Binding to Small ZnS Quantum Dots

Subscriber access provided by - Access paid by the | UCSB Libraries

C: Physical Processes in Nanomaterials and Nanostructures

Three Ligands with Biomedical Importance: Binding to Small ZnS Quantum Dots Alexander A Balaeff, and Aleksey E Kuznetsov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00936 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

1

Three Ligands with Biomedical Importance: Binding to Small ZnS Quantum Dots Alexander Balaeff,a Aleksey E. Kuznetsov*b a

Department of Physics, University of Central Florida, Orlando, FL 32826 – USA b

Instituto de Química, Universidade de São Paulo – SP,

Av. Prof. Lineu Prestes, 748 – Butantã, CEP: 05508-000, São Paulo - SP – Brasil

Abstract We have performed the first systematic DFT study of the coordination of three biomedically significant ligands, acetylcysteine, dihydrolipoic acid, and dopamine, to a small quantum dot (QD), Zn6S6. An exhaustive search for the global-minima structures of the ligands and of the ligand-QD species identified several isomers of each compound close in energy to the global minimum structures. The isomeric variety is explained by the presence of several functional groups in the ligands and thus by numerous possibilities for their coordination to the QD. The global minimum structures of the three ligand-QD complexes were further studied with a larger basis set and implicit water effects. The three complexes were shown to be stable, with the anionic ligand species coordinated to the QD being generally more stable than the neutral compounds. The QD distortions due to the coordination with the anionic ligands were much more pronounced than with the neutral ligands. Charge transfer from both the neutral and anionic ligands to the QD upon the ligand coordination was detected.

E-mail: [email protected]

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

2

1. Introduction Numerous unique properties of semiconductor quantum dots (QDs)1-12 enable QD utilization for photovoltaics,13,14 waveguides,15-17 and fluorescent bio-labels.18 In the latter role, QDs are used for studies of living cells, for example, as agents for drug delivery into the cell and/or for monitoring the conditions inside the cell.18-20 The variety of applications makes profound investigations of the mechanisms of the QD fluorescence highly important. The QD fluorescent spectra show dependence on the QD size, shape, chemical composition, and the type of the ligands covalently attached to the QD surface.2-12,21,22 Learning how to fine-tune the QD fluorescence will greatly advance the biomedical and other QD applications. Motivated by this, we decided to investigate the structural and electronic properties of a small semiconductor zinc sulfide QD capped by three different ligands: N-acetylcysteine (NAC), dihydrolipoic acid (DHLA), and dopamine (DA) (Scheme 1).

Scheme 1. Ligands investigated in the study: N-acetylcysteine (NAC) (a), dihydrolipoic acid (DHLA) (b), and dopamine (DA) (c).

The choice of ligands was motivated by a series of recent studies of QDs designed for anticancer drug delivery to the living cells19 and for monitoring the intracellular levels of an important oxidant glutathione.20 Specifically, these three ligands were shown to exhibit very different levels of quenching of a QD fluorescence: from a very strong quenching (DA) to moderate quenching (NAC) to none (DHLA). Owing to the complexity of interactions between the ligands and the surfaces of the QDs and the lack of a fast method for exhaustive computational studies of such interactions (vide infra), we restricted our QD model to the small cluster, Zn6S6, capped just with one species of NAC, DHLA, or DA. Because the amount of the charge transfer from a coordinated ligand to QD might affect the fluorescence properties of the ligand-QD complex, we performed comparative computational analysis of structural and electronic properties of both neutral and anionic ligand-QD species. Also, we investigated different possible binding modes of the three ligands to the QD surface and showed how structures and properties of the ligand-QD complexes change between the free molecules and QD-bound ligands. An exhaustive search for the global-minima structures of the ligands and the ligand-QD species identified several isomers of each compound close in energy to the global minima. The isomeric variety was explained by the presence of several functional groups in the ligands and thus by numerous possibilities for the ligands’ coordination to the QD. The global minimum structures of the three ligand-QD complexes were further studied with a larger basis set and implicit water effects taken into account. The three complexes were shown to be stable, with the anionic ligand-QD species being generally more stable than their neutral counterparts. The QD distortions due to the anionic ligand ACS Paragon Plus Environment

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 coordination were found to be much more pronounced than for the neutral ligands. Charge transfer from both the neutral and the anionic ligands to the QD upon the ligand coordination was detected. The paper is organized as follows. The next section contains the theoretical background on the previous computational studies of capped semiconductor quantum dots, including CdX (X = S, Se, Te) and ZnS species. Then, we describe the computational methods employed in our study. Next, we provide the computational results and discuss the computed structural and electronic properties of the Zn6S6 QDs capped with the three ligands studied. Finally, the conclusion and perspectives follow.

2. QD Modeling Background It is well known from multiple experimental studies that ligands bound to the QD surface have a significant effect on the QD optical and electronic properties, such as linear23 and non-linear24,25 absorption/emission of light, carrier multiplication,26 charge transfer rate,27 energy relaxation28-30 and optical blinking.31-33 These and other effects are plausibly caused by changes in the QD atomic structure and electronic configuration caused by the ligand coordination. However, the precise nature of such changes and their effect on the QD electronic and optical properties remains unknown despite numerous recent studies (see, e.g., a recent review thereof by Hines and Kamat34). Simply put, one cannot reliably predict the changes in the QD optical and electronic properties caused by the coordination of the great majority of ligands on the QD surface. Our study aims to make one step towards closing this knowledge gap. Before proceeding, we will review briefly the theoretical studies that addressed the structural and electronic basis of QD-ligand interaction. The existing theoretical studies of the ligand coordination (the surface chemistry) effect on QD electronic and optical properties have been largely limited to CdnSen or CdnTen QDs (often with n ≤ 33, see below). E.g., Koposov et al.35,36 reported combined experimental and theoretical studies of the interactions between CdSe QDs passivated with trioctylphosphine oxide (TOPO) ligands and a series of Ru-polypyridine complexes. The focus of their study was the Cd33Se33 QD with d ≈ 1.3 nm. In the theoretical study, the B3LYP/Lanl2dz approach was used and the Ru-polypyridine complexes were modeled as CH3COOH or CH3COO-. Later, it was demonstrated that either one or both O-atoms from the carboxylate group can contribute in the formation of the chemical bond with the metal sites resulting in a wide range of attachments of the ligand to the QD surface.37 The carboxylic group is essentially a typical anchoring group used for controllable surface passivation of QDs, in particular, for CdSe QDs. Thus, in 2016 Kilina and co-workers using DFT and TDDFT investigated the effects of carboxylate groups on the electronic and optical properties of Cd33Se33 QDs.38 Tretiak and co-workers studied the influence of passivating ligands (amines, phosphines, phosphine oxides and pyridines) on the electronic and optical spectra of Cd33Se33 QDs.39 Energies of the most ligand orbitals were found to be located deeply inside the valence and conduction bands of the QD. All ligands were found to contribute states delocalized over both the QD surface and the ligands. Also, it is necessary to mention the comparative computational studies of CdnSen and CdnTen QDs by Kuznetsov, Beratan and co-workers.40-42 In the 2012 paper40 the first systematic B3LYP/Lanl2dz study of the structural and electronic properties of CdnXn QDs (X = Se, Te), bare and capped with NH3, SCH3, and OPH3 ligands, was performed. “Pinning” of the HOMO energies was observed for the NH3- and SCH3-capped QDs as a function of a QD



