Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet-singlet, S₀ → S₁? singlet-triplet, S₀ → T₁t and triplet-triplet, T₁ → T_n, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T₁, is strongly localized on a single phenylene ring while the S₁ and T_n states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang-Rhys analysis of the triplet emission spectrum in model systems.