Synthesis of a Polymer Bearing Several Coumarin Dyes along the

Article pubs.acs.org/JPCB

Synthesis of a Polymer Bearing Several Coumarin Dyes along the Side Chain and Study of its Fluorescence in Pure and Binary Solvent Mixtures as well as Aqueous Surfactant Solutions Niraja Kedia,† Saswati Ghosh Roy,† Priyadarsi De,† and Sanjib Bagchi*,‡ †

Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur Campus, BCKV Main P.O., Nadia 741252, West Bengal, India ‡ Presidency University, 86/1, College Street, Kolkata 700073, West Bengal, India ABSTRACT: A copolymer bearing several pendent dyes (coumarin derivatives) along the side chain has been synthesized, and its fluorescence parameters have been monitored in pure solvents and also as a function of composition of binary solvent mixtures. Fluorescence parameters (the maximum energy of fluorescence, quantum yield, and rate constant for the decay of the excited state) of the free fluorophore show significant dependence on the nature of the immediate environment around it. The value of a parameter measured in neat solvent for the fluorophore covalently bound to the polymer is different from that of the free fluorophore, indicating that the polymer chain influences the spectroscopic properties of the dye. Whereas the energy of maximum fluorescence of the free fluorophore shows a nonlinear correlation with the solvent composition of solvent mixtures, an almost linear correlation has been observed for the polymer. A significant variation of photophysical parameters of the dye dissolved in binary solvent mixtures, which is different from that of the free fluorophore, has been observed. Thus, the free fluorophore and the fluorophore bound to the polymer sense different environments in binary solvent mixtures. A dramatic variation of fluorescence intensity of the fluorophore bound to the polymer has been observed when sodium dodecyl sulfate (SDS) is added to an aqueous solution of the polymer. The results have been explained in terms of the existence of different species (polymer, polymer−SDS aggregates, micelles) in equilibrium in solution.

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INTRODUCTION It has long been known that the fluorescence characteristics of coumarin dyes are very sensitive to changes of solvent.1−3 Thus, the coumarin dyes are used extensively as good probes for the characterization of micelles,4−7 nanoparticles (silver and silica),8−10 materials prepared by sol−gel processes,11,12 and solutions of polymers.13 The preparation of polymers with coumarin moieties has attracted much attention in recent years because of their fluorescent and photoconducting properties14 as well as physiological activities.15−17 Thin films prepared from such polymers are very useful for optoelectronic devices. The processing of these thin films with high optical quality and color tunability from polymeric electroluminescence materials is much easier than preparation of films from inorganic materials. Moerner and co-workers18 have reported the observation of photorefractive effects in a new polymeric mixture consisting of a nonlinear chromophore (coumarin 153) and a sensitizer doped into a high-mobility poly(silane)-based charge-transporting polymer host. Chujo et al.19 demonstrated a novel method for the preparation of a hydrogel based on polyoxazoline by means of photodimerization of the photosensitive coumarin groups in the pendant groups of the polymer chain. It has been reported that the covalent bonding of the coumarin dyes to the polymer side chain greatly affects the fluorescence characteristics of the dyes, and this can be effectively utilized for © 2014 American Chemical Society

investigating the nature of the different microdomains present in the aqueous solution of the polymer.20 In general, in a polymer solution, because of the interplay of segment−segment and segment−solvent interactions, the fluorophoric part can remain in regions of different polarity showing different fluorescence behavior. Thus, a study of fluorescence parameters in different media can yield information regarding various interactions in polymer solution. These interactions can be modified by adding a cosolvent. Thus, it is instructive to investigate the fluorescence characteristics of a fluorophore in different binary solvent mixtures as a function of solvent composition. The interaction between water-soluble polymers and single-tailed anionic surfactants also forms an interesting field of study.21,22 The interaction between the neutral polymer poly-N-vinylpyrrolidinium-2-one (PVP) and sodium dodecyl sulfate (SDS) has been widely studied.23,24 In the present work, we have prepared water-soluble polymers bearing several pendent dyes along the polymer side chain formed by copolymerization of N,N-dimethylacrylamide (DMA) and a coumarin derivative, 7-[4 (trifluoromethyl)coumarin]methacrylamide (TCMA). Three fluorescence parameters, Received: December 30, 2013 Revised: April 4, 2014 Published: April 8, 2014 4683

dx.doi.org/10.1021/jp4127557 | J. Phys. Chem. B 2014, 118, 4683−4692

The Journal of Physical Chemistry B

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Scheme 1. RAFT Copolymerization of DMA and TCMA in DMF at 70 °C

Table 1. Results from the RAFT Mediated Copolymerization of DMA and TCMA at 70 °C in DMFa polymer

DMA in the feed (mol %)

DMA in the polymer (mol %)b

conv. (%)c

Mn,theo (g mol−1)d

Mn,GPC (g/mol) (PDI)e

CP1 CP2

95 90

94.0 85.2

78 74

17 410 17 600

22 000 (1.5) 19 800 (1.7)

a

Two copolymerizations were carried out by maintaining the ratio of c(Monomer): c (CDP): c (AIBN) = 200:1:0.1 for 3 h. bDetermined by 1H NMR spectroscopy. cMonomer conversion as obtained by gravimetric analysis on the basis of the amount of monomer charged. dMn,theo = c (monomer)/ c (CDP) × conversion × formula weight (FW) of monomer + 403.67 (FW of CDP). eDetermined by GPC using DMF as eluent.

