Evolution from Tunneling to Hopping Mediated Triplet Energy Transfer from Quantum Dots to Molecules

Article abstract of DOI:10.1021/jacs.0c07727

Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.

Full text of DOI:10.1021/jacs.0c07727

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Article
Evolution from tunneling to hopping mediated triplet
energy transfer from quantum dots to molecules
Zhiyuan Huang, Zihao Xu, Tingting Huang, Victor Gray,
Kasper Moth-Poulsen, Tianquan Lian, and Ming Lee Tang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07727 • Publication Date (Web): 24 Sep 2020
Downloaded from pubs.acs.org on September 24, 2020
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Evolution from tunneling to hopping mediated triplet energy
transfer from quantum dots to molecules
Zhiyuan Huang,$,a Zihao Xu,$,b Tingting Huang,a Victor Gray,c,d Kasper Moth-Poulsen,c Tianquan
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Lian,b* Ming Lee Tanga*
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a Department of Chemistry, University of California, Riverside, Riverside, CA, 92521, United
States
b Department of Chemistry, Emory University, Atlanta, GA, 30322, United States
c Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412
96 Gothenburg, Sweden
d Department of Chemistry – Ångström Laboratory, Uppsala University, Box 523, 751 20 Uppsala,
Sweden.
$ Authors contributed equally
Abstract
Efficient energy transfer is particularly important for multi-excitonic processes like singlet fission
and photon upconversion. Observation of the transition from short-range tunneling to long-range
hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This
is firmly supported by steady-state photon upconversion measurements, a direct proxy for the
efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When
phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene
acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon
upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling
mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon
upconversion QYs increase again to 0.468% and 0.413%, 1.5–1.6 fold higher than that with 3
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phenylene units (using the convention where the maximum upconversion quantum efficiency is
100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively
efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient
absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and
the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first
observation of the tunneling-to-hopping transition for long range triplet energy transfer between
nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should
further be explored in the context of artificial photosynthesis.
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Introduction
Efficient long-distance energy transfer in artificial photosynthetic platforms is a long-standing
goal. It is currently understood to occur via incoherent hopping or adiabatic energy transfer.1-3
Compared to exciton transport, charge transfer is significantly better understood. Solid-state
semiconductor devices and individual cytochrome proteins rely on single short (<1 nm) and long-
range (2.5 nm) electron tunneling steps, respectively4-7. Charge transfer between component
cytochromes in the photosynthetic reaction centers requires multi-step tunneling or incoherent
electron hopping5, 8-9; polycrystalline organic semiconductor thin films generally rely on both hole
and electron hopping10. In contrast, exciton transport requires the correlated transfer of both the
electron and the hole11-12. Rates of energy transfer have to be deconvoluted into hole and electron
contributions, and identification of the possible pathways involved is key to achieving efficient
energy transfer, especially for soft materials like proteins, organic materials, and colloidal
nanoparticles.
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Colloidal semiconductor nanocrystals (NCs) are promising materials in optoelectronic
applications such as photovoltaics, light emitting diodes and photodetectors, etc.13-18 Rapid energy
and charge transport over long distances is one of the prerequisites for high performance devices.
Many studies have shown photoexcited nanocrystals can efficiently transfer charge directly to
surface bound molecules,19 or across inorganic shells,20 but much fewer studies have been
performed on triplet energy transfer from NC photosensitizers. In terms of exciton transport, long-
lived triplet excitons, with diffusion lengths up to several micrometers in crystalline organic
solids,21-22 are promising for long-distance energy transfer. In comparison, the short diffusion
length of singlet excitons (tens of nanometers) limits the thickness of organic photovoltaic devices
to ~100 nm and precludes the complete absorption of sunlight.23 Though well studied in organic
donor-bridge-acceptor (D-B-A) systems,24-27 long-range triplet exciton transport relating to
semiconductor NCs has not been reported so far. Precedence lies in mechanistic charge transfer
studies in labelled cytochrome proteins, and in molecular D-B-A systems. Despite the
heterogenous nature of proteins, single step electron tunneling up to 2.6 nm has been
experimentally measured with an exponential distance constant, β ~11 nm-1.1, 9 Short conjugated
bridges in organic donor-acceptor systems retain this exponential distance-dependent tunneling for
electron transfer but report smaller β values6. We showed previously28 that triplet energy transfer
from CdSe NCs to anthracene displays this exponential distance dependence with varying lengths
of phenylene bridges. However, longer bridges show an ohmic distance-dependent hopping
mechanism.29 Weak distance dependence has also been ascribed theoretically to adiabatic electron
transfer2-3. Optimization of the structure and energetics of the donor, bridge, and acceptor to
accentuate charge hopping and avoid the strong distance-dependent charge tunneling processes are
crucial for realizing wire-like long-range charge transport.
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In this work, we demonstrate the transfer of triplet excitons from semiconductor NCs to
molecular acceptor can occur via tunneling or hopping, and the mechanism is controlled by the
triplet energy level of the intervening p-oligophenylene bridge. With CdSe NCs as triplet energy
donors, anthracene acceptors attached by a pyridine anchoring group and p-oligophenylene bridge,
we show that the triplet energy transfer mechanism changes from tunneling (with an exponential
distance dependence) to hopping. The tunneling-to-hopping transition was observed from steady-
state photon upconversion quantum efficiency (QY) measurements. The shortest three ligands
show the expected distance dependent decrease with increasing bridge length, while the last two
ligands with the longest bridge show an unexpectedly high upconversion QY with no distance
dependence. Transient absorption spectroscopy supports this interpretation with the observation
of both the triplet state of the anthracene acceptor, and the phenylene bridge. This work shows that
molecular design is critical in enabling the first demonstration of long-range triplet exciton
transport between semiconductor NCs and organic acceptors.
