Solvation dynamics of 4-aminophthalimide in a polymer (PVP ...

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Partha Dutta, Dipankar Sukul, Sobhan Sen and Kankan Bhattacharyya*

Published on 30 September 2003. Downloaded by Jawaharlal Nehru University on 27/07/2015 15:16:32.

Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. E-mail: [email protected]; Fax: (91)-33-2473-2805

PCCP

Solvation dynamics of 4-aminophthalimide in a polymer (PVP)–surfactant (SDS) aggregate

Received 11th July 2003, Accepted 16th September 2003 First published as an Advance Article on the web 30th September 2003

Solvation dynamics of 4-aminophthalimide (4-AP) in a polymer–surfactant aggregate comprising poly(vinylpyrrolidone) (PVP) and sodium dodecyl sulfate (SDS) is reported. In an aqueous solution even at a polymer concentration much higher than the cross over value (C*), a significant portion (30%) of 4-AP molecules do not bind to PVP. In the presence of a surfactant, SDS, 4-AP binds to the PVP–SDS aggregate completely at a polymer concentration greater than C*. The critical aggregation concentration (c.a.c.) of SDS for PVP is found to be 2 mM. The solvation dynamics of 4-AP in a PVP–SDS aggregate is observed to be slower compared to that in bulk water or in SDS micelles. The slow solvation dynamics is attributed to restrictions imposed on the water molecules squeezed between the polymer chain and micellar aggregates.

1. Introduction Water molecules confined in many biological and organized assemblies play a crucial role in the structure, dynamics, molecular recognition and biological function of a biological system.1–4 In a confined environment, water molecules exhibit an ultraslow component of solvation dynamics which is slower by 2–3 orders of magnitude compared to that in bulk water ( C *.

increase in ff of 4-AP is accompanied by a blue shift of the emission maximum to 535 nm. The binding constant, Kb corresponding to the following equilibrium, 4-AP þ PVP Ð ½4-AP : PVP


2. Experimental 4-Aminophthalimide (4-AP, Kodak) is purified by repeated recrystallization from a methanol–water mixture. Poly(vinylpyrrolidone) (PVP, K 90, Fluka) and sodium dodecyl sulfate (SDS, Aldrich) are used as received. The steady-state absorption and emission spectra were recorded in a JASCO 7850 spectrophotometer and a Perkin-Elmer 44B spectrofluorimeter, respectively. The emission quantum yield (ff) of 4-AP was determined using reported ff ¼ 0.01 for 4-AP in water.35 For lifetime measurements, the sample was excited at 300 nm by the second harmonic of a rhodamine 6G dual jet dye laser with 3,30 -diethyl-oxadicarbocyanine iodide (DODCI) as saturable absorber (Coherent 702-1) synchronously pumped by a CW mode locked Nd:YAG laser (Coherent Antares 76s). The emission was collected at magic angle polarization using a Hamamatsu MCP photomultiplier (2809U). Our time correlated single photon counting (TCSPC) set up consists of Ortec 935 QUAD CFD and Tennelec TC 863 TAC. The data are collected with a PCA3 card (Oxford) as a multi-channel analyzer. The typical full width at half maximum (FWHM) of the system response is about 50 ps.

3. Results 3.1 Steady state studies In an aqueous solution, the absorption and the excitation spectra of 4-AP do not change on addition of 10 wt.% PVP or 160 mM SDS or both. However, the emission properties of 4-AP change markedly on addition of PVP to an aqueous solution of 4-AP. Fig. 1 displays emission spectra of 4-AP in the presence of PVP. In an aqueous solution, the emission quantum yield (ff) of 4-AP is 0.01 and the emission maximum is at 550 nm.35 On addition of PVP to an aqueous solution of 4-AP, ff of 4-AP gradually increases to 0.025 (i.e. about 2.5 times) at 10 wt.% PVP. The

Fig. 1 Emission spectra of 5 105 M 4-AP in 10 wt.% PVP (---), in 160 mM SDS (—) and in the presence of both 10 wt.% PVP and 160 mM SDS ( ).

