Incorporating single molecules into electrical circuits. The role of the chemical anchoring group.

Constructing electronic circuits containing singly wired molecules is at the frontier of electrical device miniaturisation. When a molecule is wired between a pair of electrodes, the two points of contact are determined by the chemical anchoring groups, located at the ends of the molecule. At this point, when a bias is applied, electrons are channelled from a metallic environment through an extremely narrow constriction, essentially a single atom, into the molecule. The fact that this is such an abrupt change in the electron pathway makes the nature of the chemical anchoring groups critically important regarding the propagation of electrons from the electrode across the molecule. A delicate interplay of phenomena can occur when a molecule binds to the electrodes, which can produce profound differences in conductance properties depending on the anchoring group. This makes answering the question "what is the best anchoring group for single molecule studies" far from straight forward. In this review, we firstly take a look at techniques developed to 'wire-up' single molecules, as understanding their limitations is key when assessing a molecular wire's performance. We then analyse the various chemical anchoring groups, and discuss their merits and disadvantages. Finally we discuss some theoretical concepts of molecular junctions to understand how transport is affected by the nature of the chemical anchor group.

Charge transport in C60-based dumbbell-type molecules: mechanically induced switching between two distinct conductance states.

Single molecule charge transport characteristics of buckminsterfullerene-capped symmetric fluorene-based dumbbell-type compound 1 were investigated by scanning tunneling microscopy break junction (STM-BJ), current sensing atomic force microscopy break junction (CS-AFM-BJ), and mechanically controlled break junction (MCBJ) techniques, under ambient conditions. We also show that compound 1 is able to form highly organized defect-free surface adlayers, allowing the molecules on the surface to be addressed specifically. Two distinct single molecule conductance states (called high G(H)(1) and low G(L)(1)) were observed, depending on the pressure exerted by the probe on the junction, thus allowing molecule 1 to function as a mechanically driven molecular switch. These two distinct conductance states were attributed to the electron tunneling through the buckminsterfullerene anchoring group and fully extended molecule 1, respectively. The assignment of conductance features to these configurations was further confirmed by control experiments with asymmetrically designed buckminsterfullerene derivative 2 as well as pristine buckminsterfullerene 3, both lacking the G(L) feature.

Does a cyclopropane ring enhance the electronic communication in dumbbell-type C60 dimers?

Two C60 dumbbell molecules have been synthesized containing either cyclopropane or pyrrolidine rings connecting two fullerenes to a central fluorene core. A combination of spectroscopic techniques reveals that the cyclopropane dumbbell possesses better electronic communication between the fullerenes and the fluorene. This observation is underpinned by DFT transport calculations, which show that the cyclopropane dumbbell gives a higher calculated single-molecule conductance, a result of an energetically lower-lying LUMO level that extends deeper into the backbone. This strengthens the idea that cyclopropane behaves as a quasi-double bond.

A detailed experimental and theoretical study into the properties of C60 dumbbell junctions.

A combined experimental and theoretical investigation is carried out into the electrical transport across a fullerene dumbbell one-molecule junction. The newly designed molecule comprises two C60 s connected to a fluorene backbone via cyclopropyl groups. It is wired between gold electrodes under ambient conditions by pressing the tip of a scanning tunnelling microscope (STM) onto one of the C60 groups. The STM allows us to identify a single molecule before the junction is formed through imaging, which means unambiguously that only one molecule is wired. Once lifted, the same molecule could be wired many times as it was strongly fixed to the tip, and a high conductance state close to 10(-2) G0 is found. The results also suggest that the relative conductance fluctuations are low as a result of the low mobility of the molecule. Theoretical analysis indicates that the molecule is connected directly to one electrode through the central fluorene, and that to bind it to the gold fully it has to be pushed through a layer of adsorbates naturally present in the experiment.

Probing charge transfer in benzodifuran-C60 dumbbell-type electron donor-acceptor conjugates: ground- and excited-state assays.

