Photochromic Dye-Doped Conjugated Polymer Nanoparticles

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J. Phys. Chem. C 2009, 113, 13707–13714

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Photochromic Dye-Doped Conjugated Polymer Nanoparticles: Photomodulated Emission and Nanoenvironmental Characterization Elizabeth J. Harbron,* Christina M. Davis, Joshua K. Campbell, Rebecca M. Allred, Marissa T. Kovary, and Nicholas J. Economou Department of Chemistry, The College of William and Mary, Williamsburg, Virginia 23187-8795 ReceiVed: April 24, 2009; ReVised Manuscript ReceiVed: June 16, 2009

We present studies of fluorescence photomodulation and solvatochromism in nanoparticles of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with a photochromic spirooxazine dye. The fluorescence properties of doped nanoparticles with dyes in the spirooxazine form are identical to those in undoped control nanoparticles. UV irradiation converts some of the dyes to their visibleabsorbing merocyanine form, which is an efficient quencher of MEH-PPV fluorescence. The fluorescence intensity of the nanoparticles drops to less than 10% of its initial value and recovers when the merocyanines undergo thermal reversion to spirooxazines. The fluorescence modulation can be cycled many times without fatigue or photodegradation, and the degree of quenching is linear with merocyanine concentration. The photochromic conversion can also be used as a probe of the environment within the nanoparticles as both the kinetics of the thermal merocyanine-to-spirooxazine conversion and the merocyanine absorption spectrum are sensitive to the dye environment. The kinetics of the thermal dye reversion in the nanoparticles are first order and nearly as fast as those in THF, while those in a MEH-PPV film are biexponential and substantially slower. The position of the merocyanine absorption within the nanoparticles is likewise distinct from that in a MEH-PPV film and implies a liquid-like environment that is more polar than THF. We hypothesize that those dyes that undergo spirooxazine-to-merocyanine conversion are adhered to solution-exposed MEH-PPV segments within the nanoparticles or to the particle surface and thus have ample free volume for the photochromic conversion. These findings will be useful in designing future stimulus-responsive nanoparticle systems. Introduction Fluorescent systems in which the emission can be reversibly modulated in response to an external stimulus are the basis for important applications ranging from optical data storage to sensors. An increasing number of systems utilize a photochromic moiety as the stimulus-responsive element for control of emission intensity, color, or other properties. A photochromic molecule has two molecular forms with different properties such as absorbance, dipole moment, and shape.1 A typical photochrome responds to irradiation with ultraviolet (UV) light by converting to its alternate form, which absorbs visible light. This photogenerated form may revert back to the initial form thermally and/or upon irradiation with visible light, with the details varying by photochromic family and molecular structure. A popular strategy for fluorescence photomodulation is to combine a fluorophore with a photochrome and employ selective electron transfer (ET) or fluorescence resonance energy transfer (FRET) to modulate the fluorophore’s fluorescence.2-5 In such systems, the photochrome acts as a gate: it is an efficient FRET acceptor or ET donor/acceptor in one form, quenching the fluorophore’s emission, but in the other form lacks spectral overlap required for FRET or the proper redox potential for ET, leaving the emission unperturbed. Here, we report efficient fluorescence intensity photomodulation in which the fluorophore is a nanoparticle of the conjugated polymer poly[2-methoxy5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and * To whom correspondence should be addressed. E-mail: ejharb@ wm.edu.

the photochromic component is a spirooxazine dye doped into the nanoparticle. Conjugated polymers offer unique advantages as the fluorophore component of fluorophore-photochrome systems designed for fluorescence modulation. A single conjugated polymer chain is multichromophoric, with individual chromophores composed of several monomer units. The heterogeneity in chromophore size and, hence, absorbance enables efficient FRET among chromophores in close proximity, with the excitation energy from higher energy chromophores migrating to lower energy chromophores.6 These intrinsic energy transfer processes enhance the efficiency of fluorescence quenching by an external quencher by effectively delivering the excitation energy of many chromophores to a single quencher. This type of amplified quenching is the basis for conjugated polymer sensing applications.7 We8-10 and others11,12 have used conjugated polymers as the fluorophore in fluorophore-photochrome systems. In addition to their photophysical advantages, conjugated polymers also offer the benefit of processability and can easily be made into films or, as described below, nanoparticles. Our previous work focused on derivatives of the conjugated polymer poly(p-phenylenevinylene) (PPV) that were covalently functionalized with photochromic azobenzene dyes.8-10 This type of fluorophore-photochrome system offers a controlled architecture with known photochrome location and concentration but has the disadvantage that a new synthesis must be developed each time a substantial change in polymer backbone structure or photochromic dye is desired. This drawback can be circumvented by switching from covalently functionalized polymers

