Supramolecularly Assembled Hybrid Materials via Molecular Recognition betweenDiaminopyrimidine-Functionalized Poly(hexylthiophene) andThymine-Capped CdSe Nanocrystals

Julia De Girolamo, Peter Reiss,* and Adam Pron*DRFMC, UMR 5819-SPrAM, CEA-CNRS-UniVersity J. Fourier-Grenoble I, Laboratoire d’ElectroniqueMoleculaire 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-hydroxy-pyrimidine. An interesting feature of this polymer is its capability of the hydrogen bond assisted formationof 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 distributionof nanocrystals within the polymer matrix in an extended three-dimensional network, whereas in blends withoutspecific interaction between the constituents, phase segregation on a submicron level occurs.

Introduction

Composites of semiconductor nanocrystals with polyconju-gated molecules constitute a new family of electroactivematerials exhibiting tunable electrical, electrochemical, andspectroelectrochemical properties.1 They are also very promisingcandidates for the application in organic electronics, for example,as active “bulk heterojunction” layers in organic photovoltaiccells.2 The main problem in the preparation of these materialsis the process of phase separation promoted by the distinctlydifferent chemical nature of both constitutive components ofthe composite; semiconductor nanocrystals, introduced to theconjugated polymer matrix, have a strong tendency to formagglomerates. In this paper, we demonstrate that homogeneousmolecular composites of this type can be prepared if bothcomposite components contain functional groups capable ofmutual molecular recognition. In particular, we show thatmacromolecules of a regioregular poly(3-hexylthiophene) de-rivative containing, in addition to solubility inducing alkyl sidechains, diaminopyrimidine (ODAP) groups can assemble withthymine capped CdSe nanocrystals via multiple hydrogen bonds.This molecular recognition process forces a uniform distributionof individual nanocrystals within the polymer matrix. Molecularrecognition has previously been applied by Rotello and co-workers3 to the preparation of supramolecular assemblies ofthymine-functionalized gold nanoparticles and poly(styrene),containing diaminotriazine side groups, in the so-called “brickand mortar” approach. Literature reports on conjugated polymersfunctionalized by diaminotriazine or diaminopyrimidine moietiesare scarce,4 and to the best of our knowledge, the title compoundis the first solution processible poly(thiophene), suitable for thepreparation of composites with semiconductor nanocrystals orother types of nanomaterials via molecular recognition.

Experimental Section

The list of the chemicals and reagents used in this researchas well as the detailed synthetic procedures leading topoly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexyl-thiophene) (P2), CdSe nanocrystals and their molecular com-posites can be found in Supporting Information.

The following characterization techniques were applied:1Hand13C NMR spectra of the synthesized monomers, polymers,and ligands were recorded on a Bruker AC 200 MHz spec-trometer using chloroform-d (CDCl3) and methylsulfoxide-d6

(DMSO-d6) solvents containing tetramethylsilane (TMS) as aninternal standard. FTIR spectra were recorded on a Perkin-Elmerparagon 500 spectrometer using the attenuated total reflectance(ATR) configuration.

UV-vis spectra were recorded on HP 8452A and Cary 5000(Varian) spectrometers; photoluminescence measurements wereperformed on a Hitachi F-4500 spectrometer.

Molecular weight determinations of polymer fractions studiedwere measured on a size exclusion chromatography (SEC, 1100HP) equipped with two types of detectors: a diode array UV-vis and a refractive index detector. The column temperature andthe flow rate were fixed to 313 K and 1 mL min-1, respectively.The calibration curve was built using 10 polystyrene narrowstandards (S-M2-10* kit from Polymer Labs).

Transmission electron microscopy (TEM) images of thefunctionalized nanocrystals and the composite were obtainedwith a JEOL 4000 EX microscope operated at 300 kV; scanningelectron microscopy (SEM) analysis was carried out with a ZeissUltra-55 microscope. Thermogravimetric analyses were per-formed with a SETARAM TG 92-12 type TG/DTA system.

