Electronic, Electrochemical, and Spectroelectrochemical Properties of

Feb 6, 2009 - E-mail: [email protected] (M.Z.); [email protected] (A.P.)., † ... Michelle E. Norako , Matthew J. Greaney , and Richard L. Brutche...
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J. Phys. Chem. C 2009, 113, 3487–3493

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Electronic, Electrochemical, and Spectroelectrochemical Properties of Hybrid Materials Consisting of Carboxylic Acid Derivatives of Oligothiophene and CdSe Semiconductor Nanocrystals Rafal Pokrop,† Katarzyna Pamuła, Sylwia Deja-Drogomirecka, and Malgorzata Zagorska* Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00 664 Warszawa, Poland

Jolanta Borysiuk Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland, and Institute of Experimental Physics, Warsaw UniVersity, Hoza 69, 00-681 Warsaw, Poland

Peter Reiss and Adam Pron* INAC/SPrAM (UMR 5819 CEA-CNRS-UniV. J. Fourier-Grenoble I), Laboratoire d’Electronique Mole´culaire Organique et Hybride, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: September 19, 2008; ReVised Manuscript ReceiVed: December 8, 2008

Hybrids of CdSe nanocrystals (3.7 nm) with two types of terthiophenes, namely (4,4′′-dioctyl-2,2′:5′,2′′terthiophene-3′′-yl) acetic acid (L1) and 7-(4,4′′-dioctyl-2,2′:5′,2′′-terthiophene-3′-yl) heptanoic acid (L2), have been prepared via ligand exchange. Photoluminescence quenching studies showed that the efficiency of the ligand exchange is higher for L1 as compared to L2. A combination of electrochemical, spectroscopic, and spectroelectrochemical investigations gave access to the position of the HOMO and LUMO levels of the inorganic and organic components of the hybrids. Both types of hybrids show staggered alignment of these levels, which is appropriate for photovoltaic applications. Voltammetric oxidation of the CdSe-L2 hybrid leads to the dimerization of the capping ligands and the formation of a new composite material consisting of NCs embedded in the terthiophene dimer matrix. The organic component of this new material can be electrochemically reversibly switched between the doped (conducting) and undoped (semiconducting) states. The obtained hybrids can be blended with poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) (PDOTT) or regioregular poly(3-hexylthiophene) (P3HT), two well-known components of organic electronics devices. Introduction Hybrid materials consisting of semiconductor nanocrystals (NCs) and conjugated oligomers or polymers have emerged in the past decade as perspective components of various electronic devices such as light-emitting diodes, photovoltaic cells, and others (see, for example, refs 1, 2). The preparation of such hybrids is not trivial because of the different chemical nature and solubility parameters of NCs capped with stabilizing ligands such as trioctylphosphine oxide (TOPO), hexadecylamine (HDA), stearic acid (SA), etc., as compared to conjugated molecules and macromolecules. Simple casting from a common solvent leads to uncontrollable phase separation of both components of the hybrid on a micrometer scale.3,4 Moreover, initial ligands constitute an insulating layer, which frequently impedes the effective charge or energy transfer between the inorganic and organic constituents of the hybrid.5 Thus, several preparation approaches of such hybrid materials are focused on the stabilization of the NCs with conjugated ligands such as, for example, thiophene oligomers. Oligothiophene-capped NCs not only ensure better electronic and optoelectronic properties of the system but also facilitate mixing of NCs with poly* Corresponding author. E-mail: [email protected] (M.Z.); [email protected] (A.P.). † Present address: Department of Chemistry, University of Warsaw, Pasteura 1, 02 093 Warsaw, Poland.

(thiophenes), leading to hybrid materials of better controlled morphology. Several methods have been applied in the preparation of NCs stabilized with conjugated ligands. In the simplest one, thiophene oligomers functionalized with a complexing (carboxylic) group were used in the reaction medium as the initial CdS NCs capping agents.6 Despite obvious advantages of this procedure (facile one-step, one-pot preparation), its few drawbacks should be mentioned. First, in the preparation of NCs of controlled size, shape, and low polydispersity, the selection of ligands is a delicate matter of crucial importance. Thus, in the majority of cases, it is more advantageous to use nonconjugated capping ligands, known to yield good quality NCs, and then exchange them for the conjugated ones. This requires, of course, the functionalization of the conjugated molecule (macromolecule) by introducing appropriate anchoring functions such as carbodithioic,7 thiol,8 phosphonic acid,9 or other complexing groups. The preparation of conjugated oligomers (polymers)/CdSe NCs hybrids by grafting constitutes an alternative to the methods described above. In the most general way, the grafting approach can be described as follows. In the first step, the initial NC surface ligands (e.g., TOPO, stearic acid) are replaced by special bifunctional linker ligands containing an anchoring function and a reactive group capable of reacting with an end- or sidefunctionalized polyconjugated molecule or macromolecule. In the next step, the grafting reaction is carried out, whose chemical

