Rationalizing Photophysics of Co(III) Complexes with\nPendant Pyrene Moieties

Pendant\norganic chromophores have been used to improve the photocatalytic\nperformance of many metal-based photosensitizers, particularly in\nfirst-row metals, by increasing π conjugation in ligands and\nlowering the energy of the photoactive absorption band. Using a combination\nof spectroscopic studies and computational modeling, we rationalize\nthe excited state dynamics of a Co(III) complex containing pendant\npyrene moieties, <b>CoL</b><sub><b>1</b></sub>, where <b>L</b><sub><b>1</b></sub> = 1,1′-(4-(pyren-1-yl)pyridine-2,6-diyl)bis(3-methyl-1<i>H</i>-imidazol-3-ium). <b>CoL</b><sub><b>1</b></sub> displays higher visible absorptivity, and blue luminescence from\npyrene singlet excited states compared with <b>CoL</b><sub><b>0</b></sub> [<b>L</b><sub><b>0</b></sub> = 1,1′-(pyridine-2,6-diyl)bis(3-methyl-1<i>H</i>-imidazol-3-ium)] in which the pyrene moiety is absent.\nEmissive properties are highly influenced by the metal center, reducing\nthe fluorescence lifetime from 5.9 to 3.5 ns, and a blue shift of\n43 nm. The lower energy of the d orbitals in Co(III) compared with\nFe(II) drastically affects the character of the excited state, resulting\nin a mixture of singlet intraligand charge-transfer (<sup>1</sup>ILCT)\nand ligand-to-metal charge-transfer (<sup>1</sup>LMCT) character.\nTransient absorption experiments revealed that although the dark triplet\nintraligand pyrene (<sup>3</sup>IL<sub>Pyrene</sub>) state is present,\nit is not efficiently populated and possesses a short nanosecond-scale\nlifetime. Instead, triplet metal-centered (<sup>3</sup>MC) states\ndominate the decay path with a 2.4 ps lifetime, no photoactivity toward\nsinglet oxygen formation or triplet–triplet energy transfer\n(TTET). This work shows how various factors can influence excited-state\ndynamics.