Optical spectroscopy of platinum and palladium containing poly-ynes

We present a comparative study of two isostructural platinum and palladium poly-ynes, poly[trans(bis(tributylphosphine))M(1,4) phenylenediethynylene], where M represents either platinum or palladium. Optical excitations are associated with the π-molecular orbitals, and with strong spin-orbit coupling provided by the transition metal, both singlet and triplet excitations can be measured. The platinum and palladium polymers show similar low-temperature photoluminescence and photoinduced absorption spectra, with a blue-shift for the palladium polymer of around 5% for electronic transitions with respect to the platinum polymer. For Pt (Pd) we see strong singlet absorption peaks at 3.26 eV (3.49 eV), and weaker triplet absorption peaks at 2.43 eV (≈2.5 eV) at room temperature. Strong photoluminescence from triplet to ground state peaks at 2.38 eV (2.5 eV), and weak photoluminescence from the excited singlet peaks at 3.06 eV (3.2 eV) at low temperatures. Photoinduced absorption that peaks at 1.49 eV (1.72 eV) is due to transitions within the triplet manifold. Triplet lifetimes at low temperatures of 30 and 200-600 μs are found for the platinum and palladium polymers, respectively.