Sonu,
Amit K. Tiwari,
Sunita Kumari and
Subit K. Saha*
Department of Chemistry, Birla Institute of Technology & Science (BITS), Pilani, Rajasthan 333 031, India. E-mail: sksaha@pilani.bits-pilani.ac.in; subitksaha@gmail.com; Fax: +91 1596 244183; Tel: +91 1596 515279
First published on 15th May 2014
The steady-state fluorescence properties, steady-state fluorescence anisotropy, solvation dynamics and rotational relaxation of coumarin 490 (C-490) have been studied in cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammoniumbromide)butane (12-4-12)–cyclohexane–n-pentanol–water reverse micelles (RMs). C-490 molecule resides in the water pool of RMs. A lesser extent of hydrogen bonding interactions occurs between the –OH group of the alcohol and the C-490 molecule at the interface of the present RMs as compared to the RMs of reported conventional cationic surfactants. The effect of the more hydrophobic nature of coumarin 480 (C-480) used to study the RMs of conventional cationic surfactants cannot be ruled out. C-490 migrates to the water pool with increasing content of water molecules. Therefore, the results of steady-state fluorescence properties, rotational relaxation and solvation dynamics depend on the water loading of RMs, unlike the RMs of conventional cationic surfactants. Thus, the solvation dynamics in gemini surfactant RMs are one order of magnitude faster as compared to the RMs of conventional cationic surfactants. The faster solvation dynamics in gemini surfactant RMs as compared with the reported AOT RMs are due to the absence of hydrogen bonding interactions between the water molecules and quaternary ammonium headgroups of gemini surfactants. The polarity of the surrounding microenvironment is found to have very little effect on the rates of non-radiative processes in the water pool of gemini surfactant RMs. The rates of non-radiative processes in the present RMs are found to be higher than those in reported AOT RMs.
The study of the dynamics of water molecules confined in various biological systems is of significant interest. The structure of water molecules present inside the pool of RMs have been studied by different techniques, including SANS,11,12 FT-IR and NMR,13 etc. Solvation dynamics is one of the best techniques used for studying the structure of water molecules because it enables the observation of an ultra-slow component, which is many orders of magnitude slower than that of bulk water.14 Bulk water exhibits very fast solvation dynamics.15 The solvation dynamics of bulk water with solvation time of less than 1 ps and single exponential decay have been studied by fluorescent probe molecules, namely, coumarin 343 (C-343) and coumarin 480.16 However, the dynamics of solvents are found to be slower in the presence of molecular assemblies.2,17–23 Bimodal behavior (fast and slow components of solvation) of water has been observed in different molecular assemblies.23–31
Solvation dynamics depend upon the nature of the molecular assemblies.3,32–35 Shirota et al.32 reported solvation dynamics in the micelles of cationic and anionic conventional surfactants in aqueous media. They reported that the dynamics are slower in the micelles of anionic surfactants than in those of cationic surfactants with quaternary ammonium headgroups in aqueous media. The slow solvation dynamics in the case of anionic surfactants is due to hydrogen bonding interactions between the water molecules and surfactant headgroups. Such hydrogen bonding does not occur in the case of cationic surfactants, which do not contain any hydrogen bonding site. Hazra et al.36 reported the solvation dynamics of C-480 in the water-in-oil microemulsions of various conventional surfactants. They observed that C-480 is located at the micellar interfacial region of cationic surfactant cetyltrimethylammonium bromide (CTAB), unlike AOT microemulsions,37 and the solvation dynamics was found to be independent of water content of the microemulsion. Corbeil et al.34 also reported that C-343 remains at the interface of the RMs of CTAB, and that the location is independent of water loading.
