Synthesis and Properties of Ladder-Type BN-Heteroacenes and Diazabenzoindoles Built on a Pyrrolopyrrole Scaffold

A simple, three-step synthesis of BN-heteroacenes and diazabenzoindoles based on the pyrrole­[3,2-<i>b</i>]­pyrrole scaffold has been developed. The incorporation of BN units has proven to be effective in modulating the electronic properties and molecular geometries of the π-conjugated backbone, creating a new type of heteroarenes. The unique planar structure and high rigidity of BN-compounds result in very high absorption coefficients and high fluorescence quantum yields, and, at the same time, very small Stokes shifts. A striking difference has been observed for a second type of derivatives: diazabenzoindoles, which remain virtually nonfluorescent, despite having a similar, rigid structure. The former class of heterocycles is characterized by a strong absorption around 400 nm and intense fluorescence observed in the 395–426 nm region, which results in very small Stokes shifts of less than 900 cm^–1

Photochemical Conversion of Phenanthro[9,10‑<i>d</i>]imidazoles into π‑Expanded Heterocycles

We discovered that phenanthro­[9,10-<i>d</i>]­imidazoles bearing a 2-halogenoaryl substituent at position 2 undergo swift photochemically driven direct arylation, leading to barely known phenanthro­[9′,10′:4,5]­imidazo­[1,2-<i>f</i>]­phenanthridines. The reaction is high-yielding, and it does not require any sensitizer or base. The discovered process is tolerant of a variety of substituents present both at positions 1 and 2; i.e., strongly electron-donating and electron-withdrawing substituents are tolerated as well as various heterocyclic units. Steric hindrance does not affect this process. The evidence gathered here indicates that S_RN1 mechanism is operating in this case with the formation of radical anion as a critical step, followed by heterolytic cleavage of a carbon–halogen bond. Also TfO groups were shown to undergo cyclization, which allows the use of salicylaldehydes in the construction of heterocyclic systems. Efficiency of this photochemically driven direct arylation has been demonstrated by the synthesis of two systems possessing 13 and 17 conjugated rings, respectively. Phenanthro­[9′,10′:4,5]­imidazo­[1,2-<i>f</i>]­phenanthridines are blue-emitters, and they exhibit strong fluorescence in solution and in the solid state in direct contrast to their precursors

χ‑Shaped Bis(areno)-1,4-dihydropyrrolo[3,2‑<i>b</i>]pyrroles Generated by Oxidative Aromatic Coupling

A synthesis of dihydropyrrolo­[3,2-<i>b</i>]­pyrroles fused with two peripheral arenes or heterocyclic units has been realized through the concise route. These nearly planar compounds were prepared starting from assembling the central core via condensation of 2-aryl or 2-heteroarylbenzaldehydes with aromatic amines and diacetyl, followed by double intramolecular oxidative aromatic coupling. This two-step procedure afforded the desired products in overall yields of 5–36%, and it tolerates structural diversity of starting materials. All the final dyes exhibit strong blue fluorescence in solution

Oxidative Aromatic Coupling of <i>meso</i>-Arylamino-porphyrins

Strategic placement of the bis-arylamino group at the <i>meso</i>-position of porphyrins allowed fusion of these two moieties <i>via</i> aromatic dehydrogenation. By placing two di(naphthalene-2-yl)amine or bis(3,5-dimethoxyphenyl)amine groups at positions 5 and 15 of the porphyrin, the oxidative aromatic coupling was directed toward closing one six-membered ring. The extension of the porphyrin chromophore leads to significant change in linear optical properties, such as a bathochromic shift of absorption and broadening of the Q-band

