Andreas Sperlich

An Isolable Radical Anion Based on the Borole Framework

The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis and activation of element–element bonds (frustrated Lewis pairs). The combination of boron with p-donating substituents (e.g. BF3) and its incorporation into organic p-conjugated systems allows the empty pz orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B2H6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. Likewise, hydrogen atom abstraction from N-heterocyclic carbene(NHC)-stabilized boranes (NHC-BH3) can lead to neutral, persistent boryl radicals of the type NHC-BH2C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes= 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li([12]crown-4)2][BMes3] (1) or [K([18]crown-6)(thf)2][Mes2BB(Ph)Mes] (2). [7] Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl2,3,4,5-tetraphenylborole (3). Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. Encouraged by these recent results on the radical anion [3]C , which indicated the presence of a highly unusual C4B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent. The synthesis of MesBC4Ph4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin–boron exchange reaction was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX2; X=Cl, Br). However, 4 was obtained in 41% yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). A more efficient alternative was found to be the salt-elimination reaction of MesBCl2 with 1,4-

Reversible and Irreversible Interactions of Poly(3-hexylthiophene) with Oxygen Studied by Spin-Sensitive Methods

Understanding of degradation mechanisms in polymer:fullerene bulk-heterojunctions on the microscopic level aimed at improving their intrinsic stability is crucial for the breakthrough of organic photovoltaics. These materials are vulnerable to exposure to light and/or oxygen, hence they involve electronic excitations. To unambiguously probe the excited states of various multiplicities and their reactions with oxygen, we applied combined magneto-optical methods based on multifrequency (9 and 275 GHz) electron paramagnetic resonance (EPR), photoluminescence (PL), and PL-detected magnetic resonance (PLDMR) to the conjugated polymer poly(3-hexylthiophene) (P3HT) and polymer:fullerene bulk heterojunctions (P3HT:PCBM; PCBM = [6,6]-phenyl-C(61)-butyric acid methyl ester). We identified two distinct photochemical reaction routes, one being fully reversible and related to the formation of polymer:oxygen charge transfer complexes, the other one, irreversible, being related to the formation of singlet oxygen under participation of bound triplet excitons on the polymer chain. With respect to the blends, we discuss the protective effect of the methanofullerenes on the conjugated polymer bypassing the triplet exciton generation.

Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu(3)N@C(80) (Lu(3)N@C(80)-PCBEH) show an open circuit voltage (V(OC)) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC(61)BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (J(SC)) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the J(SC) in blends containing the high voltage absorber Lu(3)N@C(80)-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu(3)N@C(80)-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC(61)BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu(3)N@C(80)-PCBEH are responsible for the reduced photocurrent.

Direct detection of photoinduced charge transfer complexes in polymer fullerene blends

We report transient electron paramagnetic resonance (trEPR) measurements with sub-microsecond time resolution performed on a P3HT:PCBM blend at low temperature. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g-values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e. originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes which form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.

Fullerene Dimers (C60/C70) for Energy Harvesting

A new family of fullerene-based compounds, namely, soluble [60]- and [70]fullerene homodimers and the [60]/[70]heterodimer linked through 2-pyrazolino-pyrrolidino bridges, has been synthesised by simple procedures and in high yield. Electrochemical studies confirm their suitability to act as electron acceptors in combination with poly(3-hexylthiophene-2,5-diyl) (P3HT). Their optical properties in solution and in the solid state were studied. A significantly stronger absorption in [70]fullerene-containing dimers relative to [60]homodimer in solution in the visible range was observed. Furthermore, in all donor-acceptor blends studied an efficient charge transfer was observed by means of photoluminescence (PL), photoinduced absorption and light-induced electron spin resonance spectroscopy. The [70]homodimer was found to be a distinctive species, being the strongest PL quencher and most efficient acceptor with the longest lifetime of the charge-separated (polaron) states. As a consequence, bulk-heterojunction solar cells based on this novel [70]homodimer blended with P3HT demonstrated the highest quantum and power conversion efficiencies of 37 and 1 %, respectively, compared to those of [60]fullerene dimers.

