Triplet energy migration pathways from PbS quantum dots to surface-anchored polyacenes controlled by charge transfer

Guohui Zhao ab, Zongwei Chen a, Kao Xiong c, Guijie Liang *d, Jianbing Zhang *c and Kaifeng Wu *a
aState Key Laboratory of Molecular Reaction Dynamics and Dynamics Research Center for Energy and Environmental Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China. E-mail: kwu@dicp.ac.cn
bUniversity of the Chinese Academy of Sciences, Beijing 100049, China
cSchool of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China. E-mail: jbzhang@hust.edu.cn
dHubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices, Hubei University of Arts and Science, Xiangyang, Hubei 441053, China. E-mail: guijie-liang@hotmail.com

Received 3rd November 2020 , Accepted 9th December 2020

First published on 10th December 2020


Abstract

Sensitization of molecular triplets using PbS quantum dots (QDs), followed by efficient triplet fusion, has been developed as a novel route to near-infrared-to-visible photon upconversion. Fundamentally, however, the mechanisms of triplet energy transfer (TET) from PbS QDs to surface-anchored polyacence acceptors remain highly debated. Here we study and side-by-side compare the kinetic pathways of TET from photoexcited PbS QDs to surface-anchored tetracene and pentacene derivatives using broad-band transient absorption spectroscopy spanning multiple decades of timescales. We find that the TET pathways are dictated by charge-transfer energetics at the QD/molecule interface. Charge transfer from QDs to tetracene was strongly endothermic, and hence spectroscopy showed one-step transformation from QD excited states to tetracene triplets in 302 ns. In contrast, hole transfer from QDs to pentacene was thermodynamically favoured and was confirmed by the formation of pentacene cation radicals in 13 ps, which subsequently evolved into pentacene triplets through a 101 ns electron transfer process. These results not only are consistent with a recently-established framework of charge-transfer-mediated TET, but also provide a route to manipulate triplet sensitization using lead-salt QDs for efficient upconversion of near-infrared photons.


Introduction

Colloidal semiconductor nanocrystals or quantum dots (QDs) have recently been developed as efficient and versatile sensitizers for molecular triplets that are important for many applications.1–10 One of these applications takes advantage of long-lived molecular triplets for low-threshold triplet-fusion photon upconversion,2,3,11–23 a process useful in fields ranging from solar energy conversion and photoredox catalysis to bio-imaging.24–27 Compared to traditional organic and organometallic sensitizers, QD sensitizers have negligible intersystem crossing energy loss, in principle allowing for large anti-Stokes shifts in photon upconversion.6,8

From a practical standpoint, upconversion of infrared (IR) and near-infrared (NIR) photons is of paramount importance. Mainstream solar cells cannot use photons beyond ca. 1100 nm. Bio-imaging and organic photoredox catalysis also would benefit from NIR photons with large penetration depths.27Among various types of QD sensitizers developed to date, lead-salt (PbS, PbSe) QDs are the only family capable of sensitizing NIR-to-visible photon upconversion.2,3,17,28–30 Importantly, they are also the only family of QDs that have been demonstrated to harvest molecular triplets derived from singlet fission (the opposite process of triplet fusion), which can potentially double the efficiency of solar energy conversion and/or light emission.31–33

A fundamental process underlying the above-mentioned triplet sensitization and harvesting applications is triplet energy transfer (TET) between inorganic QDs and organic molecules. So far, mechanistic studies on TET from PbS QDs to surface-anchored tetracene or pentacene derivatives have resulted in contradictory models. There is evidence suggesting hole-transfer-mediated TET,34 but the same phenomenon of delayed triplet formation has been assigned to surface-state-mediated TET in other studies.35,36 In addition, it was also reported that hole transfer from QDs competed with rather than mediated TET.29

Significant progress has recently been made towards understanding the mechanisms of TET from QDs to molecules, mainly using QDs with spectral signatures in the visible region.1,9,22,37–41 For example, it has been shown that TET from lead halide perovskite QDs to molecules proceeds either directly or in a step-wise fashion mediated by charge-separated states,38,39 depending upon the energetics for charge transfer. Definitive evidence for TET from trap states has also been provided for self-trapped excitons in CuInS2 QDs14 as well as broadly-distributed surface-trapped excitons in CdSe QDs.42 Building upon these latest findings, it is now possible to re-examine and elucidate the mechanisms of TET from NIR lead-salt QDs to molecular acceptors.

