Modulated photophysics and rotational-relaxation dynamics of coumarin 153 in nonionic micelles: the role of headgroup size and tail length of the surfactants

Abstract

Here, the effect of variation in the headgroup size and the alkyl tail length of nonionic surfactants on the photophysics and rotational-relaxation dynamics of the laser dye coumarin 153 (C153) has been explored. The variation in the headgroup size, that is, poly(ethylene oxide) (PEO) chain length of the nonionic micellar systems, has been implemented using a variety of surfactants belonging to Triton X family, whereas surfactants from the Tween family provides an access to the variation of the tail length (alkyl chain) of the surfactants. Our spectroscopic study reveals a remarkable influence on the photophysics of C153 upon binding with the micelles. The strength of the dye-micelle binding interaction is found to be largely controlled by the variation of the headgroup size and the length of the tail of the surfactants. To this end, the dye-micelle binding interaction has been quantitatively assayed from emission studies and carefully interpreted based on the micellar hydration model. The estimation of the micropolarity of the dye binding site is further employed to corroborate this argument. Furthermore, all these line of arguments are effectively substantiated from time-resolved fluorescence experiments with particular emphasis on the modulation of rotational dynamics of the dye within the micellar micro-heterogeneous environments.

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1. Introduction

Over the past few years, self-organization of various types of molecular assemblies have attracted immense attention in widespread areas of research within chemistry and biology.^1–3 The broad-spectrum of applications of various organized assemblies, e.g., micelles, cyclodextrins, and vesicles, has provided the impetus for the notable thrust of research surrounding them.^1–11 Amphiphilic surfactant molecules self-aggregate to form micellar assemblies, which have been central to extensive research activities for years, particularly in the context of ‘host–guest’ chemistry in which a self-organized molecular assembly, the host, can accommodate a suitable guest without forming covalent bonds. This intriguing characteristic of ‘host–guest’ complexation has triggered potential research prospects including applications in targeted drug delivery, development of energy storage devices, miniature model systems for mimicking the mode of action of enzymes, and simpler model system for mimicking the complex biological membranes.^1–12

The present work is focused on studying the differential interactions of an environment-sensitive molecular probe, namely, coumarin 153 (C153), with a series of nonionic micellar systems with varying poly(ethylene oxide) (PEO) chain lengths (within the Triton X series of nonionic surfactants) and alkyl chain lengths (within the Tween series of nonionic surfactants). The chemical structures of these molecules are illustrated in Scheme 1 and a simplified paradigm depicting the structures of the micellar units is presented in the ESI, Scheme S1.†¹¹ The variation in the PEO/alkyl chain length of the surfactant molecules is expected to influence the compactness of the headgroup arrangement of the micellar units,^13–15 and thus the degree of water penetration to the palisade layer of the micelles. In the present study, we have used C153 as a molecular reporter, which binds to the palisade layer of the studied micellar systems, and we monitored the spectral changes of the dye due to change in PEO chain length and alkyl chain length. C153 is known to exhibit intramolecular charge transfer (ICT) characteristics as reflected through its remarkable sensitivity of spectral properties to solvent polarity,¹⁶ which has subsequently paved the way for extensive application of C153 as a molecular reporter for a host of biological, biomimicking, and supramolecular assemblies.^16–19 Although the structural aspects, namely, size, shape, and hydration of nonionic micelles as a function of the PEO and alkyl chain length, have been extensively addressed in the literature,^13,14,20,21 the lacuna surrounding the water–micelle interactions are far from being completely resolved. The main objective of the present study involves investigation on the role of the headgroup size and alkyl tail length in controlling the interaction of the environment-sensitive probe molecule C153 with these nonionic micellar systems.

Scheme 1 Chemical Structures of (a) coumarin 153 (C153), (b) Tween 20 and (c) Triton X surfactants.

2. Experimental section

2.1. Materials

Coumarin 153 (C153, Scheme 1) and the surfactants from Triton X (TX) series (namely, TX165, TX100, and TX114), and Tween (TW) series (namely, TW20, TW40, and TW60) were used as received from Sigma-Aldrich, USA. Tris buffer was purchased from Sigma-Aldrich, USA, and 0.01 M Tris–HCl buffer of pH 7.4 was prepared using deionized triple distilled Milli pore water (Milli-Q water purification system).

