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Phasor plots (plots of the Fourier sine transform vs. the Fourier cosine transform, for one or several angular frequencies) of the fluorescence intensity decay are being increasingly used in studies of homogeneous and heterogeneous systems. In this work, the phasor approach is applied to monomer-excimer systems with a focus on mixed decays, i.e. decays with both monomer and excimer contributions. The phasors of mixed decays fall on a straight line delimited by the excimer and monomer phasors. With a fixed concentration, and by continuously increasing the emission wavelength, the phasor moves from pure monomer to pure excimer. At a certain intermediate wavelength, which is concentration-dependent, the phasor crosses the universal semicircle and the decay becomes single exponential. This is an interesting case of an exponential decay not corresponding to a single fluorescent species. All results are demonstrated experimentally with pyrene in methylcyclohexane at room temperature, for which a full phasor picture emerges, combining in a single diagram monomer, excimer and mixed decays measured at several concentrations, showing the graphical power of the phasor approach.

Three anionic fluorene-based alternating conjugated polyelectrolytes (CPEs) have been synthesized that have 9,9-bis(4-phenoxy-butylsulfonate) fluorene-2,7-diyl and 1,4-phenylene (PBS-PFP), 4,4′-biphenylene (PBS-PFP2), or 4,4″-p-terphenylene (PBS-PFP3) groups, and the effect of the length of the oligophenylene spacer on their aggregation and photophysics has been studied. All form metastable dispersions in water, but can be solubilized using methanol, acetonitrile, or dioxane as cosolvents. This leads to increases in their emission intensities and blue shifts in fluorescence maxima due to break-up of aggregates. In addition, the emission maximum shifts to the blue and the loss of vibronic structure are observed when the number of phenylene rings is increased. Debsity Functional Theory (DFT) calculations suggest that this is due to increasing conformational flexibility as the number of phenylene rings increases. This is supported by increasing amplitude in the fast component in the fluorescence decay. The nonionic surfactant n-dodecylpentaoxyethylene glycol ether (C12E5) also breaks up aggregates, as seen by changes in fluorescence intensity and maximum. However, the loss in vibrational structure is less pronounced in this case, possibly due to a more rigid environment in the mixed surfactant-CPE aggregates. Further information on the aggregates formed with C12E5 was obtained by electrical conductivity measurements, which showed an initial increase in specific conductivity upon addition of surfactants, while at higher surfactant/CPE molar ratios a plateau was observed. The specific conductance in the plateau region decreased in the order PBS-PFP3 < PBS-PFP2 < PBS-PFP, in agreement with the change in charge density on the CPE. The reverse process of aggregate formation has been studied by injecting small volumes of solutions of CPEs dissolved at the molecular level in a good solvent system (50% methanol-water) into the poor solvent, water. Aggregation was monitored by changes in both fluorescence and light scattering. The rate of aggregation increases with hydrophobicity and concentration of sodium chloride but is only weakly dependent on temperature.

Novel near-infrared luminescent compounds based on platinum(II) 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins are described. These compounds have high photostability and display light emission, in particular simultaneous fluorescence and phosphorescence emission in solution at room temperature, in the biologically relevant 700–850 nm red and near-infrared (NIR) spectral region, making them excellent materials for biological imaging. The simultaneous presence of fluorescence and phosphorescence emission at room temperature, with the phosphorescence strongly quenched by oxygen whereas fluorescence remains unaffected, allows these compounds to be used as ratiometric oxygen sensors in chemical and biological media. Both steady-state (fluorescence vs phosphorescence intensities) and dynamic (dependence of phosphorescence lifetimes upon oxygen concentration) luminescence approaches can be used. Photocytotoxicity studies against human melanocytic melanoma cells (A375) indicate that these compounds dis...

A variational Monte Carlo method for bosonic lattice models is introduced. The method is based on the Baeriswyl projected wavefunction. The Baeriswyl wavefunction consists of a kinetic energy based projection applied to the wavefunction at infinite interaction, and is related to the shadow wavefunction already used in the study of continuous models of bosons. The wavefunction at infinite interaction, and the projector, are represented in coordinate space, leading to an expression for expectation values which can be evaluated via Monte Carlo sampling. We calculate the phase diagram and other properties of the bosonic Hubbard model. The calculated phase diagram is in excellent agreement with known quantum Monte Carlo results. We also analyze correlation functions., minor changes compared to previous version

Conjugated polymers (CPs) have proved to be promising chemosensory materials for detecting nitroaromatic explosives vapors, as they quickly convert a chemical interaction into an easily-measured high-sensitivity optical output. The nitroaromatic analytes are strongly electron-deficient, whereas the conjugated polymer sensing materials are electron-rich. As a result, the photoexcitation of the CP is followed by electron transfer to the nitroaromatic analyte, resulting in a quenching of the light-emission from the conjugated polymer. The best CP in our studies was found to be poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). It is photostable, has a good absorption between 400 and 450 nm, and a strong and structured fluorescence around 550 nm. Our studies indicate up to 96% quenching of light-emission, accompanied by a marked decrease in the fluorescence lifetime, upon exposure of the films of F8T2 in ethyl cellulose to nitrobenzene (NB) and 1,3-dinitrobenzene (DNB) vapors at room temperature. The effects of the polymeric matrix, plasticizer, and temperature have been studied, and the morphology of films determined by scanning electron microscopy (SEM) and confocal fluorescence microscopy. We have used ink jet printing to produce sensor films containing both sensor element and a fluorescence reference. In addition, a high dynamic range, intensity-based fluorometer, using a laser diode and a filtered photodiode was developed for use with this system.

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction., 18 pages, 9 figures