Photoluminescent Cu(I) HETPHENs Featuring Bulky Alkyl-Substituted Phenanthrolines

Successful excited-state-enhancing
substituent effect strategies
applied in homoleptic Cu(I) metal-to-ligand charge transfer (MLCT)
chromophores have been adapted to the heteroleptic phenanthroline
(HETPHEN) platform, leveraging 2,9-mesityl-1,10-phenanthroline (mesPhen)
in conjunction with a series of seven distinct 2,9- and 2,3,4,7,8,9-substituted
phenanthrolines. The newly conceived CuHETPHEN complexes feature MLCT
lifetimes ranging from 62 to 443 ns, all of which exhibit unprecedented
room-temperature photoluminescence and demonstrate quantitative adherence
to the energy gap law. TD-DFT calculations successfully modeled the
systematic variation in electronic transition intensities, accounting
for the experimentally observed UV–vis absorption bands across
the entire series of molecules while providing detailed structural
explanations for the variations in spectral profiles. These heteroleptic
Cu(I) diimine chromophores exhibit thermally activated delayed fluorescence
(TADF), with delayed fluorescence occurring between closely spaced 1MLCT and 3MLCT excited manifolds, having energetic
separations of ΔE = 713–1009 cm–1,
as demonstrated here for the first time. Nanosecond and ultrafast
transient absorption spectroscopy verified the MLCT nature of these
excited states and extracted the time constants for the initial pseudo-Jahn–Teller
distortion and intersystem crossing throughout the entire series of
molecules. Finally, the triplet photosensitization properties of these
Cu(I) HETPHENs are demonstrated in a number of model photochemical
transformations.