Electronic Excited-State Behavior of Rhodamine 3B in AOT Reverse

J. Phys. Chem. B 2010, 114, 10417–10426

10417

Electronic Excited-State Behavior of Rhodamine 3B in AOT Reverse Micelles Sensing Contact Ion Pair to Solvent Separated Ion Pair Interconversion Jose´ A. B. Ferreira* and Sı´lvia M. B. Costa UniVersidade Te´cnica de Lisboa, Instituto Superior Te´cnico, Centro de Quı´mica Estrutural, Complexo Interdisciplinar, AV. RoVisco Pais 1, 1049-001 Lisboa, Portugal ReceiVed: January 20, 2010; ReVised Manuscript ReceiVed: June 29, 2010

The amphiphile Aerosol OT (1,4-bis(2-ethylhexyl)sodium sulfosuccinate, AOT) forms, in reverse micellar nanoaggregates of water, RM, in isooctane, ion pairs (IPs) with the cationic fluorescent probe dye, rhodamine 3B, (3,6-bis(ethylamino)-9-[2-(ethoxycarbonyl)phenyl]-9H-xanthen-9-ylium, R3B), as either contact ion pairs, CIPs, or solvent (water) separated ion pairs, SSIPs. The ground-state AOT R3B ion pairs’ equilibria as well as the dynamics of R3B electronic excited states show the progressive hydration of AOT- R3B+ toward solvent separated ion pairs, SSIPs as the characteristic reverse micelle parameter w0 ) [H2O]/[AOT] increases. The apparent limiting hydration constant of R3B ion pairs, Khyd ) 2.8 ( 0.2, corresponds to full hydration of AOT, consistent with 1-3 water molecules per AOT polar head. Transient relaxations at w0 ) 0.2, with a 375 ( 15 ps decay at 550 nm decrease to 115 ( 15 ps at w0 ) 7.2 turning into corresponding rises at 588 nm. At higher w0, water induced dynamics becomes faster. The lifetime is longer in RM with smaller w0, due to the presence of CIPs that inhibit intrinsic nonradiative decay processes, which in contrast shorten the decay times at higher w0, due to the presence of SSIPs. The pairs’ electronic excited-state properties are sensitive to viscosity and local polarity of the surrounding environment of the interfacial regions of AOT reverse micellar nanoaggregates. Introduction The structure and dynamic properties of water influence the natural environment for biomolecular assembly and determine most of the structure-function relationships found in reactions within lipid-bound enzymes1-4 or in manmade nanostructured biosystems.5-7 The interconversion of bound into free water at interfaces8,9 and cavities10,11 ubiquitously continues to be an object of scrutiny due to its relevance to biological water solvation. Dynamics in the femtosecond time-scale12 arising from extended hydrogen-bonded networks contrasts to slower subnanosecond contributions attributed to disruption of such networks and to water binding in biological systems.13 Water-in-oil microemulsions or reverse micelles, RM, are essentially constituted by quasi spherical nanoaggregates14 wherein water coexists coated by the amphiphilic layer.15 Increasing amounts of water included in the nanoaggregates are responsible for the increase in the overall size resulting in the encapsulation of such water. The molar ratio of water to the amphiphile, S, determines the aggregate size16 and is expressed by the general eq 1.

w0 ) [H2O]/[S]

(1)

The most widely explored reverse micelle system is based on the chiral amphiphile 1,4-bis(2-ethylhexyl)sodium sulfosuccinate, AOT. The physicochemical properties of water shells inside reverse micelles17 vary with increasing distance from the interfacial molecules’ polar heads as water changes state from bound to free within tenths of a nanometer. Accordingly, the local polarity of reverse micelles18 is intrinsically determined by the anionic, cationic, or nonionic amphiphiles.19 * Corresponding author. E-mail: [email protected].

Ionic solvation20 depends on the local water structure nearby and counterions can determine ion pairing21 in reverse micelles.22 Therefore, local molecular probes are interrogated concerning these aspects, since the solvation properties of cations23 and anions24 in RM have been shown to be different either from bulk polar liquids’ or from high salt concentration solutions’. Hydration of AOT-Na+ ion pairs has been addressed with scrutiny through a plethora of experimental and theoretical approaches. The number of water molecules bound to the AOT anionic polar heads assigned is 2 to 3. Water interacts weakly in a reverse micelle in the presence of the ammonium cation in contrast with Na+.23 In AOT reverse micelles, at w0 ) 1.7, the presence of the NH4+ counterion, smaller than the sodium ion, shows the impact of counterion nature on water motion, as stressed through the solvation dynamics probed by the transient Stokes shift of Coumarin 343.23 However, in aqueous AOT reverse micelles, no evidence for ion pairing of AOT- with NH4+ was found compared with Li+, Na+, and K+ that extensively form ion pairs with the AOT-.25 Confinement effects in the water pools in AOT reverse micelles can be induced by interactions of multicharged anionic probes with polar amphiphile head groups, specifically manifested at very low w0 in contrast with intermediate ones. Guests in a reverse micelle can alter the water structure and even impact micelle formation since a giVen number of water molecules is required to constitute probe solvation shells.26,27 Fluorescence anisotropies in AOT reverse micelles reflect the water droplet size confinement as well as dynamic interactions.28 At low water content, the fluorescence depolarization is dominated by micelle rotation as a whole dragging the guest within, whereas at higher water contents it senses increased mobility. In inhomogeneous environments, ionic or hydrophobic probes sense differently the local interactions respectively established

10.1021/jp100571t  2010 American Chemical Society Published on Web 07/28/2010

10418

J. Phys. Chem. B, Vol. 114, No. 32, 2010

on the basis of molecular affinities in nanosized aggregates either in water-in-oil or oil-in-water microemulsions.29-32 Molecular dynamics simulations have shown that structural alterations significant to the solvation shell occur with the increase of the number of water molecules that can solvate the AOT polar heads since interatomic radial distribution functions drastically change with the amount of water present at low water contents in RM.33 Crystal violet (CV) forms ion pairs with AOT in the interfacial region of reverse micelles, as shown by several spectroscopic techniques and by quantum chemical calculations.34 The association free energy decreases sharply as a function of w0, indicating the formation of solvent separated ion pairs, SSIPs, which arise from contact ion pairs, CIPs. Likewise, R3B (3,6-bis(ethylamino)-9-[2-(ethoxycarbonyl) phenyl]-9H-xanthen-9-ylium) has a specific interaction with AOT, making it possible to study the phenomenon of ion association/ hydration in AOT RM. R3B is an ester and thus insensitive to the pH of the reverse micelle contrasting with acid/base rhodamine B that remains bound at the interface.35 In AOT RM, given the combination of local heterogeneity and confinement, the binding of AOT- to either R3B+ or to CV+ should be different. The evolution of electronic excited states will be affected since R3B+ intrinsic energy barrier dynamics imply a clear contrast with the CV+ barrierless process.36 R3B is thus specifically sensitive to CIPs to SSIPs interconversions, assessed either by its intrinsic radiationless processes37,38 or by IPs’ ICT-solvatochromism driven hydration dynamics.39 In mixtures of weakly polar toluene in the full range of miscibility with water, R3B ion pairs with ClO4- and the corresponding cationic species were studied by electronic spectroscopy. The fluorescence quantum yield variation with the amount of water present in mixtures has shown that the fluorescence quenching process of the ion pair by water molecules is diffusion controlled with a rate constant oscillating within the limits of slip and stick boundary conditions.40 The driving force for the excited-state conversion of the ion pair into cationic species is the photoinduced unbalance of electrostatic forces changing the interionic distances in the water cluster solvation sphere created around the ion pair by local dielectric enrichment. Equilibrium constants of ion pair dissociation found in these clusters were of the order of magnitude of Na+Cl- ion pairs solvated in water clusters, as shown by molecular dynamics simulations.41 Since the R3B positively charged π-system is larger than Na+ and the interatomic maximum distance within R3B also exceeds the length of one AOT anion polar head, it is likely that the probe can displace Na+ ions forming ion pairs with AOT. In AOT RM, the balance of hydrophobic interactions, hydration and ion pair formation leads to AOT R3B species in the form of loose ion pairs. A continuous distribution of differently solvated ion paired species separated by water molecules (SSIPs) is expected. R3B gives rise to distinct chemical species providing a direct way of studying the specific hydration processes of AOT-R3B+. The purpose of the present contribution is thus focused on the hydration process of AOT anions within isooctane reverse micelles through the ion pairs formed with the fluorescent probe R3B, as indicated in Scheme 1. Experimental Methods Materials. Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) (Sigma, Ultrapure), isooctane (Sigma, spectroscopic grade), water bidistilled over quartz, and rhodamine 3B (perchlorate

