Cross-conjugation, where two separate conjugated branches are linked to the same carbon atom that participates in conjugation with each branch independently, for neutral molecules was seen unfavorably due to its impedance of linear conjugation. However, the advancement of molecular electronics quickly changed those perceptions. Chemically synthesized circuits where the desired electronic function is controlled through careful consideration of the substituents, has pushed cross-conjugation to the forefront. Where linear conjugation will act as the “wires” for the electronics, cross-conjugation can step into the roles of switches, logic gates, or even memory elements. Combining cross-conjugation with polycyclic aromatic systems may provide the best of both worlds into one compound.<br/>In this talk, the synthesis and characterization of a novel class of cross-conjugated p-extended systems incorporating both porphyrin and polycyclic aromatic hydrocarbons (PAHs) will be presented. With the intention of uniting PAHs and cross-conjugation, our group began with a β, β’-fused pentacene quinone porphyrin. Functionalization of the quinone would maintain the cross conjugation but would also allow us to expand the π conjugation into larger PAHs. Through a modified Ramirez reaction, the ketone groups of the quinones were transformed into halogenated methine groups. There are several advantages to this reaction, the first being that the halogenated methines are prime candidates for carbon-coupling reactions. Another advantage will allow for the symmetrical addition to the methine bridges, while maintaining the cross conjugation to the pentacene moiety. The Suzuki-Miyaura coupling reaction was utilized to attach benzyl groups to the methine bridges. Finally, the benzyl groups can be fused to the porphyrin core through a Scholl type reaction resulting in an expanded π PAH with a cross conjugated moiety at the center. The optoelectronic characteristics of these compounds as well as the intermediates were studied using UV-Vis, fluorescence and phosphoresce spectroscopies, cyclic voltammetry (CV), spectroelectrochemistry, transient spectroscopy, and TDDFT/DFT calculations.

Synthesis of β, β’-fused polycyclic hydrocarbon porphyrins

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ACS Fall 2022

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