Complementary Lock-and-Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer

Article abstract of DOI:10.1002/anie.201701929

Owing to the difficulty in comprehensively characterizing nanocrystal (NC) surfaces, clear guidance for ligand design is lacking. In this work, a series of bidentate bis(pyridine) anthracene isomers (2,3-PyAn, 3,3-PyAn, 2,2-PyAn) that differ in their binding geometries were designed to find the best complementary fit to the NC surface. The efficiency of triplet energy transfer (TET) from the CdSe NC donor to a diphenylanthracene (DPA) acceptor mediated by these isomers was used as a proxy for the efficacy of orbital overlap and therefore ligand binding. 2,3-PyAn, with an intramolecular N–N distance of 8.2 ?, provided the best match to the surface of CdSe NCs. When serving as a transmitter for photon upconversion, 2,3-PyAn yielded the highest upconversion quantum yield (QY) of 12.1±1.3 %, followed by 3,3-PyAn and 2,2-PyAn. The TET quantum efficiencies determined by ultrafast transient absorption measurements showed the same trend.

Full text of DOI:10.1002/anie.201701929

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701929
German Edition: DOI: 10.1002/ange.201701929
Nanocrystals
Complementary Lock-and-Key Ligand Binding of a Triplet Transmitter
to a Nanocrystal Photosensitizer
Xin Li, Alexander Fast, Zhiyuan Huang, Dmitry A. Fishman, and Ming Lee Tang*
Abstract: Owing to the difficulty in comprehensively charac-
terizing nanocrystal (NC) surfaces, clear guidance for ligand
design is lacking. In this work, a series of bidentate bis-
(pyridine) anthracene isomers (2,3-PyAn, 3,3-PyAn, 2,2-
PyAn) that differ in their binding geometries were designed
to find the best complementary fit to the NC surface. The
efficiency of triplet energy transfer (TET) from the CdSe NC
donor to a diphenylanthracene (DPA) acceptor mediated by
these isomers was used as a proxy for the efficacy of orbital
overlap and therefore ligand binding. 2,3-PyAn, with an
intramolecular N–N distance of 8.2 ꢀ, provided the best
match to the surface of CdSe NCs. When serving as a trans-
mitter for photon upconversion, 2,3-PyAn yielded the highest
upconversion quantum yield (QY) of 12.1 ꢀ 1.3%, followed
by 3,3-PyAn and 2,2-PyAn. The TET quantum efficiencies
determined by ultrafast transient absorption measurements
showed the same trend.
of PbS nanocrystals and tetracene derivatives was shown to
efficiently upconvert near-infrared into visible light under one
sun conditions.[4] In this process, low-energy photons har-
vested by the NCs are funneled through surface-anchored
transmitters to free annihilators in solution. Two annihilators
in their triplet excited state collide, converting two low-energy
photons into one high-energy photon through triplet–triplet
annihilation (TTA). In this hybrid system, triplet energy
transfer (TET) from the NC to transmitter molecules is the
bottleneck limiting the photon upconversion QY. Therefore,
designing a transmitter ligand with a high binding affinity for
the NC donor in a geometry that facilitates orbital overlap is
crucial in increasing the rate and the overall quantum
efficiency of TET.
A series of isomeric bidentate bis(pyridine) anthracene
transmitter ligands were designed to simultaneously fulfill
these two requirements (Figure 1a). The bidentate nature of
these transmitters implies that the anthracene core is parallel
to the NC surface if the ligand binds via the two pyridine
N atoms. This increases the orbital overlap between the NC
donor and the acene acceptor, which is necessary for efficient
Dexter energy transfer. In contrast, previous work solely
involved monodentate transmitters with conjugated cores
that could be either perpendicular to or parallel with the NC
surface.[4b,5] Pyridine (Py) was selected to lock the anthracene
core in a fixed geometry because it is widely used as
a coordination ligand in organometallic complexes.[6] In
T
he design of ligands for nanocrystals is challenging because
the high surface area to volume ratio modifies the bulk lattice
in ways that are difficult to predict.[1] Ligand development
requires an understanding of the binding mechanism(s), as
well as the nature of the available binding sites. The latter is
difficult to characterize,[2] owing to the disordered nature of
the organic–inorganic interface and the inherent distribution
in the size and shape of colloidally synthesized nanocrystals.
