Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States

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Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States of Anthracene Ligand Molecules Geoffrey B. Piland, Zhiyuan Huang, Ming L. Tang, and Christopher J. Bardeen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12021 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

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The Journal of Physical Chemistry

Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States of Anthracene Ligand Molecules Geoffrey B. Piland, Zhiyuan Huang, Ming Lee Tang, Christopher J. Bardeen* Department of Chemistry University of California, Riverside Riverside, CA 92521 *e-mail: [email protected]

Abstract The combination of CdSe semiconductor nanocrystals with 9-anthracene carboxylic acid ligands can sensitize triplet-triplet annihilation on an emitter molecule, diphenylanthracene. This hybrid system has recently been shown to upconvert visible light (532 nm) to ultraviolet light (420 nm) [Nano Lett. 15, 5552-5557 (2015)]. In the current paper, time-resolved photoluminescence measurements are used to characterize the kinetics of energy transfer from the CdSe exciton state to the triplet state of the anthracene ligand. We find that the 9-anthracene carboxylic acid binds to the CdSe according Poisson statistics with a maximum number of 2-3 per nanocrystal. The CdSe-to-ligand energy transfer rate is 1.5×107 s-1. The overall energy transfer efficiency appears to be limited by the presence of fast nonradiative decay channels in the nanocrystals and the low coverage of anthracene ligands resulting from the specific ligand exchange conditions used in this paper. Possible strategies for improving this component of the hybrid upconversion system are discussed in light of these results.

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Introduction In organic molecules, conversion of the lowest excited singlet state (S1) into the lowest triplet state (T1) (intersystem crossing, ISC) is spin forbidden, making relaxation from S1 to T1 relatively slow. Conversely, conversion of a singlet into a pair of triplets (singlet fission) and its inverse (triplet-triplet annihilation or triplet fusion) are spin-allowed and can occur much more rapidly.1-2 These 12 and 21 exciton fission and fusion processes have attracted considerable attention as possible strategies for improving solar energy conversion efficiencies.3 For example, triplet-triplet fusion or annihilation (TTA) provides a way to upconvert a pair of low energy photons into a high energy photon that can be absorbed in a photovoltaic cell. 4-8 One challenge in using TTA for upconversion (UC) is how to create the triplet states in the first place. The spin-forbidden nature of the S0-T1 transition prevents direct optical excitation of the triplet states. The usual strategy is to employ a second organic molecule as a triplet sensitizer. The sensitizer molecule has a low energy S0-S1 transition and undergoes rapid ISC to populate its T1 state. Triplet energy transfer (TET) from the sensitizer can then populate the T1 state of the emitter molecule. When the T1 states of two emitter molecules have been populated, they can undergo fusion to generate an excited singlet state that emits the high energy photon. Unfortunately, there are few molecular sensitizers that are able to absorb light in the near infrared, the most relevant region of the solar spectrum for standard photovoltaic materials like Si and CdTe. Recently, we demonstrated that inorganic semiconductor nanocrystals (NCs) could be used as upconversion sensitizers out to 980 nm.9

In that work, it was found that the

upconversion efficiency was low (50%) quantum yields and monoexponential decays50, and suppressing the 134 ps decay component has the potential to significantly increase the energy transfer efficiency while simplifying the photophysics. In general, longer-lived NC excitons should provide more opportunity for the NC9-ACA energy transfer to take place, leading to more 9-ACA triplet states and higher UC yields. In summary, the kinetics of energy transfer from a NC exciton state to the triplet state of a conjugated organic ligand have been characterized for the first time. This transfer represents the first step in a sequence of events that enables hybrid organic-inorganic systems to upconvert a pair of low energy photons into one high energy photon. We find that the CdSe-to-ligand energy transfer occurs on a timescale of 70 ns and its overall efficiency appears to be limited by the presence of fast nonradiative decay channels in the NCs and the low coverage of anthracene ligands. It is likely that different ligand exchange conditions and NC surface properties lead to different overall triplet energy transfer rates, and optimizing both should improve energy transfer efficiency and thus UC efficiency.

