χ-Shaped bis(areno)-1,4-dihydropyrrolo[3,2-b]pyrroles generated by oxidative aromatic coupling

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.

Enhanced Gain in Organic Photodetectors Using the Polymer with Singlet Open-Shell Ground State

Photodetectors are critical components in intelligent optoelectronic systems, and photomultiplication-capable devices are essential for detecting weak optical signals. Despite significant advances, developing photomultiplication-type organic photodetectors with high gain and low noise current simultaneously remains challenging. In this work, a new conjugated polymer PDN with singlet open-shell ground state is introduced in active layers for electron capture, and the corresponding PDN-based photodetectors exhibited an enhanced photoelectric gain and decreased dark current density at a low forward bias. At 1.5 V, the PDN-based ternary photodetector has the external quantum efficiency (EQE) up to 2552.3 % and the specific detectivity of 1.4×1014  Jones at 710 nm calculated by the measured noise current, with the gain 22 times higher than that of the control group. This study provides an approach for exploiting polymers with singlet open-shell ground state to enhance the gain of organic photodetectors.

Relating Design and Optoelectronic Properties of 1,4-Dihydropyrrolo[3,2-b]pyrroles Bearing Biphenyl Substituents

Understanding the influence of peripheral functionality on optoelectronic properties of conjugated materials is an important task for the continued development of chromophores for myriad applications. Here, π-extended 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) chromophores with varying electron-donating or electron-withdrawing capabilities were synthesized via Suzuki cross-coupling reactions, and the influence of functionality on optoelectronic properties was elucidated. First, chromophores display distinct differences in the UV-vis absorbance spectra measured via UV-vis absorbance spectroscopy in addition to changes in the onset of oxidation measured with cyclic voltammetry and differential pulse voltammetry. Solution oxidation studies found that variations in the electron-donating and -withdrawing capabilities result in different absorbance profiles of the radical cations that correspond to quantifiably different colors. In addition to fundamental insights into the molecular design of DHPP chromophores and their optoelectronic properties, two chromophores display high-contrast electrochromism, which makes them potentially compelling in electronic devices. Overall, this study represents the ability to fine-tune the optoelectronic properties of DHPP chromophores in their neutral and oxidized states and expands the understanding of structure-property relationships that will guide the continued development of DHPP-based materials.

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.

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.

On-surface synthesis of a nitrogen-embedded buckybowl with inverse Stone-Thrower-Wales topology

Curved π-conjugated polycyclic aromatic hydrocarbons, buckybowls, constitute an important class of materials with wide applications in materials science. Heteroatom doping of buckybowls is a viable route to tune their intrinsic physicochemical properties. However, synthesis of heteroatom-doped buckybowls is a challenging task. We report on a combined in-solution and on-surface synthetic strategy toward the fabrication of a buckybowl containing two fused nitrogen-doped pentagonal rings. We employ ultra-high-resolution scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations to characterize the final compound. The buckybowl contains a unique combination of non-hexagonal rings at its core, identified as the inverse Stone–Thrower–Wales topology, resulting in a distinctive bowl-opening-down conformation of the buckybowl on the surface. Our controlled design of non-alternant, heteroatom-doped polycyclic aromatic frameworks with established bottom-up fabrication techniques opens new opportunities in the synthesis of carbon nanostructures with the perspective of engineering properties of graphene-based devices.

Heteroatom doping of buckybowls is a viable route to tune their intrinsic physico-chemical properties, but their synthesis remains challenging. Here, the authors report on a combined in-solution and on-surface synthetic strategy towards the fabrication of a buckybowl containing two fused nitrogen-doped pentagonal rings.

The role of steric hindrance in the intramolecular oxidative aromatic coupling of pyrrolo[3,2-b]pyrroles

The presence of steric hindrance triggers different reaction pathways in the intramolecular oxidative aromatic coupling of tetraaryl-pyrrolo[3,2-b]pyrroles and leads to the formation of a fluorene moiety and a new cationic π-system linked together by a spiro carbon atom. Computational studies elegantly rationalized these results. These previously unknown functional dyes emit red light with reasonable efficiency.