Ferreira and Costa SCHEME 1: Model Hydration of AOT R3B Ion Pairs in RM

Nanagg-1+(H2O)mAOTnagg-R3B+ T Nanagg-1+(H2O)m-kAOTnagg-(H2O)kR3B+

salt, Radiant Dyes Chemie, laser grade) were used. The standard for fluorescence quantum yields was rhodamine 6G chloride (Spectra Physics) in ethanol (Merck, spectrophotometric grade). Samples. AOT 0.1 M in isooctane contained an amount of water that was determined by Karl Fischer titration and corresponded to w0 ) 0.22 ( 0.02. Water needed to obtain higher w0 reverse micelles was added quantitatively. The probability that a RM aggregate with m water molecules may occur at a given w0, can be estimated by the Poisson distribution using the average number of water molecules per micelle, assuming linearity between the aggregation number, nagg and w015,42 at low water content.15,43-45 That probability, at w0 ) 0.22, is maximum for m ) 6 and negligible at m higher than 11 water molecules. R3B is not soluble in dry aliphatic hydrocarbons and a residual solubility in isooctane was checked through fluorescence and no dissolved amount was observed since the dye adsorbs to quartz in the presence of pure isooctane. AOT reverse micelles with R3B were prepared from initial ethanol stock solutions of probe from which 5 µL aliquots were taken and evaporated until dryness using a stream of nitrogen followed by addition of 5 mL of each w0 water:AOT (0.1 M): isooctane reverse micelles. The R3B concentration range was such that essentially no more than one probe can exist in a given micelle, enabling dynamic exchanges of the ClO4- counteranion. Using the value of the monomer to dimer association constant, K ) 3400 M-1 of rhodamine 3B in water,46 we obtain a monomer percentage higher than % 99.85 for a total dye concentration 7 × 10-6 M. This is precisely the limit we have considered acceptable for the absence of dimers. Considering that in the absence of probe, the concentration of RM aggregates decreases with w0 and since nagg increases from 16.115 at constant CAOT, the use of Poisson’s distribution as a method confirms that the probability of finding one single R3B molecule in a given micellar aggregate in the whole w0 range is 104 to 102 times higher than that of finding two R3B molecules in the same micellar aggregate. The latter conditions are compatible with the use of bulk concentration CR3B < 7 µM, and that R3B used was within 0.3 < CR3B < 0.7 µM. Moreover, it is well-known that in micelles the populations of monomeric guest molecules predominate over dimers or over higher aggregates relatively to bulk. Solutions were considered thermodynamically equilibrated regarding the formation of reverse micelle aggregates including probe during one day protected from light. In all cases, the suitability of compounds and samples was checked by electronic absorption and fluorescence. Optical-path quartz cuvettes (10 mm) were used in all photometric determinations at T ) 296 K. Steady-State Electronic Absorption and Emission. R3B in reverse micelles absorption spectra were recorded with a JASCO V-560 UV/vis spectrophotometer using a water:AOT (0.1 M):isooctane microemulsion solution as a blank correction at each w0. Respective emission and excitation spectra were obtained in the appropriate wavelength ranges with subtracted blanks using cutoff filters with a Perkin-Elmer LS-50B spectrofluorometer. R3B Absorption and Emission Solvatochromism. Different polarity and viscosity were assessed by eq 247-49

Excited-State Behavior of Rhodamine 3B v ν¯ a,e ) ν¯ a,e + aa,e

(

(n2 - 1) ε-1 n2 - 1 + b a,e ε+2 2n2 + 1 n2 + 2

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10419

)

F¯ ) (FRM-1 + FR3B-1)-1

(2)

which expresses the variation of spectral maxima wavenumbers of either absorption (a) or emission (e), left-hand side, that shift from the corresponding values in vacuum (v), the first term on the right-hand side, through the second and third terms referring respectively to the contributions of the electronic and orientation polarization. These variations are dominated by the coefficients aa,e and ba,e, revealing the dependence on the Onsager reaction field factor, function of cavity radius, and refractive index of the medium (n) in contrast with the dependence on the orientation polarization, expressed in terms of the Debye contribution, function of the optical and static dielectric constants. Steady-State Fluorescence Anisotropy. R3B fluorescence anisotropy50 in the micellar systems at infinite time corresponds to the anisotropy value taking into account all depolarization mechanisms. Experimentally, r values are expressed by eq 3.

r)

fλex,λem,| - fλex,λem,⊥ fλex,λem,| + 2fλex,λem,⊥

(3)

where f represents the intensity of light emitted in the parallel and perpendicular planes to the excitation light respectively according to the subscripts. The fundamental fluorescence anisotropy50,51 of a molecular electronic excited state, r0, depends on the relative orientation of the absorption and emission dipoles expressed by angle R, as averaged in eq 4.

r0 )

〈3 cos2 R - 1〉 5

Fluorescence Quantum Yields. Fluorescence quantum yields, Φf (λex ) 522 nm), were calculated, eq 8, through corrected emission spectra including that of rhodamine 6G in ethanol solution.54 Compensation for different sodium D-line refractive indexes (n) relatively to standard (s), were made according to Parker.55

Φf ) φs

1 - 10-ODs,λex Is,λex,λem n2 1 - 10-ODλex Iλex,λem ns2

∫ fλ ,λ ∫ fs,λ ,λ ex

ex

em

(8)

em

Moreover, at w0 ) 0.2, at wavelengths other than λex ) 522 nm, the compensation for different excitation intensities was taken into account by means of the source profile (I). Previous studies of the R3B ion pair with ClO4- have shown40 the equilibrium of two chemical species, CIP and SIP. The fluorescence quantum yields of each species, ΦCIP and ΦSIP, can reflect the emissions arising from CIPs and from SSIPs with AOT contributing to the total fluorescence quantum yield, Φf. The apparent hydration constant Khyd(w0), can thus be expressed using the total fluorescence quantum yield, Φf, eq 8 inserted in eq 9, that considers equal electronically excited amounts of each species at the excitation wavelength in close agreement with absorption and excitation spectra.

Φf )

ΦCIP + Khyd(w0)ΦSIP 1 + Khyd(w0)

(9)

With eq 10,

(4)

The value of fluorescence anisotropy r will be determined by fluorescence lifetime τ, and by depolarization time, F, eq 5.

Khyd(w0) )

ΦCIP - Φf Φf - ΦSIP

(10)

and, with the apparent molar hydration free energy, ∆Ghyd(w0), eq 11,

r0 3τ )1+ r F

(5) ∆Ghyd(w0) ) -RT ln Khyd(w0)

Rotational correlation times, Fc, eq 6, are applicable to the stick limit of rotational diffusion52,53 of a sphere with hydrodynamic volume Vh in a medium with shear viscosity η; kB is the Boltzmann constant, and T is the absolute temperature.

Fc ) (6Drot)-1 )

(7)

ηVh kBT

(6)

Considering that the rotation of AOT RM in the isooctane nonpolar phase contributes to the rotational depolarization of R3B fluorescence, eq 6 can then be used to estimate the micelles rotational correlation times. Rotational depolarization times of R3B fluorescence can be determined by identifying FR3B with Fc. Assuming that processes other than rotation can be neglected, the combination of the two rotational depolarization contributions (rotational motion of both R3B and the host RM) enables comparison of fluorescence anisotropy values with estimates obtained using as interpolating formula eq 7.

