The synthesis and photophysical chemistry of endohedral metallofullerene derivatives
Creators: name: Kopcha, William P.
Publisher: name: No Publisher Supplied
Year: 2023

description: Buckminsterfullerenes have been the subject of intense study as electron acceptors due to their low (not very negative) reduction potential and small reorganization energy (λ) upon gaining an electron. When paired with porphyrins, which have a low (not very positive) oxidation potential and small reorganization energy upon losing an electron, the electron donor-acceptor conjugate often falls squarely in the coveted Marcus inverted region, leading to the fast formation of a high-energy charge-separated state after photoexcitation and slow decay back to the ground state. In this dissertation, we explore the structure-property relationships of the photophysical behavior of fullerenes, both empty and containing entrapped metal clusters (endohedral metallofullerenes, or EMFs), while also delving into some of the synthetic approaches used to functionalize fullerenes for specific applications. We examine the differences in charge-separation rates of host-guest complexes of C60 and C70 in a cationic bis-porphyrin cage, finding that the charge separation (CS) from the porphyrin S2 extremely rapid (&lt; 0.2±0.1 ps). CS from the porphyrin S1 is faster for C70 than C60, owing to the tighter fit of the larger fullerene in the cage. The charge recombination (CR), however, is slowed for C70 vs. C60 due to the greater conjugated surface area for charge delocalization and the influence of the positively-charged pyridinium linkers in close proximity to the fullerenes’ surfaces. In the next chapter, we analyze the CR kinetics of the same system by varying the solvent, showing that the solvent reorganization energies of the host-guest complex in various polar solvents differ much more than the CS state energies, causing the CR dynamics to be governed by reorganization energy rather than thermodynamic driving force. Moreover, we find an unusually large reorganization energy for the porphyrin nanocage of 0.58±0.11 eV, pushing the CR process into the Marcus normal region and causing the solvent with the highest reorganization energy (acetonitrile) to display the longest CS-state lifetime, and find strong evidence that both the restriction of movement in the porphyrin macrocycle by the linkers and deformation-induced offset of the PES along the nuclear coordinate by a large guest are responsible for this phenomenon. In the following chapter, in a conjugate of zinc porphyrin and Lu3N@C80, it was found that energy transfer in toluene from the porphyrin to the EMF is faster when they are separated by only a phenyl ring vs. an azobenzene linker, but charge transfer in benzonitrile is faster through the azobenzene, likely due to stronger coupling of the donor-acceptor pair by virtue of the delocalization of the LUMO, but not the HOMO, over both the porphyrin and the azobenzene linker. Finally, we examine various synthetic challenges not addressed in the previous sections, including the creation of new C60-based “cores” for further elaboration with various flexible chemistries by appending six malonyl diesters bearing different chains to the surface of fullerene, variations on the compounds from Chapter 4 made by condensing a nitrosoarene with an aniline and in using the Adler-Longo instead of Lindsey porphyrin synthesis, and palladium-catalyzed cross-couplings in the context of asymmetric linkers for porphyrin-fullerene conjugates as well as porphyrinic starting materials for larger cage structures.