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

Abstract

This article describes the multistep synthesis and photophysical properties of three highly fluorescent dyes based on the 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) scaffold and their resulting chelation to a BF2 fragment. These dyes possess functionalization at the 3,5‐positions of the phenol ring with an ethynyl‐extended fragment bearing TMS, p‐tBuC6H4, or p‐NnBu2C6H4 groups. All of the new compounds were characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. The optical properties of the HBO dyes reveal the presence of enol and keto bands as a result of a strong excited‐state intramolecular proton transfer (ESIPT). This ESIPT process is highly dependent on the nature of the electronic substituents and the polarity of the solvent. Upon coordination to a BF2 fragment, typical singlet emission is observed with λem ranging from 439 to 553 nm and quantum yields from 0.03 to 0.36. If aromatic amines are involved, strong aggregates are observed that could be dissociated upon protonation of the lone electron pair.

Computational Molecular Electronic Spectroscopy with TD-DFT

“…8), the transition state is higher in energy and the enol minimum on the ES surface applies once vibrational corrections are included, indicating that dual emission is feasible. These conclusions fit the corresponding experimental data perfectly [123]. Figure 8 also shows that the transition states for the proton transfer are located at very different geometries for the GS and the ES, e.g., at respective O-H distances of 1.410 and 1.185 Å , for the first dye, indicating that a simple vertical TD-DFT calculation performed on the GS transition state would fail to deliver valuable insights.…”

“…7 for the typical enol/keto tautomerism, the structures of the absorbing and emitting species are strongly different, which advantageously yields very large Stokes shifts [121,122]. Additionally, if the ES reaction is not quantitative, one can obtain emissions from both tautomers and hence reach dual fluorescence with a single compound [123]. This can be further optimized to design single-molecule white light emitting units [124], as ESIPT quantum yield tends to increase when going from solution to solid state.…”

“…Houari et al explored the GS and ES PES of two hydroxyphenylbenzoxazole (HBO) dyes, differing only by their end groups [123,139]. The alkylsubstituted system only shows emission from the keto tautomer experimentally, whereas the amino-substituted compound displays (dual-)emission from both enol and keto tautomers [123]. Houari et al obtained the PES of both the GS and the ES (see Fig.…”

“…There are numerous applications of TD-DFT and wavefunction approaches to rationalizing excited-state proton transfer and, for the sake of consistency, we summarize here some of the works that have been performed with an approach similar to that used in the previous section, i.e., cLR-PCM/TD-M06-2X [124,[139][140][141]. Houari et al explored the GS and ES PES of two hydroxyphenylbenzoxazole (HBO) dyes, differing only by their end groups [123,139]. The alkylsubstituted system only shows emission from the keto tautomer experimentally, whereas the amino-substituted compound displays (dual-)emission from both enol and keto tautomers [123].…”

White Emitters by Tuning the Excited‐State Intramolecular Proton‐Transfer Fluorescence Emission in 2‐(2′‐Hydroxybenzofuran)benzoxazole Dyes

“…This feature has been observed in related compounds in protic solvents in which the E* emission is usually enhanced 1d. 8b, 13d Switching the terminal t Bu group in HBBO dye 4 to a more electron‐donating methoxy group in HBBO dye 5 does not seem to have a significant influence on the optical properties.…”