Synthesis and Properties of Fluorenone-Containing Cycloparaphenylenes and Their Late-Stage Transformation.

Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor-acceptor chromophores were obtained by incorporating fluorenone or 2-(9H-fluoren-9-ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late-stage functionalization of the fluorenone-based rings by high-yielding Knoevenagel condensations. The structures were confirmed by X-ray crystallographic analyses, which revealed that replacing a phenylene for a fused-ring-system acceptor introduces additional strain. The donor-acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi-redox systems undergoing reversible or quasi-reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units.

Synthesis of 1,3-Dibromopyrene as Precursor of 1-, 3-, 6-, and 8-Substituted Long-Axially Symmetric Pyrene Derivatives.

Pyrene derivatives bearing substituents at positions 1, 3, 6, and 8 find numerous applications, as exemplified by their use in lasers, sensors, and bioimaging probes. However, these derivatives typically have point-symmetric or short-axially symmetric structures, whereas long-axially symmetric derivatives remain underexplored because of the difficulty in obtaining their precursor, 1,3-dibromopyrene. To address this problem, we herein synthesized 1,3-dibromopyrene from 1-methoxypyrene in an overall yield (71 % over four steps) considerably exceeding those of existing methods. 1,3-Dibromopyrene was converted into 13OPA, a long-axially symmetric pyrene dye with electron-donor (alkoxy) groups at positions 1 and 3 and electron-acceptor (formyl) groups at positions 6 and 8. 13OPA exhibited photophysical properties distinct from those of its point-symmetric and short-axially symmetric isomers, featuring a broad and strongly redshifted absorption, strong fluorescence with reduced sensitivity to protic solvents, and small dipole moment change upon photoexcitation. The derivatization of 13OPA into a Schiff base and its functionalization via Lewis acid-base pairing were also demonstrated. Thus, our work expands the design scope of pyrene-based molecules, particularly those used as emitters.

Supramolecular Organic Ferroelectric Materials from Donor-Acceptor Systems.

Organic ferroelectric (FE) materials, though known for more than a century, are yet to reach close to the benchmark of inorganic or hybrid materials in terms of the magnitude of polarization. Amongst the different classes of organic systems, donor (D)-acceptor (A) charge-transfer (CT) complexes are recognized as promising for ferroelectricity owing to their neutral-to-ionic phase transition at low temperature. This review presents an overview of different supramolecular D-A systems that have been explored for FE phase transitions. The discussion begins with a general introduction of ferroelectricity and its different associated parameters. Then it moves on to show early examples of CT cocrystals that have shown FE properties at sub-ambient temperature. Subsequently, recent developments in the field of room temperature (RT) ferroelectricity, exhibited by H-bond-stabilized lock-arm supramolecular-ordering (LASO) in D-A co-crystals or other FE CT-crystals devoid of neutral-ionic phase transition are discussed. Then the discussion moves on to emerging reports on other D-A soft materials such as gel and foldable polymers; finally it shows very recent developments in ferroelectricity in supramolecular assemblies of single-component dipolar or ambipolar π-systems, exhibiting intra-molecular charge transfer. The effects of structural nuances such as H-bonding, balanced charge transfer and chirality on the observed ferroelectricity is described with the available examples. Finally, piezoelectricity in recently reported ambipolar ADA-type systems are discussed to highlight the future potential of these soft materials in micropower energy harvesting.

NBN- and BNB-Phenalenyls: the Yin and Yang of Heteroatom-doped π Systems.

NBN- and BNB-doped phenalenyls are isoelectronic to phenalenyl anions and cations, respectively. They represent a pair of complementary molecules that have essentially identical structures but opposite properties as electron donors and acceptors. The NBN-phenalenyls 1-4 considered here were prepared from N,N'-dimethyl-1,8-diaminonaphthalene and readily available boron-containing building blocks (i. e., BH3⋅SMe2 (1), p-CF3-C6H4B(OH)2 (2), C6H5B(OH)2 (3), or MesBCl2/iPr2NEt (4)). Treatment of 1 with 4-Me2N-2,6-Me2-C6H2Li gave the corresponding NBN derivative 5. The BNB-phenalenyl 6 was synthesized from 1,8-naphthalenediyl-bridged diborane(6), PhNH2, and MesMgBr. A computational study reveals that the photoemission of 1, 4, and 5 originates from locally excited (LE) states at the NBN-phenalenyl fragments, while that of 2 is dominated by charge transfer (CT) from the NBN-phenalenyl to the p-CF3-C6H4 fragment. Depending on the dihedral angle θ between its Ph and NBN planes, compound 3 emits mainly from a less polar LE (θ >55°) or more polar CT state (θ <55°). In turn, the energetic preference for either state is governed by the polarity of the solvent used. An equimolar aggregate of the NBN- and BNB-phenalenyls 3 and 6 (in THF/H2O) shows a distinct red-shifted emission compared to that of the individual components, which originates from an intermolecular CT state.

