##STR1## The specification identifies dithiolenes which give good performance as the critical region of an optical switch. The dithiolenes have the formula shown in FIG. 1 wherein: (A) the R groups, any two of which may be the same or different, are selected so as to tune the absorption bands of the molecule for use at the operational wavelength and/or to act as bridges to join molecules together: (B) the X groups, any two of which may be the same or different, are selected from S and Se: (C) M is selected from Ni, Pt, Pd and Cu: (D) C=-1 or 0: and (E) (i) when C=-1, Y is a cation to render the molecule electrically neutral: preferably Y is selected from ammonium, substituted ammonium, Na
+ , Li
+ and K
+ , (ii) when C=0, Y is absent (since the molecule is neutral there is no change to balance). The dithiolene may be diluted with an organic polymer selected from polyacrylates, polymethacrylates, such as polymethymethacrylate, polycarbonates, polyisobutylene, polyethylene terephthalate, polystyrene and polysiloxane.