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

Vertically π‑Expanded Coumarins: The Synthesis and Optical Properties

A regioselective synthesis of naphtho­[2,1,8-<i>def</i>]­coumarins has been realized through a concise route that involves the intramolecular Friedel–Crafts reaction of benzo­[<i>f</i>]­coumarins. Tetracyclic, planar products were prepared starting from assembly of the suitably substituted coumarin via the Pechmann reaction of 2-naphthols with acetone-1,3-dicarboxylates, followed by an intramolecular Friedel–Crafts reaction. In contrast to earlier report, the main product of the condensation reaction performed at 130 °C was the corresponding sulfonic acid and not the phenol itself. The one-pot process afforded the desired phenol in 39% yield. This reaction has been extended to some naphthalenediols. The model 5-hydroxy-naphtho­[2,1,8-<i>def</i>]­coumarin was transformed into corresponding dimer using various pathways including intermolecular oxidative aromatic coupling. Photophysical studies revealed that 5-hydroxy-naphtho­[2,1,8-<i>def</i>]­coumarin has the most bathochromically shifted both absorption and emission among all π-expanded coumarins bearing one OH functionality. In general, all prepared coumarin-phenols as well as their hexyl ethers displayed moderate to strong greenish-yellow fluorescence, except of dimer that emits at 552 nm. The fluorescence of these dyes was strongly dependent on polarity of the solvent. Computational studies supported interpretation of optical properties for the selected compounds

Tetraaryl‑, Pentaaryl‑, and Hexaaryl-1,4-dihydropyrrolo[3,2‑<i>b</i>]pyrroles: Synthesis and Optical Properties

Efficient conditions for the synthesis of tetra-, penta-, and hexasubstituted derivatives of 1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrrole were developed. The tetraaryl derivatives were obtained in a novel one-pot reaction among aromatic aldehydes, aromatic amines, and butane-2,3-dione. After a thorough examination of various reaction parameters (solvent, acid, temperature) <i>p</i>-toluenesulfonic acid was identified as the crucial catalyst. As a result, 1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles were obtained in the highest yields reported to date. The scope and limitation studies showed that this new method was particularly efficient for sterically hindered aldehydes (yields 45–49%). Pentaaryl- and hexaaryl-1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles were prepared from tetraaryl-1,4-dihydropyrrolo­[3,2-<i>b</i>]­pyrroles via direct arylation by employing both electron-poor and electron-rich aromatic and heteroaromatic haloarenes. Strategic placement of electron-withdrawing substituents at the 2-, 3-, 5-, and 6-positions produced an acceptor–donor–acceptor type fluorophore. The resulting multiply substituted heteropentalenes displayed intriguing optical properties. The relationship between the structure and photophysical properties for all compounds were directly compared and thoroughly elucidated. All synthesized products displayed strong blue fluorescence and exhibited moderate to large Stokes shifts (3000–7300 cm^–1) as well as high quantum yields of fluorescence up to 88%. Two-photon absorption cross-section values measured in the near-IR region were surprisingly high (hundreds of GM), given the limited conjugation in these propeller-shaped dyes

Correction to Bright, Color-Tunable Fluorescent Dyes Based on π‑Expanded Diketopyrrolopyrroles

Correction to Bright, Color-Tunable Fluorescent Dyes Based on π‑Expanded Diketopyrrolopyrrole

Tetraphenylethylenepyrrolo[3,2‑<i>b</i>]pyrrole Hybrids as Solid-State Emitters: The Role of Substitution Pattern

Two hybrid dyes possessing tetraphenylethylene moieties weakly conjugated with a pyrrolo­[3,2-<i>b</i>]­pyrrole core have been synthesized. Both dyes display a weak emission in solution, however, in the solid state a ∼100-fold increase in the fluorescence quantum yield is observed. The position of the molecular rotors about the core greatly influences the photophysical characteristics. The variances in emission properties were assigned to entirely different changes in dihedral angles upon excitation, which in turn have substantial effects on radiative rate constants, allowed transitions, and HOMO/LUMO distribution

Optical Behavior of Substituted 4‑(2′-Hydroxyphenyl)imidazoles

A set of tetraarylimidazoles bearing a 2-hydroxyphenyl substituent at position 4, as well as their models lacking intramolecular hydrogen bonds, was efficiently synthesized. Structural investigations proved that the hydrogen bond strength for 4-(2′-hydroxyphenyl)­imidazoles is weaker than that for analogous 2-(2′-hydroxyphenyl)­imidazoles as estimated from dihedral angles and bond distances. Photophysical investigations revealed that these compounds have other properties than those observed for imidazoles bearing a 2-hydroxyphenyl substituent at position 2. They exhibit a negligible fluorescence quantum yield regardless of the solvent polarity. Additionally, dual fluorescence is observed in nonpolar solvents. Plausibly, although a hydrogen bond is present within their chemical structure in the solid state, it is not clear if excited-state intramolecular proton transfer occurs. The presence of OH groups triggers the radiationless deactivation channel if compared with model imidazole possessing a 2-methoxyphenyl group