(11)

according to Scheme 2,

K)

[SSIP] [CIP][H2O]n

(12)

Equation 12 leads to identification of Khyd(w0) with K [H2O]n, eq 13, derived with eq 1. K and n can then be estimated.

∆Ghyd(w0) ) -RT ln K - nRT ln {w0[AOT]}

(13)

Fluorescence Lifetimes. Fluorescence decays56 were acquired in a single-photon counting (SPC) instrument with λex ) 295 nm, corresponding to frequency doubled light delivered by a cavity-dumped rhodamine 6G dye laser (Coherent 701-2), synchronously pumped by a mode-locked Ar+ laser (Coherent Innova 400-10). The instrumental response function was ∼30 ps at fwhm. The emitted light either at λem ) 550 nm or at λem ) 588 nm was collected through a properly positioned polarizer

10420

J. Phys. Chem. B, Vol. 114, No. 32, 2010

Ferreira and Costa

SCHEME 2: Hydration of a Contact Ion Pair, CIP, by Uptake of n Water Molecules Yielding Water Separated Ion Pair, SSIP K

CIP + nH2O {\} SSIP

at the magic angle, Rma ) 54.7°. The fluorescence decay results were stored in a multichannel analyzer operating with 1024 channels at 11.12 ps/channel. Instrumental response function deconvoluted decays, at the two emission wavelengths, eq 14, amplitudes, ai and fluorescence lifetimes, τi, were obtained through a global Marquardt based algorithm and minimum leastsquares routines. Statistical analysis of the quality of the numerical adjustments was considered acceptable within 1.0 < χ2 < 1.5, as well as by the critical inspection of residuals and of respective autocorrelation functions.57 3

fλex,λem(t) ) Iλex,λem(t) X

∑ i)1

-1

ai,λex,λeme-tτi

3

∑ ai,λ ,λ i)1

ex

-1 em

(14) Quantum Chemical Calculations. AM1 semiempirical calculations58,59 were performed56 to assess the variation of AOT-R3B+ interionic distances with the number of water molecules in optimized geometries. The latter were obtained initially from molecular structures or from molecular mechanics60 with the MM+ force field. The corresponding energy minima were verified by comparison of results attained after repeating the processes using different initial conformations (See Supporting Information, Chart S1 and page 4SI). Results and Discussion AOT R3B Steady-State Absorption and Emission as a Function of w0. Micellization of R3B induces specific solvatochromic shifts and broadenings reflected in electronic absorption and emission patterns. Counterions contribute to the spectral shift and broadening, as they are the source of additional interactions other than those arising from water solvation. The association of AOT with R3B is thus progressively reflected in the electronic absorption and emission maxima (Table 1). The values of the steady-state electronic absorption maximum of R3B in AOT RM are represented in Figure 1A. The variation of the electronic absorption maximum of R3B in AOT RM is compared with results for liquids spanning a wide range of dielectric constants, revealing opposite trends depending on the w0 range. At low dielectric constants a blue shift occurs in R3B λabs-1 followed, in contrast, by a red shift at higher dielectric media. In homogeneous liquid media the broadenings follow a clear functional dependence on the medium polarizability which correlates linearly with Onsager reaction field function of induced polarization. The region where blue shift occurs can be assigned to CIP hydration while that of red shift should reveal the effect of increasing interionic distances in SSIPs. A similar trend is found in the correlation between pair energy and Gurney cosphere diameter related to effective IPs radius determining the solvation energy variation associated with the CIP conversion into SSIP and further to separated ion pair, SIP.20 Therefore, the simultaneous presence of highly polarizable sulfur atom and the nearby negatively charged oxygen atoms in the proximity of

TABLE 1: R3B Spectroscopy in AOT (0.1 M) Reverse Micelles in Isooctane at T ) 296 Ka w0

λa

λemb,c

rb,d

Φfb,e

Κhyd(w0)f

0.2 1.2 2.2 3.2 5.2 7.2 10 15 20 50 Wg W:th 1-Cl-ni

562.0 561.2 560.8 560.4 561.3 561.8 562.2

585.2 585.8 586.9 587.5 588.4 588.6 588.8

0.138 0.155 0.167

562.4 562.8 558.6 562.6 565.6

588.0 588.4 583.9 587.5 589.5

0.185 0.179 0.045

0.58 0.53 0.48 0.46 0.44 0.42 0.42 0.42 0.44 0.44 0.28 0.41 0.85

0.9 1.3 1.8 2.2 2.6 3.1 3.1 3.1 2.6 2.6 ∞ 3.4 0

0.178 0.177 0.183

a Absorption, λa, and emission, λem, spectral maxima wavelengths, steady-state fluorescence anisotropies, r, eq 3, fluorescence quantum yields, Φf, eq 8, and Khyd(w0), eq 10. b λex ) 522 nm. c (0.5 nm. d (0.01. e (0.05. f (10%. g Water (W).37 h Water/toluene (W:t), CW ) 11.9 mM.40 i 1-Cl-naphthalene (1-Cl-n).40,47

few water molecules at short distances of AOT-, should contribute to additional stabilization of the positively charged π-system of R3B. Oppositely, at higher interionic distances the progressive enrichment of the water solvation layer around R3B will induce the interactions responsible for the π-cation solvatochromism.47 In CV+, the increase in the blue shoulder region relatively to the absorption maximum is associated with the close proximity of counterions61 whereas the position of the main absorption peak of CV+-H2O closely approaches the position of the main absorption peak of CV+ solvated in water/methanol mixtures.62 In water/toluene mixtures, the presence of the ClO4counterion nearby R3B+ in water clusters produces the increase of the electronic absorption probability in the inflection of the shoulder next to the main π-system Gaussian band.40 In Figure 1B are shown the emission spectra of R3B in AOT RM. Emission maxima are red-shifted relatively to the values observed either at the lowest w0 or in water where the fully hydrated solvated cation is present.47-49 There is a sharp decrease of the emission intensity at low w0 values noticed in the higher energy spectral region. In the different environments of AOT RM the presence of AOT- confers special features to R3B ion pairs absorption and emission in contrast either with R3B ion pair in the presence of clustered water40 in water/toluene mixtures or in aqueous media, where only water molecules are present in the close vicinity of R3B cation.37 Solvatochromic effects in the absorption and emission spectra sensed by R3B in the different polarities and surroundings it experiences as w0 changes in AOT RM can be distinguished from those effects expected from the formation of different kinds of ion pairs. The variation of absorption and emission spectra as a function of polarity and viscosity in pure isotropic liquids47 enables the understanding of R3B absorption and emission properties in polar liquid mixtures composed of either low polarity40 or high polarity48 components and as well in confined media, in particular, aqueous nearby cyclodextrin rims.49 The dependence of R3B λem-1 spectral shift on the orientation polarization is stronger than that of λabs-1. Although Coulombic forces should dominate the interaction of R3B with the AOT anion polar head, hydrophobic forces due the amphiphile alkyl chains in AOT RM will further contribute to the relaxation processes, as can be expected from coupling of a higher number of vibrational modes arising from the molecular conformational degrees of freedom in R3B IPs. The orientation polarization

