Pyrene Molecules Research Articles

Electronic states of pyrene single crystal and of its single molecule inserted in a molecular vessel of cyclodextrin

Highly purified single crystals of pyrene were made by a gas phase crystal growth method from 180 times of zone-refined pyrene. The absorption spectra of the single crystal have been transformed from the reflection spectra between 2.5 and 6.5eV at 2, 77K and room temperature. The dry powder of β-cyclodextrin including pyrene single molecule were prepared in vacuum to investigate the electronic states of the isolated molecule. The absorption spectra of the single molecule show similar spectra to those of the single crystal. The pyrene molecule keeps its electronic character even in the single crystal.

Surface immobilized optical probes: pyrene molecules covalently attached to silica and indium-doped tin oxide

We report on the covalent attachment of pyrene to quartz and indium-doped tin oxide (ITO) surfaces, and its characterization by steady-state and time-resolved emission spectroscopy. We estimate the chromophore surface coverage on the basis of their electrochemical oxidation mechanism in acidic solution. Electrochemical oxidation of immobilized pyrene results in the formation of radical cations, which are converted into hydroxypyrene and, after a second oxidation step, are transformed into isomeric forms of dihydroxypyrene/pyrenedione redox couples. Our findings demonstrate that surface-bound pyrene can be used as a spectral probe in environments that are isolated from oxidizing conditions and agents.

Ultrafast Dynamics of Pyrene Excimer Formation in Y Zeolites

Pyrene excimer emission in alkali-metal-exchanged zeolites is readily observed to grow in using an ultrafast streak camera with 3.3 ps time-resolution. Rise-time constants range from 7 to 14 ps depending on the composition of the zeolite. These results show that two pyrene molecules in doubly occupied cavities must move from their pre-excitation orientation into the face-to-face orientation required for the excimer.

Ultrafast Dynamics of Pyrene Excimer Formation in Y Zeolites

Pyrene excimer emission in alkali-metal-exchanged zeolites is readily observed to grow in using an ultrafast streak camera with 3.3 ps time-resolution. Rise-time constants range from 7 to 14 ps depending on the composition of the zeolite. These results show that two pyrene molecules in doubly occupied cavities must move from their pre-excitation orientation into the face-to-face orientation required for the excimer.

Thermal characterization of poly(ethylene glycol)–poly(,-lactide) block copolymer micelles based on pyrene excimer formation

Poly(ethylene glycol)–poly(d,l-lactide) (PEG–PDLLA) block copolymers were prepared by anionic ring-opening polymerization, resulting in block sizes effectively controlled by initial monomer/initiator ratios and low molecular weight distributions (<1.12). A pyrene derivative (1-pyrenyl carbonyl cyanide—Py) was conjugated to the end of the hydrophobic block (PDLLA) in a quantitative manner, with coupling efficiencies >95%. The so-obtained PEG–PDLLA–Py copolymers displayed fluorescent properties that were associated with the pyrene monomers, when placed in good solvents for both the hydrophilic and hydrophobic blocks. When placed in selective solvents, these copolymers self-assembled into micelles in the 30-nm range, also with low particle size distributions (<0.09), within which Py could be readily entrapped in the hydrophobic PDLLA core. Py entrapment resulted in the formation of excimers, as evident from fluorescence measurements. Observation of excimer formation/dissociation further conveyed information on the physicochemical properties of the core. Thermal characterization of these systems showed that an increase in the temperature resulted in changes in the properties of excimer fluorescence, an occurrence attributed to a higher mobility of the otherwise glassy PDLLA. This, in turn, greatly affected the inter-molecular distance between pyrene molecules, a crucial factor for excimer formation. The glass transition of the PDLLA block, ∼38 °C, defined the onset for increasing chain mobility and whence excimer dissociation. Excimer fluorescence appeared to be time-dependent. Based on these observations, chain exchange processes were clearly evidenced through the time-dependent dissociation of excimers into unimers, a process that was influenced by changes in temperature.

