Spirobifluorene Bridged Ir(III) and Os(II) Polypyridyl Arrays: Synthesis, Photophysical Characterization, and Energy Transfer Dynamics

The synthesis, characterization, photophysics, and time-dependent density functional theory (TD-DFT) calculations of spirobifluorene-bipyridine based iridium(III), osmium(II), and mixed Ir/Os complexes are presented. The preparation of the reference and mixed complexes proceeded step-by-step and microwave irradiation facilitated the complexation of osmium. The absorption of the target heterobimetallic derivative, Ir-L-Os, is described by linear combination of half of the absorption spectra of the homobimetallic analogues, Ir-L-Ir and Os-L-Os, due to the occurrence of mixed ligand and metal based transitions when the spirobifluorene-(bpy)2 bridging ligand L is linked to the metal, confirming a negligible interaction between the substituted metallic chromophores. TD-DFT calculations on monometallic, homo- and hetero-bimetallic complexes fully disentangled the origin of the absorption features. Noticeably, in the mixed Ir-L-Os complex an almost quantitative energy transfer from the 3Ir to the 3Os MLCT state is occurring, with a rate constant of 4.1 × 108 s-1 and nearly exclusively via a Dexter-type mechanism mediated by the orbitals of the spiroconjugated ligand. This result, together with the outcomes of the TD-DFT calculations, supports the existence of spiroconjugation and evidences the interesting role of this kind of bridge in the energy transfer dynamics of the arrays. In all the complexes, moreover, the ligand fluorescence is heavily quenched by energy transfer processes toward the metallic appended units; the rate constant is estimated in the order of 1010 s-1 for Ir-L-Os and higher than 1012 s-1 for the other complexes. In the heterometallic array, both at room temperature and at 77 K, all photons are thus funneled to the emissive Os 3MLCT state, which acts as energy trap for the antenna cascade.