Tailoring Homogeneous Hydrogel Nanospheres by Facile Ultra-Sonication Assisted Cross-Linked Copolymerization for Rhodamine B Dye Adsorption

The present paper describes the design of shape-oriented hydrogel nanospheres using a facile ultrasonication-supported crosslinked copolymerization technique. The effect of variable monomer concentration on the homogeneity of hydrogel nanospheres was investigated. The chitosan-cl-poly(MMA) hydrogel nanospheres were well characterized using various techniques such as FTIR, XRD, TGA, SEM, and TEM. The chitosan-cl-poly(MMA) hydrogel nanospheres were studied for their swelling behavior and could potentially be used as a novel adsorbent for rhodamine B dye remediation from aqueous media. The study found that utilizing chitosan-cl-poly(MMA) nanohydrogel spheres at the optimal pH 5 increased RhB dye adsorption capacity from 7.9 to 17.8 mg/g (pH 2 to 5), followed by a slight reduction. Furthermore, when nanohydrogel concentration increased, adsorption capacity dropped from 18.03 to 2.8 mg/g, but adsorption percentage climbed from 90.2% to 97.8%. At an initial dye concentration of 140 mg/L, rhodamine B adsorption achieved 204.3 mg/g in 60 min. The rhodamine B dye adsorption study includes adsorption kinetics, isotherm, and thermodynamics analyses. The interpretation of the adsorption study revealed that Langmuir isotherms fit best with a qmax value of 276.26 mg/g, which is in close approximation with the experimental value, whereas pseudo-second-order kinetics explains the adsorption process rate. The interaction of RhB dye with chitosan-cl-poly(MMA) hydrogel nanospheres involves multiple forces such as electrostatic interactions, hydrogen bonding, van der Waals forces, etc.

Fluorescence lifetime measurements using photon pair correlations generated via spontaneous parametric down conversion (SPDC)

We have used photon pair correlations generated via spontaneous parametric downconversion (SPDC) to measure the fluorescence lifetime of the organic dye rhodamine 6 G, demonstrating that fluorescence lifetime measurements can be achieved using a continuous wave (CW) laser, without pulsed or modulated lasers. Our entangled photon method, quantum fluorescence lifetime (Q-FL) measurements, uses one photon to excite fluorescence and the resulting fluorescence photon is timed and referenced to the arrival time of the other entangled photon. Thus, we can exploit the short timescale of photon pair correlations to conduct experiments that are typically carried out with pulsed lasers and we show that the inherent timing of the photons is fast enough to resolve the nanosecond scale fluorescence lifetime of the sample. This measurement paves the way towards using the time correlations of entangled photons for fluorescence imaging; capitalizing on the presence of fast, sub-100 ps correlations that have not been demonstrated classically.

Studies of the transition between amplified spontaneous emission and optical lasing in ultrahigh-Q polymeric micro-pedestals

In this work, we demonstrate the properties of Rhodamine B-doped polymeric cylindrical microlasers to perform either as gain amplification devices through amplified spontaneous emission (ASE) or as optical lasing gain devices. A study based on different %wt concentrations of microcavity families with distinct geometrical features demonstrates the characteristic dependence on either gain amplification phenomena. Principal component analysis (PCA) discriminates the relationship between the main ASE and lasing properties and the geometrical aspects of the cavity families. ASE and optical lasing thresholds were found, respectively, as low as 0.2 μJcm^-2 and 0.1 μJcm^-2 passing the best-reported microlaser performances in literature for cylindrical cavities, even in comparison with those based on 2D patterns. Moreover, our microlasers showed ultrahigh Q-factor of ∼3 × 10⁶, and for the first time, to the best of our knowledge, a visible emission comb constituted by above a hundred peaks at 40 μJcm^-2 with a registered free spectral range (FSR) of 0.25 nm corroborated through the whispery gallery mode (WGM) theory.

A comparison on the biochemical activities of Fluorescein disodium, Rose Bengal and Rhodamine 101 in the light of DNA binding, antimicrobial and cytotoxic study

Biochemical activities of Fluorescein, Rose Bengal and Rhodamine 101 were studied by DNA binding, antibacterial and cytotoxic studies. DNA binding studies were done using spectroscopic, thermodynamic and molecular modeling techniques. Antibacterial activities were investigated against a gram-negative bacteria Escherichia coli and a gram-positive bacteria Staphylococcus aureus. Cytotoxic activities were studied against Wi-38 cell line. We observed these dyes bound to minor groove of DNA and structural diversity of dyes affect the phenomenon. No significant antibacterial and cytotoxic activities of these dyes were found in our observations.

