Chemical and Photoluminescence Properties of Purified Poly (2

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J. Phys. Chem. B 2007, 111, 12738-12747

Chemical and Photoluminescence Properties of Purified Poly(2-methoxyaniline-5-sulfonic acid) and Oligomer P. C. Innis,*,† F. Masdarolomoor,† L. A. P. Kane-Maguire,† R. J. Forster,‡ T. E. Keyes,‡ and G. G. Wallace† Intelligent Polymer Research Institute, ARC Centre of Excellence for Electromaterials Science, UniVersity of Wollongong, Northfields AVenue, Wollongong, NSW 2522, Australia, and School of Chemical Sciences, National Centre for Sensor Research, Dublin City UniVersity, Dublin, Ireland ReceiVed: July 10, 2007; In Final Form: August 28, 2007

Both the chemical and the electrochemical synthesis of poly(2-methoxyaniline-5-sulfonate) (PMAS) in aqueous media have been found to give two distinct polymer fractions with molecular weights of approximately 8-10 and 2 kDa, respectively. It is now possible to isolate the pure high molecular weight (HMWT) PMAS and low molecular weight (LMWT) PMAS oligomer and to study their individual and combined photochemistry and redox chemistry. The HMWT PMAS fraction was confirmed to be an emeraldine salt by its characteristic redox and pH switching behavior, in contrast to the oligomeric LMWT PMAS, which was inert under the same conditions. Mixtures of these two fractions exhibit photoluminescence arising from the oligomeric LMWT PMAS fraction. The observed LMWT PMAS emission was modulated by the presence of the conducting HMWT PMAS emeraldine salt via a static resonant energy transfer arising from quenching at 460 nm when excited at 355 nm. The nonlinear fluorophore-quencher behavior suggests that the two PMAS fractions are strongly associated. The behavior fitted the static Perrin quenching model in which the oligomeric LMWT PMAS fluorophore is diffusionally restricted by the presence of HMWT PMAS quencher.

Introduction To date, photochemical and photoelectrochemical studies on inherently conducting polymers have largely concentrated on poly(phenylenevinylene)s, polyphenylenes, and polythiophenes, which have exciting potential applications in areas such as light emitting diodes, photovoltaics, and photoelectrochemical cells.1,2 In contrast, relatively few photobehavioral studies have been carried out on polyaniline (PAn, 1). Initial studies focused on the fully reduced (y ) 1) leucoemeraldine base (LB) form,3,4 which exhibits a strong emission centered at ca. 410 nm when irradiated at 300-350 nm. These excitation wavelengths are close in energy to the π-π* absorption band of LB at ca. 330 nm associated with its benzenoid repeat units. The emission observed for LB at 410-420 nm equates to ca. 3.0 eV. Combining this value with corresponding cyclic voltammetry data, using established procedures,5 led to the suggestion that the thermally equilibrated photoexcited state (*LB) should be a powerful reducing agent (eq 1).6

*LB + S f EB + Sred

(1)

Early studies3 suggested that the half-oxidized (y ) 0.5)

emeraldine base (EB) form of PAn does not photoluminesce. Emission from the EB excited state was believed to be selfquenched by adjacent quinoid groups in its structure (see

structure 1). However, several recent studies by MacDiarmid7 and others8-10 have reported photoluminescence from EB in N-methylpyrrolidinone (NMP) and weak emission for ES films derived from NMP7,10 and m-cresol solvents.8 As with LB, the emitting regions in photoactivated emeraldine base were considered to be the benzenoid-like groups along the polymer backbone. More recently, photoluminescence- and photoredox-induced reactions have been reported for the fully sulfonated polyaniline, poly(2-methoxyaniline-5-sulfonic acid) (PMAS) 3 in its emeraldine salt (ES) state.6 These studies were carried out on commercially available PMAS prepared via chemical oxidation of the MAS monomer. It is now known that PMAS synthesized via either chemical or electrochemical oxidation contains two distinct molecular weight fractions that can be separated by membrane dialysis.11,12 Very recently, high-purity (>95%) samples of each of these PMAS fractions were isolated for the first time, using a cross-flow dialysis system for separation following chemical polymerization.13 The high molecular weight (HMWT) PMAS fraction has Mw of 8-10 kDa, while the low molecular weight (LMWT) PMAS fraction is an oligomer with Mw of ca. 2 kDa.

* Corresponding author. Tel.: +61 2 4221 3127. Fax: +61 2 4221 3114. E-mail: [email protected]. † University of Wollongong. ‡ Dublin City University.

