Intriguing Effects of Halogen Substitution on the Photophysical Properties of 2,9-(Bis)halo-Substituted Phenanthrolinecopper(I) Complexes

Three
new copper­(I) complexes [Cu­(LX)2]+(PF6–) (where LX stands for 2,9-dihalo-1,10-phenanthroline
and X = Cl, Br, and I) have been synthesized in order to study the
impact of halogen substituents tethered in the α position of
the chelating nitrogen atoms on their physical properties. The photophysical
properties of these new complexes (hereafter named Cu-X) were characterized in both their ground and excited states. Femtosecond
ultrafast spectroscopy revealed that early photoinduced processes
are faster for Cu-I than for Cu-Cl or Cu-Br, both showing similar behaviors. Their electronic absorption
and electrochemical properties are comparable to benchmark [Cu­(dmp)2]+ (where dmp stands for 2,9-dimethyl-1,10-phenanthroline);
furthermore, their optical features were fully reproduced by time-dependent
density functional theory and ab initio molecular dynamics calculations.
All three complexes are luminescent at room temperature, showing that
halogen atoms bound to positions 2 and 9 of phenanthroline are sufficiently
bulky to prevent strong interactions between the excited Cu complexes
and solvent molecules in the coordination sphere. Their behavior in
the excited state, more specifically the extent of the photoluminescence
efficiency and its dependence on the temperature, is, however, strongly
dependent on the nature of the halogen. A combination of ultrafast
transient absorption spectroscopy, temperature-dependent steady-state
fluorescence spectroscopy, and computational chemistry allows one
to gain a deeper understanding of the behavior of all three complexes
in their excited state.