Pyrene Chromophores Research Articles

Elaboration of Covalently Linked Polyoxometalates with Ruthenium and Pyrene Chromophores and Characteriation of Their Photophysical Properties

Keggin and Dawson-type polyoxometalates (POMs) decorated by organometallic [cyclometalated ruthenium(II) polypyridine complex] or organic (pyrene) chromophores were prepared by postfunctionalization of hybrid disilylated POM platforms. The connection is made in a very efficient and modular way via Sonogashira coupling reactions, which provide a rigid linkage between the POM and the photoactive centers. Electronic properties have been inferred from electrochemical and photophysical studies and reflect poor electronic interactions between both partners. The presence of the POM leads to luminescence quenching of the chromophores, which was attributed to an intramolecular electron transfer from the chromophore to the POM. The rate of this process is much faster in the POM-pyrene than in the POM-Ru system. It depends on the driving force dictated by the redox potentials of both partners but also in the case of the POM-Ru system on the presence of the metallacycle, which acts as a molecular insulator and delays the intramolecular electron transfer. In the POM-Ru system, a comparative study of the luminescence quenching showed that the electron transfer is still more important in the covalently bonded hybrids than in systems where the POM and the ruthenium complexes are assembled via electrostatic interactions.

Tunable Emission of Static Excimer in a Pyrene‐Modified Polyamidoamine Dendrimer Aggregate through Positive Solvatochromism

Significant aggregation is observed in pyrene-modified zero- and first-generation polyamidoamine (PAMAM) dendrimers above their critical aggregation concentration (CAC, >10(-6) M). The pyrene units are attached to the dendrimer skeleton through imine bonds, which play a pivotal role in enhancing the aggregation propensity of the PAMAM dendrimers. Scanning electron microscopy studies suggest that pyrene-modified PAMAM dendrimers aggregate into doughnut-shaped assemblies. As a result of aggregation, the pyrene chromophores are pre-arranged in a face-to-face geometry in the ground state, and readily generate pyrene "static excimer" on photoexcitation. The static pyrene excimer emits with an unprecedented quantum yield of 0.62 ± 0.01 in dichloromethane, and also exhibits remarkable positive solvatochromism from 498 to 638 nm, which leads to the highest bathochromic shift for pyrene excimer emission in solution reported so far. Lippert-Mataga analysis of the system suggests that general and specific solvent effects play a crucial role in the positive solvatochromism exhibited by the system. Luminescence quenching studies on both monomer and aggregate systems were carried out in the presence of various metal ions, and the results imply that pyrene-modified PAMAM dendrimer can be utilized for selective detection of Hg ions in the presence of a wide variety of transition, alkali, and alkaline earth metal ions. This report presents the first dendrimer-based chromophoric system exhibiting positive solvatochromism over a range of 140 nm, and shows that pyrene-modified PAMAM dendrimers can be effectively utilized to generate wavelength-tunable emitting systems displaying bluish green, greenish yellow, and orange-red colors at room temperatures.

Engineering supramolecular photoactive nanomaterials by hydrogen-bonding interactions

The photophysical properties of molecules containing anthracene, pyrene, or phenyleneethynylene chromophores bearing complementary triple H-bonding terminal units, namely, 2,6-di(acetylamino)pyridine (donor–acceptor–donor, DAD) and uracyl (acceptor–donor–acceptor, ADA) have been investigated as a function of solvent polarity. For asymmetric systems, presenting only one H-bonding unit, a solvatochromic effect is found, suggesting a charge-transfer character of the lowest electronic excited state. Systematic absorption and emission studies carried out as a function of temperature show that phenylene-ethynylenes having linear geometry and H-bonding functionalities at both ends undergo reversible self-aggregation in cyclohexane (CHX), leading to the formation of spherical nanoparticles, as evidenced by wide-field fluorescence microscopy (WFM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A combination of an anthracene derivative bearing only one ADA terminal functionality and a linear phenylene-ethynylene derivative possessing two DAD terminal groups in CHX (2:1 molecular ratio) leads to the formation of vesicular nanostructures. The interaction of linear phenylene-ethynylenes possessing two terminal 2,6-di(acetylamino)pyridine functionalities with that bearing bis uracylic units gives origin to nanofibers, while the assembly of the former with bisuracylic units exhibiting bent geometry leads to the formation of helical nanofibers. The length of these fibers can be controlled by addition of the anthracene derivative having only one uracyl group which effectively blocks the extent of H-bonding, prompting the formation of shorter nanorods.

