No abstract

Photoinduced electron transfer (ET) reactions between amines and a series of coumarins have been investigated using fluorescence-quenching measurements in aqueous P123 triblock copolymer micellar solutions. Fluorescence spectral characteristics and fluorescence anisotropy measurements indicated a nearly similar microenvironment for all of the coumarins used in P123 micelles. Substantial quenching of coumarin fluorescence in the presence of amines has been observed. The quenching rates (k(q)(TR)) are largely reduced in the P123 micelle as compared to those in other micelles (sodium dodecyl sulfate (SDS), Triton-X 100 (TX-100), cetyl trimethyl ammonium bromide (CTAB), and dodecyl trimethyl ammonium bromide (DTAB)), which is probably due to larger coumarin-amine separations in the micellar phase. The k(q)(TR) values, when plotted against free energy changes (DeltaG degrees), follow a Marcus predicted bell-shaped correlation. The estimated activation energy for the ET reactions follow an inverse bell-shaped correlation with DeltaG degrees. Present results indicate that the appearance of Marcus inversion is primarily related to the changes in the activation barrier, as predicted from the Marcus ET theory. As the k(q)(TR) values are higher than the estimated bimolecular diffusional rate constant, the role of reactant diffusion on the quenching kinetics in the P123 micelle is negligible. The appearance of Marcus inversion at unexpectedly lower exergonicity has been rationalized on the basis of slow solvent relaxation and by the application of the two-dimensional ET (2DET) theory. Critical analysis of the present results establishes that the inversion in the ET rates at high exergonicity is not due to the alteration in the diffusion parameters of the reactants, as has been suggested in some recent reports. Instead, it is evident that the inversion in quenching rates at high exergonicity is due to the alteration in the activation barrier for the ET reactions.

Ultrafast photoinduced intermolecular electron transfer (ET) dynamics involving 7-aminocoumarin derivatives as electron donor and pyridinium moiety of surfactant molecules in cetylpyridinium chloride (CPC) micelle as electron acceptor has been investigated to understand the role of separation and orientation of reactants on micellar ET reactions. Unlike in noninteracting micelles (like Triton-X-100, sodium dodecyl sulfate, dodecyltrimethylammonium bromide, etc.), where surfactant-separated donor-acceptor pairs are understood to give the ultrafast ET component with the shortest time constant in the range of approximately 4 ps, in CPC micelles with pyridinium moiety as the intrinsic acceptor the ultrafast ET component is found to be in the subpicosecond time scale (of around 240 fs). This time scale is very similar to the values reported in the cases of ultrafast ET reactions involving coumarin dyes in electron-donating solvents. The ultrafast ET times in CPC micelles are significantly faster than the diffusive solvation dynamics in the micellar media. Correlation of the observed ET rates in the present cases with the free-energy changes of the reactions shows the inverse-bell-shaped correlation, predicted by Marcus ET theory. Interestingly, the onset of the Marcus inversion appears at a relatively lower exergonicity, which is attributed to the nonequilibrium solvent configuration during the ultrafast ET reaction, as envisaged from two-dimensional ET (2DET) model. Along with the ultrafast ET component, there are also slower ET components in these systems, which are attributed to those close-contact donor-acceptor populations in the micelles that have relatively weaker electronic coupling due to improper orientation of the interacting donor-acceptor pairs. The present results suggest that, along with the shifting of Marcus inversion at lower exergonicity, the ET rates can also be maximized in a micellar media by using surfactant molecule as an intrinsic reactant.

