Supramolecularly Assembled Hybrid Materials via Molecular

Sep 14, 2007 - A solution processible poly(3-hexylthiophene) derivative containing O4-substituted diaminopyrimidine (ODAP) terminated side groups has ...
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J. Phys. Chem. C 2007, 111, 14681-14688

14681

Supramolecularly Assembled Hybrid Materials via Molecular Recognition between Diaminopyrimidine-Functionalized Poly(hexylthiophene) and Thymine-Capped CdSe Nanocrystals Julia De Girolamo, Peter Reiss,* and Adam Pron* DRFMC, UMR 5819-SPrAM, CEA-CNRS-UniVersity J. Fourier-Grenoble I, Laboratoire d’Electronique Mole´ culaire Organique et Hybride, CEA Grenoble, 17 Rue des Martyrs 38054 Grenoble Cedex 9, France ReceiVed: May 30, 2007; In Final Form: August 1, 2007

A solution processible poly(3-hexylthiophene) derivative containing O4-substituted diaminopyrimidine (ODAP) terminated side groups has been prepared by post-polymerization functionalization of the precursor copolymer, namely, regioregular poly(3-hexylthiophene-co-3-(6-bromohexyl)thiophene), with 2,4-diamino-6-hydroxypyrimidine. An interesting feature of this polymer is its capability of the hydrogen bond assisted formation of organic-inorganic composites with 1-(6-mercaptohexyl)thymine functionalized semiconductor nanocrystals (CdSe). The composites, obtained in a simple one-pot preparation step, exhibit a homogeneous distribution of nanocrystals within the polymer matrix in an extended three-dimensional network, whereas in blends without specific interaction between the constituents, phase segregation on a submicron level occurs.

Introduction

Experimental Section

Composites of semiconductor nanocrystals with polyconjugated molecules constitute a new family of electroactive materials exhibiting tunable electrical, electrochemical, and spectroelectrochemical properties.1 They are also very promising candidates for the application in organic electronics, for example, as active “bulk heterojunction” layers in organic photovoltaic cells.2 The main problem in the preparation of these materials is the process of phase separation promoted by the distinctly different chemical nature of both constitutive components of the composite; semiconductor nanocrystals, introduced to the conjugated polymer matrix, have a strong tendency to form agglomerates. In this paper, we demonstrate that homogeneous molecular composites of this type can be prepared if both composite components contain functional groups capable of mutual molecular recognition. In particular, we show that macromolecules of a regioregular poly(3-hexylthiophene) derivative containing, in addition to solubility inducing alkyl side chains, diaminopyrimidine (ODAP) groups can assemble with thymine capped CdSe nanocrystals via multiple hydrogen bonds. This molecular recognition process forces a uniform distribution of individual nanocrystals within the polymer matrix. Molecular recognition has previously been applied by Rotello and coworkers3 to the preparation of supramolecular assemblies of thymine-functionalized gold nanoparticles and poly(styrene), containing diaminotriazine side groups, in the so-called “brick and mortar” approach. Literature reports on conjugated polymers functionalized by diaminotriazine or diaminopyrimidine moieties are scarce,4 and to the best of our knowledge, the title compound is the first solution processible poly(thiophene), suitable for the preparation of composites with semiconductor nanocrystals or other types of nanomaterials via molecular recognition. * Corresponding authors. E-mail: [email protected] (A.P.).