Excited-State Behavior of Rhodamine 3B

Figure 1. R3B in AOT RM electronic transitions47-49 as a function of w0. (A) Absorption spectra maximum wavenumbers, λabs-1, in liquids and RM as a function of water content. Legend: (9) w0 ) 0.2, 1.2, 2.2, 3.2, 5.2, 7.2, 10, 20, and 50. Top x-axis: (0) 1-Cl-naphthalene (nD ) 1.6326; εr ) 4.72; ηs/cP ) 3.075), 1-Cl-n; glycerol triacetate (nD ) 1.4301; εr ) 7.11; ηs/cP ) 16.2), t; 1-decanol (nD ) 1.4372; εr ) 8.1; ηs/cP ) 11.9), d; dichloromethane (nD ) 1.4242; εr ) 9.08; ηs/cP ) 0.406), dcm; 2-methyl-2-propanol (nD ) 1.388; εr ) 12.4; ηs/cP ) 3.316), 2m2p; 1-hexanol (nD ) 1.418; εr ) 13.3; ηs/cP ) 4.87), h; 1,5-pentanediol (nD ) 1.449; εr ) 25; ηs/cP ) 123), 15pd; tetraethyleneglycol (nD ) 1.460.; εr ) 40.2; ηs/cP ) 52), teg; 1,2,3-propanetriol (nD ) 1.476; εr ) 43.2; ηs/cP ) 985), g; propylene carbonate (nD ) 1.422; εr ) 43.1; ηs/cP ) 3.37), p; dimethylsulfoxide (nD ) 1.478; εr ) 46.7; ηs/cP ) 2.198), dmso; water (nD ) 1.333; εr ) 80.4; ηs/cP ) 0.966), W. The function f′ (ε) ) 2(εr - 1)/(2εr + 1) is employed to compare the variations within AOT RM with those in the selected set of isotropic liquids. (B) Emission spectra at w0 ) 0.2, 1.2, 2.2, 3.2, 5.2, 7.2, 10, 20, and 50 and in water.

contributes to different extents depending on the relative strength on the effects of long-range dielectric relaxation as compared to local interaction at angstroem distance involved in the structure around ions. The excitation, absorption, and emission of AOT associated with R3B at w0 ) 0.2 highlight that a minute quantity of water within the nanoaggregates leads to spectral variations. R3B fluorescence emissions at different wavelengths show the evolution from predominantly excited CIPs at the lower wavelengths to predominantly excited SSIPs at the higher wavelengths as shown in Figure 2. In fact, while at low water content expressed by w0, the emission maximum occurs in the spectral region where the emission maximum in pure water is found, at high water content the emission maximum is further

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10421

Figure 2. R3B in AOT RM at w0 ) 0.2 fluorescence emissions compared with absorption spectrum. (A) Excitation spectra normalized at the fluorescence intensity at λem ) 522 nm. Legend: λem ) 545 nm, λem ) 555 nm, λem ) 562 nm, λem ) 570 nm, λem ) 580 nm, and λem ) 620 nm. Inset: Enlarged view of the regions of R3B absorption and excitation maxima. (B) Emission spectra normalized at the fluorescence intensity corresponding to λem ) 582 nm. Legend: λex ) 500 nm, λex ) 504 nm, λex ) 508 nm, λex ) 510 nm, λex ) 512 nm, λex ) 516 nm, λex ) 518 nm, λex ) 520 nm, λex ) 522 nm, and λex ) 524 nm. Inset: Enlarged view of the regions of emission maxima.

red-shifted giving evidence of local polarity effects. In highly viscous media, the contribution that arises from longer time relaxations produces an enhanced red shift of the fluorescence emission of R3B in contrast with solvation times in the subnanosecond range associated with ion solvation. The marked dependence of the emission solvatochromic shifts on the Debye orientation function reflects the influence of dielectric relaxation of far and of surrounding molecules at the solvation sphere.47-49 AOT R3B Steady-State Fluorescence Anisotropy as a Function of w0. The variations of r with w0 arise from the sensible fluorescence lifetimes and fluorescence depolarization times that are induced by different local environments. Inclusion of R3B in AOT micellar aggregates is reflected in the values of the steady-state fluorescence anisotropies (Table 1). Steadystate fluorescence anisotropy, r, eq 3, of R3B in AOT:isooctane (0.1 M) reverse micelles varies as a function of water content, w0, from r ) 0.138 ( 0.01 at w0 ) 0.2 showing an inflection in the range 3.2 < w0 < 5.2 and reaching r ) 0.179 ( 0.01 at w0 ) 50. In Figure 3 the values of r are shown. Estimates of R3B fluorescence rotational depolarization times are compatible with apparent local viscosities approaching those

10422

J. Phys. Chem. B, Vol. 114, No. 32, 2010

Figure 3. R3B in AOT RM steady-state fluorescence anisotropies, r, as a function of w0: (0) experimental r, eq 3; (×): r minimum leastsquares fit (eqs 4 and 5) to eq 3 (η ) 19 ( 3 cP, decreasing monotonically in the whole range of w0); (O) calculated r, eqs 4-6; (]) calculated r eqs 4-7; (2) r minimum least-squares fit (eqs 4-7) to eq 3 (η(w0)/cP ) 122 (5.2), 39 (7.2), 30 (10), 23 (20), and 20 (50), whereas at w0 ) 0.2, w0 ) 1.2, and w0 ) 2.2 no apparent local viscosity values, η, could describe the results). Either the calculated or the fitted equations included τ ) Φf/kf, Φf (Table 1), kf ) 2 × 108 s-1,40 r0 ) 0.373,50 RR3B ) 0.67 nm,40 RRM/nm ) 0.175w0 + 1.5,15,16 and ηiso ) 0.5 cP.

found in previous studies. Acridine orange (AO) in AOT RM at w0 ) 10 fluorescence depolarization yields close η values,63 while the trend shown by calculated r values including micelles’ rotational contribution evolves toward the expected limiting anisotropy value50,51 at high w0. The local viscosities obtained by Hasegawa et al. through the fluorescence depolarization of other xanthene dyes in AOT RM clearly indicate the dependence on molecular structure.64 While fluorescein is anionic, rhodamine B is either zwitterionic or cationic and rhodamine 6G is cationic, the number of substituent alkyl groups increasing also in this order. As expected, this trend indicates that the cationic form should bind more tightly to the AOT polar head. In fact, the viscosities obtained (e.g., at w0 ) 10) while increasing in the order: fluorescein, rhodamine B, and rhodamine 6G clearly reflect the influence of structure envisaging that the rhodamine 3B cation, being an even more hydrophobic molecule than rhodamine 6G, is expected to interact strongly with hydrophobic AOT anion, eventually affecting the RM properties at low water content. Specific friction effects’ contributions to depolarization mechanism thus arisen from the association of R3B with AOT suggest that R3B CIPs generating progressively SSIPs in AOT RM while involving the individualized molecular entities (R3B cation and AOT counterions) inherently allow fluxes of solvating molecules not always coplanar relative to the π-system. During CIPs’ hydration process yielding SSIPs, the translational and rotational polarization fluxes will change direction, progressively redirecting force lines orthogonally to the interion plane with increasing interionic distance relatively to the ion pair charge gravity center. The variation of the depolarization times of rhodamine 6G, located in the electric double layer of aqueous micelles in the vicinity of surfactant (SDS) counterions is influenced by SSIP and CIP dynamics.65 The rotational correlation times of rhodamine 6G and of rhodamine 6G base in protic and in aprotic liquids in the presence of high iodide concentration reflect the influence of donor-acceptor interactions established within SSIP.66 Hydrogen bonding may contribute to higher depolarization times arising from specific dielectric friction involving SSIP entities.