Characterizing pyrene–Ag + exciplex formation in aqueous and ethanolic solutions

The primary mechanism responsible for the dynamic fluorescence quenching of polycyclic aromatic compounds (PACs) by Ag + ions is enhanced intersystem crossing from the lowest singlet excited state to the lowest triplet excited state. For some PACs, however, exciplex formation with Ag + in polar organic solvents has also been reported. Quenching of pyrene fluorescence by Ag + in two polar solvent systems (aqueous and ethanolic solutions) was examined here using steady-state and time-resolved fluorescence techniques. In both solvents, quenching led to the formation of a pyrene–Ag + exciplex, ( 1 Py⋯ Ag + ) ∗ , which rapidly equilibrated with excited singlet pyrene molecules ( 1 Py ∗ ) and Ag + ions. The exciplex and pyrene monomer emission spectra strongly overlapped, with the dominant exciplex peaks in water and ethanol red shifted from the 0–0 transition of pyrene by 5.08 and 2.56 kJ/mol, respectively. Rate constants for the formation and dissociation of the exciplex were much larger than the radiative and nonradiative decay rate constants for both the excited state monomer and exciplex. The short exciplex fluorescence lifetimes and very low quantum yields observed in the two solvents can be attributed to enhanced nonradiative decay processes for the exciplex. The emission quantum yield of the exciplex formed in aqueous solution was approximately an order of magnitude larger than that in ethanolic solution, which is likely to be attributable to the higher polarity of water versus ethanol.

Thermal characterization of poly(ethylene glycol)–poly( d, l-lactide) block copolymer micelles based on pyrene excimer formation

Poly(ethylene glycol)–poly( d, l-lactide) (PEG–PDLLA) block copolymers were prepared by anionic ring-opening polymerization, resulting in block sizes effectively controlled by initial monomer/initiator ratios and low molecular weight distributions (<1.12). A pyrene derivative (1-pyrenyl carbonyl cyanide—Py) was conjugated to the end of the hydrophobic block (PDLLA) in a quantitative manner, with coupling efficiencies >95%. The so-obtained PEG–PDLLA–Py copolymers displayed fluorescent properties that were associated with the pyrene monomers, when placed in good solvents for both the hydrophilic and hydrophobic blocks. When placed in selective solvents, these copolymers self-assembled into micelles in the 30-nm range, also with low particle size distributions (<0.09), within which Py could be readily entrapped in the hydrophobic PDLLA core. Py entrapment resulted in the formation of excimers, as evident from fluorescence measurements. Observation of excimer formation/dissociation further conveyed information on the physicochemical properties of the core. Thermal characterization of these systems showed that an increase in the temperature resulted in changes in the properties of excimer fluorescence, an occurrence attributed to a higher mobility of the otherwise glassy PDLLA. This, in turn, greatly affected the inter-molecular distance between pyrene molecules, a crucial factor for excimer formation. The glass transition of the PDLLA block, ∼38 °C, defined the onset for increasing chain mobility and whence excimer dissociation. Excimer fluorescence appeared to be time-dependent. Based on these observations, chain exchange processes were clearly evidenced through the time-dependent dissociation of excimers into unimers, a process that was influenced by changes in temperature.

Formation of the Pigment Apparatus in Etiolated Barley Leaves under the Influence of Levulinic Acid

The effects of levulinic acid (LA) on the synthesis of pigments and the membrane system of etioplasts were studied in etiolated leaves of barley (Hordeum vulgare L.). Growing in the solution of LA during a six-day period, which started one day after the soaking of seeds, resulted in a retardation of leaf growth, more than a twofold decrease in the level of carotenoids and protochlorophyllide (Pd) in the leaf tissue, and suppression of the synthesis of long-wave form of Pd655; these effects depended on the LA concentration. In etioplasts isolated from the seedlings treated with 50 μM LA and containing predominantly a short-wave form of Pd with a peak of fluorescence at 632 nm (–196°C), there was a membrane fraction whose location in the sucrose density gradient was identical to that of prolamellar bodies (PLB) in the control plants. The content of Pd and carotenoids in this fraction calculated on a protein basis was 2.46 and 1.3 times lower than in control seedlings, while the relative content of Pd oxidoreductase (POR) essentially did not change. Thus, the suppression of Pd synthesis did not affect translocation of POR from the cytoplasm to the membranes of etioplasts. In the PLB membranes, there was no transfer of energy from the molecules of lipophilic fluorescent probe pyrene (excitation at 337, 278, and 296 nm) to Pd; however, under pigment deficiency, the production therein of pyrene excimer form at the expense of energy transfer from protein tryptophanyls (excitation at 278 and 296 nm) became more efficient, which indicated changes in protein–lipid interactions. The obtained results suggest that the short-wave form of Pd632 accumulating in etioplasts under the suppressed synthesis of tetrapyrroles is not a free pigment.

