Asparagusic Acid - A Unique Approach toward Effective Cellular Uptake of Therapeutics: Application, Biological Targets, and Chemical Properties

The synthetic approaches towards unique asparagusic acid and its analogues as well as its chemical use, the breadth of its biological properties and their relevant applications have been explored. The significance of the 1,2-dithiolane ring tension in dithiol-mediated uptake and its use for the intracellular transport of molecular cargoes is discussed alongside some of the challenges that arise from the fast thiolate-disulfide interchange. The short overview with the indication of the available literature on natural 1,2-dithiolanes synthesis and biological activities is also included. The general review structure is based on the time-line perspective of the application of asparagusic acid moiety as well as its primitive derivatives (4-amino-1,2-dithiolane-4-carboxylic acid and 4-methyl-1,2-dithiolane-4-carboxilic acid) used in clinics/cosmetics, focusing on the recent research in this area and including international patents applications.

Distortion isomers of cis-PtP2X2 and cis-PtP2XY derivatives – structural aspects

In this review, the structural parameters of distortion isomers of cis-monomeric Pt(II) coordination complexes with inner coordination spheres: Pt(PL)2X2 (X = OL, NL, SL, Br, I); Pt(PL)2(η2-X2L) (X = O2L, N2L, S2L, OSL, NSL, NSeL); Pt(η2-P2L)X2 (X = Br, I); Pt(η2-P2L)(η2-X2L) (X = O2L, OSL, NSL); Pt(η2-P2L)(NL)(Cl) and Pt(PL)(η2-P,SiL)(H) are analyzed. None of the distortion isomers with cis-configuration has a trans-partner. The distortion isomers differ mostly by the degree of distortion in the Pt-L and L-Pt-L angles. Some of the isomers also differ by crystal packing. The total mean values of Pt-P (monodentate) and Pt-P (bidentate) bond distances are 2.279 Å and 2.244 Å, respectively. The mean value of Pt-P (monodentate) (trans to H¯) of 2.320 Å is the highest one because of higher trans-influence of H¯ over PP3. The total mean values of Pt-X (trans to P) elongate quite well with the covalent radius of the X in the sequence: 1.57 Å (X = H) < 2.062 Å (O2L) < 2.095 Å (OL) < 2.108 Å (NL) < 2.154 Å (N2L) < 2.329 Å (Cl) < 2.342 Å (S2L) < 2.347 Å (SL) < 2.480 Å (Br) < 2.616 Å (I).

Stability of functionalized platform molecules on Au(111)

Trioxatriangulenium (TOTA) platform molecules were functionalized with methyl, ethyl, ethynyl, propynyl, and hydrogen and sublimated onto Au(111) surfaces. Low-temperature scanning tunneling microscopy data reveal that >99% of ethyl-TOTA and methyl-TOTA remain intact, whereas 60% of H-TOTA and >99% of propynyl-TOTA and ethynyl-TOTA decompose. The observed tendency toward fragmentation on Au(111) is opposite to the sequence of gas-phase stabilities of the molecules. Although Au(111) is the noblest of all metal surfaces, the binding energies of the decomposition products to Au(111) destabilize the functionalized platforms by 2 to 3.9 eV (190-370 kJ/mol) and even render some of them unstable as revealed by density functional theory calculations. Van der Waals forces are important, as they drive the adsorption of the platform molecules.

Controlling Heat Release from a Close-Packed Bisazobenzene-Reduced-Graphene-Oxide Assembly Film for High-Energy Solid-State Photothermal Fuels

A closed-cycle system for light-harvesting, storage, and heat release is important for utilizing and managing renewable energy. However, combining a high-energy, stable photochromic material with a controllable trigger for solid-state heat release remains a great challenge for developing photothermal fuels (PTFs). This paper presents a uniform PTF film fabricated by the assembly of close-packed bisazobenzene (bisAzo) grafted onto reduced graphene oxide (rGO). The assembled rGO-bisAzo template exhibited a high energy density of 131 Wh kg^-1 and a long half-life of 37 days owing to inter- or intramolecular H-bonding and steric hindrance. The rGO-bisAzo PTF film released and accumulated heat to realize a maximum temperature difference (DT) of 15 °C and a DT of over 10 °C for 30 min when the temperature difference of the environment was greater than100 °C. Controlling heat release in the solid-state assembly paves the way to develop highly efficient and high-energy PTFs for a multitude of applications.