[email protected] (P.R.) and

The list of the chemicals and reagents used in this research as well as the detailed synthetic procedures leading to poly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexylthiophene) (P2), CdSe nanocrystals and their molecular composites can be found in Supporting Information. The following characterization techniques were applied: 1H and 13C NMR spectra of the synthesized monomers, polymers, and ligands were recorded on a Bruker AC 200 MHz spectrometer using chloroform-d (CDCl3) and methylsulfoxide-d6 (DMSO-d6) solvents containing tetramethylsilane (TMS) as an internal standard. FTIR spectra were recorded on a Perkin-Elmer paragon 500 spectrometer using the attenuated total reflectance (ATR) configuration. UV-vis spectra were recorded on HP 8452A and Cary 5000 (Varian) spectrometers; photoluminescence measurements were performed on a Hitachi F-4500 spectrometer. Molecular weight determinations of polymer fractions studied were measured on a size exclusion chromatography (SEC, 1100 HP) equipped with two types of detectors: a diode array UVvis and a refractive index detector. The column temperature and the flow rate were fixed to 313 K and 1 mL min-1, respectively. The calibration curve was built using 10 polystyrene narrow standards (S-M2-10* kit from Polymer Labs). Transmission electron microscopy (TEM) images of the functionalized nanocrystals and the composite were obtained with a JEOL 4000 EX microscope operated at 300 kV; scanning electron microscopy (SEM) analysis was carried out with a Zeiss Ultra-55 microscope. Thermogravimetric analyses were performed with a SETARAM TG 92-12 type TG/DTA system. Results and Discussion Structural and Spectroscopic Features of Diaminopyrimidine-Functionalized Poly(3-hexylthiophene) P3HT-co-P3(ODAP)HT (P2). Multiple hydrogen bonds between 1-(6mercaptohexyl)thymine-capped CdSe nanocrystals and diaminopyrimidine-functionalized poly(alkylthiophene) (P2) are the

10.1021/jp0741758 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007

14682 J. Phys. Chem. C, Vol. 111, No. 40, 2007 SCHEME 1: Interactions between Poly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine) hexyl-thiophene) (P2) and 1-(6-mercaptohexyl)thymine Capped CdSe (CdSe-4) via Hydrogen Bonding

driving force for the formation of supramolecular assemblies between both components of the composite, as depicted in Scheme 1. In this perspective, the chain microstructure of P2 (and especially its regioregularity) and the content of the diaminopyrimidine-containing substituents are of crucial importance. The reaction pathway leading to P2 is shown in Scheme 2, with the detailed synthetic procedures being described in Supporting Information. The applied synthetic strategy is conceptually similar to that reported by Mouffouk et al.5 who prepared a poly(alkylthiophene) bearing covalently linked biotin groups through the postfunctionalization of poly(3-hexylthiophene-co-3-(6-hydroxyhexylthiophene) with biotin hydrazide. Two specific features of the Grignard metathesis method used for the preparation of the precursor polymer (P1)6 must be pointed out. First, its composition can be adjusted by changing the co-reagents ratio. One must however note that in the resulting copolymer the ratio

De Girolamo et al. of 3-hexyl-2,5-thienylene units to 3-(6-bromohexyl)-2,5-thienylene units is always higher than the molar ratio of 3 to 2 in the reaction mixture (cf. Supporting Information). Second, the applied synthesis method leads to a rather large polydispersity in the molecular mass of the resulting copolymer (Mn ) 9200 g/mol, polydispersity index (PDI) ) 2.55 relative to polystyrene standards). The polydispersity coefficient can be significantly reduced by sequential fractionation using the same set of solvents as previously established for the fractionation of regioregular poly(hexylthiophene)7 (cf. Supporting Information for macromolecular parameters of the obtained fractions). For the functionalization of P1 with ODAP groups to give P2, we used the most abundant dichloromethane fraction (Mn ) 12 800 g/mol, PDI ) 1.73 relative to polystyrene standards) showing the ratio of 3-hexylthiophene units to 3-(6-bromohexyl)thiophene ones 6:1 as determined by 1H NMR. The postfunctionalization reaction (transformation of P1 into P2) is quantitative as shown by NMR spectra (Figure 1) since the signals at 3.43 and 1.90 ppm, originating from the 6-bromohexyl group, are not present in the spectrum of P2. Instead, new lines in the chemical shift range of 4.0-5.5 ppm appear which are characteristic of the presence of ODAP groups. It should also be noted that both P1 and P2 are highly regioregular; the only aromatic proton gives rise to a clear singlet at 6.98 ppm with no satellite peaks originating from nonregioregularity.8 Nevertheless, it must be stressed that the postfunctionalization reaction, followed by the polymer precipitation in methanol and its redissolution in chloroform, results in P2 with 9:1 co-mers ratio (as determined from 1H NMR) which is higher than that in its precursor P1 (6:1). This is caused by decreasing solubility of the postfunctionalized polymer with increasing ODAP content. In fact, during the postfunctionalization procedure and the product purification, we recover only this fraction of P2 which is soluble in chloroform, that is, enriched in 3-hexylthiophene units. This puts the upper limit of 3-(ODAP)hexylthiophene to 3-hexylthiophene groups as 1:9 in P2 which is still soluble in chloroform. In Figure 2a,b, the FTIR spectra of P1 and P2 are compared. The postfunctionalization reaction results in a significant modification of the spectrum. The presence of ODAP groups

SCHEME 2: Synthesis Pathway for the Preparation of P3HT-co-P3(ODAP)HT (P2)a

a

See Supporting Information for detailed synthetic procedures and characterization data.