Gemini surfactants are a special class of surfactants having two hydrophobic chains and two hydrophilic headgroups connected with a spacer group.38 Gemini surfactants have superior properties than conventional surfactants. They have low critical micelle concentration (CMC), better solubilizing ability of numerous water-insoluble compounds, stronger biological activity, etc.38,39 They have widespread applications in chemistry, medicine, biology, etc.40 Gemini surfactants, like conventional surfactants, also form RMs in the presence of primary alcohols with medium chain length.34,41–44 Zhao et al.41–44 studied the various physical properties of the RMs of cationic gemini surfactants. They described the different states of water in the RMs of cationic gemini surfactants using the FT-IR spectroscopy.41 However, to the best of our knowledge the solvation dynamics in the RMs of gemini surfactants has not been reported. Recently, we studied the effect of a hydroxyl group-substituted spacer group of cationic gemini surfactants on the solvation dynamics and rotational relaxation of C-480 in aqueous micelles.23
Considering the superior surface active properties of gemini surfactants than conventional surfactants, we realized that the RMs of gemini surfactants are also very interesting systems for studying solvation dynamics. In the present work, we have studied the dynamics of the solvent relaxation of C-490 in the RMs of a cationic gemini surfactant, 1,4-bis(dodecyl-N,N-dimethylammoniumbromide)butane (12-4-12). (Molecular structures for C-490 and 12-4-12 are given in Scheme 1). RMs were prepared in cyclohexane (CHX) as a nonpolar solvent and n-pentanol as a co-surfactant. To the best of our knowledge, this is the first report on the solvation dynamics and rotational relaxation in RMs formed by a cationic gemini surfactant. The changes in the steady-state fluorescence, fluorescence anisotropic properties, solvation dynamics and rotational relaxation of C-490 were monitored by changing the size of the water pool of RMs. The effect of water loading on these properties was compared with that in the RMs of CTAB. The present study also compares the time scales of solvation dynamics in the gemini surfactant RMs and the reported AOT, both aqueous and non-aqueous, RMs.3 This study also suggests possible reasons for this difference. We also studied the effect of water loading in the pool of RMs on the rates of ICT processes of C-490.45,46
Absorption spectra were recorded using a Jasco V-630 UV-visible spectrophotometer. Steady-state fluorescence measurements were performed using a Horiba Jobin Yvon Fluoromax-4 scanning spectrofluorometer. The excitation and emission slit widths used for the fluorescence measurements were 1 nm each. The emission and excitation spectra were corrected for instrument sensitivity. The steady-state fluorescence anisotropy measurements were performed with the same steady-state spectrofluorometer fitted with a polarizer attachment; the details are available elsewhere.23,48,49 The relative fluorescence quantum yields were determined with respect to quinine sulphate in 0.1 N sulphuric acid as standard (ϕf = 0.55), and by calculating the area under the corrected fluorescence bands of both C-490 in different environments and quinine sulphate.50
The excited singlet state lifetimes were determined from intensity decays using a Horiba Jobin Yvon Fluorocube-01-NL picosecond time-correlated single-photon counting (TCSPC) experimental setup. A picosecond diode laser of 375 nm (NanoLED 375L, IBH, UK) was used as a light source. The fluorescence signals were detected at a magic angle (54.7°) polarization using a TBX photon detection module (TBX-07C). The typical full width at half maximum (FWHM) after deconvolution using liquid scattering of the system response was about 165 ps for a 375 nm laser.
The decays were analyzed by IBH DAS-6 decay analysis software. The goodness of fits was analyzed by the χ2 criterion and visual inspection of the residuals of the fitted function to the data.
The time-resolved fluorescence anisotropy r(t) was calculated using eqn (1):
![]() | (1) |
![]() | (2) |
〈τr〉 = a1rτ1r + a2rτ2r | (3) |
The average excited singlet state lifetime 〈τ〉 for a bi-exponential decay was calculated by eqn (4):44
〈τ〉 = a1τ1 + a2τ2 | (4) |
![]() | (5) |
![]() | (6) |
〈τs〉 = a1sτ1s + a2sτ2s | (7) |
All the spectroscopic measurements were carried out at 298.15 ± 1 K.
