Geminate proton recombination at the surface of SDS and CTAC micelles probed with a micelle-anchored anthocyanin (original) (raw)
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The Dynamics of Ultrafast Excited State Proton Transfer in Anionic Micelles †
The Journal of Physical Chemistry A, 2003
Fast excited state proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) micelles have been investigated using the photoacid 4-methyl-7-hydroxyflavylium (HMF) chloride as probe. The acidbase kinetics of excited HMF are straightforward in water, with biexponential fluorescence decays reflecting ultrafast deprotonation of the excited acid (AH + )* (k d ) 1.5 × 10 11 s -1 or ca. 6 ps) and diffusion-controlled protonation of the excited base A* (k p ) 2.3 × 10 10 L mol -1 s -1 at 20°C). In aqueous micellar SDS solutions, the kinetics are much more complex; triple exponential fluorescence decays are observed at all pH values and temperatures examined. The longest decay time (τ 1 ) 760 ps at 22°C), observed only for (AH + )* and uncoupled from the acid-base equilibrium, is assigned to excitation of HMF in orientations incapable of prompt transfer of the proton to water, i.e., that must rotate to expose the acidic OH group to water (k rot ) 1.2 × 10 9 s -1 or ca. 800 ps at 22°C). The other two decay times, τ 3 and τ 2 , are due to emission from the species involved in the acid-base reaction at the micelle surface. Deprotonation of (AH + )* is slightly slower in SDS micelles (k d ) 3.4 × 10 10 s -1 or ca. 20 ps) than in water. Two processes are operative in the back protonation of A*: (i) pH-independent unimolecular reprotonation in the initially formed geminate compartmentalized pair (A*‚‚‚H 3 O + ) (k r ) 8.8 × 10 9 s -1 ) and (ii) pH-dependent bimolecular protonation of A* via entry of an aqueous phase proton into the micelle (k p ) 1.6 × 10 11 M -1 s -1 ). Dissociation of the geminate pair (k diss ) 1.6 × 10 9 s -1 ) forms A* at the micellar surface. The present study thus provides a rather detailed kinetic picture of the initial steps involved in an ultrafast excited state proton transfer process at the surface of a typical anionic micelle.
Excited State Proton Transfer in Reverse Micelles
Journal of the American Chemical Society, 2002
The aqueous phase of water/AOT reversed micelles having varying diameters was probed by a single free diffusing proton that was released form a hydrophilic photoacid molecule (2-naphthol-6,8disulfonate). The fluorescence decay signals were reconstructed through the geminate recombination algorithm, accounting for the reversible nature of the proton-transfer reactions at the surface of the excited molecule and at the water/detergent interface. The radial diffusion of the proton inside the aqueous phase was calculated accounting for both the entropy of dilution and the total electrostatic energy of the ion pair, consisting of the pair-energy and self-energy of the ions. The analysis implied that micellar surface must be modeled with atomic resolution, assuming that the sulfono residue protrudes above the water/hydrocarbon interface by ∼2 Å. The analysis of the fluorescence decay curves implies that the molecule is located in a solvent with physical-chemical properties very similar to bulk water, except for the dielectric constant. For reversed micelles with rmax g 16 Å, the dielectric constant of the aqueous phase was ∼70 and for smaller micelles, where ∼60% of the water molecule is in contact with the van der Waals surface of the micelle, it is as low as 60. This reduction is a reflection of the increased fraction of water molecule that is in close interaction with the micelle surface.
Chemical Physics Letters, 2004
The effects of ionic and nonionic micelles on the excited state proton transfer processes of 2-hydroxy 1-naphthaldehyde (HNL) have been reported in this Letter. Deprotonation of HNL is considerably retarded in neutral and anionic micelles than that in cationic type as evinced from the increased neutral emission. Increased anion emission of HNL in cationic micelle is more due to the abundance of hydroxyl ions in the environment as well as less nonradiative deactivation. Anion emission is found to decrease in anionic micelle due to less formation of the conformer along with increase in nonradiative decay. The nonradiative processes from neutral to ionic form and ionic to ground state are less affected in neutral micelle as compared to HNL in ionic micelles.
2013
Department of Chemistry, West Bengal State University, Barasat, Kolkata-700 126, India <em>E-mail:</em> ranjan.das68@gmail.com Fax : 91-33-25241977 <em>Manuscript received online 19 July 2012, revised 21 August 2012, accepted 22 August 2012</em> The photo physics of 2-(2<em>' </em>-furyl)-3-hydroxychromone (FHC) was explored in three different non-ionic micelles of Triton X-100, Brij-58 and Tween-20. FHC exhibits a dual emission, attributable to the excited normal (<strong>N*</strong>) and tautomer (<strong>T*</strong>) forms resulting from an excited state intramolecular proton transfer (ESIPT) reaction (<strong>N*→T*</strong>). The ESIPT dynamics of FHC in the non-ionic micelles demonstrates a dependence on the hydrophile-lipophile balance (HLB) parameter of the surfactants by an increase in the kinetic constant of ESIPT reaction (<em><sup>k</sup></em>PT) with a decrease in HLB. A c...