10.1021/jp075395o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007

Properties of Purified PMAS and Oligomer

J. Phys. Chem. B, Vol. 111, No. 44, 2007 12739

Figure 1. UV-visible spectra of various aqueous mixtures of high molecular weight PMAS emeraldine salt and the oligomeric LMWT PMAS (total [PMAS] ) 50 µg/mL, 1.23 × 10-4 M in each case).

The formation of the two different molecular weight fractions for PMAS is a direct result of the insolubility of the MAS monomer at pH 0-1, in which acidic medium polyaniline emeraldine salts are typically prepared. Because of this insolubility, the synthesis of PMAS under aqueous conditions has to be carried out at pH 3.5 or higher. The oligomeric LMWT PMAS fraction formed during the PMAS synthesis is believed to be based on a phenazine-like14 structure similar to products reported for the polymerization of aniline at high pH’s.15-19 The present Article investigates the photoluminescence of the separate high and low molecular weight PMAS fractions as well as various mixtures. The influence of these two fractions on the unique redox and photoluminescence behavior of PMAS is elucidated. Experimental Section Materials. PMAS emeraldine salt was prepared via the chemical synthesis method.11,13 It was then fractionated via either (i) dialysis using a cellulose membrane with a 12 000 Da cutoff or (ii) a recently described cross-flow dialysis system.13 The PMAS sample from purification method (i) is identified hereafter as membrane dialyzed PMAS DPMAS, while the two pure fractions from method (ii) are labeled high molecular weight HMWT PMAS (Mw 8-10 kDa, PDI 1.4) and low molecular weight LMWT PMAS (Mw 2 kDa, PDI 1.1). Ammonium persulfate and hydrazine hydrate were purchased from Aldrich Chemical Co. UV-Vis Spectral and Redox Studies. All investigations were performed on aqueous solutions containing either 10 or 50 µg polymer/mL (2.46 × 10-5 or 1.23 × 10-4 M, based upon a dimer repeat unit) prepared in Milli-Q water. Oxidation and reduction of each of the aqueous PMAS fractions were carried out using 0.10 M ammonium persulfate and hydrazine hydrate as oxidant and reductant, respectively, and the subsequent UVvis spectral changes were monitored using a Shimadzu UV1601 spectrophotometer. Photoluminescence Studies. Photoluminescence (PL) and PL lifetime measurements were performed on aqueous solutions of the PMAS fractions at concentrations up to 150 µg/mL (3.69 × 10-4 M based on a dimer repeat unit), unless otherwise stated. Fluorescence spectra were recorded using a JY Spex fluorescence spectrophotometer. Fluorescence lifetime measurements were made on a PicoQuant FluoTime 100 time-correlated single photon counting spectrometer (TCSPC) using a PDL-800B pulsed diode laser controller, and employing 280, 370, and 450

nm pulsed laser sources with cut-on filters of 400, 475, and 530 nm. TCSPC analysis was performed using PicoQuant FluoFit software. Results and Discussion UV-Vis Spectroscopy. Previous membrane dialyzed DPMAS has recently been shown11-13 to be a mixture of high and low molecular weight fractions, and typically exhibits an electrical conductivity of the order of 10-3 S/cm. More recently, pure (>95%) HMWT PMAS (8-10 kDa) and LMWT PMAS (2 kDa) samples have been obtained using a cross-flow dialysis purification method13 with conductivities in the order of 10-1 and 10-5 S/cm, respectively.20 The UV-vis spectra of aqueous solutions of the HMWT PMAS and LMWT PMAS fractions, as well as a range of mixtures, are now shown in Figure 1. Pure aqueous HMWT PMAS (50 µg/mL, i.e., 1.23 × 10-4 M based on a dimer repeat unit) exhibited an absorption band at ca. 315 nm and a weak shoulder at 360 nm that may be assigned as π-π* bands, together with a strong band at 474 nm assigned as a low wavelength polaron band. Significant absorption was also observed at wavelengths g1000 nm. These features are consistent with a polyaniline emeraldine salt in an “extended coil” conformation for the PMAS polymer chains.21-23 In contrast, an aqueous solution of pure LMWT PMAS of the same concentration exhibited no bands in the visible region, showing only a single broad peak at ca. 282 nm. For each of the aqueous mixtures of high and low molecular weight PMAS whose UV-vis spectra are shown in Figure 1, the total concentration of PMAS was kept constant at 50 µg/ mL (1.23 × 10-4 M). This approach therefore provides a simple and quantitative method for the rapid estimation of the polymer composition without the need to resort to time-consuming chromatographic (GPC) analysis.12,13 For example, the UVvis spectrum of aqueous DPMAS shown in Figure 2a indicates that this sample is composed of approximately 70% w/v HMWT PMAS and 30% w/v LMWT PMAS. Effect of pH. The pH of aqueous HMWT PMAS was ca. pH 3.5. In more acidic aqueous solutions (1.0 M HCl), the π-π* band blue-shifted from 315 to 296 nm (Figure 2a), while the position of the other bands remained virtually unchanged. This suggests only minor structural changes in the PMAS polymer chain in the presence of added acid, presumably arising from more extensive protonation of the sulfonate substituents on the polymer backbone.

