Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals

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Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency Peng Cui, Patrick K. Tamukong, and Svetlana V Kilina ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00350 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Nano Materials

Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency Peng Cui, Patrick K. Tamukong, and Svetlana Kilina* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States Corresponding Author: E-Mail: [email protected] Abstract Semiconductor quantum dots (QDs) functionalized by metal-organic dyes show a great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rates – key processes governing the efficiency of energy conversion – are strongly affected by the QD-dye interactions, insights on which is challenging to obtain experimentally. We use density functional theory (DFT) and constrained-DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD-dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD-dye interaction in terms of their binding conformations and the donoracceptor electronic couplings. While the QD-dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an electron acceptor is observed in structures bonded via the iso-cyanate ligands. Such strong electronic couplings also are responsible for significant stabilization of the dye’s occupied orbitals deep inside in the valence band of the QD making the hole transfer from the photoexcited QD to the dye thermodynamically unfavorable in structures bound via iso-cyanates. Our results suggest that the most probable binding conformations are those occurring via two carboxylate linkers, which exhibit very weak electronic couplings contributing to the electron transfer from the photoexcited CdSe QD to the N719 dye, but provide the most favorable conditions for the hole transfer. Overall, our computational work provides an insightful view about the surface chemistry of CdSe regarding the donor-acceptor interaction, energy level alignment and charge transfer between CdSe and

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dye molecule, which can guide the rational design of QD-based materials for energy conversion applications.

Keyword: Charge transfer, nanocomposites, quantum dots, metal-organic complexes, donor-acceptor electronic couplings, electronic structure, and first principle calculations.

Introduction New photocatalytic applications have been recently proposed based on semiconductor quantum dots (QDs) and various organic and metal-organic dyes functionalizing their surface.1 The dye molecules covalently attached to the surface provide an interface for efficient charge separation and electron transfer to or from the QD, which changes the oxidation state of the dye forcing it to work as an oxidizer. On the other hand, usage of nanomaterials provides abilities for fabricating ultrathin, light, and flexible photocatalytic materials2 with size-tunable electronic and optical properties.3 Part of the inspiration for these materials stems from the Gratzel cells,4 which consists of TiO2 surface covered by Ru(II)-bipyridine dye (Ru(II)bpy) playing the role of the photosensitizer. In the appropriately modified Ru(II)bpy, the oxidizing equivalents stored in the complex following the electron transfer into TiO2 surface can be used to drive catalytic oxidation of various molecules.5 However, the Ru(II)bpy, as well as its derivatives and other organic dyes and metal-organic complexes, typically have a relatively narrow absorption window and are not stable against oxidation under photocatalytic conditions. An alternative approach suggests substitution of the bulk semiconductor by nanocomposites – e.g., arrays of QDs – and using them, rather than a dye, as a light-harvesting and charge generating component that can provide a rapid transfer of the photoexcited

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hole from the QD to the dye and convert the dye into the oxidizer.6,7 The QD used as a photosensitizer offers better photo-stability and size-tunable broader absorption. Moreover, a unique ability of strongly confined QDs to generate multiple excited electron-hole pairs from a single photon, so-called carrier multiplication8-9, 10 promises an improved performance for water photo-oxidation, which requires four electrons for creating one O2 molecule. Interestingly, a similar idea of using QDs as a photosensitizer instead of molecular dyes has been recently suggested for CdSe QD-sensitized solar cells,11 while the surface dye is used as an electron donor activating the hole transfer from the QD to the dye to prevent the electron−hole recombination and enhance photocurrent.12,13 Thus, QD/dye assemblies have a great promise to serve as a key element for both solar-to-electrical and solar-to-chemical energy conversion. Despite this promise, the efficiency of QD/dyes composites as photo-oxidizer agents or photovoltaic materials is still relatively low.14,

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This problem is likely due to

high sensitivity of electronic and optical properties of QDs to the surface effects, including surface defects and surface passivation by ligands, which is very challenging to control. The current updates on these issues can be found in recent review papers both from experimental2,16-20 and theoretical perspectives.21,

