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Triplet Energy Transfer at Hybrid Organic-Lanthanide Nanoparticle Interfaces

Periodic Reporting for period 1 - TET-Lanthanide (Triplet Energy Transfer at Hybrid Organic-Lanthanide Nanoparticle Interfaces)

Reporting period: 2018-03-01 to 2020-02-29

The generation, control and transfer of triplet excitons in molecular and hybrid systems is a hot topic in a variety of discipline, ranging from physics and chemistry to materials science and biology. This interest is driven by a range of triplet exciton-based applications, including light emission, photon upconversion, photocatalysis, sensing and photodynamic therapy. So far, molecular triplet excitons are manipulated through heavy-metal based spin-orbit coupling or control of the singlet-triplet energy splitting. Both approaches mainly tackle the luminescent harvesting of triplets and place strict constraints on the design of a molecular system. To overcome this limitation, it has recently been demonstrated to couple molecular triplets to semiconducting quantum dots and transfer energy between them. However, this approach is also limited only to energy transfer of triplets and the challenge of more broadly controlling the properties of triplet excitons remains unanswered.

This project aims to develop a new platform, based on coupling lanthanide nanocrystals with molecular triplet excitons, to control molecular triplet dynamics for optoelectronics and photochemistry. In this project, we couple molecular triplet excitons to lanthanide-doped nanocrystals. The coupled system allows for the direct generation of molecular triplet excitons with near-infrared excitation and luminescent harvesting of the dark triplet excitons via transfer to lanthanide nanocrystals. Furthermore, we explore and investigate the fundamental science of the lanthanide nanoparticle-molecule coupled system. The coupled systems enable to overcome many of the limitations of using lanthanide nanocrystals or molecular triplet excitons individually, and open up new possibilities for optoelectronics, molecular sensing, upconversion, photocatalysis and bio-imaging.
In this project, we developed a new lanthanide nanoparticle-molecule hybrid material system for luminescence harvesting of molecular triplets. The work performed within the entire period of project is as follows:

Sample preparation: Lanthanide-doped nanoparticles were synthesized by co-precipitation method. Ligand exchange reaction was performed to prepare solution-phase molecule-coupled nanoparticles. For preparation of solid-state film, the oleate ligand on the particle surface was first removed by acid-treatment method and then the resulting ligand-free lanthanide nanoparticles were mixed with molecule. The mixture was eventually drop-casted onto a substrate to form a blended film.

Steady-state optical characterization: Samples were characterized by conventional steady-state techniques including absorption and photoluminescence spectroscopy. Sensitive absorption measurements of solid-state film were performed using photothermal deflection spectroscopy (PDS) to rule out scattering effect. PDS can allow for absorption measurement down to 0.00002OD. Magnetic field-dependent absorption and photoluminescence measurements was performed to study the effect of magnetic fields on the coupling between the triplets and the lanthanides. This can reveal important information about nature of the coupling and triplet transfer mechanism.

Transient optical characterization: Samples were measured by transient luminescence and transient absorption spectroscopy. The lifetime measurements of the lanthanide emission and organic molecules were performed by time-resolved photoluminescence spectroscopy from ps-ms timescales, using a combination of time-correlated single photon counting (TSCPC), intensified CCD (iCDD) and Multi Channel Scaling (MCS) methods. The dynamics of triplet transfer from lanthanide nanoparticles to the molecules under 980nm excitation was monitored by transient absorption spectroscopy. These methods provided quantitative measurements of triplet transfer efficiency. the transient spectroscopies were also performed under magnetic fields to study the effect of the coupling and hence learn more about the underlying mechanism.

In summary, we have developed a novel approach to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic nanoparticles. This coupling can be achieved without any change to the molecular structure of the organic molecules and enables an unprecedented level of control over triplet dynamics. Specifically, we show that,

(1) The molecular triplets can be directly generated on the molecules via photon absorption by coupling to lanthanide-doped nanoparticles.

(2) Singlet excitons on molecules can rapidly (sub 10 ps) and efficiently (>99%) be converted to triplet excitons by coupling to lanthanide nanoparticles. These rates are ~2000 times faster than conventional intersystem crossing (ISC) and thus outcompete any decay pathways, leading to high efficiency.

(3) The efficient (>99%) transfer of triplet energy from molecules to the lanthanide ions and subsequent efficient luminescent harvesting of triplets was achieved.

(4) The excited states generated on the lanthanide ions can be transferred to the molecular triplet level with no energy loss penalty, which overcomes the inherent limitation associated with the energy losses in triplet sensitization via ISC in conventional systems. Even in this first un-optimized demonstration, we achieve NIR to visible upconversion quantum efficiencies up to 16% in the solid state (vs a maximum theoretical value of 50%).
These results establish the fundamental science for a new methodology to generate, control and harvest triplet excitons, one that offers far more opportunities than heavy-metal based spin-orbit coupling, the control of singlet-triplet energy splitting or triplet energy transfer with quantum dots, which are the dominant paradigms today. This will open up new avenues for triplet sensitization, photocatalysis, optoelectronics, biomedicine therapeutics, sensing, and photon frequency conversion driven by optically bright triplet excitons. We thus believe that our results will be of great interest to a broad range of researchers across chemistry, physics, materials science and biology.

The project provides important results that will impact a highly competitive research field. The European scientific and wider community will benefit from this work since it has the potential to deliver an entirely new platform for optoelectronics and photochemistry.
triplet transfer