| Abstract
| - Physical methods reveal the effects D2-flattening distortions have on the spectral properties of Cu(NN)2+ systems. In solids, lattice forces can impose a ground-state distortion, but they can also inhibit excited-state distortions as evidenced by the fact that the solids [Cu(tmp)2]BPh4 and [Cu(phen)2]BPh4 are both photoluminescent. The structural bases of different cooperative inter- and intraligand stereoelectronic effects in Cu(NN)2+ systems are also now apparent.
- For a variety of reasons, relating the photophysical properties of a copper phenanthroline to a structure in solutionis problematic. To elucidate some of the issues involved, in this paper we describe the crystal and molecularstructures of a series of Cu(NN)2+-containing systems along with spectral data obtained from the solids themselves.The NN ligands investigated are tmp (3,4,7,8-tetramethyl-1,10-phenanthroline), dpdmp (2,9-diphenyl-4,7-dimethyl-1,10-phenanthroline), dptmp (2,9-diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline), and dipp (2,9-diisopropyl-1,10-phenanthroline). The results show that a flattening distortion can have a large impact on the spectroscopicproperties of a Cu(NN)2+ system, whereas a typical rocking distortion has comparatively little effect. The reflectancespectra of orange or orange-red salts that have approximately perpendicular phenanthroline ligands exhibit absorptionbands in the neighborhood of 460 nm along with a shoulder at longer wavelength. In the other limit, when apronounced flattening distortion occurs and the dihedral angle between ligands is 20° or more off perpendicular,the reflectance spectrum exhibits two distinct visible bands with intense absorption occurring at 525 nm or evenlonger wavelength. If the phenanthroline ligand lacks bulky substituents in the 2,9 positions, the compound mayeven be purple, depending on the counterion. Cu(NN)2+ complexes that contain phenyl substituents in the 2,9positions and exhibit long-wavelength absorption in solution probably adopt a flattened structure in the groundelectronic state. In most other systems ground-state flattening is a solid-state effect induced by lattice forces.However, a flattening distortion is an intrinsic attribute of the emissive excited state, although intra- or intermolecularforces can inhibit the effect. In the case of the Cu(dptmp)2+ system, intramolecular steric interactions opposeflattening because the methyl groups in the 3,8 positions control the torsion angles of the neighboring phenylgroups. In the case of [Cu(tmp)2]BPh4, packing interactions induce a small flattening in the crystal, but they alsoconstrain the degree of distortion that can occur in the excited state. As a consequence [Cu(tmp)2]BPh4 exhibitsa weak photoluminescence in the solid phase (τ = 15 ns). This is the first report of emission from abis(phenanthroline)copper(I) system that does not have bulky substituents in the 2 and/or 9 positions of the ligand.The [Cu(tmp)2]BPh4 system crystallizes in space group P21/n with a = 17.4883(4) Å, b = 9.86860(10) Å, c =26.3747(6) Å, α = 90°, β = 97.7021(8)°, γ = 90°, V = 4510.8(3) Å3, and Z = 4. For 12 948 unique data withFo2> 2σ(Fo2), R = 6.5%. The [Cu(dpdmp)2]PF6 system crystallizes in space group P2/n with a = 16.0722(13)Å, b = 8.1100(7) Å, c = 16.8937(10) Å, α = 90°, β = 93.947(5)°, γ = 90°, V = 2196.8(5) Å3, and Z = 2. For2833 unique data with Fo2> 2σ(Fo2), R = 6.0%. The [Cu(dptmp)2]PF6·THF system crystallizes in space groupP1̄ with a = 12.8486(4) Å, b = 13.7341(1) Å, c = 15.1678(3) Å, α = 99.5819(14)°, β = 96.7263(13)°, γ =97.3311(12)°, V = 2591.3(2) Å3, and Z = 2. For 13 753 unique data with Fo2> 2σ(Fo2), R = 7.4%. Finally, the[Cu(dipp)2]TFPB system crystallizes in space group P1̄ with a = 14.2523(3) Å, b = 16.0496(4) Å, c = 17.5801(3) Å, α = 112.4150(13)°, β = 105.7480(13)°, γ = 99.6078(11)°, V = 3408.7(3) Å3, and Z = 2. For 8774 uniquedata with Fo2> 2σ(Fo2), R = 9.3%.
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