Stereoelectronic Effects on the Binding of Neutral Lewis Bases to CdSe Nanocrystals

Article abstract of DOI:10.1021/jacs.8b02927

Using ³¹P nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri-n-butylphosphine (Bu₃P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct ³¹P NMR signals for free and bound phosphine ligands allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric profile achieve higher surface coverages (Bu₃P = 0.5 nm^-2, Me₂P-n-octyl = 2.0 nm^-2, NH₂Bu = >3 nm^-2) and have greater relative binding affinity for the nanocrystal (binding affinity: Me₃P > Me₂P-n-octyl ～ Me₂P-n-octadecyl > Et₃P > Bu₃P). Among phosphines, only Bu₃P and Me₂P-n-octyl support a colloidal dispersion, allowing a relative surface binding affinity (K_rel) to be estimated in that case (K_rel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu₃P from the nanocrystals (K_rel: H₂NBu ～ N-n-butylimidazole > 4-ethylpyridine > Bu₃P ～ HNBu₂ > Me₂NBu > Bu₃N). The affinity for the CdSe surface is greatest among soft, basic donors and depends on the number of each ligand that bind. Sterically unencumbered ligands such as imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is a consequence of the high atom density of binary semiconductor surfaces. The observed behavior is distinct from the self-assembly of straight-chain surfactants on gold and silver where the ligands are commensurate with the underlying lattice and attractive interactions between aliphatic chains strengthen the binding.

Full text of DOI:10.1021/jacs.8b02927

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Article
Stereoelectronic Effects on the Binding of
Neutral Lewis Bases to CdSe Nanocrystals
Nicholas C. Anderson, Peter E. Chen, Aya K Buckley, Jonathan De Roo, and Jonathan S. Owen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02927 • Publication Date (Web): 10 May 2018
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Stereoelectronic Effects on the Binding of Neutral Lewis
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Bases to CdSe Nanocrystals
Nicholas C. Anderson, Peter. E. Chen, Aya K. Buckley, Jonathan De Roo, Jonathan S. Owen*
Department of Chemistry, Columbia University, New York, New York, 10027.
Email: jso2115@columbia.edu, Telephone: (212) 851-5879
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Abstract
Using 31P nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between
tri-n-butylphosphine (Bu3P) and various amine and phosphine ligands for the surface of chloride
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terminated CdSe nanocrystals. Distinct P NMR signals for free and bound phosphine ligands
allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric
profile achieve higher surface coverages (Bu3P = 0.5 nm-2, Me2P-n-octyl = 2.0 nm-2, NH2Bu = > 3
nm-2) and have greater relative binding affinity for the nanocrystal (binding affinity: Me3P > Me2P–
n-octyl ~ Me2P–n-octadecyl > Et3P > Bu3P). Among phosphines, only Bu3P and Me2P–n-octyl
support a colloidal dispersion, allowing a relative surface binding affinity (Krel) to be estimated in
that case (Krel = 3.1). The affinity of the amine ligands is measured by the extent to which they
displace Bu3P from the nanocrystals (Krel: H2NBu ~ N-n-butylimidazole > 4-ethylpyridine > Bu3P
~ HNBu2 > Me2NBu > Bu3N). The affinity for the CdSe surface is greatest among soft, basic donors
and also depends on the number of each ligand that bind. Sterically unencumbered ligands such as
imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as
alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is
a consequence of the high atom density of binary semiconductor surfaces. The observed behavior
is distinct from the self assembly of straight chain surfactants on gold and silver where the ligands
are commensurate with the underlying lattice and attractive interactions between aliphatic chains
strengthen the binding.
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Introduction
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The photoluminescence quantum yield and chemical stability of II-VI semiconductor
nanocrystals depends critically on the binding affinity of their surface ligands.1 A deeper
understanding of surface coordination chemistry would aid the design of ligands that effectively
stabilize nanocrystals in cellular environments and solid state lighting applications, while
maximizing their photoluminescence quantum yield (PLQY). However, it is challenging to directly
monitor ligand binding to surfaces in solution.
