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Nanocrystal size and quantum yield in the upconversion of green to violet light with CdSe and anthracene derivatives Zhiyuan Huang, Xin Li, Benjamin D. Yip, Justin M. Rubalcava, Christopher J. Bardeen, and Ming L. Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03731 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Chemistry of Materials

Nanocrystal size and quantum yield in the upconversion of green to violet light with CdSe and anthracene derivatives Zhiyuan Huang, Xin Li, Benjamin D. Yip, Justin M. Rubalcava, Christopher J. Bardeen, and Ming L. Tang* Abstract: We investigate the parameters affecting the upconversion of visible light in a hybrid organic- inorganic system based on CdSe nanocrystal (NC) sensitizers, 9-anthracene carboxylic acid (9-ACA) transmitter and diphenylanthracene (DPA) annihilator. Seven wurtzite CdSe nanocrystals ranging in photoluminescence quantum yield (PLQY) from 2.0 % to 11 %, and in diameter from 2.7 nm to 5.1 nm were used in combination with the 9-ACA and DPA molecules. Using 2.7 nm diameter CdSe NCs, we show that the upconversion quantum yield has a quadratic to linear dependence on NC sensitizer concentration. Additionally, upconversion efficiency correlates positively with 9-ACA surface coverage on CdSe NCs and NC PLQY but negatively with particle size. This work clearly illustrates the path towards designing nanocrystal-molecule combinations with high upconversion efficiency that have potential applications in the fields of bioimaging, solar energy conversion and data storage.

Introduction Photon upconversion refers to the absorption of low energy photons, and their subsequent emission as higher energy photons.1 Compared to two-photon fluorescence or second harmonic generation, which depends strongly on peak intensity2,3 and pulsed lasers for high efficiency,4 photon upconversion can be accomplished using cw lasers or even incoherent light, like the sun. Photon upconversion is of particular interest for solar energy conversion, because it allows photovoltaics to harvest the unused sub-bandgap photons, and can contribute to the overall power conversion efficiency.5-9 In terms of multiphoton imaging, exciting biological samples in the near infra-red (NIR) window with upconversion-based probes can minimize tissue autofluorescence and photodamage, as well as probe photobleaching.10-12 Until now, there were mainly two processes to achieve upconversion: lanthanide ion-based upconversion,13 and molecular triplet-triplet annihilation (TTA)-based upconversion.14,15 Lanthanide ion-based upconversion shows large anti-Stokes shifts and sharp emission lines, but low absorption cross sections,16 limited tunability and therefore restricted spectral coverage. In TTA upconversion, while various molecules with high absorption cross-sections can have spectral ranges extending from the ultraviolet (UV) to the NIR,17 it remains challenging to upconvert photons at wavelengths longer than 800 nm. In addition to lower photostability, organic dyes that absorb at these long wavelengths tend to undergo rapid internal conversion to the ground state, making them less effective as sensitizers.

Figure 1. (a) In this hybrid system, the CdSe nanocrystals sensitizers are functionalized with a 9-ACA transmitter ligand. DPA serves as the annihilator. The green arrow indicates photo-excitation of CdSe NCs of different sizes, followed by energy transfer from CdSe to 9-ACA, then triplet-triplet energy transfer to DPA. Two DPA triplets annihilate to one singlet state and emit an upconverted photon (blue arrow). (b) Absorption (solid line) and emission (dashed line) spectra of DPA and CdSe NCs with 7 sizes measured in hexane at RT. Blue and green arrows indicate excitation wavelengths.

Recently, we discovered hybrid molecule-nanocrystal photon upconversion.18 We found that semiconductor nanocrystals can effectively sensitize molecular triplet states which can undergo TTA and upconvert photons. Semiconductor nanocrystal sensitizers