10.1021/jp9037864 CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Harbron et al. modulation of nanoparticle fluorescence and (2) to use the SO-MC conversion and the MC absorbance as probes of the dye environment within the doped nanoparticles. The probe study provides evidence for a polar, solution-like environment with sufficient free volume for the SO f MC conversion to occur within the nanoparticles. These results will guide us in the design of future stimulus-responsive nanoparticles. Experimental Methods

to doped dye-polymer systems. Doped polymer films represent the simplest such system but often suffer from heterogeneity due to phase separation. As an alternative, doped polymer nanoparticles offer a fairly well-defined system without complicated synthesis. McNeill and co-workers have developed an aqueous reprecipitation method13,14 for the preparation of small (5 nm and up), surfactant-free, spherical conjugated polymer nanoparticles.15,16 Conjugated polymers are dissolved in an organic solvent that is miscible with water, and this precursor solution is injected rapidly into water to form a stable aqueous suspension of polymer nanoparticles. Nanoparticle size can be controlled by the concentration of the precursor polymer solution,16 and the resulting brightly fluorescent nanoparticles can be studied as aqueous suspensions, drop-cast onto solid substrates for microscopy, or dispersed in an optically transparent polymer host for single particle fluorescence studies.17 The versatility of conjugated polymer nanoparticles has been extended by doping them with other fluorescent polymers and small molecules.13,18-20 The hydrophobicity of the dopants ensures that they are incorporated into the nanoparticles during nanoparticle preparation rather than partitioning between nanoparticles and aqueous solution. Highly efficient FRET is observed in cases where the dopant absorbance overlaps the emission of the host conjugated polymer, with the result that the fluorescence color is that of the dopant rather than the host polymer.18-20 Conjugated polymer nanoparticles have great potential for various photonic applications and offer advantages relative to small molecule fluorophores including their good photostability and brightness.15 We seek to manipulate the emission properties of conjugated polymer nanoparticles but in a reVersible fashion. By employing photochromic dyes as dopants, we create conjugated polymer nanoparticles with photoswitchable emission intensity. We present herein nanoparticles prepared from the conjugated polymer MEH-PPV and a photochromic spirooxazine (SO) dye that undergoes Cspiro-O bond cleavage to form a merocyanine (MC) derivative upon UV irradiation (Scheme 1). The MC form of the dye is an efficient quencher of MEH-PPV fluorescence, while the SO form is unable to act as a quencher and leaves the polymer fluorescence unperturbed. Thus, nanoparticle fluorescence intensity can be photocontrolled through the SOMC conversion. Li and co-workers have also used a reversible quenching strategy in nanoparticles composed of nonfluorescent polymers, perylene diimide fluorophores, and photochromic spiropyran dyes, which are structurally similar to spirooxazines.21 Our goals in studying the SO-doped MEH-PPV nanoparticles are twofold: (1) to demonstrate efficient photo-

Materials. All chemicals were obtained from Acros and were used as received. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with an average molecular weight of 260 000 and a polydispersity of 4.3 was obtained from American Dye Source (Quebec, Canada) and used as received. The spirooxazine dye 9′-methoxy-1,3,3-trimethylspiro(indoline2,3′-[3H]-naphth[2,1-b][1,4]oxazine) (SO) was synthesized according to literature procedures.22-24 Sample Preparation. Undoped and doped MEH-PPV nanoparticles were prepared according to a literature procedure.18 A 1 mg/mL solution of MEH-PPV in anhydrous THF was stirred overnight under argon. The concentrated polymer solution was then filtered through a 0.7 µm filter to remove aggregates and diluted with THF to a concentration of 0.04 mg/mL (40 ppm) to form the precursor solution for undoped nanoparticles. For doped nanoparticles, the desired amount of a 0.1 mg/mL (100 ppm) SO dye solution in anhydrous THF was added to a diluted polymer solution such that the final precursor solution contained 0.04 mg/mL polymer and up to 30 wt % dye. For each nanoparticle preparation, the precursor solution was sonicated to ensure homogeneity, and a 1 mL portion of this solution was injected into 8 mL of sonicating ultrapure water. After an addititional 2 min of sonication, the THF in the water/THF/ nanoparticle mixture was removed by rotary evaporation. The clear, slightly colored aqueous nanoparticle suspension was then filtered through a 0.22 µm filter. For atomic force microscopy (AFM) analysis, the aqueous nanoparticle suspension was diluted with water and drop-cast onto a freshly cleaned glass coverslip. SO-doped polymer films were prepared by spincoating from doped precursor solutions in anhydrous THF. Characterization. Absorption and fluorescence measurements were made with Varian Cary 50 and Varian Eclipse instruments, respectively. Septum-capped semimicro cuvettes (Starna, ∼1 mL sample volume) were used for all spectroscopic experiments, and samples were degassed with argon prior to study. An excitation wavelength of 471 nm was used for all fluorescence experiments. The spirooxazine f merocyanine conversion was induced by irradiating with a 365 nm pencil lamp (Spectroline) for 10 s, which was determined to be sufficient for reaching the photostationary state in SO-doped nanoparticles. One minute of thermal recovery in the dark was more than sufficient to return the dyes to their starting state. The nanoparticle size distributions were determined by AFM using a Digital Instruments Dimension 3100 in tapping mode. Results and Discussion Fluorescence Photomodulation. The ideal photochromic dopant for conjugated polymer nanoparticles designed for photomodulation cannot quench the host polymer in one form, is a very efficient quencher in its alternate form, and is responsive to irradiation wavelengths that do not adversely affect the host polymer. The methoxy-substituted spirooxazine depicted in Scheme 1 was chosen as the dopant for MEH-PPV nanoparticles because it both meets all of these criteria and is of a photochromic class that is particularly noted for high photo-

