Rotational Isomers in Stacked Macrocycles:  Synthesis and Spectroscopic Properties of Peripherally Substituted (μ-Oxo)bis(phthalocyaninatosilicon) Compounds

The synthesis and spectroscopic properties of a series of soluble octaalkoxy-substituted silicon phthalocyanine monomers, (RO)₈PcSi(OSiMe₂tBu)L, and μ-oxo-bridged dimers, [(RO)₈PcSi(OSiMe₂tBu)]₂O (R = H₁₇C₈-, H₂₅C₁₂-, (±)-3,7-Me₂-C₈H₁₅-, (±)-2-Et-C₆H₁₂-; L = -OSiMe₂tBu, -OSiMe₃, -F, -OH), are reported. The optical absorption spectra of all dimers show strong solvatochromic behavior whereas the monomers do not. In nonaromatic solvents like, e.g., dichloromethane, the Q-band of the dimers is split into at least five broad bands between 560 and 760 nm, and proton NMR spectra show a strong splitting (Δδ = 1 ppm) of the aromatic signal at low temperature, indicating a torsion angle of ∼20−30° between the rings. In aromatic solvents such as, e.g., benzene, toluene, or 1-methylnaphthalene, thermochromism of the dimers is observed. This is shown by temperature-dependent optical absorption spectroscopy to be due to the presence of a second isomer exclusively found in solutions of aromatic solvents. It shows a rather different absorption spectrum with a very intense and sharp Q-band at 639 nm, a weak shoulder at ∼670 nm, and a very broad and weak absorption at ∼830 nm. Proton NMR spectra of this isomer show, even at −88 °C, only one signal for the aromatic protons, demonstrating a fully staggered or eclipsed conformation of the two rings. The ratio between the two isomers is dependent on the nature and concentration of the aromatic solvent molecules and the steric demand of the alkyl chains R attached to the macrocycle. The assignment of the absorption bands in both isomers is discussed in relation to exciton coupling theory and inter-ring π−π interactions between the aromatic solvent and the macrocycles, which are likely to account also for the strong aromatic solvent-induced NMR shifts (ASIS) observed for the monomers as well as for the dimers.