Designing Ï-Conjugated Polymeric Nano- and ... - ACS Publications

Article pubs.acs.org/Macromolecules

Designing π‑Conjugated Polymeric Nano- and Microstructures via Light Induced Chemistry Eva Blasco,†,‡ Basit Yameen,§ Alexander S. Quick,†,‡ Peter Krolla-Sidenstein,∥ Alexander Welle,†,‡ Martin Wegener,⊥,# and Christopher Barner-Kowollik*,†,‡ †

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76128 Karlsruhe, Germany ‡ Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Laboratory of Nanomedicine and Biomaterials, Department of Anaesthesiology, Brigham and Women’s Hospital, Harvard Medical School. 75 Francis Street, Boston, Massachusetts 02115, United States ∥ Institut für Funktionelle Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ⊥ Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Straße 1, 76128 Karlsruhe, Germany # Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: We report the synthesis and characterization of a new class of photoreactive conjugated polymers (Mn= 9300 g mol−1, Đ = 1.24; Mn= 9100 g mol−1, Đ = 1.23; Mn= 8800 g mol−1, Đ = 1.24) consisting of functionalized polythiophene-containing photoresponsive groups, i.e. a polythiophene containing 4-hydroxy2,5-dimethylbenzophenone (DMBP) (“photoenol”) and/or maleimide groups. We evidence the chemical versatility and platform character of the generated light reactive polythiophenes in a range of examples by creating variable polythiophene functional 2D and 3D structures and morphologies. Single-chain nanoparticles and polymeric networks are formed by cross-linking of DMBP and maleimide containing polythiophene via UV light irradiation by varying the polymer concentration. Maleimide-functionalized polythiophene is employed for spatially resolved functionalization of surfaces and the coating of preformed 3D polymeric microstructures via direct laser writing (DLW). The generated polythiophene functional materials are carefully characterized via size exclusion chromatography, dynamic light scattering, and time-of-flight secondary ion mass spectrometry as well as FT-IR spectroscopy techniques.

■

INTRODUCTION π-Conjugated polymers have been widely studied due to their potential applications relying on their conductivity or photo- or electroluminescence, such as organic solar cells1−4 and organic light-emitting diodes,5,6 chemical sensors,7,8 and, more recently, biological applications.9 Among the π-conjugated polymers, polythiophene has recently received increasing attention mainly due to the Grignard metathesis (GRIM) polymerization that has emerged as a rapid and powerful method for the preparation of regioregular polythiophene with low polydispersities and adjustable molecular weights.10,11 Moreover, precision architectures based on polythiophene can be readily prepared by incorporating appropriated functional groups after polymerization. Preparation of nanoparticles (NPs) from π-conjugated polymers has also recently attracted attention.12,13 These polymeric NPs are commonly prepared by miniemulsion or © XXXX American Chemical Society

precipitation methods. However, the particle size strongly depends on the preparation conditions, such as polymer concentration, surfactant concentration or addition rate, among others. One way for achieving precision polymer nanoparticlessimilar to approaches that nature takesis the folding of one macromolecule to form highly defined and compact single-chain NPs (SCNPs). To the best of our knowledge, only few examples based on single-chain conjugated polymeric nanoparticles have been described so far. For example, Harth and co-workers reported ABA triblock copolymers composed of two block copolymers (A), copolymerized from styrene and vinylbenzosulfone cross-linker monomer and a center block (B), based on fluorene and Received: September 15, 2015 Revised: November 14, 2015

A

DOI: 10.1021/acs.macromol.5b02024 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. 2D and 3D Structures Prepared from (Photo)functionalized Polythiophene: (A) Single-Chain Nanoparticle (SCNP) Formation; (B) Preparation of a Polythiophene-Based Polymeric Networks; (C) Polythiophene Surface Functionalization with Spatial Control; (D) Polythiophene Functionalization of 3D Preformed (Direct Laser Writing) Polymeric Microstructures

fluorene/thiophene copolymers.14 The formation of the nanoparticles via an intramolecular chain collapse process was performed by thermal cross-linking via the benzosulfone functionality of the copolymers with dimensions of 5−10 nm. These well-defined NPs presented increased photoluminescence depending on the length of block A. Liu and cowokers also reported the formation of single chain NPs from poly[9,9bis((6-N,N,N-trimethylammonium)hexyl)fluorene-alt-co-2,1,3benzoxadiazole dibromide] (PFBD) with an average diameter of 4 nm.15 These NPs were subsequently conjugated with streptavidin, proving their potential application as specific extracellular labeling and imaging probes. On the other hand, the preparation of stable conjugated polymeric films is still a challenge, especially in applications requiring a multilayer design such as OLEDs or solar cells.1−4 It is well-known that a decrease in the solubility of the conductive polymer can improve the multilayer processing and increases the stability of the polymer. During the last years, one of the strategies employed consists of the thermocleavage of side chains of the polythiophene to improve the solubility of these polymers.16,17 Side chains are essential to solubilize the polymer and allow for the solution processing of the active layer. However, higher solubility of the polymers also led to a poor solvent resistance of the film, which is detrimental to the multilayer device configuration during solvent based fabrication. By using thermocleavable side chains, the stability of polymeric film can be improved, however one of the main disadvantages of the above method are the required high temperatures. An alternative strategy not much explored yet consists of the preparation of polythiophene containing crosslinkable functional groups,18,19 which we take in the current study. Recently, conjugated polymers have been in the focus of interest for tuning the surface electronic properties of a variety of materials. Consequently, the interest in developing new strategies for the preparation of conjugated polymer brushes has increased due to their importance in variable applications

