Efficient Near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes

We
report on the photophysical studies of two cationic near-UV
emitters based on bis-pincer Ir­(III) carbene complexes: [IrnBu(CNHCMeCCNHC)2]­X, where IrnBu(CNHCMeCCNHC) is (4,6-dimethyl-1,3-phenylene-κC2)­bis­(1-butylimidazol-2-ylidene) and X = I– or PF6–). The compounds are highly
emitting in deaerated CH3CN solution with emission maxima
at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and
0.38, for [IrnBu(CNHCMeCCNHC)2]I and IrnBu(CNHCMeCCNHC)2]­PF6, respectively.
In order to gain deeper understandings into their structural and electronic
features, as well as to ascertain the nature of the excited states
involved into the electronic absorption processes, density functional
theory (DFT) and time-dependent DFT (TD-DFT) calculations have been
performed on the ground and excited states of the closely related
complex [IrMe(CNHCMeCCNHC)2]+. In the solid state, an emission at low
energy is observed (λmax = 500 nm) for both complexes.
However, the intensity of the emission at high energy versus the intensity
of the new emission at low energy is dependent on the nature of counterions.
The origin of this emission is not completely clear, but the experimental
data point to the formation of trapping sites induced by aggregation
processes involving the interaction between the cationic emitter and
the counterion.