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Photoluminescence spectroscopy has been used to study the binding of amines and phosphines
to CdSe nanocrystals2-10 and bulk CdSe11-13 surfaces, where ligand binding can raise (or lower) the
PLQY. For example, changes to the PLQY of a single crystal placed in an atmosphere of gaseous
amine can be analyzed using the Langmuir model.11-13 Binding constants extracted (H3N < H2NMe
< HNMe2 > NMe3) in this manner parallel the gas phase proton affinity of the amine (with the
exception of NMe3). A similar strategy was used to analyze ligand binding to colloidal CdSe
nanocrystals in solution.14 In both cases, the PLQY is assumed to be proportional to the fractional
surface coverage, which ignores several complications including changes to the recombination
mechanism,9 side reactions involving acidic impurities15 or displacement of atoms from the crystal
surface.16 The method used to analyze single crystals also convolves the ligand donor strength and
surface coverage and none measure changes to the number of accessible surface sites caused by
lateral steric interactions between ligands. It is therefore unclear how to explain binding affinities
that do not follow the ligand donor strength (e.g. HNMe2 > NMe3), or the relatively weak affinity
of N,N,N’,N’-tetramethylethylenediamine16 and bis(diphenylphosphine)ethane13, both of which are
strong donors and have the potential ability to chelate the surface.
Nuclear magnetic resonance (NMR) spectroscopy can distinguish ligands bound to the
nanocrystal surface from those freely diffusing in solution. Surface bound ligands display broad
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spectral linewidths and are typically downfield of the signals from “free” ligands. Particularly for
anionic ligands like alkylphosphonates and carboxylates, ligand exchange and binding can be
assessed quantitatively.17-19 However, NMR spectroscopy has been less useful for studying the
exchange of neutral two electron donor ligands, e.g. n-alkylamines (L-type ligands)20-22 because
they undergo rapid self-exchange that causes coalescence of signals from the free and bound
forms.23-24
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We recently reported the synthesis of CdSe nanocrystals with both tri-n-butylphosphine (Bu3P)
and tri-n-butylphosphonium chloride ([Bu3P–H+][Cl-]) ligands (CdSe–CdCl2/Bu3P/[Bu3P–
H]+[Cl]-). These chloride terminated nanocrystals could be prepared from carboxylate terminated
CdSe nanocrystals (CdSe–Cd(O2CR)2) and chlorotrimethylsilane (Me3SiCl).25 At room
temperature the exchange of the phosphine ligands is slow and distinct 31P NMR signals for bound
and free Bu3P are observed. Moreover, displacement of the phosphine ligands could be monitored
with 31P NMR spectroscopy in situ. This controlled ligand exchange reactivity presents the
opportunity to directly study the stereoelectronic factors that control the surface binding affinity of
L-type ligands.
Results
To simplify our study, we first eliminate the oleic acid impurity that produces [Bu3P–H+][Cl-]
by pretreatment of the nanocrystals with Me2Cd according to a previously described method
(Scheme 1).15 After removing the solvent and any unreacted Me2Cd, the carboxylate ligands were
cleaved using Me3SiCl and Bu3P according to our previous study.26 Unlike CdSe–
CdCl2/Bu3P/[Bu3P–H]+[Cl]-, which precipitates from pentane solution, CdSe–CdCl2/Bu3P is
soluble in pentane and precipitates from methyl acetate or acetonitrile. A 31P NMR spectrum
verifies that the isolated nanocrystals are free from [Bu3P–H]+[Cl]- (δ = 11 ppm), and UV-Visible
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absorption spectroscopy shows that the nanocrystal retain the characteristic absorption spectrum of
colloidal CdSe nanocrystals without significant changes to the nanocrystal size or size distribution
(Figure S1).
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PBu3
O2CR
1) CdMe2, 12 hrs.
2) Me3SiCl, PBu3, 24 hrs.
CdCl2
+ Me3Si–O2CR
+ Me3Si–Me
CdO2CR
H–O2CR
CdSe
Cd
Se
O2CR = oleate
Scheme 1. Synthesis of CdSe-CdCl2/Bu3P free of [Bu3PH+][Cl-].
To estimate the relative binding affinity of several L-type ligands, we monitored their ability to
displace Bu3P from CdSe–CdCl2/Bu3P (Figures S2 – S12, Table S4). For example, as n-
octylamine is added (0 – 10 equiv./PBu3), surface bound Bu3P (δ = -11 ppm) is liberated producing
the sharp signal of the “free” phosphine (δ = -31 ppm) (Figure 1).27 A broadened and shifted P
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NMR signal is observed when 1 – 2.5 equiv. of amine are added, suggesting that free Bu3P
undergoes dynamic exchange with a fraction of the surface bound ligands under these conditions.
