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Direct observation of triplet energy transfer from semiconductor nanocrystals

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Science  22 Jan 2016:
Vol. 351, Issue 6271, pp. 369-372
DOI: 10.1126/science.aad6378

A different way to put triplets in play

Most molecules adopt a singlet spin configuration: All their electrons are arranged in pairs. Unpaired triplet states engage in a variety of useful reactions but are hard to produce. Quantum mechanics dictates that photo-excitation from singlet to triplet states is inefficient. Instead, chemists rely on sensitizers, which populate the triplet states of their neighbors through energy transfer after absorbing light themselves. Mongin et al. now show that certain nanoparticles can act as triplet sensitizers.

Science, this issue p. 369

Abstract

Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.

Semiconductor nanocrystals represent an important class of stable light-emitting materials that can be systematically tuned as a result of size-dependent quantum confinement, producing intense absorptions and photoluminescence ranging from the ultraviolet (UV) to the near-infrared (near-IR) (1, 2). Their prominence continues to expand, owing to extensive optoelectronic, photochemical, and biomedical applications (39). Substantial research effort has been expended on funneling energy into these nanomaterials to produce enhanced photoluminescence via Förster transfer and on exploiting the energized semiconductor nanocrystals to deliver or accept electrons from substrates (1014), sometimes en route to solar fuels photosynthesis (1518). Tabachnyk et al. and Thompson et al. independently demonstrated the reverse triplet energy transfer process to that described here, wherein molecular organic semiconductors transfer their triplet energy to PbSe and PbS nanocrystals in thin films that interface both materials (19, 20). However, the extraction of triplet excitons from semiconductor quantum dots and related inorganic nanomaterials remains largely unexplored. Semiconductor nanocrystals potentially offer considerable advantages over molecular photosensitizers in terms of facile preparative synthesis, photostability, size-tunable electronic and photophysical properties, high molar extinction coefficients, and trivial postsynthesis functionalization. Moreover, the inherently large (and energy-consuming) singlet-triplet energy gaps characteristic of molecular sensitizers can be circumvented by using nanomaterials with ill-defined spin quantum numbers and closely spaced (1 to 15 meV) excited-state energy levels (2124). The broadband light-absorption properties of inorganic semiconductors are extendable into the near-IR region and can potentially be exploited for numerous triplet excited-state reactions, thus enabling stereoselective photochemical synthesis, photoredox catalysis, singlet oxygen generation, photochemical upconversion, and excited-state electron transfer. Here we provide definitive experimental evidence that triplet energy transfer proceeds rapidly and efficiently from energized semiconductor nanocrystals to surface-anchored molecular acceptors. Specifically, CdSe nanocrystals are shown to serve as effective surrogates for molecular triplet sensitizers and can readily transfer their triplet excitons to organic acceptors at the interface with near-quantitative efficiency.

The nanoparticle-to-solution triplet exciton transfer strategy that we implemented is shown schematically in Fig. 1; this diagram depicts all of the relevant photophysical processes and the associated energy levels promoting material-to-molecule triplet exciton migration. We employed oleic acid (OA)–capped CdSe nanocrystals (CdSe-OA) as the light-absorbing triplet sensitizer in conjunction with 9-anthracenecarboxylic acid (ACA) and 1-pyrenecarboxylic acid (PCA) as triplet acceptors in toluene. The carboxylic acid functionality enables adsorption of these chromophores on the CdSe surface through displacement of the OA capping ligands; subsequent washing steps isolate the desired CdSe/ACA or CdSe/PCA donor/acceptor systems. Selective green light excitation of CdSe/ACA or CdSe/PCA sensitizes triplet exciton migration from the semiconductor to the surface-bound molecular acceptor. We directly visualized this interfacial Dexter-like triplet-triplet energy transfer (TTET) by monitoring the kinetic growth of the characteristic triplet-to-triplet (T1 → Tn) absorptions in ACA and PCA (wavelength of maximum absorbance = 430 nm) using ultrafast transient absorption (TA) spectroscopy (25, 26). The long-lived ACA and PCA localized triplets furthermore enabled exothermic triplet energy transfer to freely diffusing 2-chlorobisphenylethynylanthracene (CBPEA) and dioxygen.

Fig. 1 Illustration of nanocrystal-to-solution triplet energy transfer, the associated energy levels, and the various TTET and decay pathways investigated in this study.

PDT, photodynamic therapy.

