Sensing Micelle Hydration by Proton-Transfer Dynamics of a 3

Article pubs.acs.org/Langmuir

Sensing Micelle Hydration by Proton-Transfer Dynamics of a 3Hydroxychromone Dye: Role of the Surfactant Headgroup and Chain Length Ranjan Das,*,†,‡ Guy Duportail,‡ Ludovic Richert,‡ Andrey Klymchenko,‡ and Yves Mély‡ †

Department of Chemistry, West Bengal State University, Barasat, Kolkata 700126, India Laboratoire de Biophotonique et Pharmacologie, UMR 7213 du CNRS, Faculté de Pharmacie, Université de Strasbourg, 67401 Illkirch, France

‡

S Supporting Information *

ABSTRACT: The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of 2-(2′-furyl)-3hydroxychromone (FHC) was studied in micelles by timeresolved fluorescence. The proton-transfer dynamics of FHC was found to be sensitive to the hydration and charge of the micelles, demonstrated through a decrease of the ESIPT rate constant (kPT) in the sequence cationic → nonionic → anionic micelles. A remarkably slow ESIPT with a time constant (τPT) of ∼100 ps was observed in the anionic sodium dodecyl sulfate and sodium tetradecyl sulfate micelles, whereas it was quite fast (τPT ≈ 15 ps) in the cationic cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In the nonionic micelles of Brij-78, Brij-58, Tween-80, and Tween-20, ESIPT occurred with time constants (τPT ≈ 35−65 ps) intermediate between those of the cationic and anionic micelles. The slower ESIPT dynamics in the anionic micelles than the cationic micelles is attributed to a relatively stronger hydration of the negatively charged headgroups of the former than the positively charged headgroups of the latter, which significantly weakens the intramolecular hydrogen bond of FHC in the Stern layer of the anionic micelles compared to the latter. In addition, electrostatic attraction between the positively charged −N(CH3)3+ headgroups and the negatively charged 4-carbonyl moiety of FHC effectively screens the intramolecular hydrogen bond from the perturbation of water molecules in the micelle−water interface of the cationic micelles, whereas in the anionic micelles, this screening of the intramolecular hydrogen bond is much less efficient due to an electrostatic repulsion between its negatively charged −OSO3− headgroups and the 4-carbonyl moiety. As for the nonionic micelles, a moderate level of hydration, and the absence of any charged headgroups, causes an ESIPT dynamics faster than that of the anionic but slower than that of the cationic micelles. Furthermore, the ESIPT rate decreased with a decrease of the hydrophobic chain length of the surfactants due to the stronger hydration of the micelles of shorter chain surfactants than those of longer chain surfactants, arising from a less compact packing of the former surfactants compared to the latter surfactants.

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INTRODUCTION Self-organized molecular assemblies, such as micelles and vesicles, serve as critical models for unraveling the structure, dynamics, and reactivity of complex biological systems.1−3 Micelles are self-organized molecular assemblies of amphiphilic molecules that comprise a hydrophobic core and a hydrophilic shell. The core of a micelle is formed by hydrocarbon chains (hydrophobic tail) with polar and/or charged headgroups (hydrophilic head) projecting outward into the bulk water.4 Depending on the nature of the surfactant headgroup, micelles can be ionic or nonionic. The hydrophilic shell of the micelles is called the Stern layer, if the micelle is ionic, or the palisade layer if the micelle is nonionic. In ionic micelles, the Stern layer5 (Figure 1a) is the interface between the hydrophobic core surface and the hydrodynamic shear surface, consisting of the ionic headgroups, a few counterions, and water molecules © 2012 American Chemical Society

associated with these ionic headgroups, whereas for the nonionic micelles, the palisade layer (Figure 1b) defines the interfacial region between the core surface and the shear surface comprising hydrophilic, uncharged poly(ethylene oxide) headgroups and water molecules which are hydrogen-bonded to these oxyethylene chains. Micelles are also envisaged as nanoreactors or nanometerscale containers, which may drastically modulate the reactivity of the entrapped solutes due to their location with respect to the micelle−water interface.6 Whereas the reactivity and dynamics of photoinduced intermolecular proton-transfer reactions in micelles and reverse micelles have been reasonably Received: January 7, 2012 Revised: April 16, 2012 Published: April 19, 2012 7147

dx.doi.org/10.1021/la300103a | Langmuir 2012, 28, 7147−7159

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Figure 1. (Top) (a) Ionic and (b) nonionic micelle interfaces. (Bottom) (A) Scheme of irreversible ESIPT reaction in micelles. N, T and N*, T* represent the normal and proton-transferred tautomer forms of 2-(2′-furyl)-3-hydroxychromone in the ground and excited states, respectively. kPT is the rate constant of the ESIPT reaction. λexc = 330 nm, λN* = 410−424 nm, and λT* = 522−537 nm, where λexc, λN*, and λT* represent the excitation wavelength and short- and long-wavelength emission bands, respectively.

addressed,7−10 only a few studies concerning the dynamics of the excited-state intramolecular proton-transfer (ESIPT) process in reverse micelles11 and micelles12,13 have been reported to date. A dramatic reduction in the ESIPT rate was observed for 3-hydroxyflavone (3HF) in the polar domains of the Aerosol-OT (AOT)/n-heptane reverse micelle compared to nonpolar and polar aprotic solvents such as acetonitrile and was attributed to the formation of intermolecularly hydrogenbonded 3-HF−AOT complexes, causing a significant disruption of the intramolecular hydrogen bond within the complexbound 3HF molecules.11 As for the micelles, Petrich et al.12 observed a slower ESIPT dynamics for curcumin in neutral Triton X-100 (TX-100) micelle with an ESIPT time constant of ∼80 ps in comparison to a faster ESIPT dynamics with a time constant of ∼50 ps in cationic dodecyltrimethylammonium bromide (DTAB) and anionic sodium dodecyl sulfate (SDS) micelles. The slower ESIPT dynamics in the neutral micelle was attributed to the presence of a high number of C−O groups in the backbone of TX-100, which may interact with curcumin

through hydrogen bonding, and the absence of such a hydrogen-bonding network led to a faster proton-transfer dynamics in DTAB and SDS micelles.12 On the other hand, Mandal et al.13 found no difference in the ESIPT dynamics of 3HF among cationic, anionic, and neutral micelles and obtained a nearly uniform ESIPT time constant of ∼50 ps similar to that of methanol. These works in reverse micelles11 and micelles12,13 revealed a common feature of specific hydrogen-bonding interaction between the dye and the surfactant molecule being responsible for the slower proton-transfer dynamics, and none of them addressed the role of micelle hydration and surfactant headgroup charge in the ESIPT dynamics. In view of the studies in reverse micelles11 and micelles,12,13 it is quite pertinent to address the issues concerning the sensitivity of the ESIPT dynamics to micelle hydration, the charge of the surfactant headgroup, and the hydrophobic tail length, which have not yet been explored. This is a particularly interesting topic worth addressing because (i) the interfacial region (Figure 1) of the micelles is the preferred site for solubilization, even for 7148