12740 J. Phys. Chem. B, Vol. 111, No. 44, 2007

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Figure 2. UV-vis spectra of 50 µg/mL (1.23 × 10-4 M) solutions of: (a) high molecular weight PMAS emeraldine salt, (b) dialyzed PMAS, and (c) oligomeric low molecular weight PMAS in water, 1.0 M HCl(aq), and 1.0 M NaOH(aq).

However, in aqueous 1.0 M NaOH, significant changes were observed in the absorption spectrum of HMWT PMAS. As seen in Figure 2a, the π-π* transition broadened and underwent a red shift to 358 nm, while the 474 nm polaron band disappeared. Additionally, the near-infrared absorption disappeared, and a new strong peak appeared at 750 nm. These observations are inconsistent with alkaline dedoping of PMAS to its emeraldine base form, which would be expected to give rise to a characteristic exciton band at ca. 600 nm.24,25 The spectral changes are believed to be due to a change in conformation of the PMAS chains from an “extended coil” to a “compact coil” arrangement.26 Similar observations were noted for the dialyzed, non-fractionated, DPMAS sample upon addition of 1.0 M HCl or 1.0 M NaOH (Figure 2b). In contrast, LMWT PMAS was found to be inert to changes in pH. It exhibited only a single broad absorption band at ca. 282 nm in water, 1.0 M HCl, and 1.0 M NaOH (Figure 2c). Redox Switching of PMAS. Among inherently conducting organic polymers, polyaniline is unique in that it has three readily accessible oxidation states. Similar behavior is confirmed here for the fully sulfonated polyaniline HMWT PMAS, as shown from the spectral studies in Figures 3 and 4. Treatment of 10 µg/mL (2.46 × 10-5 M) aqueous HMWT PMAS emeraldine salt with 0.10 M ammonium persulfate caused the disappearance, over 30 min, of the bands of the original emeraldine salt and their replacement by new bands at ca. 290 and 550 nm (Figure 3). These spectral changes are very similar to those previously observed3 for the oxidation of

unsubstituted polyaniline emeraldine salt, indicating oxidation of HMWT PMAS to its fully oxidized pernigraniline base form. Reduction of aqueous HMWT PMAS was achieved by treatment with 0.10 M hydrazine hydrate. As seen in Figure 4, within 1 min the bands of the original emeraldine salt were replaced by a new, strong band at 405 nm and a shoulder at ca. 315 nm. Interestingly, this spectral behavior upon reduction of sulfonated HMWT PMAS is different from that reported for unsubstituted polyaniline emeraldine salts, where the leucoemeraldine base product shows only a single peak at ca. 320 nm.27 The nature of the two forms of reduced leucoemeraldine base PMAS, corresponding to the two absorption bands at 315 and 405 nm, as well as their interconversion by changes in temperature and solvent, will be described elsewhere.28 The ability of the HMWT PMAS fraction to undergo facile oxidation and reduction provides strong evidence that it is indeed in the middle half-oxidized emeraldine salt state of the polymer. This has also been confirmed from cyclic voltammetry studies described in an earlier report.13 In contrast, the oligomeric LMWT PMAS underwent negligible UV-vis spectral changes upon treatment with aqueous 0.10 M ammonium persulfate or 0.10 M hydrazine. It was also insensitive toward acid and base treatment (see Figure 2c). This inertness to redox and pH switching confirms that the oligomeric LMWT form of PMAS is not an emeraldine salt. This also explains why mixtures of LMWT and HMWT PMAS {as in commercially available PMAS or samples that have been subjected to standard dialysis (e.g., DPMAS)} exhibit UV-vis



Figure 3. UV-vis spectral changes upon oxidation of 10 µg/mL (2.46 × 10-5 M) aqueous HMWT PMAS emeraldine salt to pernigraniline base using 0.10 M ammonium persulfate.

