From Hexaoxy-[6]Pericyclynes toCarbo-Cyclohexadienes,Carbo-Benzenes, and Dihydro-Carbo-Benzenes: Synthesis, Structure, and Chromophoric and Redox Properties

Strong absorber vs. strong emitter in extended π-conjugated systems: a carbo-benzene – benzothiadiazole chromophore

In the search for photoluminescent carbo-benzenes, with the view to measuring the two-photon absorption cross-section by the TPEF method, thienylbenzothiadiazole groups are envisaged as intrinsic fluorophoric substituents through acetylenic linkers.

Carbo-mer of Barrelene: A Rigid 3D-Carbon-Expanded Molecular Barrel

After extensive studies of 1D and 2D skeletal carbo-mers based on C₈ π-conjugating dialkynylbutatriene units (DABs: ∼C≡C-(R)C=C=C=C(R)-C≡C∼) bridging sp or sp² centers in carbo-butene, carbo-xylylene or carbo-benzene derivatives, 3D versions are envisaged through carbo-barrelenes and partially reduced derivatives thereof where two or three DAB blades span a bridge between sp³ carbinol vertices or ether thereof. For R=Ph, stable representatives were synthesized through a pivotal [6]pericyclynedione, and extensively characterized by spectroscopic, electrochemical and crystallographic methods. Density functional theory calculations allow detailed analysis of structural and electronic features of the 7 Å high C₂₆ barrel-shaped molecules, and show that they can behave as cages for ionic species. Beyond aesthetical concerns, the results could open prospects of applications in host-guest supramolecular chemistry and single molecule charge transport.

Steric/π-Electronic Insulation of the carbo-Benzene Ring: Dramatic Effects of tert-Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

Hexa-tert-butyl-carbo-benzene (C₁₈ tBu₆ ) and three phenylated counterparts (C₁₈ tBu_m Ph_6-m ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C₁₈ core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference (m=0). Anchoring tBu substituents at the C₁₈ macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo-benzenes reported previously were obtained as dark-reddish/greenish solids, crystals and solutions of C₁₈ tBu₆ happen to be yellow (λ_max =379 vs. 472 nm for C₁₈ Ph₆ ). In comparison to C₁₈ Ph₆ , the reduction of C₁₈ tBu₆ remains reversible, but occurs at twice as high an absolute potential (E_1/2 =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C₁₈ ring symmetry is the nearest to D_6h for m=6, which indicates a maximum geometric aromaticity. According to calculated nucleus-independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C₁₈ tBu₆ : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C₁₈ H₆ and C₁₈ F₆ , and -13.5 ppm for C₁₈ Ph₆ . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ_max , and chemical hardness, η=E_LUMO -E_HOMO , are evidenced.

Carbo-biphenyls and Carbo-terphenyls: Oligo(phenylene ethynylene) Ring Carbo-mers

Ring carbo-mers of oligo(phenylene ethynylene)s (OPEn, n=0-2), made of C₂ -catenated C₁₈ carbo-benzene rings, have been synthesized and characterized by NMR and UV-vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT-optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo-terphenyls (n=2) are reversibly reduced at ca. -0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo-benzene (-0.83 V/SCE), thus revealing efficient inter-ring π-conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (-0.30 V/SCE) for a carbo-quaterphenyl congener (n=3). Increase with n of the effective π-conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT-calculated excited states, one of them restricting to a HOMO→LUMO main one-electron transition.

A difluorenyl-carbo-cyclohexadiene: prospective chromophore for two-photon absorption

For the purpose of outlining structure-property relationships for two-photon absorption (2PA), a "s-locked" carbo-cyclohexadiene with two fluorenyl substituents has been envisaged for comparison with previously studied aromatic carbo-benzene and non-aromatic carbo-quinoid congeners. A representative where the C10-π-conjugated fluorenyl moieties are also connected by a C8-π-insulating 3,6‐dimethoxy‐3,6‐bis(trifluoromethyl)octa‐1,4,7‐triyn-1,8-diyl edge has thus been synthesized in four steps from known C8F triyne and C10 triynyldial, through a [8F+10] cyclization process. In spite of a relatively strong absorbance (e = 84 800 L.mol-1.cm-1 at 634 nm), the non-vanishing green fluorescence (at 533 nm) of the chromophore should allow measurements of the 2PA cross section by both the TPEF and Z-scan methods.