Results and Discussion

Structural and Spectroscopic Features of Diaminopyri-midine-Functionalized Poly(3-hexylthiophene) P3HT-co-P3-(ODAP)HT (P2). Multiple hydrogen bonds between 1-(6-mercaptohexyl)thymine-capped CdSe nanocrystals and diamino-pyrimidine-functionalized poly(alkylthiophene) (P2) are the

* Corresponding authors. E-mail: peter.reiss@cea.fr (P.R.) andadam.pron@cea.fr (A.P.).

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

driving force for the formation of supramolecular assembliesbetween both components of the composite, as depicted inScheme 1. In this perspective, the chain microstructure ofP2(and especially its regioregularity) and the content of thediaminopyrimidine-containing substituents are of crucial im-portance. The reaction pathway leading toP2 is shown inScheme 2, with the detailed synthetic procedures being describedin Supporting Information.

The applied synthetic strategy is conceptually similar to thatreported by Mouffouk et al.5 who prepared a poly(alkyl-thiophene) bearing covalently linked biotin groups through thepostfunctionalization of poly(3-hexylthiophene-co-3-(6-hydroxy-hexylthiophene) with biotin hydrazide. Two specific featuresof the Grignard metathesis method used for the preparation ofthe precursor polymer (P1)6 must be pointed out. First, itscomposition can be adjusted by changing the co-reagents ratio.One must however note that in the resulting copolymer the ratio

of 3-hexyl-2,5-thienylene units to 3-(6-bromohexyl)-2,5-thi-enylene units is always higher than the molar ratio of3 to 2 inthe reaction mixture (cf. Supporting Information). Second, theapplied synthesis method leads to a rather large polydispersityin the molecular mass of the resulting copolymer (Mn ) 9200g/mol, polydispersity index (PDI)) 2.55 relative to polystyrenestandards). The polydispersity coefficient can be significantlyreduced by sequential fractionation using the same set ofsolvents as previously established for the fractionation ofregioregular poly(hexylthiophene)7 (cf. Supporting Informationfor macromolecular parameters of the obtained fractions).

For the functionalization ofP1 with ODAP groups to giveP2, 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-bromohex-yl)thiophene ones 6:1 as determined by1H NMR. The post-functionalization reaction (transformation ofP1 into P2) isquantitative as shown by NMR spectra (Figure 1) since thesignals at 3.43 and 1.90 ppm, originating from the 6-bromohexylgroup, are not present in the spectrum ofP2. Instead, new linesin the chemical shift range of 4.0-5.5 ppm appear which arecharacteristic of the presence of ODAP groups. It should alsobe noted that bothP1 andP2 are highly regioregular; the onlyaromatic proton gives rise to a clear singlet at 6.98 ppm withno satellite peaks originating from nonregioregularity.8 Never-theless, it must be stressed that the postfunctionalization reaction,followed by the polymer precipitation in methanol and itsredissolution in chloroform, results inP2with 9:1 co-mers ratio(as determined from1H NMR) which is higher than that in itsprecursorP1 (6:1). This is caused by decreasing solubility ofthe postfunctionalized polymer with increasing ODAP content.In fact, during the postfunctionalization procedure and theproduct purification, we recover only this fraction ofP2 whichis soluble in chloroform, that is, enriched in 3-hexylthiopheneunits. This puts the upper limit of 3-(ODAP)hexylthiophene to3-hexylthiophene groups as 1:9 inP2 which is still soluble inchloroform.

In Figure 2a,b, the FTIR spectra ofP1andP2are compared.The postfunctionalization reaction results in a significantmodification of the spectrum. The presence of ODAP groups

SCHEME 1: Interactions betweenPoly(3-hexylthiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexyl-thiophene) (P2) and 1-(6-mercaptohexyl)thymineCapped CdSe (CdSe-4) via Hydrogen Bonding

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.

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in P2 is manifested by the appearance of several new bandswhich are absent in the nonfunctionalized polymer. In additionto broad peaks at approximately 3460, 3330 (N-H stretchings),and 3160 cm-1 (C-H stretching in the pyrimidine ring), a strongband appears at 1580 cm-1 (CdN stretching), and a bandappears at 796 cm-1 attributed to out-of-plane deformations ofthe pyrimidine ring.