such as solar cells1−4 or biosensors.7,8 There exist two main strategies for the preparation of polymeric brushes, i.e. graftingfrom (the polymerization is initiated at the surface) and grafting-to (the polymer is grafted to the surface). In recent work, our group has reported a facile route for the fabrication of conjugated polymer brushes via the grafting-to strategy. Here, cyclopentadiene−maleimide based Diels−Alder ligation between the Cp end groups of P3HT-Cp and surface anchored maleimide groups were employed.20 It is worth noting that in nearly every reported example addressing polythiophene functionalization, thermally induced reactions have been employed. Often, however, light-induced reactions are more attractive, since irradiation provides ready access to temporal and spatial control. Recently, our group has explored a highly efficient modular ligation chemistry which proceeds via a UV-light-triggered Diels−Alder reaction. This reaction consists of an addition of hydroxy-o-quinodimethanes derivatives (“photoenols”) generated by photoisomerization of o-methylphenyl ketones or aldehydes with a dienophile, such as maleimide groups. This strategy has been proven to be a powerful and facile way for rapid polymer−polymer conjugations21−24 as well as surface conjugation.25 In the current study, we present a synthetic platform technology to prepare 2D and 3D structures from functionalized polythiophene via a light induced reaction (Scheme 1). In a first approach, a polythiophene containing 4-hydroxy-2,5dimethylbenzophenone (DMBP) (termed as “photoenol”) and maleimide groups were employed for the formation of singlechain nanoparticles (A) and polymeric networks (B). Crosslinking of DMBP and maleimide containing polythiophene was performed by irradiation with UV light. By selecting the appropriate polythiophene concentration it was possible to tune cross-linking from single-chain nanoparticles to crosslinked polymeric films. Second, maleimide-functionalized polythiophene was employed for spatially resolved functionalization of surfaces (C) and preformed direct laser written 3D polymeric microstructures via a Michael addition reaction (D). B


Article

Macromolecules Scheme 2. Synthetic Strategy for the Synthesis of Target (Photo)functional Polythiophene Copolymers: (a) tBuMgCl, Ni(dppp)Cl2, THF; (b) TBAF, THF; (c) SOCl2, CH2Cl2; (d) TEA, THF; (e) Toluene, 80 °C

Table 1. Molecular Weight and Composition of the Synthesized Copolymers

a

polymer

Mna

ĐM a

% 1(n)b

% 2(m)b

% maleimide(m′)b

% DMBP(m″)b

3 3′ 7 7′ 8

8100 7100 9300 9100 8800

1.21 1.25 1.24 1.23 1.24

77 55 77 56 77

23 45 − − −

− − 12 22 23

− − 11 22 −

Mn and ĐM were determined by SEC using PS standards. bCalculated by 1H NMR (see text).

■

composition of the copolymers was studied by 1H NMR spectroscopy (refer to Figure S2 in the Supporting Information section). By comparing the resonances corresponding to hexyl chains of monomer 1 and the ones associated with the silyl ether of monomer 2, a ratio of 77:23 and 55:45 was deduced for copolymers 3 and 3′ respectively, which is in agreement with the monomer feed (Table 1). Subsequently, the tert-butyl dimethyl silyl ether groups were deprotected to afford polythiophene copolymers featuring pendant hydroxyl groups, 4 and 4′. Completion of the reaction was asserted by 1H NMR spectroscopy by the disappearance of the resonances at 0.97 and 0.14 ppm corresponding to the methyl and tert-butyl groups of the silyl ether (refer to Figure S3 in the Supporting Information section). The copolymers 4 and 4′ were subsequently esterified with a 1:1 mixture of a protected maleimide-containing carboxylic acid 5 and DMBP carboxylic acid 6. In the last step, the maleimide group was deprotected by a heat induced retro-Diels−Alder reaction in quantitative yield giving the targeted copolymers 7 and 7′ with maleimide and DMBP as pendant groups. 1H NMR spectroscopy confirmed the successful functionalization of both polymers (see Figure 1) by the appearance of resonances associated with the protons of the maleimide group at 6.6 ppm

RESULTS AND DISCUSSION Synthesis of the Functional Polythiophene Derivatives. The targeted polymers consist of polythiophene containing photoresponsive moieties as pendant groups, i.e. maleimide functions and/or DMBP, enabling UV-induced Diels−Alder reactions. The strategy employed for the synthesis of the polymers is depicted in Scheme 2. In the initial step, two statistical copolymers (3 and 3′) of 2,5-dibromo-3-hexylthiophene 1 and a 2,5-dibromothiophene compound 2, bearing a protected hydroxyl group connected to the thiophene ring at 3position, were prepared by GRIM polymerization. Two ratios of the monomers 1 and 2, i.e. 75:25 and 60:40, were employed. Molecular weight distributions were determined by size exclusion chromatography (SEC) using PS standards (refer to Table 1 and Figure S1 in the Supporting Information). It was observed that the solvent employed for the quenching of the polymerization plays an important role. When employing methanol, the resulting SEC traces showed a shoulder at higher molecular weights, while with a 5 N HCl solution a symmetric trace was obtained (refer to Figure S1 in the Supporting Information). This phenomenon has been previously reported by Yokozawa and co-workers and is attributed to a disproportionation process during the quenching.26 The C


Article

Macromolecules

benzophenone derivative 5 in solution showed an absorption band with the maximum at 290 nm, while the polythiophene presented a strong band centered at 450 nm. In order to perform the light-induced reaction under optimum conditions, a lamp emitting radiation with the maximum at 312 nm (36 W) was employed. Within this wavelength range, polythiophene exhibited a minimum of absorption while the photoreactive group still presented a strong absorption (Figure 2).

Figure 1. 1H NMR spectrum of copolymer 7 in CDCl3.