Spin signatures of photogenerated radical anions in polymer-[70]fullerene bulk heterojunctions: high frequency pulsed EPR spectroscopy.

Charged polarons in thin films of polymer-fullerene composites are investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz (X-band) and 130 GHz (D-band). The materials studied were poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester (C(60)-PCBM), and two different soluble C(70)-derivates: C(70)-PCBM and diphenylmethano[70]fullerene oligoether (C(70)-DPM-OE). The first experimental identification of the negative polaron localized on the C(70)-cage in polymer-fullerene bulk heterojunctions has been obtained. When recorded at conventional X-band EPR, this signal is overlapping with the signal of the positive polaron, which does not allow for its direct experimental identification. Owing to the superior spectral resolution of the high frequency D-band EPR, we were able to separate light-induced signals from P(+) and P(-) in PHT-C(70) bulk heterojunctions. Comparing signals from C(70)-derivatives with different side-chains, we have obtained experimental proof that the polaron is localized on the cage of the C(70) molecule.

Resonant Addressing and Manipulation of Silicon Vacancy Qubits in Silicon Carbide

Several systems in the solid state have been suggested as promising candidates for spin-based quantum information processing. In spite of significant progress during the last decade, there is a search for new systems with higher potential [D. DiVincenzo, Nat. Mater. 9, 468 (2010)]. We report that silicon vacancy defects in silicon carbide comprise the technological advantages of semiconductor quantum dots and the unique spin properties of the nitrogen-vacancy defects in diamond. Similar to atoms, the silicon vacancy qubits can be controlled under the double radio-optical resonance conditions, allowing for their selective addressing and manipulation. Furthermore, we reveal their long spin memory using pulsed magnetic resonance technique. All these results make silicon vacancy defects in silicon carbide very attractive for quantum applications.

Magnetic field and temperature sensing with atomic-scale spin defects in silicon carbide

Quantum systems can provide outstanding performance in various sensing applications, ranging from bioscience to nanotechnology. Atomic-scale defects in silicon carbide are very attractive in this respect because of the technological advantages of this material and favorable optical and radio frequency spectral ranges to control these defects. We identified several, separately addressable spin-3/2 centers in the same silicon carbide crystal, which are immune to nonaxial strain fluctuations. Some of them are characterized by nearly temperature independent axial crystal fields, making these centers very attractive for vector magnetometry. Contrarily, the zero-field splitting of another center exhibits a giant thermal shift of −1.1 MHz/K at room temperature, which can be used for thermometry applications. We also discuss a synchronized composite clock exploiting spin centers with different thermal response.

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer–Fullerene Bulk Heterojunctions

Understanding charge separation and charge transport is crucial for improving the efficiency of organic solar cells. Their active media are based on organic molecules and polymers, serving as both light-absorbing and transport layers. The charge-transfer (CT) states play an important role, being intermediate for free carrier generation and charge recombination. Here, we use light-induced electron paramagnetic resonance spectroscopy to study the CT dynamics in blends of the polymers P3HT, PCDTBT, and PTB7 with the fullerene derivative C60-PCBM. Time-resolved EPR measurements show strong spin-polarization patterns for all polymer-fullerene blends, confirming predominant generation of singlet CT states and partial orientation ordering near the donor-acceptor interface. These observations allow a comparison with charge separation processes in molecular donor-acceptor systems and in natural and artificial photosynthetic assemblies, and thus the elucidation of the initial steps of sequential CT in organic photovoltaic materials.

Photoinduced C70 radical anions in polymer:fullerene blends

Photoinduced polarons in solid films of polymer-fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH-PPV. As acceptors we employed PC60BM as reference and various soluble C70-derivates: PC70BM, two different diphenylmethano-[70]fullerene oligoether (C70-DPM-OE) and two dimers, C70-C70 and C60-C70. Blend films containing C70 revealed characteristic spectroscopic signatures not seen with C60. Light-induced ESR showed signals at g\geq2.005, assigned to an electron localized on the C70 cage. The formation of C70 radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C70-based P3HT and MEH-PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C70 blends.