Time Resolved Spectroscopy of Solar Energy Harvesting Complexes

Abstract

Porphyrin and fullerene molecules are one of the most extensively studied electron donor-acceptor pairs in the history of artificial photosynthesis. Their unique properties allow them to undergo photoinduced electron transfer to form a charge-separated state (P.+C60.-), that shows potential for them to perform as the active materials for solar energy harvesting. A class of calix[4]arene-linked bis-porphyrin (“jaws” porphyrin) was previously constructed by our collaborators, Boyd et al., and shown to bind with fullerenes (C60 and C70) with high affinity. Understanding the photophysics of these self-assembled complexes, particularly the charge recombination timescale in order to direct future targeted molecular development is the focus of the research reported in this thesis. The central goal of these studies is to evaluate the effectiveness of ferrocene derivatives as secondary donors to increase the lifetime of the charge-separated state. Femtosecond transient absorption spectroscopy (fsTRA) was utilized to measure the excited state lifetimes of the transient intermediate species subsequent to photoexcitation of the porphyrin donor. This technique uses two ~100 fs laser pulses separated by a temporal delay. The first, a narrow-band pulse, is used to pump the porphyrin to a chosen excited state. The second broadband supercontinuum pulse probes the absorption spectrum of the sample as it evolves. The 3 dimensional data (absorption difference, time, and wavelength) is carefully analysed to extract and model the kinetics of the excited species. Porphyrin dynamics dominate the UV-Vis region, while the decay profile of C60- shows a fairly isolated signal in the near-IR. Three types of ferrocene-porphyrin-fullerene triads are discussed in this thesis, and all of them exploit the bis-porphyrin “jaws” host – fullerene guest self-assembled complex. : 1) a fully self-assembled system in which the ferrocene derivative is attached via coordination to the metal centre of the porphyrin, 2) a series of compounds in which ferrocene derivatives with different spacer lengths are covalently attached to a meso-position on the porphyrin macrocycle, and 3) compounds in which the ferrocene derivative is covalently attached to the calix[4]arene linker that forms the “hinge” of the bis-porphyrin “jaws.” Two other secondary donors – a covalently bound N-trityl group and a Ru-porphyrin attached to the bis-porphyrin fullerene were also investigated. Increased lifetimes of the fullerene radical anion were observed for all triads (up to ~3 ns) compared to parent bis-porphyrin fullerene dyads (no ferrocene present). However, the experimental results did not show active participation of the secondary donors in the electron transfer process. Instead, we believe that the observed lifetime differences can be ascribed to changes in the porphyrin-fullerene donor-acceptor electronic coupling strength induced by the ferrocene derivative. Large electronic coupling strengths (>400 cm-1) , estimated from charge-transfer absorption spectra of the complexes, suggest extremely tight coupling between electronic orbitals of the donor and acceptor, resulting in fast back electron transfer in the systems explored here. We also performed fsTRA on solid porphyrin-C60 crystals ground with KBr to give thin, semi-transparent discs. The relationship between variations in crystal packing and their charge recombination dynamics is discussed. Finally, a complete nanosecond transient absorption system was constructed and tested for measuring species in the longer timescales. It employs a novel supercontinuum probe based on the nonlinear optical effects in photonic crystal fibers. The construction and benchmark testing of this new instrument upon a well-known porphyrin and on an electron transfer compound is also presented.