This investigation focuses on a series of pseudotetrahedral complexes of the form Cu(NN)2 +, where NN denotes a 1,10-phenanthroline ligand with alkyl substituents in the 2 and 9 positions and the counterion is PF6 -. In these copper(I) systems, steric effects are of considerable interest because the electronic configuration predisposes the reactive charge-transfer excited state to undergo a flattening distortion or to add a fifth ligand. Both effects lead to emission quenching and a shorter excited-state lifetime. Bulky substituents inhibit these processes, but the spatial distribution of the atoms involved is more important than the total molecular volume in determining the influence of a substituent. According to the results of this study, the effective size decreases in the following order: sec-butyl > neopentyl > n-octyl ≈ n-butyl > methyl. In conjunction with the electrochemical data, the absorption and the emission spectra reveal three kinds of steric effects: (1) Clashes between substituents on opposite phenanthroline ligands hinder D 2 flattening distortions in the oxidized form of the complex and in the charge-transfer excited state of the Cu(NN)2 + system itself. (2) Steric interactions connected with a highly branched substituent, like the neopentyl group, destabilize the Cu(NN)2 + ground state. (3) Finally, the presence of bulky groups disfavors expansion of the coordination number. The complex with sec-butyl substituents is noteworthy because it exhibits the longest excited-state lifetime (∼400 ns in CH2Cl2) ever measured for a Cu(NN)2 + system in fluid solution. In addition, it exhibits a luminescence lifetime of 130 ns in acetonitrile which is ordinarily a potent quencher of photoexcited Cu(NN)2 + systems.
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)°. Steric interactions involving γ 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, P1̄, a = 10.2755(10) Å, b = 13.9750(12) Å, c = 16.4354(12) Å, α = 79.376(7)°, β = 86.989(7)°, γ = 69.981(7)°, 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.
Water-soluble, cationic metalloporphyrins that bind to DNA show promise as artificial nucleases and as sensitizers for photodynamic therapy, but fundamental questions remain about the binding motifs and sequence specificities. To address these issues, we have studied the interactions of Cu(T4) with a series of oligonucleotides that form hairpin structures (H2T4 = meso-tetrakis(4-(N-methylpyridiniumyl))porphyrin). Each oligonucleotide is a 16-mer with a central run of four thymine (T) bases and complementary ends that can combine to form a specific sequence of six adenine−thymine (AT) and guanine−cytosine (G⋮C) base pairs. The techniques employed include thermal melting as well as circular dichroism (CD), absorbance, and emission spectroscopies. The number of G⋮C base pairs in the stem is the most important factor that determines the melting temperature of the hairpin, and in every case investigated, the uptake of Cu(T4) stabilizes the hairpin. Depending on the nature of the adduct that forms, Δε varies from −22 to +17 M-1 cm-1 in the Soret region of the CD spectrum, and the emission intensity from Cu(T4) changes by an order of magnitude. The results yield several useful insights regarding the binding interactions. One is that robust hydrogen bonding within a B-form duplex promotes intercalative binding of Cu(T4). Thus, if the composition is at least 50% G⋮C base pairs, intercalation will occur even in the absense of a G⋮C step. On the other hand, a run of four AT base pairs defines a groove-binding site with an affinity comparable to that for intercalation at a G⋮C step. Finally, at least in solutions containing excess oligonucleotide, there is no sign that either loop binding or hemiintercalation is a prevalent mode of interaction between Cu(T4) and hairpin hosts.
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