Page 4 of 21

4 size. Later, the B3LYP/Lanl2dz study of the structural and electronic properties of small bare and capped CdnSen and CdnTen QDs (n = 3, 4, 6, 9) was done.41 The binding of S-containing long-chain ligands (SCH2COOH–, SCH2CH2CO2H–, and SCH2CH2NH2–) to small CdnXn species as well as ligand binding effects on QD structure and electronic properties was investigated. Also, the comparative B3LYP/Lanl2dz studies of the structural and electronic properties of bare and NH3-, SCH3-, and OPH3-capped Cd33Se33 and Cd33Te33 QDs were performed.42 The effects of the capping ligands, coordinated to the QDs via N, S, and O atoms, on the stabilization/destabilization of the QD HOMO and LUMO, and on the vertical ionization potentials and electron affinities were studied. Recently, Kilina et al. published an excellent review on the theoretical perspectives of surface chemistry of semiconductor QDs.43 The review focus was on CdSe, PbSe, and Si QDs, where calculations successfully explained experimental trends sensitive to surface defects, doping, and ligands. Voznyy and co-workers in 2016 reported DFT studies of the electronic and optical properties of tetrahedral CdSe magic clusters (average diameter ∼1.5 nm) protected by carboxyl and amine ligands.44 Azpiroz and Angelis in 2015 presented an integrated computational study combining ab initio molecular dynamics and excited state calculations including thousands of excitations, aimed at understanding the impact of aromatic dithiocarbamate surface ligand on the optoelectronic properties of CdSe QDs.45 Also in 2015, Kravtsova et al. reported ab initio computer design of QDs based on CdTe and CdTe doped with atoms of transition elements (Co, Mn).46 In the 2014 review, Prezhdo and co-workers discussed the results of ab initio TDDFT and non-adiabatic molecular dynamics simulations of photoinduced dynamics of charges, excitons, plasmons, and phonons in semiconductor and metallic QDs.47 Gamelin and co-workers in 2016 reported TDDFT investigation of the electronic structures of copper-doped CdSe QDs.48 Also, it is of interest to mention the 2015 DFT study of CdSe/CdS core-shell QDs, where the focus was on dependence of the properties of these QDs on core types and interfaces between the core and the shell, as well as on the core/shell ratio.49 Computational studies of Zn-containing QDs are not that numerous comparing with the theoretical studies of CdX QDs but have also covered many of their structural, electronic, and surface chemistry feautures.50-57 Thus, Hoang and coworkers in 2012 reported quantum surface effects observed in activated ZnS nanocolloids and explained those using TDDFT approach.50 Feigl et al. reported the theoretical equilibrium morphologies of the ZnS QDs in the wurtzite phase as a function of size, as determined by DFT calculations.51 In 2017, Longo and co-workers reported experimental and theoretical investigations on the optical and photocatalytic properties of ZnS QDs.52 Also in 2017, Zope and co-workers reported the DFT study on the structural and electronic properties of ZnS “bubble” clusters,53 focusing on the hollow cages whose spontaneous formation was in 2003 reported by Spano et al. based on the results of classical molecular dynamics simulations.54 In the study of Zope et al. the hollow ZnS cages were modeled as ZnxSx [x = 12, 16, 24, 28, 36, 48, 108] species, and an onion-like structure was modeled as Zn96S96. They species studued were calculated to have large HOMO−LUMO gaps, between 2.5 and 3.3 eV. The hexagonal wurtzite phase of the bulk ZnS has a higher band gap (3.77 eV) compared to the cubic fcc zinc blende phase (3.68 eV),53 or 3.91 eV vs. 3.68 eV (at 300 K).55 Theoretical calculations of bulk-like ZnS clusters, reported in 2012, predicted a band gap in the range from 2.6 to 3.3 eV.56 Using DFT approach, Yong et al. in 2014 systematically investigated the possibility of the formation of different low-density framework materials based on highly stable ZnnSn (n = 12, 16) clusters.57


Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 Thus, as can be seen, no significant attention so far has been devoted to theoretical studies of structural and electronic properties of ligand-capped ZnS QDs and this manuscript aims to fill that knowledge gap.

3. Computational Methods All the calculations were performed using the Gaussian 09 package.58 The geometries of the bare and capped Zn6S6 QDs were first optimized without symmetry constraints. The resulting structures were assessed using vibrational frequency analysis to probe if they are true minimum-energy structures. If imaginary frequencies were found, further optimizations along those modes (without symmetry constraints imposed) were performed. We studied both singlet and triplet structures for all the QD-ligand complexes and performed global minimum-energy searches for the capped QDs checking different modes of the ligand coordination to the Zn6S6 QD. The ligand mode coordination to the QD was varied manually (as only a few realistic choices existed for each ligand) and the geometry optimization for each mode of coordination was further done using the Gaussian software. Calculations were done using the split-valence polarized basis sets 6-31G* and 6-31+G* augmented by the diffuse functions on heavier atoms,59 and with the hybrid B3LYP functional.60 With implicit solvent effects taken into account, we optimized the geometries of bare and capped QDs at the B3LYP/6-31G* and B3LYP/6-31+G* levels of theory using the self-consistent reaction field IEF-PCM method61 (UFF default model used in the Gaussian 09 package, with the electrostatic scaling factor α set to 1.5) with water as a solvent (dielectric constant ε = 78.39). We chose to compare the calculation results for the two basis sets and two phases (gas phase and implicit water) because the more expensive approach, namely the one employing the larger basis set (B3LYP/631+G*) and implicit water, might not necessarily be the best one. Thus, we wanted to evaluate how the augmentation of the basis set with the diffuse functions on heavier atoms and the effects of implicit solvent would affect the quality of the results obtained. Below, we discuss the solvent-phase electronic energies (E0) without zero-point corrections calculated at the two levels of theory (see above). TDDFT calculations were performed with and without implicit solvent effects included, using both the TD-B3LYP/6-31G* and TD-B3LYP/6-31+G* approaches with corresponding optimized QDs geometries. We calculated 30 states for each species studied. The ligand binding energies (BE) were calculated as follows: BE = [(E(Zn6S6) + E(L) -E(Zn6S6-L))], where E(Zn6S6-L), E(Zn6S6), and E(L) are the energies of the complex, bare QD, and the ligand, respectively, calculated at the same level. Throughout the paper, positive BE values indicate that ligands are bound to the QD. The charge analysis was performed using the Natural Bond Orbital (NBO) scheme with the ‘pop=nbo’ command as implemented in the Gaussian 09 package.62,63 The NBO scheme is considered to be a robust method of analysis compared, for example, to the Mulliken approach. Molecular structures and frontier orbitals were visualized using the OpenGL version of Molden 5.0 visualization software.64

4. Results and Discussion ACS Paragon Plus Environment


Page 6 of 21

6 Table 1. Electronic properties of the neutral and anionic Zn6S6-L species, calculated with the implicit effects from H2O, at the B3LYP/6-31+G* level of theory. Property

Species Bare Zn6S6

NAC

NAC−

DHLA

DHLA−

DA

DA−

EHOMO, A.U.