have been explored to this end [ 7,8 ] and in particular cobalt-based oxides and their binary oxides with nickel are reported to be potential candidates. [ 9 ] For a favorable OER, the M  OH bond strength should be moderate enough as per the Sabatier's principle of balanced intermediate adsorption in catalysis. [ 4,10,11 ] Cobalt in Co 3 O 4 spinel occupies two different sites, tetrahedral for Co 2+ and both tetrahedral and octahedral for Co 3+ . [ 12 ] Incorporation of nickel in the Co 3 O 4 spinel structure results in the octahedral substitution, which is reported to improve the electronic conductivity and effective surface area and add benefi cial effect on oxygen evolution kinetics. [ 13,14 ] Additionally, nickel-based oxides and hydroxides that are prepared by wet chemical routes are found to produce hierarchical nanostructures, and as a consequence provide high surface area for catalytic reactions. [ 15,16 ] Microstructuring of materials into thin fi lms, [ 17 ] 3D cages, [ 18 ] nanorod, [ 19 ] wires, [ 20 ] and porous structures [ 21 ] are known to enhance the active surface area. The oxygen vacancies and nonstoichiometry introduced during fabrication and annealing processes can further improve the conductivity and lower the hydroxyl adsorption energy. [ 22,23 ] Rational design of morphology is a promising approach to promote material's performance. Building hierarchical hollow structures thus holds promise for more effi cient electrocatalyst which can render large surface area, better electronic conductivity and porosity for electrochemical processes. [ 14,24 ] Great efforts have been devoted to develop such porous and hollow nanostructure, which can not only enhance the desired activity, but also impart new functionalities. [ 24,25 ] Recently, metal-organic framework (MOF) has been demonstrated as excellent precursors and templates for fabrication of 3D structures of metal oxides for highly effi cient supercapacitors, Li ion batteries, and oxygen reduction reactions. [ 26 ] Following similar fabrication methodology, MOF derived coreshell structured NiCo 2 O 4 -Co 3 O 4 has been prepared and used to study supercapacitor and catalytic oxygen evolution behavior. [ 18 ] There is, however, a need for systematic investigations to arrive at an optimum nickel-cobalt oxide for OER based on this fabrication route. Such investigations assume importance in realizing a competitive OER catalyst of this class for use in solar to hydrogen conversion devices.In this paper, we report a simple template based fabrication route for non-stoichiometric Ni  Co metal oxide nanocages with mesoporous structure, and rich in Ni and Co redox centers, and Nonstoichiometric Ni x Co 3− x O 4− y 3D nanocages are fabricated through metalorganic framework template route and their electrocatalytic oxygen evolution reaction (OER) characteristics have been investigated. Substitution of Ni in Co 3 O 4 spinel structure improves the intrinsic catalytic activity. Enhanced OER activity stems from the presence of nonstoichiometry and low co...

Photoinduced intermolecular electron transfer (ET) dynamics between various 7-aminocoumarin acceptors and N,N-dimethylaniline (DMAN) donor has been studied in copolymer-surfactant supramolecular assemblies prepared in aqueous 1% P123 triblock copolymer micellar solution with varying concentration of surfactants (sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium chloride (CTAC), and triton-X-100 (TX100)). The aim of the present study is to modulate the reaction environment, especially the degree of micellar hydration inside the P123 micelle by the addition of the surfactants, which can modulate the ET reaction through the changes in the ET rates and the reaction exergonicity. Within the limited surfactant to copolymer molar ratios (n) used in the present study, fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) investigations indicate that the copolymer-surfactant supramolecular assemblies retain their micellar structure, although the micellar size gradually decreases with n. The redox potentials of the electron donor and acceptors are also found to change with n, although the extent of the effect is different for SDS, CTAC, and TX100 cosurfactants. In the presence of CTAC, the estimated exergonicity (-ΔG(0)) of the ET reaction is found to increase with an increase in n compared with that in pure P123, whereas it decreases marginally with SDS and remains almost the same for TX100. Substantial quenching of coumarin fluorescence is observed in the presence of DMAN in all copolymer-surfactant micellar aggregates because of ET reaction. The ET rate is seen to increase gradually with an increase in SDS and CTAC concentration in the supramolecular assembly, although it remains unaffected on the addition of TX100. The increased ionic strength in the Corona region of the copolymer-surfactant supramolecular aggregates due to the addition of the ionic surfactants has been envisaged for the increase in the ET rates. A correlation of the quenching rate constants with the free-energy changes (ΔG(0)) of the ET reactions shows the typical bell-shaped curve as predicted by Marcus outersphere ET theory. A substantial shift along the exergonicity axis (~0.3 eV) for the appearance of the Marcus correlation is observed in some cases, although the extent of such shift depends on both the nature of the cosurfactant and the amount of cosurfactant used in the copolymer-surfactant supramolecular assembly. Therefore, these preliminary results suggest a possibility of not only modulating the ET rates but also tuning the appearance of Marcus inversion along the exergonicity scale by suitably tuning the reaction environment inside the copolymer-surfactant supramolecular assemblies with a relatively more hydrophilic cosurfactant.

No abstract

No abstract

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Df; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Df $0.31, the photophysical properties of the dye follow good linear correlations with Df. For Df >$0.31, however, the photophysical properties, especially the fluorescence quantum yields (F f ), fluorescence lifetimes (s f ) and nonradiative rate constants (k nr ), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the s f values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Df >$0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.