Cationic Iridium(III) Complexes with Two Carbene-Based Cyclometalating Ligands: Cis Versus Trans Isomers

A series of cationic
iridium­(III) complexes with two carbene-based cyclometalating ligands
and five different N^N bipyridine and 1,10-phenanthroline ancillary
ligands is presented. For the first timein the frame of a
rarely studied class of bis­(heteroleptic) iridium complexes with two
carbene-based cyclometalating ligandsa pair of cis and trans
isomers has been isolated. All complexes (trans-1–5 and cis-3) were characterized by 1H NMR, 13C NMR, 31P NMR, and HRMS (ESI-TOF); in addition, crystal
structures of cis-3 and trans-4 are reported and discussed.
Cyclic voltammetric studies show that the whole series exhibits highly
reversible oxidation and reduction processes, suggesting promising
potential for optoelectronic applications. Ground-state DFT and TD-DFT
calculations nicely predict the blue shift experimentally observed
in the room-temperature absorption and emission spectra of cis-3, compared to the trans complexes.
In CH3CN, cis-3 displays a 4-fold increase in photoluminescence quantum yield (PLQY)
with respect to trans-3, as a consequence of drastically slower nonradiative rate constant.
By contrast, at 77 K, the emission properties of all the compounds,
including the cis isomer, are much more similar, with a pronounced
hypsochromic shift for the trans complexes. A similar behavior is
found in solid state (1% w/w poly­(methyl methacrylate) matrix), with
all complexes displaying PLQY of ∼70–80%, comparable
emission lifetimes (τ ≈ 1.3 μs), and a remarkable
rigidochromic shift. To rationalize the more pronounced nonradiative
deactivation (and smaller PLQY) observed for photoexcited trans complexes,
comparative temperature-dependent emission studies in the range of
77–450 K for cis-3 and trans-3 were made
in propylene glycol, showing that solvation effects are primarily
responsible for the observed behavior.