Photoelectron spectroscopy of pyrene cluster anions, (pyrene) n− ( n=1–20)

Pyrene molecular anion and its homogeneous clusters, (pyrene) n − ( n=2–20), have been studied by anion photoelectron spectroscopy. The adiabatic electron affinity (EA a) of pyrene was directly determined to be 0.406 ± 0.010 eV. This EA a value agrees with that calculated using the method of density functional theory, showing that the excess electron occupies the lowest unoccupied molecular orbital of pyrene molecule. Photoelectron spectra of its cluster anions suggest that the excess electron in the pyrene dimer anion is delocalized over the two pyrene moieties, whereas in larger clusters the excess electron is predominantly localized on a single pyrene molecule.

Chiral Morphologies and Interfacial Electronic Structure of Naphtho[2,3-a]pyrene on Au(111)

The adsorption of the two-dimensionally chiral naphtho[2,3-a]pyrene molecule has been studied on Au(111). Both structural and electronic properties of the naphtho[2,3-a]pyrene (NP)/Au(111) interface have been measured. Ultraviolet and X-ray photoelectron spectroscopy have been employed to measure the energies of the molecular orbitals of the NP film with respect to the gold Fermi level. A Schottky junction with a large interface dipole (0.99 eV) is formed between Au(111) and NP. Temperature-programmed desorption was used to determine that adsorbed NP has a binding energy of 102.2 kJ/mol. Chiral domains have been observed with scanning tunneling microscopy due to the spontaneous phase separation of the 2-D enantiomers. Two distinct structural polymorphs have been observed, one of which has homochiral paired molecular rows. Models of the 2D structure are proposed that are in excellent agreement with experimental measurements.

Engineering of layer-by-layer coated capsules with the prospect of materials for efficient and directed electron transfer.

Intermolecular electron transfer is investigated in a dye-doped polyelectrolyte (PE) multilayer film. Hollow PE capsules, with a mean diameter of 2 microm, were prepared by stepwise adsorption of a pyrene (PY)-labeled polyanion and various polycations onto charged colloids and subsequent dissolution of the colloidal core. The high concentration of dye molecules within the capsule wall and the control of the medium polarity on a nanometer length scale are proposed to facilitate light-induced charge separation over distances of a few nanometers. In particular, a PY-labeled poly(styrene sulfonate) (PSS-PY) has been synthesized and used as polyanion for the polyelectrolyte capsule preparation. A polarity gradient across the wall of the PE shells is assumed to be achieved by adsorbing diverse polycations at different film positions. The high effective film area followed by high optical density of the PE capsule solution enables time-resolved optical spectroscopy. Using pulsed excited state absorption (ESA) the transient absorption peaks of the radical anion and cation state of pyrene were measured, respectively. In the presence of additional electron donor (or acceptor) molecules in the capsule solution the pyrene anion (cation) is observed in the ESA spectra, while both transient states are seen if no additional molecules are present. These results are interpreted as an electron transfer from pyrene to the donor (acceptor) molecule or between two pyrene molecules. An asymmetry of the electron donor and electron acceptor efficiency was observed when multilayer shells were used that are supposed to carry an internal polarity gradient.

Molecular weight effect on swelling and viscous flow of polymeric glass exposed to organic vapor: A steady state fluorescence study

Polymeric glasses formed from linear poly(methyl methacrylate) (PMMA) chains in various molecular weights were exposed to chloroform vapor to study swelling and viscous flow mechanisms. In situ steady state fluorescence (SSF) experiments were performed to monitor the vapor uptake and viscous flow processes. Direct illumination of the disc shape films were performed to excite the pyrene (P) molecules embedded inside the PMMA films. Variation in P intensity, I P, was monitored during swelling and viscous flow of the PMMA material exposed to chloroform vapor. It was observed that PMMA film swells like a crosslinked gel at early times by obeying the Li-Tanaka equation. At later times, i.e.longer than terminal relaxation times, τ t, swollen PMMA material flows linearly with time. Swelling time constant, τ c, terminal relaxation time, τ t, and viscosity η of PMMA films during vapor uptake were measured and found to be strongly correlated with the molecular weight (M) of PMMA. The exponent of M for τ t and η were measured and found to be around 3.

Fluorescence of the Protein of the Prolamellar Bodies of Etioplasts

In the high-purity membranes of the prolamellar bodies (PLB) of etioplasts, the main protein component of which is protochlorophyllide oxidoreductase (POR), “protein” fluorescence in the 320–350-nm region with a maximum in the excitation spectrum at 278–280 nm has been revealed. The microsurrounding of protein fluorophors in an acidic medium (pH 4.0) is the most hydrophobic. Transfer of energy from the protein to pyrene present in the lipid of the PLB membranes is observed, but fluorescence of protochlorophyllide (Pd) is absent when pyrene is excited via protein. A change in the content of Pd and carotenoids in the membranes of PLB influences the degree of excimerization of the acceptor molecules of pyrene (inverse dependence). The character of interaction of POR with a membrane with allowance for the data on its primary and secondary structures is discussed.