(proton “j”) and the aromatic protons of the benzophenone moiety between 7.0 and 7.8 ppm (protons “q”−“w”). A comparison of resonance integrals for protons “f ” at 0.94 ppm corresponding to the methyl group of the thiophene of comonomers 1 and protons “n” and “i” at 4.12 and 3.85 ppm corresponding to the methylene groups of comonomer 2 after different functionalization allowed for the calculation of the composition of both copolymers (refer to Table 1). For copolymer 7, 77 mol % of monomer 1 and 12 mol % and 11 mol % of maleimide and DMBP-containing thiophene respectively, were estimated. These values are in good agreement with the initial monomer feed ratio of 75:25. Similarly, for polymer 7′ a 56 mol % of monomer 1 and 22 mol % of both maleimide and DMBP groups were calculated and corresponded to the initial monomers 1 and 2 feed ratio of 60:40. By following the same strategy, copolymer 4 with the protected-maleimide was synthesized by reacting with 5, and subsequent deprotection by a heat induced retro-Diels−Alder reaction in quantitative yields, affording copolymer 8. 1H NMR spectroscopy confirmed the successful functionalization (see Figure S4) by the appearance of resonances associated with the protons of the maleimide group at 6.6 ppm (proton “k”). Similarly to the previous copolymers 7 and 7′, a comparison of the resonance integrals of protons “f ” at 0.94 ppm corresponding to the methyl group of the thiophene of comonomers 1 and protons “i” at 3.80 ppm corresponding to the methylene groups of comonomer 2 after different functionalization agreed with the expected copolymer composition, i.e., 77 mol % of monomer 1 and 23 mol % of maleimide groups. A. Formation of Single Chain NPs. As noted above, the cross-linking of the polymeric chains was performed by employing a UV-light triggered Diels−Alder reaction between the “photoenol” moiety 4-hydroxy-2,5-dimethylbenzophenone (DMBP) and the maleimide function. However, the UVinitiated reaction has not been employed before with polymers exhibiting a strong absorption in the UV region such as polythiophenes. Thus, before applying the approach to crosslink the polymers, the UV spectra of the photoresponsive unit, i.e., 4-hydroxy-2,5-dimethylbenzophenone and polythiophene were recorded in order to select a suitable light source. The

Figure 2. UV−vis absorption spectra of the DMBP unit and copolymer 7 in dichloromethane c = 5·10−3 mol L−1. The gray region indicates the emission range of the lamp employed for the photoreactions.

The preparation of SCNPs requires irradiation of highly diluted solution in order to ensure that only an intramolecular cross-linking takes place. On the basis of previous experiments of our group, the concentration of the polythiophene solutions was selected to be close to 0.02 mg mL−1.27 At higher concentrations intermolecular cross-linking was detected via SEC. Chloroform was selected as a suitable solvent due to the good solubility of the functional polythiophene copolymers. THF was also assessed as solvent for the photoreaction, however the SEC traces evidenced intermolecular cross-linking of the material during the SCNP formation (refer to Figure S5 in the Supporting Information). The formation of the NPs was followed by SEC, DLS, and AFM. For the SEC analysis, the photoreaction was performed and analyzed after different times. A clear increase in the retention time (decrease of molecular weight) was observed after irradiation in both cases (refer to Figure 3). The decrease of the apparent molecular weight can be explained by the decrease of the hydrodynamic radius, as the formed SCNPs are significantly smaller than the starting linear macromolecules. No evidence of intermolecular coupling was observed under these conditions, since no peak appeared in the region of higher molecular weight. In the case of polymer 7, a gradual increase in the retention time was observed until 2 h of irradiation. Only slight changes were detected after this time. The polymers were never irradiated for more than 4 h, since new peaks at lower molecular weight appeared, indicating the possible degradation of the material. For polymer 7′, no further changes were observed in the SEC traces after irradiation for 1 h indicating that the collapse of the chains was complete. The faster formation of the single-chain NPs from copolymer 7′ can be due to the higher density of photoreactive groups in the copolymer raising the probability of reaction. The degree of D


Article

Macromolecules

Figure 3. SEC traces of the copolymers 7 (a) and 7′ (b) after different irradiation times (λmax = 312 nm) in chloroform (c = 0.02 mg mL−1).

height map images of the SCNPs prepared from the functionalized polythiophene 7 and 7′ evidencing the presence of single chain NPs as well as larger aggregates. The size of the NPs observed by this technique cannot be directly compared to the hydrodynamic diameters measured by DLS, due to the differences between the particles in the solid state and solution in addition to the apparent size increase of the NPs due to the broadness of the AFM tip. This phenomenon was already observed in the previous work of our group.27 Nevertheless, the AFM images also suggest the formation of NPs. Clearly, SEC and DLS analysis are more conclusive. Since it is well-known that polythiophene is strongly fluorescent, the fluorescence spectra together with the UV− vis spectra were recorded before and after irradiation (refer to Figure S6a). The UV−vis spectrum of the linear polythiophene, as discussed before, exhibits a strong band with a maximum at around 450 nm. After collapse, a blue shift was observed, which can be due to the new conformation. The fluorescence spectrum of the single-chain NPs was measured as well revealing that the single-chain NPs are still fluorescent after the collapse of the polythiophene chains (refer to Figure S6b). The fluorescence spectrum exhibits a band with a maximum at 580 nm. This property makes the resulting NPs potential candidates as imaging agents. It must be noted that the fluorescence intensity of NPs is lower than for the precursor linear polymer, which is most probably due to a change in the shape of the polymer chain, being in a more compact conformation after the collapse. B. Preparation of Polythiophene Networks. Preparation of polythiophene based films was carried out by employing the same strategy described in part A (refer to Figure 6a). Compared to intramolecular cross-linking during NP fabrication, concentrated solutions of copolymer 7′ were required in

collapse (Table 2) of the macromolecular chains was calculated by employing the apparent molecular weight obtained by SEC against PS standards, being in both cases close to 30%. Table 2. Degree of Collapse of the Irradiated Copolymers 7 and 7′ at Different Irradiation Times polymer

time (h)

Mn (g mol‑1)

collapse (%)

7

0 1 2 3 0 1 2 3

9300 8400a 7600a 6600a 9100 6900a 6300a 6300a

− 10 19 30 − 24 31 32

7′

a

Apparent molecular weight.

A similar trend was observed by DLS measurements. For these experiments, the copolymers 7 and 7′ as well as their corresponding SCNPs prepared via photo-cross-linking were dissolved in THF and subjected to DLS. A decrease of the hydrodynamic diameter was observed in both cases which confirmed the formation of the SCNPs (Figure 4). The DLS of the single chain nanoparticles (continuous line) showed in both cases a small peak at higher hydrodynamic diameters (Dh), which may be due to some remaining polymers that have not collapsed. In addition, the generated SCNPs were imaged by AFM. For these experiments, the SCNPs were absorbed onto freshly cleaved mica by drop casting diluted solutions of the SCNPs in chloroform. Figure 5a shows the height map image of the mica surface employed, while parts b and c of Figure 5 depict the

Figure 4. Number distribution of the polymer precursors and their corresponding SCNPs. E


Article

Macromolecules

Figure 5. AFM topography images (scan size of 1 × 1 μm2) of the mica substrate (a) and the single-chain NPs from copolymer 7 (b) and from copolymer 7′ (c).