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Results and discussion
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Figure 1. (a) Illustration of energy diagram of energy transfer process involved in CdSe NC
sensitized photon upconversion. Red arrows denote triplet exciton tunneling while orange yellow
arrows denote hopping. (b) Absorption (black) and fluorescence (red) spectra of transmitter ligands
with different length of phenylene bridges. The spectra were obtained with hexane at room
temperature.
The mechanism of CdSe nanocrystal (NC) sensitized photon upconversion is presented in
Figure 1. This three-component upconversion platform comprises of 2.4 nm diameter CdSe NCs
as the light absorber, anthracene with a phenylene bridge and pyridine binding group as the
transmitter, and 9,10-diphenylanthracene (DPA) as the emitter. Photon upconversion is initiated
by photoexcitation of CdSe NCs with a 488 nm continuous wave laser. After intersystem crossing
within CdSe NCs, the triplet exciton is transferred to the surface anchored anthracene by tunneling
or hopping through the phenylene bridge. Subsequent TET occurs from the transmitter to the
emitter DPA. Two DPA triplets then annihilate to form a singlet, followed by the fluorescence of
DPA at 430 nm (Figure S1). In this hybrid upconversion system, each component is rationally
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designed. As upconversion QYs negatively correlate with the NC size,30-31 small CdSe NCs are
used, resulting in high upconversion quantum yields. Furthermore, without the transmitters in
Scheme 1, the native oleate ligand shell hinders energy transfer from CdSe NCs to DPA, leading
to a low upconversion QY. The transmitter ligands funnel the triplet energy from CdSe NCs to
DPA and have been reported to enhance upconversion (QYs) as much as 3 orders of magnitude.32
Lastly, DPA is selected as the light emitter due to its high fluorescence QY (90%).33
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Scheme 1. The structures of transmitter ligands with different length of phenylene bridges.
The transmitter ligands are designed with different lengths of phenylene units to vary the
bridge triplet energies between CdSe and anthracene. The structures of the 5 transmitters are
presented in Scheme 1. The 9-phenylanthracene core possesses an excited triplet state, T1, with the
energy similar to that of DPA (1.77 eV),34-35 and lower than that of CdSe (2.47 eV). Pyridine serves
as the anchoring group binding anthracene to the surface of CdSe NCs, and the phenylene bridges
as the rigid spacer to control the distance between CdSe NCs and anthracene. As transmitter
ligands are loaded on CdSe NCs by a solution-based ligand exchange reaction (see SI), ethylhexyl
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side chains are incorporated on PyP2PAn, PyP3PAn and PyP4PAn to maintain solubility. As
shown in Table 1, all transmitters possess high fluorescence QYs (> 65%), meaning reduced
nonradiative decay. These ethylhexyl side chains introduce steric bulk and rigidity and may
explain the higher fluorescence QY of PyP2PAn (79.6%) compared to PyP1PAn (74.1%). The
absorption and fluorescence spectra of transmitters are shown in Figure 1b and S3. From these
spectra, we can see that while the presence of the phenylene bridges does not modify the singlet
electronic states of anthracene core (also confirmed by TA spectra and kinetics in Figure S5-S6),
growth in the number of phenylene units increases the absorption at 300 nm from PyP0PAn to
PyP4PAn. As the number of phenylene units, n, increases from 0 to 4, the corresponding lowest
excited triplet state, T1, of this bridge decreases from 3.68 eV for pyridine36, 2.84 eV for biphenyl,
2.53 eV for p-terphenyl35, 37 to 2.37 and 2.34 eV26, 38 respectively for the two longest transmitters
(Figure 1a). With this design, the triplet exciton is expected to transfer from CdSe NCs to the
anthracene core by superexchange through the phenylene bridge when the bridge’s T1 energy
exceeds the bandedge exciton of CdSe NCs (n=0, 1, 2). On the other hand, when n=3 or 4
(PyP3PAn and PyP4PAn), the bridge’s T1 state can be thermally populated, which allows triplet
exciton transport from CdSe NCs to anthracene by hopping mechanism with a weak distance
dependence.
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Table 1. Key parameters of pyridine transmitter ligands. ΦFLT, the fluorescence quantum yield;
Copt, the optimal concentration of transmitters during ligand exchange; ΦUC, upconversion
quantum efficiency; ΦTET, the efficiency and, kTET, the rate constant of triplet energy transfer from
CdSe nanocrystal to transmitters.
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(μs-1)c
Bridge T1
(eV)
Copt (mM)
Φ
(%)
Φ
(%)a
Φ
(%)b
TET
Ligand
k
FLT
UC
TET
PyP0PAn
PyP1PAn
PyP2PAn
PyP3PAn
PyP4PAn
3.68
2.84
2.53
2.37
2.34
90.2
21.7
7.85
1.70
2.55
5.37
11.6
30.1
54.0
74.1
79.6
79.1
68.6
4.51
0.284
0.468
0.413
11.7
0.737
1.21
2.68
0.139
0.229
0.202
1.07
aΦ of 100% is defined as the emission of one high energy photon per two absorbed low energy
UC
photons. bCdSe/PyP0PAn Φ is measured by TR-PL and calculated as shown in SI section 9 and
TET
Table S2. For CdSe/PyP1PAn to CdSe/PyP4PAn, Φ is calculated from Φ assuming all other
TET
UC
c
efficiencies are the same according to Eq.1. Similarly, k
is measured by TR-PL for
TET
CdSe/PyP0PAn. For CdSe/PyP1PAn to CdSe/PyP4PAn kTET is calculated by Eq. 2.
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Figure 2. Dependence of upconversion quantum efficiencies and triplet energy transfer rate
constants on the length of phenylene bridges. The solid blue line is the linear fit of the kTET for the
first three data points, with the slope (damping coefficient) of 0.7236±0.0005 Å-1.