10.1021/jp808351h CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

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SCHEME 1

nature depends on the type of the reactive groups in the linker ligand and in the molecule (macromolecule) being grafted. Using this approach, Zhang et al.11 grafted, via Heck coupling, vinyl terminated regioregular poly(3-hexylthiophene) to CdSe nanocrystals previously surface functionalized with 2-(4-bromo-2,5di-n-octyl-phenyl)-ethanethiol through the ligand exchange process. In the research presented here, we used the ligand exchange method to functionalize the NCs’ surface with various conjugated molecules. In particular, we have fabricated CdSe NCs capped with dialkylterthiophenes containing a carboxylic acid complexing function and determined their redox and electronic properties. An interesting feature of the developed systems should be pointed out: oligothiophenes complexed on the NCs’ surface retain their capability of electrochemical coupling. Thus, a hybrid material, in which CdSe NCs are molecularly dispersed within the polyconjugated molecules matrix, can be prepared via postligand exchange electrochemical coupling. Experimental Section Preparation of TOPO-Capped CdSe NCs. A modification of the procedure described in ref 12 was used. 0.128 g (1 mmol) of cadmium oxide (CdO), 3.52 g (9.12 mmol) of trioctylphosphine oxide (TOPO), 4.88 g (20 mmol) of hexadecylamine (HDA), and 0.51 g (2 mmol) of dodecylphosphonic acid were consecutively added to a three-neck flask equipped with a condenser, a thermometer, and a magnetic stirrer and connected to a vacuum/argon line. Under stirring, the reaction mixture was degassed three times at 100 °C by freezing-pumping followed

by filling with argon. Next, it was heated to 300 °C and kept at this temperature until the reddish-brown color disappeared. A solution of the selenium precursor was prepared in a separate flask by mixing selenium powder (0.24 g, 3 mmol) and trioctylphosphine (TOP) (6.23 g, 16.8 mmol). In the next step, the Se precursor solution was quickly injected into the reaction flask containing the Cd precursor. The growth of NCs was carried out at 270 °C for 15 min. The reaction mixture was then cooled to 100 °C, and the formed NCs were precipitated with a 1:10 v:v mixture of butanol and methanol. The NCs were then separated by centrifugation and purified by consecutive dispersion in chloroform and precipitation with methanol. After drying under argon flow, 0.425 g of quasi-monodisperse spherical CdSe NCs of 3.7 nm diameter size was obtained. Synthesis of Terthiophene Ligands. The general pathway for the synthesis of the three terthiophene ligands studied in this research is shown in Scheme 1. Synthesis of (4,4′′-Dioctyl-2,2′:5′,2′′-terthiophene-3′-yl) Acetic Acid (L1). The detailed description of the preparation of the intermediate compound (4,4′′-dioctyl-2,2′:5′,2′′-terthiophene3′-yl) ethyl acetate can be found in ref 13. Its hydrolysis yields L1 (Scheme 1). In a typical hydrolysis reaction, 0.182 g (0.326 mmol) of (4,4′′-dioctyl-2,2′:5′,2′′-terthiophene-3′-yl) ethyl acetate was placed in a flask equipped with a condenser and a magnetic stirrer together with 30 mL of 1 M NaOH solution in ethanol. The reaction mixture was left under vigorous stirring at room temperature for 24 h. The reaction solvent was then evaporated, and 15 mL of diethylether and ca. 30 mL of 1 M HCl were added. The two-phase mixture was left for 20 min