Ligands that are structurally complementary to the nano-
crystal surface are critical in enhancing exciton delocalization
within the hybrid structure. Herein, we examine the conse-
quence of subtle perturbations in the structure of ligands that
serve as transmitters for triplet excitons from nanocrystal
donors. Much like Fisherꢀs lock-and-key mechanism for the
binding of substrates to an enzyme active site,[3] only one of
the transmitter ligands complements the nanocrystal surface,
resulting in a high photon upconversion quantum yield (QY)
that arises from efficient triplet energy transfer (TET).
Conjugated organic ligands can drastically enhance
energy transfer from a nanocrystal (NC) donor to an acceptor.
This is especially evident in a hybrid complex consisting of NC
sensitizers and acene annihilators. For example, a combination
Figure 1. a) The distances between the pyridine N atoms in the three
isomers are shown together with the {0001} facet of wurtzite CdSe
NCs for which the distances between neighboring cations are given.
b) Absorption and emission spectra of the three bis(pyridine) anthra-
cene isomers and 2.4 nm diameter CdSe NCs. All spectra were
measured in n-hexane at room temperature. c) Possible binding geo-
metries and the energy transfer in this hybrid photon upconversion
platform. The energy diagram depicts the triplet excitonic states of the
CdSe NC, the anthracene transmitter, and the 9,10-diphenylanthracene
annihilator.
[*] X. Li, Z. Huang, Prof. M. L. Tang
Chemistry, University of California, Riverside
501 Big Springs Road, Riverside, CA 92521 (USA)
E-mail: minglee.tang@ucr.edu
A. Fast, Dr. D. A. Fishman
Chemistry, University of California, Irvine (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201701929.
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addition, Py shows a better binding affinity for the NC surface
than the previously used carboxylic acid (COOH) group.[7]
Finally, both COOH and Py groups do not significantly
quench the NC exciton or perturb NC photoluminescence,
unlike thiols.[8] This is important in minimizing losses in this
TTA-based photon upconversion platform.
optimized conditions, m = 6.6 and 7.8 for 2,3-PyAn and 3,3-
PyAn, respectively, while for 2,2-PyAn, the average number
of bound ligands per NC is six times greater with m = 36.6
(Table 1). The isolated CdSe/PyAn complexes were mixed
with 2.1 mm DPA and excited with a 532 nm continuous wave
(CW) laser with a power density of 12.7 Wcmꢁ2. Rhodami-
ne 6G in ethanol was used as the reference (see Figure S2b
and the Supporting Information for details).[5c]
As shown in Figure 1a, the three transmitters differ in the
connectivity of the pyridines with the anthracene core. One
isomer is unsymmetric, with one Py connected in the meta and
the other in the ortho position (2,3-PyAn), while the other
two isomers are exclusively linked at either the meta or the
ortho positions (3,3-PyAn and 2,2-PyAn, respectively). The
intramolecular N–N distances vary from 9.6 ꢁ for 3,3-PyAn
to 8.2 ꢁ for 2,3-PyAn and 6.8 ꢁ for 2,2-PyAn. The atomic
scale differences between the pyridine groups provide the
possibility that only one isomer might match the unique Cd2+–
Cd2+ distances on the NC surface. As transmitter ligands for
photon upconversion, the 2,3-PyAn isomer yielded the high-
est upconversion QY of 12.1 ꢀ 1.3%, followed by 3,3-PyAn
(8.1 ꢀ 0.7%) and then 2,2-PyAn (2.5 ꢀ 0.7%). Transient
absorption (TA) measurements reflected the trends observed
in the steady-state upconversion experiments, where the
efficiency of TET is highest for 2,3-PyAn at 42.0% and lowest
for 2,2-PyAn at 23.1%. From these results, we concluded that
the 2,3-PyAn isomer best complements the wurtzite CdSe NC
by “locking” to its surface. Figure 1b shows the absorption
and emission spectra for all three bis(pyridine) anthracene
isomers and the 2.4 nm diameter CdSe NCs used in this study.
