Azobenzene-Appended Bis-Cyclometalated Iridium(III) Bipyridyl Complexes

Abstract

A new synthetic route toward photochromic azobenzeneappended bipyridyl ligands has been developed, and 20 new cationic iridium(III) bis-cyclometalated complexes containing up to four azobenzene fragments on their structure have been synthesized and characterized electronically and spectroscopically. These compounds incorporate photochromic azobenzene units in their neutral pyridyl ligands 1−5, and in four of them azobenzenes have been also introduced a posteriori on their phenylpyridyl ligands by palladium-catalyzed cross-coupling. UV−vismonitored light-triggered trans-to-cis isomerization of the azobenzeneappended ligands revealed a strong inhibition of this process upon coordination to the d 6 -metal ion. TD-DTF calculations revealed that this inhibition could be a consequence of a metal to ligand charge transfer relaxation process.

Structure-switching M3L2 Ir(iii) coordination cages with photo-isomerising azo-aromatic linkers

“…The UV-visible spectrum of C1 in CH 2 Cl 2 (Fig. S54 † ) is similar to that of reported for mononuclear [Ir(ppy) 2 (N) n ] + complexes (N = AZB-decorated pyridyl-donor ligand(s)), 28 with ligand-centred transitions at λ max 274 nm and 293 nm localized on the ppy ligands, and a pyridylazophenyl π → π* transition in the region of 300–390 nm, which overlaps with the spin-allowed mixed metal-to-ligand and ligand-to-ligand charge transfer transitions. The weak band at 400 nm is likely linked to the spin-forbidden n → π* pyridylazophenyl-centred transitions, which overlap with both the spin-allowed and spin-forbidden mixed charge transfer transitions.…”

“… 33 Mononuclear [Ir(C^N) 2 (NN) n ] + complexes with AZB-decorated ligands have been reported to have greatly reduced or even entirely suppressed E → Z photo switching due to low energy MLCT bands, which provide a pathway for relaxation of the π* azo excited state, but these studies featured very little reduction of the π → π* band on irradiation. 28 This is clearly not the case here ( Fig. 7 ) thus there is no evidence for competing photochemical pathways substantially suppressing E → Z isomerisation of these [{Ir(C^N) 2 } 3 (L) 2 ] 3+ coordination cages.…”

“…[Ir III (C^N) 2 (A) n ] + species where A is either two monodentate or one bidentate ligand(s) represent an important subclass of emissive materials, 34 and azobenzenes can themselves be weakly emissive. Freixa et al showed that [Ir III (C^N) 2 (A) n ] + complexes bearing cyclometallated 35 or bipyridyl appended 28 AZB units have negligible luminescence owing to competitive electron transfer from the mixed CT triplet to the azo group. The emission properties of cages C1 , C1-Me , C2 and C3 were determined in DCM solution, Fig.…”

“… 17 DFT studies of similar mononuclear complexes indicate that the HOMOs of these coordination cages are likely located on the [Ir(C^N) 2 ] moieties, whereas the LUMOs lie on the high-energy azobenzene fragment. 28 Therefore, the presence of the AZB units attached to coordinating pyridines in C1 , C1-Me , C2 and C3 implicate large HOMO–LUMO gaps and account for the deep-blue emissions exhibited by these cages. To the best of our knowledge, C1 , C1-Me , C2 and C3 exhibit the bluest emissions reported for metallosupramolecular cages.…”

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties

“…Most of the studied azobenzene-modifed metal complexes were ferrocene–azobenzene, bis or tris(bipyridine), terpyridine metal complexes with azobenzene-conjugation, metalladithiolenes with azobenzene groups, and azobenzene-containing tris-cyclometalated complexes. 27−35 The center metal ions of these complexes were all the d-block group VIII. They displayed reversible trans-to-cis photoisomerization.…”