namely, the energy of maximum fluorescence E(F), quantum yield φ, and lifetime τ of the fluorophore bound to the polymer have been studied, and the results have been compared with those of the free fluorophore. To investigate the influence of the interaction between the fluorophore units, a study of two samples of copolymers differing in the content of fluorophore unit has also been done. Different media used in the study include ten pure solvents and three binary solvent mixtures, namely, 1,4-dioxane/water, N,N-dimethylformamide (DMF)/ water, and benzene/methanol. The study has also been extended to the aqueous polymer−SDS system to reveal the polymer−surfactant interaction.

tion, comonomer compositions were altered and two reactions were performed using CDP and AIBN (1:0.1) in DMF at 70 °C. The ratio of monomers/CDP was fixed at 200:1 with different proportions of both monomers DMA and TCMA. The characterization results for these copolymerizations are shown in Table 1. The dn/dc values were determined for each polymer and the measured dn/dc values were used in the analysis of molecular masses and molecular mass distributions. Although number-average molecular masses determined from GPC (Mn,GPC) match well with the theoretical molecular mass (Mn,theo) calculated based on monomer conversion, the PDI values were reasonably high. The 1H NMR spectrum of the purified copolymer, poly(DMA-co-TCMA), displayed in Figure 1, indicates copolymers without any trace of unreacted

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MATERIAL AND METHODS Materials. The organic solvents, namely, methanol, ethanol, acetone, acetonitrile, N,N-dimethylformamide, 1,4-Dioxane, tetrahydrofuran, ethyl acetate, benzene (all of spectroscopic grade), and SDS were obtained from Sigma-Aldrich and were used as received. Anhydrous N,N-dimethylformamide (DMF, Aldrich, 99.9%), TCMA, (Aldrich), and CDCl3 (Cambridge Isotope, 99% D) were used without further purification. 4,4′Azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized twice from methanol. N,N-Dimethylacrylamide (DMA, Aldrich, 99%) was passed through a basic alumina column to remove the inhibitor prior to polymerization. The chaintransfer agent (CTA), 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid (CDP), was synthesized as previously reported.25 Triple-distilled water was used for preparing aqueous solutions. RAFT Copolymerization of DMA with TCMA. Trithiocarbonates have been used as a versatile class of CTA agents for the polymerization of varieties of vinyl monomers.26−28 The CDP-mediated reversible addition fragmentation chain-transfer (RAFT) polymerization of acrylamide monomers with controlled molecular mass and narrow polydispersity (PDI) was reported in the literature.29 Therefore, we have chosen CDP as CTA for the copolymerization of DMA and TCMA (Scheme1) at 70 °C in the presence of AIBN as radical initiator in DMF as solvent maintaining the ratio of c(monomer):c(CDP):c(AIBN) = 200:1:0.1, where c stands for molar concentration of the species within the parentheses. The molecular mass was determined by gel permeation chromatography (GPC) using DMF as eluent. During the copolymeriza-

Figure 1. 1H NMR spectrum of poly(DMA-co-TCMA) (CP2 in Table 1) in CDCl3.

monomer. The compositions of the copolymers were determined using 1H NMR spectroscopy by comparison of resonance signals related to the two comonomers. The copolymer compositions given in Table 1 were determined from the ratio of the peak intensities of the main chain −CH− H atoms plus side chain −CH3 H atoms of the DMA units (a + b H atoms) at δ = 2.25−3.45 ppm to the “c” H atoms of TCMA units at δ = 6.65 ppm. Steady-State Spectral Measurements. The absorption study was carried out with the help of a CARY 300 Bio UV− 4684



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RESULTS AND DISCUSSION DFT Calculations. The molecular structure of TCMA, as optimized by DFT calculations, is shown in Figure 2a. The

visible spectrophotometer. Fluorimetric measurements were done on a PerkinElmer LS55 and a Horiba Jobin Yvon Fluoromax-3 spectrofluorimeter. The molar transition energy E (in kilocalories per mole) of maximum fluorescence and absorption were calculated from the wavelength λ of the maximum fluorescence and absorbance using eq 1: E /(kcal mol−1) = hcνN (A) = 28590/(λ/nm)

Article

(1)

The quantum yield φ was calculated using eq 2: ⎛ I ⎞⎛ A ⎞⎛ n 2 ⎞ Φ = ΦR ⎜ ⎟⎜ R ⎟⎜ 2 ⎟ ⎝ IR ⎠⎝ A ⎠⎝ nR ⎠