Systems | λabsmax (nm) | λflmaxa (nm) | λflmaxb (nm) | ET(30) (kcal mol−1) |
---|---|---|---|---|
a λexc = 375 nm.b λexc = 412 nm. | ||||
CHX + n-pentanol (2.4%) | 356 | 457 | 460 | 39.8 |
n-Pentanol | 382 | 477 | 478 | 49.1 |
Water | 366 | 496 | 498 | 63.1 |
w0 = 0 | 387 | 474 | 477 | 47.3 |
w0 = 2 | 387 | 477 | 480 | 48.7 |
w0 = 20 | 385 | 482 | 485 | 51.3 |
w0 = 25 | 385 | 482 | 485 | 51.3 |
w0 = 30 | 385 | 482 | 485 | 51.3 |
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Fig. 1 UV-visible absorption spectra of C-490 in water, CHX + n-pentanol (2.4%) mixed solvent (bulk solvent) and in RM systems. |
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Fig. 2 Steady-state fluorescence spectra of C-490 in water, bulk solvent and RM systems (a) λexc = 375 nm and (b) λexc = 412 nm. |
To investigate the effect of excitation wavelength on fluorescence properties, the fluorescence spectra were also recorded at λexc = 412 nm (Fig. 2b). At this excitation wavelength, molecules residing in the pool of RMs are mostly excited. Therefore, emission characteristics reflect the properties of probe molecules entrapped in the pool of the RMs alone.2 It can be seen from Fig. 2b that the fluorescence intensities of C-490 in all the RMs are much higher than those in the bulk solvent. The fluorescence peak maxima in all the systems are approximately red-shifted by 2–3 nm as compared to the excitation at 375 nm. These results indicate that the probe molecules are partitioned to the interior of the RMs. Fig. 3 displays the excitation spectra of C-490 in RMs at w0 = 30 monitored at two different emission wavelengths, i.e. 435 and 550 nm. The excitation spectrum monitored at 435 nm resembles the absorption spectrum of C-490 in bulk solvent, while the spectrum at 550 nm resembles the absorption spectrum of C-490 at w0 = 30. These observations further demonstrate that C-490 molecules are located in two different environments in the RMs.2,36,37
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Fig. 3 Excitation spectra of C-490 recorded in w0 = 30 and monitored at (a) λems = 435 nm and (b) 550 nm, and absorption spectra of C-490 in (c) bulk solvent and (d) w0 = 30. |
A red shift in the fluorescence peak maximum and a gradual decrease in fluorescence intensity were observed with an increase in w0 from 0 to 30. Hazra et al.36 studied the RMs of conventional surfactants using C-480 as a probe. They observed the location of probe molecules at the interface of the RMs of cationic surfactant CTAB. The different location of C-490 as compared to C-480 is because C-480 is a more hydrophobic probe having a tertiary amine substituent, which is not a hydrogen bond donor. However, C-490 has a primary amine substituent, which is a hydrogen bond donor. The red shift of the fluorescence band is because of the stabilization of the emitting state due to an increase in the static polarity of the pool of RMs with increasing water content, and the decrease in intensity is due to an increase in the rate of non-radiative processes in the more polar environment of the pool. The nature of water molecules becomes more like bulk water with an increasing number of water molecules in the RM pool.41 The decrease in intensity can be explained by introducing the model suggested by Jones II et al.45,56 In 7-aminocoumarin derivatives, a planar highly fluorescent ICT excited state deactivates to a non-fluorescent twisted intramolecular charge transfer (TICT) state.43,57 The conversion of an ICT to a TICT state depends on the static polarity of the environment, and this conversion becomes feasible with an increase in the polarity of the medium.37,58,59 Fluorescence intensity decreases with the increasing polarity of the environment due to the increasing population of the TICT state. In C-490 the main non-radiative path for ICT is the generation of a TICT state.37 Hazra et al.37 also observed red-shifted fluorescence spectra with a decrease in intensity in AOT RMs for C-490.
The steady-state fluorescence anisotropies of C-490 in all the RM systems were determined.48,49 Anisotropy decreases with a decrease in the rigidity of the environment.50,60 The steady-state anisotropy values of C-490 in all the RM systems are shown in Fig. 4. With an increase in w0, the steady-state anisotropy gradually decreases. It indicates that the microenvironment around C-490 becomes less rigid with increasing water content in RMs. In the RMs of a cationic gemini surfactant, Zhao et al.41 observed that with increasing w0 the amount of bulk-like water per surfactant rapidly increases; therefore, RMs swell. At higher w0, there is a decrease in the microviscosity of the environment around C-490, which is reflected by the lowering of steady-state anisotropy values. An increase in the polarity and a decrease in the microviscosity of the environment around C-490 for changing w0 from 0 to 30 indicate that the probe molecules are entrapped in the water pool of the RMs.