Properties of Hybrid Materials with constant stirring, and in the next step the product was extracted with diethylether. The combined organic fractions were washed with 1 M HCl solution. Finally, after evaporation of the diethylether, the product was purified by crystallization from hexane to give 0.17 g of L1 (98% reaction yield). 1H NMR (CDCl3, 400 MHz, ppm): 7.08 (s, 1H), 7.02 (d, 1H, J ) 1.6 Hz), 7.01 (d, 1H, J ) 1.2 Hz), 6.92 (d, 1H, J ) 1.2 Hz), 6.80 (d, 1H, J ) 1.2 Hz), 3.79 (s, 2H), 2.62-2.55 (m, 4H), 1.67-1.58 (m, 4H), 1.38-1.22 (m, 20 H), 0.91-0.86 (m, 6H). FTIR (KBr, cm-1): 3103 (w), 3012 (w), 2957 (m), 2923 (vs), 2852 (s), 2721 (w), 2620 (w), 1700 (s), 1535 (m), 1469 (m), 1415 (m), 1376 (w), 1313 (w), 1241 (w), 1221 (m), 1188 (m), 863 (w), 820 (m), 749 (m), 720 (m), 673 (w). UV-vis (CHCl3): λmax ) 260 nm, 352 nm. Mass spectrometry: m/z ) 530.23 (M+). Anal Calcd for C30H42O2S3: C, 67.88; H, 7.97; S, 18.12. Found: C, 67.70; H, 7.90; S, 17.90. Synthesis of 7-(4,4′′-Dioctyl-2,2′:5′,2′′-terthiophene-3′-yl)methyl Heptanoate (L3). 7-(3-Thienyl) heptanoic acid (see Scheme 1), one of the reagents used in the synthesis of L3, was prepared according to the method developed by Ba¨uerle et al.14 7-(4,4′-Dioctyl-2,2′:5′,2′′-terthiophene-3′-yl)methyl heptanoate was obtained with 80% reaction yield via Suzuki coupling, in a way identical to that described in ref 13 for the preparation of the ethyl acetate derivative of terthiophene (vide supra). 1H NMR (CDCl3, 400 MHz, ppm): 6.99 (d, 1H, J ) 1.6 Hz), 6.95 (s, 1H), 6.94 (d, 1H, J ) 1.2 Hz), 6.88 (d, 1H, J ) 1.2 Hz), 6.79 (d, 1H, J ) 1.2 Hz), 3.66 (s, 3H), 2.72 (t, 2H, J ) 7.8 Hz), 2.62-2.54 (m, 4 H), 2.30 (t, 2H, J ) 7.6 Hz), 1.68-1.58 (m, 8H), 1.40-1.25 (m, 24), 0.92-0.86 (m, 6H). FTIR (KBr, cm-1): 3097 (w), 2926 (s), 2854 (s), 1740 (s), 1529 (w), 1464 (m), 1376 (w), 1251 (w), 1196 (m), 1171 (m), 1087 (w), 1018 (w), 828 (m), 725 (m). Mass spectrometry: m/z ) 614.33 (M+). Anal. Calcd for C36H54O2S3: C, 70.35; H, 8.79; S, 15.64. Found: C, 70.29; H, 9.91; S, 15.95. Synthesis of 7-(4,4′′-Dioctyl-2,2′:5′,2′′-terthiophene-3′-yl)heptanoic Acid (L2). L2 was obtained from 7-(4,4′′-dioctyl-2,2′: 5′,2′′-terthiophene-3′-yl)methyl heptanoate via hydrolysis of the ester group with a reaction yield of 85% (Scheme 1). The hydrolysis reaction was carried out in a way similar to that in the case of L1 but under reflux. The product was purified by silica gel column chromatography using ethyl acetate/methylene chloride (1:4) as an eluent. 1H NMR (CDCl3, 400 MHz, ppm): 6.99 (d, 1H, J ) 1.6 Hz), 6.95 (s, 1H), 6.94 (d, 1H, J ) 1.2 Hz), 6.88 (d, 1H, J ) 1.2 Hz), 6.78 (d, 1H, J ) 1.2 Hz), 2.72 (t, 2H, J ) 7.6 Hz), 2.62-2.54 (m, 4H), 2.35 (t, 2H, J ) 7.6 Hz), 1.70-1.58 (m, 8H), 1.4-1.22 (m, 24), 0.91-0.85 (m, 6H). FTIR (KBr, cm-1): 3432 (w), 3103 (w), 2925 (vs), 2853 (vs), 1708 (s), 1529 (m), 1463 (m), 1426 (m), 1377 (w), 1304 (w), 1267 (w), 1219 (w), 1121 (w), 965 (w), 861 (m), 825 (m), 752 (m), 725 (m), 673 (w). Mass spectrometry: m/z ) 600.31 (M+). Anal. Calcd for C35H52O2S3: C, 69.95; H, 8.72; S, 16.01. Found: C, 69.84; H, 8.86; S, 15.90. Ligand Exchange Procedure. Ten milligrams of CdSe NCs containing initial TOPO ligands and 20 mg of a terthiophene ligand (L1, L2) were dispersed in 5 mL of chloroform and then shaken for 2-3 days. The NCs were then precipitated with 30 mL of methanol and isolated by centrifugation. The dispersion and precipitation procedure was repeated several times to ensure complete removal of the excess of terthiophene ligands. The resulting NCs were then dried under dynamic vacuum. Characterization Techniques. 1H spectra were registered using a Varian Mercury (400 MHz) spectrometer, and Fourier transform infrared spectroscopy (FTIR) was performed using a BIO-RAD FTS-165 spectrometer. Solution and solid-state