Figure 3 shows the solid-state UV-vis spectrum of a thinfilm of P2. It is known that for polythiophene derivatives theposition of theπ-π* transition in the neutral undoped polymeris strongly dependent on the molecular mass. With an increasingdegree of polymerization (DPn), it is being bathochromicallyshifted, and its vibrational structure becomes more pro-nounced.7,9 A vibrational structure is also observed in thespectrum ofP2 of DPn ) 71. The positions of the 0-0, 0-1,and 0-2 transitions, determined by the second derivativeanalysis (d2A/dλ2), are 602, 562, and 516 nm, respectively; thatis, they occur at almost the same wavelengths as the cor-responding transitions in regioregular poly(3-hexylthiophene)

of slightly lower degree of polymerization (DPn ) 61).10 Theenergy of the 0-0 transition, that is, the transition from theground state to the relaxed excited state, is in poly(thiophene)derivatives inversely proportional to the conjugation length.Thus, close energetic similarity of this transition inP2 and inregioregular poly(3-hexylthiophene) of comparable DPn clearlyindicates that the introduction of bulky ODAP groups throughalkylene spacers does not perturb the conjugation of the polymerat least at the 9:1 ratio of the co-mers.

Briefly concluding this part of the paper, complementaryspectroscopic investigations unequivocally indicate that we havesucceeded in the preparation of a highly regioregular (and byconsequence highly conjugated) solution-processible poly-thiophene containing side groups capable of participating in themolecular recognition process with appropriate complementarygroups.

Functionalization of CdSe Nanocrystals with 1-(6-mer-captohexyl)thymine (CdSe-4).The ligand exchange processleading to nanocrystals capped with 1-(6-mercaptohexyl)thymineis depicted in Scheme 3.

As it has already been stated, the exchange between initialstearate ligands and 1-(6-mercaptohexyl)thymine (4) is ratherslow and inefficient. It can be significantly improved by usingmicrowave radiation. In Figure 4, the1H NMR spectrum of“free” 1-(6-mercaptohexyl)thymine (4) is compared with thatof 5.7 nm CdSe nanocrystals after the ligand exchange (CdSe-

Figure 1. Upper panel:1H NMR spectrum ofP1 (n ) 6 m ) 1, i.e.,co-mers ratio 6:1). Lower panel:1H NMR spectrum of the correspond-ing postfunctionalized copolymerP2 (n ) 9 m ) 1, i.e., co-mers ratio9:1).

Figure 2. Relevant sections of the FTIR spectra ofP1 (a) and ofP2(b), co-mers ratio 6:1 and 9:1, respectively.

Figure 3. UV-vis spectrum ofP2 of DPn ) 71 (co-mers ratio 9:1).

Figure 4. Upper panel: partial1H NMR spectrum of 1-(6-mercapto-hexyl)thymine functionalized 5.7 nm CdSe nanocrystals(CdSe-4),recorded in DMSO-d6. Lower panel: partial1H NMR spectrum of 1-(6-mercaptohexyl)thymine (4), recorded in DMSO-d6.

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4). Even with the assistance of microwave radiation, the ligandexchange is not complete, since in the spectrum ofCdSe-4linesattributable to the initial (stearate) ligands can be found inaddition to the thymine ligands. We ascribe the lower degreeof ligand exchange with respect to results reported for otherthiol-type ligands (up to 90%)11 to the decreasing solubility ofthe nanocrystals during the exchange reaction. The latter maybe due to the possibility of dimerization of thymine groupssticking out of the surface of neighboring nanocrystals (videinfra), leading to the formation of larger aggregates, which thenprecipitate. It is clear that nanocrystals incorporated in theinterior of aggregates are less accessible for further ligandexchange.

At the same time, it is not necessary to achieve a nearlycomplete replacement of stearate ligands with (4), as even arather small number of thymine groups per nanocrystal shouldin principle be sufficient for the preparation of supramolecularcomposite assemblies via molecular recognition of the comple-mentary diaminopyrimidine-containing macromolecule (P2).

Coordination of the deprotonated thiolate form of4 ismanifested by the absence of the signal attributable to the SHgroup in the spectrum ofCdSe-4(compare spectra in Figure4). From the integration of the peak corresponding to the methylgroup of the stearate at 0.86 ppm and the peak originating fromthe proton connected to the thymine ring at 7.52 ppm (not shownin Figure 4), it is possible to determine the degree of the ligandexchange which accounts for approximately 70%.

