Book Description

Interactions such as pi-pi stacking in organic pi-systems, proton coupling, and hydrogen bonding can be utilized to prepare nanostructures with excellent mechanical and electronic properties. In particular, self-assembled donor-acceptor complexes have received much recent attention because of their potential applications in optoelectronics such as photovoltaic cells and organic light emitting diodes. Porphyrins and their derivatives have been extensively used as building blocks for nanomaterials because of their optical, electronic, catalytic and biochemical properties. The pi-pi stacking and metal coordination ability of porphyrins make them very suitable for the formation of self-assembled structures. The first part of this work involves the photophysical characterization of self-assembled donor-acceptor complexes, where porphyrin derivatives are used as electron donors together with electron acceptors such as naphthalene diimide, perylene diimide, or fullerenes. In the second part, the tautomerization of N-confused tetraphenylporphyrin in various solvents was probed using time-resolved fluorescence spectroscopy.The first part of the work described here is the photophysical characterization of a self-assembled chiral bolaamphiphiles comprised of central tetraphenylporphyrin (H2TPP) with naphthalenediimide (NDI)-lysine moieties flanked on both ends. The strong intermolecular pi-pi interactions between the H2TPP and NDI chromophores leads to the formation of monolayer rings which further stack to form self-assembled nanorods. The effect of local nanostructure of assemblies on the photophysical properties was probed using steady-state fluorescence, time-resolved fluorescence and femtosecond transient absorption spectroscopic studies. This work demonstrates the importance of controlling local nanostructure in modulating the photophysical properties of optoelectronic materials. The second project involves the photophysical characterization of self-assembled donor-acceptor complexes containing an H2TPP and NDI-lysine amphiphile. Strong intermolecular pi-pi interactions between the TPP and NDI chromophores, together with the proton transfer from the lysine head group to the porphyrin ring leads to self-assembly in chloroform. Photoinduced electron transfer in the nanorods was probed using steady state fluorescence, time-resolved fluorescence and femtosecond transient absorption spectroscopic studies. The third portion of this dissertation involves the photophysical characterization of donor-acceptor systems containing covalently bound porphyrin-fullerene dyads, where different numbers (1-4) of C60-fullerenes were attached to a porphyrin core to enhance electron transfer efficiency. In this work, the fluorescence quantum yield measurements, time-resolved fluorescence and femtosecond transient absorption spectroscopic studies were used to study electron-transfer from the porphyrin core to the fullerene group(s). The fourth project involved studies of the equilibrium of the two tautomers of N-confused tetraphenylporphyrins (NCTPP). The steady-state absorption and fluorescence spectra of NCTPP have been found to significantly depend on the nature of the solvent. The previously reported photophysical studies indicated that both NCTPP tautomers can coexist in certain solvents, presumably in equilibrium with one another, while other solvents exclusively prefer one tautomer or the other. In order to gain a better understanding of the NCTPP tautomerization, NCTPP was studied by time-resolved fluorescence spectroscopy in a series of solvents having different solvent parameters. Also examined in this work was the origin of a moderately strong excited state solvent isotope effect in both steady-state and dynamic measurements. In the final project, the spectroscopic properties of a novel self-assembled ZnNCTPP-PDI traid were examined. Donor-acceptor dyads using H2TPP have been examined for years, but studies of D-A dyads based on NCTPP have not been significantly explored. In this project a non-covalently linked D-A complex formed from a zinc N-confused tetraphenylporphyrin dimer [(ZnNCTPP)2] and a pyridyl substituted perylenediimide (Py-PDI-Py) were examined. The coordination of pyridyl group on the PDI with zinc results in the dissociation of the (ZnNCTPP)2 dimer and results in the formation of a ZnNCTPP-Py-PDI-Py-ZnNCTPP triad. Steady-state and time-resolved fluorescence measurements were performed to probe the formation of the triad as well as photoinduced electron transfer in this traid.