) (1, 3 and 5wt%) to improve their physical, mechanical, thermal and photo-chemical properties. The incorporation of SiNPs and the interaction with the PU matrix was confirmed by FTIR, while their dispersion was observed by Field Emission Scanning Electron Microscopy (FE-SEM). The thermal stability of PU nanocomposites was improved due to the positive interaction of SiNPs with PU matrix. The incorporation of SiNPs leads to the formation of more thermal stable semicrystalline structure. Stress-strain experiments showed an increase on Young‘s modulus. Surprisingly, the photo-reversible ability of nanocomposites measured by UV–vis exhibited faster photo-dimerization process in PCL2000-PUs based nanocomposites than in PCL530-PUs ones, contrarily to the behavior of PUs without SiNPs. These nanocomposites are promising systems to design versatile and sustainable coatings with improved thermal stability and tunable mechanical performance.

Synthesis, crystal structure and photochemistry of Hexakis(butan-1-aminium) heptamolybdate(VI) tetrahydrate

The synthesis, crystal structure, spectral characterization, photochemistry, electrochemical and thermal studies of the hexakis(butan-1-aminium) heptamolybdate(VI) tetrahydrate (1) are reported. Dissolution of a mixed mono-hepta compound (BuNH3)8[(Mo7O24)(MoO4)] ⋅3H2O in water results in its transformation to the title compound viz., (BuNH3)6[Mo7O24] ⋅4H2O 1 (BuNH3= butan-1-aminium). The structure of the title compound consists of two crystallographically unique [Mo7O24]6− anions, twelve independent (BuNH3)+ cations and eight unique lattice water molecules, all of which are interlinked with the aid of three varieties of H-bonding interactions. Solar irradiation of 1 results in the formation of a bis(μ 2-oxo) bridged diheptamolybdate product. Electrochemical studies reveal the role of 1 in the photodimerization process. Thermal decomposition of 1 results in the formation of crystalline α-MoO3. Synthesis, structure, photochemistry and redox characteristics of hexakis(butan-1-aminium) heptamolybdate(VI) tetrahydrate are reported.

Photoresponsive Wettability in Monolayer Films from Sinapinic Acid

Sinapinic acid is an interesting material because it is both antioxidant and antibacterial agent. In addition, when illuminated with ultraviolet light, it can exhibit the so-called photodimerization process. In this paper, we report on the investigation of monolayer films from 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SinA) deposited onto poly(allylamine hydrochloride), PAH, films. SinA monolayers were prepared by using the layer-by-layer (LbL) self-assembly technique. Adsorption kinetics curves were well fitted by a biexponential function suggesting that the adsorption process is determined by two mechanisms: nucleation and growth of aggregates. By using wetting contact angle analysis, we have found that SinA monolayers exhibit photoresponsive wettability under UV irradiation (365 nm); that is, wettability decreases with increasing UV irradiation time. The photoresponse of wettability was attributed to photodimerization process. This hypothesis was supported by the dependence of surface morphological structure and absorption on UV irradiation time. The mechanism found in the well-known transcinnamic acid crystals is used to explain the photodimerization process in SinA monolayers.

Evidence for Point Transformations in Photoactive Molecular Crystals by the Photoinduced Creation of Diffuse Diffraction Patterns

Time-resolved diffuse X-ray scattering is one powerful method for monitoring the progression from the creation of local structural changes inside a crystalline material up to the transformation of the whole crystalline bulk. In this work, we study the mechanism of phototransformation of a molecular crystal by time-resolved diffuse X-ray scattering. Here, an optical excitation source, like a pulsed laser, initiates structural transformations which are monitored by X-ray scattering techniques. We have studied the dimerization process of the molecular switch α-styrylpyrylium (trifluoromethanesulfonate) TFMS, in particular for understanding whether cooperative effects influence the changes of the structure in the bulk and its periodicity. Upon illumination with optical light, α-styrylpyrylium TFMS instantaneously photoswitches. Depending on the optical fluence, X-ray diffuse planes are observed prior to phototransformation of the bulk. In the early stages of transformation, the analysis reveals systems of randomly distributed islands of product clusters with gradual growth in size and amount. The degree of transformation follows the optical excitation profile, i.e., the spatial absorption of the laser beam. In the present studies, no influence of cooperativity on the photodimerization process has been found.

