DOI: 10.1002/cphc.200600760 Nonlinear Optical Properties of Ferrocene- and Porphyrin–[60]Fullerene Dyads Evangelia Xenogiannopoulou, [a] Miroslav Medved, [a] Kostas Iliopoulos, [a] Stelios Couris,* [a] Manthos G. Papadopoulos,* [b] Davide Bonifazi,* [a, c] Chlo Sooambar, [c] Aurelio Mateo-Alonso, [c] and Maurizio Prato* [c] Introduction The nonlinear optical (NLO) response is described by a set of properties which can be used for designing materials for pho- tonic applications. [1] Our approach for developing novel pho- tonic materials relies on the discovery and preparation of new organic modules with enhanced nonlinearities. In this context, hollow carbon cages (e.g. C 60 and C 70 ) have attracted the inter- est of the scientific community because of their fascinating physical and chemical properties. [2–4] The highly symmetric structure of C 60 , its ability to undergo multiple addition reac- tions, and its exceptional electron accepting characteristics (e.g. it can accept up to 6 electrons) are by far the most impor- tant properties. [3] With its strong electron-accepting properties and remarkably small reorganization energy (ca. 0.23 eV), [2] C 60 is one of the most popular chromophores that have been in- corporated into multicomponent molecular architectures. [5–10] Therefore, fullerenes easily undergo charge-transfer interac- tions both in the ground and excited states when brought in contact with electron rich moieties via intra-molecular or inter- molecular charge transfer. [2,4] The NLO response of fullerenes, due to the large p-conjugat- ed surface, are the subject of numerous investigations several of which aim at photonic applications, for example, in the field of optical switches and optical limiters. [11] In order to enhance the non-resonant third order NLO response of fullerenes and to tune their optical properties in specific spectral regions of interest, C 60 and C 70 have been chemically modified with a large variety of electron-donating organic moieties. [12–20] In that respect, the enhancement of the charge delocalization over the fullerene cage in connection with charge transfer have been shown to be an efficient source to enhance the third- order optical nonlinear response of fullerene derivatives. Spe- cifically, evidence for the enhancement of the nonlinear optical behaviour as a consequence of the charge transfer interaction has been observed with C 60 –poly(aminonitrile) polymers and a [2+4]-adduct with a Cu–phthalocyanine. [13] All these experi- ments provided strong evidence for an exceptional enhance- ment of the second hyperpolarizability mainly due to the de- localization of charge, from the electron rich moiety to the electron poor carbon cage, producing partially negatively- charged fullerene moieties. A series of novel [60]fullerene–ferrocene and [60]fullerene–por- phyrin dyads, in which a fullerene and an electron donating moiety are attached through a flexible triethylene glycol linker are synthesized and their nonlinear optical (NLO) response stud- ied. Specifically, the third-order susceptibility c (3) of all fullerene derivatives are measured in toluene solutions by the optical Kerr effect (OKE) technique using 532 nm, 35 ps laser pulses and their second hyperpolarizability g are determined. All fullerene dyads studied exhibit enhancement of their NLO response compared to pristine fullerenes which has been attributed to the formation of a charge separated state. All experimentally measured hyperpo- larizability g values are also calculated by the semiempirical methods AM1 and PM3. A good correlation is found between the theoretical and experimental values, suggesting that simple semi- empirical methods can be employed for the designing and opti- mization of the fullerene-containing dyads displaying improved nonlinear responses. [a] Dr. E. Xenogiannopoulou, Dr. M. Medved, K. Iliopoulos, Prof. Dr. S. Couris, Dr. D. Bonifazi Institute of Chemical Engineering and High Temperature Chemical Processes (ICEHT) Foundation for Research and Technology-Hellas (FORTH) P.O. Box 1414, 26504 Patras (Greece) and Department of Physics, University of Patras, 26500 Patras (Greece) Fax: (+ 30)2610965223 E-mail: couris@iceht.forth.gr [b] Dr. M. G. Papadopoulos Institute of Organic and Pharmaceutical Chemistry National Hellenic Research Foundation (NHRF) 48 Vas. Konstandinou str., 116 35 Athens (Greece) Fax: (+ 30)2107273831 E-mail: mpapad@eie.gr [c] Dr. D. Bonifazi, Dr. C. Sooambar, Dr. A. Mateo-Alonso, Prof. Dr. M. Prato Dipartimento di Scienze Farmaceutiche Università degli Studi di Trieste Piazzale Europa 1, 34127 Trieste (Italy) Fax: (+ 39)04052572 E-mail: prato@units.it dbonifazi@units.it Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. 1056 # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2007, 8, 1056 – 1064