Unforeseen 1,2-Aryl Shift in Tetraarylpyrrolo[3,2-b]pyrroles Triggered by Oxidative Aromatic Coupling

Tetraarylpyrrolo[3,2-b]pyrrole-BODIPY dyad: a molecular rotor for FRET-based viscosity sensing

With the aim to develop a FRET-based viscosity sensor, two dyad molecules, 4 and 5, comprising tetraarylpyrrolo[3,2-b]pyrrole (TAPP) (donor) and naked boron-dipyrromethene (BODIPY) dyes (acceptor), were designed. Dyads were synthesized via acid-catalyzed multicomponent reactions followed by Sonogashira coupling. In both dyads, the BODIPY and TAPP moieties are linked through phenylethynyl groups, which allow free rotation of the BODIPY dyes; that is, they can act as molecular rotors. This was supported by X-ray crystallographic and DFT-optimized structures. Spectroscopic studies also confirmed the presence of both TAPP and BODIPY dyes in dyads with no electronic interactions that are suitable for fluorescence resonance energy transfer (FRET). Very high energy transfer efficiency (ETE >99%) from the donor TAPP moiety to the acceptor BODIPY moiety on excitation at the TAPP part was observed. However, due to the non-fluorescent nature of naked BODIPY dyes, no fluorescence emission was observed from the BODIPY moiety in both dyads. With increasing solvent viscosities, emission from the BODIPY moieties increases due to the restricted rotation of the BODIPY moieties. Plotting the logarithms of the fluorescent intensity of dyad 5 and the viscosity of the solution showed a good linear correlation obeying a Förster-Hoffmann equation. Non-fluorescent dyad 5 in methanol became greenish-yellow fluorescent in a methanol/glycerol (1:1) solvent. Furthermore, with an increase in the temperature of the methanol/glycerol (1:1) system, as the viscosity decreases, the fluorescence also starts decreasing. Thus, dyad 5 is capable of sensing the viscosity of the medium via a FRET-based "Off-On" mechanism. This type of viscosity sensor with a very large pseudo-Stokes shift and increased sensitivity will be useful for advancing chemo-bio sensing and imaging applications.

Switching the regioselectivity of acid-catalyzed reactions of arylnaphtho[2,1-b]furans via a [1,2]-aryl shift

The [1,2]-aryl shift reaction was used to synthesize naphtho[2,1-b]furans as promising fluorescent scaffolds for organic electronics. The target compounds are furan analogues of phenanthrene formally accessed by isosteric replacement of the CHCH moiety with an oxygen atom. The straightforward and robust approach involving a [1,2]-aryl shift as a key step provides easy access to a wide range of naphtho[2,1-b]furans with the possibility of late-stage functionalization. Efficient switching of the regioselectivity of acid-catalyzed reactions of arylnaphtho[2,1-b]furans via a [1,2]-aryl shift has been demonstrated. A one-pot protocol involving sequential intramolecular condensation/[1,2]-aryl shift/intermolecular oxidative aromatic coupling to provide access to binaphtho[2,1-b]furan analogues of BINOL was developed. The advantage of these compounds lies in the strong variation in chemical properties and spectral performance depending on the nature and position of the aryl substituent, which facilitates the synthesis of compounds with desired spectral characteristics and opens up prospects for their further use in electronics, biotechnologies and organic synthesis.

From Serendipity to Precision: Decoding the Enigma of Rearrangement in Scholl-Type Reactions for Programmable Cyclization

Rearrangements in the Scholl reaction have traditionally been serendipitous, lacking a systematic approach for synthesizing rearranged and cyclized products. This paper introduces a strategic pathway to achieve rearranged-cyclized thienotetrahelicene derivatives over direct-cyclized chrysenothiophene derivatives by finely modifying the reaction conditions and tuning the electronic properties in Scholl-type reaction precursors, tetraarylthiophenes. Through careful design principles, we demonstrate the programmable synthesis of these distinct products.