On the geometrical structure and spectral properties of pyrene monomer and sterically constrained intramolecular pyrene dimers

The combined theoretical and spectroscopic study of pyrene molecule and its two model sterically constrained dimers ( DPyM: di(1-pyrenylmethyl) ether and DPyS: 2,3-di(4-(1-pyrenemethoxymethyl)-phenyl)butane) is presented. Based on quantum chemical calculations, the origin of the vibronic structure of pyrene fluorescence spectra was analyzed using Franck–Condon analysis. The DPyM linked by a short spacer cannot reach the parallel arrangement of the pyrene chromophores. The elongation of the central part in DPyS increases the distance between the chromophores by about 5 Å and therefore the quasi-parallel arrangement of pyrenes is possible from the structural point of view. The analysis of the selected frontier molecular orbitals and their energy levels with respect to the isolated pyrene molecule indicates that the intramolecular interaction in DPyS plays still non-negligible role for the space orientation of pyrene cores contributing to the excimer formation. The experimental fluorescence energy of DPyS is blue shifted with respect to DPyM.

Fluorescent and Electrochemical Sensing of Polyphosphate Nucleotides by Ferrocene Functionalised with Two ZnII(TACN)(pyrene) Complexes

The [Fc-bis{Zn(II)(TACN)(Py)}] complex, comprising two Zn(II)(TACN) ligands (Fc=ferrocene; Py=pyrene; TACN=1,4,7-triazacyclononane) bearing fluorescent pyrene chromophores linked by an electrochemically active ferrocene molecule has been synthesised in high yield through a multistep procedure. In the absence of the polyphosphate guest molecules, very weak excimer emission was observed, indicating that the two pyrene-bearing Zn(II)(TACN) units are arranged in a trans-like configuration with respect to the ferrocene bridging unit. Binding of a variety of polyphosphate anionic guests (PPi and nucleotides di- and triphosphate) promotes the interaction between pyrene units and results in an enhancement in excimer emission. Investigations of phosphate binding by (31)P NMR spectroscopy, fluorescence and electrochemical techniques confirmed a 1:1 stoichiometry for the binding of PPi and nucleotide polyphosphate anions to the bis(Zn(II)(TACN)) moiety of [Fc-bis{Zn(II)(TACN)(Py)}] and indicated that binding induces a trans to cis configuration rearrangement of the bis(Zn(II)(TACN)) complexes that is responsible for the enhancement of the pyrene excimer emission. Pyrophosphate was concluded to have the strongest affinity to [Fc-bis{Zn(II)(TACN)(Py)}] among the anions tested based on a six-fold fluorescence enhancement and 0.1 V negative shift in the potential of the ferrocene/ferrocenium couple. The binding constant for a variety of polyphosphate anions was determined from the change in the intensity of pyrene excimer emission with polyphosphate concentration, measured at 475 nm in CH(3)CN/Tris-HCl (1:9) buffer solution (10.0 mM, pH 7.4). These measurements confirmed that pyrophosphate binds more strongly (K(b)=(4.45+/-0.41) x 10(6) M(-1)) than the other nucleotide di- and triphosphates (K(b)=1-50 x 10(5) M(-1)) tested.

Synthesis of novel push–pull-type solvatochromic 2′-deoxyguanosine derivatives with longer wavelength emission

We synthesized novel push–pull-type fluorescent guanosine derivatives, CN G and Ac G containing 1,6- and 2,7-disubstituted pyrene chromophores. 1,6-Disubstituted pyrene derivatives, 1,6- CN G (3b) and 1,6- Ac G (3c) , exhibited highly solvatochromic fluorescence emission at longer wavelength (∼540 nm). The environmentally sensitive fluorescent deoxyguanosines such as 3b and 3c can be used as powerful tools for structural studies of nucleic acids and molecular diagnostics.

