Highly Efficient Intrinsic Phosphorescence from a σ‑Conjugated Poly(silylene) Polymer

We have observed highly efficient intrinsic phosphorescence of a neat σ-conjugated polymer, poly(biphenyl-4-ylmethylsilylene) (PBMSi). At low temperatures, PBMSi solid films featured ∼15% phosphorescence quantum yield, which is unusually high for purely organic conjugated polymers and is comparable to that of organometallic polymers. Exciton dynamics in PBMSi was studied by ultrafast fluorescence and time-gated delayed emission measurements. It was shown that the phosphorescence of PBMSi originates from the radiative decay of triplets on the π-conjugated biphenyl group constituting the lowest triplet state, T1, which is populated under the excitation of the σ-conjugated polymer backbone, i.e., with energy well below the lowest singlet excited state of the biphenyl group itself. The nature of the excited states in PBMSi was further investigated by performing quantum-mechanical calculations of the model compound. The calculations showed that the lowest singlet excited state has charge-transfer (CT) character involving different parts of the same macromolecule. Energetically this state lies very close to the CT triplet excited state. We argue that the intramolecular CT state is responsible for the strongly enhanced intersystem crossing (ISC) in PBMSi due to the small positive CT singlet–triplet energy splitting, which is itself a consequence of a weak exchange interaction of a spatially separated electron and hole in the CT state. This study suggests a new molecular-level engineering approach for enhancement of the ISC, enabling efficient conversion of primary excited singlets into triplets in conjugated polymers without involving a heavy atom effect while leaving the rate of radiative T1 → S0 transition virtually unaffected.