Molecular orientation of thiol-derivatized tetraphenylporphyrin on gold studied by XPS and NEXAFS

Tetraphenylporphyrins bearing four linkers consisting of thioacetyl-functionalized carbon chains were immobilized on a gold surface via thiolate-gold bonds using two different preparation routes. The structure of these molecular layers was characterized in detail with synchrotron radiation based core-level spectroscopy, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results show that the geometry of the molecular layers and the number of linkers that bind to the gold surface depends on the preparation schemes. The deprotection of the linkers through removal of the terminal acetyl group before the molecular adsorption (deprotected systems) resulted in porphyrins bound to the gold surface with on the average three linkers, their normal axis being tilted ∼38° with respect to the surface normal. On the other hand, porphyrin layers prepared directly with the acetyl group still in place on the linkers (protected systems) are made of molecules bound to the gold surface via two linkers on the average. The resulting orientation is more upright, with the normal axis of the porphyrin plane tilted ∼50° with respect to the surface normal. Moreover, NEXAFS measurements revealed that the acetyl deprotected porphyrin layers have a higher degree of ordering than the protected systems.     

Peptides adsorption on TiO2 and Au: Molecular organization investigated by NEXAFS, XPS and IR

We present a spectroscopic study of the structure of two peptides deposited on Au and TiO₂:

- PeptA, (EAK16-RGD) bringing the adhesive RGD sequence linked to EAK16;

- PeptB, having RGD linked to a “scrambled” sequence of the EAK16 peptide.

Previously reported NEXAFS investigations on thin films of the self-assembling peptide EAK16 deposited on Au and TiO₂, revealed molecular order and orientation for both substrates.IR spectra show a β-sheet conformation for PeptA and a random structure for PeptB. Angular-dependent NEXAFS measurements reveal an ordered structure with preferential molecular orientation only for PeptA. XPS analysis indicates that PeptA is adsorbed on TiO₂ in a larger amount than PeptB.     

Growth mode and molecular orientation of phthalocyanine molecules on metal single crystal substrates: A NEXAFS and XPS study

We report on near-edge X-ray absorption and X-ray photoemission experiments for the large π-conjugated organic molecule metal-free phthalocyanine (H₂Pc) on two different single crystal surfaces. Results from H₂Pc deposited on Ag(1 1 1) at 300 K show a clear linear dichroism indicating that the film grows as islands with the molecules lying essentially flat. On a Ni₃Al(1 1 1) and a cooled Ag(1 1 1) substrate, however, the molecules form a homogeneous film and have an average tilt angle of 45-60°. The different film structure is the result of a different molecule-substrate interaction.     

Surface anchoring mode‐dependent hydrolysis reactions of hydrazone groups on gold examined by pH‐dependent Raman spectroscopy

We conducted a comparative study of the pH‐dependent anchoring behaviors of 3‐methyl‐2‐benzothiazolinone hydrazone (3M2BH) and benzophenone hydrazone (BPH) on gold nanoparticles (AuNPs) by means of interfacial Raman spectroscopy. We found that several bands of 3M2BH in the highly alkaline pH region disappeared as the colloidal conditions became more neutral and acidic. The vibrational band at 919, 1174, and 1222 cm⁻¹ at pH 10.0 disappeared below pH 9.2, which may be because of the hydrolysis reactions that cleave the labile N―NH₂ group of 3M2BH, indicating a rather perpendicular orientation via the sulfur atom at the surfaces. A fairly high transition pH value was assumed to be because of the interaction of the N―NH₂ group in the vicinity of the surfaces. Several characteristic bands, including 1584 and 1617 cm⁻¹, also exhibited different intensities, suggesting that the adsorbates on Au surfaces underwent structural transformations of the N―NH₂ group after the pH value became neutral or acidic. These changes were not observed for BPH, presumably because of the direct and robust binding of the hydrazone onto Au surfaces. Our results revealed that the pH‐dependent cleavage reactions may vary depending on the surface anchoring modes of the adsorbates. Copyright © 2015 John Wiley & Sons, Ltd.     

The molecular orientation of DNA bases on H-passivated Si(1 1 1) surfaces investigated by means of near edge X-ray absorption fine structure spectroscopy

Layers of the DNA bases adenine, cytosine, and guanine were deposited onto hydrogen passivated Si(1 1 1) surfaces. The average tilt angles of these molecules with respect to the substrate surface were determined by the angular dependence of the Near Edge X-ray Absorption Fine Structure (NEXAFS) of the carbon K-edge. The interpretation of the NEXAFS spectra was assisted by a semi-empirical approach to the calculation of the π^∗-transition region which employs density functional theory calculations and core level photoemission data.     

Coverage dependent organic–metal interaction studied by high-resolution core level spectroscopy: SnPc (sub)monolayers on Ag(1 1 1)

We study the electronic structure of tin-phthalocyanine (SnPc) molecules adsorbed on a Ag(1 1 1) surface by high-resolution photoelectron spectroscopy. We particularly address the effect of different SnPc coverages on the interaction and charge transfer at the interface. The results give evidence for a covalent molecule–substrate interaction, which is temperature and coverage dependent. The valence and core level spectra as well as the work function measurements allow us monitoring subtle differences in the strength of the interface interaction, thus demonstrating the sensitivity of the methods. The results consistently show the effect of charge exchange between substrate and molecules which obviously leads to a net charge transfer into the SnPc molecules, and which is increased with decreasing coverage. Surprisingly, the Sn3d core levels are neither effected by variations of charge transfer and interaction strength, nor by a possible “Sn-up” or “Sn-down” orientation, which have been observed for sub-monolayers.