The adsorbed structures and assembly behavior of porphyrin derivatives on Cu substrates were studied using low-temperature scanning tunneling microscopy (STM). The STM images showed that a porphyrin molecule with four tert-butyl-phenyl groups (H-2-TBPP) forms twin domains of a square superstructure. The deformation of H-2-TBPP required to explain the STM images forms a pair of corresponding chiral conformations. The conformation determines which of the twin domains is formed. In contrast, porphyrin molecules with two tert-butylphenyl and two vinyl-phenyl groups (H-2-TBPVPP) showed no form of ordered assembly due to the strong interaction between the vinyl phenyl and substrate. STM observation showed that cis- and trans-H-2-TBPVPP undergo different conformational changes, leading to mirror and two-fold symmetry, respectively. (C) 2004 Elsevier B.V. All rights reserved.