Covalent Attachment of Pyrene onto the Layer Surfaces of Silylated Magadiite at a High Concentration without Aggregation

Abstract Magadiite containing covalently attached pyrene chromophores at a high concentration was prepared by the reaction between silylated magadiite possessing amino groups and 1-pyrenebutanoic acid succinimidyl ester. The monomeric pyrene content without aggregation reached a very high value of 0.57 pyrene/nm2 in the nanosheet state.

Polypseudopeptides with Variable Stereochemistry: Synthesis via Click-Chemistry, Postfunctionalization, and Conformational Behavior in Solution

Polypseudopeptides with well-defined stereochemistries have been synthesized from readily available amino-acid-based building blocks by connecting (l,l)- or (l,d)-dipeptide AB-monomers carrying azide and alkyne termini via triazole amide-isosteres efficiently formed in the course of the “click” reaction. Deprotection of the thus-prepared lysine-based polypseudopeptides of both all-(l)- and (d)-alt-(l)-stereochemistries afforded water-soluble polymers with ionizable amino side chains, which could be fully labeled with pyrene chromophores via quantitative amide bond formation. The conformational behavior of the deprotected as well as the pyrene-labeled polymers was investigated using UV/vis, CD, and fluorescence spectroscopies. On one hand, the free polyamines display pH-dependent conformations in water. On the other hand, the pyrene-labeled polypseudopeptides change their conformation in response to varying organic solvent composition. Whereas the strictly alternating polypseudopeptides structurally resemble channel-forming peptides, such as the Gramicidin family, the incorporation of (d)-configured amino acids as well as triazole amide-isosteres should lead to interesting new materials for bioapplications.

Photophysical characterization of oligopyrene modules for DNA-based nanosystems

The photophysics of free pyrenedicarboxamide (Py-DCA) in solution as well as of single-stranded and double-stranded oligonucleotides (ss and ds ONs) containing 1-7 pyrene building blocks per strand were studied by steady-state and time-resolved fluorescence spectroscopy. It was found that the fluorescence quantum yield Phi(F) of free Py-DCA chromophore in solution is rather high (Phi(F) = 0.44). However, after incorporation of the chromophore into a ss ON the monomeric chromophore fluorescence is quenched more than 40-fold due to electron-transfer reactions with ON bases. An increase of the number n of neighboring pyrenes in an ON results in Phi(F) growth up to 0.25 at n = 6. Starting from n = 2, all fluorescence belongs mainly to excimer formed by pyrene chromophores. Sections composed of multiple pyrenes may be considered as robust functional entities that may serve as independent modules in DNA-based, functional nano-architectures.

Copper and zinc bis(thiosemicarbazonato) complexes with a fluorescent tag: synthesis, radiolabelling with copper-64, cell uptake and fluorescence studies

The synthesis of new copper(II) bis(thiosemicarbazonato) complexes with an appended pyrene chromophore and their zinc(II) analogues is reported. The new proligands and their copper(II) and zinc(II) complexes were characterised by a combination of NMR, EPR, high performance liquid chromatography, mass spectrometry, electronic spectroscopy and electrochemical measurements. The new copper(II) complexes are fluorescent as a consequence of an appended pyrene substituent that is separated from the sulphur coordinating to the metal ion by five bonds. The emission from the pyrene substituent is concentration- and solvent-dependent with characteristic formation of excimer aggregates. A radioactive (64)Cu complex has been prepared. Cell permeability, intracellular distribution and importantly the ability to cross the nuclear membrane to target DNA were investigated using confocal fluorescence microscopy in a human cancer cell line under normal oxygen conditions and hypoxic conditions. In both cases, there was no evidence of uptake of the copper(II) bis(thiosemicarbazonato) complexes in the area of the cell nucleus.

