Probing the Influence of Size and Composition on the Photoelectron

May 23, 2012 - *E-mail: [email protected]. .... Marc-Oliver Winghart , Ji-Ping Yang , Michael Kühn , Andreas-Neil Unterreiner , Thomas J. A. Wolf ...
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Probing the Influence of Size and Composition on the Photoelectron Spectra of Cadmium Chalcogenide Cluster Dianions⊥ Katerina Matheis,† Andreas Eichhöfer,‡ Florian Weigend,†,‡ Oli T. Ehrler,†,§ Oliver Hampe,*,†,‡ and Manfred M. Kappes†,‡,§ †

Institute of Physical Chemistry, ‡Institute of Nanotechnology, and §Center for Functional Nanostructures, Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany S Supporting Information *

ABSTRACT: We have synthesized a series of compounds comprising the halide-free cadmium chalcogenide cluster dianions [Cd 8Se(SePh) 16]2−, [Cd17Se4(SePh)28]2−, [Cd17S4(SPh)28]2−, and [Cd17Se4(SPh)28]2− to study their size- and composition-dependent electronic properties free of matrix effects. Toward this end, photoelectron spectra of the isolated dianions electrosprayed from solution were recorded at several detachment photon energies. Together with quantum chemical calculations, these data reveal a systematic correlation between electronic properties such as electron affinities or repulsive Coulomb barriers and the composition/size of these semiconducting cluster complexes. We infer that the excess negative charges are localized at the apical sites of these near-tetrahedral molecules.

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INTRODUCTION One of the aims in research1−5 into semiconductor nanoparticles is to understand and tailor their electro-optical properties toward potential applications, such as in biolabeling and imaging.6,7 In particular, the light-emitting properties (such as light amplification/lasing8,9) have been shown to depend not only on particle size but also on the chemical nature of the surfactant/surface termination and the extent of charging.10 The latter is typically inferred (or influenced) by chemical electron transfer11 or electrochemical charge injection.12 As many potential applications in nanotechnology require a level of understanding still not yet available for the nanoobjects in question,13 one must apply techniques that can extend condensed-phase studies to the gas phase and can thus allow isolated nanoparticles to be investigated in the absence of particle−particle interactions or other “matrix” effects. In this context, electrospray ionization mass spectrometry (ESI-MS) of charged clusters has rapidly developed into a powerful approach that can provide for routine gas-phase isolation.14−20 The next level of work in this area now calls for laserspectroscopic probes of isolated cluster ions to more systematically interrogate electronic properties such as charge (de)localization. Cadmium chalcogenide clusters and nanoparticles have been studied to a great extent as model systems for semiconducting materials.21,22 Advances in devoloping new synthetic routes have made it possible to obtain monodisperse semiconducting clusters with defined chemical stoichiometries and structures such as [Cd32E14(E′R)36(L)4] (with E, E′ = Se or S; L = neutral ligand),23−26 [Cd54S32(SPh)48(H2O)4]4−,27 and [Cd54S28(SPh)52(dmf)4].28 For gas-phase studies by way of © 2012 American Chemical Society

electrospray ionization, it is desirable to start with charged species in solution. However, known heteroleptic cadmium chalcogenide cluster anions such as [Cd4(SPh)6X4]2− (X = Cl, Br, I),18 [Cd8Se(SePh)12Cl4]2−,19 and [Cd32Se14(SePh)36X4]4− (X = Cl, I, Br)29 can readily undergo ligand exchange in solution, leading to a distribution with respect to the number of X− and SPh−/SePh− ligands as evidenced by mass spectrometry. To circumvent possible ambiguities with respect to isomeric structures in condensed- and gas-phase species, it therefore appears desirable to produce complexes with a single type of peripheral ligand. Known examples include [Cd8S(SePh)16]2−,30 [Cd17Se4(SePh)28]2−,31 and [Cd17S4(SPh)28]2−.32 In this article, we report on the targeted synthesis, singlecrystal X-ray diffraction analysis, and gas-phase UV photoelectron spectroscopy of four halide-free cadmium chalcogenide cluster dianions, namely, [Cd8Se(SePh)16]2−, [Cd17Se4(SePh)28]2−, [Cd17S4(SPh)28]2−, and [Cd17Se4(SPh)28]2−. The aim of this study was both to improve the understanding of the valence electronic structure of these cluster anions and to probe their photodetachment mechanism based on a systematic variation in composition (here, Se versus S) combined with precise structural information. Electrospray ionization mass spectrometry coupled with photoelectron spectroscopy in conjunction with a quantum chemical description is the approach taken in this work.33−35 Received: March 30, 2012 Revised: May 23, 2012 Published: May 23, 2012 13800