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The upconversion quantum efficiencies of this three-component system were measured as
a function of the transmitter spacer length. Upconversion QYs are optimized by varying the surface
loading of transmitters (see SI, Figure S2) and the optimal upconversion QYs are listed in Table
1. In Figure 2, the optimal upconversion QYs are plotted as a function of the distance between the
CdSe NC donor and anthracene acceptor, defined as the distance from the ligand binding site
(nitrogen in pyridine) to the center of anthracene. The upconversion QY for the shortest transmitter
(PyP0PAn) is 11.6%. This is an increase by two orders of magnitude compared with the free CdSe
NCs with no transmitters (0.1%). As the phenylene bridge units in the transmitter ligands increase,
the upconversion QYs drop exponentially to 4.51% for PyP1PAn and 0.284% for PyP2PAn. To
our surprise, the upconversion QYs of PyP3PAn and PyP4PAn (n=3 and 4) increase compared to
the shorter PyP2PAn (n=2) ligand and become weakly dependent on spacer length. This result
suggests that the mechanism of exciton transfer changes from exciton tunneling at n<3 to hopping
or adiabatic energy transfer at n>3. Although the upconversion efficiency depends on the
efficiencies of multiple steps, we will show below that in these systems with different transmitters,
the TET rate from the NC to transmitter (kTET) is the key parameter that is different while other
parameters remain nearly constant. As a result, the observed distance dependence in the
upconversion efficiency reflects that distance dependence of kTET. Similar distance dependence
was observed in previously reported triplet energy transfer rates 25-28, 39 and charge transfer rates40-
41 through related molecular bridges.
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The upconversion QY ( 푈퐶) depends on the product of the quantum efficiencies of TET
∅
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from CdSe NCs to transmitters(
), triplet-triplet annihilation of emitters ( TTA ) and emitter
∅
∅
푇퐸푇
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fluorescence (
, as shown in Eq (1):
∅퐹퐿)
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(1)
∅
푈퐶 = ∅푇퐸푇 ∙ ∅푇푇퐴 ∙ ∅퐹퐿
We have assumed that the quantum efficiency of triplet energy transfer from the transmitter to the
emitter is nearly 100% for all transmitters, independent of spacer length. The validity of such
assumption will be further discussed below with the help of transient absorption studies. TTA and
∅
depend only on the emitter, which are kept identical in these measurements. Thus, the
∅
퐹퐿
transmitter dependent upconversion efficiency reflects the distance dependence of
.
푇퐸푇 is
∅
∅
푇퐸푇
determined by the relative rates of triplet energy transfer (kTET) and the intrinsic decays of the NC,
according to Eq. (2):
푎푖푘푇퐸푇
3
푖 = 1
∑
=
(2)
∅
푇퐸푇
푘
푇퐸푇 + 푘퐶푑푆푒,푖
In Eq. (2) kCdSe,i, and ai are the rate constant and amplitude of the ith component of the multi-
exponential intrinsic decay of CdSe NCs obtained from TR-PL as shown in Table S2. In the limit
that kTET is small compared to kCdSe, Eq. 2 can be approximated to
퐶푑푆푒, which means
푇퐸푇 = 푘푇퐸푇/푘
∅
that
depends linearly on kTET, and by Eq. 1 푈퐶 also depends linearly on kTET.
∅
∅
푇퐸푇
In the following kinetics study, the samples are prepared with the same NC and mediator loading
as in the upconversion measurement, the only difference is that DPA (annihilator), which absorbs
strongly in the NC emission and mediator absorption regions, have been removed to allow time-
resolved photoluminescence (PL) and transient absorption (TA) studies of the TET process
between the NC and mediator. In CdSe/PyP0PAn sample, the NC PL decay is significantly faster
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than in pristine CdSe samples (Figure S7a), from which
can be determined to be 54.0 μs-1.
푘TET
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6
7
8
9
Using Eq. 2,
can be determined to be 30.1%, as shown in Table 1. From the measured
∅
∅
∅
푈퐶
TET
) and
and reported DPA FL =90%,33 TTA can be calculated to be 42.8% according
∅ ∅
(11.6%
푇퐸푇
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to Eq. 1. This corresponds well to TTA values reported in literature42-43. However, for emitters with
∅
longer spacer lengths (n=1-4), kTET becomes much slower than the intrinsic decay of NCs, and it
is difficult to extract a contribution from TET by comparing the total decay rates in NC/transmitter
complexes (kCdSe+kTET ≈ kCdSe) with the decay rate of free NCs (kCdSe), as shown in Figure S7b.
Alternatively, for CdSe/PyP1PAn to CdSe/PyP4PAn, the 푇퐸푇 can be calculated from measured
∅
according to Eq. 1 using
and 퐹퐿 determined above. Then the rate constant of TET from
∅
∅
푈퐶
∅
TTA
CdSe NCs to anthracene ( 푇퐸푇) can be calculated from Eq. 2. The results are listed in Table 1 and
푘
plotted in Figure 2.