Large Footprint Pyrene Chromophores Anchored to Planar and Colloidal Metal Oxide Thin Films

Sensitization and binding of a large footprint pyrene chromophore to planar (sapphire) and colloidal metal oxide films (TiO2 and ZrO2) is investigated. The model compound combines a 1-pyrenyl-ethynylenephenylene unit with a new adamantane-tripodal linker that binds to the surface. The linker design, combining a large footprint (approximately 2 nm2) of the tripodal linker with the meta position of the COOH anchoring groups, was suggested from atomistic models, and it aims to provide improved spacing control. The pyrene chromophore unit provides a probe of sensitizer-sensitizer interactions through its propensity to form excimers, unless neighboring pyrene units are sufficiently spaced (>or=3.5 A). Absorption and fluorescence studies, and a comparison with a pyrene-rigid rod model compound, suggest that the new tripodal anchor group allows spacing control on planar surfaces. On colloidal films, the linker provides spacing control at low surface coverage but sensitizer-sensitizer interactions are still observed on colloidal films at high surface coverage. Implications for the functionalization of metal oxide films in hybrid molecule-metal oxide semiconductor material systems are discussed.

Detection of RNA Hybridization by Pyrene‐Labeled Probes

Powerful pyrene probes: Two kinds of pyrene-labeled oligonucleotides (HNA- and RNA-skeleton probes) were explored. The enhanced fluorescence intensity in the monomer region and the disappearance of aggregate/excimer emission in duplexes has been successfully used to detect the hybridization of oligonucleotides. By covalently attaching pyrene chromophores with different linkers onto altritol nucleotides or ribonucleotides, and by varying the number of these pyrene modified altritol nucleotides and ribonucleotides in HNA (hexitol nucleic acid) and RNA, respectively, we have explored the general applicability of pyrene absorbance and especially fluorescence as a probe to monitor RNA hybridization. The results reveal that the backbone of the probes, the number of pyrene units attached and the nature of the tether can all substantially affect the absorbance and fluorescence properties of the probes both in single strand and double strand form. Moreover, the strength of hybridization is also affected. The disappearance of pyrene aggregate/excimer emission and simultaneous increase in monomer emission intensity of the multipyrene-labeled probes has been successfully used to monitor the hybridization of oligonucleotides, including a hairpin structure. Differences in optical response between the HNA- and RNA-skeleton probes upon hybridization indicate that the interaction of pyrene with the nucleobases in both types of duplexes is different.

Energy Transfer in a π-Conjugated Liquid Crystalline Molecule with Two Chromophores of Rigid Biphenyl Core and Pyrene Head Moieties

We investigate the energy transfer of a highly ordered pi-conjugated liquid crystalline molecule containing two chromophores of biphenyl and pyrene moiety, in which the molecule was prepared by esterification of fluorinated biphenyl compound and 1-pyrenebutanol. DSC, POM and XRD results show that this molecule forms a highly ordered smectic liquid crystalline mesophase, in which the pi-conjugated molecule was strongly packed together over large-areas. We show that the liquid crystalline bi-chromophoric systems with highly ordered structure exhibit the energy hopping and transfer to the chromophore with the energetically lowest S1 state. From the single-molecule spectroscopy of the biphenyl and pyrene chromophore in solution, we show an intramolecular energy transfer from biphenyl to pyrene moieties. In solid thin film, highly ordered structure of the liquid crystal plays a significant role in the formation of the excimer and perfect energy transfer between the chromophores.

Calculations of interfacial interactions in pyrene-Ipa rod sensitized nanostructured TiO2

