Ferrocenyl D–π–A conjugated polyenes with 3-dicyanomethylidene-1-indanone and 1,3-bis(dicyanomethylidene)indane acceptor groups: Synthesis, linear and second-order nonlinear optical properties and electrochemistry

Second-order nonlinear optical chromophores incorporating the ferrocenyl group as an electron donor and 3-dicyanomethylidene-1-indanone and 1,3-bis(dicyanomethylidene)indane acceptor groups, connected by a conjugated polyenic bridge of varied length (2[n] and 3[n], respectively) have been synthesized. The electronic absorption spectra of these compounds display in the visible region bands attributable to π–π* and metal-to ligand charge transfer (MLCT) transitions. The energies of these transitions are close to those reported earlier for ferrocenyl D–π–A chromophores with the strongest acceptor groups, e.g., with the 3-dicyanomethylidene-2,3-dihydrobenzothiophene-1,1-dioxide group (1[n]) [V. Alain, M. Blanchard-Desce, C.-T. Chen, S.R. Marder, A. Fort, M. Barzoukas, Synt. Met. 81 (1996) 133]. The solid-state structure of 2[3], determined by X-ray diffraction shows a significant reduction of the bond length alternation (BLA), 0.05 Å, suggesting high first hyperpolarizability. However, a centrosymmetrical packing of molecules of this compound in the crystal excludes its second harmonic generation ability. The μβ values of 2[n] and 3[n], determined by the EFISH technique at 1907 nm are high and increase with the increasing length of the conjugated π-bridge. The highest value of μβ (8720 × 10−48 esu) was determined for 3[4], which is close to that reported for 1[4] (11 200 × 10−48 esu), the highest value found for a ferrocenyl D–π–A chromophore until now.