Figure 4. UV-vis spectral changes after treating 10 µg/mL (2.46 × 10-5 M) HMWT PMAS emeraldine salt with 0.10 M aqueous hydrazine for 1 min.

spectral changes similar to those demonstrated for pure HMWT PMAS upon redox (Figures 3 and 4) or alkaline treatment (e.g., Figure 2a-c). The observed chemical inertness and absence of electrical conductivity of the oligomeric LMWT PMAS20 is similar to that of the phenazine-like oligomeric products recently reported for the syntheses of unsubstituted polyaniline at elevated pH values.15-19 These reported byproducts were clearly not the redox active polyaniline that is well described in the literature for syntheses carried out at low pH. They were of low molecular weight17,18 and possessed distinctly different chemical and physical properties from those reported for polyaniline emeraldine salts. A recent study of material isolated at the early stages of oxidative polymerization of aniline in base revealed the presence of phenazine-like structures suggested to form as a result of ortho-coupling18 of aniline monomers with an accompanying intramolecular cyclization. These initial oligomeric products were nonconducting (2.4 × 10-10 S/cm), and their presence was observed to lower the final conductivity of the polyaniline emeraldine salt (0.078 S/cm) with respect to polyaniline synthesized in more acidic media. Mechanistic studies on the formation of phenazine-like defects have been reported by Viva et al. for the electrosynthesis of poly(2methoxyaniline) at pH 0 and monomer concentrations of 30-

SCHEME 1

60 mM.14 They reported that the event of head-to-head orthocoupling (Scheme 1) resulted in the formation of a phenazinelike defect (4), which inhibited further chain propagation, thereby limiting molecular weight. Modeled resistance behavior predicted higher film resistance (or lower conductivity) with increasing phenazine incorporation. This behavior was consistent


Figure 5. Fluorescence map of aqueous DPMAS (1.23 × 10-4 M based on dimer), λex ) 290 nm.

with reported conductivities of polyaniline-like materials with such impurities present.13,18,20 The chemical and physical properties of the oligomeric LMWT PMAS formed in the present study that are similar to those of the above phenazines suggest that the oligomeric PMAS may possess ortho-coupled structures analogous to (4) in Scheme 1. Other researchers have reported the formation of phenazine-like products during the synthesis and aging of polyaniline via cross-linking.29-32 However, because the LMWT PMAS oligomers are at most decamers, based upon GPC analysis of this fraction,13 the presence of such cross-linking defects was considered to be unlikely. Fluorescence Studies. Previous fluorescence studies on PMAS were carried out on a sample purified by standard dialysis (12 kDa cellulose membrane).13 This DPMAS form of PMAS was shown from the above UV-vis spectral studies to contain approximately 70% w/v HMWT PMAS and 30% w/v LMWT PMAS. A 3D photoluminescence study has now been carried out to map the emission responses for this mixed material. As seen in Figure 5, excitation at three different wavelengths (290, 355, and 520 nm) caused photoluminescence (PL) emissions. Excitation at ca. 290 nm resulted in emission peaks centered at 355 and 520 nm, 355 nm excitation caused emissions at only 520 nm (and a weak shoulder at 590 nm), while 520 nm excitation gave an emission peak only at 590 nm. The intense photoluminescence resulting from what was (at the time) believed to be the conducting emeraldine salt form of the DPMAS sample was surprising, because the emeraldine salt state of unsubstituted polyaniline is reported to be a quencher.33,34 Such quenching has been attributed to the interaction of singlet excitons in the photoexcited state with polarons.33,35,36 To explain this unexpected photoluminescence observed for the mixed DPMAS, we have carried out detailed PL studies on pure HMWT PMAS emeraldine salt and oligomeric LMWT PMAS fractions obtained via the recently developed cross-flow dialysis method.13 Excitation wavelengths of 290, 355, and 520 nm were employed. As seen in Figure 5, the emissions from these various excitation sources were interrelated, with emissions from lower wavelength excitation stimulating the other emissons at higher wavelengths. Results presented in this Article focus on 355 nm excitation of the polymer π-π* transition, as this band was relatively independent of the oxidation state and protonation levels of the polymer. The PL spectra for aqueous solutions of pure HMWT PMAS emeraldine salt, oligomeric LMWT PMAS fractions, and dialyzed DPMAS are shown in Figure 6. In all spectra, a Raman