The displacement of surface bound phosphines does not influence the nanocrystal size or size
distribution, as described previously. 25
The relative surface binding affinities of the amines could be ranked by comparing the amount
of Bu3P displaced in the presence of tri-n-alkyl, di-n-alkyl, and n-alkylamines (1 or 50 equiv. /
Bu3P). One equiv. of n-butylamine more effectively displaces Bu3P than does di-n-butylamine
which is more effective than tri-n-butylamine. Amines with methyl substituents displace more
Bu3P than amines with long chain substituents (e.g. affinity of Me2NBu > Bu3N). These substituent
effects do not follow the gas phase proton affinities nor the pKa of the conjugate acids (pKa(R3N–
H+)), which are within 1 pKa unit in water.28 Instead they can be explained by the relative steric
bulk of the incoming ligand, with the bulkiest ligands being the weakest competitors.
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Figure 1. (A) Ligand exchange equilibrium between CdSe-CdCl2/Bu3P and free Bu3P. (B) A
series of 31P NMR spectra of CdSe-CdCl2/Bu3P and increasing equivalents of added n-octylamine
in benzene-d6. Prior to addition of n-octylamine, the 31P NMR spectrum shows resonances for
Bu3P bound to the CdSe nanocrystal (δ = -13 ppm) and phosphine complexes of CdCl2 (δ = -9
ppm) that are associated with the nanocrystal (see ref 25). Free Bu3P appears at δ = -32 ppm. We
assign the broadened resonance that shifts down field from free Bu3P to a population of ligands in
rapid dynamic exchange with the surface.
Similar effects were observed upon titration with tri-n-alkyphosphines, although in this case the
surface coverage of both the incoming and outgoing ligands could be extracted from the 31P NMR
spectrum. In the presence of 1 equiv. of triethylphosphine (Et3P, δ = -19 ppm), Bu3P is displaced
from the surface and the broad signal from bound phosphines shifts downfield by 5 – 10 ppm
(Figure 2). Although the signals of bound Bu3P and Et3P overlap, their surface coverages may be
determined from the amount of Bu3P and Et3P that remain free. Interestingly, in the presence of
Et3P (1 equiv.), the total number of bound phosphines increases from 30 ± 5 Bu3P/nanocrystal to
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36 ± 8 R3P/nanocrystal (23 ± 5 Et3P and 13 ± 3 Bu3P). At higher concentrations of Et3P more Bu3P
is displaced, however the nanocrystals begin to precipitate from the solution. Similar results are
obtained with trimethylphosphine (Me3P): in the presence of 1 equiv. of Me3P a higher total
phosphine coverage is achieved (45 ± 8 phosphines per nanocrystal). On the other hand, the
coverage does not increase when Bu3P is added, implying that the smaller phosphines can access a
greater number of surface sites. We conclude that the smaller phosphines have a greater affinity
for the nanocrystal.
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Figure 2. P NMR spectra of CdSe–CdCl2/Bu3P (0.5 mM nanocrystals, 14.8 mM Bu3P, black,
bottom) with triethylphosphine (δ = -19 ppm) at 1:1 equivalents (blue) and 50:1 equivalents (red).
The new broad resonance at δ = -6 ppm is Et3P bound to the nanocrystal.
We then explored the binding of P,P-dimethyl-n-octylphosphine (Me2P-n-octyl) with the
hypothesis that this ligand would provide a stable colloidal dispersion and allow us to measure the
coverage of a pure Me2P-n-octyl ligand shell. Indeed, stable dispersions of Me2P-n-octyl bound
nanocrystals (CdSe–CdCl2/Me2P-n-octyl) could be synthesized by completely displacing Bu3P
ligands from CdSe–CdCl2/Bu3P or upon reaction of CdSe–Cd(O2CR)2, with Me2P-n-octyl and
Me3SiCl (see Supporting Information, Figure S13). By either method, the Me2P-n-octyl surface
coverage is 2.0 – 2.2 nm-2 (90 ± 15 Me2P-n-octyl per nanocrystal, d = 3.8 nm, see Supporting
Information), ~4x greater than the coverage of Bu3P ligands. From the saturation coverages of Bu3P
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and Me2P-n-octyl, a competitive binding model could be used to measure the relative affinity (see
Supporting Information).29 The ratio of binding constants (Krel = K1/K2) for the incoming (L1) and
outgoing (L2) ligand can be calculated from the molar concentration of the free ligands ([Li]) and
their fractional coverage (ꢀi) in a solution of both competing ligands according to Equation 1.
