Photoluminescence of a triangular Hg$+ cluster

The luminescence of mercury clusters in the gas phase has been studied extensively in the past [ 11. Mercury vapor shows many emission bands including the ultraviolet (A,,=335 nm) and green (1,,,=485 nm) luminescence which originate from excited dimers Hg2 and trimers Hg,, respectively. In contrast to the gas phase very little is known about the emission of mercury clusters in the condensed phase. We decided to explore this possibility and selected the compound Hg,(p-dppm)S(SO,) [2] (dppm=diphenylphosphiriomethane) which contains the triangular moiety Hgi+ for the present study. With regard to the nature of its electronic structure and excited state this complex is charao terized by several interesting features. Although the mercury has the unusual oxidation number + ! the bonding pattern of the cluster is very simple. In the ground state the Hg$+ unit is kept together by one bond (see below). In the emitting excited state the electronic structure should be similar to that of the ground-state trimers M, (M= alkali, Cu, Ag, and Au) which were the subject of numerous studies [ 3-51. Moreover, the electron configuration of mercury in the Hg$+ cation is formally intermediate between thatofHg(0) (dios2) andHg(II) (d”s’). The emission behavior of d’Os* and dl” cluster complexes [ 6,7] including those of Hg(II) [8] has been studied in some detail quite recently. Finally, the nature of electronically excited Hg!+ is also of interest with regard to the photochemical activity of mercury cluster compounds. While the dimer Hg$+ photodissociates in solution [ 91, the trimer Hg!+ is expected to undergo a Jahn-Teller distortion (see below) which might lead to the ejection of one mercury atom. Recently, this type of photoreactivity has been indeed observed. The photolysis of 0sl,Hg,C2(CO& is accompanied by a reversible release of one mercury atom [ lo]. Unfortunately, the electronic structure of this large cluster which contains a HgJ triangle has not been studied.