Layer-By-Layer Construction of Flatly Adsorbed Porphyrin Monoayers on a Au(111) Surface
Porphyrin derivatives are well-known to have unique optical, electrochemical, and catalytic properties and then have extensively used as a building block in construction of molecular devices . In order to construct effective molecular photodevices using optical properties of porphyrins, ordered porphyrin layer with an orientation parallel to the solid surface should be required because a transition moment of porphyrin is parallel to a porphyrin ring. In order to construct molecular catalysts, on the other hand, flatly adsorbed porphyrin layer also should be required because substrate binds along the direction perpendicular to the porphyrin ring.
The self-assembly technique is one of the best methods to chemisorb and to arrange the molecule in order onto the solid substrate surface and the self-assembled monolayers (SAMs) on gold have been widely used because of their high stability, high packing density, and high orientation. We succeeded to flatly chemisorb the porphyrin derivatives with 4 thiol groups as binding groups on the Au(111) surface. But these porphyrin derivatives were too difficult to synthesize and very unstable.
In this report, the highly oriented SAMs of anchor molecules, such as 4-mercaptopyridine (4-PySH), 3-mercapto-propionic acid (3-MPA), and 4-mercaptobenzoic acid (4-MBA), were constructed on the Au(111) surface as a first and then, metallo-porphyrin was fixed on those SAMs using a coordinate bonding to achieve the construction of flatly adsorbed porphyrin monolayers, as shown in Fig. 1, and electro-cayalytic and photo-electrochemical activities of these porphyrin layers were investigated.
The Au(111) substrates were annealed and quenched. After that, they were immersed in various solutions containing anchor molecules such as 4-PySH, 3-MPA, and 4-MBA with various concentrations for certain periods. Their self-assembly processes were investigated by electrochemical measurements for the reductive desorption and X-ray photoelectron spectroscopy (XPS) measurements in order to find the preparation conditions of the densely-packed and well-oriented SAMs.
Results and Discussion.
Figure 2 shows linear sweep voltammograms (LSVs) of Au(111) electrodes modified with the 3-MPA SAMs, which were prepared by the dipping of the Au(111) substrate into the ethanol solutions containing 1 mM 3-MPA for several dipping periods. We can clearly see cathodic peaks, due to the reductive desorption of 3-MPA, around -0.7 V. With increasing of the dipping period, the peak potential shifted to negative and the surface coverage of 3-MPA increased and full width of half maximum (fwhm) of the cathodic peak, which shows degree of the orientation of the SAM, decreased. XP spectrum of the 3-MPA SAM prepared by the dipping period of 10 h showed that this 3-MPA SAM is well-packed and highly oriented. Constructing of the flatly adsorbed metallo-porphyrin layers on these SAMs was confirmed by XPS. Characteristics of prepared flatly adsorbed metallo-porphyrin monolayers are now under investigation.
Fig. 1 Schematic illustration of flatly adsorbed metallo-porphyrin monolayer formed on the anchor SAM on Au(111).
Fig. 2 LSVs of the 3-MPA SAM modified Au(111) electrodes, which were prepared for several dipping periods, measured in a deaerated 0.1 M KOH solution with a scan rate of 50 mV s-1.