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3489 UV-vis-NIR spectra were recorded on a Varian Cary 5000 spectrometer covering the 175-3300 nm spectral range. Photoluminescence quenching studies were performed on a Jobin Yvon Fluorolog-3 spectrometer equipped with a double monochromator. Transmission electron microscopy (TEM) images of the prepared hybrids and their composites with regioregular poly(3hexylthiophene) and poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) were studied using a JEOL JEM 3010 microscope. Thin layers were deposited on the substrate by casting from a diluted chloroform solution. Voltammetric investigations of free L2 and of L2 complexed on the surface of CdSe NCs (CdSe-L2) were carried out in 0.1 M Bu4NBF4 solution in methylene chloride with either platinum or ITO working electrode, a platinum mesh counter electrode, and an Ag/0.1 M Ag+ reference electrode. UV-vis-NIR spectroelectrochemical studies were performed on thin layers deposited on the ITO electrode in the course of electrochemical cycling of CdSe-L2. Results and Discussion Charge or energy transfer-induced photoluminescence (PL) quenching of CdSe-TOPO NCs is a convenient, although indirect, measure of the capability of a given conjugated ligand of exchanging the initial capping ligands. It should be noted that the quantitative study of this phenomenon requires the determination of the inorganic core share in the total mass of NCs, to determine the exact NCs content in the solution. This can be done using thermogravimetry (TG), assuming that in inert atmosphere the organic part of the NC undergoes decomposition to volatile products and that the inorganic core remains intact, as far as its mass is concerned. For the NCs studied (3.7 nm diameter), the contribution of the inorganic core to the total mass of the NCs was 65%. Knowing the exact content of NCs in a given solution, it is possible to follow their PL decrease induced by the addition of small aliquots of conjugated ligands to this solution.9 The results of such titration experiments carried out for the ligands L1 and L2 are shown in Figure 1. The difference in the efficiency of quenching between L1 and L2 can have two origins. Either L1 is a better coordinating ligand for CdSe NCs, or the energy transfer between the NCs and L1 is favored as compared to the case of NCs capped with L2. Taking into account that the conjugated part of the ligand is separated from the NC surface by a longer aliphatic spacer in L2, the latter explanation is more plausible. Efficient ligand exchange in the case of L1 and L2 is confirmed by the UV-vis absorption spectra of NCs redispersed in chlorofom after the exchange process (Figure 2). The resulting spectrum of CdSe-L1 contains features characteristic of nanocrystals and terthiophenes. A clear excitonic peak at 575 nm can be distinguished, which is attributed to the presence of quasimonodisperse, spherical NCs of 3.7 nm size, together with a pronounced shoulder from higher energy transition and a distinct absorbance tail increasing toward lower wavelengths. The presence of terthiophene is manifested by an additional band, superimposed on the increasing absorption tail characteristic of the nanocrystals. This band is ascribed to the π-π* transition in the π-conjugated system. The π-π* transition band in the spectrum of CdSe-L2 is better resolved. The maximum of this peak is hypsochromically shifted by 12 nm as compared to the case of the spectrum of free L2 (they are located at 342 and 354 nm for CdSe-L2 and L2, respectively).The observed shift can be considered as spectroscopic evidence of the conformational changes induced by binding of the ligand molecule on the nanocrystal’s surface. It should also be pointed out that the

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Figure 1. (a) Photoluminescence intensity of CdSe NCs upon addition of L1. (b) The decrease of the CdSe NCs’ photoluminescence peak intensity induced by titration with conjugated terthiophene ligands, λexc ) 500 nm. (The amount of CdSe core NCs in the cuvette used for the experiments was 0.65 µmol, and its concentration in chloroform solution was 0.2 mM.) Ligand L1 was added in aliquots of 0.29 µmol, whereas L2 was added in aliquots of 0.28 µmol.