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hydrophobic molecules,14 (ii) the hydrogen bond donating abilities are different for the cationic, anionic, and neutral micelles15,16 due to a differential solvation of the ionic or polar headgroups in the Stern or palisade layer of the micelle−water interface, (iii) the Stern layer of the anionic sodium dodecyl sulfate micelles with negatively charged −OSO3− headgroups was shown recently by dielectric spectroscopy17,18 to be strongly hydrophilic compared to the essentially hydrophobic surface17 of the cationic alkyltrimethylammonium halide micelles with positively charged −N(CH3)3+ headgroups, implying a very strong and poor hydration of the anionic and cationic surfactants, respectively, and (iv) the palisade layer of the nonionic micelles comprises at least two water molecules per ethylene oxide (EO) unit tightly associated with the poly(ethylene oxide) (PEO) chains19 and thus demonstrates a level of hydration intermediate between those of the cationic and anionic micelles. In this context, the fluorescent dye 2-(2′furyl)-3-hydroxychromone (N, Figure 1A) is a potential candidate for sensing micelle hydration through the modulation of its ESIPT dynamics because (i) the ESIPT reaction (Figure 1A) was found to be sensitive to the hydrogen bond donating ability or H-bond acidity of the polar, protic solvents20 and (ii) the Stern or palisade layer (Figure 1a,b) of the micelle−water interface is most likely the preferred site for localization of the FHC dye in micelles, as it is highly polar and possesses hydrophilic 4-carbonyl and 3-hydroxy moieties. Additionally, the dye possesses a furan ring which increases the partial negative charge on the 4-carbonyl oxygen21 through a possible conjugation with the chromone ring and thus favors a strong intermolecular H-bonding interaction with water as well as other protic solvents.20 Herein, we present steady-state as well as time-resolved intensity and anisotropy data for demonstrating the occurrence of an excited-state intramolecular proton-transfer reaction on a distinctly different time scale in the cationic, anionic, and nonionic micelles, which enables us to address the role of the hydration and charge of the surfactant headgroups and the hydrophobic chain length in modulation of the ESIPT dynamics.

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[M] = ([S] − cmc)/Nagg

(1)

The solutions were then allowed to equilibrate for 1 h in the dark prior to the steady-state and time-resolved measurements. The quantum yields (Φ, ΦN*, and ΦT*) of the dye were determined with respect to a solution of 3-hydroxyflavone in toluene as a reference (Φ = 0.29).23 The quantum yield was calculated on the basis of the following equation: ΦS = ΦR (ISAR /IR A S)(nS/nR )2

(2)

where the subscripts S and R refer to the sample and reference compound, respectively. I is the integrated area under the corrected emission spectrum. A is the absorbance of the solution at the excitation wavelength (A < 0.05), and (nS/nR)2 is the correction for the refractive index. For the determination of the fluorescence quantum yield (ΦN*, ΦT*) of each of the two emission bands, separately, the fluorescence spectra were decomposed into two separate bands using an iterative nonlinear least-squares method based on the Fletcher−Powell algorithm.24 The shape of the individual short- and long-wavelength emission bands was approximated by a log-normal function,25 which accounts for the asymmetry of the spectral bands. All of the spectroscopic measurements were performed at 25 °C in cuvettes of 1 cm optical path. Absorption and fluorescence spectra were recorded on a Cary 4 spectrophotometer (Varian) and a FluoroMax 3.0 (Jobin Yvon, Horiba) spectrofluorometer, respectively. Time-resolved fluorescence measurements were performed with the time-correlated, single-photon-counting technique using the excitation pulses at 315 nm provided by a pulse-picked frequency-tripled Ti−sapphire laser (Tsunami, Spectra Physics) pumped by a Millenia X laser (Spectra Physics) as described elsewhere.26 The emission was collected through a polarizer set at the magic angle and an 8 nm band-pass monochromator (Jobin-Yvon H10) at 420 and 540 nm, respectively. The single-photon events were detected with a microchannel plate photomultiplier (Hamamatsu) coupled to a pulse preamplifier HFAC (Becker-Hickl) and recorded on a SPC-130 board (Becker-Hickl). The instrumental response function was recorded using a polished aluminum reflector, and its full width at half-maximum was ∼40 ps. Integrated counts of 106 were collected for all of the lifetime measurements. For time-resolved anisotropy measurements, the fluorescence decay curves were recorded at vertical and horizontal positions of the polarizer and analyzed by the following equations:

EXPERIMENTAL SECTION

2-(2′-Furyl)-3-hydroxychromone (FHC) was synthesized as previously described.20,22 The surfactants cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), DTAB, SDS, sodium tetradecyl sulfate (STDS), Brij-58, Tween-20, and Tween-80 obtained from Sigma-Aldrich and Brij-78 from Fluka were used as received. The stock solutions of the surfactants were prepared in deionized water (from a Millipore Milli-Q nanopure water system) at the concentration range of 80−250 mM to maintain the micelle concentration in the range of 0.7−3.0 mM. A small quantity (2 μL) of the FHC stock solution in methanol (3.5 mM) was transferred to the micellar solution to yield a solution with an FHC concentration of 7 μM, used for the measurements of UV−vis and steady-state and timeresolved fluorescence decays. The total methanol contents of all the final solutions of FHC in the micelles were kept at 8.2) the fluorescence spectrum of FHC with an excitation at 420 nm displayed a single emission band at ∼500 nm assigned to an emission from the photoexcited anionic form of the dye.29 As a consequence, the emission of the anionic form of FHC in micelles can be detected only when the anion is formed in the ground state and is photoexcited directly at a wavelength of ≥420 nm. Since excitation wavelengths of 330 and 315 nm were used for recording the steady-state and time-resolved emission spectra, respectively, only the normal form is selectively photoexcited (N, Figure 1A) and the anionic form of FHC, which absorbs at ∼430−440 nm

Figure 2. Normalized absorption spectra of FHC.

absorption spectra of FHC in CTAB, SDS, and Brij-78 micelles as well as in water. The ground-state absorption maxima of FHC undergo a red shift28 of ∼10−12 nm in micelles compared to an aprotic polar solvent such as acetonitrile (ACN), with an additional small shoulder around 322−324 nm. The red shift of the absorption peak in micelles is similar to the moderate red shift observed earlier20 in alcohols relative to the polar aprotic solvents and may therefore be attributed to an intermolecular H-bonding interaction of the 4-carbonyl group of FHC with water molecules in the micelle−water interface. Moreover, the ground-state absorption maximum was blueshifted by ∼2−4 nm in various micelles28 relative to water, indicating incorporation of FHC in the restricted micellar environment. Noticeably, in SDS and STDS anionic micelles, in addition to the main absorption band28 at ∼360 nm, another very weak

Figure 3. Steady-state emission spectra of FHC (7 μM) in micelles at 298 K. The excitation wavelength is 330 nm. 7150


