Highly conjugated multiporphyrins: synthesis, spectroscopic and electrochemical properties

A series of meso-to-meso ethynyl-bridged multiporphyrin arrays have been synthesized using Sonogoshira palladium-catalyzed cross-coupling reactions involving the appropriate ethynylporphyrin and iodoporphyrin precursors. The absorption spectra of these multiporphyrins show splitting of the Soret bands and significant red shifts of the Q bands as compared to the combination of the corresponding components. These conjugated multiporphyrins also show red shifts in their emission spectra as the pi-conjugation is expanded. In the electrochemical measurements, the porphyrins dimer 7 shows two 1 - e- oxidations at E(1/2) = +0.63 and +0.76 V for the first electron abstraction from the two porphyrin rings, indicating electronic communication between the two porphyrin units. The porphyrin trimer 4 exhibits the first and second 1 - e- oxidations at E(1/2) = +0.68 and +0.77 V, respectively, which correspond to the two outer porphyrins. The cyclic voltammogram of pentamer 5 shows two overlapping 1 - e- couples at E(1/2) = +0.56 and +0.66 V, and one 2 - e- couple at E(1/2) = +0.86 V, for the four outer porphyrin units. These results demonstrate that in the porphyrin trimer and pentamer the individual peripheral porphyrin units are electrochemically coupled via a central porphyrin core. The UV-Vis-NIR spectra of the oxidized species of these multiporphyrins exhibit a broad intervalence charge transfer (IVCT) band in the region from 1200 to 3000 nm. The present work shows that a central porphyrin unit appended with ethynyl bridges affords strong electronic interactions between the peripheral porphyrin rings over a distance of about 15 A.