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Figure 1. Absorption spectra of SO dye in dilute methanol solution before (SO form, solid) and after (MC form, dotted) UV irradiation. Inset: Spectral overlap of the MC absorption spectrum (dotted) with the emission of MEH-PPV nanoparticles suspended in water (solid).

stability.25-27 The thermally stable SO form of the dye absorbs with a λmax value of 336 nm in dilute methanol solution (Figure 1) and has no spectral overlap with the MEH-PPV nanoparticle emission, which has its maximum fluorescence at 590 nm, as shown in the inset of Figure 1. Upon irradiation with UV light, the SO dye converts to its MC form, which retains the absorption in the UV region but also has an additional band in the visible region of the spectrum that overlaps strongly with the nanoparticle emission (Figure 1 and inset). The MC form of the spirooxazine is generally accepted to have the quinoidal structure depicted in Scheme 1, as opposed to the zwitterionic structure of the merocyanine form of the more common spiropyran.28 Because the MC excited state is more polar than its quinoidal ground state, the MC absorption exhibits positive solvatochromism, with the spectrum shifting to the red with increasing solvent polarity. In the methanol solution shown in Figure 1, MC absorbs with a λmax value of 606 nm. In room temperature solution in the dark, photogenerated MC reverts quickly to its SO form (first order t1/2 ∼ 2 s in methanol) in a process commonly called thermal decoloration or thermal bleaching. A thermally bistable photochrome, in which the photogenerated form remains stable until it is irradiated, is desirable for applications like optical data storage. However, the fast thermal bleaching of dyes like SO is required for applications such as ophthalmic lenses in which the MC form is desirable only so long as the external stimulus (light) is present. For this work, the rapid thermal fading of the SO dye facilitates easy study of repeated cycling. SO-doped MEH-PPV nanoparticles and undoped MEH-PPV control nanoparticles were prepared by an aqueous precipitation method that involves rapid injection of a dilute THF precursor solution containing the polymer and dye (if present) into water.16,18 The THF is removed by rotary evaporation, and the resulting aqueous nanoparticle dispersions are clear and faintly orange-colored with no difference in appearance between the doped and undoped nanoparticles. These results are consistent with literature descriptions of other conjugated polymer nanoparticles.15,16 Figure 2 shows the distribution of spherical nanoparticle heights for undoped control nanoparticles and 28 wt % SO-doped nanoparticles as measured by AFM. For both samples, the vast majority of nanoparticles fall in the 5-13 nm range. With a greater number of diameters above 13 nm than the control nanoparticles, the doped nanoparticles appear to be more prone to forming larger particles or aggregates, but these represent a small percentage of the nanoparticles. If analysis is restricted to only those particles with diameters up to 13 nm, both doped and control samples have an average particle size of 8 nm. Given the known molecular weight of the polymer,

Figure 2. Histograms of nanoparticle heights (left) obtained from AFM images (right) for control (top) and 28 wt % SO-doped (bottom) nanoparticles.

Figure 3. Absorption (left) and fluorescence (right) spectra of undoped control and 28 wt % SO-doped MEH-PPV nanoparticles suspended in water.

particle size, and typical organic density,17 it is likely that many of these particles consist of a single polymer chain or just a few smaller chains on the lower end of the molecular weight distribution.15 Absorption and fluorescence spectra of SO-doped MEH-PPV nanoparticles and undoped MEH-PPV control nanoparticles are shown in Figure 3. Spectra of the undoped control nanoparticles are consistent with those in the literature,15,29 with the polymer absorbing at ca. 490 nm and fluorescing with a λmax value of 590 nm. The MEH-PPV absorption spectrum is sensitive to conjugation length: tightly coiled polymer chains have shorter conjugation lengths and blue-shifted absorption λmax values relative to more extended chains, which have longer conjugation lengths and red-shifted absorptions.6 The bluer λmax value of the nanoparticle absorption relative to that for MEH-PPV dissolved in THF (499 nm) or cast as a film (ca. 505 nm) reflects the shorter conjugation lengths of the polymer in the tightly coiled nanoparticles, as described previously.15 The polymer absorption is the same in both doped and undoped nanoparticles, and both exhibit slight batch-to-batch variations that may be due to the polydispersity of the polymer or small differences in size distributions. The only distinction between the doped and undoped absorption spectra is in the UV region, where the SO dye absorption adds to the polymer’s own UV absorption. The fluorescence spectra of SO-doped MEH-PPV nanoparticles are indistinguishable from their undoped counterparts, with no differences in spectral position, shape, or brightness. Successful fluorescence photomodulation requires that the SO dyes respond to UV irradiation to generate MC dyes, which can then act as quenchers. Figure 4 shows the absorption spectra of MEH-PPV nanoparticles doped with 28 wt % SO before