Pyrene chromophores carrying different rigid rod spacer groups (ethynylene, ethynylene-phenylene-ethynylene, and ethynylene-bicyclo[2.2.2]octylene-ethynylene) and bound to TiO2 nanostructured materials via an isophthalic acid (Ipa) anchor group have been investigated using quantum chemical calculations in order to elucidate structural and electronic properties of dye-sensitized semiconductor structures capable of long-range photoinduced interfacial electron transfer. The calculations are used to study firstly the effect of the anchor and spacer groups on the electronic properties of the pyrene-dyes, secondly the binding of isophthalic acid to nanostructured TiO2, and thirdly the interfacial electronic interactions for dye-sensitized nanostructured TiO2 relevant to dye-sensitized solar cell applications. Together, these calculations provide theoretical insights into the effect of incorporating rigid rod anchor-cum-spacer group motifs in sensitizers for e.g. solar cell applications. In particular, the calculations help to rationalize the strong influence of the rods on the photophysical properties of the sensitizers in terms of electronic interactions between the individual chromophore, spacer, and anchor segments, as well as to provide information about interfacial electronic interactions of interest for the capabilities of the rods to act as efficient mediators of photoinduced interfacial charge separation.

Preparation of silylated magadiite thin-film-containing covalently attached pyrene chromophores

Thin-film of magadiite (Mag) silylated by [2-(perfluorohexyl)-ethyl]trichlorosilane was prepared by casting the chloroform/ethylacetate nanosheet solution of it. The addition of alkylamine and perfluoroalkyl groups to silylated magadiite greatly changed the surface morphology of the resulting sample and the transparent thin-film with relatively smooth surface was obtained. Amino groups were introduced into it by using 3-aminopropyltriethoxysilane after removal of alkylamine. The reaction of 1-pyrenebutanoic acid succimidyl ester with the thin-film of silylated magadiite-containing amino groups in N, N-dimethylforamide (DMF) resulted in the covalent attachment of pyrene chromophores to it.

Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant

Miniemulsion polymerization of styrene (St) in the presence of a hydrophobe (hexadecane:HD) using a cationic polymerizable surfactant ( N , N -dimethyl- N - n -dodecyl- N-2-methacryloyloxyethylammonium bromide:C 12Br) and a cationic initiator (2,2′-azobis(2-amidinopropane) dihydrochloride:V50), called St/C 12Br/V50 hereafter, proceeded efficiently compared with that using sodium dodecyl sulfate (SDS) and potassium persulfate (KPS), i.e., St/SDS/KPS, providing monodisperse polystyrene latex particles with a narrower particle size distribution. In St/C 12Br/AIBN, where an oil-soluble initiator, i.e., 2,2′-azobisisobutyronitrile (AIBN), was used in place of V50, little changes in polymerization kinetics or in particle size distribution were observed, while a significant drop in polymerization rate and a broad particle size distribution were observed with St/SDS/AIBN. A polymerizable pyrene derivative (1-pyrenylmethyl methacrylate: PyMMA) was quantitatively incorporated into monodisperse latex particles in St/PyMMA/C 12Br/V50 compared to pyrene (Py) in St/Py/C 12Br/V50. Contrary to our expectation, however, increased excimer emission was observed with St/PyMMA/C 12Br/V50 particles, indicating less evenly distributed pyrene chromophores in the particles. The fluorescence lifetime of pyrene chromophores in St/Py/C 12Br/V50 particles was determined to be 286 ns, which was 17 times longer than that of pyrene in THF solution.

Photoinduced energy transfer in bichromophoric co-assemblies based on amphiphilic poly(amidoamine) dendrons

Spherical co-assemblies were generated from an equivalent-molar-ratio mixture of amphiphilic poly(amidoamine) dendrons that were modified with focal carbazole and/or pyrene chromophores from the first to the third generation (D nCz I and D nPy, where n = 1 , 2, 3 denotes the generation of dendrons) in water at a concentration of 3.0 × 10 −6 M , lower than the individual critical aggregation concentrations (CAC) of either D nCz I or D nPy. UV–vis absorption and fluorescence emission spectra indicated that fluorescence resonance energy transfer (FRET) from carbazole to pyrene units occurred efficiently within the co-assemblies. Further study implied that part of the carbazole and/or pyrene units stacked homogeneously in the co-assemblies via π – π stacking and the dendritic branches served as connectors due to the hydrogen-bonding interaction.

