592 research outputs found
A multideterminant assessment of mean field methods for the description of electron transfer in the weak coupling regime
Multideterminant calculations have been performed on model systems to
emphasize the role of many-body effects in the general description of charge
quantization experiments. We show numerically and derive analytically that a
closed-shell ansatz, the usual ingredient of mean-field methods, does not
properly describe the step-like electron transfer characteristic in weakly
coupled systems. With the multideterminant results as a benchmark, we have
evaluated the performance of common ab initio mean field techniques, such as
Hartree Fock (HF) and Density Functional Theory (DFT) with local and hybrid
exchange correlation functionals, with a special focus on spin-polarization
effects. For HF and hybrid DFT, a qualitatively correct open-shell solution
with distinct steps in the electron transfer behaviour can be obtained with a
spin-unrestricted (i.e., spin-polarized) ansatz though this solution differs
quantitatively from the multideterminant reference. We also discuss the
relationship between the electronic eigenvalue gap and the onset of charge
transfer for both HF and DFT and relate our findings to recently proposed
practical schemes for calculating the addition energies in the Coulomb blockade
regime for single molecule junctions from closed-shell DFT within the local
density approximation
Tuning of the Photovoltaic Parameters of Molecular Donors by Covalent Bridging
The synthesis of donor-acceptor molecules involving triarylamines and dicyanovinyl blocks is described. Optical and electrochemical results show that rigidification of the acceptor part of the molecule by a covalent bridge leads to a ca. 0.20 eV increase of the band gap due to a parallel increase of the lowest unoccupied molecular orbital level. A preliminary evaluation of these compounds as donor materials in organic solar cells shows that although this structural modification reduces the light-harvesting properties of the donor molecule, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the open-circuit voltage and fill factor
Light-induced reversible modification of the work function of a new perfluorinated biphenyl azobenzene chemisorbed on Au (111)
This work was financially supported by EC through the Marie-Curie ITN SUPERIOR (PITN-GA-2009-238177) and IEF MULTITUDES (PIEF-GA-2012-326666), the ERC project SUPRAFUNCTION (GA-257305), the Agence Nationale de la Recherche through the LabEx project Chemistry of Complex Systems (ANR-10-LABX-0026_CSC), and the International Center for Frontier Research in Chemistry (icFRC). The work in Mons is further supported by the Interuniversity Attraction Poles Programme (P7/05) initiated by the Belgian Science Policy Office, and by the Belgian National Fund for Scientific Research (FNRS). J.C. is an FNRS research director. The synthesis team in Switzerland acknowledges financial support by the Swiss National Science Foundation (SNF) and the Swiss Nanoscience Institute (SNI)
Polarizability and Internal Charge Transfer in Thiophene-Triphenylamine Hybrid pi-Conjugated Systems
Extended star-shaped conjugated systems consisting of dicyanovinyl electron-acceptor units connected to a triphe-nylamine core by means of thiophene (T), thienylenevinylene (TV), and bithiophene (BT) conjugating spacers have been synthesized. The analysis of the electronic properties of the molecules by UV-vis absorption spectroscopy, cyclic voltammetry, and theoretical calculations shows that the electronic properties of the systems depend on the length and rigidity of the conjugating spacer
A New Photomechanical Molecular Switch Based on a Linear {\pi}-Conjugated System
We report the electronic transport properties of a new photo-addressable
molecular switch. The switching process relies on a new concept based on linear
{\pi}-conjugated dynamic systems, in which the geometry and hence the
electronic properties of an oligothiophene chain can be reversibly modified by
the photochemical trans-cis isomerization of an azobenzene unit fixed in a
lateral loop. Electron transport measurements through self-assembled monolayers
on gold, contacted with eGaIn top contact, show switching with a conductance
ratio up to 1E3. Ab initio calculations have been used to identify the most
energetically stable conformations of the molecular switch, the corresponding
calculated conductances qualitatively explain the trend observed in the
photo-switching experiments.Comment: Full manuscript and supporting information, J. Phys. Chem.
C,published on line (2017
A New Photomechanical Molecular Switch Based on a Linear {\pi}-Conjugated System
We report the electronic transport properties of a new photo-addressable
molecular switch. The switching process relies on a new concept based on linear
{\pi}-conjugated dynamic systems, in which the geometry and hence the
electronic properties of an oligothiophene chain can be reversibly modified by
the photochemical trans-cis isomerization of an azobenzene unit fixed in a
lateral loop. Electron transport measurements through self-assembled monolayers
on gold, contacted with eGaIn top contact, show switching with a conductance
ratio up to 1E3. Ab initio calculations have been used to identify the most
energetically stable conformations of the molecular switch, the corresponding
calculated conductances qualitatively explain the trend observed in the
photo-switching experiments.Comment: Full manuscript and supporting information, J. Phys. Chem.
C,published on line (2017
Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers
We have measured the ratio, r = of the formation cross
section, of singlet () and triplet () excitons
from oppositely charged polarons in a large variety of -conjugated
oligomer and polymer films, using the photoinduced absorption and optically
detected magnetic resonance spectroscopies. The ratio r is directly related to
the singlet exciton yield, which in turn determines the maximum
electroluminescence quantum efficiency in organic light emitting diodes (OLED).
We discovered that r increases with the conjugation length, CL; in fact a
universal dependence exists in which depends linearly on ,
irrespective of the chain backbone structure. These results indicate that
-conjugated polymers have a clear advantage over small molecules in OLED
applications.Comment: 5 pages, 4 figure
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