Room-temperature NIR phosphorescence of new iridium (III) complexes with ligands derived from benzoquinoxaline

A new series of new iridium (III) complexes (1-5) bearing ligands derived from benzoquinoxaline were designed and synthesized. X-ray structural analyses of 1 reveal a distorted octahedral geometry around the Ir atom in which the pyrazolate chelate is located opposite to the cis-oriented carbon donor atoms of benzoquinoxaline, while the benzoquinoxaline ligands adopt an eclipse configuration and their coordinated nitrogen atoms and carbon adopt trans- and cis-orientation, respectively. Complexes 1-5 exhibit moderate NIR phosphorescence with peak maxima located at around 910-930 nm. As supported by the TDDFT approach, the transition mainly involves benzoquinoxaline (3)pi-pi* intraligand charge transfer (ILCT) and metal (Ir) to benzoquinoxaline charge transfer (MLCT) of which the spectroscopy and dynamics of relaxation have been thoroughly investigated. The relatively weak NIR emission can be tentatively rationalized by the low energy gap of which the radiationless deactivation may be governed by nearly temperature-independent, weak-bonding motions in combination with a minor channel incorporating small torsional motions associated with phenyl ring in the benzoquinoxaline sites.