Abstract: The influence of electronic symmetry on triplet state delocalization in linear zinc porphyrin oligomers is explored by electron paramagnetic resonance techniques. Using a combination of transient continuous wave and pulse electron nuclear double resonance spectroscopies, it is demonstrated experimentally that complete triplet state delocalization requires the chemical equivalence of all porphyrin units. These results are supported by density functional theory calculations, showing uneven delocalization in a porphyrin dimer in which a terminal ethynyl group renders the two porphyrin units inequivalent. When the conjugation length of the molecule is further increased upon addition of a second terminal ethynyl group that restores the symmetry of the system, the triplet state is again found to be completely delocalized. The observations suggest that electronic symmetry is of greater importance for triplet state delocalization than other frequently invoked factors such as conformational rigidity or fundamental length-scale limitations

Highly conjugated and crystalline organic compounds are relative newcomers to the field of semiconducting materials. Their properties are best understood through molecular orbital theory, considering both the energy levels of the single molecules and the spatial relationships of the molecules within the lattice. The functional effectiveness of these materials therefore depends upon their solid-state packing, in addition to more traditionally considered properties such as HOMO-LUMO gap, presenting a new challenge in organic synthesis. Pentacene and its relatives have emerged as useful compounds for these applications, and so the synthesis of substituted linear acenes is of great current interest. Prominent strategies for this include Friedel-Crafts acylations, Knoevenagel condensations, and Diels-Alder cycloadditions. The latter of these, in the form of the trapping of an o-quinodimethane by benzoquinone, has been applied to the synthesis of a highly arene-substituted derivative of pentacene.