Electronic and optical properties of vertex sharing homo- and heterobiicosahedral gold clusters [M1M2Au 23(PH3)10(SCH3)5Cl 2]2+ (M1/M2 = Au/Au, Pt/Hg) are investigated by means of density functional computations, focusing on superatom assembly. In the homo clusters, the projected density of states onto the spherical harmonics reveals that each icosahedral unit behaves as a superatom with the Au(6s6p) electrons and electron shell closings of S and P orbitals followed by unoccupied D orbitals; each unit satisfies an 8-electron superatom condition. Because the Au(5d) band appears between the S and P orbitals, the electronic structure is characterized as S, Au(5d)-band, and P. The lowest electronic excitation in the homo cluster is the P to D transition. In the hetero biicosahedral cluster, the central Au atoms of each icosahedron are replaced by Pt and Hg. The HOMO and LUMO are well localized to the PtAu 12 and HgAu12 units, respectively, forming a cluster-assembled heterojunction in a single stable structure. The absorption spectrum of the heterobiicosahedral cluster is characterized by charge transfer from PtAu12 to HgAu12 in the visible region, accompanied by a weak charge transfer in the opposite direction. Despite low numbers of charge carriers, the heterobiicosahedral cluster shows a large dipole moment.
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