Figure 5 shows typical absorption and photoluminescence(PL) spectra ofCdSe-4. While the UV-vis spectrum does notexhibit any modification as compared with that of the originalstearate-capped nanocrystals, the intensity of the PL peak isreduced to 8% of the initial value without affection of the linewidth (30.8 nm at full width at half maximum, fwhm). A similarbehavior has also been observed for the exchange with otherthiol ligands. The observed PL decrease has been explained bythe hole accepting character of these molecules whose highestoccupied molecular orbital (HOMO) levels are located withinthe energy band gap of the CdSe nanocrystals.12

It is known that thymine molecules can dimerize via theformation of hydrogen bonds.13 In the case of 1-(6-mercapto-hexyl)thymine capped CdSe nanocrystals (CdSe-4), interactionsbetween ligands of neighboring nanocrystals, combined withthe low-size polydispersity of individual nanocrystals, maypromote the formation of large-range ordered aggregates. InFigure 6, a scanning electron microscope (SEM) image ofCdSe-4, deposited by casting on an ITO (indium-tin oxide) coated

glass substrate, is shown. An ordered superstructure can be foundconsisting of domains of hexagonally packed nanocrystals.

P3HT-co-P3(ODAP)HT-CdSe Nanocrystal Composites(P2:CdSe-4).The concept of the formation of a supramolecularassembly between semiconductor nanocrystals and the func-tionalized conjugated polymer (Scheme 1) must imply strongernanocrystal ligand-polymer interactions than the ligand-ligandones. For this reason, we have studied separately the interactionsbetween two mercaptohexylthymine molecules (4-4 dimeriza-tion) and the interactions between the postfunctionalizedpolymer and mercaptohexylthymine molecules (P2-4 associa-tion). 1H NMR is a suitable method for such investigationsbecause the chemical shift of the imino proton of thyminedepends on its involvement in hydrogen bonding14 (seeFigure 7).

The dimerization and association constants can be determinedfrom the concentration dependence of this chemical shift,following the procedure described in detail in ref 15. On thebasis of the data presented in Figure 7, the calculated constantsare 8.3 and 850 M-1 for 4-4 dimerization andP2-4association,respectively. The value of the latter is comparable to theassociation constants found for other similar systems.13 Althoughthe obtained association constant characterizes the “free”thymine ligand and P2, it also constitutes a rough estimation ofthe interactions of thymine capped nanocrystals with thepolymer. As theP2-4 association constant is 2 orders ofmagnitude higher than the4-4 dimerization one, the process

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

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

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of molecular recognition betweenP2 and the functionalizednanocrystals can be used as the driving force for the formationof composites aiming at the uniform distribution of nanocrystalswithin the polymer matrix.

Classical solution casting methods cannot be applied for thepreparation ofP2:CdSe-4composites because it is very difficultto find a common solvent for its components.P2, similar topoly(alkylthiophene)s, dissolves in chloroform and relatedsolvents whereas 1-(6-mercaptohexyl)thymine capped nano-crystals can only be dispersed in solvents which interact withthe solute via hydrogen bonds such as, for example, DMSOand DMF. These are however solvents, which do not dissolvethe polymeric part of the composite. The use of a mixed solventis also of limited use in this case, since it inevitably involvesthe use of a hydrogen-bond accepting component which wouldperturb the ligand-polymer interaction.

Having these limitations in mind, we developed a one-potcomposite preparation method, in which chloroform is used asthe sole nanocrystal and polymer dissolving medium. Themethod consists of the preparation of a molecular polymer 1-(6-mercaptohexyl)thymine (P2-4) conjugate by dissolving bothcomponents in chloroform. In the next step, a dispersion ofstearate capped nanocrystals in chloroform is added, and thewhole mixture is kept under stirring for an extended period oftime (usually 48 h). During this time, a dynamic equilibriumbetween all components is established. As these are used in astrongly diluted regime, coordination of the thiol groups of (P2-4) to the surface of CdSe nanocrystals can take place without