Ferreira and Costa

Figure 4. R3B in AOT RM IPs interconversion apparent hydration free energy, ∆Ghyd(w0), eqs 9-13: (9) interpolation lines a, b, and c. Inset: Logarithmic variations of Khyd(w0) with w0 (∆) and with the volume of bound AOT, VAOT (O).15,16

AOT Hydration Free Energy from R3B Fluorescence Quantum Yields. The decrease in the fluorescence quantum yield observed from lower w0 values can be attributed to progressive hydration of the contact ion pairs (CIP) formed between the cationic rhodamine molecule and the AOT anions, converting into water separated ion pairs (SSIP). In fact, as given in Table 1, the variation of fluorescence quantum yield Φf (λex ) 522 nm) in the range 0.2 < w0 < 50 shows that the progressively dominant hydrated equilibrium populations of directly excited CIPs convert to increasingly hydrated populations of excited SSIPs, as was also shown by the corresponding absorption, emission, and excitation spectra. The introduction of the experimental results ΦCIP ) 0.8540 and ΦSIP ) 0.2837,40 in eq 9 enables Khyd(w0) to be obtained by direct solution of eq 10 at each w0 using the corresponding experimental value of the total fluorescence quantum yield, Φf, at each w0 (Table 1). The corresponding estimates of ∆Ghyd(w0) are shown in Figure 4. Noteworthy, there are three clearly defined free energy regions that emerge from the interpretation given. At w0 > 5.2 the plateau attained corresponds exactly to the value of the solvation energy of AOT anion given by Fourier transform infrared spectroscopy of water67 corresponding to two to three water molecules per AOT polar head. At 1.2 < w0 < 5.2 a straight line is obtained using the logarithm of w0. The interpretation of slope and intercept can be straightforward according to the following: the contact ion is fully hydrated, and CIP is achieved by uptake of a given number n of water molecules by the ion pair solvation sphere, yielding the water separated ion pair SSIP, according to Scheme 2. Thermodynamics of local environments of RM organized systems is dependent on the fine details of the interactions established. In contrast, the equilibrium constants of CV+ ion pair formation with Cl- in low polarity media are of the order of 106 M-1 61 whereas CV+ ion pair formation with the AOT anion in AOT RM as a function of w0 corresponds to ion pair association free energies within -14 kJ mol-1 at w0 ) 0.7 to 1 kJ mol-1 at w0 ) 33,34 energy comparable to the average solvation energy per water molecule hydrating AOT. The dissociation constants corresponding to ion pair association free energies of CV+ ion pair formation with the AOT anion are

Excited-State Behavior of Rhodamine 3B comparable with Khyd(w0) that in the whole w0 range is within 0.9-3. The apparent hydration constant in the limit of full hydration reaches Khyd ) 2.8 ( 0.2, which corresponds to the variation of hydration free energy, ∆∆Ghyd/kJ mol-1 ) -2.5 ( 0.2. In water/toluene mixtures, the dissociation constant of rhodamine 3B ion pair with ClO4- into the cationic form within water clusters, K ) 0.354,40 has virtually the same value of the dissociation constant corresponding to the complex between acridine orange and the AOT anion, AO-AOT, K ) 0.35 ( 0.0363 in AOT RM. The values of K provide effective means of estimating the free energy associated with the thermodynamic event of the solvation of AOT-(H2O)nR3B+. From eq 13, K ) 3.6 M-1 and n ) 0.5 are obtained. In the limit, a single water molecule flip into the interionic region within the ion pair can be enough to convert the photochemistry of CIP into the photochemistry of SIP. In AOT RM, R3B+ can sit within two Na+ sites close to at least two anions specifically at very low w0 and thus it can be considered that more than one sodium ion can be displaced. This originates weaker cationic solvation of R3B+ relatively to Na+, possibly yielding a transient lack of electroneutrality on the RM aggregate wherein R3B resides, upon electronic excitation.68 From w0 ) 1 to w0 ) 2 it was shown33 that the existing water clustered at the RM aggregates evolves toward increasingly solvated Na+ ions altering the ionic population from dominant CIPs contribution toward increasing SSIPs populations. The number of weakly hydrated Na+ ions increases sharply in this narrow range. The spread of water molecules in the interfacial double layer evolves from w0 ) 3 to w0 ) 7.5 and, at w0 ) 10 where the ion populations are dominated by SSIPs, the AOT coordination number close to 1 contrasts with nearly 3 Na+ ions at the CIPs’ lattice at the lower w0. Comparatively, from low (0.1 M) to high (4 M) salt concentration, solvated Li+ only suffers a slight decrease from 4 to 3.5 water molecules while a variation from 9 to 6 occurs for the highest ionic radius, Cs+, stressing the effect of different sizes on average hydration number.69 Smaller radii ions that have higher polarizing power interact strongly with water molecules within the tetrahedral structure, originating hydrophilic hydration shells. In contrast, relatively larger ions behave similarly to uncharged hydrophobic solutes, interacting more weakly with surrounding water molecules that form a disordered cage encapsulating the ion. Ion pairs AOT-(H2O)nR3B+ are embedded within RM aggregates. At w0 ) 0.2, on average, a monodispersed model aggregate is estimated with 6 water molecules present in the close vicinity of AOT. The amplitude of variation of the total fluorescence quantum yield, Φf, is from Φf ) 0.58 at w0 ) 0.2 to Φf ) 0.44 at w0 ) 50. The Φf variation as a function of excitation wavelength, at w0 ) 0.2, yields a similar amplitude, Figure 5. A plateau at Φf ) 0.44 sharply evolves to a higher one at Φf ) 0.58 with corresponding excitation energy difference of 5 kJ mol-1, of the order of magnitude of the difference of hydration free energy, ∆Ghyd(w0) at w0 ) 0.2 relative to w0 ) 50, ∆∆Ghyd, which may reveal the effect of the direct photodissociation of CIPs yielding SSIPs. The spectral evolution corresponding to the electronic excitedstate emitting populations as a function of emission energy indicates the conversion of excited CIPs into emitting SSIPs. Consequently, the variation of the electronic excitation energy maximum at the lowest w0 indicates that excited-state solvation

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10423

Figure 5. R3B in AOT RM at w0 ) 0.2 fluorescence quantum yields, Φf, as a function of excitation wavelength, λex (9). Top x-axis: λex ) 500 nm, λex ) 504 nm, λex ) 508 nm, λex ) 510 nm, λex ) 512 nm, λex ) 516 nm, λex ) 518 nm, λex ) 520 nm, λex ) 522 nm, and λex ) 524 nm. Bottom x-axis corresponds to the top x-axis abscissa converted to energy units.

of R3B IPs in RM can reflect the turning point from predominance of tangential to longitudinal polarization fluxes. Increasingly ICT-like characteristics are shown by the chromophore excited state in contrast with the ground state. Since only weak electronic states’ mixing should exist between the R3B cation and AOT anion, dynamic solvation effects around the cation should be expected, opposite to around and in between cation and anion and also in contrast with a dipolar π-system such as Nile red.70 Conversely, in water/toluene mixtures it was shown that the variation of the excitation peaks of R3B collected along the higher energy wings of the emission spectra40 reflects the electronic excited-state solvation of contact ion pairs embedded in water clusters. This essentially shows that ion-ion interactions’ effects on the solvation energy71 can be directly distinguished from ion-molecule interaction effects in pure liquids, as is clearly shown by the fully solvated cation in water.37,47-49 Whether to expect ion pairs to be more prevalent in RM than in bulk solutions can be discussed considering ion pairs equilibrium populations. The extent may be addressed using Fuoss and Kraus’ model for the association constant.20,56,72 Equilibrium constants in general21 can be different in RM than in bulk (aqueous) solutions depending on how the interactions between ions can be modulated by molecular structure, hydration or by the curvature of interfaces as MD simulations have shown.73 In a limiting case, at infinite curvature radius, in lamellar structures of AOT, the hydration number found is 2.6 water molecules per AOT.67 While the ratio of CIP to SSIP populations obtained from fluorescence quantum yields (eq 13) varies linearly with w0 values (0.2 < w0 < 10), the apparent hydration constant of CIP to SSIP as a function of w0, Khyd(w0), displays a variation represented by two straight lines when represented against the volume of bound AOT Figure 4 (Inset), at low water content at 0.2 < w0 < 3.2 or at the growing micelles interfacial regions (3.2 < w0 < 10). At w0 > 10 in water/AOT (0.1 M)/isooctane the plateau observed in the fluorescence quantum yields remains for the higher w0 and can be attributed to the invariance of the intermolecular binding of R3B cations and the AOT counterions. AOT Solvation Dynamics from R3B Transient Fluorescence. Solvation dynamics in AOT RM is associated with distinct rise and decay times in the nanosecond time scale. The