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bands exhibits a significant variation depending on the nature of the micelles. The N* band is weak in comparison to the T* band (Figure 3) in CTAB and TTAB cationic micelles,28 resulting in a low IN*/IT* value of ∼0.13. The intensity of the N* band was found to increase relative to that of the T* band in going from the cationic to the nonionic micelles of Tween20, Tween-80, Brij-58, and Brij-78, with a consequent higher28 IN*/IT* value in the range of ∼0.21−0.33. In the anionic micelles of SDS and STDS, the N* band is most intense in comparison to the T* band (Figure 3), with a large IN*/IT* value of ∼0.82−0.93. The low IN*/IT* ratio in the cationic micelles may further be ascribed to a fast ESIPT dynamics. On the other hand, the higher IN*/IT* values for both the nonionic and the anionic micelles may be due either to a slower protontransfer dynamics20 or to the formation of solvated structures30 with disrupted intramolecular H-bond unfavorable for an ESIPT reaction. In strongly hydrated micelles, a strong FHC−water hydrogen-bonding interaction with a concomitant disruption of the intramolecular H-bond may be favored, leading to an increase in intensity of the N* emission band. This model was previously proposed by Sengupta30 and Kasha31 for 3-hydroxyflavone to interpret the higher intensity of its N* band in protic solvents. However, this model would require the presence of a ground-state heterogeneity (i.e., the presence of differently solvated dye molecules), which is absent for FHC in micelles, as a significantly large population of the intramolecularly hydrogen-bonded normal form is coexistent with a negligibly small population of its anionic form in the ground state, as evident from the UV−vis absorption spectra (Figure 2). Moreover, the steady-state fluorescence spectra were recorded with an excitation wavelength of 330 nm, which selectively photoexcites the normal form of the FHC dye (N, Figure 1A), and as a consequence, the observation of a stronger N* band in the nonionic and anionic micelles may be attributed to a slower ESIPT dynamics relative to that of the cationic micelles, which is verified further from the time-resolved fluorescence measurements. Positions of the N* and T* Band Maxima and Their Difference (νN* − νT*). The N* band undergoes a red shift of ∼2−8 nm, while the T* band is blue-shifted by ∼2−15 nm in micelles28 compared to the polar aprotic solvents. Moreover, further red and blue shifts of the N* and T* bands, by ∼4−11 and ∼13−27 nm, respectively, were observed in water compared to micelles. As a consequence, their difference (νN* − νT*) increases sequentially in the order water < micelles < aprotic solvents. The lower value of νN* − νT* in micelles28 compared to dimethylformamide (DMF) or ACN may be attributed to intermolecular H-bonding interactions20,32 between FHC and water molecules in the Stern or palisade layer of the micelle−water interface. Indeed, the oxygen atom of the 4-carbonyl group in FHC, being the site of intermolecular H-bonding interaction, possesses an effective negative charge21 in the N* state, while in the T* state this negative charge is compensated by the transferred proton (Figure 1), leading to a selective stabilization of the N* state and destabilization of the T* state by water molecules in the micelle−water interface. A comparison28 of the νN* − νT* parameter in micelles to those in a number of polar protic solvents provides valuable insights into the micellar environment of the dye. For the anionic micelles, the difference in the band maxima (∼4500 cm−1) is found to be close to that of a strong H-bond donating solvent such as methanol (∼4885 cm−1), whereas for the

in micelles (Figure 2), is unlikely to be photoexcited for interfering with the steady-state and time-resolved fluorescence spectra. As a consequence, any possible involvement of the anionic form of FHC in the proton-transfer reaction dynamics is ruled out. Steady-State Emission Spectroscopy. FHC displays a dual fluorescence with two highly resolved emission bands (Figure 3) in all of the cationic, anionic, and nonionic micelles, while in water the long-wavelength band is not well resolved and appears as a shoulder in the steady-state emission spectrum. The demonstration of dual emission bands in micelles and water suggests the occurrence of an ESIPT reaction.20,29 The short-wavelength band corresponds to an emission of the excited normal (N*) form, while the long-wavelength band originates from the proton-transferred tautomer (T*) (Figure 1A). The excitation spectra monitored at the short-wavelength (∼420 nm) and long-wavelength (∼540 nm) emission bands in micelles are identical and closely resemble the corresponding absorption spectrum in the 300−400 nm region (Figure 4),

Figure 4. Normalized absorption and excitation spectra of FHC in micelles.

demonstrating that the dual emission bands originate from the same ground-state precursor. Two spectral features of the emission spectra of FHC in micelles, namely, the (i) ratio of intensities of the N* and T* emission bands (IN*/IT*) and (ii) positions of the N* and T* band maxima and their difference (νN* − νT*), are noteworthy when compared to those in water and a number of polar protic and aprotic solvents. Intensity Ratio (IN*/IT*) of the N* and T* Bands: Role of the ESIPT Dynamics. The ratio of intensities (IN*/IT*) of the short-wavelength (N*) and the long-wavelength (T*) emission 7151


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Table 1. Time-Resolved Fluorescence Parameters for the Two Emission Bands of FHC in Micellesa micelle

τ1N*, ns

a1N*

τ2N*, ns

a2N*

χN2

τ1T*, ns

a1T*

τ2T*, ns

a2T*

χT2

STDS SDS CTAB TTAB DTAB Brij-78 Brij-58 Tween-80 Tween-20

0.097 0.106 0.014 0.016 0.032 0.034 0.041 0.053 0.066

0.99 0.99 1.00 0.99 1.00 0.92 0.91 0.93 0.92

0.84 0.85

0.01 0.01

0.88

0.01

0.42 0.46 0.57 0.37

0.08 0.09 0.07 0.08

1.03 1.04 0.94 0.97 1.10 1.01 1.06 1.14 0.96

0.092 0.101 0.015 0.017 0.025 0.036 0.045 0.049 0.055

−0.51 −0.49 −0.49 −0.51 −0.48 −0.49 −0.50 −0.49 −0.49

0.61 0.88 0.76 0.94 0.65 1.41 1.43 1.58 1.57

0.49 0.51 0.51 0.49 0.52 0.51 0.50 0.51 0.51

1.06 1.02 1.03 1.07 1.12 1.11 0.97 1.04 1.21

τ1 and τ2 are the short-lived and long-lived decay times, respectively. a1 and a2 are the relative amplitudes. χN2 and χT2 are the reduced χ2 values of fits to the time-resolved decays of the short- and long-wavelength emission bands, respectively. The error for all values of τ1 and τ2 is ±10% due to the precision of the measurements. Amplitudes were normalized according to |a1| + |a2| = 1. a

cationic and nonionic micelles, it (∼5100−5300 cm−1) closely resembles the environment of a much weaker H-bond donating solvent such as ethanol (∼5400 cm−1). Moreover, several studies33−35 on the solubilization of fluorescence probes in micelles revealed their preferential location in the Stern or palisade layer, resembling closely the probe environment in short-chain alcohols.33,34 Thus, the similarity28 of the νN* − νT* parameter in micelles to those in ethanol and methanol indicates the preferential location of FHC in the Stern (or palisade) layer of the ionic (or nonionic) micelles. The νN* − νT* parameter in water is even smaller28 (∼3800 cm−1) than that in the micelles, suggesting that a conducive environment for strong H-bonding interaction between FHC and water molecules in the micelle−water interface is likely to be accompanied by a smaller νN* − νT* value, whereas a weaker H-bonding interaction results in a larger νN* − νT* value. These observations led us to conjecture that (i) FHC is likely located in the Stern layer of the ionic micelles, (ii) the dye environment in the anionic micelles is polar and potentially favorable for Hbonding interactions, whereas it is comparatively less polar and less favorable for H-bonding interactions in the Stern layer of the cationic micelles, and (iii) FHC is likely located in the palisade layer of the nonionic micelles, where the environment is less polar and less favorable for H-bonding interactions than that of the Stern layer of the anionic micelles. To check how the ESIPT dynamics of FHC is influenced by the H-bonding interactions in the Stern or palisade layer of the micelle−water interface, time-resolved fluorescence measurements were performed. Time-Resolved Fluorescence Spectroscopy. TimeResolved Intensity Decay. In all of the micelles, the timeresolved intensity decays36 of FHC monitored at the shortwavelength emission band (∼420 nm) are predominantly monoexponential, comprising a short-lived decay component (τ1N*) and an almost negligible long-lived decay component (τ2N*) (Table 1). On the other hand, the decays monitored at the long-wavelength (∼540 nm) emission band are always characterized by a fast rise component (τ1T*) and a slower decay component (τ2T*). Moreover, the rise time (τ1T*) is found to be identical (Table 1) within experimental error to the short-lived decay time (τ1N*), demonstrating that the fast decay of the normal form (N*) is coupled with the fast rise of the tautomer form (T*) in the excited state, and points to the existence of a precursor−successor type of relation between the primarily excited normal (N*) and the tautomer (T*) forms. This precursor−successor relation between the N* and T* forms furthermore confirms the formation of the tautomeric

form from the primarily excited normal form through an ESIPT reaction (N* → T*, Figure 1). The long-lived decay components (τ2N* and τ2T*, Table 1) may further be ascribed to the lifetime of the primarily excited normal and the protontransferred tautomer forms, respectively. As the short-wavelength emission band ascribed to the excited normal (N*) form of FHC is characterized by a very low quantum yield (ΦN*) (∼4 × 10−2 to 7 × 10−3) and a fast decay component (τ1N*) (∼14−106 ps, Table 2), we can Table 2. Kinetic Parameters of ESIPT and the Radiative Process in Micellesa micelle

