An excursion in the second-order nonlinear optical properties of platinum complexes

Electrooptical devices and optical communications need compounds with second-order nonlinear optical (NLO) properties as construction blocks. Typically, to show a large quadratic hyperpolarizability, a compound should be asymmetric, be characterized by a large variation between the dipole moments in the excited and ground states, and have low energy electronic transitions. For example, a large quadratic hyperpolarizability can be reached when an electron-donor moiety is linked by means of a π-conjugated spacer to an acceptor group. In the field of second-order nonlinear optics, particularly attractive are coordination metal complexes because they are often characterized by low-energy and high-intensity intraligand (IL), ligand-to-metal (LM), and metal-to-ligand (ML) charge transfer (CT) electronic transitions, that can be regulated by the choice of the metal and ligands. Among them, platinum(II) complexes are captivating. During this excursion, we will see the quadratic hyperpolarizability of many of them. After some basic NLO notions, the review presents various classes of NLO-active platinum compounds having mono- and bidentate nitrogen ligands (pyridine, bipyridine, phenanthroline), dithiolenes, cyclometalated ligands (imines, phenylpyridine, phenylbipyridine, and dipyridylbenzene), alkynyl ligands. It is shown how to regulate the quadratic hyperpolarizability with suitable ligands and how to photomodulate the NLO properties by means of a dithienylethene (DTE) moiety. Some examples of complexes with redox-switchable NLO properties are also given. Although the accent is on the quadratic hyperpolarizability measured in solution, it is also shown how some NLO-active platinum complexes can be nano-organized to afford thin films with nonlinear optical properties. The report is not comprehensive, but it allows to get into the field of NLO-active platinum compounds and to understand their potential.