Control of Energy Flow Dynamics between Tetracene Ligands and

Jan 24, 2018 - We have prepared a series of samples with the ligand 6,13-bistri(iso-propyl)silylethynyl tetracene 2-carboxylic acid (TIPS-Tc-COOH) att...
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Control of Energy Flow Dynamics between Tetracene Ligands and PbS Quantum Dots by Size Tuning and Ligand Coverage Daniel M. Kroupa, Dylan H. Arias, Gerard M. Carroll, Devin B. Granger, Jeffrey L. Blackburn, John E Anthony, Matthew C. Beard, and Justin C. Johnson Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04144 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Control of Energy Flow Dynamics between Tetracene Ligands and PbS Quantum Dots by Size Tuning and Ligand Coverage

Daniel M. Kroupa,1 Dylan H. Arias, 1 Jeffrey L. Blackburn, 1 Gerard M. Carroll, 1 Devin B. Granger,2 John E. Anthony,2 Matthew C. Beard,1 Justin C. Johnson1,* 1

National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, CO 80401 2

Department of Chemistry, University of Kentucky, Lexington, KY 40506

Keywords: quantum dots, photoluminescence, triplet, energy transfer, upconversion

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Abstract: We have prepared a series of samples with the ligand 6,13-bistri(iso-propyl)silylethynyl tetracene 2-carboxylic acid (TIPS-Tc-COOH) attached to PbS quantum dot (QD) samples of three different sizes in order to monitor and control the extent and timescales of energy flow after photoexcitation. Fast energy transfer (~1 ps) to the PbS QD occurs upon direct excitation of the ligand for all samples. The largest size QD maintains the microsecond exciton lifetime characteristic of the as-prepared oleate terminated PbS QDs. However, two smaller QD sizes with lowest exciton energies similar to or larger than the TIPS-Tc-COO- triplet transition energy undergo their decay processes on nanosecond timescales due to barriers between the ground state triplet transition energy and the QD core or trap state energies. For the intermediate size QD, energy can be recycled many times between ligand core, but the triplet remains the dominant excited species at long times, living for ~3 s for fully exchanged QDs and up to 30 s for partial ligand exchange, which is revealed as a method for controlling the triplet lifetime. A unique upconverted luminescence spectrum is observed that results from annihilation of triplets after exclusive excitation of the QD core.

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Many recent investigations of colloidal quantum dots (QDs) employ functional rather than passive organic molecules as ligands, representing a paradigm shift in the field of QD-based optoelectronics.1-2 Many of these investigations aim to demonstrate resonant effects via energy or charge-transfer between the QD core electronic states and those of the ligand. Among the emergent phenomena that have been demonstrated as a result of the electronic coupling are, most notably: enhanced QD absorption,3-4 long-lived charge separation,5-6 and photon upconversion.7-8 These organic-inorganic “hybrid” materials combine the strong absorption and band gap tunability of QDs with the versatility of organic chromophore design, and as such have become highly attractive as functional materials. However, the nature of the interactions that occur between the organic and inorganic semiconductor sub-units is challenging to predict based upon their individual properties, and the need for systematic studies of hybrid systems is paramount in order to provide robust design principles. The mechanism of aliphatic ligand binding to cadmium and lead chalcogenide nanocrystalline surfaces is now quite well understood, and these principles have been applied to exchange the as-prepared ligand/QD to various types of ligand/QD species.9 The degree of ligand exchange can be at least semi-quantitatively determined via FTIR and NMR spectroscopic methods, and a fairly clear picture of the facet-specific binding motifs has been obtained.10-13 There are several challenges associated with performing these ligand exchange reactions with unconventional ligands, especially in the solution-phase. Typically, the subsequent solubility and overall stability of the ligand-exchanged QDs is often considerably poorer than the as-prepared ligand/QD complex, creating challenges relating to aggregation and degradation of optical properties with time. Furthermore, whereas some amount of unbound ligand may always be present due to thermodynamic equilibrium in solution, the large excess of photoactive ligand 3 ACS Paragon Plus Environment