extensive crosslinking, avoiding precipitation. The compositeis then obtained by casting on a glass substrate followed byslow evaporation of the solvent. During this operation, enhancedcoordination of the thiol groups ofP2-4 on the nanocrystalsrenders the composite phase insoluble. Uncomplexed polymertogether with excess ligands are removed by repeated washingsusing chloroform. The composition of the obtained purifiedcomposite was determined by means of thermogravimetricanalysis, heating 3.8 mg ofP2:CdSe-4from 30 to 1000°C(20 °C/min) under N2 atmosphere. Figure 8 depicts the ther-mogravimetric curves of the used 5.7 nm stearate capped CdSenanocrystals, of the copolymerP2, and of the composite. Thecurve of the nanocrystals (Figure 8a) shows a weight loss of19.2% at 600°C, which can be attributed to the desorption/decomposition of the organic surface capping layer. The latteraccounts for 19.2% of the total mass, which is in good agreementwith the expected mass fraction of 5.7 nm nanocrystals, havingapproximately 1326 core and 447 surface CdSe units. A secondfeature at high-temperature (800°C) indicates the startingthermal decomposition of the nanocrystals. The curve of thepolymerP2 (Figure 8b) shows two major weight losses, whichare assigned to the alkyl substituents’ decomposition (startingat 400 °C), followed by the decomposition of the poly-(thiophene) backbone (beginning at 600°C). A similar ther-mogravimetric behavior has been observed for other poly-(alkylthiophene)s.16 The thermal decomposition of 1-(6-mercaptohexyl)thymine4 (not shown) occurs at 360-400 °C.Finally, the curve of the composite (Figure 8c) exhibits severaldistinct weight losses, which can be deconvolved using thethermogravimetric data of the individual components. In thelower temperature part up to 600°C, the observed weight lossof 20.6% of the initial mass is assigned to the decompositionof nanocrystals’ surface ligands and of the alkyl substituents ofP2. In the differential thermogravimetry (DTG) curve, one notesthe presence of a peak at 367°C, which has also been observedin the thermal decomposition of 1-(6-mercaptohexyl)thymine4. Bearing in mind these data, it can safely be concluded thatat 800°C the overwhelming majority of the organic compoundshas been decomposed. The total weight loss at this temperatureis 37.3%, which comprises the mass of the polymer fractionand the mass of the nanocrystals’ surface ligands (stearate and1-(6-mercaptohexyl)thymine). The remaining mass fraction of62.7% corresponds to the inorganic part of the nanocrystals.On the basis of the TGA data of the nanocrystals, the massfraction of the surface ligands in the composite can be estimated

Figure 6. SEM image of a thin layer ofCdSe-4, drop-cast from DMSOonto an ITO substrate.

Figure 7. Left: Partial1H NMR spectra of 1-(6-mercaptohexyl)thymine (4) in CDCl3 recorded for increasing concentrations (4-4 dimerization).Right: Chemical shift of the imino proton of4 as a function of its concentration (inset) and upon addition of P2 (P2-4 association); [4] ) 5 mM).

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as 12%, resulting in 25.3% of the initial mass corresponding toP2 and a nanocrystal/polymer ratio of 3:1. Thus, during theprocess of supramolecular assembly of the components, acomposite 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 withthe ratios generally used for the fabrication of active layers inphotovoltaic devices, namely, 1:1-9:1 (w/w) nanocrystal/polymer.2c,d

Figure 9 shows SEM and TEM images of the obtainedP2:CdSe-4 composite, respectively. Both techniques reveal un-equivocally the presence of individual nanocrystals, uniformlydistributed within the polymer matrix, building up a sponge-like structure. The apparent absence of aggregates containingexclusively one of the components (nanocrystals or polymer)indicates the excellent homogeneity of the composite material.

Figure 10 depicts PL spectra ofP2:CdSe-4deposited on aglass slide, recorded with different excitation wavelengths under

otherwise identical experimental conditions. A broad peak withits maximum located at 664 nm and shoulders at 610, 713, 747,and 815 nm can be distinguished. The shape and position ofthis peak clearly indicates that it originates from the emissionof the polymer part of the composite, as the narrow PL peak ofthe nanocrystals is centered at 637 nm (cf. Figure 5). Thisassumption is further corroborated when the excitation wave-length is changed from 500 to 450 nm, revealing that the PLintensity is decreasing when preferentially the nanocrystals areexcited . A comparison of the PL spectra ofP2:CdSe-4withthose of regioregular P3HT17 reveals a number of similarfeatures. First, the emission maximum is significantly shiftedto longer wavelengths (by ca. 90 nm) in the solid state withrespect to its position in solution spectra. Second, the peakposition and fine structure is characteristic of highly regioregularP3HT. In case of a “perfect” bulk heterojunction between theconjugated polymer and the nanocrystals, no fluorescence shouldbe detectable because of rapid charge-transfer processes at the