For Dexter energy transfer, 푇퐸푇 decays exponentially with an increase in distance between donor
푘
and acceptor, following Eq. (3):44-45
푘푇퐸푇 = 푘0 ∙ 푒 ―훽푑
(3)
where k0 is the pre-factor and β is the damping coefficient that describes the exponential decay of
the electronic coupling between CdSe NCs and anthracene. Here, the effective transfer rate is
calculated and plotted in Figure 2. The effective transfer rate is the product of TET rate for a single
transmitter and the total number of transmitters per CdSe NC. In this study, due to the relatively
weak binding of the pyridine group, the binding of the transmitter to CdSe NC is reversible, which
means there is a population distribution between the bound and free transmitter. This is confirmed
by the observation of ligand detachment on UV-Vis absorption spectra after washing
CdSe/PyP0PAn complex a second time (Figure S2f). Therefore, the exact number of bound
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transmitters per NC is difficult to determine. However, it’s been shown in previous work that an
optimal number of bound transmitters exists, because transmitter triplets can annihilate at high
surface density which is detrimental to upconversion QYs.46-47 Therefore, the optimal number of
transmitters should be as high as possible to maximize the TET efficiency, but not surpass the
loading that could initiate triplet-triplet annihilation of transmitters. Considering the similar
structure of anthracene core for different transmitters, i.e. similar threshold surface density that
transmitter triplet-triplet annihilation occurs, we assume the number of bound transmitters with
optimal upconversion QYs would be similar, and the effective rate is used in Figure 2 to represent
the dependence of rate on distance. Fitting the kTET of PyP0PAn, PyP1PAn, and PyP2PAn yields
an exponential distance dependence, as shown in Figure 2, with a damping coefficient of 0.724
Å-1. This value is comparable with previously reported values for triplet24, 26-28 or charge transfer40-
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in D-B-A systems with the p-oligophenylene bridge, considering the damping coefficient of
triplet transfer is the sum of that for electron and hole transfer.48-49 This superexchange mechanism
is expected because of the electronic coupling between the CdSe donor and anthracene acceptor is
approximately exponentially dependent.
The TET rate constants of PyP3PAn and PyP4PAn (n=3 and 4) increase compared with
the shorter PyP2PAn (n=2) ligand, as shown in Figure 2. As the length of phenylene bridge
increases, the extended conjugation decreases the energy of T1 state of phenylene bridge, bringing
it near resonance with the donor state (Figure 1a). The triplet exciton then transfers by incoherent
hopping through the T1 state of the phenylene bridge instead of tunneling, which gives rise to the
weak distance dependence observed for PyP3PAn and PyP4PAn. Alternatively, a weak distance
dependence can also occur in the adiabatic energy transfer limit with strong electronic coupling
between CdSe NC donor and molecular acceptor. Close examination of the vibrational fine
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structure in the absorption spectra (Fig. S3) shows a red shift in the anthracene core on PyP0PAn
and PyP1PAn when bound on the surface of CdSe NCs (compared to the free ligands in solution).
This is accompanied by a slight red shift in the NC PL for CdSe/PyP0PAn and CdSe/PyP1PAn
(Fig. S4). These shifts in both anthracene absorption and NC bandedge PL could be due to the
delocalization of CdSe exciton to anthracene for the two shortest ligands, indicating a strong CdSe-
transmitter coupling. However, no change is observed for CdSe/PyP2PAn-CdSe/PyP4PAn
complexes, suggesting weak coupling between NC and ligands (Figure S3, S4). Strong electronic
coupling between CdSe, the bridge and anthracene is prevented by the 30-35º torsional angle
between the last phenyl unit and anthracene40, 50. Therefore, the weak distance dependence for
PyP3PAn and PyP4PAn is ascribed to incoherent hopping. Further support of this pathway is the
presence of the bridge triplet state as the intermediate between the triplet states of the donor and
acceptor, which is directly observed in transient absorption spectroscopy to be discussed below.
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Figure 3. (a) Nanosecond transient absorption of CdSe/PyP0PAn. The sample was pumped at 510
nm to avoid direct excitation of PyP0PAn. The 510 nm region is not shown due to pump scattering.
(b) The excited state absorption of PyP0PAn triplet state at the 400-500 nm region. (c) The
comparison of CdSe exciton bleach probed at 500 nm with (blue) and without (red) surface bound
PyP0PAn, and the corresponding PyP0PAn triplet kinetics probed at 446 nm (green), which is an
isosbestic point for the CdSe NC transient spectra. The CdSe/PyP0PAn XB is scaled (light blue)
and compared with the triplet kinetics. (d) Comparison of the triplet lifetime of pristine PyP0PAn
and CdSe sensitized probed at 440 nm. No obvious decay observed for pristine PyP0PAn in this
100 microseconds time window. The CdSe sensitized triplet is fitted to a single exponential decay
e-t/.
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Transient absorption (TA) spectroscopy is used to confirm triplet exciton transfer from CdSe NCs
to transmitters. Samples of CdSe NCs capped with transmitter ligands with optimal ligand loadings
are prepared for TA measurements (without DPA). Figure 3a shows the TA spectra of
CdSe/PyP0PAn measured with 510 nm excitation at an excitation energy density of 200 J/cm2,
which selectively excites CdSe NCs. The positive features represent the excited state absorption
(ESA), and the negative features the ground state bleach (GSB). The ESA of the T1 state of
anthracene lies around 440 nm, overlapping with the ESA of CdSe NCs. After 500 ns, when the
CdSe ESA has decayed completely, the T1-Tn transition of PyP0PAn peaked at 440 nm can be
clearly observed (Figure 3b). The kinetics of triplet growth can be obtained at 446nm, at which the
CdSe NC has negligible contribution to the TA signal (Figure 3c). Compared with free CdSe NCs,
the exciton bleach (XB) kinetics of CdSe/PyP0PAn show a faster recovery (Figure 3c), consistent
with exciton quenching by TET from the NC to PyP0PAn. The NC XB bleach and PyP0PAn triplet
growth shows good agreement with each other, with the exception of early delay times (< 1 ns),
during which there is noticeable decay of NC XB signal without the corresponding growth of
PyP0PAn triplet. We attribute this to surface electron traps caused by the ligand exchange process
used for attaching the transmitter molecules.51 The kinetics of exciton bleach probed at 500 nm
with/without mediator was fit to extract the rate of TET and the fitting parameters are listed in SI
Table S2. The TET rate is determined to be 31.5 s-1, and from which the triplet yield is determined
to be ~60% (see SI Section 9 for details). Both values are comparable to those determined from
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PL quenching (kTET ~54 s-1, and
) and shown in Table 1. Unfortunately, as shown in
∅푇퐸푇~30%
Figure 3b, the triplet absorption signal in CdSe/PyP0PAn, the complex with the highest 푇퐸푇, is
∅
already small (~1 mOD). Similar TA studies conducted on the transmitter ligands with longer
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phenylene bridges (n=1, 2, 3 and 4) become difficult due to much lower 푇퐸푇 and smaller triplet
∅
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signal. Furthermore, due to the slower TET rate, and the exciton bleach kinetics shows negligible
difference with/without the transmitter (Figure S7d). Therefore, the rate and efficiency of TET is
only measured by time-resolved spectroscopy for the shortest ligand, PyP0PAn, by both TA
spectra and time-resolved photoluminescence (Figure S7). For all other ligands, these values are
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calculated from steady-state 푈퐶 measurements.