CdSe-OA suspended in toluene was prepared as described in the supplementary materials. The first exciton band in these samples was located at 505 nm (2.46 eV; molar extinction coefficient at 505 nm, 59,200 M−1 cm−1); using an established empirical equation (27), the average diameter of these nanoparticles was estimated to be 2.4 nm, in good agreement with transmission electron microscopy results (fig. S1). The CdSe-OA photoluminescence features, including the spectrally narrow band-gap “bright state” and the lower-energy “trap state” emissions, are shown in Fig. 2A (28). Triplet excitons derived from these excited states (2.40 eV and 2.30 to 1.40 eV, respectively) are suitable for exothermic TTET to ACA (lowest triplet state energy, ET = 1.83 eV) and PCA (ET = 2.00 eV) (25, 29). We observed no evidence of triplet energy transfer from the CdSe-OA nanoparticles to anthracene (ET = 1.85 eV), 9,10-diphenylanthracene (ET = 1.77 eV), or pyrene (ET = 2.10 eV) acceptors (29), despite thermodynamic expectations that such transfers should be exothermic. Unlike ACA and PCA, these acceptors lack the carboxylate functionality to coordinate directly to the nanoparticle surface. In these cases, the putative bimolecular transfer rate through the intervening OA layer appears to be slower than the excited-state lifetime of the nanoparticle.

Fig. 2 Ultrafast spectroscopic evidence for triplet energy transfer from optically excited CdSe nanocrystals to surface-bound ACA.

(A) Normalized electronic absorption (solid lines) and emission spectra (dashed lines) of ACA (blue) and CdSe nanocrystals (green) in toluene (OD, optical density). (B and C) Ultrafast TA difference spectra of CdSe-OA nanocrystals in toluene solution upon selective excitation of CdSe, using 500-nm pulsed laser excitation [0.05 μJ per pulse, 100 fs full width at half maximum (FWHM)], in (B) the absence and (C) the presence of surface-anchored ACA in toluene (ΔA, change in absorbance; 〈k〉, average rate constant). The inset in (C) shows TA kinetics monitored for the growth of 3ACA at 441 nm. (D) Ground-state recovery of CdSe monitored by kinetics at 490 nm, illustrating quantitative quenching of CdSe in the presence of surface-anchored ACA. Complementary data for PCA are shown in figs. S2 to S4.

Photoluminescence from CdSe-OA was quantitatively quenched by ACA and PCA (fig. S5). The nanocrystals bearing surface-anchored molecular acceptors were purified by successive precipitation and centrifugation cycles, after which the final ratio of acceptor to CdSe was determined to be ~12:1 (fig. S6). Ultrafast TA experiments were performed on the quantitatively quenched CdSe-OA/ACA materials to establish the mechanism and time scale for the semiconductor-to-molecule triplet exciton transfer process. Control experiments with CdSe-OA in toluene were also performed in the absence of ACA (Fig. 2B). In all cases, symmetric decay of the transient signal was observed over the first few picoseconds, which is consistent with multi-exciton annihilation within the nanocrystals (30), and was confirmed by laser power dependence experiments (fig. S7). With surface-anchored ACA (Fig. 2C), decay of the CdSe excited state was observed within 2 ns, coinciding with the growth of an absorption band centered at 433 nm, which was assigned to the T1 ⟶ Tn transitions of ACA (25). These results confirm direct TTET (no intermediates) from the selectively excited CdSe nanocrystals to the surface-anchored ACA chromophores. The possibility of an electron transfer mechanism was eliminated, based on the absence of the ACA radical cation band that would be expected near 750 nm (figs. S8 to 11A) (31). Singlet-singlet energy transfer is not thermodynamically favorable, and no transient signals corresponding to singlet ACA (1ACA*) were observed. Transient kinetics were monitored at 441 nm, where an isosbestic point was present in the CdSe-OA control difference spectra (Fig. 2B). In the presence of ACA (Fig. 2C), this isosbestic point shifts because of the overlapping ACA T1 → Tn absorption band (fig. S12); thus, any new absorption feature observed at 441 nm (Fig. 2C, inset) can be attributed to triplet ACA (3ACA*). The quantum efficiency for TTET from CdSe-OA to surface-anchored ACA was determined to be 0.92 from TA experiments (fig. S12B). Therefore, most of the excited states produced within CdSe can be extracted as long-lived molecular triplets that are suitable for subsequent chemical reactions. Variable rates of energy transfer were anticipated, depending on the initially populated CdSe excited states. Accordingly, following a published precedent (11, 32), a stretched exponential function was determined to best model the interfacial TTET dynamics (eqs. S1 to S3). Kinetic analysis at 441 nm revealed a rise time associated with the 3ACA* absorption with an average rate constant of 2.2 × 109 s−1. Additionally, the rise of 3ACA* was correlated to the ground-state recovery of CdSe (TA probe wavelength: 480, 490, and 510 nm) (Fig. 2D and fig. S13). Rate constants between 2.0 × 109 and 2.8 × 109 s−1 (table S1) were measured, in good agreement with the formation rate of 3ACA*. TTET also occurred, with similar efficiencies but slower kinetics, when PCA was used as the surface-bound molecular acceptor (figs. S3 to S5). This is consistent with the lower TTET driving force associated with that process.

