Photophysical and laser characteristics of pyrromethene 567 dye: Experimental and theoretical studies

Narrow-band laser performance of alcohol solutions of pyrromethene 567 (PM567) and rhodamine 6G (RH6G) dye was investigated using a home-made GIG-configured dye laser, excited by the second-harmonic radiation (at 532 nm) of a pulsed Nd:YAG laser. Higher laser efficiency was observed with PM567 dye (∼23% peak) in comparison to the commonly used RH6G dye (16.5%), in spite of much lower fluorescence quantum efficiency of the PM567 (0.83) vis-à-vis RH6G (0.98) dye solutions in ethanol. First principle-based electronic structure calculations were performed on PM567 dye in the ground (S ₀) and excited states (S ₁) using density functional theory to elucidate the structure and photophysical properties of the dye.     

Single-crystal EPR studies of transition-metal ions in inorganic crystals at very high frequency

Electron paramagnetic resonance (EPR) single-crystal rotation studies at very high frequency (249.9 GHz) of transition metal ions with electron spins greater than one-half are reported. At 249.9 GHz, the spectra are in the high-field limit despite large zero-field splittings. This leads to a considerable simplification of the spectra, and aids in their interpretation. Single-crystal 249.9 GHz EPR spectra of Ni²⁺ in Ni₂CdCl₆· 12H₂O, Mn²⁺ (0.2%) in ZnV₂O₇, and Fe³⁺ (2%) in CaYA10₄ were recorded at 253 K in an external magnetic field of up to 9.2 T, along with those at X-band and Q-band frequencies at 295 K and lower temperatures. The goniometer used at 249.9 GHz for single-crystal rotation is based on a quasi-optical design and is an integral part of a special Fabry-Pérot resonator. The values of the spin-Hamiltonian parameters were estimated from a simultaneous fitting of all of the observed line positions at several microwave frequencies recorded at various orientations of each crystal with respect to the external magnetic field with least-squares fitting in conjunction with matrix diagonalization. Estimates of zero-field splitting parameterD at room temperature are: for Ni²⁺, about ?31 GHz (site I) and about ?7 GHz (site II); for Mn²⁺, about 6 GHz; and for Fe³⁺, about 29 GHz.     

Photophysical characterization of pyrromethene 597 laser dye in silicon-containing organic matrices

Samples of dipyrromethene-BF₂ dye PM597 incorporated in copolymers of 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) with methyl methacrylate (MMA) and 2-hydroxyethylmethacrylate (HEMA), and in terpolymers of MMA, HEMA and TMSPMA are characterized. The absorption cross-section spectra, stimulated emission cross-section spectra, and the excited-state absorption cross-section at 527 nm are determined. The fluorescence quantum distributions and fluorescence lifetimes are measured. The photo-degradation is studied under cw laser excitation conditions and quantum yields of photo-degradation are extracted. PM597 solid state samples are compared with PM597 in liquid ethyl acetate solution. The fluorescence quantum yield of PM597 is higher in doped samples (around 70%) compared to PM597 in ethyl acetate (43%). The excited-state absorption cross-section was found to be negligibly small. The photo-stability is considerably larger in the polymeric samples compared to the liquid solutions. PACS 42.55.-f; 78.45.+h; 78.55.-m; 78.40.Me     

Potentiodynamic studies on corrosion of copper by chloride ions and its inhibition by inorganic and organic ions in aqueous buffer solution

Electrochemical oxidation of copper in aqueous phosphate buffer solution leads to the formation of passive films, which breakdown in the presence of aggressive chloride (Cl^?) ions. The addition of Cl^? ions in the medium causes shift in current and potential for both cathodic and anodic peaks. At low Cl^? ion concentrations, the corrosion current is greatly enhanced and continues to increase up to a certain concentration, after which it decreases. The effect of inorganic ions, chromate (CrO ₄ ^2? ) and tungstate (WO ₄ ^2? ), and organic ions, acetate (CH₃COO^?) and oxalate (C₂O ₄ ^2? ), on the formation and breakdown of passive films in aqueous buffer solution containing 0.10 M Cl^? ions have also been investigated. All of the ions inhibit corrosion of copper in the presence of aggressive Cl^? ions following the order of overall inhibition efficiency of CrO ₄ ^2? > C₂O ₄ ^2? > WO ₄ ^2? > CH₃COO^? under the experimental conditions.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Photophysical properties of a surfactive long-chain styryl merocyanine dye as fluorescent probe

