PHOTOPHYSICS AND PHOTOCHEMISTRY OF 22-PI AND 26-PI ACETYLENE CUMULENE PORPHYRINOIDS

The aromatic 26pi 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 22pi 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 18pi porphycene 5. The absorption spectrum of 8 has the most red-shifted and intensified visible bands (epsilon(max) = 119 600 at 889 nm in dichloromethane) of this particular series of (4n + 2)pi 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 22pi and 26pi porphyrinoids do not phosphoresce. The triplet energy (E(T)) of 7 was therefore measured by way of reversible energy transfer to oxygen yielding singlet molecular oxygen, O2(1DELTA(g)), indirect detection by LIOAS of the increase in triplet yield induced by O2(3SIGMA(g)-)-enhanced S --> T intersystem crossing afforded the E(T) of 6, and an upper limit of E(T) could be obtained by energy-transfer experiments from O2(1DELTA(g)) to 8. The quantum yields of fluorescence, triplet formation, and the E(T) 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(1DELTA(g)). Compound 7, with the highest E(T) of the three, produces O2(1DELTA(g)) with a quantum yield of PHI(DELTA) = 0.06 through a reversible energy-transfer mechanism. Although this value is about six times smaller than PHI(DELTA) of 5, the phototherapeutic activities of both 5 and 7 toward tumors in mice are comparable. The PHI(DELTA) values for 6 and 8 are lower than 10(-3); hence, these porphyrinoids do not function as photodynamic sensitizers.