2.2. Instrumentations and methods

Steady-state spectral measurements. The absorption and fluorescence emission spectra were acquired on a Cary 100 UV-vis spectrophotometer and a Fluorolog 3-111 fluorometer, respectively. All spectroscopic measurements were performed with freshly prepared solutions containing a low concentration of the dye (∼2.0 μM) to avoid inner-filter effects. Appropriate spectral background corrections were done with a similar set of solutions omitting the dye.

Steady-state fluorescence anisotropy. Steady-state fluorescence anisotropy (r) measurements were carried out on a Fluorolog 3-111 fluorometer. The fluorescence anisotropy (r) is defined as follows:²²where, I_VV and I_VH denote emission intensities collected at vertical and horizontal emission polarizations, respectively, following excitation through a vertically polarized light. The correction factor for detector sensitivity of the instrument (G) is defined through eqn (2).

Time-resolved measurements. Fluorescence lifetimes were obtained by the method of time-correlated single photon counting (TCSPC) following excitation of the samples at λ_ex = 405 nm (IBH-NanoLED-405L) using a picosecond diode, and the signals were collected using a Hamamatsu MCP Photomultiplier (Model R-3809U-50) at the magic angle polarization of 54.7° to eliminate any considerable contribution from fluorescence depolarization.^22,23 The decays were deconvoluted using DAS-6 decay analysis software. The average lifetime 〈τ_f〉 was calculated using the following equation:^22,23where, α_i represents the normalized amplitude of the i^th decay time constant, τ_i.^22,23

For time-resolved fluorescence anisotropy decay measurements, the polarized fluorescence decays for parallel [I_∥(t)] and perpendicular [I_⊥(t)] emission polarizations were collected following excitation by vertically polarized light. The anisotropy decay function, r(t), was then constructed according to the following relationship:^22,23

The correction factor, G, was determined from the long time tail matching technique.²³

3. Results and discussion

3.1. Steady-state spectral study

In aqueous buffer medium, C153 shows a broad, structureless absorption band at λ_abs ∼ 425 nm. In all the studied micellar systems, the absorption profile of C153 is found to be slightly blue-shifted with slight enhancement of maximum absorbance, as displayed in Fig. 1, for the representative cases of Tween 20 and Triton X 165. This indicates that C153 experiences a reduced polarity in the ground-state within the micellar environment in comparison to that in an aqueous buffer medium.¹⁶

Fig. 1 Examples of absorption profiles of C153 with added (a) TW20 and (b) TX165. Curves (i) → (v) represent 0, 6, 16, 20, and 30 μM TW20 in (a), and curves (i) → (vi) represent 0, 1, 1.5, 2, 2.5, and 3.5 mM TX165 in (b).

The change in interaction of C153 with the nonionic micellar systems is more dramatically reflected on the medium polarity-sensitive emission profile of the dye. As displayed in Fig. 2 and 3, the emission profile of C153 undergoes remarkable modification on the addition of the nonionic surfactants in terms of a large blue-shift (λ_em ∼ 548 nm in aqueous buffer, ∼536 nm in TW20, ∼535 nm in TW40, ∼533 nm in TW60, and ∼525 nm in TX114, ∼527 nm in TX100, ∼535 nm in TX165) coupled with a significant increase in emission intensity in all the studied micellar assemblies. This blue-shift clearly indicates that the dye molecule experiences a reduced polarity within the micellar microheterogeneous environments compared to the bulk aqueous buffer phase. The enhancement of fluorescence intensity probably suggests the impartation of motional restriction on the dye molecules, leading to the reduction of nonradiative decay rates, and thus a consequent increase of radiative decay channel, which is manifested through the enhanced fluorescence yield. This argument is reinforced in the subsequent section. The narrowing of the emission profile of C153 (Fig. 2 and 3) within the micelle-bound state as compared to that in aqueous buffer solution further indicates the changes in the C153-micelle interaction.

Fig. 2 Examples of emission profiles of C153 with added (a) TW20, (b) TW40, and (c) TW60 in an aqueous buffer medium. Curves (i) → (xvii) represent 0, 2, 4, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50, 54, and 60 μM TW20 in (a), curves (i) → (xvii) represent 0, 1, 2, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, and 30 μM TW40 in (b), and curves (i) → (xvii) represent 0, 1, 2, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, and 30 μM TW60 in (c).

