Iron(ii) coordination complexes with panchromatic absorption and nanosecond charge-transfer excited state lifetimes

• Published in: Nature chemistry 2019-12, Vol.11 (12), p.1144-1150
• Main Authors: Braun, Jason D., Lozada, Issiah B., Kolodziej, Charles, Burda, Clemens, Newman, Kelly M. E., van Lierop, Johan, Davis, Rebecca L., Herbert, David E.
• Format: Article
• Language: English
• Subjects: 639/638/439/943; 639/638/911/406/939; Absorption; Analytical Chemistry; Biochemistry; Charge transfer; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Coordination compounds; Energy conversion; Excitation; Heavy metals; Inorganic Chemistry; Iron; Ligands; Molecular orbitals; Nitrogen atoms; Orbital stability; Organic Chemistry; Phenanthridine; Physical Chemistry; Quinoline; Solar energy; Solar energy conversion
• ISSN: 1755-4330; 1755-4349
• DOI: 10.1038/s41557-019-0357-z

Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is critical. Here, we present a sensitizer design in which iron( ii ) centres are supported by frameworks containing benzannulated phenanthridine and quinoline heterocycles paired with amido donors. These complexes exhibit panchromatic absorption and nanosecond charge-transfer excited state lifetimes, enabled by the combination of vacant, energetically accessible heterocycle-based acceptor orbitals and occupied molecular orbitals destabilized by strong mixing between amido nitrogen atoms and iron. This finding shows how ligand design can extend metal-to-ligand charge-transfer-type excited state lifetimes of iron( ii ) complexes into the nanosecond regime and expand the range of potential applications for iron-based photosensitizers. Replacing rare elements in benchmark photosensitizers with iron would facilitate the large-scale implementation of solar energy conversion, but iron complexes generally do not exhibit sufficiently long-lived photoexcited states. Now, it has been shown that iron( ii ) complexes with carefully designed ligands can absorb broadly across the visible light spectrum and have charge-transfer excited states with nanosecond lifetimes.