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A facile avenue to conductive polymer brushes via cyclopentadienemaleimide Diels–Alder ligation
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Cyclopentadienyl end-capped poly(3-hexylthiophene) was employed to fabricate conductive surface tethered polymer brushes via a facile route based on cyclopentadienemaleimide Diels–Alder ligation. The efficient nature of the Diels–Alder ligation was further combined with a biomimetic polydopamine-assisted functionalization of surfaces, making it an access route of choice for P3HT surface immobilization.
O N A
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O Si O Si O (1)
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This journal is © The Royal Society of Chemistry [year]
O O THF, 4 ambient conditions O
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Polymer brushes assisted tailoring of interfaces has led to the development of materials with controlled surface properties such as wettability, biofouling resistance, adhesion, and stimuli responsiveness.1 Recently, surface-anchored polymer chains of organic conductive polymers have been the focus of interest for tuning the surface electronic properties of a variety of materials.2 Beside the improved organic photovoltaic cell performance and superior electrostatic charge dissipation characteristics associated with surface-anchored organic conjugated materials,3 thin films of conductive polymers have also been employed for (bio)sensing, and for the fabrication of electric field responsive scaffolds for nerve regeneration and photothermal therapy.4 Consequently, the development of facile strategies for the fabrication of conductive polymer brushes has been a focus of current research activities. Grafting-from (a polymerization process initiated from an initiator-functionalized substrate surface) and grafting-to (covalent grafting of pre-synthesized polymer chains onto an appropriately functionalized surface) approaches are the two main strategies employed for the fabrication of polymer brushes; the grafting-from approach can lead to a higher grafting densities. Both approaches have been explored for the fabrication of conductive polymer brushes, too. The grafting-from strategies have generally benefited from the catalyst transfer polycondensation (CTP) based living chaingrowth processes, originally developed for the synthesis of conductive polymers.2 Kiriy and coworkers have demonstrated the viability of the Grignard Metathesis (GRIM) based surface initiated Kumada Catalyst Transfer Polycondensation (SI-KCTP) method for the fabrication of regioregular poly(3-alkylthiophene) (P3AT) polymer brushes. Exploiting the living chain-growth mechanism, flat silicon (Si) substrates and silica nanoparticles were functionalized with the Ni based KCTP initiator moieties, which were subsequently employed for the SI-KTCP of the 2bromo-5-magnesiohalo-3-alkylthiophene, a Grignard based monomer.5 Despite the success of the above strategy, its scope is
HN O Si O Si O
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(5) B Si P THF, TEA Si P Au D NH Au D NH2 2 PET A PET A Si (7) Au (11) PET (14)
O O N
Si (8) Au (12) PET (15) PDA = Polydopamine
O Si P Au D NH PET A Si (9) Au (13) PET (16)
THF, 4, ambient conditions
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Scheme 1 Cp-maleimide ligation for P3HT polymer brush fabrication via silane chemistry (A) and via PDA coating (B, the possible acylation via catechol –OH groups is ommited for brevity).
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limited, firstly by the highly demanding conditions for the fabrication of surfaces with air and humidity sensitive Ni based KTCP initiator moieties (a limitation also associated with other transition metal catalyzed CTP processes)2, and secondly by the Grignard nature of the monomer. The latter limitation further restricts the application of SI-KCTP to substrates which are reactive towards the Grignard reagent. Alternate strategies are being explored to circumvent these limitations. Contrary to the grafting-from strategies based on SI-CTP processes, endtethering polymers via grafting-to strategies can offer facile surface grafting conditions, courtesy of a variety of highly efficient ligation reactions.6 However, the synthesis of an appropriately end functionalized polymer and a suitably functionalized surface can be challenging. In the context of P3AT, the GRIM method has emerged as a facile route to P3ATs with a variety of end groups. Recently, employing the GRIM method, we have reported the synthesis of cyclopentadienyl (Cp) end-capped poly(3-hexylthiophene) (P3HT-Cp).7 The Cp moiety is known for its facile Diels–Alder cycloaddition with dienophiles, which function efficiently under ambient conditions and, for certain dienophiles (e.g., maleimide), without the need of a catalyst.6 Capitalizing on this opportunity, we herein [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
Basit Yameen,a Cesar Rodriguez-Emmenegger,a,b Corinna M. Preuss,a Ognen Pop-Georgievski,c Elisseos Verveniotis,d Vanessa Trouillet,e Bohuslav Rezek d and Christopher Barner-Kowollika*
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demonstrate a cyclopentadiene-maleimide based Diels–Alder ligation between the Cp end groups of P3HT-Cp and surface anchored maleimide groups as a facile route to conductive polymer brushes. A Si substrate was functionalized with maleimide functional groups by employing a two step approach (Scheme 1A). The surface amino groups of the (3-aminopropyl)triethoxysilane (APTES) functionalized Si substrate (1) were reacted with 4maleimidobutyroyl chloride (2) to afford the maleimidefunctionalized Si substrate (3). The maleimide functionalized substrate 3 was subsequently immersed in a THF solution of P3HT-Cp (4, Mn 6.5 kDa, Đ 1.2, 5 mg·mL-1) and left to react overnight under ambient conditions. The resulting P3HT brush functionalized Si substrate (5) was rinsed with copious amounts of THF to remove any physically absorbed P3HT.
