Heteroleptic mer -[Cr(N ∩ N ∩ N)(CN) 3 ] complexes: synthetic challenge, structural characterization and photophysical properties.

The substitution of three water molecules around trivalent chromium in CrBr 3 ·6H 2 O with the tridentate 2,2':6',2''-terpyridine (tpy), N , N '-dimethyl- N , N '-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer -[Cr(L)Br 3 ] complexes. Stepwise treatments with Ag(CF 3 SO 3 ) and KCN under microwave irradiations provide mer -[Cr(L)(CN) 3 ] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN 3 C 3 ] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr III . Room-temperature ligand-centered UV-excitation at 18 170 cm -1 ( λ exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr( 2 E → 4 A 2 ) phosphorescence in the red to near infrared domain 13 150-12 650 cm -1 (760 ≤ λ em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer -[Cr(dqp)(CN) 3 ] in frozen acetonitrile.