Emission Tuning of Luminescent Copper(I) Complexes by Vapor-Induced Ligand Exchange Reactions

We have synthesized
two luminescent mononuclear Cu­(I) complexes, [Cu­(PPh2Tol)­(THF)­(4Mepy)2]­(BF4) (1) and [Cu­(PPh2Tol)­(4Mepy)3]­(BF4) (2) (PPh2Tol = diphenyl­(o-tolyl)­phosphine,
4Mepy = 4-methylpyridine, THF = tetrahydrofuran), and investigated
their crystal structures, luminescence properties, and vapor-induced
ligand exchange reactions in the solid state. Both coordination complexes
are tetrahedral, but one of the three 4Mepy ligands of complex 2 is replaced by a THF solvent molecule in complex 1. In contrast to the very weak blue emission of the THF-bound complex 1 (wavelength of emission maximum (λem) =
457 nm, emission quantum yield (Φem) = 0.02) in the
solid state at room temperature, a very bright blue-green emission
was observed for 2 (λem = 484 nm, Φem = 0.63), suggesting a contribution of the THF ligand to
nonradiative deactivation. Time-dependent density functional theory
calculations and emission lifetime measurements suggest that the room-temperature
emissions of the complexes are due to thermally activated delayed
fluorescence from the metal-to-ligand charge transfer excited state.
Interestingly, by exposing the solid sample of THF-bound 1 to 4Mepy vapor, the emission intensity drastically increased and
the emission color changed from blue to blue-green. Powder X-ray diffraction
measurements revealed that the emission change of 1 is
due to the vapor-induced ligand exchange of THF for 4Mepy, forming
the strongly emissive complex 2. Further emission tuning
was achieved by exposing 1 to pyrimidine or pyrazine
vapors, forming green (λem = 510 nm) or orange (λem = 618 nm) emissive complexes, respectively. These results
suggest that the vapor-induced ligand exchange is a promising method
to control the emission color of luminescent Cu­(I) complexes.