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Due to these problems, the

exact mechanisms and ways of controlling the charge transfer from the photoexcited QD to the dye or in reverse order are still under debates.23,24 On the other hand, it is well understood that a delicate interplay between processes taking place at the QD surface and the interface governs the efficiency of light conversion to electrical or chemical energy in QD-based devices.21 Thus, it was shown that the transfer rate of the photoexcited carrier from the QD to the dye, as well as from the dye to the QD, varies depending on the QD

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size,6 QD’s chemical composition,15, 25 the complex type,26 its binding structure,7, 27 and the overall interaction with the QD surface28, 29 and solvent.30 Two main criteria crucial for the direction and the rate of charge transfer via the QD/dye interface are the optimal energy alignment of electronic states associated with the dye and the QD, which act either as an electron donor or an acceptor, and the degree of the electronic coupling between donor’s and acceptor’s orbitals. The energy alignment between the QD’s and the dye’s states governs thermodynamic conditions for the charge transfer direction in these systems (either an electron or a hole), while the electronic couplings between the QD and the dye determine the efficiency and rates of charge transfer. Both parameters depend on the interaction between the QD surface and the dye. In particular, it was experimentally shown that a specific type of a molecular linker – phosphonic acids,31 carboxylic acids,31 silianes,32 phenyldithiocarbamates,33 and their derivatives29, 34, 35, 36 – mediating adsorption of a metal complex and organic dyes to a semiconductor QD surface is responsible for the complicated QD-dye interaction, which influences both the energy alignment and electronic couplings between the donor and acceptor states in the QD/dye composites. Not only the type of the anchoring group, but also its protonation has been shown to have a strong effect on the electronic properties and conditions for electron transfer37,

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of Ru(II) complexes adsorbed on TiO2

substrates39-40 and CdSe QDs.7, 27 In addition to the type of a linker group, the dye adsorption geometry on the semiconductor surface has a strong influence on the electronic structure of the composite, which is associated with creation of the dipolar fields of different magnitudes and orientations due to different adsorption modes.41 Taking into account that the intrinsic

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dipole moments of nanostructures are much higher than those of their bulk counterparts, the effect of different binding configurations on the QD vs. dye orbital alignment and their couplings is expected to be even more pronounced in QD/dye composites. Whereas most studies in these lines have mainly focused on the binding geometry of various dyes on TiO2 surfaces,40, 42-46 much less investigations have been focused on CdSe QDs.7, 17, 19, 27

and ZnS capped CdSe QDs.47, 48 Our previous calculations using density functional

theory (DFT) and time-dependent DFT (TDDFT)28 have demonstrated that modification of the ligands of the Ru(II)bpy) dye, specifically in their charge, is instrumental in adjustments of the energy levels associated with the CdSe QD and the dye in nanocomposites. These studies, however, pinpoint the origin of the effect only qualitatively given a limited number of considered structures. The details of mechanism of the interaction between the CdSe QD and Ru(II) dyes are yet to be fully investigated. Here we report the systematic DFT-based studies to investigate a degree of sensitivity of the QD/dye composite energetic alignment and related QD-dye electronic couplings to binding configurations of Ru(II)-polypyridyl complex, namely N719 dye, to the surface of the 1.5 nm CdSe QD. Using constrained DFT, we represent the QD-dye electronic couplings by the charge transfer integrals based on donor-accepter diabatic wavefunction overlaps, which are proportional to the charge transfer rate according to Fermi’s golden rule.49 Our calculations evidence that the strength of the binding between the QD and the dye governed by their linking group attachment does not provide large donor-acceptor electronic couplings. A change in the binding geometry of the N719 dye at the CdSe QD via carboxylate or iso-cyanate linkers has the direct consequences on the energetic alignment of the dye’s orbitals with respect to the QD’s states, as well as the

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donor-accepter electronic couplings, which could either foster or inhibit both the direction and rates of the charge transfer between the QD and the dye.