Rigid electron donor-acceptor conjugates (1-3) that combine π-extended benzodifurans as electron donors and C60 molecules as electron acceptors with different linkers have been synthesized and investigated with respect to intramolecular charge-transfer events. Electrochemistry, fluorescence, and transient absorption measurements revealed tunable and structure-dependent charge-transfer processes in the ground and excited states. Our experimental findings are underpinned by density-functional theory calculations.

Quinone methide generation via photoinduced electron transfer.

Photochemical activation of water-soluble 1,8-naphthalimide derivatives (NIs) as alkylating agents has been achieved by irradiation at 310 and 355 nm in aqueous acetonitrile. Reactivity in aqueous and neat acetonitrile has been extensively investigated by laser flash photolysis (LFP) at 355 nm, as well as by steady-state preparative irradiation at 310 nm in the presence of water, amines, thiols, and ethyl vinyl ether. Product distribution analysis revealed fairly efficient benzylation of the amines, hydration reaction, and 2-ethoxychromane generation, in the presence of ethyl vinyl ether, resulting from a [4 + 2] cycloaddition onto a transient quinone methide. Remarkably, we found that the reactivity was dramatically suppressed under the presence of oxygen and radical scavengers, such as thiols, which was usually associated with side product formation. In order to unravel the mechanism responsible for the photoreactivity of these NI-based molecules, a detailed LFP study has been carried out with the aim to characterize the transient species involved. LFP data suggest a photoinduced electron transfer (PET) involving the NI triplet excited state (λ(max) 470 nm) of the NI core and the tethered quinone methide precursor (QMP) generating a radical ions pair NI(•-) (λ(max) 410 nm) and QMP(•+). The latter underwent fast deprotonation to generate a detectable phenoxyl radical (λ(max) 390 and 700 nm), which was efficiently reduced by the radical anion NI(•-), generating detectable QM. The mechanism proposed has been validated through a LFP investigation at 355 nm exploiting an intermolecular reaction between the photo-oxidant N-pentylnaphthalimide (NI-P) and a quaternary ammonium salt of a Mannich base as QMP (2a), in both neat and aqueous acetonitrile. Remarkably, these experiments revealed the generation of the model o-QM (λ(max) 400 nm) as a long living transient mediated by the same reactivity pathway. Negligible QM generation has been observed under the very same conditions by irradiation of the QMP in the absence of the NI. Owing to the NIs redox and recognition properties, these results represent the first step toward new molecular devices capable of both biological target recognition and photoreleasing of QMs as alkylating species, under physiological conditions.

Resting energy expenditure in HIV/AIDS patients: Development and validation of a predictive equation.

BACKGROUND: Accurate measurements of resting energy expenditure (REE) are important for determining nutritional needs in HIV patients. Indirect calorimetry (IC) is a noninvasive method that reflects REE but can be costly and is frequently calculated with predictive equations. Research suggests that REE obtained by predictive equations in people living with HIV/AIDS (PLWH) is inaccurate. The aim of the study is to develop and validate a new predictive equation of REE based on a population of PLWH. METHODS: Cross-sectional study including 164 PLWH (82 to develop and 82 to validate the equation). Multiple linear regression was used to determine the relationship between variables and to develop the new predictive equation. Intraclass correlation coefficient (ICC) and Bland-Altman methods were used to evaluate agreement between the new predictive equation and indirect calorimetry. RESULTS: A new predictive equation with an accuracy of 67% when compared with IC was developed. This equation included as covariates: fat free mass, antiretroviral therapy status and age. CONCLUSION: A new equation to predict energy expenditure in PLWH was developed and validated. This formula can be used to estimate REE if IC is not available.

Zinc and selenium indicators and their relation to immunologic and metabolic parameters in male patients with human immunodeficiency virus.

