Isomerism, Diradical Signature, and Raman Spectroscopy: Underlying Connections in Diamino Oligophenyl Dications.

A diradical dication of a 4,4'-di(bis(1,4-methylphenyl)amino)-p-terphenyl oligomer has been characterized in solid-state by Raman spectroscopy and thermo-spectroscopy together with quantum chemical calculations. The diradical character has been evaluated on the basis of the Raman spectra and as a function of temperature. A complete understanding of the nature of the changes in solid state has been provided based on a pseudo-Jahn-Teller effect, which is feasible owing to the fine balance between quinoidal/aromatic extension among consecutive rings and steric crowding. This study contributes to the further comprehension of the molecular and electronic structures of these particular diradical molecules with strong implications on the understanding of the nature of chemical bonds in the limits of high electronic correlation or π-conjugation.

Fluorenyl Based Macrocyclic Polyradicaloids.

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

Stable 2D anti-ferromagnetically coupled fluorenyl radical dendrons.

We report the first class of stable, two-dimensional (2D) anti-ferromagnetically coupled dendritic polyradicaloids. A kinetically blocked fluorenyl radical was used to build up the first and second generation dendrons FR-G1 and FR-G2 containing three and seven fluorenyl units, respectively. The neighboring fluorenyl radicals in FR-G1 and FR-G2 show moderate anti-ferromagnetic exchange interaction, resulting in a doublet and quartet ground state, respectively, with a small doublet-quartet energy gap. From FR-G1 to FR-G2, the energy gap decreased and the two-photon absorption was enhanced owing to more extended 2D π-conjugation. Both compounds showed multiple redox waves due to their polyradical character.

Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods.

The description of low-lying ππ* states of linear acenes by standard electronic structure methods is known to be challenging. Here, we broaden the framework of this problem by considering a set of fused heteroaromatic rings and demonstrate that standard electronic structure methods do not provide a balanced description of the two (typically) lowest singlet state (La and Lb) excitations. While the Lb state is highly sensitive to correlation effects, La suffers from the same drawbacks as charge transfer excitations. We show that the comparison between CIS/CIS(D) can serve as a diagnostic for detecting the two problematic excited states. Standard TD-DFT and even its spin-flip variant lead to inaccurate excitation energies and interstate gaps, with only a double hybrid functional performing somewhat better. The complication inherent to a balanced description of these states is so important that even CC2 and ADC(2) do not necessarily match the ADC(3) reference.

In Silico Design of Monomolecular Drug Carriers for the Tyrosine Kinase Inhibitor Drug Imatinib Based on Calix- and Thiacalix[n]arene Host Molecules: A DFT and Molecular Dynamics Study.

The use of functionalized calix- and thia-calix[n]arenes is proposed as the basis for our in silico design of a suitable drug carrier for the tyrosine kinase inhibitor, Imatinib. Their mutual electronic properties and interaction energies, Eint, were assessed with the use of Density Functional Theory (DFT) methods under the NBODel methodology. Three structural variables for the host molecules were considered: R = {SO3H (1), t-Bu (2), i-Pr (3), COOH (4), (CH2)2OH (5), (CH2)2NH2 (6)}; b = {CH2, S}; n = {5, 6, 8}, and two possible orientations for the insertion of Imatinib within the macrocycle cavity: pyridine moiety pointing inward (N1) and piperazine pointing inward (N2). In total, we explored 72 different assemblies. Initial molecular mechanics geometry optimizations with the UFF potential were undertaken for every host-guest complex, with further optimization at the B97D/6-31G(d,p) level of theory. Using the same optimized structures, Molecular Dynamics (MD) simulations were carried out on all 72 assemblies using the General Amber Force Field and the AMBER 12 MD package. Electronic parameters were fitted using the RESP method, and the complexes were run for 100 ns. Potential of mean force was obtained for the most stable systems using umbrella sampling and the Weighted Histogram Analysis Method. Calix[n]arenes families 1 and 5 (R = SO3H and (CH2)2OH, respectively) with n = 6 constitute the most promising candidates to become drug carriers within our parameter space due to their more negative Eint values and increased flexibility to allow the inclusion of the drug.