Distinct Ligand- and Metal-Centered Phosphorescence in a Terbium Carbazolyl Complex

Carbazolide complexes of lanthanum and terbium with cyclooctatetraenediide (COT) and THF coligands of the type [(^dtbpCbz)LnCOT(thf)_n] (n = 2 for La, 1 for Tb) were synthesized by salt metathesis reactions. The THF molecules were found to be labile, and drying under vacuum led to their partial removal with concomitant formation of the dinuclear complexes [(^dtbpCbz)₂Ln₂(COT)₂(thf)]. The luminescence of both lanthanum and terbium complexes was investigated, and at cryogenic temperatures, strongly temperature-dependent phosphorescence was observed. The terbium complexes show the expected element-characteristic emission with narrow lines between 480 and 700 nm upon excitation at 370 nm. Beyond that, broad emission was induced selectively by excitation at lower energy. Related phosphorescence was found for the lanthanum complex, which implies intra- or inter-ligand excitation as source for the latter. This interpretation was corroborated by TD-DFT computations.