Oxygen-Enhanced Upconversion of Near Infrared Light from Below the Silicon Band Gap

Oxygen-enhanced upconversion of near infrared light from below the silicon band gap

Elham M. Gholizadeh,1 Shyamal K. K. Prasad,1 Zhi Li Teh,2 Thilini Ishwara,1 Sarah Norman,1 Anthony J. Petty II,3 John E. Anthony,3 Shujuan Huang,2 and Timothy W. Schmidt1∗

1ARC Centre of Excellence in Exciton Science, School of Chemistry, UNSW Sydney, NSW 2052, Australia 2School of Photovoltaic and Renewable Energy Engineering, UNSW Sydney, NSW 2052, Australia 3Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States

∗To whom correspondence should be addressed; E-mail: [email protected].

Abstract Photochemical upconversion is a strategy for converting infrared light into more ener- getic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. While systems have been developed for upconverting light from the biological tissue window near 800 nm, they remain susceptible to quenching by oxygen. Here we demonstrate an upconversion composition using semiconductor nanocrystal sensitizers that employs molecular triplet states below the singlet oxygen energy. We show that, contrary to the usual expectation, the admission of oxygen enhances the intensity of upconverted light and signiﬁcantly speeds up the photochemical processes involved. Further, we demonstrate photochemical upconversion from below the silicon band gap in the presence of oxygen. These results establish a new strategy for cir- cumventing the problem of oxygen in photochemical upconversion and lay the foundation for an expansion of this process into new applications.

1 ν 1. h 3. 4. hν 5. O2 × 2 2.

5. 4. 1O2 1. 2. 3.

PbS NC TTCA V79 V79

Figure 1: A cartoon showing the energy ﬂow and mechanism of NC sensitized PUC. 1. Photon absorption by the PbS-NC. 2. Triplet energy trapping by the TTCA ligand. 3. Triplet energy transfer to the V79. 4. Triplet-triplet annihilation between two triplet-state V79 molecules. 5. Upconverted photon emission. An alternative energy transfer pathway involving singlet oxygen is shown.

Transforming low-energy light into higher energy light has numerous applications not lim- ited to photocatalysis,[1] photoelectrochemistry,[2] photovoltaics,[3, 4] and biological imaging.[5] One highly adaptable strategy to achieve this is by photochemical upconversion (PUC) in organic molecules, also called triplet-triplet annihilation upconversion or triplet fusion upconversion.[6,

7] PUC has advantages over other upconversion strategies in that it is highly spectrally ﬂexible and can operate under low light levels. PUC proceeds with absorption of light by sensitizer species which generates molecular triplet states. After triplet energy transfer to emitter molecules, the latter can undergo triplettriplet annihilation to an emissive singlet state which emits upconverted light. To access the biological window, sensitizers are required that will absorb ∼ 800 nm light.[1] Recently, semicon-

2 ductor nanocrystals (NCs), which have excited state energies tunable across the visible and near infrared (nIR) spectrum, have been shown to be efﬁcient sensitizers for PUC.[8–12] Moreover, molecular ligands have been shown to prolong the lifetime of NC excited states and mediate the transfer of triplet energy to emitter molecules.[9, 13–15] The mechanism of semiconductor nanocrystal sensitized upconversion is illustrated in Figure 1.

However, molecular triplet states are sensitive to oxygen. Dioxygen has a triplet ground state, but upon interaction with molecular triplet states, can be placed in an excited singlet state

1 ( ∆g, 0.98 eV). In this case, not only is the triplet energy lost, but the singlet oxygen species thus produced can cause damage to organic molecules. This is the basis of photodynamic cancer therapy.[16] As such, there are several strategies which have been developed to mitigate oxygen as a problem in PUC, especially in biological applications.[17–20]

But, one strategy that is almost entirely unexplored is to “ﬂy under the O2 radar” – utilizing

1 triplet state energies that lie under the ∆g O2 energy of 0.98 eV.[21] Indeed, photosynthetic organisms utilize such molecules to rapidly quench chlorophyll triplet states and prevent singlet oxygen production.[22] While nature employs carotenoids to carry out this task, such molecules do not have the required energetics to carry out PUC: The triplet state is lower than half the excited singlet state energy.[23] Moreover, carotenoids exhibit a dark state which quenches

ﬂuorescence.[24] It is thus of great interest to identify a species which is capable of performing PUC from a triplet state lower in energy than singlet oxygen. One such chromophore is violanthrone

