Mono- and dinuclear sesquifulvalene complexes, organometallic materials with large nonlinear optical properties

Abstract

Currently, the most important class of nonlinear optical (NLO) compounds providing second harmonic generation (SHG) comprises of electron donating and accepting groups which are electronically coupled. These kinds of dipolar compounds have potential in photonic applications. In this article the synthesis and the NLO activities of dipolar mono- and dinuclear sandwich-like complexes are reviewed. These structures are composed of sesquifulvalenes and metal–ligand fragments ML n and M′L′ n which stabilize the aromatic units of the sesquifulvalene moiety. In these complexes the five-membered ring of the sesquifulvalene complexes is coordinated to ML n =Fe(C 5H 5), Fe(C 5Me 5), Ru(C 5H 5) and Co(C 4Ar 4) and acts as an electron donor; hence, its electron donating capability is varied by changing the ML n entity. The electron withdrawing capability of the seven-membered ring is changed by the coordination of Cr(CO) 3 and RuCp + or by keeping it uncoordinated. In addition, cationic borabenzene complexes are introduced as electron accepting groups. The electronic communication between the donor and acceptor is facilitated by a direct coupling or by introducing acetylenic and olefinic bridges between the donor and acceptor, e.g. E-1,2-ethendiyl, E, E-1,3-diendiyl, E, E, E,-1,5-triendiyl, 2,5-thiendiyl, 2,5′-dithiendiyl. Structural as well as UV–vis spectroscopic studies indicate a distinct mutual influence of the donor and the acceptor of the sesquifulvalene complexes. The NLO properties of these mono- and dinuclear sandwich-type complexes are studied by means of hyper-Rayleigh scattering experiments. They prove that most of the ferrocenyl derivatives demonstrate fluorescence in the region of the SHG due to two-photon absorption whereas all of the ruthenium congeners show SHG which is quantified by determination of the first hyperpolarizability. The measured β-values are large and their magnitude can be partly explained by resonance enhancement. Taking the two-level model into account the calculated values of the static hyperpolarizability are still very large indicating that mono- and bimetallic sesquifulvalene complexes are powerful NLO-chromophores.