Electronic and Vibrational Structure of Complexes of

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Electronic and Vibrational Structure of Complexes of Tetracyanoquinodimethane with Cadmium Chalcogenide Quantum Dots Laura C. Cass, Nathaniel K. Swenson, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: This paper describes the use of visible, near-infrared, and mid-infrared steadystate optical spectroscopy to study the geometries in which tetracyanoquinodimethane (TCNQ) adsorbs to the surfaces of highly cadmium enriched and near-stoichiometric CdSe quantum dots (QDs) in the formation of QD-TCNQ charge transfer (CT) complexes. Several TCNQ molecules are spontaneously reduced by chalcogenides on the surface of each CdSe QD. The degree of CT depends on the geometry with which the TCNQ adsorbs and the degree of distortion of TCNQ’s geometry upon adsorption. Comparison of the electronic and vibrational spectra of CdSe QD-TCNQ complexes with those of CT complexes of TCNQ with molecular reductants (including molecular chalcogenides) and computer simulations of the geometries and vibrational spectra of the TCNQ-chalcogenide CT complexes show that (i) the Cd-enriched CdSe QDs reduce a factor of 7.4 more TCNQ molecules per QD than nearly stoichiometric CdSe QDs because surface selenides are more accessible in the Cdenriched QDs than in the near-stoichiometric QDs and (ii) TCNQ interacts with surface selenides through several adsorption modes that result in different amounts of charge transfer and different degrees of geometric distortion of TCNQ. This study provides a framework for determining the range of adsorption geometries of small molecules on QD surfaces, and for optimizing QD surfaces to adsorb molecules in configurations with maximal electronic coupling between the QD and the adsorbate.

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INTRODUCTION This paper describes the use of visible, near-infrared, and midinfrared steady-state optical spectroscopy to determine the set of geometries in which tetracyanoquinodimethane (TCNQ) adsorbs to the surfaces of CdSe quantum dots (QDs) in the formation of QD-TCNQ charge transfer (CT) complexes, and the dependence of these geometries on the surface chemistry of the QDs. Metal-chalcogenide QDs have high absorption cross sections, for instance, factors of ten to 100 greater than that of the metal-to-ligand charge transfer absorption band of ruthenium(II)tris(bipyridine) (Ru(bpy)32+),1 for bandgapresonant excitations across the long-UV, visible, and nearinfrared regions of the solar spectrum.2 This optical bandgap can be tuned with both the material and size of the QD.2 They also have the ability to accumulate multiple simultaneous excitons or charge carriers within a single nanoparticle. These properties make QDs potentially important as chromophores and redox centers within photovoltaic and photocatalytic active materials. For both types of devices, efficient extraction of charge carriers from the QDs is critical to the performance of the device. We have shown previously that, in order for ultrafast (tens of picoseconds or faster) charge transfer between a QD and a molecule to occur, the molecule must permeate the ligand shell of the QD and adsorb, at least transiently, to its inorganic surface.3,4 It is therefore important to know what types of surface chemistries promote adsorption of potential molecular redox partners, in particular, adsorption of molecules in geometries that facilitate large electronic coupling, and how chemical, structural, and electronic heterogeneity in QD surface © 2014 American Chemical Society

translates to heterogeneity in adsorption geometries, degree of CT, and vibrational and electronic spectra of the adsorbate. To explore this issue, we chose the CdSe QD-TCNQ donor−acceptor system. Upon adsorption of TCNQ onto the surfaces of the QDs in certain geometries, TCNQ is spontaneously reduced by surface chalcogenides (in this study, selenides, and, in a previous study of PbS QDs,5 sulfides).6,7 The electronic and vibrational spectra in the nitrile region of TCNQ in this adsorbed state are sensitive to the number of electrons (which can be integer or noninteger) that TCNQ acquires from its reductant, the distribution of this electron density within the TCNQ molecule,5,8,9 and the geometry of TCNQ in the adsorbed state,10−12 specifically, which part of the molecule is in direct contact with the surface and to what degree the molecule is distorted from planarity. TCNQ is then a reporter of its own interaction with the QD surface. We show that certain types of surface chemistries for the QD, specifically, the preparations of QDs in which selenides are most accessible, are more conducive to adsorption of TCNQ in tight-binding, highly electronically coupled geometries than are others. In order to understand the electronic and vibrational spectra of TCNQ within CdSe QD-TCNQ mixtures, we first studied mixtures of TCNQ with molecular reductants, including molecular chalcogenides. Our analysis is aided by computer simulations of the geometries and vibrational spectra of TCNQ Received: June 16, 2014 Published: July 15, 2014 18263