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr₃ with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.

Discrimination of Invasive Human Skin Tumor Using an Ultrafast ATP-Proton AND-Gate Probe

Cancer cells undergo unscheduled proliferation resulting from dysregulation of the cell cycle, and hence, evaluation in tumor is of keen interest to examine the invasiveness and recurrence of cancer in the lesion. Molecular probes capable of discriminating actively growing tumor from resting ones remain unexplored despite their vast importance. Here, we describe a novel strategy to visualize invasive areas in tumor with a fluorescence probe that implements synergistic fluorescence response toward the slightly acidic environment of tumor and an ATP-abundant nature of actively growing cells. The probe has been designed for ultrafast detection of ATP with high specificity. We demonstrate its utility in visualizing invasive areas in tumor by distinguishing basal cell carcinomas and squamous cell carcinomas at their early stages by two-photon microscopy.

Improved One- and Multiple-Photon Excited Photoluminescence from Cd2+-Doped CsPbBr3 Perovskite NCs

Metal halide perovskite nanocrystals (NCs) attract much attention for light-emitting applications due to their exceptional optical properties. More recently, perovskite NCs have begun to be considered a promising material for nonlinear optical applications. Numerous strategies have recently been developed to improve the properties of metal halide perovskite NCs. Among them, B-site doping is one of the most promising ways to enhance their brightness and stability. However, there is a lack of study of the influence of B-site doping on the nonlinear optical properties of inorganic perovskite NCs. Here, we demonstrate that Cd²⁺ doping simultaneously improves both the linear (higher photoluminescence quantum yield, larger exciton binding energy, reduced trap states density, and faster radiative recombination) and nonlinear (higher two- and three-photon absorption cross-sections) optical properties of CsPbBr₃ NCs. Cd²⁺ doping results in a two-photon absorption cross-section, reaching 2.6 × 10⁶ Goeppert-Mayer (GM), which is among the highest reported for CsPbBr₃ NCs.

The Role of the Core Attachment Positioning in Triggering Intramolecular Singlet Exciton Fission in Perylene Diimide Tetramers

Previous studies have proposed that the presence of a flexible π-bridge linker is crucial in activating intramolecular singlet exciton fission (iSEF) in multichromophoric systems. In this study, we report the photophysical properties of three analogous perylene diimide (PDI) dendritic tetramers showing flexible/twisted π-bridged structures with α- and β-substitutions and a rigid/planar structure with a β-fused ring (βC) connection to a benzodithiophene-thiophene (BDT-Th) core. The rigidity and enhanced planarity of βC lead to significant intramolecular charge transfer and triplet formation via an intersystem crossing pathway. Steady-state spectroscopic measurements reveal similar absorption and emission spectra for the α-tetramer and the parent PDI monomer. However, their fluorescence quantum yield is significantly different. The negligible fluorescence yield of the α-tetramer (0.04%) is associated with a competitive nonradiative decay pathway. Indeed, for this twisted compound in a high polar environment, a fast and efficient iSEF with a triplet quantum yield of 124% is observed. Our results show that the α-single-bond connections in the α compound are capable of interrupting the coupling among the PDI units, favoring iSEF. We propose that the formation of the double triplet (¹[TT]) state is through a superposition of singlet states known as [S₁S₀]_[TT]CT, which has been suggested previously for pentacene derivatives. Using steady-state and time-resolved spectroscopic experiments, we demonstrate that the conformational flexibility of the linker itself is necessary but not sufficient to allow iSEF. For the case of the other twisted tetramer, β, the strong π-π co-facial interactions between the adjacent PDI units in its structure lead to excimer formation. These excimer states trap the singlet excitons preventing the formation of the ¹[TT] state, thus inhibiting iSEF.