Innis et al. scattering peak was observed at ca. 410 nm. The LMWT fraction exhibited a strong, broad emission with a peak maximum at ca. 525 nm. On the other hand, the HMWT PMAS sample displayed a very weak PL emission, as expected for an emeraldine salt. The weak emission band observed for this HMWT PMAS fraction at ca. 515 nm is believed to be due to a trace impurity of oligomeric LMWT PMAS. Dialyzed DPMAS presented a similar, but more intense, PL spectrum to that observed for the HMWT PMAS sample, with a medium intensity emission band at ca. 525 nm (Figure 6). This band can be attributed to the 30% w/v LMWT PMAS present in this sample. The origin of the emissions shown in Figure 6 was probed in more detail by a study of the LMWT PMAS emission in the presence of added HMWT PMAS. This was performed by comparing the PL spectra of mixtures containing increasing amounts of HMWT PMAS from 0 to 100 µg/mL (0 to 2.5 × 10-4 M) while maintaining a constant LMWT PMAS concentration of 50 µg/mL (1.24 × 10-4 M). As seen in Figure 7, the LMWT PMAS oligomer emission decreased markedly as the HMWT PMAS emeraldine salt concentration was increased. Two characteristic quenching behaviors were observed: from 400 to 500 nm, with a shoulder at ca. 460-470 nm; and from 500 nm and higher, with a maximum at 520 nm. The distortion of the 460-479 nm emission band with loss of intensity in the high-energy end of the band is attributed to absorbance quenching by the polaron absorption band of the conducting HMWT PMAS emeraldine salt (Figure 1). Emissions above 500 nm are associated with the extended free carrier tail exhibited by the polymer at longer wavelengths (Figure 1). As discussed below, there is almost certainly an element of absorbance quenching of the LMWT PMAS emission by the (HWMT) PMAS fraction as a result of an inner filter effect (trivial quenching process), whereby the photon released by the luminophore is simply reabsorbed by an adjacent chromophore. This type of quenching does not usually affect emission intensity outside the absorbance region of the chromophore, resulting in the distortion of the observed emissions. In radiative, or trivial energy transfer, the luminophore emits via it usual mechanism, but the emitted photon is reabsorbed before it reaches the fluorimeter detector. Interestingly, the reduction of luminescence intensity of the oligomeric LMWT PMAS upon addition of increasing amounts of HMWT PMAS emeraldine salt, Figure 8, exhibited a distinctly nonlinear behavior, even though preliminary studies showed that these systems were maintained at concentrations that obey the Beer-Lambert law. Clearly, other quenching mechanisms contribute to the observed responses. Dynamic fluorescence quenching, arising from a molecular collision, can be modeled using the Stern-Volmer equation (eq 2), where I and Io are the fluorescence intensities with and without a quencher, and the concentration of the quencher is denoted as [Q]. A plot of Io/I versus [Q] will yield a slope of kqτo, where the collision quenching rate constant, kq, can be determined if the fluorescence lifetime, τo, can be measured.

Io Φ o ) ) 1 + kqτo[Q] I Φ

(2)

If the fluorophore can form a stable complex with a quenching molecule, the resultant ground state of the fluorophorequencher complex is said to be statically quenched. In this case, the Stern-Volmer equation is modified, where the slope of a plot of Io/I versus [Q] will be proportional to the association constant, Ka, of the fluorophore-quencher complex. In the static



Figure 6. Photoluminescence spectra of aqueous HMWT PMAS emeraldine salt, oligomeric LMWT PMAS, and dialyzed DPMAS ([PMAS] ) 100 µg/mL (2.46 × 10-4 M), λex ) 355 nm).

Figure 7. Photoluminescence spectra of various aqueous mixtures of LMWT PMAS (1.26 × 10-4 M) with increasing amount of HMWT PMAS emeraldine salt (0 to 2.51 × 10-4 M), λex ) 355 nm.

(complexation) quenching model, the lifetime of the fluorophore will be unchanged, because the fluorophores that are not complexed (and therefore able to emit after excitation) will have normal excitation/emission properties. This model assumes that the resulting complex is non-luminescent and the fluorescence intensity of the sample is reduced because the quencher is essentially reducing the number of fluorophores that can emit. The Stern-Volmer plots of the observed fluorescence data for both static and dynamic models gave a poor fit to the model, as the behavior was nonlinear (Figure 8). Alternatively, because the quencher is polymeric and therefore diffusionally restricted in fluid media, the Perrin model (eq 3)37-39 may provide a better interpretation of the fluorescence behavior. In eq 3, Io and I are as described above, while the term c is the quenching volume of action. The Perrin model applies if diffusion is sufficiently restricted such that the time taken for collision between donor and acceptor exceeds the lifetime of the excited state. Further, quenching behavior only occurs within a set volume containing the excited-state species.

