Phosphorescent Chromophores as Structural Tectons for Coordination-Driven Self-Assembly Leading to Emergent Photophysical Properties.
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Coordination-driven self-assembly is a synthetic methodology that allows for the creation of metallacycles and metallacages from component Lewis-acid acceptors and Lewis-basic donors as building blocks, or tectons. By using tectons that are emissive, specifically phosphorescent, metallacycles and cages are assembled that hold these chromophoric building blocks in specific orientations vis-à-vis one another. In doing so, photophysical properties of the assembly can be different than those of the parent chromophore, and new properties emerge that are absent in the parent. Ruthenium-pyridyl bonding was used as the driving force to assemble a truncated tetrahedral cage with bis-(2,2′-bipyridine)ruthenium(II) at each corner of the cage as the Lewis-acid acceptor. This cage proved to be more absorptive than the parent chromophore. The emission observed was weaker than the parent and decayed bi-exponentially due to the emergence of a second triplet excited state close in energy to that of the emissive state. Similar features were observed using cyclometalated iridium as the parent chromophore to assemble both truncated tetrahedra and metallosquares. These assemblies, although still less emissive than the parent chromophore, were more emissive than the ruthenium analogue, allowing for a more accurate measure and detailed analysis to be performed. Finally, a Lewis-basic donor based on a platinum-alkynyl motif was synthesized. It was observed that the quantum yield could be controlled by the methylation of a 3 or 4-pyridyl functional group, giving evidence for the existence of a metallo-cumulene resonance structure. This tecton was then used to assemble a triangle, which had an increase in the radiative rate constant leading to a higher quantum yield than the parent chromophore. A modified version of this tecton was also used to assemble a tetrahedron which, depending on the choice of metal (Zn or Fe) used for the vertices, showed energy transfer from the platinum tecton to the first-row transition metal node.