Bulky ligands protect molecular ruby from oxygen quenching

Chromium(iii) complexes can show phosphorescence from the spin-flip excited doublet states E-2/T-2(1) in the near-infrared with high photoluminescence quantum yields and extremely long lifetimes in the absence of dioxygen. The prototype molecular ruby, [Cr(ddpd)(2)](3+) (ddpd = N,N & PRIME;-dimethyl-N,N '-dipyridine-2-ylpyridine-2,6-diamine), has a photoluminescence quantum yield and a luminescence lifetime of 13.7% and 1.1 ms in deaerated acetonitrile, respectively. However, its luminescence is strongly quenched by (3)O(2)via an efficient Dexter-type energy transfer process. To enable luminescence applications of molecular rubies in solution under aerobic conditions, we explored the potential of sterically demanding ddpd ligands to shield the chromium(iii) center from O-2 using steady state and time-resolved photoluminescence spectroscopy. The structures of the novel complexes with sterically demanding ligands were investigated by single crystal X-ray diffraction and quantum chemically by density functional theory calculations. The O-2 sensitivity of the photoluminescence was derived from absolutely measured photoluminescence quantum yields and excited state lifetimes under inert and aerobic conditions and by Stern-Volmer analyses of these data. Optimal sterically shielded chromium(iii) complexes revealed photoluminescence quantum yields of up to 5.1% and excited state lifetimes of 518 mu s in air-saturated acetonitrile, underlining the large potential of this ligand design approach to broaden the applicability of highly emissive chromium(iii) complexes.

Automated summary

Chromium (III) complexes exhibit phosphorescence under anaerobic conditions but are quenched in the presence of oxygen. This study explores the use of sterically demanding ligands to shield the chromium (III) center from oxygen, enabling luminescence applications of these complexes under aerobic conditions. Optimal sterically shielded complexes showed high photoluminescence quantum yields and longer excited state lifetimes, demonstrating the potential of this ligand design approach.