(2)

where I, A, and n denote the integrated intensity of fluorescence, absorbance, and the refractive index, respectively. The subscript R indicates reference. Solution of quinine sulfate monohydrate (Sigma-Aldrich) in 0.1 M H2SO4 (φ = 0.577) was used as standard references for the quantum yield measurements.30 Time-Resolved Fluorescence Studies. Fluorescence decay measurements were obtained from a time-correlated single-photon counting (TCSPC) technique using a Horiba Jobin Yvon time-resolved spectrofluorimeter and a 377 nm Nano LED. The fluorescence decay was measured at the emission maximum. The decay curves were analyzed using DAS-6 decay analysis software provided with the instrument and fitted to the multiexponential decay equation I(t)= ∑aie−t/(τ), where I(t) denotes the fluorescence intensity at time t and the pre-exponential factor ai is the contribution to the time-resolved decay of the component with lifetime τ. The decay parameters ai and τi were recovered using a nonlinear curve-fitting procedure. Fitting with χ2 values around 1 was taken as acceptable. The quantity Bi = ai(τ)/∑ ai(τ) represents the fraction of contribution of steady-state fluorescence intensity due to the species with decay constants τ. Characteristics of Polymers. The polymer molecular masses and molecular mass distributions were determined by GPC in DMF at 35 °C using a flow rate of 1.0 mL min−1 (Viscotek pump; columns, two ViscoGel I-Series G4000). The detection consisted of a Viscotek refractive index detector operating at λ = 660 nm and a Viscotek model 270 series platform, consisting of a laser light-scattering detector (operating at 3 mW, λ = 670 nm with detection angles of 7° and 90°) and a four-capillary viscometer. For the copolymers, dn/dc was determined and molecular masses were calculated by the triple-detection method. The 1H NMR spectroscopy was conducted with a Bruker Avance 400 spectrometer operating at 400 MHz. The dynamic light-scattering (DLS) study was done in a Malvern Zetasizer Nano isntrument equipped with a 4.0 mW HeNe laser operating at λ = 633 nm. Density Functional Theory Calculations. Density Functional Theory (DFT) calculations were carried out on the coumarin derivative by using Gaussian 03 software.31 Calculations was performed using the B3LYP functional. The 6-31G basis set was used for all atoms. Geometry optimization was done followed by frequency calculation to ascertain that the optimized structure was a true minima. The default criteria for geometry optimizations were used in the process. Timedependent DFT (TD-DFT) calculations were done using the optimized geometry.

Figure 2. Optimized molecular geometry of TCMA (a) and its HOMO (b) and LUMO (c) structures.

electronic distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the dye are shown in panels b and c of Figure 2, respectively. Note that the HOMO is characterized by a large electron density on nitrogen relative to that on carbonyl oxygen. On the other hand, the electron density on the carbonyl oxygen is relatively larger in the LUMO. Thus, the HOMO → LUMO transition is associated with an intramolecular charge transfer (ICT). The HOMO−LUMO gap has been found to be 2.9 eV for the free TCMA. The ground-state dipole moment is found to be μ = 5.7 D. Fluorescence Studies. The free fluorophore (TCMA) shows an absorption maximum at λ = 340 nm, and its value is almost independent of the solvent used. However, the fluorescence band shows solvatochromism; a red band shift is observed with an increase in the polarity of the solvent. Table 2 lists the values of E(F), the energy of maximum fluorescence, measured in ten different solvents. A hypsochromic shift of the fluorescence band maximum with a decrease in solvent polarity for coumarin dyes is well-documented in the literature.1−3 This is consistent with an ICT emission state of the dye; a more polar solvent brings about more stabilization of the ICT state. The E(F) value of coumarin derivatives is known to depend on the solvent’s dipolarity/polarizability and the hydrogen-bond donor (HBD) ability.20 To analyze the role of different modes of solute−solvent interaction on the E(F) values of TCMA, a correlation of this parameter has been sought with the solvent’s dipolarity/polarizability, hydrogen-bond donor (HBD) ability, and hydrogen-bond acceptor ability represented by the Kamlet−Taft solvatochromic parameters π*, α, and β, respectively. The following correlation equation has been observed. E(F )/kcal mol−1 = (73.0 ± 0.7) − (2.7 ± 0.4)α − (2.5 ± 0.8)β − (4.1 ± 0.9)π *;

n = 10, R2 = 0.96 (3)

The negative sign of all the regression coefficients in eq 3 indicates that the excited state is more stabilized than the 4685



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Table 2. E(F), φ, and τ in Various Pure Solvents for Free Fluorophore

a

solvent

ET(30) (kcal mol−1)a

E(F) (kcal mol−1)

φ

τ (ps)

1. water 2. methanol 3. ethanol 4. acetonitrile 5. acetone 6. N,N-dimethylformamide 7. 1,4-dioxane 8. tetrahydrofuran 9. ethyl acetate 10. benzene

63.1 55.4 51.9 45.6 42.2 43.2 36.0 37.4 38.1 34.3

64.4 65.7 66.9 68.2 68.4 67.6 70.1 69.4 69.7 70.3

0.14 0.35 0.40 0.21 0.16 0.19 0.06 0.14 0.12 0.03

961 1892 1725 984 906 1595 1132 693 697 950

Reference 32.

ground state. Equation 3 also suggests that the dipolarity/ polarizability mode of interaction is the most important. The value of Δ μ = (μ1 − μ0) for TCMA has been obtained by using the Lippert equation involving absorption and fluorescence energies in different solvents. The optical transition leads to an increase in the dipole moment by Δμ = 7.6 D. Using the value of μ0 = 5.7 D obtained from DFT calculations, the value of μ1 for the free fluorophore has been estimated to be 13.3 D. For the solvents used in the study, the maximum energy of fluorescence E(F) for TCMA is linearly related to the empirical polarity parameter ET(30)32,33 as shown in Figure 3. Table 2

Figure 4. (a) Quantum yield φ versus ET(30) of the monomer (■) and the polymers CP1 (□), and CP2 (*) in pure solvent. (b) Knr and Kr versus ET(30) for monomer (■, □), CP1(▲, Δ), and CP2 (●, ○) in pure solvents. Numbers correspond to the solvent number in Table 2 in both cases.

