1,233 research outputs found
Photoexcited triplet states of twisted acenes investigated by Electron Paramagnetic Resonance
Twisting of the acene backbone out of planarity in twisted acenes leads to a variation in their optical and electronic properties. The effect of increasing twist angles on the properties of the photoexcited triplet states of a series of anthracene-based helically tethered twisted acenes is investigated here by Electron Paramagnetic Resonance (EPR) spectroscopy. Increasing signal intensities with increasing twist angles indicate increased intersystem crossing efficiencies for the twisted molecules compared to the untethered reference compound. Variations in the electron spin polarisation observed in the transient EPR spectra, in particular for the compound with the shortest tether, imply changes in the sublevel population kinetics depending on molecular geometry. Changes in the zero-field splitting parameters and in the proton hyperfine couplings for compounds with short tethers and therefore higher twist angles point towards a slight redistribution of the spin density compared to the parent compound. The experimental results can be explained by considering both an increase in twist angle and a related decrease in the dihedral angle between the phenyl side groups and the acene core. The observation of a clear excitation-wavelength dependence suggests preferential excitation of different molecular conformations, with conformers characterised by higher twist angles selected at higher wavelengths
Elucidating the structural composition of a Fe-N-C catalyst by nuclear and electron resonance techniques
FeâNâC catalysts are very promising materials for fuel cells and metalâair batteries. This work gives fundamental insights into the structural composition of an FeâNâC catalyst and highlights the importance of an inâdepth characterization. By nuclearâ and electronâresonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of αâiron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in FeâNâC catalysts prepared by pyrolysis at 900â°C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with endâon bonded oxygen as one of the axial ligands
Lock-in detection for pulsed electrically detected magnetic resonance
We show that in pulsed electrically detected magnetic resonance (pEDMR)
signal modulation in combination with a lock-in detection scheme can reduce the
low-frequency noise level by one order of magnitude and in addition removes the
microwave-induced non-resonant background. This is exemplarily demonstrated for
spin-echo measurements in phosphorus-doped Silicon. The modulation of the
signal is achieved by cycling the phase of the projection pulse used in pEDMR
for the read-out of the spin state.Comment: 4 pages, 2 figure
Probing the wave function and dynamics of the quintet multiexciton state with coherent control in a singlet fission material
High-spin states play a key role in chemical reactions found in nature. In artificial molecular systems, singlet fission produces a correlated triplet-pair state, a spin-bearing excited state that can be harnessed for more efficient solar-energy conversion and photocatalysis. In particular, triplet-pair states with overall quintet character (total spin S=2) have been discovered, but many of the fundamental properties of these biexciton states remain unexplored. The net spin of these pair states makes spin-sensitive probes attractive for their characterization. Combined with their surprisingly long spin coherence (of order microseconds), this opens up techniques relying on coherent spin control. Here we apply coherent manipulation of triplet-pair states to (i) isolate their spectral signatures from coexisting free triplets and (ii) selectively couple quintet and triplet states to specific nuclear spins. Using this approach, we separate quintet and triplet transitions and extract the relaxation dynamics and hyperfine couplings of the fission-borne spin states. Our results highlight the distinct properties of correlated and free triplet excitons and demonstrate optically induced nuclear spin polarization by singlet fission
An organic borate salt with superior pâdoping capability for organic semiconductors
Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (optoâ) electronics. Beyond singleâcomponent molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes2B+; Mes: mesitylene) and the tetrakis(pentaâfluorophenyl)borate anion [B(C6F5)4]â is demonstrated as pâtype dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes2B+, and the positive charge on the polymer is stabilized by [B(C6F5)4]â. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C6F5)4]â even enables the stabilization of polarons and bipolarons in poly(3âhexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes2B+[B(C6F5)4]â is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors
Two-point function of strangeness-carrying vector-currents in two-loop Chiral Perturbation Theory
We calculate the correlator between two external vector-currents having the
quantum-numbers of a charged kaon. We give the renormalized expression to two
loops in standard chiral perturbation theory in the isospin limit, which, as a
physical result, is finite and scale-independent. Applications include a low
energy theorem, valid at two loop order, of a flavor breaking combination of
vector current correlators as well as a determination of the phenomenologically
relevant finite -counterterm combination by means of inverse
moment finite energy sum rules. This determination is less sensitive to
isospin-breaking effects than previous attempts.Comment: 24 pages, revtex, 4 figures, 2 tables, revised version, one ref.
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QED Corrections to Neutrino Electron Scattering
We evaluate the O(alpha) QED corrections to the recoil electron energy
spectrum in the process nu_l + e --> nu_l + e (+gamma), where (+gamma)
indicates the possible emission of a photon and l=e, mu or tau. The soft and
hard bremsstrahlung differential cross sections are computed for an arbitrary
value of the photon energy threshold. We also study the O(alpha) QED
corrections to the differential cross section with respect to the total
combined energy of the recoil electron and a possible accompanying photon.
Their difference from the corrections to the electron spectrum is investigated.
We discuss the relevance and applicability of both radiative corrections,
emphasizing their role in the analysis of precise solar neutrino electron
scattering experiments.Comment: 14 pages + 10 figures. Minimal changes, published versio
Deoxyribonucleic Acid Encoded and Size-Defined Ï-Stacking of Perylene Diimides
Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of Ï-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a âtoolboxâ for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobicâhydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular Ï wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spinâorbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision
Photogeneration of Spin Quintet TripletâTriplet Excitations in DNA-Assembled Pentacene Stacks
Singlet fission (SF), an exciton-doubling process observed in certain molecular semiconductors where two triplet excitons are generated from one singlet exciton, requires correctly tuned intermolecular coupling to allow separation of the two triplets to different molecular units. We explore this using DNA-encoded assembly of SF-capable pentacenes into discrete Ï-stacked constructs of defined size and geometry. Precise structural control is achieved via a combination of the DNA duplex formation between complementary single-stranded DNA and the local molecular geometry that directs the SF chromophores into a stable and predictable slip-stacked configuration, as confirmed by molecular dynamics (MD) modeling. Transient electron spin resonance spectroscopy revealed that within these DNA-assembled pentacene stacks, SF evolves via a bound triplet pair quintet state, which subsequently converts into free triplets. SF evolution via a long-lived quintet state sets specific requirements on intermolecular coupling, rendering the quintet spectrum and its zero-field-splitting parameters highly sensitive to intermolecular geometry. We have found that the experimental spectra and zero-field-splitting parameters are consistent with a slight systematic strain relative to the MD-optimized geometry. Thus, the transient electron spin resonance analysis is a powerful tool to test and refine the MD-derived structure models. DNA-encoded assembly of coupled semiconductor molecules allows controlled construction of electronically functional structures, but brings with it significant dynamic and polar disorders. Our findings here of efficient SF through quintet states demonstrate that these conditions still allow efficient and controlled semiconductor operation and point toward future opportunities for constructing functional optoelectronic systems
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