Assembled Hybrid Materials via Molecular Recognition

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Figure 3. UV-vis spectrum of P2 of DPn ) 71 (co-mers ratio 9:1).

Figure 1. Upper panel: 1H NMR spectrum of P1 (n ) 6 m ) 1, i.e., co-mers ratio 6:1). Lower panel: 1H NMR spectrum of the corresponding postfunctionalized copolymer P2 (n ) 9 m ) 1, i.e., co-mers ratio 9:1).

Figure 4. Upper panel: partial 1H NMR spectrum of 1-(6-mercaptohexyl)thymine functionalized 5.7 nm CdSe nanocrystals (CdSe-4), recorded in DMSO-d6. Lower panel: partial 1H NMR spectrum of 1-(6mercaptohexyl)thymine (4), recorded in DMSO-d6.

Figure 2. Relevant sections of the FTIR spectra of P1 (a) and of P2 (b), co-mers ratio 6:1 and 9:1, respectively.

in P2 is manifested by the appearance of several new bands which are absent in the nonfunctionalized polymer. In addition to broad peaks at approximately 3460, 3330 (N-H stretchings), and 3160 cm-1 (C-H stretching in the pyrimidine ring), a strong band appears at 1580 cm-1 (CdN stretching), and a band appears at 796 cm-1 attributed to out-of-plane deformations of the pyrimidine ring. Figure 3 shows the solid-state UV-vis spectrum of a thin film of P2. It is known that for polythiophene derivatives the position of the π-π* transition in the neutral undoped polymer is strongly dependent on the molecular mass. With an increasing degree of polymerization (DPn), it is being bathochromically shifted, and its vibrational structure becomes more pronounced.7,9 A vibrational structure is also observed in the spectrum of P2 of DPn ) 71. The positions of the 0-0, 0-1, and 0-2 transitions, determined by the second derivative analysis (d2A/dλ2), are 602, 562, and 516 nm, respectively; that is, they occur at almost the same wavelengths as the corresponding transitions in regioregular poly(3-hexylthiophene)

of slightly lower degree of polymerization (DPn ) 61).10 The energy of the 0-0 transition, that is, the transition from the ground state to the relaxed excited state, is in poly(thiophene) derivatives inversely proportional to the conjugation length. Thus, close energetic similarity of this transition in P2 and in regioregular poly(3-hexylthiophene) of comparable DPn clearly indicates that the introduction of bulky ODAP groups through alkylene spacers does not perturb the conjugation of the polymer at least at the 9:1 ratio of the co-mers. Briefly concluding this part of the paper, complementary spectroscopic investigations unequivocally indicate that we have succeeded in the preparation of a highly regioregular (and by consequence highly conjugated) solution-processible polythiophene containing side groups capable of participating in the molecular recognition process with appropriate complementary groups. Functionalization of CdSe Nanocrystals with 1-(6-mercaptohexyl)thymine (CdSe-4). The ligand exchange process leading to nanocrystals capped with 1-(6-mercaptohexyl)thymine is depicted in Scheme 3. As it has already been stated, the exchange between initial stearate ligands and 1-(6-mercaptohexyl)thymine (4) is rather slow and inefficient. It can be significantly improved by using microwave radiation. In Figure 4, the 1H NMR spectrum of “free” 1-(6-mercaptohexyl)thymine (4) is compared with that of 5.7 nm CdSe nanocrystals after the ligand exchange (CdSe-

