Molybdenum-molybdenum quadruple bonds supported by 9,10-anthraquinone carboxylate ligands. Molecular, electronic, ground state and unusual photoexcited state propertiesElectronic supplementary information (ESI) available: Table S1, Fig. S1-S12. CCDC 972771. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc00341a

The preparation of trans -Mo 2 (T i PB) 2 (O 2 C-2-AnQ) 2 , I , and Mo 2 (DAniF) 3 (O 2 C-2-AnQ), II , where, O 2 C-2-AnQ = 9,10-anthraquinone-2-carboxylate, T i PB = 2,4,6-triisopropylbenzoate and DAniF = N , N ′-di-( p -anisyl)formamidinate are described together with electronic structure calculations employing density functional theory on model compounds. Compounds I and II have been further characterized by 1 H NMR spectroscopy, MALDI-TOF mass spectrometry, electrochemical studies, UV-vis-NIR and steady state emission spectroscopy, time-resolved (fs and ns) transient absorption and infrared spectroscopy. Compound II has been structurally characterized by a single crystal X-ray crystallography study. Both compounds show metal-to-ligand charge transfer transitions, 1 MLCT, corresponding to Mo 2 δ to L π* (L = O 2 C-2-AnQ) and have S 1 states with relatively long lifetimes ( τ I ∼ 8 ps; τ II ∼ 2 ps). Femtosecond time-resolved IR studies, fs-TRIR indicate that for I , despite its relatively small HOMO-LUMO gap, and II the negative charge in the S 1 state resides on one anthraquinone carboxylate ligand. For II , fs-TRIR spectroscopy detects the Mo 2 3 δδ* state with τ ∼ 100 ns despite the presence of a lower energy 3 MLCT state. This, together with earlier findings indicates a significant kinetic barrier to the interconversion of MLCT and Mo 2 δδ* states. Supporting ligands are used to tune the relative energies of the Mo 2 δ and L π* in paddlewheel complexes resulting in compounds with unusual photoexcited state properties.