Photoisomerization of Self-Assembled Monolayers of Azobiphenyls: Simulations Highlight the Role of Packing and Defects

We present surface hopping simulations of the photodynamics of self-assembled monolayers (SAMs) of 4'-(biphenyl-4-ylazo)-biphenyl-4-thiol (ABPT) on Au(111). We show that trans → cis photoisomerization is suppressed because of steric hindrance in a well-ordered SAM. Photoisomerization is instead viable in the presence of defects. Two particularly important defects are the boundaries between domains of trans-ABPT molecules leaning in different directions (a line defect) and single cis molecules embedded in a SAM of trans (a point defect). Our findings explain the cooperative behavior observed during the photoisomerization of a trans-ABPT SAM, leading to large domains of pure cis and trans isomers. The line and point defects are predicted to produce different patterns of cis-ABPT molecules during the early stages of the photoconversion.

Monitoring the reversible photoisomerization of an azobenzene-functionalized molecular triazatriangulene platform on Au(111) by IRRAS

Spectroscopic evidence of a reversible, photoinduced trans ↔ cis photoisomerization is provided for an azobenzene-functionalized triazatriangulene (TATA) platform on Au(111). As shown by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), these molecules form a well-ordered self-assembled monolayer (SAM) on Au(111). The surface-adsorbed azo-TATA platforms are also investigated by infrared reflection absorption spectroscopy (IRRAS); a methoxy marker group at the upper phenyl ring of the azo moiety is employed to monitor the switching state. The IRRAS data are analyzed by comparison with theoretical and transmission IR spectra as well as bulk and surface-enhanced Raman spectroscopic (SERS) data. IRRAS shows that the methoxy group is oriented perpendicular to the surface in trans- and tilted with respect to the surface normal in cis-configuration. This indicates that the photoswitching capability of the azobenzene moieties is retained on the gold surface. The lifetime of the cis-configuration is, however, reduced by a factor of ∼10(3) with respect to the homogeneous solution.

Different formation kinetics and photoisomerization behavior of self-assembled monolayers of thiols and dithiolanes bearing azobenzene moieties

Self-assembled monolayers (SAMs) containing azobenzene moieties are very attractive for a wide range of applications, including molecular electronics and photonics, bio-interface engineering and sensoring. However, very little is known about the aggregation and photoswitching behavior that azobenzene units undergo during the SAM formation process. Here, we demonstrate that the formation of thiol-based SAMs containing azobenzenes (denoted as AzoSH) on gold surfaces is characterised by a two-step adsorption kinetics, while a three-step assembly process has been identified for dithiolane-based SAMs containing azobenzenes (denoted AzoSS). The H-aggregation on the AzoSS SAMs was found to be remarkably dependent on the time of self-assembly, with less aggregation as a function of time. While photoisomerization of the AzoSH was suppressed for all different assembly times, the reversible trans-cis photoisomerization of AzoSS SAMs formed over 24 hours was clearly observed upon alternating UV and Vis light irradiation. We contend that detailed information on formation kinetics and related optical properties is of crucial importance for elucidating the photoswitching capabilities of azobenzene-based SAMs.

Formation of highly ordered and orientated gold islands: effect of immersion time on the molecular adlayer structure of pentafluorobenzenethiols (PFBT) SAMs on Au(111)

Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the [11(-)0] substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, β, γ, δ, and ε were found. The α-, β-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→β→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.

Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes: coordination to gold surfaces

The zwitterionic dithiocarboxylates 1(+)-CS(2)(-)-4(+)-CS(2)(-) were prepared by reacting the corresponding N-heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (1), 1,3-diisopropylimidazol-2-ylidene (2), 1,3-dibenzylimidazol-2-ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS(2). In the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2(+)-CS(2)(-)-4(+)-CS(2)(-) were structurally characterised by single-crystal X-ray diffraction studies. The chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the case of 1(+)-CS(2)(-) and 2(+)-CS(2)(-), revealing the formation of almost contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order.

Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers: a study based on density functional theory

Based on the analysis of optical absorption spectra, it has recently been speculated that the excitonic coupling between individual azobenzene-functionalized alkanethiols arranged in a self-assembled monolayer (SAM) on a gold surface could be strong enough to hinder collective trans-cis isomerization-on top of steric hindrance [Gahl et al., J. Am. Chem. Soc., 2010, 132, 1831]. Using models of SAMs of increasing complexity (dimer, linear N-mers, and two-dimensionally arranged N-mers) and density functional theory on the (TD-) B3LYP/6-31G* level, we determine optical absorption spectra, the nature and magnitude of excitonic couplings, and the corresponding spectral shifts. It is found that at inter-monomer distances of about 20 Å and above, TD-B3LYP excitation frequencies (and signal intensities) can be well described by the frequently used point-dipole approximation. Further, calculated blue shifts in optical absorption spectra account for the experimental observations made for azobenzene/gold SAMs, and hint to the fact that they can indeed be responsible for reduced switching probability in densely packed self-assembled structures.

Azobenzene-containing triazatriangulenium adlayers on Au(111): structural and spectroscopic characterization

Adlayers of different azobenzene-functionalized derivatives of the triazatriangulenium (TATA) platform on Au(111) surfaces were studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS), and cyclic voltammetry (CV). The chemical composition of the adlayers is in good agreement with the molecular structure, i.e., different chemical groups attached to the azobenzene functionality were identified. Furthermore, the presence of the azobenzene moieties in the adlayers was verified by the vibration spectra and electrochemical data. These results indicate that the molecules remain intact upon adsorption with the freestanding functional groups oriented perpendicularly to the TATA platform and thus also to the substrate surface.

Adamantane-Based Tripodal Thioether Ligands Functionalized with a Redox-Active Ferrocenyl Moiety for Self-Assembled Monolayers

Self-assembled monolayers (SAMs) can decorate surfaces with `smart´ functional units possessing reversible stimulus-response behavior for optical, thermal, magnetic or redox-chemical stimuli. An independent performance of individual functional groups in such a film is desirable, which can be, in particular, ensured by fairly large lateral separations between tailgroups in the SAM. Adsorbate molecules with multiple attachment points are very promising in this context owing to their large surface footprint, which covers a surface area exceeding the lateral dimensions of the functional groups. To address these design constraints, novel tridentate long-chain tripodal thioether ligands with central adamantine units and a redox-active ferrocenyl tailgroup, 1-[4-(ferrocenylethynyl)phenyl]-3,5,7-tri[(4-n-octylsulfanyl)phenyl]adamantine (T8) and 1-[4-(ferrocenylethynyl)phenyl]-3,5,7-tri[(4-n-dodecylsulfanyl)phenyl]adamantine (T12), were synthesized and used as tripodal adsorbate molecules for the fabrication of redox-active ferrocenyl-terminated SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and sum frequency generation spectroscopy. The data suggest that T8 and T12 form almost contamination-free, well-aligned and fairly densely-packed SAMs on Au(111) with laterally separated ferrocenyl units. The SAMs show a homogeneous binding chemistry, an important requirement for high fidelity SAMs. SFG results indicate lateral interactions between neighboring molecules via the long-chain binding units.

COOH-terminated SAMs on gold fabricated from an azobenzene derivative with a 1,2-dithiolane headgroup

Well-defined and homogeneous, contamination-free self-assembled monolayers (SAMs) were fabricated by the chemisorption of lip-NH-p-C₆H₄-N=N-p-C₆H₄-COOH (lip = α-lipoyl) onto gold. This adsorbate species is composed of a 1,2-dithiolane-based headgroup, an azobenzene-based (and hence photochromic) spacer unit and a carboxylic acid functional group. The SAM constituents are covalently attached to the substrate by the bidentate thiolate anchor groups and exhibit a strongly tilted binding configuration.