14684 J. Phys. Chem. C, Vol. 111, No. 40, 2007 4). Even with the assistance of microwave radiation, the ligand exchange is not complete, since in the spectrum of CdSe-4 lines attributable to the initial (stearate) ligands can be found in addition to the thymine ligands. We ascribe the lower degree of ligand exchange with respect to results reported for other thiol-type ligands (up to 90%)11 to the decreasing solubility of the nanocrystals during the exchange reaction. The latter may be due to the possibility of dimerization of thymine groups sticking out of the surface of neighboring nanocrystals (vide infra), leading to the formation of larger aggregates, which then precipitate. It is clear that nanocrystals incorporated in the interior of aggregates are less accessible for further ligand exchange. At the same time, it is not necessary to achieve a nearly complete replacement of stearate ligands with (4), as even a rather small number of thymine groups per nanocrystal should in principle be sufficient for the preparation of supramolecular composite assemblies via molecular recognition of the complementary diaminopyrimidine-containing macromolecule (P2). Coordination of the deprotonated thiolate form of 4 is manifested by the absence of the signal attributable to the SH group in the spectrum of CdSe-4 (compare spectra in Figure 4). From the integration of the peak corresponding to the methyl group of the stearate at 0.86 ppm and the peak originating from the proton connected to the thymine ring at 7.52 ppm (not shown in Figure 4), it is possible to determine the degree of the ligand exchange which accounts for approximately 70%. Figure 5 shows typical absorption and photoluminescence (PL) spectra of CdSe-4. While the UV-vis spectrum does not exhibit any modification as compared with that of the original stearate-capped nanocrystals, the intensity of the PL peak is reduced to 8% of the initial value without affection of the line width (30.8 nm at full width at half maximum, fwhm). A similar behavior has also been observed for the exchange with other thiol ligands. The observed PL decrease has been explained by the hole accepting character of these molecules whose highest occupied molecular orbital (HOMO) levels are located within the energy band gap of the CdSe nanocrystals.12 It is known that thymine molecules can dimerize via the formation of hydrogen bonds.13 In the case of 1-(6-mercaptohexyl)thymine capped CdSe nanocrystals (CdSe-4), interactions between ligands of neighboring nanocrystals, combined with the low-size polydispersity of individual nanocrystals, may promote the formation of large-range ordered aggregates. In Figure 6, a scanning electron microscope (SEM) image of CdSe4, deposited by casting on an ITO (indium-tin oxide) coated

De Girolamo et al.

Figure 5. UV-vis absorption and photoluminescence (λex ) 450 nm) spectra of CdSe-4.

glass substrate, is shown. An ordered superstructure can be found consisting of domains of hexagonally packed nanocrystals. P3HT-co-P3(ODAP)HT-CdSe Nanocrystal Composites (P2:CdSe-4). The concept of the formation of a supramolecular assembly between semiconductor nanocrystals and the functionalized conjugated polymer (Scheme 1) must imply stronger nanocrystal ligand-polymer interactions than the ligand-ligand ones. For this reason, we have studied separately the interactions between two mercaptohexylthymine molecules (4-4 dimerization) and the interactions between the postfunctionalized polymer and mercaptohexylthymine molecules (P2-4 association). 1H NMR is a suitable method for such investigations because the chemical shift of the imino proton of thymine depends on its involvement in hydrogen bonding14 (see Figure 7). The dimerization and association constants can be determined from the concentration dependence of this chemical shift, following the procedure described in detail in ref 15. On the basis of the data presented in Figure 7, the calculated constants are 8.3 and 850 M-1 for 4-4 dimerization and P2-4 association, respectively. The value of the latter is comparable to the association constants found for other similar systems.13 Although the obtained association constant characterizes the “free” thymine ligand and P2, it also constitutes a rough estimation of the interactions of thymine capped nanocrystals with the polymer. As the P2-4 association constant is 2 orders of magnitude higher than the 4-4 dimerization one, the process

SCHEME 3: Preparation of CdSe Nanocrystals Functionalized with 1-(6-mercaptohexyl)thymine (CdSe-4) via Ligand Exchange

Assembled Hybrid Materials via Molecular Recognition

Figure 6. SEM image of a thin layer of CdSe-4, drop-cast from DMSO onto an ITO substrate.

