Switchable electronic coupling in model oligoporphyrin molecular wires examined through the measurement and assignment of electronic absorption spectra

The use of a quinone functionality in the linkage unit of laterally bridged oligoporphyrins as a switch for controlling electronic coupling between the termini is examined. The quinone-bridged bisporphyrin P(2)TA-O(2) was synthesized by condensation of 2 equiv of the dione 2,3-dioxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin with 2,3,5,6-tetraamino-1,4-benzoquinone. The electronic absorption spectra of P(2)TA-O(2) and its conjugated benzenoid analogue P(2)TA are measured and assigned, in conjunction with the spectra of the fragment monomers and porphyrin-bridge compounds. Band homologies and CASPT2 calculations are used to make the assignments. Chemically, the dimer in one case is bridged by a through-conjugated, pi-delocalized 1,4,5,8-tetraazaanthracene molecule. This is shown to display significant inter-porphyrin coupling, with an observed difference in the exciton couplings of the B(x) and B(y) bands being ca. 0.18 eV. However, the other dimer is bridged using a derivative in which the central ring is converted to a cross-conjugated, pi-localized quinonoid form; this molecule displays no observable inter-porphyrin coupling. This scenario provides a paradigm for the use of molecular electronic devices in sensing, control, and high-capacity relatively low-speed data storage applications.