-0.21699

-0.25497

-0.23644

-0.24682

-0.22234

-0.22081

-0.16347

ELUMO, A.U.

-0.05800

-0.02980

-0.00343

-0.01075

-0.00930

-0.01535

0.00470

4.30

6.13

6.34

6.42

5.80

5.59

4.58

3.81

5.85

5.78

5.44

5.46

4.87

4.04

∆EHOMO-LUMO, eV Optical gap, eV

Capped EHOMO, A.U.

-0.21747

-0.20425

-0.21852

-0.20281

-0.21539

-0.18769

ELUMO, A.U.

-0.05571

-0.04935

-0.05486

-0.04701

-0.05234

-0.04971

∆EHOMO-LUMO, eV

4.40

4.22

4.45

4.24

4.44

3.76

Optical gap, eV

3.90

3.52

3.96

3.67

3.90

3.88

EBinding Zn6S6-L, eV

1.73

13.34

4.94

19.67

13.63

23.38

4.1. The Bare Quantum Dot Zn6S6 and Free Ligands We calculated the Zn6S6 QD in C1 symmetry but essentially the final structure has the C3v symmetry (see Fig. S1) similar to the Cd6Se6 QD studied in our earlier work.41 The calculated intralayer bond distances are in a good agreement with the experimental values observed for bulk ZnS polymorphs, 2.34 Å,55 and in a reasonably good agreement with the calculated Zn-S bond distances reported recently for larger ZnnSn clusters by Zope et al.53 Agreement with the latter is encouraging given the fact that the Zope et al. relied on a basis set different from ours and used the generalized gradient approximation of Perdew, Burke, and Ernzerhof which differs from our computational approach. The calculated HOMO/LUMO band gaps for Zn6S6 are 4.58 and 4.30 eV with the B3LYP/6-31G* and B3LYP/631+G* approaches, respectively (cf. Table 1 and Tables S1-S2 in Supporting Information). The TDDFT optical gaps calculated with the two methods are 4.08 and 3.81 eV, respectively. The calculated band gaps are noticeably larger than those calculated for the wurtzite or the zinc blende phases of the bulk ZnS (3.91 eV and 3.68 eV, respectively55), or the numbers calculated for larger atomic clusters (between 2.48 and 3.27 eV53). The discrepancy between our and the earlier results is not surprising, being attributable to the small size of the QD studied here. Interestingly, our calculated optical gaps, 4.08/3.81 eV, are relatively close to the values of 4.38 and 4.01 eV reported by Zope et al. for Zn12S12 and Zn16S16 clusters, respectively.53 It is possible that if our calculations were conducted with bigger clusters, there results would be more in line with those of the earlier studies. Finally, our calculated NBO charges at Zn and S centers were found to be very close for both the B3LYP/6-31G* and B3LYP/6-31+G* computational approaches: 1.069 and 0.993 for Zn and -1.069 and -0.993 for S. ACS Paragon Plus Environment

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

Now let us consider the free ligands used in this study. The structures of the neutral and anionic forms of the three ligands calculated at the B3LYP/6-31+G* level of theory with implicit effects of water along with selected bond distances and NBO charges are shown in Supporting Information (Fig. S2). The neutral molecule of N-acetylcysteine (NAC) (Fig. S2a) has the thiol group essentially in trans position relative to the carboxyl group, with a possibility of hydrogen bond formation between the N-atom and the thiol moiety (see short SH-N distance in Fig. S2a). Interestingly, the neutral NAC molecule also possesses a low-lying isomer wherein the SHand carboxyl groups are in cis-position relative to each other, and the hydrogen of the thiol moiety makes a hydrogen bond with the carbonyl oxygen of the CO2H-group. The cis-isomer is 1 kcal/mol higher than the trans isomer at the B3LYP/631+G* level of theory with implicit water (see Table S1). In the anionic form, where H+ is abstracted from the CO2Hgroup, the structure of the acetylcysteine backbone changes significantly: the amide moiety forms a distorted sevenmembered cycle with the thiol moiety, with the hydrogen bond of 2.087 Å between the C=O-group and the thiol hydrogen (Fig. S2a). The C-O bond distances in the CO2- moiety become equalized, a relatively short (2.064 Å) hydrogen bond appears between one of the oxygens of the CO2- moiety and the N-H-group, and a relatively short (ca. 2.6 Å) distance appears between another oxygen of the CO2- moiety and one of hydrogens of the CH2-group to which the SH is bound. Like the neutral species, the NAC– anion has a low-lying isomer where the proton is abstracted from the S-atom. That isomer is raised by 7.4 kcal/mol from the ground state at the B3LYP/6-31+G* level (both with implicit water effects, cf. Table S1). Comparison of the calculated NBO charges for NAC and NAC– shows that in the anion, as expected, the negative charge is mainly accumulated on the oxygens of the carboxyl group, making charges of those atoms more negative by 0.10 – 0.16 e. The S-atom and the oxygen of the amide moiety become slightly more negatively charged in the anion, too, by 0.07 and 0.1 e, respectively. For the dihydrolipoic acid (DHLA), both the neutral and the anionic forms have a slightly distorted non-planar backbone (Fig. S2b). In the anion, the C-S (tail) bond distance becomes elongated by 0.007 Å compared to the neutral molecule, whereas the other C-S bond distance remains the same. As expected, the carboxyl C-O bond distances become essentially equalized in the anion. The NBO charges on the S-atoms in both the neutral and the anionic species are the same, and only the negative charges on the carboxyl oxygens increase noticeably in the anionic species, by ca. 0.2 and 0.15 e (Fig. 2b). In addition to the ground state, the anionic species DHLA– has two low-lying isomers, where a proton is abstracted either from the tail S atom (isomer I) or the middle S atom (isomer II), see Table S1. These isomers are higher in energy than the ground state by 4.7 and 5.3 kcal/mol, respectively, at the B3LYP/6-31+G* level with implicit water (Table S1). It is worth noting that isomers I and II have similar HOMO/LUMO gaps and optical gaps (if calculated in implicit water, with the exception of the optical gap at the B3LYP/6-31G* level for isomer I). Those gaps are noticeably smaller than the corresponding gaps for the ground state species (see Table S1). For dopamine (DA), both the neutral and the anionic species have a “scorpion”-like structure (Fig. S2c), with the NH2- and the para-OH-groups in trans position to each other. In the neutral molecule, there is a hydrogen bond between ACS Paragon Plus Environment


