Progress and perspectives on strategies to control photochemical properties in Metallo-Dithiolene Donor-Acceptor systems

In this mini-review recent progress in studies on heteroleptic d8-metal dithiolene complexes with a D-M-A donor–π–acceptor electronic structure showing non-linear and linear optical properties, are presented. The ligands, formally dithiolenes, consist in a variety of donors (dithiolato) and acceptors (dithioxamide and dithioxamidate), differing for electronic properties and/or structural features, varying from rigid and planar systems to conformationally-flexible ones. It is shown how the components of these D-M-A systems modulate both the energy and topology of the frontier molecular orbitals involved in the charge transport mechanism to reach a fine tuning of the optical properties and achieve high values of the quadratic hyperpolarizability. Further tailoring of the ligands by integrating specific functional groups in their periphery, such as NH and/or a quinoxaline ring, is presented as a versatile tool to achieve a reversible response to external stimuli. Accordingly, selected cases where exchange color proton and silver ions tunable properties, switching of the nonlinear optical response (with a 2.5–6.5 contrast for HCl exchange) and also uncommon emission color reversibly tunable by different stimuli, such as proton and silver ions, are presented. In the bulk, crystals gave a poor or null second harmonic generation (SHG) response, while a stable and large response has been obtained in a limited number of cases of complexes dispersed in polymethylmethacrylate films. Reported results are addressed to stimulate further work to exploit the potential of this class of complexes as molecular sensors and switches and to optimize the processing procedures to achieve good SHG responses in the bulk.