Figure 6. (a) Schematic representation of the UV-cross-linking process. (b) Photograph of the cross-linked film (right) and the metallic ring employed during the irradiation (left). (c) Photograph of the cross-linked film immersed in a THF solution. (d) FTIR spectrum of the polythiophene film. (e) High resolution FT-IR microscopy image (32 × 32 μm2) of the dropcast polythiophene (Mn = 9100 g mol−1, Đ = 1.23) film (ν = 1650−1750 cm−1).

order to achieve intermolecular cross-linking. In a first attempt, solutions of 10 mg mL−1 of the photoreactive polythiophene in chloroform were irradiated. A macroscopic circular metallic ring was placed on top of a microscope cover glass and the solution was placed in the inside hole and exposed to UV light (refer to Figure 6b). After irradiation for 3 h, the success of the crosslinking was checked by washing the resulting film with pure

solvent. An almost completely soluble polymer was observed, indicating a nonefficient cross-linking. In order to improve the cross-linking, longer irradiation times were also assessed. However, in all the cases the film was partially soluble in organic solvents such as chloroform or THF. Such a low efficiency of the cross-linking may be explained by the decrease of the mobility caused by the fast evaporation of chloroform. F


Article

Macromolecules

Figure 7. (a) UV induced Diels−Alder reaction between a photoenol functionalized surface and maleimide-containing polythiophene. (b) Schematic representation of the polythiophene functionalization of the surfaces with spatial control employing a micropatterned shadow mask. (c) ToF−SIMS images of S− and SH− due to polythiophene immobilization in a zigzag pattern defined by the applied photomask.

stretching band corresponding to the functionalized thiophene moieties carrying the maleimide and “photoenol” units. The high resolution FT-IR microscopy image (see Figure 6e) corresponding to the mentioned CO band evidence the formation of a homogeneous and smooth surface. C. Surface Functionalization. After evidencing the UVinduced cross-linking of the functionalized copolymers 7 and 7′, the next goal was to immobilize polythiophene on a surface. Because of the employment of a UV-induced reaction, a spatially resolved polythiophene functionalization can be achieved opening a new range of possibilities for variable

For this reason, an alternative solvent with a higher boiling point allowing the irradiation of the solution for longer time without evaporation was tried. New solutions of 10 mg mL−1 of the photoreactive polythiophene in toluene were prepared, placed in the metallic ring and irradiated for 3 h. Subsequently, the film was washed with THF and toluene. The film remained insoluble indicating successful cross-linking (see Figures 6b,c). The polythiophene cross-linked film was characterized by highresolution Fourier transform infrared microscopy (HRFTIRM). Figure 6d shows the FTIR spectrum of the film, where it was possible to detect at 1700 cm−1 the CO G


Article

Macromolecules

Figure 8. (a) Functionalization of polymeric microstructure with maleimide containing polythiophene via Michael addition. (b) SEM image of a cuboid structured fabricated via DLW by employing a tetrafunctional thiol and a tetrafunctional alkyne-based photoresist. (c) FTIR spectra of the microstructure before (black) and after (red) polythiophene functionalization.

of preformed polymeric structures. A polymeric photoresist developed by our team28 consisting of a tetrafunctional thiol, a tetrafunctional alkyne and a photoinitiator (Figure S7) was employed for the preparation of simple 3D structures, i.e., cuboids, which were subsequently functionalized with polythiophene (Figure 8a). The polymeric microstructures were fabricated via direct laser writing (DLW). In this technique, a femtosecond pulsed laser beam is focused into a resist, which is photopolymerized via a multiphoton process. In the current case, radical thiol−yne coupling occurred allowing the crosslinking of the photoresist and the formation of stable solid 3D microstructures (Figure 8b). Importantly, during the crosslinking process and due to an increase of the viscosity, not all thiol and alkynes groups are consumed and can therefore be used for further surface modification. Making use of these residual thiols, the cuboid microstructures were functionalized with the maleimide-containing polythiophene via a Michael addition reaction. During this procedure, the cuboids were immersed into a copolymer 8 solution in chloroform and a base (triethylamine) which catalyzed the reaction. The microstructures were subsequently thoroughly washed and dried in an argon stream. The functionalization was monitored via FTIR since SH groups exhibit a characteristic band at 2550 cm−1 due to a stretching vibration. FTIR spectra of the structures recorded before and after functionalization (Figure 8c) revealed the extinction of the thiol band due to the consumption of the groups during the functionalization with polythiophene. Furthermore, the appearance of a new band at above 3000 cm−1 corresponding to Csp2−H stretching was also detected due to the presence of Csp2 in the aromatic rings of the polythiophene, while the polymeric photoresist contains only Csp3. The clear changes in the FTIR spectra of the fabricated

applications. In order to carry out the above strategy, the photoresponsive group, i.e., the 2-formyl-3-methylphenoxy derivative (the photoenol) and maleimide moieties were incorporated in the polymer and on the surface, respectively. The “photoenol” moiety was first immobilized onto a silicon wafer following a procedure previously described.25 In a first step, the silicon wafers were carefully cleaned, activated and subsequently silanized with the corresponding 2-formyl-3methylphenoxy derivative (for details refer to the Experimental Section). Next, the UV-induced Diels−Alder reaction was employed to graft the maleimide-functionalized polythiophene 8 onto the photoresponsive surface (Figure 7a). The irradiation conditions used (λmax = 312 nm, 3 h) were the optimum ones identified in the previous approaches (parts A and B). To perform the photoreaction, the silicon wafers were placed in a vial containing a polythiophene 8 solution in chloroform, degassed and exposed to UV light. To prove the spatial control over the functionalization, micropatterning of a flat substrate using a photomask was carried out (Figure 7b). After removing the mask and washing the surfaces, the patterns were revealed by time-of-flight secondary ion mass spectrometry (ToF− SIMS), a surface sensitive analytical method providing images containing chemical information generated by collecting mass spectra at a high lateral resolution. Figure 7c depicts the ToF− SIMS images of the patterned surfaces. S− and SH− were exclusively detected in the UV exposed areas, and not in the nonirradiated regions. Since sulfur is only present in the polythiophene polymer, the presence of sulfur derivative based patterns confirms the successful spatial functionalization via the UV-induced Diels−Alder reaction. D. Functionalization of 3D Preformed Polymers. In the last approach, copolymer 8 was employed for the functionalization H


