Excimer Fluorescence Emission Research Articles

Photoexcimerisation of pure acriflavine dye in water and alcohol

The light emission properties of acriflavine have been extensively utilised in its applications as dye laser active media, as photosensitizer, as fluorescent and FRET (Fluorescence Resonance Energy Transfer) agent for sensing mechanism etc. However, studies on the photochemical and photophysical properties of acriflavine and its aggregates are very limited and little is known about its emission characteristics in different media. The present study was undertaken to acquire a comprehensive understanding of the aggregation process and emission characteristics of the solution of pure acriflavine dye in water and alcohol, namely the propanol, methanol, ethanol and butanol. This study includes confirming the photoexcimerisation of acriflavine by concentration dependent UV–Visible spectroscopy, steady state fluorescence spectroscopy and time resolved fluorescence spectroscopy at room temperature. The present study shows the monomer and excimer fluorescence emission of acriflavine in the range 495–528 nm and 549–587 nm respectively (monomer and excimer emission peak wavelength varies for different solvents). The nature of the excimer emission band of acriflavine around 575 nm for all the solvents has been tested through temperature dependent steady state fluorescence spectroscopy and also from time resolved fluorescence spectra it has been found that the excimer band does not originate from any triplet excited state. The pH dependent emission spectra for fixed concentration of acriflavine in the solvents reveals red shifts in the monomer and the excimer fluorescence maxima. The excimerisation of acriflavine in water, propanol and methanol is found as static process from excited dimers, whereas in ethanol and butanol it is the result of dynamic diffusion process between a photoexcited and a ground state monomer. The excimerisation process of the acriflavine has been quantified by evaluating the monomer-excimer equilibrium concentration i.e., the critical concentration in the cited solvents. At the critical concentration the monomer and the excimer are found to be fluoresced with equal intensity. The acriflavine solutions specifically at the critical concentration in the cited solvents will enhance its photophysical and photochemical applications by extending them up to the new range of excimer emission radiation at the wavelength in-between 549 nm and 587 nm. Hence, this report shows that, by using acriflavine solutions of certain concentrations in any one of the cited solvents as an active medium of a dye laser and thereby obtaining both the monomer and excimer emission band, the wavelength of the laser can be tuned from 495 nm to 587 nm.

Enhanced excimer fluorescence emission of pyrene derivatives: Applications in artificial light-harvesting systems

Excimer emission usually has a significant Stokes shift, making it an ideal candidate for constructing artificial light-harvesting systems (LHSs). However, the emission of excimers is altered by intermolecular interactions. Precise modulation of the spatial distance and interaction between luminescent units is the key to achieving enhanced emission of excimers. In the current study, electrostatic assembly of polyanionic PSS with cationic bolaamphiphile (PRB) containing pyrene was used to achieve close stacking of pyrene, resulting in significantly enhanced excimer emission. Furthermore, the morphology of the assemblies changed from nanodisks to nanospheres. In particular, the quantum yield of the PRB with PSS increased from 0.5% to 23.1%. In contrast, the amphiphilic molecule containing pyrene with a single hydrophilic head also showed enhanced fluorescence after being co-assembled with PSS. Nevertheless, the fluorescence quantum yield of excimers was much lower than that of the co-assemblies formed with PRB. The assembly formed by PSS and PRB acted as an energy donor. It underwent Förster resonance energy transfer with Nile red, achieving high energy conversion efficiency (ΦET = 49.6%) and antenna effect (9.8) at a donor/acceptor molar ratio of 12.5:1.

Dimension-dependent fluorescence emission and photoelectric performances of a 3D pyrene-based metal−organic framework

A pyrene-based metal-organic framework [Ca2 (TBAPy) (μ2-OH2)2] (1, H4TBAPy ​= ​1,3,6,8-tetrakis (p-benzoic acid)pyrene), directed by ionic liquids, has been synthesized. 1 has non-interpenetrated open 3D framework constructed by infinite rod-shaped Ca-carboxylate secondary building units (SBUs) and rectangle TBAPy ligands. The temperature-dependent fluorescence emission clearly shows that 1 can maintain the excimer fluorescence emission in a broad temperature range (20–450 ​K). Photoelectronic measurements reveal that the average light photocurrent density (Ilight) of 1 is apparently higher than that of other 2D TBAPy-based MOFs. The dimension-dependent fluorescence emission and photoelectric performances of several TBAPy-based MOFs were well investigated based on the steric configuration and the interplanar distance of the coupled TBAPy ligands, as well as the photocurrent conduction path.

