Sterically Hindered 2‐(2’‐Hydroxyphenyl)benzoxazole (HBO) Emitters: Synthesis, Spectroscopic Studies, and Theoretical Calculations

Abstract

We describe various synthetic pathways to introduce sterically hindered substituents (mesityl, 2,4,6-triisopropylphenyl, anthracene) to the proton donor side of excited-state intramolecular proton transfer (ESIPT)-capable 2-(2'-hydroxyphenyl) benzoxazole (HBO) fluorophores. Two original synthetic approaches were investigated in order to synthesize seven HBO derivatives. Optimization studies concluded that electron rich and bulky phosphine ligands are required to ensure completion of the Suzuki-Miyaura cross-coupling reaction involving a bulky aromatic boronic acid. The photophysical properties of all dyes revealed a strong influence of the nature of the solvent on the optical properties, as protic solvents tend to stabilize enol tautomers and induce dual emission. Our studies confirmed that, unlike the majority of organic dyes, ESIPT fluorophores do not undergo π-stacking in the solid-state. The nature of the excited-states was explored by ab initio calculations.

Heteroaryl‐Substituted Bis‐Anils: Aggregation‐Induced Emission (AIE) Derivatives with Tunable ESIPT Emission Color and pH Sensitivity

“…The observed emission band can be assigned to the radiative S 1 ‐S 0 transition of the EK * tautomer, generated upon ESIPT process (see Figure S49 for a schematic representation of excited‐state dynamics, and theoretical calculations below). One single quantitative proton transfer, i. e. a single EK * band is observed, which goes in strong contrast with the majority of non‐AIE ESIPT dyes where the addition of protic solvent such as ethanol is accompanied by the stabilization of the H‐bonded enol tautomer [24] . This difference can be explained by the formation of aggregates which limits the influence of solvatation on the ESIPT process.…”

“…One single quantitative proton transfer, i. e. a single EK* band is observed, which goes in strong contrast with the majority of non-AIE ESIPT dyes where the addition of protic solvent such as ethanol is accompanied by the stabilization of the H-bonded enol tautomer. [24] This difference can be explained by the formation of aggregates which limits the influence of solvatation on the ESIPT process. It is also worth noting that this series of bis-anils AIEgens display a fine-tuning of the maximum emission wavelength, with fluorescence color spanning from orange to near infra-red (NIR) (λ em = 610-715 nm), depending on the electronic nature of the peripheral substitution.…”

Influence of ethynyl extension on the dual-state emission properties of pyridinium-substituted ESIPT fluorophores

“…This a strong contrast with the vast majority of not only HBO dyes but all dyes showing ESIPT in general, where strong non-radiative processes typically occur in solution upon formation of the tautomeric excited species, quenching the fluorescence. 18 It is worth pointing out that the presence of an additional ethynyl-TIPS fragment does not lead to a further increase of the QY. Stabilization of the excited-state and subsequent radiative deactivation enhancement seems to be solely driven by resonance effects and not cooperatively with molecular rigidification.…”

“…It is worth noting that this coupling occurs only at the iodo position, to provide 5 and 6 in excellent overall yields. The HBO scaffold was then constructed, using classical conditions 18 to yield 7 and 8 in 90-91%, respectively. Scheme 1.…”

“…17 Another typical feature of ESIPT is the occurrence of strong fluorescence quenching processes in solution, owing to rotational and vibrational deactivations induced by the proton transfer, which is counter-balanced by a strong solid-state emission, largely explained by the absence of π-stacking interactions. 18 The possibility to use ESIPT fluorophores as DSE dyes looks attractive, with the view to maximize the range of possible applications for one single dye, in the context of sustainable development, but one need to tackle first the weak fluorescence intensity in solution issue, as a prerequisite. In order to increase the photoluminescent quantum yield (QY) in solution of ESIPT dyes, several strategies have been proposed, such as cyclization of the organic backbones 19 or introduction of ethynyl-extended aryl 20 or silyl moieties 21 at the proton donor side.…”