of molecular recognition between P2 and the functionalized nanocrystals can be used as the driving force for the formation of composites aiming at the uniform distribution of nanocrystals within the polymer matrix. Classical solution casting methods cannot be applied for the preparation of P2:CdSe-4 composites because it is very difficult to find a common solvent for its components. P2, similar to poly(alkylthiophene)s, dissolves in chloroform and related solvents whereas 1-(6-mercaptohexyl)thymine capped nanocrystals can only be dispersed in solvents which interact with the solute via hydrogen bonds such as, for example, DMSO and DMF. These are however solvents, which do not dissolve the polymeric part of the composite. The use of a mixed solvent is also of limited use in this case, since it inevitably involves the use of a hydrogen-bond accepting component which would perturb the ligand-polymer interaction. Having these limitations in mind, we developed a one-pot composite preparation method, in which chloroform is used as the sole nanocrystal and polymer dissolving medium. The method consists of the preparation of a molecular polymer 1-(6mercaptohexyl)thymine (P2-4) conjugate by dissolving both components in chloroform. In the next step, a dispersion of stearate capped nanocrystals in chloroform is added, and the whole mixture is kept under stirring for an extended period of time (usually 48 h). During this time, a dynamic equilibrium between all components is established. As these are used in a strongly diluted regime, coordination of the thiol groups of (P24) to the surface of CdSe nanocrystals can take place without

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14685 extensive crosslinking, avoiding precipitation. The composite is then obtained by casting on a glass substrate followed by slow evaporation of the solvent. During this operation, enhanced coordination of the thiol groups of P2-4 on the nanocrystals renders the composite phase insoluble. Uncomplexed polymer together with excess ligands are removed by repeated washings using chloroform. The composition of the obtained purified composite was determined by means of thermogravimetric analysis, heating 3.8 mg of P2:CdSe-4 from 30 to 1000 °C (20 °C/min) under N2 atmosphere. Figure 8 depicts the thermogravimetric curves of the used 5.7 nm stearate capped CdSe nanocrystals, of the copolymer P2, and of the composite. The curve of the nanocrystals (Figure 8a) shows a weight loss of 19.2% at 600 °C, which can be attributed to the desorption/ decomposition of the organic surface capping layer. The latter accounts for 19.2% of the total mass, which is in good agreement with the expected mass fraction of 5.7 nm nanocrystals, having approximately 1326 core and 447 surface CdSe units. A second feature at high-temperature (800 °C) indicates the starting thermal decomposition of the nanocrystals. The curve of the polymer P2 (Figure 8b) shows two major weight losses, which are assigned to the alkyl substituents’ decomposition (starting at 400 °C), followed by the decomposition of the poly(thiophene) backbone (beginning at 600 °C). A similar thermogravimetric behavior has been observed for other poly(alkylthiophene)s.16 The thermal decomposition of 1-(6mercaptohexyl)thymine 4 (not shown) occurs at 360-400 °C. Finally, the curve of the composite (Figure 8c) exhibits several distinct weight losses, which can be deconvolved using the thermogravimetric data of the individual components. In the lower temperature part up to 600 °C, the observed weight loss of 20.6% of the initial mass is assigned to the decomposition of nanocrystals’ surface ligands and of the alkyl substituents of P2. In the differential thermogravimetry (DTG) curve, one notes the presence of a peak at 367 °C, which has also been observed in the thermal decomposition of 1-(6-mercaptohexyl)thymine 4. Bearing in mind these data, it can safely be concluded that at 800 °C the overwhelming majority of the organic compounds has been decomposed. The total weight loss at this temperature is 37.3%, which comprises the mass of the polymer fraction and the mass of the nanocrystals’ surface ligands (stearate and 1-(6-mercaptohexyl)thymine). The remaining mass fraction of 62.7% corresponds to the inorganic part of the nanocrystals. On the basis of the TGA data of the nanocrystals, the mass fraction of the surface ligands in the composite can be estimated

Figure 7. Left: Partial 1H NMR spectra of 1-(6-mercaptohexyl)thymine (4) in CDCl3 recorded for increasing concentrations (4-4 dimerization). Right: Chemical shift of the imino proton of 4 as a function of its concentration (inset) and upon addition of P2 (P2-4 association); [4] ) 5 mM).

