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

Abstract

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.

Optical Spectra of Phthalocyanines and Related Compounds

“…Similar changes have been observed in aromatic solvents by changing the temperature of the solutions [228,229]. All the spectra are shown with their Q-band intensity normalized band decreases with increasing polarity of the solvent.…”

“…Highly solvatochromic behaviors have been reported for μ-oxo dimers of Si IV phthalocyanines but not for the corresponding monomers [227][228][229][230]. In aromatic solvents such as benzene or toluene, their optical absorption spectra show a sharp, single Q-band at approximately 640 nm, which is characteristic of μ-oxo dimers (see Sect.…”

Synthesis and Optical Limiting Properties of Axially Bridged Phthalocyanines: [(tBu4PcGa)2O] and [(tBu4PcIn)2O]

“…The shapes of the UV/Vis spectra are solventdependent. [15] An additional absorption at 738 nm was observed for 3 in n-hexane. Comparison of the UV/Vis spectra of a thin film and dilute solution of 3 and 4 in CHCl 3 reveals that the absorption spectra of thin films of 3 and 4 are apparently broader.…”

Synthesis of silicon tetrapyrazinoporphyrazines

“…Peripherally substituted dihydroxysilicon phthalocyanines R n PcSi(OH) 2 can be silylated owing to their good solubility, but under much more moderate conditions as we have shown recently [22]. The reaction is quantitative at room temperature within a few hours [22]. The reaction is quantitative at room temperature within a few hours [22].…”