A time-resolved powder diffraction study of in-situ photodimerization kinetics of 9-methylanthracene using a CCD area detector and parametric Rietveld refinement

The [4π + 4π] photodimerization process of the 9-substituted anthracene derivative crystalline 9-methylanthracene (9-MA) was investigated using time-resolved X-ray powder diffraction. The study was carried out in-situ using a CCD area detector. Sequential and parametric Rietveld refinement was applied for quantitative phase analysis. The results of traditional sequential Rietveld refinement showed that the evolution of the dimerization process can be described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The parameters of the JMAK equation were obtained successfully by parametric Rietveld refinement and suggest that the reaction follows heterogeneous nucleation and one-dimensional growth with a decreasing nucleation rate.

Single‐Crystal‐to‐Single‐Crystal Transformation from 1 D Staggered‐Sculls Chains to 3 D NbO‐Type Metal‐Organic Framework through [2+2] Photodimerization

A new approach to synthesize 3D structures through a [2+2] photodimerization process has been carried out by transforming two isostructures of 1D staggered-sculls chains [M(1,2-chdc)(bpe)(2)(H(2)O)(2)]⋅H(2)O (M = Zn, Mn) into 3D NbO-type metal-organic frameworks through UV irradiation in single-crystal-to-single-crystal mode. (see figure; bpe = trans-1,2-(bis(4-pyridyl)ethene, chdc =cyclohexanedicarboxylate).

Photochemical Behavior of High Quantum Yield SWNTs Functionalized with Anthracene Salts

Two different fluorescent single walled carbon nanotubes (SWNT) were prepd. First by a reaction of oxidized SWNT-K salt with 1,3-bis(9-anthracenylmethyl)imidazolium chloride (bamim)Cl to provide a fluorescent system SWNT-(bamim), via ion exchange and formation of KCl. Second, by phys. mixing of non-functionalized SWNTs with (bamim)Cl salt to provide SWNT/[bamim]Cl in which anthracene units adhere to the walls of the nanotube via π-π stacking interactions. The SWNT-[bamim] material exhibits a partially reversible photodimerization process that was investigated using UV/Vis spectroscopy, fluorescence emission spectroscopy, and HR-TEM. This study establishes the concept of photoinduced/photoswitchable org.-SWNT conjugated systems based on anthracene dimerization as a prototypical reaction. Hierarchical nanoscale hybrid structures based on the above materials and an understanding of their optical properties may facilitate the construction of SWNT based materials.

R-1,c-2,t-3,t-4-1,2-Bis(4-methoxyphenyl)-3,4-bis(5-phenyl-1,3,4-oxadiazol-2-yl)cyclobutane

The title compound, C34H28N4O4, is a derivative of di­phenyl­oxa­diazo­lylcyclo­butane (s-DPC), and although it is a syn-head-to-head photodimer, it does not display molecular mirror symmetry in the crystalline state. The cyclo­butane ring has a puckered conformation, with a dihedral angle of 30°, which is the largest among tetra­aryl-substituted cyclo­butanes reported so far. The four C—C single bonds of the cyclo­butane ring differ from each other. One is the same length as a typical C—C single bond, while the others are longer. The photodimerization of the corresponding monomer yields only 1% of s-DPC, suggesting that the head-to-head reaction is a minor photodimerization process.