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needed to enable the complete ligand exchange can lead to a considerable concentration of free ligand that can interfere with photophysical studies. Therefore, purification techniques that can physically separate ligand exchanged QDs from excess free ligand efficiently are critical to clear and convincing spectroscopic assignments. A few recent reports of the exchange of conventional ligands for polyacene-based ligands at nanocrystal surfaces have arisen from the field of triplet-triplet annihilation based upconversion (TTA-UC).7, 14-16 Polyacenes are considered good partners for near-IR absorbing QDs in both TTA-UC and singlet fission (SF) schemes. However, bilayers have traditionally been used, creating uncertainty about the activity at the ill-defined polyacene/QD interface.17-19 Among the polyacene ligands studied, substituted pentacenes have been shown to undergo triplet energy transfer (TET) from the ligand to the QD and harbor long-lived triplet excitons after initial excitation of the PbS counterpart.15 The relatively slow arrival of triplets was ascribed to the need for a charge-transfer intermediate. However, spectral congestion and the lack of a systematic adjustment of ligand/QD resonance energies hampered a true mechanistic analysis. Tetracene-based ligands have also been exchanged for native ligands on PbS QDs.20 Quantum yields for TTUC as high as 8% are achieved upon the formation of a CdS shell on the PbS QD surface. Some dynamics are inferred from the TTUC trends, but a true mechanistic picture including fs to s dynamics remains to be developed. Here we exchange the native oleate (OA-) ligands at PbS QD surfaces through the addition of excess 6,13-bis(triisopropylsilylethynyl)-tetracene-2-carboxylic acid (TIPS-TcCOOH) in the solution phase to produce samples with varying degrees of electronic coupling, in particular between the triplet state T1 of TIPS-Tc-COO- and the 1S-exciton of PbS QDs (Fig. 1B). TIPS-Tc is a tetracene derivative that possesses several features that contrast with its parent 4 ACS Paragon Plus Environment

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and make it a desirable choice for these studies: (i) high solubility in many organic solvents; (ii) good photostability; and (iii) strong photoinduced absorption features assignable to population in S1 and T1 states.21 We prepared oleate (OA-) capped PbS QDs with lowest 1S-exciton absorption bands peaking at 0.95 eV, 1.25 eV, and 1.45 eV (Figure S1). Each of the OA-/QD samples had a core diameter size distribution around 5-6%, as approximated by the first exciton peak half-width at half maximum absorption.22 In a suspension of dichloromethane (DCM), the native OAligands of the PbS QDs were exchanged through the addition of excess TIPS-Tc-COOH as in scheme in Figure 1A and following our previously developed procedures.23 After ligand exchange, the lowest exciton absorption band exhibited a slight blue-shift, while the absorption at higher energies gained a series of peaks due to the TIPS-Tc-COO- ligand (Figure 2A). The ligand absorption has sharp vibronic bands characteristic of all acene molecules.24 The bands are identical to those observed for solvated TIPS-Tc-COOH not attached to QDs. The energy level diagram in Figure 1B shows approximate positions of states for measurements made on isolated TIPS-Tc-COOH or PbS QDs with OA ligands. Interactions between the components after ligand exchange is likely to shift the energies due to interfacial fields and possible hybridization of states due to strong coupling.3 Thus, the relative positions of levels should serve as a tentative qualitative guide for the expected photophysics.

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Figure 1. (A) Ligand exchange reaction involving a 1:1 X-type ligand exchange of the as-synthesized oleate terminated PbS QD with TIPS-Tc-COO- ligand. Excess ligand drives the reaction to completion and Gel Permeation Chromatography is used to purify the product. (B) Energy levels, in units of eV and with respect to vacuum, are measured herein for TIPS-Tc-COOH singlet levels. PbS QD levels are taken from ref. 23 The T1 energy level is approximate, based on ref. 21

Following a single round of conventional precipitation, centrifugation, resuspension (PCR) purification to remove the majority of unbound acid species from the ligand exchange reaction mixture (OAH and TIPS-Tc-COOH), gel permeation chromatography (GPC) purification was employed to remove residual unbound TIPS-Tc-COOH to isolate the TIPS-TcCOO-/QD complex of interest. After GPC purification of the ligand exchanged and 1x PCR purified TIPS-Tc-COO-/QD sample, the relative absorption strength of the TIPS-Tc-COO(H) compared with the PbS QDs was reduced by a factor of roughly 5 times (Figure 2A). There is also a ~2 nm red shift in the position of the TIPS-Tc-COOH absorption bands after GPC purification (Figure S2). When normalized at the peak of their lowest exciton band (Figure 2B), the samples exhibit a systematic trend in ligand absorption strength. The largest QD sample, 6 ACS Paragon Plus Environment

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expected to have the lowest effective surface:bulk ratio and thus smallest available area for ligand binding, has the weakest TIPS-Tc-COO- absorption. Correspondingly, the smallest QD sample has the strongest ligand absorption peaks. The interaction of TIPS-Tc-COO- with the varying amounts of available surface binding sites produces the slight red shift and the trend in ligand absorption strength.