Page 8 of 21

8 the proton of the para-OH-group and the meta-OH-group of the benzene ring, which is, interestingly, absent in the anionic species. The C-O bond distance for the para-OH-group in the anion is ca. 0.02 Å longer than in the neutral species, whereas the C-O bond distance for the meta-OH group is shorter by 0.08 Å in the anion due to its increased double bond character. Also, the benzene moiety in the anion is less planar than in the neutral molecule because of the partial phenoxide character of the anion. The C-NH2 bond distances remain essentially the same in both the neutral and the anionic species. The NBO charges on the N-atom and the para-O atom are the same in the neutral molecule and in the anion, whereas the charge on the meta-O is 0.14 e greater in the anion. Regarding isomers, the anion DA- has a low-lying isomer where the proton is abstracted from the para-O atom. That isomer is by 0.32 kcal/mol higher than the ground state at the B3LYP/6-31+G* theory level with implicit water (see Table S1). Thus, the neutral and anionic species in each of the three ligands have several isomers with relatively close energies. For the anions, the isomers arise from the presence of several various functional groups in the species. The isomerization may result in several possible modes of coordination with the Zn6S6 QD which is supported by our computational findings (vide infra). The calculated HOMO/LUMO gaps and optical gaps for the ligands are generally significantly larger than those for the QD (cf. Tables 1 and Supporting Information), and that would certainly remain the case if a more realistic, larger QD model were used in our study (vide supra). For all the three ligands except DHLA– the HOMOs are lower in energy than the Zn6S6 QD HOMO, and the LUMOs are noticeably higher in energy than the QD LUMO (Table 1 and Fig. S3).

4.2. Capped Quantum Dot Now let us consider what happens upon the neutral and the anionic ligand coordination to the QD. Figure 1 presents the structures of the global-minimum neutral ligand-QD species and Figure 2 shows the global-minimum structures of the anionic ligand-QD species, calculated at the B3LYP/6-31+G* level with implicit water effects.


Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9

Figure 1. The lowest-energy neutral structures of ligand-Zn6S6 species, ligand: N-Acetylcysteine (NAC) (a), Dihydrolipoic acid (DHLA) (b), and Dopamine (DA) (c), calculated at the B3LYP/6-31+G* level with implicit effects of water. Color codes: dark grey for Zn, brown for C, dark yellow for S, blue for N, dark red for O, light grey for H. Bond distances are given in Å. NBO charges are given in e, bold italics.



Page 10 of 21

10

Figure 2. The lowest-energy anionic structures of ligand-Zn6S6 species, ligand: N-Acetylcysteine (NAC) (a), Dihydrolipoic acid (DHLA) (b), and Dopamine (DA) (c), calculated at the B3LYP/6-31+G* level of theory with implicit effects of water. Color codes: dark grey for Zn, brown for C, dark yellow for S, blue for N, dark red for O, light grey for H. Bond distances are given in Å. NBO charges are given in e, bold italics.

Analysis of the calculated structural and electronic properties of the neutral ligand-Zn6S6 species (see Fig. 1 and Figs. S2-S4 and Tables 1 and S1-S2) shows the following. (i) Acetylcysteine prefers to coordinate to the QD with its S-atom forming a relatively long bond (ca. 2.7 Å) with one of the QD Zn-atoms. A weak hydrogen bond is also formed between the NAC NH-group and the QD S-atom adjacent to the Zn-center coordinating with the SH-group of the ligand. The ligand structure becomes very different from the one calculated for the free molecule (cf. Fig. S2). Dihydrolipoic acid coordinates to the QD with its carboxyl group making a relatively short (ca. 2.1 Å) dative bond between the carbonyl oxygen and the QD Zn-center and a short (ca. 2.2 Å) hydrogen bond between the OH-group and the QD neighboring S-center. The ligand structure does not change much upon coordination (cf. Fig. S2), except for the SHgroup attaining a relative cis-conformation with the H-atoms that is pointing away from the QD. Dopamine prefers to coordinate to the QD through a relatively short (ca. 2.3 Å) dative bond N-Zn. The structure of the ligand does not change much upon coordination. ACS Paragon Plus Environment

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11 (ii) The QD-Ligand binding energies increase steadily from NAC, for which the BE is relatively small, 1.73 kcal/mol, through DHLA (4.94 kcal/mol) to DA where the BE jumps to 13.63 kcal/mol. The large jump in BE can be explained by the fact that the N-atom of DA carries a larger partial electronic charge than the O-atoms of DHLA (see Fig. S2) and has smaller electronegativity than the O atoms, hence possessing a higher electron pair donating ability. (iii) The QD structure becomes significantly distorted upon the ligand coordination. Thus, the intralayer Zn-S bond distances in the layer to which the ligand is coordinated change noticeably compared to the free QD (cf. Figs. S1 and 1). Generally, the Zn-S bonds adjacent to the Zn-center with coordinated ligand are elongated significantly, whereas the other Zn-S bonds in that layer might either elongate or shorten, compared to the free QD. Interestingly, the interlayer Zn-S bond distances change significantly upon the ligand coordination. The bonds close to the Zn-center coordinated to the ligand shorten by ca. 0.03 – 0.07 Å compared to the free QD. The Zn-S bond shortening possibly indicates the increasing covalent character of these bonds due to the donation of some electron density from the ligand to the coordinated Zncenter and further redistribution of the electron density along the bonds within the QD. In principle, the distortion could be less pronounced in a larger QD. The effect of the QD size on the degree of distortion by the coordinated ligand merits a further study. (iv) The NBO analysis shows that, indeed, some charge transfer from the ligand to the QD occurs upon the ligand coordination. The charge on the Zn-center where the ligand coordinates decreases by 0.23 e for the NAC coordination, by 0.14 e for DHLA coordination, and by 0.16 e for DA coordination (see Fig. 1). Also, a slight decrease or increase of the charge can occur on the S-centers next to the ligand-coordinated Zn-center, by ca. 0.02 - 0.03 e. Regarding the ligands, the largest change of the NBO charge occurs on the S-atom of NAC upon coordination: the charge increases by 0.13 e and becomes positive due to lower electronegativity of S compared to O and N and thus greater ligand-QD charge transfer. For the O-atoms of coordinated DHLA, the NBO charges even slightly increase, and for the N-atom of DA the charge remains essentially the same (cf. Figs. S2 and 1). Again, the question of how much the calculated magnitude of charge transport depends on the size of the QD model used is very interesting and merits a separate study. (v) The HOMO/LUMO gaps of the QD increase by 0.10 – 0.15 eV upon coordination of each of the three ligands (Tables 1, S1-S2 and Fig. 3). The optical gaps for the ligand-coordinated QD are also larger than for the free QD, by 0.09 – 0.15 eV. The HOMO and LUMO energies of the ligand-coordinated QD do not change noticeably compared to the free QD (Table 1), which suggests large contributions of the QD to these orbitals. Comparing with the ligands, the HOMO/LUMO gaps and the optical gaps of the ligated QD are much smaller than those of the free ligands, by ca. 1.7 – 2.5 eV for the HOMO/LUMO gaps and by ca. 1 – 2 eV for the TDDFT gaps (Table 1 and Fig. S4). (vi) Similar to the free ligands, the ligand-QD species have low-lying isomers, see Supporting Information for the further discussion.

