Beyond Common Energy Transfer: Intramolecular Electron Transfer Cascade Controls Triplet Population of a Long-Lived...

An iron-anthracene dyad was recently used to populate a microsecond-lived nonluminescent (dark) triplet state, but with a surprisingly low triplet population yield. In-depth spectroscopic experiments highlight that direct energy transfer does not take place, but rather an intramolecular electron-transfer occurs, generating the corresponding reduced iron center and oxidized anthracene moiety, and the final triplet dark state is populated following charge recombination. This electron-transfer cascade reaction mechanism provided the unique opportunity to control the energy level of the charge-separated state relative to the energy of the triplet state by changing the solvent polarity. As such, the triplet formation yield increased from 5% in acetonitrile to 75% in dichloromethane. This outlines an unreported mechanistic pathway for first-row transition metal complexes to populate long-lived excited states and provides design guidelines that differ between d⁵ and prototypical d⁶ photosensitizers. The d⁵ electronic configuration enables population of the final triplet energy acceptor via a cascade of electron transfer that does not formally require intersystem crossing or spin-flip transitions, thus also minimizing energy loss channels. Although the energy of the final triplet state is important, our findings highlight that the redox potentials of the excited photosensitizer and final energy acceptor moiety are pivotal to efficiently populate dark triplet states.