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

From Stilbenes to carbo-Stilbenes: an Encouraging Prospect

Beyond previously described carbo-naphthalene and carbo-biphenyl, a novel type of bis-carbo-benzenic molecules is envisaged from the stilbene parent. The synthesis, structure, spectroscopic and electrochemical properties of two such carbo-stilbenes are described at complementary experimental and computational DFT levels. In the selected targets, the bare skeletal carbo-mer of carbo-stilbene is decorated by 8 or 10 phenyl groups, 0 or 2 tert-butyl groups, and 2 n-octyl chains, the later substituents being introduced to compensate anticipated solubility issues. As in the parent stilbene series, isomers of the phenylated carbo-stilbenes are characterized. The cis- and trans-isomers are, however, formed in almost equal amounts and could not be separated by either chromatography or crystallization. Nevertheless, due to a slow interconversion at the NMR time scale (up to 55 °C) the 1H NMR signals of both isomers of the two carbo-stilbenes could be tentatively assigned. The calculated structure of the cis-isomer exhibits a helical shape, consistent with the observed magnetic shielding of phenyl p-CH nuclei residing inside the shielding cone of the facing C18 ring. The presence of the two isomers in solution also gives rise to quite broad UV-vis absorption spectra with main bands at ca 460, 560 and 710 nm, and a significant bathochromic shift for the decaphenylated carbo-stilbene vs the di-tert-butyl-octaphenylated counterpart. Square wave voltammograms do not show any resolution of the two isomers, giving a reversible reduction wave at -0.65 or -0.58 V/SCE, and an irreversible oxidation peak at 1.11 V/SCE, those values being classical for most carbo-benzene derivatives. Calculated NICS values (NICS(1)=-12.5±0.2 ppm) also indicate that the aromatic nature of the C18 rings is not markedly affected by the dialkynylbutatriene (DAB) connector between them.

Theoretical analysis of expanded porphyrins: Aromaticity, stability, and optoelectronic properties

Expanded porphyrin systems are capable of binding a variety of substrates due to their increased cavity size and aromatic nature, holding important applications as magnetic resonance imaging contrast agents and as sensitizers for photodynamic therapy. It is there of fundamental interest to know the photoelectrical properties of expanded porphyrins using quantum chemistry calculations. In this work, we theoretically designed and screened a series of expanded porphyrins by incorporating terthiophene (TTH) and dithienothiophene (DTT) moieties. Our calculations showed that all the designed molecules exhibit excellent optoelectronic performance than the reference molecule. It is suggested that the porphyrin molecule with TTH moiety has better stability than the one with DTT moiety. Finally, we demonstrated that molecule 2 features with TTH moiety and the inverted selenophene ring outperform other molecules because it exhibits increased HOMO-LUMO gap, planar geometry, and strengthened aromaticity. We expect that this work can provide theoretical guidelines for the design of novel porphyrin materials.

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.

Stability, Aromaticity, and Photophysical Behaviors of Macrocyclic Molecules: A Theoretical Analysis

The macrocyclic molecules with terthiophene (TTH) isomers unit exhibit intriguing properties in terms of aromaticity, stability, and absorption. In this work, we theoretically designed a series of macrocyclic molecules featured with TTH and dithienothiophene (DTT) π-conjugated building units, which are used to permute pyrrole unit in porphyrin skeleton. Density functional theory and time-dependent DFT methods are used to evaluate the performance of the designed molecules. Our simulations show that molecules 1–3 exhibit excellent optoelectronic performance. Specifically, the molecule with the DTT unit is more stable than the one with TTH unit in terms of aromaticity and aromatic stabilization energy. This is because DTT unit enhances the coplanarity of the molecular material, facilitating electronic communication. Calculation of vertical electronic excitations suggests the absorption feature of these molecules is mainly contributed by the electronic excitations of higher occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO)+1 and HOMO-1 → LUMO. Judging from the key parameters determining the overall performance, 3 stands out because of its good planarity, large HOMO–LUMO gap, and strong aromaticity among all molecules. Interestingly, molecule 1 has the current density flow distributes around the outer section of TTH unit; in contrast, molecule 3 with DTT unit has the current density flow located at the inner section of DTT, which is beneficial for stability and aromaticity. Second-order perturbation energies are calculated to rationalize this observation. We expect that these research results can provide valuable insights into the rational design of novel molecular materials for a variety of applications.

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.