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In this chapter, we discuss the essence of hyperfine-field induced magnetic field effects in organic semiconductors originating from spindependent reactions between electron and hole polarons. The basic picture we employ is that of two spin-1/2 polarons, each localized on a site in the semiconductor, where one of the polarons incoherently hops to the other site, forming a spin-0 bipolaron or singlet exciton, or a spin-1 triplet exciton. We describe this process within the framework of the stochastic Liouville equation. We introduce the useful simplifications where the hyperfine coupling of the electronic spin with the surrounding nuclear spins is replaced by a classical random hyperfine field (semiclassical approximation) and where the hopping rate is assumed to be much smaller than the hyperfine precession rate (slow-hopping approximation). Within the resulting theoretical framework we discuss the magnetic field effects measured in the conduction and electroluminescence of unipolar and bipolar devices of organic semiconductors at low magnetic field, focusing in particular on the line shapes of the effects. We conclude with a theoretical discussion of a huge organic magnetoresistance that could be realized in donor-acceptor copolymers.

Room-temperature magnetoresistance of the order of $10%$ has been observed in organic semiconductors. We predict that even larger magnetoresistance can be realized in suitably synthesized doped conjugated polymers. In such polymers, ionization of dopants creates free charges that recombine with a rate governed by a competition between an applied magnetic field and random hyperfine fields. This leads to a spin-blocking effect that depends on the magnetic field. We show that the combined effects of spin blocking and charge blocking, the fact that two free charges cannot occupy the same site, lead to a magnetoresistance of almost two orders of magnitude. This magnetoresistance occurs even at vanishing electric field and is therefore a quasiequilibrium effect. The influences of the dopant strength, energetic disorder, and interchain hopping are investigated. We find that the dopant strength and energetic disorder have only little influence on the magnetoresistance. Interchain hopping strongly decreases the magnetoresistance because it can lift spin-blocking and charge-blocking configurations that occur in strictly one-dimensional transport. We provide suggestions for realization of polymers that should show this magnetoresistance.

It is still an open question whether the statistical ratio of 1:3 for the formation of singlet vs. triplet excitons in organic semiconductors can be violated. In this work, we use a two-site and a multi-site model to investigate this problem. We show that spin mixing by hyperfine fields leads to a violation of the statistical ratio if singlet and triplet excitons are formed with a different rate. This only occurs if the exciton formation rate is slow enough compared to the hyperfine precession frequency. The violation of the statistical ratio can be suppressed by applying an external magnetic field. In the multi-site model we demonstrate how disorder and Coulomb interactions affect the fraction of singlet excitons formed. © 2010 Elsevier B.V. All rights reserved.

We present a first-principles scenario where a realistic three-magnetic-center metallic cluster acts as a prototypic magnetic-logic element within the frame of a unified optically induced spin manipulation. We find that the spins of the energetically low-lying triplet states of a Ni3 Na2 cluster are always localized at a single magnetic center and that controlled spin flips and transfers are possible within a hundred femtoseconds with suitable static external magnetic field and laser pulses. The magnetic state or the position of the spins and the static magnetic field can be used as input bits while the output bit is the final state of the magnetic centers, thus the gates AND, OR, XOR (CNOT), and NAND can be built.

Our report “ultrahigh magnetoresistance at room temperature in molecular wires” (1) presents measurements on onedimensional molecular chains confined inside the nanochannels of zeolite L crystals. In these measurements, we observed signals that were interpreted as an exceptionally large (~1000%) response of the conductance through the molecular chains to an external magnetic field of a few millitesla. The explanation of the results was based on a room-temperature Pauli spin blockade effect, intrinsic to the hopping transport through the molecules. The observed magnetic field scale of a few millitesla could be explained by the typical magnitude of the random nuclear magnetic field in the molecular environment. The shape of the conductance versus magnetic field dependence was found to be in close agreement with similar curves observed in bulk organic semiconductors, in which the effect is referred to as “organic magnetoresistance” or “OMAR.” The exceptionally large effect in our case was ascribed to the one-dimensional nature of electron transport along the molecular chains.In follow-up research by some of the coauthors, suspicion arose with regard to data collected by the first author Rabindra N. Mahato, which led to a thorough investigation by the co-authors. This investigation has revealed inappropriate data handling by Dr. Mahato, such that the experimental results are not accurately represented in the paper. This makes it, in our eyes, impossible to solidly underpin the conclusions made in the report. All co-authors have therefore concluded that the paper should be immediately retracted. Dr. Mahato has agreed to this Retraction.

