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J. Phys. Chem. C 2008, 112, 4104-4110
Single-Molecule Spectroscopy Reveals the Conformational Heterogeneity of Conducting Polymers Encapsulated within Hollow Silica Spheres Encarna Peris,† Jordi Hernando,*,‡ Francesc X. Llabre´ s i Xamena,† Niek F. van Hulst,§,| Jose´ L. Bourdelande,‡ and Hermenegildo Garcı´a*,† Instituto de Tecnologı´a Quı´mica CSIC-UPV, UniVersidad Polite´ cnica de Valencia, 46022 Valencia, Spain, Departament de Quı´mica, UniVersitat Auto` noma de Barcelona, 08193 Cerdanyola del Valle` s, Spain, ICFO-Institut de Cie` ncies Foto` niques, Mediterranean Technology Park, 08860 Castelldefels, Spain, and ICREA-Institucio´ Catalana de Recerca i Estudis AVanc¸ ats, 08015 Barcelona, Spain ReceiVed: September 13, 2007; In Final Form: December 24, 2007
Poly-2,6-naphthylidenevinylene (PNV) has been synthesized inside microporous hollow silica spheres of a uniform diameter of 600 nm. The spheres obtained by acid hydrolysis followed by basic condensation of phenyltrimethoxysilane and final aerobic calcination at 550 °C have a specific surface area (SBET) of 482 m2 g-1, have an average pore size of 6.8 Å, and are nonfluorescent. Adsorption of 2,6-bis(bromomethyl)naphthalene as PNV precursor and subsequent room-temperature treatment with potassium tert-butoxide renders PNV inside the hollow spheres. Use of small monomer amounts allows preparation of silica spheres with only one or very few encapsulated polymers, as revealed by single-molecule fluorescence spectroscopy. Fluorescence intensity and polarization experiments on individual PNV molecules within the spheres uncover different spectroscopic behaviors arising from the heterogeneous distribution of polymer chain conformations. Tightcoiled PNV molecules fluoresce from a small number of emitting sites due to efficient excited-state energy funneling, whereas multiple emitter behavior is found for extended polymer chains. Coexistence of both extended and coiled segments in individual polymers accounts for the intermediate fluorescing properties encountered for a significant portion of PNV molecules within the particles.
Introduction Poly-para-phenylenevinylene (PPV) and its derivatives are among the most important conducting polymers due to their potential use in organic transistors and in electroluminescence devices.1,2 However, in spite of the interesting properties of these compounds and their applicability to develop technology at the nanometer scale, they suffer from a poor stability due to the lability of the CdC bonds that tend to undergo oxidative degradation.3-5 Encapsulation within porous inorganic matrixes can be a viable methodology to enhance the stability of an organic molecule, particularly when there is a tight fit between the incorporated guest and the inert walls of the host.6 In addition, preparation of polymers inside micro-/mesoporous inorganic matrixes has attracted interest due to the remarkable properties of the intimate composite material resulting from the encapsulation.7-9 Recently, we have reported the in situ synthesis of PPV encapsulated within basic CsY zeolite10,11 as well as that of the 2,5-dimethoxy derivative of PPV inside the intergallery spaces of montmorillonite.12 We have shown that incorporation of these moieties within the internal voids of a rigid, inorganic host can serve to stabilize the polymer against the attack of oxygen and moisture under conditions in which a film of pure polymers becomes promptly degraded.10,12 For applications that require the formation of crack-free films with perfect surface coverage and high uniformity in surface * Corresponding authors. E-mail: (Garcia)
[email protected]; (Jordi)
[email protected]. † Universidad Polite ´ cnica de Valencia. ‡ Universitat Auto ` noma de Barcelona. § ICFO-Institut de Cie ` ncies Foto`niques. | ICREA-Institucio ´ Catalana de Recerca i Estudis Avanc¸ ats.