ΦN*

τ1N*(τPT), ns

kRN* × 10−8, s−1

kPT × 10−10, s−1

HLB

STDS SDS CTAB TTAB DTAB Brij-78 Brij-58 Tween-80 Tween-20

0.042 0.052 0.007 0.007 0.014 0.012 0.014 0.017 0.020

0.097 0.106 0.014 0.016 0.032 0.034 0.041 0.053 0.066

4.33 4.90 5.00 4.37 4.37 3.53 3.41 3.21 3.03

1.03 0.94 7.14 6.25 3.13 2.94 2.44 1.88 1.52

15.3 15.7 15.0 16.7

a ΦN* is the quantum yield of the N* form. τ1N* is the short-lived decay component of the N* form. kRN* and kPT are, respectively, the radiative rate constant of the N* form and ESIPT rate constant. The error for all values of kRN* and kPT is ±10% due to the precision of the measurements. HLB is the hydrophile−lipophile balance number of a micelle.62

unambiguously conjecture that the time-resolved decay of the N* form in micelles is dominated by the rate constant (kPT) of the ESIPT reaction so that the short-lived component (τ1N*) may be regarded as a measure of the time constant of the excited-state intramolecular proton-transfer (τPT) reaction, τ1N* ≈ τPT (Table 2), similar to the time-resolved data for FHC in polar solvents.20 The long-lived decay component (τ2) is systematically observed for the T* band, but for the N* band, it either is not observed or shows a negligibly small amplitude (Table 1), suggesting an irreversible ESIPT20 for the FHC dye in all of the micelles (Figure 1). To evaluate quantitatively the effect of differently charged micelles on the ESIPT dynamics, we used the following model20 of irreversible ESIPT, where the quantum yield of the N* band (ΦN*) is related to the shortlived decay component (τ1N*) of the N* form by the following relations: 7152


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ΦN * = kR N *τ1N *

(6)

(τ1N *)−1 = kR N * + kNR N * + kPT

(7)

kRN*

anisotropy decays in micelles must be due to the rotational diffusion of the FHC dye bound to the micelles. In this respect, the biexponential anisotropy decay could possibly be related to the FHC dye bound to two different sites of a micelle, namely, the hydrophobic core and the Stern or palisade layer of the micelle−water interface. However, this two-site model38 would require a biexponential time-resolved fluorescence decay with two lifetime components associated with the dye’s location in two different sites of the micelle, which is, however, in sharp contrast to the observed monoexponential fluorescence decay of the ESIPT tautomer (T*) of FHC with only one lifetime component, τ2T* (Table 1). In addition, as (i) FHC has poor solubility in alkane-like solvents, but possesses a moderate to good solubility in water and other polar protic solvents and (ii) FHC is highly polar on account of its reasonably high groundand excited-state dipole moments and charge distributions,21 it must be located39 in the Stern or palisade layer of the micelle− water interface due to the higher polarity and water content of this region in comparison to the nonpolar hydrophobic core. As a consequence, the observed biexponential anisotropy decays may be ascribed to two different kinds of motion of the FHC dye in micelles according to the two-step wobbling in a cone model.40−42 In this model, the dye molecule is proposed to undergo a lateral diffusion on the curved micelle surface and a fast wobbling motion in an imaginary cone coupled to the overall rotation of the micelle. The observed fast (φ1) and slow (φ2) rotational correlation times (Table 3) are related to the time constants for lateral diffusion (τL), wobbling motion (τW), and the overall rotation of the micelle (τM) by the following relations:

kNRN*

where and are the radiative and nonradiative rate constants of the N* form, respectively, and kPT is the protontransfer rate constant. Estimations of ΦN* from the steady-state fluorescence spectra and τ 1N * from the time-resolved fluorescence measurements (Table 1) allow us to obtain the values of the radiative decay constant kRN* from eq 6 for FHC in different micelles. Furthermore, on the basis of the very low quantum yield of the N* band and its associated short-lived decay component (Table 1), we may assume that the ESIPT reaction is much faster than the radiative and nonradiative deactivation processes, so that kPT ≫ kRN* + kNRN*. Then eq 7 may be simplified as

(τ1N *)−1 = kPT

(8)

The time-resolved decay of the N* band is characterized by a fast component, τ1N* ≈ 15 ps (Table 1) in the CTAB and TTAB cationic micelles, implying a fast ESIPT reaction with a rate constant kPT ≈ (6−7) × 1010 s−1 (Table 2). On the contrary, this short-lived decay component (τ1N*) of the cationic micelles becomes significantly longer (τ1N* ≈ 100 ps) in the SDS and STDS anionic micelles with an ESIPT rate constant of ∼1.0 × 1010 s−1, which indicates a significantly slower intramolecular proton-transfer dynamics in the anionic micelles than the cationic micelles. For the nonionic micelles, the ESIPT rate constant (kPT ≈ (1.5−3) × 1010 s−1, Table 2) is intermediate between those of the cationic and the anionic micelles, suggesting a moderate retardation of the protontransfer dynamics in these micelles compared to the cationic micelles. The observation of a 7-fold difference in the time scale of ESIPT dynamics between the cationic and anionic micelles or a 3-fold difference between the cationic and nonionic micelles (Table 2) is consistent with the variation of the IN*/IT* ratio,28 reflecting the distinctly different H-bonding environments of the dye in different types of micelles. This may be attributed to a difference in the extent of hydration and charge of the surfactant headgroups of the cationic, anionic, and nonionic micelles, sensed by the FHC dye, because of its possible location in the Stern or palisade layer of the micelle− water interface. For a further clarification of the dye’s location in the micelle−water interface of differently charged micelles, time-resolved anisotropy measurements were carried out. Time-Resolved Anisotropy Decay. The time-resolved anisotropy decays36 for the long-wavelength emission band in micelles were adequately described by a biexponential function (eq 9) as evident from the satisfactory values of the reduced χ2 r(t ) = r0[α1exp( −t /φ1) + α2 exp( −t /φ2)]

1/φ1 = 1/τW + 1/τL + 1/τM

(10)

1/φ2 = 1/τL + 1/τM

(11) 40−42

According to the above-mentioned model, the rotational anisotropy decay function may be expressed as r(t ) = r0[β exp(−t /φ2) + (1 − β )exp(−t /φ1)]

(12)

where β = S2. The generalized order parameter (S) is a measure of the spatial restriction of the dye motion within the micelle and satisfies the condition 0 ≤ S2 ≤ 1. If the motion is isotropic, S = 0, and if it is completely hindered, |S| =1. Moreover, the time constant for the wobbling motion (τW) of the dye may be regarded as a measure of the dynamics of the dye-perturbed local micelle structure,38 which depends on the nature of packing of the surfactant headgroups around the intercalated dye in micelles. Furthermore, the time constants for the lateral diffusion (τL) of FHC in SDS and CTAB micelles were determined from eq 11 and compared to those of three other well-known fluorescent dyes.43 τL values for the FHC dye in the anionic and cationic micelles agree quite well (Table 4) with those of the other fluorescent dyes, thereby confirming the successful treatment of the observed anisotropy data using the two-step wobbling in a cone model for evaluation of the generalized order parameter (S). Probe Location in Micelles. If the FHC molecules were located in the hydrophobic interior of the micelles, their wobbling motion would have been almost isotropic with a very low value of the order parameter (|S| ≈ 0) because the interior of the micelle is known to resemble a liquid hydrocarbon. In contrast, quite high values of the order parameter (|S| ≥ 0.77, Table 3) were observed, which is consistent with the location of