Structure and dynamics of poly(methacrylic acid) and its interpolymer complex probed by covalently bound rhodamine-123

The dynamics and structural characteristics of polymethacrylic acid bound rhodamine-123 (PMAA-R123) and its interpolymer complex formed through hydrogen bonding between the monomeric units with poly(vinylpyrrolidone) were investigated using single molecular fluorescence studies. The time resolved fluorescence anisotropy decay of PMAA-R123 under acidic pH exhibits an associated anisotropy decay behavior characteristic of two different environments experienced by the fluorophore with one shorter and another longer rotational correlation time. The anisotropy decay retains normal bi-exponential behavior under neutral pH. Fluorescence correlation spectroscopic investigation reveals that the attached fluorophore undergoes hydrolysis under basic condition which results in the release of the fluorophore from the polymer backbone. Shrinkage in the hydrodynamic radius of PMAA is observed on addition of the complementary polymer PVP which is attributed to the formation compact solubilized nanoparticle like aggregates. The size of particle further decreases on the addition of NaCl. The detailed results show that these complexes have potential for use as drug-delivery system under physiological conditions.

Development of a Simple and Rapid Method for In Situ Vesicle Detection in Cultured Media

Extracellular membrane vesicles (EMVs) are biogenic secretory lipidic vesicles that play significant roles in intercellular communication related to human diseases and bacterial pathogenesis. They are being investigated for their possible use in diagnosis, vaccines, and biotechnology. However, the existing methods suffer from a number of issues. High-speed centrifugation, a widely used method to collect EMVs, may cause structural artifacts. Immunostaining methods require several steps and thus the separation and detection of EMVs from the secretory cells is time-consuming. Furthermore, detection of EMVs using these methods requires specific and costly antibodies. To tackle these problems, development of a simple and rapid detection method for the EMVs in the cultured medium without separation from the secretory cells is a pressing task. In this study, we focused on the Gram-negative bacterium Shewanella vesiculosa HM13, which produces a large amount of EMVs including a cargo protein with high purity, as a model. Curvature-sensing peptides were used for EMV-detection tools. FAAV, a peptide derived from sorting nexin protein 1, selectively binds to the EMVs even in the presence of the secretory cells in the complex cultured medium. FAAV can fully detect the EMVs within a few minutes, and the resistance of FAAV to proteases enables it to withstand prolonged use in the cultured medium. Fluorescence/Förster resonance energy transfer was used to develop a method to detect changes in the amount of the EMVs with high sensitivity. Overall, our results indicate the potential applicability of FAAV for in situ EMV detection in cultured media.

Synthesis of novel xanthene based analogues: Their optical properties, jack bean urease inhibition and molecular modelling studies

In this work, a series of the rhodamine 6G based derivatives 5a-5g, were synthesized. The structural framework of the synthesized compounds was established by using ¹H NMR, ¹³C NMR, FT-IR, and LC-MS analytical methods. The spectroscopic properties of the target compounds were determined by using absorption and fluorescence study in four different solvents. Furthermore, the synthesized derivatives were assessed for in-vitro screening against jack bean urease inhibition and in-silico molecular docking study. The result reveals that all the compounds exhibit good urease inhibitory activity against this enzyme but among the series, the compound 5a &5c with an IC₅₀ values of 0.1108 ± 0.0038 μM and 0.1136 ± 0.0295 μM shows to be most auspicious inhibitory activity compared to a standard drug (Thiourea) having IC₅₀ value 4.7201 ± 0.0546 μM. Subsequently, the molecular docking experiment was analysed to distinguish the enzyme-inhibitor binding interaction.

Visible-Light-Mediated Synthesis of AzaBenzannulated Perylenediimide-Based Light-Harvesting Dyads

Intramolecular imine photocyclization has been explored for grafting on the bay region of perylenediimide (PDI) different electro- and photoactive chromophores to achieve new AzaBenzannulated-PDI (AzaBPDI) dyads. Triphenylamine (TPA), fluorene (Fl), perylenemonoimide (PMI), and perylenediimide (PDI) units have been successfully assembled to AzaBPDI using this straightforward one-pot synthesis starting from the easily accessible 1-aminoPDI. This original procedure was compared to the well-known Pictet-Spengler reaction and appears to be an attractive alternative in terms of versatility and efficiency with higher yields obtained. The optical and electrochemical properties of these molecular systems demonstrated large absorption capabilities in the visible range, good accepting abilities with low LUMO levels, and efficient electronic interactions between chromophoric units such as energy or electron transfers. In addition, with their large dihedral angle estimated by theoretical calculations, those dyads should present interesting applications in various organic optoelectronic devices. In particular, the PMI-AzaBPDI and PDI-AzaBPDI dyads presenting low LUMO levels, a broad absorption in the visible range, and a twisted conformation make them good candidates as non-fullerene acceptors in organic solar cells.

Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

Recent advances in the determination of crystal structures and studies of optical properties of gold nanoclusters in the size range from tens to hundreds of gold atoms have started to reveal the grand evolution from gold complexes to nanoclusters and further to plasmonic nanoparticles. However, a detailed comparison of their photophysical properties is still lacking. Here, we compared the excited state behaviors of gold complexes, nanolcusters, and plasmonic nanoparticles, as well as small organic molecules by choosing four typical examples including the Au₁₀ complex, Au₂₅ nanocluster (1 nm metal core), 13 diameter Au nanoparticles, and Rhodamine B. To compare their photophysical behaviors, we performed steady-state absorption, photoluminescence, and femtosecond transient absorption spectroscopic measurements. It was found that gold nanoclusters behave somewhat like small molecules, showing both rapid internal conversion (<1 ps) and long-lived excited state lifetime (about 100 ns). Unlike the nanocluster form in which metal–metal transitions dominate, gold complexes showed significant charge transfer between metal atoms and surface ligands. Plasmonic gold nanoparticles, on the other hand, had electrons being heated and cooled (~100 ps time scale) after photo-excitation, and the relaxation was dominated by electron–electron scattering, electron–phonon coupling, and energy dissipation. In both nanoclusters and plasmonic nanoparticles, one can observe coherent oscillations of the metal core, but with different fundamental origins. Overall, this work provides some benchmarking features for organic dye molecules, organometallic complexes, metal nanoclusters, and plasmonic nanoparticles.

Dynamic Optics with Transparency and Color Changes under Ambient Conditions

Mechanochromic materials have recently received tremendous attention because of their potential applications in humanoid robots, smart windows, strain sensors, anti-counterfeit tags, etc. However, improvements in device design are highly desired for practical implementation in a broader working environment with a high stability. In this article, a novel and robust mechanochromism was designed and fabricated via a facile method. Silica nanoparticles (NPs) that serve as a trigger of color switch were embedded in elastomer to form a bi-layer hybrid film. Upon stretching under ambient conditions, the hybrid film can change color as well as transparency. Furthermore, it demonstrates excellent reversibility and reproducibility and is promising for widespread application.

Photon Counting Data Analysis: Application of the Maximum Likelihood and Related Methods for the Determination of Lifetimes in Mixtures of Rose Bengal and Rhodamine B

It is often convenient to know the minimum amount of data needed to obtain a result of desired accuracy and precision. It is a necessity in the case of subdiffraction-limited microscopies, such as stimulated emission depletion (STED) microscopy, owing to the limited sample volumes and the extreme sensitivity of the samples to photobleaching and photodamage. We present a detailed comparison of probability-based techniques (the maximum likelihood method and methods based on the binomial and the Poisson distributions) with residual minimization-based techniques for retrieving the fluorescence decay parameters for various two-fluorophore mixtures, as a function of the total number of photon counts, in time-correlated, single-photon counting experiments. The probability-based techniques proved to be the most robust (insensitive to initial values) in retrieving the target parameters and, in fact, performed equivalently to 2-3 significant figures. This is to be expected, as we demonstrate that the three methods are fundamentally related. Furthermore, methods based on the Poisson and binomial distributions have the desirable feature of providing a bin-by-bin analysis of a single fluorescence decay trace, which thus permits statistics to be acquired using only the one trace not only for the mean and median values of the fluorescence decay parameters but also for the associated standard deviations. These probability-based methods lend themselves well to the analysis of the sparse data sets that are encountered in subdiffraction-limited microscopies.

