Rationalizing Photophysics of Co(III) Complexes with Pendant Pyrene Moieties
Jéssica Toigo, Ka‐Ming Tong, Rida Farhat, Saeid Kamal, Eva M. Nichols, Michael O. Wolf
Abstract
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 1, where L 1 = 1,1′-(4-(pyren-1-yl)pyridine-2,6-diyl)bis(3-methyl-1 H -imidazol-3-ium). CoL 1 displays higher visible absorptivity, and blue luminescence from pyrene singlet excited states compared with CoL 0 [ L 0 = 1,1′-(pyridine-2,6-diyl)bis(3-methyl-1 H -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 ( 1 ILCT) and ligand-to-metal charge-transfer ( 1 LMCT) character. Transient absorption experiments revealed that although the dark triplet intraligand pyrene ( 3 IL Pyrene ) state is present, it is not efficiently populated and possesses a short nanosecond-scale lifetime. Instead, triplet metal-centered ( 3 MC) 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.