Here we study and side-by-side compare the kinetic pathways of TET from photo-energized PbS QDs to surface-anchored tetracene and pentacene derivatives using broad-band transient absorption (TA) spectroscopy across multiple decades of timescales. By carefully examining the TA signatures of molecular cation radicals and triplets, we concluded that pentacene triplet sensitization by PbS QDs was clearly mediated by a hole-transfer state, whereas tetracene triplets were sensitized in one step. This conclusion is consistent with the recently-established framework of charge-transfer-mediated TET from photoexcited QDs to molecular acceptors and will guide the system design for efficient NIR-to-visible photon upconversion.

Results and discussion

System design

We synthesized PbS QDs with their first exciton absorption peak at ∼800 nm using a recently-developed cation-exchange method;43 see the ESI for details. QDs prepared with this method are capped with chloride and oleic acid ligands that effectively passivated their surface states. A representative transmission electron microscopy (TEM) image of the QDs is shown in Fig. 1a, indicating an average diameter of 3.3 ± 0.4 nm (Fig. S1). As for the molecular acceptors, we studied both tetracene and pentacene derivatives that are common triplet acceptors for lead-salt QDs. The molecular structures of 5-tetracene carboxylic acid (Tc) and 6,13-bis(triisopropylsilylethynyl)pentacene-2-carboxylic acid (Pc) are shown in Fig. 1b and c, respectively. Both molecules are functionalized with the carboxyl group which helps to anchor molecules onto the cation sites on QD surfaces. The Pc molecule is also functionalized with two triisopropylsilylethynyl (TIPS) groups to enhance its ambient stability. Synthetic methods for these molecules can be found in the literature.44,45 They were attached onto QD surfaces using a simple ligand functionalization method (see the ESI). In addition to functionalizing molecules with anchoring groups, using short, inorganic ligand-capped46 or even ligand-free QDs47 might be another way to enhance the QD–molecule interactions, but the surface chemistry and excited state dynamics of those QDs are not sufficiently well understood yet for our current model study.
image file: d0nr07837a-f1.tif
Fig. 1 (a) Representative transmission electron microscopy (TEM) image of the PbS QDs. (b and c) Molecular structures of (b) 5-tetracene carboxylic acid (Tc) and (c) 6,13-bis(triisopropylsilylethynyl)pentacene-2-carboxylic acid (Pc). (d) Schematic energy level alignment between QDs and Tc and Pc determined from the cyclic voltammogram. Ee and Eh are the lowest electron and hole energy levels in the conduction and valence bands, respectively. Eox is the oxidation potential energy for ground-state molecules, whereas Ered and Ered,T are the reduction potential energies for the Tc and Pc cations to form singlet and triplet excited states, respectively. ET is the triplet energy.

The redox potentials of the QDs and Tc and Pc molecules were evaluated by cyclic voltammetry (CV); see the ESI for details. The results are summarized in Fig. 1d. The measured lowest energy electron and hole levels (Ee and Eh, respectively) yield a bandgap of ∼1.7 eV that is higher than the optical gap (∼1.55 eV), reflecting on the electron–hole binding in the QDs. The oxidation potential energies of ground-state Tc and Pc molecules (Eox) were directly obtained from CV, whereas the reduction potential energies for the Tc and Pc cations to form singlet or triplet excited states (Ered and Ered,T, respectively) were estimated by adding the respective singlet or triplet transition energies to Eox.39,48,49 The obtained singlet Ered should be lower than the reduction potential energy of ground-state molecules by the electron–hole binding energy that is on the order of 0.5 to 1 eV for small molecules.50–52 Therefore, electron transfers from photoexcited PbS QDs to ground-state Tc and Pc are both strongly energetically disfavoured (Fig. 1d). In contrast, the energetics of hole transfers from PbS QDs to them are different; while hole transfer is strongly disallowed for Tc, it can be operative for Pc. We note that, due to the uncertainties associated with CV measurements (up to 100 s of meV) and the Coulomb energy changes accompanying charge transfer (CT) that are not explicitly included in the “single-particle” representation, Fig. 1d serves only as a semi-quantitative guide for interfacial CT. Nonetheless, the strong difference between the hole transfer energetics of Tc and Pc is well consistent with the fact that Pc is much more susceptible to oxidation than Tc.

