Carlos

An unprecedented catalytic system composed of the Wilkinson catalyst [Rh(PPh₃)₃Cl] and CF₃COOH enabled the highly regioselective cross-coupling of aromatic amines with a variety of heteroarenes through dual CH bond cleavage. This protocol provided a facile and rapid route from readily available substrates to (2-aminophenyl)heteroaryl compounds, which may be conveniently transformed into highly extended π-conjugated heteroacenes. The experimental studies and calculations showed that thianaphtheno[3,2-b]indoles have large HOMO–LUMO energy gaps and low-lying HOMO levels, and could therefore potentially be high-performance organic semiconductors. Herein we report the first use of a rhodium(I) catalyst for oxidative CH/CH coupling reactions. The current innovative catalyst system is much less expensive than [RhCp*Cl₂]₂/AgSbF₆ and could open the door for the application of this approach to other types of CH activation processes.

An old favorite that can’t be beat: An innovative system composed of the Wilkinson catalyst [Rh(PPh₃)₃Cl] and CF₃COOH enabled the highly regioselective oxidative CH/CH cross-coupling of aromatic amines with heteroarenes (see scheme). The products could be conveniently transformed into highly extended π-conjugated heteroacenes with large HOMO–LUMO energy gaps and low-lying HOMO levels as promising organic semiconductors.

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)-dimesitylborylvinyl groups (4 V–5 V), and five thiophene rings with two terminal aryldimesitylboryl groups (5 B), as well as an analogue of 5 V with a central EDOT ring (5 VE), have been synthesized via Pd-catalyzed cross-coupling reactions in high yields (66–89 %). Crystal structures of 4 V, 5 B, bithiophene 2 V, and five thiophene-derived intermediates are reported. Chromophores 4 V, 5 V, 5 B and 5 VE have photoluminescence quantum yields of 0.26–0.29, which are higher than those of the shorter analogues 1 V–3 V (0.01–0.20), and short fluorescence lifetimes (0.50–1.05 ns). Two-photon absorption (TPA) spectra have been measured for 2 V–5 V, 5 B and 5 VE in the range 750–920 nm. The measured TPA cross-sections for the series 2 V–5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V–5 V with the related compounds 5 B and 5 VE, giving insight into the structure–property relationship for this class of chromophore. DFT and TD-DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two-photon-excited fluorescence applications.

Large two-photon absorption: Oligothiophene-based quadrupolar chromophores with BMes₂ terminal groups have been synthesized in high isolated yields (see scheme). These compounds have large two-photon absorption (TPA) cross-sections in the NIR region. Experimental results show that the TPA cross-section increases with increasing length of the bridge. DFT and TD-DFT calculations complement the experimental findings and contribute to their interpretation. (V=with terminal (E)-dimesitylborylvinyl groups; VE=with terminal (E)-dimesitylborylvinyl groups and with a central 3,4-ethylenedioxythiophene ring)

The synthesis, structural, and photophysical properties of a new series of original dyes based on 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) is reported. Upon photoexcitation, these dyes exhibit intense dual fluorescence with contribution from the enol (E*) and the keto (K*) emission, with K* being formed through excited-state intramolecular proton transfer (ESIPT). We show that the ratio of emission intensity E*/K* can be fine-tuned by judiciously decorating the molecular core with electron-donating or -attracting substituents. Push–pull dyes 9 and 10 functionalized by a strong donor (nNBu₂) and a strong acceptor group (CF₃ and CN, respectively) exhibit intense dual emission, particularly in apolar solvents such as cyclohexane in which the maximum wavelength of the two bands is the more strongly separated. Moreover, all dyes exhibit strong solid-state dual emission in a KBr matrix and polymer films with enhanced quantum yields reaching up to 54 %. A wise selection of substituents led to white emission both in solution and in the solid state. Finally, these experimental results were analyzed by time-dependent density functional theory (TD-DFT) calculations, which confirm that, on the one hand, only E* and K* emission are present (no rotamer) and, on the other hand, the relative free energies of the two tautomers in the excited state guide the ratio of the E*/K* emission intensities.

La synthèse, les études structurales et photophysiques d’une nouvelle série de fluorophore basée sur une architecture 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) sont décrites. Après photoexcitation, ces composés présentent une intense émission duale issue de l’émission simultanée des états excités des formes énol (E*) et kéto (K*). La forme K* est formée après un transfert de proton intramoléculaire dans l’état excité (ESIPT). Nous montrons que le ratio des intensités d’émission E*/K* peut être finement modulé par un choix judicieux de décoration sur le squelette moléculaire avec des substituents fortement donneur ou attracteur. Les colorants push–pull 9 et 10 fonctionnalisés par un donneur fort (nNBu₂) sur la partie phénol et un accepteur fort (CF₃ et CN respectivement) sur la partie benzoxazole présentent une émission duale intense, et ceci plus particulièrement dans des solvants apolaires tels que le cyclohexane, cas où la séparation entre les deux pics d’émission est la plus importante. Tous ces composés présentent en plus, une importante émission à l’état solide, que ce soit dans une matrice de KBr ou dans un film polymère, avec un rendement quantique de fluorescence allant jusqu’à 54 %. Une sélection pertinente de substituents permet d’obtenir une émission blanche en solution ou à l’état solide. Enfin, ces résultats expérimentaux ont été confirmés par TD-DFT (time-dependent density functional theory); ces études montrent non seulement que seules les émissions des espèces E* et K* sont présentes durant le processus (pas de rotamère), mais aussi que l’énergie libre relative des deux tautomères dans l’état excite guide le ratio des intensités d’émission E*/K*.