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have the following advantages: 1) large absorption cross sections19 compared to molecules, and especially when compared to lanthanide ions; 2) the ability to absorb light efficiently in the NIR;20-22 3) optical properties which are easily tuned based on the size and material properties of the nanoparticle.23 Note that none of these properties are available simultaneously in the rare earth and molecular TTA systems. In fact, the tunable optical properties of nanoparticles, when combined with the vast selection of potential molecular annihilators, allow photon upconversion with this hybrid system to be engineered at arbitrary wavelengths. However, much work needs to be done to investigate the fundamental processes underlying photon upconversion in this hybrid system. Here, we examine the nanocrystal-based parameters affecting the upconversion of green to violet light using wurtzite CdSe NCs to sensitize the triplet states of anthracene derivatives, as depicted in Figure 1a. We have hypothesized18 that the relatively small Böhr radius of CdSe (4-5 nm) requires a transmitter ligand, such as 9-anthracenecarboxylic acid (9-ACA), directly anchored on the surface of these NCs to mediate the energy transfer to the annihilator, diphenylanthracene (DPA) in this case. We investigate the dependence of solvent, transmitter ligand density, NC photoluminescence quantum yield (PLQY), size and concentration on upconversion. This work sheds light on the mechanism of triplet energy transfer (TET) from CdSe NCs to organic molecules, and provides a guide to understanding and optimizing upconversion. Experimental section The 9-ACA transmitter ligands were attached to the CdSe NC sensitizers via ligand exchange. Ligand exchange was performed by stirring CdSe NCs and 9-ACA mixture for 12h in a combination of good solvents. For this ligand exchange, we found it was impossible to use a single solvent because 9-ACA is very polar, in contrast to the extremely hydrophobic CdSe NCs that have long, aliphatic hydrocarbon chains. We found a toluene to acetonitrile volume ratio of 11 to 3 to be optimal. Using this ratio, the NCs remain in solution and the concentration of 9-ACA is maximized. After ligand exchange, the NCs remain soluble in toluene or hexane, implying most of the native octadecylphosphonic acid (ODPA) ligands remain on the particle surface. This is in line with observations by the Hens group that carboxylic acid ligands cannot displace phosphonic acid ligands effectively.29 The optimum time frame for ligand exchange was found to be 12 hrs, as samples stirred for shorter or longer times under identical conditions did not show any higher upconversion. Upconversion samples were prepared by redispersing the CdSe pellet into hexane with designated amount of DPA. Upconversion fluorescence spectra were recorded at right angles to focused 488 nm and 532 nm excitation beams. Details about the preparation of upconversion sample: for all particle diameters, 20 nmol of CdSe NCs was mixed with 3 mL methanol and spun down on a centrifuge at 7000 rpm for 5 min. The supernatant was removed and the CdSe NC pellet was redispersed in 1.1 mL toluene and 6 ml of 9-ACA in a mixture of acetonitrile: toluene = 3:11(v/v) solution (for a final concentration of 0.0216 M). The mixture was stirred for 12h. Acetone (twice the volume of the ligand exchange solution) was added to precipitate the particles. The resulting solution was centrifuged at 7830 rpm for 15 min, and the pellet was redispersed in hexane containing the designated amount of DPA. Note that the solubility of 9-ACA reaches a limit of 0.0216M in 3:11(v/v) acetonitrile: toluene. Upconversion samples were prepared in a nitrogen glovebox with air-free Starna cuvettes. The upconversion quantum yield was calculated using R6G in ethanol as the reference.18 Results and discussion

Figure 2. Log-log plot of the upconversion signal versus absorption at excitation wavelengths, showing the transition from quadratic (slope=2) to linear (slope=1) regimes. Measurements were conducted with 2.7 nm CdSe in dry and degassed hexanes at room temperature, with excitation wavelengths of 19.8 W/cm2 488 nm (black solid square) and 12.7 W/cm2 532 nm (red hollow square). The concentration of DPA is 2.15 mM.


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In molecular upconversion systems, Monguzzi et al. has shown that there is a quadratic dependence on sensitizer absorption and excitation density when TTA is limited (equation 1), and a linear dependence when TTA is saturated (equation2),24

S A = 0.2 S A = 0.2

γ TT ktr / k AT

[ ]2 [α ( E ) I exc ]2 k AS kDT + ktr

[Eq.(1)]

k 1 [ T tr ]α ( E ) I exc S k A kD + ktr

[Eq.(2)]