Fig. 3 Examples of emission profiles of C153 with added (a) TX165, (b) TX100, and (c) TX114 in aqueous buffer medium. Curves (i) → (xi) represent 0, 1, 1.5, 2, 2.5, 3, 3.5, 4, 6, 8, 10 mM TX165 in (a), curves (i) → (xi) represent 0, 0.25, 0.31, 0.38, 0.44, 0.5, 0.56, 0.63, 0.69, 0.75, 0.81 mM TX100 in (b), and curves (i) → (xii) represent 0, 0.25, 0.31, 0.38, 0.44, 0.5, 0.56, 0.63, 0.75, 0.88, 1, 1.63 mM TX114 in (c).

3.2. Assessing the probable location of C153 within the nonionic micelles using micropolarity study

The micropolarity of the immediate vicinity of C153 inside the micelle-encapsulated state can be assayed by comparing the spectroscopic properties of C153 in this environment with those in bulk solvents or solvent mixture of known polarity. Although such a comparative method does not claim to yield a precise determination of the polarity of the given microheterogeneous environment in relation to that of a bulk homogeneous fluid, the results are, in general, reliable.^1,24–26 Thus, the spectroscopic properties of C153 was monitored in a solvent mixture of water–1,4-dioxane, which has known polarity values depending on the relative compositions (Fig. S1 in the ESI†). The polarity equivalent index, E_T(30), for a water–1,4-dioxane mixture is obtained from the literature.²⁶ A calibration curve was then constructed by observing the variation of the emission wavelength of C153 against the polarity index, E_T(30) (Fig. S1 in ESI†). Then, the polarity of the dye microenvironment within the micellar assemblies was determined by the direct interpolation of the emission wavelength value of micelle-bound C153 on the calibration curve, Fig. 4 (a detailed description of the protocol is mentioned in the ESI†). The as-obtained micropolarity values in various micellar environments are shown in Table 1.

Fig. 4 Plot of the variation of λ_em (nm) of C153 with polarity equivalent parameter E_T(30) (kcal mol⁻¹) in different compositions of a water–1,4-dioxane reference solvent mixture. The polarity of the dye binding site in micellar environments is indicated on the calibration curve as specified in the figure legends for Tween series in panel a (TW20: , TW40: , and TW60: ) and Triton X series in panel b (TX114: , TX100: , and TX165: ). The goodness of the linear fit is judged by the correlation coefficient, r² = 0.99.

Table 1 Binding constant (K), and free energy change (ΔG) for C153-micelle binding interaction. The micropolarity values on E_T(30) scale at the dye binding sites within the nonionic micellar environments, CMCs, and the hydrodynamic radii (r_h) of the micellar systems are also indicated

System	K	ΔG (kJ mol^−1)	ET(30) (kcal mol^−1)	CMC	rh (nm)
Tween 20	(9.7 ± 1.1) × 10^5	−33.6 ± 0.25	55.5	12 μM	4.8
Tween 40	(5.5 ± 1.3) × 10^6	−37.8 ± 0.51	55.0	11 μM	5.0
Tween 60	(6.4 ± 1.4) × 10^6	−38.2 ± 0.48	54.5	10 μM	8.4
Triton X 165	(1.4 ± 0.5) × 10^5	−28.8 ± 0.76	55.1	0.25 mM	3.8
Triton X 100	(4.1 ± 1.1) × 10^5	−30.8 ± 0.68	51.2	0.10 mM	4.1
Triton X 114	(10.5 ± 1.5) × 10^5	−33.8 ± 0.32	50.2	0.14 mM	4.5

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In general, the micropolarity of C153 binding sites within the micellar assemblies was found to be considerably lower than that of the aqueous buffer phase (E_T(30) = 63.1 kcal mol⁻¹),²⁶ though it had striking similarity to that of methanol (E_T(30) = 55.5 kcal mol⁻¹).²⁶ This in turn appears to conform to a probable location of the dye in the palisade layer of the micellar systems.^28,29 Within the Tween series, the dye microenvironment was found to be most polar in Tween 20, least polar in Tween 60, and Tween 40 being in between; thus, the polarity of the dye microenvironment decreases with increasing alkyl chain length of the Tween series micelles. However, a reverse trend is noted within the Triton X series, that is, the polarity of the dye microenvironment increases with PEO chain length, a maximum in Triton X165 and minimum in Triton X114. The change in the polarity of the dye microenvironment thus occurs in the order: TW60 < TW40 < TW20 within the Tween series, and TX114 < TX100 < TX165 within the Triton X series. This observation can be rationalized using the water penetration model for micellar structures. The extent of water penetration into micellar units is largely controlled by the compactness of the headgroup arrangement; the less compact headgroup being more vulnerable to water penetration. Increasing the degree of micellar compactness, and thus decreasing the degree of water penetration with an increase in alkyl chain length, thus justifies the aforementioned trend of polarity within the Tween series micelles. However, with an increase in PEO chain length, the micelle undergoes a greater extent of water penetration (reduced headgroup compactness), which accounts for the aforementioned trend of polarity along the Triton X series.^30,31 The results of this micropolarity estimation around the microenvironment of the micelle-bound dye were found to be strongly substantiated using fluorescence anisotropy measurements, as discussed in the subsequent section.