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ꢅ
ꢈ [ꢉ ]
ꢆ ꢇ
ꢆ
ꢁꢂꢃꢄ
=
=
(1)
ꢅ
ꢇ
ꢈ [ꢉ ]
ꢇ ꢆ
The ꢀi of Me2P-n-octyl and Bu3P is calculated by dividing the coverage of each phosphine in the
mixture by the saturation coverage of the pure ligand shell. From this analysis Me2P-n-octyl has a
surface binding affinity ~3x greater than Bu3P. Because Ki is normalized by the number of binding
sites accessible to each ligand, the difference in affinity reflects the binding characteristics on a per
ligand basis. Thus, the 3x greater affinity of Me2P-n-octyl may be attributed to a weaker repulsive
interaction of its substituents with neighboring phosphines or the nanocrystal surface. On the other
hand, Eq. 1 assumes that a single binding constant applies at all coverages. If the acidity of the
nanocrystal decreases as the number of surface bound ligands grows the binding affinity will
decrease. This effect can influence the magnitude of intermediate coverages obtained in equimolar
solutions of Me2P-n-octyl and Bu3P will increase the affinity of Me2P-n-octyl and reduce the
affinity of Bu3P relative to their affinity at saturation. More detailed investigations of the binding
energy as a function of the coverage and ligand structure are required to assess these issues.
A wide range of ligands were surveyed using the approaches described above. The relative
affinity of the tri-n-alkylphosphines is Me3P > Me2P-n-octyl > Et3P > Bu3P while the affinity of
the amine ligands is H2NBu > Bu3P ~ HNBu2 > Me2NBu > NBu3. Previous studies that used the
photoluminescence intensity from single crystals12, 30 and colloidal nanocrystals14 to monitor ligand
binding report similar trends in the affinity of mono-, di-, and trisubstituted n-alkylamines despite
the fact that they do not deconvolute variations in the ligand coverage from the relative binding
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affinities. In addition to the ligands described above, a variety of bulky and/or electron deficient
ligands displace little or no Bu3P from the nanocrystals even at high concentration, including
triethylphosphite, triphenylphosphine, diphenylphosphine, tetradecanol, furan, thiophene,
tetrahydrofuran, diethylether, n-pentylisocyanide, and di-n-butylsulfide.
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To assess the effect of ligand basicity and structure on the displacement reactivity, the pKa of
the conjugate acid and the Tolmann cone angle of each ligand are plotted in Figure 3.31-32 Ligands
that effectively compete with Bu3P for the nanocrystal surface are highlighted. Both a small cone
angle and a high ligand basicity are key to a high affinity for the surface. Sterically unencumbered
ligands with low basicity, such as n-pentylisocyanide (pKa(R–NºC–H+) = 0.86, H2O, R =
cyclohexyl)33 are weak competitors. However, a weak basicity is partly overcome if the donor atom
is soft, as in the case of tetrahydrothiophene (pKa(Et2S–H+) = -6.7, H2O)34, which displaces Bu3P
at high concentration. The special affinity of soft ligands helps explain the poor binding of the hard
Bu3N ligand (pKa(Et3N–H+) = 10.7, H2O)34, which is a stronger Brønsted base than its isostructural
phosphine (pKa(Bu3P–H+) = 8.4, H2O)32. Bu3N also has a greater cone angle than Bu3P, owing to
the shorter M–N bond and the larger C–E–C angle, which increases its steric profile. Thus, soft,
basic ligands with a small steric profile bind with the greatest affinity.
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Figure 3. (left) pKa versus Tolmann cone angles for amines and phosphines in the ligand binding
series. The green area contains strong binders. (right, top) Relative binding affinities of all
molecules studied, with molecules of greatest affinity on the right. Molecules in brown do not
support stable colloidal dispersion on their own. (right, bottom) Molecules that do not displace
significant quantities of Bu3P at high concentration. In all cases, R = n-alkyl.