The E(F) values for the copolymer sample in a given solvent do not depend on the percentage of fluorophore present in the sample, but the value in a particular solvent is different from that of the free fluorophore in that solvent as can be seen from Figure 3. A plot of E(F) for polymer versus ET(30) in different solvents is linear, but the slope of the plot is lower than that for the free TCMA. Thus, it appears that the fluorophore in the free state is relatively more sensitive toward a change in solvent polarity as represented by the parameter ET(30) than that bound to the polymer chain. For aqueous solutions, the value of E(F) comes at a value for the polymer sample that is higher than that of free monomer, indicating that the fluorophore in the polymer environment senses a polarity less than that of pure water, the probe being in a relatively less polar region. On the other hand, for solutions in non-HBD solvents, particularly for nonpolar solvents, the probe unit in the polymer senses a more polar environment. For example, the E(F) value of the free probe in benzene is 70.3 kcal mol−1, whereas that of the probe covalently attached to the polymer is 68.2 kcal mol−1. The values of the quantum yield in a given solvent also show an interesting variation as one goes from the free monomer to the dye bound to the polymer. Thus, the value of φ in non-HBD solvents is greater for the dye bound to polymer, while the reverse is found for HBD solvents. Thus, the variation of φ with the polarity parameter for the fluorophore bound to polymer is noticeably different from that for a free fluorophore. The decay

Figure 3. E(F) versus ET(30) in pure solvents for the free fluorophore (■) and the polymer (●). Numbers correspond to the solvent number in Table 2.

also lists the values of quantum yield φ and lifetime τ for TCMA in different neat solvents. Figure 4a shows plots of φ as a function of the polarity parameter ET(30). An increasing trend of φ with increase in solvent polarity has been observed for a non-HBD environment around the solute (TCMA), while the reverse trend can be observed for the HBD solvents. Alcohols are characterized by high values of φ. Values of the decay constant by radiative and nonradiative pathways (kr and knr, respectively) have been calculated using the measured value of φ and τ. Figure 4b shows a plot of kr and knr as a function of solvent polarity. Note that while the kr values are almost independent of the nature of solvent, those for knr show significant solvent dependence. Here also the variation for nonHBD solvent is different from that for HBD solvents. Alcohols are characterized by a lower value of knr. Significant changes in the fluorescence parameters of the fluorophore have been observed when it is covalently bound to the polymer side chain. 4686



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Figure 5. (a) E(F) versus X(water) and ET(30) for the monomer (■), CP1(□), and CP2 (*); (b) φ versus X(water) and ET(30) for the monomer (■), CP1(□), and CP2 (*); and (c) knr and kr versus X(water) for the monomer (■, □), CP1(▲, Δ), and CP2 (●, ○) in 1,4-dioxane/water solvent mixtures.

of fluorescence for a polymer solution can be expressed by a three-exponential equation, and the values of kr and knr have been calculated using the average lifetime values. Here again the value of kr is practically independent of the nature of the solvent, but the knr values show an interesting solvent dependence (Figure 4b). The variation with solvent polarity of the parameters φ and knr is not very prominent. For water as solvent, the value of φ practically remains the same for the two cases. The results indicate that the polymer in solution exists in a conformation where the choromophoric part of the fluorophore does not sense the same environment as present in solution of the free fluorophore in a neat solvent. Interactions of a polymer solution, namely, solvent−solvent, solvent−segment, and segment−segment interactions, presumably lead to a situation where the fluorophore bound to a polymer side chain is not fully exposed to the solvents. The results indicate that various interactions in a polymer−solvent system are possibly important in modifying the photophysical properties of the fluorophore. Binary Solvent Mixtures. The results for neat solvents prompted us to undertake studies where the solvent− fluorophore interaction is varied by adding a cosolvent. As

the fluorescence parameters depend on the HBD capability of a solvent, we studied three HBD/non-HBD binary solvent mixtures, namely, 1,4-dioxane/water, N,N-dimethylformamide/water, and benzene/methanol. The variation of fluorescence parameters as a function of solvent composition for 1,4-dioxane/water mixtures is shown in Figure 5. The values of the energy of maximum fluorescence E(F) for the free TCMA does not vary linearly over the entire mole fraction range. It is customary to express the value of E(F) in a binary solvent mixtures as the average of the E(F) values in the component solvents (1 and 2), weighted with respect to the mole fraction X of the component solvents.34,35 According to eq 4 E(F ) = X1E(F )1 + X 2E(F )2

(4)

According to this equation, E(F) would vary linearly with the mole fraction of one component solvent over the entire composition range. A linear variation of E(F) with solvent composition has often been explained by assuming that the local composition of the solvent mixture around the solute is similar to that in the bulk (ideal solvation).34,35 The “ideal” line for the case of the free fluorophore is shown in Figure 5a as a 4687



