Article pubs.acs.org/Langmuir
Surface Grafting of Vinyl-Functionalized Poly(fluorene)s via Thiol− Ene Click Chemistry Andrew R. Davis and Kenneth R. Carter* Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Dr., Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Thiol−ene chemistry is used for the surface grafting of vinyl-functionalized poly(fluorene) derivatives onto substrates containing free surface thiol groups. The grafting reaction proceeds in a matter of minutes under UV irradiation without photoinitiator, and the resulting surface-bound, solvent-impervious conjugated polymers retain their characteristic optoelectronic properties. End-chain grafted poly(fluorene)s reach greater surface densities than their sidechain grafted counterparts and show less blue-shifting of photoluminescence upon grafting, suggesting that chain endgrafted conjugated polymers experience less disruption of their extended conjugation and adopt a more brush-like surface conformation. Surface grafted poly(fluorene)s showed facile photopatterning, and thin film transistors with semiconducting polymers directly grafted to the dielectric layer showed performances directly comparable to conventional self-assembled layers of performance-improving alkylsilanes. increasing success in recent years using “grafting to” approaches, primarily employing reactive silanes or alkyne− azide chemistry, to attach conjugated polymer chains directly to a reactive surface.20−23 Inspired by the successes of alkyne−azide click chemistry compared to the general difficulties of conjugated polymer grafting, we investigate the thiol−ene reaction as a means to functionalize surfaces with conjugated polymers. Photoinitiated thiol−ene chemistry is highly efficient and tolerant of impurities, giving the reaction a wide range of applicability and interest.24−27 The efficiency of this reaction may be useful in accomplishing the historically difficult task of grafting conjugated polymers to surfaces. Recent studies in our group have demonstrated that conjugated polymers can easily be functionalized to include alkene moieties, and these polymers were shown to be amenable to rapid initiator-free cross-linking reactions with multifunctional thiols to form robust electronically active networks.28,29 Growing from this work, we report here on the successful photoinduced thiol−ene surface grafting of end-group and side-chain functionalized conjugated poly(fluorene)s and investigate the effect of that surface tethering on their optoelectronic properties.
1. INTRODUCTION Over the past two decades, the structural and chemical details of organic/organic and organic/inorganic interfaces in organic electronic devices have been found to have immense impact on device performance.1−3 This has led to considerable research into surface modification techniques that improve ordering at the interface, beneficially shift metal work functions, or improve device longevity. These effects have been observed through the use of self-assembled monolayers (SAMs)4−6 or with thin interlayer materials such as zwitterionic species,7 conjugated polyelectrolytes,8,9 or inorganic materials.10 For this reason, the covalent bonding of semiconducting polymers directly to inorganic and organic surfaces is of particular interest due to a potentially high degree of control over surface ordering and chemistry. Typically, high coverage grafting of conjugated polymers to surfaceseither by bonding along the polymer chain or by bonding as a brush from the chain endis difficult due to their relatively rigid nature. Our group has shown initial success growing poly(fluorene) via Yamamoto polymerization from a surface bound monomer11 and polyacetylene via tungsten-catalyzed microwave polymerization from an acetylene-functionalized surface.12 Some additional surface-initiated polymerization success has been seen with the growth of polyvinylcarbazole brushes via free radical polymerization13 as well as poly(thiophene) and poly(phenylene) brushes via Kumada-type polymerizations.14−19 Furthermore, very few studies quantitatively report grafting densities, and experiences in our laboratory and others have often yielded patchy, nonuniform surface coverage of conjugated polymers.14,17 There has been modest but © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. All materials were purchased from Sigma-Aldrich Company and used without further purification unless otherwise Received: January 6, 2014 Revised: March 30, 2014 Published: April 1, 2014 4427
dx.doi.org/10.1021/la5000588 | Langmuir 2014, 30, 4427−4433
Langmuir
Article
Scheme 1. Synthesis of Alkene-Functionalized Poly(fluorene) Derivatives for Surface Grafting via Thiol−Ene Reactiona
a Reaction conditions: (i) tetrabutylammonium bromide (0.01 equiv), 50 mL of 1:1 toluene:50% aqueous NaOH, reflux 17 h; (ii) Pd(PPh3)4 (0.03 equiv), 10 mL of 3:1 toluene:2 M aqueous K2CO3, Aliquat 336, reflux 48 h. The degree of polymerization (DP) and glass transition temperatures (Tg) are reported as measured by gel permeation chromatography and differential scanning calorimetry, respectively.
diol) ester (328 mg, 0.65 mmol) were added to a round-bottom flask containing Pd(PPh3)4 (22 mg, 0.02 mmol) under Ar. Solids were dissolved in 10 mL of a 3:1 mixture of toluene:2 M potassium carbonate with a few drops of Aliquat 336 added as a phase transfer agent, and the reaction flask was placed under vacuum and backfilled with N2 (×3). Reaction was refluxed for 48 h, turning dark purple and beginning to fluoresce under UV after 1 h. Product was isolated in 60% yield as a light yellow powder following precipitation from acidic methanol. 1H NMR (400 MHz, CDCl3) δ: 7.85−7.28 (m, 12H, aromatic); 5.64 (m, 2H, vinyl), 4.91−4.87 (m, 4H, vinyl); 2.15 (b, 8H, alkyl); 1.91 (b, 4H, alkyl); 1.14−0.79 (b, 30H, alkyl). Mn via GPC in tetrahydrofuran: 3.5 kg/mol vs PS standards (DP = 6), PDI: 2.7. 2.5. Polymerization of Styrene End-Capped Poly(fluorene) (xPF). End-capped poly(dihexylfluorene) (xPF) was synthesized as previously reported.28 Briefly, 2,7-dibromo-9,9-dihexyl-9H-fluorene was polymerized via Ni(0)-mediated Yamamoto coupling with 0.125 equiv of 4-bromostyrene added as an end-capping agent. Product was isolated in good yield as a gray powder after precipitation from acidic methanol. DP = 13 via 1H NMR; Mn via GPC 10 400; PDI 2.31. 2.6. Mercaptosilanization. Highly doped Si substrates with 200 nm of thermally grown SiO2 were sequentially rinsed with water, acetone, and isopropanol. Substrates were then treated with O2 plasma for 2 min and immediately immersed in 5 vol % 3-mercaptopropyltriethoxysilane in anhydrous toluene. Si substrates were allowed to react for 4 h at 40 °C. Functionalized surfaces were then removed, rinsed with toluene and ethanol, and dried under N2. 2.7. Thiol−Ene Surface Grafting and Patterning of Poly(fluorene) Derivatives. Solutions of xPF and pPF in CHCl3 (5 mg/ mL) were spin-coated on thiol-functionalized Si substrates at 3000 rpm. The polymer-coated surfaces were then placed under N2 at 100 °C and exposed to 254 nm light for 2 min through a quartz cover dish. UV exposed films were rinsed in CHCl3 for 30 s to reveal the surface grafted layer. For photopatterning, an arbitrary photomask was placed over the spin-coated films prior to UV exposure. 2.8. Device Fabrication. Top-contact thin film transistors were fabricated on highly doped Si/SiO2 (200 nm) substrates. The substrates were sequentially cleaned using H2O, acetone, and IPA, and they were then exposed to O2-plasma for 2 min to clean the surface and install reactive hydroxyl groups. Surfaces were functionalized with 3-mercaptopropyltriethoxysilane as above. pPF was then spin-coated at 3000 rpm (5 mg/mL CHCl3 solution) and either grafted as described above or placed under vacuum for nongrafted devices. Au source/drain were then deposited by thermal evaporation via shadow mask (W/L = 30) at pressures