The absorption spectra share similar features, being governed
mainly by the anthracene core. However, from 3,3-PyAn to
2,3-PyAn to 2,2-PyAn, the absorption maximum shows
a 4 nm blue shift, as observed in a related series of phenyl-
pyridine isomers.[9] Details of the synthesis can be found in the
Supporting Information.[10]
Steady-state photon upconversion measurements were
conducted to evaluate the effect of the binding geometry on
TET (see the Supporting Information, Figure S2a). The
energy diagram in Figure 1c illustrates the overall upconver-
sion process. TET from the NC to the bound anthracene
transmitter is exergonic by roughly 0.57 eV. In order for
upconversion to occur, TET must occur from the CdSe NC
photosensitizer to a bound PyAn transmitter to a 9,10-
diphenylanthracene (DPA) annihilator. The latter is com-
monly used in organic–organic upconversion schemes
because of its long-lived, low-lying triplet state and relatively
high fluorescence QY (90%).[11] The upconversion QY in our
study is defined as described in Equation (1),
Table 1: The quantum yield of photon upconversion (FUC), the TET
efficiency (from both upconversion and TA spectroscopy, FTET(UC) and
FTET(TA), respectively), and the average number of bound ligands per
CdSe (m) for each CdSe/PyAn complex.[a]
2,3-PyAn
3,3-PyAn
2,2-PyAn
FUC [%]
FTET(UC) [%]
FTET(TA) [%]
m
12.1ꢀ1.3
25.8
42.0
8.1ꢀ0.7
17.3
38.1
2.5ꢀ0.7
5.34
23.1
6.6
7.8
36.6
ttrap [ps]
51.1
35.2
40.6
tq [ns]
0.785
0.785
1.28
1.04
1.05
0.962
0.81
0.80
1.25
tTET [ns]
kTET [ꢀ109 sꢁ1
]
[a] Based on the TA data, analysis of the kinetics of the NC donor
provides the decay time constants of surface trapping and quenching on
the NC induced by the PyAn ligands, ttrap and tq. Analysis of the kinetics of
TL1!TLn on the PyAn transmitters gives the time constant and rate of TET
(tTET and kTET, respectively) from the CdSe NCs.
As the transmitter ligand shuttling triplet excitons from
CdSe donors to DPA acceptors, 2,3-PyAn performed best out
of all three bis(pyridine) anthracene isomers. The upconver-
sion QY for 2,3-PyAn was as high as 12.1 ꢀ 1.3%, comparable
to the 14.3% record with 9-anthracenecarboxylic acid (9-
ACA) as transmitter.[5c] The excitation density (Ith) denoting
the transition from the quadratic to the linear regime for this
2,3-PyAn transmitter is 146.8 mWcmꢁ2 for a 68.7 mm solution
of functionalized CdSe NCs in 2.1 mm DPA in hexanes at
room temperature. The upconversion QYs for the other two
isomers, 3,3-PyAn and 2,2-PyAn, were 8.1 ꢀ 0.7% and 2.5 ꢀ
0.7%, respectively. Yanai, Kimizuka, and co-workers have
shown that monodentate PyAn transmitters with just one
pyridine moiety for binding to the NC result in an upconver-
sion QY of 1.4%.[12] This shows that at least two of the
bidentate ligands here outperform their monodentate con-
gener, probably by increasing orbital overlap with the CdSe
NC donor. Although 3,3-PyAn and 2,3-PyAn have a similar
binding affinity to wurtzite CdSe NCs, the upconversion QY
of the latter is almost twice as large as that of the former. This
indicates that its binding geometry matches the CdSe surface
lattice best and tightly locks the anthracene core to the
surface. While 2,2-PyAn may bind more strongly to the NCs
(as indicated by a higher number of surface-bound ligands),
the upconversion QY is low, quite close to that for the
monodentate ligand. This suggests that 2,2-PyAn may bind to
the surface in a monodentate fashion. This can also explain
the higher number of surface-bound ligands as the mono-
dentate geometry has a smaller footprint than the bidentate
binding geometry, thus allowing the NC surface to accom-
modate more ligands.