Bright Fluorescent p-Phenylene-bridged Triarylmethyl Highly Stable Diradical.

Two units of highly stable luminescent triarylmethyl radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), were bridged by p-phenylene linker. The photoluminescence quantum yield (PLQY) of PyBTM-PhPyBTM was at most 0.4 % in various organic solvents. Adding two mesityl groups on the terminals did not improve the PLQY. In the MesPyBTM-PhPyBTM-Mes, the mesityl group did not worked as an electron donor unlike the previously reported monoradical MesPyBTM. However, adding two n-hexyl groups on the bridging p-phenylene did greatly improve it, and the PLQY of the PyBTM-(Hex2Ph)PyBTM was 7 % in dichloromethane and acetone, and 12 % in poly(methyl methacrylate) (PMMA) film. Twisting p-phenylene linker by hexyl groups hindered the π-conjugation and suppressed the non-radiative decay of the excited state.

Design and Photonics of Merocyanine Dyes.

Merocyanines, thanks to their easily adjustable electronic structure, appear to be the most versatile and promising functional dyes. Their D-π-A framework offers ample opportunities for custom design through variations in both donor/acceptor end-groups and the π-conjugated polymethine chain, and leads to a broad range of practical properties, including noticeable solvatochromism, high polarizability/hyperpolarizabilities, and the ability to sensitize various physicochemical processes. Accordingly, merocyanines are applied and extensively studied in various fields, such as light-converting materials for optoelectronics, nonlinear optics, optical storage, solar cells, fluorescent probes, and antitumor agents in photodynamic therapy. This review encompasses both classical and novel more important publications on the structure-property relationships in merocyanines, with particular emphasis on the results by A.  I. Kiprianov and his followers in Institute of Organic Chemistry in Kyiv, Ukraine.

Comparative Investigation of Ultrafast Excited-State Electron Transfer in Both Polyfluorene-Graphene Carboxylate and Polyfluorene-DCB Interfaces.

The Photophysical properties, such as fluorescence quenching, and photoexcitation dynamics of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3'-N,N'-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and anionic graphene carboxylate (GC) have been discovered for the first time via steady-state and time-resolved femtosecond transient absorption (TA) spectroscopy with broadband capabilities. The steady-state fluorescence of PFN is quenched with high efficiency by the GC acceptor. Fluorescence lifetime measurements reveal that the quenching mechanism of PFN by GC is static. Here, the quenching mechanisms are well proven via the TA spectra of PFN/GC systems. For PFN/GC systems, the photo electron transfer (PET) and charge recombination (CR) processes are ultrafast (within a few tens of ps) compared to static interactions, whereas for PFN/1,4-dicyanobenzene DCB systems, the PET takes place in a few hundreds of ps (217.50 ps), suggesting a diffusion-controlled PET process. In the latter case, the PFN+•-DCB-• radical ion pairs as the result of the PET from the PFN to DCB are clearly resolved, and they are long-lived. The slow CR process (in 30 ns time scales) suggests that PFN+• and DCB-• may already form separated radical ion pairs through the charge separation (CS) process, which recombine back to the initial state with a characteristic time constant of 30 ns. The advantage of the present positively charged polyfluorene used in this work is the control over the electrostatic interactions and electron transfers in non-covalent polyfluorene/quencher systems in DMSO solution.

Silylium-Catalyzed Activation of Donor- Acceptor Strained Rings and Annulation with Indoles.

Leveraging the unique reactivity profile of donor-acceptor aminocyclopropanes and cyclobutanes allows the preparation of complex nitrogen-substituted molecules. While most reports focus on donor-acceptor strained rings with two geminal carbonyl groups as acceptors, mono carbonyl acceptor systems, despite their synthetic relevance, have been considerably less studied. Herein we describe catalytic annulation reactions ofaminocyclopropane and aminocyclobutane monoesters employing silylium catalysis to activate these less reactive donor-acceptor systems.