Static quenching behavior, conforming to the Perrin model, is not expected to exhibit changes in lifetime of the fluorophore. The model assumes that once the quencher resides within the effective quenching volume, the excited state is completely extinguished, while fluorophores outside this volume have unimpeded behavior.

ln

()

Io ) c[Q] I

(3)

A plot of the Perrin model is shown in Figure 9 and exhibits an excellent fit at selected emission wavelengths of 460 and 520 nm. Significantly, quenching is far more efficient around 460 nm associated with the nonspin paired electron of the mobile polaron charge carrier sited upon the conducting HMWT PMAS emeraldine salt backbone. The interactions of the HMWT PMAS with the LMWT PMAS are characteristic of a nonradiative energy transfer. The association of the LMWT PMAS and


Innis et al.

Figure 8. Stern-Volmer plot of the fluorescence quenching ratio of aqueous mixtures of oligomeric LMWT PMAS (1.26 × 10-4 M) with increasing amounts of HMWT PMAS emeraldine salt (0 to 2.51 × 10-4 M), λex ) 355 nm.

Figure 9. Perrin plot of the fluorescence quenching ratio of aqueous mixtures of oligomeric LMWT PMAS (1.26 × 10-4 M) with increasing amounts of HMWT PMAS emeraldine salt (0 to 2.51 × 10-4 M), λex ) 355 nm.

HMWT PMAS fractions in solution is not surprising given the very significant effort required to separate these polymer fractions.13 Influence of Oxidation and Reduction. Treatment of an aqueous LMWT PMAS with 0.10 M ammonium persulfate oxidant caused little change in its PL spectrum. This is consistent with the UV-vis spectral studies above, which showed that this oligomeric form of PMAS did not undergo oxidation. In contrast, similar treatment of aqueous HMWT PMAS emeraldine salt caused oxidation to pernigraniline, as evidenced by the UV-visible spectral changes in Figure 3. In unsubstituted polyaniline, the fully oxidized pernigraniline state has been reported not to fluoresce.3 Unfortunately, oxidized HMWT PMAS solutions were too unstable to permit reproducible PL spectra to be obtained. A PL reduction study of various aqueous PMAS samples treated with 0.10 M hydrazine is shown in Figure 10. Not surprisingly, the LMWT PMAS sample exhibited the same PL response as seen in water, because the UV-vis spectral studies above showed it to be inert to reduction. On the other hand, the UV-vis spectral changes in Figure 4 confirmed rapid

reduction of HMWT PMAS emeraldine salt to the leucoemeraldine base form upon hydrazine treatment. This leucoemeraldine base HMWT PMAS exhibited strong photoluminescence at ca. 430 nm. This strong photoluminescence contrasts with the extremely weak emission of the initial emeraldine salt and parallels the behavior of polyaniline where the reduced leucoemeraldine base also photoluminesces strongly.3,4,7,27,40 A similar PL spectrum was observed upon reduction of dialyzed DPMAS with 0.10 M hydrazine (Figure 10), confirming its high content of HMWT PMAS, Figure 10. The weaker, broad emission observed at longer wavelengths for reduced HMWT PMAS is believed to arise from a trace impurity of the LMWT oligomer. This later feature is clearer for reduced DPMAS due to its higher level of the LMWT PMAS impurity. From these results, it can be concluded that the unexpected photoluminescence of commercially available, as-synthesized PMAS reported previously6 does not originate from the conductive HMWT PMAS emeraldine salt, but rather from a low molecular weight, redox inactive component present in the sample. In turn, the mechanism for the fascinating photoelectron-



Figure 10. Photoluminescence spectra of aqueous HMWT PMAS emeraldine salt, oligomeric LMWT PMAS, and dialyzed DPMAS ([PMAS] ) 150 µg/mL (3.69 × 10-4 M)) samples after treatment with 0.10 M hydrazine, λex ) 355 nm.

