Max von Delius

Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability

Fullerene dimerization has been linked to short circuit current (Jsc) losses in organic solar cells comprised of certain polymer–fullerene systems. We investigate several polymer–fullerene systems, which present Jsc loss to varying degrees, in order to determine under which conditions dimerization occurs. By reintroducing dimers into fresh devices, we confirm that the photo-induced dimers are indeed the origin of the Jsc loss. We find that both film morphology and electrical bias affect the photodimerization process and thus the associated loss of Jsc. In plain fullerene films, a higher degree of crystallinity can inhibit the dimerization reaction, as observed by high performance liquid chromatography (HPLC) measurements. In blend films, the amount of dimerization depends on the degree of mixing between polymer and fullerene. For highly mixed systems with very amorphous polymers, no dimerization is observed. In solar cells with pure polymer and fullerene domains, we tune the fullerene morphology from amorphous to crystalline by thermal annealing. Similar to neat fullerene films, we observe improved light stability for devices with crystalline fullerene domains. Changing the operating conditions of the investigated solar cells from Voc to Jsc also significantly reduces the amount of dimerization-related Jsc loss; HPLC analysis of the active layer shows that more dimers are formed if the cell is held at Voc instead of Jsc. The effect of bias on dimerization, as well as a clear correlation between PL quenching and reduced dimerization upon addition of small amounts of an amorphous polymer into PC60BM films, suggests a reaction mechanism via excitons.

Self-assembly of dynamic orthoester cryptates

The discovery of coronands and cryptands, organic compounds that can accommodate metal ions in a preorganized two- or three-dimensional environment, was a milestone in supramolecular chemistry, leading to countless applications from organic synthesis to metallurgy and medicine. These compounds are typically prepared via multistep organic synthesis and one of their characteristic features is the high stability of their covalent framework. Here we report the use of a dynamic covalent exchange reaction for the one-pot template synthesis of a new class of coronates and cryptates, in which acid-labile O,O,O-orthoesters serve as bridgeheads. In contrast to their classic analogues, the compounds described herein are constitutionally dynamic in the presence of acid and can be induced to release their guest via irreversible deconstruction of the cage. These properties open up a wide range of application opportunities, from systems chemistry to molecular sensing and drug delivery.

Nitrogen Directs Multiple Radical Additions to the 9,9′‐Bi‐1‐aza(C60‐Ih)[5,6]fullerene: X‐ray Structure of 6,9,12,15,18‐C59N(CF3)5

Azafullerenes, in which a cage carbon atom is replaced by a nitrogen atom, are the only type of heterofullerenes that can be made in practical amounts, and this makes it possible to probe the effects of cage-atom substitution on the physical and chemical properties. 4] Herein, we report a unique type of isomerism in azafullerenes bearing trifluoromethyl groups, which is attributed to the directing role of the nitrogen atom, and the first X-ray structure of the C59N derivative prepared directly from (C59N)2. Figure 1 shows the negative-ion atmospheric-pressure chemical ionization (NI APCI) mass spectra of the products of the high-temperature trifluoromethylation of (C59N)2 with CF3I obtained either in a sealed ampoule (top) or in a flow tube (bottom). The thermolysis of (C59N)2 yields a radical monomer C59NC which readily adds up to 15 (bottom spectrum) or even 19 CF3 groups per cage (top spectrum) to form closed-shell species C59N(CF3)n, where n is an odd number. In these reactions, a nonsoluble dark-brown solid dimer (C59N)2 is quantitatively converted into a volatile, thermally stable orange crystalline material, which is highly soluble in many organic and fluoroorganic solvents. When trifluoromethyl groups were added to C60 under similar conditions, the closed-shell species with even n values up to 22 were formed (see the Supporting Information). The product in the flow tube was separated by chromatography into four main fractions: I) C59N(CF3)11-15, II) C59N(CF3)9, III) C59N(CF3)7, and IV) C59N(CF3)5 (inset in Figure 1). This one-step HPLC process yielded a 98 % compositionally pure sample of C59N(CF3)5 as proven by NI APCI mass spectrometry, which detects also a single isomer with Cs symmetry (see the F NMR spectrum in Figure 2, top). The absence of terminal CF3 groups (seen as quartets in the F NMR spectra) implies that five CF3 groups should be arranged in a para 5 (or p) loop rather than as a ribbon of edge-sharing p-C6(CF3)2 hexagons as most commonly observed in the C60(CF3)n compounds. [5] The absorption spectrum (see the Supporting Information) is similar to that of Cs-C60(CF3)6. [6] The most probable addition pattern that agrees with the spectroscopic data and was previously observed for azafullerenes features an isolated pyrrole moiety on the fullerene core (Figure 1). Interestingly, C60X6 compounds with a similar addition pattern of skew-pentagonal pyramid (SPP) are formed abundantly, if not regioselectively, in various room-temperature reactions (see references in the Supporting Information). However, SPP-Cs-C60(CF3)6 was found only as a minor isomer in the high-temperature synthesis, whereas C1-C60(CF3)6 with a ribbon addition pattern (para -meta-para ; pmp) was at least ten times more abundant. 6] The Cs isomer is also 14.4 kJ mol 1 less stable than pmp-C1-C60(CF3)6 at the DFT Figure 1. Negative-ion APCI mass spectra of the C59N(CF3)n products obtained in a sealed glass ampoule at 530 8C for 24 h (top) and a hot flow tube at 500 8C for 3 h (bottom). The HPLC trace (inset) of the crude product (100% toluene eluent).

NMR Studies on Li+, Na+ and K+ Complexes of Orthoester Cryptand o-Me2-1.1.1

Cryptands, a class of three-dimensional macrobicyclic hosts ideally suited for accommodating small guest ions, have played an important role in the early development of supramolecular chemistry. In contrast to related two-dimensional crown ethers, cryptands have so far only found limited applications, owing in large part to their relatively inefficient multistep synthesis. We have recently described a convenient one-pot, template synthesis of cryptands based on O,O,O-orthoesters acting as bridgeheads. Here we report variable-temperature, 1H-1D EXSY and titration NMR studies on lithium, sodium, and potassium complexes of one such cryptand (o-Me2-1.1.1). Our results indicate that lithium and sodium ions fit into the central cavity of the cryptand, resulting in a comparably high binding affinity and slow exchange with the bulk. The potassium ion binds instead in an exo fashion, resulting in relatively weak binding, associated with fast exchange kinetics. Collectively, these results indicate that orthoester cryptands such as o-Me2-1.1.1 exhibit thermodynamic and kinetic properties in between those typically found for classical crown ethers and cryptands and that future efforts should be directed towards increasing the binding constants.