∅
The fit of the triplet decay kinetics shown in Figure 3d also reveals that the triplet lifetime
of PyP0PAn is shortened from over 100 s in solution to ~80 s when grafted on CdSe NC surface
(Figure 3d and S4). With such a long triplet life time and high DPA concentration (2.15 mM) in
this upconversion system, the TET efficiency from transmitters to DPA can be estimated based on
푘푇퐸푇2[퐷푃퐴]
, where kTET2 is the bimolecular TET rate constant between transmitter and
Φ푇퐸푇2
=
푘푇퐸푇2[퐷푃퐴] + 푘푇
DPA (~30 s-1M-1 based on a previous study52), and kT is the transmitter triplet lifetime (80 s,
equivalent to 0.0125 s-1). The calculated TET efficiency from NC-surface bound transmitters to
DPA is estimated to be over 84%. In our analysis, it is approximated to be unity and constant for
all transmitters.53
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Figure 4. TA absorption spectra of CdSe/PyP4PAn. (a) Nanosecond transient absorption of
CdSe/PyP4PAn measured with 510 nm excitation. Sample: 4.06 μM CdSe NCs and 0.182 M
PyP4PAn dissolved in THF with no DPA (annihilator). (b) TA spectra in panel a) after subtraction
of CdSe contribution. (c) Comparison of subtracted TA spectra of CdSe/PyP4PAn (from panel b)
at 2-5 ns (red) and 5-10 s (blue) with the TA spectrum of pristine PyP4PAn at 2-5 ns (grey line).
The spectra for pristine PyP4PAn were measured with 400 nm and the full spectra at other delay
times are shown in Figure S5.
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Although challenging to measure the rate of TET in longer transmitter ligands, TA spectroscopy
is able to observe the triplet excited state in the bridge of PyP4PAn as an intermediate state between
the recovery of the CdSe NC exciton bleach and rise of the triplet state absorption of the anthracene
core. In order to observe the weak bridge state signal, the TA spectra of CdSe NC-PyP4PAn were
measured at high 510 nm excitation energy density (1200 J/cm2) using a sample that is saturated
with transmitter ligands (4.06 μM CdSe NCs and 0.182 M PyP4PAn, excessive transmitter loading
compared to steady-state upconversion measurements). The TA spectra (Figure 4a) are dominated
by the CdSe exciton features although additional spectral features of PyP4PAn can also be
observed. To better observe the PyP4PAn spectral features, the CdSe signals are removed by
subtracting the TA spectra of CdSe /PyP4PAn by the normalized TA spectra of pristine CdSe NCs.
The latter has been normalized to the same amplitude at the main exciton bleach peak (~500 nm)
as the former at each delay time. The subtracted spectra ( Figure 4b) show clearly the GSB (~ 425
nm) and singlet ESA (~ 600 nm) of PyP4PAn at early delay times (<10 ns), and PyP4PAn triplet
ESA (~425 nm) at long delay times (~ 1s). These species are assigned based on the TA spectra
of pristine PyP4PAn (Figure S5) that were measured with 440 nm excitation, which directly excites
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the singlet excited state of anthracene core (An). Although the PyP4PAn singlet state shown in
Figure 4a&b cannot be generated by one photon absorption at 510 nm, it can be generated by
multi-photon absorption under the high excitation fluence used in this study.
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The subtracted TA spectral also shows a broad peak at ~800 nm at intermediate delay times
that is attributed to the bridge triplet state, located on the 4 phenylene units in the conjugated bridge
of the PyP4PAn transmitter ligand. The peak position agrees well with low temperature
measurements of the tetra-p-phenyl triplet state by De Cola.27, 38 This species is absent in the TA
spectra of pristine CdSe NCs and pristine PyP4PAn (Figure S5e), which is more clearly seen in
the comparison of these spectra in Figure 4c. This result shows that the bridge triplet state cannot
be generated by the excitation of anthracene singlet state. In pristine PyP4PAn, anthracene triplet
state can be generated from singlet state via intersystem crossing with a time constant of ~5 ns
(Figure S8a, Table S1). In CdSe/PyP4PAn, most of anthracene triplet is formed on a much slower
time scale (~200 ns). The growth kinetics of anthracene triplet contains a minor component (~
5%) that is formed on the ns time scale, suggesting that the direct multiphoton excitation of
anthracene singlet state plays a minor role in the formation of triplets. The comparison of the
kinetics of the bridge triplet decay (probed at 813 nm) and anthracene triplet growth (probe at 443
nm) in Figure S8b shows that these kinetics are the same, suggesting that bridge triplet is an
intermediate in TET from the CdSe NC donor to anthracene acceptor. The transfer rate can be
estimated to be 2 us-1 from Figure S8b, ~10 times faster than the value in Table 1. This can be
explained by the significantly higher PyP4PAn ligand concentration (180 mM compared to 5 mM).
