Redox control of ferrocene-based complexes with systematically extended π-conjugated connectors: switchable and tailorable second order nonlinear optics

Abstract

The studies of geometrical structures, thermal stabilities, redox properties, nonlinear responses and optoelectronic properties have been carried out on a series of novel ferrocenyl (Fc) chromophores with the view of assessing their switchable and tailorable second order nonlinear optics (NLO). The use of a constant Fc donor and a 4,4'-bipyridinium acceptor and varied conjugated bridges makes it possible to systematically determine the contribution of organic connectors to chromophore nonlinear optical activities. The structures reveal that both the reduction reactions and organic connectors have a significant influence on 4,4'-bipyridinium. The potential energy surface maps along with plots of reduced density gradient mirror the thermal stabilities of the Fc-based chromophores. The first and second reductions take place preferentially at the 4,4'-bipyridinium moieties. Significantly, the reduction processes result in the molecular switches with large NLO contrast varying from zero or very small to a large value. Moreover, time-dependent density functional theory results indicate that the absorption peaks are mainly attributed to Fc to 4,4'-bipyridinium charge transfer and the mixture of intramolecular charge transfer within the two respective 4,4'-bipyridinium moieties coupled with interlayer charge transfer between the two 4,4'-bipyridinium moieties. This provides us with comprehensive information on the effect of organic connectors on the NLO properties.

Why is Ferrocene so Exceptional?

“…Ferrocenes such as the seminal example of cis ‐1‐ferrocenyl‐2‐(4‐nitrophenyl)ethylene (Figure , left) were found by Green's group, followed by other groups, to show nonlinear optical (NLO) properties, with applications in optical signal processing such as the frequency doubling of laser light.…”

Synthesis, linear and nonlinear optical properties of thermally stable ferrocene-diketopyrrolopyrrole dyads

“…[16][17][18] Depending upon the oxidation state of the metal centre, the Fc unit can turn a strong donor or acceptor, a feature that has also been exploited in the reversible redox switching of the NLO response in Fc dyads. [19][20][21][22][23][24] Ferrocene dyads generally possess low oxidation potential and upon facile charge transfer (CT) to an acceptor yield stable αferrocenyl carbocations. 17,[25][26] Further, as a specific feature of the substitution pattern of the Fc unit, the non-centrosymmetric dyads are associated with high optical nonlinearities.…”

Nonlinear optical properties of rhenium(I) complexes: Influence of the extended π-conjugated connectors and proton abstraction

“…Specific methods including redox, protonation, and photocyclization have been employed to switch the NLO responses [24][25][26]. Among them, the more appealing schemes are redox and protonation, since they can lower the donor/acceptor capacity of the typical donor-acceptor significantly (D-A) [27,28].…”