Intramolecular Charge Transfer near a Hydrophobic Surface. 2,6-p-Toluidinonaphthalene Sulfonate in a Reverse Micelle

Abstract

The behavior of an organic molecule at an interface between two bulk media is often significantly different from that in either of the bulk medium. Since the polarity and viscosity at the interface are often very much different from those of the bulk media, the structure, dynamics and reactivity of an organic or biomolecule at an interface differ markedly from those in the bulk. Interestingly most natural and biological processes occur at such interfaces or confined systems, such as proteins, biomembranes and vesicles. As a result in recent years several new experimental and theoretical techniques have been used to study such systems. Several groups have used surface second harmonic and surface sum frequency generation and picosecond total internal reflection spectroscopy to study the structure and the dynamics at various interfaces.1-3 The behavior of water molecules present at such interfaces and perturbed by local electrostatic and hydrogen bond interactions are of special interest because of the unique role played by the water molecules in many biological processes. Recently several groups used the time dependent Stokes shift 5-10 , pulsed NMR and dielectric relaxation techniques [11][12][13][14] to unravel the dynamics of the water molecules in various organized media such as cyclodextrins 5,6 , reverse micelle 4,7-9 and micelles. 10 The results of the dynamic Stokes shift studies indicate that while ordinary water molecules relax in the subpicosecond time scale, inside these organized assemblies the solvation dynamics becomes several thousand times slower and occurs in the nanosecond timescale. [5][6][7][8][9][10] In hydrocarbon solvents at concentrations above ≈1 mM certain surfactants (e.g. sodium dioctyl sulfosuccinate, AOT) form so called reverse micelles with average aggregation number 20 and radius 15 Å, in which the polar headgroups point inwards. 4,7-9,21-28 On addition of water to a solution of AOT in a hydrocarbon solvent a microemulsion is formed which consists of nanometer sized water droplets ("water pool") surrounded by a layer of the surfactant molecule. In heptane the radius (r w ) of such a water pool is about 2w 0 (in Å) where w 0 is the number ratio of the water to the surfactant molecules.4,25 Such microemulsions are simple yet interesting models of biological membranes and the water molecules confined in biological systems. In such a water pool, the water molecules at the peripheries of the pool are strongly held by the polar or ionic headgroups of the surfactants and are thus "bound", while those at the center of the pool are relatively "free". The amount of free and bound water molecules present in such a microemulsion has been estimated and tabulated by many workers. 4,22,23 Recently Bright et al. 7 and our group 8,9 separately reported that in such a water pool of a microemulsion the solvent relaxation times are about 8 ns when the pool is small (r w <10 Å) and 2 ns for bigger water pools. Such nanosecond solvation times are several thousand times slower than the subpicosecond rela...

Energy Gap Dependence of the Nonradiative Decay Rate Constant of 1-Anilino-8-naphthalene Sulfonate in Reverse Micelles

“…7,[13][14][15][16][17][18][19][20][21] As an example, Cho et al reported that ANS in reverse AOT micelles exhibited a double exponential decay: short component=0.3 -0.6 ns and long component=2.6 -5.1 ns at w 0 =4 -33.…”

“…So far, a number of spectroscopic studies have been reported on reverse AOT micelles. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, detailed discussion based on k nr has been rarely explored. The present study showed clearly that detailed analysis of the photophysical parameters was essential to discuss fluorescence characteristics of a probe molecule in reverse micelle systems.…”

“…As one of the several advantages of a reverse AOT micellar system, the size of the water pool can be controlled precisely at the nanometer level through the molar ratio of water to AOT: 7 Therefore, size effects on chemical and physical properties in nanometer dimensions can be studied by the use of reverse AOT micelles. Indeed, reverse AOT micelles have been studied by various techniques such as fluorescence spectroscopy [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] , NMR 22,23 , ESR 24,25 , IR [26][27][28] , and so on. [29][30][31][32] These studies demonstrated that polarity [8][9][10] , viscosity 11 , pH 12 , and other properties 30,32 of the water pool in the micelles were different from those in the bulk state and were dependent on the w 0 value.…”

“…Particularly, recent time-resolved fluorescence spectroscopy has provided invaluable information about the structures and dynamical behavior of water molecules inside the micelles. 7,[13][14][15][16][17][18][19][20][21] It is noteworthy that these studies are essentially based on the changes in the fluorescence properties of a probe molecule with the size of the water pool. However, we supposed that detailed discussions on the fluorescence characteristics should be based on photophysical parameters represented by the radiative and nonradiative rate constants of a probe molecule.…”

Spectrofluorimetric Characterization of Mixed Nanoaggregates Comprising a Double-Chain Cationic Surfactant and a Cationic or Non-Ionic Single-Chain Surfactant

“…This photophysical scheme, however, has been questioned, mainly in supramolecular assemblies. 2,[17][18][19][20][21] In another approach, the p ! p* emission band of this type of fluorescent probe is believed to consist of different bands, each of which is attributable to the p !…”