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filtered and layered with diethyl ether through slow diffusion by evaporation through a connected flask to give colorless crystals of 3. [Li(thf)4]2[Cd17S4(SPh)28] (4). LiN(SiMe3)2 [25 mg (0.15 mmol)] and Cd[N(SiMe3)2]2 [0.51 mL (1.25 mmol)] were dissolved in 25 mL of thf, and 0.066 mL (0.311 mmol) of S(SiMe3)2 was added to give a colorless solution. Addition of HSPh [0.27 mL (2.64 mmol)] resulted in the immediate formation of a white suspension that dissolved upon further stirring. The clear reaction solution was then heated for 2 h at 60 °C in an oil bath. Layering with diethyl ether through slow diffusion by evaporation through a connected flask gave colorless needles of 4 with a total yield of 42% (0.782 g). Elemental analysis suggested that the thf molecules in 4 were almost completely removed upon drying under a vacuum. C200H204Cd17Li2O8S32 (5686.6): calcd C 42.2, H 3.6, S 18.0. C168H140Cd17Li2S32 (5109.7): calcd C 39.5, H 2.8, S 20.1; found C 40.2, H 3.2, S 19.4%. Crystallography. Crystals suitable for single-crystal X-ray diffraction were taken directly from the reaction solution of the compound and then selected in perfluoroalkylether oil. Singlecrystal X-ray diffraction data of 1 were collected using Simonochromated synchrotron radiation (λ = 0.80 Å) on a STOE IPDS II (imaging plate diffraction system) and for 4 on a Bruker SMART APEX sealed-tube chemical crystallography system at the ANKA synchrotron source in Karlsruhe, Germany. Single-crystal X-ray diffraction data of 2 and 3 were collected with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) on a STOE IPDS II (Table S1 in the Supporting Information). Raw intensity data for 1−3 were collected and treated with the STOE X-Area software, version 1.27, and those for 4 were collected and treated with the Bruker Apex 2 software. Data for all compounds were corrected for Lorentz and polarization effects. Based on an optimized crystal description, numerical absorption corrections were applied for 1 and 2,39 whereas data for 4 were corrected semiempirically 40 for absorption. The structures were solved with the direct methods program SHELXS of the SHELXTL PC suite of programs41 and were refined with the full-matrix least-squares program SHELXL.41 Atomic form factors for λ = 0.80000 Å (15.510 keV) were obtained by the method of Brennan and Cowan42 as implemented in the X-ray Anomalous Scattering WebTool of the University of Washington Biomolecular Structure Center.43 Molecular diagrams were prepared using SCHAKAL 97.44 All Cd, Se, S, and C atoms of the cluster anions were refined with anisotropic displacement parameters, with the exceptions mentioned below, whereas C and O atoms of the solvents dme and thf showed large thermal ellipsoids and were refined isotropically. H atoms were calculated in fixed positions, with the exception of those of the solvents dme and thf in 1 and 4. In 1−4, some of the phenyl rings were refined isotropically with an optimized geometry with a disorder model. Similarity constraints (for thermal parameters) were used as necessary within the disordered groups. Crystals of 1, although measured at a synchrotron source, diffract only up to 48° (2θ). The R(int) value increases from 7.5% for the inner reflections (2− 22°) to 36.7% for reflections from 47.2° to 49.2°, and the mean intensity simultaneously decreases significantly. Only one [Li(dme)3]+ counterion could be localized in the difference Fourier map. High thermal displacement parameters of the isotropically refined C and O atoms of the dme molecules indicated strong disorder that could not be fully modeled on

EXPERIMENTAL AND THEORETICAL METHODS Syntheses. Standard Schlenk techniques were employed throughout the syntheses using a double-manifold vacuum line with high-purity dry nitrogen (99.9994%) and a MBraun glovebox with high-purity dry argon (99.9990%). The solvents tetrahydrofuran (thf), 1,2-dimethoxyethane (dme), and diethyl ether were dried over sodium benzophenone and distilled under nitrogen. Anhydrous CH3CN (acetonitrile) (H2O < 0.001%) obtained from Aldrich was degassed, freshly distilled, and stored over molecular sieves under nitrogen. PhSH and LiN(SiMe3)2 were purchased from Aldrich. S(SiMe3)2, Se(SiMe3)2,36 PhSeH,37 and Cd{N(Si(CH3)3}238 were prepared according to literature procedures. [Li(dme)3]2[Cd8Se(SePh)16] (1). LiN(SiMe3)2 [83.3 mg (0.49 mmol)] and Cd[N(SiMe3)2]2 [0.81 mL (1.99 mmol)] were dissolved in 75 mL of dme, and 0.045 mL (0.199 mmol) of Se(SiMe3)2 was added at −70 °C to give a colorless solution. Addition of HSePh [0.48 mL (4.48 mmol)] resulted in the immediate formation of a white suspension. Gradual warming of the solution in the cool bath with stirring overnight afforded a colorless solution with a white precipitate. Subsequent addition, three times, of 2 mL of CH3CN every 30 min resulted in the formation of a clear solution, which was then layered with diethyl ether through slow diffusion by evaporation through a connected flask to give colorless crystals of 1 with a total yield of 78% (0.782 g). C120H140Cd8Li2O12Se17 (4029.91): calcd C 35.8, H 3.5; found C 35.7, H 3.5%. [Li(dme)3]2[Cd17Se4(SePh)28] (2). LiN(SiMe3)2 [31 mg (0.185 mmol)] and Cd[N(SiMe3)2]2 [0.5 mL (1.23 mmol)] were dissolved in 50 mL of dme, and 0.069 mL (0.308 mmol) of Se(SiMe3)2 was added at −70 °C to give a colorless solution. Addition of HSePh [0.28 mL (2.64 mmol)] resulted in the immediate formation of a white suspension. Gradual warming of the solution in the cool bath with vigorous stirring overnight afforded a pale yellow solution with a white precipitate. Subsequent addition, six times, of 2 mL of CH3CN every 30 min resulted in the formation of a nearly clear solution, which was filtered and then carefully layered with diethyl ether through slow diffusion by evaporation through a connected flask to give colorless crystals of 2 with a total yield of 49% (0.25 g). C192H200Cd17Li2O12Se32 (7151.3): calcd C 32.3, H 2.8; found C 32.1, H 2.8%. It should be mentioned that, several times, layering resulted only in the formation of an oily product. However, repeated evaporation of the solvent and redissolution in dme/CH3CN (slightly varying ratios) usually produced at least a white microcrystalline precipitate of 2. [Li(dme)3]2[Cd17Se4(SPh)28] (3). LiN(SiMe3)2 [31 mg (0.185 mmol)] and Cd[N(SiMe3)2]2 [0.5 mL (1.23 mmol)] were dissolved in 30 mL of dme, and 0.061 mL (0.295 mmol) of Se(SiMe3)2 was added at −70 °C to give a colorless solution. Addition of HSPh [0.28 mL (2.64 mmol)] resulted in the immediate formation of a white suspension. Gradual warming of the solution in the cool bath to room temperature with vigorous stirring overnight afforded an almost colorless solution with a white microcrystalline precipitate of 3, which was filtered and washed with dme to give a total yield of 68% (0.287 g). C192H200Cd17Li2O12S4Se28 (5838.1): calcd C 39.5, H 3.5, S 15.4; found C 39.5, H 3.4, S 15.4%. Crystals suitable for X-ray analysis were grown by subsequent addition, five times, of 2 mL of CH3CN every 30 min to the reaction solution before filtering, which resulted in the formation of a nearly clear solution; this solution was then 13801

