Photon Upconversion at Organic-Inorganic Interfaces.

Photon upconversion is a process that combines low-energy photons to form useful high-energy photons. There are potential applications in photovoltaics, photocatalysis, biological imaging, etc. Semiconductor quantum dots (QDs) are promising for the absorption of these low-energy photons due to the high extinction coefficient of QDs, especially in the near infrared (NIR). This allows the intriguing use of diffuse light sources such as solar irradiation. In this review, we describe the development of this organic-QD upconversion platform based on triplet-triplet annihilation, focusing on the dark exciton in QDs with triplet character. Then we introduce the underlying energy transfer steps, starting from QD triplet photosensitization, triplet exciton transport, triplet-triplet annihilation, and ending with the upconverted emission. Design principles to improve the total upconversion efficiency are presented. We end with limitations in current reports and proposed future directions. This review provides a guide for designing efficient organic-QD upconversion platforms for future applications, including overcoming the Shockley-Queisser limit for more efficient solar energy conversion, NIR-based phototherapy, and diagnostics in vivo.

Tuning the Chiral Growth of Plasmonic Bipyramids via the Wavelength and Polarization of Light.

Circularly polarized light (CPL) is a versatile tool to prepare chiral nanostructures, but the mechanism for inducing enantioselectivity is not well understood. This work shows that the energy and polarization of visible photons can initiate photodeposition at different sites on plasmonic nanocrystals. Here, CPL on achiral gold bipyramids (AuBPs) creates hot holes that oxidatively deposit PbO2 asymmetrically. We show for the first time that the location of PbO2 photodeposition and hence optical dissymmetry depends on the CPL wavelength. Specifically, 488 and 532 nm CPL induce PbO2 growth in the middle of AuBPs, whereas 660 nm CPL induces PbO2 growth at the tips. Our observations show that wavelength-dependent plasmonic field distributions are more important than surface lightning rod effects in localizing plasmon-mediated photochemistry. The largest optical dissymmetry occurs at excitation wavelengths between the transverse and longitudinal resonances of the AuBPs because higher-order modes are required to induce chiral electric fields.

Efficient photon upconversion enabled by strong coupling between silicon quantum dots and anthracene.

Hybrid structures formed between organic molecules and inorganic quantum dots can accomplish unique photophysical transformations by taking advantage of their disparate properties. The electronic coupling between these materials is typically weak, leading photoexcited charge carriers to spatially localize to the dot or to a molecule at its surface. However, we show that by converting a chemical linker that covalently binds anthracene molecules to silicon quantum dots from a carbon-carbon single bond to a double bond, we access a strong coupling regime where excited carriers spatially delocalize across both anthracene and silicon. By pushing the system to delocalize, we design a photon upconversion system with a higher efficiency (17.2%) and lower threshold intensity (0.5 W cm-2) than that of a corresponding weakly coupled system. Our results show that strong coupling between molecules and nanostructures achieved through targeted linking chemistry provides a complementary route for tailoring properties in materials for light-driven applications.

High-Level Reverse Intersystem Crossing and Molecular Rigidity Improve Spin Statistics for Triplet-Triplet Annihilation Upconversion.

The structural factors affecting triplet-triplet annihilation (TTA) at the molecular level are not well-understood. Here, our steady-state photoluminescence and transient absorption results demonstrate that the spin statistical factor, η, decreases from 0.60 to 0.46 and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and 2,6-DPA isomers, respectively, during photon upconversion with a platinum octaethylporphyrin sensitizer. Density functional theory (DFT) shows that η depends on the energetics of hot triplet states and molecular rigidity. The significantly high conical intersection energy between the S0 and T1 states for 9,10-DPA gives its longer triplet lifetime. Time-dependent DFT calculations show that 9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossing from their T2 and T3 states, respectively, to the bright S1 state, increasing the limit of the spin statistical factor. Both factors ultimately serve to enhance the TTA efficiency. This work provides insight into designing molecules for efficient light-emitting and photon upconversion applications.

Gas-phase grafting for the multifunctional surface modification of silicon quantum dots.

Photon upconversion in systems incorporating inorganic quantum dots (QDs) is of great interest for applications in solar energy conversion, bioimaging, and photodynamic therapy. Achieving high up-conversion efficiency requires not only high-quality inorganic nanoparticles, but also precise control of their surface functional groups. Gas-phase surface functionalization provides a new pathway towards controlling the surface of small inorganic nanoparticles. In this contribution, we utilize a one-step low-temperature plasma technique for the synthesis and in-flight partial functionalization of silicon QDs with alkyl chains. The partially functionalized surface is then modified further with 9-vinylanthracene via thermal hydrosilylation resulting in the grafting of 9-ethylanthracene (9EA) groups. We have found that the minimum alkyl ligand density necessary for quantum dot solubility also gives the maximum upconversion quantum yield, reaching 17% for silicon QDs with Si-dodecyl chains and an average of 3 9EA molecules per particle.

Quantifying the Ligand-Induced Triplet Energy Transfer Barrier in a Quantum Dot-Based Upconversion System.