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Supporting Information. We observed the maximum TCNQ− signal at 3 days, see Figure S2 in the Supporting Information. We present the data taken after the QD/TCNQ mixtures stirred for 3 days in the main text. Absorbance and FT-IR Spectroscopy. We performed ground state absorption measurements on a Varian Cary 5000 spectrometer using 1 cm cuvettes, except for samples containing TOPSe and DFe, for which we used 2 mm quartz cuvettes because the signals of these samples in 1 cm cuvettes saturated the detector of the spectrophotometer. We corrected the baselines of all spectra with neat solvent prior to measurement. We measured all samples of mixtures and their corresponding control samples in the same-sized cuvette and scaled and vertically shifted the resultant spectra by the same factors to maintain the relative intensities of the peaks of the control and mixture samples. All spectra were measured in CHCl3. We acquired FT-IR spectra of the samples with a Thermo Nicolet Nexus 870 FT-IR spectrometer at a resolution of 1 cm−1 and 64 scans per sample, using OMNIC software. The sample chamber is under a constant purge of N2. The 0.5 mm path-length, sealed liquid spectrophotometer cell comprises 38.5 mm × 19.5 mm × 4 mm KCl plates and a mercuryamalgamated spacer. All samples were in CHCl3. The spectrum of each electron donor, at the same concentration as it is in the mixture with TCNQ, is subtracted from the respective spectrum of the mixture of TCNQ and the electron donor. Both spectra are normalized to a peak of the electron donor, which does not overlap with any peaks of TCNQ, prior to subtraction. Measurements of the electronic and vibrational spectra of each electron donor oxidized by AgNO3, Figures S4 and S5 in the Supporting Information, confirm that the oxidized forms of these donors do not contribute to the spectra that we present in the main text. Spectroelectrochemistry. We acquired an absorbance spectrum of electrochemically reduced TCNQ (TCNQ1−) by applying a constant potential of −0.25 V vs an Ag wire pseudoreference (0.1 M TBAP, Pt wire counter electrode, Pt mesh working electrode) for 5 min to 0.7 mM TCNQ in CH2Cl2. We acquired the absorbance spectrum of doubly reduced TCNQ (TCNQ2−) by applying a constant potential of −0.8 V vs an Ag wire pseudoreference with all other conditions the same. These spectra are truncated at 422 nm, below which there is a residual absorbance of neutral TCNQ. We also subtracted the residual TCNQ1− signal from the spectrum of TCNQ2−. Computational Details. We optimized the geometry of the TCNQ molecule as a neutral molecule, an anion, and a dianion, as well as TCNQ in complexes with sulfide (S2−) and selenide (Se2−), using Density Functional Theory (DFT). We used the B3LYP functional and a triple-ζ basis set with polarization on all atoms, 6-311+G(d,p). We used the LANL2DZ effective core potential and corresponding basis set for Cd and Se atoms where appropriate. Once we identified the energy-minimized structure for each form of TCNQ, we calculated its vibrational frequencies. We scaled the raw computed frequencies by a constant factor of 0.9679 to better match with experiment, as recommended by Andersson and Uvdal,13 and then plotted the computed frequencies as a “stick”-spectrum to create the simulated IR spectra. We performed a Natural Bond Order analysis using the NBO 5.0 software to estimate atomic charges.14

in various redox states and in complexes with model reductants. While these gas-phase calculations do not necessarily provide accurate absolute energies for nitrile vibrational modes, they do accurately predict numbers of peaks and energy spacings between peaks within these spectra for many states of TCNQ, and show us trends in vibrational spectra with amount of CT, degree of geometric distortion, and symmetry of the electron density distribution. Our study tells us that, in addition to forming weakly interacting ion pairs with TCNQ through a discrete nonadiabatic electron transfer process, the surfaces of QDs also form CT complexes with TCNQ that are characterized by new CT absorptions similar to those in molecular coordination complexes, in which electron density is shared across the donor chalcogenide and the acceptor TCNQ. These types of complexes, of which several form per QD, are interesting in themselves because they localize electrons from the QDs at specific surface sites and make them available indefinitely for use in chemical reactions. Additionally, TCNQ molecules bound within these complexes are highly electronically coupled to the QD surface and are therefore the best candidates for further reduction (to TCNQ2−) by an excitonic electron in the core of a photoexcited QD. The transient TCNQ2− state can then potentially act as a precursor for further charge separation or possibly a redox-triggered chemical reaction.

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EXPERIMENTAL AND COMPUTATIONAL METHODS Formation of CT Complexes of TCNQ and Electron Donors. We added stock solutions of 0.7 mM TCNQ in CHCl3 to vials of bis(trimethylsilyl)sulfide (TMS2S), trioctylphosphine selenide (TOPSe), and decamethylferrocene (DFe) such that the molar ratio of electron donor to TCNQ in the mixture was 10:1 for TMS2S, TOPSe, and DFe. We stirred all mixtures in the dark in a N2 box and kept these mixtures under N2 during absorbance and FT-IR experiments due to the ability of ambient atmosphere to oxidize DFe. We acquired absorbance spectra of the mixtures containing TMS2S and DFe after stirring for 7 days and 11 days, and of the mixture containing TOPSe after 1 and 4 days, see Figure S1 in the Supporting Information. We show absorbance and FT-IR data for the TMS2S and DFe samples stirred for 11 days and for the TOPSe sample stirred for 4 days in the main text. The intensities of the signals of reduced TCNQ differed by less than 10% between 7 and 11 days for the DFe sample and between 1 and 4 days for the TOPSe sample; we observed no neutral TCNQ by FT-IR for either sample. The reduced TCNQ signal in the TMS2S/TCNQ mixture grew more than 10−20% from 7 to 11 days of stirring, but we did not acquire spectra of this mixture after 11 days because the TMS2S complex with TCNQ begins to precipitate from solution and stick to the walls of the scintillation of vial after stirring for more than 11 days. We prepared the mixtures of QDs with TCNQ by first evaporating the solvent, CHCl3, from a known volume of QD sample, and then adding TCNQ stock solutions in CHCl3 of this same volume to the vial of dried CdSe QDs such that the concentrations of the QDs did not change upon addition of TCNQ. We used a 0.2 mM stock solution of TCNQ to achieve a 25:1 molar ratio of TCNQ to QD in these mixtures. We stirred all mixtures in the dark for 7 days under ambient atmosphere and measured UV−vis−NIR absorbance and FTIR spectra after 3 days and an additional UV−vis−NIR absorbance spectrum after 7 days, see Figures S2 and S3 in the 18264

