Book Description

In order to understand and mimic the processes that occur in photosynthesis, the photophysical properties of a series of porphyrinic molecules have been investigated. This work is divided into five projects involving the study of three different porphyrins and porphyrinoids: 1) free base N-confused tetraphenylporphyrin and various substituted derivatives, 2) free base N-confused porphyrin-zinc porphyrin, N-confused porphyrinpyromellitimide and N-confused porphyrin-pyrene arrays, 3) free base tetraphenylcorroles, 4) a series of porphyrin-quinone arrays to probe porphyrin excited state structure and 5) a series of porphyrin-naphthalenediimide arrays to explore energy and electron transfer as a function of linker/bridge dendron type. N-Confused tetraphenylporphyrin (NCTPP) differs from tetraphenylporphyrin (H2TPP) by having one of the pyrrolic nitrogens inverted and facing outside the macrocycle, with a C-H group inside. The inverted pyrrole ring in NCTPP results in a change in the [pi]-system of the macrocycle, leading to photophysical properties that are different than those of H2TPP. The first project was intended to investigate the photophysical properties of the two NCTPP tautomers, as well as several substituted (di- and tetra-) NCTPP derivatives using several spectroscopic techniques, including steady-state absorption and emission spectroscopy, time-correlated single photon counting, femtosecond transient absorption and nanosecond transient absorption spectroscopy. The second project consisted of the studies the photophysical characterization of pentameric N-confused porphyrin-zinc tetraphenylporphyrin artificial light-harvesting array (NCP-ZnP4), the photophysical properties of an N-confused-pyromellitimide array (NCP-Pym4), and the photophysical characterization of an N-confused porphyrin-pyrene donor-acceptor dyad (NCP-Pyr). The supramolecular systems NCP-ZnP4, NCP-Pym4 and NCP-Pyr represent the first structures where an N-confused porphyrin has been covalently incorporated into an array designed to involve highly efficient photoinduced energy and electron transfer processes. Free base corroles differ from free base porphyrin by having one of meso methine carbons replaced by a direct pyrrole-pyrrole linkage. The fourth project in this dissertation involved the photophysical characterization of four free base triphenylcorroles using steady state absorption and emission spectroscopy, and time correlated single photon counting experiments. The photophysical results indicate that similar to NCTPP, there is noticeable solvent dependence on the corrole photophysical properties. It is postulated that these differences result from the presence of two (or more) different tautomers. Although regular tetraphenylporphyrins such as H2TPP and Zntetraphenylporphyrin (ZnTPP) have been extensively used in different artificial photosynthetic systems, their incorporation into arrays presents characterization challenges, and energy and electron transfer mechanisms differ from array to array. The fifth project presented here involved an investigation into the effects of donor-acceptor orientation on the electron transfer (ET) and charge recombination (CR) rate constants. In a series of porphyrin-benzoquinone (PBQ) dyads, methyl groups on the meso phenyl groups were used to control the orientation between the donor and acceptor groups. The rate constants for ET and CR in these compounds are discussed, as well as the ramifications of these results on porphyrin photophysics. The final project in this dissertation involved the study of the photophysical properties of a series of triads, where two ZnTPP electron donor groups were linked, using a flexible dendritic spacer, to a naphthalenediimide acceptor group. The flexibility in these triads permitted the formation of several conformers, which were investigated using steady state and time-resolved spectroscopic techniques as well as semi-empirical molecular orbital calculations.