Evolution of lowest singlet and triplet excited states with number of thienyl rings in platinum poly-ynes

Soluble, rigid-rod organometallic polymers trans-$\mathrm{[-}\mathrm{Pt}({\mathrm{PBu}}_3^n{)}_2–\mathrm{C\equiv C–R–C\equiv C}{\mathrm{–]}}_{\infty }$ ($\mathrm{R=bithienyl}$ 2, terthienyl 3) have been synthesized in good yields by the CuI-catalyzed dehydrohalogenation reaction of trans-$[\mathrm{Pt}({\mathrm{PBu}}_3^n{)}_2{\mathrm{Cl}}_{\mathrm{2}}]$ with one equivalent of the diterminal alkynyl oligothiophenes $\mathrm{H–C\equiv C–R–C\equiv C–H}$ in ${\mathrm{CH}}_{\mathrm{2}}{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{/}}^{\mathrm{i}}{\mathrm{Pr}}_{\mathrm{2}}\mathrm{NH}$ at room temperature. We report the thermal properties, and the optical absorption, photoluminescence, and photocurrent action spectra of 1 (trans-$\mathrm{[–}\mathrm{Pt}({\mathrm{PBu}}_3^n{)}_2–\mathrm{C\equiv C–R–C\equiv C}{\mathrm{–]}}_{\infty },$ $\mathrm{R=thienyl}$ ), 2 and 3 as a function of the number of thiophene rings within the bridging ligand. With increasing thiophene content, the optical gap is reduced and the vibronic structure of the singlet emission changes toward that typical for oligothiophenes. We also find the intersystem crossing from the singlet excited state to the triplet excited state to become reduced, while the singlet–triplet energy gap remains unaltered. The latter implies that, in these systems, the $T_1$ triplet excited state is extended over several thiophene rings. The photoconducting properties do not depend on the size of the thiophene fragment. We discuss and compare our results with studies on oligothiophenes and related organometallic polymers.