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applied (with default parameters) to simulate the counterions. Contour plots were generated with gOpenMol.54

the basis of the data, resulting in slightly reduced occupancy factors for C97, C100, and O6. C and O atoms of the solvent thf molecules and of the Li atom in 4 also showed large thermal ellipsoids and were refined isotropically with half-occupancy. Geometrical constraints were used as necessary. The empirical formulas and formula weights for the X-ray structure analyses of 1 and 4 thus differ from the expected ones. Supplementary crystallographic data for this article were deposited with the Cambridge Crystallographic Data Centre as files CCDC-838586 (1), -838587 (2), -838588 (3), and -838589 (4). These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax (international) +44−1223/336-033; e-mail [email protected]]. X-ray powder diffraction (XRD) patterns were measured for 2 as a crystalline powder and for 1, 3, and 4 as suspensions of crystals in the mother liquor on a STOE STADI P diffractometer (Cu Kα1 radiation, germanium monochromator, Debye−Scherrer geometry) in sealed glass capillaries. The theoretical powder diffraction patterns were calculated on the basis of the atom coordinates obtained from single-crystal X-ray analysis using the program package STOE WinXPOW.45 Condensed-Phase Measurements. C, H, S elemental analyses were performed on an Elementar vario Micro cube instrument. UV−vis absorption spectra of cluster molecules in solution were measured on a Varian Cary 500 spectrophotometer in quartz cuvettes and in the solid state as mulls in nujol between quartz plates inside a Labsphere integrating sphere. Electrospray Ionization (ESI) Fourier Transform (FT) Mass Spectrometry and Photoelectron Spectroscopy. The experimental setups of the Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer46 and electrospray ionization photoelectron spectroscopy (ESI-PES) instrument47 were recently described in detail. Briefly, for both gas-phase instruments, solutions of 1−5 in 1,2-dichloroethane or acetonitrile were electrosprayed to generate the negative-ion species. Negative-ion mass spectra were externally calibrated to within 10 ppm using cesium iodide cluster anions, (CsI)nI−. For UV photodetachment, various laser wavelengths were used: the third, fourth, and fifth harmonics of a Nd:YAG laser (i.e., 355 nm/3.50 eV, 266 nm/4.66 eV, 213 nm/5.84 eV) and the output of an ArF laser (193 nm, 6.24 eV). Typical laser fluences used were 15, 10, and 1 mJ/cm2 for the third, fourth, and fifth harmonics of the Nd:YAG laser. The kinetic energies of the photodetached electrons were measured with a “magnetic bottle” time-of-flight (TOF) electron spectrometer. Typically, spectra were accumulated at a repetition rate of 10 Hz. The conversion from TOF to electron kinetic energy was calibrated against known spectra of iodide (I−). Electron binding energies were then obtained using Einstein's photoelectric equation. DFT Computations. Calculations were carried out with the program package TURBOMOLE48 using the BP-86 functional49,50 with def2-SV(P) bases51 and corresponding auxiliary bases52 for the Coulomb part. Employment of larger def2TZVP bases51 did not significantly change results. Structural parameters were optimized at this level for the dianions. Vertical detachment energies were calculated as the difference between total energies of mono- and dianions with the structural parameters of the dianions. For the calculation of excited states, time-dependent density functional theory (TDDFT, BP-86) was used. The COSMO model53 was

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RESULTS AND DISCUSSION Synthesis, Crystal Structure, and Condensed-Phase Characterization. To avoid ligand mismatch and to generate “pure” cadmium chalcogenide cluster complexes, we developed a new synthetic routeoutlined in Scheme 1that involves Scheme 1

reacting the bis(trimethylsilyl)amido precursor compounds of cadmium and lithium with appropriate equivalents of PhE′H and E(SiMe3)2 to yield [Li(dme)3]2[Cd8Se(SePh)16] (1), [Li(dme)3]2[Cd17E4(E′Ph)28] [E = Se, E′ = Se (2); E = Se, E′ = S (3)], and [Li(thf)4]2[Cd17S4(SPh)28] (4). For comparative studies, the cluster compound (NBu4)2[Cd8Se(SePh)12Cl4] (5) was synthesized as described recently.19 Compound 1 crystallizes in the tetragonal space group P4̅21c; its crystallographic details are summarized in Table S1 of the Supporting Information. A representation of the molecular structure of the cluster anion [Cd8Se(SePh)16]2− (1a) is given in Figure 1, showing that a central selenium atom is tetrahedrally surrounded by four “inner” cadmium atoms. Each of these four inner cadmium atoms is bound through three μ2-SePh− ligands to one of the four “outer” cadmium atoms, which again form a tetrahedron. In addition, each of the four outer cadmium atoms is bound to a terminal selenolato ligand, which leads to a slightly distorted tetrahedral coordination sphere for all cadmium atoms. Compounds 2 and 3 both crystallize in the orthorhombic space group Ccca; 4 crystallizes in Pbcn. (See Table S1 in the Supporting Information for details.) The anionic clusters [Cd17E4(E′Ph)28]2− [E = E′ = Se (2a); E = Se, E′ = S (3a); E = E′ = S (4a)] all exhibit pseudotetrahedral symmetry (as shown in Figure 2) with a central cadmium atom. This central Cd cadmium atom is bound to four μ4-E2− atoms, forming an 13802

