Synthesis of a series of octabutoxy- and octabutoxybenzophthalocyanines and photophysical properties of two members of the series

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[2,3-1:2',3'-q]porphyrazine, has a Q-band at a wavelength of 804 nm and an extinction coefficient of 1.9 x 10⁵ M^-1 cm^{- 1}. Its wavelength thus matches the wavelength of the output of the most common GaA1As 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, Q₁, 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 x 10^-3 and E(T) is 19.27 kcal/mol.