Fig. 2a presents the UV-vis absorption spectra of free PbS QDs and QD–Tc and QD–Pc complexes. As mentioned above, the size of the QDs was tuned such that their first exciton absorption peak was situated at ∼800 nm. We chose to study this small-size sample for two reasons. First, recent studies have established that the strong quantum confinement afforded by the small size can facilitate TET from QDs to surface acceptors by providing a strong electronic coupling and/or large driving force for TET.9,10 Second, this sample has negligible absorption at longer than 900 nm where the absorption features of relevant molecular species (cation radicals, triplets, etc.) may be present,49,53 which should greatly facilitate our analysis of TET mechanisms. In QD–molecule complexes, the absorption features of Tc and Pc can be observed and the absorption peak of QDs is well maintained. In the previous studies on PbS QD–Tc/Pc complexes, the absorption peaks of QDs were often strongly blue-shifted as a result of QD etching caused by long-duration ligand exchange procedures (see Fig. S2), which could introduce additional trap states complicating the TET mechanisms. This issue is avoided in our current study. On the basis of the absorption spectra and the extinction coefficients of QDs and molecules, we estimated that there were, on average, 18 and 20 Tc and Pc acceptors, respectively, on each QD.


image file: d0nr07837a-f2.tif
Fig. 2 (a) Absorption spectra of PbS QDs (green), QD–Tc (blue) and QD–Pc (orange) complexes dispersed in hexane. (d) PL spectra of PbS QDs (green), QD–Tc (blue) and QD–Pc (orange) complexes dispersed in hexane, collected under 745 nm excitation.

The photoluminescence (PL) spectrum of PbS QDs excited at 745 nm is presented in Fig. 2b, showing a symmetric band peaked at ∼940 nm. The PL quantum yield was ca. 50%, measured using a calibrated integrating sphere.54 In the presence of Tc and Pc, the PL of PbS QDs was strongly quenched. The net quenching mechanism should be TET from QDs to acene molecules, as the triplet energies of both Tc (ca. 1.3 eV)55 and Pc (ca. 0.9 eV)49 are lower than that of the QD exciton (ca. 1.55 eV), but the detailed TET pathways should be distinct for Tc and Pc due to their different CT energetics.

TET in PbS QD–Tc

Before we examine the mechanisms of TET from PbS QDs to Tc and Pc, we first briefly introduce the excited state dynamics in free PbS QDs which was investigated using femto- and nano-second transient absorption (TA) spectroscopy; see the ESI for details. Briefly, QDs were excited at their band edge absorption (excluding direct excitation of molecules) and the average number of photons per QD was controlled to be ≪1 in order to exclude multiexcitonic effects; pump-induced absorption changes were monitored by broad-band probe pulses from visible to NIR. The TA spectra of free QDs are presented in Fig. S3, which contain a photoinduced exciton bleach (XB) feature at ∼800 nm and broad-band photoinduced absorption (PIA) signals in the visible and NIR regions. These signals show negligible decay on the ns timescale; their average lifetime is ∼4.1 μs (52% 2.2 μs and 48% 6.2 μs; Fig. S3).

QD–molecule complexes were studied under exactly the same conductions as those used for free QDs. In the presence of surface-anchored Tc, the XB and PIA features of PbS QDs still show negligible decay within 1 ns (Fig. 3a). In addition, there is no formation of new absorption features in the range of 900–1000 nm associated with Tc cation radicals.39,49 Thus, neither TET nor hole transfer from QDs to Tc occurred within 1 ns. At later delays (ca. 10 ns to 1 μs), we observe the decay of the QD TA features accompanied by the formation of structured absorptive features in the range of 400–500 nm that can be assigned to the triplet excited state of Tc (3Tc*).29,39 The complementary nature of the kinetics of QD and 3Tc* features (Fig. 3c) is further clearly illustrated by the kinetics probed at 796 nm (XB of QDs) and 485 nm (peak of 3Tc* absorption). This behaviour is consistent with direct TET from photoexcited QDs to Tc. Fitting the kinetics in Fig. 3c reveals an average TET time constant of 302 ns (26% 28 ns and 74% 397 ns), from which we can further calculate a TET efficiency of 93%. This efficiency is consistent with the PL quenching efficiency observed in Fig. 2b (90%). At delays later than 10 μs, TET has finished and the TA spectra are dominated by the absorption of 3Tc* (Fig. 3d). The sensitized 3Tc* species are long-lived, with a lifetime of ∼78 μs.