Figure 2. (A) Visible range absorption and fluorescence of 1.18 eV TIPS-Tc-COO-/QD complexes postPCR purification but before (solid) and after (dashed) GPC purification. Fluorescence of TIPS-Tc-COOH in DCM solution is shown for comparison. (B) Near-IR range absorption (solid) and fluorescence (dashed) spectra of the three ligand-exchanged QD samples. All curves are normalized to the peak of the QD lowest exciton absorption. (C) Fluorescence spectra in the visible range after 540 nm or 750 nm excitation.

Fourier transform infrared (FTIR) spectroscopy of the as-synthesized and ligandexchanged PbS QD samples having undergone GPC purification suggest that OA- is efficiently removed from the QD surface via an exchange with TIPS-Tc-COO- (Figure S3). Proton nuclear magnetic resonance (1H NMR) spectroscopic analysis also suggests that OA- is completely removed from the surfaces of all three QD samples (Figure S4). Our analysis of the TIPS-TcCOO- exchange is consistent with previously reports where functionalized cinnamic acids replaced OA- at the surface of PbS QDs via an X-type ligand exchange scheme.4, 23 Based on the physicochemical and spectroscopic data shown here, we hypothesize that the TIPS-Tc-COOexchange proceeds through a similar 1:1 X-type ligand exchange mechanism in which the

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addition of excess acid drives the exchange to completion. Because the ligand exchange is likely highly cooperative, it is unlikely that any OA- ligand remains bound to the surface of the QDs.4 The photoluminescence (PL) spectra of the samples excited at 540 nm show characteristic fluorescence bands from the TIPS-Tc-COOH ligand (Figure 2A). For samples that have not undergone GPC purification, the fluorescence is strong and virtually identical to that of solutions of TIPS-Tc-COOH without QDs, which implies that residual free TIPS-Tc-COOH (with measured fluorescence yield ~ 0.76) leads to this strong signal. A slight change in the relative amplitudes of vibronic peaks is noticeable, which is likely due to reabsorption of fluorescence by the QDs that preferentially absorb the higher energy side of the fluorescence spectrum. After GPC purification, the fluorescence lineshape of ligand-exchanged samples remains the same but the intensity is reduced by more than one order of magnitude. By tuning the light source to 750 nm, exclusive excitation of the QD core can be accomplished (Figure 2C). Under these conditions the emission from the TIPS-Tc-COO- ligands is extremely weak, and the spectrum is altered from that seen with 540 nm excitation. Because 750 nm is well below the absorption onset of TIPS-Tc-COO(H), and the excitation source is too weak to cause two-photon absorption (roughly 200 mW cm-2), the only pathway for such emission is via triplet-triplet annihilation based upconversion (TTA-UC). This emission is observed only for the 1.45 eV and 1.18 eV samples. Photoluminescence excitation (PLE) spectroscopy shows that the quantum efficiency of PbS QD PL as a function of the excitation wavelength for each of the ligand-exchanged samples (Figure S5-S6). Prior to GPC purification, the PLE spectrum is a poor match for the sample absorptance (Figure S5B), and the sharp molecular bands near 500-550 nm are broad and weak. Here, free TIPS-Tc-COOH in solution is absorbing and re-emitting visible PL, and thus not 8 ACS Paragon Plus Environment

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contributing to the PLE spectrum. After GPC purification, features associated with the TIPS-TcCOO- ligand absorption align well with absorption features exclusive to the QD, suggesting that the majority of unbound ligands have been removed. A slightly poorer match is observed as QD size is reduced, suggesting a weak trend of either more unbound ligand or less efficient energy transfer (Figure S6). The dramatic difference in PLE before and after GPC purification underscores the need for free ligand removal in order to isolate the photophysics of the QD/ligand complex. For the 0.95 eV QD sample, both the spectrum and the yield of PL from the QD core exciton are unchanged after ligand exchange, (Figure 3A). However, the PL from the 1.18 eV or 1.45 eV PbS/TIPS-Tc-COO- QDs is found to be at least 10 times weaker than that of the assynthesized OA-/QDs (Figure 3D). For the 1.18 eV sample the shape is similar but slightly redshifted after ligand exchange (Figure 3B), but very strongly red-shifted and distorted for the 1.45 eV QDs (Figure 3C). The spectrum appears to match the low energy tail of the 1.45 eV PbS/OA- samples, implying that the sub-band gap luminescence is present while the exciton PL is quenched. The nature of the sub-band gap luminescence has been discussed at length in the literature,11, 25-27 but regardless of the origin of the states that produce it, the electrons and/or holes involved are likely too low in energy to make the triplet energy transfer to the TIPS-TcCOO- ligand kinetically competitive with QD core radiative or nonradiative decay.