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RESULTS AND DISCUSSION General Trends in the Electronic and Vibrational Spectra of TCNQ upon Its Reduction. TCNQ, in its uncomplexed form, is a planar molecule of D2h symmetry with four nitrile groups, Chart 1. The magnitude and distribution of Chart 1. TCNQ

electron density within the molecule in its various redox states determines both its electronic (Figure 1) and vibrational

Figure 2. (A−D) Stretching modes of the nitrile groups in the TCNQ molecule. Vibrations A and D are IR-silent when TCNQ is uncomplexed, or when it is complexed such that its CT partner interacts with all four nitrile groups identically. When preferential interaction of an electron donor with a subset of the nitrile groups breaks the symmetry of the system, modes A and D become IR-active. (E) Simulated IR spectra of neutral TCNQ, TCNQ1−, and TCNQ2−, as well as complexes of TCNQ with the electron donors S2− and Se2− (which we group together because peak energies in these spectra only differ by, at most, 4 cm−1). The labels on each peak indicate the corresponding mode, A−D. We show simulated spectra for the CT complex of TCNQ with S2− or Se2−, where we have constrained the distance between S2− or Se2− and TCNQ to be 2 Å, 3 Å, or 5 Å, and then optimized all other geometric degrees of freedom of the complex, as shown in panel F.

Figure 1. UV−vis−NIR absorbance spectra of TCNQ in various redox states (produced electrochemically in CH2Cl2), and of mixtures of TCNQ and a set of electron donors DFe, TMS2 S, nearly stoichiometric CdSe QDs, Cd-enriched CdSe QDs, and TOPSe, in CHCl3. The dashed lines are spectra of the electron donors without added TCNQ. The spectra are ordered by increasing degree of reduction of TCNQ from bottom to top. The Methods section describes the preparation of these samples. Arrows indicate regions that have been magnified (relative to other regions in the same spectrum).

vibrational spectrum only includes one observable peak at 2224 cm−1,17 Figure 2 (black, simulated) and Figure 3 (purple, measured), because the symmetric-asymmetric mode B has a very weak IR intensity in neutral TCNQ. TCNQ1− formed by electrochemical reduction or exposure to a very weakly interacting reductant such as decamethylferrocene (DFe), has three structured peaks at 850, 749, and 686 nm5,7,15,17−19 and an additional peak at 445 nm20,21 in its electronic spectrum, Figure 1 (blue and green traces), and two peaks at approximately 2182 and 2156 cm−1 in the nitrile stretching region of its mid-IR spectrum, Figure 2 (gray, simulated) and Figure 3 (green, measured).17 Frequency analysis of the TCNQ1− molecule assigns these peaks to the IR-active modes B and C in Figure 2. Electrochemically reduced TCNQ2− has an electronic spectrum that consists of one broad peak centered at 485 nm

(Figures 2 and 3) spectra. Calculations of the optimized geometries and charge distributions of integer redox states of TCNQ, i.e., TCNQ0, TCNQ1−, and TCNQ2−, (as described in the Methods section, with bond lengths and angles detailed in the Supporting Information, Scheme S1) show that in all three cases, the electron density is distributed symmetrically with respect to all three mirror planes of the molecule. Our vibrational frequency analysis shows that when electron density (of any magnitude) is distributed equally across all four nitrile groups, the nitrile-group stretching modes have two IR-active symmetries (B and C in Figure 2) with frequencies that are independent of the electron donor. The electronic spectrum of neutral TCNQ contains two peaks at 380 and 400 nm, Figure 1 (black trace).15−18 The 18265

