Self-assembled porphyrin microrods and observation of structure-induced iridescence† Cicero Martelli, abc John Canning, * ab Tony Khoury, d Nina Skivesen, a Martin Kristensen, a George Huyang, bd Paul Jensen, d Chiara Neto, d Tze Jing Sum, d Mads Bruun Hovgaard, a Brant C. Gibson e and Maxwell J. Crossley * d Received 27th August 2009, Accepted 19th October 2009 First published as an Advance Article on the web 5th January 2010 DOI: 10.1039/b917695k Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(C n H 2n+1 ) 3 -NO 2 P]} and microplates {based on 5,10,15,20-tetraheptylporphyrin [(C 7 H 15 ) 4 -P]} are fabricated and characterised using optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The length of the alkyl chains and the deposition surface are found to influence the optical properties and microrod self-assembly. When the deposition surface is silica (a-quartz), 5-nitro- trialkylporphyrins, (C 5 H 11 ) 3 -NO 2 P, (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P all form microrods of 0.7–0.8 micron diameter; the average length of the microrods varies from 170 microns for (C 5 H 11 ) 3 -NO 2 P to about 11 microns for (C 7 H 15 ) 3 -NO 2 P and (C 11 H 23 ) 3 -NO 2 P, whereas (C 19 H 39 ) 3 -NO 2 P with much longer alkyl chains only gives powders. Controlling the precipitation is crucial in preventing the disordered aggregation of assembled layers observed in the bulk. Very interestingly, the microrods formed from (C 7 H 15 ) 3 -NO 2 P show marked iridescent character. When (C 7 H 15 ) 3 -NO 2 P is deposited on silicon, however, longer curved microrods which do not show iridescence are produced. Single crystal X-ray crystallography of (C 7 H 15 ) 3 -NO 2 P reveals the packing of the bulk material which explains the packing topology of the layers observed by AFM but not the iridescence. The observed structural colour of the (C 7 H 15 ) 3 -NO 2 P microrods is explained by staggering of the layers to produce a corrugated surface with a period of 125 nm, as measured by AFM. Introduction A commonly used approach to self-assemble porphyrin struc- tures uses both electrostatic and hydrophobic interactions of porphyrins containing ionic substituents in aqueous solutions, 1–3 as well as organic solvents. 4 In these solutions, water soluble porphyrins can be forced to aggregate by controlling the pH, the ionic strength, and temperature. 3–7 Without a supporting matrix, H- or J-aggregates are formed, detected by the blue (H) or red (J) shift of the exciton bands. 6,8 Ionic self-assembly 9 enables the formation of porphyrin nanotubes in an aqueous solution. 10–12 The expounded mechanism relied on electrostatic interactions between two oppositely charged porphyrins [one with Sn(IV)], which in addition to the van der Waals, hydrogen-bonding, axial coordination and other weak intermolecular forces enhanced the structural stability of the system. Tunnelling electron microscopy (TEM) images showed these nanotubes to be hollow, microns in length and between 50 and 70 nm in width. Fringe analysis, together with spectrophotometer measurements, suggests that these nanotubes are stacks of offset J-aggregated porphyrins likely in the form of cylindrical lamellar sheets and X-ray diffraction studies reveal moderate crystallinity. Studies in acid were unable to rule out the role of water molecules. Another important observation of porphyrin self-assembly was the formation in alcohols of rod-like micelles on the nanoscale of a cobalt(II) porphyrin. 13 In this case, the structure is thought to be in a reverse micellular arrangement of face-to-face aggregate having a hydrophobic corona around a polar core. In addition to ionic self-assembly, porphyrin thin films have been fabricated in the holes of photonic crystal fibres, showing interaction between the tin(IV) porphyrin and the silica of the fibres. 14 Examples of self-assembly based on topology packing in two and three dimensions of porphyrins and related systems such as phthalo- cyanines and corroles have also been reported. 15,16 We report self-assembly studies of porphyrin arrays on a-quartz and silicon and show that in nonaqueous solutions the incorporation of a polar NO 2 group transforms the planar 2-D self-assembly of 5,10,15,20-tetraheptylporphyrin [(C 7 H 15 ) 4 -P] into 1-D microrods as 5-nitro-10,15,20-triheptylporphyrin [(C 7 H 15 ) 3 -NO 2 P]. These structures are characterised using optical microscopy, atomic force microscopy (AFM) and scan- ning electron microscopy (SEM). The optical transparency obtained with the microrods after drying suggests potential photonic transport applications. The observed iridescence reveals a lamellar period, suggesting novel optical functionality exploiting these periodic structures is possible. a iNANO & Department of Physics and Astronomy, University of Aarhus, DK-8000  Arhus C, Denmark b Interdisciplinary Photonics Laboratories, School of Chemistry, The University of Sydney, NSW 2006, Australia. E-mail: j.canning@usyd.edu.au c Departamento de Engenharia Mec ^ anica, Pontif´ ıcia Universidade Cat  olica do Rio de Janeiro, RJ 22453-900, Brasil d School of Chemistry, The University of Sydney, NSW 2006, Australia e Quantum Physics Victoria, School of Physics, The University of Melbourne, Vic 3010, Australia † CCDC reference number 742688. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b917695k 2310 | J. Mater. Chem., 2010, 20, 2310–2316 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/materials | Journal of Materials Chemistry Downloaded by University of Sydney on 13 February 2013 Published on 05 January 2010 on http://pubs.rsc.org | doi:10.1039/B917695K View Article Online / Journal Homepage / Table of Contents for this issue