Fringe fields emanating from magnetic domain structures can give rise to magnetoresistance in organic semiconductors. In this article, we explain these magnetic-field effects in terms of a $\ensuremath{\Delta}B$ mechanism. This mechanism describes how variations in magnetic-field strength between two polaron hopping sites can induce a difference in precessional motion of the polaron spins, leading to mixing of their spin states. In order to experimentally explore the fringe-field effects, polymer thin-film devices on top of a rough in-plane magnetized cobalt layer are investigated. The cobalt layer can be described by a distribution of out-of-plane magnetic anisotropies, most likely induced by thickness variations in the cobalt. With a magnetic field perpendicular to the cobalt layer, fringe fields are created because some domains are magnetized out of plane whereas the magnetization of other domains remains approximately in plane. By varying the distance between the polymer layer and the cobalt layer, we find that the magnetoresistance arising from these fringe fields reduces with the gradient in the fringe fields, in agreement with the $\ensuremath{\Delta}B$ mechanism.

Large negative magnetoconductance (MC) of \ensuremath{\sim}12$%$ is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (${V}_{\mathrm{bias}}$). Simultaneously, a positive magnetoefficiency (M\ensuremath{\eta}) of 9$%$ is observed at ${V}_{\mathrm{bias}}$ $=$ 2 V. At higher bias voltages, both the MC and M\ensuremath{\eta} diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive M\ensuremath{\eta} effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

We explore the magnetoelectroluminescence (MEL) of organic light-emitting diodes by evaluating the magnetic-field dependent fraction of singlet excitons formed. We use two- and multisite polaron-hopping models with spin mixing by hyperfine fields and different singlet and triplet exciton formation rates k(S) and k(T). A huge MEL is predicted when exciton formation is in competition with spin mixing and when k(T) is significantly larger than k(S). This competition also leads to a low-field structure in the MEL that is in agreement with recent experiments.

The stochastic Liouville equation is applied to the field of organic magnetoresistance to perform detailed microscopic calculations on the different proposed models. By adapting this equation, the influence of a magnetic field on the current in bipolaron, electron-hole pair, and triplet models is calculated. The simplicity and wide applicability of the stochastic Liouville equation makes it a powerful tool for interpreting experimental results on magnetoresistance measurements in organic semiconductors. New insights are gained on the influence of hopping rates and disorder on the magnetoresistance.

The reversible assembly of helical supramolecular polymers of chiral molecular building blocks is known to be governed by the interplay between mass action and the competition between weakly and strongly bound states of these building blocks. The highly co-operative transition from free monomers at high temperatures to long helical aggregates at low temperatures can be monitored by photoluminescence spectroscopy that probes the energetically lowest-lying optical excitations in the assemblies. In order to provide the interpretation of obtained spectroscopic data with a firm theoretical basis, we present a comprehensive model that combines a statistical theory of the equilibrium polymerization with a quantum-mechanical theory that not only accounts for the conformational properties of the assemblies but also describes the impact of correlated energetic disorder stemming from deformations within the chromophores and their interaction with solvent molecules. The theoretical predictions are compared to fluorescence spectra of chiral oligo(p-phenylene−vinylene) molecules in the solvent dodecane and we find them to qualitatively describe the red-shift of the main fluorescence peak and its decreasing intensity upon aggregation.

We present an ab initio theory of ultrafast nanologic elements and show that controlled spin manipulation is feasible with the inclusion of spin-orbit coupling in ?-processes. We show that in branched metallic chains with three magnetic centers both spin flips and spin transfers are possible within a hundred femtoseconds. A static external magnetic field and the magnetic state of one magnetic center serve as input poles (input bits), while the magnetic state of the cluster after a controlled laser pulse can be mapped to the output. Combining laser-induced spin-manipulation scenarios we are able to construct the NAND logic. Thus multicenter magnetic clusters can extend spin dynamics to optically triggered and functionalized magnetic transport on a subpicosecond timescale and nanometer spatial scale.