Article

Macromolecules

liquid metal primary ion source and a nonlinear time-of-flight analyzer. UHV base pressure was 97%) (TCI), 3thiophenemethanol (>96%) (TCI), tetrabutylammonium fluoride trihydrate (TBAF, 99%) (Acros Organics), dry THF (Acros Organics), [1,3-iso(diphenylphosphino)propane]dichloronickel(II) Ni(dppp)Cl2 (99%) (Acros Organics), triethylamine (TEA, 99%) (Sigma-Aldrich), tert-butylmagnesium chloride (2 M ether solution) (Sigma-Aldrich), and 2,5-dimethyl-4-hydroxy benzophenone (BASF) were used as received. The monomer 2 and 3-(exo-3,6-epoxy-1,2,3,6tetrahydrophthalimido)propanoic acid 5 were synthesized according to a procedure reported in the literature.29,30 The 2-formyl-3methylphenoxy (“photoenol”) silane was also synthesized by employing the strategy described by the group.25 Synthesis of Copolymer 3. First, 2.50 g (7.76 mmol) of 2,5dibromo-3-hexylthiophene 1 and 1.03 g (2.56 mmol) of 2 were dissolved in 100 mL of dry THF under Ar atmosphere. To this solution, 5.2 mL (10.32 mmol) of a 2 M ether solution of tertbutylmagnesium chloride was added and the reaction mixture was stirred under reflux for 90 min in an Ar atmosphere. The reaction mixture was subsequently cooled to ambient temperature and 112.5 mg (0.21 mmol) of Ni(dppp)Cl2 were added. The polymerization was allowed to proceed for 10 min before it was poured into a 5 N HCl solution to precipitate the polymer. The precipitated polymer was collected by filtration and subjected to overnight Soxhlet extraction with methanol. The copolymer 3 (Mn 8100 g mol−1, Đ 1.21) was collected by Soxhlet extraction with chloroform. The chloroform was removed via a rotary evaporator and drying overnight under vacuum at ambient temperature. The product was obtained as a dark solid. 1H NMR (CDCl3, 400 MHz) δ (ppm): 6.98 (s), 4.83 (s), 2.82 (q), 1.72− 1.65 (m), 1.59−1.35 (m), 0.97 (s), 0.91 (t), 0.14 (s). Mn = 8100 g mol−1, ĐM = 1.21. Synthesis of the Copolymer 3′. The copolymer 3′ was synthesized by following the same procedure as copolymer 3, but with a different monomer ratio, i.e., 0.60 g (1.84 mmol) of 2,5dibromo-3-hexylthiophene 1 and 0.48 g (1.22 mmol) of 2 were employed. Mn = 7100 g mol−1, Đ = 1.25. 1H NMR (CDCl3, 400 MHz) δ (ppm): 6.98 (s), 4.83 (s), 2.82 (q), 1.72−1.65 (m), 1.59−1.35 (m), 0.97 (s), 0.91 (t), 0.14 (s). Mn = 7100 g mol−1, ĐM = 1.25. Synthesis of Copolymer 4 and 4′. A 0.75 g sample of copolymer 3 or 3′ was dissolved in 100 mL of THF, and 75 mg of (TBA)F was added to the solution. The reaction mixture was heated at 60 °C overnight. The crude was concentrated using a rotary evaporator and precipitated into methanol. The precipitates were collected and subjected to an overnight Soxhlet extraction with methanol. The copolymer with pendant − OH groups was collected by extraction with CHCl3. The chloroform was removed using a rotary evaporator and drying overnight under vacuum at ambient temperature. The product was obtained as a dark solid. 1H NMR (CDCl3, 400 MHz) δ (ppm): 6.98 (s), 4.83 (s), 2.81 (q), 1.72−1.67 (m), 1.54−1.34 (m), 0.92 (t). Mn= 8200 g mol−1, ĐM = 1.25; Mn = 7800 g mol−1, ĐM = 1.26. Synthesis of 4-(4-Benzoyl-2,5-dimethylphenoxy)butanoic Acid (6). 1.32 g (5.92 mmol) of 2,5-dimethyl-4-hydroxybenzophe-

structure evidence the successful polythiophene functionalization. The method allows for an easy preparation of polymeric structures coated with a conjugated polymer which may be employed in different applications requiring an electrically active surface. It should be noted that simple cuboid were created as a proof of principle, however, more complicated geometries can be readily prepared by the DLW technique employing the thiol−yne based polymeric photoresist.28

■

CONCLUSIONS Well-defined conjugated polymers consisting of polythiophene containing photoresponsive groups as pendant entities, i.e., maleimide groups and/or DMBP, were successfully synthesized by a combination of GRIM polymerization and postfunctionalization reactions. The polymers were subsequently employed as platform materials for the formation of variable nano- and microstructures via light-induced reactions. In a first approach, single-chain NPs and polymeric films were prepared from polythiophene containing maleimide groups and DMPS via UV-induced cross-linking. The morphology can be controlled by merely adjusting the concentration of the conjugated polymer solution. By employing maleimide functionalized polythiophene, 2D-patterned surfaces were created by employing a light-triggered reaction. Finally, the same polymer was employed to functionalize 3D polymeric structures prefabricated by direct laser writing (DLW). These examples evidence the versatile nature of the photoreactive conjugated polymers.