S where SA is the concentration of annihilator molecules in the singlet excited state; γ TT the second-order rate constant for TTA; k A T the singlet decay rate constant of the annihilator; ktr is the rate constant of TET; k A the triplet decay rate constant of the annihilator; k DT the triplet decay rate constant of the sensitizer; α ( E ) the absorption coefficient of the sensitizer; and Iexc the laser intensity. In order to investigate the effect of sensitizer concentration/ absorption, we varied the amount of 2.7 nm CdSe/9-ACA with a fixed concentration of 2.15 mM DPA in hexane. Figure 2 plots the upconverted DPA emission versus nanoparticle absorption when excited by 488 and 532 nm cw lasers at constant intensity. It indicates that the upconversion photoluminescence is dependent on the light absorbed, which is proportional to NC concentration, as predicted. A higher density of 9-ACA transmitter ligands on CdSe NCs leads to higher upconversion QY. Figure 3a shows that upconversion is proportional to the concentration of 9-ACA in the ligand exchange solution. Here, five samples were prepared with 9ACA concentrations ranging from 1.6 to 15.4 mM in the ligand exchange solution. The more 9-ACA in solution, the more 9-ACA molecules bound to the CdSe NCs, and the more efficient the TET. Figure 3b shows that upconversion is maximized in solvents that minimize solvation of the 9-ACA transmitter ligand. The upconverted PL with 2.7 nm CdSe/9-ACA and 1 mM DPA in 8 different solvents is shown. Since the CdSe NC/ 9-ACA samples were all prepared in the same flask, the only variable was the solvent used for upconversion. Figure 3c shows the upconversion signal in figure 3b plotted against the solubility of 9-ACA in these 8 solvents. Good solvents for 9-ACA like THF (solubility limit: 813 mM) will dissociate the transmitter from the surface of CdSe NC. Hexane or toluene, with saturation concentrations of 0.016 and 1.909 mM respectively, are bad solvents for 9-ACA, so the 9-ACA transmitter molecules remain bound to the surface of CdSe NC, and upconversion is enhanced. The relatively low viscosity of hexane25 (see Table S2) may facilitate the diffusion of DPA and increase TTA. However, this solubility trend is not observed in halogenated solvents. It has b`een shown that aromatic fluorophores like anthracene can be quenched by CCl4 due to the formation of a ground-state complex.26-28 It is possible that in the halogenated solvents, there are two competing effects, solubility and singlet/triplet state quenching that preclude the observation of a clear trend. Finally, upconversion QYs decrease after excessive NC cleaning. If the CdSe-9ACA complex was resuspended in hexane and re-precipitated out with acetone, the upconversion QY does not increase but instead drops from 7.5% to 0.65% for 2.7 nm CdSe excited with 488nm light. This is because acetone used in the cleaning dissociates bound 9-ACA molecules from the nanoparticle surface, thus decreasing the TET rate. Note that the sensitivity to 9-ACA surface density is due to the fact that we start with ODPA coated NCs which are resistant to displacement by carboxylic acids.29

Table 1. The diameter, optical properties, and upconversion QYs of the CdSe NC sensitizers. λmax (nm) a

Size (nm)

Eg b (eV)

PLQY c (%)

Upconversion QY (%) 488 nmd

532 nmd

Abs.

Ems.

2.7

529

544

2.34

11

7.5

7.7

2.9

544

554

2.28

4.0

3.0

3.1

3.1

552

563

2.25

3.5

1.4

1.4

3.2

557

568

2.23

8.9

4.4

4.6

3.7

577

588

2.15

4.3

2.1

1.8

4.1

588

598

2.11

3.4

1.2

1.1

5.1

611

618

2.03

2.0

0.26

0.27

a

In hexanes at RT; boptical gap as given by the absorption maxima; cNC photoluminescence quantum yield (PLQY), in hexanes at RT in air against R6G in ethanol (QY= 95%); d[DPA]=1.7mM, excitation with 19.8 W/cm 2 488 nm and 12.7 W/cm2 532 nm cw lasers.



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Figure 3. All measurements were conducted at RT with 1 mM DPA and a 532 nm laser at 12.7 W/cm2. (a) Upconverted DPA emission in toluene decreases as the concentration of 9-ACA transmitter in the ligand exchange solution with 3.2 nm CdSe NCs decreases. (b) Upconversion photoluminescence spectra of 1.5 µM 2.7 nm CdSe/9-ACA in different solvents. (c) The upconversion signal versus the solubility limit of 9-ACA in the solvents in (b).