This work deals with detailed investigations of the photophysical properties of a styryl merocyanine dye, namely 1-cetyl-4-[4′-(methoxy) styryl]-pyridinium bromide (CMSPB) of molecular rotor type. The solvatochromic analyses of the data in different solvents using the Kamlet-Taft parameters (α, β, π^?) were discussed. Optical excitation of the studied merocyanine dye populates a CT S₁ state with about 22.64 folds higher dipole moment value relative to that in the S₀ state. Moreover, the effect of solvent viscosity (glycerol at various temperatures (299.0–351.0 K)) on CMSPB fluorescent properties is analyzed to understand the molecular mechanisms of the characteristic increase in CMSPB fluorescence intensity. The results indicate that CMSPB exhibits fluorescent properties typical for molecular rotors. The results show that torsional relaxation dynamics of molecular rotors in high-viscosity solvents cannot be described by the simple stick boundary hydrodynamics defined by the Debye–Stokes–Einstein (DSE) equation. The fluorescence depolarization behavior in glycerol at various temperatures (299.0–351.0 K) shows that the molecular rotational diffusion is controlled by the free volume of the medium. Furthermore, excited state studies in ethanol/chloroform mixture revealed the formation of weak complex with chloroform of stoichiometry 1:1 with formation constant of 0.004l mol^?1. Moreover, the increase of the quantum yield values in micellar solutions of CTAB and SDS relative to that of water indicates that the guest dye molecules are microencapsulated into the hydrophobic interior of host micelle. The obtained non-zero values of fluorescence polarization in micellar solution imply reduced rotational depolarization of dye molecules via association with the surfactant. Upon comparing the spectral data in micelles with those in homogeneous solvent systems, more can be learned of the structural details of the micellar environment, which have often been used as models for more complex bioaggregates. The results point out to a possible use of this dye as a fluorescence probe for microenvironmental parameters as well as in some micellar systems.     

Photophysical properties and laser characteristics of a new rigid aminocoumarin dye lasing in the blue-green region

The photophysical properties of a new dye, 7-Diethyl AminoCoumarin with a Rigid substitution in the 3-position (referred to as DARC) have been studied in three solvents: dioxane, DMF and DMSO. The dye has been found to have a fluorescence quantum efficiency (_fl) between 0.40 and 0.80 in these solvents. The dye-laser performance of this dye has also been investigated in dioxane, DMF and DMSO, under nitrogen-laser pumping and compared with that of the commercially available standard laser dye, Coumarin 515 (C-515). A tuning range of nearly 70 nm was obtained in the blue-green region with an efficiency up to 80% of that of the standard dye. The observed characteristics of the dye are explained in terms of the sructural rigidization of the dye in the 3-position which inhibits the formation of the Twisted Intramolecular Charge Transfer (TICT) conformation in the excited state leading to an enhancement of the _fl and a considerable improvement in the laser performance.Research carried out at U.D.C.T., Bombay, India     

Dielectric relaxation studies of alkanols solubilized by sodium dodecyl sulphate aqueous solutions

Considerable attention has been paid in recent years to the influence of alcohols on ionic micellar structures, alcohol co-surfactants most commonly employed in the preparation of microemulsions. Dielectric relaxation spectroscopy (DRS) is a versatile tool to monitor the dynamic process of such micellar systems. We report here the changes in the dielectric constant of the medium and the relaxation time of micellar solutions of sodium dodecyl sulphate (SDS) in water in the presence and absence of alkanols—hexan-1-ol, octan-l-ol, decan-l-ol and 1,2-ethanediol. The time domain dielectric data were obtained in the reflection mode in the frequency range of 10 MHz to 20 GHz using a HP54750A sampling oscilloscope and HP 54754A TDR plug-in-module. The sample is held at 303 K in a SMA cell with an effective pin length of 1.35 mm. We have determined the relaxation time (τ) using the Cole–Cole method. The relative viscosity ηr on the micellar solutions were also determined. The result shows that the dielectric constant increases linearly with SDS concentration in pure aqueous solutions up to 400 mM and decreases thereafter. The relaxation times are of the order of 15 ps to 25 ps and are far greater than that of Debye rotational relaxation of the monomer species. It is assigned to the dispersion step to water molecules surrounding to hydrophobic particles or the “bound water”. The addition of alcohols results in a linear increase in relaxation time in all the systems. As the concentration of SDS increases τ/ηr attains a near constant value showing that solubilization becomes more difficult at these concentrations. Our results show that the alcohol molecules are solubilized at the surface of the SDS micelle (i.e.) the micelle–water interface.     

2S12 state ions in inorganic luminescent materials

Selected examples of impurity ions with one unpaired electron in an unfilled shell are Pb³⁺ ions in II–IV compounds (ZnO, ZnSe, ZnTe), Pb³⁺ in CaWO₄,F centres in alkali halides and Ag⁰ and Au⁰ atoms in radiophotoluminescent (RPL) glasses. Comparative studies of EPR, luminescence, thermoluminescence and related phenomena were performed on systems with ²S_{$\text{1}\text{2}$} state ions as central ions and systematically varying ligand ions in the temperature range from 1.6°K to 450°K. Luminescence bands attributed to internal transitions ²P_{$\text{3}\text{2}$,$\text{1}\text{2}$}→²S_{$\text{1}\text{2}$} ground state are reported especially with respect to the corresponding EPR data. All data are summarized in a molecular orbital model for the luminescence complex.     

