A Combined Experimental-Computational Investigation to Uncover the Puzzling (Chiro-)optical Response of Pyridocyclophanes: One- and Two-Photon Spectra.

A combined experimental-computational analysis of the one- and two-photon absorption (OPA, TPA) and the electronic circular dichroism (ECD) of complex chiral shape-persistent (2,6)-pyrido[142 ]allenoacetylenic cyclophane, (P,P,P,P)-MC[142 ] enantiomers, sheds light on the origin of their peculiar spectral signatures. The study on MC[142 ], which is represented in solution by three possible conformers, indicates that two of them (chair and twist) are the prevalent conformers at room temperature; the population ratio depending on the solvent. The complex shape of the red-edge of the UV and ECD spectra is qualitatively reproduced by using vibronic calculations and assigned to progressions on the ethynyl stretchings observable only in the chair and twist conformers. The picture arising from the analysis of the linear spectra is confirmed by two-photon spectroscopies, showing that they constitute valuable tools for the exploration of high-energy electronic transitions in pyridoallenoacetylenic cyclophanes. Discrepancies among computed and experimental spectra point out that an accurate description of such complex systems with a large delocalized π framework and a dense manifold of electronic excited state remains a challenge for theoretical models.

Conformational stable alleno-acetylenic cyclophanes bearing chiral axes.

The design and synthesis of chiral cyclophanes containing signaling functional groups as an integral part of the macrocyclic framework offer promising possibilities for chiral sensing, molecular recognition, and chiral supramolecular architectures. Our research group has been involved in the construction and study of chiroptical properties of several allenic meta- and para-cyclophanes bearing anthracene and pyridine rings as spacers. A revision of our results is presented.

Rotation-locked 2,6-pyrido-allenophanes: characterization of all stereoisomers.

New 2,6-disubstituted pyrido-allenophanes with locked rotation of aromatic spacers were designed and synthesized. The synthesis was accomplished by Pd-catalyzed C(sp(2))-C(sp) Sonogashira cross-coupling reaction between 1,3-diethynylallene (DEA) and 2,6-dibromopyridine followed by an intermolecular ring closure. Because racemic DEA was employed, pyrido-allenophanes were obtained as mixtures of stereoisomers that were resolved by preparative HPLC. The conformational space of all these diastereoisomers was explored at the CAM-B3LYP/6-31+G*//AM1 level of theory. The isomers were characterized through their symmetry properties revealed in NMR, circular dichroism, and chiral stationary-phase HPLC experiments. X-ray diffraction was used to assign and to corroborate the configuration of several diastereoisomers. The unexpected encapsulation of two molecules of CHCl(3) in the crystal structures shows the potential of these conformationally hampered allenophanes as encapsulating hosts.

Theoretical and experimental exploration of the photochemistry of resveratrol: beyond the simple double bond isomerization.

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has been detected in various studies although its nature was unknown. In this work, we aim to provide a thorough description of the photochemical course of this reaction through experimental and computational approaches working in a synergetic association.

Experimental and computational exploration of indolizinyl carbene generation. A route to biindolizines.

In previous work, (E)-2-enynyl pyridines were reported to yield indolizinyl singlet carbenes through base-catalyzed E/Z isomerization followed by a 5-exo-dig pseudocoarctate cyclization. We report herein that in the presence of ethyl acrylate these carbenes undergo stereoselective cis-cyclopropanation due mainly to electrostatic interactions in the transition state. The scope of this carbene generation scheme has been further explored through the preparation of a symmetric bis(pyridylenyl)alkyne which spontaneously furnished the biindolizine core in a one-pot reaction. Computational characterization of this transformation suggests a highly asynchronous double cyclization.

Preparation and characterization of a halogen-bonded shape-persistent chiral alleno-acetylenic inclusion complex.

A chiral bidentate inclusion complex has been formed by halogen-bond interaction between the pyridyl moieties of a pyridoallenoacetylenic host and octafluorodiiodobutane. X-ray crystallography showed that the guest adopts a chiral conformation inside the molecular channels formed by stacking of the host units. A 10 ppm shielding of the (15)N NMR resonance for the pyridil units provided evidence of the formation of the halogen-bond complex in solution.

Morphological self-assembly of enantiopure allenes for upstanding chiral architectures at interfaces.

Chiroptically active allenes are employed for the construction of surface-confined nanostructures. Morphological complementarity between the homochiral units leads to self-assembly of two highly-ordered, upstanding, diastereomeric architectures. The novel, intertwined self-assembled layer structures feature reactive terminal alkynes for further functionalization and carry potential for widespread applications exploiting chiroptical amplification.