Methodology The initial geometry of the Cd33Se33 QD/N719 dye nanocomposite was optimized using Gaussian-09 software50 at the density functional theory (DFT) level of theory using the B3LYP functional with LANL2DZ/6-31g* mixed basis set, wherein LANL2DZ was used for Cd, Se and Ru atoms while the Pople 6-31g* basis described the rest of elements (C, H, S and O). Employing an effective core potentials (ECPs) basis such as LANL2DZ for transition metals, while using all-electron basis sets for all other non-transition-metal atoms has become a common practice in computations of transition metal containing systems.51 ECPs are parameterized to implicitly account for scalar relativistic (SR) effects, which increase its practical application to transition metals. Our group has been intensively used this choice of mixed basis sets coupled with the hybrid-GGA functionals such as B3LYP and PBE1PBE to ligated CdSe and PbSe QDs,52-55 as well as to different Ru(II)-

56, 57

and Ir(III)-complexes58-59, showing good agreement with their experimental

absorption and emission spectra. Therefore, we apply the same approach to study the Cd33Se33/N719 nanocomposites. After geometry optimization, the projected density of states (PDOS) is obtained using the following equation: =

1

√

ℎ

− − , 1

where is n-th ground state Kohn-Sham (KS) energy, ℎ is the percentage of the KS orbital projected on the specified fragment (either the QD or the dye), and σ is the

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broadening of the Gaussian function, chosen of 100 meV to include thermal fluctuations of ions. For the total DOS, ℎ = 1. Eqn. (1) was also used for plotting the absorption spectra, wherein and ℎ are excitation energy and oscillation strength, respectively, and is an empirical parameter for line broadening set at 100 meV, which is close to the experimental line width of Ru(II)polypyridine complexes,57 as well to the first absorption peak in 1.7 nm CdSe nanocrystals60 at room temperatures. The excitation energy and oscillation strength in Eqn. (1) are obtained by solving a linear response equation based on the Casida formalism61 in the frames of time dependent DFT (TDDFT) calculations, as implemented in Gaussian-09 software.50 Totally 150 transitions were calculated using LANL2DZ/631g* mixed basis set and long-range corrected CAM-B3LYP functional. While it is known that CAM-B3LYP functional typically overestimates the energy of optical transitions in molecular systems,21 including Ru(II)-polybipyridines,28 it more accurately describes the charge transfer (CT) states. Despite a uniform blue-shift of the spectra, the qualitative structure and profile of the spectra obtained by CAM-B3LYP is not significantly affected, as was already shown in previous simulations of ligated CdSe QDs.55 Because the CT states are of the main interest in our simulations, the CAMB3LYP functional has been chosen for excited state calculations. In general, accurate calculations of structural and spectroscopic properties of molecular systems containing transition metals require a delicate balance between describing static and dynamical correlations, which is most naturally done through a multiconfigurational wave function approach based on self-consistent-field (SCF) formalism.62 These methods include multi-configuration complete-active-space SCF

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(CASSCF)63 or the variants restricted-active-space SCF (RASSCF)64 or generalizedactive-space SCF (GASSCF),65 the configuration interaction (CI) and its multi-reference extension (MR-CI),66 the 2nd order perturbation theory based on CASSCF (CASPT2)67 and its multireference extension MS-CASPT2.68 There are also new hybrid methods, combining DFT at short-range and multiconfigurational wave functions or perturbative approaches at long-range.69 These methods have been applied to different transition metal complexes,70-71 as well as to small QDs of a few atoms in size.72,

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However, high

computational cost restricts applications of these methods to large nanocomposites. Therefore, optimal balance between accuracy and computational effort characterizes the DFT based calculations with its excited-state extension TD-DFT, which is the standard practice in simulating excited state properties of closed-shell metal-organic complexes and QDs due to its favorable computational cost, usability, and reasonable accuracy.21, 22, 74