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 1996
The excited-state proton transfer and dual emission behaviour of 3-hydroxyfiavone (3HF) have been investigated in reverse micelles of sodium bis(2-ethylhexyl) sulphosuccinate (AOT)/n-heptane at different values of water to surfactant molar ratio (Wo). The green tautomet emission (~i .... ~ 524 nm) and blue-violet normal emission (2 ...... 400 nm) originate from two different ground state populations of 3HF molecules, which are located respectively in the apolar phase and at the interphase of the reverse micelles, proximal to the AOT head groups. With increasing Wo the relative yield of the green emission band is enhanced with a concomitant decrease in that of the blue-violet emission. This is interpreted in terms of the population of 3HF molecules which are initially located in the interfacial region proximal to the polar head groups being "pushed" out into the apolar phase, where external hydrogen bonding perturbations are minimized.
An Approach to a Model Free Analysis of Excited-State Proton Transfer Kinetics in a Reverse Micelle
The Journal of Physical Chemistry C, 2018
Time resolved area normalized emission spectra (TRANES) of a photo-acid dye HPTS, confined within the water-pools of reverse micelles (RMs), are analyzed for a direct and model free observation of excited state proton transfer (ESPT) kinetics. When area normalized emission spectra of HPTS at different times are overlapped in a single window, we find population of RO-* (PTS-*) form of HPTS increases at the cost of a concomitant decreasing of the population of ROH* (HPTS*) form as time progresses. Migration of excited state population from ROH* form to RO-* form causes the emergence of an isoemissive point in TRANES, which retains throughout the entire course of ESPT process. An estimation of ESPT timescale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO-* form; both are practically same here. Emergence of an isoemissive point in TRANES is implying that there are only two kinetically and reversibly coupled emitting species (ROH* and RO-*) of HPTS are present within the water pools of RMs. Continuous spectral relaxations of HPTS due to excited state solvation dynamics apparently have no effect on the ESPT kinetics of HPTS within the RMs; otherwise spectral shifting, caused by the excited state solvation, would have destroyed the isoemissive point of TRANES.
Proton transfer reactions in nanoscopic polar domains: 3-hydroxyflavone in AOT reverse micelles
The Journal of Chemical Physics, 2010
A dramatic reduction in the excited-state intramolecular proton transfer ͑ESIPT͒ rate is observed for 3-hydroxyflavone ͑3-HF͒ within the nanoscopic polar domains of Aerosol-OT ͑AOT͒/n-heptane reverse micelle solutions. It is attributed to the formation of intermolecularly hydrogen-bonded 3-HF:AOT complexes, which cause a significant disruption of intramolecular hydrogen bonding within the complex-bound 3-HF molecules, thereby limiting the overall rate of the ESIPT process. Introduction of strong hydrogen-bonding polar solvents like water or methanol into the reverse micelles causes extensive solvation of the AOT head groups, leading to the collapse of the 3-HF:AOT complex and eventual release of intramolecularly hydrogen-bonded 3-HF molecules which are then able to undergo ultrafast ESIPT. With increasing W-value ͑W = ͓polar solvent͔ : ͓AOT͔͒, a larger number of 3-HF:AOT complexes are decimated, thus accelerating the overall ESIPT process. In contrast, in presence of solvents like acetonitrile, whose hydrogen-bonding power is inherently weak, the AOT head groups are poorly solvated, so that the 3-HF:AOT complexes are hardly affected at any W-value. Consequently the ESIPT dynamics of 3-HF in acetonitrile-containing AOT reverse micelles is nearly independent of the W-value, and always slower compared to that in water-or methanol-containing AOT reverse micelles. The results highlight the importance of hydrogen-bonding property of the polar solvent on the ESIPT of 3-HF within a nanoscopic domain.
Excited-state proton transfer of 1-naphthol in micelles
The Journal of Physical …, 1998
The fast deprotonation of 1-naphthol, which occurs in 35 ps in aqueous solution, is studied in neutral (triton X 100, reduced, TX-100R), cationic (cetyl trimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Drastically different effects on the ...
Journal of Colloid and Interface Science, 1986
The fluorescence quenching of 1-methoxynaphthalene and 1-cyanonaphthalene by protons in sodium dodecyl sulfate miceUes has been measured, The micelles notably increases the rate of the process as a consequence of the high local concentration of protons. The results obtained can be quantitatively described by the ion exchange formalism. Comparison of the pseudounimolecular quenching constant with the bimolecular quenching constant expected at the micellar interface allows an estimation of the local concentration of protons in the surroundings of the micelle incorporated probes.