Emerging Bottom-Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures.

The solution-phase synthesis is one of the most promising strategies for the preparation of well-defined graphene nanoribbons (GNRs) in large scale. To prepare high quality, defect-free GNRs, cycloaromatization reactions need to be very efficient, proceed without side reaction and mild enough to accommodate the presence of various functional groups. In this Minireview, we present the latest synthetic approaches for the synthesis of GNRs and related structures, including alkyne benzannulation, photochemical cyclodehydrohalogenation, Mallory and Pd- and Ni-catalyzed reactions.

Toward Thiophene-Annulated Graphene Nanoribbons.

Narrow thiophene-edged graphene nanoribbons (GNRs) were prepared from polychlorinated thiophene-containing poly(p-phenylene)s using the photochemical, metal-free cyclodehydrochlorination (CDHC) reaction. 1 H NMR and Raman spectroscopy confirmed the structures of the GNRs. The regioselectivity of the CDHC reaction allows the preparation of both laterally symmetrical and unsymmetrical GNRs and, consequently, the modulation of their optical and electronic properties.

Helically Coiled Graphene Nanoribbons.

Graphene is a zero-gap, semiconducting 2D material that exhibits outstanding charge-transport properties. One way to open a band gap and make graphene useful as a semiconducting material is to confine the electron delocalization in one dimension through the preparation of graphene nanoribbons (GNR). Although several methods have been reported so far, solution-phase, bottom-up synthesis is the most promising in terms of structural precision and large-scale production. Herein, we report the synthesis of a well-defined, helically coiled GNR from a polychlorinated poly(m-phenylene) through a regioselective photochemical cyclodehydrochlorination (CDHC) reaction. The structure of the helical GNR was confirmed by 1 H NMR, FT-IR, XPS, TEM, and Raman spectroscopy. This Riemann surface-like GNR has a band gap of 2.15 eV and is highly emissive in the visible region, both in solution and the solid state.

Regioselective Synthesis of Nanographenes by Photochemical Cyclodehydrochlorination.

Novel nanographenes were prepared by a photochemical cyclodehydrochlorination (CDHC) reaction. Chlorinated precursors were irradiated in acetone in the presence of a base or in pure benzene and underwent multiple (up to four) regioselective cyclization reactions to provide rigid π-conjugated molecules. Pure compounds were recovered in good yields by simple filtration at the end of the reaction. The CDHC reaction showed compatibility with both electron-poor and electron-rich substrates, thus allowing the synthesis of pyridine- and thiophene-fused nanographenes. It also enabled the synthesis of sterically hindered contorted π-conjugated molecules without causing full aromatization. A kinetic study showed that the CDHC reaction under the conditions used is a very fast process, and some reactions are completed within minutes. The CDHC reaction thus shows great potential as an alternative to other reactions involving harsher conditions for the preparation of nanographenes.

Synthesis of Carboxylate Cp*Zr(IV) Species: Toward the Formation of Novel Metallocavitands.

With the intent of generating metallocavitands isostructural to species [(CpZr)3(µ(3)-O)(µ(2)-OH)3(κO,O,µ(2)-O2C(R))3](+), the reaction of Cp*2ZrCl2 and Cp*ZrCl3 with phenylcarboxylic acids was carried out. Depending on the reaction conditions, five new complexes were obtained, which consisted of Cp*2ZrCl(κ(2)-OOCPh) (1), (Cp*ZrCl(κ(2)-OOCPh))2(µ-κ(2)-OOCPh)2 (2), [(Cp*Zr(κ(2)-OOCPh))2(µ-κ(2)-OOCPh)2(µ(2)-OH)2]·Et2O (3·Et2O), [[Cp*ZrCl2](µ-Cl)(µ-OH)(µ-O2CC6H5)[Cp*Zr]]2(µ-O2CC6H5)2 (4), and [Cp*ZrCl4][(Cp*Zr)3(κ2-OOC(C6H4Br)3(µ3-O)(µ2-Cl)2(µ2-OH)] [5](+)[Cp*ZrCl4](-). The structural characterization of the five complexes was carried out. Species 3·Et2O exhibits host-guest properties where the diethyl ether molecule is included in a cavity formed by two carboxylate moieties. The secondary interactions between the cavity and the diethyl ether molecule affect the structural parameters of the complex, as demonstrated be the comparison of the density functional theory models for 3 and 3·Et2O. Species 5 was shown to be isostructural to the [(CpZr)3(µ(3)-O)(µ(2)-OH)3(κO,O,µ(2)-O2C(R))3](+) metallocavitands.

Improving the reactivity of phenylacetylene macrocycles toward topochemical polymerization by side chains modification.

The synthesis and self-assembly of two new phenylacetylene macrocycle (PAM) organogelators were performed. Polar 2-hydroxyethoxy side chains were incorporated in the inner part of the macrocycles to modify the assembly mode in the gel state. With this modification, it was possible to increase the reactivity of the macrocycles in the xerogel state to form polydiacetylenes (PDAs), leading to a significant enhancement of the polymerization yields. The organogels and the PDAs were characterized using Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM).

On the interaction of acetone with electrophilic metallocavitands having extended cavities.

We report the synthesis and characterization of tantalum-boronate trimetallic clusters of general formula {[Cp*Ta](3)(µ(2)-RB(O)(2))(3)(µ(2)-OH)(µ(2)-O)(2)(µ(3)-OH)} (R= 4-(C(6)H(5))(C(6)H(4)) (Ta(3)-4Ph), 4-(C(6)H(5)O)(C(6)H(4)) (Ta(3)-4OPh), 4-(C(7)H(7)O)(C(6)H(4)) (Ta(3)-4OBn), 4-(C(8)H(5))(C(6)H(4)) (Ta(3)-4PhEt), and 4-(C(12)H(7))(C(6)H(4)) (Ta(3)-4Napht)). All complexes have been characterized by NMR spectroscopy and X-ray diffraction. The trimetallic species feature a large Lewis acid type cavity allowing for substrate binding in both the solid and the liquid state using a unique electrostatic interaction and a hydrogen bond. ΔH° and ΔS° values for association of acetone with the complexes vary between -2.0 and -4.1 kcal·mol(-1) and -3 and 2 cal·mol(-1)·K(-1), respectively, showing weaker binding than smaller cavitands of the same type. The barrier for acetone exchange at equilibrium is similar for all complexes, and ΔH(‡) values vary between 8.2 and 11.4 kcal·mol(-1).

H-bonding-driven gel formation of a phenylacetylene macrocycle.

An amide-containing phenylacetylene macrocycle (PAM) has been synthesized and its gelation properties were studied in different solvents. Surprisingly, this macrocycle forms organogels at low concentration in many polar and apolar solvents. XRD and FTIR analysis suggest that this macrocycle forms stable supramolecular assemblies owing to H-bonding. Scanning electron microscopy analyses show the formation of bundles of nanofibrils, demonstrating the long-range organization of this material.

Rigid organic nanotubes obtained from phenylene-butadiynylene macrocycles.

Rigid organic nanotubes were prepared from six-membered phenylene-butadiynylene macrocycles through topochemical polymerization in the xerogel state. All six butadiyne units underwent polymerization, thus creating rigid nanotubes with six polydiacetylene chains lying parallel, one relative to each other.