Julien Massue

Luminescent Materials: Locking π-Conjugated and Heterocyclic Ligands with Boron(III)

Multidisciplinary research on novel organic luminescent dyes is propelled by potential applications in plastic electronics and biomedical sciences. The construction of sophisticated fluorescent dyes around a tetrahedral boron(III) center is a particular approach that has fueled the creativity of chemists. Success in this enterprise has been readily achieved with simple synthetic protocols, the products of which display unusual spectroscopic behavior. This account is a critical review of recent advances in the field of boron(III) complexes (excluding BODIPYs and acetylacetonate boron complexes) involving species displaying similar coordination features, and we outline their potential development in several disciplines.

Synthesis of luminescent BPh2-coordinated 2-(2′-hydroxyphenyl)benzoxazole (HBO)

Triphenylborane (BPh3) has been successfully coordinated to five π-conjugated aromatic systems containing a N∧O bidentate 2-(2′-hydroxyphenyl)benzoxazole (HBO) ligand. Complexes 6–8 are directly substituted on the phenolic side of the HBO core while the structure of derivatives 9–10 includes a 4-dibutylaminophenyl module linked through an ethynyl fragment, at position 4 or 5. The crystal structure of complexes 8 and 10 reveals different solid-state molecular packing depending on the substitution, a herringbone molecular packing being observed for complex 8 while the dibutylamino fragment present in complex 10 is in favour of a lamellar structure. The optical properties are highly dependent on the nature and the position of substituents. Solvatochromic charge-transfer emissions are observed for substitution at position 3 or 5 while singlet emission is favoured when position 4 is functionalized. Solid-state fluorescence reveals that complex 8 possesses red-shifted emission when dispersed in a KBr matrix.

Substituent and Solvent Effects on the Excited State Deactivation Channels in Anils and Boranils

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

2,4 and 2,5-bis(benzooxazol-2′-yl)hydroquinone (DHBO) and their borate complexes: synthesis and optical properties

This article describes the multistep synthesis and the solution/solid-state optical properties of a series of highly fluorescent dyes based on either a 2,4 or a 2,5-bis(benzooxazol-2′-yl)hydroquinone (DHBO) scaffold. Their fluorescence emission stems from an intrinsic excited-state intramolecular proton transfer (ESIPT) photophysical process, which is highly dependent on the environment of the probe (solvent/matrix). Besides their ESIPT emission, these dyes are also excellent chelates towards boron(III) fragments such as BF2 or B(Ar)2 leading to singlet emitters with a quantum yield in solution reaching up to 51%. The solid-state emission of all these fluorophores was investigated by dispersing them as pellets in a KBr matrix revealing an intense emission in condensed matter.