Acknowledgements


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The authors acknowledge financial support from the National Science Foundation grant DMR1508099 (C.J.B), the US Army grant W911NF-14-1-0260 for instrumentation (M.L.T.) and National Science Foundation grant CHE-1351663 for supplies (M.L.T.).

Supporting Information CdSe nanocrystal synthesis, upconversion quantum yield measurements, data fitting procedures, and early and late time CdSe photoluminescence spectra. This information is available free of charge via the Internet at http://pubs.acs.org



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Figure 1. Absorption (in black) and fluorescence (in red) of a) DPA, b) 9ACA, and c) CdSe. The triplet energies of 9-ACA and DPA are marked with dashed lines (red for 9-ACA, blue for DPA).


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Figure 2. a) Time-resolved photoluminescence of NCs with ODPA and [9-ACA]LEX = 20 mM in a 1 ns time window. Approximately 55% of the photoluminescence decays with a time constant of 134 ps in this window, with the exponential fit shown in green. This early-time decay is unaffected by the presence of the 9-ACA. b) Time-resolved photoluminescence of NCs with various [9-ACA]LEX concentrations along with fits (red lines) based on a Poisson binding model given by Equation (1). The decays are plotted on a linear scale for the first 300 ns. c) The timeresolved photoluminescence data plotted on a logarithmic scale for the first 800 ns.



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Figure 3. Time-resolved photoluminescence of CdSe NCs with ODPA ligands only, ODPA ligands with 2.15 mM DPA in solution, and CdSe with benzoic acid ligands only. Neither the presence of COOH binding groups or DPA in solution has an effect on the CdSe decay.


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Figure 4. The number of bound 9ACA ligands on the NCs calculated from the Poisson binding model versus [9-ACA]LEX along with fits using Equation (2) in the text. There is a ~30% decrease in the number of bound ligands when DPA is present



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Figure 5. a) Time-resolved photoluminescence decay of 9-ACA in the presence of CdSe NCs (black, integrated from 400-470nm) compared to that of 9-ACA by itself (red). b) Photoluminescence spectra measured using an excitation wavelength of 400 nm of CdSe NCs exposed to various [9-ACA]LEX concentrations, illustrating the increasing fluorescence due to free 9-ACA in solution.


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Figure 1. Absorption (in black) and fluorescence (in red) of a) DPA, b) 9ACA, and c) CdSe. The triplet energies of 9-ACA and DPA are marked with dashed lines (red for 9-ACA, blue for DPA). 117x166mm (300 x 300 DPI)

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Figure 2. a) Time-resolved photoluminescence of NCs with ODPA and [9-ACA]LEX = 20 mM in a 1 ns time window. Approximately 55% of the photoluminescence decays with a time constant of 134 ps in this window, with the exponential fit shown in green. This early-time decay is unaffected by the presence of the 9-ACA. b) Time-resolved photoluminescence of NCs with various [9-ACA]LEX concentrations along with fits (red lines) based on a Poisson binding model given by Equation (1). The decays are plotted on a linear scale for the first 300 ns. c) The time-resolved photoluminescence data plotted on a logarithmic scale for the first 800 ns. 203x464mm (300 x 300 DPI)


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Figure 3. Time-resolved photoluminescence of CdSe NCs with ODPA ligands only, ODPA ligands with 2.15 mM DPA in solution, and CdSe with benzoic acid ligands only. Neither the presence of COOH binding groups or DPA in solution has an effect on the CdSe decay. 63x48mm (300 x 300 DPI)



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Figure 4. The number of bound 9ACA ligands on the NCs calculated from the Poisson binding model versus [9-ACA]LEX along with fits using Equation (2) in the text. There is a ~30% decrease in the number of bound ligands when DPA is present. 63x48mm (300 x 300 DPI)


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Figure 5. a) Time-resolved photoluminescence decay of 9-ACA in the presence of CdSe NCs (black, integrated from 400-470nm) compared to that of 9-ACA by itself (red). b) Photoluminescence spectra measured using an excitation wavelength of 400 nm of CdSe NCs exposed to various [9-ACA]LEX concentrations, illustrating the increasing fluorescence due to free 9-ACA in solution. 66x24mm (300 x 300 DPI)


CdSe

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Dynamics of Energy Transfer from CdSe Nanocrystals to Triplet States

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