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Figure 4. Absorption spectra of 28 wt % SO-doped MEH-PPV nanoparticles suspended in water before (solid) and after UV irradiation (dotted) and after thermal recovery (dashed). Right inset: Red-edge expansion of the three spectra. Left inset: Difference spectrum created by subtracting the spectrum prior to irradiation from the one recorded immediately after UV irradiation.

irradiation, immediately after UV irradiation, and after 1 min of recovery in the dark. The UV irradiation has no effect on the polymer absorption at 490 nm, as expected, but does induce a small increase in absorbance centered near 600 nm, which can be seen in the expansion in the Figure 4 right-hand inset. That this absorption is the spectrum of the photogenerated MC can be seen more clearly in the left-hand inset of Figure 4, which is the difference spectrum obtained by subtracting the spectrum measured prior to irradiation from that recorded immediately after to remove the polymer absorption. This spectrum is similar in shape and position to the solution-phase MC spectrum shown in Figure 1. The concentration of MC in the nanoparticles is calculated from the MC absorbance in the difference spectrum and, for this particular sample, represents just over 4% of the doped dye molecules. Typically, 1.5-5% of SO dyes in nanoparticles undergo conversion to MC. These values are comparable to those measured in SO-doped MEH-PPV films (2-7%) but substantially less than those in solution (7-10%). The low conversion in nanoparticles and films as compared to solution implies that free volume is restricted in these systems relative to solution with the consequence that fewer dyes have the required free volume to accomplish the SO f MC conversion. The relatively small number of MC dyes generated by UV irradiation of the dye-doped nanoparticles has a substantial effect on the nanoparticle fluorescence. Figure 5A shows fluorescence spectra of MEH-PPV nanoparticles doped with 28 wt % SO before irradiation, after the photostationary state concentration of MC is reached, and after 1 min of recovery in the dark. The spectra show a substantial reduction in fluorescence intensity, with the peak fluorescence decreasing to 7.5% of its initial intensity and then recovering fully. We previously quantified the magnitude of fluorescence modulation by the modulation efficiency (Emod), as shown in eq 1

Emod ) 1 -

Ipss I0

(1)

where I0 and Ipss are the fluorescence intensities before and after UV irradiation to produce the photostationary state concentration of MC dyes, respectively.9 Emod for the sample depicted in Figure 5 is 92.5%, and other samples have shown Emod values up to 95%, which is significantly higher than the highest value observed previously for azobenzene-functionalized PPVs (52%).9 The fluorescence intensity can be cycled numerous times by repeated application of UV and dark recovery periods, as

Figure 5. (A) Fluorescence spectra of 28 wt % SO-doped MEH-PPV nanoparticles suspended in water before (solid) and after UV irradiation (dotted) and after thermal recovery (dashed). Inset: Fluorescence spectra of the nanoparticles in the main graph after dilution with THF to a final composition of 67:33 v/v THF:H2O before (solid) and after UV irradiation (dotted) and after thermal recovery (dashed). (B) Fluorescence intensity of the doped nanoparticles at 590 nm after UV irradiation (low-intensity points) and thermal recovery (high-intensity points) over many cycles.

shown in Figure 5B. Robust cycling of the fluorescence intensity requires that neither the photochrome nor the conjugated polymer undergoes photodegradation or other degradation processes during the cycling period. The SO dye is particularly known for its ability to resist degradation, in contrast to its more commonly used but much less stable relative, the spiropyran.26 The conjugated polymer nanoparticles also exhibited photostability under the conditions of these experiments. Control experiments do not reproduce the modulation observed in the dye-doped nanoparticles. The aqueous suspension of dye-doped nanoparticles represented by Figure 5A was diluted with THF to swell and partially destroy the nanoparticles. The fluorescence spectra of these nanoparticles, shown in the Figure 5A inset, retain the 590 nm λmax value of the nanoparticles but also have a strong shoulder on the blue side of the emission at ca. 555 nm, which reflects a polymer population that is wellsolvated by THF. UV irradiation has no effect on the fluorescence of the THF-diluted nanoparticles: the spectra measured before and immediately after UV exposure and after a dark recovery period are all identical. No fluorescence modulation is observed in spite of the fact that 9% of the dyes undergo photoconversion, a greater than 100% increase in MC population compared to the aqueous nanoparticle suspension. Presumably, the partial destruction of nanoparticle architecture by the added THF releases some of the dyes into solution, where they have more free volume to undergo photoconversion but are no longer in close enough proximity to MEH-PPV chains to be effective quenchers. Likewise, a diluted portion of the SO/MEH-PPV THF precursor solution from which the nanoparticles are created shows no modulation (data not shown). These control experiments demonstrate that the efficient quenching observed here is the result of very close dye-polymer proximity and is not due to simple inner filter effects or other trivial explanations. The efficient fluorescence quenching depicted in Figure 5 was generated by approximately nine MC quenchers for every MEHPPV polymer chain. To explore the dependence of fluorescence quenching on MC concentration, multiple nanoparticle samples

