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Issue 4, 2017
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Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

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Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV–vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations. Each compound displays a reversible Fc+/0 redox process, together with two reversible one-electron reductions of the Hq fragment. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering at 1064 nm, and Stark (electroabsorption) spectroscopic studies on the visible absorption bands. The obtained first hyperpolarizabilities β are moderate, consistent with the relatively short π-conjugation lengths between the Fc and attached pyridinium group. A single-crystal X-ray structure has been solved for one of the complexes as its PF6 salt, revealing a centrosymmetric packing in the triclinic space group P[1 with combining macron]. Density functional theory (DFT) and time-dependent DFT calculations indicate that the lowest energy absorption bands have mainly metal-to-ligand charge-transfer character. The donor orbitals involved in the electronic transitions forming the next lowest energy ICT band also have substantial contributions from the Fe atom. Good agreement between the simulated and experimental UV–vis absorption spectra is achieved by using the PBE0 functional with the 6-311++G(d)/LANL2DZ mixed basis set, and the theoretical β values are reasonably large. Oxidation of the Fc unit is predicted to cause the βtot value to decrease by more than 80% in one of the complexes.

Graphical abstract: Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

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Publication details

The article was received on 16 Nov 2016, accepted on 19 Dec 2016 and first published on 20 Dec 2016

Article type: Paper
DOI: 10.1039/C6DT04347J
Citation: Dalton Trans., 2017,46, 1052-1064
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    Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores

    L. E. R. Buckley, B. J. Coe, D. Rusanova, S. Sánchez, M. Jirásek, V. D. Joshi, J. Vávra, D. Khobragade, L. Pospíšil, Š. Ramešová, I. Císařová, D. Šaman, R. Pohl, K. Clays, N. Van Steerteghem, B. S. Brunschwig and F. Teplý, Dalton Trans., 2017, 46, 1052
    DOI: 10.1039/C6DT04347J

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