The tetratriptycenoporphyrazines revisited

The spectroscopic behavior of octa-t-butyltetra-2,3-triptycenotetraazaporphyrin and some of its metal complexes ( Cu 2+, Zn 2+ and Co 2+) were examined. UV-visible and electron paramagnetic resonance spectroscopy indicate that these phthalocyanine derivatives form cofacial dimers in pentane solution. Modeling suggests that the lowest energy configuration of the dimer is a self-complementary embrace in which the two phthalocyanine cores are staggered at an angle of 45° relative to each other. This configuration results in a remarkably intense and sharp absorption band (~635 nm; ε = ~4.0 × 105 M-1.cm-1) arising from excitonic coupling within the dimer, a unique property for self-assembled dimers but analogous to the behavior of certain μ-oxo-dimers of silicon phthalocyanine. Introduction of methyl substituents into the bridgehead positions of the triptycene subunits prevents dimer formation.