transfer reactions recently reported6 with PMAS will require reassessment. Fluorescence Lifetime. To further elucidate the nature of the observed LMWT PMAS photoluminescence, fluorescence lifetime studies were carried out for LMWT PMAS and HMWT PMAS, as well as the mixed DPMAS sample. A time-correlated single photon counting (TCSPC) system was employed using 280, 370, and 450 nm pulsed laser sources. Cut-on filters of 400, 475, and 530 nm were used to separate out the effects of the multiple photoluminescence emission bands. Typical TCSPC responses for HMWT PMAS and LMWT PMAS are shown in Figure 11. In each instance, the upper trace relates to the TCSPC output and resultant model fit. The underlying curve relates to the background signal from the excitation laser source, which is subtracted from the TCSPC output when estimating fluorescence lifetime. The TCSPC responses in each case were collected to the same photon count intensity, with collection times varying from 120 s typically for LMWT PMAS to 400 s for HMWT PMAS. Emission responses were analyzed using PicoQuant FluoFit software employing a fitting function containing three time constants. The first-order decay function was typically a residual instrumental laser plus response of less than 0.25 ns and was rejected. For both PMAS samples, second- (τ2) and third- (τ3) order lifetime responses had lifetimes of 1-2 and 6-8 ns, respectively. A typical Stern-Volmer plot for the fluorescence lifetime, τ0/τ versus [Q], of HMWT PMAS exhibited clear independence of quencher concentration, Figure 12. This observation was also consistent for all PMAS forms as well as for the order of the modeled lifetime response. As discussed above, the observed independence for the fluorescence lifetime was inconsistent with the Stern-Volmer dynamic quenching collision model. In contrast, this independence was predicted for the Perrin model where diffusionally restricted behavior can be anticipated for the polymer in solution. The TCSPC emission responses (and fluorescence lifetimes τ2 and τ3) for HMWT PMAS emeraldine salt, oligomeric LMWT PMAS, and DPMAS were unaffected when these samples were treated with 1.0 M HCl, 1.0 M NaOH, 0.10 M (NH4)2S2O8, or 0.10 M hydrazine. This insensitivity to changes in pH and redox conditions is consistent with the earlier conclusion that the overall photoluminescence was dominated by the chemically inert LMWT PMAS fraction. The presence of the quenching HMWT PMAS fraction in mixed samples such

Figure 11. Typical TCSPC emission response curves for HMWT PMAS emeraldine salt and oligomeric LMWT PMAS using a 370 nm excitation source with no emission filters employed.

as DPMAS merely delays the time it takes to accumulate the TCSPC signal. The presence of only trace low molecular weight impurities in the HMWT PMAS sample results in a weak PL


Figure 12. Typical Stern-Volmer fluorescence lifetime ratio τo/τ dependence of τ2 on HMWT PMAS emeraldine salt quencher concentration.

signal and corresponding TCSPC output for the latter emeraldine salt, hence the need for a longer data accumulation cycle. The observed TCSPC behavior is also consistent with the occurrence of both radiative energy transfer and static quenching in a restricted environment. The fluorescence lifetime of the LMWT PMAS is independent of the HMWT PMAS concentration, Figure 12. This condition arises because the LMWT PMAS fluorophores that are not associated are able to emit after excitation in the normal fashion. The fluorescence of the sample is reduced because the quencher is essentially reducing the number of fluorophores that can emit, consistent with the increasing time required to acquire lifetime signals at higher HMWT PMAS concentrations. Conclusions The separation via cross-flow dialysis of high molecular weight and low molecular weight fractions from previously described poly(2-methoxyaniline-5-sulfonic acid) (PMAS) has permitted a detailed reassessment of the photochemical properties of this fully sulfonated polyaniline. HMWT PMAS is shown to be a polyaniline emeraldine salt, while LMWT PMAS is believed to be a phenazine-like species arising from the relatively high pH (>3.5) that has to be employed during PMAS synthesis in aqueous environments. The oligomeric LMWT PMAS fraction is shown to be the source of the unexpected photoemission recently reported for aqueous PMAS. Pure HMWT PMAS emeraldine salt does not photoluminesce and is shown to statically quench the photoluminescence of LMWT PMAS. Photoluminescence lifetimes for LMWT PMAS and its mixtures with HMWT PMAS are identical and are unaffected by pH or redox switching. This is consistent with the photoluminescence of these samples being due only to the chemically inert LMWT PMAS component and confirms a static quenching mechanism. Mixtures of the high and low molecular weight PMAS fractions display a distortion to their luminescence spectra and decreased luminescence intensity, without accompanying changes in luminescent lifetime. This behavior is consistent with absorption of emitted light from the oligomeric LMWT PMAS fraction by the HMWT PMAS emeraldine fraction via an inner filter quenching (or radiative energy transfer) effect. Results suggest that a second quenching process occurring between the HMWT PMAS and LMWT PMAS fractions further complicates the interpretation. Based on relative absorbance and emission properties of the two fractions, a second nonradiative energy transfer is likely to occur, and the quenching data correspond