NIR-II Absorption/Fluorescence of D-A π-Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron-Fused Azobenzene Complexes.

Luminescence in the second near-infrared (NIR-II, 1,000-1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron-acceptors with low-lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor-acceptor (D-A) π-conjugated polymers (CPs) with the NIR-II emission property, however, limited kinds of molecular skeletons are still available. Herein, D-A CPs involving fluorinated boron-fused azobenzene complexes (BAz) with enhanced electron-accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to -4.42 eV, which is much lower than those of conventional strong electron-acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR-II. Furthermore, owing to the inherent solid-state emissive property of the BAz skeleton, obvious NIR-II fluorescence from the film (up to λFL=1213 nm) and the nanoparticle in water (λFL=1036 nm, brightness=up to 29 cm-1 M-1) were observed, proposing that our materials are applicable for developing next-generation of NIR-II luminescent materials.

Tunable Isometric Donor-Acceptor Wurster-Type Covalent Organic Framework Photocathodes.

Covalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster-type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor-acceptor-donor (D-A-D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady-state photoluminescence revealed a tunable red-shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical studies exhibited a doubled current density (8.1 µA cm-2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of these D-A-D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of structure-property relationships in this promising class of photoactive materials.

Conversion of Ketones into Blue-Emitting Electron-Deficient Benzofurans.

A novel heavy metal-free and safe synthetic methodology enabling one-step conversion of ketones into corresponding 4,5,6,7-tetrafluorobenzofurans (F4 BFs) has been developed. The presented approach has numerous advantageous qualities, including utilization of readily available substrates, broad scope, scalability, and good reaction yields. Importantly, some of the benzofurans prepared by this method were heretofore inaccessible by any other known transformation. Importantly, furo[2,3-b]pyrazines and heretofore unexplored difuro[2,3-c:3',2'-e]pyridazine can be prepared using this strategy. Spectroscopic studies reveal that for simple systems, absorption and fluorescence maxima fall within the UV spectral range, while π-electron system expansion red-shifts both spectra. Moreover, the good fluorescence quantum yields observed in solution, up to 96 %, are also maintained in the solid state. Experimental results are supported by density functional theory (DFT) calculations. The presented methodology, combined with the spectroscopic characteristics, suggest the possibility of using F4 BFs in the optoelectronic industry (i. e., organic light emitting devices (OLED), organic field-effect transistors (OFET), organic photovoltaics (OPV)) as inexpensive and readily available emissive or semiconductor materials.

Electroactive Dyes Based on 1,8-Naphthalimide with Acetylene Linkers as Promising OLED Materials - the Relationship Between Structure and Photophysical Properties.

Four A-π-D-π-A type small organic molecules with 1,8-naphthalimide motifs were successfully synthesised. The designed compounds are built of two 1,8-naphthalimide units linked via ethynyl π-linkages with selected functionalised donor motifs i. e. 2,2'-bithiophene, fluorene, phenothiazine and carbazole derivative. The synthesis based on Sonogashira cross-coupling allowed us to obtain the presented dyes with good yields. The resulting symmetrical small molecules' optical, electrochemical and thermal properties were thoroughly investigated, and their potential applicability for the OLED devices was demonstrated. In addition, the relationship between molecular structure and properties was considered by employing experimental and theoretical studies. As a result of using various donor groups, it was possible to achieve efficient electroluminescence in the range from green (DEV4) to orange-red light (DEV3) with a maximum luminance of 3 820 cd/m2 for DEV4. Upon the insertion of an acetylene linker to the designed molecules, the free rotation of D and A fragments, and hence the effective π-electron communication within the entire molecule, is possible, which was confirmed by DFT studies. The obtained dyes are characterised by high thermal stability, reversible oxidation-reduction process, satisfactory optoelectronic properties and good solubility in organic solvents, which is advisable for the application in small molecular organic light-emitting diodes (SM-OLEDs) technology.

Amino-Ene Click Reaction of Electron-Deficient π-Conjugated Molecules with Negative Activation Enthalpies.