(9)

parameter (Table 3). In eq 9, r0 is the initial anisotropy and αi is the amplitude of the ith rotational correlation time φi such that ∑αi = 1. The biexponential anisotropy decays in micelles do not originate from the distribution37 of the dye molecule between the aqueous and micellar phases because the ratio of the amplitudes (α1/α2) of the fast (φ1) and slow (φ2) correlation times differs by 2−3 orders of magnitude from the ratio ([P]f/ [P]b) of the equilibrium concentrations of the free dye to the micelle-bound dye (Table 3) estimated from the binding constants (Kd) of FHC to the micelles. This implies that the 7153


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Table 3. Time-Resolved Anisotropy Decay Parameters of FHC in Micellesa micelle

φ1, ns

α1

φ2, ns

α2

χ2

S

τW, ns

α1/α2 × 10

[P]f/[P]b × 103

SDS STDS CTAB TTAB DTAB Brij-78 Brij-58 Tween-80 Tween-20

0.414 0.425 0.662 0.592 0.516 0.540 0.506 0.593 0.476

0.38 0.39 0.41 0.41 0.41 0.39 0.40 0.41 0.42

2.10 2.24 3.90 3.20 2.61 4.27 3.70 5.14 2.77

0.62 0.61 0.59 0.59 0.59 0.61 0.60 0.59 0.58

1.25 1.23 1.02 1.30 1.27 1.01 1.23 0.98 1.22

0.79 0.78 0.77 0.77 0.77 0.78 0.77 0.77 0.76

0.51 0.53 0.80 0.73 0.64 0.62 0.59 0.67 0.58

6.13 6.39 5.15 5.15 5.15 5.38 5.62 7.24 7.24

2.14 8.50 4.28 4.52 4.22 1.14 0.96 2.56 1.50

φ1 and φ2 are the fast and slow rotational correlation times, respectively. α1 and α2 are the relative amplitudes. χ2 is the reduced χ2 of the fit parameter to the anisotropy decays. S and τW are the generalized order parameter and the time constant for wobbling motion, respectively. [P]f and [P]b are respective concentrations of free and micelle-bound FHC, calculated at 7 μM dye concentration. The error for all values of φ1 and φ2 is ±10% due to the precision of the measurements. a

Table 4. Rotational Parameters of FHC in Micelles Based on the Two-Step Wobbling in a Cone Modela dye in micelle

φ2, ns

τL, ns

τM, ns

FHC in SDS Nile red in SDS cresyl violet in SDS rhodamine B in SDS FHC in CTAB Nile red in CTAB cresyl violet in CTAB rhodamine B in CTAB

2.10 1.99 2.05 2.33 3.90 3.09 3.83 4.22

2.80 2.62 2.72 3.22 5.23 3.87 5.10 5.81

8.30 8.30 8.30 8.30 15.4 15.4 15.4 15.4

Scheme 1. Mechanism of ESIPT in the Micelle−Water Interface

φ2, τL, and τM are the slow rotational correlation time, time constant for lateral diffusion of the dye, and time constant for the overall rotation of the micelle. φ2 and τL for different fluorescent dyes and τM for the micelles were taken from the work of Periasamy et al.43 a

the FHC dye in the Stern or palisade layer (Figure 1) of the ionic or nonionic micelles, which corresponds to a region of high degree of order in the micelle−water interface as a consequence of the ordered packing of the surfactant headgroups. The much higher degree of order near the micelle−water interface than that of the hydrophobic interior of a micelle was also established by NMR relaxation experiments.44,45 Both (i) the slower ESIPT dynamics of the FHC dye in micelles compared to an aprotic solvent such as acetonitrile (τPT < 10 ps) and (ii) the distinct ESIPT time scale for differently charged micelles are consistent with location of the FHC dye in the Stern or palisade layer of the micelle−water interface. Origin of the Slower ESIPT Dynamics in the Micelle− Water Interface. The ESIPT dynamics in all of the micelles (τPT ≈ 15−100 ps) was considerably slower than that (τPT < 10 ps) in an aprotic solvent such as acetonitrile.20 As the FHC dye is localized in the Stern or palisade layer of the micelle−water interface, and since the Stern or palisade layer of micelles has water molecules associated with the surfactant headgroups, the slower proton-transfer dynamics in micelles relative to an aprotic solvent may quite reasonably be attributed to a weakening of the intramolecular H-bond between the 3hydroxy and the 4-carbonyl group as a consequence of the formation of an intermolecular H-bond (A, Scheme 1) between a water molecule and the 4-carbonyl group of FHC. Since the intramolecular H-bond is a pathway to the ESIPT process, its weakening due to a competitive intermolecular H-bonding interaction with water molecules in the Stern or palisade layer is expected to decrease the ESIPT rate of FHC in micelles, similar

to the proton-transfer dynamics of 3-hydroxyflavone,46,47 FHC,20 and 4-(dialkylamino)-3-hydroxyflavones in protic solvents.48 Hydration and Surfactant Headgroup Sensitive ESIPT Dynamics in Different Micelles. As the slower protontransfer dynamics in micelles was attributed to a weakening of the intramolecular H-bond between the 4-carbonyl and the 3hydroxy moieties, as a consequence of the formation of an intermolecularly H-bonded complex (A, Scheme 1), the weakening of this intramolecular H-bond depends on the strength of the competitive intermolecular H-bonding interaction of the 4-carbonyl group of FHC with water molecules in the Stern or palisade layer of the micelle−water interface. In addition to water molecules, the surfactant headgroups may also be engaged in specific hydrogen-bonding interaction with the 4-carbonyl group of FHC (B, Scheme 1), which likely competes with the intramolecular H-bond, thereby weakening it. The strength of the intermolecular H-bonding interactions in the micelle−water interface depends on several factors, viz., (i) the extent of hydration of the charged or polar headgroups in the Stern or palisade layer of differently charged micelles and (ii) specific interactions of the surfactant headgroups with the 4-carbonyl group of the FHC dye. A strong hydration of the surfactant headgroups or a strong H-bonding interaction by the 7154