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Figure 6. (a) E(F) versus X(water) and ET(30) for the monomer (■), CP1(□), and CP2 (*); (b) φ versus X(water) and ET(30) for the monomer (■), CP1(□), and CP2 (*); and (c) knr and kr versus X(water) for the monomer (■, □), CP1(▲, Δ), and CP2 (●, ○) in DMF/water solvent mixtures.

quantum yield φ as a function of solvent composition is shown in Figure 5b. It appears that upon addition of water to free TCMA in 1,4-dioxane the value of φ gradually increases, passes through a maximum value, and then decreases. Plots of φ as a function of ET(30), as shown in Figure 5, indicate that for low ET(30) values (1,4-dioxane-rich region) the overall microenvironment around the fluorophore is like that of an nonHBD solvent where φ increases with ET(30). Similarly, the decrease of polarity for higher ET(30) values (water-rich region) suggests that the solvent mixture behaves grossly as HBD solvent. Here also the change in the behavior takes place at ET(30) ≈ 52 kcal mol−1. Values of φ at intermediate compositions exceed those for the pure solvent components. The parameter φ for the fluorophore bound to polymer also shows similar variation with respect to solvent composition and polarity, but only to a lesser extent. The φ values of CP1 are always found to be greater than those of CP2. Variation of knr and kr (calculated using φ and the average lifetime) as a function of solvent composition for the solvent mixture is indicated in Figure 5c. Also in this case, the sample of polymers behaves differently from the free fluorophore. The results in N,N-dimethylformamide/water mixtures are shown in Figure 6. In this case as well, addition of the HBD component (water) to non-HBD dimethylformamide brings about a modification of the local composition around the fluorophore and the changes are different for the fluorophore in free form and that bound in a polymer. The results are similar to those found for 1,4dioxane/water mixtures. This is intelligible in view of the similar nature (non-HBD) of the cosolvents (1,4-dioxane and

continuous straight line. The deviation from linearity of spectroscopic transition in energy has often been explained in terms of preferential solvation of the solute by one of the solvents.36,37 The results obtained in the present case suggest that the free fluorophore is preferentially solvated by water, particularly in the mole fraction range X(water) = 0.0−0.6. The preference of water over 1,4-dioxane can be rationalized in view of the H-bond formation between the carbonyl group of the coumarin derivative and water. The results in the mole fraction range X(water) = 0.6−1.0 suggest that the solvent composition in the microenvironment around the fluorophore is almost the same as that in the bulk. For the polymers on the other hand, an approximately linear relation of E(F) with solvent composition over the entire range has been observed (the dotted line in Figure 5a). The variation of E(F) values with solvent polarity ET(30) shows interesting features. For the free fluorophore, the variation of E(F) versus ET(30) is bilinear. The plots divide the composition range into two regions. For lower values of ET(30), characteristic of non-HBD solvents, a straight line with a slope higher than that obtained for higher ET(30) values (characterizing HBD solvents) is observed. The transition from one region to the other takes place at ET(30) ≈ 52 kcal mol−1. Thus, for the free fluorophore, the HBD capability of the solvent mixture is important in determining the E(F) values for ET(30) beyond 52 kcal mol−1. For the bound polymer, however, a linear correlation has been obtained over the entire polarity range. The slope of the straight line is equal to that for the region where the HBD capability plays an important role for the free fluorophore. The variation of 4688



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Figure 7. (a) E(F) versus X(methanol) and ET(30) for the monomer (■), CP1(□), and CP2 (*); (b) φ vs X(methanol) and ET(30) for the monomer (■), CP1(□), and CP2 (*); (c) knr and kr versus X(methanol) for the monomer (■, □), CP1(▲, Δ), and CP2 (●, ○) in benzene/ methanol solvent mixtures.

the fluorophore, particularly in the mole fraction range of 0.2− 0.8 of the HBD component. Aqueous Surfactant Solutions. Fluoroscence properties of the fluorophore in the free state and that bound to a polymer have been monitored as a function of surfactant (SDS) concentration in aqueous solution. The wavelength of fluorescence maximum remains practically unaltered as the concentration of surfactant in solution changes. The intensity of fluorescence, however, shows a significant change. Figure 8a shows the fluorescence intensity of the free fluorophore as a function of the SDS concentration. It appears that the fluorophore indicates an onset of aggregation of SDS at a concentration of c(SDS) = 8 mM. But the variation of fluorescence intensity with the concentration of surfactant differs dramatically from that observed for the free fluorophore. Figure 8b,c shows such variations. Two inflection points have been indicated. Thus, for CP1, the inflection point appears at concentration c(SDS) = 2.2 and 12.9 mM, while for CP2 the values are c(SDS) = 3.9 and 11.8 mM. The appearance of two inflection points can be explained by assuming the existence of three distinct regions in the polymer−surfactant system, as represented by eq 5. Below a certain concentration of SDS, as