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Figure 1. Molecular structure of the cluster dianion [Cd8Se(SePh)16]2− (1a) in the crystal of 1. (C and H atoms are omitted for clarity.) Figure 3. UV−vis absorption spectra of [Li(dme)3]2[Cd8Se(SePh)16] (1), [Li(dme)3]2[Cd17E4(E′Ph)28] (2 E, E′ = Se; 3 E = Se, E′ = S), and [Li(thf)4]2[Cd17S4(SPh)28] (4) in CH3CN. The maximum energy of the first optically allowed transition (Efirst trans) was derived from a Gauss fit to the corresponding lowest-energy absorption onset.

the maximum of a Gaussian fit to the low-energy onset in each spectrum. Two different systematic behaviors were observed for these values: (i) For a given cluster size, namely, [Cd17E4(E′Ph)28] 2−, the maximum shows a blue shift from 341 nm (3.6 eV) in 2 (E, E′ = Se) to 292 nm (4.2 eV) in 4 (E, E′ = S) upon substitution of the chalcogenide atom positions from pure selenium in 2 through the mixed species 3 (E = Se, E′ = S) to the cadmium sulfur cluster 4. This directly reflects the behavior of the band gaps of the corresponding bulk semiconductor materials CdSe and CdS and has been observed before, for example, for [Cd10E4(E′Ph)12(PR 3)4] (E, E′ = Te, Se, S) cluster molecules.55,56 (ii) For a given composition of the atoms E and E′, the first optically allowed transition shifts by 0.5 eV to higher energy as the size of the cluster molecule is reduced from [Cd17Se4(SePh)28]2− in 2 to [Cd8Se(SePh)16]2− in 1. This behavior of a blue shift of the first electronic absorption with reduced cluster size has been interpreted in terms of quantum confinement, for example, for larger CdSe nanocrystals 57 as well as for much smaller CdSe cluster molecules.58,59 We note that these trends were also reproduced in the solid-state spectra taken as mulls of the crystalline powders (shown in Figure S3 in the Supporting Information) with red shifts of about 10−30 nm when contrasted to the solution-phase spectra, as has been observed before for similar systems.19,60,61 The molar extinction coefficients ε of the lowest-energy absorption features amount to ( 1−1.5) × 106 M−1 cm−1, suggesting that the first transitions are strongly allowed. To shed some light on the origin of these electronic transitions and on associated cluster-size effects, the lowest 50 dipole-allowed excitations of 1a, 2a, and 4a were calculated at the TDDFT(BP86/COSMO) level and are shown in Figure 4 (broadened by Gaussians with 0.1 eV full width at halfmaximum). In all cases, the lowest-energy excitations (at ∼2.45 eV for 1a and 2.05 eV for 2a and 4a), which dominantly involve the four near-degenerate highest occupied molecular orbitals (HOMOs) located at the outermost E atoms (see below), are of very low oscillator strength and therefore probably not observable experimentally. Transitions at higher energies (above 2.8 eV for 1a and 2.3 eV for 2a and 4a) have higher

Figure 2. Molecular structure of the cluster dianion [Cd17Se4(SePh)28]2− (2a) in the crystal of 2. (C and H atoms are omitted for clarity.)

inner tetrahedral “CdSe4” unit, which is, in turn, surrounded by a shell of 24 μ2-bridging EPh− ligands and 16 cadmium atoms. The four cadmium atoms at the apex positions are finally coordinated by one terminal μ1-EPh− group. As a whole, the tetrahedral cluster skeleton consists of four fused adamantoid cages in the center with four additional barrelanoid cages (bold in Figure 2) at the corners, showing characteristics of both the sphalerite and wurzite types of structures of bulk CdSe and CdS, respectively. All 17 cadmium atoms have a distorted tetrahedral coordination. The ranges and mean values of selected bond lengths in 1− 4 are listed in Table S2 (Supporting Information). The values and trends are similar to those reported for the related anionic cluster molecules [Cd8Se(SePh)12Cl4]2−, 19 [Cd8S(SePh)16]2−, 30 [Cd17S4(SPh)28]2−,32 and [Cd17Se4(SePh) 28]2−.31 The measured powder XRD patterns of 1−4 are in good agreement with simulations based on the single-crystal data (Figures S1 and S2 in the Supporting Information), which confirms the crystalline purity. To characterize the electronic structures and optical transitions of the cluster molecules in 1−4, UV−vis absorption spectra were recorded at room temperature in CH3CN, as shown in Figure 3. To parametrize the energy of the first optically allowed electronic transition (E first trans), we considered 13803

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Figure 5. Negative-ion electrospray-FT-ICR mass spectrum of [Li(dme)3]2[Cd8Se(SePh)16] (1) sprayed from a solution in dry acetonitrile. Inset: Contrast between the experimentally observed dianion peak A (centered around m/z = 1738) and the calculated isotopomere structure for [Cd8Se(SePh)16]2− (1a). Note also the formation of a “dimer” cluster trianion (peak B around m/z = 2264.8) corresponding to [(Cd8Se(SePh)15)2SePh]3− and the singly charged species [Cd8Se(SePh)15]− (peak C around m/z = 3319).