Photochromic Dye-Doped Conjugated Polymer Nanoparticles

Figure 6. Fluorescence quenching of SO-doped nanoparticles versus MC quencher concentration expressed as dyes per polymer chain. Inset: Overlap of the MC absorption difference spectrum from 28 wt % SOdoped MEH-PPV nanoparticles with the nanoparticle fluorescence spectrum.

with different doping levels were prepared and studied. All of the samples yielded the same fluorescence spectrum as the doped nanoparticles depicted in Figure 3 and varied only in the degree of quenching. Figure 6 shows the relationship between fluorescence quenching and quencher concentration, plotted according to the Stern-Volmer equation (eq 2)

I0 ) 1 + KSV[Q] I

(2)

where I0 and I are the unquenched and quenched fluorescence intensities, KSV is the Stern-Volmer quenching constant, and [Q] is the quencher concentration.30 Due to the photoswitchable nature of the nanoparticles, I0 and I can be obtained from peak fluorescence intensity measurements of the same sample: I0 is measured when the dyes are in the nonquenching SO form, and I is recorded immediately after UV irradiation when the maximum concentration of MC quenchers is present. When the quencher concentration is calculated in terms of the number of MC dyes per polymer chain, KSV represents the number of polymer chains quenched by a single MC dye.18 The SternVolmer plot in Figure 6 is linear with some scatter (r ) 0.98), which could be due to the fact that the MC quencher concentration must be calculated from absorbance data following a second UV irradiation of the same sample used to generate the quenching data. The absorbance and fluorescence data sets must be synched with regard to time, and some error is inherent in this process. The Stern-Volmer plot yields a KSV value of 1.4, indicating that each MC quenches the fluorescence of 1.4 polymer chains on average. Since some nanoparticles likely contain only a single polymer chain, these particles may be quenched completely, while others are partially quenched. The MEH-PPV used to prepare the nanoparticles has a molecular weight of 260 000, with an average chain containing 1000 monomer units. Estimates of the chromophore length for MEHPPV range from 4 to 17 monomer units.6,31,32 If a middle-ground estimate of 10 units per chromophore is assumed, then an average polymer chain contains 100 chromophores. Thus, a single MC dye quenches 140 chromophores on average. This large chromophore-per-quencher ratio is possible because of MEH-PPV’s intrinsic energy migration process that can funnel multiple excitations to a single quencher. Such amplification of quenching is the basis for a variety of sensing applications based on conjugated polymers.7

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13711 The fluorescence quenching mechanism in these doped nanoparticles is not known and could be nonradiative energy transfer, radiative energy transfer, electron transfer, or some combination of these mechanisms. The doped MEH-PPV nanoparticles were designed to undergo efficient FRET to MC dyes. The spectral overlap between the donor fluorescence and acceptor absorbance spectra is a key contributor to the energy transfer rate and, ultimately, to the energy transfer efficiency. The Figure 6 inset shows that the spectral overlap of the absorption of MC dyes within the nanoparticles and the nanoparticle fluorescence is considerable. Sheng et al. studied nanoparticles composed of small molecules in which the fluorescence of laser dye DCM was efficiently quenched by the merocyanine form of a spirooxazine dye.33 The DCM-MC spectral overlap was qualitatively similar to that for MC and the MEH-PPV nanoparticles, and time-resolved fluorescence measurements supported a FRET mechanism for the quenching of DCM in those nanoparticles. Overlap is quantified by the spectral overlap integral, J, which depends on the corrected fluorescence intensity of the donor (FD) and the extinction coefficient of the acceptor (εa) as shown in eq 3.30

J(λ) )

∫0∞ FD(λ)εa(λ)λ4 dλ

(3)

The Fo¨rster radius (R0), which is the donor-acceptor distance at which half of the donor chromophores decay via the energy transfer pathway, is proportional to J(λ). The R0 value for the nanoparticle MEH-PPV donor-MC acceptor FRET pair is 3.9 nm, which is roughly half of the average particle diameter of 8 nm. Assuming a FRET mechanism for nanoparticle quenching, significant improvement in quenching efficiency would require substitution of the MC quencher (εa ) 4.3 × 104 M-1 cm-1)34 with an alternate dye with a larger extinction coefficient and/or greater overlap with the nanoparticle fluorescence spectrum. These changes would also improve quenching efficiency if radiative energy transfer contributes to the observed quenching. Photoinduced electron transfer must also be considered as a possible quenching mechanism for the doped MEH-PPV nanoparticles.35 The electrochemistry of spirooxazines and spiropyrans is quite similar.36 In both cases, the dyes are easily reduced only when functionalized with nitro groups that act as electron acceptors.36,37 The SO dye studied here lacks the nitro substituent; accordingly, its reduction by MEH-PPV is extremely unlikely. Spirooxazines and spiropyrans are oxidized by loss of an electron from the nitrogen in the indoline moiety, which is the left-hand side of SO, as it is depicted in Scheme 1.36 This moiety changes little upon photoinduced conversion to the MC form, and electrochemical studies have shown either no change37,38 or a small change39 in oxidation potential upon conversion from the closed spirooxazine or spiropyran to the open MC. Thus, if the dye transferred an electron to MEHPPV in the nanoparticles, it would do so from both SO and MC forms, and the extent of fluorescence quenching would be similar if not identical before and after UV irradiation. As the fluorescence intensity of nanoparticles with dyes in the SO form matches that of undoped nanoparticles, it is clear that quenching occurs only when MC dyes are present. In view of these results and the dye’s electrochemical properties, electron transfer appears to be unlikely and leaves energy transfer as the most likely quenching mechanism. Nanoenvironmental Characterization. The MC form of the SO dye serves not only as an efficient quencher of MEH-PPV