10424

J. Phys. Chem. B, Vol. 114, No. 32, 2010

Ferreira and Costa

TABLE 2: R3B Spectroscopy in AOT (0.1 M) Reverse Micelles in Isooctane at T ) 296 K: CIPs Yielding SSIPs Accompanied by Faster Solvation Dynamics (SD) as a Function of w0 Transient Fluorescence56 a AOT R3B IPsb w0 0.2 1.2 2.2 3.2 7.2 15 50 Wd 1-Cl-ne

τaaa (a; b) 3674 3258 2726 2601 2263 2215 2159

(76.0; (84.5; (96.8; (96.3; (98.1; (96.8; (97.5;

46.5) 42.6) 41.8) 39.8) 46.4) 51.4) 50.8)

τbab (a; b) 1834 (22.7; 34.3) 1477 (15.5; 34.9) 1082 (3.2; 33.0) 1002 (3.7; 32.0) 624 (1.9; 30.1) 968 (3.2; 26.7) 672 (2.5; 26.3) 1419 3330f

τSDaSD (a; b) 375 332 226 210 115 253 140

(1.3; 19.2) (-1;c 22.5) (-5;c 25.2) (-3;c 28.2) (-7;c 25.5) (-7;c 21.9) (-5;c 22.9)

a Equation 14 fitted globally with lifetimes τi and local amplitudes ai at λem ) 588 nm (a) and λem ) 550 nm (b) (se also text and Figure 6). b τi ( 15 ps, ai, %; in parentheses, a at λem ) 588 nm and b 2 at λem ) 550 nm. c % SD at λem ) 588 nm )102aSD(∑i)1 ai*SD)-1. d Water (W).37 e 1-Cl-naphthalene (1-Cl-n).40,47 f λem ) 620 nm.

interchange of solutes between RM occurs in the microsecond time range where long dielectric relaxation times are assigned to the diffusion of surfactant monomers between micelles. In general, construction of full time-resolved emission spectra and the analysis of the dynamic Stokes’ shift are very essential. Herein, the discussion of the solvation process imparts from the analysis of pairs of fluorescence decays associated with each w0 made globally with one of the decays at the blue edge of the spectral region where emission arising from CIP dominates and one at the red where SSIP emission progressively becomes superimposed. The analysis of nonexponential decays through eq 14, led to the determination of fluorescence lifetimes and respective amplitudes. The data showed a continuous variation with w0, which may reflect the influence of electronic excitedstate solvation dynamics (SD), as given in Table 2. Except at w0 ) 0.2, fast decay components occur in the blue region of the emission pattern (λem ) 550 nm) and corresponding rise components in the red region (λem ) 588 nm) as a function of w0, which are directly linked to the solvation dynamics of CIPs leading to SSIPs.56 Relative weights depend on the amount present in the corresponding ground states and also on the interconversion between the corresponding electronic excitedstate populations. Consequently, the recombination probabilities of SSIPs into CIPs will diminish with w0 since even for highly polar molecules such as water, the interionic potential screened by the local dielectric constants leads to newly born ion paired species within the hydrated ionic clusters in AOT RM. A clear evolution exists between the three fluorescence lifetimes linking them both in trend and in order. While a simultaneous decrease occurs, longer fluorescence lifetimes are dominantly observed at lower w0 in contrast with faster ones at the higher w0. The shortest lifetime of the longer ones observed at the highest w0 is larger than the intermediate lifetime at the lowest w0 and again the shortest lifetime of the intermediate ones observed at the highest w0 is larger than the shortest lifetime of the shorter ones observed at the lowest w0. At each w0, the longest lifetime should closely approach the fluorescence lifetime of the characteristic IP at that w0 and the intermediate one should closely approach the fluorescence lifetime of the characteristic SSIP also at that w0. The continuous variations of the corresponding amplitudes as a function of wavelength and of w0 are also represented in Figure 6. The wavenumbers of R3B absorption and emission shown in Figure 1 and time-resolved data are in good agreement. The

Figure 6. Solvation dynamics in AOT RM as a function of w0. AOT R3B amplitudes, ai (τi), eq 14, Table 2. Legend: (A) λem ) 550 nm; (B) λem ) 588 nm; ([) aa (τa); (9) ab (τb); (∆) aSD (τSD).

interpretation of the dynamic Stokes’ shift was taken into account for the CIPs and SSIPs in AOT RM without construction of full time-resolved emission spectra. The spectral evolution of both the amplitudes and corresponding lifetimes reflects different stages of the specific hydration processes at a given w0 and suggests that SSIPs are characteristic entities whose nature and behavior are modulated by the number of water molecules and of counterions that reside predominantly in the first and second solvation shells. R3B IPs emission in RM as a function of w0 indicates that the longer time range provides information on the solvated IPs, τa, while the shorter ones, τb, and, τSD should respectively reflect the combination of the local heterogeneity and confinement that determine local viscosity and dielectric medium transient effects. This conditions either ion solvation or ICT processes and the fast dynamic hydration associated with interconversion between CIPs and SSIPs. Radiationless processes of CIPs and SSIPs should thus be determined by local physical details of the AOT RM environment. SSIPs, as well as fully solvated cations, are expected to present fluorescence lifetimes shorter than those of contact ion pairs, CIPs. In fact, SSIPs fluorescence lifetimes can be either shorter than the fluorescence lifetime of CIP or even shorter than that of the fully solvated cation in pure water. Nevertheless, SSIPs show intermediate fluorescence lifetimes that can be longer than that of the fully separated ion pair by the water molecules in the absence of the counterion in the solvation sphere in agreement with reversible ion-pair dissociation in the excited electronic state. The presence of the AOT counterions and water on progressively larger and more definite water droplets justifies the decrease of the fluorescence lifetimes with increasing w0 whereas CIP hydration yielding SSIPs is stressed by the increase in speed

Excited-State Behavior of Rhodamine 3B of fast contributions reflecting the solvation dynamics. At high w0 this implies full ion pair interconversion between CIP and SSIPs at the interfacial regions that can be rationalized in terms of local polarity and viscosity. As the solvation sphere structure depends on local ionic strength effects, it is expected that large local ion concentration may be reflected in a variable SSIP depending on w0. In fact, solvation sphere dynamics is attested through water residence times, as observed in CIPs vs SSIPs through the variation of the transient SD contributions with w0. R3B IP with ClO4- in 1-Cl-naphthalene, presents a long fluorescence lifetime40,47 as the close vicinity of counterion contributes to block the activated radiationless process in agreement with the viscosity dragging effect in the case of the respective cation radiationless process in pure liquids,37 polar liquid mixtures,48 and molecular cavities such as in β-cyclodextrin.49 To discuss why SSIPs will show a decrease in the fluorescence quantum yield as compared to CIP, we invoke the extent of the radiationless process in each species by comparing the different dynamic situations. Since the time scale assigned to the barrier top in the excited-state activated reaction of R3B cation is ca.1 ps, the dynamic behavior exhibits variations that range from close to the Smoluchowski limit to close to the TST limit.37 In water/ethanol mixtures the probable location of the R3B cation at the nanocluster surface near the interface with solution may explain why the dielectric effects probed by the excited-state reaction can be related to the averaged dielectric interactions as described by the function f(ε) of the total polarization.47,48 The interactions of R3B with β-cyclodextrin, described by association with the hydrophobic cavity rim are clearly in line with a location within a confined molecular cluster near the surface, as shown by the transmission coefficients of the activated contribution to the S1 relaxation to S0.49 The radiationless activated process of the R3B host-guest complexes with cyclodextrin reveals a local polarization effect that can vary from that of pure water to 1-decanol due to dynamic hydrophobic interactions established within the cyclodextrin rim. In contrast with R3B, in rhodamine 123 and other rigid rhodamines with alike molecular structures, the electronic excited-state relaxations to the ground state are dominated by the activationless process and thus the corresponding cations and respective contact ion pairs present similar fluorescence lifetimes.38 In fact, while the fluorescence lifetime of R3B in 1-Cl-n is 3.73 ns where R3B exists as a contact ion pair, CIP, one finds for SSIP a fluorescence lifetime close to the values of the R3B cations solvated in ethanol/water mixtures48 or in cyclodextrins49 opposite to the contact ion pair where the variation of the ICT processs is not similarly determined by polarity and viscosity. The solvation induced dynamics noticed in the blue spectral region of the ion pair emission is consistent with the spectral shifts of the corresponding electronic excitedstate absorption as a function of w0. Nile red locally excited state, LE, generates an intramolecular charge transfer state, ICT.70,74 Nile red’s hybrid electronic structure, half rhodamine and half fluorescein (oxazine-like vs coumarin-like),75 integrates the solvatochromic properties76,77 displayed by a contact ion pair. The fluorescence lifetimes of Nile red in RM show a variation with w0 that closely resembles those obtained herein with R3B, suggesting a common essential nature of the electronic state’s solvation in polar liquids.69,78-80 Concluding Section In AOT RM nanoaggregates R3B forms CIPs with the AOT anion coexisting with SSIPs.

