Andrea La Rosa

Polymer solar cells based on diphenylmethanofullerenes with reduced sidechain length

Diphenylmethanofullerenes (DPMs) show interesting properties as acceptors in polymer bulk heterojunction solar cells due to the high open circuit voltages they generate compared to their energy levels. Here we investigate the effect of reducing the alkane sidechain length of the DPMs from C12 to C6 in the properties of the solar cell. This change leads to an increase in the electron mobility, thus allowing for a lower fullerene content, which in turn results in an increase in the short circuit current and, finally, in an increase in the efficiency of the device (from 2.3 to 2.6%) due to the higher concentration of the more absorbing polymer in the film. Atomic force microscopy images and external quantum efficiencies suggest the absence of crystallization of the fullerene to be at the origin of the slightly reduced performance of DPMs versus the standard fullerene [6,6]-phenyl-C61-butyric acid methyl ester, implying that higher efficiencies could be possible with this class of fullerenes.

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.

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.

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.