Zincafluorene complex with an empty C–Zn π orbital that captures visible light

Abstract

Zinc complexes are attractive candidates for photofunctional materials owing to the low cost and benign nature of Zn, yet directly engaging Zn orbitals in visible-light responses has remained elusive in mononuclear systems. Here, we realize this by engineering an empty C–Zn π orbital as the LUMO in carbene–zincafluorene frameworks, as demonstrated in three newly designed mononuclear Zn complexes. Selective recrystallization of one complex afforded two conformational polymorphs, enabling us to establish a clear correlation among carbene–zincafluorene coplanarity, formation of the empty C–Zn π orbital, LUMO lowering, and visible-light absorption. Natural atomic orbital analyses of the complexes indicate that tuning the carbene scaffold improves C(2p)/Zn(4p) energy-level matching and thereby increases Zn(4p) participation in the LUMO. Additionally, one complex exhibits bright room-temperature phosphorescence in the solid state with a PL quantum yield of 21% and an emission lifetime of 2.0 ms; theoretical calculations including spin–orbit coupling identify the Zn center as a key contributor to the emission. As a proof-of-concept for visible-light responsive photofunctional application, the same complex catalyzes stilbene isomerization under blue-LED irradiation. These results highlight the non-innocent role of Zn in visible-light responses and pave the way toward photoactive mononuclear Zn complexes.