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is determined from the plot (Fig. 2) of 1/Dff against 1/[PVP], and is 7.5 mM1. Thus in 10 wt.% PVP, about 70% 4-AP remains bound to PVP and 30% remains free. In the absence of polymer, on addition of SDS to an aqueous solution of 4-AP the emission maximum gradually shifts to 532 nm and ff gradually increases and reaches a plateau above 150 mM.36 The plateau indicates that at a SDS concentration > 150 mM, almost all the 4-AP molecules remain bound to the SDS micelles. The magnitude of ff at the plateau (0.025) indicates that ff of micelle bound 4-AP is 2.5 times higher than that in bulk water (0.01).36 On addition of SDS to an aqueous solution of 4-AP containing 10 wt.% PVP, ff of 4-AP increases to 0.1 at 160 mM SDS and the emission maximum of 4-AP shifts to 510 nm. It is evident that the emission intensity of 4-AP in 10 wt.% PVP and 160 mM SDS is nearly 4 times larger than those in 10 wt.% PVP alone or 160 mM SDS alone. The emission maximum of 4-AP in 10 wt.% PVP and 160 mM SDS is blue shifted by 25 nm from that in 10 wt.% PVP and 22 nm from that in 160 mM SDS. This suggests that the microenvironment of 4AP in the presence of 10 wt.% PVP and 160 mM SDS is different from that in the presence of the polymer alone or that in the SDS micelle. This indicates that even at a PVP concentration of 10 wt.% ( > C*), SDS penetrates the polymer network and the surfactant molecules surround the polymer chains. The increase in the emission intensity and the blue shift of the emission maxima of 4-AP inside the PVP–SDS aggregate suggest that the 4-AP molecules trapped between the micellar surface and the polymer chains, experience a lower polarity compared to that in the polymer alone or in SDS micelle. Note that the probe 4-AP may form hydrogen bonds through the protons of both the amino and the imino groups. One of them may be hydrogen bonded to the carbonyl group of PVP and the other may form a hydrogen bond with the sulfate oxygen of SDS. Thus 4-AP may act as a bridge between PVP and SDS micelles. This may be the reason why 4-AP binds more strongly to the PVP–SDS aggregate than to PVP alone or SDS alone.

Fig. 2 Plot of 1/Dff against 1/[PVP] in an aqueous solution containing 5 105 M 4-AP.


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Fig. 3a shows variation of ff of 4-AP in an aqueous solution containing 10 wt.% PVP on addition of SDS. In this case the plot of ff vs. [SDS] exhibits a slope change at 2 mM SDS and this is considered to be the c.a.c. of SDS for PVP. Evidently the c.a.c. (2 mM) is 4 times smaller than the c.m.c. (8 mM) of SDS. In the presence of the polymer, micellar aggregates are formed when free SDS concentration exceeds the c.m.c.23 Arai et al. reported that for complete adsorption of SDS, the weight ratio of PVP to bound SDS is 1:2.3 regardless of the concentration of PVP in solution.26 Thus, in a solution containing 10 wt.% PVP (K90), the concentration of bound SDS is 23 wt.% (i.e. about 800 mM) and hence, no micellar aggregates are formed at a SDS concentration below 800 mM. Our observation is similar to this. ff of 4-AP in SDS micelles (0.025) is 4 times lower than that (0.1) in the PVP–SDS aggregate. Thus, in a PVP solution on addition of a very large amount of SDS when micelles were formed, ff of 4-AP should decrease as the 4-AP molecules would be partitioned between the PVP–SDS aggregate and SDS micelles. However, no such decrease in ff is observed even at a SDS concentration as high as 300 mM (Fig. 3a). This suggests that in a 10 wt.% PVP solution, SDS micelles are not formed even at a SDS concentration of 300 mM. Fig. 3b describes the effect of the addition of SDS on ff of 4AP in an aqueous solution containing 0.5 wt.% PVP (i.e. 15 mM (Fig. 3b), the emission maximum of 4-AP is found to be different from that in a SDS micelle. In a solution containing 0.5 wt.% PVP, even in 300 mM SDS, the emission maximum of 4-AP is at 524 nm which is in between that in the PVP–SDS aggregate (510 nm) and SDS micelles (532 nm).

