Synthesis of Porphyrin Arrays and Investigation of Their Optical and Electrochemical Properties

Porphyrin arrays have attracted much attention in a wide range of scientific applications such as in artificial photosynthetic antenna¹, electronic devices, and single-molecule devices². Although various synthetic methods have been reported, they are applicable only to the homologous porphyrin arrays; in addition, it is difficult to synthesize the elaborate porphyrin arrays with programmable order of each unit. Thus, we synthesized porphyrin dimers and trimers that have varying sequences of the porphyrin units via the consecutive Suzuki coupling reactions using the same key porphyrin compound. The use of the consecutive Suzuki coupling reactions for the effective syntheses of elaborate oligoarenes by repetition of the same reaction at each elongation step³, has been reported. In this work, the consecutive Suzuki coupling was applied for the synthesis of porphyrin arrays of various orders.

We synthesized two porhpyrins as the starting monomers: porhyrin 2 bearing a triflate group, and the key porphyrin 3 bearing a boronic ester and a hydroxyl group. Porphyrins 2 and 3 were synthesized in three and four steps, respectively. The Suzuki coupling reaction between 2 and 3 in various combinations afforded four different dimers, as denoted by 4 in Scheme 1. After triflation of the hydroxyl group of 4, a further coupling of 4 with 3 afforded five different trimers, as denoted by 6 in Scheme 1. The porphyrin monomers were synthesized in gram scale in one batch, and the coupling reactions afforded moderate to good yields. The optical and electrochemical properties of these dimers and trimers were investigated.

The absorption and emission spectra of the dimers and trimers, possessing the same molecular formula, were different (Fig. 1) when the order of the units was different. The emission profiles of the arrays, 4-HZn and 6-HZnH, were attributed to that from the free-base porphyrin unit; however, the wavelengths and intensities of their emission were different. (Fig. 1.(b)). The differential pulse voltammograms of the monomers, 1-H and 1-Zn, and the dimer 4-HZn are demonstrated in Fig. 2. Certain redox peaks of the dimer 4-HZn were almost identical to those of the component monomers (1-H and 1-Zn); however, a few new redox peaks were observed, which indicated the emergence of new electronic states via interaction of each unit component.

(1) Nakamura, Y.; Aratani, N.; Osuka, A. Chem. Soc. Rev. 2007, 36, 831.

(2) Sedghi, G.; Garcia-Suarez, V. M.; Esdaile, L. J.; Anderson, H. L.; Lambert, C. J.; Martin, S.; Bethell, D.; Higgins, S. J.; Elliott, M.; Bennett, N.; Macdonald, J. E.; Nichols, R. J. Nat Nano 2011, 6, 517.

(3) Noguchi, H.; Hojo, K.; Suginome, M. J. Am. Chem. Soc. 2007, 129, 758.