image file: d0nr07837a-f3.tif
Fig. 3 (a and b) TA spectra of the QD–Tc complexes at indicated time delays following excitation at the QD band edge. The QD exciton bleach (XB), photoinduced absorption (PIA) and Tc triplet (3Tc*) features are indicated. (c) TA kinetics of QD–Tc complexes probed at the XB (∼796 nm; yellow squares) and at the 3Tc* (∼485 nm; blue triangles). The black solid line is a multi-exponential fit to the kinetics. τTET and τ3Tc* are the lifetimes for triplet energy transfer and 3Tc* decay, respectively. (d) Decay of the sensitized 3Tc* spectra.

Thus, we have observed direct TET from photoexcited PbS QDs to surface-anchored Tc, which is consistent with our expectations based on the energy levels in Fig. 1d. In principle, endothermic hole transfer from PbS QDs to Tc could occur, but this process is unlikely to be able to compete with direct TET due to its strong endothermicity (>0.4 eV) as compared to the exothermic direct TET (driving force ∼0.25 eV).

TET in PbS QD–Pc

It is then interesting to investigate the TET mechanism in the PbS QD–Pc system where hole transfer seems to be thermodynamically favoured. Fig. 4a–c present the TA spectra of PbS QD–Pc complexes at indicted delays from 2 ps to 150 μs following selective excitation of QDs. In the 2 to 50 ps range (Fig. 4a), we observe instantaneous formation of the XB and PIA features of QDs as well as the bleach-like features of Pc superimposed on the PIA of QDs (ca. 600 and 650 nm). Since QDs were selectively excited in this system, these instantaneous Pc features should be ascribed to Stark effect signals, i.e., shift and/or broadening of the absorption of Pc by the electric field of the QD exciton.34,35 In addition to these instantaneous features, there are subtle spectral changes in both the visible and NIR regions. In the visible region, an absorptive feature at ∼410 nm rapidly grows within 50 ps. This feature coincides with the spectral feature of Pc cation radicals (Pc+) identified in a prior spectroelectrochemistry study.34 In the NIR region, we observe the growth of a broad-band absorptive feature within 850–1000 nm which is also assignable to Pc+.53 These spectral features strongly evidence hole transfer from photoexcited PbS QDs to Pc.
image file: d0nr07837a-f4.tif
Fig. 4 (a–c) TA spectra of QD–Pc complexes at indicated time delays following excitation at the QD band edge. The QD exciton bleach (XB), Pc ground state bleach (GSB), Pc cation radical (Pc+) and Pc triplet (3Pc*) features are indicated.

In the time window of 100 ps to 1 μs (Fig. 4b), we observe strong decay of the Pc+ feature at 410 nm and the QD XB feature at 800 nm along with the complementary growth of the absorptive feature (ca. 487 and 518 nm) of the triplet excited state of Pc (3Pc*). In addition, in the range of 850–1000 nm, we also observe spectral evolution from a broad absorption band of Pc+ to two strong absorption peaks (ca. 874 and 987 nm) that are characteristic absorptions of 3Pc*.53 Taken together, these observations are consistent with a model of hole-transfer mediated TET from PbS QDs to Pc where hole transfer is followed by electron transfer from QDs to Pc cations to populate Pc triplets. At delays later than 10 μs (Fig. 4d), electron transfer and thus the net TET have finished, resulting in pure TA spectra of 3Pc*.

By fitting the growth of the TA kinetics probed at 418 and 900 nm (Pc+), we obtain a hole transfer time constant of 13 ps (Fig. 5a). In principle, the electron transfer kinetics could be obtained by following the decay of the Pc+ signals in Fig. 5a. However, due to their weak amplitudes compared to the 3Pc* signals and a strong overlap between them, it is technically challenging to accurately extract the Pc+ decay kinetics. Rather, the electron transfer and triple decay time constants are simultaneously obtained by fitting growth and decay, respectively, of the kinetics probed at 518 nm (3Pc*; Fig. 5b). By doing so, we determine an average electron transfer time of 101 ns (46% 3.8 ns, 36% 52 ns and 18% 442 ns) and a 3Pc* lifetime of 21 μs. The 3Pc* kinetics at 987 nm can also be extracted by subtracting the broad-band NIR background arising from QD PIA and/or Pc+ (assuming that the kinetics of the background signal is relatively insensitive to the wavelength). The extracted 3Pc* kinetics is fully consistent with the one probed at 518 nm (Fig. 5b).