Figure 4. Calculated [TDDFT(BP-86)/SVP/COSMO] lowest 50 excitations for [Cd8Se(SePh)16]2− (1a), [Cd17Se4(SePh)28] 2− (2a), and [Cd17S4(SPh)28] 2− (4a). Lines are broadened with Gaussian functions to simulate the experimental spectrum.

oscillator strengths and involve lower-lying occupied molecular orbitals formed by the 3p (4p) orbitals of the inner S (Se) atoms (with only small contributions from the corresponding atomic orbitals from the outer SPh/SePh groups) and unoccupied orbitals with dominant contributions from the Cd 5s orbitals. The dependence of the excitation energies on the cluster size is given by the difference in TDDFT predictions for 1a and 2a and amounts to ∼0.4 eV for the lowest-energy transitions and ∼0.5 eV for the next higher ones. This matches well with the experimental data, which displays a shift of the lowest-energy onset by 0.5 eV. The calculations support the assignment of this transition to a charge-transfer excitation from the S 3p/Se 4p atomic orbitals (leading to the valence band in more extended systems) to electronic states that essentially stem from the Cd 5s atomic orbitals59,62with the caveat that TDDFT-computed absolute energy values for such charge-transfer transitions are known to be less reliable. Analogous results were also found for similar model systems.63 The charge-transfer character of these excitations possibly explains the remaining discrepancy between measurement and calculation upon the exchange of selenium for sulfur. Gas-Phase Studies: Electrospray Mass Spectrometry and Photoelectron Spectroscopy. [Cd8Se(SePh)16]2− (1a) and [Cd8Se(SePh)12Cl4]2− (5a). The anions that were detected from electrosprayed solutions of 1 are shown in the mass spectrum of Figure 5.64 The dominant ion signal around m/z = 1738 (labeled A) is assigned to the doubly charged anion 1a. This species was subsequently probed by photoelectron spectroscopy under ion source conditions virtually identical to those used for Figure 5. Peak B in the mass spectrum belongs to a triply charged dimer with stoichiometry [(Cd8Se(SePh)15)2SePh]3−, which constitutes an interesting species in itself. Finally, peak C can be attributed to the singly charged [Cd8Se(SePh)15]−. The species for peaks B and C were not studied further. Figure 6 displays the photoelectron spectra of 1a taken at detachment wavelengths of 266, 213, and 193 nm. In all photoelectron spectra shown, the photon energy used for detachment is included as a vertical solid line, indicating the

Figure 6. Photoelectron spectra of [Cd8Se(SePh)16]2− (1a) at detachment laser wavelengths of 266 nm (4.66 eV), 213 nm (4.84 eV), and 193 nm (6.42 eV).

highest binding energy for which electron detachment was possible. Linear extrapolation of the low-energy onset of the spectrum yields an adiabatic second electron affinity of 3.01 ± 0.07 eV (AEA2, shown as vertical dashed lines in Figure 6). From a superposition/fit of several Gaussian functions (not shown here for clarity) to the discernible bands in the PE spectra, a ground-state feature can be assigned that peaks at a vertical detachment energy (VDE2) of 3.29 ± 0.07 eV. All electron affinity and detachment energy values are summarized in Table 1. At higher binding energies, all three photoelectron spectra exhibit a cutoff that appears to be steepest for the 266-nm spectrum and somewhat smoother in the 213- and 193-nm 13804

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Table 1. Adiabatic Second Electron Affnities (AEA2s), Vertical Detachment Energies (VDE2s), and Repulsive Coulomb Barriers (RCBs) for All Dianions Probed by Photoelectron Spectroscopya cluster compound 2−

[Cd8Se(SePh)12Cl4] [Cd8Se(SePh)16]2− [Cd17Se4(SePh)28]2− [Cd17Se4(SPh)28]2− [Cd17S4(SPh)28]2−

5a 1a 2a 3a 4a

AEA2 (eV)

VDE2 (eV)

RCB (eV)

VDE2b (eV)

3.12(7) 3.01(7) 3.51(10) 3.60(10) 3.63(10)

3.51(7) 3.29(7) 3.88(10) 4.00(10) 4.03(10)

0.50(10) 0.50(10) 0.40(10) 0.30(10) 0.30(10)

3.46 3.37 3.86 3.97 4.00

a Experimental errors in the last digits given in parentheses. bCalculated VDE2 values obtained from the mean values of the energies of the four degenerate HOMOs at the DFT level of the dianion, shifted by 2.32 eV; see text for details.

spectra. For example, the band peaking at a binding energy of ∼4.25 eV in the 213-nm spectrum does not appear in the 266nm spectrum. This phenomenon is very specific for multiply charged anions and is a clear indication of the prevailing repulsive Coulomb barrier (RCB). The photoelectric equation BE = hν − Ekin [relating the measured kinetic energy of the detached electron (Ekin) with its binding energy (BE) and the frequency of the impinging radiation (ν)] still holds, but with the additional boundary condition hν > BE + RCB. The gap between this cutoff and the photodetachment laser energy (indicated as double-headed arrows in all PE spectra shown) can therefore be used to parametrize the height of the RCB. For 1a, this gap amounts to 0.5 ± 0.1 eV.65 For comparison, Figure 7 displays the PE spectra of 5a at three detachment photon