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Figure 8. Thermogravimetric analysis (TGA, top row) and derivative thermogravimetric analysis (DTG, bottom row) of 5.7 nm CdSe nanocrystals (a), of the copolymer P2 (b), and of the composite P2:CdSe-4 (c).

Figure 9. SEM (left) and TEM (right) images of the composite of the copolymer with CdSe nanocrystals (P2:CdSe-4).

as 12%, resulting in 25.3% of the initial mass corresponding to P2 and a nanocrystal/polymer ratio of 3:1. Thus, during the process of supramolecular assembly of the components, a composite phase enriched with nanocrystals has been formed, as the initially used nanocrystal/polymer weight ratio was 1:1. Importantly, the obtained stoichiometry is in accordance with the ratios generally used for the fabrication of active layers in photovoltaic devices, namely, 1:1-9:1 (w/w) nanocrystal/ polymer.2c,d Figure 9 shows SEM and TEM images of the obtained P2: CdSe-4 composite, respectively. Both techniques reveal unequivocally the presence of individual nanocrystals, uniformly distributed within the polymer matrix, building up a spongelike structure. The apparent absence of aggregates containing exclusively one of the components (nanocrystals or polymer) indicates the excellent homogeneity of the composite material. Figure 10 depicts PL spectra of P2:CdSe-4 deposited on a glass slide, recorded with different excitation wavelengths under

otherwise identical experimental conditions. A broad peak with its maximum located at 664 nm and shoulders at 610, 713, 747, and 815 nm can be distinguished. The shape and position of this peak clearly indicates that it originates from the emission of the polymer part of the composite, as the narrow PL peak of the nanocrystals is centered at 637 nm (cf. Figure 5). This assumption is further corroborated when the excitation wavelength is changed from 500 to 450 nm, revealing that the PL intensity is decreasing when preferentially the nanocrystals are excited . A comparison of the PL spectra of P2:CdSe-4 with those of regioregular P3HT17 reveals a number of similar features. First, the emission maximum is significantly shifted to longer wavelengths (by ca. 90 nm) in the solid state with respect to its position in solution spectra. Second, the peak position and fine structure is characteristic of highly regioregular P3HT. In case of a “perfect” bulk heterojunction between the conjugated polymer and the nanocrystals, no fluorescence should be detectable because of rapid charge-transfer processes at the

Assembled Hybrid Materials via Molecular Recognition

Figure 10. Solid-state PL spectra of the composite P2:CdSe-4 recorded at different excitation wavelengths.

Figure 11. FTIR spectra of (a) 1-(6-mercaptohexyl)thymine (4), (b) P3HT-co-P3(ODAP)HT (P2), and (c) P2:CdSe-4 composite.

organic/inorganic interface. While the PL of the nanocrystals is completely quenched in P2:CdSe-4, the remaining emission of the polymer part most probably originates from inefficient charge transfer. The latter is limited by the long alkyl chain containing ligands on the nanocrystals’ surface, which create an insulating barrier. It has been demonstrated that the exchange of TOPO with pyridine on the surface of CdSe NCs significantly enhanced the polymer PL quenching in blends of CdSe and MEH-PPV.2a The synthesis of composites containing pyridinefunctionalized nanocrystals is currently underway. In Figure 11, IR spectra of the nanocrystal ligand 4, the polymer P2, and the composite P2:CdSe-4 are compared. Although the spectrum of the composite is complex since it embraces contributions from several components (including residual, unexchanged stearate ligands), important information concerning the interactions between the composite components can be extracted from it. First, we notice that the band at 1580 cm-1, characteristic of the CdN stretchings in the diaminopyrimidine group of P2, and the band at 1633 cm-1, which originates from the CdO stretchings in the carbonyl group of 4, merge in the P2:CdSe-4 composite, into one broad band. Similarly, bands corresponding to N-H stretchings in P2 and 4 merge into one broad band peaked at approximately 3280 cm-1 in the composite spectrum. All of these changes can be considered as a spectroscopic evidence of the hydrogen-bond

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Figure 12. SEM image of a composite using CdSe-4 and poly(3hexylthiophene) containing no diaminopyrimidine functions (ITO substrate).

