Double Jahn-Teller Distortion in AuGe Complexes Leading to a Dual Blue-Orange Emission

The tetrahedral Au^I complexes [Au(GeCl₃ )L₃ ] (L=PMe₃  (1), PMe₂ Ph (2), PMePh₂  (3), PPh₃  (4)) were synthesized by in situ treatment of the [Au(GeCl₃ )(tht)] complex with three equivalents of the corresponding tertiary phosphine. The crystal structures of complexes 1 and 3 have been determined through X-ray diffraction studies and show [GeCl₃ ]^- fragments bonded to the corresponding [Au(PR₃ )₃ ]⁺ units through covalent bonds, leading to a tetrahedral coordination environment for gold. Complexes 3 and 4 show dual phosphorescence in the solid state at 77 K and, in the case of complex 4, an interesting room-temperature mechanochromism effect is observed. Correlated MP2, SCS-MP2, SCS-CC2, and ONIOM MP2/UFF calculations suggest the existence of two largely distorted and thermally connected triplet excited states (T₁ and T₂ ) for a model system of complex 4. The AuP₃ ⁺ moiety can be distorted in two different planes leading to the two triplet excited states T₁ and T₂ , leaving the GeCl₃ ^- unit almost unaltered. The emissions of these phosphorescent Au^I Ge^II complexes is mainly ascribed to a metal (gold)-centered (³ MC) transition with some contribution from the [GeCl₃ ]^- fragments.