Coumarin Dimer Research Articles

Photocleavage of dimers of coumarin and 6-alkylcoumarins

The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin ( syn-ht-CC 1), the anti- and syn-hh dimers of 6-methylcoumarin ( anti-hh-CC 2 and syn-hh-CC 2, respectively) and the anti-hh dimer of 6-dodecylcoumarin ( anti-hh-CC 3), was studied by UV–vis and IR spectroscopy and HPLC upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φ sp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO 2 along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile.

1,1-Dimethylnaphthalenon-dimers as photocleavable linkers with improved two-photon-absorption efficiency and hydrolytic stability

Coumarins are well known for reversible dimer formation with wavelengths greater than 300 nm and dimer cleavage below 300 nm. In a photochemical [2+2]-cycloaddition a cyclobutane ring is formed. Formation as well as cleavage of the cyclobutane ring may be accomplished by a single-photon-absorption as well as by a two-photon-absorption triggered reaction. The coumarin system is of interest for various kinds of applications, ranging from drug delivery for ophthalmic implants to optical data storage. However, the two-photon-absorption coefficient of coumarin dimers is rather low falling in the range of 1 GM in the visible range. We present here a substitute for the coumarin dimer system which not only has an about one order of magnitude higher two-photon-absorption coefficient, but also overcomes several other problems of the coumarin dimer system. Coumarines and in particular coumarine dimers have a very limited solubility in common solvents and are susceptible to hydrolysis of the lactone ring, which leads to an undesired complexity in the photochemical cleavage reaction. The 1,1-dimethylnaphtalenone dimers introduced here show excellent stability, lead only to a single cleavage product, and have a two-photon-absorption coefficient of about 10 GM at 532 nm. These properties make the 1,1-dimethylnaphtalenone dimers a superior substitute over the well-known coumarin dimers in particular in applications where two-photon-absorption induced photocleavage is required.

Synthesis and Photochemistry of Coumarin-Based Self-Assembled Monolayers on Silicon Oxide Surfaces

In this study, we report on a system consisting of self-assembled monolayers (SAMs) formed by 7-(11-trichlorosilylundecyloxy)coumarin on mica and on quartz glass. For the first time, in the absence of an inert atmosphere or a stabilizing matrix, we demonstrate by means of absorption and fluorescence spectroscopy that the photochemical cycloaddition of coumarin head groups is completely reversible in SAMs. Photodimerization and photocleavage were monitored for five cycles of alternating irradiation with light of wavelengths 355 and 254 nm, respectively. SAM formation was analyzed using atomic force microscopy and contact angle measurements. The quantum yield of the single photon absorption induced photocleavage of coumarin dimers in a SAM was determined to be Phi = 0.29. Asymmetrical photocleavage after lactone ring-opening of the coumarin dimer SAM causes a change in contact angle from about 70 degrees to about 55 degrees. This may be observed, for example, as a significant change in surface wettability.

Directed Organization of Dye Aggregates in Hydrogen-Bonded Host Frameworks

Organic laser dyes coumarin 1, coumarin 2, coumarin 102, coumarin 314, and coumarin 334 have been included in crystalline guanidinium organodisulfonate (GDS) host frameworks, forming stable inclusion compounds with lamellar architectures. The GDS hosts consist of 2D quasihexagonal hydrogen-bonded sheets with topologically complementary guanidinium (G) ions and the sulfonate (S) moieties of a variety of organodisulfonates that serve as “pillars” that connect opposing GS sheets, thus generating inclusion cavities between the sheets. These host frameworks display a variety of architectural isomers, including the so-called bilayer, simple brick, zigzag brick as well as two heretofore unreported framework isomers: double zigzag brick and a “chevron” brick. These isomers vary with respect to the connectivity between opposing GS sheets and the corresponding shape of the inclusion cavities. The preference for the framework isomers reflects a systematic templating role for the guest molecules, largely based on their steric requirements. The coumarin guests exhibit a distinct arrangement in each host−guest combination, resulting in a range of fluorescence emission wavelengths that differ from that observed for the monomeric form in dilute solutions. For example, the bilayer framework, which has narrow 1D pores, enforces the alignment of coumarin guests as head-to-tail arrays resembling J-aggregates. The simple brick structure, however, has wider 1D channels that permit the formation of linear arrays of face-to-face coumarin dimers resembling H-aggregates. The coumarin guests in the zigzag brick architecture are confined within isolated inclusion cavities as face-to-face dimers. In general, the maxima of the emission bands of the coumarin dyes in the bilayer structure are blue-shifted, while those in the simple brick and zigzag brick structures are red-shifted. The fluorescence reflects the unique guest−guest packing in each framework isomer as well as interactions between the coumarin guest and the organodisulfonate pillar. The ability to include laser dyes in high concentrations in a robust host framework with regulation of intermolecular association between the dye molecules may lead to new solid-state lasing materials that overcome some of the barriers encountered for dyes embedded as dilute solutes in amorphous solid-state matrices such as polymers, zeolites, and sol−gel glasses.

