Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics

Indenoannulated Tridecacyclene: An All-Carbon Seven-Stage Redox-Amphoter

We disclose an indenoannulated tridecacyclene comprising a central cyclooctatetraene moiety with multiple adjacent pentagonal rings which is accessible in a concise synthetic sequence. The saddle-shaped geometry of the non-benzenoid polycyclic scaffold and its unique packing behavior in the solid state were characterized by X-ray crystallography. In electrochemical studies, the compound undergoes seven reversible redox events comprising five reductions and two oxidations. The dicationic and dianionic species obtained by chemical oxidation and reduction, respectively, were characterized spectroscopically in solution. Density functional theory calculations were applied to provide insights into aromaticity evolution in the respective charged species, highlighting the beneficial effect of the non-benzenoid moieties on charge stabilization.

Inducing Curvature to Pyracylene upon π‐Expansion

We disclose a successive π‐expansion of pyracylene towards boat‐shaped polycyclic scaffolds. The unique structural features of the resulting compounds were revealed by X‐ray crystallographic analysis. Depending on the extent of π‐expansion the compounds display intense bathochromically shifted absorption bands in their UV/Vis spectra and are prone to several redox events as documented by cyclic voltammetry. The experimental observations are in line with the computational studies based on density functional theory, suggesting progressive narrowing of the HOMO–LUMO gap and distinct evolution of the electronic structure and aromaticity.

Spacer-Dependent Cooperativity of Helicity in Fluorescent Bishelical Foldamers Based on L-Shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine

For the construction of helical foldamers composed of π-frameworks, the choice of appropriate π-π stacking units and π-spacers connecting them is important. The transfer of helicity between the minimal helix structural units is also an essential factor in the construction of homochiral helical foldamers. Tetramers 4 a-4 d, which have four L-shaped dibenzopyrrolo[1,2-a]naphthyridine units, were synthesized to investigate the interplay and cooperativity of the helical structures. Tetramer 4 a bridged with a biphenyl unit formed a homochiral bishelical structure with π-π stacking between the L-shaped units (3.3 Å), consisting only of (P,P)- and (M,M)-enantiomers without the (P,M)-diastereomer, owing to interplay through the axial chirality of biphenyl unit in the solid state. Similarly, in solution, thermodynamic stabilization of the two helix formations worked cooperatively to favor the bishelical form of 4 a. Furthermore, bishelical foldamer 4 a emitted intense fluorescence (Φ=0.86).

A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

Two subvalent, redox-active diborane(4) anions, [3]4- and [3]2- , carrying exceptionally high negative charge densities are reported: Reduction of 9-methoxy-9-borafluorene with Li granules without stirring leads to the crystallization of the B(sp3 )-B(sp2 ) diborane(5) anion salt Li[5]. [5]- contains a 2,2'-biphenyldiyl-bridged B-B core, a chelating 2,2'-biphenyldiyl moiety, and a MeO substituent. Reduction of Li[5] with Na metal gives the Na+ salt of the tetraanion [3]4- in which two doubly reduced 9-borafluorenyl fragments are linked via a B-B single bond. Comproportionation of Li[5] and Na4 [3] quantitatively furnishes the diborane(4) dianion salt Na2 [3], the doubly boron-doped congener of 9,9'-bis(fluorenylidene). Under acid catalysis, Na2 [3] undergoes a formal Stone-Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na2 [3] shows boron-centered nucleophilicity toward n-butyl chloride. Na4 [3] produces bright blue chemiluminescence when exposed to air.

Fulvalene-Based Polycyclic Aromatic Hydrocarbon Ladder-Type Structures: Synthesis and Properties

Polycyclic aromatic hydrocarbons (PAHs) have found strong interest for their electronic properties and as model systems for graphene. While PAHs have been studied intensively as single units, here PAHs were constructed in ladder-type arrangements using cross-conjugated fulvalene and dithiafulvalene motifs as connecting units and moving forward a convenient synthetic approach for dimerizing (thio)ketones into olefins by the action of Lawesson's reagent. Some of the PAHs can also be regarded as "super-extended" tetrathiafulvalenes (TTFs) with some of the largest cores ever explored, being multi-redox systems that exhibit both reversible oxidations and reductions. Concomitant absorption redshifts were observed when expanding the ladder-type structures from one to two to three indenofluorene units, and optical and electrochemical HOMO-LUMO gaps were found to correlate linearly. Various conformations (and solid-state packing arrangements) were studied by X-ray crystallography and computations.