During photon upconversion, quantum dots (QDs) transfer energy to molecules in solution through a long ligand shell. This insulating ligand shell imparts colloidal stability at the expense of efficient photosensitization. For the first time, we quantify the barrier these aliphatic ligands pose for triplet energy transfer in solution. Using transient absorption spectroscopy, we experimentally measure a small damping coefficient of 0.027 Å-1 for a ligand exceeding 10 carbons in length. The dynamic nature of ligands in solution lowers the barrier to charge or energy transfer compared to organic thin films. In addition, we show that surface ligands shorter than 8 carbons in length allow direct energy transfer from the QD, bypassing the need for a transmitter ligand to mediate energy transfer, leading to a 6.9% upconversion quantum yield compared with 0.01% for ligands with 18 carbons. This experimentally derived insight will enable the design of efficient QD-based photosensitizers for catalysis and energy conversion.

Lanthanide-doped inorganic nanoparticles turn molecular triplet excitons bright.

The generation, control and transfer of triplet excitons in molecular and hybrid systems is of great interest owing to their long lifetime and diffusion length in both solid-state and solution phase systems, and to their applications in light emission1, optoelectronics2,3, photon frequency conversion4,5 and photocatalysis6,7. Molecular triplet excitons (bound electron-hole pairs) are 'dark states' because of the forbidden nature of the direct optical transition between the spin-zero ground state and the spin-one triplet levels8. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin-orbit coupling9-11 or tuning of the singlet-triplet energy splitting12,13 via molecular design. Both these methods place constraints on the range of properties that can be modified and the molecular structures that can be used. Here we demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles. This allows the classically forbidden transitions from the ground-state singlet to excited-state triplets to gain oscillator strength, enabling triplets to be directly generated on molecules via photon absorption. Photogenerated singlet excitons can be converted to triplet excitons on sub-10-picosecond timescales with unity efficiency by intersystem crossing. Triplet exciton states of the molecules can undergo energy transfer to the lanthanide ions with unity efficiency, which allows us to achieve luminescent harvesting of the dark triplet excitons. Furthermore, we demonstrate that the triplet excitons generated in the lanthanide nanoparticle-molecule hybrid systems by near-infrared photoexcitation can undergo efficient upconversion via a lanthanide-triplet excitation fusion process: this process enables endothermic upconversion and allows efficient upconversion from near-infrared to visible frequencies in the solid state. These results provide a new way to control triplet excitons, which is essential for many fields of optoelectronic and biomedical research.

Evolution from Tunneling to Hopping Mediated Triplet Energy Transfer from Quantum Dots to Molecules.

Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.

On the size-dependence of CdSe nanocrystals for photon upconversion with anthracene.

In triplet-triplet annihilation based photon upconversion, controlling triplet energy transfer (TET) through the system is key to unlocking higher efficiencies. In this work, we vary the size of colloidally synthesized CdSe nanocrystals (NCs) to examine the effects on TET during photon upconversion, using steady-state measurements and transient absorption spectroscopy. As the CdSe NC size increases, the photon upconversion quantum yield (QY) decreases due to the decrease in the rate of TET from CdSe to the surface bound anthracene transmitter ligand, as expected for the Marcus description of energy transfer from the transmitter to the NC. Long microsecond transmitter lifetimes are critical to high photon upconversion QYs.

Spin-coated fluorinated PbS QD superlattice thin film with high hole mobility.

Motivated by the oleophobic and electron-withdrawing nature of perfluorocarbons, we explore the effect of a trifluoromethyl coating on lead sulfide quantum dots (PbS QDs) in thin film transistor (TFT) geometry. The low surface energy conferred by the oleophobic perfluorocarbons creates QDs packed in a primitive cubic lattice with long range order, as confirmed by grazing incidence small angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM). Hole mobilities as high as 0.085 cm2 V-1 s-1 were measured in the TFTs. No electron transport was observed. This suggests that the electron-withdrawing nature of the trifluoromethyl ligand is eclipsed by the excess holes present in the PbS QDs that likely stem from cation vacancies induced by the thiol group.

Achieving spin-triplet exciton transfer between silicon and molecular acceptors for photon upconversion.

Inorganic semiconductor nanocrystals interfaced with spin-triplet exciton-accepting organic molecules have emerged as promising materials for converting incoherent long-wavelength light into the visible range. However, these materials to date have made exclusive use of nanocrystals containing toxic elements, precluding their use in biological or environmentally sensitive applications. Here, we address this challenge by chemically functionalizing non-toxic silicon nanocrystals with triplet-accepting anthracene ligands. Photoexciting these structures drives spin-triplet exciton transfer from silicon to anthracene through a single 15 ns Dexter energy transfer step with a nearly 50% yield. When paired with 9,10-diphenylanthracene emitters, these particles readily upconvert 488-640 nm photons to 425 nm violet light with efficiencies as high as 7 ± 0.9% and can be readily incorporated into aqueous micelles for biological use. Our demonstration of spin-triplet exciton transfer from silicon to molecular triplet acceptors can critically enable new technologies for solar energy conversion, quantum information and near-infrared driven photocatalysis.