Gold-Catalyzed 1,2-Aryl Shift and Double Alkyne Benzannulation

The tandem intramolecular hydroarylation of alkynes accompanied by a 1,2-aryl shift is described. Harnessing the unique electron-rich character of 1,4-dihydropyrrolo[3,2-b]pyrrole scaffold, we demonstrate that the hydroarylation of alkynes proceeds at the already occupied positions 2 and 5 leading to a 1,2-aryl shift. Remarkably, the reaction proceeds only in the presence of cationic gold catalyst, and it leads to heretofore unknown π-expanded, centrosymmetric pyrrolo[3,2-b]pyrroles. The utility is verified in the preparation of 13 products that bear six conjugated rings. The observed compatibility with various functional groups allows for increased tunability with regard to the photophysical properties as well as providing sites for further functionalization. Computational studies of the reaction mechanism revealed that the formation of the six-membered rings accompanied with a 1,2-aryl shift is both kinetically and thermodynamically favourable over plausible formation of products containing 7-membered rings. Steady-state UV/Visible spectroscopy reveals that upon photoexcitation, the prepared S-shaped N-doped nanographenes undergo mostly radiative relaxation leading to large fluorescence quantum yields. Their optical properties are rationalized through time-dependent density functional theory calculations. We anticipate that this chemistry will empower the creation of new materials with various functionalities.

The Essence in Selectivity of Copper-Mediated Intermolecular Nucleophilic Substitution of a meta C-H Bond in 2-Methyl-N-methoxyaniline: A Theoretical Study

The detailed mechanism for NHC-Cu(I)-catalyzed intermolecular nucleophilic substitution of the C-H bonds at aniline (2-methyl-N-methoxyaniline) was studied via DFT methods to reveal the essence of the selectivity. Calculations revealed that the meta C-H functionalization proceeds via two nucleophilic attacks on the aromatic ring rather than a one-step meta C-H substitution to give the experimentally observed major product. The reaction is initiated by activation of the substrate via oxidative addition with an NHC-Cu(I) catalyst, through which an umpolung occurs at the ring. From the activated intermediate, methoxyl group transfer to benzyl forms a resting state, while a nucleophile can attack the ortho position of benzyl to form a more stable intermediate. The nucleophile group can then transfer to the meta position by a 1,2-Wagner-Meerwein rearrangement to form the final product through a proton shuttle. In contrast, other transfer processes affording ortho- or para-substituted products encounter higher activation barriers. This work investigates the relationship of product selectivity with the umpolung of the aromatic ring, as well as the priority of a nucleophilic attack at the ortho position of the aromatic, 1,2-Wagner-Meerwein rearrangement from the ortho-substituted intermediate, and proton shuttle from the meta-substituted intermediate.

BODIPY as a Multifunctional Theranostic Reagent in Biomedicine: Self-Assembly, Properties, and Applications

The use of boron dipyrromethene (BODIPY) in biomedicine is reviewed. To open, its synthesis and regulatory strategies are summarized, and inspiring cutting-edge work in post-functionalization strategies is highlighted. A brief overview of assembly model of BODIPY is then provided: BODIPY is introduced as a promising building block for the formation of single- and multicomponent self-assembled systems, including nanostructures suitable for aqueous environments, thereby showing the great development potential of supramolecular assembly in biomedicine applications. The frontier progress of BODIPY in biomedical application is thereafter described, supported by examples of the frontiers of biomedical applications of BODIPY-containing smart materials: it mainly involves the application of materials based on BODIPY building blocks and their assemblies in fluorescence bioimaging, photoacoustic imaging, disease treatment including photodynamic therapy, photothermal therapy, and immunotherapy. Lastly, not only the current status of the BODIPY family in the biomedical field but also the challenges worth considering are summarized. At the same time, insights into the future development prospects of biomedically applicable BODIPY are provided.

Saddle-shaped aza-nanographene with multiple odd-membered rings

A saddle-shaped aza-nanographene containing a central 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) has been prepared via a rationally designed four-step synthetic pathway encompassing intramolecular direct arylation, the Scholl reaction, and finally photo-induced radical cyclization. The target non-alternant, nitrogen-embedded polycyclic aromatic hydrocarbon (PAH) incorporates two abutting pentagons between four adjacent heptagons forming unique 7-7-5-5-7-7 topology. Such a combination of odd-membered-ring defects entails a negative Gaussian curvature within its surface with a significant distortion from planarity (saddle height ≈ 4.3 Å). Its absorption and fluorescence maxima are located in the orange-red region, with weak emission originating from the intramolecular charge-transfer character of a low-energy absorption band. Cyclic voltammetry measurements revealed that this stable under ambient conditions aza-nanographene underwent three fully reversible oxidation steps (two one-electron followed by one two-electron) with an exceptionally low first oxidation potential of E ox1 = -0.38 V (vs. Fc/Fc+).