■

EXPERIMENTAL SECTION

Instrumentation. NMR Spectroscopy. The structures of the synthesized compounds were confirmed via 1H- and 13C NMR spectroscopy using a Bruker Ascend 400 MHz spectrometer for hydrogen nuclei and 100 MHz for carbon nuclei. Samples were dissolved in CDCl3. The δ-scale was referenced with tetramethylsilane (δ = 0.00) as internal standard. Gel Permeation Chromatography (GPC). Chromatograms were obtained on a Polymer Laboratories PL-GPC 50 Plus Integrated System. The system is constituted of an autosampler, a PLgel 5 μm bead-size guard column (50 × 7.5 mm) followed by one PLgel 5 μm Mixed E column (300 × 7.5 mm), three PLgel 5 μm Mixed C columns (300 × 7.5 mm) and a differential refractive index detector using THF as the eluent at 35 °C with a flow rate of 1 mL min−1. The GPC system was calibrated using linear PS standards ranging from 160 to 6 × 106 g mol−1. UV/Vis Spectroscopy. UV/Vis spectra were performed using a Cary 300 Bio UV/vis Spectrophotometer (Varian). Fluorescence. Fluorescence spectra were recorded using a CARY ECLIPSE spectrofluorometer (Varian) with a quartz cell. Dynamic Light Scattering (DLS). Hydrodynamic diameters were determined via dynamic light scattering (Nicomp 380 DLS spectrometer from Particle Sizing Systems, Santa Barbara, CA; laser diode, 90 mW, 658 nm). The polymer solutions in CHCl3 were filtered (0.2 μL PTFE syringe filter) before DLS analysis. The measurements were performed in automatic mode and evaluated by a standard Gaussian and an advanced evaluation method, the latter using an inverse Laplace algorithm to analyze for multimodal distributions. Numbers given in text are the number weighted average values as calculated by the NICOMP evaluation. All measurements were determined at 90° to the incident beam. AFM Measurements. AFM was carried out at ambient conditions on a MultiMode 2 (Bruker) equipped with a Nanoscope IIIa controller. Silicon cantilevers (model NSC18 from MikroMasch), typical resonance frequency of 75 kHz, spring constant 2.8 N/m) were used for imaging in tapping mode. ToF−SIMS. This was performed on a TOF-SIMS5 instrument (ION-TOF GmbH, Münster, Germany), equipped with a Bi cluster I


Article

Macromolecules none and 1.50 g (7.69 mmol) of methyl 4-bromobutyrate were dissolved in 80 mL of butanone. Then, 0.05 g (0.30 mmol) of potassium iodide and 1.64 g (11.84 mmol) of potassium carbonate were added to the solution. The suspension was stirred and heated under reflux for 24 h. Subsequently, the mixture was filtered and concentrated. The crude product was purified by flash column chromatography on silica gel using DCM as eluent. The product was obtained as a colorless oil. Yield: 75%. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.76 (d, J = 7.3 Hz, 2H), 7.55 (t, J = 7.3 Hz, 1H), 7.44 (t, J = 7.7 Hz, 2H), 7.15 (s, 1H), 6.70 (s, 1H), 4.23−4.04 (m, 4H), 2.55 (t, J = 7.3 Hz, 2H), 2.37 (s, 3H), 2.26−2.10 (m, 4H), 1.27 (t, J = 7.1 Hz, 3H). 13C NMR (CDCl3, 101 MHz) δ (ppm): 197.99, 173.16, 158.59, 138.89, 137.78, 132.50, 132.41, 130.07, 129.99, 128.24, 123.22, 113.33, 66.76, 60.48, 30.79, 24.62, 20.61, 15.66, 14.21. An aqueous solution of KOH (1.51 g, 15 mL) was added to a solution of methyl 4-(4-benzoyl-2,5-dimethylphenoxy)butyrate (1.03 g, 2.9 mmol) in 20 mL of ethanol. The mixture was stirred and heated under reflux for 1 h. Next, the crude product was precipitated by addition of HCl until pH 2 and it was recovered by filtration. The product was crystallized from methanol and obtained as a white powder. Yield: 90%. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.76 (dd, J = 8.1, 1.1 Hz, 2H), 7.63−7.50 (m, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.15 (s, 1H), 6.70 (s, 1H), 4.10 (t, J = 6.0 Hz, 2H), 2.63 (t, J = 7.2 Hz, 2H), 2.37 (s, 3H), 2.35−2.12 (m, 4H). 13C NMR (CDCl3, 10 MHz) δ (ppm): 198.05, 178.68, 158.49, 138.85, 137.79, 132.53, 132.45, 130.16, 130.01, 128.25, 123.22, 113.29, 66.71, 66.54, 30.49, 24.60, 24.37, 20.60, 15.63. Synthesis of Copolymer 7. The acid chloride derivates were prepared by reaction of compound 5 (0.18 g, 0.76 mmol) and 6 (0.24 g, 0.76 mmol) with 0.2 mL (3.04 mmol) of thionyl chloride in 15 mL of dichloromethane. After the mixture was stirred at ambient temperature for 4 h, the solvent and the excess thionyl chloride were removed by distillation. The obtained solid was dissolved in 10 mL of dry THF and directly added to a solution of copolymer 4 (0.10 g, 0.15 mmol of OH) in 20 mL of THF and 0.2 mL (1.52 mmol) of triethylamine. The mixture was stirred overnight at ambient temperature under an argon atmosphere. After this time, the mixture was filtered off, and the obtained solution was concentrated and poured into methanol to precipitate the polymer. 1H NMR (CDCl3, 400 MHz) δ (ppm): δ 7.74 (d, J = 7.8 Hz), 7.59−7.35 (m), 7.20− 7.02(m), 6.98 (s), 6.68 (s), 6.47 (s), 5.26 (s), 5.22 (s), 4.12−4.09 (m), 3.85−3.81 (m), 2.82−2.55 (m), 2.32−2.30 (m), 2.25−2.21 (m), 2.15− 2.12(m), 1.77−1.65 (m), 1.49−1.22 (m), 0.92−0.90 (m). In a last step, the obtained polymer was dissolved in toluene and heated to 80 °C for 2 days. The solvent was removed under reduced pressure to give copolymer 7. 1H NMR (CDCl3, 400 MHz) δ (ppm): δ 7.75 (d, J = 7.8 Hz), 7.60−7.35 (m), 7.20−7.02 (m), 6.98 (s), 6.68 (s), 6.66 (s), 5.27 (s), 5.23 (s), 4.12−4.09 (m), 3.85−3.80 (m), 2.82− 2.55 (m), 2.32−2.30 (m), 2.25−2.20 (m), 2.16−2.12(m), 1.77−1.63 (m), 1.49−1.23 (m), 0.92−0.90 (m). Mn= 9300 g mol−1, ĐM= 1.24. Synthesis of Copolymer 7′. Copolymer 3′ was synthesized by following the same procedure as copolymer 7. The acid chloride derivates were prepared by reaction of compound 5 (0.27 g, 1.16 mmol) and 6 (0.36 g, 1.16 mmol) with 0.3 mL (4.64 mmol) of thionyl chloride in 10 mL of dichloromethane and added to a solution of copolymer 4′ (0.10 g, 0.40 mmol of OH) in 10 mL of THF and 0.3 mL (2.32 mmol) of triethylamine. 1H NMR (CDCl3, 400 MHz) δ (ppm): δ 7.76 (d, J = 7.8 Hz), 7.60−7.35 (m), 7.23−7.02 (m), 6.98 (s), 6.67 (s), 6.64 (s), 5.27 (s), 5.24 (s), 4.12−4.08 (m), 3.85−3.80 (m), 2.82−2.55 (m), 2.32−2.29 (m), 2.25−2.20 (m), 2.17−2.12 (m), 1.77−1.65 (m), 1.50−1.25 (m), 0.93−0.90 (m). Mn= 9100 g mol−1, ĐM= 1.23. Synthesis of Copolymer 8. The acid chloride derivates were prepared by reaction of compound 5 (0.18 g, 0.76 mmol) with 0.1 mL (1.52 mmol) of thionyl chloride in 10 mL of dichloromethane. After stirring at ambient temperature for 4h, the solvent and the excess of thionyl chloride were removed by distillation. The obtained solid was dissolved in 5 mL of dry THF and directly added to a solution of copolymer 4 (0.05 g, 0.08 mmol of OH) in 10 mL of THF and 0.1 mL (0.76 mmol) of triethylamine. The mixture was stirred overnight at