R-1,c-2,t-3,t-4-1,3-Bis(4-methoxyphenyl)-2,4-bis(5-phenyl-1,3,4-oxadiazol-2-yl)cyclobutane 1,4-dioxane solvate

The asymmetric unit of the crystal structure of C34H28N4O4.C4H8O2, which is a 1:1 inclusion complex, consists of half a mol­ecule of the title compound t-DPC and half a mol­ecule of 1,4-dioxane. The structure shows that t-DPC is centrosymmetric; its cyclo­butane ring has an exactly planar conformation, and is nearly square. The four C—C single bonds are of equal length [1.569 (4) Å] and the bond angles are 91.6 (2) and 88.4 (2)°. The major photodimerization process is a head-to-tail reaction rather than a head-to-head one.

Solid-state photodimerization of 9-methylanthracene as studied by solid-state [formula omitted] NMR

The locus of the photodimerization reaction of 9-methylanthracene in the crystal was examined by high-resolution solid-state 13 C NMR techniques. Examination of the 13 C spectra of the products showed that only the trans dimer is formed by the solid state photodimerization, while both trans and cis dimers are formed by the photodimerization in benzene solution. The 1 H T 1 relaxation curves for the monomer and the dimer at various exposure times were separately observed via well-resolved 13 C peaks. The two T 1 curves exhibit characteristic features for relaxation of a weakly coupled two-spin system. By analyzing the T 1 curves, the spin diffusion rates between the monomer and the dimer and the fraction of the dimer were obtained for various exposure times. From the result, the maximum domain size of the minor component during the photodimerization process was estimated to be ca. 0.3 μm. The heterogeneous domain structure generated by the photodimerization indicates that the reaction takes place at defects of the crystal in the monomer.

Nonsteroidal antiinflammatory drug-photosensitized formation of pyrimidine dimer in DNA

Phototoxic nonsteroidal antiinflammatory drugs (NSAIDs) may induce DNA damage in vitro upon irradiation. In this study, we investigated the ability of ketoprofen (KP), tiaprofenic acid (Tia), naproxen (NP) and indomethacin (IND) to photosensitize the formation of pyrimidine dimers and single strand breaks. Both kinds of damage were sought by analyzing DNA-drug mixtures irradiated at 313 nm by agarose gel electrophoresis. The formation of pyrimidine dimers was evidenced by using endonuclease V from bacteriophage T4 and compared to that induced by acetophenone, a well-known photosensitizer of thymine dimerization. Upon irradiation of DNA alone, pyrimidine dimers were observed while single strand breaks were not detected under our conditions. DNA, in the presence of NSAIDs, undergoes single strand breaks, the quantum yield of the DNA cleavage so induced (ΦC) varying from 5 × 10 −4 for KP to 10 −5 for IND. The formation of dimers was only increased in the presence of KP or Tia. The quantum yields of pyrimidine dimers formed by photosensitization (ΦD) were 2 × 10 −4 for KP and 10 −5 for Tia, respectively. The oxygen and concentration dependence of both processes was analyzed in the case of KP. In aerated solution, KP-photoinduced cleavage of DNA was predominant on the photodimerization process of pyrimidines, whereas in deaerated solution the cleavage was decreased and the dimerization increased. These results reflect competition between a radical process leading to DNA cleavage and a poorly efficient energy transfer between the drug and the pyrimidines at the origin of the dimerization process.

Anthracene monomer-dimer photochemistry: High density 3D optical storage memory

Ultrafast spectra and kinetics of all spectroscopic states and intermediate species in the photodimerization process of substituted anthracenes have been identified and measured by means of ultrafast spectroscopy. The two forms of this process, monomer and dimer, have been used to construct a 3D optical storage memory device capable of 1013 bits/cm3 density and very large bandwidth. Storage and accessing 3D information is based on non-linear absorption and the different structures of the monomer and dimer. This rather novel 3D memory device and its operation is described in detail.

Photo-dimerization kinetics in solid C 60 films

The kinetics of the photo-dimerization of solid C 60 under 488 nm laser irradiation is studied by Raman scattering and is identified with a first-order reaction between one monomer in the first excited triplet state and the second monomer in the ground state (e.g. 3M*+M→D). The photo-dimerization process is limited by the photoexcitation step, and the rate constant is found proportional to the incident light flux, indicating that a single photon is involved.