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molecule to be ∼14, in very close agreement with a value of ∼10 determined earlier by the small-angle neutron scattering technique.52 On the other hand, Bales et al.49,50 determined values of ∼4.5 and ∼9.0 for the number of water molecules per surfactant molecule in the polar shell of DTAB and SDS micelles, respectively, using the EPR technique. Moreover, a number of studies17,53−55 have shown that the negatively charged headgroups as well as the first one or two carbons of the alkyl chains connected to the anionic surfactant headgroups have significant amounts of water associated with them. Thus, the Stern layer of these anionic micelles becomes strongly hydrated,17 resulting in a strongly hydrophilic surface of the SDS micelles with adsorbed counterions, generally separated by a layer of water molecules.53 Stronger hydration of these anionic micelles than the cationic micelles entails a significant weakening of the intramolecular H-bond in FHC, thereby leading to a significantly slower ESIPT dynamics relative to that of the cationic micelles of CTAB and TTAB. (2) However, the significant retardation of the protontransfer dynamics of FHC in the anionic micelles relative to the cationic micelles may not be solely attributed to a stronger hydration of the former relative to the latter. Although stronger hydration of the anionic micelles relative to the cationic micelles is corroborated well from the recent works on micellar hydration by dielectric spectroscopy,17,18 EPR,49−51 and smallangle neutron scattering (SANS),52 the measurements of water activity at the micelle surface by Romsted et al.56,57 using the diazonium salt capture method or NMR measurements of the water structure at the micelle surface by El Seoud et al.58,59 have demonstrated a marginal difference in the local water activity (concentration) and structure at the micelle−water interface of the cationic and anionic micelles. These apparent contradictions of the evidence on the micellar hydration led us to propose that, in addition to the micellar hydration, the charge of the surfactant headgroups or the net micellar charge might have played a key role60 in the observed difference of the ESIPT dynamics between the cationic and anionic micelles. The oxygen atom of the 4-carbonyl group in FHC possesses a significant negative charge in the excited state (Figure 1), which is likely augmented through a possible conjugation of the furan ring with the chromone ring, while in the T* state this negative charge is compensated by the transferred proton. This is supported well by our theoretical calculations,21 showing a charge distribution of −0.84 on the oxygen atom of the 4carbonyl group in ethanol and methanol, which closely mimic the Stern layer of the micelle−water interface with respect to polarity and hydrogen bond donating ability. The negative charge on the carbonyl oxygen makes it susceptible to a strong electrostatic attraction with the positively charged −N(CH3)3+ headgroups of the cationic micelles, which likely provides an effective screening of the 4-carbonyl group from the intermolecular H-bonding perturbation of the water molecules (A, Scheme 1) in the headgroup region of the cationic micelles. The strong electrostatic attraction of the positively charged −N(CH3)3+ headgroups likely localizes the FHC dye in close proximity to the headgroup region of the Stern layer, where the approach of water molecules may be inhibited by the hydrophobic methyl groups of these positively charged headgroups. As a consequence, the intramolecular H-bond in FHC is likely to be least perturbed by the intermolecular Hbonding interaction of the water molecules in the Stern layer of these cationic micelles with the manifestation of a fast ESIPT dynamics. On the contrary, the screening of the 4-carbonyl

surfactant headgroups is likely to be accompanied by a strong intermolecular H-bonding interaction either between the 4carbonyl group of FHC and a water molecule (A, Scheme 1) or between the 4-carbonyl group and a surfactant headgroup (B, Scheme 1), which entails a significant weakening of the intramolecular H-bond in FHC. On the other hand, a weak hydration of the surfactant headgroups, or a weak specific interaction of the 4-carbonyl group with a surfactant headgroup, entails a weak intermolecular H-bonding interaction, thereby leading to a mild weakening of the intramolecular Hbond. As a consequence, stronger hydration of the micelles or stronger specific interactions of the 4-carbonyl group of FHC with the surfactant headgroups should have led to the slower ESIPT dynamics and vice versa. This expectation was indeed borne out nicely by the manifestation of a hydration and surfactant headgroup sensitive ESIPT dynamics in (A) ionic and (B) nonionic micelles. ESIPT Dynamics in the Ionic Micelles: Cationic vs Anionic Micelles. The intramolecular proton-transfer process of FHC is fastest (τPT ≈15 ps, kPT ≈ 6.3−7.1 × 1010 s−1) in the cationic CTAB and TTAB micelles, relative to its slowest dynamics (τPT ≈ 100 ps, kPT ≈ 1.0 × 1010 s−1) in the anionic SDS and STDS micelles (Tables 1 and 2). This significant difference in the time scale of the proton-transfer dynamics between the cationic and anionic micelles may be attributed to (1) different extents of hydration of the cationic and anionic micelles and (2) different charges of the surfactant headgroups or different net micellar charges of the Stern layer. (1) The Stern layer of the cationic micelles comprises positively charged −N(CH3)3+ headgroups and a few Br− counterions and is weakly or poorly solvated relative to that of the anionic SDS or STDS micelles. The region of these positively charged −N(CH3)3+ headgroups in the Stern layer of the cationic micelles is weakly solvated with water molecules because the intervening methyl groups likely inhibit the approach of water molecules close to the positively charged headgroups.16 This is nicely corroborated from recent results of Kunz et al.17 on micellar hydration by dielectric relaxation spectroscopy, which have shown the surface of the cationic alkyltrimethylammonium halide micelles to be essentially hydrophobic, implying a very poor hydration of the positively charged headgroups. Furthermore, Zana et al.49 and Bales et al.50,51 recently employed electron paramagnetic resonance (EPR) studies and a zeroth-order model to show that the cationic micelles of DTAB are significantly drier than the anionic SDS micelles. Therefore, the intramolecular H-bond of FHC is supposed to be weakly perturbed through an intermolecular H-bonding interaction with water molecules (A, Scheme 1) associated with the positively charged headgroups in the Stern layer of the cationic micelles, leading to a fast proton-transfer dynamics. On the other hand, the Stern layer of the anionic SDS or STDS micelles is more strongly hydrated17,18 than the cationic micelles, which may be attributed to a strong hydration of their negatively charged −OSO3¯ headgroups in the Stern layer of these anionic micelles.15,16 The negative charge of the −OSO3¯ headgroup is evenly distributed over its oxygen atoms through resonance, which causes them to bind a larger number of water molecules through an ion−dipole and intermolecular H-bonding interaction than the positively charged −N(CH3)3+ headgroups of the cationic micelles. This was further corroborated recently by Gambi et al.,18 who used dielectric spectroscopy to determine the number of bound water molecules per SDS surfactant 7155


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around the dye in DTAB micelles, compared to a tighter packing of the same cationic headgroups in CTAB micelles, owing to the more compact packing of the longer cetyl chains. This was also corroborated from the lower time constant for the wobbling motion (τW ≈ 0.64 ns) of the dye in DTAB micelles relative to its higher value (τW ≈ 0.8 ns) in CTAB micelles. As a consequence, there is an increased penetration of water molecules in the Stern layer of the DTAB micelles compared to the CTAB micelles, leading to an increased hydration of the former. Thus, the proton-transfer dynamics of FHC is retarded in the Stern layer of the DTAB micelles relative to the CTAB micelles. Unlike the cationic micelles, the effect of a variation of the surfactant chain length on the ESIPT dynamics in the anionic micelles is much less pronounced, and the ESIPT rate constant (kPT) decreases marginally by ∼10% in going from the longer chain STDS micelles to the shorter chain SDS micelles (Table 2), suggesting only a mild increase of the Stern layer hydration in the shorter chain SDS micelles relative to the longer chain STDS micelles. This is further evident from similar τW values (τWSDS = 0.51 ns, τWSTDS = 0.53 ns) in these micelles, indicating a similar kind of packing of the tetradecyl and dodecyl surfactant chains, which possibly gives rise to a similar extent of water penetration up to the first one or two carbon atoms along the surfactant chain near the negatively charged headgroup. ESIPT Dynamics in the Nonionic Micelles. As for the nonionic micelles, the proton-transfer process is moderately retarded in comparison to that of the cationic micelles and occurred with a rate constant (kPT ≈ (1.5−3) × 1010 s−1, Table 2) intermediate between those of the cationic and anionic micelles. The palisade layer of the nonionic micelles, where the dye is located, comprises PEO chains as the hydrophilic moiety. These PEO chains terminate into single or multiple −OH groups (Figure 5), which along with the oxyethylene ether linkages bind61 more water molecules through intermolecular H-bonding interactions in the palisade layer than the −N(CH3)3+ headgroups in the Stern layer of the cationic micelles. As a consequence, the FHC molecule becomes accessible to a greater number of water molecules in the palisade layer of the nonionic micelles than the Stern layer of the cationic micelles, leading to a moderate perturbation of the intramolecular H-bond, and hence, a moderate retardation in the ESIPT dynamics relative to that of the cationic micelles is observed. On the other hand, hydration of the negatively charged −OSO3¯ headgroups in the Stern layer of the anionic micelles is much stronger than hydration of the neutral −OH groups and poly(ethylene oxide) chains in the palisade layer of the nonionic micelles due to the strong ion−solvent (water) interactions. In addition, the nonionic poly(ethylene oxide) surfactants appear to be more tightly packed around the FHC dye than the negatively charged surfactants, as evident from their higher values of τW ≈ 0.6−0.8 ns (Table 3) relative to those of the anionic micelles, which causes a lesser extent of water penetration in the palisade layer of the nonionic micelles than the Stern layer of the anionic micelles. As a consequence, weaker hydration of the nonionic micelles than the anionic micelles weakens the intramolecular H-bond of FHC to a lesser extent than that in the anionic micelles, and thus, a faster ESIPT dynamics in the nonionic micelles than the anionic micelles is demonstrated. Moreover, a difference of the ESIPT time constant had been noted (Table 2) between the Brij (τPT ≈ 0.03−0.04 ns) and the Tween (τPT ≈ 0.05−0.06 ns) micelles. This difference in the

