FORMATION AND OPTICAL-PROPERTIES OF SELF-ORGANIZED PENTAMERIC PORPHYRIN ARRAYS

Principles of formation, electronic absorption and fluorescence spectr a are reported for self-organized pentameric arrays of tetrapyrrolic m acrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(I I)1,4-bis[5-(10, 15,20-tri-p-hexyl-phenylporphyrinyl)]-benzene ((ZnHTP P)(2)) are bound via one molecule of a tetrapyridyl-substituted free b ase of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions in the dim er with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature, Th e formation of the complexes can be followed by changes in the absorpt ion bands of (ZnHTPP)(2) characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spec tral behavior of the free bases in the pentads is determined by a non- planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substitu ted free bases. This quenching effect is assigned to a singlet-singlet energy transfer from the complexed Zn-porphyrin dimers to the free ba se subunit in the pentad.