Theoretical Comparison of a Longitudinal versus a Transverse Transport Path through Diarylethene Molecular Switches

Diarylethene molecules are regularly used in molecular junctions as light-activated switches. Two crucial parameters drive the performance of these switches: (i) ON–OFF ratios and (ii) reversibility. In this work, we first show using the theoretical NEGF–DFT method that an efficient decoupling between the molecular backbone and the electrodes, which is necessary for reversibility, unfortunately, tends to weaken the ON–OFF ratio. We then show that this trade-off situation can be avoided by considering an alternative “transverse” contact configuration of the diarylethene, which exploits the bond breaking associated with the isomerization reaction. Interestingly, this transverse contact ensures both high on–off ratios (at least by 2 orders of magnitude) and an efficient decoupling of the active unit from the gold electrodes

Unipolar Injection and Bipolar Transport in Electroluminescent Ru-Centered Molecular Electronic Junctions

Bias-induced light emission and light-induced photocurrents were used as independent probes of charge transport in carbon-based molecular junctions containing Ru­(bpy)3. The thickness, bias, and temperature dependence of both the total device current and photoemission were compared, as well as their response to bias pulses lasting from a few milliseconds to several seconds. The device current was exponentially dependent on the square root of the applied electric field, with weak dependence on thickness when compared at a constant field. In contrast, light emission was strongly dependent on thickness at a given electric field, with a thickness-independent onset for light emission and a large intensity increase when the bias exceeded the 2.7 V HOMO–LUMO gap of Ru­(bpy)3. The apparent activation energies for light emission and current were similar but much smaller than those expected for thermionic emission or redox exchange. Light emission lagged current by several milliseconds but reached maximum emission in 5–10 ms and then decreased slowly for 1 s, in contrast to previously reported solid-state Ru­(bpy)3 light-emitting devices that relied on electrochemical charge injection. We conclude that at least two transport mechanisms are present, that is, “unipolar injection” initiated by electron transfer from a Ru­(bpy)3 HOMO to the positive electrode and “bipolar injection” involving hole and electron injection followed by migration, recombination, and light emission. The unipolar mechanism is field-driven and the majority of the device is current, while the bipolar mechanism is bias-driven and involves electrode screening by PF6 ions or mobile charges. In addition, significant changes in thickness and temperature dependence for thicknesses exceeding 15 nm imply a change from injection-limited transport to bulk-limited transport. The current results establish unequivocally that electrons and holes reside in the molecular layer during transport once the transport distance exceeds the ∼5 nm limit for coherent tunneling and that redox events involving nuclear reorganization accompany transport. In addition, they demonstrate luminescence in a single organometallic layer without hole or electron transport layers, thicknesses below 30 nm, and symmetric electrodes with similar work functions

DataSheet1_A Fluorescent Alcohol Biosensor Using a Simple microPAD Based Detection Scheme.pdf

A paper-based microfluidic detection device for the detection of ethanol is demonstrated in this work. The method is based on a fluorophore consisting of short-chain conjugated molecular unit susceptible to the protonation of its terminal pyridine groups, along with a carboxyl-functionalized sidechain that acts as a binder and renders it water-soluble. The resulting fluorescent paper device yields large fluorescence changes when exposed to reactions that yield H2O2 in aqueous solutions. Using an enzyme-catalyzed rection that produces H2O2 from ethanol, we developed a two-zone, cut-out paper device containing a reaction zone in which the ethanol-containing analyte is placed, and an adjacent sensor zone where we observe a fluorescence color shift proportional to the ethanol concentration. The limit of detection of the fluidic ethanol biosensor was 0.05 v/v% and the dynamic range was 0.05–2 v/v%. This method was employed to detect the alcohol concentration of consumer vodkas using only a paper sensor and a smartphone camera.</p