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Fig. 4 Steady-state fluorescence anisotropy (r) of C-490 vs. w0. Inset (a) plot of average rotational correlation times (τc) vs. w0 and (b) non-radiative rate constant (knr) vs. w0. λexc = 375 nm. |
The average rotational correlation times for C-490 in RMs with increasing w0 were calculated by using Perrin's equation50 (eqn (8)). It ensures that the observed change in the steady-state anisotropy of C-490 with increasing w0 is not because of any change in lifetime.
τc = (〈τ〉r)/(ro − r) | (8) |
kr = ϕf/τ | (9) |
knr = (kr/ϕf) − kr | (10) |
Systems | ϕf | τ1 (ns) | a1 | τ2 (ns) | a2 | 〈τ〉 (ns) | kr × 10−9 (s−1) | knr × 10−9 (s−1) | χ2 |
---|---|---|---|---|---|---|---|---|---|
a λexc (for both steady-state and time-resolved) = 375 nm. Lifetime measured at λflmax. | |||||||||
CHX + n-pentanol | 0.604 | 2.670 | 0.20 | 4.990 | 0.80 | 4.520 | 0.133 | 0.088 | 1.05 |
w0 = 0 | 0.610 | 3.690 | 0.27 | 4.347 | 0.73 | 4.169 | 0.146 | 0.094 | 1.08 |
w0 = 2 | 0.594 | 2.950 | 0.15 | 4.127 | 0.85 | 3.950 | 0.150 | 0.103 | 1.05 |
w0 = 20 | 0.511 | 2.680 | 0.21 | 3.738 | 0.79 | 3.516 | 0.145 | 0.139 | 1.03 |
w0 = 25 | 0.504 | 2.675 | 0.24 | 3.761 | 0.76 | 3.500 | 0.144 | 0.141 | 1.06 |
w0 = 30 | 0.499 | 2.188 | 0.13 | 3.662 | 0.87 | 3.470 | 0.143 | 0.144 | 1.07 |
Water | 0.266 | 4.560 | 1.00 | — | — | 4.560 | 0.058 | 0.161 | 1.02 |
System | r0 | a1r | τ1r (ns) | a2r | τ2r (ns) | 〈τr〉 (ns) |
---|---|---|---|---|---|---|
CHX + n-pentanol (2.4%) | 0.20 | 1 | 0.194 | — | — | 0.194 |
n-Pentanol | 0.30 | 1 | 0.613 | — | — | 0.613 |
w0 = 0 | 0.36 | 0.45 | 0.691 | 0.55 | 4.692 | 2.892 |
w0 = 2 | 0.36 | 0.39 | 0.518 | 0.61 | 4.026 | 2.658 |
w0 = 25 | 0.36 | 0.58 | 0.473 | 0.42 | 3.335 | 1.675 |
w0 = 30 | 0.36 | 0.62 | 0.457 | 0.38 | 3.295 | 1.535 |
The rotational relaxation time of C-490 in pure water is reported to be 95 ps.63 At w0 = 2, the relaxation process of C-490 in the present RMs is ∼14 times slower as compared to the bulk solvent and ∼28 times slower as compared to the bulk water. These results suggest that C-490 molecules are experiencing a more rigid environment in the pool of RMs than that in bulk solvent and bulk water. Therefore, the environment around the probe in the RM pool is different from that in bulk solvent and bulk water. The decrease in the rotational relaxation time with an increase in the amount of polar solvent in the AOT RMs has also been observed in other studies.7,33
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Fig. 6 Time-resolved fluorescence decay of C-490 in 12-4-12 RMs (w0 = 2). 1 to 4 at 560 nm, 470 nm, 455 nm, and 425 nm, respectively, and 5 for instrument response function. |
The time-resolved emission spectra (TRES) were constructed by applying the procedure of Fleming and Maroncelli.51 The dynamic Stokes shifts have been observed for the emission spectra of C-490 in n-pentanol, bulk solvent and RMs. A representative TRES is shown in the inset of Fig. 7. The solvent dynamics are monitored by solvent correlation function (SCF) C(t). Fig. 7 represents a plot of C(t) with time (t) at different w0. The decay characteristics of C(t) for RMs at various w0, n-pentanol, and for bulk solvent are tabulated in Table 4. The solvation time for n-pentanol was found to be 128 ps with a single exponential. Corbeil et al.34 reported this value to be 97 ps with a probe molecule C-343. This difference observed in solvation time could be due to the use of a different probe molecule and a different technique.