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undergoes a reduction to symmetric TCNQ1− with no distortion from planar geometry and (ii) more strongly interacting complexes, where TCNQ shares electron density with its electron donor and therefore participates in CT-type absorptions. For example, the electronic and vibrational spectra of a mixture of 7 mM bis(trimethylsilyl) sulfide (TMS2S), a molecular form of the sulfide present on the surfaces of CdS and PbS QDs, and 0.7 mM TCNQ include both (i) the typical TCNQ1− features in the electronic spectrum at 850, 749, and 686 nm (Figure 1, red), and in the vibrational spectrum at 2180 and 2152 cm−1 (Figure 3, red), and (ii) typical features of a more strongly bound complex in the electronic spectrum at ∼610 nm (broad) and in the vibrational spectrum at 2203, 2194, 2120, and 2095 cm−1. We know that two distinct populations of reduced TCNQ are present in this sample because the electronic signatures of the two reduced forms of TCNQ grow in at different rates during mixing (see Figure S1A in the Supporting Information). The broad line width of the CT peaks in the vibrational spectrum of the TMS2S-TCNQ complex indicates that there is a distribution of geometries for this complex. We performed simulations of complexes of S2− (or Se2−) anions with TCNQ with results visualized in VESTA,29 Figure 2F, which show that the chalcogenide interacts asymmetrically with TCNQ; specifically, the anion associates most closely with an electropositive carbon that links the nitrile groups to the ring, Figure 2F. We also simulated the binding of the chalcogenide to TCNQ through one of the nitrile groups of TCNQ, which is a known binding structure between organoselenium species and nitriles.30 The energy required to form this structure, however, was 0.2 eV larger than the energy required to form the other binding structure we propose. This observation is consistent with the presence of four IR peaks (instead of two) that we attribute to the TMS2S-TCNQ CT complex in addition to the two peaks for weakly associated TCNQ1−. Our calculations also show that the frequencies of the four nitrile stretching modes of TMS2S-TCNQ depend on the degree to which the planar structure of the TCNQ molecule is distorted upon formation of the complex, the amount of CT from the chalcogenide to TCNQ (Figure 2E), and, in turn, the distance between the chalcogenide ion and the carbon with which it most closely associates. In general, on going from a 5-Å separation to a 2-Å separation, the amount of CT from the S(Se)2− to TCNQ increases from 1.3(1.3) electrons to 1.4(1.5) electrons, the molecule is increasingly distorted from planarity, peaks A and C shift to lower energy, and peaks B and D shift to higher energy, such that peak B becomes higher-energy than peak C. The Supporting Information contains our calculated geometries and vibrational spectra for S(Se)x‑-TCNQy‑ complexes at intermediate constrained separations. The experimental vibrational spectrum of the TMS2S-TCNQ mixture most closely resembles a combination of the TCNQ1− spectrum and the simulated spectrum for the S0.6‑TCNQ1.4‑ complex (1.4 electrons transferred to TCNQ) at a 2-Å separation. Since more than one electron is transferred to TCNQ in the ground state, the broad feature at 610 nm in the electronic absorbance spectrum of the complex is probably an LMCT band. Given the vibrational and electronic spectra of the TMS2S-TCNQ mixture, we conclude that, within the ensemble, some pairs of TMS2S and TCNQ form a strongly associated complex that results in distortion of the planar geometry of TCNQ, and some pairs form a weak, transient encounter

Figure 3. Dotted traces: Experimentally measured FT-IR spectra of TCNQ or mixtures of TCNQ and an electron donor, where the electron donors are DFe, TMS2S, nearly stoichiometric CdSe QDs, Cd-enriched CdSe QDs, and TOPSe. Solid lines: Components of multi-Lorentzian fits to these spectra. The total multi-Lorentzian fits (sums of the components shown) are not drawn here for clarity, but are shown and compared to experimental spectra in the Supporting Information. The Methods section describes the preparation of these samples.

(Figure 1, dark yellow),15,17,19 and a vibrational spectrum that contains two peaks (modes B and C) at 2153 and 2107 cm−1.17 We show the simulated vibrational spectrum for TCNQ2− in Figure 2 (gray). When TCNQ forms a CT complex with its electron donor within which charge density is delocalized over both donor and acceptor, TCNQ is in a noninteger redox state, and new features appear in the electronic and vibrational spectra. The number, frequencies, and line widths of these peaks depend on the amount of charge transferred to TCNQ from the electron donor,8,11,22,23 how symmetrically this charge density is distributed over the molecule, and the degree of geometric distortion of the TCNQ molecule upon complexation with the electron donor.10−12 In complexes of TCNQ with TiO2,24 Ti4+centered molecules,20 fluorene derivatives,25 and surfactants such as salts of dodecylsulfate,26,27 dodecylpyridinium halides, and sodium decyl- and hexylsulfates,27,28 a broad peak in the range of 520−700 nm appears in the visible spectrum; this peak has been assigned to a charge-transfer-type electronic absorption of the CT complex delocalized over both TCNQ and its CT partner. The direction of the transition dipole moment for the CT absorption depends on the amount of charge on the TCNQ molecule upon formation of the ground state complex. For CT complexes of TCNQ with metal ions where less than one electron is transferred to TCNQ in the ground state, the CT band is usually due to a metal-to-ligand CT (MLCT) absorption. For complexes where more than one electron is transferred to TCNQ in the ground state, the band is usually due to a ligand-to-metal CT (LMCT) absorption.20,24 CT Complexes of TCNQ with Molecular Chalcogenides. Some mixtures of TCNQ with electron donors contain both (i) weakly interacting complexes, where the TCNQ 18266