Real-space imaging of a phenyl group migration reaction on metal surfaces

The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.

Aryl Dance Reaction of Arylbenzoheteroles

We report a 1,2-migration (aryl dance reaction) of the aryl group on heteroles. AlCl3 can efficiently convert C3-arylheteroles to C2-arylheteroles. Depending on the electron density of the substrate, conversion from C2- to C3-arylheteroles was also possible with catalytic Zn(OTf)2. A one-pot aryl dance/acylation or bromination and arylation/aryl dance cascade was also demonstrated.

Mechanochemical Molecular Migration on Graphene

In this study, we propose that the curvature of graphene can be exploited to perform directional molecular motion and provide atomistic insights into the curvature-dependent molecular migration through density functional theory calculations. We first reveal the origin of the different migration trends observed experimentally for aromatic molecules with electron-donating and -withdrawing groups on p-doped functionalized graphene. Next, we show that the kinetic barrier for migration depends on the amount and nature of the curvature, that is, positive versus negative curvature. We find that the molecular migration on a wrinkled/rippled graphene sheet preferentially happens from the valley (positive curvature) to the mountain (negative curvature) regions. To understand the origin of such curvature-dependent molecular motion and migrational kinetic barrier trends, we develop a descriptor based on the frontier orbital orientation of graphene. Finally, based on these findings, we predict that time- and space-varying curvature can drive directional molecular motion on graphene and thus further propose that efforts should focus on exploring other two-dimensional materials as active platforms for performing controlled molecular motion.

Synthesis and fluorescence properties of unsymmetrical 1,4-dihydropyrrolo[3,2-b]pyrrole dyes

Despite being regioisomers, unsymmetrical 1,4-dihydropyrrolo[3,2-b]pyrroles 5 and 6 showed significantly different absorption and fluorescence properties due to the difference of the resonance structure between 5 and 6.

Rearrangements in Scholl Reaction

Rearrangements in Scholl reaction are mostly serendipitous. The design of molecular precursors is what seems to guide the course of rearrangement. This review consolidates different classes of precursors used in Scholl reaction and their accompanying rearrangements that include aryl migration, migration followed by cyclization and skeletal rearrangements involving ring expansion, ring contraction and both, under the reaction conditions. The attempt in collating heretofore-reported examples in this review is to guide designing appropriate precursors to predictably achieve complex molecular structures or nanographenes or defect-nanographenes via rearrangement.

Transformation of Crowded Oligoarylene into Perylene-Cored Chiral Nanographene by Sequential Oxidative Cyclization and 1,2-Phenyl Migration

Synthetic innovation for constructing sophisticated nanographenes is of fundamental significance for a variety of advanced applications. Herein, we report a distinctive method to prepare π-extended chiral nanographenes with 29 benzenoid rings and two helical breaches from a highly crowded perylene-cored oligoarylene precursor. Under Scholl's conditions, the reaction predominantly involves the regioselective and sequential cyclization in the peri- and bay regions of the perylene core, and the complanation of the 1-phenyl[5]helicene intermediate module via 1,2-phenyl migration. The resulting chiral nanographenes are configurationally stable at 180 °C due to the high diastereomerization barriers of ca. 45 kcal mol^-1 . These molecules also possess globally delocalized π-systems with low HOMO/LUMO gaps, leading to nearly panchromatic absorption, intensive electronic circular dichroism signals and deep-red circularly polarized luminescence.

Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C-C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.