thickness and roughness, the use of porous host materials of specific particle morphologies and narrow particle size is very convenient.13,14-18 Regular zeolite powders constituted by submicrometric particles tend to strongly aggregate and do not easily form defect-free films of sufficient quality for applications in nanotechnology. For this reason, it would be very important to prepare conducting PPV-type polymers incorporated inside porous hosts with well-defined morphology and particle size, such as porous silica spheres. Controlled aggregation of these particles should allow regular and high-quality thin films to be prepared. Future applications of such type of host-guest materials will strongly depend on the effect of the matrix on the photophysical and photochemical behavior of the encapsulated conjugated polymers. Indeed, changes in morphological features arising from different processing conditions19,20 as well as the influence of the surroundings21 are known to modify dramatically the properties of thin films of PPV and its derivatives employed in optoelectronic devices. Single-molecule spectroscopy (SMS) emerges as an ideal tool to analyze several of these effects for conducting polymers embedded within porous solid matrixes. On the one hand, SMS reports on the influence of polymer chain conformation22,23-32 and of the environment22,33,34 on the spectroscopic behavior of conducting polymers. On the other hand, fluorescence microscopy allows investigation of the heterogeneity of host-guest porous materials down to the molecular scale.35,36,37 In this paper we report on the preparation of a PPV derivative, the poly-2,6-naphthylydenevinylene (PNV), inside hollow porous silica spheres with a uniform diameter of 600 nm. These spheres have been found adequate to form monolayer films.
10.1021/jp0773740 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008
Hollow Silica Spheres Loaded with PNV
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4105
Figure 1. (a) SEM micrograph of the hollow silica spheres prepared. The inset shows a broken sphere, where its hollow nature is evident. (b) SEM micrograph obtained for a monolayer film prepared by drop-casting of a dispersion of the spheres in ethanol onto a glass substrate, followed by solvent evaporation.
SCHEME 1: Synthesis of Hollow Silica Spheres
By lowering the monomer loading we explore the feasibility of preparing silica spheres with only one or a very few PNV polymer chains in their interior. Confocal fluorescence microscopy with single-molecule sensitivity is used to investigate the spectroscopic properties of the resulting composite material. This information has been used to establish the chain conformation of the polymers inside the silica matrix, a feature modulating the optical behavior of the polymer. Results and Discussion Synthesis and Bulk Characterization. Hollow silica spheres were obtained by the method reported in the literature by Hah et al.38 In short, an aqueous solution of phenyltrimethoxysilane was acidified with nitric acid and stirred vigorously for a few seconds before adding an aqueous solution of NH4OH. Treatment with acid produces prompt hydrolysis of the methoxy groups, while the subsequent basic medium causes the condensation of the silanol groups to form the silica spheres (Scheme 1). The spherical shape and the hollow interior are probably imparted by the hydrophobic interaction of the siloxane and phenyl groups with water, while the efficient mixing and stirring of the solutions is responsible for the size monodispersity of the sample. The resulting phenylsiloxane spheres were calcined in air at 550 °C, which leads to combustion of the phenyl groups as well as thermally promoted condensation of a fraction of residual silanol groups. This results in the creation of porosity (due to the disappearance of phenyl groups) and a significant increase in the mechanical resistance of the particles (due to the increase in crosslinking between the silicon atoms).
Figure 1a shows SEM images of the silica spheres prepared, which present a narrow distribution of sizes with an average diameter of 600 nm. Importantly, some broken spheres are found in the images, which allows direct observation of their hollow interior (see inset in Figure 1a). However, most of the particles maintained their structural integrity after calcination, and SEM images do not reveal any appreciable change with respect to their morphology prior to calcination. The resulting particles are ideal for preparing monolayer films of a very good quality. Figure 1b shows the SEM micrograph of a film prepared by deposition of a few drops of a suspension of the spheres in ethanol onto a glass substrate followed by natural evaporation of the solvent. Clearly, this very simple and straightforward procedure is sufficient to produce a monolayer of high surface coverage in a very short period of time. Isothermal N2 adsorption measurements on the calcined silica spheres showed a type I curve typical of microporous materials, whereas isothermal Ar adsorption experiments revealed the occurrence of a rather broad distribution of pore section diameters ranging from 5 to 16 Å with an average value of 6.8 Å. Noticeably, calcination leads to a large increase in surface area (from 12 to 482 m2 g-1) and a high pore volume (0.4 cm3 g-1) of the spheres, a situation ascribed to the combustion of the phenyl groups and the subsequent creation of micropores. As a result, the hollow center of the particles becomes accessible from the exterior, as illustrated in Scheme 2. This allows envisaging the resulting silica spheres as regular, submicrometer-sized hosts for the encapsulation of guest molecules.
