Facilitating Work With Photons Via Photomechanical Crystals and Multi-Exciton Materials
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Facilitating Work With Photons Via Photomechanical Crystals and Multi-Exciton Materials

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Every year, the amount of solar energy reaching the Earth’s surface is approximately 3.4 × 1024 J — nearly 10,000 times the global energy demand. Converting the abundance of solar energy into meaningful work remains a modern challenge in global sustainability. This dissertation focuses on materials that convert photons into two types of work: electrical and (photo)mechanical.Photovoltaics used in solar cells rely on the generation of excitons from incident light and the band of acceptable wavelengths is dependent on the photovoltaic — typically silicon. Photons with energies that are too large or small to be accepted by silicon’s bandgap are either wasted as heat or transmitted/reflected entirely. Performance can be improved by converting the energy of these photons downward (singlet-fission) or upward (triplet-triplet annihilation) towards silicon’s bandgap. Both singlet-fission (SF) and triplet-triplet annihilation (TTA) are spin-allowed, multi-exciton processes that have been extensively studied for the past 60 years, and yet relatively little is known about the correlated triplet pair intermediate. To discover whether a charge-transfer (CT) or excimer state is an intermediate in TTA, solutions of pyrene sensitized with tris-(2-phenyl pyridine) iridium (III) are analyzed. Direct formation of excimers by the annihilating pyrene triplets was not observed, suggesting that TTA only forms a single excited pyrene and there is not a CT intermediate. Analogously, solid SF materials such as anthradithiophene and tetracene are blended with [6]-phenacene, whose large bandgap functions as an inert spacer for local exciton generation. Investigation of the red-shifted luminescence following singlet-fission reveals that excimer formation is a parallel pathway to SF, reinforcing the previous discovery that excimers are not formed directly in either SF or TTA. Photomechanical materials offer the potential to convert the energy of photons into expansion type work through volume changes generated by photoswitching molecules. There is presently no theoretical framework that establishes an upper limit on the efficiency of photomechanical systems in a manner similar to the Shockley-Quiesser limit for silicon photovoltaics. Using a 1-D harmonic oscillator to model a simple photomechanical cycle, this dissertation defines a maximum absorbed photon-to-work efficiency of 55.4%. Although this model neglects non-idealities of real systems, at 1.5 times the Shockley-Quiesser limit photomechanical materials have the potential to deliver a significant amount of work. Additional progress towards realizing this goal with photomechanical actuators is made through the optimization and characterization of porous alumina templates filled with 9-methylanthracene (9MA) nanocrystals. Demonstrating nearly complete conversion of the photoactive material through 9MA’s negative photochromism, these templates bend and deform. This motion is tracked using a Michelson interferometer, revealing the type of actuation in the template. Surface functionalization has a negligible impact on template loading but optimizing the solvent annealing conditions more than doubles the net loading. Reversible, linear actuation via photomechanical materials has yet to be realized, but advances made in this dissertation provide the basis for achieving this goal.

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