Figure 8. Negative-ion electrospray FT-ICR mass spectrum obtained from a solution of [Li(dme)3]2[Cd17Se4(SPh)28] (3) in dry CH3CN. Inset: Experimental and simulated isotopomere distributions for the cluster dianion [Cd17Se4(SPh)28]2− (3a).

electrospray mass spectrum of 3, with the cluster dianion 3a appearing around m/z = 2642 as virtually the only ion signal.66 The photoelectron spectra of the three Cd17 chalcogenide cluster dianions recorded at photon wavelengths of 266 nm (4.66 eV) and 213 nm (5.84 eV) are reproduced in Figure 9. Vertical detachment energies derived from the spectra amount to 4.03 ± 0.1, 4.00 ± 0.1, and 3.88 ± 0.1 eV for 2a−4a, respectively. These values, together with the adiabatic electron

Figure 7. Photoelectron spectra of [Cd8Se(SePh)12Cl4]2− (5a) at detachment laser wavelengths of 266 nm (4.66 eV), 213 nm (4.84 eV), and 193 nm (6.42 eV).

energies. Here again, a ground-state feature can be assigned, from which one can derive a vertical detachment energy (as the maximum of the Gaussian function) of VDE2 = 3.51 ± 0.07 eV and an adiabatic second electron affinity of AEA2 = 3.12 ± 0.07 eV. Also in these spectra, the double-headed arrows indicate the energy gaps between the high-energy cutoffs (of detachable electrons) and the respective detachment photon energies. For this cluster dianion, the RCB amounts to 0.5 ± 0.1 eV. [Cd17Se4(SePh)28]2− (2a), [Cd17Se4(SPh)28]2− (3a), and [Cd17S4(SPh)28]2− (4a). Figure 8 displays a negative-ion

Figure 9. Photoelectron spectra of [Cd17Se4(SePh)28]2− (2a), [Cd17Se4(SPh)28]2− (3a), and [Cd17S4(SPh)28]2− (4a) at different detachment photon energies (indicated as vertical lines in the individual panels). 13805

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species, using the optimized structural parameters of the dianion in both cases). Finally, we also attempted to rationalize the significantly larger VDE2 values for the Cd17 species compared to the Cd8 species by a simple point-charge model. Calculation of [Cd8E(E′Ph)16]2− and [Cd17E4(E′Ph)28]2− (E = S, Se; E′ = S, Se) yielded very similar electronic structures for the frontier orbitals of all eight possible variants. The highest occupied orbitals are a set of four essentially degenerate doubly occupied orbitals. The energies of the highest and lowest of these four orbitals differ by less than 0.01 eV for each of the Cd17 species and by less than 0.09 eV for the Cd8 species. They are energetically well-separated from the next-lower-lying HOMO − 4 (by more than 0.5 eV) and also from the lowest unoccupied molecular orbital (LUMO). The corresponding HOMO−LUMO gap amounts to ∼2.2 and ∼1.7 eV for Cd8 and Cd17, respectively. In all cases, the four (quasi-) degenerate HOMOs are linear combinations of nonbonding p orbitals located at the four S/Se atoms at the apex positions of the tetrahedral structures; contour plots of the four HOMOs of 1a are shown in Figure 11 as an example.

affinities, are summarized in Table 1. The heights of the Coulomb barriers (RCBs) from the cutoff on the high-energy side of the spectra (vide supra) are generally smaller than those for the two Cd8 cluster dianions: 0.30 ± 0.1 eV for 4a/3a and 0.40 ± 0.1 eV for 2a. To aid the interpretation of the PE spectra in terms of excess charge localization and to relate this property to the initial and final states for detachment of the most weakly bound electron (giving rise to the highest electron kinetic energies), we first focus on the observed differences for the two cluster sizes and the differences among the chemical compositions of the three variants [Cd17E4(E′Ph)28]2− (2a E = E′ = Se; 3a E = Se, E′ = S; and 4a E = E′ = S), hereafter denoted as Cd17. For facile comparison, Figure 10 highlights the threshold regions of the

Figure 10. Threshold regions of the 266-nm photoelectron spectra of the cluster dianions [Cd8Se(SePh)16]2− (1a) and [Cd17E4(E′Ph)28]2− (2a E, E′ = Se; 3a E = Se, E′ = S; 4a E, E′ = S). The data are vertically scaled to give the same photoelectron yield at the maximum of the band.

PE spectra of the various cluster dianions all acquired at a laser wavelength of 266 nm. (1) From Figure 9, one immediately deduces a shift by about 0.5 eV in the onset (and therefore in the second electron affinities) when going from the smaller cluster anion 1a to the larger homologue 2a. (2) For the three Cd17 cluster variants, the lowest-energy band of the PE spectra is modified upon variation of the inner and/or outer chalcogenide atoms (i.e., E versus E′). When all outer SePh− groups are replaced by SPh− groups, AEA2 changes by about 0.1 eV (see traces 2a and 3a in Figure 9 and values for AEA2 in Table 1). In contrast, further substitution of the four inner selenium atoms by additional sulfur atoms leaves the photoelectron spectrum of 4a virtually unchanged. This “chemical-substition”-induced trend already suggests that the excess charges that are responsible for the lowest-energy band in the photoelectron spectra reside on the surface of the clusters, most likely on the terminal E’Ph groups. DFT(BP86) calculations were carried out to model VDE2, that is, the energy needed to detach one electron from the dianionic species. For this purpose, the energies and spatial distributions of the highest occupied molecular orbitals of the dianionic species were investigated. Then, we studied the influences of the cluster size and E/E′ ratio on the orbital energies, as well as on VDE2 (calculated as the difference between the total energies of dianionic and monoanionic

Figure 11. Computed amplitudes at the BP86 level of the four (quasi) degenerate HOMOs of [Cd8Se(SePh)16]2− (1a) (shown as contours in gray/white, yellow/purple, red/green, and blue/orange at a value of 0.06 au). H atoms and unimportant C atoms are omitted for clarity.