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Figure 3. (A)-(C) PL spectra before and after ligand exchange for (A) 0.95 eV, (B) 1.18 eV, and (C) 1.45 eV QD samples. Dips in spectra near 1140 nm, 1350 nm, and 1400 nm are from DCM absorption. (D) PL quantum yield for as-synthesized (black) and ligand-exchanged (red) PbS QD samples. Timeresolved photoluminescence for the 1.18 eV PbS QD sample at (E) visible and (F) near-infrared detection after 540 nm excitation. Inset in (E) shows emission kinetics for TIPS-Tc-COOH in DCM , which is repeated in the main panel as a dashed line. In (F), the black trace is for the same QD sample with native OA- ligands, and the red trace is after exchange for TIPS-Tc-COO- ligands. Kinetic slices are obtained by integrating the strongest band in the emission spectrum.

Time-resolved PL (TRPL) measurements were performed with varying excitation wavelength in the visible and near-infrared spectral regions. With 540 nm excitation and detection in the visible region, isolated TIPS-Tc-COO(H) emission is characterized by monoexponential decay with a time constant of about 11 ns (Figure 3E, inset). For the samples with TIPS-Tc-COO- attached to 1.18 eV PbS QDs, the same emission feature is also detected, which is likely due to the small but highly emissive unbound ligand remaining in the sample. An additional slower decaying regime is also found (Figure 3E, red trace), which includes components with time constants of roughly 160 ns and 700 ns. We note that even at the lowest 10 ACS Paragon Plus Environment

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repetition rates possible for the experiment (100 kHz), some triplet population survives the time between consecutive laser pulses and is available for annihilation with a triplet produced from a subsequent pulse. This elongated emission component thus likely arises from the recycling of population between QD and TIPS-Tc-COO-, and the subsequent annihilation of triplets to produce emissive singlets. A similar elongated fluorescence decay profile is observed for the 1.45 eV QD/TIPS-Tc-COO- sample, but the 0.95 eV sample shows only the fast component (Figure S7A). Upon detection of PL from the QD band gap, the lifetime is monoexponential at about 1.3 s (for 1.18 eV QD/OA-) (Figure 3F, black trace). For TIPS-Tc-COO-/QDs, the lifetime is approximately biexponential with a dominant decay component of roughly 150 ns and a minor component of 2.2 s (Figure 3F, red trace). Based on the lifetimes and their respective amplitudes, the long-lived decay represents about 68% of the total emission yield. For the 0.95 eV QD sample, TRPL results (Figure S7B) are virtually identical before and after ligand exchange ( = 1.8 s, 1.4 s). The 1.45 eV QD sample (Figure S7C) has features similar to those of the 1.18 eV sample, with a monoexponential lifetime of 2.1 s before ligand exchange and multiexponential lifetime of 0.10 and 2.3 s after exchange. However, the relative yield of fast vs. slow TRPL decays is a function of detection wavelength (Figure S8). Photoexcitation of the as-synthesized OA-/QDs near the 1S exciton produced transient bleach and absorption signals in femtosecond transient absorbance (TA) pump-probe experiments that have been previously well-characterized, Figure S9.28-29 The 1S-exciton bleach decay follows monoexponential kinetics at low laser fluence and approximately biexponential kinetics as the fluence is raised toward exciton occupancy, 〈𝑁0 〉, equal to 1. The Auger decay