N,N-dimethylformamide) . The variation of E(F) values with solvent composition for benzene/methanol mixtures are shown in Figure 7. While the values for the free fluorophore is nonlinear, indicating a preferential solvation of the dye by methanol, the results for the polymer sample are approximately linear. As discussed earlier, linear variation of E(F) with solvent composition has often been explained by assuming that the local composition of the solvent mixture around the solute is similar to that in the bulk. Thus, studies in solvent mixtures also indicate that the fluorophore−solvent interaction is modified when it is bound to the polymer side chain. One interesting point to note is that the variation of photophysical parameters with solvent composition for all solvent mixtures can be broadly classified into three distinctly different regions, namely, 0.0 to 0.2, 0.2 to 0.8, and 0.8 to 1.0 mole fraction of the HBD component. The value of the photophysical parameter passes through a maximum at an intermediate solvent composition. The appearance of a maximum or minimum of a solute property in a binary solvent mixture has often been interpreted in terms of solvent−solvent interaction.38−41 The results in the present case indicate that solvent−solvent interactions play a significant role in modifying the photophysical parameters of 4689



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Figure 8. Fluorescence intensity versus log c(SDS): (a) free fluorophore, (b) CP1, and (c) CP2.

the fluorophore in the system.24 The observed value of fluorescence intensity I can be written as

represented by the first inflection point (c(SDS) = 2.2 mM for CP1 and 3.9 mM for CP2), the system consists mainly of polymer and SDS monomers in aqueous phase. No significant interaction of polymer and SDS is observed in this region (region1). Beyond this concentration, the polymer interacts cooperatively with SDS and polymer−SDS aggregates are formed. The concentration of SDS corresponding to the first inflection point c1 can be regarded as the critical aggregation concentration (CAC) of SDS for this polymer. This continues up to a concentration of SDS when all the polymers in solution interact completely with SDS. This concentration (c2) is often called the polymer saturation point (PSP). Polymer−SDS aggregates exist in the region of concentration between c1 and c2 (region 2). Beyond the concentration c2 of SDS (region 3), free SDS micelles begin to form and remain in equilibrium with polymer−SDS aggregates. The state of affairs can be represented schematically by the following equilibrium.

I = I1 + [K12I2 − I1]v(c T − c0) + (1 − K12)vI(c T − c0) (6)

In eq 6, I1 and I2 are the values of I when it exists completely in phase 1 and 2, respectively; cT is the total surfactant concentration, c0 represents the critical concentration (c1 or c2 in the present case), v the molar volume of SDS, and K12 the molar-based distribution coefficient of the solute between the two phases 1 and 2. Values of K12, I1, and I2 that fit eq 6 best in a least-squares sense can be obtained by a multiple linear regression analysis. Using the data set in the region where the equilibrium between aqueous phase and polymer−SDS aggregates can be assumed to exist (c(SDS) in the range of 2.2−3.0 mM), we have calculated (assuming c0 = 2.2 mM) the value of K12 as 65 for the polymer sample CP1. Similarly, from the data set in the range of concentration c(SDS) ≈ 12.9−15.0 mM, and assuming c0 = 12.9 mM, we get K23 = 1.3 × 104 for CP1. Similar calculations, using an appropriate concentration range and c0 values for the sample CP2, yields K12 = 3 × 103 and K23 = 1.2 × 104. The higher value of K12 obtained for CP2 indicates that the polymer−SDS aggregate formation is favored when the percentage of fluorophore is greater in the polymer sample. The value of distribution coefficients for the second equilibrium process in eq 5 has been found to be greater than that for the first equilibrium for both the polymer samples. The formation of polymer−SDS aggregates is often ascribed to a process of opening up of a polymer coil because of interaction with SDS molecules.43 The average size would diminish

aqueous phase ⇌ polymer−SDS aggregates⇌ free SDS micelles (free SDS + polymer) (region 1) (region 2) (region 3)

(5)

As stated earlier, the values of c1 and c2 depend on the percentage of fluorophore in a polymer. Values of distribution coefficients characterizing the distribution of fluorophore according to the above equilibrium can be estimated by an analysis described in a previous contribution.42 The value of the spectroscopic parameter, used to reveal equilibrium (eq 5), can be assumed to be determined by the time-average location of 4690



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chain senses a different environment than that sensed by the free fluorophore. Solvent−solvent interactions are likely to be important in such cases. The fluorescence intensity shows a dramatic change when SDS is added to the aqueous solution of the polymer. The results can be explained by assuming the existence of a successive equilibrium involving polymer, free SDS, polymer−SDS aggregates, and SDS micelles in aqueous solution. Distribution coefficients, characterizing the distribution of the fluorophore in different phases, have been estimated. The average size of the polymer in aqueous solution decreases because of addition of SDS.

because of this uncoiling process. DLS studies, as shown in Figures 9 and 10, illustrate this. Thus, the average size of a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +919434238073. Fax: +9133-25873020.

Figure 9. DLS spectra of CP1, CP1 + 5 mM SDS, and CP1 + 14 mM SDS.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.K. thanks CSIR (India) for senior research fellowship. P.D. acknowledges Department of Science and Technology (DST), India [Project SR/S1/OC-51/2010] for financial support. S.B. is grateful to UGC (India) for an emeritus fellowship.