image file: d0nr07837a-f5.tif
Fig. 5 (a) TA kinetics of QD–Pc complexes probed for Pc+ at 418 nm (blue circles; top) and 900 nm (green circles; bottom). (b) TA kinetics of QD–Pc complexes probed for 3Pc* at 518 nm (blue open circles; divided by a factor of 27) and 984 nm (yellow open circles). The black solid lines are multi-exponential fits to the kinetics. THT, τET and τ3Pc* are the lifetimes for hole transfer, electron transfer and 3Pc* decay, respectively.

In principle, the hole transfer rate can be tuned by the solvent polarity, because the energy of the charge-separated state depends sensitively on the solvent polarity. Prior studies have demonstrated this principle for covalently-linked molecular donor–acceptor systems.56 The issue with our current QD–molecule systems, however, is that we do not have as many choices for the solvents. The current solvent is hexane, in which the Tc and Pc molecules have low solubility, ensuring that the molecules prefer binding to QD surfaces than dissolving into the solution; note that there exists an adsorption–desorption equilibrium between molecules bound to QD surfaces and those dissolved in the solvent. If we choose a solvent of higher polarity, the molecules will desorb from QD surfaces, precluding a well-defined study on QD–molecule complexes.

For a hole-transfer-mediated TET mechanism, the QD quenching efficiency should mainly be determined by the first hole transfer step. Because the ps hole transfer time is much shorter than the μs lifetime of free QDs, a unity quenching efficiency would be expected, which contradicts the PL quenching efficiency of ca. 84% in Fig. 2b. One possibility is that there exists a subset of free QDs in the QD–Pc sample. This, however, is very unlikely considering a high Pc/QD ratio of 20[thin space (1/6-em)]:[thin space (1/6-em)]1. This puzzle is likely related to the strange kinetics of the Pc ground state bleach (GSB) feature that has not been discussed so far. Specifically, after hole transfer, the GSB feature does not reach it maximum amplitude; rather, it continues to grow until completion of the electron transfer process (Fig. 4b). This appears to contradict the fundamental principle of photophysics of molecules; in principle, upon accepting a hole and being oxidized, the ground state absorption of a molecule should be completely bleached. In order to resolve this contradiction, we recall that the hole transfer driving force is relatively weak (<0.1 eV) for the PbS QD–Pc system. In this case, the hole can reversely transfer from Pc to the valence band of QDs, eventually reaching a quasi-equilibrium with the forward hole transfer process. Because of this quasi-equilibrium, the effective occupation number of the hole for a Pc molecule is less than 1. Only after the electron transfer process populates the Pc triplet, the ground state absorption can be completely bleached. For the same reason, electron–hole recombination in PbS QDs competes with the electron transfer process and leads to non-unity PL quenching efficiency. Under the assumption that the rates of forward and backward hole transfer rates are much faster than those of electron–hole recombination and electron transfer, the PL quenching efficiency is determined by the electron transfer rate. The quenching efficiency calculated in this way is 97%. This is still higher than the PL quenching efficiency observed in Fig. 2b (84%). One possible reason is that the kinetic assumption above is oversimplified. Another possibility is that the samples used for PL measurements were diluted compared to those for TA measurements; this dilution could induce ligand fall-off from QD surfaces, and hence slow down the charge/energy transfer rate that scales with the molecule/QD ratio.

CT-controlled TET from PbS QDs

The comparison between PbS QD–Tc and –Pc systems clearly illustrates that the TET pathways across the interfaces of PbS QDs and molecular acceptors are dictated by CT energetics, a conclusion similar to that previously drawn for CsPbBr3 QD–molecule systems.39 As illustrated in Scheme 1, CT from photoexcited PbS QDs to Tc acceptors is too endothermic to be involved, leaving direct TET as the most viable pathway. In contrast, CT states (specifically, QD–Pc+) in the QD–Pc system are energetically accessible, which can efficiently mediate net TET. It has been established that, when both energetically allowed, CT in QD–molecule complexes often outcompetes direct TET because the former tends to have a stronger electronic coupling than the latter.38,39,41
image file: d0nr07837a-s1.tif
Scheme 1 Triplet energy migration pathways from PbS QDs to surface-anchored polyacenes controlled by charge transfer. (a) One-step TET. (b) Stepwise TET mediated by CT states. TET: triplet energy transfer; (r)HT: (reverse-)hole transfer; ET: electron transfer; QD*: QD excited state; QD–Tc(Pc)+: charge separated state; 3Tc(Pc)*: molecular triplet state.