NabsðrefÞ NemðsampleÞ
NabsðsampleÞ NemðrefÞ
FUC ¼ 2Fref
ð1Þ
where FUC and Fref are the QYs of the upconversion sample
and reference, respectively. Nabs/em(sample/ref) refers to the
photons absorbed/emitted by the sample/reference.
The optimized upconversion QYs were found by varying
the transmitter density on the NC surface (Figure S4).[5c] The
average number of bound anthracene ligands per CdSe NC,
m, was determined by UV/Vis electronic absorption spec-
troscopy (see the Supporting Information for details). Under
2
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Figure 2. Femtosecond TA difference spectra in hexanes solution of a) CdSe NCs only and b) CdSe NCs with surface-anchored 2,3-PyAn. The CdSe
NCs were selectively excited with a 505 nm 100 fs pulsed laser. Experimental delays span from 2.4 ps (black squares) to 3 ns (grey pentagons).
c) Depletion of the CdSe excited-state absorption monitored by kinetic traces at 437 nm. The dots are the raw data from TA measurements for
CdSe NCs only as well as CdSe NC/transmitter complexes with 3,3-PyAn, 2,2-PyAn, or 2,3-PyAn as the ligand. The black solid lines are the fits for
each complex. d) An energy diagram depicting the physical processes during the TA measurements. The solid lines denote the pump and probe
wavelengths, dashed lines denote the relaxation processes on the CdSe NCs, and wavy lines denote possible quenching processes induced by the
ligand, such as triplet energy transfer (TET).
TA measurements were performed to determine the
quantum efficiency of TET, FTET(TA), and the rate of TET
for each isomer. TA spectra were acquired for up to 3.1 ns for
each CdSe/PyAn complex with the highest upconversion QY.
The TA difference spectra of native CdSe NCs without ligand
exchange and the CdSe/2,3-PyAn complex in the range of
400–560 nm are shown in Figure 2a and b, respectively. The
complete TA difference spectra spanning the visible region
for all three CdSe/PyAn complexes can be found in Figure S5.
In all experiments, the pump laser power was low (220 nJ)
such that no multi-exciton annihilation occurred. This was
confirmed by power dependence experiments. All of the
experiments were conducted in the linear regime (see Fig-
ure S6).
The transient kinetics of the excited state absorption
(ESA) of CdSe NCs at 437 nm in Figure 2c reveal that the
quenching of the NCs induced by the ligand is fastest for the
2,3-PyAn transmitter compared to the 3,3-PyAn and 2,2-
PyAn isomers. As shown in Figure 2d, the ESA at 437 nm was
assigned to transitions from band edge states F =ꢀ 1L
(“bright”) and F =ꢀ 2 (“dark”) to higher-energy states on
the CdSe NCs.[13] Possible channels depopulating the ESA
include electron trapping on defect sites, stimulated emission,
and band edge recombination.[13] The multiexponential model
shown in Equation (2) was used to fit the kinetics.
similar timescales of about 40 ps and approximately 1 ns for
all three ligands. We note that the fastest decay process, ttrap
ꢂ 40 ps, is slower than that for the pristine CdSe NCs and may
correspond to charge trapping to deeper defects that were
created during ligand exchange and cleaning.[14,15] This change
is observed across all CdSe/PyAn complexes as the same
functional group is involved. In the presence of a transmitter
ligand, the new decay constant, tq, with a timescale of
approximately 1 ns, corresponds to an additional decay
pathway introduced by surface-bound PyAn transmitters,
which is most probably due to TET from F =ꢀ 2 (“dark” state
on NC) to TL1 (the triplet state on anthracene) or other decay
processes induced by the disruption of the original ligand
shell. The values for ttrap and tq are listed in Table 1 for each
ligand. Detailed fitting parameters can be found in Table S2.
As the solid line in Figure 2c shows, the fits are robust. Here,
we focus on the depleted ESA of the NCs because its decay
kinetics are directly related to the physical process of TET to
surface-bound PyAn.
