Synthesis of Porphyrin-CdSe Quantum Dot Assemblies: Controlling Ligand Binding by Substituent Effects

Article abstract of DOI:10.1021/acs.inorgchem.5b00892

Cadmium selenide quantum dots of 2.2-2.3 nm diameter were prepared by phosphorus-free methods using oleic acid as stabilizing surface ligand. Ligand exchange monitored quantitatively by ¹H NMR spectroscopy gave an estimate of 30-38 monodentate ligands per nanocrystal, with a ligand density of 1.8-2.3 nm^-2. The extent of ligand exchange with macrocycles carrying one or more functional groups was investigated, with the aim of producing nanocrystal-macrocycle conjugates with a limited number of coligands. Metal-free porphyrins are able to sequester the Cd²⁺ ions from the Cd(oleate)₂ outer layer of the nanocrystals. Zinc porphyrin complexes carrying one carboxylate function displace oleate efficiently to give porphyrin/CdSe composites with porphyrins stacked upright on the crystal surface. Porphyrins with four potential ligating sites are able to bind to the crystal surface only if the donors are at the end of sufficiently long and flexible tethers. High-dilution methods allowed the synthesis and isolation of well-defined composites of composition [CdSe{porphyrin}₂], where porphyrin = 5,10,15,20-tetrakis{3-(carboxy-n-alkyloxy)phenyl}porphyrinato zinc (n = 5 or 10) and 5,10,15,20-tetrakis{3-(11-undecenyloxythiol)phenyl}-porphyrinato zinc. Comparison of the composition data obtained by ¹H NMR spectroscopy with luminescence quenching behavior suggests a dependence of quenching efficiency on the tether length. Luminescence quenching was also observed for porphyrins that, according to ¹H NMR results, do not undergo surface ligand exchange.

Full text of DOI:10.1021/acs.inorgchem.5b00892

Article
pubs.acs.org/IC
Synthesis of Porphyrin−CdSe Quantum Dot Assemblies: Controlling
Ligand Binding by Substituent Effects
Isabelle Chambrier, Chiranjib Banerjee, Sonia Remiro-Buenamanana, Yimin Chao,
̃
Andrew N. Cammidge,* and Manfred Bochmann*
School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K.
S
* Supporting Information
ABSTRACT: Cadmium selenide quantum dots of 2.2−2.3 nm diameter were prepared by phosphorus-free methods using oleic
acid as stabilizing surface ligand. Ligand exchange monitored quantitatively by 1H NMR spectroscopy gave an estimate of 30−38
monodentate ligands per nanocrystal, with a ligand density of 1.8−2.3 nm−2. The extent of ligand exchange with macrocycles
carrying one or more functional groups was investigated, with the aim of producing nanocrystal−macrocycle conjugates with a
limited number of coligands. Metal-free porphyrins are able to sequester the Cd2+ ions from the Cd(oleate)2 outer layer of the
nanocrystals. Zinc porphyrin complexes carrying one carboxylate function displace oleate efficiently to give porphyrin/CdSe
composites with porphyrins stacked upright on the crystal surface. Porphyrins with four potential ligating sites are able to bind to
the crystal surface only if the donors are at the end of sufficiently long and flexible tethers. High-dilution methods allowed the
synthesis and isolation of well-defined composites of composition [CdSe{porphyrin}2], where porphyrin = 5,10,15,20-tetrakis{3-
(carboxy-n-alkyloxy)phenyl}porphyrinato zinc (n = 5 or 10) and 5,10,15,20-tetrakis{3-(11-undecenyloxythiol)phenyl}-
1
porphyrinato zinc. Comparison of the composition data obtained by H NMR spectroscopy with luminescence quenching
behavior suggests a dependence of quenching efficiency on the tether length. Luminescence quenching was also observed for
1
porphyrins that, according to H NMR results, do not undergo surface ligand exchange.
Semiconductor nanocrystals are stabilized by surfactant-type
surface ligands which determine the growth rate, stability, and
solubility of the nanocrystals.6 For example, while CdSe
prepared using trioctylphosphine (TOP) and trioctylphosphine
oxide (TOPO) are frequently represented with a surface
coverage of TOPO, it is now known that the surface is mainly
covered by TOP oxidation products, alkyl phosphinates and
phosphonates,7−10 which coordinate to metal surface sites as
bridging ligands.11 As such they can be difficult to displace in
ligand exchange reactions.11−13
Nanocrystals prepared under phosphorus-free conditions
using long-chain carboxylic acids such as stearic or oleic acid are
stabilized by a layer of metal carboxylate.14 The nature of ligand
binding has been explored mainly by NMR spectroscopy,15,16
as well as luminescence17 and isotopic labeling18 methods.
INTRODUCTION
■
The arrangement of nanosized objects into more complex
structures remains a challenging target.1−3 One of the
preconditions for such higher level organization is the
introduction of a limited number of attachment points. This
offers the opportunity to link up spherical nanoparticles using
covalent bonds or specific donor−acceptor interactions to
generate, for example, divalent4 or trivalent nanoparticles that
can be the building blocks for linear or branched arrangements.