Figure 2. Spectra of terthiophene-capped CdSe NCs dispersed in chloroform; for comparison, the spectra of the free ligands and of TOPO-capped NCs, recorded in the same solvent, are included. (a) Spectra of free L1, CdSe-TOPO, and CdSe-L1; and (b) spectra of free L2, CdSe-TOPO, and CdSe-L2.

π-π* transition band in CdSe-L2 is not only shifted with respect to the analogous band in free L2 but also broadened. As a result, no deconvolution of the CdSe-L2 spectrum can be carried out on the basis of the superposition of CdSe-TOPO and free L2 spectra in appropriate proportions. It should be noted that the exchange of the initial ligands for L1 results in a significant lowering of the NCs’ solubility. A similar effect, although less pronounced, is observed for L2. The limited solubility of the new systems impedes on a more quantitative determination of the ligand exchange rate by NMR spectroscopy. For many applications of NC-conjugated oligomer hybrids, including photovoltaic ones, the relative positions of the ligand and the NCs’ HOMO and LUMO levels are of crucial importance. The HOMO levels of both constituents of the hybrid can conveniently be determined from voltammetric measurements.15,16 In particular, the potential of the onset of the anodic peak corresponding to the removal of the first electron (transformation of the neutral species into radical cations) can be considered as corresponding to the HOMO level. If this potential is determined versus that of the ferrocene/ferrocenium (Fc/Fc+) redox couple, whose position with respect to the vacuum level is known (4.8 eV), the following expression for the calculation of the HOMO level can be used:17

EHOMO (eV) ) -4.8 - Eox

(1)

The LUMO level can, in turn, be determined by adding the energy of the optical band gap to the energy of the HOMO

level. For the conjugated organic molecules, this energy can be deduced from the higher wavelength onset of the optical absorption peak, while in the case of semiconductor NCs generally the position of the PL peak is used. In Figure 3, a typical cyclic voltammogram of CdSe-L2 is compared to that registered for “free” L2. The shapes and positions of the observed anodic peaks require some comments. First, the onset of the anodic peak in the voltammogram of CdSe-L2 is shifted toward higher potential by ca. 15 mV as compared to the corresponding peak in the voltammogram of “free” L2. This means that the ligand’s HOMO level is lowered upon its coordination to the electron-accepting surface of CdSe NCs. This observation is consistent with a hypsochromic shift in the maximum of the π-π* transition band, observed in the UV-vis spectrum of CdSe-L2 as compared to that of L2 (vide supra). Such shifts have been observed in other electroactive organic-inorganic hybrids.18 The onset of the NCs’ oxidation in CdSe-L2, which occurs at higher potentials, is totally obscured by the electrochemical activity of the ligand L2. For this reason, the shape of the anodic peak is in this case modified as compared to that registered for free L2. To resolve this problem, we have determined the HOMO level for the same NCs but capped with electrochemically inactive (TOPO) ligands. One must be, however, aware of the fact that the presence of conjugated ligands may affect the position of the NCs’ HOMO level, although this change is usually small.18

Properties of Hybrid Materials

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Figure 3. Cyclic voltammograms of CdSe-L2 (a) and L2 (b) dissolved in 0.1 M Bu4NBF4/CH2Cl2 electrolyte; E vs Ag/0.1 M Ag+; scan rate 50 mV/s.

Qualitatively, the energy levels’ alignment is the same in the two hybrids studied, although quantitative differences exist. The observed staggered alignment is appropriate for photovoltaic applications in bulk heterojunction configuration. After photoexcitation, efficient charge separation can be achieved by transferring holes from the HOMO level of the inorganic core to the HOMO level of the ligand or, alternatively, by transferring electrons from the LUMO level of the ligand to the LUMO level of the inorganic core.5,19

Figure 4. HOMO and LUMO levels of the inorganic and organic components of the prepared CdSe NCs-terthiophene hybrids (NCs’ size 3.7 nm). For comparison, the HOMO and LUMO levels of poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) (PDOTT, Mn ) 10.50 kDa, PI ) 1.69) and regioregular poly(3-hexylthiophene) (P3HT, Mn ) 11.4 kDa, PI ) 2.55) are added.