The kinetics of the triplet excited state formed on the
PyAn transmitters can be monitored at 445 nm, an isosbestic
point in the TA spectra. While the TL1!TLn transition of
anthracene is normally found around 433 nm, in this work, the
ESAs of the transmitter and NC overlap, resulting in an
isosbestic point at 445 nm. It is well known that the excited-
state energy levels of anthracene are very sensitive to small
changes in the molecular structure.[16] Changes in the TA
kinetics hypsochromic to this isosbestic point are dominated
by ESA of the CdSe NCs, while difference spectra bath-
ochromically shifted from 445 nm were assigned to the ESA
of the triplet excited state on the anthracene ligand. The
overall TL1!TLn transition time decay constants at approx-
imately 445 nm are 800 ps, 785 ps, and 1050 ps for 2,2-PyAn,
2,3-PyAn, and 3,3-PyAn, respectively. We notice that the
decay time constant of the ESA at 437 nm of the nanocrystal
photosensitizer in the CdSe/PyAn complexes (tq) is inversely
correlated to the growth in the absorption of the triplet
excited states on the transmitters at 445 nm (tTET). This
constitutes strong evidence that the quenching on NC induced
by PyAn is due to TET. Detailed fitting procedures are given
in the Supporting Information (Figure S8). Using TA spec-
X
n
DA ¼
i¼1 Ai expðꢁt=tiÞ
ð2Þ
Here, DA is the ESA for the CdSe NC, and Ai and ti are the
amplitude and corresponding decay constant for each com-
ponent. For the unmodified CdSe NCs, two time constants,
t1 = 1.05 ps and t2 = 34.6 ns, were needed for a satisfactory fit
of the data. Considering the timescales, the fast decay may
correspond to electron trapping to shallow defect sites on the
surface or stimulated emission, and the slower decay may be
due to direct band edge recombination, that is, F =ꢀ 1L
[14]
(“bright”)!1S3/2(h)
.
The time constants extracted are in
good agreement with previous reports.[14] Analysis of the ESA
of the CdSe NC photosensitizers functionalized with pyridine
bidentate ligands with a triexponential fit given the longest
component fixed, yielded two other decay components with
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FUC ¼ FTETðUCÞFTTAFA
ð3Þ
troscopy, Mongin and co-workers determined the kTET value
from CdSe NC to surface-bound 9-ACA to be about 2.0 ꢂ
109 sꢁ1,[5d] which is on the same order of magnitude as the
value obtained in this study. Note that the average number of
bound ligands, m, was about twelve in their case. Using time-
resolved photoluminescence measurements, Piland and co-
workers determined a rate of energy transfer of about 1.5 ꢂ
107 sꢁ1 for a similar system, which is about two orders of
magnitude smaller.[17] This lower rate of TET is perhaps due
to the lower number of surface-bound transmitter ligands (ca.
2).We have considered electron transfer from ligands to CdSe
NCs as another possible energy transfer mechanism. How-
ever, the absence of peaks at 629 nm, 681 nm, and 748 nm for
the anthracene radical cation[18] or 596 nm and 732 nm for the
anthracene radical anion,[19] rules out the possibility of
sequential charge transfer as a possible mechanism for TET
for CdSe/PyAn complexes (see Figure S5).
Here, FUC is the upconversion QY. FTET(UC) is the efficiency
of TET from CdSe to the bound bis(pyridine) isomers, FTTA
the efficiency of TTA for the DPA annihilator, and FA is the
fluorescence QY of DPA. If we assume FA = 0.9 and FTTA
=
0.52 for DPA based on previous reports,[11,22] the calculated
FTET(UC) values are 25.8%, 17.3%, and 5.3% for 2,3-PyAn,
3,3-PyAn, and 2,2-PyAn, respectively. Note that in our
system, FTET(UC) can be regarded as the overall TETefficiency
and is composed of two individual TET steps, that is, TET
from the NC light absorber to the PyAn transmitter, fTET(NC!