In the direct TET process, the TET efficiency is determined by the TET rate. In the CT-mediated TET process, depending upon the energy of the CT states, the TET efficiency can be dictated by either the first or the second CT step. In our previous study of CsPbBr3 QD–Tc complexes, the energy of the hole-transfer CT state is lower than that of the QD exciton state by more than 0.3 eV, which suppressed back transfer of the hole, and the TET efficiency was controlled by the competition between the hole transfer and QD exciton recombination.39 In our current system of PbS QD–Pc, the hole-transfer CT state has a similar energy to the QD exciton and both are under rapid inter-conversion. In this case, the TET efficiency is controlled by the competition between the QD exciton recombination and the second electron transfer step.

The current results can potentially reconcile all prior studies on PbS QD–acene systems. In ref. 29, Lian et al. proposed hole transfer from PbS QDs to a tetracene derivative that can be suppressed by CdS shells, but no evidence for tetracene cation radicals was provided and it was unclear why the shell suppressed only hole transfer but not TET. Therefore, the hole decay process could be alternatively assigned to hole trapping (introduced by ligand exchange) rather than hole transfer. Indeed, with the exception of this initial hole decay, the behavior of direct TET from PbS QDs to tetracene is consistent with our results here. In ref. 34, Castellano et al. proposed the mechanism of hole-transfer-mediated TET from PbS QDs to a pentacene derivative, which explained most of the data except that the pentacene cation radicals were not unambiguously identified presumably due to a spectral overlap issue. Therefore, in a closely-related study in ref. 35, Roberts et al. proposed that the intermediate states were surface states rather than hole-transfer CT states; power-saturation experiments and constrained density functional theory (DFT) calculations were used to support this model. In our current study, we provide clear evidence for pentacene cation radicals in the spectral range of both the visible and NIR regions. Admittedly, surface-state-mediated TET could not be completely ruled out due to the QD etching issue introduced by ligand exchange commonly encountered in prior studies.34,57 Still, CT-mediated TET should play an important role in PbS QD–pentacene systems because of the thermodynamically favored hole transfer pathway.

Conclusions

We studied the kinetic pathways of triplet exciton transfer from photoexcited PbS QDs to surface-anchored tetracene and pentacene derivatives. We found that the triplet exciton directly migrated from PbS QDs to the tetracene derivative in 302 ns, without involving any detectable charge transfer species, because charge transfer from QDs to tetracene was strongly endothermic. In contrast, the triplet exciton migrated in a stepwise fashion from PbS QDs to the pentacene derivative, with the intermediate states as the charge transfer states resulting from thermodynamically favoured hole transfer from QDs to pentacene. This mechanism was unambiguously established by the observation of pentacene cation radical signals in both the visible and NIR regions that formed in 13 ps and subsequently decayed into pentacene triplet signals within 101 ns. Compared to the prior reports on charge-transfer-mediated triplet energy transfer, the PbS QD–pentacene system also showed an interesting phenomenon of quasi-equilibrium between forward and backward hole transfer processes enabled by a relatively weak driving force. Accordingly, the net triplet energy transfer efficiency was dictated by the second electron transfer step rather than the first hole transfer step. The results presented here further elucidate the mechanisms of triplet exciton migration from photoexcited QDs to molecular species. An understanding of these mechanisms provides a route to manipulate molecular triplet sensitization using semiconductor QDs for many important applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (21975253, 61974052), the Dalian Institute of Chemical Physics (DICP I201914) and the LiaoNing Revitalization Talents Program (XLYC1807154). The 6,13-bis(triisopropylsilylethynyl)pentacene-2-carboxylic acid molecule used in this study was a general gift from F.N. Castellano at North Carolina State University.

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Footnotes

Electronic supplementary information (ESI) available: Experimental methods; Fig. S1–3. See DOI: 10.1039/d0nr07837a
These authors contributed equally to this work.

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