The role of spin in triplet-triplet annihilation upconversion

Abstract

Chemical systems that exhibit photochemical upconversion via triplet triplet annihilation have been studied using a range of fluorescence, absorption, and magnetic spectroscopy techniques. Each part of this investigation furthers understanding of how the emitter properties govern both the efficiency of the overall upconversion process, and the spin dependent annihilation step. Developing this fundamental understanding is vital to enable rational design of high performance emitters, and in turn unlock the potential of upconversion in applications across photovoltaics and light emitting diodes. Time-resolved and steady-state fluorescence emission spectroscopy was used to compare the performance of a standard upconversion system to one with a deuterated emitter. For perylene, deuteration was found to decrease the rate constant for first order nonradiative losses by 16 %, while leaving other electronic and kinetic parameters unchanged. This selective control over one of the emitter properties allowed for straightforward and direct comparison between the systems, and resulted in a 45% increase in upconverter performance under low intensity excitation. Transient absorption spectroscopy was used to characterise a range of commonly used emitter species. The family of emitters investigated were found to be largely similar in their decay kinetics, indicating that any comparative advantage comes about primarily through different intrinsic efficiencies of the annihilation step. A system with two emitter species was also studied, and its superior performance attributed to an improved rate of exciton transport, rather than annihilation events between heterogeneous emitter pairs. Finally, magnetic field effects and resonance spectroscopy were pursued as ways to probe the nature of spin mixing in the annihilation event. Fibre based experimental platforms were developed to enable these techniques for air sensitive solutions, although resonance signal remained elusive. Nonetheless, modelling the effects of static magnetic fields on upconversion emission reveals the critical importance of emitter pair orientation as the spins interact. In the absence of external magnetic fields, this orientation factor is identified as the key determinant of the annihilation outcome, rather than the individual spin states of the interacting triplets as was previously thought.