Analysis of the calculated structural and electronic properties of the anionic Zn6S6-Ligand species (see Figs. 2-3, S1-S4, and Tables 1 and S1-S2) and their comparison with the neutral species shows the following.



Page 12 of 21

12

Figure 3. The HOMO–LUMO (left) and optical gaps (right) for the bare QD and the lowest-energy QD-ligand complexes. (i) NAC- coordinates to the QD by its carboxyl group in “chelate” mode with relatively short, ca. 2.2 Å, Zn-O bond distances. Also, a long hydrogen bond (3.556 Å) is formed between the S-center of the QD and the SH-group of the ligand. This coordination mode is drastically different from the mode observed for the neutral species (see above). The structure of the ligand becomes quite distorted upon coordination, with the C-SH bond distance shortened by ca. 0.01 Å and a very short, 2.332 Å, hydrogen bond formed between the NH-group and one of the oxygens of the carboxyl group. DHLA– coordinates to the QD by its S-atom located at the carbon backbone, which is again different from the coordination mode of the neutral molecule. The S(ligand)-Zn bond formed upon the anion ligand coordination is relatively short, ca. 2.35 Å, which shows its covalent character. The ligand structure is somewhat distorted compared to the free anion (cf. Fig. 2), with the carboxyl and tail SH-groups being directed away from the quantum dot. DA–, too, coordinates to the QD in a totally different mode than the neutral molecule, forming a relatively short, 1.968 Å, bond between the para-O and the QD Zn-center. A short hydrogen bond is formed between the meta-OH group and the para-O, and the NH2-group is directed away from the QD. (ii) The binding energies between the anionic ligands and the QD again increase steadily from NAC– through DHLA– to DA– (see Table 1). The BEs are drastically higher than those calculated for the neutral species, especially for NAC and DHLA. This can be explained by the fact that anions can donate more of the unpaired electrons density to form stronger bonds compared to the neutral species. Also, for NAC– the “chelate” mode of coordination by two oxygens should be taken into account as the factor drastically increasing the binding energy (by ca. 12 kcal/mol). Thus, in general, coordination of the anionic ligand species to the QD results in much more stable complexes than coordination of the neutral species. (iii) The QD structures become very strongly distorted upon the coordination with the anionic ligands, which is especially pronounced for the coordination of NAC– and DHLA– (Fig. 2). (Again, the effect could possibly be magnified by the size of the QD studied; vide supra). With either of these two ligands, the interlayer bond between the coordinated Zn-center and the S-center in another layer becomes completely broken, elongating to 4.248 Å and 4.552 Å long for NAC– ACS Paragon Plus Environment

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13 and DHLA–, respectively. This observation could be explained by the formation of “real” covalent bonds between the Zcenter and the ligand which depletes the Z-center of the electron density needed to form the interlayer Zn-S bond. Interestingly, in the case of DHLA– coordination, a similar breakage of a Zn-S interlayer bond is observed on the opposite side of the QD. Essentially, for the NAC– and DHLA– coordination, the Zn3S3 layers of the QD attain a “saddle” configuration which is especially pronounced in the case of DHLA–. In general, the QD distortions due to the anionic ligand coordination are much more pronounced than those due to the neutral ligand coordination. (iv) Interestingly, the NBO analysis shows that the atomic charges of the QD do not change significantly upon the anionic ligands coordination. The changes of the NBO charges are even smaller than in the case of neutral ligands coordination (cf. Figs. 1 and 2). Thus, for NAC– coordinated, the NBO charge on the Zn-center bound to the ligand increases by a mere 0.06 e compared to the bare QD, while the charge of the S-center in the other layer connected to this Zn-center increases by ca. 0.09 e. There is a slight decrease of the charges of the oxygens of the carboxyl bound to the QD Zn-center, by ca. 0.05 e compared to the free anion (see Fig. S2), and a slight decrease of the charge at the S-center of the ligand, by ca. 0.05 e. The positive charges of the Zn-centers in the QD layer not coordinated to the ligand decrease slightly by ca. 0.07 e. Overall, one can conclude that charge transfer from NAC– to the QD still takes place but to a smaller extent than in the case of the neutral NAC. The situation is similar for the DHLA– and DA– coordination. Some charge transfer from the anionic ligands to the QD occurs, albeit to a smaller extent than in the case of neutral ligands. The charge transfer results in slightly decreasing positive charges on Zn-centers and slightly increasing negative charges on S-centers of the QD. In the ligands, a notable change of the partial charge takes place in DHLA– where the charge of the S-center coordinating to the QD increases by ca. 0.4 e compared to the free anion (cf. Figs. S2 and 2). The small magnitude of charge transport suggests that a larger, more realistic model of the QD would likely yield a similar value. (v) In contrast to the neutral ligands, the calculated HOMO/LUMO gaps of the anionic ligand-QD complexes are smaller than the free QD HOMO/LUMO gaps (see Table 1 and Fig. 3). The HOMO/LUMO gap decrease is especially pronounced (0.54 eV) for the DA– coordination. The optical gaps for the QD-NAC– and QD-DHLA– complexes are smaller than that of the free QD by 0.29 and 0.14 eV, respectively, whereas the optical gap of the QD-DA– complex is larger than that of the free QD by 0.07 eV. Compared with the free ligand anions, the HOMO/LUMO and optical gaps of the QD complexes are always noticeably smaller, by ca. 0.16 – 2.23 eV. Also, the HOMO/LUMO and optical gaps of each of the three anionic complexes are smaller than in the corresponding neutral complexes. The ‘closure’ of the HOMO/LUMO and optical gaps in the anionic QD complexes could be attributed to the fact that while both HOMO and LUMO energies increase compared to the bare QD, the LUMO does so to a smaller extent (cf. Fig. S3). Compared to the free ligand anions, both HOMO and LUMO energies of the DA–-QD complex are decreased (HOMO to a greater extent), whereas for NAC– and DHLA– the HOMOs of the QD complexes are raised in energy while the LUMOs are lowered in energy (Fig. S3). Compared to the neutral ligand-QD complexes, both HOMO and LUMO of the anionic ligand-QD complexes are raised in energy.



