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-dineop entyl-1,10-phenanthroline, reveals a flattened tetrahedral copper comp lex with a dihedral angle between the least-squares planes of the liga nds of only 63.4(1)degrees. Steric interactions involving gamma methyl groups of the substituents have an important role in shaping the comp lex, but lattice forces are ultimately responsible for the flattened g eometry. 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 shor ter wavelengths (20-50 nm) in the solid state as compared with fluid s olution. The emission from Cu(dnpp)(2)(+) is unique in that it maximiz es at a slightly longer wavelength in the rigid solid (670 nm vs 665 n m in CH2Cl2), The spectral data support the following conclusions rega rding structures in fluid solution: (1) The vibrationally relaxed exci ted state of Cu(dnpp)(2)(+) adopts a structure similar to that observe d in the solid. (2) However, the ground state assumes a less flattened , more tetrahedral geometry.