For gold nanoparticles monofunctionalization and binary
linkers have been realized in a number of cases.5
We were especially interested in the functionalization of
colloidal semiconductor quantum dots (QDs), in particular
CdSe, and were looking for ways of restricting the number of
surface ligands by macrocyclic attachments, using coordination
chemistry principles for the construction of stable nanocrystal−
organic conjugates. The successful functionalization of nano-
particles requires an understanding of their surface chemistry.
Received: April 22, 2015
© XXXX American Chemical Society
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Chart 1
Figure 1. (a) Typical UV−vis (blue) and fluorescence (red) spectra of CdSe nanoparticles used in this study (CHCl3 solution). (b) Transmission
electron microscopy (TEM) image of this CdSe sample.
These carboxylate ligand spheres show dynamic behavior in
solution and undergo a multitude of ligand exchange
reactions,15,19,20 although the preparation of heteroligated
nanocrystals at the synthesis stage has also been reported.21
Macrocyclic compounds with a rigid aromatic framework,
such as phthalocyanines, subphthalocyanines, and porphyrins,
are synthetically readily accessible and can easily be derivatized
to give compounds with a predetermined number of anchor
points (usually 0−4). Such ligands are expected to bind to
nanocrystal surface sites by substitution of the surfactant
ligands present from the synthesis stage. For a given
concentration, due to the chelate effect polydentate ligands
should bind to the surface of a nanocrystal significantly more
strongly than monodentate ligands. In continuation of our
synthetic and structural investigations of porphyrin and
phthalocyanine systems,22 we decided to focus on ligands of
types I−IV for the present study (Chart 1). Subphthalocya-
nines I possess a cup-shaped structure,23 which suggested a
good geometric match with nanocrystals with diameters in the
2−3 nm range; in addition they contain a functional group
perpendicular to the macrocyclic core which offers the
possibility of connecting to other ligand-decorated nano-
particles by covalent bonds. Porphyrins of type II would be
expected to act as monodentate ligands oriented perpendicular
to the nanocrystal surface. On the other hand, although the
central ring in porphyrins of type III is flattened, there is
sufficient flexibility both in the ring and in the functionalized
substituents X to allow these compounds to attach themselves
to nanocrystals flat-on, i.e., parallel to the crystal surface.
Alternatively these ligands may adopt a perpendicular
orientation bridging between two adjacent quantum dots.
The structures and properties of the CdSe−macrocyclic
1
constructs have been evaluated using a combination of H
NMR, absorption, and fluorescence spectroscopies.
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These experiments also demonstrated that phosphonate
binding cannot be assessed by 31P NMR spectroscopy since no
31P NMR signal was observed for phosphonate-exchanged
CdSe; liberation of the phosphonate by a dodecanethiol/NEt3
mixture was required in order to detect the 31P NMR signal.
For this reason the surface coverage of our ligand-exchanged
quantum dots was always assessed by purification followed by
quantitative displacement with thiolate.
RESULTS AND DISCUSSION
■
CdSe Nanoparticle Preparation. Although CdSe quan-
tum dots are frequently synthesized using trioctylphosphine
(TOP)/trioctylphosphine oxide (TOPO) as coordinating
solvent and surfactants, as discussed in the Introduction,6−10
the surface chemistry of these nanoparticles is not well-defined.
We therefore favored a modified octadecene (ODE)/oleic acid
(OA) method,24,25 especially because there were fewer
uncertainties concerning the composition of the ligand surface.
The CdSe nanocrystals used in this study were prepared using
CdO, oleic acid in octadecene (ODE), and Se powder,
following the method of Jasieniak et al.24 This method
produced highly monodisperse particles, coated exclusively
with oleate ligands. The particles are readily soluble in
chloroform, and all ligand exchange reactions described here
were carried out in that solvent. Typical examples of UV−vis
and fluorescence spectra are shown in Figure 1a. The diameter
of the nanoparticles (d = ca. 2.2−2.3 nm, confirmed by TEM,
Figure 1b)26 and the solution concentration were calculated
from the UV absorption maximum using the method by Yu et
al.27 A fluorescence quantum yield (QY) of ca. 3% was
measured by a comparative method described by Williams et
al.28 using quinine sulfate in 0.1 N H2SO4 as reference (QY =
54.6%).
Number of Ligands per Nanoparticle. The number of
oleate ligands per nanoparticle and hence the ligand density
1
was evaluated using a combination of UV−vis and H NMR
1
spectroscopies. The H NMR spectrum of the pure nano-
particles shows a broad signal at ca. δ 5.3 assigned to the cis-
CHCH protons of the bound oleate ligands. When attached
to a solid particle, the resulting slow motion and the diverse
chemical environments the molecules experience lead to
broadening of the peaks. The effect is especially pronounced
for the atoms closest to the surface whose signals can
sometimes be shifted or disappear.21,30 Several methods were
employed for monitoring and measuring surface coverage and
exchange. For the as-prepared nanoparticles, introduction of a
known quantity of an NMR integration standard (ferrocene,
sharp singlet at δ 4.16, CDCl3) allows direct calculation of
surface coverage by integration of the broad δ 5.3 ppm signal.
For 2.0 nm particles the average surface coverage is calculated
as 30 oleate ligands per particle (see Supporting Information).
For other ligands, the native oleates were exchanged by
titration; the release of oleic acid from the surface is
accompanied by the appearance of a sharp signal of free oleic
acid in solution and addition was continued until no further
sharpening of the free oleic acid signal was observed. A typical
example is shown for 11-bromoundecanoic acid in Figure 2,
and the results, presented in Table 1, show the range of
calculated results over a number of nanoparticle synthesis and
exchange sequences. The values of ligand density obtained in
Table 1 could, in principle, be affected by the ligand exchange
dynamics, especially in the case of acid−acid exchange.
However, thiols, which quantitatively displace carboxylates
Relative Ligand Binding Strength. Oleic acid (octadec-9-
enoic acid) possesses a cis-vinylene moiety which acts as a
convenient 1H NMR marker. Because of the inhomogeneity of
surface binding sites and enhanced relaxation times, the
vinylene signal of surface-bound oleate is broad,15,21 without
a detectable coupling pattern, but sharpens when oleate is
released. Preliminary qualitative experiments served to establish
the relative binding preference of carboxylic acids, phosphonic
acids, and thiols in our system and showed the sequence
RCOO− < RPO3H2 ≈ RSH < R−PO3 < RS−
2−
For example, both dodecanethiol and 11-undecenylphos-
phonic acid quantitatively displace oleate from the CdSe
surface. However, it was not possible to displace surface-bound
alkylphosphonate by thiol, unless triethylamine was added as a
base to generate equilibrium concentrations of the more
nucleophilic thiolate anion.
On the other hand, competition experiments using equimolar
mixtures of dodecanethiol and 11-undecenylphosphonic acid
gave nanoparticles which showed a S:P ratio of 3:1 in the
absence of NEt3. Under such conditions the exchange
presumably involves the hydrophosphonate anion, R′PO3H−
(eq 1). On addition of NEt3, however, the S:P ratio of the CdSe
ligand sphere increased to 11:1, demonstrating preferential
binding of thiolate under basic conditions.29
CdSe(O2CR)x + yR′PO3H2 + zR″SH
HooooooI CdSe(SR″)z(R′PO3H)y + xRCOOH
x=y+z
(1)
In these reactions 11-undecenylphosphonic acid was chosen
since the terminal −CHCH2 unit provides another 1H NMR
1
marker; its H NMR signal occurs conveniently in a clear part
of the spectrum. Upon the first additions to an excess of CdSe
in solution, the terminal double bond signals shifted slightly
upfield. Although remote from the anchoring point, the double
bond signal shows the typical broadening effect induced by
binding to the nanocrystal surface.
1
Figure 2. H NMR resonances of the vinylene protons of the oleate
ligands upon successive additions of 11-bromoundecanoic acid (0−50
equiv). The arrows indicate the decrease of the broad signal of surface-
bound oleate at δ 5.3 and its replacement with the signal for free oleic
acid at δ 5.35 as the result of surface ligand substitution.
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calculate the maximum number of oleate ions that could
comfortably be accommodated on the surface of a nanoparticle
of d = 2.3 nm as 39−45, i.e., a density of 2.4−2.8 nm−2. Our
data in Table 1 fall below this value and are therefore
consistent. The collapse pressure, i.e., the pressure at which the
monolayer becomes unstable, was established at an area per
molecule of 27 Å2. Using this value, a maximum density of 3.7
nm−2 would be obtained for a very condensed system.
Table 1. Estimated Values of Ligand Densities from Ligand
Exchange as Observed by H NMR Spectroscopy
1
ligands per QD
ligand density (nm−2
)
11-bromoundecanoic acid
dodecanethiol
33−38
32−47
14−18
30
1.95−2.25
1.9−2.8
0.8−1.1
1.8
4,5-didodecylphthalic acid
a
oleic acid
a
As-prepared nanoparticles (2.0 nm diameter) measured against
Reactions of Subphthalocyanines with CdSe QDs. In a
series of initial reactions, the binding behavior of subphthalo-
cyanine I (Chart 1) was explored. This ligand is decorated with
three meta-pyridyl substituents which give a bite angle that
should be geometrically well-matched for binding to nano-
crystals of 2−3 nm diameter, provided there are accessible
Lewis acidic surface sites. Treatment of a solution of CdSe QDs
in chloroform gave a deeply colored solution. Quenching of the
QDs fluorescence was observed. The nanoparticles were
precipitated with methanol, isolated by centrifugation, and
repeatedly washed with acetone until the washings were free of
subphthalocyanine by UV/vis spectroscopy. Subsequent
ferrocene as NMR integration standard.
from Cd chalcogenide nanoparticles,6 yielded a similar
calculated ligand density and thus confirmed the values found
by carboxylate exchange. As a multidentate ligand, 4,5-
didodecylphthalic acid is expected to bind more strongly than
monodentate ligands31 and displace two oleates, which is what
was found. All measurements show good agreement. The values
calculated are lower than that obtained by Hens and co-workers
(4.6 nm−2)15c but are in agreement with those found by
Anderson and Owen (2.0−2.9 nm−2)32 for carboxylate-ligated
CdSe nanocrystals of similar size.
1
analysis by H NMR spectroscopy showed, however, that the
Assuming a ratio of Cd:Se per nanoparticle of ca. 1.15:1,7,33
a
peaks of the residual SubPc contained in the sample were
neither broadened nor shifted, indicating that the SubPc was
not bound but probably trapped in the forest of aliphatic chains
of oleic acid. Unsubstituted SubPc also induced quenching of
QDs fluorescence (vide infra).
Carboxylate-coated nanoparticles of the type used here have
two pathways for ligand substitution, the displacement of an X−
(oleate) anion, or displacement of surface-bound CdX2. The
latter pathway would have to be operative in the case of neutral
donors like pyridine-decorated sub-Pc. Evidently, at least with
carboxylate-terminated nanoparticles, this pathway is not
competitive.
Reaction of Metal-Free Porphyrin Derivatives with
CdSe Nanoparticles. The interaction of pyridine-substituted
free-base porphyrins as well as their metal complexes with CdSe
and Cd/ZnS core−shell particles made by the TOPO route has
been intensively studied, especially by Zenkevich and
molecular formula of Cd128Se111 can be derived for nano-
particles of d = 2.3 nm. Comparing the number of oleate
ligands to the excess cadmium per CdSe quantum dot (17), a
ratio of ca. 2:1 was obtained, i.e., the surface excess of Cd2+ is
balanced by a double amount of oleate ligands as reported
earlier by Hens,15c giving an average composition of our CdSe
particles as [Cd111Se111{CdX2}17] (X = oleate).
The area per molecule of a Langmuir film of oleic acid was
measured by Tomoaia-Cotisel et al. as A0 = 41 Å2 on water (pH
= 2).34 By comparison, stearic acid yielded a lower area per
molecule of A0 = 20 Å2 under the same conditions. The kink
induced by the cis-double bond in oleic acid is believed to be
the cause of this increase in area. The authors confirmed these
results through calculations taking into account the van der
Waals radius of H and obtained an area comprised between
36.0 and 41.0 Å2. Based on these “footprint” data, we can
Figure 3. Conversion of metal-free 5,10,15,20-tetra(4-decyloxyphenyl)porphyrin (1) to 5,10,15,20-tetra(4-decyloxyphenyl)porphyrinato cadmium in
the presence of CdSe nanoparticles over time (room temperature, CH2Cl2) (dark blue line, CdSe alone, λmax 486 nm; metal-free porphyrin, λmax 420
nm; cadmium porphyrin, λmax 433 nm; time = 0, 1, 16, 24, 40, 64, 140 h).
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others.35−37 We were therefore interested in the interactions of
carboxylate-type CdSe nanocrystals with carboxylate-substi-
tuted porphyrins.
The ligand exchange process was assessed by H NMR
spectroscopy. Aliquots of a solution of 2 were added to a
solution of CdSe and NMR spectra obtained after each
addition. After ca. 20 equiv of 2 had been added, the signals
characteristic of free oleic acid in solution started to appear.
The contents of the tube was transferred to a centrifuge tube,
methanol was added to flocculate the nanoparticles, and the
mixture was centrifuged. Porphyrin 2 is soluble in methanol,
and the supernatant was colored. Further washings were
required until the UV spectrum of the supernatant confirmed
the absence of porphyrin. The nanoparticle sample was
dissolved in CDCl3; the resulting 1H NMR spectrum is
shown in Figure 5a. As expected in the case of surface-bound
molecules, the signals are very broad; in fact the signals
attributed to the phenyl ring bearing the carboxylic function are
not visible at all and broadened into the baseline. Adding an
excess of dodecanethiol releases the porphyrin and gives the
spectrum shown in Figure 5b, along with that of oleic acid.
Integration gives a ratio of 1:16 porphyrin:oleate. Based on the
average number of surface ligands per nanoparticle determined
earlier (Table 1), this equates to about two bound porphyrin
molecules per quantum dot.
1
Preliminary studies were carried out with 5,10,15,20-tetra(4-
decyloxyphenyl)porphyrin38 (1) (Figure 3), which carries only
alkyl substituents but no surface-binding functional groups. It
quickly became evident that metal-free porphyrins become
metalated in the presence of CdSe nanoparticles over time at
room temperature. The insertion of Cd2+ into the ring system
was evidenced by a visible color change; this was confirmed by
the UV−vis spectra. Figure 3 shows the conversion of the
metal-free 1 into 5,10,15,20-tetra(4-decyloxyphenyl)-
porphyrinato cadmium as a typical example. In fact all metal-
free porphyrins in this work behaved similarly, whether these
were designed to bind to the nanoparticles or not. The Soret
band shifts from 420 to 433 nm, and this is accompanied by the
characteristic Q-band reduction from 4 to 2 absorption peaks
that can be seen in the inset. It was further established that only
the excess Cd2+ on the surface of the nanoparticles reacts in this
way, as the reaction stops when these are depleted, even though
excess porphyrin-H2 is present.
The metalation of tetraphenylporphyrins by metal films has
been observed before; in these cases the macrocycle was
obviously able to bind face-on to the metal surface.39 This is
evidently not possible in this case. Metal sequestration by
porphyrins that are not capable of coordinating directly to the
nanoparticle surface is possible only if surface-bound CdX2 is
part of a solution equilibrium, which is shifted by Cd2+ uptake
by the macrocycle (eqs 2 and 3), in line with observations by
Anderson et al.16c
The addition of 20 equiv of porphyrin was deemed sufficient
as we anticipated that not more than half of the oleate ligands
would be exchanged due to steric constraints. However, free
oleic acid could be fully released in solution as judged from the
1H NMR spectrum when a very large excess of porphyrin was
added (>200 equiv), but the resulting isolated powder could
not be redispersed in any solvent.
Quantitative UV experiments showed a small bathochromic
shift in absorption of the porphyrin in the presence of CdSe
nanoparticles (Figure 6). This was only evident when CdSe was
added to an excess of a porphyrin solution.
CdSe(CdX2)x ⇌ CdSe(CdX2)x−1 + CdX2
(2)
CdX2 + porphH2 → Cd(porph) + 2HX
In order to examine the influence of the length of the tether
chain on binding ability and the number of surface-bound
porphyrins, two new Zn-porphyrins were synthesized, 3 and 4,
which bear anchoring and solubilizing alkyl chains of different
lengths. The synthetic route is shown in Scheme 1.
(3)
Consequently, the work described hereafter was carried out
using zinc metalated porphyrin molecules.
Monodentate Porphyrin Ligands and CdSe Nano-
particles. The binding of monodentate porphyrin ligands was
first examined (Figure 4). The macrocycles were functionalized
with a single carboxylic acid function that would act as the
anchoring point. 5-(4-Carboxyphenyl)-10,15,20-triphenylpor-
phyrinato zinc (2) was synthesized using a modified literature
procedure.16
Aliquots of a solution of porphyrins 3 or 4 (up to ca. 50
equiv) were added to a solution of CdSe, and the binding
1
process was followed by H NMR spectroscopy. Based on the
shift and change of shape of the vinyl oleate region signals, it
appears that the longer C11 chains of 4 are better able to induce
release of oleic acid than the shorter C6 chains of 3. When
additions were complete, the nanoparticle−porphyrin con-
jugate was isolated and extensively washed with acetone, as
described for 2. The samples were subsequently dried and
1
redissolved in CDCl3. The aromatic region of the H NMR
spectrum of 4 (Figure 7a) showed a mixture of broad and
sharper peaks. The NMR spectrum of 3 was very similar.
Integration of the whole region yielded a value identical to the
integration after addition of dodecanethiol, Figure 7b. The inset
in Figure 7 shows the signals associated with the −CH2−CO2
protons of the carboxylic acid function, in surface-bound (lower
trace) form and after release with dodecanethiol (upper trace).
Integration of the ligand signals after thiol treatment gave a
porphyrin:oleate ratio of 4:1 for the shorter chain ligand 3, and
8:1 for 4. As can be expected from steric considerations, it is
evidently easier for the longer chain ligand to penetrate the
oleate ligand shell and undergo ligand exchange. Unlike
compound 2, binding had no effect on the UV absorption
spectra of either 3 or 4.
Figure 4. Structures of monodentate porphyrin ligands 2−4.
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1
Figure 5. H NMR spectra of isolated nanoparticles following the addition of ca. 20 equiv of 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrinato
zinc (2) in the absence (a) and presence (b) of excess dodecanethiol (7.6−9.1 ppm region).
Figure 6. UV/vis spectra of the sequential addition of CdSe to porphyrin 2 showing the bathochromic shift from 419 nm (2) to 423 nm (2 + CdSe).
Tetradentate Porphyrin Ligands and CdSe Nano-
particles. While monodentate porphyrin ligands were readily
able to substitute oleate ligands, their steric requirements are
moderate since they bind to the nanoparticle surface in
“upright” position. Polydentate ligands, on the other hand, have
the potential of covering large sections of the nanocrystal
surface by lying flat, and we were interested in the question
whether or not a nanoparticle could be essentially encapsulated
by porphyrins and what binding mode would be adopted. The
tetrasubstituted porphyrins 5−9 (Figure 8) were therefore
synthesized (see Supporting Information for details). In
addition to the carboxylate functionalities, porphyrins 5 and 6
carry long chain alkenyloxy substituents to aid solubility, each
equipped with a terminal CHCH2 moiety that acts as an
additional 1H NMR marker. The metalation with Zn had to be
achieved as the last step in the synthesis as demetalation
occurred during the ester hydrolysis. The use of weakly
coordinating organic solvent such as THF or CH2Cl2 led to the
formation of an insoluble material, probably resulting from the
formation of coordination polymers from the interaction of Zn
with the carboxylate functions. Consequently, pyridine was
used as solvent in the metalation reaction as it appeared to
suppress this behavior.
Porphyrins were chosen due to the flexibility of the molecule.
It is well-known that, in the case of tetraphenylporphyrins, the
phenyl groups in the meso positions lie perpendicular to the
tetrapyrrolic ring. Such a ligand is therefore able to bind parallel
to the nanoparticle surface, as in Figure 9a. Of course, the
phenyl substituents are free to rotate, and geometries b, c, and d
will also be present.
After addition of ca. 10 equiv of 5 (i.e., 40 equiv of acid
functions), there was no evidence by 1H NMR spectroscopy for
any binding of this porphyrin 5 to the nanoparticles. The
aromatic peaks were slightly shifted downfield relative to the
starting material but not broadened, and the oleate signal did
not sharpen. This contrasts with the behavior of 2, which has a
similarly short carboxylate linker. This lack of binding could be
taken as an indication of the molecule’s attempt to lie flat on
the nanoparticle surface, without being able to break through
and displace the oleate layer. It was clear that a longer tether
was required, and compound 6 was prepared.
The reaction of 6 with CdSe caused a precipitate to form
which was clearly visible after 1 equiv of 6 was added to the
NMR tube. The supernatant was checked by UV/vis
spectroscopy and was free of both porphyrin and CdSe.
There was clearly an interaction between 6 and the nano-
particles disrupting the stability of the colloidal suspension. The
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Scheme 1. Preparation of 5-(4-Carboxyhexyl-6-oxyphenyl)-10,15,20-tris(4-decyloxyphenyl)porphyrinato Zinc (3) and 5-(4-
a
Carboxyundecyl-11-oxyphenyl)-10,15,20-tris(4-decyloxyphenyl)porphyrinato Zinc (4)
a
Reagents: (i) K2CO3, DMF, BrC10H21; (ii) K2CO3, DMF, Br(CH2)nCOOR; (iii) propionic acid, pyrrole; (iv) THF, aqueous NaOH; (v) THF,
Zn(OAc)2.
after shaking a suspension of the solid in the presence of more
porphyrin, as judged from the absence of porphyrin absorption
in the supernatant (UV/vis).
Compounds 7 and 8 (Figure 8) were synthesized to increase
the flexibility of the alkyl chains carrying the carboxylic acid
functions (for details see Supporting Information).
Aliquots of a solution of porphyrin 8 in THF-d8 were added
1
to a solution of CdSe in CDCl3 (9 mg in 0.5 mL), and the H
NMR spectra were recorded after each addition (Figure 11). All
the aromatic proton signals are very broad and almost disappear
into the baseline, which indicates that all four
−C6H4(CH2)10COOH substituents must interact with the
nanoparticles equally strongly. A precipitate was observed after
the addition of 2 equiv of porphyrin. In this case, due to the
presence of THF-d8, the oleate signals shifted upfield upon
successive additions (relative to the peak of residual CHCl3),
Figure 7. 1H NMR spectra (CDCl3) of isolated and redissolved CdSe
and sharpened, as expected for exchanging free and coordinated
nanoparticles coated with 4: (a) aromatic region; (b) after addition of
oleates. Porphyrin 7 behaved identically.
dodecanethiol. The inset shows the −CH2CO2 signals before (lower
Because of the long flexible anchoring chains in these
trace) and after addition of dodecanethiol (upper trace).
compounds it was not unexpected that the resulting [CdSe-
{porphyrin}n] assemblies should show a tendency to cross-link
solid was isolated but could not be redissolved except after
addition of dodecanethiol, which released the bound porphyrin
from the nanoparticle surface as well as any remaining oleate
ligands. A possible binding mode of 6 to the nanoparticle is
shown in Figure 10a, which may explain the loss of colloidal
stability, despite the presence of the four undecyloxy chains.
However, it can also be envisaged that 6 may bridge across two
nanoparticles, Figure 10b, which is likely to lead to the
formation of a polymeric system, Figure 10c, causing the
formation of a precipitate. Indeed it is likely that the polymeric
system is prominent in this case. It was in fact possible to bind
more porphyrin molecules to the already precipitated material
and form insoluble polymeric systems. A high-dilution strategy
was therefore applied. 2 equiv of 8 was slowly added to a very
dilute CdSe solution (0.04 mg/mL) over a 16 h period. This
produced a clear solution free of any solid precipitate. The
solvent was then removed, leaving a 5−10 mL volume, which
was transferred to a centrifuge tube. The nanoparticle assembly
was precipitated with methanol and separated by centrifugation.
The resulting solid was washed several times with acetone until
the washings were clear (UV/vis), and then dried. The dried
material was readily soluble in CDCl3. The NMR spectra of the
[CdSe{porphyrin}] assembly (Figure 12) appeared totally
featureless in the aromatic region, implying that 8 was attached
G
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Figure 8. Structures of tetradentate porphyrin ligands 5−9.
Figure 9. Schematic representation of the possible geometries adopted by a tetrasubstituted porphyrin molecule bearing 4 anchoring points. The
clear circle represents the tetrapyrrolic ring, while the dark circles indicate the anchoring points.
Figure 10. Schematic representation of possible interactions of 6 and CdSe nanoparticles.
to individual nanocrystals in tetradentate fashion. The
porphyrin and oleate ligands could be liberated upon addition
of dodecanethiol. Peak integration yielded an oleate:porphyrin
ratio of ca. 15:1, consistent with the coordination of two
tetradentate porphyrins per nanoparticle, and since all aromatic
1H NMR signals are subject to the same amount of broadening,
a face-down coordination mode is indicated. The results also
highlight the importance of retaining a large proportion of the
original oleate ligands to ensure adequate solubility.
The TEM images of these [CdSe{porphyrin}] conjugates
showed no indication of aggregation. Together with the NMR
evidence this confirms the formation of well-defined porphyr-
in:CdSe assemblies where the porphyrin is bound by
tetradentate ligation.
Since thiol ligands show the highest binding constants for
coordinating to CdSe nanocrystal surfaces, the possibility of
generating a tetrathiol porphyrin was explored. Since polythiols
are very prone to oxidation and form insoluble disulfides,
porphyrin 9 (Figure 8) was prepared as the tetra(thioacetate),
with the deprotection and formation of the −SH compound
left to the last step, just prior to the addition to the
nanoparticles. The deprotection has to be carried out under
strictly anaerobic conditions (see Supporting Information for
details). Similar compounds have been used in the literature for
binding onto gold surfaces, but the deprotection step was
always carried out in situ.40 About 2 equiv of the deprotected
porphyrin was added slowly overnight to a dilute solution of
CdSe (0.04 mg/mL) in the presence of BHT (= 4-Me-2,6-
But2C6H2OH) as antioxidant. A clear solution was obtained
H
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Figure 11. 1H NMR spectra (5−10 ppm region) recording the addition of 8 in THF-d8 to CdSe nanoparticles in CDCl3. Blue: CdSe alone. Red: 0.5
equiv added. Green: 1 equiv added. Pink: 2 equiv added. Orange: after adding excess dodecanethiol.
Figure 12. 1H NMR spectra (5−9 ppm region) resulting from the isolated [CdSe{8}2] assembly after addition of 2 equiv of 8 under high-dilution
conditions to CdSe nanoparticles. Blue: [CdSe{8}2] assembly. Red: after addition of dodecanethiol.
from which the porphyrin−nanoparticle conjugates were
isolated through flocculation and centrifugation. The solid
product could be redissolved in CDCl3. The 1H NMR
spectrum was once again totally featureless, essentially identical
to Figure 12. Upon addition of dodecanethiol, the broad oleate
proton signal at ca. 5 ppm sharpened, indicating their
displacement by the monodentate thiol, but, as expected, no
porphyrin signal emerged from the baseline. UV/vis spectros-
copy confirmed the presence of porphyrin. The data are
therefore consistent with the formation of a [CdSe{porphyrin}]
assembly in which on average about two porphyrin ligands are
irreversibly bound to the nanocrystal surface in tetradentate
fashion.
luminescence. The influence of the porphyrin ligands bearing
acid groups was evaluated through comparisons with porphyrin
ligands that are not expected to bind (i.e., 5,10,15,20-
tetraphenylporphyrinato zinc (10) and 5,10,15,20-tetra(4-
decyloxyphenyl)porphyrinato zinc (1)). The results are
shown in Figure 13. For comparison, the effect of a non-
porphyrin acid, i.e., 4-nonylbenzoic acid (11), is also plotted.
All porphyrins show a quenching effect that could not be
assigned to FRET. Zenkevich et al.41 also concluded that non-
FRET quenching of fluorescence was operative in their work on
5,10,15,20-tetrapyridylporphyrin−CdSe/ZnS quantum dot
nanocomposites. From Figure 13 it is immediately clear that
the two nonbinding ligands, 1 and 10, have a very different
profile from the other, binding ligands. Although NMR
spectroscopy demonstrated that porphyrin 5 did not form
Fluorescence Quenching. All porphyrins in this work
induced fluorescence quenching of the quantum dot
I
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Article
porphyrin in this work is compound 2, which lacks an alkyl
linker between the ring and the −COOH function. This
suggests that the proximity of the porphyrin ring to the
nanoparticle plays an important role in the quenching
efficiency.
CONCLUSION
■
The results confirm the picture of CdSe nanocrystals generated
by phosphorus-free synthesis methods of a (CdSe)x core
surrounded by a CdX2 layer, where X = oleate. While tridentate
N-donors like pyridyl-substituted subphthalocyanines proved
unable to form identifiable nanocrystal/macrocycle conjugates,
monodentate zinc porphyrinates bearing one carboxylic acid
effectively displace oleate ligands and give high surface
coverage. Defined nanoparticle assemblies with only two
porphyrins per quantum dot can be obtained under high-
dilution conditions, provided the carboxylate donors are linked
to the ring by sufficiently long tethers to enable displacement of
oleates and binding of the porphyrin parallel to the crystal
surface. For ease of quantitative determination of ligand binding
Figure 13. Comparison of the fluorescence quenching effects of
porphyrins 1−10 in this study, as well as of tris(ethynylpyridine)-
subphthalocyanine I in comparison with unsubstituted subphthalo-
cyanine. 4-Nonylbenzoic acid (11) is included for comparison
purposes.
1
by H NMR integration, it proved important to release all
bound ligands by treatment with excess alkyl thiol, to overcome
the problems of line broadening. [CdSe{porphyrin}2]
assemblies are also obtainable using porphyrin tetrathiolates;
in such cases the S-function needs to be protected and the
protection group removed only immediately prior to the
reaction with the nanocrystals, under anaerobic conditions.
However, once formed, the binding of such tetrathiolato
porphyrins is irreversible, even in the presence of excess
dodecanethiol.
stable, long-lived adducts, the fluorescence quenching effect is
on a par with binding porphyrins; at least a fleeting association
has to be present to account for the observed quenching.
However, these results illustrate that fluorescence alone cannot
be used reliably as a measure for ligand exchange processes.
The quenching data are presented as Stern−Volmer plots,
Figure 14. All plots, except those of 1 and 10, deviate
substantially from linearity. This is a characteristic feature of the
Stern−Volmer plot associated with combined dynamic and
static quenching of the fluorophore. The tether chain length
appears to influence the extent of quenching, as shown in
Figures 14b and 14c. Within the series of monocarboxylate
porphyrins, compound 3 with a 6-carbon tether is more
effective than 4 (11-carbon chain). Indeed the most effective
EXPERIMENTAL PROCEDURES
■
1-Octadecene tech. 90%, oleic acid 99%, and cadmium oxide
(Puratronics 99.998%) were sourced from Alfa-Aesar. Octadecylamine,
selenium 100 mesh 99.5+%, and 1-dodecanethiol ≥98% were obtained
from Aldrich. Methyl 5-formylsalicylate was obtained from TCI.
Figure 14. Stern−Volmer plots of CdSe/porphyrin systems for porphyrins 1−10.
J
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Article
Unless specified, solvents and chemicals were stored at ambient
laboratory conditions and used without further purification. H NMR
solution was transferred to a centrifuge tube, methanol added to
flocculate the nanoparticles, and the tube centrifuged. The solid was
separated. Acetone was added and the tube sonicated for 5 min,
followed by centrifugation. This was repeated as necessary until the
UV spectra of the washing were free of porphyrin. The solid sample
could then be redissolved in a suitable NMR solvent.
Porphyrin (9) bearing thioacetate chains must be deprotected prior
to addition to the nanoparticles. The following procedure was used. All
solvents and reagents were degassed and placed under N2 before use.
The porphyrin was dissolved in dichloromethane under N2. A few
drops of an aqueous solution of NaOH (2 M) and methanol were
added to the porphyrin with stirring at room temperature. A
precipitate formed. When no porphyrin remained in solution, the
solvent was removed via a syringe. Acetic acid was added and the mix
stirred for 5 min. The acid was removed via a syringe. Methanol was
added and the mix stirred for 5 min. The solvent was removed via a
syringe. The methanol wash was repeated. Finally CH2Cl2 (1 mL) was
added. The deprotected porphyrin dissolved, and the solution was
transferred to a syringe and was slowly added to CdSe nanoparticles as
described above.
1
spectra were obtained on Bruker Avance 500 or 300 MHz NMR
spectrometers. CDCl3 was used as solvent unless specified. Spectra
were referenced to the residual solvent peaks, δ 7.26 for CDCl3. UV/
vis absorption spectra of solutions were recorded on a Hitachi U3000
spectrophotometer using the stated solvents. MALDI-TOF mass
spectra were measured using a Shimadzu Biotech MALDI mass
spectrometer using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
propenylidene]malononitrile (DCTB) as matrix. Fluorescence emis-
sion spectra were obtained using a PerkinElmer LS55 spectrophoto-
fluorimeter. Elemental microanalyses were carried out at London
Metropolitan University. Transmission electron microscopy (TEM)
images were obtained on a FEI Tecnai 20 TEM or Jeol JEM 2000-Ex
microscope. TEM samples were prepared by dipping a carbon-coated
300 mesh copper grid into a solution of CdSe nanoparticles in
dichloromethane. The solvent was evaporated, and TEM micrographs
were taken. Compound 1 was prepared from 4-hydroxybenzaldehyde,
bromodecane, and pyrrole.38 The tris(alkynylpyridine)-
subphthalocyanine42 I and 5-(4-carboxyphenyl)-10,15,20-triphenyl-
porphyrinato zinc43 (2) were prepared following literature procedures.
For the syntheses of porphyrin complexes 3−9 see the Supporting
Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
Preparation of CdSe Nanocrystals. CdSe nanocrystals were
produced by a modification of a literature procedure.24 To cadmium
oxide (300 mg, 2.34 mmol) in octadecene (20 mL) was added oleic
acid (2 mL). The mixture was stirred under vacuum for 10 min, and
then N2 was introduced. The mixture was heated to 250 °C and stirred
at this temperature until a clear solution was obtained, which was then
left to cool to ca. 120 °C. Selenium powder (100 mg, 1.27 mmol) was
added and the mixture heated to 240 °C, causing the color to change
from yellow to orange. Heating was stopped when the color of the
solution was deemed the right shade of orange for the desired
nanocrystal size (after various experimental trials). The solution was
immediately cooled on an ice bath, and toluene (10 mL) was added.
The solution was then transferred to two large centrifuge tubes with
filtration (syringe filter: 0.22 μL). The volume of both tubes was
adjusted to 30 mL with toluene. The addition of acetone (20 mL)
caused the formation of a white precipitate of unreacted starting
material. The tubes were centrifuged (1400 rpm), the orange solutions
were collected, and the white precipitate was discarded. The solution
(25 mL) was again placed in a large centrifuge tube. Methanol (25
mL) was added, followed by centrifugation. An orange oil separated at
the bottom, which was collected; the clear top solvent layer was
discarded. This process was repeated until all the solution was
processed in this way. The separated orange oils were combined and
transferred to a smaller centrifuge tube with toluene (total volume 7
mL). Methanol (7 mL) was added and the tube centrifuged. Again the
orange oil was decanted. The volume was made up to 7 mL in
dichloromethane, and the same volume of ethanol was added, followed
by centrifugation. This process was repeated until a thick orange oil or
powder was obtained. Finally, acetone (14 mL) was added, and the
tube was sonicated for several minutes and then centrifuged. This was
repeated five times. A free-flowing orange powder was finally obtained
after drying under vacuum and stored under N2. Yield of CdSe
Experimental and spectroscopic characterization data of the
syntheses of porphyrin compounds, luminescence studies, and
quantum yields. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b00892.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: A.Cammidge@uea.ac.uk.
*E-mail: m.bochmann@uea.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Leverhulme Trust (RPG-197).
We thank Dr. C. MacDonald for assistance with transmission
electron microscopy. C.B. and S.R.-B. thank the University of
East Anglia for studentships.
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