(V79, see Figure 2).[25] In this work, we describe an upconversion composition that is greatly enhanced in the presence of oxygen. The system is sensitized with PbS nanocrystals ligated to tetracene chromophores. By employing an emitter species with a triplet state below the energy of singlet oxygen, V79, we upconvert nIR to the visible spectrum. Oxygen is found to speed up the kinetics of upconversion and enhance the emission intensity. This work lays

3 V79 abs. O O V79 (delayed) PL TTCA 808 nm excitation O O abs.

V79

NC abs. NC PL

O C OH

Si TTCA 400 500 600 700 800 900 1000 1100 1200 wavelength (nm)

Figure 2: Absorption (abs.) and emission (PL) spectra of the PbS NCs, V79, and TTCA ligands. The V79 emission is PUC following 808 nm excitation of TTCA-functionalized PbS nanocrystals. Structures of V79 and TTCA are shown. the foundation for an expansion of PUC into a greater range of applications, such as where oxygen is problematic, and deeper into the infrared region, with potential applications to silicon photovoltaics.[26]

Results

The absorption and emission spectra of the PbS NCs, TTCA ligands and V79 are shown in Fig- ure 2. The NCs absorb over the entire visible spectrum, and exhibit an excitonic peak at 850 nm (1.46 eV). The peak of the photoluminescence (PL) is at 960 nm (1.29 eV). This exceeds the expected triplet energy of the TTCA chromophore (1.25 eV),[27, 28] and thus NC PL quenching is expected upon ligand exchange (see below). The upconverted emission of V79 is shown

4 in Figure 2 upon continuous 808 nm excitation in a deaerated mixture of the ligand-exchanged TTCA/NCs (OD = 1.2 cm−1) and V79 (2 mM).

The mechanism of the upconversion, evident in Figure 2, is elucidated by several experiments which probe the various energy transfer steps involved, shown in Figure 1. After photon absorption, the first energy transfer step is triplet energy transfer from the PbS NC to the TTCA ligand. To gauge the efficacy of this step, we monitored the PbS NC photoluminescence as a function of the TTCA concentration used in the ligand exchange step. The results are shown in Figure 3. As the TTCA concentration is increased, the lifetime of the PL was found to decrease, as expected.[15] The decay of the PbS NC PL was found to be well described by a single exponential, but increasing numbers of exponential functions were required to fit the decays as the concentration of the TTCA ligand was increased (see Supplementary Materials for details of the

ﬁts). This is most likely a result of inhomogeneity in the number of energy-accepting ligands on each NC and their site of attachment. The integrated PL allows a Stern-Volmer plot to be made (inset, Figure 3) which reveals a biomolecular quenching constant (2.25(14) × 1010 M−1s−1) in excess of the diffusion limit in toluene, consistent with attachment of TTCA to the NCs. At the highest concentration used, we estimate that 85 % of the initial excitation events lead to triplet-excited ligands.

The sensitizing NCs were blended with deaerated V79 solution, and the time-dependent emission in the 650-800 nm region is shown as a function of time after a 930 nm laser pulse in Figure 4. The triplet states of TTCA and V79 are dark, but their kinetics can be inferred by monitoring the upconverted photoluminescence, which is proportional to the square of the triplet-state V79 concentration. The time-dependence of the emission is well ﬁt by

3 2 2 IUC (t) ∝ [ V 79] ∝ (exp(−t/τ2) − exp(−t/τ1)) (1)

3 The rise time, τ1 is indicative of the time constant for TTCA quenching by V79 and the decay

5 0 8 7 6

) 5 / I 0 d

I 4 3

l i z e 2

a -2 1

m 0 20 40 60 80 100120 r

o [TTCA] (µM) n ( ) L -4 N C P S b

P [TTCA] (µM)

( -6 0 g 12 l o 58 115

-8 0 2 4 6 8 10 time (µs)

Figure 3: Photoluminescence (PL) decay of PbS NCs incubated with various concentrations of TTCA. The ﬁts are increasingly multiexponential as the concentration is increased, indicating inhomogeneity. Inset: Stern-Volmer plot of PbS NCs quenched by TTCA ligands. The bimolecular quenching rate constant is 2 × 1010 M−1s−1, indicating attachment of ligands and NCs.