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nm (R = 1.25 nm), respectively.2 The absorption spectra of these QDs do not overlap with the peaks of the radical anion of TCNQ in the near-infrared or with the characteristic CT band of TCNQ CT complexes near 600 nm. Both types of CdSe QDs were washed three times with methanol, which removes many of the weak, datively bound ligands such as hexadecylamine and TOPO as well as excess Cd and Se precursors that remain in solution.35 We measured the Cd:Se ratios of the two samples of QDs by ICP-AES, see the Supporting Information for further details, and obtained values of 9.5:1 for the Cdenriched sample, and 1.2:1 for the nearly stoichiometric CdSe QDs. Cd:Se ratios for Cd-enriched QDs vary with the amount of phosphonic acid impurity in the reagent-grade TOPO, and values as large as 6.5:131 have been reported. The high degree of cadmium enrichment in these samples is due to a disordered multilayer coating of Cd-phosphonate complexes on the nearly stoichiometric cores of these QDs;31,36 these complexes survive the purification procedure because of their electrostatic association with the QD surface and their relative insolubility in methanol, but are easily displaced by tighter-binding ligands for both Cd and Se and even desorb spontaneously upon dilution of the QDs in nonpolar solvents.37 The degree of Cdenrichment in the near-stoichiometric CdSe QDs (Cd:Se = 1.2:1) corresponds to 90% of one monolayer of excess Cd2+ with associated octadecylphosphonate ligands adsorbed to a stoichiometric CdSe core. Phosphorus NMR spectra of the near-stoichiometric QDs indicate that TOP and/or TOPSe are present in these samples even after purification (see Figure S6 in the Supporting Information); TOP binds to any exposed Se2− on the QD surface. Both Cd-enriched and nearly stoichiometric CdSe QDs reduce TCNQ, but the yield and primary mechanism with which they do so depends on the surface chemistry of the QDs. Reduction of TCNQ by an electron from the core of a CdSe QD with R = 1.3 nm is energetically uphill by ∼0.6 eV, as calculated from the ionization potential of an electron from the core of the QD (−5.54 eV versus vacuum38) and the reduction potential for TCNQ (−4.9 eV versus vacuum.5) We have shown previously, for complexes of TCNQ and PbS QDs, that, instead, surface chalcogenides are the source of electrons for this reduction.5 We note that none of the molecular precursors used to synthesize the QDs or ligands of the QDs, except for TOPSe, reduce TCNQ on their own (see Figures S7 and S8 in the Supporting Information). We recorded the spectrum of TCNQ reduced by TOPSe (Figures 1 and 3), and none of the spectra of TCNQ reduced by the QDs resembles this spectrum. We therefore know that the chalcogenides on the surfaces of the QDs are the electron donors for TCNQ within these samples. The characteristic peaks for TCNQ1− at 850, 749, and 686 nm and the CT absorption at ∼620 nm are present in the electronic spectra of TCNQ with both types of QDs (Figure 1). From our analysis of the spectra of molecular chalcogenides, we know that both types of QDs therefore reduce TCNQ by both a discrete 1-electron redox process, where the resulting ion pair is bound only through electrostatic interactions (mechanism (i)), and by formation of a bound CT complex, where the donor and acceptor share electrons and the geometry of TCNQ is potentially distorted (mechanism (ii)). The ratio of absorbances at 850 and 620 nm in each spectrum is one measure of the relative contributions of mechanisms (i) and (ii). This ratio is 1.6:1 for the Cd-enriched QDs and 2.6:1 for the nearly stoichiometric QDs, so we conclude that the Cd-

complex that allows for the spontaneous transfer of one full electron to TCNQ without distortion of its geometry. Another illustrative example of an LMCT complex of TCNQ with a chalcogenide is the trioctylphosphine selenide (TOPSe)TCNQ complex. The electronic and vibrational spectra of this mixture indicate that TOPSe reduces TCNQ to a noninteger oxidation state exclusively; there are no signals corresponding to undistorted TCNQ1− or TCNQ2− in these spectra. The electronic spectrum of the mixture contains one broad peak at 555 nm (Figure 1); the integrated area of this CT peak is a factor of ∼12 greater than that of the CT peak at 620 nm in the spectrum of TMS2S complexed with TCNQ, so TOPSe is a more effective reductant of TCNQ than is TMS2S. The vibrational spectrum contains four broad peaks at 2120, 2130, 2172, and 2197 cm−1, Figure 3. The presence of four, and only four, broad distinct peaks indicates (i) that TOPSe interacts asymmetrically with TCNQ, (ii) that only one type of binding geometry of the TOPSe-TCNQ complex is present in the sample, and (iii) that this complex has a large degree of conformational freedom. This vibrational spectrum most closely resembles the simulated IR spectrum of the Se0.7‑TCNQ1.3‑ complex (1.3 electrons transferred to TCNQ), where the distance between TCNQ and Se2− is constrained to be 5 Å, Figure 2E. The indication that Se2− is further from TCNQ in the TCNQ-TOPSe CT complex than is S2− within the TMS2STCNQ CT complex is reasonable since the ionic radius of Se2− is larger than S2− by ∼8%. Both the electronic spectrum of the TOPSe-TCNQ complex and the lower oxidation potential of Se2− compared to S2−, however, suggest that, counter to our simulation’s prediction, TOPSe donates more charge to TCNQ than does TMS2S. We therefore believe that the simulated vibrational spectra are more sensitive to donor−acceptor distance and degree of distortion of TCNQ than to the amount of charge transferred. Nonetheless, these simulations tell us the approximate binding geometry of chalcogenideTCNQ complexes, and reveal the qualitative relationship between the geometry of TCNQ within these complexes and the spacing and ordering of peaks within their vibrational spectra. We now use these relationships to analyze the spectra of mixtures of TCNQ with CdSe QDs. CT Complexes of TCNQ with CdSe QDs. We synthesized two types of CdSe QDs, highly Cd-enriched QDs and nearly stoichiometric QDs, using TOPO of different purities in the reaction mixtures.31,32 TOPO of 90% purity contains several phosphonate impurities, including octylphosphonic acid (OPA), that are known to govern the growth of the QDs and produce QDs that are Cd-enriched. By contrast, TOPO of 99% purity contains little or no OPA or other alkylphosphonic acid impurities and syntheses that employ 99% TOPO yield QDs which have a Cd:Se ratio of 1:1−1.2:1.31,32 Briefly, we synthesized Cd-enriched CdSe QDs by heating dry mixtures of cadmium stearate (CdSt2), HDA, and 90% TOPO under N2(g) to 320 °C and injecting TOPSe.33 We synthesized stoichiometric CdSe QDs by heating dry mixtures of CdO, octadecylphosphonate, and 99% TOPO under N2(g) to 395 °C and injecting TOPSe.34 We obtain QDs with radius R < 1.5 nm with both syntheses by immediately removing the heating mantle and cooling the reaction mixtures under N2(g) after the injection of TOPSe. The Supporting Information contains the full details of both syntheses. The band-edge energies of the Cd-enriched and nearly stoichiometric CdSe QDs are 524 nm (R = 1.30 nm) and 511 18267