PyAn), and from the PyAn transmitter to the DPA annihilator,
fTET(PyAn!DPA), as shown in Equation (4):
FTETðUCÞ ¼ fTETðNC!PyAnÞfTETðPyAn!DPAÞ
ð4Þ
Therefore, as the TA measurements only characterize the
TET process from the NC to the surface-bound transmitters,
fTET(NC!PyAn), it makes sense that FTET(TA) is larger than
FTET(UC) (Table 1). However, fET(PyAn!DPA) should be close to
1 as the concentration of DPA is 2.1 mm, and TTA is
diffusion-limited here.[17,23] The discrepancy may arise from
the fact that the upconversion QY is measured in the presence
of the annihilator whereas the fTET from TA is determined in
the absence of the DPA emitter. The DPA annihilator is an
energy sink for the triplets transferred to the PyAn trans-
mitters and increases the efficiency of TET.
The efficiencies of TET from NC to PyAn obtained from
TA measurements, FTET(TA), are 42.0%, 38.1%, and 23.1%
for 2,3-PyAn, 3,3-PyAn, and 2,2-PyAn, respectively, correlat-
ing directly with the upconversion QYs. FTET(TA) was
extracted from the kinetics at 445 nm of the CdSe/PyAn
complexes after accounting for the contribution of the NC
(Figure 3; see also Figure S8). As expected, the 2,3-PyAn
In conclusion, the binding geometry greatly affects the
efficiency of TET from the NC to bound ligands. The best
binding geometry results in a significantly higher upconver-
sion QY, as reflected in the increased efficiency of TET given
by TA measurements. This study indicates that even though
the surface chemistry of NC is ill-defined and difficult to
accurately predict, it is still oriented in a specific manner.
Careful design of surface ligands to match the NC surface can
maximize exciton delocalization between inorganic NCs and
organic molecules. The use of bidentate ligands here suggests
a new approach in the design of ligands for nanocrystal
surfaces to maximize charge or energy transfer across these
hybrid surfaces.
Figure 3. The TET efficiency measured by transient absorption spec-
troscopy, FTET(TA), (black circles) for each bis(pyridine) anthracene
transmitter correlates with the corresponding photon upconversion QY
(gray squares).
isomer with the highest upconversion QY has the highest
FTET(TA) value, and it quenches the CdSe NC donor the fastest
(lowest tq). In addition, the 2,2-PyAn transmitter with the
lowest upconversion QY has the lowest FTET(TA). However,
we noticed that 2,2-PyAn quenches the CdSe NC donor more
effectively than the 3,3-PyAn isomer (Figure 2c and Table 1)
despite the fact that it has a lower FTET(TA) and upconversion
QY. This observation is consistent with reports on the
superlinear dependence of NC emission on ligand function-
alization, and that NC photoluminescence can only be related
to ligand binding at high surface coverage, and not at the low
density of transmitter ligands in this work.[7,20]
Acknowledgements
M.L.T. thanks the U.S. Army W911NF-16-1-0523 for instru-
mentation and CHE-1351663 for supplies and personnel.
D.A.F. acknowledges NSF grant 1532125.
Conflict of interest
The authors declare no conflict of interest.
Keywords: anthracenes · bidentate ligands · CdSe nanocrystals ·
Another way of calculating the quantum efficiency of
TET is by considering the photon upconversion QY obtained
under CW irradiation, which is a convolution of the following
factors in Equation (3).[21]
ligand design · upconversion
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Manuscript received: February 22, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Nanocrystals
X. Li, A. Fast, Z. Huang, D. A. Fishman,
M. L. Tang*
&&&& — &&&&
Complementary Lock-and-Key Ligand
Binding of a Triplet Transmitter to
a Nanocrystal Photosensitizer
Transmitter optimization: A series of
bidentate bis(pyridine) anthracene iso-
mers that differ in their binding geome-
tries were designed to find the best fit to
the surface of CdSe nanocrystals. The
efficiency of triplet energy transfer (TET)
from the CdSe NC donor to a diphenyl-
anthracene acceptor via these transmit-
ters was used as a proxy for the efficacy of
orbital overlap and ligand binding.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!