OPTICAL-EXCITATIONS IN TRANSITION-METAL CONTAINING POLY-YNES

We have measured the optical properties of poly[trans(bis(tributylphosphine))platinum(1,4) phenylenediethynylene)]. In this polymer, conjugated phenylenediethynylene units are linked by square-planar coordinated platinum to form a linear chain. We consider that in contrast to hydrocarbon conjugated polymers, the presence of the heavy metal atom on the chain allows transitions between the singlet and triplet manifolds. We see pi conjugation that extends through the metal sites. Strong singlet pi-pi* absorption peaks at 3.26 eV, and weaker triplet absorption peaks at 2.43 eV at room temperature. Strong photoluminescence from triplet to ground state peaks at 2.38 eV, and weak photoluminescence from the excited singlet peaks at 3.06 eV at low temperatures. We also see photoinduced absorption which peaks at 1.49 eV, and which shows the same temperature and temporal dependence as the strong luminescence, with a lifetime of 30 musecs at 10 K. We associate this feature with a triplet to triplet transition. We consider that these measurements provide a direct probe of the exchange energy for the excitons which are primarily located on the pi-conjugated section of the chain.