Selective access to p-dialkyl-carbo-benzenes from a [6]pericyclynedione: the n-butyl nucleophile model for a metal switch study

The synthesis, spectroscopic properties, comparative electrochemical behavior in CHCl3 vs CH2Cl2, and X-ray crystal structure of p-di-n-butyl-tetraphenyl-carbo-benzene are described. The selectivity of preparation of the ultimate [6]pericyclynediol precursor has been examined by comparing the reactivity of the [6]pericyclynedione substrate with four n-Bu-MXn nucleophiles involving more or less halogenated metal centers MXn (0 ≤ n ≤ 2): Li, MgBr, MgCl, CeCl2/LiCl. The cerium reagent is found to be the most efficient, giving approximately twice the yield given by Grignard reagents in the target diadduct (90% vs 51%–53%). The dibutyl-carbo-benzene product happens to be soluble in both CHCl3 and CH2Cl2: cyclic voltammograms of either solution exhibit almost identical peak potentials with reduced reversibility of the redox processes in CHCl3.

Extended germa[N]pericyclynes: synthesis and characterization

We herein describe the syntheses and characterization of extended germa[N]pericyclynes, which are macrocycles composed of germanium-butadiyne units. The obtained novel extended germa[4]-[8]pericyclynes were characterized by X-ray crystallography, UV-Vis spectroscopy, fluorescence and phosphorescence emission spectroscopy, and cyclic voltammetry, and exhibited characteristic absorptions and emissions. Density functional theory (DFT) calculations suggested smaller HOMO-LUMO gap energy compared to that of general germapericyclynes.

Hexaaryl-carbo-benzenes revisited: a novel synthetic route, crystallographic data, and prospects of electrochemical behavior

Hexaphenyl-carbo-benzene and a substituted homologue were prepared via an efficient route, and characterized by crystallography and voltammetry with the view of studying charge transport properties within thin films.

Truncated Transition Densities for Analysis of (Nonlinear) Optical Properties of carbo-Chromophores

The optical properties of several quadrupolar carbo-benzene derivatives are investigated at various levels of calculation (TDDFT and CASPT2) and analyzed using a new theoretical tool here disclosed: The "visualization" of the transition dipole moment from the transition density truncated to the main monoexcitations involved in the electronic transition (TTD). The experimental or calculated one-photon UV-visible absorption spectra of the carbo-benzene derivatives fit with the Gouterman model originally proposed for porphyrins, where the first four excited states involve linear combinations of monoexcitations of the same four frontier molecular orbitals. The relative intensities of the absorption bands are analyzed from the transition dipole moments calculated from the TTDs and an analogy between porphyrins and carbo-benzenes is argued. The two-photon absorption (TPA) cross section related to the third-order nonlinear optical response is calculated for each two-photon-allowed excited state |f⟩ from the contribution of all possible intermediate excited states |i⟩ using the "sum-over-state" (SOS) scheme. The quadrupolar carbo-benzene derivatives fit into the three-level model, as their TPA cross section exhibits a dominant contribution of one of the intermediate excited states. The origin of TPA efficiency (enhancement) upon carbo-merisation of the C-C link to the para-substituents is discussed from the excitation energies of the intermediate and final excited states and from the two corresponding transition dipole moments (μ0i and μif). The latter may be calculated from the TTDs.

Difluorenyl carbo-Benzenes: Synthesis, Electronic Structure, and Two-Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores

The synthesis, crystal and electronic structures, and one- and two-photon absorption properties of two quadrupolar fluorenyl-substituted tetraphenyl carbo-benzenes are described. These all-hydrocarbon chromophores, differing in the nature of the linkers between the fluorenyl substituents and the carbo-benzene core (C-C bonds for 3 a, C-C=C-C expanders for 3 b), exhibit quasi-superimposable one-photon absorption (1PA) spectra but different two-photon absorption (2PA) cross-sections σ2PA. Z-scan measurements (under NIR femtosecond excitation) indeed showed that the C≡C expansion results in an approximately twofold increase in the σ2PA value, from 336 to 656 GM (1 GM = 10(-50) cm(4) s molecule(-1) photon(-1)) at λ = 800 nm. The first excited states of Au and Ag symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum-over-state estimations of σ2PA(λi), in which λi = 2 hc/Ei, h is Planck's constant, c is the speed of light, and Ei is the energy of the 2PA-allowed transition. The calculated σ2PA values of 227 GM at 687 nm for 3 a and 349 GM at 708 nm for 3 b are in agreement with the Z-scan results.

Stepwise synthesis and characterization of germa[4], [5], [8], and [10]pericyclynes

The stepwise syntheses of germa[N]pericyclynes, including [5]pericyclynes, and their characterization are described. The yields of germa[4] and [8]pericyclynes were improved significantly compared to those obtained in previous studies. The routes reported herein afforded the novel germa[5] and [10]pericyclynes, which were characterized by X-ray crystallography, UV-Vis spectroscopy, and fluorescence emission spectroscopy. A unique fluorescence emission was observed for the large germa[10]pericyclyne ring.

