Triplet Excited-State Energetics and Dynamics in Molecular "Roller Wheels"

We report the photoinduced excited-state dynamics of a series of novel Pt(II)-bisacetylide containing conjugated small molecules that possess geometric shapes resembling "roller wheels," but that differ in lengths of the linear conjugated chromophores, i.e., the "rollers". We measure excited-state evolution and triplet formation by ultrafast pump probe transient absorption followed by quantifying the triplet decay lifetimes using nanosecond flash photolysis transient absorption techniques. Two of these complexes of relatively longer chromophore lengths and smaller bandgaps do not show phosphorescence emission. Thus, the triplet energy is approximated by quenching,utilizing fullerene derivatives with varying lowest unoccupied molecular orbital energies. Furthermore, O-3(2) quenching experiments are performed to quantify the minimum triplet excited-state quantum yields for these complexes. Finally, our fullerene and O-3(2) transient absorption quenching data demonstrate that the estimations of excited-state energies by cyclic voltammetry are not always accurate for these low band-gap organic materials, which require more extensive photophysical studies.