Seeing white: A series of 2-(2′-hydroxybenzofuran)benzoxazole dyes that exhibit efficient excited-state intramolecular proton transfer (ESIPT) emission both in solution and in the solid state is reported. Fine-tuning the substituents around the π-conjugated core provides broad, dual enol/keto (E*/K*) emission bands to result in the production of white light (see figure).

Detection of trace amounts of explosive materials is significantly important for security concerns and pollution control. Four multicomponent metal–organic frameworks (MOFs-12, 13, 23, and 123) have been synthesized by employing ligands embedded with fluorescent tags. The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density in the MOFs. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by single-crystal XRD. Single-crystal structures of the MOFs-12 and 13 showed the formation of three-dimensional porous networks with the aromatic tags projecting inwardly into the pores. These electron-rich MOFs were utilized for detection of explosive nitroaromatic compounds (NACs) through fluorescence quenching with high selectivity and sensitivity. The rate of fluorescence quenching for all the MOFs follows the order of electron deficiency of the NACs. We also showed the detection of picric acid (PA) by luminescent MOFs is not always reliable and can be misleading. This attracts our attention to explore these MOFs for sensing picryl chloride (PC), which is as explosive as picric acid and used widely to prepare more stable explosives like 2,4,6-trinitroaniline from PA. Moreover, the recyclability and sensitivity studies indicated that these MOFs can be reused several times with parts per billion (ppb) levels of sensitivity towards PC and 2,4,6-trinitrotoluene (TNT).

Electronic tagging: Four multicomponent metal–organic frameworks (MOFs) have been synthesized by employing ligands embedded with fluorescent tags (see figure). The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density. The phase purity of the MOFs was established by PXRD, NMR spectroscopy, and finally by single-crystal XRD.

A butterfly-like phosphorescent platinum(II) binuclear complex can undergo a molecular structure change in which the Pt–Pt distance shortens upon photoexcitation, which leads to the formation of two distinct excited states and dual emission in the steady state, that is, greenish-blue emission from the high-energy excited state at the long Pt–Pt distance and red emission from the low-energy excited state at the short Pt–Pt distance. This photoinduced molecular structure change has a strong dependence on the molecule’s surrounding environment, allowing its application as self-referenced luminescent sensor for solid–liquid phase change, viscosity, and temperature, with greenish-blue emission in solid matrix and rising red emission in molten liquid phase. With proper control of the surrounding media to manipulate the structural change and photophysical properties, a broad white emission can be achieved from this molecular butterfly.

The white brothers: A molecular “butterfly”, that is, a phosphorescent Pt^II binuclear complex, can flap its “wings” and generate dual (white) emission upon photoexcitation. This photoinduced molecular structure change results from the shortening of the Pt–Pt distance in the excited state and is phase-dependent allowing application of the complex as a self-referenced luminescent sensor for phase change, temperature, and viscosity.

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl- and 5-hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺, are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂⋅AlCl₃, rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7-position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺. All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄⁻ resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl₄] with 1-pentyne exclusively yielding the cyclotrimerised product, 1,3,5-tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn-1,2-carboboration of 3-hexyne. DFT calculations at the M06-2X/6-311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2-carboboration relative to 1,1-carboboration.

1,2-1,2-Testing: Novel N,O-chelated borenium cations react with 3-hexyne to give trisubstituted alkenes as a rare example of a transition metal-free 1,2-carboboration (see figure).

We designed, synthesized, and characterized a new Zr-based metal–organic framework material, NU-1100, with a pore volume of 1.53 ccg⁻¹ and Brunauer–Emmett–Teller (BET) surface area of 4020 m²g⁻¹; to our knowledge, currently the highest published for Zr-based MOFs. CH₄/CO₂/H₂ adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g⁻¹, which corresponds to 43 g L⁻¹. The volumetric and gravimetric methane-storage capacities at 65 bar and 298 K are approximately 180 v_STP/v and 0.27 g g⁻¹, respectively.

Natural-gas vehicle: A new Zr-based metal–organic framework material, NU-1100, with a pore volume of 1.53 ccg⁻¹ and Brunauer–Emmett–Teller (BET) surface area of 4020 m²g⁻¹ was designed, synthesized, and characterized. CH₄/CO₂/H₂ adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions (see figure).

In this study the scope of the 1,1-carboboration reaction was extended to the preparation of mixed heterole-based conjugated π-systems. Two arylbis(alkynyl)phosphane starting materials 2 were synthesized bearing two thiophene isomers at the alkyne units and the bulky tipp-substituent (tipp=2,4,6-triisopropylphenyl) at the phosphorous atom. The bis(thienylethynyl)phosphanes 2 were converted into the corresponding 2,5-thienyl-substituted 3-borylphospholes 4 in a double 1,1-carboboration reaction sequence employing the strongly electrophilic B(C₆F₅)₃ reagent under mild reaction conditions. Subsequent Suzuki–Miyaura type cross-coupling yielded the corresponding 3-phenylphospholes 7 in a one-pot procedure from phosphanes 2 in high yields. Phospholes 7 were converted into the respective phosphole oxides 8. A photophysical characterization of derivatives 7 and 8 was carried out. The results presented here demonstrate the suitability of the 1,1-carboboration reaction for the preparation of phosphole-/thiophene-based, light-emitting systems.

Pulling a crowd: Highly substituted thiophene–phosphole–thiophene monomers and their P-oxides are accessible in high overall yields by a one-pot 1,1-carboboration/Suzuki–Miyaura-type cross-coupling sequence and subsequent H₂O₂ oxidation (see scheme, tipp=2,4,6-triisopropylphenyl). The photophysical properties were determined in solution, in the solid state, and in a poly(methyl methacrylate) matrix.