Quantitative Prediction and Measurement of Exciton Diffusion in Photoactive Framework Materials

Light-harvesting materials are currently at the forefront of research worldwide, with researchers developing more efficient methods of using the Sun's light to create green fuels and materials. Of the many avenues of research currently being undertaken, permanently porous framework materials such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have shown great potential for light-harvesting applications. This potential stems from their uniform, repeating structure, that can be modified to preferentially capture certain gases over others, while their components can be functionalised to improve light absorption and introduce catalytic active sites. The goal of many researchers is to use framework materials to mimic the photosynthesis process observed in nature, with the framework funnelling solar energy to active sites to drive reactions such as water splitting. The absorption of solar energy leads to the generation of excitons, Coulombically bound electron-hole pairs, which diffuse through the material and drive catalytic reactions. In order to successfully mimic the photosynthesis process frameworks need to be constructed with chromophores capable of absorbing a significant portion of the solar emission spectrum, while also being able to transport excitons to active sites on or in the framework. The implementation of framework materials as light-harvesting complexes has been limited by the lack of knowledge into how their chemical and physical structure effects their exciton diffusion properties. The work presented in this thesis aims to fill this knowledge gap by providing a comprehensive analysis of the exciton diffusion properties in a selection of framework materials. This analysis included using a combination of computational chemistry calculations and steady state spectroscopy to predict the diffusion properties before validating our findings with ultrafast spectroscopic measurements to experimentally determine the exciton diffusion values. Our research has shown that the exciton diffusion properties of a family of porphyrin MOFs were significantly affected by changes to their physical construction. Specifically, we demonstrated that changes in ligand orientation and positioning lead to the enhancement of exciton diffusion in some frameworks over others, despite their chromophore building blocks remaining unchanged. Furthermore, we have shown that the change in exciton diffusion caused by these changes in structure can be accurately predicted using a combination of steady state spectroscopy and computationally, due to the precise, repeating structure of the frameworks. We then showed how the photophysical properties of these porphyrin MOFs could be systematically altered through the inclusion of transition metals of differing atomic weights. Heavy transition metals were shown to increase intersystem crossing (ISC) within the framework, due to an increase in spin-orbit coupling, thus decreasing the diffusion length of singlet excitons. In addition, we showed that the increase in ligand symmetry caused by the inclusion of closed shell metal ions decreases resonant spectral overlap, and in turn, decreases the rate of exciton diffusion within these porphyrin frameworks. These findings show that nature and rate of energy transfer occurring within the frameworks can be modulated in a predictable way via the inclusion of different transition metals. We also investigated a pair of porphyrin COFs whose construction introduced distinct energy transfer pathways that were not present in the porphyrin MOFs. These COFs formed in 2D sheets, as opposed to the 3D pores of the MOFs mentioned above. Our initial findings suggested that the short inter layer distance of the COFs would lead to the fastest exciton diffusion, greater than those observed in the MOFs. These findings were not supported by our ultrafast spectroscopic analysis however as we observed minimal evidence of the expected rapid exciton diffusion. We instead observed that both COFs contained complex photophysical kinetics that were likely caused by the presence of multiple excited state. We propose that these complex kinetics stem from the COFs containing multiple morphologies, which introduce traps states that prevent excitons from undergoing fast diffusion. Lastly, we examined a truxene-based MOF that appeared to show signatures of rapid exciton diffusion in spite of negligible spectral overlap supporting resonant energy transfer. We provided experimentally viable mechanisms to explain these fast fluence dependent kinetics. From our analysis we demonstrated that the fast kinetics could be explained by two mechanisms, the first being a Dexter energy transport mechanism that was mediated by the excited states of the metal nodes of the MOF, and the second being that the MOF was undergoing a third order photophysical process, exciton-charge annihilation. The findings presented in this work show the importance of both the chemical and structural properties of frameworks for light-harvesting applications, as small changes in either could greatly affect their exciton diffusion properties. Our findings also show that predicting exciton diffusion in frameworks is possible but can potentially provide inaccurate results without validation from time-resolved ultrafast spectroscopic measurements.