1 2,7-Fluorene-bridged Complexes Containing Electroactive "Fe(η η η 5 - C 5 Me 5 )( κ κ κ 2 -dppe)C≡C-" Endgroups: Molecular Wires and Remarkable Nonlinear Electrochromes Floriane Malvolti, a Cédric Rouxel, b Amédée Triadon, a Guillaume Grelaud, a,c Nicolas Richy, a Olivier Mongin, a,b Mireille Blanchard-Desce, b,d Loic Toupet, e Fazira I. Abdul Razak, c Robert Stranger, c Marek Samoc, f,g Adam Barlow, c Marie P. Cifuentes, c Mark G. Humphrey, c, * and Frédéric Paul a, * a Institut des Sciences Chimiques de Rennes - UMR CNRS 6226 - Université de Rennes 1 - Campus de Beaulieu - Bât. 10C - 35042 Rennes Cedex (France). b Chimie et Photonique Moléculaire - UMR CNRS 6510 - Université de Rennes 1 - Campus de Beaulieu - Bât. 10C - 35042 Rennes Cedex (France). c Research School of Chemistry, Australian National University, Canberra, ACT 2601 (Australia). d Institut des Sciences Moléculaires - UMR 5255 – Université de Bordeaux, 351 cours de la Libération, 33405 Talence (France). e Institut de Physique de Rennes (IPR) - UMR CNRS 6251 - Université de Rennes I - Campus de Beaulieu - 35042 Rennes Cedex (France) f Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200 (Australia). g Advanced Materials Engineering and Modeling Group, Faculty of Chemistry, Wroclaw University of Technology, 50370 Wroclaw (Po- land). Supporting Information Placeholder ABSTRACT: The 2,7-fluorenyl-bridged Fe(η 5 -C 5 Me 5 )(κ 2 -dppe)[C≡C(2,7-C 13 H 6 /Bu 2 )C≡C]Fe(η 5 -C 5 Me 5 )(κ 2 -dppe) (1a), its ex- tended analogue Fe(η 5 -C 5 Me 5 )(κ 2 -dppe)[C≡C(1,4-C 6 H 4 )C≡C(2,7-C 13 H 6 /Bu 2 )C≡C(1,4-C 6 H 4 )C≡C](η 5 -C 5 Me 5 )(κ 2 -dppe) (1b) and the corresponding mononuclear complexes Fe(η 5 -C 5 Me 5 )(κ 2 -dppe)[C≡C(2-C 13 H 7 /Bu 2 )] (2a) and Fe(η 5 -C 5 Me 5 )(κ 2 -dppe)[C≡C(1,4- C 6 H 4 )C≡C(2-C 13 H 7 /Bu 2 )] (2b), which model one half of these molecules, have been synthesized and characterized in their various redox states. The molecular wire characteristics of the dinuclear complexes were examined in their mixed-valent states, with pro- gression from 1a[PF 6 ] to 1b[PF 6 ] resulting in a sharp decrease in electronic coupling. The cubic nonlinear optical properties of these species were investigated over the visible and near-IR range, a particular emphasis being set on their multiphoton absorption properties; the complexes are shown to function as redox-switchable nonlinear chromophores at selected wavelengths, and the more extended derivatives shown to exhibit the more promising NLO performance. INTRODUCTION The 2,7-fluorenyl unit, with its high fluorescence quantum yield 1 and large multi-photon absorption cross-sections, 2 was identified very early as an outstanding building block for the construction of molecules for electronic 3,4,5 and photonic 6 applications. For example, this unit has been incorporated in organic and organometallic (discrete or polymeric) structures designed for optoelectronics, 7 optical limiting 8,9 or bio- imaging. 10 In these applications, the central fluorenyl linker is most often functionalized at the 2,7-positions with unsatu- rated π-conjugated arms terminated by electron-releasing substituents. 11,12 In a continuation of studies aimed at identifying efficient organoiron-based molecular wires, 13,14 we considered ex- ploiting the electron-transfer properties of such a planar and conjugated unit in Fe(II)/Fe(III) dinuclear mixed valent (MV) alkynyl complexes featuring “Fe(η 5 -C 5 Me 5 )(η 2 - dppe)C≡C-” endgroups, such as 1a + or 1b + (Chart 1), with the related mononuclear complexes 2a-b[PF 6 ] n (n = 0, 1)