Modular Synthesis of Benzene-Centered Porphyrin Trimers and a Dendritic Porphyrin Hexamer † Olivier Mongin, Cyril Papamicae ¨l, Nicolas Hoyler, and Albert Gossauer* Institut fu ¨ r Organische Chemie der Universita ¨ t, Universita ¨ t Freiburg Schweiz, Ch. du Muse ´ e 9, CH-1700 Fribourg, Switzerland Received May 12, 1998 Rigid, star-shaped D 3 -symmetric arrays have been synthesized in which three porphyrin macrocycles are attached to the 1, 3, and 5 positions each of a benzene core through linkers consisting of collinear repetitive phenylethynyl units. Using the same methodology, a dendridic porphyrin hexamer having an external diameter of ca. 10 nm has been also obtained. By successive substitution of the three benzene positions, both a porphyrin trimer, the three linkers of which are of different length, and a starlike porphyrin, in which the complexed metal ions are different from each other, have been synthesized. The latter is the first example of a prochiral arrangement of metal ions in a D 3 - symmetric ligand. To investigate their capability of forming ordered self-assembled monolayers on gold substrates, some of the porphyrin trimers and the dendritic porphyrin hexamer described in this work bear meta-thioanisole units at the apical positions. Analogously to similar multipor- phyrin systems described in the literature, in which, however, the chromophores were arranged collinearly, the interaction between the chromophores of the multiporphyrin arrays described in this work is negligible, in the ground state, while effective energy transfer takes place in the singlet excited state. Introduction Owing to their ready accessibility, 1 large molecular size, 2 and rigid planar geometry as well as their electronic properties and their ability to complex almost any kind of metal ions, porphyrins are irreplaceable building blocks for the syntheses of both straight-chain and branched extended molecular systems designed to per- form a great diversity of functions as, for instance, enzyme-mimetic catalysts, 3-5 optical switches, 6 molecular photonic wires, 7-10 molecular optoelectronic gates, 11 and light harvesting arrays. 12-19 On the other hand, the prospective application potential of branched, monodis- perse, sequence-specific oligomers of polycyclic aromatic macromolecular systems for the construction of nano- structures has been emphasized recently in a compre- hensive review article 20 as well as by Mu ¨ llen’s work on polyphenylene dendrimers. 21,22 As a matter of fact, the use of dendrimers in molecular recognition and self- assembling systems is attracting increasing attention in supramolecular chemistry. 23 Moreover, metal-donor atom interactions in dendrimers containing complexed metal ions increase still more the possibilities for their practical applications in diverse areas including material science and mimicking of the essential features used by living systems in energy-transfer processes. 24 With this in mind, the syntheses of a series of benzene- centered porphyrin trimers as well as of a dendritic porphyrin hexamer, all of them possessing D 3h symmetry, have been carried out in the present work by means of the building block approach developped by Lindsey et al. 25 for covalent assembly of multiporphyrin arrays and * To whom correspondence should be addressed. Phone: 41-26-300 8770. Fax: 41-26-300 9739. E-mail: albert.gossauer@unifr.ch. † Dedicated to Prof. A. Ian Scott of the Texas A & M University on the occasion of his 70th birthday. (1) Dolphin, D., Ed. The Porphyrins; Academic Press: New York, 1979; Vol. 1, Chapters 3-6. (2) Webb, L. E.; Fleischer, E. B. J. Am. Chem. Soc. 1965, 87, 667- 669. (3) Milgrom, L. R. J. Chem. Soc., Perkin Trans. 1 1983, 2535-2539. (4) Davila, J.; Harriman, A.; Milgrom, L. R. Chem. Phys. Lett. 1987, 136, 427-430. (5) Bonnett, R.; Ioannou, S.; Pearson, C.; Petty, M. C.; Roger-Evans, M.; Wilkins, R. F. J. Mater. Chem. 1995, 5, 237-242. (6) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, 63-65. (7) Crossley, M. J.; Burn, P. L. J. Chem. Soc., Chem. Commun. 1991, 1569-1571. (8) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-9760. (9) Sessler, J. L.; Capuano, V. L.; Harriman, A. J. Am. Chem. Soc. 1993, 115, 4618-4628. 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