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REFERENCES

(1) Jones, G., II; Jackson, W. R.; Halpern, H. A. Medium Effects on Fluorescence Quantum Yields and Lifetimes for Coumarin Laser Dyes. Chem. Phys. Lett. 1980, 72, 391−395. (2) Jones, G., II; Jackson, W. R.; Kanoktanaporn, S.; Halpern, A. L. Solvent Effects on Photophysical Parameters for Coumarin Laser Dyes. Opt. Commun. 1980, 33, 315−320. (3) Jones, G., II; Jackson, W. R.; Choi, C.; Bergmark, W. R. Solvent Effects on Emission Yield and Lifetime for Coumarin Laser Dyes: Requirements for a Rotatory Decay Mechanism. J. Phys. Chem. 1985, 89, 294−300. (4) Winnik, F. M.; Regismond, S. T. A. Fluorescence methods in the Study of the Interactions of Surfactants with Polymers. Colloids Surf., A 1996, 118, 1−36. (5) De Paula, R.; De Hora, A. E.; Machado De Miranda, J. A. 3Benzoxazol-2-yl-7-(N,N-diethylamino)-Chromen-2-one as a Fluorescence Probe for the Investigation of Micellar Microenvironments. J. Photochem. Photobiol., A 2004, 165, 109−114. (6) Das, K.; Jain, B.; Gupta, P. K. Photophysics of Coumarin 500 and Coumarin 151 in AOT Reverse Micelles. Chem. Phys. Lett. 2005, 410, 160−164. (7) Tamoto, Y.; Segawa, H.; Shirota, H. Solvation Dynamics in Aqueous Anionic and Cationic Micelle Solutions: Sodium Alkyl Sulfate and Alkyltrimethylammonium Bromide. Langmuir 2005, 21, 3757− 3764. (8) Takahashi, Y.; Shimada, R.; Maeda, A.; Kojima, K.; Uchida, K. Photophysics of Coumarin 4 Doped-Amorphous Silica Glasses Prepared by the Sol-Gel Method. J. Lumin. 1996, 68, 187−192. (9) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U.; Orel, B. Study of Laser Action of Coumarine-153 Incorporated in Sol−Gel Made Silica/ Poly(propylene oxide) Nanocomposite Gels. Chem. Phys. Lett. 2001, 345, 381−385. (10) Takahashi, Y.; Maeda, A.; Kojima, K. Optical pH Sensing Characteristics in Dye-Doped Sol−Gel Coating Film Based on the Energy Transfer. Jpn. J. Appl. Phys. 2003, 42, 4369−4377. (11) Yamaguchi, A.; Amino, Y.; Shima, K.; Suzuki, S.; Yamashita, T.; Teramae, N. Local Environments of Coumarin Dyes within Mesostructured Silica−Surfactant Nanocomposites. J. Phys. Chem. B 2006, 110, 3910−3916.

Figure 10. DLS spectra of CP2, CP2 + 5 mM SDS, and CP2 + 14 mM SDS.

polymer CP1 in aqueous solution is ca. 28 nm (Figure 9). The size drastically diminishes (to ca. 6 nm) as a small quantity of SDS (5 mM) is added to it. Addition of an excess of SDS does not bring about any significant change in the size. Thus, it can be assumed that polymer−SDS aggregate formation leads to a decrease in the average size of the polymer in solution. The DLS studies of CP2 solution in the presence and absence of SDS show similar features (Figure 10). Thus, fluorophore molecules become more exposed to water. Enhancement of fluorescence intensity of the fluorophore on the formation of polymer−SDS aggregate may be due to the formation of the exposed fluorophores. Finally, when micellization of SDS takes place after PSP, the strong interaction between the micelle and the fluorophore results in a further increase in the fluorescence intensity. This is supported by the observation that the fluorescence intensity of free TCMA increases abruptly after the onset of micellization of SDS. The value of the distribution coefficient of the free fluorophore has been estimated using eq 6, and its value is 1.9 × 103. This also indicates a strong fluorophore−micelle interaction.

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CONCLUSIONS Fluorescence properties in a pure solvent of a coumarin probe are significantly modified when it is covalently bounded to a polymer chain. This depends on the polarity and the HBD capability of the medium. The polarity sensed by the fluorophore bound to the polymer is different from that sensed by the free fluorophore in a given solvent. Interactions in polymer−solvent systems are modified in HBD and non-HBD solvent mixtures, and the fluorophores bound to a polymer 4691



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(32) Reichardt, C.; Welton, T. Solvents and Solvent Effects in Organic Chemistry, 4th ed., Wiley-VCH: Weinheim, Germany, 2011. (33) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. (Washington, DC, U.S.) 1994, 94, 2319−2358. (34) Marcus, Y. Ion Solvation; Wiley: Chichester, U.K., 1985. (35) Ben-Naim, A. Preferential Solvation in Two-Component Systems. J. Phys. Chem. 1989, 93, 3809−3813. (36) Banerjee, D.; Laha, A. K.; Bagchi, S. Studies of Solvation in Homogenous Media by a Spectroscopic Method: A Ketocyanine Dye in Neat and Mixed Binary Solvents. J. Photochem. Photobiol., A 1995, 85, 153−159. (37) Shannigrahi, M.; Pramanik, R.; Bagchi, S. Studies of Solvation in Homogeneous and Heterogeneous Media by Electronic Spectroscopic Method. Spectrochim. Acta., Part A 2003, 59, 2921−2933. (38) Kovacs, H.; Laaksonen, A. Molecular Dynamics Simulation and NMR Study of Water Acetonitrile Mixtures. J. Am. Chem. Soc. 1991, 113, 5596−5605. (39) Vaisman, I.; Berkowitz, M. L. Local Structural Order and Molecular Associations in Water−DMSO Mixtures. Molecular Dynamics Study. J. Am. Chem. Soc. 1992, 114, 7889−7896. (40) Ferrario, M.; Haughney, M.; McDonald, L. R.; Klein, M. Molecular-Dynamics Simulation of Aqueous Mixtures: Methanol, Acetone, and Ammonia. J. Chem. Phys. 1990, 93, 5156−5166. (41) Pramanik, R.; Das, P. K.; Bagchi, S. Solubility and Solvation Interaction in Neat and Mixed Binary Solvents. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 1999, 38A, 906−912. (42) Shannigrahi, M.; Bagchi, S. Influence of a Neutral Polymer (PVP) on the Solvatochromic Properties of SDS Micelles. J. Phys. Chem. B 2005, 109, 14567−14572. (43) Nagarajan, R.; Kalpakcir, B. Viscosimetric Investigation of Complexes between Poly-ethylene Oxide and Surfactant Micelles. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1982, 23, 41−42.