J. Phys. Chem. B, Vol. 114, No. 32, 2010 10425 The contrast between CIPs and SSIPs clearly arises in steadystate electronic absorption and emission, as indicated through specific solvatochromism as a function of w0. Steady-state fluorescence anisotropy shows special features at low w0 that are suggested to arise from specific friction effects reflected on rotational diffusion, in line with the specific solvation of R3B AOT IPs. Fluorescence quantum yields as well as the transient fluorescence indicate that while SSIPs become further hydrated the fluorescence quantum yields remain unchanged due to evolving dynamics. The variation of free energy estimated from fluorescence quantum yields and the transient fluorescence data as a function of w0 enable the study of hydration characteristics of ion pairs formed and are explained on the basis of charge, molecular size, and dimensions of the reverse micelles containing water molecules characteristic of situations from incomplete water separated ion pairs to fully hydrated ones thus individualizing ion pair and cation photochemistry. Special emphasis, highlighted by the results obtained at very low w0, can be made by pointing to a comprehensive interpretation of the AOT R3B ion pair hydration by a limited number of water molecules. In fact, since low water content aggregates are a means of isolating a small number of water molecules interacting with the ion pair formed by AOT- with R3B+ the existence of a tightly bound SSIP AOT-H2OR3B+ growing at the expense of solvating water molecules with increasing w0 can contribute to explain either the variation of the fluorescence depolarization or the transient photokinetics of R3B in AOT RM in isooctane. Supporting the hypothesis that a single water molecule interacting with CIP can generate SSIP, the optimized IP geometries also suggest a dissociation path in SSIP AOT-H2OR3B+. This is likely to occur promoted by interaction of the hydrogen atoms of the water molecule with the sulfonate group of the AOT anion, yielding a hydration process accompanied by the increase of interionic distance which finally leads to dissociation. Yet, in contrast with a description based on two individualized fluorescent species, SSIPs in AOT RM reveal a full description of the system dynamic evolution in the whole w0 range highlighting that hydration dynamic influences are felt in the AOT RM nanoaggregates. Acknowledgment. Financial support is due to CQE_G4/IST, FCT: PPCDT/QUI/64658/2006. Prof. L. F. V. Ferreira, Prof. J. M. G. Martinho, and Dr. A. Fedorov are respectively acknowledged for the kind gift of R3B, the use of SPC, and time-resolved measurements. Dr. P. M. R. Paulo is gratefully acknowledged for helpful discussions as is the contribution of anonymous Reviewers. Supporting Information Available: Fluorescence decays of R3B in AOT RM, AOT-(H2O)nR3B+ optimized geometries, and calculated interionic distances within ion pairs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lee, A. G. Biochim. Biophys. Acta, Biomembr. 2003, 1612, 1. (2) Kuffel, A.; Zielkiewicz, J. J. Phys. Chem. B 2008, 111, 209. (3) Biasutti, M. A.; Abuin, E. B.; Silber, J. J.; Correa, N. M.; Lissi, E. A. AdV. Colloid Interface Sci. 2008, 136, 1. (4) (a) Abbyad, P.; Childs, W.; Shi, X. H.; Boxer, S. G. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 20189. (b) Hanson, G. T.; McAnaney, T. B.; Park, E. S.; Rendell, M. E. P.; Yarbrough, D. K.; Chu, S. Y.; Xi, L. X.; Boxer, S. G.; Montrose, M. H.; Remington, S. J. Biochemistry 2002, 41, 15477. (5) Zheng, M.; Huang, X. Biofunctionalization of Gold Nanoparticles. In Biofunctionalization of Nanomaterials; Kumar, C. S. S. R., Ed.; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, 2005; p 99. (6) Yanagida, T.; Ishii, Y. Biosystems 2003, 71, 233.

10426

J. Phys. Chem. B, Vol. 114, No. 32, 2010

(7) Vogler, E. A. AdV. Colloid Interface Sci. 1998, 74, 69. (8) Fukuma, T.; Higgins, M. J.; Jarvis, S. P. Biophys. J. 2007, 92, 3603. (9) Sy´kora, J.; Slavı´cˇek, P.; Jungwirth, P.; Barucha, J.; Hof, M. J. Phys. Chem. B 2007, 111, 5869. (10) Williams, M. A.; Goodfellow, J. M.; Thornton, J. M. Protein Sci. 1994, 3, 1224. (11) Zhong, D.; Douhal, A.; Zewail, A. H. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14056. (12) Park, S.; Moilanen, D. E.; Fayer, M. D. J. Phys. Chem. B 2008, 112, 5279. (13) Bhattacharyya, K. Chem. Commun. 2008, 2848. (14) Abel, S.; Sterpone, F.; Bandyopadhyay, S.; Marchi, M. J. Phys. Chem. B 2004, 108, 19458. (15) (a) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480. (b) Ueda, M.; Schelly, Z. A. Langmuir 1988, 4, 653. (c) Izquierdo, C.; Moya´, M. L.; Usero, J. L.; Casado, J. Monatsh. Chem. 1992, 123, 383. (16) Nicholson, J. D.; Clarke, J. H. R. In Surfactants in Solution v.3; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p1663. (17) Costa, S. M. B.; Vela´zquez, M. M.; Tamai, N.; Yamazaki, I. J. Lumin. 1991, 48-49, 341. (18) Biswas, R.; Rohman, N.; Pradham, T.; Buchner, R. J. Phys. Chem. B 2008, 112, 9379. (19) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1996, 184, 570. (20) Fuoss, R. M. J. Am. Chem. Soc. 1978, 100, 5576. (21) Marcus, Y.; Hefter, G. Chem. ReV. 2006, 106, 4585. (22) (a) Eastoe, J.; Fragneto, G.; Robinson, B. H.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1992, 88, 461. (b) Stahla, M. L.; Baruah, B.; James, D. M.; Johnson, M. D.; Levinger, N. E.; Crans, D. C. Langmuir 2008, 24, 6027. (23) (a) Ritter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062. (b) Corbeil, E. M.; Riter, R. E.; Levinger, N. E. J. Phys. Chem. B 2004, 108, 10777. (24) Sando, G. M.; Dahl, K.; Owrutsky, J. C. J. Phys. Chem. A 2004, 108, 11209. (25) Owrutsky, J. C.; Pomfret, M. B.; Barton, D. J.; Kidwell, D. A. J. Chem. Phys. 2008, 129, 024513. (26) Correa, N. M.; Levinger, N. E. J. Phys. Chem. B 2006, 110, 13050. (27) Baruah, B.; Swafford, L. A.; Crans, D. C.; Levinger, N. E. J. Phys. Chem. B 2008, 112, 10158. (28) (a) Dutt, G. B. J. Phys. Chem. B 2008, 112, 7220. (b) Bose, D.; Sarkar, D.; Girigoswami, A.; Mahata, A.; Ghosh, D.; Chattopadhyay, N. J. Chem. Phys. 2009, 131, 114707. (29) Dutt, G. B. J. Chem. Phys. 2008, 129, 014501. (30) Hirose, Y.; Yui, H.; Sawada, T. J. Phys. Chem. B 2004, 108, 9070. (31) Belleteˆte, M.; Lachapelle, M.; Durocher, G. J. Phys. Chem. 1990, 94, 5337. (32) Agmon, N. J. Phys. Chem. A 2002, 106, 7256. (33) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2000, 104, 1033. (34) Oliveira, C. S.; Bastos, E. L.; Duarte, E. L.; Itri, R.; Baptista, M. S. Langmuir 2006, 22, 8718. (35) Visser, A. J. W. G.; Vos, K.; Hoek, A. V.; Santema, J. S. J. Phys. Chem. 1988, 92, 759. (36) (a) Bagchi, B.; Fleming, G. R.; Oxtoby, D. W. J. Chem. Phys. 1983, 78, 7375. (b) Bagchi, B.; Fleming, G. R. J. Phys. Chem. 1990, 94, 9. (37) (a) Ferreira, J. A. B.; Costa, S. M. B.; Ferreira, L. F. V. J. Phys. Chem. A 2000, 104, 11909. (b) Ferreira, J. A. B.; Costa, S. M. B. J. Chem. Phys. 2004, 120, 8095. (38) Ferreira, J. A. B.; Costa, S. M. B. Chem. Phys. 2006, 321, 197. (39) Chandra, A.; Bagchi, B. Proc. Indian Acad. Sci. (Chem. Sci.) 1989, 101, 83. (40) Ferreira, J. A. B.; Costa, S. M. B. Chem. Phys. Lett. 1999, 307, 139. (41) Makov, G.; Nitzan, A. J. Phys. Chem. 1992, 96, 2965. (42) Manoj, K. M.; Jayakumar, R.; Rakshit, S. K. Langmuir 1996, 12, 4068. (43) Yu, Z. J.; Zhon, N. F.; Newman, R. D. Langmuir 1992, 8, 1885. (44) Yu, Z. J.; Newman, R. D. Langmuir 1994, 10, 2553. (45) Hirai, M.; Hirai, R. K.-H.; Yabuki, S.; Takizawa, T.; Hirai, T.; Kobayashi, K.; Amemiya, Y.; Oya, M. J. Phys. Chem. 1995, 99, 6652.