ambient temperature under an argon atmosphere. After this time, the mixture was filtered off and the obtained solution was concentrated and poured into methanol to precipitate the polymer. 1H NMR (CDCl3, 400 MHz) δ (ppm): 6.98 (s), 6.48 (s), 5.22 (s), 3.85−3.81 (m), 2.82−2.55 (m), 1.77−1.65 (m), 1.49−1.22 (m), 0.92−0.90 (m). In a last step, the obtained polymer was dissolved in toluene and heated to 80 °C for 2 days. The solvent was removed under reduced pressure to give copolymer 8. 1H NMR (CDCl3, 400 MHz) δ (ppm): 6.98 (s), 6.68 (s), 5.23 (s), 3.85−3.80 (m), 2.82−2.55 (m), 1.77−1.63 (m), 1.49−1.23 (m), 0.92−0.90 (m). Mn= 8800 g mol−1, ĐM= 1.24 Single-Chain Nanoparticle Formation. The copolymers 7 and 7′ (4 mg) were each dissolved in 200 mL of chloroform in a flask sealed with a septum and flushed with nitrogen for 1 h. The solutions were then irradiated different times in a custom-built photoreactor with a 36 W compact fluorescence lamp with a maximum emission wavelength at 312 nm. After irradiation, the solvent was removed and the NPs were directly characterized by different techniques. Polymeric Network Formation. Copolymer 7′ (5 mg) was dissolved in 0.5 mL of toluene in a small vial sealed with a septum and flushed with nitrogen. A droplet of the solution was placed inside a metallic ring place into a cover glass and irradiated under the same conditions as before for 3 h. The formed film was rinsed with toluene and THF. Polythiophene Surface Functionalization of Si Wafers. Prior to surface activation, the silicon wafers (p-type, boron doped, (100) from Si-Mat Silicon Materials, Landsberg, Germany) were cleaned with chloroform, acetone, and ethanol. The wafers were rinsed thoroughly with fresh solvent and sonicated 5 min several times with each solvent. After cleaning, the silicon wafers were activated by immersion in Piranha solution (H2SO4 95%/H2O2 35% 3:1 vol/vol) at 90 °C for 1 h. After extensive rinsing with deionized water, they were dried under a stream of argon. Functionalization of Silicon Wafers with “Photoenol” Units. The activated silicon wafers were placed in a flask containing a solution of silane functionalized tetrazole (TET) in dry toluene (3 mg in 1 mL). The flask was heated to 50 °C overnight. Subsequently, the wafers were rinsed thoroughly with fresh toluene and chloroform and sonicated for 5 min. The wafers were finally dried in a stream of argon. Functionalization of Silicon Wafers with Polythiophene. The tetrazole functionalized silicon wafers were placed in a sealed vial containing a polthythiophene solution in chloroform (1 mg of copolymer 8 in 4 mL of chloroform) and degassed for 15 min. The vials were introduced into a custom built photoreactor (see Figure S9 in the Supporting Information) and irradiated for 3 h. Subsequently, the wafers were rinsed thoroughly with fresh chloroform and sonicated for 5 min. The wafers were finally dried in a stream of argon. Direct Laser Writing (DLW) Experiments. Photoresist Preparation. Tetraalkyne derivative (1.00 g, 1.08 mmol), tetrathiol derivative (1.05 g, 2.16 mmol) and DETC (5 mg, 0.25 wt %) were added to a glass vial equipped with a stirring bar and stirred overnight under ambient (yellow light) conditions. Preparation of the Cuboid Structures. The photoresist was drop cast onto glass substrates with a dimension of 22 mm × 22 mm × 0.17 mm. Cuboids structures of 50 × 50 × 5 μm3 were fabricated via DLW. Development of the sample was performed by washing for 15 min with acetone and water. Functionalization of the Cuboids Microstructures. A substrate containing the cuboid structures was immersed in a solution of copolymer 8 (10 mg) and 0.2 mL of triethylamine in 5 mL of chloroform during 24 h. Afterward, the substrate was subsequently developed in chloroform (20 min), acetone (5 min), and water (5 min).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02024. J