Systems | Δν (cm−1) | a1s | τ1s (ns) | a2s | τ2s (ns) | 〈τs〉 (ns) | Missing component |
---|---|---|---|---|---|---|---|
CHX + n-pentanol (2.4%) | 852 | 0.68 | 1.797 | 0.32 | 1.756 | 1.784 | — |
n-Pentanol | 918 | 1 | 0.128 | — | — | 0.128 | — |
w0 = 0 | 816 | 0.60 | 0.612 | 0.40 | 1.828 | 1.098 | 0.053 |
w0 = 2 | 691 | 0.65 | 0.514 | 0.35 | 1.561 | 0.880 | 0.172 |
w0 = 25 | 386 | 0.71 | 0.356 | 0.29 | 1.624 | 0.724 | 0.703 |
w0 = 30 | 331 | 0.76 | 0.203 | 0.24 | 0.682 | 0.318 | 0.710 |
The average solvation time of C-490 in the bulk solvent was found to be 1.78 ns. This solvation time is greater as compared to pure n-pentanol. The slower solvation process in the mixed solvent system as compared to pure solvent was also observed by Hazra et al.36 The data in Table 4 reveal that C(t) consists of two components for the RMs, one with a longer lifetime and another with a shorter lifetime. The bimodal decay arises due to the presence of bound and free water molecules, and there is a dynamic exchange between them.29 In the present work, the bulk solvent in RMs consists of 2.4% n-pentanol. The presence of n-pentanol affects solvent reorganization in RMs. Corbeil et al.34 also observed the effect of n-pentanol for RMs of conventional surfactants. It is evident from the data in Table 4 that the solvation time corresponding to the slow component in RMs at w0 = 0 is close to that in the bulk solvent. However, the solvation time corresponding to the fast component in RMs with major weightage is very different from that in bulk solvent. The solvation time in RMs is also very different from that in pure n-pentanol. It is much faster in pure n-pentanol than that in RMs. Hence, the observed solvation dynamics in RMs is not exclusively the dynamics of n-pentanol in bulk solvent. The increase in the solvation time of n-pentanol in the vicinity of the cationic headgroups of gemini surfactant is clear because there is an electrostatic interaction between the lone pair of electrons of the oxygen atom of the hydroxyl group of n-pentanol and the positively charged nitrogen atom of the gemini surfactant.36
The solvation time for the slow component at w0 = 30 is reduced by more than half as compared to w0 = 0. On comparison with the steady-state fluorescence spectra of C-490 in bulk solvent and in RMs at w0 = 30, we found that the band in RMs is 25 nm red-shifted as compared to bulk solvent. These results indicate that C-490 molecules are moving towards the polar core of the RMs with increasing water loading. The number of free water molecules becomes greater at w0 = 30 than that at lower w0, and the solvation dynamics becomes faster.3,33 It has been found that the relative contribution of the fast component gradually increases with a subsequent decrease in the contribution of the slow component of solvation with an increasing water content in RMs (Table 4). Moreover, the solvation time for the fast components, along with the average solvation time, gradually decreases. Hazra et al.3 during their study on non-aqueous AOT RMs observed that the average solvation time decreases with the increasing content of polar solvents and concluded that the probe molecules C-490 are approaching the core of the RMs. However, in another study on the microemulsions of conventional cationic surfactant CTAB with C-480 as a probe, they noticed that the solvation dynamics at the interface is independent of water loading.36 However, in the present study with RMs of cationic gemini surfactant, the solvation dynamics mostly occurs in the water pool of the RMs; thus, the dynamics is dependent on water loading. Although the location of the probe molecules could be a reason for this difference in solvation dynamics, Corbeil et al.34 noticed water loading-independent dynamics even with a more polar dye (C-343) as compared to C-480 and C-490. Clearly, in the case of C-343, there are electrostatic interactions between the quaternary ammonium headgroup of CTAB and the –COO− functional group of C-343.34 However, the difference in the structure of RMs of gemini surfactant and those of conventional cationic surfactants is also responsible for the dependence of solvation dynamics on water loading, as discussed in the following paragraph.