Carbazole-Fused Polycyclic Aromatics Enabled by Regioselective Scholl Reactions

The synthesis of new carbazole-fused polycyclic aromatics with interesting geometry and useful properties was explored using Scholl reactions. As found from the Scholl reactions of substrates having two carbazole units linked at different positions through o-phenylene, oxidative coupling of carbazole units occurred in a regioselective manner with new carbon-carbon bonds preferably formed at C3 and C4 in N-alkyl carbazoles. A new N-containing aromatic bowl was characterized by single-crystal X-ray crystallography, and new p-type organic semiconductors exhibited field effect mobility of up to 0.070 cm²  V^-1  s^-1 in solution-processed thin-film transistors.

Scholl reaction as a powerful tool for the synthesis of nanographenes: a systematic review

Nanographenes, or extended polycyclic aromatic hydrocarbons, have been attracting increasing attention owing to their widespread applications in organic electronics. However, the atomically precise fabrication of nanographenes has thus far been achieved only through synthetic organic chemistry. Polycyclic aromatic hydrocarbons (PAHs) are popular research subjects due to their high stability, rigid planar structure, and characteristic optical spectra. The recent discovery of graphene, which can be regarded as giant PAH, has further stimulated research interest in this area. Chemists working with nanographene and heterocyclic analogs thereof have chosen it as their preferred tool for the assembly of large and complex architectures. The Scholl reaction has maintained significant relevance in contemporary organic synthesis with many advances in recent years and now ranks among the most useful C-C bond-forming processes for the generation of the π-conjugated frameworks of nanographene or their heterocyclic analogs. A broad range of oxidants and Lewis acids have found use in Scholl-type processes, including Cu(OTf)2/AlCl3, FeCl3, MoCl5, PIFA/BF3-Et2O, and DDQ, in combination with Brønsted or Lewis acids, and the surface-mediated reaction has found especially wide applications in PAH synthesis. Undoubtedly, the utility of the Scholl reaction is supreme in the construction of nanographene and their heterocyclic analogues. The detailed analysis of the progress achieved in this field reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted cyclodehydrogenation and developing new reagents. In this review, we highlight and discuss the recent modifications in the Scholl reaction for nanographene synthesis using numerous oxidant systems. In addition, the merits or demerits of each oxidative reagent is described herein.

Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene Containing a Central Pyrrolo[3,2-b]pyrrole Core

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C-Br and C-Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons-the inverse Stone-Thrower-Wales topology. The presence of both five- and seven-membered rings in the final structure is responsible for interesting properties such as a perpendicularly aligned dipole moment, absorption and fluorescence in the orange-red region, weak emission originating from the charge-transfer character of a low-energy absorption band, and a high lying HOMO. In the solid state slipped convex-to-convex π-π stacking dominates.

Scholl Reaction of Perylene-Based Polyphenylene Precursors under Different Conditions: Formation of Hexagon or Octagon?

A planar dibenzo-peri-hexacene derivative (2) was synthesized via FeCl₃ -mediated Scholl reaction from a cyclopenta-fused perylene (CP) based polyphenylene precursor (1). However, an unexpected octagon-containing, negatively curved molecule (3) was obtained in nearly quantitative yield when 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and methanesulfonic acid (MeSO₃ H) were used. Similar results were observed when two smaller-sized precursors containing one (4) or two CP units (5) were tested. X-ray crystallographic analysis also revealed that there is no close π-π stacking between neighboring π-conjugated skeletons. DFT calculations suggest a radical cation mechanism in the presence of FeCl₃ while an arenium ion pathway for the DDQ/MeSO₃ H mediated Scholl reaction, which can well explain the selective formation of hexagons and octagons under different conditions. The obtained compounds showed tunable optical and electrochemical properties.

Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor-donor-acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.

Method for the Large-Scale Synthesis of Multifunctional 1,4-Dihydro-pyrrolo[3,2-b]pyrroles

