Dual Fluorescence and Phosphorescence Emissions from Dye-Modified ()-Bismuth Pincer Thiolate Complexes.

We report the synthesis, characterization, and photophysical properties of four new dye-modified ()Bi pincer complexes with two mercaptocoumarin or mercaptopyrene ligands. Their photophysical properties were probed by UV/vis spectroscopy, photoluminescence (PL) studies, and time-dependent density functional theory (TD-DFT) calculations. Absorption spectra of the complexes are dominated by mixed pyrene or coumarin π → π*/n(pS) → pyrene or coumarin π* transitions. While unstable toward reductive elimination of the corresponding disulfide under irradiation at room temperature, the complexes provide stable emissions at 77 K. Under these conditions, coumarin complexes and exhibit exclusively green phosphorescence at 508 nm. In contrast, the emissive properties of pyrene complexes and depend on the excitation wavelength and on sample concentration. Irradiation into the lowest-energy absorption band exclusively triggers red phosphorescence from the pyrenyl residues at 640 nm. At concentrations < 1 μM, excitation into higher excited electronic states results in blue pyrene fluorescence. With increasing (1-100 μM), the emission profile changes to dual fluorescence and phosphorescence emission, with a steady increase of the phosphorescence intensity, until at ≥ 1 mM only red phosphorescence ensues. Progressive red-shifts and broadening of steady-state excitation spectra with increasing sample concentration suggest the presence of static excimers, as we observe it for concentrated solutions of pyrene. Crystalline and powdered samples of indeed show intermolecular association through π-stacking. TD-DFT calculations on model dimers and a tetramer of support the idea of aggregation-induced intersystem crossing (AI-ISC) as the underlying reason for this behavior.