Intermediate States in Solar Light Harvesting: Correlated Triplet Pair Dynamics in Singlet Fission & Photochemical CO2 Reduction with Manganese Complexes

Abstract

As the global demand for energy has been on the steady increase, scientists have developed ways to exploit renewable sources aiming to reduce the usage of fossil fuels. Considering large portions of abundant solar energy available at the surface is often wasted, numerous strategies have been cultivated for harnessing the power of the sun. This thesis focuses on two materials for solar energy harvesting: singlet fission materials which can be utilized in organic photovoltaic devices, and Earth abundant manganese complexes in solar fuel synthesis via CO2 reduction. In part I, we focus on the correlated triplet pair intermediate in singlet fission while part II emphasizes electro- and photochemical behaviours of CN-bridged di-manganese complex and its solvato intermediate.

There has been renewed scientific interest in comprehensive studies on singlet fission as the potential advantages of utilizing two triplet excitons generated from a single solar photon were realized for the organic photovoltaic devices. This two-for-one conversion of the absorbed photon is accomplished through a critical intermediate, correlated triplet pair state, that bridges singlet and the triplet scaffolds. In part I of the thesis, we examine the role that the correlated triplet pair state plays on mediating various steps in singlet fission. To begin with, we assess the effect of intermolecular coupling on the rate of correlated triplet pair formation in a number of functionalized pentacene nanoparticles exhibiting different extents of exciton delocalization. We demonstrate that the correlated triplet pairs are formed faster when coupling is stronger. Moving forward, we interrogate the steps beyond formation of the correlated triplet pair intermediate. We include an additional step in single fission and incorporate spatially separated triplet pair intermediates as the electronic coupling must be lost prior to becoming two independent triplets. Utilizing the ultrafast spectroscopy on the thin films of bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) at cryogenic temperatures, we unequivocally demonstrate that the correlated triplet pair separation is mediated by thermally activated triplet energy transfer. Furthermore, we report a second regime of triplet transport in pentacene film where the transport is driven by disorder present in the material.

In part II of this thesis we examine photochemical solar fuel synthesis via CO2 reduction, an approach harvesting solar energy while recycling atmospheric CO2. CO2 is one of the primary sources of greenhouse gas contributing to climate change on a global scale. CO2 reduction not only removes unutilized waste, it also transforms into more usable precursors. We report CN-bridged di-manganese complex with significantly improved photostability compare to the benchmark complex, Mn(bpy)(CO)3Br. This di-manganese shows both electrocatalytic and photochemical activities in reducing CO2 into CO. In addition, the di-manganese complex can reduce CO2 photochemically in the absence of electron source. In pursuit of gaining further insights, we characterize the photo-evolution of the complex with various spectroscopic techniques and computational analysis. These studies revealed a participation of an intriguing solvato intermediate that stabilizes the complex through the π-π interaction of the ligand motifs.