Suppression of Polyfluorene Photo-Oxidative Degradation via

Letter pubs.acs.org/JPCL

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes Kyle A. Luck,† Heather N. Arnold,† Tejas A. Shastry,† Tobin J. Marks,*,†,‡ and Mark C. Hersam*,†,‡,§ †

Department of Materials Science and Engineering, ‡Department of Chemistry, and §Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are wellknown dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN−SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

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interactions assist with on-chain ketone defect formation.37−40 This deformation chemistry is initiated by C−H activation of monoalkylated fluorene units on the polymer backone41 and is completed by free radical propagation under photo-oxidative conditions.31 This interpretation is supported by infrared (IR) spectroscopic measurements conducted in situ that have detailed the appearance of a ketone (carbonyl) vibrational mode during photo-oxidation.42−44 In addition, photoluminescence (PL) experiments on a dilute model poly(fluorene-cofluorenone) solution45 and single-polymer molecules46 reveal that the low-energy emission band can be directly stimulated, undermining the excimer emission theory. Furthermore, suppressing intermolecular interactions in the solid state by isolating polyfluorene chains in composite polymer matrixes results in enhanced polyfluorene spectral stability when the composite is subjected to photo-oxidative stress.31,47,48 Recently, single-molecule spectroscopy experiments on isolated solid-state polyfluorene chains unveiled previously unseen blueshifted electroluminescent emission bands relative to their PL spectra,49 which further suggests that the current understanding of polyfluorene photophysics remains incomplete. Here we examine the photophysics of poly [(9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), a blue-emitting alternating fluorene copolymer.12,18 Specifically, PFN thin films are irradiated with

hile many classes of conjugated polymers offer exceptional optoelectronic properties, polyfluorenes and fluorene copolymers distinguish themselves with extraordinary fluorescence properties.1−4 Furthermore, synthetic advances have produced polyfluorene derivatives with diverse optical absorption and emission characteristics that span the entire visible spectrum.5−8 Consequently, these materials have shown significant promise for organic photovoltaics9−12 and organic light-emitting diodes,13−20 both as photoactive and interfacial components. In a parallel line of research, polymer wrapping of single-walled carbon nanotubes (SWCNTs) has facilitated a range of applications in electronic and optoelectronic devices including inverters,21 thin-film transistors,22 light-emitting diodes,23 and photovoltaics.24 While various polymer classes interact strongly with SWCNTs, polyfluorene dispersants have attracted disproportionate attention due to their selective SWCNT dispersion characteristics,25 including diameter and electronic type discrimination,26,27 which is achieved through solvent selection and/or side-chain asymmetry,21 side-chain length tuning,28 and copolymer formulation.29 Despite these many attributes and extensive literature precedent, polyfluorenes remain plagued by undesired green band emission that appears under mild photo-oxidative conditions, ultimately limiting the durability of polyfluorenes in optoelectronic applications.30−33 Initially, polyfluorene green band emission was attributed to intermolecular interactions, which give rise to excimer emission in the solid state,34−36 particularly due to the absence of the green band emission in dilute polyfluorene solutions.30 However, more recent work suggests that intermolecular © XXXX American Chemical Society