group from the intermolecular H-bonding interactions of water molecules is likely to be absent in the Stern layer of the anionic micelles, owing to a strong electrostatic repulsion between their negatively charged −OSO3¯ headgroups and the negatively charged carbonyl oxygen atom of the 4-carbonyl group. FHC may protrude slightly out of the negatively charged headgroup region and into the water-rich region of the Stern layer, thereby reducing its electrostatic repulsions with the negatively charged −OSO3¯ headgroups and exposing it to the intermolecular Hbonding perturbation of the water molecules. As a result, the intramolecular H-bond in FHC is significantly weakened through an intermolecular H-bonding interaction of the 4carbonyl group with water molecules in the Stern layer of these anionic micelles, followed by the demonstration of a significantly retarded ESIPT dynamics compared to that of the positively charged cationic micelles. The possible contribution of the charge on the surfactant headgroups, in electrostatic interactions between FHC and ionic micelles, becomes clearly evident in the distinctly different τW values between the anionic and cationic micelles because the time constant for the wobbling motion (τW) may be regarded as the time constant for the relaxation of a dye-perturbed local micelle structure due to intermolecular interactions between the dye and its local environment in the micelle−water interface.38 τW for the FHC dye increases considerably from ∼0.5 ns in the anionic SDS and STDS micelles to ∼0.8 ns in the cationic CTAB and TTAB micelles (Table 3) because electrostatic attraction between the positively charged −N(CH3)3+ headgroups and the negatively charged 4-carbonyl moiety causes the motions of the FHC dye to be more strongly coupled to the dynamics of the cationic micelles, whereas in SDS and STDS micelles, the electrostatic repulsion between the negatively charged −OSO3¯ headgroups and the 4-carbonyl moiety reduces the coupling of the dye motions to the dynamics of the anionic micelles. Effect of the Surfactant Chain Length on ESIPT Dynamics in Ionic Micelles. In addition, the ESIPT rate of FHC was found to decrease by nearly 55% (Table 2) in going from the longer chain cationic micelles of CTAB [CH3(CH2)15N+(CH3)3Br−] to the shorter chain cationic micelles of DTAB [CH3(CH2)11N+(CH3)3Br−]. Since both CTAB and DTAB possess identical positively charged −N(CH3)3+ headgroups, the difference in the ESIPT rate is likely to originate from a difference of the surfactant chain length (Figure 5). The loose packing of the shorter length dodecyl chains leads to a loose packing of the cationic headgroups

Figure 5. Cationic, anionic, and nonionic micelles. 7156


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proton-transfer kinetics may be attributed to (i) the higher hydrophilicity and lower hydrophobicity of the Tween micelles, especially the Tween-20 micelle, compared to the Brij micelles, and (ii) the presence of multiple terminal −OH groups (Figure 5) in the palisade layer of the Tween micelles compared to the single terminal −OH group of the Brij micelles. The hydrophile−lipophile balance (HLB) parameter for the Tween-20 micelle is higher (∼16.7) compared to that (∼15.3−15.7) for the Brij micelles (Table 2). The HLB value describes the relative ratio of the hydrophilic to the hydrophobic portions within a surfactant molecule,62 and measured on a scale of 0−20, higher HLB values correspond to the surfactants possessing more hydrophilic character. Therefore, the intramolecular H-bond in FHC is likely to be perturbed to a greater extent through a stronger hydration of these Tween micelles, especially the Tween-20 micelle in comparison to the Brij-micelles, due to the lower hydrophobicity or higher hydrophilicity of the Tween micelles than the Brij micelles. As a consequence, the ESIPT kinetics is retarded to a greater extent in the strongly hydrophilic Tween20 micelle compared to the more hydrophobic Brij micelles. However, it is also noted that although the HLB values of Brij78 and Brij-58 micelles are similar to that of the Tween-80 micelle (Table 2), the ESIPT kinetics is faster in the former than the latter because of the presence of multiple terminal −OH groups in the Tween-80 micelles compared to only one terminal −OH group in the Brij micelles. In addition to the water molecules, these terminal −OH groups of the Tween-80 micelles may also participate in the formation of an intermolecular H-bond with the 4-carbonyl group of FHC (B, Scheme 1) in the palisade layer of the micelle−water interface, thereby weakening the intramolecular H-bond between the 3-hydroxy and 4-carbonyl groups, which leads to a decrease of the ESIPT rate relative to that of the Brij micelles, for which this kind of additional perturbation of the intramolecular H-bond is likely to be less owing to the presence of only one terminal −OH group. Effect of the Surfactant Chain Length on ESIPT Dynamics in Nonionic Micelles. The effect of a variation of the surfactant chain length (Figure 5) on the proton-transfer dynamics was slightly more pronounced in the nonionic than the anionic micelles since the ESIPT rate decreases by ∼20% in going from the Brij-78 to Brij-58 or Tween-80 to Tween-20 micelles. A decrease of τW (Table 3) in going from the Brij-78 or Tween-80 micelles to the Brij-58 or Tween-20 micelles, respectively, indicates a relatively looser packing of the poly(ethylene oxide) headgroups in the Brij-58 or Tween-20 micelles than the Brij78 or Tween-80 micelles. This leads to an increased hydration of the palisade layer of the Brij-58 or Tween-20 micelles relative to their longer chain counterparts, followed by a moderate retardation of the ESIPT dynamics in going from the nonionic micelles of Brij-78 to Brij-58 or Tween-80 to Tween-20.

(i) Hydration of the Stern layer of the cationic micelles is weakest because hydrophobic methyl groups of the positively charged −N(CH3)3+ headgroups inhibit the approach of water molecules close to the headgroup region. As a consequence, the intramolecular H-bond of FHC is only weakly perturbed through an intermolecular H-bonding interaction with water molecules of the Stern layer, which leads to a fast proton-transfer dynamics. As for the anionic micelles, hydration of their Stern layer is strongest owing to a strong hydration of their negatively charged −OSO3¯ headgroups, which causes a significant retardation of the proton-transfer dynamics in the anionic micelles relative to the weakly hydrated cationic micelles. (ii) In addition to the hydration effect, the charge of the surfactant headgroups plays a key role in modulating the proton-transfer dynamics in the ionic micelles. A strong electrostatic attraction between the negatively charged 4carbonyl moiety of FHC and the positively charged −N(CH3)3+ headgroups effectively screens the intramolecular H-bond from the intermolecular H-bonding perturbation of the water molecules in the Stern layer of the cationic micelles, thereby leading to a fast ESIPT dynamics. On the contrary, a strong electrostatic repulsion between the negatively charged 4-carbonyl moiety and the negatively charged −OSO3¯ headgroups of the anionic micelles exposes the intramolecular Hbond of the FHC dye to a strong intermolecular Hbonding perturbation of the water molecules in the micelle−water interface and leads to the occurrence of a significantly slower ESIPT dynamics than that of the cationic micelles. (iii) The palisade layer of the nonionic micelles comprises poly(ethylene oxide) chains as the hydrophilic moieties, which can bind more water molecules than the positively charged −N(CH3)3+ headgroups of the cationic micelles, but less water molecules than the negatively charged −OSO3¯ headgroups of the anionic micelles. Thus, the nonionic micelles are hydrated to a greater extent than the cationic micelles, but to a lesser extent than the anionic micelles. Therefore, the ESIPT dynamics in the nonionic micelles is slower than that in the poorly hydrated cationic micelles, but faster than that in the strongly hydrated anionic micelles. Additionally, the ESIPT kinetics of FHC in the Brij micelles was found to be faster relative to that of the Tween micelles. The slower proton-transfer dynamics in the Tween micelles is attributed to their lower hydrophobicity and higher hydrophilicity than the Brij micelles and multiple terminal −OH groups per surfactant molecule for these micelles. (iv) Finally, the ESIPT rate was found to decrease with a decrease in the surfactant chain length for all types of micelles. Whereas the effect of a variation of the surfactant chain length on the ESIPT dynamics was most and least pronounced for the cationic and anionic micelles, respectively, it is intermediate for the nonionic micelles. The decrease of the ESIPT rate in the shorter chain micelles compared to the longer chain micelles may be attributed to an increased hydration of the former micelles, owing to a looser packing of the shorter chain surfactant molecules than the longer chain surfactants.

