FaFrit

Understanding charge separation and charge transport at a molecular level is crucial for improving the efficiency of organic photovoltaic (OPV) cells. Under illumination of Bulk Heterojunction (BHJ) blends of polymers and fullerenes, various paramagnetic species are formed including polymer and fullerene radicals, radical pairs, and photoexcited triplet states. Light-induced Electron Paramagnetic Resonance (EPR) spectroscopy is ideally suited to study these states in BHJ due to its selectivity in probing the paramagnetic intermediates. Advanced techniques like pulsed EPR and ENDOR spectroscopy allow the determination of hyperfine coupling tensors, while high-frequency EPR allows the EPR signals of the individual species to be resolved and their g-tensors to be determined. The magnetic resonance parameters of the various polymer donors reveal details about the delocalization of the positive polaron which is important for the efficient charge separation in BHJ systems. Time-resolved EPR can contribute to the study of the dynamics of charge separation, charge transfer and recombination in BHJ by probing the unique spectral signatures of charge transfer and triplet states. EPR also has the potential to allow characterization of intermediates and products of BHJ degradation.

Light-induced EPR spectroscopy is ideally suited to study charge separated states in BHJ since it selectively probes the paramagnetic charge carriers and excited states. This review focusses on the application of advanced EPR techniques to characterize the electronic structure of positive and negative polarons as well as dynamics of spin-dependent charge separation and charge recombination processes in organic photovoltaic materials.

Carbon–heteroatom bond formation is a long-standing hot topic in organic chemistry owing to the ubiquitous presence of these bonds in organic molecules. A major pathway to form them is the heterofunctionalization of unsaturated moieties mediated by transition metals or radical initiators. Recently, visible-light catalysis has become a powerful method for the synthesis of many chemical bonds, and carbon–heteroatom bond formation has attracted the particular attention of photochemists. In this review, we highlight recent advances in visible-light photocatalysis for the formation of C–N, C–O, C–S, and C–P bonds through the hydro- or carbo-heterofunctionalization reactions of unsaturated partners.

An overview of the visible-light-induced heterofunctionalization of unsaturated compounds is provided, and its usefulness as a powerful tool for the synthesis of complex molecules is demonstrated.

Cesium EncapsulationIn their Communication on page 2582 ff., A. Yu. Rogachev, T. Hirao, M. A. Petrukhina, and co-workers report unprecedented double concave encapsulation of a metal ion by two bowl-shaped sumanenyl carbanions.

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly–disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π–π stacking interactions, metal-coordination, van der Waals interactions and host−guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Fluorescent supramolecular polymers are attracting increased attention currently due to their dynamic fluorescent properties and potential applications. Recent studies on fluorescent supramolecular polymers depending on the type of main driving forces for supramolecular polymerization, including multiple hydrogen bonding interactions, electrostatic interactions, π–π stacking interactions, metal-coordination, van der Waals interactions and host−guest interactions are discussed.

Recent advances in dye-sensitized solar cells (DSSC), including the use of nanocarbons, resulted in breakthroughs relevant to both the academic and industrial sectors. Their relevance encompasses, on one hand, the development and integration of novel nanocarbon/dye hybrids for superior light-harvesting and improved charge injection properties and, on the other hand, the integration of nanocarbons into each part of DSSCs for enhancing the charge collection efficiency as well as the dye and electrolyte regeneration processes. In the context of the earlier, control over the electron injection and recombination rates is realized on the molecular level. In the context of the latter, i) the charge transport and charge collection efficiencies in the photoelectrode are increased, ii) the charge recombination processes are reduced, when used as a nanocarbon-based buffer layer, iii) the ion diffusion and catalytic effects into (quasi) solid-state electrolytes are improved, and iv) devices using Pt-free counter electrodes with similar device efficiencies are realized. Leading examples of nanocarbons are graphene, carbon black, multi-walled carbon nanotubes, and single-walled carbon nanotubes. In this review, the focus is on a novel carbon allotrope, namely single-walled carbon nanohorns (SWCNH), which is discussed in depth and in comparison with other nanocarbons.

Research on nanocarbon-based DSSCs is on the rise. SWCNHs stand out given their unique production, functionalization, and dispersibility. They are applied as i) SWCNH/dye hybrids, ii) buffer layers, iii) active components in photo- and counter electrodes, and iv) additives in solid-state electrolytes. This review summarizes the recent achievements concerning their use with respect to those realized with other nanocarbons.

Two types of ruthenocenes and a ferrocene coordinated by rac-9H-cyclopenta[1,2-c:4,3-c′]diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono-helicene ruthenocene 1 and its iron analogue 1_Fe with one [7]helicene ligand bound to the central metal, and bis-helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac-2 and meso-2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.

Jumbo sandwiches: Ruthenocenes and a ferrocene were successfully synthesized from [7]helicene(s) possessing a cyclopentadienyl anion at the center of its skeleton that coordinates to the metal atom. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism responses.

Intrinsically exchangeable dynamic covalent bonds that can be triggered by readily usable stimuli offer easy incorporation of their dynamic properties in various molecular systems, but the library of such bonds is still being developed. Herein, we report the dynamic covalent chemistry of 2,2,6,6-tetramethylpiperidine-1-sulfanyl (TEMPS) dimers derived from thermally reversible homolytic dissociation of disulfide linkages. High air stability of TEMPS was observed even at 100 °C, affording facile employment of thermal dissociation–association equilibria and adjustable bond exchange properties under atmospheric conditions. We also established an efficient synthetic route for a modifiable derivative of the dimer that enabled incorporation of dynamic properties into linear and network polymer structures. The obtained polymers showed controllable molecular weights, temperature-dependent swelling properties, healing ability, and recyclability, reflecting the thermally tunable dynamics of the dimer.

Air-stable radicals derived from thermally reversible homolytic dissociation of disulfide linkages were utilized to implement dynamic-covalent properties into linear and network polymer structures. The polymers show various inducible functions such as damage-healing and temperature-dependent swelling.

A Cu^I-catalyzed azide–alkyne cycloaddition reaction between an azide-terminated Ni^II-templated [2]pseudorotaxane with an oxy-ether tris(amino) macrocycle (MC) and an alkyne-terminated triphenylene stopper unit produces a new [2]rotaxane (ROT) with multiple functional groups. A detailed mass spectrometry study and various 1D and 2D NMR spectroscopy studies confirm the interpenetration of the axle in the macrocycle of ROT. The tri-acetylation of the tris(amine) moiety of ROT produces AcROT with three tertiary amide functionalities. Prior to this selection, a detailed study of the dynamic properties was undertaken with the tertiary amide functionalized macrocycle MC1 and the tri-methylated derivative of MC, MC2. A ¹H NMR study of AcROT at room temp. reveals multiple signals for most of the protons owing to the existence of multiple conformers and co-conformers in solution through tertiary amide bond rotations. Furthermore, the ethereal pocket of the heteroditopic macrocycle was explored to demonstrate the reversible locking and unlocking of the dynamic behaviour of AcROT through the addition and removal, respectively, of alkali-metal ions.

A new [2]rotaxane based on a tris(amino) ether macrocyclic wheel is functionalized with three tertiary amide groups and exhibits dynamic behaviour through rotamer formation as well as locking and unlocking properties in the presence of Na⁺ ions and 18-crown-6 inputs, respectively.