OBJECTIVES: Micronutrient deficiencies are common among people living with HIV (PLWHIV). The clinical and immunologic consequences of micronutrient deficiencies have been poorly explored in the context of human immunodeficiency virus (HIV) infection. The aim of this study was to determine the prevalence of zinc and selenium deficiency (dietary intake and serum concentrations) and analyze their associations with absolute CD4+ T-cell counts, inflammation markers, and metabolic disorders in a cohort of antiretroviral-experienced HIV-infected individuals. METHODS: The zinc and selenium intakes of 124 HIV-infected men were estimated using 3-d food records. In a subcohort of 45 individuals, serum zinc and selenium concentrations and proinflammatory cytokines were determined. Body composition, bone mineral density (BMD), CD4+ T-cell counts, lipid profile, glucose, and blood pressure were determined and were associated with zinc and selenium dietary intake and serum concentrations. RESULTS: Of the PLWHIV studied, 58% had suboptimal intake of zinc and 8% demonstrated suboptimal intake of selenium. Serum deficiencies for zinc and selenium were 23.9% and 65.9%, respectively. Zinc and selenium intake were correlated with increased muscle mass. Selenium intake was associated with increased BMD of the lumbar region. An inverse correlation between serum selenium concentration and several proinflammatory cytokines (interleukin-1ß, interleukin-6, and tumor necrosis factor-α) was found. CONCLUSION: Suboptimal zinc and selenium intake and serum concentration deficiencies are highly prevalent in treated HIV-positive individuals and are associated with body composition, BMD, and inflammation. Clinical trials should be designed to explore the effect of zinc and selenium supplementation on metabolic, inflammatory, and immunologic parameters on the HIV-positive population.

Accuracy of Predictive Equations for Energy Expenditure in Mexicans Living With HIV/AIDS With and Without Antiretroviral Therapy.

INTRODUCTION: Determination of the resting energy expenditure (REE) is essential for planning nutrition therapy in patients with human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) to help to improve their nutrition status. We aim to evaluate the agreement and accuracy of prediction equations that estimate the REE in a Mexican population with a diagnosis of HIV/AIDS with and without antiretroviral therapy (ART). METHODS: A cross-sectional study in Mexican patients with HIV/AIDS with and without ART. Weight, height, and body composition measured with dual-energy x-ray absorptiometry were evaluated. The REE was determined with indirect calorimetry and estimated using the Mifflin-St Jeor (MSJ), Harris-Benedict (HB), Schofield 1 and 2, Cunningham, Melchior 91, Melchior 93, and Batterham equations. The Bland-Altman method assessed agreement between the real and estimated values, and the percent difference between these values was used to assess the prediction accuracy. RESULTS: Sixty-five adults without ART and 102 adults with ART were included. The mean REE (kcal/kg) was 24.8 ± 2.4 and 23.8 ± 3.6 in patients without and with ART, respectively. Good agreement and reliability were observed in the HB (intraclass correlation coefficient [ICC], 0.75; P < .05), Batterham (ICC, 0.79; P < .05), Schofield 1 (ICC, 0.74; P < .05), and Schofield 2 (ICC, 0.78; P < .05) results in individuals without ART. In individuals with ART, good agreement and reliability were observed with the HB equation (ICC, 0.76; P < .05). The MSJ equation showed good agreement with poor reliability (ICC, 0.05; P < .05). CONCLUSION: The equations with the best agreement and accuracy were Schofield 2, Batterham, and HB in individuals without ART and HB and MSJ in the population with ART.

Fingerprinting Electronic Molecular Complexes in Liquid.

Predicting the electronic framework of an organic molecule under practical conditions is essential if the molecules are to be wired in a realistic circuit. This demands a clear description of the molecular energy levels and dynamics as it adapts to the feedback from its evolving chemical environment and the surface topology. Here, we address this issue by monitoring in real-time the structural stability and intrinsic molecular resonance states of fullerene (C60)-based hybrid molecules in the presence of the solvent. Energetic levels of C60 hybrids are resolved by in situ scanning tunnelling spectroscopy with an energy resolution in the order of 0.1 eV at room-temperature. An ultra-thin organic spacer layer serves to limit contact metal-molecule energy overlap. The measured molecular conductance gap spread is statistically benchmarked against first principles electronic structure calculations and used to quantify the diversity in electronic species within a standard population of molecules. These findings provide important progress towards understanding conduction mechanisms at a single-molecular level and in serving as useful guidelines for rational design of robust nanoscale devices based on functional organic molecules.