Photophysics of Oligothiophenes End‐Capped with Platinum(II) Auxochromes.

A series of four oligothiophenes end‐capped with −Pt(PBu3)2Cl moieties on both ends of the oligomers was synthesized, and their excited state properties were investigated. The observation of low fluorescence quantum yield (<2 %) for the oligomers indicates the strong effect of platinum on the intersystem crossing (ISC) efficiency. No phosphorescence was detected for any of the oligomers; however, strong triplet‐triplet absorption was observed by nanosecond transient spectroscopy for oligomers with more than one thiophene unit. The oligomers displayed short triplet lifetimes (∼1–2 μs) compared to the unmetallated oligomers, due to large spin‐orbit coupling induced by the platinum atom. The lower limits of the ISC yields were indirectly determined by measuring the singlet oxygen quantum yields. Femtosecond–picosecond transient absorption studies revealed that the ISC rate ranges from 1012–1010 s−1, decreasing with increasing oligomer length. Electrochemical studies showed that the oligomers exhibit relatively low oxidation potentials (ca. 0.1 V vs. Fc/Fc+). Quenching of the oligomers' triplet state absorption, simultaneously with the rise of their corresponding cationic radical absorption band in nanosecond transient spectra in the presence of methyl viologen, as an electron acceptor, established that the electron transfer occurs from their triplet state. [ABSTRACT FROM AUTHOR]