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times for the three QD sizes are approximately 25 ps, 58 ps, and 78 ps (Figure S9B). After ligand exchange there is a small blue shift and minor broadening in the bleach spectra for each QD sample (Figure 4A, cf. dashed vs. solid traces), which we propose is a result of dipolar ligand-induced confinement of the excitonic wavefunction in the QD core. The transient spectral features of free photoexcited TIPS-Tc-COOH in solution, (Figure 4B-C, dashed curves), include singlet excited state absorption (ESA) near 450 nm, 950 nm, and 1280 nm, and bleach and stimulated emission that follow the steady-state absorption and fluorescence, respectively. Excitation at 540 nm incites transitions in both the TIPS-Tc-COO- ligands and the QD cores. In order to visualize the spectral changes vs. time, we show in Figure 4 spectral slices at specific times, or decay associated spectra (DAS), that result from global fitting to an exponential decay model. The DAS are a combination of spectral features from multiple species, and the interpretation of these spectra relies on comparison to known spectra (dashed curves in Figure 4). Full TA data sets are shown in Figures S10-S12. For the visible spectrum, the most prominent feature is the S1-Sn absorption of TIPS-Tc-COO- at about 450 nm (Fig 4B cf. magenta dashed trace vs. red, black and blue traces). The majority of the amplitude of this feature is lost quickly: 900 fs in the 0.95 eV QDs, 1.0 ps in the 1.18 eV QDs, and 1.5 ps in the 1.45 eV QDs. Concomitant with the ESA decay is the loss of stimulated emission at 600 nm. Broad absorptions associated with QD ESA rise within 1 ps and decay on a 25-100 ps time scale (Auger decay) and a few hundred ps time scale before becoming constant into the ns regime. In the NIR, the two known S1-Sn bands for TIPS-Tc-COO- at 950-1000 nm and 1200-1300 nm appear with the excitation pulse and decay with the same time constants as the 450 nm ESA (Figure 4C, black and blue traces). QD bleach features remain after the TIPS-Tc-COO- singlet ESA decay, and they decay slightly over a few ns before stabilizing. 12 ACS Paragon Plus Environment

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Figure 4. (A) 5 ns time delay spectra for the three QD samples passivated with OA- (dashed) and TIPSTc-COO- ligands (solid). (B) Visible and (C) near-IR transient spectra associated with the fast decay of the singlet for the ligand-exchanged QD samples. Dashed lines show the slowly decaying singlet spectra of TIPS-Tc-COOH in solution. Near-IR features corresponding to TIPS-Tc-COO- for the ligandexchanged 0.95 eV QD samples could not be extracted due to spectral overlap with QD bleach and ESA. (D) Rise of triplet absorption after 800 nm excitation, probed at 530 nm. The absorption for the 0.95 eV sample represents residual QD ESA and not triplet absorption. (E) Bleach recovery kinetics probed at the peak of the lowest exciton feature for each QD sample. (F) Triplet rise and decay kinetics for the fully

and partially ligand-exchanged QDs. The pulse arrival time in (D)-(F) is shifted to 0.01 s for clarity on the logarithmic scale. After the first few ps, all of the dynamics and spectral features become the same irrespective of excitation wavelength, further corroborating the notion of fast and complete energy transfer from the photoexcited TIPS-Tc-COO- to the QDs upon its direct excitation. The excited state dynamics associated with energy transfer from QD core to the TIPS-Tc-COO- was investigated using 800 nm pulses that exclusively excite the QD core for all samples (Figure 4DF). The lowest exciton bleach and the broad ESA features initially produced after photoexcitation largely match those of the OA-/QD sample, Figures S9-11. However, overlapping with the broad ESA are negative features that resemble the TIPS-Tc-COO- bleach spectrum (Figure S13A). These features are most prominent for higher power excitations but are 13 ACS Paragon Plus Environment

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clearly present for all samples and excitation wavelengths used here, either resonant with the QD 1S exciton or higher-lying absorptions (Figure S13B). The decay of these features was not investigated thoroughly, but they appear to be mostly absent from the spectra within the Auger lifetime. Similar peaks were previously assigned to a dipole coupling effect between the QD exciton and a TIPS-Pc-COO- transition dipole, reducing its oscillator strength and producing a bleach,15 and we tentatively assign these spectral features to the same phenomenon. The long-time excited state dynamics are distinctly different for the three different QD sizes studied here. For the 1.18 eV TIPS-Tc-COO-/QD sample, there is a rise component of about 150 ns (Figure 4D, black trace), corresponding to the visible ESA features that resemble the T1-Tn absorptions of TIPS-Tc (Figure S14, black trace) and indicating energy transfer from the QD 1S-exciton to the T1 state of the ligand. The triplet feature has a lifetime of about 8 s. It is worthy to note that the T1-Tn spectra more closely resemble the triplet spectrum derived from sensitization of the TIPS-Tc-COO-/QD complex (Figure S14, magenta) than from free TIPS-Tc-COOH, suggesting the triplet spectra of the bound ligands are unique. Furthermore, strong T1-Tn peaks of bound TIPS-Tc-COOH are found only in the visible range compared with TIPS-Tc,21 for which narrow bands in the near-infrared were reported. A broad visible range ESA due to excitations in the QD does overlap with the ligand triplet absorptions, but is relatively weak compared with T1-Tn bands.30 Concomitant with the rise of the triplet ESA features is the decay of the QD ESA and 1Sexciton bleach features in the NIR (Figure 4E, black trace). The QD decay is multiexponential with decay components around 150 ns and a slower decay in the microsecond range. The 1.45 eV QD sample has a TIPS-Tc-COO- triplet feature that rises multiexponentially (Figure 4D, blue trace), but it is difficult to assign the faster rise unequivocally to triplet formation due tooverlap 14 ACS Paragon Plus Environment

