D–A dyads and A–D–A triads based on ferrocene: push–pull dyes with unusual behaviours in solution

Ferrocene has been extensively used for the design of chromophores with reversible electrochemical properties. In this work, a series of twelve D–A dyads and twelve symmetrically substituted A–D–A triads have been developed based on twelve different electron acceptors. The photophysical characteristics of the different dyes have been studied using various techniques including UV-visible absorption and photoluminescence spectroscopy. Their electrochemical behaviours have also been determined. To evidence the specificities of the triads relative to those of their donor–acceptor (D–A) dyad analogues, comparison between these two families of dyes (D–A and A–D–A) has been made. The solvatochromic behaviour of the twenty-four dyes has been examined in 22 solvents of different polarities. Parallel to solvatochromism, suppression of the push–pull effect by oxidation of the ferrocene unit using a strong oxidizing agent, i.e. nitrosonium tetrafluoroborate, drastically modified the optical properties of the different dyes. To support the experimental results, theoretical calculations were carried out. Interestingly, in this work, an unprecedented modification of the UV-visible absorption spectra in solution was observed over time for all ferrocene-based push–pull dyes in various solvents. Evidence of this behaviour for ferrocene-based push–pull dyes is unprecedented in the literature.