type interactions between 1-(6-mercaptohexyl)thymine capped CdSe nanocrystals and ODAP functionalized poly(3-hexylthiophene). In order to confirm the influence of the molecular recognition process between diaminopyrimidine functions of P2 and 1-(6mercaptohexyl)thymine nanocrystal surface ligands on the observed morphology of the supramolecular assemblies, we have performed a control experiment, in which P2 was replaced by regioregular poly(3-hexylthiophene) of a similar DPn but without diaminopyrimidine side groups. A radically different morphology was obtained as presented in Figure 12 showing the SEM image of the resulting composite. Phase segregation on a submicrometer level occurs, leading to polymer-rich zones (dark) and areas consisting essentially of nanocrystals (CdSe4, light). Very similar results have been obtained when studying blends of poly(3-hexylthiophene) with CdSe nanocrystals containing different types of small capping ligands, such as pyridine, EDOT, or hexylthiophene.18 We believe that the presented approach of building up hybrid composites using supramolecular chemistry provides a simple and versatile way for morphology control in this emerging class of materials. An important advantage over covalently bound systems is the fact that the temperature sensitive hydrogen bonds between the organic and the inorganic components are of a reversible nature. Therefore, temperature control during the composite processing can be used to optimize its morphology.3a Photophysical studies of the presented composites are currently underway in order to evaluate the efficiency of exciton dissociation and charge transport properties. Conclusions To summarize, we have tested the molecular recognition concept in the preparation of composites of poly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexylthiophene) with 1-(6-mercaptohexyl)thymine ligands, binding to the surface of CdSe nanocrystals. The obtained composites exhibit a homogeneous distribution of nanocrystals (ca. 75% in weight) within the polymer matrix in an extended three-dimensional network, which is an important step toward morphologycontrolled organic/inorganic nanostructured materials for optoelectronic devices. Acknowledgment. The authors thank Patrice Rannou for his assistance with size exclusion chromatography and thank Myriam Protie`re for the synthesis of CdSe nanocrystals.

14688 J. Phys. Chem. C, Vol. 111, No. 40, 2007 Supporting Information Available: Syntheses of 3-(6bromohexyl)thiophene (1), 2,5-dibromo-3-(6-bromohexyl)thiophene (2), 2,5-dibromo-3-hexylthiophene (3), Poly(3-hexylthiophene-co-3-(6-bromohexyl)thiophene) (P1), Poly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexylthio-phene) (P2), CdSe nanocrystals, 1-(6-mercaptohexyl)thymine (4), CdSe nanocrystals capped with 1-(6-mercaptohexyl)thymine (CdSe4), and of P2:CdSe-4 composites as well as the macromolecular parameters of P1 obtained by size exclusion chromatography. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Querner, C.; Reiss, P.; Bleuse, J.; Pron A. J. Am. Chem. Soc. 2004, 126 (37), 11574-11582. (b) Querner, C.; Reiss, P.; Sadki, S.; Zagorska, M.; Pron, A. Phys. Chem. Chem. Phys. 2005, 17, 3204-3209. (c) Querner, C.; Reiss, P.; Zagorska, M.; Renault, O.; Payerne, R.; Genoud, F.; Rannou, P.; Pron, A. J. Mater. Chem. 2005, 15, 554-563. (d) Querner, C.; Benedetto, A.; Demadrille, R.; Rannou, P.; Reiss, P. Chem. Mater. 2006, 18 (20), 4817-4826. (e) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Fre´chet, J. M. J. AdV. Mater. 2003, 15 (1), 58-61. (f) Zhang, Q.; Russell, T. P.; Emrick, T. Chem. Mater. 2007, 19, 3712-3716. (2) (a) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B. 1996, 54 (24), 17628-17637. (b) Gur, I.; Fromer, N. A.; Alivisatos, A. P. J. Phys. Chem. B 2006, 110 (50), 25543-25546. (c) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295 (5564), 2425-2427. (d) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126 (21), 6550-6551. (e) Sun, B. Q.; Snaith, H. J.; Dhoot, A. S.; Westenhoff, S.; Greenham, N. C. J. Appl. Phys. 2005, 97, 014914. (f) Advincula, R. C., Dalton Trans. 2006, 23, 2778-2784. (g) Locklin, J.;

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