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Pyridine and tri-n-octylphosphine oxide (TOPO) have been reported to stabilize nanoparticle
dispersions, although recent studies argue otherwise.35-38 To shed light on the issue we studied the
displacement of Bu3P from CdSe–CdCl2/Bu3P in pyridine and TOPO solution. Despite its
moderate basicity, pyridine (pKa(pyridine–H+) = 5.2, H2O)39 effectively displaces the much more
basic and soft Bu3P donor ligand. In the presence of 1 equiv. of pyridine, the nanocrystals begin to
precipitate. However, 1 equiv. of 4-ethylpyridine displaces 30% of the Bu3P and maintains a stable
dispersion. Higher concentrations of 4-ethylpyridine also induce precipitation. Similar results are
observed with 1-n-butylimidazole, which outcompetes Bu3P for the nanocrystal surface and
displaces a greater quantity of Bu3P than does pyridine, consistent with its greater basicity
(pKa(imidazole–H+) = 7.0, H2O) and small steric profile. TOPO, on the other hand, does not
displace Bu3P, even at high concentrations (0.3 M). Moreover, the reaction of CdSe-Cd(O2CR)2
with Me3SiCl in pyridine or TOPO solution caused precipitation of the nanocrystals. We conclude
that pyridine and 1-butylimidazole bind the nanocrystal surface effectively but do not stabilize a
colloidal dispersion, even in a neat solution of the ligand. On the other hand, TOPO does not
compete with Bu3P, nor does it stabilize a colloidal dispersion.
The relatively high affinity of the pyridine and imidazole ligands, and the influence of steric
properties on the coverage of alkylphosphines and amines suggests that the competitive binding
equilibrium is determined by the number of each competitor that binds as well as the relative
surface–ligand bond dissociation energy (BDE(S–L)). The surface coverages vary widely
depending on the steric properties of the ligand. The coverage of phosphines increases 4-fold on
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exchanging Bu3P for Me2P-n-octyl (0.5 nm-2 vs. 2 nm-2). These coverages are somewhat insensitive
to the solution concentration and appear near the maximum coverages for these ligands. n-
Alkylamines, on the other hand, display concentration dependent binding - poor colloidal stability
is observed as the amine concentration is lowered - and their rapid degenerate exchange prevents
the coverage from being directly measured in situ using 1H NMR spectroscopy. A lower bound for
their saturation coverage can be estimated by precipitating the nanocrysatls from concentrated
amine solution, drying them under vacuum, and measuring the amine content of the isolated
product (see Supporting Information). A range of coverages determined this way suggest the
saturation coverage of n-alkylamines is greater than > 3 nm-2. This is similar to typical coverages
of n-alkylcarboxylates following careful purification (3 – 3.5 carboxylates/nm-2).15-16, 25 Thus,
ligands with a smaller effective cross-sectional area can form a greater number of surface ligand
bonds and, in principle, compensate for a weak surface–ligand interaction. This helps explain the
affinity of relatively weak donors such as pyridine and n-pentylisocyanide. On the contrary, strong
donors, such as trialkylamines and N-heterocylic carbenes (NHCs) (pKa(NHC–H+) ~ 23)40 may
form a strong surface–ligand bond, but achieve low coverages when their substituents are bulky
(e.g. mesityl). Nonetheless, NHCs form especially stable monolayers on gold surfaces that are
resistant to displacement by sterically unencumbered thiols.41 In all cases, to understand the binding
affinity one must assess both the saturation coverage as well as the BDE(S–L).
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The precipitation caused by displacing Bu3P with pyridine confirms a recent study of
stoichiometric CdSe nanocrystals bound only by n-alkylamine ligands.15 That study suggested that
previously reported dispersions stabilized by pyridine are aided by acidic impurities that contribute
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electrostatic stabilization.10,
The same study also reported that stoichiometric CdSe
nanocrystals stabilized by Bu3P alone (CdSe–Bu3P) were unstable to aggregation, which is at odds
with the stability of CdSe–CdCl2/Bu3P herein.15, 25 Interestingly, adding CdCl2 to partially
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aggregated CdSe–Bu3P15 forms a clear, stable dispersion that is indistinguishable from the CdSe–
CdCl2/Bu3P used in this study (See Supporting Information). The mechanism by which CdCl2
improves the colloidal stability is unclear and the subject of current investigations in our lab.