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Figure 7. (A) Decay of MC peak absorbance as a function of time during thermal MC f SO recovery for a 30 wt % SO-doped polystyrene film (black), 30 wt % SO-doped MEH-PPV film (green), 30 wt % SOdoped MEH-PPV nanoparticles (blue), and SO dye in dilute THF solution (red). Inset: Expansion of the decay during the first 15 s. (B) MC absorption difference spectra created by subtracting the spectrum prior to irradiation from the one recorded immediately after UV irradiation for each sample. The samples are the same as in part A except for the red spectrum, which is the dye in 99:1 v/v THF:H2O. Inset: Polynomial fits of the peak top (550-630 nm) for each spectrum.

nanoparticle fluorescence but also as a reporter of the environment inside the nanoparticles. Both the thermal decoloration rate for the MC f SO process and the position of the MC absorption spectrum have been shown to be highly influenced by the dye environment.34 After the MC form of the dye is generated by UV irradiation, the kinetics of thermal decoloration are measured by monitoring the absorbance at the λmax value of the MC dye absorption as the dye decays back to the SO form. Figure 7A shows the MC thermal decoloration kinetics for four different environments: THF solution, nanoparticle suspension, MEH-PPV film, and polystyrene film. In solution, thermal decoloration occurs with first-order kinetics (i.e., monoexponential decay), indicating a homogeneous environment for the conversion. The first-order rate constant, 0.25 s-1 in the THF solution shown in Figure 7A, has a relatively weak dependence on solvent polarity compared to some SO derivatives.34 The rate is, however, sensitive to rigidity and free volume in the dye matrix.40 Any photochromic molecule requires some minimal amount of free volume in order to complete its transformation. In solid matrices such as polymer films, there is typically a distribution of free volume such that some dyes have ample free volume to convert to the other form, some are inhibited but can convert, and others may not convert at all. This distribution of free volume typically manifests itself as biexponential thermal decoloration kinetics for photochromic dyes in rigid polymers below their glass transition temperature (Tg), as exemplified by the thermal decoloration of MC in a film of polystyrene (Tg ) 100 °C41) in Figure 7A. With a Tg value in the 57-67 °C range,42 MEH-PPV is likewise in its glassy state at room temperature and also displays biexponential kinetics, albeit

Harbron et al. somewhat faster than those measured in polystyrene. Remarkably, the thermal decoloration kinetics in aqueous suspensions of doped nanoparticles are first order and nearly as fast as those in homogeneous solution. The Figure 7A inset shows the kinetics for all four environments at early times, and decoloration in the nanoparticles is only slightly slower than that in THF solution (k ) 0.20 vs 0.25 s-1, respectively). In addition to the fit of the decay kinetics, the thermal decoloration process is also characterized by T1/2, the time required for the absorbance of the colored form to decay to half of its initial value.1 The T1/2 values for thermal decoloration of MC are 34 s in the polystyrene film, 15 s in the MEH-PPV film, 3 s in the MEHPPV nanoparticles, and 2.7 s in THF solution. Solution-like MC decoloration kinetics and low T1/2 times have also been recorded in other solid systems including hybrid organic-inorganic matrixes,43 silica particles,44 and mesoporous organosilica films45 but generally not in organic polymers below Tg.40 The thermal decoloration kinetics measured here clearly demonstrate that the nanoparticle dye environment is solution-like in its free volume and completely distinct from the more constrained environment within the MEH-PPV film. The MC absorption spectra provide another means of characterizing the environment within the nanoparticles and yield additional information about the solution-like environment suggested by the kinetic results. The position and shape of the MC absorption spectra are influenced by the polarity of the dye environment in polymer matrixes as well as in solution, albeit to different extents.40,46 Thus, both the polarity and the nature of the medium (solution vs solid matrix) must be considered. Figure 7B shows the MC difference spectra in the MEH-PPV nanoparticle suspension, in MEH-PPV and polystyrene films, and in a THF/H2O mixture. To highlight the differences in peak position and shape of the somewhat noisy spectra, polynomial fits of the peak tops are shown in the inset to Figure 7B. The MEH-PPV and polystyrene films share an asymmetric peak shape, with a shoulder on the high-energy side of the main peak. This peak shape is typical of spirooxazine-derived merocyanines and is attributed to the vibronic nature of the most stable trans isomer of MC, which is depicted in Scheme 1.25 With a 580 nm shoulder and 613 nm peak, MC in the MEH-PPV film is red-shifted relative to the polystyrene film, which has a 572 nm shoulder and 602 nm peak. This difference is consistent with the positive solvatochromism, i.e., a red shift with increasing polarity, that has been observed for spirooxazinederived MC in both solution and polymer films.34,46 MEH-PPV’s static dielectric constant has been estimated to be 4,47 while polystyrene has a value of 2.5,48 and the observed solvatochromic shifts are qualitatively consistent with this difference. The absorption spectrum of MC in MEH-PPV nanoparticles is significantly different from that measured in MEH-PPV films in terms of position and shape. It has an unstructured peak shape with a λmax value of 597 nm, similar in position to that observed in the polystyrene film and quite blue-shifted from that in the MEH-PPV film. Clearly, the dye environment is very different within nanoparticles than in films prepared from the same polymer, MEH-PPV. A blue shift for the MC dye studied here generally indicates a decrease in polarity as described above. However, this generalization does not take important matrix effects into account, and the unusual blue shift most likely reflects a change of medium. In solution, the absorption λmax value of this MC dye ranges from the 550s for nonpolar solvents like cyclohexane to greater than 600 nm for polar solvents like methanol.34 Anhydrous THF yields a λmax value of 574 nm, which shifts to 597 nm when water is added to make a 99:1

