Helicene Bis(pyrazol‐1‐yl)pyridine Ligands for Luminescent Transition‐Metal Complexes

We describe herein the synthesis of 2,6‐bis(pyrazol‐1‐yl)pyridine (1‐bpp) ligands substituted by helicene units in the 4‐position of the pyridine ring. The two‐step procedure afforded the molecules bpp[ n ]helicene ( n = 4, 5 and 6) and methyl‐bpp[4]helicene, abbreviated as 4 , 5 , 6 and 4m respectively, with overall good yields. This series of ligands shows fluorescence in the violet region, with a red shift of the emission when increasing the number of aromatic rings in the helicene unit. The tridentate fluorophores have been reacted with Re(CO) 5 Cl and [Ru(terpy)Cl 3 ] to prepare a series of transition metal complexes formulated as [Re I ( n )(CO) 3 Cl] ( n = 4 and 5 ) and [Ru II ( n )(terpy)](PF 6 ) 2 ( n = 4 , 4m , 5 and 6 ). The compounds [Re I ( 4 )(CO) 3 Cl] · (CH 2 Cl 2 ) 0.5 (H 2 O) and [Ru II ( 4m )(terpy)](PF 6 ) 2 crystallized in the centrosymmetric space groups P 2 1 / n and P 1 respectively, and thus present both M and P helicene units in their crystal structures. The absorption and emission properties of the complexes have been studied. All complexes show luminescence at 77 K in frozen DCM solution. The tricarbonyl complex [Re I ( 5 )(CO) 3 Cl] shows a strong emission band with two peaks at 540 nm and 580 nm, which could be attributed to ligand‐localized π–π* phosphorescence due to the extended π system of the ligand. The heteroleptic Ru II complexes show emission bands above 600 nm attributed to the terpyridine‐based 3 MLCT state.