Given the high binding affinities and increased surface coverages of sterically unencumbered
ligands observed above, we sought to stabilize stoichiometric CdSe nanocrystals in the absence of
CdCl2 using P,P-dimethyl-n-octadecylphosphine (Me2P-n-octadecyl). CdSe–Me2P-n-octadecyl
was prepared from CdSe–NH2Bu15 via ligand exchange. Addition of Me2P-n-octadecyl to CdSe–
NH2Bu in C6D6 does not displace n-butylamine, as expected from the relative binding affinities
measured above (Figure 3), until the primary amine is removed under vacuum with heat (See
Supporting Information). Binding of the Me2P-n-octadecyl ligand can be monitored by the
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appearance of a broad P NMR resonance (δ = -38 ppm, Δδ = 15 – 20 ppm) that increases in
intensity as the amine ligands are desorbed. Following complete removal of NH2Bu, stable
colloidal dispersions are obtained. The Me2P-n-octadecyl coverage reaches 2 nm-2, similar to the
coverage of phosphine ligands in CdSe–CdCl2/Me2P-n-octyl. We conclude that the higher ligand
coverage and the long n-octadecyl chain provide greater colloidal stability to CdSe–Me2P-n-
octadecyl compared to CdSe–Bu3P.
Interestingly, Me2P–n-octadecyl undergoes slow degenerate ligand exchange with CdSe–
Me2P-n-octadecyl on the NMR timescale. Even at temperatures as high as 390 K, the average
Me2P–n-octadecyl exchange rate constant is lower than 10-3 s-1 (see Supporting Information). This
suggests that phosphines form stronger surface ligand bonds than n-alkylamines, which undergo
fast degenerate exchange on the 1H NMR timescale at room temperature.23-24 Similarly, phosphines
are known to bind aqueous Cd2+ more tightly than isostructural amines.49-50 Thus, we tentatively
conclude that tri-n-alkylphosphine ligands have a greater BDE(S–L) than primary n-alkylamine
ligands, yet their affinity for the surface is lower because primary n-alkylamines achieve higher
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surface coverages, as depicted in the table of contents graphic. To properly assess their relative
BDE(S–L), it will be important to measure the relative binding of these ligands at the same surface
coverage, and ideally at multiple coverages for the reasons related to the surface coverage
dependent affinity discussed above.
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In all cases described herein, the surface ligand coverages are significantly lower than the aerial
density of atoms on the CdSe surface (5.4 – 6.2 nm-2) and the packing density of crystalline alkane
chains (4.9 nm-2). These low coverages suggest that repulsive interactions between ligands can
block adjacent binding sites and many binding sites will remain uncoordinated. While surface
coverages higher than the areal density of crystalline alkanes or binding sites on the crystal surface
are sometimes reported, these values may reflect the formation of multilayers or the presence of
free ligands, rather than the number of surface–ligand bonds.51 On the other hand, the highly curved
surfaces of very small nanocrystals can accommodate a greater number of surface ligands. For
example, pyramidal CdSe clusters with 1.7 – 2.5 nm edge lengths have 1.5 – 2x increased volume
available for their ligands compared to a flat facet and one benzoate or n-butylamine ligand can
bind every available coordination site.52 These high ligand coverages are thought to stabilize their
so called “magic” sizes. However, as the particle size increases and the curvature drops, the packing
of ligands must fall below that of crystalline n-alkanes (4.9 nm-2). Thus, the high atom density of
surfaces causes steric interactions between ligands that reduces their packing on the nanocrystal
surface and lowers their surface binding affinity.
Self-assembled monolayers (SAMs) pack with aerial densities (4 – 4.6 nm-2) just below those
of crystalline alkanes.53-56 On the Au(111) surface, thiolate SAMs assume high symmetry,
crystalline structures that are commensurate with the underlying lattice, but much less densely
packed (4.6 nm-2) than the surface atoms (12 atoms nm-2). Van der Waals interactions between
chains within the SAM strengthen the binding and increase as the chain length grows.57-58 However,
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the Si(111) surface has an aerial density of atop sites (7.8 nm-2) that is greater than the maximum
packing density of alkane chains. Each surface atom on Si(111) presents a single dangling bond
that can be terminated by a Si–H or Si–Me bond, however larger functional groups, such as ethyl,
do not form a complete monolayer.59 Moreover, theoretical and experimental work has shown that
the rotation of methyl groups on Si(111) is hindered by steric interactions with neighboring
methyls.60 In both cases interactions between neighboring ligands dictate the coverage and
structure of these surface layers.
