Nonlinear optical properties, dithiolenes

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand square-planar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL CT excited state (14, 129). 

Since Chapter 6 (352) in this volume is devoted to the luminescence and photochemistry of dithiolene complexes, these aspects will not be developed here. In this section, we restrict ourselves to the review of dithiolene complex-based compounds exhibiting optical properties, such as strong infrared (IR) absorption and second- and third-order nonlinear optical properties. In the last part of this section, we will review the use of some dithiolene complexes for optical data storage (printers, developer, CD-ROM, etc.). Contrary to what was observed throughout the two previous sections, no particular dithiolene complex really dominates the domain of the molecular materials exhibiting interesting optical properties. 

The extensive jt-delocalized system of metal-dithiolene complexes is also responsible for the nonlinear optical properties (NLO) which have been recently reviewed . The interaction of radiation with the matter induces an instantaneous displacement (polarization Pq = /X = aE, where a is the linear polarizability) of the electronic density away from the nucleus at small field (linear optics). At high fields (laser light) the polarizability of the molecule can be driven beyond the linear regime and a nonlinear polarization is induced (NLO) = aE + fiE" + y E + and for the bulk material... 

Finally, in Section IV dithiolene complexes with interesting optical properties, such as strong near-IR (NIR) absorption, nonlinear optical behavior, and the use of some dithiolene complexes for optical data storage will be reviewed. In Section V, attempts toward coupling conducting, and magnetic or optical properties will be discussed briefly. 

Some heteroleptic dithiolene complexes studied for their third-order NLO properties and involving the dmit or the mnt ligands are shown in Scheme 27 (455, 456). Only the dmit-containing complexes exhibit a large third-order optical nonlinearity, due to their larger planar conjugated system compared to the mnt analogues. 

Third-order NLO effects, of interest for potential applications, for example in optical limiting ° impose no symmetry constraints. Thus, studies on this type of nonlinearity have addressed both homoleptic and heteroleptic 1,2-dithiolene complexes. Among the members of the former class, study on TBA salts of monoanionic nickel Al5(l,2-dithiolene) complexes with ligands such as phdt , dddt , pddt , cddt, and -substituted 1,2-diphenyl-1,2-ethyl-enedithioate ° have been published. Underhill and co-workers have reported on the nonlinear properties of unsymmetrically substituted w(l,2-dithiolene) complexes, such as iw[l-butyl-2-phenylethene-l,2-dithiolato(2 )-S,S ]nickel(II), embedded in PMMA-based composites (PMMA=polymethylmethacrylate), or in sol-gel materials.As regards the latter class, some works have dealt with complexes containing mnt and dmit ligands. ... 

---