Page 14 of 21

14 (vi) Similar to the neutral complexes, the anionic ligand-QD species possess low-lying isomers, see Supporting Information for the discussion. On a final note, we would like to point out that it is tempting to attribute the experimentally observed difference in QD fluorescence quenching between the three ligands to the difference in MO lineup between the ligands and the QD (Fig. S3). One can see that DA– is the only species which has MOs inside the HOMO/LUMO gap of the free QD. Those orbitals could offer an alternative relaxation pathway for the photoexcited holes in the QD, thus providing the basis for the observed fluorescence quenching by the DA ligand. It is less clear from our calculations why the NAC ligand would exhibit partial fluorescence quenching while the DHLA ligand would not. A rigorous explanation of the differences in fluorescence quenching will likely require conducting TDDFT computations of the vertical spectrum and oscillator strength using a model with a larger QD (vide infra), likely including the Mn dopant. Such a task, based on a rigorous approach described, falls beyond the scope of the present manuscript. We would like to offer this explanation of the QD fluorescence quenching differences between the three ligands as an interesting possibility. Yet, one certainly has to be cautious not to overinterpret our calculation results and test this explanation more rigorously in model calculations involving a bigger quantum dot.

5. Conclusions and Perspectives We have performed the first systematic DFT study of the coordination of three biomedically significant ligands, acetylcysteine, dihydrolipoic acid, and dopamine, to the model quantum dot, Zn6S6. We used two different basis sets with increasing quality and conducted calculations in both the gas phase and implicit water. We have performed an exhaustive search for the global-minima structures of the ligands and of the ligand-QD species and used the global minimum structures obtained with the larger basis set and implicit water effects for the analysis of the general trends and features of the complexes. Our research findings can be summarized as follows. (i) The three ligands can form stable complexes upon coordination to the ZnS quantum dots in both neutral and anionic form; (ii) The ligands have in general several relatively close-lying isomers, which endow the ligands with numerous possibilities for coordination to the QD. (iii) For both neutral and anionic ligand-QD complexes, the ligand-QD binding energies increase steadily from NAC through DHLA to DA (Tables 1 and S1). The binding energies for the anionic species are much greater than for the neutral species, due to the fact that anions donate more of the unpaired electrons density to the QD to form stronger bonds compared to the neutral species. The greater binding energies imply that the anionic species form much more stable QD complexes than the neutral species; (iv) The ligand-QD coordination mode is very different between the neutral and the anionic ligand-QD complexes;


Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 (v) The QD structure becomes significantly distorted upon the ligand coordination in either neutral or anionic form, especially for NAC– and DHLA– (Fig. 2). The Zn-S bond distances change noticeably compared to the free QD both in the layer coordinated with the ligand and between the two ZnS layers included in our model. The Zn-S bond shortening may indicate the increasing covalent character of these bonds due to electron density donation to the Zn-center coordinated with the ligand; (vi) The ligand coordination results in charge transfer from the ligand to the QD. Interestingly, more charge is transferred to the QD from the neutral ligands than from the anionic ligands; (vii) The neutral ligand coordination results in a 0.10 – 0.15 eV increase of both the HOMO/LUMO and the optical gaps compared to the free QD (Fig. 3). In contrast, the anionic ligand coordination results in a decrease of the QD HOMO/LUMO gap (up to 0.54eV in case of DA–), and either a slight decrease (in NAC– and DHLA–) or a slight increase (in DA–) of the optical gap (Fig. 3). The HOMO/LUMO of the ligand-QD complex reside mostly on the QD, whereas the occupied/unoccupied ligand-localized orbitals have lower/higher energies than the frontier orbitals; (viii) Both B3LYP/6-31G* and B3LYP/6-31+G* approaches with implicit effects from solvent yield comparable results and can both be recommended for computational studies of such systems; (ix) The experimentally observed drastic difference in QD fluorescence quenching between the three ligands can be attributed to the difference in MO lineup between the ligands and the free QD (Fig. S4), although such an interpretation is subject to verification by more rigorous calculations involving a bigger, more realistic quantum dot. Based on the results obtained so far, we can formulate the following questions to be considered in the follow-up studies: (i) Could other implicit solvents, for instance, non-polar benzene, toluene, etc. affect the coordination modes of the ligands towards the QD? Knowing this, one could design further experimental studies of these systems. (ii) How would the results of our calculations change if a larger, more realistic model of QD, for instance, Zn9S9 or Zn33S33 were involved? For example, would the QD be distorted by the ligand coordination to the same extent as calculated here? How would the orbital lineup between the QD and the ligands and the HOMO/LUMO and the optical gaps of the ligand-QD complexes change? Etc. (iii) What would occur upon coordination of several ligands to the QD surface? What ligand coordination modes to the larger QD would remain possible? Will the ligands cooperate with each other or destabilize each other’s binding to the QD? Answers to these questions could help significantly in understanding the experimentally observed properties of the QD-ligand complexes.

Supporting Information Available (free of charge): data for all the structures calculated for the Zn6S6 QD with three different ligands, at the B3LYP/6-31G*//B3LYP/6-31+G* levels of theory, in the gas phase and with implicit effects from water; data for global-minima structures, at the B3LYP/6-31G*//B3LYP/6-31+G* levels of theory, in the gas phase and with implicit effects from water; the QD-vs-ligand HOMO and LUMO lineup for the global-minima structures of the three ligands; the HOMO/LUMO gaps and optical gaps for the global-minima structures.



Page 16 of 21

16

Acknowledgements The authors greatly appreciate the computational resources of the centers Centro Nacional de Processamento de Alto Desempenho em São Paulo, Núcleo de Tecnologia da Informação, and Centro Nacional de Processamento de Alto Desempenho – UFC, as well as those of the Extreme Science and Engineering Discovery Environment (XSEDE). The latter was supported by National Science Foundation grant number ACI-1053575. The work of Dr. Aleksey E. Kuznetsov was supported by the University of São Paulo – SP. The authors appreciate fruitful discussions with S. Santra and A. Gesquiere (U. Central Florida).

References (1) Brus, L. Electronic wave functions in semiconductor clusters: experiment and theory. J. Phys. Chem. 1986, 90, 25552560. (2) Wu, M.; Mukherjee, P.; Lamont, D. N.; Waldeck, D. H. Electron transfer and fluorescence quenching of nanoparticle assemblies. J. Phys. Chem. C 2010, 114, 5751-5759. (3) Markus, T. Z.; Wu, M.; Wang, L.; Waldeck, D. H.; Oron, D.; Naaman, R. Electronic structure of CdSe nanoparticles adsorbed on Au electrodes by an organic linker: Fermi level pinning of the HOMO. J. Phys. Chem. C 2009, 113, 1420014206. (4) Jose, R.; Zhanpeisov, N. U.; Fukumura, H.; Baba, Y.; Ishikawa, M. Structure−property correlation of CdSe clusters using experimental results and first-principles DFT calculations. J. Am. Chem. Soc. 2006, 128, 629-636. (5) Park, Y.-S.; Dmytruk, A.; Dmitruk, I.; Kasuya, A.; Takeda, M.; Ohuchi, N.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. Size-selective growth and stabilization of small CdSe nanoparticles in aqueous solution. ACS Nano 2010, 1, 121-128. (6) Park, Y.-S.; Dmytruk, A.; Dmitruk, I.; Kasuya, A.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. Aqueous phase synthesized CdSe nanoparticles with well-defined numbers of constituent atoms. J. Phys. Chem. C 2010, 114, 1883418840. (7) Kasuya, A.; Sivamonah, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B. et al. Ultra-stable nanoparticles of CdSe revealed from mass spectrometry. Nat. Mater. 2004, 3, 99-102. (8) Donakowski, M. D.; Godbe, J. M.; Sknepnek, R.; Knowles, K. E.; de la Cruz, M. O.; Weiss, E. A. A quantitative description of the binding equilibria of para-substituted aniline ligands and CdSe quantum dots. J. Phys. Chem. C 2010, 114, 22526-22534. (9) Ji, X.; Copenhaver, D.; Sichmeller, C.; Peng, X. Ligand bonding and dynamics on colloidal nanocrystals at room temperature: the case of alkylamines on CdSe nanocrystals. J. Am. Chem. Soc. 2008, 130, 5726-5735. (10) Bullen, C.; Mulvaney, P. The effects of chemisorption on the luminescence of CdSe quantum dots. Langmuir 2006, 22, 3007-3013.


Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 (11) Munro, A. M.; Jen-La Plante, I.; Ng, M. S.; Ginger, D. S. Quantitative study of the effects of surface ligand concentration on CdSe nanocrystal photoluminescence. J. Phys. Chem. C 2007, 111, 6220-6227. (12) Markus, T. Z.; Itzhakov, S.; Alkotzer, Y. I.; Cahen, D.; Hodes, G.; Oron, D.; Naaman, R. Energetics of CdSe quantum dots adsorbed on TiO2. J. Phys. Chem. C 2011, 115, 13236-13241. (13) Wang, Z.; Zhang, N.; Brenneman, K.; Wu, T. C.; Jung, H.; Biswas, S.; Sen, B.; Reinhardt, K.; Liao, S.; Stroscio, M. A. et al. Optoelectronic applications of colloidal quantum dots. In Quantum Dot Devices; Wang, M. Z., Ed.; Springer: New York, NY, 2012; pp 351–367. (14) Koleilat, G. I.; Levina, L.; Shukla, H.; Myrskog, S. H.; Hinds, S.; Pattantyus-Abraham, A. G.; Sargent, E. H. Efficient, stable infrared photovoltaics based on solution-cast colloidal quantum dots. ACS Nano 2008, 2, 833–840. (15) Gordillo, H.; Suárez, I.; Abargues, R..; Rodríguez-Cantó, P.; Albert, S.; Martínez-Pastor, J. P. Polymer/QDs nanocomposites for waveguiding applications. J. Nanomater. 2012, 2012, 33. (16) Schaller, R. D.; Petruska, M. A.; Klimov, V. I. Tunable near-infrared optical gain and amplified spontaneous emission using PbSe nanocrystals. J. Phys. Chem. B 2003, 107, 13765–13768. (17) Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Single-exciton optical gain in semiconductor nanocrystals. Nature 2007, 447, 441–446. (18) Fontes, A.; de Lira, R. B.; Seabra, M. A. B. L.; da Silva, T. G.; de Castro Neto, A. G.; Santos, B. S. Quantum dots in biomedical research. In Biomedical Engineering - Technical Applications in Medicine; Hudak, R.; Penhaker, M.; Majernik, J., Eds.; InTech: 2012; pp 269–290. (19) Mitra, R. N.; Doshi, M.; Zhang, X. L.; Tyus, J. C.; Bengtsson, N.; Fletcher, S.; Page, B. D. G.; Turkson, J.; Gesquiere, A. J.; Gunning, P. T., et al. An activatable multimodal/multifunctional nanoprobe for direct imaging of intracellular drug delivery. Biomaterials 2012, 33, 1500-1508. (20) Banerjee, S.; Kar, S.; Perez, J. M.; Santra, S. Quantum dot based OFF/ON probe for detection of glutathione. J. Phys. Chem. C 2009, 113, 9659-9663. (21) Yang, H.; Santra, S.; Holloway, P. H. Syntheses and applications of Mn-doped II-VI semiconductor nanocrystals. J. Nanosci. Nanotechn. 2005, 5, 1364–1375. (22) Scholes, G. D. Controlling the optical properties of inorganic nanoparticles. Adv. Funct. Mater. 2008, 18, 1157-1172. (23) Ning, Z.; Molnár, M.; Chen, Y.; Friberg, P.; Gan, L.; Ågren, H.; Fu, Y. Role of surface ligands in optical properties of colloidal CdSe/CdS quantum dots. Phys. Chem. Chem. Phys. 2011, 13, 5848–5854. (24) Zeng, Y.; Kelley, D. F. Two-photon photochemistry of CdSe quantum dots. ACS Nano 2015, 9, 10471–10481. (25) Makarov, N. S.; Lau, P. C.; Olson, C.; Velizhanin, K. A.; Solntsev, K. M.; Kieu, K.; Kilina, S.; Tretiak, S.; Norwood, R. A.; Peyghambarian, N. et al. Two-photon absorption in CdSe colloidal quantum dots compared to organic molecules. ACS Nano 2014, 8, 12572–12586. (26) Ten Cate, S.; Sandeep, C. S.; Liu, Y.; Law, M.; Kinge, S.; Houtepen, A. J.; Schins, J. M.; Siebbeles, L. D. Generating free charges by carrier multiplication in quantum dots for highly efficient photovoltaics. Acc. Chem. Res. 2015, 48, 174– 181.



