Conformational Change in Molecular Assembly of Nickel(II) Tetra(n‑propyl)porphycene Triggered by Potential Manipulation Soichiro Yoshimoto,* ,† Teppei Kawamoto, ‡,∥ Toru Okawara, §,⊥ Yoshio Hisaeda, § and Masaaki Abe* ,§,# † Priority Organization for Innovation and Excellence, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡ Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan § Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744, Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan *S Supporting Information ABSTRACT: Metal-coordinated porphyrin and related com- pounds are important for developing molecular architectures that mimic enzymes. Porphycene, a structural isomer of porphyrin, has shown unique properties in semiartificial myoglobin. To explore its potential as a molecular building block, we studied the molecular assembly of nickel(II) tetra(n- propyl)porphycene (NiTPrPc), a metalloporphycene with introduced tetra n-propyl moieties, on the Au(111) electrode surface using in situ scanning tunneling microscopy. Because of the low molecular symmetry of NiTPrPc, the molecular assembly undergoes unique phase transitions due to conforma- tional change of the n-propyl moieties. The phase transitions can be precisely controlled by the electrode potential, demonstrating that the latter can play an important role in the porphycene molecular assembly on Au surface. This new discovery indicates possible uses of this porphycene framework in molecular engineering. ■ INTRODUCTION Porphyrin and related compounds are key components in important biological processes such as dioxygen transport, photosynthesis, and enzyme catalysis, where they constitute the active centers, that is, “hemes” in metalloproteins. 1−3 They are also considered promising building blocks in designing molecular structures. 4−7 Porphycene (Pc), a structural isomer of porphyrin, has attracted attention for creating new functional and molecular nanostructures. 8−10 For example, when myoglobin (a well- known O 2 storage hemeprotein) was reconstituted with iron or cobalt porphycene, the O 2 binding affinity was remarkably enhanced due to the lower symmetry (D 2h ) of the porphycene framework. 11,12 Porphycene has also attracted interest in nanoscience due to the cis−trans tautomerization of the inner protons in its free base form (H 2 Pc) that could be induced by the scanning tunneling microscope (STM) tip. 13−16 Recently, a 2D molecular assembly of free-base tetraphenylporphycene (H 2 TPPc) on Cu(111) in ultrahigh vacuum (UHV) was formed, showing highly ordered adlayers. 17 Because of the low symmetry of the porphycene framework, molecular assemblies of porphycene compounds on surfaces offer us the ability to design and fabricate unique molecular architectures. The coordination of various metal ions to the porphycene framework could tune its properties, such as the intermolecular and molecule−substrate interactions. These metalloporphycene compounds are espe- cially attractive in surface electrochemistry studies because their 2D molecular assemblies on a surface may enable the design of new nanostructures for energy conversion by controlling the redox potentials of the coordinated metal ions. 18 However, to the best of our knowledge, there has been no report about 2D assemblies of metalloporphycenes. Here we report for the first time a 2D molecular assembly of nickel(II) tetra(n-propyl)porphycene (NiTPrPc, Chart 1) on Au(111) under electrochemical conditions. The unique molecular arrangements in the NiTPrPc adlayer and their phase transitions were clearly revealed by electrochemical scanning tunneling microscopy (EC-STM). ■ EXPERIMENTAL SECTION NiTPrPc was synthesized as described in a previous paper. 19 Au(111) single-crystal electrodes were prepared by Clavilier’s method. 20 The Au Received: October 17, 2016 Revised: November 28, 2016 Published: November 29, 2016 Chart 1. Chemical Structure and CPK Model of NiTPrPc Article pubs.acs.org/Langmuir © 2016 American Chemical Society 13635 DOI: 10.1021/acs.langmuir.6b03782 Langmuir 2016, 32, 13635−13639