Photophysics and Photochemistry of 22π and 26π Acetylene-Cumulene Porphyrinoids
The aromatic 26π acetylene-cumulene porphyrinoid 8 has been synthesized by a reaction sequence involving reductive carbonyl coupling of the dialdehyde 12 utilizing low-valent titanium in the crucial step. Like its lower 22π homologs 6 and 7, the new tetrapyrrolic macrocycle 8 is centrosymmetric in t...
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| Autores principales: | , , , , , , , |
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| Formato: | JOUR |
| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_00027863_v114_n25_p9969_Martire |
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| Sumario: | The aromatic 26π acetylene-cumulene porphyrinoid 8 has been synthesized by a reaction sequence involving reductive carbonyl coupling of the dialdehyde 12 utilizing low-valent titanium in the crucial step. Like its lower 22π homologs 6 and 7, the new tetrapyrrolic macrocycle 8 is centrosymmetric in the crystal and has a practically planar ring framework. The photophysical and photochemical properties of 6-8 have been studied and compared to those of the related 8π porphycene 5. The absorption spectrum of 8 has the most red-shifted and intensified visible bands (εmax = 119600 at 889 nm in dichloromethane) of this particular series of (4 + 2)π porphyrinoids. None of the compounds exhibit any photoreactivity. The photophysical properties were determined by a combination of techniques, including steady-state thermal lensing, flash photolysis, laser-induced optoacoustic spectroscopy (LIOAS), and steady-state and time-resolved NIR spectroscopy. The 22π and 26π porphyrinoids do not phosphoresce. The triplet energy (ET) of 7 was therefore measured by way of reversible energy transfer to oxygen yielding singlet molecular oxygen, O2(1Δg), indirect detection by LIOAS of the increase in triplet yield induced by O2(3Σg-)-enhanced S ⟶T intersystem crossing afforded the ET of 6, and an upper limit of ET could be obtained by energy-transfer experiments from O2(1Δg) to 8. The quantum yields of fluorescence, triplet formation, and the ET values dropped significantly on going from 5 to 6-8 (all measurements at room temperature). The triplet energies of 6, 7, and 8 are all below the energy of O2(1Δg). Compound 7, with the highest ET of the three, produces O2(1Δg) with a quantum yield of ΦΔ= 0.06 through a reversible energy-transfer mechanism. Although this value is about six times smaller than ΦΔ of 5, the phototherapeutic activities of both 5 and 7 toward tumors in mice are comparable. The ΦΔ values for 6 and 8 are lower than 10-3; hence, these porphyrinoids do not function as photodynamic sensitizers. © 1992, American Chemical Society. All rights reserved. |
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