The energies of the HOMOs are mainly determined by the cluster size. For the larger cluster, they are ca. 0.5 eV lower than for the smaller one (see Table 2). In contrast, the effect of varying E′ from S to Se is much smaller (the HOMO energy is lower for E′ = S by ca. 0.1 eV). The influence of varying E is even smaller but still observable in computation and experiment. These findings for the relative energies of the four HOMOs at the DFT level, eB̅ P, are in line with the experimental trends for VDE2. They also agree with the trends in orbital energies of the four HOMOs as calculated at the Hartree−Fock level, eH̅ F (the latter can be related to the VDE to first order by Koopmans' theorem). eB̅ P values can also be used as part of the input to obtain a more accurate prediction of VDE2 by forming the difference between the total energies of the monoanion and dianion, ΔEBP. These data, namely, −eB̅ P, −eH̅ F, and ΔEBP values, are contrasted with experimental values in Table 2. Note that, in contrast to relative numbers, which are quite similar for the four sets (−eB̅ P, −eH̅ F, ΔEBP, experiment), the absolute values are (not unexpectedly given their disparate reference states) very different. Therefore, the most pragmatic way to 13806

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decrease of the repulsive Coulomb interaction of the excess charge with increasing cluster size. The composition-dependent shift in VDE2 (∼0.1 eV) for the Cd17 clusters with S occupying the apical position compared to Se might well have its origin in the slightly higher electron affinity of S (2.07 eV) compared to Se (2.02 eV), which would be reflected in variations in K. From our experiments and calculations of dianionic and monoanionic species, it is evident that the second excess electron is bound to the cluster (i.e., there is a bound electronic ground state). Nevertheless, the first excess electron leads to a repulsive electrostatic potential for charging the cluster further with a second excess electron, resulting in the so-called Coulomb barrier (RCB) at intermediate electron−monoanion separations. Going beyond the simple electrostatic picture used in the previous section for describing VDE2, one can approximate the RCB height by taking the second excess electron as a point charge and calculating the interaction energy with the monoanion electron density while allowing for relaxation. Specifically, for 2a, we set the point charge (1− elementary charge) at various positions along paths A−D and calculated the repulsion energy self-consistently; see Figure 12.

Table 2. Negatives of the Averages of the Energies of the Four (Quasi-) Degenerate HOMOs (−e ̅ in eV) for [Cd17E4(E′Ph)28]2− and [Cd8E(E′Ph)16]2− at the DFT(BP86) and HF Levels and BP86 Energy Differences of Mono- and Dianionic Species, ΔEBPa n(Cd) shift 17 17 17 17 8 8 8 8

E Se S Se S Se S Se S

E′

−eB̅ P

ΔEBP

Se Se S S Se Se S S

2.32 3.86 3.87 3.97 4.00 3.38 3.37 3.34 3.34

1.39 3.83 3.85 3.94 3.97 3.45 3.43 3.51 3.52

−eH̅ F −0.56 3.87 3.90 3.94 3.99 3.40 3.37 3.42 3.44

expt 3.88 4.00 4.03 3.29

Data shifted by a constant value (line “shift”) to minimize differences from experimental data.

a

correlate experimental and calculated VDEs is to shift each of the calculated data sets by a constant that minimizes the differences from experiment. Shifting the data sets for −eB̅ P, − eH̅ F, and ΔEBP by 2.32, −0.56, and 1.39 eV, respectively, leads to the smallest differences from the experimental data, as indicated by the right-hand side of Table 2. Under these conditions, the average differences amount to 0.02, 0.03, and 0.05 eV for the three data sets −eB̅ P, −eH̅ F, and ΔEBP, respectively, which brings the predictions to within the absolute experimental uncertainty (see Table 1). It is interesting to note that the larger VDE2 values for the larger clusters can, to first order, be rationalized by a simple electrostatic point-charge model. As noted above, the excess negative charge is located mainly at the four E atoms of the terminal ligands, specifically, 0.5 electrons per atom for the dianions and 0.25 per atom for the monoanions. These four atoms form a tetrahedron, with edge length R amounting to ca. 21 au for the Cd8 species and 29 au for the Cd17 species. VDE2 can be estimated as the difference between the total energies between dianion and monoanion. In a simple picture, these total energies are determined (relative to that of a neutral tetrahedron) by favorable charge localization at the apexes minus unfavorable Coulomb repulsion. By summing over all six repulsive two-body Coulomb interactions that occur among the fractional localized charges (under the additional assumption of a dielectric constant of ε = 1), the VDE2 values can then be determined in atomic units (au) as

Figure 12. Interaction energies of a point charge (1− elementary charge) with [Cd17Se4(SePh)28]− (2a) along paths A−D, computed self-consistently for 15 points per path, versus distance of the point charge from the cluster center. A8 and B8 (dashed lines) denote the curves for the respective paths for [Cd8Se(SePh)16]− (1a), A8,Cl and C8,Cl (dotted lines) denote those for [Cd8Se(SePh)12Cl4]− (5a). Also shown (1/d) is the interaction energy of two bare negative elementary charges.

