We have developed a new route to silicon-centered phthalocyanines and phthalocyanine-like compounds that is robust and flexible, and of considerable potential usefulness. This route entails insertion of silicon into the metal-free macrocycle. It has been developed in the course of preparing four new and two known metal-free, six new dihydroxysilicon, and six new bis-trihexylsiloxysilicon octabutoxy- and octabutoxybenzophthalocyanines. One of the siloxysilicon compounds, that with the ligand 5,9,12,16,19,23,28,32-octabutoxy-33H,35H-dibenzo[b,g]dinaphtho[2,3-l:2‘,3‘-q]porphyrazine, has a Q-band at a wavelength of 804 nm and an extinction coefficient of 1.9 × 105 M-1 cm-1. Its wavelength thus matches the wavelength of the output of the most common GaAlAs diode laser. The compound and its analog in which the benzo rings are trans to each other instead of cis have no tendency to aggregate in benzene up to a concentration of 150 μM. The triplet state of the cis isomer has an absorption maximum at 640 nm and a lifetime in deaerated benzene solution of 105 μs, while the triplet state of the trans isomer has a maximum at 660 nm and a lifetime of 72 μs. Both isomers have a triplet quantum yield, Q t, of ca. 0.20 and a singlet oxygen quantum yield, Q Δ, of ca. 0.20, photochemical properties that are consistent with potentially efficient photosensitization action in photodynamic therapy of tumors. For both sensitizers, energy transfer from the sensitizer triplet to ground state of dioxygen is reversible at appropriate concentrations. For the cis isomer, the equilibrium constant for the energy transfer process, K e, is 0.012 ± 0.001, and the triplet state energy calculated from this, E T, is 21.29 kcal/mol (E T derived from phosphorescence measurements is 21.26 kcal/mol). For the trans isomer, K e is 3.68 × 10-3 and E T is 19.27 kcal/mol.
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...
Single crystal structures have been determined for the three cofacial, oxygen-bridged, silicon phthalocyanine oligomers, [((CH(3))(3)SiO)(2)(CH(3))SiO](SiPcO)(2-4)[Si(CH(3))(OSi(CH(3))(3))(2)], and for the corresponding monomer. The data for the oligomers give structural parameters for a matching set of three cofacial, oxygen-bridged silicon phthalocyanine oligomers for the first time. The staggering angles between the six adjacent cofacial ring pairs in the three oligomers are not in a random distribution nor in a cluster at the intuitively expected angle of 45° but rather are in two clusters, one at an angle of 15° and the other at an angle of 41°. These two clusters lead to the conclusion that long, directional interactions (LDI) exist between the adjacent ring pairs. An understanding of these interactions is provided by atoms-in-molecules (AIM) and reduced-density-gradient (RDG) studies. A survey of the staggering angles in other single-atom-bridged, cofacial phthalocyanine oligomers provides further evidence for the existence of LDI between cofacial phthalocyanine ring pairs in single-atom-bridged phthalocyanine oligomers.
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