The difference in the effect of SDS on ff of 4-AP in 0.5 wt.% PVP and 10 wt.% PVP may arise from the following reasons. Firstly, as indicated by the time resolved emission data (discussed in the next section) in 0.5 wt.% PVP and SDS a significant amount of 4-AP remains free in bulk water with a low ff (0.01). This shows that in the presence of 0.5 wt.% PVP, 4-AP binds less strongly to the PVP–SDS aggregate compared to 10 wt.% PVP. Secondly, the microenvironment in the PVP–SDS complex at a PVP concentration 0.5 wt.% (i.e. below C*) may be different from that for 10 wt.% PVP (i.e. above C*). It seems that in 0.5 wt.% PVP, even at 300 mM SDS 4-AP molecules remain distributed among three environments bulk water, PVP–SDS aggregate and SDS micelles. 3.2 Time-resolved studies Usually, for a solvation probe the time resolved studies exhibit a decay at the blue end of the emission spectra, while at the red end a rise precedes the decay. The rise at the red end roughly follows the decay at the blue end. In the case of solvation, the decay at the blue end becomes complete much before that at the red end.1–4,37 Thus the decay at the red end is longer than that at the blue end. However, in an aqueous solution containing 10 wt.% PVP, the emission decay of 4-AP at the red end does not exhibit a rise and appears to be faster than that at the blue end (Fig. 4a). Similar behavior is observed in the presence of the polymer–surfactant aggregate at a PVP concentration below C* (e.g. in 0.5 wt.% PVP and 15 mM SDS). This shows that 4-AP does not exhibit solvation dynamics in 10 wt.% PVP or PVP–SDS aggregate at a PVP concentration below C*. In these two cases, 4-AP does not exhibit solvation dynamics for the following reason. In both cases a significant amount of 4-AP (30% in the case of 10 wt.% PVP) does not bind to PVP (or PVP–SDS) aggregate and remains free in bulk water. The emission spectrum of free 4-AP in bulk water is red shifted compared to that for the few 4-AP molecules inside the nonpolar interior of PVP or PVP–SDS aggregate and also the emission decay of free 4-AP in bulk water is faster than that for 4-AP confined in a non-polar environment. To summarize, in 10 wt.% PVP alone or in a PVP–SDS aggregate at a PVP concentration less than C*, a significant amount of 4-AP remains in bulk water in unbound form and this obscures detection of solvation dynamics. In the presence of 10 wt.% PVP and 160 mM SDS, the temporal characteristics of the emission decays of 4-AP resemble those of other solvation probes. Note, under this condition,

Fig. 3 Variation of quantum yield (ff) of 5 105 M 4-AP in PVP with increasing concentration of SDS. The initial part of the variation is shown in the inset. (a) 10 wt.% PVP and (b) 0.5 wt.% PVP.


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Fig. 5 Time resolved emission spectra of 5 105 M 4-AP in 10 wt.% PVP in the presence of 160 mM SDS at 0 ps (L), 150 ps (X), 500 ps (*) and 3000 ps (N).

Fig. 4 (a) Fluorescence decays of 4-AP in 10 wt.% PVP at (i) 475 nm and (ii) 600 nm. (b) Fluorescence decays of 4-AP in 10 wt.% PVP in the presence of 160 mM SDS at (i) 450 nm, (ii) 495 nm and (iii) 625 nm.

all the 4-AP molecules remain bound to the PVP–SDS aggregate and the concentration of free probe is negligible. In this case, a distinct rise is observed in the fluorescence decays at the red end of the emission spectrum while at the blue end, only fast decays are observed (Fig. 4b). The decay at the blue end (at 450 nm) may be fitted to a biexponential with components of 550 ps (65%) and 7.45 ns (35%). On the other hand at the red end (at 605 nm) a decay of 8.20 ns component is preceded by a distinct growth with a component of 235 ps. Such a wavelength dependence clearly indicates that the 4-AP molecules undergo solvation dynamics in the polymer–surfactant aggregate. From the parameters of best fit to the emission decays and using the steady state emission spectra, time resolved emission spectra (TRES, Fig. 5) of 4-AP in PVP– SDS aggregate have been constructed following the procedure described by Fleming and Maroncelli.37 The solvation dynamics is described by the decay of the response function C(t) which is defined as, CðtÞ ¼