The measured triplet energy transfer rate constant agrees with our previous studies on similar
system, such as CdSe-ACA and CdSe-T6.51, 54 This triplet transfer involves a real intermediate
localized on the bridge, and is likely controlled by low-frequency vibrational modes in the
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tetraphenyl linker55, or by coupling to nuclear motions of the toluene solvent. Such long distance
hopping mediated energy transfer has been shown to be more efficient than coherent
superexchange at long distances56-57. The results here mirror that observed for charge transfer58,
and for triplet transfer in organics24, 59, and in d6 metal diamine donor-acceptor complexes bridged
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2+
2+
by phenylene units26-27, 38. For the Ru(bpy)3 (bpy = 2,2’-bipyridine) donor and Os(bpy)3
acceptor pair26, phenylene bridges posed an energetically insurmountable tunneling barrier with
an attenuation factor of 0.50 Å-1 (similar to the first three ligands here where β = 0.72 Å-1); but for
the Ir(bpy)32+ donor and Ru(bpy)32+ acceptor couple, a weak distance dependence with of β=0.07
Å-1 was attributed to the fact that transfer to the bridge triplet states was exergonic27.
Conclusions
In conclusion, a systematic study of triplet exciton transfer pathways is enabled by a series of
carefully designed transmitter ligands with increasing phenylene bridge units. The upconversion
quantum efficiency first decreases exponentially with the increasing distance between the CdSe
NC sensitizer and transmitter from zero to two phenylene units (1.4 nm), then remains unchanged
with two, three and four units. This result suggests a change in the triplet exciton transfer pathway
from a tunneling to hopping mechanism. The hopping mechanism is likely enabled by the
gradually lowered triplet state energy of an increased number of phenylene units with a bridge
triplet state in resonance with the CdSe donor and anthracene acceptor triplet energy levels. The
TA spectroscopy provides a direct observation of triplet excited state on the transmitter and on the
bridge. The small donor-bridge gap here of 0.1 eV has been shown to be undesirable for long range
energy transfer in organic D-B-A systems60. This study suggests that energy transfer will be more
efficient with a larger driving force between the donor and bridge, easily achieved by increasing
the conjugation length of the bridge, using oligophenylene vinylenes, for example. This work
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provides a guide to design hybrid NC-molecule system for efficient and long-range exciton
transport for artificial photosynthesis.
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Supporting Information
Experimental methods, synthesis, and data fitting procedures are outlined in the Supporting
Information. This material is available free of charge at http://pubs.acs.org.
Author Information
Corresponding Author:
tlian@emory.edu.
mltang@ucr.edu
Acknowledgement
MLT acknowledges Air Force Office of Scientific Research (AFOSR) Award FA9550-19-1-0092
for equipment and DOE DE-SC0018969 for salary support. TL acknowledges the financial support
by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Solar
Photochemistry Program under Award Number (DE-FG02-12ER16347 and DE-SC0008798). VG
acknowledges financial support from Sixten Gemzéus Stiftelse.
References
(1) Winkler, J. R.; Gray, H. B., Long-Range Electron Tunneling. J. Am. Chem. Soc. 2014, 136
(8), 2930-2939.
(2) Renger, T.; Marcus, R. A., Variable-Range Hopping Electron Transfer through Disordered
Bridge States:ꢀ Application to DNA. J. Phys. Chem. A 2003, 107 (41), 8404-8419.
(3) Newton, M. D.; Smalley, J. F., Interfacial bridge-mediated electron transfer: mechanistic
analysis based on electrochemical kinetics and theoretical modelling. Phys. Chem. Chem. Phys.
2007, 9 (5), 555-572.
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2
3
4
5
6
(4) Winkler, J. R.; Nocera, D. G.; Yocom, K. M.; Bordignon, E.; Gray, H. B., Electron-transfer
kinetics of pentaammineruthenium(III)(histidine-33)-ferricytochrome c. Measurement of the rate
of intramolecular electron transfer between redox centers separated by 15.ANG. in a protein. J.
Am. Chem. Soc. 1982, 104 (21), 5798-5800.
7
(5) Parson, W. W., Long live electronic coherence! Science 2007, 316 (5830), 1438-1439.
(6) Wenger, O. S., Photoinduced electron and energy transfer in phenylene oligomers. Chem.
Soc. Rev. 2011, 40 (7), 3538-3550.
(7) Skourtis, S. S.; Waldeck, D. H.; Beratan, D. N., Fluctuations in Biological and Bioinspired
Electron-Transfer Reactions. Annu. Rev. Phys. Chem. 2010, 61 (1), 461-485.
(8) Page, C. C.; Moser, C. C.; Chen, X.; Dutton, P. L., Natural engineering principles of electron
tunnelling in biological oxidation–reduction. Nature 1999, 402 (6757), 47-52.
(9) Winkler, J. R.; Gray, H. B., Electron Flow through Metalloproteins. Chem. Rev. 2014, 114
(7), 3369-3380.
(10) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. L., Charge
transport in organic semiconductors. Chem. Rev. 2007, 107 (4), 926-952.
(11) Solomon, G. C.; Bergfield, J. P.; Stafford, C. A.; Ratner, M. A., When "small" terms matter:
Coupled interference features in the transport properties of cross-conjugated molecules. Beilstein
J. Nanotechnol. 2011, 2, 862-871.
(12) Wohlthat, S.; Solomon, G. C.; Hush, N. S.; Reimers, J. R., Interference-induced electron-
and hole-conduction asymmetry. Theor. Chem. Acc. 2011, 130 (4-6), 815-828.
(13) Alivisatos, A. P., Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996,
271 (5251), 933-937.