Figure 8. Thermogravimetric analysis (TGA, top row) and derivative thermogravimetric analysis (DTG, bottom row) of 5.7 nm CdSe nanocrystals(a), of the copolymerP2 (b), and of the compositeP2:CdSe-4(c).

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

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organic/inorganic interface. While the PL of the nanocrystalsis completely quenched inP2:CdSe-4, the remaining emissionof the polymer part most probably originates from inefficientcharge transfer. The latter is limited by the long alkyl chaincontaining ligands on the nanocrystals’ surface, which createan insulating barrier. It has been demonstrated that the exchangeof TOPO with pyridine on the surface of CdSe NCs significantlyenhanced the polymer PL quenching in blends of CdSe andMEH-PPV.2a The synthesis of composites containing pyridine-functionalized nanocrystals is currently underway.

In Figure 11, IR spectra of the nanocrystal ligand4, thepolymer P2, and the compositeP2:CdSe-4 are compared.Although the spectrum of the composite is complex since itembraces contributions from several components (includingresidual, unexchanged stearate ligands), important informationconcerning the interactions between the composite componentscan be extracted from it. First, we notice that the band at 1580cm-1, characteristic of the CdN stretchings in the diaminopy-rimidine group of P2, and the band at 1633 cm-1, whichoriginates from the CdO stretchings in the carbonyl group of4, merge in theP2:CdSe-4composite, into one broad band.Similarly, bands corresponding to N-H stretchings inP2 and4 merge into one broad band peaked at approximately 3280cm-1 in the composite spectrum. All of these changes can beconsidered as a spectroscopic evidence of the hydrogen-bond

type interactions between 1-(6-mercaptohexyl)thymine cappedCdSe nanocrystals and ODAP functionalized poly(3-hexyl-thiophene).

In order to confirm the influence of the molecular recognitionprocess between diaminopyrimidine functions ofP2 and 1-(6-mercaptohexyl)thymine nanocrystal surface ligands on theobserved morphology of the supramolecular assemblies, we haveperformed a control experiment, in whichP2 was replaced byregioregular poly(3-hexylthiophene) of a similar DPn but withoutdiaminopyrimidine side groups. A radically different morpho-logy was obtained as presented in Figure 12 showing the SEMimage of the resulting composite. Phase segregation on asubmicrometer level occurs, leading to polymer-rich zones(dark) and areas consisting essentially of nanocrystals (CdSe-4, light). Very similar results have been obtained when studyingblends of poly(3-hexylthiophene) with CdSe nanocrystalscontaining different types of small capping ligands, such aspyridine, EDOT, or hexylthiophene.18

We believe that the presented approach of building up hybridcomposites using supramolecular chemistry provides a simpleand versatile way for morphology control in this emerging classof materials. An important advantage over covalently boundsystems is the fact that the temperature sensitive hydrogen bondsbetween the organic and the inorganic components are of areversible nature. Therefore, temperature control during thecomposite processing can be used to optimize its morphology.3a

Photophysical studies of the presented composites are currentlyunderway in order to evaluate the efficiency of exciton dis-sociation and charge transport properties.

Conclusions

To summarize, we have tested the molecular recognitionconcept in the preparation of composites of poly(3-hexyl-thiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexylthiophene)with 1-(6-mercaptohexyl)thymine ligands, binding to the surfaceof CdSe nanocrystals. The obtained composites exhibit ahomogeneous distribution of nanocrystals (ca. 75% in weight)within the polymer matrix in an extended three-dimensionalnetwork, which is an important step toward morphology-controlled organic/inorganic nanostructured materials for op-toelectronic devices.

Acknowledgment. The authors thank Patrice Rannou forhis assistance with size exclusion chromatography and thankMyriam Protiere for the synthesis of CdSe nanocrystals.

