Title

Photon Upconversion Based on Triplet-Triplet Annihilation

Date of Award

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Felix Castellano

Second Advisor

H. Lu (Committee Member)

Third Advisor

Alexander Tarnovsky (Committee Member)

Fourth Advisor

John Laird

Abstract

Upconversion (UC), a process that can convert two or more photons with low energy to a single photon of higher energy, has the potential for overcoming the thermodynamic efficiency limits of sunlight-powered devices and processes, including photovoltaic cells, photocatalytic reactions, and photoelectrosynthetic processes. The UC phenomenon is traditionally synonymous with second-harmonic generation using nonlinear inorganic crystals and the sequential excitation of inorganic host materials doped with rare-earth metals, but also includes sensitized UC relying on the fusion of two energized triplet molecules. Sensitized UC, exhibits several distinctive features making it directly amenable for sunlight powering. Sensitized UC can be accomplished at much higher efficiencies with non-coherent continuous-wave excitation at extremely low power densities (< 1 mW cm-2). In this process, the photon energy absorbed by a sensitizer is transferred to an acceptor through triplet-triplet energy transfer, and two excited acceptor molecules undergo annihilation, producing (upconverted) singlet fluorescence. The first part of my research is kinetic study of triplet-triplet annihilation (TTA) process. The delayed fluorescence kinetic analysis reported by Schmidt and coworkers was used to reveal the maximum possible efficiency from a model red-to-yellow upconverting composition and this treatment was applied to the anthryl triplet absorption decay transients of triplet anthracene measured for the PdOEP/anthracene composition. The resulting parameters were obtained by the fits for various excitation pulse power. From this analysis, approximately 50% of the anthryl triplets that decay by TTA produce singlet fluorescence, consistent with the notion that spin statistics does not impose efficiency limits on upconversion photochemistry. The second part of my research is investigating unconversion compositions for TTA photon upconversion. Upconversion system using the red-light absorbing platinum(II) tetraphenyltetranaphthylporphyrin (PtTPTNP) as a triplet sensitizer with yellow emitter rubrene and PDI as the acceptor in both solution and polymer films was investigated. In deaerated toluene solution, stable and high quantum efficiency (6%) was observed. Apart from that, another Cd (II) texaphyrin (TXP) was used as a new NIR upconversion senstizer with rubrene as the acceptor to upconvert NIR (670-88 nm) incident photons into yellow fluorescence through sensitized triplet-triplet annihilation. The quadratic-to-linear power dependence of the upconverted rubrene fluorescence with respect to 750 nm incident laser power density was observed to conform to two extreme kinetic limits (weak and strong annihilation regimes). In deaerated DCM solution, stable quantum efficiency (1.6%) was observed. The third part of my research is to investigate more host materials for UC apart from organic solvent. Moldable polyurethane materials (Clearflex 50) prepared from their corresponding liquid precursors produce high efficiency, regenerative green-to-blue photochemical upconverting solids when impregnated with benchmark palladium(II) octaethylporphyrin (PdOEP) sensitizers and 9,10-diphenylanthracene (DPA) acceptor/annihilator molecules. The cured rubbery materials promote sufficient diffusion at room temperature to support the sequential bimolecular reactions necessary for both triplet sensitization and triplet-triplet annihilation occurring between the precisely doped chromophores. Similar to photochemical upconversion processes occurring in solution, the newly conceived rubbery upconverting polymers are shown to exhibit quadratic-to-linear incident light power dependence for the first time. Once the linear power regime is achieved, the highest possible quantum efficiencies for photochemical upconversion are realized. Quantum yields of upconversion in current polymer samples measured in...