Reversible switching of the first hyperpolarisability of an NLO-active donor–acceptor molecule based on redox interconversion of the octamethylferrocene donor unit Michael Malaun, a Zoe R. Reeves, a Rowena L. Paul, a John C. Jeffery, a Jon A. McCleverty,* a Michael D. Ward,* a Inge Asselberghs, b Koen Clays b and Andr´ e Persoons b a School of Chemistry,University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS. E-mail: mike.ward@bristol.ac.uk; jon.mccleverty@bristol.ac.uk b Laboratory of Chemical and Biological Dynamics, Center for Research on Molecular Electronics and Photonics, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Received (in Cambridge, UK) 5th October 2000, Accepted 15th November 2000 First published as an Advance Article on the web 15th December 2000 Compound 1, containing an octamethylferrocene donor linked to a nitrothiophene acceptor via an ethenyl linker, shows a static first hyperpolarisability b 0 of 95(±10) 3 10 230 esu, which is reduced to 10(±2) 3 10 230 esu on oxidation of the octamethylferrocene unit; this provides for a simple redox-based switching of the NLO characteristics of the compound. Compounds displaying non-linear optical (NLO) properties are of considerable interest because of their possible applications in the emerging technologies of optoelectronic and photonic devices. 1 Second-order NLO effects, including second-har- monic generation, and especially electro-optic modulation are important in interfacing massive amounts of electronic data to wideband optical communication. At the molecular level, the efficiency for electro-optic modulation is determined by the second-order non-linear polarisability, also called the first hyperpolarisability, b. A large value of b is generally associated with molecules which have a donor/conjugated bridge/acceptor (D-p-A) structure, such that there is a long-range charge- transfer transition from one end to the other and consequently a substantial difference between the ground-state and excited- state dipole moments. The ability to switch the NLO response of a molecule ‘on’ and ‘off’ reversibly by a simple controllable perturbation would add significant value to the utility of NLO molecules, from the point of view of developing molecular photonic devices whose properties can be switched by modifying one of the component parts. 2 Despite the large number of molecules with large first hyperpolarisabilities, there are remarkably few examples in which such reversible switching has been demonstrated. Of these the majority depend on isomerisation or tautomerisation of the molecule, such that the nature of the conjugated bridge linking the donor and acceptor termini undergoes a substantial change. 3 A more appealing method of controlling the second- order NLO response of a molecule would be a reversible redox change, in which either the donor (D) unit is oxidised or the acceptor (A) unit is reduced. The result in either case would be a loss of the charge-transfer capability and a consequent drop in the hyperpolarisability b. 2 To date there is a single example of this in the literature, from the group of Coe, comprising a {Ru(NH 3 ) 5 } 2+ donor linked to a viologen-like acceptor; the value of b decreased by an order of magnitude on one-electron oxidation of the Ru terminus. Subsequent re-reduction of the Ru terminus completely restored the SHG properties of the compound. 4 We describe here a new molecule for second-order NLO applications (1) which contains an octamethylferrocene donor unit and a nitrothiophene acceptor, linked by an ethenyl bridge. The reversible octamethylferrocene–octamethylferrocenium couple, at modest potential, provides an ideal route for redox- based switching of the hyperpolarisability. Compounds which contain ferrocene units as the electron donor, with a single conjugated side-arm linked to an acceptor unit, have been exceptionally popular for studying SHG in the last decade. 5 The redox activity of the ferrocene donor unit however has not until now been exploited for switching purposes; use of the octamethylferrocene unit in 1 will both enhance its electron- donor properties compared to ferrocene, and will reduce the redox potential to make redox-based switching more facile. Compound 1 was prepared in good yield† by a Wittig reaction between 1A,2,2A,3,3A,4,4A,5-octamethylferrocenylme- thyltriphenylphosphonium bromide 6 and 5-nitro-2-thiophene- carbaldehyde in tetrahydrofuran (THF); the crystal structure is in Fig. 1.‡ The cyclopentadienyl ring of the donor, the ethenyl bridge, and the nitrothiophene acceptor are essentially coplanar which will clearly optimise end-to-end charge-transfer via the conjugated system. The electronic spectrum in CH 2 Cl 2 (Fig. 2) Fig. 1 Crystal structure of 1 together with selected bond distances. The structure of the complex cation of [1] + (PF 6 )·CH 2 Cl 2 is essentially identical, and the bond distances for this are given in square parentheses after the corresponding value for 1. Fe–C (average) 2.05 [2.10], N(1)–O(1) 1.242(4) [1.232(3)], N(1)–O(2) 1.228(4) [1.235(3)], N(1)–C(1) 1.423(4) [1.428(3)], C(1)–C(2) 1.354(5) [1.356(4)], C(2)–C(3) 1.405(4) [1.401(3)], C(3)–C(4) 1.377(4) [1.379(3)], C(1)–S(1) 1.719(3) [1.718(2)], C(4)–S(1) 1.738(3) [1.731(2)], C(4)–C(5) 1.442(4) [1.446(3)], C(5)–C(6) 1.348(4) [1.336(3)], C(6)–C(7) 1.449(4) [1.461(3)] Å. Fig. 2 Electronic spectra of 1 and [1] + (PF 6 ) in CH 2 Cl 2 . This journal is © The Royal Society of Chemistry 2001 DOI: 10.1039/b008056j Chem. Commun., 2001, 49–50 49