A twist on the copper center in the crystal structure of [Cu(dnpp)(2)]PF6 and the charge-transfer excited state? (dnpp equals 2,9-dineopentyl-1,10-phenanthroline)

The X-ray structure of [Cu(dnpp)(2)]PF6, where dnpp denotes 2,9-dineopentyl-1,10-phenanthroline, reveals a flattened tetrahedral copper complex with a dihedral angle between the least-squares planes of the ligands of only 63.4(1)degrees. Steric interactions involving gamma methyl groups of the substituents have an important role in shaping the complex, but lattice forces are ultimately responsible for the flattened geometry. Crystal data: [Cu(C22H28N2)(2)] PF6, triclinic, <P(1)over bar>, a = 10.2755(10) Angstrom, b = 13.9750(12) Angstrom, c = 16.4354(12) Angstrom, alpha = 79.376(7)degrees, beta = 86.984(7)degrees, gamma = 69.981(7)degrees, Z = 2. Spectral measurements involving four other Cu(NN)(2)(+) systems, where NN denotes a 2,9-dialkyl-1,10-phenanthroline ligand, reveal that the room-temperature emission maxima fall at shorter wavelengths (20-50 nm) in the solid state as compared with fluid solution. The emission from Cu(dnpp)(2)(+) is unique in that it maximizes at a slightly longer wavelength in the rigid solid (670 nm vs 665 nm in CH2Cl2), The spectral data support the following conclusions regarding structures in fluid solution: (1) The vibrationally relaxed excited state of Cu(dnpp)(2)(+) adopts a structure similar to that observed in the solid. (2) However, the ground state assumes a less flattened, more tetrahedral geometry.