Structural and Photophysical Studies of Cu(NN)₂⁺ Systems in the Solid State. Emission at Last from Complexes with Simple 1,10-Phenanthroline Ligands

Abstract

For a variety of reasons, relating the photophysical properties of a copper phenanthroline to a structure in solution is problematic. To elucidate some of the issues involved, in this paper we describe the crystal and molecular structures 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 spectroscopic properties of a Cu(NN)2+ system, whereas a typical rocking distortion has comparatively little effect. The reflectance spectra of orange or orange-red salts that have approximately perpendicular phenanthroline ligands exhibit absorption bands in the neighborhood of 460 nm along with a shoulder at longer wavelength. In the other limit, when a pronounced flattening distortion occurs and the dihedral angle between ligands is 20 degrees or more off perpendicular, the reflectance spectrum exhibits two distinct visible bands with intense absorption occurring at 525 nm or even longer wavelength. If the phenanthroline ligand lacks bulky substituents in the 2,9 positions, the compound may even be purple, depending on the counterion. Cu(NN)2+ complexes that contain phenyl substituents in the 2,9 positions and exhibit long-wavelength absorption in solution probably adopt a flattened structure in the ground electronic 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 intermolecular forces can inhibit the effect. In the case of the Cu(dptmp)2+ system, intramolecular steric interactions oppose flattening because the methyl groups in the 3,8 positions control the torsion angles of the neighboring phenyl groups. In the case of [Cu(tmp)2]BPh4, packing interactions induce a small flattening in the crystal, but they also constrain the degree of distortion that can occur in the excited state. As a consequence [Cu(tmp)2]BPh4 exhibits a weak photoluminescence in the solid phase (tau = 15 ns). This is the first report of emission from a bis(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 P2(1)/n with a = 17.4883(4) A, b = 9.86860(10) A, c = 26.3747(6) A, alpha = 90 degrees, beta = 97.7021(8) degrees, gamma = 90 degrees, V = 4510.8(3) A3, and Z = 4. For 12,948 unique data with Fo2 > 2 sigma(Fo2), R = 6.5%. The [Cu(dpdmp)2]PF6 system crystallizes in space group P2/n with a = 16.0722(13) A, b = 8.1100(7) A, c = 16.8937(10) A, alpha = 90 degrees, beta = 93.947(5) degrees, gamma = 90 degrees, V = 2196.8(5) A3, and Z = 2. For 2833 unique data with Fo2 > 2 sigma(Fo2), R = 6.0%. The [Cu(dptmp)2]PF6....

Synthesis, Structure, and Spectroscopic and Electrochemical Properties of Copper(II/I) Complexes with Symmetrical and Unsymmetrical 2,9‐Diaryl‐1,10‐phenanthroline Ligands

“…[32][33][34][35] For 7 and 10, there is a shoulder in the 550-600 nm range, which can be attributed to the distortion of the complex molecule from D 2d symmetry caused by intramolecular π-stacking interactions, as in some known Cu I complexes, for example, [Cu(phen) 2 ]ClO 4 (580 nm) [28] and [Cu(tmp) 2 ]PF 6 (595 nm). [36] This lower-energy shoulder decreases with increasing steric bulk of the ligands (well resolved for 10, much weaker for 11, and disappeared for 12; see Figure 5b), as more sterically demanding ligands may enforce the D 2d symmetry.…”

Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution

“…In particular, cuprous diimine complexes have been considered as potential substitutes for ruthenium (II) and osmium(II) systems. 18 McMillin and coworkers 3,[19][20][21][22][23][24] performed pioneering work for elucidating the unique photophysics and photochemistry of [Cu I (NN) 2 ] + complexes and developed a fundamental picture of the photophysics occurring on the nanosecond timescale. 6 Their experimental observations led to remarkable insights into the properties of these systems in their excited states.…”

“…McMillin recognized the potential for JahnTeller distortion causing a structural rearrangement from the tetrahedral ground state to square planar or trigonal bipyramidal geometries following a metal-to-ligand-charge-transfer (MLCT) excitation in [Cu I (NN) 2 ] + complexes. 3,[23][24][25] Also, he proposed the model for exciplex quenching by Lewis bases to explain solvent-dependent luminescence and excited state lifetimes. 3,19,26 Despite the lack of any direct structural evidence, the exciplex formation hypothesis was supported by the correlation between accessibility of the cuprous center by the solvent and the luminescence lifetime, and was widely accepted.…”

Homoleptic tetraazaphenanthrene-based copper(I) complexes: Synthesis, spectroscopic characterization, crystal structures and computational studies

“…In particular, copper(I) diimine complexes have been considered as potential substitutes for ruthenium(II) and osmium(II) systems. Pioneering work toward elucidation of the unique photophysical and photochemical properties of [Cu I (N^N) 2 ] + complexes has been reported by McMillin and co-workers over the last 35 years [9][10][11][12][13][14][15]. Owing to the significant similarities in absorption spectra and photophysical behavior there have been recent attempt to replace Ru(II) with Cu(I) diimine complexes in dye-sensitized solar cells (DSSCs) [16][17][18][19][20].…”