Porphyrin dimers bridged by an electrochemically switchable unit.

In the past decade much effort has been devoted to the synthesis and studies of molecular systems comprising porphyrin units bridged by well-defined p-conjugated spacers at the mesoor b-positions. These conjugated oligoporphyrin systems are expected to have potential applications in molecular wires and electronic devices due to their unique optical, physical, and chemical properties. Photophysical studies on electron transfer of a series of ferrocene–oligoporphyrin–fullerene triads show that a meso–meso butadiyne-bridged oligoporphyrin acts as an efficient molecular wire for long-range charge transfer over 65 &. More recently, the conductivity measurements on butadiyne-bridged porphyrin polymers indicate that the formation of doublestrand 4,4’-bipyridyl ladder complex by addition of bipyridine to the single-strand porphyrin polymer leads to an increase in conductivity by an order of magnitude. EPR studies performed by Therien and co-workers on meso– meso ethyne-linked porphyrin oligomers show that this type of conjugated molecules is able to mediate charge migration over a distance of about 75 &. The synthesis of meso– meso, b–b, b–b triply-linked porphyrin tapes reported by Osuka represents a milestone in the area of porphyrin molecular wires. The fully conjugated porphyrin tapes exhibit remarkable properties including very low optical HOMO– LUMO gap, low oxidation potential and rigid structure, and they may find use in a variety of molecular electronics. Despite extensive studies on the conjugated porphyrin arrays, oligoporphyrin molecular wires that show switchable conductance states by chemical or electrochemical means are rare. One example is the porphyrin arrays in which the individual macrocyles are laterally-linked by a quinonoid unit at the b,b’-positions of the porphyrin rings. In this system, the electronic communication between porphyrin units can be modulated by quinonoid/benzenoid conversion using chemical or electrochemical means. Another example of switchable porphyrin wires is the systems where proquinoidal units bridge two porphyrins. It is well known that the reversible conversion between quinone and hydroquinone can be achieved by chemical or electrochemical methods. Incorporation of a quinone unit into the central part of a porphyrin dimer via acetylene linkers would allow the interporphyrin interaction to be switched off and on in a controllable fashion and this type of molecules may have the potential for the application in switchable molecular wires. Therefore, we set out to design and synthesize a series of porphyrin dimers Ni22, Ni23, and Ni24 to explore their potential as redox-controllable switching molecules. The synthetic routes to porphyrin dimers Ni22, Ni23, and Ni24 were outlined in Scheme 1. [11] The coupling reaction of porphyrin Ni1 with the diiodide gave porphyrin dimer Ni22. [12] Demethylation of Ni22 to give Ni23 employing excess BBr3 was unsuccessful. However, the demethylation can be achieved by the reaction of excess BBr3 with the free base of the zinc analogue Zn22, obtained by a procedure similar to that for Ni22. [13] Subsequent nickel insertion afforded porphyrin dimer Ni23. The oxidation reaction of dimer Ni23 with PbO2 gave porphyrin Ni24. [14] For comparison purposes, porphyrin Ni5 was synthesized by employing a procedure similar to that for Ni22. The molecular structures of these porphyrins were confirmed by various spectroscopic methods such as NMR, UV/Vis, IR spectroscopy, and mass spectrometry. Figure 1 shows the crystal structure of compound Ni5. The bond lengths and angles fall in the normal range. The porphyrin ring adopts an essentially planar conformation [a] C.-L. Mai, Y.-L. Huang, Prof. C.-Y. Yeh Department of Chemistry National Chung Hsing University Taichung 402 (Taiwan) Fax: (+886) 4-2286-2547 E-mail : [email protected] [b] Dr. G.-H. Lee, Prof. S.-M. Peng Department of Chemistry National Taiwan University Taipei 106 (Taiwan) Supporting information for this article is available on the WWW under http://www.chemistry.org or from the author.