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non-Planar Conjugated Organic Molecules

Non-planar conjugated organic molecules (NPCOMs) contain π-conjugation across their length and also exhibit asymmetry in their conformation. In other words, certain molecular fragments in NPCOMs are either twisted or curved out of planarity. This conformational asymmetry in NPCOMs leads to non-uniform charge-distribution across the molecule, with important photophysical and electronic consequences such as altered thermodynamic stability, chemical reactivity, as well as materials properties. Majorly, NPCOMs can be classified as having either Fused or Rotatable architectures. NPCOMs have been the focus of significant scientific attention in the recent past due to their exciting photophysical behavior that includes intramolecular charge-transfer (ICT), thermally activated delayed fluorescence (TADF) and long-lived charge-separated states. In addition, they also have many useful materials characteristics such as biradical character, semi-conductivity, dynamic conformations, and mechanochromism. As a result, rational design of NPCOMs and mapping their structure-property correlations has become imperative. Researchers have executed conformational changes in NPCOMs through a variety of external stimuli such as pH, temperature, anions-cations, solvent, electric potential, and mechanical force in order to tailor their photophysical, optoelectronic and magnetic properties. Converging to these points, this review highlights the lucrative electronic features, photophysical traits and upcoming applications of NPCOMs by a selective survey of the recent scientific literature.

Star-Shaped Non-Fullerene Small Acceptors for Organic Solar Cells

Over the past decade, organic solar cells (OSCs) have received considerable attention from the scientific community and are considered one of the most important sources of low-cost electricity production. Recently, OSC-based on star-shaped small-molecule (SM) non-fullerene acceptors (NFAs) have developed rapidly, and the highest power conversion efficiency (PCE) has exceeded 10 %. The star-shaped SM NFAs not only have three-dimensional charge-transport characteristics similar to fullerenes but also have a strong light absorption capacities and easily tunable energy levels. They are potential candidates as outstanding acceptor materials. In this Review, research progress in of star-shaped SM NFAs OSCs is reviewed specifically. Moreover, the influence of molecular structure, central unit, and peripheral linking group on OSC performance has been evaluated systematically. This Review could stimulate inspiration for designing high-performance OSC acceptor materials in the future.

Mechanochromism, Twisted/Folded Structure Determination, and Derivatization of (N-Phenylfluorenylidene)acridane

(N-Phenylfluorenylidene)acridane (Ph-FA) compounds with electron-withdrawing and -donating substituents (H, MeO, Ph, NO₂ , Br, F) at the para position of the phenyl group were successfully synthesized by Barton-Kellogg reactions of N-aryl thioacridones and diazofluorene. By using the substituent on the nitrogen atom to alter the electronic properties, both the folded and twisted conformers of p-NO₂ -C₆ H₄ -FA could be crystallographically characterized, which enabled the charge transfer from the electron-donating acridane moiety to the electron-accepting fluorenylidene moiety to be understood. Ground-state mechanochromism, thermochromism, vapochromism, and proton-induced chromism were demonstrated between the folded and twisted conformations of the conformers. Protonation and chemical oxidation of Ph-FA gave two stable acridinium compounds, namely, the fluorenylacridinium and acridinium radical cations. The present study will contribute to the development of functional dyes and organic semiconductors.

Subnaphthalocyanines as Electron Acceptors in Polymer Solar Cells: Improving Device Performance by Modifying Peripheral and Axial Substituents

A new class of subnaphthalocyanines bearing various peripheral and axial substituents have been synthesized for use as electron acceptors in solution-processed bulk-heterojunction polymer solar cells. The resulting solar cells exhibit modest photovoltaic performance with contributions from both the polymer donor and subnaphthalocyanine acceptor to the photocurrent.

Low-Cost Perovskite Solar Cells Employing Dimethoxydiphenylamine-Substituted Bistricyclic Aromatic Enes as Hole Transport Materials

The synthesis, characterization and photovoltaic performance of series of novel molecular hole transport materials (HTMs) based on bistricyclic aromatic enes (BAEs) are presented. The new derivatives were obtained following a simple and straightforward procedure from inexpensive starting reagents mimicking the synthetically challenging 9,9'-spirobifluorene moiety of the well-studied spiro-OMeTAD. The novel HTMs were tested in mixed cations and anions perovskite solar cells (PSCs) yielding a power conversion efficiency (PCE) of 19.2 % under standard global 100 mW cm^-2 AM1.5G illumination using 9-{2,7-bis[bis(4-methoxyphenyl)amino]-9H-fluoren-9-ylidene}-N² ,N² ,N⁷ ,N⁷ -tetrakis(4-methoxyphenyl)-9H-thioxanthene-2,7-diamine (coded as KR374). The power conversion efficiency data confirms the easily attainable heteromerous fluorenylidenethioxanthene structure as valuable core for low-cost and highly efficient HTM design and paves the way towards cost-effective PSC technology.

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk-Heterojunction Solar Cells

Four hexachlorosubphthalocyanines SubPcCl6 -X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution-processed bulk-heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone-shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7-Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems.

A potential perylene diimide dimer-based acceptor material for highly efficient solution-processed non-fullerene organic solar cells with 4.03% efficiency

A highly efficient acceptor material for organic solar cells (OSCs)--based on perylene diimide (PDI) dimers--shows significantly reduced aggregation compared to monomeric PDI. The dimeric PDI shows a best power conversion efficiency (PCE) approximately 300 times that of the monomeric PDI when blended with a conjugate polymer (BDTTTT-C-T) and with 1,8-diiodooctane as co-solvent (5%). This shows that non-fullerene materials also hold promise for efficient OSCs.

Small molecule organic semiconductors on the move: promises for future solar energy technology

This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.