Synthesis, photochemistry, and electrochemistry of a series of phthalocyanines with graded steric hindrance

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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 I 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 O-2, by beta-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 pi 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 double electron exchange of TT energy transfer is more stringent compared to the single electron transfer (ET).

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