⎛ 0.252 0.52 ⎞ 9 VDE 2 = 6⎜ − +K ⎟+K=− R ⎠ 8R ⎝ R

Path A points to one of the μ2-coordinated Se atoms and is in the plane spanned by the Se−C and Se−Cd bonds, pathe B and C point to H atoms of the phenyl rings, and path D points to the center of a phenyl ring. A8 and B8 are the corresponding paths for 1a; and A8,Cl and C8,Cl are the corresponding paths for 5a. For each of these paths, the energy is plotted versus the distance from the cluster center. Additionally, we show the interaction energy of two bare point charges (1/d) for comparison. As expected, at long ranges, the shapes of all curves are similar; differences between paths become relevant for distances of less than ca. 2 nm from the cluster center (or ca. 1 nm from the cluster surface). Here, the increase in energy with decreasing distance is slower than 1/d, and at a point specific for each path, the slope becomes negative. The resulting barrier heights for 2a are lowest for paths B and C (ca. 0.7 eV): For these paths, the “surface” of the cluster is reached

For the purposes of argument, we assume that the difference in the intrinsic apex charging energies of singly and doubly charged species, K, is independent of cluster composition and size. Correspondingly, the difference in VDEs of two differentsized tetrahedral species is VDE 2(Cd17) − VDE 2(Cd8) = −

9⎛ 1 1 ⎞ ⎜ ⎟ − 8 ⎝ 29 21 ⎠

= 0.0148 au = 0.402 eV

Thus, this simple model predicts the VDE value of the Cd17 species to be 0.4 eV lower than that of Cd8, which is close to the experimentally observed difference of 0.5 eV. The increase of the VDE with cluster size is thus to a large part due to a 13807

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(4) (Table S1). Ranges and mean values for selected bond distances (pm) for 1−4 (Table S2). Measured (black) and simulated (gray) X-ray powder patterns for 1−4 (Figures S1 and S2). UV−vis absorption spectra of 1−4 as mulls in nujol (Figure S3). Negative-ion electrospray-FT-ICR mass spectra of [N(nC4H9)4]2[Cd8Se(SePh)12Cl4] (5), 2, and 4 (Figures S4− S6). This material is available free of charge via the Internet at http://pubs.acs.org.

at a comparatively large distance from the center for which the electron density at the H atoms is also quite low. Path D, in contrast, is less favorable because of high electron density above the phenyl ring. The path directly approaching the “final” position of the electron at the Se atom, path A, shows a higher barrier (ca. 0.9 eV), because these atoms are closer to the cluster center. The respective barriers for smaller 1a are somewhat higher: 0.8 eV for the approach through the phenyl ring (B8) and ca. 1.25 eV for the direct approach to Se. It is thus probable that the electron does not enter/leave its final position in the molecule (at Se) in a direct manner along path A or path A8, respectively, but instead that this passage is indirect through a phenyl ring. This is even more evident for 5a: For the path through a phenyl ring, C8,Cl, the barrier height is about the same as that for 1a, 0.8 eV. The path with direct access to the Cl atom, A8,Cl, with a barrier height of 1.4 eV, is highly improbable. It is of interest to contrast the lowest-barrier pathways (B and B8) with the outcome of the photoelectron spectroscopic results for the Coulomb barrier. Again, there is good agreement between relative numbers for the two different cluster sizes: 0.7 and 0.8 eV (theory) for 2a and 1a, respectively, versus 0.4 and 0.5 eV from experiment. Additionally, for 1a and 5a, the calculations yield virtually identical values for “indirect” electron-transfer paths (consistent with experiment), that is, for the approach of the electron to the cluster through a phenyl ring and not directly to the final position at the Se/Cl atoms.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Deutsche Forschungsgemeinschaft. A.E. thanks Prof. Dr. A.K. Powell for generous support and E. Tröster for her assistance in the practical work. Infrastructure support from the Helmholtz Association (POF NanoMikro) is also appreciated.

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DEDICATION This work is dedicated to Prof. Dr. Dieter Fenske on the occasion of his 70th birthday.

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CONCLUSIONS We report a study of the photoelectron spectra of a variety of ligand-stabilized cadmium chalcogenide cluster dianions isolated in the gas phase versus their size and composition. Toward this end, several variants of two cadmium chalcogenide cluster sizes comprising eight and seventeen cadmium atoms were synthesized. Size effects were studied by contrasting [Cd8Se(SePh)16]2− and [Cd17Se4(SePh)28]2−. Over this size range, the (second) electron affinity increases by ∼0.5 eV, whereas the repulsive Coulomb barrier decreases slightly by ∼0.1 eV. Composition dependence was probed with respect to the type of chalcogenide atom by studying variants of [Cd17E4(E′Ph)28]2− (with E, E′ = Se or S). By using appropriate synthetic strategies, we prepared variants in which S/Se atoms in the outer section of the cluster were exchanged. The photoelectron spectra of these cluster dianions showed small but significant composition-dependent differences: ∼0.1 eV for both detachment energy and Coulomb barrier. We interpreted these results in terms of DFT-based quantum chemical calculations, which (after appropriate reference shifting) provide a reasonable description of electron affinities as well as repulsive Coulomb barrier heights. In contrast, upon chemically exchanging chalcogenide atoms (Se/S) in the inner section of the Cd17 cluster dianions, observations and calculations showed no differences, within error. These findings suggest a picture in which the electron photodetachment process (as determined by the electron binding energy and Coulomb barrier) is governed to a large extent by electrostatic repulsion between the excess negative charge density localized at the four apical positions of the tetrahedral clusters.

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AUTHOR INFORMATION

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for [Li(dme)3]2[Cd8Se(SePh)16] (1), [Li(dme)3]2[Cd17Se4(SePh)28] (2), [Li(dme)3]2[Cd17Se4(SPh)28] (3), and [Li(thf)4]2[Cd17S4(SPh)28] 13808

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