Photocurrent Generation in Noncovalently Assembled Multilayered Thin Films

Multilayered photocurrent generating thin films were fabricated by templated noncovalent assembly via stepwise assembly of molecular components. Each of films I-IV contained an underlying self-assembled monolayer (SAM) consisting of an alkanethiol linked covalently to a 2,6-dicarboxypyridine ligand that served as a binding site for attaching additional molecular components. The SAM subsequently was functionalized by sequential deposition of Cu(II), Co(II), or Fe(III) ions followed by a variety of substituted 2,6-dicarboxypyridine ligands as a means to incorporate one or more layers of pyrene chromophores into the film. The films were characterized by contact angle measurements, ellipsometry, grazing incidence IR, cyclic voltammetry, and impedance spectroscopy after deposition of each layer, confirming the formation of ordered, stable layers. Following incorporation into a three-electrode system, photoexcitation resulted in the generation of a cathodic photocurrent in the presence of methyl viologen and an anodic photocurrent in the presence of triethanolamine. Using this strategy, systems were fabricated that produced up to 89 nA/cm(2) of reproducible photocurrent.

Synthesis and Photoluminescence Properties of Pyrene-Incorporated Organic-Inorganic Polymer Hybrids

Pyrene-incorporated organic-inorganic polymer hybrids were synthesized from methacryloyl monomers, tetramethoxysilane (TMOS) and pyrene via the in situ polymerization method. The photoluminescence properties of the incorporated pyrene chromophore were studied by using hybrids those were obtained by changing the ratio of inorganic moieties. As compared with organic polymers or a silica glass, the excimer emission of pyrene chromophore increased in the polymer hybrids. The excimer emission can be affected by changing the organic/inorganic ratio or the species of organic polymers. In the case of employing hydrophilic polymers, the excimer emission intensity of pyrene increased. Also, the excimer emission intensity of pyrene increased in the polymer hybrids having low organic polymer content. Furthermore, the sensitivity against oxygen of these excimer emissions in the polymer hybrids was found to be tuned by the organic/inorganic ratio in the polymer hybrids or the oxygen permeability of the organic polymers employed for hybridization.

Synthesis of 5‐(2‐Pyrenyl)‐2′‐deoxyuridine as a DNA Modification for Electron‐Transfer Studies: The Critical Role of the Position of the Chromophore Attachment

Abstract5‐(2‐Pyrenyl)‐2′‐deoxyuridine (2PydU, 2) has been prepared as a new thymidine analogue in which the 2‐position of the pyrene chromophore is connected covalently to the 5‐position of uridine through a single C–C bond. The synthesis of 2 starts with the conversion of pyrene (3) into 2‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)pyrene (4) by using an Ir catalyst that was prepared in situ from [IrCl(cod)]2 and 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine (dtbpy) in the presence ofNaOMe. The subsequent Suzuki–Miyaura cross‐coupling of 4 with 5‐iodo‐2′‐deoxyuridine (5) was performed by using1,1′‐bis[(diphenylphosphanyl)ferrocene]dichloropalladium(II) as the catalyst in a THF/MeOH/H2O mixture as the solvent. The modified nucleoside 2 was characterized by absorption and fluorescence spectroscopy. The results were compared with the strongly electronically coupled 5‐(1‐pyrenyl)‐2′‐deoxyuridine (1PydU, 1). Finally, the nucleoside 2 was converted into the corresponding phosphoramidite 7 as a DNA building block. The DNA set 8a–8d was synthesized to explore the optical properties of the 2PydU label in duplex DNA with respect to the counterbase opposite the 2PydX modification site and in comparison with 1PydU‐modified DNA. The studies with the 2PydU nucleoside and the 2PydU‐modified DNA clearly allow the conclusion to be drawn that the pyrene and the uridine moieties, as the two aromatic groups in this modified nucleoside, are only weakly electronically coupled. Accordingly, the 2PydU label behaves optically like a pyrene derivative, in contrast to the 1PydU label. The chromophore shows the ability for Watson–Crick base‐pairing inside the DNA, as revealed by the absorption and fluorescence spectra. 2PydU represents an optical label for DNA that changes its absorption properties upon DNA hybridization and undergoes fluorescence quenching by charge transfer.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)