In addition to ligand coverage, the TET may also depend on the properties of the NCs, specifically NC size and PLQY. The absorption and emission spectra of CdSe NCs investigated are shown in Figure 1b. Details on ligand exchange and sample preparation can be found in the supporting information (SI). Proton NMR reveals that the 9-ACA transmitter ligand has a density around 0.1/nm2 on the CdSe NC surface, in contrast to 1.3/nm2 for the native ODPA ligands29 (see Table S1). NMR shows no trend with respect to particle size, probably because NMR cannot distinguish between the free and bound 9-ACA ligands.30,31 Figure 4a shows the upconversion photoluminescence spectra with 532 nm excitation (see Figure S2 for 488 nm excitation). Upconversion QYs at 488nm and 532 nm excitation are the same for each sample, in accordance with the Kasha-Vavilov rule.32 Figure 4b shows the upconversion QY of each sample plotted against CdSe NC PLQY, with particle size listed. Taken by itself, this data suggests that upconversion efficiency increases with the increase in NC PLQY. When the upconversion yields are divided by the PLQY values in Table 1, we find that there is a systematic dependence on NC diameter, as shown in Figure 4c, implying that the energy of the CdSe NC exciton does affect TET to the anthracene derivatives.

Figure 4. (a) Upconverted DPA photoluminescence spectra using CdSe NC sensitizers excited with a 12.7 W/cm2 532 nm cw laser. (b) Plot of upconversion QY versus CdSe NC PLQY, with the diameter (nm) of each NC labelled. (c) Plot of the UCQY normalized by PLQY versus size of CdSe NCs. All measurements were done with 488 nm (black cross) and 532 nm (red hollow square) excitation at RT with 2 µM CdSe and 1.7 mM DPA in dry, degassed hexane.

The dependence of upconversion yield on PLQY was investigated in more detail using NCs with the same size but different PLQY values. For example, 3.2 nm and 3.1 nm diameter CdSe NCs, with PLQY = 8.9% and 3.5 % respectively, give a corresponding upconversion QY of 4.4% and 1.4%. The particle diameters are quite similar, but the NC with lower PLQY showed lower upconversion QY. Indeed, 2.9 nm CdSe NCs size with low PLQY gave a lower upconversion QY than 3.2 nm and 2.7 nm CdSe samples with high PLQY (see Table 1). The PLQY depends inversely on the non-radiative decay rate. Non-radiative decay processes includes relaxation to dark exitonic states through intersystem crossing,33 as well as trapping by the mid-gap states and surface states. While relaxation to the dark exitonic states with triplet character33-35 may aid TET, nonradiative trapping to other states would compete with TET and decrease the efficiency of triplet energy transfer.36 The nonradiative decay rate likely stems from the presence of surface defects, dangling bonds, unpassivated sites, nanoparticle stoichiometry, etc37-41 whose exact role in determining the PLQY is not well understood. An increased NC PLQY may indicate fewer trap states to impede energy transfer, or a NC surface that better binds 9-ACA.42 Upconversion QY correlates inversely with size of the NCs. Particle size affects the bandgap energy of CdSe NCs, which in turn relates to the driving force for energy transfer from particle to 9-ACA ligand. Here, 5.1 nm diameter CdSe have a bandgap of 2.03 eV, and the triplet state energy of 9-ACA is 1.83 eV.43 If we assume that 9-ACA’s highest occupied molecular orbital (HOMO) and the


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valence band of CdSe NC equilibrate, TET from the 5.1 nm CdSe still has a driving force of 0.20 eV. Thus our results indicate that driving energies greater than 0.2 eV result in energy transfer, which is in good accordance with molecular upconversion systems.44 Conclusion To conclude, the nanocrystal-based parameters affecting the upconversion QY of wurtzite CdSe/9-ACA/DPA sensitizer/ transmitter/annihilator were investigated using 7 CdSe NCs of different sizes and PLQYs ranging from 2.0% to 11%. In TTA-based upconversion, one of the steps that limits efficiency is the TET from sensitizer to transmitter or annihilator.44 Our results indicate that more 9-ACA ligand on the CdSe surface leads to higher TET efficiency and higher upconversion QYs. In addition, upconversion QYs correlate with NC PLQYs and inversely with NC size. Therefore, NCs with high PLQYs and small sizes should be utilized. This work provides a guide to understanding, designing and synthesizing functional hybrid nanoparticle-molecular photon upconversion systems. ASSOCIATED CONTENT Supporting Information. Experimental details, data, and spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] Notes Authors declare no competing financial interest ACKNOWLEDGEMENTS The authors acknowledge financial support from the US Army W911NF-14-1-0260 for instrumentation (M.L.T.), and the National Science Foundation, CHE-1152677 (C.J.B.) and CHE-1351663 for supplies (M.L.T.). REFERENCES

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