| 1. |
- Antonio Ribeiro, Luiz, et al.
(författare)
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Polaron stability in molecular semiconductors: theoretical insight into the impact of the temperature, electric field and the system dimensionality
- 2015
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 17:14, s. 8973-8982
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Tidskriftsartikel (refereegranskat)abstract
- A semi-empirical Holstein-Peierls model is used to study the temperature effects on the polaron stability in organic semiconductors at a molecular scale. The approach takes into account both intra- and intermolecular electron-lattice interactions and is aimed at describing charge transport in the system. Particularly, we present a systematic numerical investigation to characterize the influence of both temperature and electric field on the stability as well as mobility of the polaron. It is found that the parameter space for which the polaron is dynamically stable is quite limited and the variations in some of these parameters strongly depend on the temperature. The electric field can play a role in further localizing the charge causing a compression of the lattice distortions associated with the polaron, increasing thereby its stability, up to a field strength of approximately 2.0 mV angstrom(-1). Considering field strengths higher than this critical value, the polaron is annihilated spreading charge through the lattice. Furthermore, we have studied the polaron mobility as a function of the anisotropy of the system, going from a one-dimensional system via a highly anisotropic two-dimensional system to a uniform two-dimensional system. There is a clearly observed mobility edge for the polaron; it exhibits a high mobility in the one-dimensional system but as the coupling in the second dimension is turned on the polaron slows down and becomes immobile in the uniform system. The results provided by this transport mechanism are in good agreement with experimental observations and may provide guidance to improve the charge transport in organic optoelectronic devices.
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| 2. |
- Arpa Gonzalez, Enrique Manuel, et al.
(författare)
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A Proof-of-Principle Design for Through-Space Transmission of Unidirectional Rotary Motion by Molecular Photogears
- 2023
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Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765.
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Tidskriftsartikel (refereegranskat)abstract
- The construction of molecular photogears that can achieve through-space transmission of the unidirectional double-bond rotary motion of light-driven molecular motors onto a remote single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a proof-of-principle design of such photogears that is based on the possibility of using stereogenic substituents to control both the relative stabilities of two helical forms of the photogear and the double-bond photoisomerization reaction that connects them. The potential of the design was verified by quantum-chemical modeling through which photogearing was found to be a favorable process compared to free-standing single-bond rotation ("slippage"). Overall, our study unveils a surprisingly simple approach to realizing unidirectional photogearing. A stereochemical approach to transmitting the directional double-bond rotary motion of light-driven molecular motors through space onto a remote single-bond axis is put forth and successfully tested by means of quantum-chemical modeling. A key result in the assessment of the approach is that the desired photogearing process is favorable compared to the undesired, free-standing single-bond rotation process ("slippage") with which it competes.**image
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| 3. |
- Arpa Gonzalez, Enrique Manuel, et al.
(författare)
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Photochemical formation of the elusive Dewar isomers of aromatic systems: why are substituted azaborines different?
- 2024
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084.
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Tidskriftsartikel (refereegranskat)abstract
- Photochemical reactions enabling efficient transformation of aromatic systems into energetic but stable non-aromatic isomers have a long history in organic chemistry. One recently discovered reaction in this realm is that where derivatives of 1,2-azaborine, a compound isoelectronic with benzene in which two adjacent C atoms are replaced by B and N atoms, form the non-hexagon Dewar isomer. Here, we report quantum-chemical calculations that explain both why 1,2-azaborine is intrinsically more reactive toward Dewar formation than benzene, and how suitable substitutions at the B and N atoms are able to increase the corresponding quantum yield. We find that Dewar formation from 1,2-azaborine is favored by a pronounced driving force that benzene lacks, and that a large improvement in quantum yield arises when the reaction of substituted 1,2-azaborines proceeds without involvement of an intermediary ground-state species. Overall, we report new insights into making photochemical use of the Dewar isomers of aromatic compounds. Quantum-chemical calculations combined with molecular-dynamics simulations reveal mechanisms for improving the quantum yields by which aromatic compounds form their non-aromatic Dewar isomers, with potential implications in solar-energy storage.
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| 4. |
- Barta, P., et al.
(författare)
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Temperature evolution of the electronic band structure of the undoped and doped regioregular analog of poly(3â€alkylthiophenes) : A spectroscopic and theoretical study
- 1994
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 100:2, s. 1731-1741
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Tidskriftsartikel (refereegranskat)abstract
- The undoped and ferric chloride p‐doped ‘‘head‐to‐head’’ ‘‘tail‐to‐tail’’ analog of poly(3‐decylthiophene) obtained from substituted bithiophenes, i.e., poly(4,4’didecyl‐2,2’bithiophene)‐PDDBT has been investigated. The samples were studied by means of ultraviolet photoelectron spectroscopy,optical absorption measurements, and polymerbands structure calculations. Experiments were carried out at different temperatures between 100 °C and −180 °C. The results indicate that PDDBT is nonplanar in the undoped state. No temperature effects have been observed in the π‐electron properties. In the doped state, however, lowering of the temperature results in an increase of the system planarity that modifies the electronic band structure. This effect has been shown to be fully reversible with temperature.
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| 5. |
- Baughman, R.H., et al.
