Quinoxalino[2,3-b′]porphyrins behave as π-expanded porphyrins upon one-electron reduction: Broad control of the degree of derealization through substitution at the macrocycle periphery

The synthesis and redox properties of a series of free-base and metal(II) quinoxalino[2,3-b′]porphyrins and their use in an investigation of the substituent effects on the degree of communication between the porphyrin and its β,β′-fused quinoxalino component are reported. ESR, thin-layer spectroelectrochemistry, and quantum chemical calculations of the resultant radical anions from one-electron reduction indicate that localization of the unpaired electron across both the porphyrin and the fused quinoxalino group can be controlled, the system as a whole behaving as a highly polarizable π-expanded porphyrin radical anion. ESR studies on the radical anions of zinc(II) quinoxalino[2,3-b′]porphyrin derivatives indicate that nitrogen-atom spin distribution changes as a function of chemical substitution: 27% quinoxaline character when the porphyrin ring bears a 7-nitro substituent, 34% quinoxaline character in the unsubstituted parent, and 51-61% nitroquinoxaline character when the quinoxalino unit has one or more nitro groups. Close analogies are found between the calculated and observed nitrogen-atom spin distributions, indicating that the calculations embody the key chemical effects. The calculations also indicate that the nitrogen-atom spin distributions closely parallel the important total porphyrin, quinoxaline, and nitro spin distributions, indicating that the observed quantities realistically depict the change in the nature of the delocalization of the radical anion as a function of chemical substitution. The profound effects observed indicate long-range communication of the type that is essential in molecular electronics applications.