On heating, syn-diporphyrin zinc complexes fused with bis(ethano)dimethoxyanthracene, the crystal structure of which showed a unique trimeric assembly, extruded one ethylene molecule at 240-310 °C to give a porphyrin-naphthoporphyrin diad, and the second retro-Diels-Alder reaction and concomitant decomposition of the methoxy groups occurred at 280-350 °C to give the anthraquinone-fused diporphyrin, while the first thermal conversion of the anti-diporphyrin zinc complex occurred in a much lower temperature range (180-230 °C). Copyright © 2003 American Chemical Society.