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

The steric strain around copper(I) in typical[Cu(NNR)2]+complexes, where NNRis a diimine ligand substitutedin alpha-positions of the nitrogen atoms by R, is known to stronglyimpact the excited-state properties. Generally speaking, the larger theR, the longer the emission lifetime and the higher the quantum yield.However, the stability of the coordination scaffold can be at stake ifthe steric strain imposed by R is too large. In this work, we explore away offine-tuning the steric strain around Cu(I) to reach a balancebetween high emission quantum yield and stability in a highly bulkycopper(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-phenan-throline) as the boundary of two least and most sterically strainedstructures, we designed and characterized the nonsymmetrical ligand 2-isopropyl-9-tert-butyl-1,10-phenanthroline (L1) andcorresponding complex [Cu(L1)2]+(Cu1). The key experimentalfindings are that Cu1 exhibits a rigid tetrahedral geometry in theground 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 ofeither [Cu(dipp)2]+or [Cu(dtbp)2]+. This peculiar behavior is rationalized through the in depth analysis of the ultrafast dynamics ofthe excited state measured with optical transient absorption spectroscopy and theoretical calculations performed on the ground andexcited state of Cu1. Our mainfindings are that the obtained complex is significantly more stable than [Cu(dtbp)2]+despite thesterically strained coordination sphere. The nonsymmetrical nature of the ligand translates into a strongly distorted structure in theexcited state. The distortion can be described as a rocking motion of one ligand, entailing the premature extinction of the excitedstate via several deactivation channels