Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Experimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τ_f), fluorescence lifetime (τ_f), radiative rate constant (k_f) and nonradiative rate constant (k_nr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n² ‐ 1)/ (2n²+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: ( 1 ) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt₂ substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; ( 2 ) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt₂ group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; ( 3 ) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and ( 4 ) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.     

Enthalpic relaxation of convective desiccated trehalose-water glasses

Minimizing molecular mobility for desiccation preservation of biologics close to ambient temperature using trehalose glasses require quantitative characterization of their enthalpic relaxation at various end water contents. Differential scanning calorimetry (DSC) was used to characterize three different water contents: 0%, 1.5% and 10% over a wide range of aging temperatures. Results showed the characteristic time (τ) varies both with the water content and the aging temperature. τ increased with lowered aging temperature but showed a non-monotonous relationship as a function of water content. Fragility of trehalose glasses was analyzed using thermophysical parameters obtained from relaxation studies. The study showed trehalose to be a fragile glass former at all water contents, with 0% water samples showing a relatively stronger glass. A compromise between molecular mobility and glass fragility led to an optimal water content close to 1.5% and an aging temperature close to room temperature. This would ensure a τ value of 9000 h, which corresponds to a storage period of a year.     

Size-selective synthesis and stabilization of gold organosol in C(n)TAC: enhanced molecular fluorescence from gold-bound fluorophores

Gold nanoparticles of variable sizes have been synthesized in toluene employing two-phase (water-toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of a series of cationic surfactants of a homologous series having the general formula C(n)TAC. The solubility features of the gold particles in the organic solvent have been accounted qualitatively by calculating the van der Waals interaction potential between the particles. The effect of thermal energy and medium dielectric constant on the stability of metal particles has been studied by measuring the surface plasmon resonance. The stabilization of surfactant-mediated gold particles as hydrosol or organosol has been elucidated by considering the double-layer interaction as a function of the dielectric constant of the solvent medium. The influence of the counterion of the phase transfer reagent and stabilizing ligand on the photochemical stability of the gold colloids has been investigated. The fluorescence probe 1-methylaminopyrene (MAP) was considered for the surface functionalization of the gold particles, and it has been found that there is an enhancement of molecular fluorescence from the gold-probe assembly.     

Synthesis of cetyltrimethylammonium tribromide (CTMATB) and its application in the selective oxidation of sulfides to sulfoxides

The bright yellow crystalline cetyltrimethylammonium tribromide (CTMATB) reagent has been synthesized from the reaction of CTMAB and KBr with H₂MoO₄·H₂O, H₂O₂ and H₂SO₄ in the molar ratio 1:2:0.01:4:0.93. CTMATB selectively oxidizes a variety of dialkyl and alkyl aryl sulfides to the corresponding sulfoxides in high yields under mild conditions.     

Effect of environmental factors and carbohydrate on gellan gum production

Submerged culture fermentation studies were carried out in batch mode for optimizing the environmental parameters and carbon source requirement by Pseudomonas elodea for the production of gellan gum. The maximum production of gellan gum was obtained with 16-h-old culture and 8% inoculum at 30°C and pH 7.0 after 52 h of incubation (6.0 g/L). Of the various carbon sources tested, 2% sucrose, glucose, and soluble starch yielded considerably high amounts of gellan. Studies on the concentration of various carbohydrates on gellan gum production indicated that the optimum concentration of glucose and starch was 3%, whereas for sucrose it was 4%. The addition of glucose in the medium above 3% had a detrimental effect on gellan yield. The investigation of intermediate two-step addition of glucose under identical conditions of fermentation showed an enhanced production of gellan (8.12 g/L) as compared with the control (6.0 g/L). To optimize the recovery of gellan from fermented broth, different solvents were tested for precipitation of gellan gum. Among the various solvents tested, tetrahydrofuran gave better recovery of gellan (82%) as compared with the conventional solvent isopropanol (49%).     

