UC Riverside

The sequential absorption of two low energy photons, followed by the emission of one higher energy photon, is known as photon upconversion. This two-photon process has potential applications in biological imaging, solar energy conversion and photocatalysis etc. Two existing strategies to realize upconversion are an inorganic system based on lanthanide doping, and an organic system based on triplet-triplet annihilation. This dissertation introduces a novel hybrid molecule-nanocrystal system that upconverts photons across visible to near-infrared. It combines the advantages of high photostability and low excitation intensity, and overcomes the limits in the previous systems. Here, the absorption of low energy photons by the semiconductor nanocrystals (NCs) is followed by the energy transfer to molecular triplet states. Triplet-triplet annihilation then occurs to create high energy singlet states that emit the upconverted light. I show the introduction of functionalized acene molecules on NC surfaces greatly enhances the upconversion quantum yields (QYs) by up to three orders of magnitidue in different systems. As one of the efficiency limiting steps in upconversion, TET is systematically studied by modifying both NC donor and molecular acceptor. TET is characterized by steady state upconversion, time-resolved transient absorption and photoluminescence spectroscopies. TET is dependent on the anchoring groups of the transmitter molecules due to different binding affinities, docking geometries, sterics, intra-molecular spin-orbit couplings and molecule-NC triplet-triplet couplings. Transmitters with phosphonic acid, carboxylic acid and imidazole as binding groups show the best upconversion performance. For the NC donors, small sizes lead to high upconversion QYs due to the large driving force. While the growth of inorganic shells passivates surface traps, TET from core-shell NCs to transmitters is dependent on the shell composition and thickness. TET is improved when submonolayer shells suppress charge transfer and decrease exciton-phonon coupling.