(14) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P., Light-emitting diodes made from cadmium
selenide nanocrystals and a semiconducting polymer. Nature 1994, 370 (6488), 354-357.
(15) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V., Prospects of Colloidal
Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110 (1), 389-458.
(16) Carey, G. H.; Abdelhady, A. L.; Ning, Z.; Thon, S. M.; Bakr, O. M.; Sargent, E. H., Colloidal
Quantum Dot Solar Cells. Chem. Rev. 2015, 115 (23), 12732-12763.
(17) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.;
Sargent, E. H., Ultrasensitive solution-cast quantum dot photodetectors. Nature 2006, 442 (7099),
180-183.
(18) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X.,
Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 2014,
515 (7525), 96-99.
(19) Zhu, H.; Yang, Y.; Wu, K.; Lian, T., Charge Transfer Dynamics from Photoexcited
Semiconductor Quantum Dots. Annu. Rev. Phys. Chem. 2016, 67 (1), 259-281.
(20) Zhu, H.; Lian, T., Wavefunction engineering in quantum confined semiconductor
nanoheterostructures for efficient charge separation and solar energy conversion. Energy Environ.
Sci. 2012, 5 (11), 9406-9418.
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9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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31
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33
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(21) Najafov, H.; Lee, B.; Zhou, Q.; Feldman, L. C.; Podzorov, V., Observation of long-range
exciton diffusion in highly ordered organic semiconductors. Nat. Mater. 2010, 9 (11), 938-943.
(22) Irkhin, P.; Biaggio, I., Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion
Length in Rubrene Single Crystals. Phys. Rev. Lett. 2011, 107 (1), 017402.
(23) Peumans, P.; Yakimov, A.; Forrest, S. R., Small molecular weight organic thin-film
photodetectors and solar cells. J. Appl. Phys. 2003, 93 (7), 3693-3723.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 25
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7
8
9
(24) Vura-Weis, J.; Abdelwahed, S. H.; Shukla, R.; Rathore, R.; Ratner, M. A.; Wasielewski, M.
R., Crossover from Single-Step Tunneling to Multistep Hopping for Molecular Triplet Energy
Transfer. Science 2010, 328 (5985), 1547-1550.
(25) Lafolet, F.; Welter, S.; Popović, Z.; Cola, L. D., Iridium complexes containing p-phenylene
units. The influence of the conjugation on the excited state properties. J. Mater. Chem. 2005, 15
(27-28), 2820-2828.
10
11
12
13
14
15
16
17
18
19
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21
22
23
24
25
26
27
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35
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38
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43
44
45
46
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52
53
54
55
56
57
58
59
60
(26) Welter, S.; Salluce, N.; Belser, P.; Groeneveld, M.; De Cola, L., Photoinduced electronic
energy transfer in modular, conjugated, dinuclear Ru(II)/Os(II) complexes. Coord. Chem. Rev.
2005, 249 (13), 1360-1371.
(27) Welter, S.; Lafolet, F.; Cecchetto, E.; Vergeer, F.; De Cola, L., Energy Transfer by a
Hopping Mechanism in Dinuclear IrIII/RuII Complexes: A Molecular Wire? ChemPhysChem
2005, 6 (11), 2417-2427.
(28) Li, X.; Huang, Z.; Zavala, R.; Tang, M. L., Distance-Dependent Triplet Energy Transfer
between CdSe Nanocrystals and Surface Bound Anthracene. J. Phys. Chem. Lett. 2016, 7 (11),
1955-1959.
(29) Ho Choi, S.; Kim, B.; Frisbie, C. D., Electrical Resistance of Long Conjugated Molecular
Wires. Science 2008, 320 (5882), 1482-1486.
(30) Huang, Z.; Li, X.; Yip, B. D.; Rubalcava, J. M.; Bardeen, C. J.; Tang, M. L., Nanocrystal
Size and Quantum Yield in the Upconversion of Green to Violet Light with CdSe and Anthracene
Derivatives. Chem. Mater. 2015, 27 (21), 7503-7507.
(31) Mahboub, M.; Maghsoudiganjeh, H.; Pham, A. M.; Huang, Z.; Tang, M. L., Triplet Energy
Transfer from PbS(Se) Nanocrystals to Rubrene: the Relationship between the Upconversion
Quantum Yield and Size. Adv. Funct. Mater. 2016, 26 (33), 6091-6097.
(32) Huang, Z.; Li, X.; Mahboub, M.; Hanson, K. M.; Nichols, V. M.; Le, H.; Tang, M. L.;
Bardeen, C. J., Hybrid molecule–nanocrystal photon upconversion across the visible and near-
infrared. Nano Lett. 2015, 15 (8), 5552-5557.
(33) Morris, J. V.; Mahaney, M. A.; Huber, J. R., Fluorescence quantum yield determinations. 9,
10-Diphenylanthracene as a reference standard in different solvents. J. Phys. Chem. 1976, 80 (9),
969-974.
(34) Gray, V.; Dzebo, D.; Lundin, A.; Alborzpour, J.; Abrahamsson, M.; Albinsson, B.; Moth-
Poulsen, K., Photophysical characterization of the 9,10-disubstituted anthracene chromophore and
its applications in triplet–triplet annihilation photon upconversion. J. Mater. Chem. C 2015, 3 (42),
11111-11121.
(35) Murov, S. L.; Carmichael, I.; Hug, G. L., Handbook of Photochemistry, 2nd ed.; CRC Press:
Boca Raton, FL, 1993.
(36) Japar, S.; Ramsay, D. A., Triplet‐singlet absorption in pyridine. J. Chem. Phys. 1973, 58
(12), 5832-5833.
(37) Berlmann, I. B., Handbook of Fluorescence Spectra of Aromatic Compounds. Academic
Press: London, 1965.
