Alok K. Ray

Design and development of a new pyrromethene dye with improved photostability and lasing efficiency: theoretical rationalization of photophysical and photochemical properties.

In an attempt to develop a photostable and efficient pyrromethene compound for use in liquid dye lasers, three congeners of the commercially available pyrromethene 567 (PM567) laser dye were synthesized and their photophysical properties, lasing efficiencies, and photochemical stabilities were studied. In general the presence of an aryl group at C-8 of the pyrromethene chromophore increased the photostability. One of the congeners possessing a C-8 trimethoxyphenyl moiety showed significantly improved lasing parameters than PM567. Compared to PM567, the photochemical stability of the new dye was 2-fold, while it showed an equivalent lasing efficiency to that of PM567 at a significantly lower concentration. The increased photostability of these new dye molecules could be explained by theoretical calculation on their capacity to generate singlet oxygen ((1)O(2)) and probability of reaction with (1)O(2). Our calculations were in agreement with the experimental results and indicated that a systematic design of new derivatives of pyrromethene chromophore might lead to improved laser dye molecules.

Molecular Encapsulation of Fluorescent Dyes Affords Efficient Narrow-band Dye Laser Operation in Water

A water-based narrow-band high-efficiency dye laser was designed by means of a supramolecular host-guest chemical approach. The lasing characteristics of rhodamine B and sulforhodamine B (Kiton Red S) dyes in aqueous solution with the macrocyclic host cucurbit[7]uril (CB7) as additive were investigated in a narrow-band dye laser setup. Significant improvements in both photostability and thermo-optical properties of the aqueous CB7-complexed dye systems were observed as compared to the uncomplexed dyes in ethanol solution. The tuning curves for the new dye-CB7-water systems were constructed by measuring the laser output at different wavelengths, which showed similar peak efficiencies and red-shifted gains compared to the ethanolic solutions of the dyes, while dye laser operation revealed comparable pump threshold energies and slope efficiencies. The combined results render the dye-CB7-water system an attractive active medium for high-repetition rate dye laser operation.

Characterisation of zirconium nitride coatings prepared by DC magnetron sputtering

Abstract The composition, thickness and microhardness of zirconium nitride (ZrN) coatings deposited at different nitrogen partial pressures using DC magnetron sputtering technique have been studied. Proton elastic backscattering technique involving the 14N(p,p)14N reaction has been utilised for the determination of composition and thickness of these coatings.

Solvent dependent nonlinear refraction in organic dye solution

Nonlinear refraction in rhodamine-B (Rh-B) organic dye solution at 510.6 nm was studied using the Z-scan technique with 40 ns laser pulses from a copper vapor laser. The observed nonlinear refraction is of thermal origin resulting from nonradiative energy transfer from the dye molecules to the solvent molecules and is therefore strongly dependent on the thermal properties of the solvent. Using this scheme, the thermal properties of an aqueous dye solution were compared, with that using water-surfactant mixture as the solvent.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Supramolecular Interaction of Coumarin 1 Dye with Cucurbit[7]uril as Host: Combined Experimental and Theoretical Study

Molecules of the coumarin family have fluorescence characteristics that are highly sensitive to their environment, and thus, they have been used as fluorescent sensors in chemical and biological systems. However, the very poor fluorescence yield of most coumarin dyes in aqueous media limits their applications. We have adopted a supramolecular strategy to improve the fluorescence intensity of coumarin dye through its interaction with the relatively new host cucurbit[7]uril (CB[7]). The virtually nonfluorescent coumarin 1 (Φ(f) = 0.04) was converted into a highly fluorescent (Φ(f) = 0.52) entity in water upon addition of the nonfluorescent host CB[7]. Various spectroscopy techniques, namely, UV-vis absorption and steady-state and time-resolved fluorescence spectroscopies, established the formation of a strong 1:1 dye-CB[7] inclusion complex with a high binding constant of (1.2 ± 0.1) × 10(5) M(-1) for the dye. The stable inclusion complex of the neutral molecule was supported by density-functional-theory- (DFT-) based quantum chemical calculations. Energy decomposition analysis of various interaction factors in the host-guest complex revealed that key components providing stability to the complex were electrostatic, polarization, and charge-transfer energies. These new results on the formation of a strong inclusion complex of the versatile fluorophore coumarin 1 with the nontoxic host CB[7] could lead to the design of efficient molecular-scale biological probes, sensors, and photostable aqueous UV dye lasers.

Congeners of Pyrromethene-567 Dye: Perspectives from Synthesis, Photophysics, Photostability, Laser, and TD-DFT Theory

In an attempt to develop photostable and efficient BODIPY (PM) dyes for use in liquid dye lasers, three new congeners of widely used laser dye, PM567, were synthesized and their photophysical properties in various organic solvents, laser performances, and photostabilities in a selected solvent, 1,4-dioxane, have been investigated using a frequency doubled Q-switched (10 Hz) Nd:YAG laser at 532 nm. The results of photostability study in nonpolar 1,4-dioxane revealed the remarkable enhancement in stability of the novel dyes compared to that of PM567 as well as improved laser performances. Cyclic voltammetry study strongly supports the observed enhancement in photostability of the novel dyes compared to that of PM567. The observed properties of the novel dyes in relation to those of PM567 have been rationalized by extensive use of DFT and TD-DFT using the B3LYP/6-31G(d) method of theory.

Rational Design of Boradiazaindacene (BODIPY)‐Based Functional Molecules

Three boradiazaindacene (BODIPY) dyes with different-coloured (greenish-yellow, orange and red) fluorescence and good Stokes shifts were synthesised starting from the greenish-yellow BODIPY dye PM546. The high Stokes shifts of the dyes are due to the release of the steric strain in their excited states relative to that in the highly twisted ground states. One of these compounds might be a useful water-soluble fluorophore, whereas the other two are promising H(+) sensors.

Spectral characteristics of a binary dye-mixture laser

We have investigated laser action in a binary mixture of dyes, Rh-6G and DCM, resulting in tunable laser emission over an extended frequency region. The two dyes absorb the same pump radiation but fluoresce over frequency ranges that are shifted with respect to each other, thereby resulting in extended tunability. Following a time-dependent analysis of a rate-equation model that describe the operation of such a laser, theoretical estimates for optimum dye concentrations and the corresponding extension of the laser tuning range have been obtained.

Laser emission from self-assembled active photonic crystal matrix

Three-dimensionally ordered photonic crystals were grown using self-assembly technique from Rhodamine-B dye doped polystyrene micro-spheres resulting in a stop band at 611 nm overlapping the emission spectrum of the dye. When excited at a wavelength away from the stop band, using a frequency-doubled Nd:YAG laser, the crystal showed angle- dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement at the band edge, in reflection and transmission geometries. Spectral narrowing, a sharp threshold and a highly directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix.