Variable-gap conjugated oligomers grafted to CdSe nanocrystals

Article abstract of DOI:10.1021/cm301351j

Three linear asymmetrically functionalized conjugated molecules composed of five or six aromatic rings were synthesized, bearing a terminal phosphonic acid group, with the objective of enabling their grafting onto inorganic CdSe nanocrystals. These chromo

Full text of DOI:10.1021/cm301351j

Article
pubs.acs.org/cm
Variable-Gap Conjugated Oligomers Grafted to CdSe Nanocrystals
Romain Stalder,†,∇ Dongping Xie, Renjia Zhou, Jiangeng Xue, John R. Reynolds,*
†,∇
‡
‡
,†,§
,
†
and Kirk S. Schanze*
†
The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and
‡
Engineering and Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-7200, United
States
§
School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics,
Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
*
S Supporting Information
ABSTRACT: Three linear asymmetrically functionalized
conjugated molecules composed of five or six aromatic rings
were synthesized, bearing a terminal phosphonic acid group,
with the objective of enabling their grafting onto inorganic
CdSe nanocrystals. These chromophoresoligo(phenylene
ethynylenes), oligothiophenes, or donor−acceptor−donor
oligothiophenes with a benzothiadiazole acceptorwere
designed with decreasing HOMO−LUMO energy gaps such
that increasing amounts of light could be absorbed toward the
longer wavelengths up to 600 nm. Electrochemical measure-
ments show that the energy offsets between the HOMO and
LUMO energies of the organic molecules and the energy bands
of the CdSe nanocrystals are well-suited for charge transfer
between the organic and inorganic components. The characteristics of each component’s excited state are studied by fluorescence
spectroscopy and the interaction between the conjugated molecules and the CdSe nanocrystals in dilute solutions is monitored
by photoluminescence quenching. In the latter experiments, where ester and acid derivatives are compared, the pronounced
difference in luminescence quenching supports the ability of the phosphonic acid groups to strongly anchor onto the surface of
the nanocrystals. Moreover, these results suggest that charge transfer likely occurs between the organic and the inorganic
compounds, and appropriate ratios for the corresponding organic/inorganic hybrids preparation are identified. The preparation
by direct ligand exchange and the photophysical properties of the hybrids are described, and spectroscopic analysis estimates that
the nanocrystals are covered, on average, with 100−200 electroactive organic molecules. The incident photon-to-electron
conversion efficiency reflects the solution absorption of the hybrids because it shows the response from both organic and
inorganic components.
KEYWORDS: conjugated oligomers, nanocrystals, hybrid materials, donor−acceptor, phosphonic acid
INTRODUCTION
limiting factor to the efficiency of the latter type of hybrid solar
■
cells is the unfavorable phase segregation in the active layers. By
The interest in organic/inorganic hybrid materials has
flourished, in part, because of the evolution of the field in
12,13
varying the shape of the inorganic NCs,
inorganic chemists
1
,2
have offered solutions to this morphology issue: blends of
three-dimensional branched NCs with unfunctionalized poly-
mers afforded power conversion efficiencies up to 2.2% with
solar energy conversion, whether applied to photocatalysis,
3
,4
5−7
solar fuels, or photovoltaics.
Dye-sensitized solar cells
(
DSSCs) based on metal oxide/adsorbed dyes have been at the
forefront of efficient solar cells combining organic and
inorganic materials, with efficiencies reaching 10%. Unlike
9
,14
poly(3-hexylthiophene),
2.1% with poly(phenylene vinyl-
1
5,16
8
ene), and up to 3.2% with polymers taking advantage of the
donor−acceptor approach.
1
7,18
Since NCs are often coated
the solid/liquid-based DSSCs, solid-state hybrid photovoltaics
rely on blends of electroactive organic materials and inorganic
semiconductors. Early reports by Alivisatos and co-workers of
photovoltaic devices based on hybrid systems combining a
conjugated polymer and cadmium selenide (CdSe) nanocryst-
als (NCs) in thin film blends have sparked considerable
research efforts on organic semiconductor/chalcogenide NC
with trialkylphosphine oxide or alkylcarboxylate surfactants,
depending on the colloidal NC synthesis method employed,
they are inherently surrounded by an insulating layer of
aliphatic molecules, which has been determined earlier to be
Received: May 2, 2012
Revised: July 28, 2012
Published: August 17, 2012
9
hybrids. Since NCs do not disperse well within the
1
0,11
unfunctionalized polymer matrix and tend to aggregate,
a
©
2012 American Chemical Society
3143
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Scheme 1. Synthesis of the OPE-A Electroactive Ligand from Hydroquinone and 1,4-Diiodobenzene
detrimental to the electronic interaction between the organic
photophysical properties were investigated. Thin films of the
hybrids were also used as the active layer of hybrid organic−
inorganic solar cells, and the incident photon-to-electron
conversion efficiency (IPCE) measurement clearly showed
the photocurrent contribution from both bound oligomers and
CdSe NCs for each hybrid.
19
and inorganic components of the hybrids. Subjecting the NCs
to a solvent treatment aimed at replacing the original
surfactants also contributed to increased efficiencies of hybrid
1
0,20−23
solar cells.
As a means of controlling both the morphology of the hybrid
active layer and the NCs surfactant composition, conjugated
2
4,25
30
polymers that feature functional groups such as amines,
RESULTS
26−28
25,28,29
■
phosphine oxides,
thiols,
and carbodithioic acids
Electroactive Ligand Synthesis. The oligomers that we
employed were designed as electroactive and photoactive rods
bearing a single phosphonic acid functional group at one end.
Each oligomer backbone was functionalized with two linear
alkyl chains for solubility purposes. In the case of the
oligothiophenes, the terminal ring opposite to that bearing
the phosphonic acid group was functionalized with a linear alkyl
chain at the α position, in order to prevent any side reaction
upon photoexcitation of the electron-rich thiophenes. To
achieve the well-defined monofunctionalized asymmetrical
oligomers, the approach used in this study consists of building
the unfunctionalized and the functionalized conjugated units
separately, then coupling them in the last steps of the synthesis.
As described in Scheme 1, the synthesis of the OPE oligomer
bearing one phosphonic acid group (OPE-A) begins with the
alkylation of hydroquinone to 1, followed by diiodination of the
benzene ring to afford 2, which is the precursor to the central
benzene ring in OPE-A.
were introduced. Although they provided better control of the
dispersion of the NCs in the polymer matrix, little enhance-
ment of the overall power conversion efficiency was observed.
A related approach consists in using discrete conjugated
oligomers in place of polymers, allowing for greater molecular
control of the hybrids formation, because of the well-defined
structure of the oligomers. In most previous studies, the
oligomer’s functional groups enable their grafting onto the
31
inorganic NCs: carbazoles with phosphonate groups, perylene
32
diimide with dicarboxylic acid groups, oligoanilines with
3
3,34
carbodithioic acid groups,
oligo(phenylene vinylene)s with
3
5,36
phosphine oxide groups,
oligo(phenylene ethynylene)s
3
7
38
with thiol groups
and oligothiophenes with thiol,
30
39−41
42
carbodithioic acid, carboxylic acid,
phosphonate, and
anchoring groups have been reported.
Some report the further electropolymerization of the attached
4
3−45
phosphonic acid
3
1,39,40,46
ligands,
but most systems are treated as discrete
inorganic core/organic-shell-type entities to be characterized
and processed as such into optoelectronic devices.
Here, we report the synthesis of three monofunctionalized
conjugated molecules as electroactive ligands: a five-ring
oligo(phenylene ethynylene) (OPE), a six-ring oligothiophene
The two other precursors to OPE-A, the extended phenylene
ethynylene moieties 4 and 7, were synthesized from 1,4-
diiodobenzene: under Sonogashira cross-coupling conditions,
phenyl acetylene and propargyl alcohol were added to yield 3,
which was subsequently deprotected under basic conditions to
afford di(phenylene ethynylene) 4 bearing one terminal alkyne.
In a nickel-mediated Arbuzov-type reaction with one equivalent
of triethylphosphite, 1,4-diiodobenzene was converted to
diethyl (4-iodophenyl)phosphonate 5. Similar conditions as
described for 3 were applied to transform 1,4-diiodobenzene
into intermediate 6, which is then coupled with 5 to generate
the di(phenylene ethynylene) 7, bearing a phosphonate group
on one side and a terminal alkyne on the other end. Precursors
2, 4, and 7 were then reacted in a one-pot synthesis to yield the
phosphonate-monofunctionalized oligo(phenylene ethynylene)
OPE-E. The symmetrical side products (the non-functionalized
and the difunctionalized OPEs) were readily removed by
column chromatography, because of their substantial polarity
difference. The phosphonate ester of OPE-E was finally treated
with trimethylsilyl bromide (TMSBr) and hydrolyzed with
methanol. Subsequent dissolution in a minimum of dichloro-
methane and reprecipitation in methanol afforded the pure
(
T6), and a five-ring benzothiadiazole (BTD)-containing
oligothiophene (T4BTD). Each was functionalized with one
phosphonic acid anchoring group, which has been shown to
bind strongly to CdSe NC surfaces from both experimental and
4
3,44,47,48
theoretical standpoints.
By varying the nature of the
aromatic rings in the molecular structures, the position of their
HOMO and LUMO energy levels is adjusted with respect to
that of the CdSe NCs, as measured electrochemically, especially
as the donor−acceptor approach is employed for the first time
in this type of organic oligomer/CdSe NC hybrids. The
interaction of the conjugated molecules and the NCs in
solution was monitored by photoluminescence quenching
experiments. The hybrids were then prepared and purified:
we chose the direct ligand exchange method to replace the
superficial aliphatic ligands inherent to the NC synthesis with
the newly synthesized monofunctional electroactive ligands.
This afforded oligomer-capped CdSe NC hybrids, for which the
3
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Scheme 2. Synthesis of the T6-A and T4BTD-A Electroactive Ligands from Dibromo-bithiophene and Dibromo-2,1,3-
benzothiadiazole, Respectively
phosphonic acid-functionalized OPE-A, in 8% overall yield
from diiodobenzene.
In donor−acceptor chemistry, an electron-rich unit and an
electron-deficient unit are linked together in conjugation,
4
9,50
In the synthesis of the first thiophene-based oligomera
sexithiophene (T6) bearing one phosphonic acid group (T6-
A), shown in Scheme 2the oligothiophene core is extended
from the 5,5′-dibromo-2,2′-bithiophene starting compound
into a symmetrical dialkylated quaterthiophene 10 by reacting
the dibromobithiophene with the Grignard derivative of 2-
bromo-3-hexylthiophene under nickel-catalyzed Kumada cross-
coupling conditions in high yields.
Dibromination of 10 at the 5,5‴-positions using N-
bromosuccinimide (NBS) in dimethylformamide affords 11
with little purification as the insoluble product can be filtered
and recrystallized from a hexanes/ethanol mixture. In the
following step targeting a monocoupled product, 11 is reacted
with 1.5 equiv of 2-(5-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-
typically via palladium-catalyzed cross-coupling.
Following
the latter approach, we synthesized the second thienylene
oligomera five-ring oligomer consisting of one central BTD
unit flanked by two thiophene rings on each side and bearing
one phosphonic acid (T4BTD-A), also shown in Scheme 2
by reacting 4,7-dibromo-2,1,3-benzothiadiazole with 2 equiv of
2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane under the same Suzuki cross-coupling conditions than that
used for the synthesis of the pentathiophene 12, to afford
compound 15 in high yields. Subsequent dibromination by
NBS in chloroform yields the symmetrical precursor 16. As for
the conversion of 11 into 12 during the synthesis of the T6-A
oligomer, the stoichiometry of the reaction of 16 with 2-(5-
hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
needed to be adjusted in order to optimize the ratio of targeted
monocoupled compound 17 to the unreacted and dicoupled
byproduct. Unlike for the synthesis of the sexithiophene, for
which 1.5 equiv of the thienyl borolane were used, in this case,
we reacted 16 with 0.80 equiv of the borolane. We expected
such a stoichiometry to still afford 17 in acceptable yields while
being able to recover the valuable starting material 16 rather
than the unreactive dicoupled byproduct. After purification of
the reaction by flash chromatography, the monocoupled
product 17 was obtained in 26% yield while the recovered
starting compound 16 accounted for 53% of the material.
Although slightly lower, the yield for the latter asymmetrical
coupling was comparable to that when 1.5 equiv of the
borolane were used. Therefore, the stoichiometry for this type
of asymmetrical cross-coupling can be chosen, depending on
the interest for a specific byproduct. Compound 17 was then
coupled to tin compound 14 to install the phosphonate group
onto the oligomer, affording T4BTD-E in 64% yield. Starting
with 3 g of 4,7-dibromobenzothiadiazole, 1.34 g of T4BTD-E
were obtained. The phosphonate was hydrolyzed as done
previously, using TMSBr to yield the phosphonic acid
monofunctionalized T4BTD-A in 14% overall yield.
1,3,2-dioxaborolane, in a Suzuki cross-coupling reaction in
toluene. We selected Pd dba and tri(ortho-tolyl)phosphine as
2
3
palladium source and phosphine ligand, respectively, and
tetraethylammonium hydroxide as boron-activating base.
Under such conditions and after purification by column
chromatography, yields of 31% for the targeted monocoupled
product 12 and 23% for the dicoupled symmetrical
sexithiophene byproduct were obtained. Moderate yields are
expected in this asymmetrical synthesis step, because of the
necessary stoichiometry and the purification process. The
phosphonate functionality was installed separately on a
thiophene ring by the nickel-mediated Arbuzov-type conversion
of 2-bromothiophene into diethyl thiophen-2-ylphosphonate
1
3, followed by stannylation at the 5-position of the thiophene
ring to afford compound 14. Monobrominated pentathiophene
2 was reacted with the stannylation product 14 under Stille
1
coupling conditions using the same palladium source and
phosphine ligands as for the previous Suzuki coupling, which
yielded the phosphonate-monofunctionalized sexithiophene
T6-E. The increased polarity of the oligomer induced by the
presence of the phosphonate group facilitated purification by
column chromatography. Starting with 2.67 g of dibromo-
bithiophene, 650 mg of T6-E were obtained. The last step to
the phosphonic acid T6-A involved treatment of the
phosphonate T6-E using TMSBr in DCM followed by
hydrolysis with methanol. This afforded T6-A in 5% overall
yield from the dibromo-bithiophene starting material.
The CdSe nanocrystals used in this study were synthesized
51
according to a previously reported procedure with some
52
modification. The as-synthesized CdSe nanocrystals were
capped by a mixture of oleic acid (OA) and trioctylphosphine
oxide (TOPO), given that the molar feed ratio during NC
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synthesis was ∼1:1 in the two ligands. (See the Supporting
Information for CdSe NC synthesis details.)
Photophysics of the Free Oligomers. With OPE-A, T6-
A, and T4BTD-A on hand, the UV−visible absorption and
fluorescence spectra were obtained, as shown in Figure 1. All
The photoluminescence of each oligomer was measured in
ester and acid form, in dilute chloroform solution, and the
results are also summarized in Table 1. The oligomer solutions
exhibit intense fluorescence with a quantum efficiency near or
above 50%. The peak emission wavelengths red-shift in
sequence from OPE to T4BTD: the peak fluorescence is at
4
34 nm for OPE-A, 565 nm for T6-A, and 676 nm for T4BTD-
A, which is consistent with the absorption results. The emission
band broadens for the T4BTD oligomer, reflecting the charge
transfer nature of the excited state.
Fluorescence lifetimes were determined for both acids and
esters; in each case, the decays followed first-order (single
exponential) kinetics. There is little difference between the acid
form and the ester form for each oligomer, as expected for the
dilute solutions used where little aggregation is expected. While
the OPE and T6 oligomers show short lifetimes of ca. 0.9−1.0
−
1
ns (with radiative rate, k ≈ 0.6−0.8 ns ), the T4BTD
r
oligomers exhibit significantly longer lifetimes of ca. 5.6 ns. The
longer lifetime for T4BTD results from a significantly lower
Figure 1. Normalized UV−visible absorption (a) and photo-
luminescence (b) of OPE-A (black line), T6-A (blue line), T4BTD-
A (red line), and the CdSe NCs (dashed line) in chloroform solution.
−1
radiative rate (k ≈ 0.16 ns ); this is again consistent with the
r
D−A nature of the fluorescent excited state, with a magnitude
5
5,56
typical of bis(diaryl)benzothiadiazoles.
For applications
such as hybrid photovoltaics, where the photogenerated exciton
should be dissociated into separated charged species, a longer
lifetime is beneficial, because it increases the probability for
charge separation to occur.
Redox Properties. The redox properties of each oligomer
were investigated using cyclic and differential pulse voltamme-
try (CV and DPV) in solution. For each oligomer, a small
amount of material was dissolved in a dry and degassed
dichloromethane electrolyte solution containing 0.1 M
the absorption spectra feature a strongly allowed long
wavelength absorption band, which shifts systematically from
3
83 nm for OPE-A to 426 nm for T6-A to 508 nm for T4BTD-
−1
−1
A. High molar absorptivities of 30 000−50 000 M cm were
recorded in CHCl solutions, as summarized in Table 1. From
3
the absorption onset of OPE-A and T6-A in solution, a
relatively large energy gap of 2.8 and 2.4 eV, respectively, is
calculated, as expected of oligomers with homogeneous π-
electronic systems. The BTD-based oligomer, on the other
hand, features a longer wavelength absorption onset corre-
sponding to a smaller HOMO−LUMO gap of 2.0 eV. This is
due to the D−A interaction attributable to the mixing of the
tetrabutylammonium hexafluorophosphate (TBAPF ), to
6
achieve a concentration of 1 mM in oligomer. All measure-
ments were performed in an argon-filled glovebox, and the
background current for an oligomer-free electrolyte is shown in
Figure S1 in the Supporting Information. All potentials are
53
BTD acceptor unit with the flanking bithiophene donors. The
molar absorptivity of T4BTD-A is also lower in comparison to
the OPE and T6 and is consistent with the D−A transition.
The CdSe NCs have a first excitonic absorption peak at 624
nm characteristic of the quantum confinement effect, and the
absorption increases steadily toward the UV region of the
spectrum. The peak position for the NC absorption is in
accordance with their size (see Supporting Information, Figure
S10, for NCs TEM images), and an optical energy gap of 1.9 eV
is calculated. The fluorescence quantum yield is comparatively
low for the CdSe NCs (Table 1), presumably due to the
formation of defect states at the NCs surface during the
+
reported against the Fc/Fc standard. For the OPE-A oligomer
in solution, the oxidative CV (see Figure S1-b in the Supporting
Information) shows one irreversible oxidation process with an
onset at 0.78 V, and no further oxidation wave at higher
potentials was observed. In the cathodic CV scans, no
electrochemical process was observed other than the beginning
of electrolyte breakdown at a potentials negative of −2.0 V, as
evident from the background CV shown in Figure S1-a in the
Supporting Information. We conducted similar CV experiments
with T6-A, for which the oxidative CV shows two quasi-
reversible processes centered at half-wave potentials of 0.33 and
0.56 V (see Figure S1-c in the Supporting Information). Similar
52
removal of excess TOPO and oleic acids.
Table 1. Optical Data of the Oligomer Esters and Acids and CdSe NC in CHCl Solution
3
a
εabs (M−1 cm−1
Stokes shift (cm−1
c
c
λmax abs (nm)
optical ΔE (eV)
)
λmax Fl (nm)
)
ΦFl
τFl (ns)
d
d
d
d
e
OPE-E
381
383
424
426
504
508
624
2.8
2.8
2.4
2.4
2.1
2.0
1.9
42 700
33 800
55 600
48 700
30 000
21 000
436
434
3100
3068
5854
5775
5026
4892
641
0.79
0.53
0.54
0.49
0.71
0.71
0.91
0.89
0.86
0.85
5.60
5.55
OPE-A
T6-E
537/564
539/565
675
T6-A
T4BTD-E
T4BTD-A
CdSe NC
e
676
b
e
f
632 000
650
0.001
1.26
a
b
c
From low-energy onset of absorption in the solution UV−vis spectra. Calculated by the method described in ref 54. Φ and τ are fluorescence
Fl
Fl
d
e
quantum efficiency and lifetime, respectively. Quinine sulfate in 0.1 M sulfuric acid (Φ = 0.54) as standard. Ru(bpy) Cl in H O (Φ = 0.056,
degassed) as standard. Average lifetime from a multiexponential decay.
F
3
2
2
F
f
3
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Chemistry of Materials
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to OPE-A, no reduction was observed when the potentials were
scanned cathodically of 0 V up to −2.0 V, after which the
electrolyte breakdown started. The absence of reduction
processes for OPE-A and T6-A is not surprising, since both
oligomers are electron-rich chromophores that would require
even more negative potentials to accommodate the addition of
an electron. For T4BTD-A, the oxidative CV shows one
reversible oxidation process centered at a half-wave potential of
absence of a voltammogram wave attributable to the reduction
of the oligomer for OPE-A or T6-A prevented the electro-
chemical estimation of LUMO energies for the latter two
compounds. Since the optical energy gap of the BTD-based
oligomer is within 0.05 eV of its electrochemical energy gap, we
therefore used the corresponding optical energy gaps listed in
Table 1 to estimate the energies of the LUMOs for OPE-A and
T6-A, which are at −3.04 and −2.92 eV, respectively, vs
vacuum.
0.50 V (see Figure S1-d in the Supporting Information). In
contrast to the latter two oligomers, the reductive CV of
T4BTD-A features a single reversible reduction centered at a
half-wave potential of −1.66 V. This is consistent with the D−A
nature of the chromophore, which results in a deeper LUMO
energy level.
The energy diagram in Figure 2 also shows the range of
energies that one can estimate for the conduction and valence
bands (CB and VB, respectively) of the CdSe NCs used in this
study. For bulk CdSe, the CB energies are set between −4.3 eV
52
and −4.5 eV, and between −6.2 eV and −6.3 eV for the VB.
The input signal of a DPV experiment involves short pulses
with a stepwise increase of the base potential, and since the
recorded current in this experiment is the difference between
currents measured before and after each pulse, the overall
output signal in a DPV is free of the capacitive current, which is
otherwise part of the output in a CV experiment. Therefore, the
differential current recorded in a DPV is mostly Faradaic,
involving the actual redox process of adding or removing an
electron at the electrode surface. This results in well-defined
oxidation (reduction) onsets during the oxidative (reductive)
DPV, which is why we employed the latter electrochemical
technique to measure the energies of the frontier molecular
orbitals of all three oligomers, using the same solutions as for
the CV experiments. From the oxidative DPV, oxidation onsets
for OPE-A, T6-A, and T4BTD-A were measured at 0.78, 0.22,
and 0.40 V, respectively. As previously described, only T4BTD-
A exhibits a reduction process in reductive DPV experiments,
with an onset of reduction at −1.55 V (see Figure S1 in the
Supporting Information). In order to convert the voltage values
calibrated against the Fc/Fc+ standard into energy values
Energy band measurements have been reported for CdSe
nanoparticles in the 3−4-nm-diameter range, setting the CB
60
around −3.5 eV and the VB around −5.5 eV. For the 6-nm-
diameter CdSe NCs used in this study, it is thus safe to estimate
that the CB is found between −3.8 eV and −4.3 eV, and with
an optical bandgap of 1.9 eV, the VB is found between −5.7 eV
and −6.2 eV. For the three oligomers, the LUMO levels are
thus likely higher than the VB the NCs. Although the HOMO
level of OPE-A is rather deep (−5.84 eV), likewise the HOMO
levels of the organic molecules are likely higher than the VB of
the NCs. Such energetic offsets would result in staggered
energy gaps for each organic oligomer/CdSe NCs complex,
which is analogous to that described as type II heterojunctions
in solid-state semiconductor physics. In terms of the expected
photoelectrochemical behavior, this type of heterojunction
suggests that photoexcitation of either the oligomers or the
NCs will lead to electron transfer from the oligomer (as an
electron donor) to the CdSe NCs (as an electron acceptor).
Photoluminescence Quenching. Photoluminescence
(
PL) quenching is a powerful tool to probe the electronic
+
against vacuum, the value for Fc/Fc against vacuum must be
interactions between and among chromophores and electro-
active species. This technique was used in particular by Frechet
and co-workers to decipher between charge- and energy-
transfer processes in a system composed of 4 nm CdSe NCs
+
57
known. Since Fc/Fc is +0.380 V vs SCE, and the energy of
58
+
SCE is 4.7 eV vs vacuum, we used a Fc/Fc redox standard
set at −5.1 eV versus vacuum, a value recently discussed by
59
45
Bazan and co-workers. Figure 2 depicts the position of the
and phosphonic acid functionalized pentathiophenes. They
observed significant photoluminescence quenching of the
pentamer’s fluorescence in solution upon the addition of
CdSe NCs, and, likewise, significant quenching of the NCs’
emission upon addition of the pentamer. The luminescence of
shorter thiophene trimers was also quenched by CdSe NCs, but
the emission of the NCs increased with the trimer
concentration. This was accounted for by the difference in
staggered energy gaps between the NCs and the pentamer
(
(
type II heterojunction), compared to straddling energy gaps
type I heterojunctions) between the NCs and the wider-
energy gap trimer. Ruling out the possibility of energetic surface
defect passivation by the phosphonic acid anchoring group
itself, the dual luminescence quenching was explained by an
electron-transfer mechanism from the thiophene pentamers to
the NCs. Similar observations were reported by Advincula and
co-workers for phosphonic acid-functionalized thiophene
dendrons, and this type of experiment was used by others as
well. In the dilute solutions typically used for fluorescence
experiments, fluorescence quenching upon the interaction of
two different species requires them to be in close proximity,
regardless of the quenching mechanism. The synthesis of CdSe
NCs involves the use of surfactants usually composed of long
alkyl chains and a polar functional group, with which the NC
surface is coated after the reaction is over. The surfactants used
Figure 2. Band structure diagram comparing the HOMO and LUMO
levels of OPE-A, T6-A, and T4BTD-A, and their offsets relative to the
CdSe NCs valence- and conduction-band energy ranges.
HOMO and LUMO energy levels for the three oligomers: from
the DPV oxidation onsets, OPE-A has a HOMO energy level at
11
−
5.84 eV, T6-A at −5.32 eV, and T4BTD-A at −5.50 eV, all vs
vacuum.
We calculate the LUMO energy of T4BTD-A from the
reductive DPV onset to be at −3.55 eV, giving an electro-
chemical energy gap of 1.95 eV, which is close to the optical
energy gap value of 2.0 eV measured spectroscopically. The
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Figure 3. (a) Evolution of the fluorescence in chloroform of OPE-E (left) and OPE-A (right) upon the addition of CdSe NCs into the solution, and
b) evolution of the peak fluorescence intensities for the ester (solid lines) and acid (dashed lines) forms of OPE (black line), T6 (blue line), and
(
T4BTD (red line) upon the incremental addition of CdSe NCs.
in the synthesis of the CdSe NCs for this study are oleic acid
intensities for all six experiments: for all three chromophores,
the fluorescence of the ester form decreases by 5%−20% upon
addition of the CdSe NCs, whereas the fluorescence of the acid
form is quenched by more than 95% under the same
conditions. In these experiments, diffusional quenching can
be ruled out, because of the low concentrations employed;
therefore, any evolution in the PL intensity would be due to
oligomer/NC complex formation. The PL quenching results
suggest that the phosphonate ester forms have, at best, a weak
interaction and, thus, there is little binding of the oligomers to
the NCs, while the acid forms bind to the NCs, likely via
surface binding of the phosphonic acid moiety.
(OA) and trioctylphosphine oxide (TOPO), in a feed ratio
close to one. There is thus an inherent insulating layer of
aliphatic surfactants (OA and TOPO) coating each NC, which
has been shown to be detrimental to their electronic interaction
19
with conjugated polymers. The exact nature and distribution
of the aliphatic surfactants at the surface of the NCs is not
straightforward, because it depends on the nature and purity of
that used during NC synthesis, and the purification process that
followed. Nevertheless, the use of functional groups such as
phosphonic acids or carboxylates, which bind strongly to the
NCs surface, have been shown to displace the aliphatic native
surfactants, during a ligand exchange process, which results in
In the experiments described above, no luminescence
61
new molecules anchored to the NC surface.
increase was observed when carefully monitoring the 610−
With this in mind, the three phosphonic acid-functionalized
oligomers in this study were designed to undergo efficient
ligand exchange with native surfactants on the surface of CdSe
NCs. For each chromophore solution, we monitored the
evolution of the PL intensity upon addition of aliquots of CdSe
NCs in solution, and compared the evolution for the
phosphonate ester derivative, with respect to the corresponding
phosphonic acid. Figure 3a shows the PL evolution for the
OPE phosphonate ester (OPE-E, left) and the OPE
phosphonic acid (OPE-A, right) in dilute chloroform solution
6
30 nm range for any enhancement of the emission from the
CdSe in the oligomer/CdSe mixture. This was not surprising,
considering the low concentration of CdSe NCs in solution (up
to 100 nM, after the final increment), and was not sufficient to
decipher between either electron- or energy-transfer mecha-
nisms. This point was further addressed by performing the
reverse experiment, i.e., studying the PL quenching of CdSe
emission by the oligomers. Thus, when solutions of CdSe NCs
were selectively excited at 630 nm, the emission intensity was
recorded as aliquots of oligomer solution were added. This was
only possible for the OPE and T6 oligomers, since, for
T4BTD, the absorption of the oligomer and CdSe overlap
significantly. For OPE and T6, the procedure is described in
detail in the Supporting Information. After mixing the oligomer
and CdSe NCs, the solutions were stirred and excited at the
CdSe peak absorption wavelength (which does not overlap with
the absorption of OPE or T6). The resulting emission was
recorded and plotted against that of oligomer:CdSe ratio
mixtures. The photoluminescence of CdSe decreased upon
addition of OPE-A, whereas the same amount of OPE-E had
no influence on the CdSe emission, as displayed in Figure S5 in
the Supporting Information. Similar results were observed for
the T6 derivatives (see Figure S6 in the Supporting
Information).
(
5 μM) as 20 μM CdSe in chloroform was added, in
increments. The increments were chosen so that each addition
doubled the concentration of CdSe NCs in the oligomer
solution up to an oligomer:CdSe ratio of 50:1. The
concentrations of the starting solutions (pure oligomer, 5 μM
and pure CdSe NCs, 20 μM) were such that only microliters of
CdSe solution were added to the fixed volume of 2 mL of
oligomer solution, thereby negating the effect of dilution on the
PL intensity. For the phosphonate OPE-E, the addition of the
CdSe reduced the oligomer fluorescence by 20% at the 50:1
ratio. When the acid form OPE-A was subjected to the same
procedure, very strong luminescence quenching was observed:
the fluorescence is essentially fully quenched when the molar
ratio of OPE-A:CdSe equals 50:1 in solution.
The same set of experiments was carried out for the ester and
acid forms of T6 and T4BTD, respectively, for which the
evolution in the fluorescence intensities is displayed in the
Supporting Information (Figures S3 and S4, respectively). The
results for T6-A versus T6-E and T4BTD-A versus T4BTD-E
follow the same trend as that shown in Figure 3a for OPE.
Figure 3b shows the evolution of the peak fluorescence
In summary, the intensity of the PL quenching in all cases
suggests that the phosphonic acid group anchors the oligomers
onto the CdSe NCs, to an extent where a 50:1 oligomer:NC
ratio is sufficient to achieve complete quenching of the excited
state of the oligomer by the NC. The quenching likely occurs
via a charge-transfer process rather than energy transfer as the
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emission of both species is quenched for OPE-A:CdSe and T6-
A:CdSe.
nm toward shorter wavelengths, and peaking at 325 nm with a
shoulder at 370 nm, which shows the contribution of the bound
OPE-A. The T6 hybrid (blue line) has a broad absorption band
centered at 426 nm from the contribution of the bound T6-A
oligomers. Similarly, the absorption profile of the T4BTD
hybrid shows the contribution of the NCs-bound T4BTD-A
peaking at 360 and 508 nm. In all three spectra, the
contribution of the NCs to the overall absorption is observed
as a peak or shoulder around 625 nm. The optical properties of
the hybrids are summarized in Table 2. Extremely weak
fluorescence was observed in dilute solutions of the hybrids in
chloroform, with quantum yields below 0.1% at 433 nm for the
OPE hybrid solution, 564 nm for the T6 hybrid solution, and
Preparation and Characterization of Hybrids. In order
to prepare and isolate CdSe NCs that are coated with the
oligomers (oligomer/NC hybrids), solutions of the two
components were mixed and the resulting NCs isolated by
precipitation/centrifugation. From the PL quenching experi-
ments, a 50:1 ratio of oligomer:NC was found to be sufficient
to completely quench the luminescence of the organic
chromophore. Thus, we rationalized that such a ratio or higher
would be suitable for the synthesis of the NC hybrids. The
preparation consists in the exchange of the OA and TOPO
ligands of the as-synthesized NCs with OPE-A, T6-A, or
T4BTD-A by mixing in chloroform, followed by precipitation
of the oligomer/NC hybrid in an appropriate solvent and
centrifugation to remove the supernatant containing any
unbound species. Experimentally, 10 mg of the oligomer was
dissolved in 5 mL of degassed chloroform, to which was added
a solution of the NCs in chloroform at the appropriate
concentration for an oligomer:NCs ratio in excess of 200:1.
The mixture was stirred vigorously in the absence of light at
room temperature for 30 min, after which it was precipitated in
a poor solvent for the oligomer/NC hybrid, but a good solvent
for the unbound surfactants and excess oligomer. For the T6-
A/CdSe NC system, ethyl acetate was used to precipitate the
hybrids, while methanol was suitable to precipitate the OPE-A/
CdSe NC and T4BTD-A/CdSe NC systems. After centrifuga-
tion of the suspension and removal of the supernatant
containing unbound species, the precipitates were redissolved
in chloroform and precipitated once again in the proper
solvent. This was repeated several times, while recording the
UV−visible absorption spectrum of the chloroform solutions in
each step. Since unbound oligomers remained in the super-
natant, which was removed after each precipitation, the overall
absorption profile of the redissolved precipitates featured less
absorption contribution from the oligomers. Once the relative
absorption intensities of the NCs versus that of the oligomer
stabilized, the chloroform solution containing the redissolved
oligomer/NC hybrid was considered free of unbound
oligomers. The UV−visible absorption spectra of the resulting
washed hybrids are shown in Figure 4.
6
76 nm for the T4BTD hybrid solution.
Seeking evidence for the dynamics of the hybrids
fluorescence, we measured the fluorescence lifetime of each
hybrid in solution. As recorded in Table 2, two components
contribute to the average lifetime of the hybrids fluorescence:
one from the ligands emission and the other from the CdSe
emission. Compared with the fluorescence lifetime of the free
ligands or CdSe NCs (Table 1), we observed that the
fluorescence of each hybrid solution has almost identical
lifetime with their corresponding free oligomer and CdSe NCs.
This observation suggests that the fluorescence of the hybrids
results from trace amounts of free ligands and CdSe NCs, while
the hybrids themselves are nonemissive, presumably because of
the charge transfer between the components as described
earlier. This finding also points to the fact that the quenching
62,63
process is very rapid, occurring on a time scale of <100 ps.
The efficient quenching, along with the absorption profiles of
the hybrids, supports the strong surface binding and interaction
between the oligomers and the NCs.
Transmission electron microscopy (TEM) images of the
hybrids displayed in the Supporting Information (Figure S10)
show that the hybrids are prone to aggregation, compared to
the TOPO/OA-coated NCs, which unfortunately makes
dispersity analysis of the hybrids complicated. But, with the
hybrids synthesized, and the presence of surface-bound
oligomers established, a semiquantitative estimation of the
average number of oligomers at the NCs surface was carried
out. First, thermogravimetric analysis (TGA) was employed to
determine a total weight loss difference between the pristine
NCs (TOPO and OA-coated) and those functionalized with
the electroactive oligomers. In principle, during the ligand
exchange process, if a native surfactant such as TOPO (MW =
Compared to the pure CdSe NC solution absorption which
is displayed as a dashed line, the absorption profile of the OPE
hybrid (black line) has an absorption band emerging out at 410
4
15 g/mol) or OA (MW = 282 g/mol) is replaced by OPE-A
(
(
MW = 815 g/mol), T6-A (MW = 827 g/mol), or T4BTD-A
MW = 797 g/mol), then an oligomer/NC hybrid should have
a higher organic content by weight than the pristine NC.
Displayed in the Supporting Information (Figure S7), the TGA
thermograms for a CdSe sample before ligand exchange
(
dashed line) and after ligand exchange with OPE-A (black
line), T6-A (blue line), or T4BTD-A (red line) are plotted.
Weight loss differences for the hybrids were 4%−8% greater,
compared to the pristine CdSe sample (T ≈ 500 °C). This
result implies that the ligand exchange process increases the
organic content in the hybrid, supporting the presence of a
higher-molecular-weight species bound to the surface of the
NCs. One obvious limitation to this method is that it is, in fact,
very difficult to determine the exact number of native
surfactants before ligand exchange. Thus, the results from the
TGA experiments are, at best, qualitative.
Figure 4. Absorption spectra of the OPE-based hybrid (black line),
the T6-based hybrid (blue line) and the T4BTD-based hybrid (red
line) along with the spectrum of free CdSe NCs (dashed line) in
chloroform solution (spectra were normalized at the CdSe band
maximum (625 nm)).
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Chemistry of Materials
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Table 2. Optical Data of the Three Oligomer/CdSe NC Hybrids in CHCl Solution
3
a
a
λmax abs (nm)
λmax Fl (nm)
fluorescence quantum efficiency, ΦFl (%)
fluorescence lifetime, τFl (ns)
OPE hybrid
T6 hybrid
325/370/625
426/625
433
564
676
<0.1
<0.1
<0.1
0.88(450 nm)/1.24(650 nm)
0.83(560 nm)/1.23(650 nm)
5.2(680 nm)/1.48(650 nm)
T4BTD hybrid
360/508/625
a
ΦFl and τFl are fluorescence quantum efficiency and lifetime, respectively.
A second approach to estimate the number of surface-bound
oligomers consists in a careful comparison of the hybrids’
absorption with that of the pristine NCs and the free
1
1
oligomer. The absorption spectra of the latter three species
in the case of T6 is shown in Figure 5. The relative absorption
Figure 6. Incident photon-to-electron conversion efficiency of OPE-
A/CdSe (black line), T6-A/CdSe (blue line), and T4BTD-A/CdSe
(red line) hybrids films in a ITO/PEDOT:PSS/oligomer:CdSe/ZnO/
Al device architecture under AM1.5 illumination.
The IPCE for the OPE hybrid peaks at 350 nm to 10% with
little response below 3% at wavelengths higher than 500 nm,
except for a small shoulder at 620 nm, which is likely due to the
contribution of CdSe NC absorption. For the T6 hybrid, a
broad band with a maximum IPCE of ∼12% appears in the
IPCE from 400 nm to 550 nm likely corresponding to light
absorbed by the T6 ligands bound on the CdSe NC surface. A
shoulder at 620 nm is observed, signifying contribution of the
CdSe NC absorption to the overall photocurrent.
Figure 5. Absorption profiles of the T6-A:CdSe hybrid (solid line),
the free T6-A (dotted line), the free CdSe (dashed line), and the sum
of the latter two (dash-dotted line).
intensity of the free oligomer (dotted line) and the free NCs
(
dashed line) was adjusted such that the sum of their
absorption spectra (dash-dotted line) resulted in a profile for
The IPCE for the T4BTD hybrid is broadened further to 650
nm, with two bands centered at 371 and 505 nm, and the IPCE
maximum is ∼13%. This parallels the absorption of the T4BTD
electroactive ligands which are bound onto the NCs. A slight
shoulder at ∼650 nm can be distinguished, likely corresponding
to the NC contribution to the photocurrent. In summary, the
IPCE spectra for three categories of hybrids indeed indicated
the photocurrent contribution both from oligomers and CdSe
NCs.
which the intensities at the respective absorption maxima (at
4
26 and 624 nm) matched that of the hybrid (solid line).
This was achieved for an absorbance of 0.912 at 426 nm for
T6-A and 0.087 at 624 nm for the NCs. From Beer’s law,
concentrations of 18.7 μM and 136 nM, respectively, were
calculated, using the extinction coefficients listed in Table 1,
resulting in an oligomer:NC ratio of 137. The same spectral
analysis and calculations were applied to the OPE and T4BTD
hybrids (see Supporting Information, Figures S8 and S9,
respectively), yielding ratios of 200 OPE-A and 140 T4BTD-A
per NC, respectively. These ratios are, of course, average values
and remain an approximation of the number of oligomers
bound to the NCs, but they suggest that a significant coverage
of the NCs was achieved using each phosphonic acid derivative.
For 6-nm-diameter NCs, this corresponds to one oligomer
DISCUSSION
■
Hybrid photovoltaics bring together materials that are
synthesized and characterized with very different methods,
although both organic and inorganic parts present energetic
similarities that justify their interaction as hybrid materials. The
field has benefitted from research efforts in the design of each
part of the hybrid, for instance with tailored NC sizes, shapes,
composition, and surface treatments for improved opto-
2
bound onto every 1 nm of NC surface, on average.
With varied HOMO−LUMO gaps, the three oligomers were
designed such that an increasing amount of light toward the
higher wavelengths could be absorbed by the photoactive and
electroactive ligands, in going from OPE to T6 to T4BTD. In
order to study the possibility of these oligomers/NCs hybrids
for photovoltaic (PV) application, we then fabricated hybrid PV
devices with a device structure of ITO/PEDOT:PSS/
oligomer:CdSe/ZnO/Al. Here, the oligomer:CdSe is the active
layer, and ZnO NCs serve as an optical spacer and hole-
65
electronic performance. On the organic materials side, one
improvement came from functionalizing an electroactive
molecule or polymer to improve their interaction with the
11
NCs.
Following this, the molecules in this study were function-
alized with phosphonic acid groups, for which the binding
strength to CdSe NCs had been previously reported with
6
4
43−45
blocking layer. The incident photon-to-electron conversion
efficiency (IPCE) was recorded for each hybrid as displayed in
Figure 6.
oligothiophenes.
Each molecule in this study was made
asymmetrical so that grafting onto the CdSe surface can achieve
a hybrid complex of CdSe NC “core” and electroactive ligand
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Chemistry of Materials
shell”. The synthesis of such asymmetrical oligomers can be
Article
“
CONCLUSION
■
tedious and low-yielding (see below), which is a major
disadvantage when put in the perspective of the sought-after
application. In this study, we investigated three variations of a
synthetic route to afford asymmetrical molecules. First, the
OPE oligomer was built by synthesizing three separate
segments of the target molecule, and combining them in a
final one-pot synthesis. Second, the T6 oligomer was
synthesized starting from a symmetrical core then converted
to an asymmetrical precursor (compound 12) using 1.5 equiv
of an end-capping thienyl-borolane. Third, the T4BTD
oligomer was synthesized also from a symmetrical core but
reacted with only 0.8 equiv of the end-capping thienyl-borolane
in the asymmetrical step to give compound 17. The remaining
aryl bromide sites in both cases could finally be used to install
the phosphonate functionality. Importantly, the overall yields
for the synthesis of each oligomer remains ∼10%, regardless of
the route employed, and the choice of the synthetic route can
be done based on considerations for valuable side products.
Another improvement from the field of conjugated organic
materials came with the advent of the donor−acceptor
approach to the smaller HOMO−LUMO energy gap of the
organic semiconductors. D−A systems were used also in
hybrids, where fully conjugated D−A polymers were introduced
in conjunction with CdSe NC to achieve 3% power conversion
Through synthetic tailoring of the conjugated chromophore,
the absorption of the organic components of the hybrids
described here was extended to 600 nm, and the presence of
one phosphonic acid group on the chromophore allows for
strong anchoring of the electroactive ligands onto the CdSe
NCs surface. Under the conditions identified during the
photoluminescence quenching experiments, hybrids were
prepared and purified, leading to composites with significant
coverage of the NCs by more than 100 electroactive ligands.
This effectively increased the absorption of the hybrids in the
visible region of the spectrum especially as the donor−
acceptor−donor (D−A−D) molecule was used, and the
photocurrent response of hybrids thin films reflects the
increased absorption at wavelengths longer than 400 nm. We
anticipate that the type of phosphonic acid-functionalized
conjugated molecules described here are good candidates as
active layer components for efficient hybrid organic−inorganic
solar cells, especially as the absorption is further increased by
taking advantage of the donor−acceptor (D−A) approach.
ASSOCIATED CONTENT
Supporting Information
■
*
S
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
7
efficiency. It is understood that adjusting the position of the
HOMO and LUMO energy levels of the organic part, with
respect to the CB and VB of the inorganic part, can help
increase the efficiency of charge separation the organic/
inorganic interface. The D−A approach also allows the organic
material to absorb more light toward the longer wavelengths.
The molecules in this study were designed with varying
HOMO−LUMO gaps from 2.8 eV for the OPE oligomer to
AUTHOR INFORMATION
Corresponding Author
■
*E-mail addresses: reynolds@chemistry.gatech.edu (J.R.R.),
kschanze@chem.ufl.edu (K.S.S.).
Author Contributions
These authors contributed equally to this work.
Notes
∇
The authors declare no competing financial interest.
2.0 eV for the T4BTD oligomer, absorbing light up to 600 nm.
With the latter molecule, the D−A−D structure is used for the
first time in designing deep-LUMO, small-energy-gap (ex-
tended absorption) electroactive ligands for grafting onto
inorganic structures.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the U.S. Department of
Energy Solar Energy Technologies Program (DE-FG36-
08GO18020), for financial support. R.S. acknowledges the
University Alumni Awards Program for a fellowship.
We carefully studied the interactions between the electro-
active ligands and CdSe NCs. The communication between the
components characterizing photoinduced charge transfer is
clearly facilitated by the anchoring groups on the organic
ligands. Transient absorption was measured for the hybrid
materials in an attempt to monitor charge-separated states;
however, we were unable to observe any transients on the
nanosecond time scale. Previous reports of the transient
absorption of CdSe NCs revealed that the relative photo-
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