Synthesis of Indolo[3,2-b]carbazole-Based Boron Complexes with Tunable Photophysical and Electrochemical Properties

Push-Pull Carbazole-Based Dyes: Synthesis, Strong Ultrafast Nonlinear Optical Response, and Effective Photoinitiation for Multiphoton Lithography

The present work reports on the ultrafast nonlinear optical (NLO) properties of a series of D-π-Α and D-A push-pull carbazole-based dyes and establishes a correlation between these properties and their efficiency for potential photonic and optoelectronic applications such as multiphoton lithography (MPL). The ultrafast NLO properties of the studied dyes are determined by two distinct experimental techniques, Z-scan and pump-probe optical Kerr effect (OKE), employing 246 fs laser pulses at 515 nm. The results indicate that chemical functionalization of the carbazole moiety with various strong electron-donating and/or electron-withdrawing groups, such as benzene, styrene, 4-bromostyrene, nitrobenzene, trimethyl isocyanurate, methyl, and indane-1,3-dione, can result in a controlled and significant enhancement of the NLO absorptive and refractive responses. In the context of potential applications, the efficiency of carbazole-based organic materials as photoinitiators (PIs) for MPL applications is demonstrated. The fabricated woodpile microstructure using chemically functionalized carbazole as a PI demonstrates improvements in both feature size and MPL efficiency compared to that using unfunctionalized carbazole as a PI. This is attributed to the efficient charge transfer resulting from chemical functionalization, which leads to a substantial increase (approximately 1 order of magnitude) in the values of the imaginary part of the second-order hyperpolarizability (Imγ) and the two-photon absorption cross section (σ). The achieved feature size of 280 nm is comparable to that obtained with other widely used PIs in MPL applications. Additionally, owing to the strong NLO properties of the studied functionalized carbazole, they could also be promising candidates for further applications in photonics and optoelectronics.

Construction of Boron Difluoride Complexes with Asymmetric N,N'-Bidentate Ligands

Boron difluoride (BF2) complexes with asymmetrical N,N'-bidentate ligands have received increasing attention due to their fascinating properties and broad applications. They are generally constructed in two steps: ligand formation, followed by boron complexation. This review focuses on categorizing these BF2 complexes based on the key synthetic strategies that have been applied in the ligand formation steps. The post-functionalization, properties and applications of different types of BF2 complexes are presented. Their challenges and opportunities are also discussed. This should help the future rational design and synthesis of BF2 complexes with intriguing properties and practical applications.

Difluoro Dipyridomethene Boron Complexes: Synthesis, Characterization, and Ab Initio Calculations

Molecular engineering studies on the meso-cyano difluoro dipyridomethene boron complexes are presented and two series (a and b) of novel fluorophores are extensively studied. Halogenated derivatives were reacted under Suzuki-Miyaura or Sonogashira cross coupling reactions to introduce electron-donating or electron-withdrawing functional groups on positions 1 and 2 of the aromatic ligand. All derivatives were obtained in 14-90% yields and studied in detail by structural, photophysical, and computational analyses. Both series display excellent emissive properties in solution with blue to orange fluorescence emission upon blue light absorption and promising features as solid emitters. All the spectroscopic measurements are supported and confirmed by first-principles theoretical calculations combining TD-DFT and CC2. Series b, featuring an aryl substituent onto position 1 of the aromatic core, showed significantly large Stokes shifts values.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

A series of boron difluoride complexes of azinylcarbazoles: synthesis and structure-property relationships