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On the surfaces of II-VI and III-V nanocrystals, the areal densities of surface atoms are equal to
or lower than Si(111), but still greater than the crystalline alkanes in most cases. Even straight
chain ligands such as NH2Bu will not bind every available site as the surfaces grow beyond a few
nanometers. In addition, amine and phosphine ligands typically form a single dative bond while
the [100] surface presents two dangling bonds per surface atom. Thus, the surfaces of colloidal
nanocrystals will contain many vacant coordination sites if organic ligands are the exclusive
surface binding agent. Nanocrystals stabilized solely by sterically bulky ligands (e.g. Bu3P), can,
therefore, be expected to contain even greater numbers of vacant coordination sties. As a result,
there is a significant driving force to displace large bulky ligands and increase the number of
surface ligand bonds. Thus, the steric size of the ligand strongly influences its ligand binding
affinity.
Conclusion
The stereoelectronic properties of amines and phosphines were surveyed using competitive
binding experiments. Soft, electron rich donor ligands bind the surface most tightly making
phosphines a better ligand than their isostructural amines. However, the surface coverage of ligands
is very sensitive to their steric bulk with coverages varying by 6x or more depending on the ligand
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structure. The large difference in the number of surface–ligand bonds has a significant impact on
the competitive binding equilibrium, such that analyzing donor strength of the ligand alone is not
sufficient to determine which ligands have a high binding affinity. Hence a strong Lewis base may
therefore be readily displaced from the surface by weaker Lewis base with a smaller steric profile.
The displacement of Bu3P by pyridine, a much harder and weaker Lewis base, is a good example
of this reactivity. The impact of steric bulk on the coverage and competitive binding is expected
for all the binary semiconductor crystals whose surface atoms are more densely packed than
crystalline alkane chains, particularly as the nanocrystal grows larger than a few nm.
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Acknowledgements
This work was supported by the Department of Energy (Grant No. DE-SC0006410). Nicholas
Anderson and Peter Chen were supported by National Science Foundation Graduate Research
Fellowships (Grant No. DGE07-07425 and DEG11-44155, respectively). Jonathan De Roo
acknowledges support from the Belgian American Education Foundation (BAEF), Fulbright and
the European Union's Framework Program for Research and Innovation Horizon 2020 (2014-2020)
under the Marie Skłodowska-Curie Grant Agreement COMPASS No. 691185. The authors
acknowledge Ponisseril Somasundaran for helpful discussions.
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General Considerations. All manipulations were performed using standard Schlenk techniques or
within a nitrogen atmosphere glovebox unless otherwise indicated. Pentane, toluene, methyl
acetate, diethyl ether and tetrahydrofuran were purchased anhydrous from Sigma Aldrich and
shaken over activated alumina, filtered, and stored over 4 Å molecular sieves in an inert atmosphere
glovebox at least 24 h prior to use. Diphenylphosphine (99%), N,N,′N,′N-tetramethylethylene-1,2-
diamine (TMEDA) (98%), triethylphosphite (99%), tri-n-octylphosphine (97%), triethylphosphine
(99%), and tri-n-butylphosphine (99%) were purchased from Strem and used without further
purification. CdMe2 was purchased from Strem and vacuum distilled prior to use. CAUTION:
Dimethylcadmium is extremely toxic and because of its volatility and air-sensitivity should only be
handled by a highly trained and skilled scientist. N,N-Dimethylbutylamine (98%), furan (99%),
thiophene (98%), n-butylamine (98%), di-n-butylamine (98%), trimethylphosphine (99%), , n-
pentylisocyanide, di-n-butylsulifide (98%), trichloromethylsilane (98%), tri-n-butylamine (99%),
n-octylamine (99%), benzene-d6 (99.9%) and pyridine (99.5%) were purchased from Sigma
Aldrich and dried over CaH2, distilled, and stored in a nitrogen glovebox. Toluene-d8 was
purchased from Cambridge Isotopes and dried over CaH2, distilled, and stored in a nitrogen
glovebox. Tri-n-octylphosphine oxide (99%) was purchased from Sigma Aldrich and recrystallized
from acetonitrile as reported previously.(34)
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CdSe–Cd(O2CR)2. Carboxylate terminated CdSe nanocrystals (CdSe–Cd(O2CR)2) are
synthesized and treated with Me2Cd to remove acidic impurities as previously described.61
CdSe–CdCl2/Bu3P. All manipulations are conducted on a Schlenk line at room temperature. In a
typical synthesis, a benzene-d6 stock solution of CdSe–Cd(O2CR)2 (1.0 ml, 0.5 – 2.0 mmolar
carboxylate, [CdSe] = 1.6 – 6.5 mmolar, [nanocrystal] = 4 – 16 µmolar) was transferred to a 50 ml
Schlenk tube with a magnetic stir bar. The solution was diluted to a total volume of 5 ml with
toluene to which Bu3P (0.506 g, 0.624 ml, 2.5 mmol) was added. Me3Si–Cl (6.0 – 24 mmol, 12
equiv.) was added and the solution stirred for 24 hours. After this time, the volatiles were removed
under vacuum and the red solid dissolved in pentane (5 ml) and a methyl acetate was added to
precipitate the nanocrystals, which were separated by centrifugation (7000 RPM for 5 minutes).
