Copper phenanthrolines are attractive as potential photosensitizers because of the ready availability of the metal, but efficient nonradiative decay including a solvent-induced quenching phenomenon ordinarily limits their utility. However, the present studies show that the addition of methyl substituents in the 3,8-positions of 1,10-phenanthroline can enhance the protective effect that bulky groups in the 2,9-positions have on the reactive charge-transfer excited state of a bis-ligand copper(I) derivative. Thus, the photoexcited Cu(dbtmp)(2)(+) complex has a lifetime of 920 ns in dichloromethane, whereas the parent complex without the methyl substituents has a lifetime of only 150 ns under the same conditions (dbtmp = 2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline). In dichloromethane, the complex with the 2,9-diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline ligand also exhibits a long lifetime (480 ns). Even more importantly, the latter combination of substituents appears to eliminate the problem of solvent-induced exciplex quenching.

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....

No abstract

For a series of copper(II) porphyrins, we report EPR data from solid solutions as well as E 0 values for the first ring oxidation, emission spectra, and luminescence lifetimes in methylene chloride. Although the EPR parameters are fairly constant, the potentials vary by almost 700 mV, and the room-temperature lifetimes range from 300 ns for Cu(TCl2PP) to 15 ns for Cu(TMeOPP), where TCl2PP denotes 5,10,15,20-tetra(2‘,6‘-dichlorophenyl)porphyrin and TMeOPP denotes 5,10,15,20−tetra(4‘-methoxyphenyl)porphyrin. The data show that the variation in the lifetime of the emitting π−π* state is not due to the thermal population of another excited state of either d−d or charge-transfer parentage. However, the results are consistent with a model originally introduced by Asano et al. who proposed that an important vibronic distortion occurs in the emitting trip-doublet and trip-quartet states when the excitation involves the a2u orbital of the porphyrin (Asano, M.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1988, 89, 6567−6576). In view of the fact that the distortion is unique to the copper systems, we suggest that it involves movement toward a sitting-atop structure, consistent with the role the d10 configuration is likely to have in the excited-state wave function.

The metal-to-ligand charge-transfer excited states of Cu(NN)(2)(+) systems tend to be good reducing agents but poor oxidants for kinetic and thermodynamic reasons. However, this report demonstrates that reductive electron-transfer quenching is an important pathway for ferrocenes that react with the photoexcited states of Cu(dipp)(2)(+) and Cu(tptap)(2)(+) in methylene chloride (dipp = 2,9-diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-tetraphenyl-1,4,5,8-tetraazaphenanthrene). In the case of the dipp complex the bulky isopropyl substituents inhibit structural relaxation within the excited state, and the self-exchange rate for reductive quenching is quite favorable, k() approximately 2 x 10(8) M(-)(1) s(-)(1). Even in the absence of a significant kinetic barrier to reaction, however, for energetic reasons only extensively methylated ferrocene derivatives with relatively negative reduction potentials are capable of transferring an electron to the excited state. In contrast, every ferrocene derivative investigated, except diacetyl ferrocene, reacts with the charge-transfer excited state of the tptap complex by an electron-transfer mechanism. This is mainly due to a difference in driving force which is about 0.5 V greater for the tptap complex. This system also has a favorable self-exchange rate, k() approximately 5 x 10(7) M(-)(1) s(-)(1), evidently because the juxtapositioned phenyl substituents inhibit low-symmetry distortions within the ground state as well as the excited state. Although energy transfer to ferrocene is also possible, this is a less competitive process with the tptap complex because the zero-zero energy of the reactive excited state is rather low ((3)E(00) approximately 1.7 V).