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Fig. 1 Monitoring of P3HT brush fabrication (via silane chemistry, Scheme 1A, Si substrates 1, 3, and 5) by C 1s (left) and S 2s (right) by high resolution XPS analysis. 20
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All the surface chemical transformations (Scheme 1) were characterized by X-ray photoelectron spectroscopy (XPS). The successful functionalization of the Si substrate with the maleimide groups (3) was ascertained by the appearance of a C1s signal for the N-C=O bond at 288.8 eV (Fig. 1). The subsequent Diels–Alder ligation with P3HT-Cp led to a decrease in the intensity of N-C=O bond signal and an increase in the intensity of the signal assigned to C-C and C-H bonds, which evidences the successful grafting of the P3HT chains at the surface. The presence of the sulfur S 2s signal at 228.5 eV solely in the XPS spectrum of 5 confirms the successful surface functionalization with P3HT (Fig. 1). The XPS analysis of a negative control sample 6, prepared by immersing substrate 1 in the THF solution of the P3HT-Cp 4 under the similar conditions as employed for the fabrication of substrate 5, showed no change when compared to the XPS analysis of 1. In addition to the identical C 1s spectra for 1 and 6, the negative control sample 6 did not display any trace of sulphur (Fig.1), which confirmed the success of the surface grafting of P3HT chains via covalent ligation of the terminal Cp groups of P3HT-Cp 4 and the surface maleimide groups. We further extended the scope of the Cp-maleimide ligation for the surface grafting of P3HT chains by combining it with polydopamine’s (PDA) proven versatility to functionalize virtually any kind of surface (Scheme 2).8 The versatility of the 2 | Journal Name, [year], [vol], 00–00
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approach was demonstrated by coating PDA onto three different substrates, namely, Si, gold coated Si substrate (Au), and View Article Online poly(ethylene terephthalate) (PET), a flexible The Si DOI:substrate. 10.1039/C3CC44683B 9 substrate was coated with PDA, resulting in the amino and catechol group functionalized10 Si substrate 7, which was subsequently reacted with 2 to achieve the maleimide functionalized Si substrate 8. The treatment of the substrate 8 with the P3HT-Cp 4 under ambient conditions gave PDA assisted P3HT functionalized Si substrate. XPS analysis (Fig. S1, ESI) confirmed the maleimide functionalization via the appearance of the C 1s orbital signal for the N-C=O bond at 288.8 eV. The subsequent successful P3HT grafting is evidenced by an increase in the C 1s orbital signal corresponding to the C-C and C-H bonds, which is consistent with the observation made for the substrate 3 and 5. The appearance of the S 2p orbital signal originating from the sulphur constituent of the P3HT confirms the covalent grafting. The C 1s orbital signal in the XPS analysis of the negative control sample 10, prepared by treating 7 with 4 under the conditions similar to 9, was absolutely identical to the sample 7 with no trace of sulphur at the surface, confirming the covalent grafting of P3HT-Cp 4 via Cp-maleimide conjugation with the surface maleimide groups of the substrate 8. The XPS results for the Au (11-13) and PET (14-16) substrates were identical to the above described XPS analysis of the substrates 79, which is a consequence of the identical chemical nature of the PDA coating and subsequent surface chemistry. For the sake of brevity the XPS data for the Au (11-13) and PET (14-16) substrates are only provided in the SI section (Fig. S2, ESI). In addition to the chemical characterization via XPS, spectroscopic ellipsometry (SE) was employed to estimate the thickness changes associated with all the surface functionalization steps on Si substrates 1, 3, 5 fabricated via silane chemistry (Scheme 1A). The ellipsometric analysis of the Au substrates (11-13) was used to estimate the thickness changes associated with the PDA assisted P3HT brush fabrication (refer to Table S1; in-depth details of the employed SE method is provided in the ESI). The thickness of the P3HT grafted at the surface of Si substrate 5 was estimated to be 4.3 nm. The grafting density of the P3HT chains was estimated by employing the expression σ = hρNA/Mn, where h is the ellipsometric thickness of the P3HT film, ρ is the density of the polymer (1.1 g·cm-3),11 NA is Avogadro’s number, and Mn is the number average molecular weight of P3HT-Cp (Mn 6.5 kDa). This estimation led to a σ value of 0.44 chain·nm-2. Ellipsometric analysis performed for