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result in the distortion of the TCNQ molecule from planarity that is indicated by the IR spectra we measure (see the Supporting Information). (ii) It has been shown repeatedly that neutral TCNQ does not readily coordinate to Cd2+ or other divalent cations;8,9,40 rather, TCNQ must be prereduced chemically (i.e., added as a lithium salt)9,18,21,40 or the reaction driven electrochemically41 to induce coordination. (iii) The Cd2+ on the surfaces of CdSe QDs is ligated by either octylphosphonate or octadecylphosphonate, which TCNQ does not displace. Figures S7−S9 in the Supporting Information show that we saw no evidence of binding of TCNQ to Cd2+ in mixtures of TCNQ with either of these cadmium salts. We believe that TCNQ molecules form CT complexes with surface Se ions much more readily in mixtures with Cdenriched QDs than in mixtures with nearly stoichiometric QDs because selenides are more accessible in Cd-enriched QDs. The ligands for selenides in Cd-enriched QDs are Cd-phosphonate molecules that are in exchange on and off the surface of the QD.31 When these Cd-phosphonate molecules transiently desorb, TCNQ has access to the underlying stoichiometric CdSe surface which, since it served as an adsorption site for Cd2+, is most likely terminated with Se ions. In nearstoichiometric QDs, the collection of phosphonate impurities in 90%-pure TOPO are not present in the reaction mixture, so multilayers of Cd-phosphonate do not form.31 Instead, 31P NMR spectra in Figure S6 show any exposed selenides on the surfaces of these QDs are passivated by TOP that survives the purification procedure because it is bound to the QD surface (no TOP survives the purification of Cd-enriched QDs).35 Neutral TCNQ should not displace TOP from Se2− (see a comparison of adsorption constants to Se2− in the Supporting Information), so the chalcogenide is much less accessible in nearly stoichiometric QDs than in Cd-enriched QDs, and therefore the Cd-enriched CdSe QDs are much more effective reductants of TCNQ than are the nearly stochiometric QDs.