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v/v THF:H2O mixture. As shown in Figure 7B, the spectrum measured in the THF/H2O mixture matches the λmax value of that from nanoparticles, although the spectral width and shape are moderately different. These results indicate that the MC dyes within the nanoparticles are in a solution-like environment that is more polar than THF. The dyes may very well be adhered to segments of MEH-PPV that are partially or fully exposed to water. The solution-like thermal decoloration kinetics (Figure 7A), which are nearly identical to those measured in THF, provide strong support for the idea that the dyes are in contact with the surrounding aqueous environment. Taken together, the kinetic data and spectral shifts point toward a highly solutionlike environment that is more polar than THF and has ample free volume for the SO-MC conversion. Aggregation of MC dyes is the most likely alternative explanation for the observed spectral shifts. The manifestation of dye aggregation in absorption spectra is well-known: Jaggregates shift a dye spectrum to the red, and H-aggregates shift it to the blue. Theoretically, the blue shift in λmax and increase in intensity in the 400-500 nm range of the MC spectrum upon going from MEH-PPV film to nanoparticles could be explained by H-aggregates. However, spirooxazinederived merocyanines with a quinoidal structure are far less likely to aggregate than those derived from spiropyrans, which have a zwitterionic structure.49,50 Aggregation of quinoidal merocyanines is generally not observed, even in systems such as liposomes in which aggregation might be expected.51 We searched for signs of dye aggregation in the doped MEHPPV nanoparticles in their fluorescence properties, absorption kinetics, and absorption spectra. It is known that nitrospiropyranderived merocyanines can become fluorescent when aggregated52 or when encapsulated in hydrophobic pockets.53,54 In contrast, spirooxazine-derived merocyanines do not normally exhibit fluorescence55 due to the different photophysics of the SO-MC conversion, which occurs exclusively in the singlet manifold.26 We varied the fluorescence excitation wavelength in an attempt to observe MC fluorescence in the SO-doped nanoparticles but detected only the same MEH-PPV nanoparticle fluorescence spectrum shown in Figure 3 under all conditions. Aggregation would also affect the thermal decoloration kinetics and MC absorption spectra as a function of dye concentration. We varied the dye doping level widely and measured kinetic rate constants ranging from 0.17 to 0.21 s-1. There was no clear correlation of rate constant magnitude with dye concentration, and no deviations from first-order kinetics (eq 4, where A0, At, and A∞ are the absorbances at times 0, t, and infinity, respectively) were observed. Systems exhibiting first-order kinetics give linear plots with slope ) -k when ln(At - A∞) is plotted as a function of time according to eq 4.

Figure 8. Comparison of kinetic data (A) and MC difference spectra (B) for samples having MC concentrations among the highest (9.0 MC/ chain) and lowest (1.6 MC/chain).

(

ln

)

At - A∞ ) -kT A0 - A∞

(4)

Figure 8A shows kinetic data from samples with MC concentrations among the highest (9.0 MC/chain) and lowest (1.6 MC/ chain) that we measured. Both plots are linear and have essentially the same slope, with the major difference being the increased scatter observed for the low-concentration data set. The fact that the thermal decoloration kinetics for all nanoparticle samples are first order makes a strong argument against aggregation. A lack of correlation with dye concentration was likewise observed for the MC absorption spectra, which had λmax values ranging from 597 to 604 nm in different samples.

