Homomolecular Photon Upconversion in a Perylene-Decorated Iron(III) Complex

Abstract

Classical upconversion employs sensitizers and annihilators as two distinct molecular species and is, therefore, classified as heteromolecular. Homomolecular upconversion, the unification of both roles inside one molecular species, is attractive from both fundamental and applied perspectives yet remains vastly underrepresented. Herein, we report a molecular compound (FePer), composed of an FeIIIcarbene complex that acts as red-light absorber and a doublet-state energy donor to covalently attached perylene triplet-acceptors. This compound enables homomolecular photon upconversion, a behavior made possible because the perylene fluorescence falls within a spectral window, where the FeIIIcarbene complex absorbs only very weakly. Consequently, FePer features three photoactive excited states: the fluorescent S1state of perylene, a luminescent doublet ligand-to-metal charge transfer (2LMCT) excited state on the FeIIIcarbene complex, and a dark T1state on perylene, from which the upconversion process originates. As result of reverse doublet-triplet energy transfer between the FeIII2LMCT and the perylene T1excited states, FePer exhibits2LMCT luminescence with a lifetime of 150 ns, significantly exceeding previous FeIIIcomplexes. Mechanistic investigations reveal a linear relationship between the upconverted luminescence intensity and sample concentration, which is not commonly observed in heteromolecular upconversion systems, as well as the ability of FePer to perform photon upconversion even in a frozen matrix, where diffusion cannot occur. These findings are consistent with a unimolecular mechanism, tentatively involving a doublet-triplet annihilation process. Our results highlight the untapped potential of first-row transition-metal complexes with doublet states, particularly in combination with photoactive chromophores featuring singlet and triplet states.