Figure 10. Solid-state PL spectra of the compositeP2:CdSe-4recordedat 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-4composite.

Figure 12. SEM image of a composite usingCdSe-4and poly(3-hexylthiophene) containing no diaminopyrimidine functions (ITOsubstrate).

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Supporting Information Available: Syntheses of 3-(6-bromohexyl)thiophene (1), 2,5-dibromo-3-(6-bromohexyl)-thiophene (2), 2,5-dibromo-3-hexylthiophene (3), Poly(3-hexyl-thiophene-co-3-(6-bromohexyl)thiophene) (P1), Poly(3-hexyl-thiophene-co-3-(6-oxy-2,4-diaminopyrimidine)hexylthio-phene)(P2), CdSe nanocrystals, 1-(6-mercaptohexyl)thymine (4), CdSenanocrystals capped with 1-(6-mercaptohexyl)thymine (CdSe-4), and ofP2:CdSe-4composites as well as the macromolecularparameters ofP1 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.; Frechet, 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. B2006, 110(50), 25543-25546. (c) Huynh, W. U.; Dittmer,J. J.; Alivisatos, A. P.Science2002, 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.;

Patton, D.; Deng, S. X.; Baba, A.; Millan, M.; Advincula, R. C.Chem.Mater. 2004, 16 (24), 5187-5193.

(3) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.;Russell, T. P.; Rotello, V. M.Nature2000, 404(6779), 746-748. (b) Uzun,O.; Frankamp, B. L.; Sanyal, A.; Rotello, V. M.Chem. Mater.2006, 18(23), 5404-5409.

(4) Emge, A.; Ba¨uerle, P.;Synth. Met.1999, 102 (1-3), 1370-1373(5) Mouffouk, F.; Higgins, S. J.; Brown, S. J.; Sedghi, N.; Eccleston,

B.; Reeman, S. Chem. Commun.2004, (20), 2314-2315.(6) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D.AdV. Mater.

1999, 11 (3), 250-253.(7) Trznadel, M.; Pron, A.; Zagorska, M.; Chrzaszcz, R.; Pielichowski,

J. Macromolecules1998, 31 (15), 5051-5058.(8) Chen, T. A.; Wu, X. M.; Rieke, R. D.J. Am. Chem. Soc.1995,

117 (1), 233-244.(9) Pokrop, R.; Verilhac, J. M.; Gasior, A.; Wielgus, I.; Zagorska, M.;

Travers, J. P.; Pron, A.J. Mater. Chem.2006, 16 (30), 3099-3106.(10) Verilhac, J. M.; Pokrop, R.; LeBlevennec, G.; Kulszewicz-Bajer,

I.; Buga, K.; Zagorska, M.; Sadki, S.; Pron, A.,J. Phys. Chem. B2006,110 (27), 13305-13309.

(11) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi,M. G. J. Chem. Phys.1997, 106 (23), 9869-9882.

(12) Wuister, S. F.; Donega, C. D.; Meijerink, A.J. Phys. Chem. B2004, 108 (45), 17393-17397.

(13) Beijer, F. H.; Sijbesma, R. P.; Vekemans, J.; Meijer, E. W.;Kooijman, H.; Spek, A. L.J. Org. Chem.1996, 61 (18), 6371-6380.

(14) Fielding, L.Tetrahedron2000, 56 (34), 6151-6170.(15) Deans, R.; Cuello, A. O.; Galow, T. H.; Ober, M.; Rotello, V. M.

J. Chem. Soc., Perkin Trans.2000, 2, 1309-1313.(16) Adhikari, A. R.; Huang, M.; Bakhru, H.; Chipara, M.; Ryu, C. Y.;

Ajayan, P. M.Nanotechnology2006, 17, 5947-5953.(17) Xu, B.; Holdcroft, S.Macromolecules1993, 26, 4457-4460.(18) Aldakov, D.; Chandezon, F.; De Bettignies, R.; Firon, M.; Reiss,

P.; Pron, A.Eur. Phys. J.: Appl. Phys.2006, 36 (3), 261-265.

14688 J. Phys. Chem. C, Vol. 111, No. 40, 2007 De Girolamo et al.