Hindered rotational barriers in conjugated donor-acceptor substituted systems: calculations vs. experiments

Donor-Acceptor Assembly of Amphiphilic Molecules Based on 9,10-Distyrylanthracene Derivatives with Terminal Naphthalene Groups

Amphiphilic rod-coil compounds have excellent photophysical properties and can be assembled into supramolecular nanostructures of different sizes in water or polar solvents. Herein, we synthesized the amphiphilic compounds 2N-DSA, 4N-DSA, and 6N-DSA with 9,10-distyrylanthracene (DSA) as the core and a naphthalene unit as the terminal group that connected DSA through a tetraethylene glycol chain. These compounds have excellent aggregation-induced emission (AIE) properties in aqueous solution and are assembled into worm-like fragments or different sizes of spherical assemblies, defending the volume ratio of the rod to coil segments. Notably, the donor-acceptor interaction between DSA and electron- deficient compounds 2,4,6-trinitrophenol (TNP), 2,4,5,7-tetranitrofluorenone (TNF), and tetraethylene glycol dinitrobenzoate (TGDNB) forms a charge transfer complex, which can be used as a nanoreactor to improve the yield of the Suzuki coupling reaction about 8-10 times. The experimental results reveal that the synergy effect of the donor-acceptor, intermolecular π-π stacking, and hydrophobic-hydrophilic interactions significantly influences the morphology of aggregates and the efficiency of the nanoreactor.

Conjugated donor–acceptor substituted systems involving the 1,3-indandione-derived electron accepting moieties

Conjugated donor–acceptor molecules are the focus of research owing to their unusual photo- and electro-physical properties. At the same time, several unusual features of these compounds are difficult to explain or predict. Here we present our results on the synthesis, X-ray structures and D-NMR spectra providing a deeper insight into the conjugation within the derivatives involving the 1,3-indandione-derived series of compounds with varying electron acceptor strength and conjugating bridge length. The X-ray structures show the presence of several intermolecular short contacts strongly affecting the molecular geometries. In solution, the coalescence temperatures corresponding to the rotation of the phenylamino moiety of all derivatives do not exceed 246 K indicating the unhindered rotation at room temperature. Using B3LYP/aug-cc-pVDZ, the calculated model chemistry barriers to rotation, dipole moments and first hyperpolarizabilities are within experimental error. We conclude that neglecting the electron donating properties of bridges themselves and internal rotation about the single bonds taking part in conjugation can result, for instance, in misinterpretation of their room temperature NMR spectra and overestimation of the computed molecular dipole moments by more than 5 D.

Determination of barriers to rotation using the D-NMR technique within a series of push–pull derivatives sheds light on the inability of stronger acceptors to considerably increase the dipole moments and nonlinear optical response.

Dipole moments of conjugated donor-acceptor substituted systems: calculations vs. experiments

We find that quantum mechanical calculations using B3LYP/aug-cc-pVTZ model chemistry involving anharmonic correction on simple conjugated organic compounds without rotating moieties provide the dipole moment values and molecular geometries with high accuracy. In the presence of one or two conjugated electron donating or accepting substituents capable of hindered rotation, the calculated dipole moments reproduce the experimental results equally well only in the cases when the experiments were done at the temperatures at which rotation of substituents remains hindered. In order to reproduce the experimental dipole moments determined at higher temperatures, a model assuming free (unhindered) rotation should be applied. In these cases, the contribution of each rotamer is equal and using anharmonic correction is not necessary. The APFD functional produces similar results and the M062X functional yields larger deviations from the experimental data. The other methods, like HF and MP2, are the least accurate with the basis sets usually employed for interpreting the experimental data.

B3LYP functional can reproduce the experimental dipole moments of conjugated organic molecules with high accuracy.