The Tetraarylpyrrolo[3,2-b]pyrroles-From Serendipitous Discovery to Promising Heterocyclic Optoelectronic Materials

Progress in organic optoelectronics requires compounds possessing a suitable combination of photophysical and electronic properties. Another key constraint encompasses the availability of feasible, and hopefully scalable, synthetic procedures for preparing the molecular scaffolds of interest. A multicomponent reaction of aromatic aldehydes, aromatic amines, and butane-2,3-dione that was discovered in 2013 gives straightforward access to previously unavailable 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles. These dyes are examples of heteropentalenes-a class of 10-π-electron aromatic compounds. The unsurpassed variety of aromatic aldehydes and primary aromatic amines, which are commercially available or easy to prepare, allows for potential routes to thousands of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles that are currently unknown. This synthetic procedure offers a means for preparing the pyrrolopyroles in gram quantities and isolating them by simple filtration. Typically, the construction of an aromatic core is merely the first phase in a long procedure toward multistep functionalization. Conversely, the synthesis of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles leads to preinstalled substituents in frames with C₂ symmetry, which "opens Sesame" to a wealth of structural possibilities. In addition, steric hindrance of the aldehyde components, rather than presenting a problem, is beneficial for increasing the yields of the products. This feature provides invaluable routes for the synthesis of a broad range of π-extended systems possessing the pyrrolo[3,2-b]pyrrole core in just a few steps. Indeed, this approach has enabled the preparation of a large number of previously unknown ladder-type heteroacenes possessing additional rings based on carbon-carbon, carbon-nitrogen, and nitrogen-nitrogen double bonds as well as nitrogen-boron single bonds. This set of chromophores includes planar and curved structures bearing up to 14 conjugated rings. 1,2,4,5-Tetraarylpyrrolo[3,2-b]pyrroles manifest broad absorption bands between about 300 and 450 nm, strong violet-blue or blue fluorescence with typical quantum yields of ∼60%, significant Stokes shifts ranging between 3000 and 5800 cm^-1, and emission while in the solid state. Should the two peripheral aryl groups have an electron-deficient character, the two-photon absorption cross section also becomes pronounced, i.e., ∼400 GM. Perhaps the most important feature of these dyes is their strong solvatofluorochromism, which predestines their value as environment-sensitive probes. Extension of the π-conjugation of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles enables further modifications of their photophysical properties, such as shifting the emission bathochromically, increasing the Stokes shift beyond 10 000 cm^-1, and attaining solvatofluorochromism for curved, butterfly-shaped analogues without a decrease in emission intensity when the solvent polarity is increased. Common features of these chromophores include a significant difference between the geometries of their relaxed ground and relaxed excited states as well as strong electronic coupling through their aromatic cores. Past and future intense exploration of the wide chemical space built around the pyrrolo[3,2-b]pyrrole skeleton offers unprecedented opportunities for comprehensive elucidation of how photoexcitation increases the electronic coupling through biaryl linkages.

Synthesis, Structure, and Properties of Near-Infrared [b]Phenanthrene-Fused BF2 Azadipyrromethenes

A new class of phenanthrene-fused BF₂ azadipyrromethene (azaBODIPY) dyes have been synthesized through a tandem Suzuki reaction and oxidative ring-fusion reaction, or a palladium-catalyzed intramolecular C-H activation reaction. These phenanthrene-fused azaBODIPY dyes are highly photostable and display markedly redshifted absorption (up to λ=771 nm) and emission bands (λ≈800 nm) in the near-infrared region. DFT calculations and cyclic voltammetry studies indicate that, upon annulation, more pronounced stabilization of the LUMO is the origin of the bathochromic shift of the absorption and high photostability.

Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption

Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ_2PA ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ_2PA =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S₀ →S₁ transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.

Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9'-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.

Extension of Pyrrolopyrrole π-System: Approach to Constructing Hexacyclic Nitrogen-Containing Aromatic Systems

A facile three-step approach to synthesizing quinoline-fused pyrrolopyrroles is reported. The crucial step in this synthesis is the condensation of 2-aminophenyl substituted pyrrolopyrroles with aromatic aldehydes. The resulting hexacyclic ladder-type dyes strongly absorb UV radiation and exhibit fluorescence at 450-510 nm. The presence of pyridine-type and pyrrole-type nitrogen atoms is important for the electronic properties of this almost planar heterocycle. These heteroatoms, along with the addition of moderate electron-withdrawing and electron-accepting substituents, provide a means for fine-tuning of the emission characteristics of the polycyclic conjugates.

Strong solvent dependence of linear and non-linear optical properties of donor–acceptor type pyrrolo[3,2-b]pyrroles

The pyrrolo[3,2-b]pyrrole core was determined to be an efficient linker allowing the conjugation of peripheral benzene rings. The resulting dipolar compounds displayed strong solvatochromism of fluorescence.