Furthermore, it is pertinent here to justify the use of a water–1,4-dioxane mixture as the reference solvent mixture for the micropolarity determination instead of a water–alcohol mixture. Because the estimated micropolarity values for micelle-encapsulated C153 are apparently close to those of alcoholic solvents (e.g., MeOH and EtOH have E_T(30) (kcal mol⁻¹) = 55.5 and 51.9, respectively),²⁶ the use of a water–alcohol mixture for micropolarity determination was rationally avoided because this would have led to an estimated E_T(30) value within the micelles near to the end of the calibration curve. On the contrary, the application of a water–1,4-dioxane solvent mixture eliminated this difficulty because this mixture covers a wider range of polarities, and thus ensures more reliable results.

3.3. C153-micelle binding

The C153-micelle binding constant (K) is determined by the method described by Almgren et al.³² using the following relationship:here, I₀, I_x and I_∞ represent the fluorescence intensity of the fluorophore in the absence of surfactant, at an intermediate concentration of surfactant, and under conditions of saturation of interaction, respectively.

The micellar concentration [M] is expressed as follows:

here, [S] represents the surfactant concentration, CMC is the critical micellar concentration and N_agg is the aggregation number of the surfactant.

The double reciprocal plots of 1/[I_x − I₀] versus 1/[M], leading to the estimation of C153-micelle binding constants in all the studied micellar environments using eqn (5), are displayed in Fig. S2 and S3 of the ESI,† and the free energy change (ΔG) for the binding process has been subsequently evaluated from the relationship as follows:

ΔG = −RT ln K (7)

The negative free energy change evidences the spontaneity of the dye-micelle binding process in all the cases under investigation (Table 1).^33–37 Of particular interest in the present context is to note the relative orders of magnitude of the binding constants in the two different series of micellar assemblies (Table 1). The size of the headgroup (Triton X series) and alkyl tail length (Tween series) of the micellar assemblies are found to have a profound influence on the C153-micelle binding constant (K) in the following order: K_TX165 < K_TX100 < K_TX114 (that is, an inverse relationship with the PEO chain length within the Triton X series) and K_TW20 < K_TW40 < K_TW60 (that is, proportionate variation with the surfactant tail length within the Tween series). The CMC values determined from the variation of the emission intensity of C153 with added surfactants are shown in Table 1 (representative examples are given in Fig. S4 of the ESI†).

3.4. Steady-state fluorescence anisotropy

As depicted in Fig. 5, the increasing fluorescence anisotropy of C153 with increasing surfactant concentration provides a clear signature for the impartation of motional restriction on the dye molecules within the micelle-entrapped state as compared to the free dye (in aqueous buffer medium).^{9,10,12–23,36,38}

Fig. 5 Variation of the steady-state fluorescence anisotropy (r) of C153 as a function of (a) Tween, and (b) Triton X concentrations (the plot for TX165 is shown in the inset because of the difference in scale on the abscissa). Each data point is an average of 15 individual measurements. The error bars are within the marker symbols if not apparent.

Furthermore, it is intriguing to note that the differential variation of fluorescence anisotropy of C153 as a function of the headgroup size or alkyl tail length of the micellar systems enables us to assess the differential degrees of motional restriction imposed on the fluorophore. Within the Tween series, the anisotropy varies as TW60 < TW40 < TW20, that is, an increasing degree of motional confinement is imposed on the dye molecules with increasing length of the alkyl tail of the surfactants, while the pattern of variation is typically reversed within the Triton X series, TX165 < TX100 < TX114, that is, the degree of motional restriction on the dye molecules varies inversely with the size of the headgroup (PEO chain length). The variation of fluorescence anisotropy of C153 within the micellar aggregates is thus found to bear a commendable harmony to the pattern of variation of C153-micelle binding strength (Section 3.3). This can be rationalized on the grounds of the compactness of the micellar headgroups, which increases with increasing alkyl chain length (in Tween series), and decreases with increasing PEO chain length (in Triton X series). Thus, the micellar hydration model can be invoked to aptly rationalize the findings. Fig. 5 shows that the increment of steady-state fluorescence anisotropy (r) of C153 with increasing concentration of the Tween series of surfactants is regular in nature, that is, a steady increase with surfactant concentration followed by saturation (Fig. 5a). However, the pattern of variation with increasing concentration of the Triton X series of surfactants is slightly different, the change of anisotropy (r) is not very significant in the lower surfactant concentration region, which is then followed by a drastic increase of anisotropy before attaining saturation, and thus it conforms to a sigmoidal nature of variation (Fig. 5b).

