Disentangling electron- and electric field-induced ring-closing reactions in a diarylethene derivative on Ag(111)

Using scanning tunneling microscopy and spectroscopy we investigate the adsorption properties and ring-closing reaction of a diarylethene derivative (C5F-4Py) on a Ag(111) surface. We identify an electron-induced reaction mechanism, with a quantum yield varying from $10^{-14}-10^{-9}$ per electron upon variation of the bias voltage from $1-2$ V. We ascribe the drastic increase in switching efficiency to a resonant enhancement upon tunneling through molecular orbitals. Additionally, we resolve the ring-closing reaction even in the absence of a current passing through the molecule. In this case the electric-field can modify the reaction barrier, leading to a finite switching probability at 4.8 K. A detailed analysis of the switching events shows that a simple plate-capacitor model for the tip-surface junction is insufficient to explain the distance dependence of the switching voltage. Instead, describing the tip as a sphere is in agreement with the findings. We resolve small differences in the adsorption configuration of the closed isomer, when comparing the electron- and field-induced switching product

Synthesis and properties of photochromic difurylethenes for mechanically controlled break junctions

Twenty-one new photochromic furan-based diarylethenes with various side-substituents were synthesized and their switching properties were investigated. The influence of central fluorinated ring size (four- and five-membered) on the structural and photochemical properties of molecular switches was shown. More strained difurylcyclobutenes exhibited larger quantum yields of the ring-opening reaction than cyclopentene-based switches. Single crystals of 8 new diarylethenes were obtained and examined by means of X-ray analysis; unusual cycloalkene double bond twisting deformation in isoxazole-based switch was observed. Four difurylethenes with different anchoring groups were applied as molecular switches in mechanically controlled break junctions

Basic enemies of photochromism : irreversible transformation of fluorinated diarylethenes to polyenic enamines and enols

Non-photochemical degradation of perfluorinated photochromic diarylethenes (DAE) under Knoevenagel, Sonogashira or Wittig conditions was discovered. This base promoted formation of strongly colored non-photochromic byproducts has an impact in the field of molecular electronics due to the basic conditions often employed during deacylation and desilylation of the protected thiol anchoring groups of functionalized DAE. The products were identified as seven-membered ring systems of the bicyclo[5.3.0]deca-1,7-diene type. Their formation was rationalized by a tentative two-step reaction mechanism.publishe

Photochromic difurylethene bisaldehyde as potential building block for [AA] and [AB]₂ macrocyclization

Catalyst- and template-free condensation of photochromic difurylethene bisaldehyde with o-phenylenediamine led to selective formation of macrocyclic Schiff base [AB]2 product. Application of the same difurylethene bisaldehyde in the McMurry reaction resulted in cyclic dimer formation in moderate yield. No linear oligomers were observed in both approaches.publishe

Nanoscopic Visualization of Soft Matter Using Fluorescent Diarylethene Photoswitches

The in situ imaging of soft matter is of paramount importance for a detailed understanding of functionality on the nanoscopic scale. Although super-resolution fluorescence microscopy methods with their unprecedented imaging capabilities have revolutionized research in the life sciences, this potential has been far less exploited in materials science. One of the main obstacles for a more universal application of super-resolved fluorescence microscopy methods is the limitation of readily available suitable dyes to overcome the diffraction limit. Here, we report a novel diarylethene-based photoswitch with a highly fluorescent closed and a nonfluorescent open form. Its photophysical properties, switching behavior, and high photostability make the dye an ideal candidate for photoactivation localization microscopy (PALM). It is capable of resolving apolar structures with an accuracy far beyond the diffraction limit of optical light in cylindrical micelles formed by amphiphilic block copolymers.publishe

Nonuniform STM Contrast of Self-Assembled Tri- n -octyl-triazatriangulenium Tetrafluoroborate on HOPG