Triazine Dendrimers for Drug Delivery: Evaluation of Solubilization Properties, Activity in Cell Culture, and In Vivo Toxicity of a Candidate Vehicle

Three criteria are evaluated to assess the potential of a dendrimer based on triazines, 1, for use as a vehicle for drug delivery. These criteria are: (1) its ability to solubilize small hydrophobic guests as measured spectrophotometrically; (2) its ability to deliver a drug in vitro as evaluated using a gene reporter assay; and (3) its in vivo toxicity in mice as determined by autopsy and screens of liver and kidney function. Vehicle 1 solubilizes pyrene to a similar extent to dendrimers based on poly(arylether)s, 4, encapsulating approximately 0.2 molecules of pyrene per dendrimer. This activity is approximately 10-fold greater than that of the more polar poly(propyleneimine) and poly(amidoamine) dendrimers, 2 and 3. Gas-phase computational models reveal that both 1 and 4 have cores that are accessible to solvent, suggesting that these dendrimers can occupy much greater volumes than 2 and 3 whose cores are confined toward the interior of the structure. Electrostatic potential maps can be used to rationalize differences in solubilization between 1 and 4. Precipitation results from mixing cationic 1 with the anionic indomethacin, but not with methotrexate, suggesting that the composition of the drug may dictate the scope of delivery applications. Dendrimer 1 solubilizes 10-hydroxycamptothecin and a novel bisindolemethane; approximately four and five molecules of drug per dendrimer are solubilized, respectively. In cell-culture experiments using a luciferase reporter gene assay, the dendrimer:bisindolemethane conjugate shows comparable activity to the bisindolemethane delivered in aqueous DMSO, suggesting that the dendrimer does not preclude delivery of the molecule to an intracellular target. Preliminary toxicology studies of 1 in mice show that this molecule has no adverse toxicity to the kidneys or the liver in single doses delivered intraperitoneally up to 10 mg/kg.

Determination of Initial and Long-Term Microstructure Changes in Ultrahigh Molecular Weight Polyethylene Induced by Drawing Neat and Pyrenyl Modified Films

Deformation processes in gel-crystallized ultrahigh molecular weight polyethylene (UHMWPE) films with draw ratios (DR) as high as 96 have been investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and positron annihilation lifetime spectroscopy (PALS). In addition, low concentrations of pyrene molecules have been introduced at the time of film preparation from the gels or afterward by sorption after film preparation, and the polarization of their electronic absorption and fluorescence spectra at different draw ratios has been measured over a large temperature range extending to below the glass transition. The pyrene-doped films have been irradiated to introduce covalently attached 1-pyrenyl groups, and these films at two draw ratios have been employed to investigate over large temperature ranges (1) the steady-state fluorescence intensity and (2) the rates of diffusion of N,N-dimethylaniline (DMA). These data have been correlated with the XRD, DSC, and PALS information obtained on the unmodified films. On the basis of analyses of this body of information, a novel deformation model that explains the decreased crystallinity and increased mean free volumes in gel-crystallized UHMWPE at low draw ratios is proposed. It involves "stretch" and "flip" motions of microfibrils present in the undrawn films. The high crystallinity content and stiffer chains due to drawing UHMWPE films result in weak alpha- and beta-relaxation processes, slower diffusion of DMA than in undrawn films, and orientation factors for doped pyrene molecules that are constant over a large temperature range. The overall picture that emerges allows several aspects of the morphology of UHMWPE, a polymer of fundamental importance in materials research, to be understood.

Studying flocculation mechanism of chitosan with pyrene‐fluorescence probe method

AbstractA peak/ratio in pyrene‐fluorescence spectrum was employed to measure the polarity of micro‐environment of chitosan adsorbing pyrene molecules. The authors have in the first time detected the pyrene‐fluorescence spectrum of chitosan with different degrees of deacetylation (D. D.), and found the relationship between the flocculation of bentonite colloid by chitosan and the peak/ratio values of different molecular weight (M.W.) and D.D. of chitosan. We find that M.W. plays a key role in the flocculation, but D. D. has limited effect on it. From micro‐environmental structure of view, it can be proved that the inter‐particle bridging rather than charge neutralization dominates the flocculation with chitosan.