6 iue4 ilnhoe7 msina ucino ieatra90n ae pulse laser nm 930 a ex- two after of time difference concentration. the V79 of of of function square function a as the time a by rise inverse described as The well Inset: emission is ponentials. emission 79 The Violanthrone mM). ([V79]=1.5 4: Figure

0 V79 PL intensity (arb.) 0 2 5 5 0

7 1/τ (µ −1) 5 1 s 0 0 0 0 0 0 0 0 0 ...... 0 0 0 0 0 0 0 0 0 4 4 5 5 6 6 7 7 8 tim 0 5 0 5 0 5 0 5 0 0 . 1 0 7 0 e 0 0 . ( 2 0 µs) . 4 0 1 . 6 2 0 5 . [V 8 1 7 . 0 9 ] ( 1 1 . 2 5 m 0 1 M . 4 ) 1 . 6 1 1 7 . 8 5 2 . 0 2 . 2 2 0 0 3 time τ2 is the lifetime of V79 triplets. The inverse rise time 1/τ1 is plotted as a function of V79 concentration in the inset to Figure 4. The slope of 1.70(23) × 107 M−1s−1 indicates the bimolecular rate constant for 3TTCA quenching by V79. The lifetime of the 3TTCA triplet state is taken as the (inverse of the) intercept, 25.1 ± 2.1 µs, and the triplet energy transfer efﬁciency is estimated to be 45 % at [V 79] = 2 mM.

The efﬁciency of the upconversion process can be broken down into the efﬁciency of se- quential energy transfer steps.

η η η Φ η = trap TET TTA PL (2) UC 2

From the experiments above, it was shown that ηtrap ∼ 0.85 and ηTET ∼ 0.45. We estimate

ΦPL ∼ 0.25 for dilute V79, although this could be further diminished under the present con- ditions. Under cw excitation at 808 nm we measured ηUC = 0.031 %. From this result, we estimate that ηTTA ∼ 0.65 %. Power-dependence experiments indicate a slope of 1.3 on a log- log plot of UC emission against incident excitation power, which indicates that this quantity is not saturated (Supplementary Figure 3).

1 Violanthrone is known to luminesce in the presence of singlet oxygen ( O2),[25] and given

1 the probable generation of O2 by NC/TTCA after absorption of light in the presence of oxygen, we investigated the effect of oxygen on the present UC system. Upconverting samples were opened to the atmosphere and agitated to saturate with O2. The effect on upconversion yield and kinetics was found to be dramatic. The dynamics of V79 emission following the 930 nm laser pulse in the presence and absence of O2 is shown in Figure 5. The rise time in the ﬁts in Figure 5 (Equation 1) decreases from 15 to 10 µs in the presence of O2. The concentration of O2 in toluene at 298 K is approximately 1.76 mM with a quenching rate constant of 1.9 × 1010 M−1s−1.[29] As such, it is clear that the quenching of TTCA triplets by O2 will outpace that by V79, and after 100 ns essentially all of the excitation quanta will

8 O2 V79 PL intensity (arb.)

600 650 700 750 800 850 wavelength (nm)

× 10 deaerated V79 PL intensity (arb.) w/ O2

0 0 25 50 75 100 125 150 175 200 225 250 time (µs)

Figure 5: Violanthrone 79 emission as a function of time after a 930 nm laser pulse, with and without O2. Upon admission of O2, the rise time and decay are both found to shorten dramat- ically. Inset: The time-integrated upconverted V79 emission with and without O2. There is a more than ﬁve-fold increase in upconversion due to oxygen.

9 1 be present as O2. These packets of energy are then transferred to V79 on a 10 µs time-scale, leading to upconverted emission. The peak of the UC emission occurs earlier in the presence of oxygen, and peaks with about 10 times the intensity. But, the time-integrated intensity is ﬁve-

3 times higher in the presence of O2 due to the increased rate of decay of V79. Nevertheless, the effect of O2 is striking, and contrary to the usual expectations of the role of O2 in photochemical upconversion. In samples with lower V79 concentrations, where 3TTCA quenching is far from complete, we observed as much as a 35-fold increase in upconversion upon admission of oxygen.

What is clear, is that the role of oxygen in PUC changes when the annihilating triplet state energy is low enough. The V79 triplet energy has not been measured, but we take 0.98 eV as an upper limit based on its ability to quench singlet oxygen. We found that the addition of

2 mM TIPS-pentacene (0.86 eV), or β-carotene (0.94 eV) both significantly quenched the V79 upconversion. As such, it is imputed that 0.94 < ET (V 79) < 0.98 eV. While one of the principal motivations for PUC is the enhancement of photovoltaics (PV), and given the dominance of crystalline silicon in the PV market, it is of great interest to demonstrate PUC from below the silicon band gap of 1.12 eV. The sensitization of TTCA ligands re- quires photon energies above 1.25 eV.[27] But, should singlet oxygen be generated directly from excitation of PbS NCs, this would lead to upconverted light from excitation energies approach- ing 0.98 eV.[30] We synthesized a batch of PbS NCs with absorption maxima near the silicon band gap (Figure 6). Irradiation of these NCs in the presence of V79 and O2 with 1140 nm light resulted in upconverted emission as shown in Figure 6. The excitation was filtered with a 200 µm silicon wafer. At this wavelength, the extinction coefficient of silicon is less than 1 cm−1,[31] and so the majority of the (non-reflected) beam is transmitted. The time-resolved signal indicates generation of 3V79 on a 600 ns time-scale, with decay on a 10 µs time-scale. The data is well fit to Equation 1, indicating energy transfer from NCs to V79 mediated by

