Photophysics of Heteroleptic Iridium(III) Complexes Of Current Interest; a Closer Look on Relaxation Dynamics

The excited-state dynamics of heteroleptic iridium(III) complexes have been investigated using femto-nanosecond timeresolved luminescence and transient absorption spectroscopy. Two prototypical examples illustrated here are iridium(III) bis[2-(2,4-difluorophenyl)pyridinato-N,C^2′] quinaldinate (1) and iridium(III) bis[2-(2,4-difluorophenyl)pyridinato-N,C^2′]- [2-(6-methylbenzoxazol-2-yl)phenolate] (2). Upon photoexcitation at, for example, 400 nm, the emission decay, monitored at the region of phosphorescence, for both 1 and 2 consists of a system response limited rise (<300 ps) and a single exponential decay. Further single wavelength as well as full spectrum of femto-picosecond transient absorption acquired in CH₂Cl₂ at room temperature reveals an ultrafast S₁→T_n intersystem crossing (<1 ps) and a rapid T_n→T₁ internal conversion/vibrational relaxation within a time period of <10 ps. The results lead us to conclude that following Franck-Condon excitation, the time scale required to populate the lowest triplet state should be <10 ps.