Since the d-elements in our system have even number of electrons and the closed shell

structure (d10 for Cd and d6 for Ru(II) with the full occupation of three lowest t2g states in the octahedral field of the ligand) and also do not poses high symmetry that lifts degeneracy of electronic levels, TD-DFT, is expected to be an efficient approach balancing accuracy and computational cost in simulating these systems. This expectation is in line with good agreement between experimental and TD-DFT calculated spectra of pristine 1.7 nm CdSe QDs60 and Ru(II)-bipyridines,57 as well as other Ru(II)-

56, 57

and

Ir(III)-complexes.58-59 To permit an accurate orbital interpretation of an optical transition, we computed the natural transition orbitals (NTOs)75 using Gaussian09 software.50 NTOs redistribute the hole (occupied) and electron (unoccupied) electron density obtained from the

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transition density matrix, while presenting many-body excited states in terms of an electron-hole pair. The obtained NTOs were visualized and plotted using Gaussview,76 with the contour isovalue set as 0.02 e-/au3. This value is typically used for the electrondensity plots of different types of molecular systems, since it provides good resolution for the shape of molecular orbitals. Solvation effects are included using the polarizable continuum medium model (CPCM) implemented in Gaussian09 software packages.50 Acetonitrile is used for incorporating the solvation effect in the geometry optimization, while the electronic structure, and excited state calculations of N719 dye functionalized Cd33Se33 QDs are calculated in less polar solvent dichloromethane with preserved vacuum geometry. This approach was used because a strong polar solvent has the tendency to disfavor the conditions for the hole transfer from the excited QD to the dye via strong stabilization of dye occupied orbitals with respect to the QD’s VB edge.28 To study reasonable conditions allowing for the hole transfer to occur, we focus our electronic structure calculations only on a weakly polar solvent as an intermediate case, where the dye orbitals are not too much stabilized versus the QD states providing favorable thermodynamic conditions for the hole transfer, while the binding geometries are also not strongly perturbed by the solvent. Note, we hold the vacuum geometry for the electronic structure calculations in dichloromethane to eliminate the effect of geometry reorganization due to the solvent, which allows us to distinguish the solvent effect on the electronic structure. This allows for a better comparison of DFT results with those of the constrained DFT that are conducted in vacuum.

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The electronic couplings are calculated from first principles using constrained density functional theory (CDFT) within the formulation proposed by Wu and Van Voorhis,77 as implemented in NWChem-6.5 software package.78 CDFT includes the effects of polarization and orbital relaxation through the evaluation of many-body wavefunctions describing localized charges. These wavefunctions (diabatic states) are obtained by modifying the Kohn−Sham (KS) equations to include a spatially dependent potential, whose strength is varied selfconsistently to satisfy the constraint of the desired number of electrons localized within the volume of a given fragment, which is either the QD (the electron donor) or the dye (the electron acceptor) in our case. Thus, the CDFT potential is just the sum of the usual KS potential and the constraining potential, which is defined via the selfconsistent calculation.79, 80 CDFT and the electronic donor-acceptor couplings from the Marcus theory can be connected by recalling that two diabatic states, the initial and final , are sufficient to characterize the kinetics of the electrontransfer process. These many-body diabatic states are represented via the Slater determinants built from Kohn-Sham orbitals obtained from CDFT. In our calculations, we consider only the case of QD-to-dye electron transfer (et), where the final constrained state, , assumes a hole on the QD and an extra electron on the dye, as illustrated in Fig. 1. The initial state, , is an excited QD’s state approximated as an non-interacting electron-hole pair corresponding to the Slater determinant of non-constrained KS orbitals obtained by promotion of an electron from the highest occupied orbital (HOMO) to the lowest unoccupied orbital (LUMO) with both originated from the QD. While such a single-particle approximation is not valid for molecular systems due to strong electron-hole couplings, it is reasonable for representing

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the lowest energy excitations in QDs, as discussed elsewhere.21,

