The effect of conformational isomerism on the optical properties of bis(8-oxyquinolato) diboron complexes with a 2,2'-biphenyl backbone

A fluorescent bis(8-oxyquinolato) diborinic complex with a central 2,2'-biphenyl backbone 1 and its octafluoro analogue 2 were synthesized to study the optical-structural relationship of sterically encumbered molecules featuring close intramolecular π-stacking interactions involving chromophore units. The crystal structure of 1 revealed a unique π-stacked arrangement of two pendant phenyl groups and two 8-oxyquinolato ligands (Q) located in the inner part of the complex. Unlike 1, the closely related complex 2 features conformational isomerism, and two major forms, namely 2-syn and 2-anti, are observed in solution to a varying extent depending on the solvent polarity. Form 2-syn, a geometrical analogue of 1, is preferable in polar solutions, whereas its rotational isomer 2-anti featuring π-stacking interactions between the terminal phenyl group and Q ligand dominates in benzene and chloromethane solutions. The observed conformational equilibria strongly affect the optical properties of the system, specifically leading to a significant increase of the quantum yield of emission (from 22% in MeCN to 38% in benzene) accompanied by a bathochromic shift (Δλ = 10 nm) of absorption and hypsochromic shifts (Δλ = -8 nm) of emission spectra with decreasing solvent polarity. This effect was ascribed to the variation in frontier orbital distributions.

Ground-State Charge-Density Distribution in a Crystal of the Luminescent ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline

This contribution is devoted to the first electron density studies of a luminescent oxyquinolinato boron complex in the solid state. ortho-Phenylenediboronic acid mixed with 8-hydroxyquinoline in dioxane forms high-quality single crystals via slow solvent evaporation, which allows successful high resolution data collection (sin θ/λ = 1.2 Å^-1) and charge density distribution modeling. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and to the solvent species exhibiting anharmonic thermal motion. The experiment and theory compared rather well in terms of atomic charges and volumes, except for the boron centers. Boron atoms, as expected, constitute the most electron-deficient species in the complex molecule, whereas the neighboring oxygen and carbon atoms are the most significantly negatively charged ones. This part of the molecule appears to be very much involved in the charge transfer occurring between the acid fragment and oxyquinoline moiety leading to the observed fluorescence, as supported by the time-dependent density functional theory (TDDFT) results and the generated transition density maps. TDDFT calculations indicated that p-type atomic orbitals contributing to the HOMO-1, HOMO, and LUMO play the major role in the lowest energy transitions, and enabled further comparison with the charge density features, which is discussed in details. Furthermore, the results confirmed the known fact the Q ligand character is most important for the spectroscopic properties of this class of complexes.

Tuning of the colour and chemical stability of model boranils: a strong effect of structural modifications

An improved approach to luminescent diphenylborinic complexes with functionalized salicydeneaniline ligands was developed. A strong effect of structural modifications on their stability and optical properties was established.