The complete set of electrochemical and spectroscopic experiments (potentials vs Fc/Fc+: EL1 ) 0.57 V, EL2 ) 0.49 V, onset of the absorption band λL1 ) 415 nm, λL2 ) 425 nm), combined with the electrochemical determination of the HOMO level of TOPO-capped NCs of the same size (not shown), enabled us to establish the relative positions of the energy levels in all three hybrids studied. They are schematically shown in Figure 4.

Another interesting feature of the ligand L2 should be pointed out. It retains its capability of electrochemical coupling even after its coordination to the NC’s surface. The irreversible nature of the cyclic voltammogram presented in Figure 3 seems to suggest that the oxidation of the terthiophene is followed by the coupling of the formed radical cations to form longer chain species.20 The electrochemical coupling process is clearly evidenced in the consecutive cyclic voltammetry scans shown in Figure 5. Anodic oxidation of CdSe-L2, dispersed in the 0.1 M Bu4NBF4/CH2Cl2 electrolyte, results in the deposition of a reaction product on the surface of the ITO electrode, which shows features of an electroactive oligomer longer than L2. In particular, a redox couple appears at lower potentials than the onset of the electrochemical oxidative coupling and grows in intensity with the number of cycles. The presence of the redox

Figure 5. Electrochemical deposition of CdSe-L2 on an ITO electrode to give a hybrid material consisting of 3.7 nm CdSe NCs embedded in an organic oligothiophene matrix. Ten consecutive scans are presented; ITO working electrode; 0.1 M Bu4NBF4/CH2Cl2 electrolyte; E vs Ag/0.1Ag+; scan rate 50 mV/s.

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Figure 6. Absorption spectra of the electrodeposition product of CdSeL2 on ITO surface, recorded for increasing electrode potentials. Electrolyte 0.1 M Bu4NBF4/acetonitrile; E vs Ag/0.1Ag+.

couple is usually attributed to the anodic doping and the cathodic dedoping of the deposited electroactive oligomer or polymer. To reveal the presence of NCs in the electrodeposited composite, the ITO electrode was carefully washed with methylene chloride and then transferred to an electrolytic cell containing 0.1 M Bu4NBF4/acetonitrile and no CdSe-L2. The spectrum registered for the open circuit potential (Eoc) shows the features characteristic of both poly(dialkylterthiophene) and 3.7 nm CdSe NCs (see Figure 6). First, at Eoc, the oligomer is in its reduced (undoped) state, which is manifested by a strong peak at 390 nm due to the π-π* transition in the conjugated polythienylene backbone. The peak is bathochromically shifted as compared to the corresponding peak in L2. It is, however, hypsochromically shifted with respect to the analogous peak in chemically obtained poly(7-(4,4′′-dioctyl-2,2′:5′,2′′-terthiophene-3′-yl)heptanoic acid) (PTTHA) (409 nm). Two origins of this hypsochromic shift can be proposed. The first one involves a lower polymerization degree of L2 coordinated to the CdSe surface as compared to “free” L2. It is known that the position of the π-π* transition peak in polyterthiophenes is strongly dependent on the polymerization degree.21 PTTHA shows a DPn value of 3 (Mn ) 1.9 kDa with polystyrene standards). This may imply that, in the case of CdSe-L2, the electrodeposition is a dimerization process because the position