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with QD ESA features at early times. A clear secondary rise of about 50 ns is observed, and the subsequent triplet lifetime is about 3 s. In contrast, the 0.95 eV PbS QDs show no rise in the spectral features associated with population of the triplet state, and the 1S-exciton lifetime of the TIPS-Tc-COO-/QD samples remains virtually unperturbed from the OA-/QD sample at about 1.5 s. Partial ligand exchange was accomplished by adding a sub-equivalent amount of TIPSTc-COOH in DCM to a 1.45 eV OA-/QD sample. 1H NMR spectra, Figure S15, show broadened peaks consistent with both OA- and TIPS-Tc-COO- ligands bound to the QD surface through a dynamic exchange process, although the extreme broadening of TIPS-Tc-COO- ligands makes it difficult to assign an extent of exchange. The TA dynamics of the partially exchanged sample after 545 nm excitation reveal a fast (~1 ps) energy transfer process from the singlet state of the bound TIPS-Tc-COO- to the 1S-exciton of the PbS QDs, which allows us to conclude that partial exchange does not affect the initial energy transfer process. Similarly, on much longer timescales, the energy transfer time from the PbS QD core to the TIPS-Tc-COO- triplet remains roughly equivalent to that of the fully exchanged samples (Figure 4F). However, the TIPS-TcCOO- triplet for the partially exchanged sample has a lifetime of > 30 s, approaching the lifetime of the free ligand in solution (~50 s) and considerably longer than the ~5 s lifetime of the fully exchanged sample. The lifetime extension suggests the suppression of triplet hopping processes that can occur on the surface of the densely-packed TIPS-Tc-COO- ligands for fully exchanged samples. Such hopping may expedite the discovery of unpassivated sites that result in nonradiative recombination to the ground state.31 We tested the possibility that dynamic exchange could be lengthening the triplet lifetime through desorption of a surface ligand harboring a triplet excitation by measuring the triplet lifetime of a fully exchanged 1.45 eV 15 ACS Paragon Plus Environment

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TIPS-Tc-COO-/QD sample in the presence of excess TIPS-Tc-COOH (Figure S16). The lifetime shows slight elongation (4.8 s vs. 3.0 s for no excess ligand), which hints that the dynamic exchange has a small influence on the lifetime but rules out ligand desorption as the primary mechanism for triplet lifetime elongation. Overall, the spectroscopic data lead to a clear kinetic picture based on the decay and rise of features assigned to species associated with the TIPS-Tc-COO- singlet and triplet states or the PbS QD core 1S-exciton or sub band gap states (summarized in Figure 5). In all cases, direct photoexcitation of the lowest singlet excited state of the ligand leads to fast energy transfer (within ~ 1.5 ps) to the 1S-exciton of the QDs that likely occurs by a dipole-dipole energy transfer mechanism. This is directly observed in the loss of S1-Sn absorptions from TIPS-TcCOO- as well as the good agreement between absorptance and PLE spectrum when monitoring the QD emission, Figure S5-S6. For the largest QD size (smallest band gap), the energy of the 1S-exciton is smaller than that of any ground state transition in TIPS-Tc-COO(H) (including S0T1), and thus once the energy migrates to the QD the carrier dynamics are essentially identical to samples with OA- ligand surface passivation. For the smaller QD sizes (larger bandgaps), the 1S-exciton energy is either roughly resonant with or larger than that of the S0-T1transition energy of TIPS-Tc-COO(H), and energy transfer from the QD to the ligand is allowed. Due to large spin-orbit coupling in the PbS, the exciton is of mixed spin character,32-33 and a Dexter-type mechanism is likely to be responsible for energy transfer back to the pure triplet state of the ligand. The relatively slow rate of triplet transfer (~5 s-1) despite intimate coupling (i.e., no aliphatic chain is included with the -COOH linker) is likely due to the intrinsic energetic barrier to the transfer of triplet energy between the two partners, which is estimated to be at least 70 meV. For the 1.45 eV QDs there is ostensibly no energetic barrier, and concomitantly there is 16 ACS Paragon Plus Environment