4106 J. Phys. Chem. C, Vol. 112, No. 11, 2008 SCHEME 2: Structural Changes Produced by Calcination of the Silica Spheres
To encapsulate conducting polymers in the interior of silica spheres, we followed the same strategy previously applied by us for other porous matrixes,10-12 which consists of the inclusion of suitable monomers and the subsequent in situ synthesis of the polymer chains within the host. In this work, we aimed at preparing silica spheres with poly(2,6-naphthylydenevinylene) (PNV), an electron-rich derivative of PPV. To reach this goal we used 2,6-bis(bromomethyl)naphthalene as precursor for the Gilch synthesis of PNV, which is summarized in Scheme 3. The procedure employed consisted of the incipient wetness impregnation of the spheres with a 2,6-bis(bromomethyl)naphthalene solution in THF with a total volume corresponding to the pore volume of the spheres followed by solvent evacuation under vacuum, potassium tert-butoxide impregnation (also under incipient wetness conditions with THF solution), and polymerization at room temperature. The formation of PNV within the monomer-impregnated spheres after the base treatment was confirmed spectroscopically. The red-orange color of the PNV@sphere samples was demonstrated to arise from the occurrence of a structured broad absorption band peaking at 405 nm, which is shown in the diffuse reflectance UV-vis (DRUV-Vis) spectrum depicted in Figure 2a. Similar absorption bands at ∼420 nm have been previously reported for films of PNV39 and for similar soluble PNV derivatives.40 Clearly, this band is absent in the absorption spectrum of the monomer (see Figure 2a), thus confirming the arylvinylene conjugation of the polymer formed. The Raman spectrum of the PNV@sphere samples shows several bands in the 2000-1300 cm-1 region (Figure 2b). We ascribe these bands to the aromatic groups of the polymer contained in the spheres, since the silica matrix does not give rise to any signal in this spectralwindow.CombustionchemicalanalysisofthePNV@spheres indicates a C content of 4 wt % that corresponds to 0.28 mmoles of napthylidenevinylene groups per gram of silica. PNV@sphere samples also display strong luminescence emission upon light absorption, a typical feature of PPV-type polymers. Figure 3 shows the fluorescence spectrum recorded for PNV@spheres upon excitation at 405 nm, in which the structured emission characteristic of PNV-type polymers is SCHEME 3: Synthesis of PNV Embedded into Silica Spheres
Peris et al. observed.39,40 Time-resolved measurements demonstrate that the emission arising from PNV@spheres possesses a very short lifetime, the temporal profile of the fluorescence signal being just slightly longer than the temporal resolution of our lifetime measurement instrument (see inset in Figure 3). A tentative monoexponential fit of the deconvoluted emission decay yields a fluorescence lifetime value of τ ∼ 200 ps. All these spectroscopic features confirm the synthesis of PNV polymer within the silica particles. Fluorescence Microscopy Measurements. Scanning confocal fluorescence microscopy with single-molecule sensitivity41 was employed to investigate the optical properties of PNV@spheres at the nanoscale (see Supporting Information). With this instrument we could acquire both the transmission and fluorescence signals arising from individual silica particles deposited onto a glass substrate by drop-casting. Several batches of PNV@spheres with decreasing concentration of impregnated monomers were synthesized and subsequently analyzed by fluorescence microscopy. Ultimately, this should allow the preparation of silica spheres carrying only one or a very few conducting polymer chains, a requirement needed for singlepolymer detection within particles of 600 nm in diameter by means of diffraction-limited microscopy (lateral spatial resolution ∼ 250 nm). Figure 4 displays the transmission and fluorescence images recorded for PNV@spheres prepared at different synthetic conditions: (a) 0, (b) 150, and (c,d) 7.5 mg of monomer per gram of spheres. Our 600 nm wide silica spheres render enough optical contrast to be detected in transmission, regardless of the polymer concentration. This allows us to correlate features observed in the fluorescence images with the location of the silica spheres in the samples. Importantly, when the synthetic procedure to obtain PNV@spheres was carried out with cmonomer ) 0, the resulting particles showed no detectable luminescence upon excitation at 437 nm (Figure 4a). Together with the absence of monomer emission within our spectral detection window (λdet > 500 nm), the results from this blank experiment demonstrate that polymer formation accounts for all the fluorescence observed for the silica spheres in our confocal microscopy experiments, in good agreement with the steadystate fluorescence measurements previously discussed. Moreover, since the aqueous suspensions of PNV@spheres were sonicated prior to deposition onto glass, we believe that emission should arise from polymers lying in the interior of the silica particles, whose small pore size (average pore section diameter of 6.8 Å) prevents the polymeric chains from migrating out of the host. Indeed, when employing the same procedure to deposit PNV-loaded MCM-41 particles onto glass, we did observe evacuation of most of the polymer chains from the mesoporous matrix into the aqueous solvent due to the much larger pore size of the host (34 Å). PNV@spheres with a high concentration of monomer precursors display very intense and homogeneous fluorescence emis-
Hollow Silica Spheres Loaded with PNV
Figure 2. (a) Diffuse reflectance UV-vis (plotted as the KubelkaMunk function) and (b) Raman spectra of the PNV@sphere sample. The absorption spectrum in THF of the monomer 2,6-bis(bromomethyl)naphthalene used in the synthesis of the polymer is also shown in (a) (dashed line).