Surface States Mediate Triplet Energy Transfer in Nanocrystal-Acene Composite Systems.

Hybrid organic:inorganic materials composed of semiconductor nanocrystals functionalized with acene ligands have recently emerged as a promising platform for photon upconversion. Infrared light absorbed by a nanocrystal excites charge carriers that can pass to surface-bound acenes, forming triplet excitons capable of fusing to produce visible radiation. To fully realize this scheme, energy transfer between nanocrystals and acenes must occur with high efficiency, yet the mechanism of this process remains poorly understood. To improve our knowledge of the fundamental steps involved in nanoparticle:acene energy transfer, we used ultrafast transient absorption to investigate excited electronic dynamics of PbS nanocrystals chemically functionalized with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) ligands. We find photoexcitation of PbS does not lead to direct triplet energy transfer to surface-bound TIPS-pentacene molecules but rather to the formation of an intermediate state within 40 ps. This intermediate persists for ∼100 ns before evolving to produce TIPS-pentacene triplet excitons. Analysis of transient absorption lineshapes suggests this intermediate corresponds to charge carriers localized at the PbS nanocrystal surface. This hypothesis is supported by constrained DFT calculations that find a large number of spin-triplet states at PbS NC surfaces. Though some of these states can facilitate triplet transfer, others serve as traps that hinder it. Our results highlight that nanocrystal surfaces play an active role in mediating energy transfer to bound acene ligands and must be considered when optimizing composite NC-based materials for photon upconversion, photocatalysis, and other optoelectronic applications.

PbS/CdS Core-Shell Quantum Dots Suppress Charge Transfer and Enhance Triplet Transfer.

A sub-monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor, 5-CT. While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the 5-CT triplet (with rates of 5.91±0.60 ns-1 and 1.03±0.09 ns-1 respectively), ultrafast hole transfer occurs only from PbS QDs to 5-CT. Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the 5-CT triplet and thus enhances TET from 5-CT to the rubrene emitter, further bolstering the upconverison QY.

Complementary Lock-and-Key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer.

Owing to the difficulty in comprehensively characterizing nanocrystal (NC) surfaces, clear guidance for ligand design is lacking. In this work, a series of bidentate bis(pyridine) anthracene isomers (2,3-PyAn, 3,3-PyAn, 2,2-PyAn) that differ in their binding geometries were designed to find the best complementary fit to the NC surface. The efficiency of triplet energy transfer (TET) from the CdSe NC donor to a diphenylanthracene (DPA) acceptor mediated by these isomers was used as a proxy for the efficacy of orbital overlap and therefore ligand binding. 2,3-PyAn, with an intramolecular N-N distance of 8.2 Å, provided the best match to the surface of CdSe NCs. When serving as a transmitter for photon upconversion, 2,3-PyAn yielded the highest upconversion quantum yield (QY) of 12.1±1.3 %, followed by 3,3-PyAn and 2,2-PyAn. The TET quantum efficiencies determined by ultrafast transient absorption measurements showed the same trend.

Triplet transport in thin films: fundamentals and applications.

Triplet excitons are key players in multi-excitonic processes like singlet fission and triplet-triplet annihilation based photon upconversion, which may be useful in next-generation photovoltaic devices, photocatalysis and bioimaging. Here, we present an overview of experimental and theoretical work on triplet energy transfer, with a focus on triplet transport in thin films. We start with the theory describing Dexter-mediated triplet energy transfer and the fundamental parameters controlling this process. Then we summarize current experimental methods used to measure the triplet exciton diffusion length. Finally, the use of hierarchically ordered structures to improve the triplet diffusion length is presented, before concluding with an outlook on the remaining challenges.

On the efficacy of anthracene isomers for triplet transmission from CdSe nanocrystals.

The effect of isomeric substitutions on the transmitter for triplet energy transfer (TET) between nanocrystal (NC) donor and molecular acceptor is investigated. Each isomeric acceptor is expected to bind in a unique orientation with respect to the NC donor. We see that this orbital overlap drastically affects the transmission of triplets. Here, two functional groups, the carboxylic acid and dithiocarbamate, were varied between the 1-, 2- and 9-positions of the anthracene ring to give three ACA and three ADTC isomers. These six anthracene isomers served as transmitters for triplets between CdSe NC sensitizers and 9,10-diphenylanthracene annihilators for photon upconversion. The photon upconversion quantum yield (QY) is the highest for 9-ACA (12%), lowest for 9-ADTC (0.1%), around 3% for both 1-ACA and 1-ADTC, and about 1% for the 2-isomers. These trends in QYs are reflected in the rates of TET given by ultrafast transient absorption spectroscopy where a maximum of 3.8 × 107 s-1 for 9-ACA was measured. Molecular excited state energy levels were measured both in solution and polymer hosts to correlate structure to TET. This work confirms that anthracene excited states levels are very sensitive to molecular substitution, which in combination with orbital overlap, critically affect Dexter-based TET.