Irreversible cis-to-trans photoisomerization behavior was observed for a stilbazole– porphyrin (1), and its zinc complex (2). trans-1 and trans-2 did not isomerize to the corresponding cis-isomer under irradiation with UV light, although stilbazoles readily undergo the trans-to-cis isomerization upon UV irradiation. In contrast, cis-to-trans photoisomerization was observed for both cis-1 and cis-2; the isomerization of cis- ...