In situ characterization of nanoparticle biomolecular interactions in complex biological media by flow cytometry

Nanoparticles interacting with, or derived from, living organisms are almost invariably coated in a variety of biomolecules presented in complex biological milieu, which produce a bio-interface or 'biomolecular corona' conferring a biological identity to the particle. Biomolecules at the surface of the nanoparticle-biomolecule complex present molecular fragments that may be recognized by receptors of cells or biological barriers, potentially engaging with different biological pathways. Here we demonstrate that using intense fluorescent reporter binders, in this case antibodies bound to quantum dots, we can map out the availability of such recognition fragments, allowing for a rapid and meaningful biological characterization. The application in microfluidic flow, in small detection volumes, with appropriate thresholding of the detection allows the study of even complex nanoparticles in realistic biological milieu, with the emerging prospect of making direct connection to conditions of cell level and in vivo experiments.

Gold-catalyzed allene cycloisomerization for pyrrole synthesis: towards highly fluorinated BODIPY dyes

A novel synthetic strategy toward highly fluorinated BODIPY dyes with exceptional photostabilities relying on sustainable gold catalysis has been developed. A key to the tailored pyrrole precursors is the gold catalysis performed in ionic liquids as the reaction medium, allowing a facile recycling of the catalysts. The dyes prepared are well-matching with the spectral windows of popular rhodamine dyes and possess high brightness while showing a distinctly higher photostability than the rhodamines especially in aprotic solvents.

Quantum yield regeneration: influence of neutral ligand binding on photophysical properties in colloidal core/shell quantum dots

This article describes an experiment designed to identify the role of specific molecular ligands in maintaining the high photoluminescence (PL) quantum yield (QY) observed in as-synthesized CdSe/CdZnS and CdSe/CdS quantum dots (QDs). Although it has been possible for many years to prepare core/shell quantum dots with near-unity quantum yield through high-temperature colloidal synthesis, purification of such colloidal particles is frequently accompanied by a reduction in quantum yield. Here, a recently established gel permeation chromatography (GPC) technique is used to remove weakly associated ligands without a change in solvent: a decrease in ensemble QY and average PL lifetime is observed. Minor components of the initial mixture that were removed by GPC are then added separately to purified QD samples to determine whether reintroduction of these components can restore the photophysical properties of the initial sample. We show that among these putative ligands trioctylphosphine and cadmium oleate can regenerate the initial high QY of all samples, but only the "L-type" ligands (trioctyphosphine and oleylamine) can restore the QY without changing the shapes of the optical spectra. On the basis of the PL decay analysis, we confirm that quenching in GPC-purified samples and regeneration in ligand-introduced samples are associated chiefly with changes in the relative population fraction of QDs with different decay rates. The reversibility of the QY regeneration process has also been studied; the introduction and removal of trioctylphosphine and oleylamine tend to be reversible, while cadmium oleate is not. Finally, isothermal titration calorimetry has been used to study the relationship between the binding strength of the neutral ligands to the surface and photophysical property changes in QD samples to which they are added.

Investigation of the nonlinear absorption spectrum of all-trans retinoic acid by using the steady and transient two-photon absorption spectroscopy

This paper reports for the first time a complete study on the steady and transient excited state dynamics induced by 2PA for ATRA.

Peptide-functionalized luminescent iridium complexes for lifetime imaging of CXCR4 expression

The chemokine receptor 4 (CXCR4) is over-expressed in 23 types of cancer in which it plays a role in, among others, the metastatic spread. For this reason it is a potential biomarker for the field of diagnostic oncology. The antagonistic Ac-TZ14011 peptide, which binds to CXCR4, has been conjugated to luminescent iridium dyes to allow for CXCR4 visualization. The iridium dyes are cyclometalated octahedral iridium(III) 2-phenylpyridine complexes that can be functionalized with one, two or three targeting Ac-TZ14011 peptides. Confocal microscopy and fluorescence lifetime imaging microscopy (FLIM) showed that the peptide-iridium complex conjugates can be used to visualize CXCR4 expression in tumor cells. The CXCR4 receptor affinity and specific cell binding of the mono-, di- and trimeric peptide derivatives were assessed by using flow cytometry. The three derivatives possessed nanomolar receptor affinity and could distinguish between cell lines with different CXCR4 expression levels. This yields the first example of a neutral iridium(III) complex functionalized with peptides for FLIM-based visualization of a cancer associated membrane receptor.