Received: September 11, 2016 Accepted: October 10, 2016

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and the absorption spectra (Figure 1b) feature a band that extends from the UV into the blue region of the visible spectrum.18 The broad absorption band and low boiling point solvent used for processing suggest that such thin films are in the glassy phase with negligible β phase formation.50 Subsequently, PL spectroscopy was employed to verify the blue emission band from pristine PFN, as shown in Curve 1 in Figure 1c. This film exhibits a vibronic feature at 425 nm, a progression at 448 nm, and two shoulders at 472 and 502 nm, in agreement with literature precedent.18 One minor distinction in this PFN spectrum is that the second vibronic peak is nearly as intense as (rather than being significantly weaker than) the first peak. The near parity in peak heights can be attributed to the contrasting solvent system (methanol with trace glacial acetic acid versus chloroform) used to disperse the PFN, which induces slightly different torsional angles in the spun-cast polymer chains.25,51 Furthermore, these PL peaks are indicative of glassy phase emission18 given the slight red shift compared to previously reported solution-phase PFN PL, the lack of postprocessing treatments to enhance film crystallinity, and (N,N-dimethylamino)propyl side chains that likely interfere with the known van der Waals stacking interactions between the n-octyl side chains.25 Following initial characterization, the PFN thin films were exposed to UV radiation in ambient conditions (see the Supporting Information for details), and the resulting PL spectra were recorded to track the emission spectral evolution. Figure 1c shows that the PFN PL develops a broad featureless green band emission centered at 550 nm following UV irradiation, as expected. Additionally, a slight decrease in green band emission intensity following 10 min of UV exposure is observed, possibly due to a reduction in the effective conjugation length of the polymer backbone.52 Overall, the peak PL intensity is reduced by ∼90% following 30 min of UV exposure. The appearance of green band emission following UV exposure is likely due to ketone defect formation, which requires oxygen, and is believed to proceed via a free radical propagation reaction mechanism that is accelerated by UV light.31 C−H activation at the monoalkylated tertiary sites on the central (C9) carbon atom of the fluorene unit likely initiates these reactions.53 While synthetic procedures have been devised to remove C9−H defects from monomer feedstocks, these monoalkylated sites remain after polymerization.40 Moreover, residual metal catalyst from the polymerization process can initiate C−H activation at these sites.53 This activation process introduces free radicals on the polymer backbone, leading to subsequent reactions that ultimately culminate in ketone defect formation. Furthermore, radicals generated via C−H activation induce additional radical formation on alkyl carbons α to C9, which can subsequently be converted to ketone defects. Ultimately, these defects ensure that the degree of polyfluorene oxidation is not limited by the number of intrinsic monoalkylated sites resulting from polymerization.31,40 After verifying PFN green band emission, the effects of polymer−SWCNT wrapping on polyfluorene photo-oxidative degradation were probed. While spectral stability has been demonstrated with isolated polyfluorene chains in the solid state, completely isolated polyfluorene chains are impractical for optoelectronic device integration. On the other hand, wrapping PFN around SWCNTs, which might inhibit both interchain interactions and the aforementioned C−H activation relative to the loosely packed glassy phase, may offer a pathway

ultraviolet (UV) radiation under ambient conditions, leading to the expected green band emission from photo-oxidative degradation. Then, PFN-wrapped SWCNT dispersions are prepared to minimize interchain interactions. The resulting drop-cast PFN−SWCNT films show significantly suppressed green band emission following UV exposure, suggesting that intermolecular interactions play a primary role in green band evolution in pristine PFN. These results are then extended to technologically relevant spun-cast PFN thin films incorporating variable PFN−SWCNT content, which reveals that green band emission remains suppressed for SWCNT loadings down to ∼23 wt %. These effects are also visually illustrated as a function of UV illumination. Overall, this study helps elucidate the effect of intermolecular interactions on green band emission in polyfluorenes and presents a SWCNT-based strategy for suppressing photo-oxidative degradation as is required for environmentally stable organic optoelectronic devices. Figure 1a shows the PFN copolymer structure, where one repeat unit incorporates two n-octyl side chains and the other

Figure 1. Structural and optical properties of PFN. (a) PFN chemical structure. (b) Optical absorption spectrum of a PFN thin film. (c) PL spectra of a PFN thin film, where Curve 1 is taken pre-exposure and Curve 2 is taken after 30 min of UV exposure. The inset shows the normalized PL spectra (where each curve is normalized to its peak PL intensity).

repeat unit contains two (N,N-dimethylamino)propyl side chains. PFN was dissolved in methanol with trace glacial acetic acid, and thin films were prepared via spin-casting (see the Supporting Information for details). Optical absorbance spectroscopy was used to characterize the PFN thin films, 4224