A series of boron difluoride (BF₂) complexes of azinylcarbazoles 1b-1h were synthesized, and the effects of the structure of azine moieties on the photophysical and electrochemical properties of the BF₂ complexes were clarified. UV-vis analysis of 1b with quinoline, 1c with isoquinoline, and fully fused 1d revealed that fusion with a benzene ring to a pyridylcarbazole BF₂ complex (1a) resulted in red shifts of longest-maximum absorption wavelengths (λ_max). UV-vis analysis of 1e and 1f with pyrimidine, 1g with pyridazine, and 1h with pyrazine revealed that substitution of a carbon atom to a nitrogen atom in 1a also resulted in red shifts of λ_max. The fluorescence quantum yields (Φ_f) decreased from 1a to 1b-1h, and especially, the fluorescence of 1e, 1g, and 1h was quenched in solution. At 77 K, the emission intensities of 1b-1h were significantly increased compared with those at ambient temperature, and they also exhibited phosphorescence with relatively narrow energy gaps between the singlet and triplet excited states. These results on the emission at 77 K indicate that the quench of fluorescence from 1e, 1g, and 1h at ambient temperature originates from both internal conversions and intersystem crossing. In the solid state, all of the complexes including 1e, 1g, and 1h exhibited emission. Distinctive aggregation-induced emission properties were observed for 1e-1h. Electrochemical measurements revealed that the replacement of the pyridine moiety in 1a with azine moieties reduced electrochemical gaps mainly due to a decrease in the LUMO levels. The effects of azine moieties on electronic structures were also discussed based on theoretical calculations.

Phenothiazine-, Dihydroacridine-, and Acridone-Based Boron Difluoride Complexes: Synthesis and Structure-Property Relationships

Boron difluoride complexes of phenothiazine, 9,10-dihydroacridine, and acridone derivatives with a pyridyl group were synthesized. Their structures, dynamic conformational behaviors, and photophysical properties, including solid-state fluorescence, were disclosed. The effects of the oxidation state of the sulfur atom in the phenothiazine derivatives on their properties were also clarified.

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

Access to tetracoordinate boron-doped polycyclic aromatic hydrocarbons with delayed fluorescence and aggregation-induced emission under mild conditions

Boron-doped polycyclic aromatic hydrocarbons (PAHs) have attracted ongoing attention in the field of optoelectronic materials due to their unique optical and redox properties. To investigate the effect of tetracoordinate boron in PAHs bearing N-heterocycles (indole and carbazole), a facile approach to four-coordinate boron-doped PAHs was developed, which does not require elevated temperature and pre-synthesized functionalized boron reactants. Five tetracoordinate boron-doped PAHs (NBNN-1–NBNN-5) were synthesized with different functional groups. Two of them (NBNN-1 and NBNN-2) could further undergo oxidative coupling reactions to form fused off-plane tetracoordinate boron-doped PAHs NBNN-1f and NBNN-2f. The investigation of photophysical properties showed that the UV/vis absorption and fluorescence emission are significantly red-shifted compared to those of the three-coordinate boron-doped counterparts. In addition, the emission of NBNN-1–NBNN-3 consisted of prompt fluorescence and delayed fluorescence. The compounds NBNN-1f and NBNN-2f showed aggregation-induced emission.

A series of tetracoordinate boron-doped polycyclic aromatic hydrocarbons have been synthesized under mild conditions, featuring delayed fluorescence and aggregation-induced emission.

Recent advances in the synthesis of luminescent tetra-coordinated boron compounds

Tetra-coordinated boron compounds offer a plethora of luminescent materials. Different chelation around the boron center (O,O-, N,C-, N,O-, and N,N-) has been explored to tune the electronic and photophysical properties of tetra-coordinated boron compounds. A number of fascinating molecules with interesting properties such as aggregation induced emission, mechanochromism and tunable emission by changing the solvent polarity were realised. Owing to their rich and unique properties, some of the molecules have shown applications in making optoelectronic devices, probes and so on. This perspective provides an overview of the recent developments of tetra-coordinated boron compounds and their potential applications.