Innis et al. well to a static quenching model in a diffusionally restricted polymer environment. The nonlinear fluorophore-quencher behavior is successfully described via a Perrin quenching model. The static quenching behavior suggests strong association of the two PMAS fractions and also sheds further light on why extraordinary purification methods are required for their separation. The UV-visible and photoluminescence spectra of LMWT PMAS are unaffected by changes in pH or treatment with oxidizing or reducing agent, confirming its chemical inertness. In contrast, aqueous HMWT PMAS is oxidized to its pernigraniline base form by persulfate ions and reduced to PMAS leucoemeraldine base by hydrazine, consistent with it being an emeraldine salt. The fully reduced HMWT PMAS leucoemeraldine base (LB) strongly photoluminesces, in common with the LB form of unsubstituted polyaniline. Acknowledgment. The assistance of the Australian Research Council under the ARC Centres of Excellence and QEII Fellowship (Innis) programs are gratefully acknowledged. Additional support was also provided through the SFI (Ireland). Prof Wallace acknowledges the support of the Walton Fellowship Program (Ireland). References and Notes (1) Heeger, A. J. J. Phys. Chem. 2001, 105, 8477 and references cited therein. (2) Friend, R. H.; Greenham, N. C. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R., Reynolds, J. R.; Marcel Dekker: New York, 1998 and references cited therein. (3) Thorne, J. R. G.; Masters, J. G.; Williams, S. A.; MacDiarmid, A. G.; Hochstrasser, R. M. Synth. Met. 1992, 49-50, 159. (4) Kim, K.; Lin, L. B.; Ginder, J. M.; Gustafson, T. C.; Epstein, A. J. Synth. Met. 1992, 49-50, 423. (5) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1998, 84, 85. (6) Kane-Maguire, L. A. P.; Causley, J. A.; Kane-Maguire, N. A. P.; Wallace, G. G. Curr. Appl. Phys. 2004, 4, 394. (7) Shimano, Y. J.; MacDiarmid, A. G. Synth. Met. 2001, 123, 251. (8) Cochet, M.; Buisson, J.-P.; Wery, J.; Jonusauska, G.; Falques, E.; Lefrant, S. Synth. Met. 2001, 119, 389. (9) Chen, S.-A.; Chuang, K.-R.; Chao, C.-I.; Lee, H.-T. Synth. Met. 1996, 82, 207. (10) Wan, M.; Yang, J. J. Appl. Polym. Sci. 2005, 55, 339. (11) Guo, R.; Barisci, J. N.; Innis, P. C.; Too, C. O.; Wallace, G. G.; Zhou, D. Synth. Met. 2000, 114, 267. (12) Zhou, D.; Innis, P. C.; Wallace, G. G.; Shimizu, S.; Maeda, S.-I. Synth. Met. 2000, 114, 287. (13) Masdarolomoor, F.; Innis, P. C.; Ashraf, S.; Wallace, G. G. Synth. Met. 2005, 153, 181. (14) Viva, F. A.; Andrade, E. M.; Florit, M. I.; Molina, F. V. Phys. Chem. Chem. Phys. 2002, 4, 2293. (15) Li, N. B.; Zhang, S. T.; Ding, P. D. Chin. Chem. Lett. 2000, 11, 681. (16) Venancio, E. C.; Wang, P.-E.; MacDiarmid, A. G. Synth. Met. 2006, 156, 357. (17) Trchova, M.; Sedenkova, I.; Konyushenko, E. N.; Stejskal, J.; Holle, P.; Ciric-Narjanovic, G. J. Phys. Chem. B 2006, 110, 9461. (18) Stejskal, J.; Sapurina, I.; Trchova, M.; Konyushenko, E. N.; Holler, P. Polymer 2006, 47, 8253. (19) Liu, X.-X.; Zhang, L. L.; Li, Y.-A.; Bian, L.-J.; Hou, Y.-Q.; Su, Z. Polym. Bull. 2006, 57, 825. (20) Masdarolomoor, F. Ph.D. Thesis, University of Wollongong, 2007. (21) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. (22) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 69, 85. (23) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995, 7, 4435. (24) Stafstro¨m, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Phys. ReV. Lett. 1987, 59, 1464. (25) McManus, P. M.; Yang, S. C.; Cushman, R. J. J. Chem. Soc., Chem. Commun. 1985, 1556. (26) Strounina, E. V.; Shepherd, R.; Kane-Maguire, L. A. P.; Wallace, G. G. Synth. Met. 2003, 135-136, 289. (27) Ram, M. K.; Mascetti, G.; Paddeu, S.; Maccioni, E.; Nicolini, C. Synth. Met. 1997, 89, 63. (28) Pornputtkul, Y. Ph.D. Thesis, University of Wollongong, 2005.

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