An amino-ene click reaction is a type of aza-Michael addition reaction that is congruent with click chemistry in terms of its reaction efficiency and rate under mild conditions. The amino-ene click reaction is increasingly recognized as a prominent synthetic tool to form C-N bonds in the context of organic materials chemistry and polymer chemistry. Herein, an unconventional amino-ene click reaction with negative activation enthalpies, in which an electron-deficient π-conjugated molecule, such as a naphthalenediimide, reacts with an amine faster at lower temperatures is reported. The detailed study of the reaction mechanism reveals that the amino-ene click reaction proceeds via a pre-equilibrium reaction, the key to which is the formation of a stable reaction intermediate due to the solvation and charge delocalization on the π-core. By optimizing the reaction conditions, it was demonstrated that the amino-ene click reaction proceeded faster at 273 K than at 347 K, which was easily observed visually.

A Widely Applicable and Versatile Method for the Ring-Opening 1,3-Carbocarbonation of Donor-Acceptor Cyclopropanes.

A 1,3-carbocarbonation of 2-substituted cyclopropane 1,1-dicarboxylates introduces various saturated or unsaturated carbon residues at the 1- and 3- position of the former three-membered ring. Under copper catalysis, ring-opening attack with a Grignard reagent proceeded smoothly; the intermediate was converted to the final product by reaction with appropriate carbon-based electrophiles under basic conditions. As nucleophiles, Grignard reagents derived from sp3 -, sp2 -, and sp-hybridized carbon residues were successfully employed, whereas various aliphatic bromides and EBX derivatives (for sp moieties) served as electrophiles.

Asymmetrical Platinum and Rhodium Dinuclear Complex Strongly Bound to Filled d z 2 ${{_{{\rm z}{

Heterometallic extended metal atom chains (EMACs) aligned with three types of metal were rationally synthesized by forming unbridged metal-metal bonds based on the interactions between highest occupied and lowest unoccupied molecular orbitals at the d z 2 ${{_{{\rm z}{^{2}}}}}$ orbital. These chains form pentanuclear structures aligned as Rh-Pt-M-Pt-Rh with relatively large formation constants of 5.0×1013  M-2 for M=Pt and 6.3×1011  M-2 for M=Pd, while retaining their backbones in solution. In the case of M=Cu, the original Cu(+2) atoms were reduced to Cu(+1) during the synthetic process. Cu(+1) has an unprecedented trigonal bipyramidal coordination geometry. The reported synthesis based on asymmetrical dinuclear complexes provides a guideline for the synthesis of hetero-EMACs to allow several analogs through judicious combinations realized by tuning the number of metal nuclei and metal species.

From Terminal to Spiro-Phosphonium Acceptors, Remarkable Moieties to Develop Polyaromatic NIR Dyes.

Phosphonium-based compounds gain attention as promising photofunctional materials. As a contribution to the emerging field, we present a series of donor-acceptor ionic dyes, which were constructed by tailoring phosphonium (A) and extended π-NR2 (D) fragments to an anthracene framework. The alteration of the π-spacer of electron-donating substituents in species with terminal -+ PPh2 Me groups exhibits a long absorption wavelength up to λabs =527 nm in dichloromethane and shifted the emission to the near-infrared (NIR) region (λ=805 nm for thienyl aniline donor), although at low quantum yield (Φ<0.01). In turn, the introduction of a P-heterocyclic acceptor substantially narrowed the optical bandgap and improved the efficiency of fluorescence. In particular, the phospha-spiro moiety allowed to attain NIR emission (797 nm in dichloromethane) with fluorescence efficiency as high as Φ=0.12. The electron-accepting property of the phospha-spiro constituent outperformed that of the monocyclic and terminal phosphonium counterparts, illustrating a promising direction in the design of novel charge-transfer chromophores.

Silicon- and Germanium-Functionalized Perylene Diimides: Synthesis, Optoelectronic Properties, and Their Application as Non-fullerene Acceptors in Organic Solar Cells.

Organic solar cells have been continuously studied and developed through the last decades. A major step in their development was the introduction of fused-ring non-fullerene electron acceptors. Yet, beside their high efficiency, they suffer from complex synthesis and stability issues. Perylene-based non-fullerene acceptors, in contrast, can be prepared in only a few steps and display good photochemical and thermal stability. Herein, we introduce four monomeric perylene diimide acceptors obtained in a three-step synthesis. In these molecules, the semimetals silicon and germanium were added in the bay position, on one or both sides of the molecules, resulting in asymmetric and symmetric compounds with a red-shifted absorption compared to unsubstituted perylene diimide. Introducing two germanium atoms improved the crystallinity and charge carrier mobility in the blend with the conjugated polymer PM6. In addition, charge carrier separation is significantly influenced by the high crystallinity of this blend, as shown by transient absorption spectroscopy. As a result, the solar cells reached a power conversion efficiency of 5.38 %, which is one of the highest efficiencies of monomeric perylene diimide-based solar cells recorded to date.