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We also model a

competing process of the electron-hole recombination (re), illustrated in Fig. 1, for which the initial state, , is represented as a constrained diabatic state with a hole localized at the QD and an electron located at the dye, while the final state, , is a ground state computed by non-constrained DFT. To get these initial (for re) or final (for et) charge localized diabatic states, the geometry is optimized under the constrained conditions. Afterwards, the initial and final wavefunctions are imported into the charge transfer module81 to obtain the charge transfer couplings by solving the secular equation: − −

− = 0 2 −

Where E is the eigenvalue and = | , = || , = || , = || with H being an electronic KS Hamiltonian. The same functional and basis set that have been applied in the geometry optimization and electronic structure calculations (B3LYP and LANL2DZ/6-31g*) are used for CDFT and electronic coupling calculations for et and re processes in vacuum.

Results and Discussion Binding Geometries of Cd33Se33/N719 Composites. The N719 dye was attached to the most reactive QD’s surface28 with 2-coordinated cadmiums through different linkage groups – namely, carboxylates and/or iso-cyanates – leading to four distinct classes of binding configurations, as depicted in Scheme 1. Three-dimensional views of a few representative configurations in each class are illustrated in Fig. 2, with all structures shown in the Supplemental Information (SI), Fig. S1. In class I, the dye is attached to the QD surface via two carboxylate groups in the mono-dentate position resulting in three

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distinct binding geometries named as 2O-A, 2O-B, and 2O-C. In class II, a single deprotonated carboxylic group binds to two surface cadmiums in its bridging attachment, named as 1O-D and 1O-E configurations. In class III, there are four configurations named by 1S/2O, 2S/1O, 1S/1O-A, and 1S/1O-B, where the dye is attached to the surface via both carboxylate and iso-cyanate groups. Finally, in class IV, two iso-cyanates bind to the QD surface via their S atoms providing one conformation named by 2S. Note that we do not include conformations that are absolutely symmetric with respect to the considered structures, while involve a different 2, 2´-bipyridine ligand. In our notations of configurations, the numbers 1 or 2 preceding the letters O and S denote that there are either one or two of carboxylate (O) or/and iso-cyanate (S) anchoring group(s) involved in binding, while letters from A to E differentiate the position of an anchoring group on the same 2, 2´-bipyridine ligand, non-adjacent rings, or adjacent rings of different 2, 2´bipyridine ligands. Due to variations in number of deprotonated carboxyl groups, the Cd33Se33/N719 conformations vary in their overall charge, as depicted in Fig. 2. Notably, the difference in observed geometries due to the change in degree of protonation of carboxylic groups aligned with literature reports showing that binding configurations of Ru(II) complexes on the TiO2 substrate39-40 and the CdSe QDs7,

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are dictated by the

degree of protonation of anchoring groups. Note that the considered QD/dye models do not include surface ligands, such as TOPO and TOP which are typically passivate the surface of the colloidal QDs, as well as QD/Ru(II)-polypyridine composites.6,

7, 27

Our previous calculations28 of CdSe QD

functionalized by Ru(II)-bipyridines and the Black Dye have shown that neighboring surface ligands, such as phosphine oxides (the reduced model of TOPO) fully passivating

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the QD, negligibly affects the relative energy alignment and the nature of orbitals associated with the dye and the QD, as well as its binding energy. As such, our previous results justify the simplified model containing only the dye and the QD we use here to save computational efforts. Passivation of the QD surface by a single dye also can be considered as a reasonable model for the QD/dye composites. In fact, it was experimentally shown that self-assembly of only one functionalized porphyrin dye molecule at the CdSe/ZnS QD not only modifies the photoluminescence intensity but also creates a few energetically clearly distinguishable electronic states, opening additional effective relaxation pathways with a pronounced sensitivity to the specific nature of the respective dye.47, 48 Additionally, it is expected that the qualitative trends in the dye-QD interactions at the local Cd sites are insignificantly affected by increasing the QD size from 1.5 nm to 2-4 nm, which are the most typical sizes of CdSe QD in experiments. This expectation is rationalized by insignificant changes in the lattice symmetry of the reactive facets and edges, while only the number of surface Cd sites increases with the QD size. At low concentrations of adsorbed dyes, these changes are expected to lead to relatively small perturbations in the dye-QD interactions. In fact, our previous calculations of the binding energy between the Ru(II)polypyridines and PbSe QDs have shown very similar trends for 1 nm and 2 nm QDs.53 Table 1 shows the binding energies, band gaps, Cd-S, and Cd-O bond lengths of the different Cd33Se33/N719 configurations, both in vacuum and in acetonitrile solvent. In vacuum, the strength of QD-dye interactions for all configurations with two carboxylate anchoring groups in class I are significantly larger than for other classes, while class IV with attachment via two iso-cyanates shows the weakest interaction that is smaller by 2.5-

