Osmium Complex-Chromophore Conjugates with Both Singlet-to-Triplet Absorption and Long Triplet Lifetime through Tuning of the Heavy-Atom Effect

Os(II) complexes showing singlet-to-triplet absorption are of growing interest as a new class of triplet sensitizers thatcircumvent energy loss during intersystem crossing, and they enableeffective utilization of input photon energy in various applications,such as photoredox catalysis, photodynamic therapy, and photonupconversion. However, triplet excited-state lifetimes of Os(II)complexes are often too short (Tau<1 mu s) to transfer their energy toneighboring molecules. While the covalent conjugation of chromo-phores has been known to extend the net excited-state lifetimesthrough an intramolecular triplet energy transfer (IMET), heavy-atom effects of the central metals on the attached chromophore unitshave rarely been discussed. Here, we investigate the relationshipbetween the spin-density contribution of the heavy metals and the nettriplet excited-state lifetimes for a series of Os(II) and Ru(II) bis(terpyridine) complexes modified with perylene units.Phosphorescence lifetimes of these compounds strongly depend on the lifetimes of the perylenyl group-localized excited states thatare shortened by the heavy-atom effect. The degree of heavy-atom effect can be largely circumvented by introducingmeta-phenylenebridges, where the perylene unit retains its intrinsic long excited-state lifetime. The thermal activation to the short-lived excited statesis suppressed, thanks to sufficient but still small energy losses during the IMET process. Involvement of the metal center was alsoconfirmed by the prolonged lifetime by replacing Os(II) with Ru(II) that possesses a smaller spin-orbit coupling constant. Theseresults indicate the importance of ligand structures that give a minimum heavy-atom effect as well as the sufficient energy gap among the excited states and fast IMET for elongating the triplet excited-state lifetime without sacrificing the excitation energy.

Automated summary

The relationship between the spin-density contribution of heavy metals and the net triplet excited-state lifetimes for Os(II) and Ru(II) complexes modified with perylene units was investigated. It was found that the lifetimes of perylenyl group-localized excited states are shortened by the heavy-atom effect, which can be largely circumvented by introducing meta-phenylene bridges.