Pulse train fluorescence technique for measuring triplet state dynamics

We report on a method to study the dynamics of triplet formation based on the fluorescence signal produced by a pulse train. Basically, the pulse train acts as sequential pump-probe pulses that precisely map the excited-state dynamics in the long time scale. This allows characterizing those processes that affect the population evolution of the first excited singlet state, whose decay gives rise to the fluorescence. The technique was proven to be valuable to measure parameters of triplet formation in organic molecules. Additionally, this single beam technique has the advantages of simplicity, low noise and background-free signal detection.

Pulsed fluorescence measurements of trapped molecular ions with zero background detection

Sensitive methods have been developed to measure laser-induced fluorescence from trapped ions by reducing the detection of background scattering to zero levels during the laser excitation pulse. The laser beam diameter has been reduced to approximately 150 microm to eliminate scattering on trap apertures and the resulting laser-ion interaction is limited to a volume of approximately 10(-5) cm which is approximately 0.03-0.15 of the total ion cloud volume depending on experimental conditions. The detection optics collected fluorescence only from within the solid angle defined by laser-ion interaction volume. Rhodamine 640 and Alexa Fluor 350 ions, commonly used as fluorescence resonance energy transfer (FRET) fluorophores, were generated in the gas phase by using electrospray ionization and injected into a radiofrequency Paul trap where they were stored and exposed to Nd:YAG laser pulses at 532 and 355 nm for times up to 10 m. Fluorescence emitted by these ions was investigated for several trap q(z) values and ion cloud temperatures. Analysis of photon statistics indicated an average of approximately 10 photons were incident on the PMT detector per 15 ns pulse for approximately 10(3) trapped ions in the interaction volume. Fluorescence measurements displayed a dependence on trapped ion number which were consistent with calculations of the space charge limited ion density. To investigate the quantitative capability of these fluorescence techniques, the laser-induced fragmentation of trapped Alexa Fluor 350 ions was measured and compared with a rate equation model of the dynamics. Decay of the fluorescence signal as well as the parent ion number compared closely with quantitative predictions of the photofragmentation model.

Photophysics of Rhodamine B Interacting with Silver Spheroids

Deactivation processes of Rhodamine B (RB) in water and interacting with silver sols were studied by using conventional fluorescence, laser-induced fluorescence, and pulsed laser photoacoustics. Results showed that the emission efficiencies, absorption cross sections, and nonradiative deactivation pathway of RB interacting with silver spheroids of different sizes are comparable to those obtained for the dye in aqueous solutions. Copyright 1999 Academic Press.

Demonstration of droplet size and vaporization rate measurements in the near field of a two-phase jet with droplet lasing spectroscopy

Droplet lasing spectroscopy has been applied to the measurement of droplet size and evaporation rate in a spray. A single droplet, doped with laser dye, was injected along the centerline of a liquid spray. Filters were used to block the strong elastic-scattering signal. The lasing emission from the doped droplet could be detected against the background with mass loadings of liquid in the spray as high as 20%. An analysis of the spectrum of droplet lasing was used to evaluate the droplet diameter. The evaporation rate of the droplet was obtained from consecutive lasing spectra that were obtained from the same droplet. An error analysis of the drop size and drop evaporation measurements was carried out and showed that accurate measurements of evaporation rates were feasible.

Spectroscopy and laser characteristics of copper-vapor-laser pumped pyrromethene-556 and pyrromethene-567 dye solutions

We measured the basic optical properties of Pyrromethene-567 (P567) and Pyrromethene-556 (P556) dye solutions that are relevant to their application as dye lasers. The fluorescence spectra of methanol solutions show mirror images in relation to the absorption spectra, with Stokes shifts of 29.5 and 37.5 nm, respectively, for the two dyes. The central fluorescence peaks were at 546 and 535 nm, with widths of ~40 and ~50 nm (FWHM). The quantum yields were 97% ? 5% and 78% ? 5% for P567 and P556, respectively. Fluorescence lifetimes of 6.0 ? 0.2 ns were obtained for both dyes in methanol. Laser action, obtained by pumping with the green emission line (510.6 nm) from a copper-vapor laser, was measured in a Hänsch-type cavity. Tunability ranged from 531 to 590 nm for P567 and from 522 to 590 nm for P556. Lasing thresholds were ~0.27 and ~0.16 mJ/pulse, with 25% and 27% slope efficiencies for P567 and P556, respectively. Spectroscopy and lasing were studied in other solvents as well.