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nm imply successful PFN−SWCNT dispersion preparation. Thin films from the ∼1:1 PFN/SWCNT (or ∼50 wt % SWCNT) dilute dispersion solution were then prepared via drop-casting, and the effects of UV exposure were subsequently investigated with PL spectroscopy, as shown in Figure 2c. The pre-exposure spectrum displays a vibronic peak at 420 nm and a progression at 442 nm. The slight peak position changes and nearly identical intensity between the vibronic peak and progression can be attributed to chain conformational changes induced by SWCNT wrapping because changes to polymer film morphology perturb optoelectronic properties.60 In contrast to Figure 1c, the postexposure PFN−SWCNT PL spectrum does not exhibit any measurable green band formation. The spectral shape remains unchanged, with some photobleaching occurring from the UV irradiation.34 Although initially less bright than pristine PFN, there is a significantly smaller 7.3% reduction in peak PL intensity following 30 min of UV exposure. While the drop-cast PFN−SWCNT film exhibits enhanced spectral stability relative to spun-cast pristine PFN, spin-casting is a more realistic technique for preparing polymer thin films in organic optoelectronic devices.61,62 With this in mind, a series of ∼70 nm PFN thin films incorporating variable SWCNT wt % content were prepared via spin-casting, up to the maximum SWCNT content possible given a device-relevant thickness constraint of ∼70 nm,63 as verified by profilometry (Dektak 150 Stylus Surface Profiler). The PL results from these blend films are outlined in Figure 3. While overall brightness decreases with increasing SWCNT wt % content, there is a concurrent enhancement in film spectral stability. In particular, the maximum ∼23 wt % condition in Figure 3f exhibits a ∼13% reduction in peak intensity following 30 min of UV exposure, nearly at parity with the 7.3% peak intensity reduction observed for the drop-cast 50 wt % SWCNT dispersion films in Figure 2c. Notably, the pre-exposure PL spectra in Figure 3b−e all exhibit a weaker vibronic progression intensity than the initial vibronic peak. Because film morphology influences optoelectronic properties,60 these spectral changes could be the result of the polymer wrapping process enabling effective intermixing of pristine PFN and PFN−SWCNTs. To further illustrate the blend film results shown in Figure 3, Figure 4 presents photographs of PFN thin films with variable SWCNT content under UV illumination (Life Technologies Gibco BRL TFX-35 M UV Transilluminator), including the impact of different exposure times. The results in Figures 3 and 4 show that increasing the SWCNT content reduces film brightness but enhances polyfluorene spectral stability, evidenced by the ∼90% decrease in peak PL intensity for pristine PFN versus the ∼13% decrease for PFN with ∼23 wt % SWCNT content. The reduction in brightness with increasing SWCNT content is probably due in part to displacement of PFN by SWCNTs. Additionally, the SWCNT π plasmon band absorbs some of the UV excitation light.64 Because the polymer emission band and (6,5) SWCNT absorption bands do not overlap, Förster resonance energy transfer (FRET) is unlikely to occur.65 However, direct exciton transfer from the polymer to SWCNT is possible.66 The spectral stability results observed in Figures 2c and 3f suggest that the PFN wrapping conformation induced by the SWCNTs suppresses green band formation, likely inhibiting the alkyl chain to ketone conversion chemistry. This interpretation further suggests that intermolecular interactions in the glassy phase play a significant role in green band evolution. One possible explanation for the green band

toward enhanced spectral stability in a manner that is more amenable to optoelectronic applications. Figure 2a schemati-

Figure 2. Materials and characterization of PFN−SWCNT dispersions. (a) Illustration of PFN−SWCNT wrapping. (b) Optical absorption spectrum (solution) of a diluted PFN−SWCNT dispersion, with polymer (PFN) and SWCNT (S11 and S22) absorption peaks assigned. (c) PL spectra of a drop-cast PFN thin film incorporating ∼50 wt % SWCNT content, where Curve 1 is taken pre-exposure and Curve 2 is taken after 30 min of UV exposure. The inset shows the normalized PL spectra (where each curve is normalized to the peak PL intensity).

cally illustrates PFN−SWCNT wrapping. Molecular dynamics (MD) simulations suggest that there is a favorable van der Waals interaction when the polymer helically wraps around the SWCNT in a geometry where the sp2-hybridized backbone of the polymer contacts the sp2-hybridized nanotube walls, with most alkyl tails pointing radially outward from the nanotube surface.25,28 Additionally, MD calculations show that the alkyl tails tend to interdigitate, further stabilizing the helical polymer conformation.25,28 Consistent with these theoretical results, several experimental studies have reported that the PFN structural relatives poly(9,9-dioctylfluorene) (PFO)25,54 and PFO copolymers23,55 effectively disperse SWCNTs. To understand the photophysical implications of polymer wrapping, PFN−SWCNT dispersions were prepared following established procedures, including horn sonication and centrifugation (see the Supporting Information for details).28,56 Optical absorbance spectroscopy was then used to verify successful dispersion preparation.26,57,58 For example, Figure 2b illustrates the optical absorption characteristics of a PFN−SWCNT dispersion. The 988 nm S11 peak and 576 nm S22 peak are well-matched with the van Hove optical transitions reported in the literature for (6,5) chirality SWCNTs.59 These spectral features and the PFN peak at 391 4225