nðtÞ nð1Þ nð0Þ nð1Þ

where n(0), n(t) and n(1) denote the observed emission energies (frequencies) at time zero, t and infinity. The total Stokes shift, Dn ¼ n(0)–n(1), is found to be 710 cm1 (Table 1). The decay parameters of C(t) for 4-AP in PVP–SDS aggregate are given in Table 1. The decay of C(t) is shown in Fig. 6. The average solvation time (htsi ¼ a1t1 + a2t2) is found to be 380 ps. It may be recalled that in previous work,36 we found that the solvation dynamics of 4-AP in 160 mM SDS is very fast with ts ¼ 80 ps. Thus in a PVP–SDS aggregate the solvation dynamics of 4-AP is about 5 times slower than that in SDS micelle.

this work, in 10 wt.% PVP about 70% 4-AP binds to PVP and the rest (30%) remains unbound. In the presence of 160 mM SDS and 10 wt.% PVP, ff of 4-AP increases about 10 times relative to bulk water. It is quite clear that in the presence of high concentration ( > C*) of polymer, the surfactant (SDS) molecules penetrate the pseudo-network of the polymer chains. It should be pointed out that if SDS micelles were formed at a SDS concentration > c.m.c. (8 mM) and if 4-AP were partitioned between the polymer–surfactant aggregate and micelle, ff should have decreased with an increase in concentration of the surfactant beyond the c.m.c. But, even at a concentration of 300 mM SDS, in the presence of 10 wt.% PVP, no decrease in ff has been found. This clearly indicates that SDS micelles are not formed even at 300 mM SDS in the presence of high concentration of polymer (10 wt.%). The solvation dynamics of 4-AP in PVP–SDS aggregate is nearly 400 times slower compared to bulk water. Nandi and Bagchi17 proposed that in an aqueous solution of a protein the slow relaxation component arises from dynamic exchange between the bound and free water molecules. Thus, the water molecules bound and trapped in-between the surfactant molecules and the polymer chain may give rise to a slow component of solvation dynamics. The solvation dynamics of 4-AP in PVP–SDS aggregate (htsi ¼ 380 ps) detected in this work is about 3 times faster than that observed for TNS in PVP–SDS aggregate (htsi ¼ 1290 ps).16 However, the time scale of solvation dynamics of 4-AP in PVP–SDS aggregate and TNS in PEG-SDS aggregate (htsi ¼ 430 ps)38 is similar. It should be noted that TNS does not bind to PEG. The similar time scale of solvation dynamics for 4-AP in PVP–SDS aggregate and for TNS in PEG-SDS aggregate, may be due to the absence of strong interaction between the probe and the polymer, in both the cases.

Table 1 Decay parameters of C(t) of 4-AP in the presence of 10 wt.% PVP and 160 mM SDS Dn/cm1

4. Discussion In an aqueous solution containing 10 wt.% PVP, 4-AP exhibits a 2.5 times increase in ff compared to bulk water. As shown in 4878


710 a

10%.

b

a1

t1a /ps

a2

t2a /ps

htsia b /ps

0.60

200

0.40

650

380

htsi ¼ a1t1 + a2t2 .


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Fig. 6 Decay of response function, C(t) of 5 105 M 4-AP in 10 wt.% PVP in the presence of 160 mM SDS. The points denote the actual values of C(t) and the solid line denotes the best fit to a biexponential decay. The initial parts of the decays of C(t) are shown in the inset.

5. Conclusion The neutral solvation probe, 4-AP does not bind very strongly with the polymer PVP even at a high concentration of PVP ( > C *). However, 4-AP binds to the PVP–SDS aggregate when the polymer concentration exceeds C * and shows a 4-fold increase in ff compared to that in the polymer. For 4-AP, the solvation dynamics in a PVP–SDS aggregate (htsi ¼ 380 ps at a PVP concentration > C *) is about 400 times slower compared to bulk water and 3 times faster than that for TNS in a PVP–SDS aggregate (htsi ¼ 1290 ps at a PVP concentration