Oxasmaragdyrin±Ferrocene and Oxacorrole±Ferrocene Conjugates: Synthesis, Structure, and Nonlinear Optical Properties Sundararaman Venkatraman, [a] Rajeev Kumar, [a] Jeyaraman Sankar, [a] Tavarekere K. Chandrashekar,* [a] Kaladevi Sendhil, [b] C. Vijayan, [b] Alexandra Kelling, [c] and Mathias O. Senge [c] Introduction Research on the development of molecule-based electronic devices has gained momentum in recent years. [1] Discrete molecular conjugates are created by a covalent linkage of either a redox-active or a photoactive moiety to macrocycles having strong p-electron conjugation. Such conjugates are expected to show strong electronic interactions between the molecular components. [2] Specifically, the ferrocenyl groups have been used as redox-active groups with many macrocy- cles containing conjugated p electrons, for example with porphyrins to synthesize porphyrin±ferrocene conjugates. [3] The linkage between the porphyrin macrocycle and the fer- rocenyl moiety plays an important role in inducing electron- ic interactions. There are many reports of one or more fer- rocenyl moieties being linked to a porphyrin macrocycle through various spacer groups. [4] In such molecules the strength of electronic interaction depends on the nature of the spacer group, its length and its orientation. In most cases the spacer group has been observed to hinder the electronic contact, leading to weak communication between the molec- ular components. Therefore, for effective electronic commu- nication, direct covalent linkage of the ferrocenyl groups with the porphyrin p system is a necessary requirement. There are a few reports in the literature [5] of ferrocenyl groups being covalently linked directly to a porphyrin p system, without any spacer. Hendrickson and Wollmann [6] were the first to synthesize a porphyrin±ferrocene conjugate by refluxing pyrrole and ferrocene carboxaldehyde in pro- pionic acid. However, the formation of atropisomers hin- [a] S. Venkatraman, R. Kumar, J. Sankar, Prof.Dr. T.K. Chandrashekar Department of Chemistry, Indian Institute of Technology Kanpur 208016 (India) Fax:(+ 91)512-2597282/2597436/2590007 E-mail:tkc@iitk.ac.in [b] K. Sendhil, Prof.Dr. C. Vijayan Photonics Laboratory, Department of Physics Indian Institute of Technology–Madras, Chennai 600036 (India) [c] A. Kelling, Prof. Dr. M. O. Senge Institut f¸r Chemie, Universit‰t Potsdam Karl-Liebknecht-Strasse 24±25, 14476 Golm (Germany) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. Abstract: Ferrocenyl macrocyclic con- jugates involving 22p oxasmaragdyrins and 18p oxacorroles have been synthe- sized and characterized. The direct co- valent linkage of the ferrocenyl moiety to the meso position of the macrocycle is achieved by simple oxidative cou- pling of appropriate precursors with trifluoroacetic acid as catalyst. The electronic coupling between the ferro- cenyl moiety and the macrocyclic p system is apparent from: a) the red shifts (293±718 cm 1 ) of the Soret and Q-bands in the electronic absorption spectra of ferrocenyl conjugates; b) the shift of oxidation potentials (50± 130 mV) of both the ferrocene and the corrole rings to the positive potentials; and c) considerable shortening of the C C bond which connects the ferro- cene and the meso-carbon atom of the macrocycle. The single-crystal X-ray structure of oxasmaragdyrin±ferrocene conjugate 9 reveals the planarity of the 22p skeleton with very small deviations of the meso-carbon atoms. The meso- ferrocenyl substituent has a small dihe- dral angle of 388, making way for mixing of the molecular orbitals of the ferrocene and the macrocycle. Howev- er, the other two meso substituents are almost perpendicular to the mean plane, defined by the three meso carbon atoms. Classical C H¥¥¥O and nonclassical C H¥¥¥p interactions lead to a two-dimensional supramolecular network. Ferrocene±smaragdyrin con- jugate 9 bonds to a chloride ion in the protonated form and a rhodium(i) ion in the free base form. Nonlinear optical measurements reveal a larger nonlinear refractive index (5.83î10 8 cm 2 W 1 ) and figure of merit (2.28î 10 8 cm 3 W 1 ) for the rhodium smarag- dyrin±ferrocene conjugate 19 than for the others, suggesting its possible appli- cation in optical devices. Keywords: conjugation ¥ ferrocenyl conjugates ¥ iron ¥ nonlinear optics ¥ porphyrinoids ¥ supramolecular chemistry Chem. Eur. J. 2004, 10,1423±1432 DOI: 10.1002/chem.200305558 ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1423 FULL PAPER