A 2,2′-bipyridine
with bulky triphenylamine substituents
in the 6 and 6′ positions of the ligand (6,6′-ditriphenylamine-2,2′-bipyridine,
6,6′-diTPAbpy) was generated. Despite the steric bulk, the
ligand readily formed bis(homoleptic) complexes with copper(I) and
silver(I) ions. Unfortunately, efforts to use the 6,6′-diTPAbpy
system to generate heteroleptic [Cu(6,6′-diTPAbpy)(bpy)]+ complexes were unsuccessful with only the [Cu(6,6′-diTPAbpy)2](PF6) complex observed. The 6,6′-diTPAbpy
ligand could also be reacted with 6-coordinate metal ions that featured
small ancillary ligands, namely, the [Re(CO)3Cl] and [Ru(CO)2Cl2] fragments. While the complexes could be formed
in good yields, the steric bulk of the TPA units does alter the coordination
geometry. This is most readily seen in the [(6,6′-diTPAbpy)Re(CO)3Cl] complex where the Re(I) ion is forced to sit 23°
out of the plane formed by the bpy unit. The electrochemical and photophysical
properties of the family of compounds were also examined. 6,6′-diTPAbpy
exhibits a strong ILCT absorption band (356 nm, 50 mM–1 cm–1) which displays a small increase in intensity
for the homoleptic complexes ([Cu(6,6′-diTPAbpy)2]+; 353 nm, 72 mM–1 cm–1, [Ag(6,6′-diTPAbpy)2]+; 353 nm, 75
mM–1 cm–1), despite containing
2 equiv of the ligand, attributed to an increased dihedral angle between
the TPA and bpy moieties. For the 6-coordinate complexes the ILCT
band is further decreased in intensity and overlaps with MLCT bands,
consistent with a further increased TPA-bpy dihedral angle. Emission
from the 1ILCT state is observed at 436 nm (τ = 4.4
ns) for 6,6′-diTPAbpy and does not shift for the Cu, Ag, and
Re complexes, although an additional 3MLCT emission is
observed for [Re(6,6′-diTPAbpy)(CO)3Cl] (640 nm,
τ = 13.8 ns). No emission was observed for [Ru(6,6′-diTPAbpy)(CO)2Cl2]. Transient absorption measurements revealed
the population of a 3ILCT state for the Cu and Ag complexes
(τ = 80 ns). All assignments were supported by TD-DFT calculations
and resonance Raman spectroscopic measurements.