(12) Sabatini, C. A.; Pereira, R. V.; Gehlen, M. H. Fluorescence Modulation of Acridine and Coumarin Dyes by Silver Nanoparticles. J. Fluoresc. 2007, 17, 377−382. (13) Costela, A.; Moreno, I. G.; Barroso, J.; Sastre, R. Laser Performance of Coumarin 540A Dye Molecules in Polymeric Host Media with Different Viscosities: From Liquid Solution to Solid Polymer Matrix. J. Appl. Phys. 1998, 83, 650−660. (14) Brahmbhatt, D. I.; Hirani, B. R. Synthesis and Characterization of Coumarin−Trioxane−Urea Copolymers. High Perform. Polym. 1996, 8, 295−300. (15) Yen, T.; Davar, M.; Rembaum, A. Potentialities of a New Class of Anticlotting and Antihemorrhagic Polymers. J. Macromol. Sci., Chem. 1970, 4, 693−714. (16) Patel, H.; Patel, S. Coumarin Polymers Derived from Salicylaldehyde-Formaldehyde Polymer. J. Macromol. Sci., Chem. 1984, A21, 343−352. (17) Chen, Y.; Chen, Y.-H.; Wang, J.-H. Radical Copolymerization Behaviors of Styrene and Coumarin. J. Polym. Res. 1994, 1, 295−303. (18) Silence, S. M.; Scott, J. C.; Hache, F.; Ginsburg, E.; Jenkener, R.; Miller, R.; Twieg, R.; Moerner, W. E. Poly(Silane)-Based HighMobility Photorefractive Polymers. J. Opt. Soc. Am. B 1993, 10, 2306− 2312. (19) Chujo, Y.; Sada, K.; Saegusa, T. Polyoxazoline Having a Coumarin Moiety as a Pendant Group: Synthesis and Photogelation. Macromolecules 1990, 23, 2693−2697. (20) Jones, G., II.; Rahman, M. A. Fluorescence Properties of Coumarin Laser Dyes in Aqueous Polymer Media. Chromophore Isolation in Poly(methacrylic acid). J. Phys. Chem. 1994, 98, 13028− 13037. (21) Interactions of Surfactants with Polymers and Proteins; Goddard, D. E.; Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (22) Winnik, F. M.; Regismond, S. T. A.; In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 77, pp 267−316. (23) Zana, R.; Lang, J.; Lianos, P. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985. (24) Turro, N. J.; Baretz, B. H.; Kuo, P.-L. Photoluminescence Probes for the Investigation of Interactions Between Sodium Dodecylsulfate and Water-Soluble Polymers. Macromolecules 1984, 17, 1321−1324. (25) Moad, G.; Chong, Y. K.; Postma, A.; Rizzardo, E.; Thang, S. H. Advances in RAFT Polymerization: The Synthesis of Polymers with Defined End-Groups. Polymer 2005, 46, 8458−8468. (26) Wang, R.; McCormick, C. L.; Lowe, A. B. Synthesis and Evaluation of New Dicarboxylic Acid Functional Trithiocarbonates: RAFT Synthesis of Telechelic Poly(n-butyl acrylate)s. Macromolecules 2005, 38, 9518−9525. (27) Venkataraman, S.; Wooley, K. L. Synthesis and Characterization of Block Copolymers Containing Poly(di(ethylene glycol) 2-ethylhexyl ether acrylate) by Reversible Addition−Fragmentation Chain Transfer Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5420−5430. (28) De, P.; Gondi, S. R.; Roy, D.; Sumerlin, B. S. Boronic AcidTerminated Polymers: Synthesis by RAFT and Subsequent Supramolecular and Dynamic Covalent Self-Assembly. Macromolecules 2009, 42, 5614−5621. (29) Lai, J. J.; Nelson, K.; Nash, M. A.; Hoffman, A. S.; Yager, P.; Stayton, P. S. Dynamic Bioprocessing and Microfluidic Transport Control with Smart Magnetic Nanoparticles in Laminar-Flow Devices. Lab Chip 2009, 9, 1997−2002. (30) Demas, J. N.; Crosby, G. A. Measurement of Photoluminescence Quantum Yields. Review. J. Phys. Chem. 1971, 75, 991−1024. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. 4692


Synthesis of a Polymer Bearing Several Coumarin Dyes along the

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