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CONCLUSIONS The ESIPT dynamics of the FHC dye is significantly sensitive to the hydration of the micelles and the charge of the surfactant headgroups, with an inverse correlation of its kinetic constant (kPT) to the micellar hydration. Whereas the ESIPT is fastest in the Stern layer of the poorly hydrated CTAB and TTAB cationic micelles, it is slower in the palisade layer for the moderately hydrated neutral micelles and slowest in the Stern layer of the strongly hydrated anionic micelles of SDS and STDS. 7157


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(11) Basu, S.; Mondal, S.; Mandal, D. Proton transfer reactions in nanoscopic polar domains: 3-Hydroxyflavone in AOT reverse micelles. J. Chem. Phys. 2010, 132, 034701−034705. (12) Adhikary, R.; Carlson, P. J.; Kee, T. W.; Petrich, J. W. Excitedstate intramolecular hydrogen atom transfer of curcumin in surfactant micelles. J. Phys. Chem. B 2010, 114, 2997−3004. (13) Mondal, S.; Basu, S.; Mandal, D. Ground- and excited-state proton-transfer reaction of 3-hydroxyflavone in aqueous micelles. Chem. Phys. Lett. 2009, 479, 218−223. (14) (a) Mukherjee, P.; Cardinal, J. R. Benzene derivatives and naphthalene solubilized in micelles. Polarity of microenvironments, location and distribution in micelles, and correlation with surface activity in hydrocarbon−water systems. J. Phys. Chem. 1978, 82, 1620− 1627. (b) Ganesh, K. N.; Mitra, P.; Balasubramanian, D. Solubilization sites of aromatic optical probes in micelles. J. Phys. Chem. 1982, 86, 4291−4293. (15) Vitha, M. F.; Carr, P. W. Study of the polarity and hydrogenbond ability of dodecyltrimethylammonium bromide micelles by the Kamlet−Taft solvatochromic comparison method. J. Phys. Chem. B 1998, 102, 1888−1895. (16) Shannigrahi, M.; Bagchi, S. Use of fluorescence probes for characterization of solvation properties of micelles: A linear solvation energy relationship study. J. Phys. Chem. B 2004, 108, 17703−17708. (17) Buchner, R.; Baar, C.; Fernandez, P.; Schrödle, S.; Kunz, W. Dielectric spectroscopy of micelle hydration and dynamics in aqueous ionic surfactant solutions. J. Mol. Liq. 2005, 118, 179−187. (18) Lanzi, L.; Carla, M.; Lanzi, L.; Gambi, C. M. C. A new insight on the dynamics of sodium dodecyl sulfate aqueous micellar solutions by dielectric spectroscopy. J. Colloid Interface Sci. 2009, 330, 156−162. (19) Asami, K. Dielectric properties of water in Triton X-100 (nonionic detergent)−water mixtures. J. Phys.: Condens. Matter 2007, 19, 376102−376106. (20) Das, R.; Klymchenko, A. S.; Duportail, G.; Mely, Y. Unusually slow proton transfer dynamics of a 3-hydroxychromone dye in protic solvents. Photochem. Photobiol. Sci. 2009, 8, 1583−1589. (21) Table SI.2 in the Supporting Information. (22) Shvadchak, V. V.; Klymchenko, A. S.; de Rocquigny, H; Mely, Y. Sensing peptide−oligonucleotide interactions by a two-color fluorescence label: Application to the HIV-1 nucleocapsid protein. Nucleic Acids Res. 2009, 37, 1−12. (23) Ormson, S. M.; Brown, R. G.; Vollmer, F.; Rettig, W. Switching between charge- and proton-transfer emission in the excited state of a substituted 3-hydroxyflavone. J. Photochem. Photobiol., A 1994, 81, 65− 72. (24) Yushchenko, D.A..; Shvadchak, V. V.; Bilokin, M. D.; Klymchenko, A. S.; Duportail, G.; Mely, Y.; Pivovarenko, A. G. Modulation of dual fluorescence in a 3-hydroxyquinolone dye by perturbation of its intramolecular proton transfer with solvent polarity and basicity. Photochem. Photobiol. Sci. 2006, 5, 1038−1044. (25) Siano, D. B.; Metzler, D. E. Band shapes of the electronic spectra of complex molecules. J. Chem. Phys. 1969, 51, 1856−1861. (26) Godet, J.; Ramalanjaona, N.; Sharma, K. K.; Richert, L.; de Rocquigny, H.; Darlix, J. L.; Duportail, G.; Mely, Y. Specific implications of the HIV-1 nucleocapsid zinc fingers in the annealing of the primer binding site complementary sequences during the obligatory plus strand transfer. Nucleic Acids Res. 2011, 39, 6633−6645. (27) Goel, T.; Mukherjee, T.; Rao, B. J.; Krishnamoorthy, G. J. Fluorescence dynamics of double- and single-stranded DNA bound to histone and micellar surfaces. J. Phys. Chem. B 2010, 114, 8986−8993. (28) Table SI.1 in the Supporting Information. (29) Klymchenko, A. S.; Demchenko, A. P. 3-Hydroxychromone dyes exhibiting excited-state intramolecular proton transfer in water with efficient two-band fluorescence. New J. Chem. 2004, 28, 687−692. (30) Sengupta, P. K.; Kasha, M. Excited state proton-transfer spectroscopy of 3-hytdroxyflavone and quercetin. Chem. Phys. Lett. 1979, 68, 382−385. (31) McMorrow, D.; Kasha, M. Intramolecular excited-state proton transfer in 3-hydroxyflavone. Hydrogen-bonding solvent perturbations. J. Phys. Chem. 1984, 88, 2235−2243.

ASSOCIATED CONTENT

S Supporting Information *

Tables listing the photophysical characteristics of 2-(2′-furyl)-3hydroxychromone in micelles and solvents and dipole moments and charge distributions on the carbonyl oxygen and hydroxyl oxygen in the ground state and first excited state of the normal and ESIPT tautomer forms of FHC and figures showing the time-resolved fluorescence decay of FHC in cationic, anionic, and nonionic micelles and time-resolved anisotropy decay of FHC in CTAB, SDS, and Tween-80 micelles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.D. acknowledges the Université de Strasbourg for nominating him to the position of Invited Professor in the Laboratoire de Biophotonique et Pharmacologie. He also acknowledges his colleagues Dr. Mukut Chakraborty, Dr. Suman Biswas, and Dr. Chirantan Roy Choudhury of West Bengal State University for their kind support.

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