(författare)
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Negative Poisson's ratios for extreme states of matter
- 2000
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 288:5473, s. 2018-2022
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Tidskriftsartikel (refereegranskat)abstract
- Negative Poisson's ratios are predicted for body-centered-cubic phases that likely exist in white dwarf cores and neutron star outer crusts, as well as those found for vacuumlike ion crystals, plasma dust crystals, and colloidal crystals (including certain virus crystals). The existence of this counterintuitive property, which means that a material laterally expands when stretched, is experimentally demonstrated for very low density crystals of trapped ions. At very high densities, the large predicted negative and positive Poisson's ratios might be important for understanding the asteroseismology of neutron stars and white dwarfs and the effect of stellar stresses on nuclear reaction rates. Giant Poisson's ratios are both predicted and observed for highly strained coulombic photonic crystals, suggesting possible applications of large, tunable Poisson's ratios for photonic crystal devices.
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| 6. |
- Berggren, Magnus, et al.
(författare)
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Controlling inter-chain and intra-chain excitations of a poly(thiophene) derivative in thin films
- 1999
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Ingår i: Chemical Physics Letters. - : Elsevier. - 0009-2614 .- 1873-4448. ; 304:1-2, s. 84-90
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Tidskriftsartikel (refereegranskat)abstract
- The decay of photoexcitations in polythiophene chains has been studied in solid solutions of the polymer from room temperature to 4 K. A strong blue shift of the emission spectrum is observed in the polymer blend, as compared to the homopolymer. Dispersion of the polythiophene suppresses the non-radiative processes, which are suggested to be correlated to close contacts of polymer chains. Quantum chemistry modeling of the excited state distributed on two chains corroborate this conclusion.
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| 7. |
- Björk, Jonas, et al.
(författare)
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Adsorption of large hydrocarbons on coinage metals : a van der Waals density functional study
- 2014
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Ingår i: ChemPhysChem. - : Wiley-VCH Verlagsgesellschaft. - 1439-4235 .- 1439-7641. ; 15:13, s. 2851-2858
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of organic molecules onto the close-packed facets of coinage metals is studied, and how accurately adsorption heights can be described by using recent advances of the van der Waals density functional (vdWDF), with optPBE/vdWDF, optB86b/vdWDF, vdWDF2, and rev/vdWDF2 functionals is illustrated. The adsorption of two prototypical aromatic hydrocarbons is investigated, and the calculated adsorption heights are compared to experimental literature values from normal incident X-ray standing wave absorption and a state-of-the-art semi-empirical method. It is shown that both the optB86b/vdWDF and rev/vdWDF2 functionals describe adsorption heights with an accuracy of 0.1 Å, compared to experimental values, and are concluded as reliable methods of choice for related systems.
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| 8. |
- Björk, Jonas, et al.
(författare)
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Mechanisms of halogen-based covalent self-assembly on metal surfaces
- 2013
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:15, s. 5768-5775
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Tidskriftsartikel (refereegranskat)abstract
- We computationally study the reaction mechanisms of halogen-based covalent self-assembly, a major route for synthesizing molecular nanostructures and nanographenes on surfaces. Focusing on biphenyl as a small model system, we describe the dehalogenation, recombination, and diffusion processes. The kinetics of the different processes are also investigated, in particular how diffusion and coupling barriers affect recombination rates. Trends across the periodic table are derived from three commonly used close-packed (111) surfaces (Cu, Ag, and Au) and two halogens (Br and I). We show that the halogen atoms can poison the surface, thus hindering long-range ordering of the self-assembled structures. Finally, we present core-level shifts of the relevant carbon and halogen atoms, to provide reference data for reliably detecting self-assembly without the need for atomic-resolution scanning tunneling microscopy.
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| 9. |
- Björk, Jonas, et al.
(författare)
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Unraveling the Mechanism of the Covalent Coupling Between Terminal Alkynes on a Noble Metal
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:6, s. 3181-3187
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of the newly reported route for surface-assisted covalent coupling of terminal alkynes on Ag(111) is unraveled by density functional theory based transition state calculations. We illustrate that the reaction path is fundamentally different from the classical coupling schemes in wet chemistry. It is initiated by the covalent coupling between two molecules instead of single-molecule dehydrogenation. The silver substrate is found to play an important role stabilizing the intermediate species by chemical bonds, although it is hardly active electronically in the actual coupling step. The dimer intermediate is concluded to undergo two subsequent dehydrogenation processes expected to be rate-limiting according to the comparatively large barriers, which origin is discussed.
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| 10. |
- Björk, Jonas, et al.
(författare)
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Zipping Up: Cooperativity Drives the Synthesis of Graphene Nanoribbons
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 133:38, s. 14884-14887
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the cooperative effects controlling the synthesis of a graphene nanoribbon on the Au(111) surface starting from an anthracene polymer using density functional calculations including van der Waals interactions. We focus on the high-temperature cyclodehydrogenation step of the reaction and find that the reaction proceeds by simultaneously transferring two H-atoms from the anthracene units to the Au surface, leaving behind a C C bond in the process. This step is significantly more favorable than the three other potential reaction paths. Moreover, we find that successive dehydrogenations proceed from one end of the polyanthracene and propagate step-by-step through the polymer in a domino-like fashion.
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