In the present work, the dependency of solvent relaxation time of C-490 on w0 is because of the larger droplets, which are formed in the case of RMs of 12-4-12 gemini surfactant42 as compared to the RMs of conventional surfactants. Larger droplets are suitable for solubilizing more water.42 Zhao et al.42 reported that the variation in the size of water pool with increasing w0 is more sensitive for the RMs of gemini surfactants as compared to conventional surfactants.42 It is worth noting that the solvation dynamics does not depend on the chain length of alcohol used to form RMs for CTAB–n-heptanol–cyclohexane–water and CTAB–n-pentanol–cyclohexane–water RMs, as reported by Corbeil et al.34 The –OH groups of alcohol molecules interact with the headgroups of gemini surfactant. With the increasing size of the water pool, the interfacial curvature is reduced, which allows more water molecules to associate with the ionic heads of gemini surfactant. Therefore, a part of the alcohol molecules are expelled from the interface.41 Based on the results obtained by Zhao et al.42 on gemini surfactant RMs, we can suggest that in the present case the surfaces of larger droplets formed by 12-4-12 gemini surfactant require smaller amounts of alcohol in the gaps between the headgroups of the surfactant as compared to conventional surfactants. A lesser extent of hydrogen bonding interaction between –OH groups of the alcohol and the C-490 molecule is expected at the interface of the present RMs of gemini surfactant than those of conventional surfactants. Consequently, the solvation time in Gemini surfactant RMs is faster as compared to conventional surfactants. Because the location of the probe molecules in the present gemini surfactant RMs is different from that in the reported RMs of conventional surfactants, the timescale of solvation dynamics is also different in these two cases. The solvation dynamics in the pool of the present gemini surfactant RMs are faster than those in the bulk solvent, which is different from the RMs of conventional surfactants.34 The solvation time for pure water is 0.5 ps.37 Thus, the water dynamics are many times slower in the pool of RMs as compared to bulk water. It is worth noting that the contribution from Br− ions to the measured solvation dynamics in the pool of RMs is considered negligible.36 However, some of the n-pentanol molecules present at the interface would also contribute to the salvation dynamics.
It is reported that solvation time depends upon the nature of molecular assemblies.32 The solvation time observed in the present study with cationic gemini surfactant RMs is different from that observed by other researchers in AOT RMs using the same probe C-49037 and by different probe molecules.33 The solvation time of solvent molecules in organised assemblies depends on the interactions between the solvent molecules and the organised assemblies.32 Hydrogen bonding, electrostatic interaction, counter-ion interaction, etc., make the relaxation process of a solvent many times slower in organised assemblies as compared to bulk solvent. We observed faster solvation as compared to the solvation (at w0 = 4 and w0 = 32 the average solvation times are 2.97 and 1.17 ns, respectively) in AOT RMs using the same probe C-490 reported by Hazra et al.37 They reported the average solvation time in nanosecond timescales but we have reported the average solvation time in sub-nanosecond timescales with different water content in the RMs except at w0 = 0, where there is no water molecule added externally. Clearly, the solvation time will be different because the headgroups of AOT and gemini surfactant are different. In AOT RMs, there is strong hydrogen bonding between the water molecules and the oxygen atoms of the AOT surfactant. Hydrogen bonding between water molecules and the quaternary ammonium headgroups of gemini surfactant is absent in the RMs of gemini surfactant. However, there are n-pentanol molecules present at the interface of the RMs of the gemini surfactant but the hydrogen bonding of water molecules with n-pentanol may not be as strong as with the oxygen atoms of AOT surfactant.41 Zhao et al.41 on the basis of their FT-IR study confirmed the weak interaction between n-hexanol and water molecules at the interface of RMs.
The method proposed by Fee and Maroncelli51,66 was used for calculating time zero frequency (eqn (11)) for C-490 in RMs because the excitation wavelength 375 nm and absorption maxima ∼385 nm in RMs are very close:
νp,md(t = 0) ≈ νp,md(abs) − [νnp,md(abs) − νnp,md(ems)] | (11) |
Footnote |
† Electronic supplementary information (ESI) available: 1H NMR data of synthesized gemini surfactant (12-4-12) and plot of ET(30) versus fluorescence maxima of C-490 in dioxane–water mixtures. See DOI: 10.1039/c4ra02708f |
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