Figure 8B shows MC difference spectra for the same two samples represented by the data in Figure 8A. The lowconcentration sample is much noisier, as expected, but does not show any other difference that can be correlated with concentration. In the absence of aggregation, the idea that the dyes are adhered to solvent-exposed segments of MEH-PPV within the nanoparticles or to the particle surface is the most likely explanation for the observed solution-like behavior. Conclusions We have demonstrated efficient fluorescence intensity photomodulation in MEH-PPV nanoparticles doped with photochromic spirooxazine dyes. When the dyes are in their thermally stable spirooxazine form, the fluorescence of the dye-doped nanoparticles is as bright as that of undoped MEH-PPV nanoparticles. Upon UV-induced conversion to the highly conjugated merocyanine form of the dye, the MEH-PPV fluorescence is quenched to less than 10% of its initial value. The stability of both the photochromic dye and the MEH-PPV nanoparticles enables repeated cycling of the modulation. The mechanism by which the MC dyes quench the MEH-PPV fluorescence is most likely some form of energy transfer, and future time-resolved work will be required to determine the mechanism or combination of mechanisms. The kinetics of the thermal MC f SO decoloration reaction in the nanoparticles are remarkably solution-like and completely distinct from the more complex kinetics observed in polymer films. The solvatochromic absorption of the MC form of the dye indicates that it is in an environment that is more polar than THF and again completely different from the environment in a MEH-PPV film. Together, the kinetics and absorption spectra report on an environment that is surprisingly solution-like with ample free volume for the photochromic transformation to take place. This environment may well be composed of dyes adhered to waterexposed segments of MEH-PPV or to the particle surface, and its existence likely depends on nanoparticle size and polymer structure. The small size of the nanoparticles studied here

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increases the likelihood that dyes will be in solution-exposed environments. Identification of this environment in dye-doped MEH-PPV nanoparticles will enable us to design future dyedoped nanoparticle systems that take advantage of the free volume within the nanoparticle to carry out molecular transformations in response to an external stimulus. Acknowledgment. We gratefully acknowledge support of this work by NSF through CAREER award CHE-0642513. We thank Changfeng Wu and Prof. Jason McNeill of Clemson University for experimental advice and Olga Trofimova and Amy Wilkerson at the Surface Characterization Lab of the William and Mary Applied Research Center for assistance with AFM measurements. References and Notes (1) Bouas-Laurent, H.; Du¨rr, H. Pure Appl. Chem. 2001, 73, 639. (2) Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343. (3) Raymo, F. M.; Tomasulo, M. Chem. Soc. ReV. 2005, 34, 327. (4) Gust, D.; Moore, T. A.; Moore, A. L. Chem. Commun. 2006, 1169. (5) Cusido, J.; Deniz, E.; Raymo, F. M. Eur. J. Org. Chem. 2009, 2009, 2031. (6) Schwartz, B. J. Annu. ReV. Phys. Chem. 2003, 54, 141. (7) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. ReV. 2007, 107, 1339. (8) Harbron, E. J.; Vicente, D. A.; Hoyt, M. T. J. Phys. Chem. B 2004, 108, 18789. (9) Grimes, A. F.; Call, S. E.; Vicente, D. A.; English, D. S.; Harbron, E. J. J. Phys. Chem. B 2006, 110, 19183. (10) Lewis, S. M.; Harbron, E. J. J. Phys. Chem. C 2007, 111, 4425. (11) Finden, J.; Kunz, T. K.; Branda, N. R.; Wolf, M. O. AdV. Mater. 2008, 20, 1998. (12) Li, X.; Tian, H. Macromol. Chem. Phys. 2005, 206, 1769. (13) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Guntner, R.; Scherf, U. Nat. Mater. 2003, 2, 408. (14) Landfester, K.; Montenegro, R.; Scherf, U.; Gu¨ntner, R.; Asawapirom, U.; Patil, S.; Neher, D.; Kietzke, T. AdV. Mater. 2002, 14, 651. (15) Szymanski, C.; Wu, C.; Hooper, J.; Salazar, M. A.; Perdomo, A.; Dukes, A.; McNeill, J. J. Phys. Chem. B 2005, 109, 8543. (16) Wu, C.; Szymanski, C.; McNeill, J. Langmuir 2006, 22, 2956. (17) Grey, J. K.; Kim, D. Y.; Norris, B. C.; Miller, W. L.; Barbara, P. F. J. Phys. Chem. B 2006, 110, 25568. (18) Wu, C.; Zheng, Y.; Szymanski, C.; McNeill, J. J. Phys. Chem. C 2008, 112, 1772. (19) Wu, C.; Peng, H.; Jiang, Y.; McNeill, J. J. Phys. Chem. B 2006, 110, 14148. (20) Kong, F.; Wu, X. L.; Huang, G. S.; Yuan, R. K.; Chu, P. K. Thin Solid Films 2008, 516, 6287. (21) Zhu, L.; Wu, W.; Zhu, M.-Q.; Han, J. J.; Hurst, J. K.; Li, A. D. Q. J. Am. Chem. Soc. 2007, 129, 3524. (22) Son, Y.-A.; Park, Y.-M.; Choi, M.-S.; Kim, S.-H. Dyes Pigm. 2007, 75, 279. (23) Kim, S.-H.; Suh, H.-J.; Cui, J.-Z.; Gal, Y.-S.; Jin, S.-H.; Koh, K. Dyes Pigm. 2002, 53, 251. (24) Deligeorgiev, T.; Minkovska, S.; Jejiazkova, B.; Rakovsky, S. Dyes Pigm. 2002, 53, 101. (25) Lokshin, V.; Samat, A.; Metelitsa, A. V. Russ. Chem. ReV. 2002, 71, 893.

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