A group of twelve new and three known silicon phthalocyanines having axial ligands and peripheral groups
which provide varying amounts of steric protection to the ring face and ring periphery has been assembled.
These are SiPc[OSi(n-C6H13)3]2, 1, (known), SiPc[OSi(i-C4H9)2(n-C18H37)]2, 2, SiPc(OEt)8[OH]2, 5, SiPc(OEt)8[OSi(CH3)3]2, 6, SiPc(OnBu)8[OH]2, 8, (known), SiPc(OnBu)8[OSi(n-C6H13)3]2, 9, (known), SiPc(OnBu)8[OSi(i-C4H9)2(n-C18H37)]2, 10, SiPc(dib)4(OnBu)8[OH]2, 15, SiPc(dib)4(OnBu)8[F]2, 16, SiPc(dib)4(OnBu)8[OSi(n-C6H13)2]2, 17, SiPc(dib)4(OnBu)8[OSi(i-C4H9)2(n-C18H37)]2, 18, SiPc(dib)4(OnBu)8[OSi8O12(C5H9)7]2,
19, SiPc(dib)4(OnBu)8[OH]2, 22, SiPc(dib)4(OiBu)8[OSi(n-C6H13)3]2, 23, and SiPc(dib)4(OiBu)8[OSi8O12(C5H9)7]2, 24. Syntheses are given for the twelve members of the group that are new. Photophysical and
voltammetric investigations of six representative members of the group, 1, 2, 10, 18, 19, and 24, have been
carried out. The results show that compounds 1 and 2 (no butoxy substituents at the 1 and 4 positions) have
significantly larger values of the first oxidation potential (E
+1) than those compounds (10, 18, 19, and 24)
that do carry these substituents. The values of E
-
1 (first reduction potential) show very little in the way of
structural dependence. Alkoxy substitution at the 1,4 positions affects the HOMO energies, and therefore, the
addition of an electron from an electrode to the LUMO of a 1,4 substituted silicon phthalocyanine will not
be a sensitive function of the substitution pattern. The removal of an electron from the HOMO in an oxidation
step on the other hand would be expected to be energetically less demanding for those compounds wherein
the HOMO is higher lying. This orbital energy effect of substitution makes it clear why the E
+1 values for
compounds 1 and 2 are significantly lower. Substitution of dibenzobarreleno (dib) at the 2,3 positions has
only minor effects on the HOMO energy, as shown by the similarities in the position of the Q-band maximum.
However, it is very likely that the steric effects of the dibenzobarreleno substituents and the [OSi8O12(C5H9)7]
axial cages contribute to the observed trends in E
+1. Bimolecular rate constants for quenching of the triplet
states of the six target compounds by O2, by β-carotene, and by chloranil were measured. The first two
compounds quench by triplet−triplet (TT) energy transfer, whereas the last is an electron transfer (ET) reactant.
All rate parameters were sensitive to the steric crowding of the phthalocyanine π system, but with different
degrees. The least sensitive was the ET reaction with chloranil. Thus, it appears that although steric crowding
of the triplet state of the silicon phthalocyanines is very effective at reducing the rate constants of exoergic
electron exchange energy transfer (TT) reactions, even for a small molecule such as oxygen, it is much less
effective at discriminating against electron transfer (ET) processes. These differences may be accounted for
on the concept that the overlap requirement for the d...
