title: Synthesis of luminescent ethynyl-extended regioisomers of borate complexes based on 2-(2'-hydroxyphenyl)benzoxazole.
abstract: A series of thirteen luminescent tetrahedral borate complexes based on the 2-(2'-hydroxyphenyl)benzoxazole (HBO) core is presented. Their synthesis includes the incorporation of an ethynyl fragment by Sonogashira cross-coupling reaction, with the goal of extending the conjugation and consequently redshifting their emission wavelength. Different regioisomers, substituted in the 3-, 4-, or 5-position of the phenolate side of the HBO core, were studied in order to compare their photophysical properties. The complexes were characterized by X-ray diffraction and NMR, UV/Vis, and emission spectroscopy in solution and in the solid state. In all cases, complexation to boron leads to a donor-acceptor character that impacts their photophysical properties. Complexes with a 3- or 5-substituted fragment display mild to pronounced internal charge transfer (ICT), a feature strengthened by the presence of p-dibutylaminophenylacetylene in the molecular structure, protonation of the nitrogen atom of which leads to a significant blueshift and an increase in quantum yield. On the contrary, when the ethynyl module is grafted on the 4-position, narrow, structured, symmetrical absorption/emission bands are observed. Moreover, the fact that protonation has little effect on the emission maximum wavelength reveals singlet excited-state decay. Solid-state emission properties reveal a redshift compared to solution, explained by tight packing of the π-conjugated systems and the high planarity of the dyes. Subsequent connection of these complexes to other photoactive subunits (BODIPY, Boranil) provides dyads in which efficient cascade energy transfer is observed.
authors: Laboratoire de Chimie Moléculaire et Spectroscopies Avancées (LCOSA), ICEEPS-LCOSA, UMR CNRS 7515, École Européenne de Chimie, Polymères et Matériaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex 02, France. massue@unistra.fr
authors: Julien
authors: J
authors: Massue
authors: Denis
authors: D
authors: Frath
authors: Pascal
authors: P
authors: Retailleau
authors: Gilles
authors: G
authors: Ulrich
authors: Raymond
authors: R
authors: Ziessel
mesh_terms: D001583:Benzoxazoles / Q000737:chemistry*; D001881:Borates / Q000138:chemical synthesis* / Q000737:chemistry; D001895:Boron / Q000737:chemistry; D001896:Boron Compounds / Q000737:chemistry; D018360:Crystallography, X-Ray; D004735:Energy Transfer; D005456:Fluorescent Dyes / Q000138:chemical synthesis* / Q000737:chemistry; D049449:Luminescence; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: C433838:2-(2'-hydroxyphenyl)benzoxazole; C095489:4,4-difluoro-4-bora-3a,4a-diaza-s-indacene; D001583:Benzoxazoles; D001881:Borates; D001896:Boron Compounds; D005456:Fluorescent Dyes; D001895:Boron