Effect of the Steric Molecular Structure of Azobenzene on the Formation of Self-Assembled Monolayers with a Photoswitchable Surface Morphology

The growth processes of self-assembled monolayers (SAMs) of two azobenzene disulfides formed on flat gold surfaces were studied to confirm the effect of the intermolecular interactions between azobenzene molecules on the light-triggered surface morphologies of the SAMs. Scanning tunneling microscopy (STM), atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible (UV–vis) absorption spectroscopy were employed to study the SAMs and their growth processes. The SAMs studied were of bulky-substituted azobenzene disulfide (Et-2S), and nonsubstituted azobenzene disulfide (Me-2S), formed on a gold-covered substrate, and had a twisted and a planar structure, respectively. STM-based imaging of the initial stage of the self-assembly of the Et-2S molecules revealed that cleavage of the disulfide bond occurred on the gold surface, and phase-separated domains composed of azobenzenethiolate and dodecanethiolate were formed. Time-dependent AFM-based imaging illustrated the mechanism through which the Et-2S SAM grewit was through the formation of dendritic aggregates and islandseventually resulting in phase-separated domains with a wormlike structure. This wormlike structure showed noticeable changes in its surface morphology upon irradiation with UV and visible light. On the other hand, while the growth process for the Me-2S SAM was similar to that of the Et-2S SAM, the final Me-2S SAM had smooth domains whose morphology did not exhibit photoswitchability. The TD and XP spectra of the SAMs showed that the number of adsorbed Et-2S molecules reached a point of saturation after a 24 h long immersion while the number of Me-2S molecules increased even after a 336 h long immersion. Furthermore, the area occupied by the azobenzene moiety in the Et-2S SAM was constant regardless of the immersion time, whereas that in the Me-2S SAM decreased with the immersion time. These results indicated that the strength of the interactions between the azobenzene molecules significantly influenced the aggregate-forming ability in SAMs

Cationic Self-Assembled Monolayers Composed of Gemini-Structured Dithiol on Gold: A New Concept for Molecular Recognition Because of the Distance between Adsorption Sites

Cationic self-assembled monolayers (SAMs) composed of quaternary ammonium (QA) sulfur derivatives have been synthesized to control the distance between charged headgroups on gold substrates. Two molecules bearing resembling molecular structures, “gemini”-structured didodecyl dithiol (HS-gQA-SH) and didodecyl disulfide (QA-SS-QA), were utilized in this study, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared-reflection adsorption spectroscopy (FTIR−RAS). In the HS-gQA-SH SAM, the orientation and distance between QA groups are specified by the covalent bonding with ethylene spacer, while those of the QA-SS-QA SAM are determined by the electric repulsion between charged headgroups, that is, QA groups in the QA-SS-QA SAM are more randomly located, being more distant than with those in the HS-gQA-SH SAM. We found that l-tartaric acid, a probe molecule with two carboxyl groups having the distance of an ethylene unit, exhibits a strong affinity on the HS-gQA-SH SAM. In contrast, no specific binding was observed on the QA-SS-QA SAM. These results imply the possibility to build up a molecular recognition system on surfaces because of the control of the distance between the charged headgroups by using the gemini-structured molecular design.</i

Characterization of the Interactions between Alq₃ Thin Films and Al Probed by Two-Color Sum-Frequency Generation Spectroscopy

We present the investigation of the vibrational and electronic states of tris(8-hydroxyquinoline) aluminum (Alq3)/Al (Alq3 on Al) interfaces by using two-color infrared-visible sum frequency generation (SFG) spectroscopy. The visible wavelength dependence of the SFG spectra of the 2 nm thick Alq3/Al consists of the vibrational bands derived from the Alq3 at the Al interfaces. The intensities of the peaks derived from the ring stretching modes of the quinolate ligands were significantly enhanced due to the double resonance effect. In contrast, the SFG electronic spectrum obtained from the output photon energy dependence of the SFG peak amplitudes derived from the CC bands of the Al on Alq3 interfaces does not show the wavelength dependences, indicating that the electronic-resonance associated with the π–π* transitions in the quinolate rings are almost vanished at the Al deposited on the Alq3. The disappearance of the electronic-resonance of the CC stretching modes must be caused by the perturbation of the HOMO and LUMO of pristine Alq3 by the interaction with the Al. The spectral features of the two-color SFG spectra of the Al/LiF/Alq3 system show quite different behavior from those of Alq3/Al and Al/Alq3. The shift of the CC stretching modes toward lower frequencies is indicative of the formation of the Alq3 anionic states upon reaction with Li at the interface. Additional broad bands around 1335 and 1450 cm–1, which show the weak excitation wavelength dependence, might be due to the existence of the Li-reacted graphitic carbon-like Alq3

Formation and Superlattice of Long-Range-Ordered Self-Assembled Monolayers of Pentafluorobenzenethiols on Au(111)

The formation and surface structure of pentafluorobenzenethiol (PFBT) self-assembled monolayers (SAMs) on Au(111) formed under various experimental conditions were examined by means of scanning tunneling microscopy (STM). Although it is well known that PFBT molecules on metal surfaces do not form ordered SAMs, we clearly revealed for the first time that the adsorption of PFBT on Au(111) at 75 °C for 2 h yields long-range, well-ordered self-assembled monolayers having a (2 × 5√13)R30° superlattice. Our results will provide new insight into controlling the structural order of PFBT SAMs, which will be very useful in precisely tailoring the interface properties of metal surfaces in electronic devices

Surface Structure, Adsorption, and Thermal Desorption Behaviors of Methaneselenolate Monolayers on Au(111) from Dimethyl Diselenides

To understand the effect of headgroups (i.e., sulfur and selenium) on surface structure, adsorption states, and thermal desorption behaviors of self-assembled monolayers (SAMs) on Au(111), we examined methanethiolate (CH₃–S, MS) and metheneselenolate (CH₃–Se, MSe) monolayers formed from dimethyl disulfide (DMDS) and dimethyl diselenide (DMDSe) molecules by ambient vapor-phase deposition. Scanning tunneling microscopy imaging revealed that DMDS molecules on Au(111) after a 1 h deposition form MS monolayers containing a disordered phase and an ordered row phase with an inter-row spacing of 1.51 nm, whereas DMDSe molecules form long-range-ordered MSe monolayers with a (√3 × 3√3)<i>R</i>30° structure. X-ray photoelectron spectroscopy measurements showed that MS or MSe monolayers chemisorbed on Au(111) were formed via S–S bond cleavage of DMDS or Se–Se bond cleavage of DMDSe. On the other hand, we monitored three main desorption fragments for MS and MSe monolayers using TDS monomers (CH₃S⁺, CH₃Se⁺), parent mass species (CH₃SH⁺, CH₃SeH⁺), and dimers (CH₃S–SCH₃⁺, CH₃Se–SeCH₃⁺). Interestingly, we found that thermal desorption behaviors of MSe monolayers were markedly different from those of MS monolayers. All desorption peaks for MSe monolayers were observed at a higher temperature compared with MS monolayers, suggesting that the adsorption affinity of selenium atoms for the Au(111) surface is stronger than that of sulfur atoms. In addition, the desorption intensity of dimer fragments for MSe monolayers was much lower than for MS monolayers, indicating that selenolate SAMs on Au(111) did not undergo their dimerization efficiently during thermal heating compared with thiolate SAMs. Our results provide new insight into understanding the surface structure and thermal desorption behavior of MSe monolayers on Au(111) surface by comparing those of MS monolayers