Electron-Deficient Polycyclic π-System Fused with Multiple B←N Coordinate Bonds

An extensive polycyclic π-system with 23 fused rings is synthesized via a highly efficient borylation reaction, in which four B-N covalent bonds and four B←N coordinate bonds are formed in one pot. B←N coordinate bonds not only lock the backbone into a near-coplanar conformation but also decrease the LUMO energy level to around -3.82 eV, demonstrating the dual utility of this strategy for the synthesis of extensive rigid polycyclic molecules and the development of n-type conjugated materials for organic electronics and organic photovoltaics.

Predicting Absorption and Emission Maxima of Polycyclic Aromatic Azaborines: Reliable Transition Energies and Character

Polycyclic aromatic azaborines have potential applications as luminophores, novel fluorescent materials, organic light-emitting diodes, and fluorescent sensors. Additionally, their relative structural simplicity should allow the use of computational techniques to design and screen novel compounds in a rapid manner. Herein, the absorption and emission maxima of twelve polycyclic aromatic BN-1,2-azaborine analogues containing the N-BOH moiety were examined to determine a methodology for reliably predicting both the energy and character (local excitation [LE] vs charge transfer [CT]) of the absorption and emission maxima for these compounds. The necessity of implicit solvation models was also investigated. The cam-QTP(01) functional with a small, double-ζ quality basis set provides reliable data compared to EOM-CCSD/cc-pVDZ single-point computations. Of note, commonly used functionals for these applications (B3LYP and ωB97xD) struggle to provide reliable results for both the energy and LE character of the transitions relative to EOM-CCSD computations.

Fundamental studies to emerging applications of pyrrole-BF2 (BOPHY) fluorophores

This review highlights the up-and-coming pyrrole-BF₂ (BOPHY) fluorophores, with a focus on synthetic procedures, photophysical properties – including structure–property analyses – as well as emerging applications.

Rigid π-Extended Boron Difluoride Complex with Mega-Stokes Shift for Bioimaging

The rational design of a rigid π-extended ligand, suitable for the formation of four-coordinate boron complexes, has led to the synthesis of the fused hexacyclic structure of carbazolo[2,1-c]phenanthridine. The photophysical characterization of the novel fluorophore revealed a significant Stokes shift whose intramolecular charge transfer origin has been corroborated by computational calculations. The usefulness of the reported N,N-difluoroboryl complex as fluorescent probe with large Stokes shift has been demonstrated for cancer cells imaging.

Design and Characterization of New D-A Type Electrochromic Conjugated Copolymers Based on Indolo[3,2-b]Carbazole, Isoindigo and Thiophene Units

Two new donor–acceptor (D–A) type organic conjugated random copolymers were successfully synthesized by three-component Stille coupling polymerization of indolo[3,2-b]carbazole (ICZ), isoindigo (IID) and thiophene units, namely PITID-X (X = 1 and 2), with the controlled monomer feed ratios of 3:1:4 and 1:1:2, respectively. The strategy of incorporating different alkyl-branched donor/acceptor units and raw material feed ratios facilitated the improvement of optical properties, solubility, conjugated structure, and electrochromic performance. Cyclic voltammetry, UV-vis-NIR absorption spectra, kinetic and colorimetric measurements of the spray-coated films were recorded in the fabricated three-electrode cells. The results showed that PITID-2, whose optical/electrical properties were better than that of PITID-1, was the candidate electrochromic material due to low band gap of 1.58 eV accompanying the color changing from cyan (neutral state) to gray (oxidized state). The copolymer also illustrated fast bleaching/coloration response time of 2.04/0.33 and 1.35/1.50 s in a 4 s time interval, high coloration efficiency of 171.52 and 153.08 cm² C⁻¹ and stable optical contrast of 18% and 58% at the wavelength of 675 and 1600 nm, respectively.

Optoelectronic, Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max =427-464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL =525-578 nm) with quantum yields of up to 18 % and lifetimes of 1.1-1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1 =0.62-1.02 V vs. Fc/Fc⁺ ) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Mechanofluorochromic behaviors of triphenylamine functionalized salicylaldimine difluoroboron complexes

The solid-state emission of D–A type difluoroboron complexes is tuned by external forces.