Design, synthesis, spectroscopy, and photophysical behavior of highly coupled porphyrin donor -acceptor systems

This thesis describes the synthesis, spectroscopy and electron transfer kinetics of porphyrin-containing donor-spacer-acceptor (D-Sp-A) assemblies. These compounds can be separated into two discrete categories (i) linear electron transfer (ET) arrays for the study of “through bond” electron transfer reactions and (ii) stacked ET arrays for the study of “through space” electron transfer. The ET dynamics of a directly attached, D-A dyad (PZnPI) provide a benchmark to which the behavior of the bridged arrays may be compared. D-A electronic coupling is minimized because steric interactions between the β-hydrogens of the porphyrin macrocycle and the diimide acceptor carbonyl oxygens impose a large D-A dihedral angle. The photoinduced charge separation and thermal charge recombination reactions of PZnPI, as studied by time-resolved transient absorption spectroscopy, confirm that the donor and acceptor moieties function as independent units in these reactions. This behavior can be directly contrasted to that undergone in the D-Sp-A compounds. The conjugated aryl(ethynyl) bridge plays an important role in determining the ground, excited, and charge-separated state structure. The effect of the bridge on the ET dynamics is discussed and explained. Strong Coulombic interactions of the donor radical cation with the acceptor radical anion result in delocalization of charge over the bridging unit in the charge-separated state and the ET dynamics observed in these D-Sp-A systems highlight the importance of the conjugated bridge in mediating ET reactions. A second class of ET arrays considered in this thesis are those in which a quinone acceptor is stacked over a porphyrin donor, and held in place by n naphthalene pillars. The singly stacked [5-[8-[2″ ,5″-dimethoxyphenyl]ethynyl]napthyl]10,20-diphenylporphinato]zinc (II) and the doubly stacked [5-(8′-[4″-(8 ″′-[2″″,5″″ -dimethoxyphenyl]-1″′-naphthyl)-phenylethynyl]-napthyl)10,20-diphenylporphinato]zinc(II) were prepared by a sequence of cross coupling reactions, in which the key step is the Suzuki cross-coupling of a halogenated pillar to a boronated or “Suzuki” porphyrin. The potential for the singly stacked compound in this series to undergo an ultrafast proton coupled electron transfer (PCET) reaction to form a stable and long-lived charge neutral diradical species is discussed