Microenvironment Engineering in ortho- and para-Dendronized Metalloporphyrin-Fullerene Conjugates Involving a trans-2-Bisaddition Pattern

Synthesis and Atropisomerism of Cascaded Tetraphenylporphyrin-[60]Fullerene Hybrids

Flexible, linked dendritic tetraphenylporphyrin (TPP)-fullerene hybrids were synthesized. They were designed to gain insight into and mimic the primary events in the natural photosynthetic reaction center. These multiporphyrin moieties are based on a light-harvesting concept. Moreover, they incorporate multiple redox components aligned along a redox gradient. Newkome-type dendrons were added to these TPP-fullerene hybrids. In principle they can mediate pH-dependent water solubility, which, however, could not be observed in this case. A protecting-group strategy using tert-butyldiphenylsilyl groups allows convergent synthesis of the dendritic compounds. The dendritic multiporphyrins were synthesized separately and can be used as individual building blocks. Atropisomerism was observed in the dendritic compounds, and single atropisomers could be assigned to the corresponding peaks of a characteristic pattern in the NMR spectra. Deprotection of the Newkome-type dendrons was shown to be feasible under mild conditions that leave the redox gradient intact.

A charge-transfer challenge: combining fullerenes and metalloporphyrins in aqueous environments

A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.