Bulky and Stable Copper(I)-Phenanthroline Complex: Impact of Steric Strain and Symmetry on the Excited-State Properties

The steric strain
around copper­(I) in typical [Cu­(NNR)2]+ complexes, where NNR is a diimine
ligand substituted in α-positions of the nitrogen atoms by R,
is known to strongly impact the excited-state properties. Generally
speaking, the larger the R, the longer the emission lifetime and the
higher the quantum yield. However, the stability of the coordination
scaffold can be at stake if the steric strain imposed by R is too
large. In this work, we explore a way of fine-tuning the steric strain
around Cu­(I) to reach a balance between high emission quantum yield
and stability in a highly bulky copper­(I) complex. Taking stable [Cu­(dipp)2]+ and unstable [Cu­(dtbp)2]+ (where dipp and dtbp are, respectively, 2,9-diisopropyl-1,10-phenanthroline
and 2,9-di-tert-butyl-1,10-phenanthroline) as the
boundary of two least and most sterically strained structures, we
designed and characterized the nonsymmetrical ligand 2-isopropyl-9-tert-butyl-1,10-phenanthroline (L1) and corresponding complex
[Cu­(L1)2]+ (Cu1). The key experimental findings
are that Cu1 exhibits a rigid tetrahedral geometry in the ground state,
close to that of [Cu­(dtbp)2]+ and with an intermediate
stability between that of [Cu­(dipp)2]+ and [Cu­(dtbp)2]+. Conversely, the nonsymmetrical nature of ligand
L1 leads to a shorter emission lifetime and smaller quantum yield
than those of either [Cu­(dipp)2]+ or [Cu­(dtbp)2]+. This peculiar behavior is rationalized through
the in depth analysis of the ultrafast dynamics of the excited state
measured with optical transient absorption spectroscopy and theoretical
calculations performed on the ground and excited state of Cu1. Our
main findings are that the obtained complex is significantly more
stable than [Cu­(dtbp)2]+ despite the sterically
strained coordination sphere. The nonsymmetrical nature of the ligand
translates into a strongly distorted structure in the excited state.
The distortion can be described as a rocking motion of one ligand,
entailing the premature extinction of the excited state via several
deactivation channels.