A thorough investigation has enabled the optimization of the synthesis of 1,4-dihydro-pyrrolo[3,2-b]pyrroles. Although salts of such metals as vanadium, niobium, cerium, and manganese were found to facilitate the formation of 1,4-dihydro-pyrrolo[3,2-b]pyrroles from amines, aldehydes, and diacetyl, we confirmed that iron salts are the most efficient catalysts. The conditions identified (first step: toluene/AcOH = 1:1, 1 h, 50 °C; second step: toluene/AcOH = 1:1, Fe(ClO₄)₃·H₂O, 16 h, 50 °C) resulted in the formation of tetraarylpyrrolo[3,2-b]pyrroles in a 6–69% yield. For the first time, very electron-rich substituents (4-Me₂NC₆H₄, 3-(OH)C₆H₄, pyrrol-2-yl) originating from aldehydes and sterically hindered substituents (2-ClC₆H₄, 2-BrC₆H₄, 2-CNC₆H₄, 2-(CO₂Me)C₆H₄, 2-(TMS-C≡C)C₆H₄) present on anilines can be appended to the pyrrolo[3,2-b]pyrrole core. It is now also possible to prepare 1,4-dihydropyrrolo[3,2-b]pyrroles bearing an ordered arrangement of N-substituents and C-substituents ranging from coumarin, quinoline, phthalimide to truxene. These advances in scope enable independent regulations of many desired photophysical properties, including the Stokes shift value and emission color ranging from violet-blue through deep blue, green, yellow to red. Simultaneously, the optimized conditions have finally allowed the synthesis of these extremely promising heterocycles in amounts of more than 10 g per run without a concomitant decrease in yield or product contamination. Empowered with better functional group compatibility, novel derivatization strategies were developed.

Aryl Migration on Graphene

The design and synthesis of molecular systems able to carry out movements resembling macroscopic objects is an exciting and challenging endeavor. Molecules that can walk covalently on a track have been demonstrated, and we now report how aryl groups that can migrate over a graphene surface. Specifically, we describe a system comprised of covalently functionalized aryl groups on graphene that undergo continuous aryl shifts. The dynamic aryl shift allows the aryl groups on graphene to effectively migrate step-by-step wherein each step involves reversible bond breaking and making that is initiated by a combination of an activated arene and p-doping of the graphene surface. Raman spectroscopic mapping of the distribution of the covalent attachment revealed that activated 4-methoxyphenyl groups migrate several microns from regions of high functionalization to regions with no prior functionalization.

Negatively Curved Nanographene with Heptagonal and [5]Helicene Units

Negatively curved nanographene (NG) 4, having two heptagons and a [5]helicene, was unexpectedly obtained by aryl rearrangement and stepwise cyclodehydrogenations. X-ray crystallography confirmed the saddle-shaped structures of intermediate 3 and NG 4. The favorability of rearrangement over helicene formation following radical cation or arenium cation mechanisms is supported by theoretical calculations. NG 4 demonstrates a reversible mechanochromic color change and solid-state emission, presumably benefiting from its loose crystal packing. After resolution by chiral high-performance liquid chromatography, the circular dichroism spectra of enantiomers 4-(P) and 4-(M) were measured and showed moderate Cotton effects at 350 nm (|Δε| = 148 M^–1 cm^–1).

Synthetic Applications of Oxidative Aromatic Coupling-From Biphenols to Nanographenes

Oxidative aromatic coupling occupies a fundamental place in the modern chemistry of aromatic compounds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chemistry of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is especially true for heterocoupling, that is, oxidative aromatic coupling with the chemoselective formation of a C-C bond between two different arenes. Analysis of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted (cyclo)dehydrogenation, and developing new reagents.

Phosphindole fused pyrrolo[3,2-b]pyrroles: a new single-molecule junction for charge transport

A new family of phosphindole fused ladder-type heteroacenes with a pyrrolo[3,2-b]pyrrole core were synthesized and characterized, which show good luminescence efficiency, high thermostability and tunable conductance.

FeCl3-promoted tandem 1,4-conjugate addition/6-endo-dig cyclization/oxidation of propargylamines and benzoylacetonitriles/malononitriles: direct access to functionalized 2-aryl-4H-chromenes

An efficient and concise procedure has been developed for the synthesis of functionalized 2-aryl-4H-chromenes based on a tandem reaction of propargylamines and benzoylacetonitriles/malononitriles in the presence of FeCl3 as an environmentally friendly promoter. This reaction involves a highly efficient tandem sequence consisting of 1,4-conjugate addition, 6-endo-dig cyclization, and oxidation. This protocol tolerates a variety of functional groups, thereby providing a practical and efficient method for the fabrication of 2-aryl-4H-chromene skeletons.