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.
