Encumbering the Copper(I)-Diimine Coordination Sphere with a Benzyl Group: Impact on the Stability and Photoredox Properties

Cu(I)-bisdiimine complexes, [Cu(NN)2]+ (where
NN represents a diimine such as phenanthroline or bipyridine derivatives),
are considered promising photosensitizers for various photochemical
applications. However, their effectiveness is subject to several key
challenges. In particular, controlling steric strain around the copper(I)
center by introducing appropriate substituents (R) at the α-position
of the nitrogen atoms is crucial for optimizing the excited-state
properties of the complex. In brief, increasing the size of R leads
to longer emission lifetimes and higher quantum yields. Additionally,
the energy of the singlet excited state rises with increasing steric
bulk, enhancing photoinduced reactivity. However, excessive steric
strain from bulky substituents can significantly destabilize the coordination
sphere. To balance complex stability with increased steric bulk around
the metal center, we have developed two novel nonsymmetrical ligands
featuring branched alkyl chains and benzyl groups at the α-position
of the nitrogen atoms. Our findings demonstrate that intramolecular
π-stacking interactions between the benzyl group and the opposing
phenanthroline ligand contribute to stabilizing the coordination sphere.
Furthermore, the flexibility of the benzyl group reinforces the tetrahedral
geometry around copper(I), resulting in an increased singlet excited-state
energy compared to benchmark complexes. Notably, we show that this
enhancement in excited-state energy translates into greater excited-state
reactivity.