Figure 3. Fluorescence emission spectrum (λexc ) 405 nm) obtained for the PNV@spheres sample. The inset shows the temporal decay of the signal at the emission maximum (λem ) 507 nm, curve b), together with the temporal profile of the excitation lamp of our instrument (curve a).
sion, which arises from the whole volume of the particles (Figure 4b). As the fluorescence intensity line cut through one of such particles plotted in Figure 4f demonstrates, the fluorescence features in Figure 4b show diameters similar to the corresponding signals in the transmission image (fwhm ∼ 700 nm) and they are much larger than the diffraction-limited fluorescent spots (fwhm ∼250 nm) previously reported even for very long individual PPV-type polymers (more than 1000 monomers per chain).22 This leads us to conclude that the emission from the PNV@spheres in Figure 4b stems from multiple PNV polymers covering the whole interior of the hollow silica spheres. Therefore, we cannot discriminate the fluorescence arising from separate PNV chains in this case. In contrast, PNV@spheres prepared with low concentrations of monomers show clearly different emission features (Figure 4, parts c and d): (i) lower fluorescence intensities are detected, which are, on average, up to 8 times less than those of spheres in Figure 4b; (ii) smaller fluorescence spots are measured, whose diameter is narrower than that for the particles and fits the diffraction limited-resolution of our microscope, as shown in Figure 4f; (iii) although the diameter of the particles is only slightly larger than the z-resolution of our microscope (∼500 nm), different optimal focal distances are found for distinct spheres, which points toward the location of the emitting polymers at different particular heights; (iv) very low or even no fluorescence is displayed by some of the spheres regardless of the focal distance (see the arrow in Figure 4c); (v) spheres
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4107 with more than one defined fluorescent spot are observed (see the overlays of transmission and fluorescence signals in Figure 4e); and (vi) fluorescent spots showing typical features of single quantum emitters are obtained, such as reversible (blinking) and irreversible (photobleaching) dark states (see squares in Figure 4d). All these observations are consistent with the detection of individual PNV chains in the interior of the hollow silica spheres. Therefore, we conclude that, by properly tuning the amount of monomer precursors used in the synthesis, loading of submicrometric silica spheres with one or a very few conducting polymer chains can be attained. Ultimately, this allows individual polymers to be detected by single-molecule fluorescence spectroscopy (SMS), a powerful technique to report on the influence of the surrounding environment on the emissive properties and morphology of conjugated polymers.22-34 For the PNV@sphere sample whose transmission and fluorescence images are shown in Figure 4, parts c and d, a detailed analysis of the emission arising from individual particles was performed. In particular, our attention was focused on spheres displaying one (or two) well-defined and separated diffractionlimited fluorescence spots with smaller size than the sphere particle. This ensures the detection of individual polymers. However, contribution of multiple close-by PNV chains to some of such spots cannot be totally ruled out. Figure 5 displays the temporal dependence of the intensity (I) and degree of polarization (P) of the fluorescence arising from three individual PNV chains in the interior of silica spheres. Each of these three sets of plots illustrates the different emissive behaviors observed for 53 single conducting polymers. Several of the molecules detected (25%) showed a continuous decrease of the emission intensity in time without apparent changes in the degree of polarization (type E, Figure 5a). An opposite behavior was measured for a similar fraction of PNV chains (26%), which displayed a discrete number of intensity levels in their intensity trajectory (type C, Figure 5b). Moreover, changes in P between different intensity levels and intra- and interlevel reversible long dark states (e.g., at t ) 25 s) were observed. Interestingly, most of the monitored polymer