Page 18 of 21

18 (27) Sharma, S. N.; Pillai, Z. S.; Kamat, P. V. Photoinduced charge transfer between CdSe quantum dots and pphenylenediamine. J. Phys. Chem. B 2003, 107, 10088–10093. (28) Peterson, M. D.; Cass, L. C.; Harris, R. D.; Edme, K.; Sung, K.; Weiss, E. A. The role of ligands in determining the exciton relaxation dynamics in semiconductor quantum dots. Annu. Rev. Phys. Chem. 2014, 65, 317–339. (29) Liu, J.; Kilina, S. V.; Tretiak, S.; Prezhdo, O. V. Ligands slow down pure-dephasing in semiconductor quantum dots. ACS Nano 2015, 9, 9106–9116. (30) Kilina, S.; Velizhanin, K. A.; Ivanov, S.; Prezhdo, O. V.; Tretiak, S. Surface ligands increase photoexcitation relaxation rates in CdSe quantum dots. ACS Nano 2012, 6, 6515–6524. (31) Frantsuzov, P. A.; Volkán-Kacsó, S. N.; Jankó, B. R. Universality of the fluorescence intermittency in nanoscale systems: experiment and theory. Nano Lett. 2013, 13, 402–408. (32) Busby, E.; Anderson, N. C.; Owen, J. S.; Sfeir, M. Y. Effect of surface stoichiometry on blinking and hole trapping dynamics in CdSe nanocrystals. J. Phys. Chem. C 2015, 119, 27797–27803. (33) Miller, J. B.; Dandu, N.; Velizhanin, K. A.; Anthony, R. J.; Kortshagen, U. R.; Kroll, D. M.; Kilina, S.; Hobbie, E. K. Enhanced luminescent stability through particle interactions in silicon nanocrystal aggregates. ACS Nano 2015, 9, 9772– 9782. (34) Hines, D. A.; Kamat, P. V. Recent advances in quantum dot surface chemistry. ACS Appl. Mater. Interfaces 2014, 6, 3041–3057. (35) Koposov, A. Y.; Cardolaccia, T.; Albert, V.; Badaeva, E.; Kilina, S.; Meyer, T. J.; Tretiak, S.; Sykora, M. Formation of assemblies comprising Ru–polypyridine complexes and CdSe nanocrystals studied by ATR-FTIR spectroscopy and DFT modeling. Langmuir 2011, 27, 8377–8383. (36) Koposov, A. Y.; Szymanski, P.; Cardolaccia, T.; Meyer, T. J.; Klimov, V. I.; Sykora, M. Electronic properties and structure of assemblies of CdSe nanocrystal quantum dots and Ru-polypyridine complexes probed by steady state and time-resolved photoluminescence. Adv. Funct. Mater. 2011, 21, 3159–3168. (37) Hedrick, M. M.; Mayo, M. L.; Badaeva, E.; Kilina, S. First-principles studies of the ground- and excited-state properties of quantum dots functionalized by Ru(II)–polybipyridine. J. Phys. Chem. C 2013, 117, 18216–18224. (38) Tamukong, P. K.; Peiris, W. D. N.; Kilina, S. Computational insights into CdSe quantum dots’ interactions with acetate ligands. Phys. Chem. Chem. Phys. 2016, 18, 20499-20510. (39) Fischer, S. A.; Crotty, A. M.; Kilina, S. V.; Ivanov, S. A.; Tretiak, S. Passivating ligand and solvent contributions to the electronic properties of semiconductor nanocrystals. Nanoscale 2012, 4, 904-914. (40) Kuznetsov, A. E.; Balamurugan, D.; Skourtis, S. S.; Beratan, D. N. Structural and electronic properties of bare and capped CdnSen/CdnTen nanoparticles (n = 6, 9). J. Phys. Chem. C 2012, 116, 6817-6830. (41) Lim, E.; Kuznetsov, A. E.; Beratan, D. N. Effects of S-containing ligands on the structure and electronic properties of CdnSen/CdnTen nanoparticles (n=3, 4, 6, and 9). Chem. Phys. 2012, 407, 97-109. (42) Kuznetsov, A. E.; Beratan, D. N. Structural and electronic properties of bare and capped Cd33Se33 and Cd33Te33 quantum dots. J. Phys. Chem. C 2014, 118, 7094-7109.


Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 (43) Kilina, S. V.; Tamukong, P. K.; Kilin, D. S. Surface chemistry of semiconducting quantum dots: theoretical perspectives. Acc. Chem. Res. 2016, 49, 2127−2135, and references therein. (44) Voznyy, O.; Mokkath, J. H.; Jain, A.; Sargent, E. H.; Schwingenschlögl, U. Computational study of magic-size CdSe clusters with complementary passivation by carboxylic and amine ligands. J. Phys. Chem. C 2016, 120, 10015−10019. (45) Azpiroz, J. M.; De Angelis, F. Ligand induced spectral changes in CdSe quantum dots. ACS Appl. Mater. Interfaces 2015, 7, 19736−19745. (46) Kravtsova, A. N.; Lomachenko, K. A.; Suchkova, S. A.; Pankin, I. A.; Fayn, M. B.; Bugaev, A. L.; Soldatov, A. V. Doped CdTe-based quantum dots. Bull. Russ. Acad. Sci. Phys. 2015, 79, 1413–1416. (47) Neukirch, A. J.; Hyeon-Deuk, K.; Prezhdo, O. V. Time-domain ab initio modeling of excitation dynamics in quantum dots. Coord. Chem. Rev. 2014, 263–264, 161–181. (48) Nelson, H. D.; Li, X.; Gamelin, D. R. Computational studies of the electronic structures of copper-doped CdSe nanocrystals: oxidation states, Jahn-Teller distortions, vibronic bandshapes, and singlet-triplet splittings. J. Phys. Chem. C 2016, 120, 5714–5723. (49) Kocevski, V.; Rusz, J.; Eriksson, O.; Sarma, D. D. First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals. Sci. Rep. 2015, 5:10865. (50) Ngo, T. H.; Bui, H. V.; Pham, V. B.; Tran, T. H.; Ngac, A. B.; Hoang, N. N. Emission characteristics of SPAN-80 activated ZnS nanocolloids. J. Luminescence 2012, 132, 2135–2142. (51) Feigl, C. A.; Barnard, A. S.; Russo, S. P. Size- and shape-dependent phase transformations in wurtzite ZnS nanostructures. Phys. Chem. Chem. Phys. 2012, 14, 9871–9879. (52) La Porta, F. A.; Nogueira, A. E.; Gracia, L.; Pereira, W. S.; Botelho, G.; Mulinari, T. A.; Andrés, J.; Longo. E. An experimental and theoretical investigation on the optical and photocatalytic properties of ZnS nanoparticles. J. Phys. Chem. Solids 2017, 103, 179–189. (53) Bhusal, S.; Rodriguez Lopez, J. A.; Ulises Reveles, J.; Baruah, T.; Zope, R. R. Electronic and structural study of ZnxSx [x = 12, 16, 24, 28, 36, 48, 96, and 108] cage structures. J. Phys. Chem. A 2017, 121, 3486−3493. (54) Spano, E.; Hamad, S.; Catlow, C. R. A. Computational evidence of bubble ZnS clusters. J. Phys. Chem. B 2003, 107, 10337-10340. (55) Springer Handbook of Electronic and Photonic Materials; Kasap, S.; Capper, P., Eds.; Springer US, 2007; LXVIII. (56) Chemical Modelling; Springborg, M., Ed.; RSC Publishing, 2012; Vol. 9, pp 135−169. (57) Yong, Y.; Li, X.; Hao, X.; Cao, J.; Li, T. Theoretical prediction of low-density nanoporous frameworks of zinc sulfide based on ZnnSn (n = 12, 16) nanocaged clusters. RSC Adv. 2014, 4, 37333−37341. (58) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H. et al. Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. (59) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A. 6-31G* basis set for third-row atoms. J. Comp. Chem. 2001, 22, 976-984. (60) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 56485652. ACS Paragon Plus Environment


Page 20 of 21

20 (61) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105, 2999-3093. (62) Foster, J. P.; Weinhold, F. Natural hybrid orbitals. J. Am. Chem. Soc. 1980, 102, 7211-7218. (63) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint. Chem. Rev. 1988, 88, 899-926. (64) Schaftenaar, G.; Noordik, J. H. Molden: a pre- and post-processing program for molecular and electronic structures. J. Comput.-Aided Mol. Design 2000, 14, 123-134.


Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21

TOC Graphic


Binding to Small ZnS Quantum Dots

Recommend Documents