The T1 → Tn absorption centered near 430 nm that was observed in the ultrafast TA experiments (Fig. 2C), which is characteristic of 3ACA*, appeared as a prompt signal in nanosecond flash photolysis (Fig. 3A). Complementary data for PCA show similar results and are presented in Fig. 3B. The transient signals of the acceptor T1 → Tn decay to the ground state over the next several milliseconds, illustrating that excitons can indeed be harvested from these semiconductor nanomaterials, resulting in an excited-state lifetime enhancement of six orders of magnitude.

Fig. 3 Kinetic profiles and quenching studies of ACA and PCA triplet states populated from excited CdSe nanocrystals.

TA difference spectra of a toluene solution of (A) CdSe/ACA (8 μM) measured from 2 μs to 5 ms and (B) CdSe/PCA (8 μM) measured from 2 μs to 10 ms after a 505-nm laser pulse (1 mJ, 5 to 7 ns FWHM; 〈τ〉, average lifetime). The insets show TA decay kinetics at 430 nm (gray squares) and their respective fits to eq. S1, illustrating the triplet decay. (C and D) TA difference spectra (excitation wavelength, 505 nm; 5 to 7 ns FWHM; 1 mJ) measured at selected delay times after the laser pulse in (C) CdSe/ACA (5 μM) and CBPEA (6 μM) and (D) CdSe/PCA (5 μM) and CBPEA (6 μM) in deaearated toluene at room temperature. The insets show TA decay kinetics at 430 nm (red circles) and the rise and decay at 490 nm (blue squares), with their respective biexponential fit lines (solid and dashed), illustrating the triplet energy transfer reaction between 3ACA* and CBPEA.

The CdSe/ACA and CdSe/PCA materials also sensitized the production of 1O2* when the solutions were aerated, evidenced by its characteristic photoluminescence centered at 1277 nm in the near-IR region (fig. S14). This is a unique example of 1O2* sensitization through metal chalcogenide nanocrystals enabled by a mechanism distinct from that of Förster transfer (3335). To further illustrate that excitons could be transferred away from the CdSe interface into the bulk solution, a second molecular triplet acceptor (CBPEA) was added to the CdSe/acceptor solution. CBPEA possesses a low-lying triplet state (ET < 1.61 eV), facilitating exothermic TTET from energized acceptor chromophores, and is readily discernible by its distinct T1 → Tn excited-state absorption, centered at 490 nm (36). Figure 3C presents the TA difference spectra measured at 3 μs and 200 μs after 505-nm pulsed nanosecond laser excitation of the CdSe/ACA solution in the presence of 6 μM CBPEA (Fig. 3D shows the complementary data for PCA as the acceptor). At early delay times, the 3ACA* absorption spectrum dominates (Fig. 3C, blue line), eventually giving way quantitatively (quantum efficiency for TTET ∼ 1.0) to a spectrum characteristic of triplet CBPEA (3CBPEA*) at longer delay times (Fig. 3C, red line). By 3 ms after the laser pulse, the sample completely relaxed to the ground state (Fig. 3C, green line). These data demonstrate that the formation of 3ACA* on the CdSe surface is followed by diffusion-controlled triplet-triplet energy transfer to CBPEA (fig. S15; rate constant for energy transfer, 1.2 × 109 M−1 s−1), effectively transferring the triplet exciton into the bulk solution. The dynamics of this energy migration were captured in the single-wavelength absorption transients measured at 430 and 490 nm after the 505-nm excitation pulse (Fig. 3C, inset). The promptly formed transient absorption signal at 430 nm from 3ACA* (red squares) exhibits an excited-state decay that is kinetically correlated with the formation of 3CBPEA* (blue circles) at 490 nm. These experiments illustrate that bimolecular excited-state chemistry readily proceeds from the CdSe/acceptor materials, exhibiting behavior characteristic of a donor/acceptor TTET molecular system.

The results presented here provide proof-of-concept that excitons can be extracted from this particular semiconductor through direct TTET, but it stands to reason that this general strategy is probably also applicable to a plethora of associated materials. In this regard, molecular triplet-triplet annihilation processes can be sensitized by energized semiconductor nanocrystals (23, 37, 38). Although this investigation specifically targeted mechanistic insights into the TTET process along with chemically relevant triplet exciton decay pathways, we expect related photochemistry to promote similar triplet energy transfer phenomena in solid-state optoelectronic devices.

Supplementary Materials

www.sciencemag.org/content/351/6271/369/suppl/DC1

Materials and Methods

Figs. S1 to S15

Table S1

References (3941)

References and Notes

Acknowledgments: This work was supported by the Air Force Office of Scientific Research (grant FA9550-13-1-0106) and the Ultrafast Initiative of the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under contract no. DE-AC02-06CH11357. M.Z. was supported by NSF (grant CHE-1465052).
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