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faster TET (~20 s-1) (Figure 4D, blue trace). However, this is a relatively modest hastening, and we postulate that the slower TET results from a reduced driving force for “trapped excitons” that have electrons or holes occupying sub-band gap states.27 Evidence for such states was found in the steady-state PL spectra, Figure 3C. The distribution of QD core states energies is further revealed in wavelength-dependent TRPL kinetics, which show a shifted equilibrium between ligand triplet and QD excitations at long delay time due to the varying energy barrier between the states (Figure S8A). Upon 540 nm excitation, fast triplet formation could also arise from singlet fission (SF) on the surface of the QD. A recent study reports both a fast (< 1 ps) and slower (20 ps) SF process for TIPS-Tc films. Our investigations of monolayer TIPS-Tc-COOH attached to mesoporous TiO2 reveal SF times of roughly 40 ps.34 In the ligand-exchanged QD samples studied here, the lack of a clear signature of triplets on the requisite timescales argues against the likelihood that long-lived triplets are arising from SF. The more probable scenario is that energy transfer to the QD from the ligand singlet state outcompetes the slower SF process that would otherwise occur in the disordered ligand monolayer. QD/ligand systems designed to take advantage of SF will need to take the likelihood of fast singlet energy transfer into account, which has been pointed out in other contexts.35 Once the excitation energy resides in the triplet state of the TIPS-Tc-COO-, it has the possibility of being recycled for the 1.18 eV and 1.45 eV QDs due to the near resonance of the lowest QD transition energy and triplet transition energy. In fact, our data suggest an apparent equilibrium forms between the excited state residing on the ligand and the QD. The apparent lifetimes of both the triplet state, observed via transient absorption probing of the T1-Tn induced absorption, and the 1S-exciton, observed via PL decay, are equivalent, supporting the notion of 17 ACS Paragon Plus Environment

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an equilibrium. Furthermore, the slow component of the PL lifetime for the QD is longer than values for the associated OA-/QD samples, Figure 3F. This behavior is qualitatively reproduced by a kinetic model that includes the possibility for forward and reverse triplet energy transfer modulated by an energy barrier, Figure S8C.

Figure 5. Control of energy flow within the TIPS-Tc-COO-/PbS QD system. In part (A) the energy flow pathways are shown for the 1.45 eV TIPS-Tc-COO-/PbS sample. Excitation of the TIPS-Tc-COOproduces a singlet on the ligand that within 1.4 ps transfers to the 1S-exciton of the QD. The energy then transfers back to the ligand as a triplet state within ~50 ns. The lifetime of the triplet is 3 s and can be lengthened by the reducing the amount of ligand on QDs. Part (B) is for the 1.18 eV PbS QDs. Here the energy of the 1S exciton is just slightly smaller than the triplet energy and the energy transfer from the QD to the triplet is much slower (150 ns). Energy on the triplet can recycle back to the QD and this recycling lengthens the lifetime of the QD exciton. Part (C) is for the smallest band gap QD (0.95 eV) in this case once the energy is transferred from the ligand to the QD it cannot go back to the triplet. In both A and B there is evidence for triplet-triplet annihilation up-conversion (TTA-UC) from the triplet to the singlet state of the ligand under steady-state excitation conditions.

Interestingly, the return of the triplet exciton to the ground state can be controlled by the extent of ligand exchange. Partial exchange of OA- ligands for TIPS-Tc-COO- is likely to leave surface-bound TIPS-Tc-COO- molecules isolated from each other. Thus, triplets that arrive at these sites from the QD core will not undergo fast hopping to other bound TIPS-Tc-COO-