molecules (49%) presented a combination of those two behaviors (type M, Figure 5c), with regions of both continuous intensity decay (t ) 0-5 s) and multistep emission intensity (t ) 530 s) coexisting in the intensity time trajectory of the polymer chain. In addition, sudden changes in the degree of polarization of emission are concomitant with discrete intensity level transitions, as also observed in Figure 5b. SMS experiments on PPV-type molecules have investigated the relationship between polymer morphology and the different type of fluorescence intensity time trajectories displayed by individual chains.23,25-28,30 PPV-type polymers are usually visualized as arrays of effective chromophores comprising segments of ∼5-20 monomers over which excited-state energy delocalizes upon light absorption. The mutual distance and orientation of such chromophores are controlled by the polymer chain conformation. Therefore, polymer morphology governs the intrachain interaction between chromophores, which ultimately accounts for the emissive properties of the system. If the intrachain coupling is weak, the multiple chromophores in the polymer absorb and emit independently, a situation ascribed to extended conformations of the chain rendering large interchromophoric distances.25,26,28,30 Individual long PPV-type polymers with such rodlike conformations then display continuously decreasing fluorescence intensity time trajectories due to the gradual photodegradation of the multiple independent emitting sites in the chain.25,26,28,30 Conversely, tight-coiled
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Peris et al.
Figure 4. (a-d) Transmission (left) and fluorescence (right) images of hollow silica spheres containing PNV polymers and deposited onto a glass substrate. The four sets of images correspond to distinct PNV@sphere samples prepared with different amounts of monomer precursors: (a) 0, (b) 150, and (c,d) 7.5 mg of monomer per gram of spheres. All these samples were measured under the same experimental conditions (λexc ) 437 nm, power density ∼ 4 kW cm-2, scan area ) 10 × 10 µm2, pixel rate ) 1 kHz). The fluorescence intensity scale is also equal for all displayed images (Imin ) 2 kcount s-1; Imax ) 30 kcount s-1). The white squares in the fluorescence image of (d) zoom in on particular fluorescence features as indicated by the arrows. (e) Overlays of the transmission and fluorescence signals for particles displaying more than one emission spot. (f) Fluorescence intensity cuts through two of the imaged particles, which were prepared with high (blue) and low (red) amounts of monomer precursor. From this plot, fwhm values of 730 and 223 nm are derived for the emission signals of such particles, respectively.
polymer conformations reducing the separation between intrachain chromophores increase their mutual interaction and allow excited-state energy to funnel to few low-energy sites from which emission eventually arises.22-26,28 As a result, their fluorescence intensity time trajectories present stepwise behavior due to the sequential photobleaching of the limited number of emitting sites in the polymer chain.22-26,28 In accordance to these previous works, type E PNV molecules detected in our experiments must correspond to polymer chains with extended conformations. The temporal fluorescence intensity profile observed for type E molecules indicates the occurrence of multiple chromophoric sites (ca. >10 chromophores) and, therefore, a large polymerization degree (ca. >100 monomers).42 Further proof for the occurrence of multiple simultaneous emitters in type E polymers is given by the P distribution retrieved for this type of molecules (Figure 6). A narrow distribution around P ) 0 is found, which contrasts with the behavior expected for a set of randomly oriented single individual emitting dipoles which are excited with circularly polarized light.43 This indicates strong depolarization of the emission arising from type E PNV polymers, as previously observed for extended conformations of other PPV-type conjugated polymers.25,26 In the case of type C PNV molecules, we ascribe this species to long polymers with a tight-coiled conformation, thus allowing for efficient funneling of excited-state energy to a few emitting sites. Although short polymers (
