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 (<i><b>trans</b></i><b>-1–5</b> and <i><b>cis</b></i><b>-3</b>) were characterized by ¹H NMR, ¹³C NMR, ³¹P NMR, and HRMS (ESI-TOF); in addition, crystal structures of <i><b>cis-</b></i><b>3</b> and <i><b>trans</b></i><b>-4</b> 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 <i><b>cis</b></i><b>-3</b>, compared to the trans complexes. In CH₃CN, <i><b>cis-</b></i><b>3</b> displays a 4-fold increase in photoluminescence quantum yield (PLQY) with respect to <i><b>trans</b></i><b>-3</b>, 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 <i><b>cis-</b></i><b>3</b> and <i><b>trans</b></i><b>-3</b> were made in propylene glycol, showing that solvation effects are primarily responsible for the observed behavior