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ligands because the distance and dielectric barrier reduce the wavefunction overlap and thus the transfer rate. More systematic studies of ligand exchange should aid in developing quantitative models of how triplet energy moves on the QD surface. The slight red shift of bound TIPS-Tc-COO- absorption observed after GPC purification could be due to two effects: (1) electronic coupling between the QD and ligand; or (2) the change in dielectric environment from being primarily provided by solvent to being strongly influenced by the PbS QD. The change in fluorescence line shape of the TIPS-Tc-COO- ligand with 750 nm excitation vs. resonant 540 nm excitation is intriguing, and suggests that upconverted luminescence is occurring from molecules either attached to or near the QD surface and as such is in competition with the known energy transfer from S1 back to the QD. Although the upconverted PL yield was not measured, it is clearly several orders of magnitude weaker than directly excited TIPS-Tc-COOH fluorescence, which would be consistent with the low steadystate population of S1 for surface-adsorbed TIPS-Tc-COO-. Somewhat counterintuitively, both the lifetime and the relative amplitude of the longlived triplet signature are unaffected by the excitation fluence (Figure S17). This is due to relatively fast Auger recombination in the QDs ( < 100 ps) that reduces multiexcitons to single excitons in advance of their transfer to the triplet state of the ligand. Thus, under fast pulsed excitation, only one triplet is likely to exist in the ligand shell at any time, and the typical power dependence of TTA-UC signatures is absent. Although the rough estimate of time between photon absorption events under steady-state illumination confirms that some triplets on bound ligands of isolated QDs can participate in TTA-UC (see Supporting Information), the very small yield allows for the possibility that a minority of undetected aggregated species could also contribute. QD aggregation is known to occur in a variety of contexts,36-37 and the proximity of 19 ACS Paragon Plus Environment

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multiple excitations in a QD cluster would be expected to enhance TTA-UC. Alternatively, the desorption of electronically excited ligands from the QD surface may enhance the triplet lifetime and reduce the singlet energy transfer efficiency, thus enhancing the TTA-UC yield. Ligand desorption appears to produce a marginal increase in triplet lifetime when excess ligand is added (Figure S16), but the degree to which desorption occurs in samples purified of free ligand remains unclear. These effects are under further investigation, but for practical applications of TTA-UC, an “annihilator” molecule is typically employed that is capable of accepting triplet energy from the ligand and subsequently emitting,2 ameliorating the loss mechanisms inherent in the systems studied here. Photoinduced charge-transfer, as demonstrated in a variety of QD-ligand systems, remains a possible competing pathway with energy transfer. Shortly after TIPS-Tc-COOexcitation, no spectral bands associated with a TIPS-Tc cation intermediate (Figure S17) are detected during the decay of the singlet. Instead, it is likely that the arrangement and distance between TIPS-Tc-COO- and QD favor energy transfer over electron transfer. Upon QD core excitation, longer time dynamics could reflect electron or hole transfer to the ligand. A previous study on a PbS/TIPS-pentacene-COOH sample suggested that a charge-separated species involving a TIPS-Pc cation and a negatively charged QD serves as an intermediate between the PbS QD 1S-exciton species and the triplet on a TIPS-Pc-COO- ligand.15 The energy level alignment in the present case, shown in Figure 1B, makes fast electron or hole transfer after excitation of the QD 1S-exciton unlikely for all sizes investigated here, and transfer of a hot hole or electron remains possible but unlikely given fast competing hole cooling.38 The TIPS-TcCOOH anion spectrum does have overlapping bands with that of the long-lived species assigned to T1 (Figure S18), and slow transfer of an electron to the ligand is a possible interpretation of 20 ACS Paragon Plus Environment

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the TA data. However, the correlated loss of QD bleach during the formation of the long-lived molecular species (Figure 4E) is inconsistent with a remaining hole in the valence band of the PbS QD core and more consistent with exciton transfer, as is the observation of upconverted PL. Clearly, the interplay between the various photophysical pathways in this system with two photoactive components can be complex, but the continued advances toward understanding and control leaves open the possibility to tune the kinetics for a variety of useful applications. Notes The authors declare no competing financial interest. Acknowledgments D.M.K, D.H.A., J.L.B., G.M.C., M.C.B., and J.C.J acknowledge the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences under contract DE-AC36-08GO28308 with NREL. J.E.A and D.B.G thank the National Science Foundation (DMREF-1627428) for support of acene synthesis. We thank Nick Anderson (NREL) for assistance with electrochemical experiments. Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental methods, synthesis and characterization of TIPS-Tc-COOH, absorption of assynthesized samples, FTIR spectra, NMR spectra, PLE spectra, TIPS-Tc redox spectra, and complete TA data sets.

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Graphical Table of Contents

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TOC graphic 254x190mm (96 x 96 DPI)

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