Effects of Dilution, Polarization Ratio, and Energy Transfer on Photoalignment of Liquid Crystals Using Coumarin-Containing Polymer Films

Orientation of a nematic liquid crystal, E-7, was investigated using coumarin-containing polymethacrylates to elucidate the roles played by the dilution of coumarin and the polarization ratio of irradiation. Dilution of coumarin by inert moieties had adverse effects on a nematic cell’s number density of disclinations and its orientational order parameter in the parallel but not the perpendicular regime. In addition, both dilution of coumarin and a decreasing polarization ratio resulted in a lower extent of coumarin dimerization at crossover, Xc. The significantly reduced Xc in a homopolymer comprising triphenylamine and coumarin was attributed to the dilution of coumarin and the diminished polarization ratio caused by competing absorption with simultaneous triplet energy transfer from triphenylamine to coumarin moieties.

Single- and two-photon absorption induced photocleavage of dimeric coumarin linkers: Therapeutic versus passive photocleavage in ophthalmologic applications

Phototriggered chemical reactions are elegant and spatially selective. However, in application a general problem is that aside of the desired changes, e.g. induced by laser, undesired changes may occur, e.g. induced by daylight or sunlight. One such example which we are studying here is the phototriggered release of drugs from materials suitable for polymeric intraocular lenses (IOL). Through such an IOL sunlight needs to pass for several years without affecting the drug loading, but at the therapeutically required moment the drug release shall be triggered also by light. We studied this using a novel coumarin dimer (7,7′-[3-( tert-butyldimethylsilyloxy)propoxy]-dicoumarin) synthesized from the monomer in a photochemical [2π + 2π] cycloaddition reaction. The cleavage of the obtained photodimer by single-photon (SPA) and two-photon absorption (TPA) excitation was investigated in acetonitrile solution and in polymethylmethacrylate in order to exclude major matrix entrapment effects. The cleavage quantum yields and TPA cross-section were determined to be Φ ≈ 0.13 and δ TPA ≈ 2.7 GM, respectively. It was found that 10% undesired drug release would require more than 16 years of continuous exposure to sunlight. Release of 90% of drug under therapeutic conditions would be completed within seconds. This shows that photochemically triggered drug release is capable to become a powerful tool for drug delivery in ophthalmology.

Quantitative Assessment of Coumarin-Containing Polymer Film's Capability for Photoalignment of Liquid Crystals

The photoalignment of a nematic fluid, E-7, and a glassy-nematic oligofluorene, F(MB)5, was investigated on films of Polymers 1 and 2 in the parallel regime. Polarized absorption spectroscopy and computational chemistry were employed to characterize coumarin monomer's and dimer's molar extinction coefficients and to locate absorption dipoles as parallel to their long molecular axes. Moreover, their orientational order parameters, Sm and Sd, were experimentally determined as functions of the extent of dimerization. Higher Sd and Yd, coumarin dimer's mole fraction, were achieved in films of Polymer 1 than in Polymer 2 because of the greater coumarin mobility of the former. The ability of a coumarin-containing photoalignment film to orient a spin-cast F(MB)5 film was found to improve with increasing YdSd to an extent comparable to that of a rubbed polyimide film. Because of the relatively short lengths of its constituent molecules, E-7 was oriented equally well on both polymer films regardless of the YdSd va...

Photodimerization of Coumarin Functionalized Poly(alkyl Acrylate) and Poly(alkyl Methacrylate) Random Copolymers: Influence of Copolymer Composition on Photocrosslinking

Coumarin‐containing poly(alkyl (meth)acrylates) were prepared via 70–80% esterification of hydroxy‐containing acrylic copolymers, then solution cast into thin films and photocrosslinked via the dimerization of coumarin derivatives with UVA light. The coumarin‐modified polymers crosslinked upon exposure and exhibited gel fractions between 74 and 99%. Coumarin dimerization efficiency increased with higher polymer mobility at the irradiation temperature. The effects of light intensity and irradiation time in photo‐dimerized systems followed the Bunsen‐Roscoe reciprocity law indicating that coumarin photodimerization depended only on dose. Thus, low intensities are overcome with longer times. This is an important advantage over photoinitiated free radical crosslinking which depends on irradiation intensity to the ½ power.

Coumarin Dimer and Sesquiterpene Lactones from Carpesium lipskyi Winkl.