(38) Lafolet, F.; Welter, S.; Popovic, Z.; De Cola, L., Iridium complexes containing p-phenylene
units. The influence of the conjugation on the excited state properties. J. Mater. Chem. 2005, 15
(27-28), 2820-2828.
(39) Nienhaus, L.; Wu, M.; Geva, N.; Shepherd, J. J.; Wilson, M. W. B.; Bulović, V.; Van
Voorhis, T.; Baldo, M. A.; Bawendi, M. G., Speed Limit for Triplet-Exciton Transfer in Solid-
State PbS Nanocrystal-Sensitized Photon Upconversion. ACS Nano 2017, 11 (8), 7848-7857.
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Page 23 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
(40) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R.,
Making a Molecular Wire:ꢀ Charge and Spin Transport through para-Phenylene Oligomers. J. Am.
Chem. Soc. 2004, 126 (17), 5577-5584.
(41) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M.
R., Conformationally Gated Switching between Superexchange and Hopping within Oligo-p-
phenylene-Based Molecular Wires. J. Am. Chem. Soc. 2005, 127 (33), 11842-11850.
(42) Gray, V.; Dreos, A.; Erhart, P.; Albinsson, B.; Moth-Poulsen, K.; Abrahamsson, M., Loss
channels in triplet–triplet annihilation photon upconversion: importance of annihilator singlet and
triplet surface shapes. Phys. Chem. Chem. Phys. 2017, 19 (17), 10931-10939.
(43) Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F., Low power, non-
coherent sensitized photon up-conversion: modelling and perspectives. Phys. Chem. Chem. Phys.
2012, 14 (13), 4322-4332.
(44) Dexter, D. L., A Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21 (5),
836-850.
(45) You, Z.-Q.; Hsu, C.-P., Theory and calculation for the electronic coupling in excitation
energy transfer. Int. J. Quantum Chem. 2014, 114 (2), 102-115.
(46) Huang, Z.; Simpson, D. E.; Mahboub, M.; Li, X.; Tang, M. L., Ligand enhanced
upconversion of near-infrared photons with nanocrystal light absorbers. Chem. Sci. 2016, 7 (7),
4101-4104.
(47) Okumura, K.; Mase, K.; Yanai, N.; Kimizuka, N., Employing Core-Shell Quantum Dots as
Triplet Sensitizers for Photon Upconversion. Chemistry – A European Journal 2016, 22 (23),
7721-7726.
(48) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P., A connection between
intramolecular long-range electron, hole, and triplet energy transfers. J. Am. Chem. Soc. 1989, 111
(10), 3751-3753.
(49) Closs, G. L.; Piotrowiak, P.; MacInnis, J. M.; Fleming, G. R., Determination of long-distance
intramolecular triplet energy-transfer rates. Quantitative comparison with electron transfer. J. Am.
Chem. Soc. 1988, 110 (8), 2652-2653.
7
8
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10
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12
13
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(50) Walther, M. E.; Grilj, J.; Hanss, D.; Vauthey, E.; Wenger, O. S., Photoinduced Processes in
Fluorene-Bridged Rhenium-Phenothiazine Dyads - Comparison of Electron Transfer Across
Fluorene, Phenylene, and Xylene Bridges. Eur. J. Inorg. Chem. 2010, (30), 4843-4850.
(51) Jin, T.; Lian, T., Trap state mediated triplet energy transfer from CdSe quantum dots to
molecular acceptors. J. Chem. Phys. 2020, 153 (7), 074703.
(52) Schmidt, T. W.; Castellano, F. N., Photochemical upconversion: the primacy of kinetics. J.
Phys. Chem. Lett. 2014, 5 (22), 4062-4072.
(53) Xu, Z.; Huang, Z.; Li, C.; Huang, T.; Evangelista, F. A.; Tang, M. L.; Lian, T., Tuning the
Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-
Visible Photon Upconversion Systems. ACS Appl. Mater. Interfaces 2020, 12 (32), 36558-36567.
(54) Xu, Z.; Jin, T.; Huang, Y.; Mulla, K.; Evangelista, F. A.; Egap, E.; Lian, T., Direct triplet
sensitization of oligothiophene by quantum dots. Chem. Sci. 2019, 10 (24), 6120-6124.
(55) Davis, W. B.; Ratner, M. A.; Wasielewski, M. R., Conformational gating of long distance
electron transfer through wire-like bridges in donor-bridge-acceptor molecules. J. Am. Chem. Soc.
2001, 123 (32), 7877-7886.
(56) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R., Molecular-wire behaviour
in p-phenylenevinylene oligomers. Nature 1998, 396 (6706), 60-63.
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Page 24 of 25
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6
(57) Davis, W. B.; Wasielewski, M. R.; Ratner, M. A.; Mujica, V.; Nitzan, A., Electron transfer
rates in bridged molecular systems: A phenomenological approach to relaxation. J. Phys. Chem. A
1997, 101 (35), 6158-6164.
(58) Scott, A. M.; Miura, T.; Ricks, A. B.; Dance, Z. E. X.; Giacobbe, E. M.; Colvin, M. T.;
Wasielewski, M. R., Spin-Selective Charge Transport Pathways through p-Oligophenylene-
Linked Donor-Bridge-Acceptor Molecules. J. Am. Chem. Soc. 2009, 131 (48), 17655-17666.
(59) Colvin, M. T.; Ricks, A. B.; Wasielewski, M. R., Role of Bridge Energetics on the Preference
for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor
Molecules. J. Phys. Chem. A 2012, 116 (9), 2184-2191.
(60) Davis, W. B.; Ratner, M. A.; Wasielewski, M. R., Dependence of electron transfer dynamics
in wire-like bridge molecules on donor-bridge energetics and electronic interactions. Chem. Phys.
2002, 281 (2-3), 333-346.
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