3.5. Fluorescence decay measurements

Representative examples of fluorescence decay transients of C153 in TW20 and TX100 micellar environments are displayed in Fig. 6 (the decay transients obtained in other micellar assemblies are displayed in Fig. S5 of the ESI†) and the corresponding fitting parameters are summarized in Table S1 in ESI.† The dye (C153) was seen to exhibit a biexponential decay pattern both in the aqueous buffer solution and the micellar systems investigated, signifying a degree of associated heterogeneity. By virtue of its intramolecular charge transfer (ICT) photophysics, C153 is known to experience a remarkable increase in dipole moment following photoexcitation.¹⁶ In addition to this, the fluorescent probe used C153 contains several heteroatoms such that the unequal hydration of C153 in the excited-state is also possible, which adds on to the extent of heterogeneity. Thus, the biexponential decay behavior of C153 in aqueous buffer may not be surprising and can be ascribed to the structure of the fluorescent probe. Therefore, for such heterogeneous systems, it is nontrivial to assign a specific mechanistic model to individual decay components. Thus, instead of emphasizing on the magnitude of each decay component, we chose to use the average lifetime to study the dynamic behavior of the micelle-bound C153. To enable a qualitative assessment for the modulation of the fluorescence decay behavior of C153 within the hydrophobic micellar environments, the data were compared with those obtained in a water–1,4-dioxane mixture. The decay parameters of C153 in a water–1,4-dioxane mixture are displayed in Table S2 in ESI.† In an aqueous medium, C153 exhibits a biexponential decay comprising of a fast major component and a relatively slow minor component. With increasing 1,4-dioxane proportion in the mixture, that is decreasing polarity, the contribution of the longer lifetime component increases (along with an increase in the average lifetime). In a micelle-bound condition, C153 is found to exhibit a similar trend in fluorescence decay behavior, that is, with increasing surfactant concentration, the contribution of the longer decay component increases accompanying an increase in the average lifetime (Table S1†). Nevertheless, the decay behavior of C153 in a micellar environment is not superimposable with that in a bulk water–1,4-dioxane mixture. This is not surprising given the microheterogeneous nature of micellar environments as compared with the bulk homogeneous medium of a water–1,4-dioxane mixture. It is seen from the data compiled in Table S3† that for the Tween series, with increasing alkyl chain length, the average lifetime increases, while the reverse pattern is observed in the case of variation of PEO chain length within the Triton X series. For a Tween series micelle, with increasing alkyl chain length, the palisade layer of the micelle experiences enhanced rigidity and consequently a greater degree of hydrophobicity due to decreased water penetration, while the reverse interaction scenario prevails for Triton X series micelles with increasing PEO chain length. This accounts for the observed trend in the variation of the average lifetime of C153 in the various micellar assemblies (Table S3†).

Fig. 6 Representative examples of fluorescence decay transients of C153 in (a) TW20 and (b) TX100 micellar environments. Curves (i) → (v) represent 0, 6, 22, 30, and 38 μM TW20 in (a), and 0, 0.13, 0.25, 0.38, and 0.5 mM TX100 in (b). IRF: instrument response function. The black solid lines are the fitted curves.

With a view to delve deeper into the modification of excited-state photophysical properties of C153 within the studied micelles, the radiative (k_r) and nonradiative (k_nr) decay rate constants for C153 have been estimated using the following equations^22,27 and the respective parameters are shown in Table S3†:

where, Φ_f is the fluorescence quantum yield, and 〈τ_f〉 is the average fluorescence lifetime. As surfactant concentration increases for both the Tween and Triton X series micelles, the radiative decay rate is found to increase with the concomitant reduction of nonradiative decay rate.²⁷ These data also corroborate to the observed modifications in the steady-state emission profile of C153, in the sense that an increase of k_r and decrease of k_nr can be connected with the increment of emission intensity of the dye as a function of surfactant concentration.