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Preparation of new C2-symmetric tetraphosphine ligands for Rh-catalyzed asymmetric hydrogenation of arylenamides

A new type of chiral tetraphosphine ligands 1 were prepared from enantiopure anti-head-to-head coumarin dimer 5 and then utilized in Rh-catalyzed asymmetric hydrogenation reaction of arylenamides, affording the corresponding amides with up to 85% ee. Some chiral bisphosphine ligands derived from coumarin dimer 5 were also evaluated in the same reaction.

The photoinduced cleavage of coumarin dimers studied with femtosecond and nanosecond two-photon excitation

The photocleavage of a coumarin derivative dimer is a promising mechanism for laser controlled drug release in medical applications. We studied the efficiency of such reaction after two-photon excitation, comparing femtosecond and nanosecond pulses in the range from 400 to 800 nm. The photocleavage reaction is remarkably more efficient for shorter wavelengths. The spectral profile of the two-photon absorption (TPA) spectrum resembles the single-photon absorption (SPA) spectrum. It was also observed that the photocleavage reaction triggered with femtosecond pulses is about 4.9 × 10 4 times more efficient than the reaction with nanosecond pulses at the same wavelength and beam area.

Synthetic Models Related to Furanocoumarin-CYP 3A4 Interactions. Comparison of Furanocoumarin, Coumarin, and Benzofuran Dimers as Potent Inhibitors of CYP3A4 Activity

Furanocoumarin derivatives (dimers and monomers) present in commercially available grapefruit juice have the capacity to inhibit the activity of human CYP3A4. Such interactions are believed to result from the mechanism-based inhibition of CYP3A4 activity in the intestine. The aim of this work was to synthesize and test a series of dimers with a view to determining the relationship between structure and inhibitory activity and determining whether they might make suitable probes of CYP3A4 activity. We prepared a series of furanocoumarin, coumarin, and benzofuran derivatives that have inhibitory effects on the activity of human CYP3A4. A synthetic benzofuran dimer, which is more accessible than furanocoumarin dimers, exhibited activity against CYP3A4 comparable to that of furanocoumarin dimers.

Photodimerized 7-hydroxycoumarin with improved solubility in PMMA: Single-photon and two-photon-induced photocleavage in solution and PMMA films

tert-Butyldimethylsilyl-chloride (TBS) revealed to be a photostable protecting group for the photodimerization of 7-hydroxycoumarin in a [2 + 2]-cycloaddition. TBS-functionalized coumarin dimers show an about 100-fold increased solubility in organic solvents enabling them to be easily incorporated into polymeric films, e.g., PMMA. In the described photochemical dimerization reaction almost pure anti-head-to-head isomer is obtained. The single- and two-photon absorption-induced cycloreversion reactions in acetonitrile as well as in PMMA matrix were investigated and the two-photon absorption cross sections and quantum yields were determined to be around 1 GM and about 0.36, respectively. The only product obtained upon photocleavage of the dimer is the TBS-protected 7-hydroxycoumarin monomer. The TBS-protecting group withstands the high light intensities required for two-photon absorption-induced photocleavage without any noticeable degradation. The mild deprotection conditions for tert-butyldimethylsilyl-ethers (TBS-ethers), the chemical stability of the compound as well as its significantly improved solubility in organic solvents and its miscibility with acrylic polymers, make this a very useful compound for potential applications in 3D volumetric optical data storage and photocontrolled drug delivery.

Photodeformable Spherical Hybrid Nanoparticles

We report the first synthesis of spherical nanoparticles of a bridged polysilsesquioxane. The hard, brittle nanoparticles are prepared by a simple sol-gel polymerization without surfactants or templates. Particle size ranges from 50 to 100 nm. The organic component of these hybrid nanoparticles is composed of a photolabile coumarin dimer. Irradiation with UV light dissociates the photodimer resulting in rupture of the cross-links and subsequent deformation and eventual "melting" of the nanoparticles.

New Insight into Photoalignment of Liquid Crystals on Coumarin-Containing Polymer Films

Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the parts of the reacted and the unreacted coumarin moieties. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and the unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction.

Development of a Light-Deactivatable PSA Via Photodimerization

ABSTRACT Photoreactive pressure-sensitive adhesives (PSAs) were developed to create photodeactivatable resins. Coumarin-functionalized poly(2-ethylhexyl acrylate-co-hydroxyethyl acrylate) was prepared via traditional free-radical polymerization followed by quantitative hydroxyl group esterification and studied as model photoactive PSAs. The polymers were solution-cast into films and photocrosslinked via dimerization of the coumarin derivatives with ultraviolet-A (UVA) light irradiation (> 300 nm). Ultraviolet-Visible (UV-Vis) spectroscopy indicated that approximately 60% of the coumarin groups photodimerized when exposed to 22 J cm−2 of UVA irradiation. The formation of reversible coumarin crosslinks gelled the model PSA and reduced peel strength from 1.62 to 0.05 N/mm. UVC irradiation photocleaved the coumarin dimers, reducing the crosslink density and raising the peel strength to 0.10 N/mm. This reversibility of the coumarin photodimerization and consequent peel-strength modulation may provide a mechanism for the repeated use of these model adhesives.

