635 research outputs found
The coupled electronic oscillators vs the sum-over-states pictures for the optical response of octatetraene
A coupled electronic oscillator (CEO) analysis of the third harmonic generation (THG) spectrum for octatetraene is presented. The dominant oscillators and their couplings are identified using tree diagrams. The correspondence between the dominant oscillators in the CEO picture and the relevant excited states in the sum-over-states (SOS) description is demonstrated. The important channels in the SOS are related to the dominant oscillator pathways in the CEO picture. © 1996 American Institute of Physics.published_or_final_versio
Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals
PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a highly soluble C60 derivative that is extensively used in organic solar cells, enabling power conversion efficiencies above 10%. Here we report, for the first time to the best of our knowledge, the photoluminescence of high-quality solvent-free PCBM crystals between room temperature and 4 K. Interestingly, the PL spectra of these crystals become increasingly structured as the temperature is lowered, with extremely well-resolved emission lines (and a minimum line width of âŒ1.3 meV at 1.73 eV). We are able to account for such a structured emission by means of a vibronic coupling model including FranckâCondon, JahnâTeller and HerzbergâTeller effects. Although optical transitions are not formally forbidden from the low-lying excited states of PCBM, the high symmetry of the electronically active fullerene core limits the intensity of the 0â0 transition, such that HerzbergâTeller transitions which borrow intensity from higher-lying states represent a large part of the observed spectrum. Our simulations suggest that the emissive state of PCBM can be considered as a mixture of the T1g and Hg excited states of C60 and hence that the Hg state plays a larger role in the relaxed excited state of PCBM than in that of C60
Hierarchical Self-Assembly of Supramolecular Helical Fibres from Amphiphilic C3-Symmetrical Functional Tris(tetrathiafulvalenes)
The preparation and self-assembly of the enantiomers of a series of C3-symmetric compounds incorporating three tetrathiafulvalene (TTF) residues is reported. The chiral citronellyl and dihydrocitronellyl alkyl chains lead to helical one dimensional stacks in solution. Molecular mechanics and dynamics simulations combined with experimental and theoretical circular dichroism support the observed helicity in solution. These stacks self-assemble to give fibres that have morphologies that depend on the nature of the chiral alkyl group and the medium in which the compounds aggregate. An inversion of macroscopic helical morphology of the citronellyl compound is observed when compared to analogous 2-methylbutyl chains, which is presumably a result of the stereogenic centre being further away from the core of the molecule. This composition still allows both morphologies to be observed, whereas an achiral compound shows no helicity. The morphology of the fibres also depends on the flexibility at the chain ends of the amphiphilic components, as there is not such an apparently persistent helical morphology for the dihydrocitronellyl derivative as for that prepared from citronellyl chains
Long-lived quantum coherence in photosynthetic complexes at physiological temperature
Photosynthetic antenna complexes capture and concentrate solar radiation by
transferring the excitation to the reaction center which stores energy from the
photon in chemical bonds. This process occurs with near-perfect quantum
efficiency. Recent experiments at cryogenic temperatures have revealed that
coherent energy transfer - a wavelike transfer mechanism - occurs in many
photosynthetic pigment-protein complexes (1-4). Using the Fenna-Matthews-Olson
antenna complex (FMO) as a model system, theoretical studies incorporating both
incoherent and coherent transfer as well as thermal dephasing predict that
environmentally assisted quantum transfer efficiency peaks near physiological
temperature; these studies further show that this process is equivalent to a
quantum random walk algorithm (5-8). This theory requires long-lived quantum
coherence at room temperature, which never has been observed in FMO. Here we
present the first evidence that quantum coherence survives in FMO at
physiological temperature for at least 300 fs, long enough to perform a
rudimentary quantum computational operation. This data proves that the
wave-like energy transfer process discovered at 77 K is directly relevant to
biological function. Microscopically, we attribute this long coherence lifetime
to correlated motions within the protein matrix encapsulating the chromophores,
and we find that the degree of protection afforded by the protein appears
constant between 77 K and 277 K. The protein shapes the energy landscape and
mediates an efficient energy transfer despite thermal fluctuations. The
persistence of quantum coherence in a dynamic, disordered system under these
conditions suggests a new biomimetic strategy for designing dedicated quantum
computational devices that can operate at high temperature.Comment: PDF files, 15 pages, 3 figures (included in the PDF file
Evidence for Strong and Weak Phenyl-C61-Butyric Acid Methyl Ester Photodimer Populations in Organic Solar Cells
In polymer/fullerene organic solar cells, the photochemical dimerization of phenyl-C61-butyric acid methyl ester (PCBM) was reported to have either a beneficial or a detrimental effect on device performance and stability. In this work, we investigate the behavior of such dimers by measuring the temperature dependence of the kinetics of PCBM de-dimerization as a function of prior light intensity and duration. Our data reveal the presence of both âweaklyâ and âstronglyâ bound dimers, with higher light intensities preferentially generating the latter. DFT simulations corroborate our experimental findings and suggest a distribution of dimer binding energies, correlated with the orientation of the fullerene tail with respect to the dimer bonds on the cage. These results provide a framework to rationalize the double-edged effects of PCBM dimerization on the stability of organic solar cells
Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons
Graphene nanoribbons (GNRs) with atomically precise width and edge structures
are a promising class of nanomaterials for optoelectronics, thanks to their
semiconducting nature and high mobility of charge carriers. Understanding the
fundamental static optical properties and ultrafast dynamics of charge carrier
generation in GNRs is essential for optoelectronic applications. Combining THz
spectroscopy and theoretical calculations, we report a strong exciton effect
with binding energy up to 700 meV in liquid-phase-dispersed GNRs with a width
of 1.7 nm and an optical bandgap of 1.6 eV, illustrating the intrinsically
strong Coulomb interactions between photogenerated electrons and holes. By
tracking the exciton dynamics, we reveal an ultrafast formation of excitons in
GNRs with a long lifetime over 100 ps. Our results not only reveal fundamental
aspects of excitons in GNRs (gigantic binding energy and ultrafast exciton
formation etc.), but also highlight promising properties of GNRs for
optoelectronic devices.Comment: 26 pages, 4 figures, 5 pages Supplementary Informatio
Stable 6H organic-inorganic hybrid lead perovskite and competitive formation of 6H and 3C perovskite structure with mixed A cations
We thank the Chinese Scholarship Council for Ph.D. Studentship support (to JT). Computational resources have been provided by the Consortium des Ăquipements de Calcul Intensif (CĂCI), funded by the Fonds de la Recherche Scientifique de Belgique (F.R.S.-FNRS) under grant no. 2.5020.11. D.B. is an FNRS Research Director.We report the synthesis and properties of a new organicâinorganic hybrid lead perovskite (OIHP), azetidinium lead bromide (AzPbBr3), possessing the 6H perovskite structure (space group P63/mmc with a = 8.745 Ă
and c = 21.329 Ă
). This compound has a band gap of 2.81 eV and remains stable for >6 months in the ambient environment. DFT simulations are in fairly good agreement with experiments and indicate that AzPbBr3 is a direct band gap semiconductor. A partial solid solution with the cubic (3C) perovskite methylammonium lead bromide (Az1âxMAxPbBr3) is possible. In Az-rich 6H compositions the lattice volume and band gap are invariant with x (â€0.3), whereas in the MA-rich 3C phase (0.8 †x †1.0) the lattice parameters and band gap increase with increasing Az content. Although the relatively large band gap of AzPbBr3 makes it unsuitable for photovoltaic applications, the results indicate Az+ is a suitable alternative organic A cation for band gap tuning of OHIPs.PostprintPeer reviewe
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