Article

Macromolecules

■

(20) (a) Yameen, B.; Rodriguez-Emmenegger, C.; Preuss, C. M.; Pop-Georgievski, O.; Verveniotis, E.; Trouillet, V.; Rezek, B.; BarnerKowollik, C. Chem. Commun. 2013, 49, 8623−8625. (b) Yameen, B.; Zydziak, N.; Weidner, S. M.; Bruns, M.; Barner-Kowollik, C. Macromolecules 2013, 46, 2606−2615. (21) Gruendling, T.; Oehlenschlaeger, K. K.; Frick, E.; Glassner, M.; Schmid, C.; Barner-Kowollik, C. Macromol. Rapid Commun. 2011, 32, 807−812. (22) Glassner, M.; Oehlenschlaeger, K. K.; Gruendling, T.; BarnerKowollik, C. Macromolecules 2011, 44, 4681−4689. (23) Winkler, M.; Mueller, J. O.; Oehlenschlaeger, K. K.; Montero de Espinosa, L.; Meier, M. A. R.; Barner-Kowollik, C. Macromolecules 2012, 45, 5012−5019. (24) Oehlenschlaeger, K. K.; Mueller, J. O.; Heine, N. B.; Glassner, M.; Guimard, N. K.; Delaittre, G.; Schmidt, F. G.; Barner-Kowollik, C. Angew. Chem., Int. Ed. 2013, 52, 762−766. (25) Pauloehrl, T.; Delaittre, G.; Winkler, V.; Welle, A.; Bruns, M.; Boerner, H. G.; Greiner, A. M.; Bastmeyer, M.; Barner-Kowollik, C. Angew. Chem., Int. Ed. 2012, 51, 9181−9184. (26) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004, 25, 1663−1666. (27) Altintas, O.; Willenbacher, J.; Wuest, K. N. R.; Oehlenschlaeger, K. K.; Krolla-Sidenstein, P.; Gliemann, H.; Barner-Kowollik, C. Macromolecules 2013, 46, 8092−8101. (28) Quick, A. S.; de los Santos Pereira, A.; Bruns, M.; Bückmann, T.; Rodriguez-Emmenegger, C.; Wegener, M.; Barner-Kowollik, C. Adv. Funct. Mater. 2015, 25, 3735−3744. (29) Gandini, A.; Silvestre, A. J. D.; Coelho, D. Reversible click chemistry at the service of macromolecular materials. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2053−2056. (30) Nam, S.; Jiang, X.; Xiong, Q.; Ham, D.; Lieber, C. M. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 21035−21038.

Additional spectroscopic data including SEC traces, NMR spectra, UV−vis and fluorescence spectra, and chemical composition of the photoresist employed as well as the description of the custom-built photoreactor employed (PDF)

AUTHOR INFORMATION

Corresponding Author

*(C.B.-K.) E-mail: [email protected]. Fax: +49 721 608 5740. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS C.B.-K. and M.W. acknowledge funding from the Karlsruhe Institute of Technology (KIT) via the Helmholtz association (STN program). C.B.-K additionally acknowledges funding by the German Research Council (DFG) via their large equipment granting scheme. E.B. acknowledges the Alexander von Humboldt Foundation for financial support.

■

REFERENCES

(1) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324−1338. (2) Wang, H.-J.; Chen, C.-P.; Jeng, R.-J. Materials 2014, 7, 2411− 2439. (3) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533− 4542. (4) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109, 5868−5923. (5) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121−128. (6) Gong, X.; Moses, D.; Heeger, A. J. Polymer-Based Light-Emitting Diodes (PLEDs) and Displays Fabricated from Arrays of PLEDs. In Organic Light Emitting Devices; Wiley-VCH Verlag GmbH & Co. KGaA: 2006; pp 151−180. (7) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339−1386. (8) Xia, L.; Wei, Z.; Wan, M. J. Colloid Interface Sci. 2010, 341, 1−11. (9) Feng, L.; Zhu, C.; Yuan, H.; Liu, L.; Lv, F.; Wang, S. Chem. Soc. Rev. 2013, 42, 6620−6633. (10) (a) Loewe, R. S.; Ewbank, P. C.; Liu, J.; Zhai, L.; McCullough, R. D. Macromolecules 2001, 34, 4324−4333. (b) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526−3528. (c) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macromolecules 2005, 38, 8649−8656. (11) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Catalyst-Transfer Polycondensation. J. Am. Chem. Soc. 2005, 127, 17542−17547. (12) Pecher, J.; Mecking, S. Chem. Rev. 2010, 110, 6260−6279. (13) Tuncel, D.; Demir, H. V. Nanoscale 2010, 2, 484−494. (14) Adkins, C. T.; Muchalski, H.; Harth, E. Macromolecules 2009, 42, 5786−5792. (15) Liu, J.; Feng, G.; Liu, R.; Tomczak, N.; Ma, L.; Gurzadyan, G. G.; Liu, B. Small 2014, 10, 3110−3118. (16) Søndergaard, R. R.; Norrman, K.; Krebs, F. C. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1127−1132. (17) Bundgaard, E.; Hagemann, O.; Bjerring, M.; Nielsen, N. C.; Andreasen, J. W.; Andreasen, B.; Krebs, F. C. Macromolecules 2012, 45 (8), 3644−3646. (18) (a) Lai, C.; Guo, W.; Tang, X.; Zhang, G.; Pan, Q.; Pei, M. Chin. J. Chem. 2011, 29, 1985−1988. (b) Gadgil, B.; Damlin, P.; Ä ar̈ italo, T.; Kvarnström, C. Electrochim. Acta 2014, 133, 268−274. (19) Yameen, B.; Puerckhauer, T.; Ludwig, J.; Ahmed, I.; Altintas, O.; Fruk, L.; Colsmann, L.; Barner-Kowollik, C. Small 2014, 10, 3091− 3098. K


Designing Ï-Conjugated Polymeric Nano- and ... - ACS Publications

Recommend Documents

Designing Ï-Conjugated Polymeric Nano- and ... - ACS Publications