10 excitation V79 PL <1.12 eV

NC abs. NC PL

solvent feature

700 800 900 1000 1100 1200 1300 1400 1500 wavelength (nm)

pulsed 1140 nm sample

650-800 nm 200 µm upconversion Si wafer

Si iCCD detector V79 PL intensity (arb.)

0 0 5 10 15 20 25 time (µs)

Figure 6: Absorption and emission spectra of PbS nanocrystals and the upconverted V79 emission following 1140 nm excitation below the silicon band gap of 1.12 eV. The emission occurs after 600 ns and decays on a 10 µs time-scale.

11 O2. Less intense upconversion was observed in deaerated solutions. This is the ﬁrst report of PUC from excitation energies below the band gap of silicon, opening the door to applications in silicon PV in the near future.

Discussion

While the presently achieved efficiencies are low, there is a clear pathway to improvement.[32] Within an optimized device structure, a Lambertian back reflector can improve the light harvesting efficiency by greater than a factor of two, leading to a more than doubling of exciton density within a thinner device structure. This is expected to improve the TTA efficiency by nearly an order of magnitude.[32, 33] V79 is the only example so far shown to give rise to upconversion from such low energies. It is therefore far from optimized: the PL quantum yield is not impressive (25 %), and the efficiency with which triplet pairs access the emissive singlet in TTA is not known. However, given the rather sluggish quenching of TTCA triplets by V79, it is likely that its annihilation rate constant is also rather low. The triplet diffusion rate is greatly increased where the exciton can hop, rather than being carried by a diffusing molecule.[10] It is therefore expected that much higher efficiencies will be obtained within an optimized, solid state device structure. Indeed, chemical solutions are not desirable in a PV device.

Applications for nIR upconversion go beyond PV. Nanoparticles comprising NCs and a emitter such as V79 could potentially be used in biological imaging, in vivo. Further, without requiring an excitation source it is possible that nanoparticles with the appropriate electronic structure could be used to detect and image singlet oxygen, which has been recently shown to regulate vascular tone and blood pressure in inﬂammation.[34] Annealed nanoparticles of

V78, an stearate ester derivative of violanthrone, were found to exhibit J-aggregate like spec- tral behaviour.[35] However, their observed photoluminescence lifetime of just 20 ps suggests that an alternative non-radiative decay pathway is operative in the solid state. Endothermic

12 singlet ﬁssion is one possibility.[28] Crystalline rubrene also suffers from competition between endothermic singlet ﬁssion and emission, and in upconvertors incorporating this material, the

PL quantum yield has been increased by doping with dibenzotetraphenylperiflanthene.[10] It is thus possible that such difficulties can also be overcome in violanthrone derivatives. By employing chromophores with low-energy triplet states, it has been demonstrated that sensitized upconversion is not quenched by oxygen. Moreover, due to the rapid diffusion of O2, upconversion of nIR light from the tissue window near 800 nm is significantly enhanced. While harnessing this phenomenon may lead to applications in oxygen sensing, the clear message is that oxygen need not be detrimental to PUC. Further, it has been demonstrated that, by using semiconductor nanocrystals and V79, nIR light from under the silicon band gap can be upconverted, thus opening the door for PUC to be employed in silicon-based photovoltaics, sensing and imaging applications.

Methods

Detailed methods are given in the Supporting Material. TTCA was synthesized according to the method in Reference 15. The PbS nanocrystals were synthesised according to the method in Reference 36. V79 was obtained from Sigma-Aldrich and used as supplied.

Data Availability

The data that support the ﬁndings of this study are available from the corresponding author upon reasonable request.

13 Acknowledgements

This work was supported by the Australian Research Council (Centre of Excellence in Exciton Science CE170100026).

Author Contributions

EMG, SKKP, SN and TI performed the measurements. SH and ZLT synthesized the PbS material. JEA and AJP synthesized the TTCA ligands. TWS conceived the experiments and wrote the manuscript.

Competing Financial Interests

There are no competing ﬁnancial interests.

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Supplementary Information

Detailed Materials and Methods Supplementary Figures 1 to 3

Supplementary Table 1 and 2