Effects of Sterics and Electronic Delocalization on the Photophysical, Structural, and Electrochemical Properties of 2,9-Disubstituted 1,10-Phenanthroline Copper(I) Complexes

The syntheses, crystal structures, electronic absorption spectra, electrochemical properties, and photophysical
properties of a series of copper(I) bis(phenanthroline) complexes are reported. The phenanthroline ligands that
have been prepared and investigated are the following:  dop (2,9-di-(2-methylphenyl)-1,10-phenanthroline), xop
(2-(2-methylphenyl)-9-(2,6-dimethylphenyl)-1,10-phenanthroline), dpep (2,9-diphenylethynyl-1,10-phenanthroline),
and dmesp (2,9-dimesityl-1,10-phenanthroline). The complex [Cu(dop)2](PF6)·Et2O crystallizes in space group
P1̄with a = 11.854(3) Å, b = 14.705(3) Å, c = 15.866(4) Å, α = 107.81(2)°, β = 106.72(2)°, γ = 97.56(2)°,
V = 2447.6(10) Å3, and Z = 2. For 5739 unique data with F > 4.0σ(F), R = 7.52%. The complex
[Cu(xop)2](PF6)·3/2CH3OH crystallizes in space group C2/c with a = 23.096(6) Å, b = 23.387(6) Å, c =
17.873(7) Å, β = 100.08(3)°, V = 9505(5) Å3, and Z = 8. For 5631 unique data with F > 4.0σ(F), R = 6.02%.
The complex [Cu(dpep)2](PF6) crystallizes in space group P1̄ with a = 13.327(7) Å, b = 14.114(7) Å, c =
15.175(5) Å, α = 87.23(4)°, β = 66.48(3)°, γ = 61.84(4)°, V = 2273(2) Å3, and Z = 2. For 4851 unique data
with F > 4.0σ(F), R = 5.47%. The complex [Cu(dmesp)(dpep)](PF6) crystallizes in space group Pbca with a =
14.547(6) Å, b = 22.868(6) Å, c = 30.659(10) Å, V = 10199(6) Å3, and Z = 8. For 2281 unique data with F >
4.0σ(F), R = 9.43%. The electrochemical, spectral, and structural properties of [Cu(dop)2]+ and [Cu(xop)2]+
demonstrate that the copper coordination environment is more sterically encumbered and more rigid in these two
complexes than the coordination environment in the comparison molecule [Cu(dpp)2]+ (dpp = 2,9-diphenyl-1,10-phenanthroline). A larger energy gap is predicted for [Cu(dop)2]+ and [Cu(xop)2]+ based on these data, and
consequently, a blue-shifted emission is observed relative to [Cu(dpp)2]+. The room-temperature excited-state
lifetimes in dichloromethane and methanol of the dop and xop complexes are shown to be shorter than the dpp
complex, and these results are interpreted as due to a reduction in ligand π-electron delocalization in the former
two complexes. The complexes [Cu(dpep)2]+ and [Cu(dmesp)(dpep)]+ are shown to have increased ligand π-electron
delocalization relative to [Cu(dpp)2]+; however, neither complex displays room-temperature steady-state emission
in dichloromethane.