Fluorescent Janus ring siloxanes for detection of Au(III) and l-cysteine

Two new all-cis pyrene-substituted cyclic tetrasiloxanes (JR-01 and JR-02) are synthesized and studied as selective metal ion sensors. Both compounds exhibit solvent- and concentration-dependent excimer fluorescence emission at 482 nm. They also show highly selective fluorescence quenching by Au(III) ion with detection limits of 0.9 and 0.8 μM for JR-01 and JR-02, respectively. Data from the Stern-Volmer's plots at various temperatures, zetasizers, NMR, and UV–Vis spectroscopy indicate a plausible mechanism involving the formation of a non-emissive Au(III) complex. An application of JR-01 as Au(III) sensor in real water samples reveals a good sensing efficiency which is comparable to the analysis result from ICP-QMS. In addition, when an excess amount of l-cysteine is added to the JR-01/Au(III) mixture, the fluorescence signal is restored and the 1H NMR data suggests that JR-01 remains intact. As an extension of this study, we successfully demonstrate the detection of l-cysteine by fluorescence turn-on process using the JR-01/Au(III) mixture with a detection limit of 14.0 μM.

Estimation of CYP3A4*1B single nucleotide polymorphism using target-assembled in-situ detection by synthetic DNA-mounted excimers

CYP3A4*1B is a single nucleotide polymorphism of CYP3A4 and is associated with prostate cancer which exhibits higher nifedipine oxidase activity in liver. This research provides details of the effects of structural variation and medium effects for the recently reported split-oligonucleotide (tandem) probe system for excimers-based fluorescence detection of DNA. In this approach the detection system is split at a molecular level into signal-silent components, which must be assembled correctly into a specific 3-dimensional structure to ensure close proximity of the excimer partners and the consequent excimer fluorescence emission on excitation. The model system consists of two 11-mer oligonucleotides, complementary to adjacent sites of a 22-mer DNA target. Each oligonucleotide probeis equipped with functions able to form an excimer on correct, contiguous hybridization. The extremely rigorous structural demands for excimer formation and emission required careful structural design of partners for excimer formation, which are here described. This study demonstrates that the excimer formed emitted at ~480 nm with alarge Stokes shift (~130 - 140 nm).

A perylene monoimide probe based fluorescent micelle sensor for the selective and sensitive detection of picric acid.

A hydroxyl functionalized perylene monoimide probe (PMI-OH) was prepared and self-assembled with the nonionic surfactant Triton X-100 (TX100) to fabricate a fluorescent micelle sensor for the selective and sensitive detection of picric acid (PA), a common explosive and environmental pollutant. The synthesized PMI-OH probe exhibited excimer fluorescence emission, and the intensity of the excimer fluorescence emission was significantly enhanced after the PMI-OH probe formed micelles with TX100. The obtained PMI-OH@TX100 micelles presented excellent photoluminescence properties and had a maximum fluorescence emission at 630 nm. The red fluorescence of the PMI-OH@TX100 micelles was quenched upon introduction of the nitro explosive PA due to electron transfer from the donor (PMI-OH) to the acceptor (PA). The fluorescence quenching of the fluorescent micelle sensor was proportional to the concentration of PA in the range of 2 to 10 μM. The limit of detection was 500 nM using 3σ/k. Thus, the developed PMI-OH@TX100 micelle sensor has great potential to detect PA in ordinary samples.

Host-guest interactions in nor-seco-cucurbit[10]uril: novel guest-dependent molecular recognition and stereoisomerism.

The unique monomer and excimer fluorescence emissions of pyrene were first exploited as distinctly photophysical signals to identify the possible diastereomers of guests within nor-seco-cucurbit[10]uril (NS-CB[10]) cavities. Further experiments revealed that balancing the hydrophilic and hydrophobic effects of the guest in aqueous solution can improve the molecular recognition and binding ability of NS-CB[10].

Hybrid polysilsesquioxanes for fluorescence resonance energy transfer

New hybrid polymers containing pyrene units bound to linear silsesquioxane chains (LPSQ-Py) through rigid ethene linkers exhibit almost exclusively excimer fluorescence emission, both in very diluted solutions (10−8 mol/L in THF and CHCl3) and in the solid state. They are also thermally stable and exhibit good thin film forming ability. Comparative studies with their octahedral silsesquioxane analogue (POSS-Py) elucidated the role of the specific double-strand architecture of LPSQ-Py backbone and distribution of side Py groups for their unique photo-luminescent properties. The rigid fluorescent rods of LPSQ-Py make interesting candidates for optoelectronic devices. Their ability to act as antennae for energy transfer to red-emitting acceptors has been studied.

