Structural and Photophysical Properties of Lanthanide Nitrate 1:1 Complexes with planar tridentate nitrogen ligands analogous to 2,2′:6′,2′‐terpyridine

The ligand 2,6‐bis(1‐methylbenzimidazol‐2‐yl)pyridine (mbzimpy, 1 ) reacts with Eu III to give [Eu(mbzimpy)(NO 3 ) 3 (CH 3 OH)] [ 4 ] whose crystal structure (EuC 22 H 21 N 8 O 10 , a = 7.658(3) Å, b = 19.136(2) Å, c = 8.882 Å, β = 104.07(1)°, monoclinic, P 2 1 , Z = 2) shows a mononuclear structure where Eu III is ten‐coordinate by a meridional tridentate mbzimpy ligand, three bidentate nitrates, and one CH 3 OH molecule, leading to a low‐symmetry coordination sphere around the metalion. Essentially the same coordination is found in the crystal structure of [Eu(obzimpy)(NO 3 ) 3 ] ( 8 ) (EuC 35 H 45 N 8 O 9 , a = 9.095(2) Å, b = 16.624(2) Å, c = 26.198(6) Å, β = 95.85(1)°, monoclinic, P 2 1 / c , Z = 4) obtained by reaction of 2,6‐bis(1‐octylbenzimidazol‐2‐yl)pyridine (obzimpy, 2 ) with Eu III . Detailed photophysical studies of crystalline [Ln(mbzimpy)(NO 3 ) 3 (CH 3 OH)] and [Ln(obzimpy)(NO 3 ) 3 ] complexes (Ln = Eu, Gd, Tb, Lu) show that 1 and 2 display 1 ππ* and 3 ππ* excited states very similar to those observed in 2,2′:6′,2″‐terpyridine, leading to efficient ligand to Ln III intramolecular energy transfer. Spectroscopic results show that an extremely efficient mbzimpy‐to‐Eu III transfer occurs in [Ln(mbzimpy)(NO 3 ) 3 (CH 3 OH)] and in the case of Tb III , a Tb III ‐to‐mbzimpy back transfer is also implied in the deactivation process. The origin of these peculiar effects and the influence of ligand design by going from mbzimpy to obzimpy are discussed. 1 H‐NMR and luminescence data indicate that the structure found in the crystal is essentially maintained in solution.