Positive Charge Carriers on Isolated Chains of MEH-PPV with Broken Conjugation:Optical Absorption and Mobility

Pulse radiolysis of oxygen-saturated dilute solutions of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]s in benzene showed charge-transfer from the solvent radical cation to the polymer, yielding positive charge carriers (radical-cations or "holes") on the isolated chains. The charge-transfer reactions are diffusion controlled with (time-dependent) rate coefficients that are determined by the conformation adopted by the polymer chains. The radical cation of the fully conjugated polymer exhibited an absorption maximum at 1.32 eV. The absorption maximum shifted to higher energies with decreasing conjugated fraction, in a way consistent with thermal equilibration of the positive charge over the conjugated segments of different length. The mobility of the positive charge along the fully conjugated MEH-PPV chains was determined to be 0.42 cm(2) V-1 s(-1). The introduction of conjugated breaks decreased the charge mobility strongly, which is attributed to the disruption of the T conjugation and/or the conformational disorder caused by the flexibility of the chains at the sites of conjugation breaks

Energy landscape of self-assembled superlattices of PbSe nanocrystals

Self-assembly of nanocrystals (NCs) into superlattices is an intriguing multiscale phenomenon that may lead to materials with novel collective properties, in addition to the unique properties of individual NCs compared with their bulk counterparts. By using different dispersion solvents, we synthesized three types of PbSe NC superlattices--body-centered cubic (bcc), body-centered tetragonal (bct), and face-centered cubic (fcc)--as confirmed by synchrotron small-angle X-ray scattering. Solution calorimetric measurements in hexane show that the enthalpy of formation of the superlattice from dispersed NCs is on the order of -2 kJ/mol. The calorimetric measurements reveal that the bcc superlattice is the energetically most stable polymorph, with the bct being 0.32 and the fcc 0.55 kJ/mol higher in enthalpy. This stability sequence is consistent with the decreased packing efficiency of PbSe NCs from bcc (17.2%) to bct (16.0%) and to fcc (15.2%). The small enthalpy differences among the three polymorphs confirm a closely spaced energy landscape and explain the ease of formation of different NC superlattices at slightly different synthesis conditions

Broadband and picosecond intraband absorption in lead-based colloidal quantum dots

Using femtosecond transient absorption spectroscopy, we demonstrate that lead chalcogenide nanocrystals show considerable photoinduced absorption (PA) in a broad wavelength range just below the band gap. The time-dependent decay of the PA signal correlates with the recovery of the band gap absorption, indicating that the same carriers are involved. On this basis, we assign this PA signal to intraband absorption, that is, the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. We confirm our experiments with tight-binding calculations. This broadband response in the commercially interesting near- to mid-infrared range is very relevant for ultra-high-speed all-optical signal processing. We benchmark the performance with bulk Si and Si nanocrystals

High Electronic Conductance through Double-Helix DNA Molecules with Fullerene Anchoring Groups

Determining the mechanism of charge transport through native DNA remains a challenge as different factors such as measuring conditions, molecule conformations, and choice of technique can significantly affect the final results. In this contribution, we have used a new approach to measure current flowing through isolated double-stranded DNA molecules, using fullerene groups to anchor the DNA to a gold substrate. Measurements were performed at room temperature in an inert environment using a conductive AFM technique. It is shown that the π-stacked B-DNA structure is conserved on depositing the DNA. As a result, currents in the nanoampere range were obtained for voltages ranging between ±1 V. These experimental results are supported by a theoretical model that suggests that a multistep hopping mechanism between delocalized domains is responsible for the long-range current flow through this specific type of DNA.status: publishe

Charge transport properties in discotic liquid crystals: a quantum-chemical insight into structure-property relationships.

We describe at the quantum-chemical level the main parameters that control charge transport at the molecular scale in discotic liquid crystals. The focus is on stacks made of triphenylene, hexaazatriphenylene, hexaazatrinaphthylene, and hexabenzocoronene molecules and derivatives thereof. It is found that a subtle interplay between the chemical structure of the molecules and their relative positions within the stacks determines the charge transport properties; the molecular features required to promote high charge mobilities in discotic materials are established on the basis of the calculated structure-property relationships. We predict a significant increase in the charge mobility when going from triphenylene to hexaazatrinaphthylene; this finding has been confirmed by measurements carried out with the pulse-radiolysis time-resolved microwave conductivity technique.Journal Articleinfo:eu-repo/semantics/publishe