Theoretical investigation the electroluminescence characteristics of pyrene and its derivatives

The chromophore of pyrene molecule has been studied in detail by using ab initio (HF, DFT and MP2) and the semi-empirical methods (AM1, PM3 and ZINDO). For its molecular structure, HOMO, LUMO, molecular orbital energies and electronic spectra, the theoretical calculations using AM1 (for optimized structure) with ZINDO (for excitation energy) showed results consistent with experimental finding. Further investigation on 1-, 2- and 4-mono-substituted derivatives of pyrene with the effect of the substitution position and the substituents showed: (1) LUMO is decreased when the substituent is an electron acceptor group; (2) HOMO is increased when the substituent is an electron donor group. However, both electron donor and electron acceptor substituents are effective in reducing the energy gap between HOMO and LUMO in the mono-substituted pyrene. In terms of electronic spectra, the transition π→π ∗ shows a larger red shift if the substitutions on the pyrene ring are in the 1- and 4-positions. The red shift is even larger if the substituents contain both electron donor and electron acceptor groups. Nevertheless, a slight blue shift would occur, since the substituents are large groups and distort the planar structure of the parent pyrene molecule. This decreases the overlap of the π orbital and hinders the electron transition from π to π ∗. According to the above conclusion, semi-empirical AM1 and ZINDO methods in this work predict several electron luminescence materials among pyrene derivatives, and the results agree very well with the experimental findings. Presumably, the procedures of theoretical calculation employed in this study can be successfully applied to investigation on the electroluminescence characteristics of other materials, and further, to design novel materials for organic light emitting diode.

Comparison of Energy Deposition Modes in Polyethylene Films by Megaelectronvolt Range Neutrons, Electrons, Protons, and α Particles as Monitored by Covalent Attachment of Doped Pyrene Molecules

The selectivity and efficiency of the covalent attachment of pyrene molecules to chains of polyethylene (PE) films by various forms of ionizing radiation (neutrons, electrons, protons, and α partic...

Crystal Structures of (Pyrene)10(I3-)4(I2)10 and [1,3,6,8-Tetrakis(methylthio)pyrene]3(I3-)3(I2)7: Structural Trends in Fused Aromatic Polyiodides

We report here the synthesis, crystal structures, and electronic band structures of (pyrene)10(I3-)4(I2)10, 1, and of [1,3,6,8-tetrakis(methylthio)pyrene]3(I3)3-(I2)7, 2. In both systems, the organic molecules form face-to-face cationic stacks that are separated from one another by a polyiodide network. Band calculations at the extended Hückel (eH) level suggest that the stacks of pyrene molecules in 1 have undergone a Peierls distortion. This Peierls distortion is abetted by rotation of the pyrene molecules within the face-to-face stacks. Within the chains, the pyrene HOMO bands are 3/4-filled. Band calculations on 2, at both the LDA-DFT and eH levels, suggest that 2 is a Mott insulator. The various intermolecular interactions in these crystals are assessed. Strongest are the I···I contacts. These contacts are rationalized as being Lewis acid−Lewis base (charge-transfer) interactions between HOMO and LUMO orbitals. HOMO interactions also are responsible for the stacking sequence observed in 1. In 1, the strength of intermolecular interactions follows the order inorganic−inorganic (i.e., I···I) > organic−organic (i.e., π − π) > organic−inorganic (i.e., C−H···I). Thus, 1 belongs to the family of two-component systems (here, organic and inorganic) where the interface of the two components has a relatively high free energy. An analogy is made to other two-component systems, such as block copolymers. The topology of the interface is analyzed, and a relationship between the dimensionality of the organic network and the ratio of the inorganic volume to the total volume is presented.

Characterization of n-hexadecylalkylamine-intercalated graphite oxides as sorbents

Adsorption properties of graphite oxides hydrophobized by n-hexadecylamine, (C16) x GO, were investigated using pyrene molecules as a model of nonionic organic contaminants. A large quantity of pyrene (28.5 mg/g) was adsorbed from a water–ethanol mixture (1:2) containing 2 mM of pyrene when (C16) 0.6GO was used. The isotherm of pyrene adsorption was better described by Freundlich equation rather than Langmuir equation, which indicated a single adsorption mechanism was involved. The change in the amount of adsorbed pyrene as a function of amine content in GO was very similar to that which occurs upon introduction of pyrene into (C16) x GO films from chloroform solution, as determined by X-ray measurements. This suggests that pyrene molecules were adsorbed not only on the outer surface but also within the interlayer space of the intercalation compound. Swelling of the intercalation compound by ethanol can render the interlayers more organophilic and make access to hexadecylamine molecules bonded to the graphite oxide layer easier for pyrene molecules, especially in (C16) x GOs with lower amine contents.