enriched QDs are more likely to form strongly bound CT complexes with TCNQ than are the nearly stoichiometric QDs. Furthermore, the sum of the integrated peaks of reduced TCNQ (of both types) is a factor of 7.4 greater for the Cdenriched QDs than for the nearly stoichiometric QDs, which means the total yield of reduction of TCNQ is 7.4 times higher for the Cd-enriched QDs than for the nearly stoichiometric QDs. We fit the vibrational spectrum of the mixture of Cdenriched QDs and TCNQ, Figure 3, to six peaks (excluding the TCNQ0 peak at 2224 cm−1). Although, given our signal-tonoise ratio, we are confident that all six peaks are spectroscopically distinguishable, many of the peaks are broad and are probably convolutions of at least two peaks. For instance, the electronic spectrum of this mixture (Figure 1) tells us that undistorted TCNQ1− is present in this sample, but the corresponding peaks at 2181 and 2151 cm−1 are obscured by the peaks corresponding to the CT complex. The presence of more than four peaks (none of which correspond to TCNQ1−) in the spectrum of the Cd-enriched QDs with TCNQ indicates that at least two geometries of the QD-TCNQ CT complex are present. The 2171 and 2163 cm−1 peaks in the experimental spectrum match the two highest energy peaks in the simulated spectrum of Se0.7‑TCNQ1.3‑ where the distance between the selenide and TCNQ molecule is constrained to be 5 Å. The 2204 and 2196 cm−1 peaks in the experimental spectrum match the two highest energy peaks in the simulated spectrum of Se0.5‑TCNQ1.5‑ where the distance between the selenide and TCNQ molecule is 2 Å. The 2-Å separation may indicate the formation of a carbon−selenium bond, which, in organoselenium molecules is 1.9 Å.30,39 The broad peaks at 2135 and 2107 cm−1 in the experimental spectrum likely encompass the two sets of lower-energy peaks of both geometries. Cd-enriched QDs therefore reduce TCNQ in three ways: (i) by donating exactly one electron to TCNQ without detectably distorting its geometry (we assume such an interaction would lead to an electrostatically bound ion pair), (ii) by forming a weakly bound CT complex with TCNQ in which TCNQ accepts slightly more than one electron and is very slightly distorted from planarity, and (iii) by forming a more strongly bound CT complex with TCNQ in which TCNQ accepts more than one electron and is more dramatically distorted from planarity. As mentioned previously, the overall yield of TCNQ reduction in mixtures with nearly stoichiometric QDs is a factor of 7.4 lower than in mixtures with Cd-enriched QDs. The adsorption constant for TCNQ on nearly stoichiometric QDs is clearly small, as the FT-IR spectrum of a mixture of TCNQ with these QDs shows that, after mixing for 3 days, the only form of reduced TCNQ is undistorted TCNQ1− (see Supporting Information, Figure S2), either freely diffusing or bound electrostatically to the QD surface. After 7 days, some of the TCNQ molecules form CT complexes with the QDs that result in the electronic spectrum in Figure 1 and the vibrational spectrum in Figure 3, where both TCNQ1− and a small amount of reduced TCNQ within a CT complex are present. The CT complex may form upon adsorption of the TCNQ to surface selenides or upon complexation of TCNQ with TOPSe that, over a period of 7 days, desorbs from the QD surface. Within all of the binding geometries for both types of QDs, we believe that TCNQ interacts directly with surface Se ions, rather than coordinating to a surface Cd ion through the nitrile group and interacting indirectly with Se, for three reasons: (i) Our simulations show that the indirect interaction does not

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CONCLUSIONS We used the electronic and vibrational signatures of TCNQ reduced by molecular reducing agents, sulfides, selenides, and metallocenes to guide our interpretation of the absorbance and FT-IR spectra of TCNQ reduced by Cd-enriched and nearly stoichiometric CdSe QDs. We also calculated the theoretical IR spectra of complexes of these molecular reducing agents bound to TCNQ and found that the number and frequency-spacing of the nitrile modes are affected by the amount of charge transferred to TCNQ and the degree of distortion of TCNQ from planarity upon CT. We conclude that three species of reduced TCNQ are present in mixtures with highly Cdenriched CdSe QDs: TCNQ1−, which forms as a result of a discrete electron transfer from Se2− on the surfaces of the QDs without formation of a bound CT complex, and two forms of more highly reduced TCNQ (TCNQx‑, where 1 < x < 2) within CT complexes of TCNQ and Se2−. The amount of charge on TCNQ, and the degree of its distortion from planarity, within these CT complexes probably depends on the steric accessibility of and the electron density on the reducing selenide−that is, whether it is coordinated to one, two or three underlying cadmium anions, how far it protrudes from the underling lattice, and whether it is located within a planar facet, or at an edge or corner between facets. The reduction of TCNQ by nearly stoichiometric CdSe QDs mainly occurs through discrete reduction to TCNQ1− without distortion of 18268