This process was repeated twice more, after which the red powder was dried overnight under
vacuum. The nanocrystals were dispersed in benzene-d6 to a CdSe concentration of 0.5 – 1.0 M, as
described previously.16
Competitive Displacement of Bu3P from CdSe–CdCl2/Bu3P. Benzene-d6 stock solutions of
various competitor ligands are prepared in a nitrogen filled glove box by diluting the ligand (0.9
mmole) with benzene-d6 (1 ml). Using a 25 µl syringe, 10 µl of this stock solution (9 µmoles of
ligand) is added to a benzene-d6 solution of CdSe-CdCl2/Bu3P (600 µl, 15 mM in Bu3P, 0.6 mM
in nanocrystal) in a J-young NMR tube to form an equimolar solution of the added ligand and Bu3P.
1
31P{1H} and H NMR spectra are acquired within 1 hour (31P{1H}: 2 sec delay with 0.1 sec
acquisition, 800 scans; 1H: 30 sec delay with 5 sec acquisition, 16 scans). The J-young tube is then
transferred to a nitrogen filled glove box where the appropriate mass of neat ligand is added to
bring the total concentration of ligand to 0.75 M (50 equiv.). The J-young tube is then sealed and
1
31P{1H} and H NMR spectra are acquired as described above. In some cases the procedure is
reapeated to bring the concentration of competitor ligand to 1.5 M (100 equiv.).
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P,P-Dimethyl-n-octylphosphine. P,P-dimethyl-n-octylphosphine was prepared on 19.7 mmole
scale from n-octylmagnesium bromide and chlorodimethylphosphine as previously described.62
31P{1H} NMR (C6D6, 162 MHz): δ = -55 ppm, (chloroform-d, 162 MHz): d = -51 ppm. 1H NMR
(chloroform-d, 400 MHz): d = 0.89 (d, 6H, -CH3), 0.91 (t, 3H, -CH3) 1.2-1.6 (b, 12H, -CH2), 1.59
(m, 2H, β-CH2), 1.98 (m, 2H, -PCH2). 31P{1H} NMR (s).
Synthesis of CdSe–CdCl2/Me2P–n-octyl. All manipulations were conducted on a Schlenk line at
room temperature. In a typical synthesis, a benzene-d6 stock solution of CdSe–Cd(O2CR)2 (1.0
ml, 0.5–2.0 mmol ligand) with a known carboxylate concentration was transferred to a 50 ml
Schlenk tube with a magnetic stir bar. The solution was diluted to a total volume of 5 ml with
toluene to which Me2P-n-octyl (0.438 g, 2.5 mmol) was added. Me3Si–Cl (0.651 – 2.607 g, 6.0 –
24 mmol, 12 equiv.) was added and the solution stirred for 24 hours. After this time, the volatiles
were distilled off under vacuum and the red solid dissolved in toluene (5 ml) and methyl acetate
was added to precipitate the nanocrystals, which were separated by centrifugation (7000 RPM for
5 minutes). This process was repeated twice more, after which the red powder was dried overnight
under vacuum. The nanocrystals were diluted in toluene-d8 to [nanocrystal] = 0.5 – 1.0 mM and
analyzed 31P{1H} and 1H NMR spectroscopies.
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Supporting Information
Variable temperature NMR spectroscopy of CdSe–CdCl2/Me2P–n-octyl, the synthesis of CdSe–
CdCl2/PBu3 from CdSe–NH2Bu, the synthesis of Me2P–n-octadecylphosphine and the synthesis
of CdSe–Me2P–n-octadecylphosphine are described in the supporting information.
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