Self-assembled porphyrin microrods and observation of structure-induced iridescence

Self-assembled microrods {based on 5-nitro-10,15,20-trialkylporphyrins [(CnH2n+1)3-NO2P]} and microplates {based on 5,10,15,20-tetraheptylporphyrin [(C7H15)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 (α-quartz), 5-nitro-trialkylporphyrins, (C5H11)3-NO2P, (C7H15)3-NO2P and (C11H23)3-NO2P all form microrods of 0.7–0.8 micron diameter; the average length of the microrods varies from 170 microns for (C5H11)3-NO2P to about 11 microns for (C7H15)3-NO2P and (C11H23)3-NO2P, whereas (C19H39)3-NO2P 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 (C7H15)3-NO2P show marked iridescent character. When (C7H15)3-NO2P is deposited on silicon, however, longer curved microrods which do not show iridescence are produced. Single crystal X-ray crystallography of (C7H15)3-NO2P 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 (C7H15)3-NO2P microrods is explained by staggering of the layers to produce a corrugated surface with a period of 125 nm, as measured by AFM.