Temperature Dependent Emission Properties of Re^I Tricarbonyl Complexes with Dipyrido-Quinoxaline and Phenazine Ligands

In this work, the emission properties of fac-[Re(CO)₃(NN)(py)]⁺, NN = 1,10-phenanthroline (phen), dipyrido[3,2-f:2’,3’-h]quinoxaline (dpq) and dipyrido[3,2-a:2’3’-c]phenazine (dppz); py = pyridine were investigated in different temperatures, ranging from 80 to 300 K, and in different solvent mixtures and in polymethyl methacrylate. The changes observed in the emission quantum yields were rationalized based on a two-level excited state model, in which the non-emissive upper state is thermally populated and decays faster than the lowest lying emissive state. fac [Re(CO)₃(dpq)(py)]⁺ is a metal-to-ligand charge transfer (MLCT) emitter as the complex with phen but exhibits smaller emission quantum yields, being more sensitive to the solvent. This behavior was rationalized by quantum-mechanical calculations including the spin-orbit coupling matrix elements, revealing that intersystem crossing from the lowest singlet excited state in fac-[Re(CO)₃(dpq)(py)]⁺ likely occurs to triplet states lying at higher energies. Similar behavior were observed for fac-[Re(CO)₃(dppz)(py)]⁺, although the later exhibits intraligand emission that are strongly quenched in fluid solutions by low-lying MLCT states. The fundamental studies carried out here provide new insights on the excited state dynamics of Re^I complexes with dipyrido-quinoxaline and phenazine ligands and can contribute for further advances on their application as luminescent probes.

Keywords:
luminescence; photophysics; spin-orbit coupling; rhenium complexes