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Figure 3. PL spectra of spun-cast PFN films incorporating variable SWCNT mass percentages. Insets show the normalized PL spectra. For the normalized plots, each curve is normalized to its peak PL intensity, allowing for better visualization of spectral shifts. For all plots, Curve 1 is taken pre-exposure, Curve 2 is taken after 10 min of UV exposure, Curve 3 is taken after 20 min of UV exposure, and Curve 4 is taken after 30 min of UV exposure. Approximate SWCNT contents are (a) 0, (b) 5, (c) 9, (d) 14, (e) 17, and (f) 23 wt %.

ultimately result in ketone defects if allowed to proceed unimpeded. In conclusion, polyfluorene green band evolution was investigated in solid-state PFN and PFN-wrapped SWCNTs. The green band evolution observed for pristine PFN is significantly suppressed when a drop-cast PFN−SWCNT film with ∼50 wt % SWCNT content is exposed to the same ambient UV exposure conditions. This suppression is attributed to the inhibition of free radical propagation reactions, potentially initiated by residual metal catalyst and driven in part by side-chain intermolecular interactions. The spectral evolution of spun-cast PFN thin films with variable PFN− SWCNT content was also explored, revealing that PFN thin films with ∼23 wt % SWCNT content achieve spectral stability nearly at parity with drop-cast PFN−SWCNT dispersion films. Overall, this work indicates that intermolecular interactions play a significant role in green band evolution and presents PFN−SWCNT composites as a pathway for stable incorporation of polyfluorenes in organic optoelectronic devices.

inhibition achieved through nanotube wrapping is residual metal catalyst removal during the dispersion/centrifugation process, which could suppress C−H activation and subsequent free radical generation on the polymer backbone under photooxidative stress. However, trace metal catalyst has been observed in SWCNT samples purified by centrifugation, which implies that some catalyst remains in the prepared dispersions.67 Another possibility is that polymer chains with monoalkylated C9 sites do not wrap SWCNTs. Because the helical conformation of the wrapped polymer is stabilized by interdigitation of the alkyl tails, polymer chains with monoalkylated C9 sites should be less stable energetically compared to chains with all dialkylated C9 sites. However, the rigorous procedures developed to purify monomer feedstocks suggest that the low density of these monoalkylated sites on the polymer chains would minimally interfere with SWCNT wrapping.40,41 A more plausible explanation is that the combination of n-octyl tail interdigitation and chain isolation afforded by polymer wrapping, in addition to potential polymer−SWCNT exciton transfer, inhibit the aforementioned C−H activation and radical propagation reactions, which would 4226

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Figure 4. Photographs of PFN thin films incorporating variable SWCNT mass percentages as a function of exposure times to UV illumination. From left to right, the approximate SWCNT contents are 0, 5, 9, 14, 17, and 23 wt %. Row 1: Pre-exposure. Row 2: 10 min of UV illumination. Row 3: 20 min of UV illumination. Row 4: 30 min of UV illumination.

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ASSOCIATED CONTENT

Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the NSF-MRSEC program (NSF DMR-112162); the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02079. Detailed experimental procedures, including PFN− SWCNT dispersion preparation, optical absorbance spectroscopy, and photoluminescence measurement details (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.J.M.). *E-mail: [email protected] (M.C.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Argonne−Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DESC0001059. The research utilized instruments in the Energy Materials Laboratory within the Department of Materials Science and Engineering at Northwestern University, which is partially supported by the NSF-MRSEC (NSF DMR-1121262). The Institute for Energy and Sustainability at Northwestern (ISEN) supplied partial equipment funding. K.A.L. and T.A.S. acknowledge graduate research fellowships from the National Science Foundation. T.A.S. also acknowledges a fellowship from the Patrick G. and Shirley W. Ryan Foundation. H.N.A. acknowledges support from a NASA Space Technology Research Fellowship (NSTRF, #NNX11AM87H). This work made use of the Keck-II facility of the NUANCE Center at 4227

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