Pokrop et al. of the π-π* transition peak is intermediate between the position recorded for the monomer and that of the trimer. On the other hand, the observed hypsochromic shift might also originate from the interactions of the coordinating carboxylic group with the NC surface, which could induce the torsion of the neighboring thienylene units with respect to each other. Such phenomenon has been reported for composites of mercaptohexylthymine-capped CdSe NCs and diaminopyrimidine-functionalized poly(alkylthiophenes).18 Evidently, the coordination of L2 on the NC surface can be considered as a steric hindrance, which limits the electropolymerization to dimerization. At Eoc, the excitonic peak, originating from NCs embedded in the composite, is seen as a shoulder of the π-π* transition peak in the terthiophene dimer. The presence of NCs is additionally corroborated by the increasing absorbance in the spectral range of 350-300 nm. The produced terthiophene dimer is electrochemically active and undergoes p-type doping typical of oligothiophenes. Upon increasing electrode potential, the π-π* transition peak bleaches and two new peaks at 750 and 1300 nm, characteristic of the doped state, appear in the nearinfrared part of the spectrum and grow in intensity (see Figure 7). In the studied potential range, the CdSe NCs remain intact, and the distinct absorbance tail increasing toward lower wavelength, characteristic of their presence, persists even at E ) 0.9 V. With increasing electrode polarization, the excitonic peak originating from NCs becomes less obscured by the decreasing intensity of the terthiophene dimer π-π* transition peak; at E ) 0.9 V, it is clearly seen as a separate peak. It should be, however, noted that the doping of the organic part of the hybrid results in a shift of the NCs excitonic peak. At E ) 0.9 V, it is located at 560 nm; that is, it is hypsochromically shifted by 15 nm as compared to the corresponding peaks in CdSe-TOPO and neutral CdSe-L2. We attribute it to a lowering of the HOMO level in NCs stabilized with positively charged (doped) conjugated ligands. This behavior is expected, because positive charge imposed on the ligand in the course of doping should make the onset of NCs’ oxidation more difficult. A similar phenomenon has also been observed in the electrochemistry and spectroelectrochemistry of tetraaniline-capped CdSe NCs.16 To summarize this part of the Article, the organic component of the electrochemically deposited hybrid can be

Figure 7. TEM images of composites of poly(3,3′′-dioctyl-2,2′:5′,2′′-terthiophene) (PDOTT, Mn ) 10.50 kDa, PI ) 1.69) with CdSe-L2 hybrid (left) and regioregular poly(3-hexylthiophene) (P3HT, Mn ) 11.4 kDa, PI ) 2.55) with the same hybrid (right). The composites were cast from chloroform solution of both components in 1:1 weight ratio.

Properties of Hybrid Materials switched between the semiconducting state and the conducting one by applying the appropriate potential. Finally, we have checked the blending of CdSe-terthiophene hybrids with high molecular weight poly(alkylthiophenes). Several methods have been elaborated to improve dispersion of NCs in conjugated polymers. Among them, replacing the initial ligands with “labile” ones such as pyridine followed by casting the NCs-polymer hybrid from a common solvent or mixture of solvents became very popular.22 However, significant and uncontrolled phase separation between the inorganic and organic components takes place unless the poly(alkylthiophene) is functionalized with an appropriate anchoring side or end group.3,11 Coating of the NCs’ surface with conjugated oligomers should facilitate their dispersion in polymer matrices consisting of their high molecular mass analogues. This aspect can be discussed in terms of the solubility parameters, δ. To a first approximation, such dispersion can be considered as a molecular solution, in which the polymer matrix is the “solvent” and the oligomer-coated NCs the “solute”. The solubility parameter of the solute is then determined by the chemical constitution of the NCs’ surface and by consequence must be close to that of the solvent, because both are chemically very similar. As a result, a homogeneous dispersion of the NCs in the polymer matrix is expected. Figure 7 shows TEM images of the composites obtained from CdSe-L2 and poly(3,3′′-dioctyl-2,2′: 5′,2′′-terthiophene) (PDOTT), which is a promising candidate for electronic organic application23 and regioregular poly(3hexylthiophene) (P3HT), respectively. Although the NCs’ distribution is not totally uniform, the observed agglomerates are rather small, and individual NCs can easily be distinguished. It should also be noted that the HOMO levels in the threecomponent composite (CdSe NCs L2 and PDOTT (or P3HT)) show the staircase type of alignment (see Figure 4). Taking into account that phase separation is a serious problem in the fabrication of composites containing semiconductor NCs and conjugated polymers,5,24 coating of the former with conjugated oligomers may be one possible solution, in addition to already existing ones such as the use of labile ligands or conjugated polymer functionalization via introduction of appropriate anchoring groups. Conclusions To summarize, we have studied the electronic and electrochemical properties of hybrid materials consisting of CdSe NCs capped with dialkylterthiophenes functionalized with carboxylic acid groups. The prepared hybrids show a staggered alignment of the HOMO and LUMO levels of their organic and inorganic components, leading to charge separation and photoluminescence quenching. Voltammetric oxidation of these hybrids leads to the dimerization of the capping ligands and the formation of a new composite material consisting of NCs embedded in the terthiophene dimer matrix. Finally, composites of high molecular

