Rationalizing Photophysics of Co(III) Complexes with Pendant Pyrene Moieties

Pendant organic chromophores have been used to improve the photocatalytic performance of many metal-based photosensitizers, particularly in first-row metals, by increasing π conjugation in ligands and lowering the energy of the photoactive absorption band. Using a combination of spectroscopic studies and computational modeling, we rationalize the excited state dynamics of a Co(III) complex containing pendant pyrene moieties, CoL, where L = 1,1'-(4-(pyren-1-yl)pyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium). CoL displays higher visible absorptivity, and blue luminescence from pyrene singlet excited states compared with CoL [L = 1,1'-(pyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium)] in which the pyrene moiety is absent. Emissive properties are highly influenced by the metal center, reducing the fluorescence lifetime from 5.9 to 3.5 ns, and a blue shift of 43 nm. The lower energy of the d orbitals in Co(III) compared with Fe(II) drastically affects the character of the excited state, resulting in a mixture of singlet intraligand charge-transfer (¹ILCT) and ligand-to-metal charge-transfer (¹LMCT) character. Transient absorption experiments revealed that although the dark triplet intraligand pyrene (³IL_Pyrene) state is present, it is not efficiently populated and possesses a short nanosecond-scale lifetime. Instead, triplet metal-centered (³MC) states dominate the decay path with a 2.4 ps lifetime, no photoactivity toward singlet oxygen formation or triplet-triplet energy transfer (TTET). This work shows how various factors can influence excited-state dynamics.