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4 times than those of other conformations. Increasing the number of iso-cyanate anchors in class III and IV elongates the Cd-S bonds for two iso-cyanate anchors in 2S/1O and 2S resulting in 2.88-2.89 Å vs 2.74-2.75 Å for a single-iso-cyanate anchor in 1S/1O-A and 1S/1OB. Increase in Cd-S bond correlates with decreasing in the QD-dye interaction in 2S/1O and 2S compounds. On the contrary, the Cd-O bond length does not significantly change upon change of configuration for classes I and II, staying of 2.27-2.28 Å. However, configurations within class III, which additionally involve the iso-cyanate ligand(s), are found to have relatively shorter Cd-O bond lengths, reduced by ~0.1 Å. This is likely associated with a redistribution of a negative charge towards moreelectronegative oxygens in the carboxylates linkage, compared to the less-electronegative sulfur in iso-cyanate anchors, when both are bound to surface cadmiums. On the other hand, reduction of the Cd-O and firmness of Cd-S bonds in class III conformations are the signatures of strong covalent bonds between the cadmiums and both carboxylate and iso-cyanate anchors, so that neither of them is detached from the QD’s surface. A polar solvent, like acetonitrile, is found to increase and distort the Cd-O bond lengths for all considered binding configurations in classes I-III, making them all varying at the range of 2.39-2.42 Å. Similarly, Cd-S bonds are also increased to 3.05-3.10 Å and even to ~5.4 Å in 1S/2O and 1S/1O-A. The last one demonstrates complete elimination of the covalent character of the bond, so that only carboxylate groups are involved in the covalent binding with surface cadmiums in these two cases. As has been discussed in our previous studies,28, 53 solvation effects result in reducing the dipole moments on the QD’s surface that leads to diminishing QD-dye binding energies and lengthening bond lengths between the QD and anchoring groups. This effect is more pronounced for iso-cyanate

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anchors than for carboxylates, with the greatest in class IV, where the N719 dye initially attached only via iso-cyanates is completely detached from the QD when the CH3CN solvent is included. Similar to vacuum case, the attachment of the dye via two carboxylate groups (class I) provides the most stable conformations followed by structures with one carboxylate anchor (class II), with both classes having the absolute values of the QD-dye binding energies at the range of 0.8-1.0 eV in acetonitrile, which agree with our previous joined experimental-computational studies.7 Overall, inclusion of the polar solvent does not change the qualitative trends in the binding energies between the QD and the dye, so that the weakest interacting systems involve iso-cyanates as a linking group, while the most stable structures are attached via two carboxylate groups. However, due to the unstable structure of 2S in acetonitrile, we consider the electronic structure of all systems in less polar solvent, dichloromethane, as an intermediate case were the binding geometries are much less perturbed by the solvent. Energetic Alignments of QD’s vs. Dye’s Electronic Levels and Nature of their Orbitals. For all binding configurations, attachment of the dye to the QD results in significant decrease in the energy gap of Cd33Se33/N719 composites compared to the bare Cd33Se33 QD. Thus the energy gap of the unpassivated Cd33Se33 is calculated of 2.8 eV in vacuum and 3.0 eV in acetonitrile solvent, which well agrees with both previous computational results82, 83 and experimental data on magic size (

Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals

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