Fluorescence and absorption spectra of Rose-Bengal dye in the presence of surfactants

The fluorescence and absorption spectra of Rose Bengal dye in aqueous solution have been studied in the presence of various nonionic, anionic and cationic surfactants. With cationic and nonionic surfactants, shifts occur in the absorption and emission peaks of the dye solution, with a large enhancement in the absorption and fluorescence intensity at the shifted λ_max. No appreciable change in the absorption and fluorescence spectra of the dye has been observed on the addition of anionic surfactants.All the changes observed in the absorption and fluorescence spectra of the dye solution with surfactants may be attributed, to binding of the surfactant with dye molecules and the disaggregation of the dye multimer forms into the monomeric form.     

Photophysical and electrochemical properties of D–π–A type solvatofluorchromic isophorone dye for pH molecular switch

The photophysical and electrochemical properties of a donor-π-accepter (D–π–A) type isophorone dye, 2-(3-(4-((2-hydroxyethyl)(methyl)amino)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile with asymmetrical structure, was investigated. It showed interestingly solvatofluorchromic property in various solvents based on intramolecular charge transfer mechanism. Its LUMO and HOMO values were obtained by electrochemical measurement and theoretical calculation. In addition, it can be also constructed a pH fluorescent molecular switch by modulation of intramolecular charge transfer with protonation/deprotonation.     

Spectral and nonlinear studies of night blue dye

Solid-state dye-doped polymer is an attractive alternative to the conventional liquid dye solution. In this Letter the spectral characteristics and the nonlinear optical properties of the dye night blue are studied. The spectral characteristics of night blue dye doped poly(methylmethacrylate) modified with additive n-butyl acetate (nBA) are studied by recording its absorption and fluorescence spectra and the results are compared with the corresponding liquid mixture. The nonlinear refractive index of the dye in nBA and dye doped polymer film were measured using z-scan technique [S.-B., Mansoor, A.A. Said, T.-H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760], by exciting with He–Ne laser. The results obtained are intercompared. Both the samples of dye night blue show a negative nonlinear refractive index. The origin of optical nonlinearity in the dye may be attributed due to laser-heating induced nonlinear effect.     

Comparative electroabsorption studies of organic and inorganic solids

Spectral changes induced by moderate electric fields provide detailed insight into the electronic states of organic and inorganic solids. Although the basic effects, Stark effect and Franz–Keldysh effect, are the same in both types of material, the electroabsorption spectra vary strongly in size and spectral lineshape due to competing interactions. The large variance of the effects is demonstrated by representative examples of high mobility semiconductors, quantum wells, π-conjugated polymers, and charge transfer transitions in single crystals, disordered films and a double-quantum well. It is shown that only high-quality samples reveal the quantum mechanics of field-induced effects which are very sensitive to disorder.     

Kiton red S dye: Photophysical, photostability, photothermal and narrow-band laser performances using different solvents

Many aspects of photophysical, photostability and laser properties of kiton red S dye remain unresolved, particularly for pumping with 578 nm radiation of CVL and 532 nm output of Nd:YAG lasers, and these are studied using different alcohol- and water-based binary solvents. Our results show that methanol is better suited for low and high-repetition-rate KRS dye lasers because of its superior laser efficiency, photostability as well as photothermal properties.     

Self-diffraction and Z-scan studies in organic dye doped thin films

Self-diffraction in Acid Red 87 (eosin Y) dye doped thin films is studied using argon ion laser (514.5 nm). Growth of self-diffraction grating is monitored by measuring intensities of various diffraction orders. This study has resulted in the observation of phase variation between the contributing beams in any diffracted order. This change of phase is measured at various stages of grating formation. Due to self-phase modulation, circular concentric rings pattern is obtained in the far field. The observed fluctuation in this pattern may be due to the phase variation between the contributing beams in any diffracted order. Z-scan technique is used to study the optical non-linearity of the sample.     

Three-body recombination of electrons and ions in the presence of two-level atoms

The distribution function of bound electrons and the recombination rate of electrons and ions in the presence of two-level atoms is considered within a diffusion model. Two cases are considered: (a) it is assumed in accordance with traditional theories that relaxation occurs as a result of binary collisions; (b) it is assumed that the anomalous drift previously discovered on the basis of a first-principles simulation takes place. It is shown that the distribution of bound electrons obtained on the basis of the theory of binary Coulomb collisions is not consistent with the results of a numerical many-particle dynamics simulation, while a kinetic model which utilizes the theory of anomalous drift is consistent with the simulation results. Zh. Tekh. Fiz. 68, 15–19 (January 1998)