Early Transition Metal Carbene Complexes as Photocatalysts: Unveiling Dual-Emission and Photo-reactivity

Photoactive transition metal complexes are playing a pivotal role in synthetic chemistry and application technologies, such as photocatalysis and light-emitting diodes, yet they often depend on precious metals. Early-transition-metal photosensitizers offer a promising, yet still highly challenging, alternative, and understanding of their excited-state behavior is crucial for the design of more efficient compounds. Here, we present a detailed photophysical and computational study of homoleptic N-heterocyclic carbene [OCO]-type zirconium and hafnium d0 complexes, revealing their potential as efficient Earth-abundant photocatalysts. Steady-state and time-resolved spectroscopy, combined with (TD-)DFT calculations, uncover dual emission pathways involving both singlet and triplet states. Variable-temperature photoluminescence studies demonstrate that these emissions arise from distinct excited states rather than thermally activated delayed fluorescence. Under UV irradiation, both complexes catalyze bromo-alkylation of alkenes with various bromoalkyls, including fluorinated reagents, to access diverse molecular scaffolds. Mechanistic investigations incorporating Stern–Volmer quenching analysis, quantum yield measurements, and variable time normalization analyses (VTNA) support a photoinduced electron transfer process within the proposed catalytic cycle.