Temperature-dependent interchromophoric interaction in a fluorescent pyrene-based metal-organic framework.

Compounds exhibiting tuneable fluorescence emission upon heating or cooling are considered smart materials as their optical properties can be exquisitely controlled by adjusting the external temperature. Herein, we report such a material, which is a porous pyrene-based metal-organic framework with a chemical formula of [Mg1.5(HTBAPy)(H2O)2]·3DMF (H4TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene), named SION-7. The bulk solid material of SION-7 can display either monomer or excimer fluorescence emission due to the temperature-dependent extent of interchromophoric interactions between the HTBAPy3- ligands within the framework. Consequently, the fluorescence emission colours gradually change from blue at low temperature (80 K) to yellow-green at high temperature (450 K). Interestingly, while kept in a relatively wide temperature range of 80-200 K, SION-7 displays a structured monomer-like spectrum and its colour changes reversibly from deep to light blue. Ex situ variable-temperature (100-350 K) single-crystal X-ray diffractometry studies revealed the impact of solvent content on the optical properties of SION-7, and illustrated the correlation between the position of the phenylene groups of the HTBAPy3- ligands at different temperatures and the interchromophoric interaction. Our study demonstrates a step forward towards the design of tuneable thermofluorochromic materials sought by optoelectronics.

Ultrasound Induced Fluorescence of Nanoscale Liposome Contrast Agents.

A new imaging contrast agent is reported that provides an increased fluorescent signal upon application of ultrasound (US). Liposomes containing lipids labelled with pyrene were optically excited and the excimer fluorescence emission intensity was detected in the absence and presence of an ultrasound field using an acousto-fluorescence setup. The acousto-fluorescence dynamics of liposomes containing lipids with pyrene labelled on the fatty acid tail group (PyPC) and the head group (PyPE) were compared. An increase in excimer emission intensity following exposure to US was observed for both cases studied. The increased intensity and time constants were found to be different for the PyPC and PyPE systems, and dependent on the applied US pressure and exposure time. The greatest change in fluorescence intensity (130%) and smallest rise time constant (0.33 s) are achieved through the use of PyPC labelled liposomes. The mechanism underlying the observed increase of the excimer emission intensity in PyPC labelled liposomes is proposed to arise from the “wagging” of acyl chains which involves fast response and requires lower US pressure. This is accompanied by increased lipid lateral diffusivity at higher ultrasound pressures, a mechanism that is also active in the PyPE labelled liposomes.

Specific cellular incorporation of a pyrene-labelled cholesterol: lipoprotein-mediated delivery toward ordered intracellular membranes.

In the aim of testing tools for tracing cell trafficking of exogenous cholesterol, two fluorescent derivatives of cholesterol, 22-nitrobenzoxadiazole-cholesterol (NBD-Chol) and 21-methylpyrenyl-cholesterol (Pyr-met-Chol), with distinctive chemico-physical characteristics, have been compared for their cell incorporation properties, using two cell models differently handling cholesterol, with two incorporation routes. In the Caco-2 cell model, the cholesterol probes were delivered in bile salt micelles, as a model of intestinal absorption. The two probes displayed contrasting behaviors for cell uptake characteristics, cell staining, and efflux kinetics. In particular, Pyr-met-Chol cell incorporation involved SR-BI, while that of NBD-Chol appeared purely passive. In the PC-3 cell model, which overexpresses lipoprotein receptors, the cholesterol probes were delivered via the serum components, as a model of systemic delivery. We showed that Pyr-met-Chol-labelled purified LDL or HDL were able to specifically deliver Pyr-met-Chol to the PC-3 cells, while NBD-Chol incorporation was independent of lipoproteins. Observations by fluorescence microscopy evidenced that, while NBD-Chol readily stained the cytosolic lipid droplets, Pyr-met-Chol labelling led to the intense staining of intracellular structures of membranous nature, in agreement with the absence of detectable esterification of Pyr-met-Chol. A 48 h incubation of PC-3 cells with either Pyr-met-Chol-labelled LDL or HDL gave same staining patterns, mainly colocalizing with Lamp1, caveolin-1 and CD63. These data indicated convergent trafficking downwards their respective receptors, LDL-R and SR-BI, toward the cholesterol-rich internal membrane compartments, late endosomes and multivesicular bodies. Interestingly, Pyr-met-Chol staining of these structures exhibited a high excimer fluorescence emission, revealing their ordered membrane environment, and indicating that Pyr-met-Chol behaves as a fair cholesterol tracer regarding its preferential incorporation into cholesterol-rich domains. We conclude that, while NBD-Chol is a valuable marker of cholesterol esterification, Pyr-met-Chol is a reliable new lipoprotein fluorescent marker which allows to probe specific intracellular trafficking of cholesterol-rich membranes.