Ferreira and Costa (46) Wong, M. M.; Schelly, Z. A. J. Phys. Chem. 1974, 78, 1891. (47) (a) Ferreira, J. A. B.; Costa, S. M. B. Chem. Phys. 2001, 273, 39. (b) Ferreira, J. A. B.; Costa, S. M. B. J. Mol. Struct. 2001, 565, 35. (c) Ferreira, J. A. B.; Costa, S. M. B. Chem. Phys. 2001, 269, 313. (d) Ferreira, J. A. B.; Costa, S. M. B. Pol. J. Chem. 2008, 82, 893. (48) Ferreira, J. A. B.; Costa, S. M. B. Phys. Chem. Chem. Phys. 2003, 6, 1064. (49) Ferreira, J. A. B.; Costa, S. M. B. J. Photochem. Photobiol. A 2005, 173, 309. (50) Johansson, L. B.-Å. J. Chem. Soc., Faraday Trans. 1990, 86, 2103. (51) Valeur, B. Molecular Fluorescence: Principles and Applications; Wiley-VCH: Verlag, 2001. (52) Wittouck, N.; Negri, R. M.; Ameloot, M.; De Schryver, F. C. J. Am. Chem. Soc. 1994, 116, 10601. (53) Jonas, J.; Adamy, S. T.; Grandinetti, P. J.; Masuda, Y.; Morris, S. J.; Campbell, D. M.; Li, Y. J. Phys. Chem. 1990, 94, 1157. (54) Olmsted, J., III. J. Phys. Chem. 1979, 83, 2581. (55) Parker, C. A. Photoluminescence in Solution; Elsevier: Amsterdam, 1968. (56) See Supporting Information (cited references therein follow article text). (57) O’Connor, D. V.; Phillips, D. Time-correlated Single Photon Counting; Academic Press: New York, 1984. (58) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (59) Rzepa, H. S.; Suner, G. A. J. Chem. Soc., Perkin Trans. 1994, 2, 1397. (60) Vaiana, A. C.; Schulz, A.; Wolfrum, J.; Sauer, M.; Smith, J. C. J. Comput. Chem. 2003, 24, 632. (61) Oliveira, C. S.; Branco, K. P.; Baptista, M. S.; Indig, G. L. Spectrochim. Acta, Part A 2002, 58, 2971. (62) Loison, C.; Antoine, R.; Broyer, M.; Dugourd, P.; Guthmuller, J.; Simon, D. Chem.sEur. J. 2008, 14, 7351. (63) Andrade, S. M.; Costa, S. M. B. Photochem. Photobiol. Sci. 2002, 1, 500. (64) Hasegawa, M.; Sugimura, T.; Shindo, Y.; Kitahara, A. Colloids Surf. A 1996, 109, 305. (65) Klein, U. K. A.; Haar, H.-P. Chem. Phys. Lett. 1978, 58, 531. (66) Bushuk, B. A.; Rubinov, A. N.; Stupak, A. P. Opt. Spectrosc. 1996, 81, 838. (67) (a) Temsamani, M. B.; Maeck, M.; El-Hassani, I.; Hurwitz, H. D. J. Phys. Chem. B 1998, 102, 3335. (b) Boissie`re, C.; Brubach, J. B.; Mermet, A.; de Marzi, G.; Bourgaux, C.; Prouzet, E.; Roy, P. J. Phys. Chem. B 2002, 106, 1032. (68) Bilski, P.; Dabestani, R.; Chignell, C. F. J. Phys. Chem. 1991, 95, 5784. (69) Du, H.; Rasaiah, J. C.; Miller, D. J. Phys. Chem. B 2007, 111, 209. (70) Datta, A.; Mandal, D.; Pal, S. K.; Bhattacharyya, K. J. Phys. Chem. B 1997, 101, 10221. (71) Shim, Y.; Kim, H. J. J. Phys. Chem. B 2008, 112, 11028. (72) Fuoss, R. M.; Kraus, C. A. J. Am. Chem. Soc. 1933, 55, 1019. (73) Chorny, I.; Dill, K. A.; Jacobson, M. P. J. Phys. Chem. B 2005, 109, 24056. (74) (a) Pradhan, T.; Biswas, R. J. Solution Chem. 2009, 38, 517. (b) Bagchi, B.; Biswas, R. AdV. Chem. Phys. 1999, 109, 207. (75) Han, W.-G.; Liu, T.; Himo, F.; Toutchkine, A.; Bashford, D.; Hahn, K. M.; Noodleman, L. ChemPhysChem 2003, 4, 1084. (76) Harder, T.; Bendig, J. Chem. Phys. Lett. 1994, 228, 621. (77) (a) Reichardt, C. Chem. ReV. 1994, 94, 2319. (b) Matyushov, D. V.; Schmid, R.; Ladanyi, B. M. J. Phys. Chem. B 1997, 101, 1035. (78) Laage, D.; Stirnemann, G.; Hynes, J. T. J. Phys. Chem. B 2009, 113, 2428. (79) Li, T. P.; Hassanali, A. A.; Kao, Y. T.; Zhong, D.; Singer, S. J. J. Am. Chem. Soc. 2007, 129, 3376. (80) Mizoshiri, M.; Nagao, T.; Mizoguchi, Y.; Yao, M. J. Chem. Phys. 2010, 132, 164510.

JP100571T