International audienceWe have assembled 4,8,12-tri-n-octyl-4,8,12-triazatrianguleniumtetrafluoroborate (TATA-BF4) on highly oriented pyrolytic graphite (HOPG) and have studied the structure and tunneling properties of this self-assembled monolayer (SAM) using scanning tunneling microscopy (STM) under ambient conditions. We show that the triazatriangulenium cations TATA(+) form hexagonally packed structures driven by the interaction between the aromatic core and the HOPG lattice, as evidenced by density functional theory (DFT) modeling. According to the DFT results, the three alkyl chains of the platform tend to follow the main crystallographic directions of HOPG, leading to a different STM appearance. The STM contrast of the SAM shows that the monolayer is formed by two types of species, namely, TATA(+) with BF4- counterions on top and without them. The cationic TATA(+) platform gives rise to a seemingly higher appearance than neutral TATA-BF4, in contrast to observations made on metallic substrates. The variation of the STM tunneling parameters does not change the relative difference of contrast, revealing the stability of both species on HOPG. DFT calculations show that TATA-BF4 on HOPG has sufficient binding energy to resist dissociation into TATA(+) and BF4-, which might occur under the action of the electric field in the tunneling gap during STM scanning

Optical properties of gold nanoparticles decorated with furan-based diarylethene photochromic molecules

The optical properties of the photochromic 1,2-bis(2-methyl-5-((4-mercaptophenyl)ethynyl)-furan-3-yl)perfluorocyclopentene (YnPhT) covalently attached to gold nanoparticles (AuNP) (d = 17 ± 1 nm) were studied in air at ambient conditions. The reversible ring-opening/closing photochromic reactions of molecules attached on AuNP surface can occur under similar light irradiation conditions as observed for free molecules in ethanolic solution. An unexpected blue shift of the local surface plasmon resonance band (LSPR) of AuNP covered by photochromic molecules was observed after UV light irradiation. A reverse red shift of LSPR was observed when the AuNP are illuminated by visible light. Moreover, increased amplitude of the LSPR was observed for AuNP covered with molecules in the closed-ring state. These observations are discussed in view of molecule – AuNP interactions. These interactions decelerate the kinetics of the ring-closing reaction of YnPhT on AuNP surfaces under UV light irradiation.publishe

Charge Transport in Azobenzene-Based Single-Molecule Junctions

Azobenzene-derivative molecules change their conformation as a result of a cis-trans transition when exposed to ultraviolet or visible light irradiation and this is expected to induce a significant variation in the conductance of molecular devices. Despite extensive investigations carried out on this type of molecule, a detailed understanding of the charge transport for the two isomers is still lacking. We report a combined experimental and theoretical analysis of electron transport through azobenzene-derivative single-molecule break junctions with Au electrodes. Current-voltage and inelastic electron tunneling spectroscopy (IETS) measurements performed at 4.2 K are interpreted based on first-principles calculations of electron transmission and IETS spectra. This qualitative study unravels the origin of a slightly higher conductance of junctions with the cis isomer and demonstrates that IETS spectra of cis and trans forms show distinct vibrational fingerprints that can be used for identifying the isomer

Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions

Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.publishe

Nanoskopische Bildgebung der Vernetzungsdichte in Polymernetzwerken mittels Diarylethen‐Photoschaltern

Die nanoskopische In‐situ‐Bildgebung weicher Polymerstrukturen ist für das Verständnis der Zusammenhänge von Struktur, Eigenschaften und Funktionalität im nm‐Bereich entscheidend. Die Vernetzung von Polymerketten legt die viskoelastischen Eigenschaften eines Gels fest, und die Korrelation zwischen mechanischen Eigenschaften und der Verteilung und Menge an Vernetzern ist für Anwendungen und ein elementares Verständnis von Polymeren auf der molekularen Ebene relevant. Hier wird eine auf superauflösender Fluoreszenzmikroskopie basierende Methodik zur Visualisierung und Quantifizierung von Vernetzungspunkten in Polymersystemen vorgestellt. Ein neuartiger Diarylethen‐Photoschalter mit stark fluoreszierender geschlossener und nicht‐fluoreszierender offener Form wird als photoschaltbarer Vernetzer in ein Polymernetzwerk eingebaut. Seine photophysikalischen Eigenschaften, sein Schaltverhalten und seine hohe Photostabilität machen ihn zu einem idealen Kandidaten für photoaktivierte Lokalisationsmikroskopie. Als Anwendungsbeispiel wurden pNIPAM‐Mikrogele untersucht, die bekanntlich hohe Variationen in ihrer internen Vernetzungsdichte aufweisen.publishe