Highly Selective and Quantitative Point-of-Care Diagnostic Method for Adrenaline.

Adrenaline, also known as epinephrine, is a neurotransmitter/hormone that is an important target in diagnostics. Development of an effective method for detecting it in the presence of other neurotransmitters is a challenging task. The electrochemical and fluorescent techniques commonly used have low selectivity in distinguishing among catecholamines. Herein, a small-molecule organic probe with an activated furfural moiety is reported to exploit the nucleophilicity of epinephrine to generate a bright-colored donor-acceptor Stenhouse adduct. Among nine common neurotransmitters or their analogues, only epinephrine was found to generate a unique colour change discernible with the naked eye, whereas the other ones remain unaffected. Under various in-field detection conditions, including solution, droplet, and paper strip-based detection, the colour change were also noticeable. The low detection limit of 1.37 nM and a limit of quantitation of 4.37 nM were achieved with simple UV/Vis methods in addition to the sub-ppm level sensing under visual conditions with naked eyes. The probe could be used for practical colorimetric measurements as a point-of-care tool without any complex and expensive machinery, making this approach accessible to all. In addition, using a simple smartphone, the determination of epinephrine concentrations is possible by using machine-learning techniques.

3-Ligated Phenothiazinyl-terephthalonitrile Dyads and Triads - Synthesis, Electronic Properties, Delayed Fluorescence and Electronic Structure.

The bromine-lithium exchange-borylation-Suzuki sequence efficiently furnishes phenothiazine-terephthalonitrile donor-acceptor dyads and triads in high yields. In contrast to most phenothiazine-acceptor conjugates the title compounds are ligated in p-position to the phenothiazine nitrogen atom. Moreover, the acceptors are either directly linked or ligated by an arylene bridge and p-anisyl N-substituents on the phenothiazine are chosen to lock the tricycle into an intra-configuration. Cyclic voltammetry reveals effects of bridging and ligation of the N-substituent. Optical spectroscopy likewise displays similar band gaps, large Stokes shifts and substantial to high quantum yields in solution, in the solid state and in PMMA matrix. Time-resolved fluorescence spectroscopy indicates quite long fluorescence decay times in solution and emission components in the microsecond time range. TADF properties are further assessed by fluorescence increase in deoxygenated solution, gated emission spectroscopy and temperature-dependent determination of phosphorescence. The nature of the electronically excited states is investigated by DFT/MRCI. While for the directly ligated dyad a singlet-triplet energy gap Δ E ( S 1 - T 1 ) ${{E}_{{({\rm S}}_{1}-{{\rm T}}_{1})}{\rm \ }}$ of 0.24 eV can be estimated and is consistently confirmed by quantum chemical calculations on the lowest energy conformer, even lower Δ E ( S 1 - T 1 ) ${{\rm \Delta }{E}_{{(S}_{1}-{T}_{1})}{\rm \ }}$ of 0.029 and 0.008 eV are estimated for the investigated dyads and the triad in the solid state and in PMMA matrix.

Unravelling the Superiority of Nonbenzenoid Acepleiadylene as a Building Block for Organic Semiconducting Materials.

Acepleiadylene (APD), a nonbenzenoid isomer of pyrene, exhibits a unique charge-separated character with a large molecular dipole and a small optical gap. However, APD has never been explored in optoelectronic materials to take advantage of these appealing properties. Here, we employ APD as a building block in organic semiconducting materials for the first time, and unravel the superiority of nonbenzenoid APD in electronic applications. We have synthesized an APD derivative (APD-IID) with APD as the terminal donor moieties and isoindigo (IID) as the acceptor core. Theoretical and experimental investigations reveal that APD-IID has an obvious charge-separated structure and enhanced intermolecular interactions as compared with its pyrene-based isomers. As a result, APD-IID displays significantly higher hole mobilities than those of the pyrene-based counterparts. These results imply the advantages of employing APD in semiconducting materials and great potential of nonbenzenoid polycyclic arenes for optoelectronic applications.