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.

Effect of 3,5‐Disubstitution on the Optical Properties of Luminescent 2‐(2′‐Hydroxyphenyl)benzoxazoles and Their Borate Complexes

“…Although weakly luminescent in CH 2 Cl 2 ( λ abs = 410 nm, λ em = 502 nm, Φ = 0.03), HBO borate complex 10 lightens up in THF and CH 3 CN ( λ abs = 388 nm, λ em = 482 nm, Φ = 0.23 and λ abs = 393 nm, λ em = 553 nm, Φ = 0.11). This behavior is reminiscent to that observed in the corresponding monoethynyl‐extended borate complex substituted by a single p ‐di‐ n ‐butylaminophenylacetylene fragment in the 3‐position; the luminescence of this species was fully quenched in CH 2 Cl 2 owing to strong ICT, but this species was emissive in other solvents 10b. To obtain more insight into this ICT process, we recorded optical data in CH 2 Cl 2 after bubbling HCl into the solution, which resulted in a significant blueshift in the absorption and emission and a major increase in the quantum yield ( λ abs = 386 nm, λ em = 448 nm, Φ = 0.36; Figure 7, b).…”

“…Excitation in the lower‐energy band of HBO borate complex 3 ( λ abs = 384 nm) leads to one emission band centered at 439 nm ( Φ = 0.21, Figure 5). This is a slight redshift relative to the emission band of the HBO borate monoethynyltrimethylsilyl‐extended at the 5‐position ( λ abs = 366 nm, λ em = 427 nm, Φ = 0.31) 10b. Finally, the excellent match between absorption and excitation spectra excludes the presence of aggregates in solution.…”

“…This purely singlet‐emitting band is not solvatochromic, a feature reminiscent to that observed for corresponding ligand 7 . Connecting a second ethynyl spacer at the 3‐position led to a significant redshift in the emission by nearly 40 nm relative to that of the corresponding monoethynyl‐extended borate complex ( λ abs = 372 nm, λ em = 431 nm, Φ = 0.30 in toluene) 10b. Unexpectedly, different photophysical data were collected for HBO borate complex 10 .…”

“…This peculiar duality (ESIPT emitters or N ∧ O chelates) renders these HBO dyes very interesting for fine‐tuning the fluorescence emission. We recently reported the synthesis and optical properties of HBO borate complexes substituted directly10a or through a single ethynyl spacer10b that display highly luminescent emission spanning from the UV region to the orange region of the spectrum depending on the electronic substitution and its position on the phenolic side. These experimental results were rationalized by time‐dependent DFT calculations on a selection of decorated HBO borate dyes 16.…”

Iodo‐Functionalized Salicylideneimine‐Boron Complexes: Synthesis and Photosensitized Degradation of Organic Water Pollutants

“…As shown in Figure S11 (in the Supporting Information), we observed significant photobleaching for 1 – 3 under our experimental conditions whereas negligible changes were observed in the absorption spectrum of 4 upon irradiation, thereby indicating its excellent photostability. It is reported that boranils are prone to deboronation . On a similar note, the observed changes in the absorption spectra could be attributed to the elimination of the BF 2 moiety, resulting in the formation of the corresponding imines.…”