Matrix Effects on Photoluminescence and Oxygen Sensitivity of a Molecular Ruby

The molecular ruby analogue [Cr(ddpd)(2)](3+) (ddpd=N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) exhibits near infrared (NIR) emission with a high photoluminescence (PL) quantum yield phi(PL) of 11 % and a lifetime of 898 mu s in deaerated water at room temperature. While ligand-based control of the photophysical properties has received much attention, influences of the counter anions and microenvironment are still underexplored. In this study, the luminescence properties of the molecular ruby were systematically examined for the counter anions Cl-, Br-, [BF4](-), [PF6](-), [BPh4](-), and [BArF24](-) in acetonitrile (MeCN) solution, in crystals, and embedded into polystyrene nanoparticles (PSNP). Stern-Volmer analyses of the oxygen quenching studies in the intensity and lifetime domain showed the highest oxygen sensitivity of the complexes with the counter anions of [BF4](-) and [BArF24](-), which also revealed the longest luminescence lifetimes. Embedding [Cr(ddpd)(2)][PF6](3) in PSNPs and shielding with poly(vinyl alcohol) yields a strongly NIR-emissive oxygen-insensitive material with a record phi(PL) of 15.2 % under ambient conditions.

Automated summary

The research examines the photophysical properties of a molecular ruby analogue and explores the influence of counter anions and microenvironment. Embedding the analogue into nanoparticles results in an oxygen-insensitive material with strong near infrared emission.