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Organic Monolayers on the Surfaces of Colloidal PbS Quantum Dots. J. Phys. Chem. C 2013, 117, 15849−15857. (4) Malicki, M.; Knowles, K. E.; Weiss, E. A. Gating of Hole Transfer from Photoexcited PbS Quantum Dots to Aminoferrocene by the Ligand Shell of the Dots. Chem. Commun. (Cambridge, U. K.) 2013, 49, 4400−4402. (5) Knowles, K. E.; Malicki, M.; Parameswaran, R.; Cass, L. C.; Weiss, E. A. Spontaneous Multielectron Transfer from the Surfaces of PbS Quantum Dots to Tetracyanoquinodimethane. J. Am. Chem. Soc. 2013, 135, 7264−7271. (6) Zhang, J. Z.; Ellis, A. B. Adsorption of TCNQ Derivatives onto the Surface of Cadmium Selenide Single-Crystals - Quenching of Semiconductor Photoluminescence by a Family of Strong Pi-Acids. J. Phys. Chem. 1992, 96, 2700−2704. (7) Boivin, M.; Lamarre, S.; Tessier, J.; Lecavalier, M. E.; Najari, A.; Dufour-Beausejour, S.; Dussault, E. B.; Collin, P.; Allen, C. N. Morphological Control of Hybrid Polymer-Quantum Dot Solar Cells with Electron Acceptor Ligands. Appl. Phys. Lett. 2012, 100, 033302− 033305. (8) Zehe, A.; Martinez, J. G. R. Molecular Wires in Future Nanoelectronics Systems. Theochem-J. Mol. Struct. 2004, 709, 215− 222. (9) Zhao, H.; Heintz, R. A.; Ouyang, X.; Dunbar, K. R.; Campana, C. F.; Rogers, R. D. Spectroscopic, Thermal, and Magnetic Properties of Metal/TCNQ Network Polymers with Extensive Supramolecular Interactions between Layers. Chem. Mater. 1999, 11, 736−746. (10) Gross-Lannert, R.; Kaim, W.; Olbrich-Deussner, B. Electron Delocalization in Molecule-Bridged Polymetallic Systems - Unique Neutral Complexes of TCNE or TCNQ and up to 4 Organometallic Fragments (C5R5)(CO)2Mn. Inorg. Chem. 1990, 29, 5046−5053. (11) Silva, M. D. D.; Diogenes, L. C. N.; Lopes, L. G. D.; Moreira, I. D.; de Carvalho, I. M. M. Synthesis and Physical Properties of Ruthenium Bipyridine Complexes with TCNQ and TCNE Ligands. Polyhedron 2009, 28, 661−664. (12) de Caro, D.; Souque, M.; Faulmann, C.; Coppel, Y.; Valade, L.; Fraxedas, J.; Vendier, O.; Courtade, F. Colloidal Solutions of Organic Conductive Nanoparticles. Langmuir 2013, 29, 8983−8988. (13) Andersson, M. P.; Uvdal, P. New Scale Factors for Harmonic Vibrational Frequencies Using the B3LYP Density Functional Method with the Triple-Xi Basis Set 6-311+G(d,p). J. Phys. Chem. A 2005, 109, 2937−2941. (14) Glendening, E. D.; J. K. B, Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2001. (15) Kim, Y. H.; Jung, S. D.; Chung, M. A.; Song, K. D.; Cho, D. W. Photoinduced Charge-Transfer Association of Tetracyanoquinodimethane with Aminobiphenyls. Bull. Korean Chem. Soc. 2008, 29, 948−952. (16) Makowski, M.; Pawlikowski, M. T. Absorption, Resonance, and near-Resonance Raman Studies of the Tetracyanoquinodimethane Neutral and Its Monoanion in Terms of Density Functional Theory and Complete Active Space Self-Consistent Field Methods. Int. J. Quantum Chem. 2006, 106, 1736−1748. (17) Bellec, V.; De Backer, M. G.; Levillain, E.; Sauvage, F. X.; Sombret, B.; Wartelle, C. In Situ Time-Resolved FTIR Spectroelectrochemistry: Study of the Reduction of TCNQ. Electrochem. Commun. 2001, 3, 483−488. (18) Fadly, M.; Elgandoor, M. A.; Sawaby, A. Solid-State Properties and Molecular-Structure of Some Divalent nd10 Cation-TCNQ Salts. J. Mater. Sci. 1992, 27, 1235−1239. (19) Rabaa, H.; Taubert, S.; Sundholm, D. Computational Studies of the Electronic Absorption Spectrum of (2,2 ′;6 ′,2 ″-Terpyridine)Pt(II)-OH 7,7,8,8-Tetracyanoquinodimethane Complex. J. Phys. Chem. A 2013, 117, 12363−12373. (20) Hartmann, H.; Sarkar, B.; Kaim, W.; Fiedler, J. Electron Transfer Reactions of (C5R5)(2)(CO)(2)Ti (R = H or Me) with TCNE or TCNQ - Spectroelectrochemical Assignment of Metal and Ligand Oxidation States in (C5Me5)2(CO)Ti(TCNX) (2−/−/O/+). J. Organomet. Chem. 2003, 687, 100−107.

TCNQ’s geometry. Cd-enriched CdSe QDs reduce a factor of 7.4 more TCNQ molecules per QD than do nearly stoichiometric CdSe QDs because, in Cd-enriched QDs, the reducing selenides are protected only by weakly bound Cdphosphonate complexes, while, in the nearly stoichiometric QDs, the selenides are passivated with tight-binding TOP. The result that TCNQ has multiple types of interactions with the surfaces of QDs is not surprising given the known structural and chemical heterogeneity of these surfaces. Importantly, however, we have identified the species that act as the reductants for TCNQ, surface selenides, and shown that we can control the range of charge transferred to TCNQ by controlling access to these selenides through selection of surface ligands. In turn, the electronic and vibrational spectra of reduced TCNQ within mixtures with QDs are reporters of the surface chemistry of the QD. We have documented the trends in these spectra as a function of the magnitude of the chalcogenide-TCNQ interaction and shown that the evolution of these spectra depend not only on the amount of charge transferred to TCNQ but also on the symmetry of the distribution of this charge and the degree of distortion from planarity of the molecule. As with our study of adsorption of methyl viologens on Cdand S-enriched surfaces of CdS QDs,37 the driving force for strong adsorption of TCNQ onto the surfaces of CdSe appears to be electrostatic attraction, more specifically, the interaction of the most electropositive region of TCNQ with the electronrich chalcogenide on the QD surface. We aim to further test this strategy with other neutral and charged small molecules and QDs with controlled surface stoichiometry to develop a general approach to maximizing the adsorption constant for small molecules on nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and additional results including Figures S1−S9, Scheme S1, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Army Research Office via the Presidential Early Career Award for Scientists and Engineers (PECASE) to E.A.W. ICP-AES and NMR data were acquired at Northwestern University’s Integrated Molecular Structure Education and Research Center (IMSERC). FT-IR data were acquired at Northwestern University Atomic and Nanoscale Characterization Experimental (NU-ANCE) Center.

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