3.6. Rotational-relaxation dynamics

The typical anisotropy decay profiles of C153 in various micellar assemblies are displayed in Fig. 7, and the relevant rotational-relaxation dynamical parameters are summarized in Table 2.

Fig. 7 Time-resolved fluorescence anisotropy decay profiles of C153 in various micellar environments of (a) Tween series (TW20: , TW40: , TW60: ), and (b) Triton X series (TX165: , TX100: , TX114: ).

Table 2 Rotational-relaxation dynamic parameters of C153 in various nonionic micellar environments

System	β	θ (±4%) (ns)
Tween 20	1.00	1.57
Tween 40	1.00	1.75
Tween 60	1.00	1.86
Triton X 165	1.00	2.14
Triton X 100	1.00	2.54
Triton X 114	1.00	3.12

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C153 is found to exhibit monoexponential anisotropy decay in an aqueous buffer as well as in all the studied micellar environments. The anisotropy decay function is described as follows:²²

in which is the limiting anisotropy, β_i represents the amplitude corresponding to the i^th rotational-correlation time, θ_i. The increase in rotational-relaxation time of C153 within the micelle-encapsulated state as compared to that in aqueous buffer phase (Table 2) implies that the dye molecules experience considerable rigidity within the micellar microheterogeneous environments. Furthermore, the variation of rotational-relaxation time of the dye in different micellar environments is noteworthy in the present context. Within the Tween series, the rotational-relaxation time is found to increase with increasing alkyl chain length, while within the Triton X series, increasing PEO chain length decreases the rotational correlation time of the dye (Table 2). This pattern of observation is found to be in good agreement with other experimental findings as discussed previously based on the micellar hydration model.

Now, several arguments may be invoked to describe the monoexponential fluorescence depolarization profile of C153 within the micellar systems. The following possibilities are thus taken into consideration:^{10,11,35,39–41} (i) rotation of the dye within the micellar units with no significant contribution from the rotation of the latter; (ii) rotation of the micellar unit encapsulating the dye with no significant contribution from the rotation of the dye itself; and (iii) both rotations are operative. The third possibility is, however, negated in view of an observed monoexponential anisotropy decay pattern of micelle-bound C153 because a biexponential decay functional can only aptly describe this possibility. Thus, to confirm one of the first two possibilities, the rotational-relaxation time of the micellar unit (θ_m) has been estimated using the Stokes–Einstein–Debye (SED) relationship as follows:^39,40

here, η is the coefficient of viscosity of the medium, r_h is the hydrodynamic radius of the micellar aggregates (as obtained from DLS measurement and summarized in Table 1), k_B is the Boltzmann constant and T is the Kelvin temperature. From the calculated θ_m values, the rotational-relaxation time of the micellar units was found to be remarkably higher than the corresponding experimental fluorescence depolarization times. This leads us to conclude that the observed rotational-relaxation dynamics of the micelle-encapsulated dye is attributable to the rotational motion of the dye only, and not the micellar units.^{10,11,35,39–41}

4. Summary

The present work reports the study of interaction of the laser dye C153 with a series of nonionic micellar assemblies varying in the size of their headgroups (Triton X series) and the length of their tail parts (Tween series) of the constituent surfactants. Apart from demonstrating the efficient binding interaction of C153 with all the micellar systems studied, the present work reveals intriguing modifications to the photophysics of the dye as induced by the microheterogeneous micellar assemblies. From the series of steady-state and time-resolved spectroscopic results, the probable location of the dye molecule is substantiated to be in the palisade layer of all the micelles under investigation, with the strength of dye-micelle interaction being largely dependent on the alkyl chain length (within the Tween series) or the PEO chain length (within the Triton X series). Such differential interaction of the dye molecule with the micellar systems is found to be aptly rationalized in terms of the micellar hydration model, that is, the varying susceptibility of micellar units to the degree of water penetration depending on the thickness of the palisade layer, and thus a micellar polarity at the C153 interaction site as a function of variation in the alkyl/PEO chain length.^30,31 Our time-resolved fluorescence depolarization study reveals that the motional confinement imposed on the micelle-encapsulated dye molecules decreases with an increase in the PEO chain length, while it increases with an increase in the alkyl chain length as subsequently corroborated from steady-state fluorescence anisotropy measurements.²²

Acknowledgements

BKP acknowledges a post-doctoral research fellowship from IISER, Bhopal and NG acknowledges a Junior Research Fellowship through CSIR-NET. SM thanks DST, Govt. of India for financial assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12568a

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