PDA based surface functionalization on the Au substrate led to a 4.7 nm thick P3HT brush layer with a grafting density of 0.48 chain nm-2. These values are consistent with those estimated for the silicon substrate 5, reflecting on the consistency of the P3HT brush formation via Cp-maleimide ligation irrespective of the adopted chemical route. The σ values as discussed above support the formation of a P3HT polymer brush of medium density, which is consistent with the previous report where a P3HT of comparable Mn (6.0 kDa) was employed for the fabrication of conductive polymer brushes (polymer brush thickness = 4.8 nm, σ =0.53 chain nm-2) via Cu catalyzed azide-alkyne ligation.11 The conductive polymer films are known to change the surface electronic properties of the materials. In the present study, the conductive P3HT brushes induced change in the surface
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electronic properties is demonstrated by the Kelvin-Probe Force Microscopy (KFM) performed on the P3HT functionalized Au substrate fabricated via the PDA route. The potential differences between the initial substrate (i.e., Au) and each deposited layer (11-13) are depicted in the Fig. 2 (top panel). The corresponding AFM topography and KFM potential map are depicted in Fig. 2 a and b, respectively (bottom panel). It is evident that every modification resulted in a shift of the electric potential vs. the bare Au substrate (Fig. 2, top panel). The PDA coated substrate (11) exhibited a surface potential of 50 mV (± 3.5), which decreased to 31 mV (± 4.5) for the maleimide functionalized substrate 12. This decrease in surface potential is consistent with the incorporated aliphatic constituent at the surface of 12 increasing the surface work function. The subsequent Cpmaleimide ligation of the P3HT-Cp at the surface (13) increased the surface potential to 77 mV (± 4), evidencing the decrease in the surface work function and a clear proof for the grafting of the π-conjugated P3HT chains at the surface.
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Fig.2 Potential differences (mV) between Au and the deposited layers for PDA assisted P3HT brush fabrication (top). The AFM topography (bottom, a) and KFM potential map (bottom, b) for the P3HT brush.
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In summary, an efficient and catalyst free Cp-maleimide ligation route for the fabrication of conductive polymer brushes of moderately high grafting density is presented. The success of the proposed strategy is demonstrated by grafting a precisely designed Cp end-capped P3HT polymer onto maleimide functionalized Si substrates. The scope of the developed strategy is further expanded by combining it with the unique ability of PDA to coat virtually any kind of surface. The change in the surface potential was demonstrated by employing a KFM analysis on PDA tethered P3HT brushes on Au substrates. The success of the Cp-maleimide ligation constitutes an important step to achieve conductive and surface anchored polymer brushes under ambient conditions. Additionally, the versatility of the PDA based biomimetic platform employed in the current study will facilitate access to a variety of substrates with controlled surface electronic properties of flexible conductive materials. B.Y. and C.R.-E. acknowledge the Alexander von Humboldt Foundation for support. C.B.-K. acknowledges funding from the Karlsruhe Institute of Technology (KIT) and the Helmholtz association via its BioInterfaces program. C. R.-E. (P205-12-G118), E.V. and B.R. (P108/12/G108), as well as O. P.-G. (P108/11/1857) acknowledge support from the Grant Agency of the Czech Republic.
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Notes and references
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a Preparative Macromolecular Chemistry, Institut für Technische Chemie View Article Online und Polymerchemie, Karlsruhe Institute DOI: of 10.1039/C3CC44683B Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany and Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Email:
[email protected], http://www.macroarc.de b Zoological Institute, Department of Cell- and Neurobiology, Karlsruhe Institute of Technology (KIT), Haid-und-Neu-Str. 9, Karlsruhe, Germany. c Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Square 2, 162 06 Prague 6, Czech Republic. d Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 16253, Prague, Czech Republic. e Institute for Applied Materials (IAM) and Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
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