Carbo-cyclohexadienes vs. carbo-benzenes: structure and conjugative properties

Ideally Cs-/C2v-symmetric chromophores, constituted by two electro-active groups conjugated through the carbo-mer of the cyclohexa-1,3-diene core, are selectively prepared by the SnCl2-mediated reduction of tailored hexaoxy-[6]pericyclynes: in the latter substrates, one of the 1,4-dioxybut-2-yne edges is "chemically locked" by two CF3 substituents preventing complete reduction to the corresponding aromatic carbo-benzenic core, which is expected to be more "π-insulating" between the electro-active ends. The bis-trifluoromethylated carbo-cyclohexadiene products are also shown to be significantly stabilized with respect to their bis-phenylated analogues. Their structural (crystal X-ray diffraction analyses), spectroscopical (NMR and UV-vis spectra), physio-optical (dichromism in solution) and electrochemical (cyclic voltammograms) properties are compared on the basis of the electron-donating/electron-withdrawing nature of the substituents. These properties are also compared with those of their aromatic carbo-benzene and flexible carbo-n-butadiene counterparts.

First example of ring carbomer of 1,4-cyclohexadiene

While a series of carbo-mers of 1,3-cyclohexadienes was reported through the use of a specifically developed synthetic strategy, no example of their 1,4-regioisomers was known. Inspired by the methodology elaborated for the preparation of the 1,3-isomers, the synthesis of the first example of carbo-mer of 1,4-cyclohexadiene is presented. Comparison of physico-chemical properties of this first representative with those of the recently described 1,4-regioisomer, especially UV-vis absorption properties, is also addressed.

“Carbo-aromaticity” and novel carbo-aromatic compounds

Recent advances in experimental and theoretical studies ofcarbo-benzene derivatives, along with the proposition of a generalization of the definition of aromaticity to the two-membered π-rings of triple bonds, suggest relevance for the notion of “carbo-aromaticity”.

First insights into the synthesis of carbo-phospholane and carbo-phospholene oxides

Fifteen-membered ring carbo-mers of five-membered rings are considered in the heterocyclic series of the phosphole oxide and less unsaturated parents. The synthesis of the first carbo-phospholane oxides is achieved by a [14+1] two-step/one-pot macrocyclization route with 86 % yield. Reduction of the latter phosphora-[5]pericyclyne with SnCl­2 allowed consistent 1H and 31P NMR characterization of the corresponding carbo-phospholene, produced with 11 % yield. The ultimate carbo-phosphole oxide could not be isolated, but preliminary results on alternative strategies towards this 14 pz-electron Hückel carbo-aromatic are reported.

En route to a dianilinyl-substituted carbo-cyclohexadiene with promising electrical properties

The macro-aromatic carbo-benzene core para-disubstituted by 4-anilinyl groups is known to be an efficient single-molecule conductor, exhibiting a conductance of 106 nS measured by the scanning tunneling microscopy-break junction technique. The linear carbo-butadiene analogue bearing the same anilinyl substituents was found to be less efficient, with a conductance of 2.7 nS. The reason of this difference could be elucidated through the study of the charge transport properties of a cyclically locked carbo-butadiene core in a carbo-cyclohexadiene derivative. In this paper, advances in the synthesis of this challenging dianilinyl-substituted carbo-cyclohexadiene are presented.

On terminal alkynylcarbinols and derivatization thereof

The chemistry of three prototypes of secondary alkynylcarbinols (ACs), recently highlighted as challenging targets in anti-tumoral medicinal chemistry, is further documented by results on n-alkyl, alkynyl and alkenyl representatives. The N-naphthyl carbamate of an n-butyl-AC is thus characterized by X-ray crystallography. A novel dialkynylcarbinol (DAC) with synthetic potential is described, namely the highly dissymmetrical triisopropylsilyl-protected version of diethynylmethanol. The latter is shown to act as a dipolarophile in a selective Huisgen reaction with benzyl azide under CuAAC click conditions, giving an alkenyl-AC, where the alkene unsaturation is embedded in a 1,4-disubstituted 1,2,3-triazole ring, as confirmed by X-ray crystallography.

Synthesis and characterization of germa[n]pericyclynes

The synthesis and characterization of novel pericyclynes comprising germanium atoms and acetylenes, germa[n]pericyclynes, are described. The prepared germa[4]-, [6]-, and [8]pericyclynes were compared by (13)C NMR spectroscopy, X-ray crystallography, cyclic voltammetry, UV-visible spectroscopy, fluorescence emission spectroscopy, Raman spectroscopy, and density functional theory calculation analyses.

Light-harvesting capabilities of low band gap donor–acceptor polymers

The ability of three new and six existing donor–acceptor polymers to absorb strongly at low photon energies is studied experimentally and theoretically, and is related to their chemical structures.