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3493 poly(alkylthiophenes) with terthiophene-capped NCs, prepared by casting from a common solution, have less tendency to phase separate as compared to composites with commonly used pyridine ligands. Acknowledgment. We thank the Faculty of Materials Science and Engineering of Warsaw University of Technology for use of the transmission electron microscope JEOL JEM 3010. R.P., K.P., S.D.-D., and M.Z. thank Warsaw University of Technology for financial support. References and Notes (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354–357. (2) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324–1338. (3) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550–6551. (4) Holder, E.; Tessler, N.; Rogach, A. L. J. Mater. Chem. 2008, 18, 1064–1078. (5) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628–17637. (6) Antoun, T.; Brayner, R.; Al terary, S.; Fievet, F.; Chehimi, M.; Yassar, A. Eur. J. Inorg. Chem. 2007, 1275–1284. (7) Querner, C.; Benedetto, A.; Demadrille, R.; Rannou, P.; Reiss, P. Chem. Mater. 2006, 18, 4817–4826. (8) Sih, B. C.; Wolf, M. J. Phys. Chem. C 2007, 111, 17184–17192. (9) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Frechet, J. M. J. AdV. Mater. 2003, 15, 58–63. (10) van Beek, R.; Zoombelt, A. P.; Jenneskens, L. W.; van Walree, C. A.; Donega, C. D.; Veldman, D.; Janssen, R. A. J. Chem.-Eur. J. 2006, 12, 8075–8083. (11) Zhang, Q.; Russell, T. P.; Emrick, T. Chem. Mater. 2007, 19, 3712– 3716. (12) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781–784. (13) Buga, K.; Pokrop, R.; Majkowska, A.; Zagorska, M.; Planes, J.; Genoud, F.; Pron, A. J. Mater. Chem. 2006, 16, 2150–2164. (14) (a) Ba¨uerle, P.; Wu¨rthner, F.; Heid, S. Angew. Chem., Int. Ed. Engl. 1990, 29, 419–420. (b) Ba¨uerle, P.; Hiller, M.; Scheib, S.; Sokolowski, M.; Umbach, E. AdV. Mater. 1996, 8, 214–218. (15) Kucur, E.; Riegler, J.; Urban, G. A.; Nann, T. J. Chem. Phys. 2003, 119, 2333–2337. (16) Querner, C.; Reiss, P.; Sadki, S.; Zagorska, M.; Pron, A. Phys. Chem. Chem. Phys. 2005, 7, 3204–3209. (17) Polec, I.; Henckens, A.; Goris, L.; Nicolas, M.; Loi, M. A.; Adriaensens, P. J.; Lutsen, L.; Manca, J. V.; Vanderzande, D.; Sariciftci, N. S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1034–1045. (18) De Girolamo, J.; Reiss, P.; Zagorska, M.; De Bettignies, R.; Bailly, S.; Mevellec, J. Y.; Lefrant, S.; Travers, J. P.; Pron, A. Phys. Chem. Chem. Phys. 2008, 10, 4027–4035. (19) Huynh, W. U.; Dittmer, J. J.; Teclemariam, N.; Milliron, D. J.; Alivisatos, A. P.; Barnham, K. W. J. Phys. ReV. B 2003, 67, 1153261– 11532612. (20) Roncali, J. Chem. ReV. 1992, 92, 711–738. (21) Pokrop, R.; Verilhac, J. M.; Gasior, A.; Wielgus, I.; Zagorska, M.; Travers, J. P.; Pron, A. J. Mater. Chem. 2006, 16, 3099–3106. (22) Huynh, W. U.; Dittmer, J. J.; Allivisatos, A. P. Science 2002, 295, 2425–2427. (23) Wu, Y.; Liu, S.; Gardener, S.; Ong, B. S. Chem. Mater. 2005, 17, 221–223. (24) Aldakov, D.; Chandezon, F.; De Bettignies, R.; Firon, M.; Reiss, P.; Pron, A. Eur. Phys. J.: Appl. Phys. 2006, 36, 261–265.

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