Study of Densities, Viscosities and Ultrasonic Speeds of Binary Mixtures Containing 1,2-Dimethoxyethane and an Alkan-l-ol at 298.15 K

The viscosity deviation (Δη), the excess molar volume (V ^E) and the ultrasonic speed (u) have been investigated from viscosity (η) and density (ρ ) measurements of binary liquid mixtures of 1,2-dimethyoxyethane with methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol, hexan-1-ol or octan-1-ol over the entire range of composition at 298.15 K. The excess volumes are negative over the entire range of composition for all of the mixtures with the exception of hexan-1-ol and octan-1-ol. The excess isentropic compressibilities (K _S ^E) and viscosity deviations are negative for all of the mixtures. The magnitudes of the negative values of V ^E decrease with the number of carbon atoms of the alkan-1-ol. The trend of increasing K _S ^E values with the chain length of the alkanol is similar to that observed in the case of V ^E. Graphs of V ^E, Δ η, K _S ^E, Δ u, L _f ^E and Z ^E against composition are presented as a basis for a qualitative discussion of the results.     

The synthesis and conformational analysis of tetrahydro-1,2,4-oxadiazines

The synthesis and variable temperature ¹H and ¹³C NMR spectra of three tetrahydro-1,2,4-oxadiazines are reported. The N(4)-Me inversion barriers are 6.8–7.0 (ax→ts) and 7.4–7.9 kcal mol^?1 (eq→ts) with ΔG° 0.6–0.9 kcal mol^?1. The N(2)-Me inversion barriers are 10.4–11.4 (ax→ts) and 11.6–13.1 kcal mol^?1 (eq→ts) with ΔGδ 1.2–1.7 kcal mol^?1. The barrier to ring inversion is ca. 12.7 kcal mol^?1. “R value” analysis shows the ring to have a 56.5±2δ dihedral angle about the C(5)-(6) bond, indicative of the expected chair conformation.     

A study on the electronic effect of <Emphasis Type="Italic">para</Emphasis> substituents in the aryloxy ring of the hydrazone ligands on the vanadium centre in a family of mixed-ligand [V<Superscript>V</Superscript>O(ONO)(OO)] complexes

[V^IVO(acac)₂] reacts with the methanolic solutions of tridentate dibasic ONO donor hydrazone ligands derived from the condensation of benzoyl hydrazine with either 2-hydroxyacetophenone (H₂L¹) or its para-substituted derivatives (H₂L^2–4) (general abbreviation H₂L), in the presence of vanillin (Hvan) in equimolar ratio under aerobic conditions generating the mixed-ligand oxovanadium(V) complexes of the type [V^VO(L)(van)], (1)–(4) in good yield. All the complexes are diamagnetic and exhibit only ligand-to-metal charge transfer (l.m.c.t.) band near 510 nm in addition to intra-ligand (π → π*) transition band near 330 nm in CH₂Cl₂ solution. ¹H-n.m.r. spectra of the complexes in CDCl₃ solution indicate the presence of two isomeric forms [(1A), (1B); (2A), (2B); (3A), (3B) and (4A), (4B)] in different ratios, which is explained by the interchange of the two binding sites of van⁻ motif between its coordinated equatorial and axial positions. Complexes display two quasi-reversible one electron reduction peaks near +0.10 V and near +0.30 V versus s.c.e. in CH₂Cl₂ solution which are attributed to the successive reduction of V^V→ V^IV and the V^IV→ V^III motifs, respectively. λ_max (for l.m.c.t. transition), and the two reduction potential values (E _1/2)^I (average of the first step anodic and first step cathodic peak potentials) and (E _1/2)^II (average of the second step anodic and second step cathodic peak potentials) of the complexes, are found to be linearly related to the Hammett constants (σ) of the substituents in the aryloxy ring of the hydrazone ligands. λ_max, (E _1/2)^I and (E _1/2)^II values show large dependence: dλ_max/dσ = 37.29 nm, d(E _1/2)^I/dσ = 0.21 V and d(E _1/2)^II/dσ = 0.21 V, respectively, on σ.     

Barbier coupling in water: SnCl2-mediated and Co(acac)2-catalyzed allylation of carbonyls

Co(acac)₂·2H₂O efficiently catalyzes SnCl₂-mediated Barbier coupling in water between carbonyls, including aromatic, aliphatic and α,β-unsaturated aldehydes, ketones, sugars and allyl bromide to afford the corresponding homoallylic alcohols in high yields. The catalyst was reused for several cycles with consistent activity.