Interdependence of pyrene interactions and tetramolecular G4-DNA assembly.

Controlling the arrangement of organic chromophores in supramolecular architectures is of primary importance for the development of novel functional molecules. Insertion of a twisted intercalating nucleic acid (TINA) moiety, containing phenylethynylpyren-1-yl derivatives, into a G-rich DNA sequence alters G-quadruplex folding, resulting in supramolecular structures with defined pyrene arrangements. Based on CD, NMR and ESI-mass-spectra, as well as TINA excited dimer (excimer) fluorescence emission we propose that insertion of the TINA monomer in the middle of a dTG4T sequence (i.e. dTGGXGGT, where X is TINA) converts a parallel tetramolecular G-quadruplex into an assembly composed of two identical antiparallel G-quadruplex subunits stacked via TINA-TINA interface. Kinetic analysis showed that TINA-TINA association controls complex formation in the presence of Na(+) but barely competes with guanine-mediated association in K(+) or in the sequence with the longer G-run (dTGGGXGGGT). These results demonstrate new perspectives in the design of molecular entities that can kinetically control G-quadruplex formation and show how tetramolecular G-quadruplexes can be used as a tuneable scaffold to control the arrangement of organic chromophores.

The Extent of Pyrene Excimer Fluorescence Emission Is a Reflector of Distance and Flexibility: Analysis of the Segment Linking the LDL Receptor-Binding and Tetramerization Domains of Apolipoprotein E3

Pyrene is a spatially sensitive probe that displays an ensemble of monomeric fluorescence emission peaks (375-405 nm) and an additional band (called excimer) at ~460 nm when two fluorophores are spatially proximal. We examined if there is a correlation between distance between two pyrenes on an α-helical structure and excimer/monomer (e/m) ratio. Using structure-guided design, pyrene maleimide was attached to pairs of Cys residues separated by ~5 Å increments on helix 2 of the N-terminal domain of apolipoprotein E3 (apoE3). Fluorescence spectral analysis revealed an intense excimer band when the probes were ~5 Å from each other with an e/m ratio of ~3.0, which decreased to ~1.0 at 20 Å. An inverse correlation between e/m ratio and the distance between pyrenes was observed, with the probe and helix flexibility also contributing to the extent of excimer formation. We verified this approach by estimating the distance between T57C and C112 (located on helices 2 and 3, respectively) to be 5.2 Å (4.9 Å from NMR and 5.7 Å from the X-ray structure). Excimer formation was also noted to a significant extent with probes located in the linker segment, suggesting spatial proximity (10-15 Å) to corresponding sites on neighboring molecules in the tetrameric configuration of apoE. We infer that oligomerization via the C-terminal domain juxtaposes the linker segments from neighboring apoE molecules. This study offers new insights into the conformation of tetrameric apoE and presents the use of pyrene as a powerful probe for studying protein spatial organization.

Aromatic Chromophore-Tethered Schiff Base Ligands and Their Iron(III)/Chromium(III) Salen and Saloph Capped Complexes

Aromatic chromophores; pyrene, phenanthrene, anthracene, naphtalene and benzene-tethered Schiff base ligands and their iron(III)/chromium(III) Salen and Saloph capped complexes have been synthesized. Compounds have been characterized by means of FT-IR Spectroscopy, (1)H-NMR Spectroscopy, Magnetic Susceptibility, Elementel Analsis, TG/DTA measurements. Their fluorescence and absorbance properties have been investigated by Luminescence Spectroscopy and UV-vis Spectroscopy. Generally, ligands show an intense excimer fluorescence emissions in acetonitrile-methanol medium while iron(III) and chromium(III) complexes exhibit low fluorescence's. Intensity compared to ligands iron and chromium centers act as an extra chromophore that quench the pyrene, phenanthrene, anthracene, naphtalene and benzene molecules' singlet state. The mechanism of quenching is attributed to a iron (or chromium)-to-pyrene (or phenanthrene, anthracene, naphtalene and benzene) electronic energy transfer process.

Fluorescence Resulting from π-stacking in Polystyrene Solutions

The fluorescence of polystyrenes solutions are reported in three different concentrations with one in semi-diluted regime and two in diluted regime. With the excitation energy of 260 nm, only the widely studied monomer fluorescence emission near 285 nm and excimer fluorescence emission near 330 nm are observed. With the excitation energies at 277nm and longer wavelengths, the unexpected emissions in the longer wavelength range 350-480 nm that was recently discovered in polystyrene fine particles are also found to appear in polystyrene solutions, but with fewer structures and major peaks in different wavelengths. The major emissions shift from the region of 320-390 nm in polystyrene fine particles to the region of 380-450 nm in solutions. The excitation spectrum of the emissions at longer wavelength reveals much broader absorption bands with fewer resolved structures in polystyrene solutions than the absorption bands recorded in fine particles. Considering the simplicity of polystyrene molecules and previous studies, aromatic-aromatic (or π-stacking) interactions are believed to be the major driving forces for the formation of phenyl dimers and multimers from which the new emissions in longer wavelengths originate. The mechanism of the formation of these phenyl multimers in polystyrene solutions remains unsolved in this study, however the new fluorescence emissions discovered will contribute to the study of π-stacking interactions, and can also be a powerful probe for monitoring molecular motions and ordered structures of polystyrene molecules or other aromatic macromolecules in solutions.

Low-temperature photoluminescence of 5CB liquid crystal

The photoluminescence spectra of the 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2–200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at Т=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=160 K. The 5CB crystal–crystal transition at Т=80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T=140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region.

Conformational analysis of the natural iron chelator myo-inositol 1,2,3-trisphosphate using a pyrene-based fluorescent mimic

myo-Inositol phosphates possessing the 1,2,3-trisphosphate motif share the remarkable ability to completely inhibit iron-catalysed hydroxyl radical formation. The simplest derivative, myo-inositol 1,2,3-trisphosphate [Ins(1,2,3)P(3)], has been proposed as an intracellular iron chelator involved in iron transport. The binding conformation of Ins(1,2,3)P(3) is considered to be important to complex Fe(3+) in a 'safe' manner. Here, a pyrene-based fluorescent probe, 4,6-bispyrenoyl-myo-inositol 1,2,3,5-tetrakisphosphate [4,6-bispyrenoyl Ins(1,2,3,5)P(4)], has been synthesised and used to monitor the conformation of the 1,2,3-trisphosphate motif using excimer fluorescence emission. Ring-flip of the cyclohexane chair to the penta-axial conformation occurs upon association with Fe(3+), evident from excimer fluorescence induced by pi-pi stacking of the pyrene reporter groups, accompanied by excimer formation by excitation at 351 nm. This effect is unique amongst biologically relevant metal cations, except for Ca(2+) cations exceeding a 1 : 1 molar ratio. In addition, the thermodynamic constants for the interaction of the fluorescent probe with Fe(3+) have been determined. The complexes formed between Fe(3+) and 4,6-bispyrenoyl Ins(1,2,3,5)P(4) display similar stability to those formed with Ins(1,2,3)P(3), indicating that the fluorescent probe acts as a good model for the 1,2,3-trisphosphate motif. This is further supported by the antioxidant properties of 4,6-bispyrenoyl Ins(1,2,3,5)P(4), which closely resemble those obtained for Ins(1,2,3)P(3). The data presented confirms that Fe(3+) binds tightly to the unstable penta-axial conformation of myo-inositol phosphates possessing the 1,2,3-trisphosphate motif.

Fluorescence Switches Triggered by Metal Ions

AbstractMolecular switches 1 with a modular architecture whose functional principle is based on conformational changes around two acetylene axes were developed. By systematic screening of various metal ions, it was found that especially Pb2+ ions effectively trigger the switching event. The reporting of this event takes place by the change of the ratio between excimer and monomer fluorescence emission of two pyrene moieties.

Electric Field Effects on Absorption and Fluorescence Spectra of Trimethylsilyl- and Trimethylsilylethynyl-Substituted Compounds of Pyrene in a PMMA Film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.

Fluorescent Ratiometry of Tetrahomodioxacalix[4]arene Pyrenylamides upon Cation Complexation

[structure: see text] C-1,2-alternate tetrahomodioxacalix[4]arene pyreneamides were synthesized. Pb(2+) coordination gave a quenched monomer and excimer fluorescence emission, while upon Ca(2+) ion binding, the receptor provides an enhanced excimer and declined monomer emission with ratiometric response. The excimer emission spectra changes are rationalized by frontier molecular orbitals that the effective Py-Py interaction induces emission intensity increases upon Ca(2+) ion complexation, whereas there is no such interaction observed upon Pb(2+) binding.