New Bisphosphine Ligands for Palladium-Catalyzed Desymmetrization ofmeso-Cyclopent-2-en-1,4-diol Biscarbamate

A new class of bisphosphine ligands (5, 6) with a cyclo­butane backbone have been designed and synthesized on the basis of a privileged C 2 scaffold of head-to-head coumarin dimer, among which ligand 5a was found to show excellent activity and enantioselectivity in Pd-catalyzed desymmetrization of the biscarbamate of meso-cyclopent-2-en-1,4-diol, affording oxazolidin-2-one in up to 90% yield with 97% ee.

New types of soluble polymer-supported bisphosphine ligands with a cyclobutane backbone for Pd-catalyzed enantioselective allylic substitution reactions.

A highly efficient and practical optical resolution of anti head-to-head racemic coumarin dimer 7 has been achieved by molecular complexation with TADDOL, (-)-8, through a hydrogen bonding interaction to afford the corresponding two enantiomers, (-)- and (+)-7, in 70 and 75 % yields, respectively, with >99 % ee. Starting from enantiopure (-)-7, a new type of C2-symmetric bisphosphine ligand (S,S,S,S)-3 with a cyclobutane backbone has been synthesized in good yield by facile transformations. The asymmetric induction efficiency of these chiral bisphosphine ligands in Pd-catalyzed asymmetric allylic substitution reactions was evaluated. Under the experimental conditions, the allylic substitution products could be obtained in excellent yields (up to 99 %) and enantioselectivities (up to 98.9 % ee). By taking advantage of the high enantioselectivity of this catalytic reaction and the easily derivable carboxylate groups on the cyclobutane backbone of ligand (S,S,S,S)-3, a new type of analogous ligand (S,S,S,S)-4 as well as the MeO-PEG-supported soluble ligand (S,S,S,S)-5 (PEG=polyethylene glycol) have also been synthesized and utilized in asymmetric allylic substitution reactions. In particular, the MeO-PEG supported (S,S,S,S)-5 b had a synergistic effect on the enantioselectivity of the reaction compared with its nonsupported precursor (S,S,S,S)-4 c, affording the corresponding allylation products 14 a and 14 b with excellent enantioselectivities (94.6 and 97.2 % ee, respectively). Moreover, the Pd complex of (S,S,S,S)-5 b could easily be recovered and recycled several times without significant loss of enantioselectivity and activity in the allylic substitution reactions.

Two New Coumarin Derivatives from the Roots ofHeracleum rapula

Two new coumarins, 13- O-[ beta- D-apiofuranosyl(1-->6)- beta- D-glucopyranosyl]-(12 R)-heraclenol ( 1) and (12 R,12" R)-diheraclenol ( 2) were isolated from the acetone extract of the fresh roots of Heracleum rapula. Their structures were determined by means of spectroscopic analysis and, in the case of compound 1, the structure elucidation was supported by acid hydrolysis. Compound 1 is a coumarin glycoside while 2 is a coumarin dimer. The inhibitory effects of 1, its aglycone ( 3), and 2 on rabbit platelet aggregation induced by PAF, AA and ADP were tested. Weak activities were observed for each compound with the percentages of inhibition in the range of 0.7 - 24.8 %.

Photoreversible Chain Extension of Poly(ethylene glycol)

AbstractSummary: Coumarin containing poly(ethylene glycol) monols and diols were prepared in a telechelic fashion using quantitative end group esterification. The coumarin modified poly(ethylene glycol) monols and diols were solution cast into films (1 to 15 μm) and chain extended via the dimerization of the coumarin derivatives with UVA (>300 nm) light irradiation. The coumarin‐modified PEG diol doubled in molecular weight, and the molecular weight distribution increased from 1.17 to 2.75 upon exposure to 110 J · cm−2 of UVA light. The chain extended poly(ethylene glycol)s were subsequently converted to their original molecular weight via the photocleavage reaction of the coumarin dimer at 254 nm with less than 2 J · cm−2. The photoreversible chain length modulation was monitored utilizing GPC, DSC and UV‐Vis spectroscopy.Molecular weight modulation of COU‐PEG‐COU.magnified imageMolecular weight modulation of COU‐PEG‐COU.