Impact of Heteroatom Substitution on Dual‐State Emissive Rigidified 2‐(2’‐hydroxyphenyl)benzazole Dyes: Towards Ultra‐Bright ESIPT Fluorophores**

Abstract

2‐(2’‐Hydroxyphenyl)benzazole (HBX) fluorophores are well‐known excited‐state intramolecular proton transfer (ESIPT) emitters largely studied for their synthetic versatility, photostability, strong solid‐state fluorescence and ability to engineer dual emission, thus paving the way to applications as white emitters, ratiometric sensors, and cryptographic dyes. However, they are heavily quenched in solution, due to efficient non‐radiative pathways taking place as a consequence of the proton transfer in the excited‐state. In this contribution, the nature of the heteroring constitutive of these rigidified HBX dyes was modified and we demonstrate that this simple structural modification triggers major optical changes in terms of emission color, dual emission engineering, and importantly, fluorescent quantum yield. Investigation of the photophysical properties in solution and in the solid state of a series of ethynyl‐TIPS extended HBX fluorophores, along with ab initio calculations demonstrate the very promising abilities of these dyes to act as bright dual‐state emitters, in both solution (even in protic environments) and solid state.

Color‐Tunable Multifunctional Excited‐State Intramolecular Proton Transfer Emitter: Stimulated Emission of a Single Dye

“…The calculated photoluminescence quantum yields (QY) in nonpolar solvents (7-24 %) are on par with some of the other literature examples of HBX dyes. [2,19] In protic polar solvents (ethanol to acetonitrile), another band is present both in absorption and emission spectra, bathochromically shifted in comparison to E* bands (at ~428 or ~460 nm). As already described in the literature, [45,46] deprotonation of the ground or excited state enol form, leads to the formation of stabilized anionic species A or A*, emissive at shorter wavelengths than K* (ca.…”

“…Additional transformations are also possible for ESIPT dyes, such as well-studied deprotonation in a polar solvent leading to the formation of highly fluorescent anionic species. [2][3][4] A simplified diagram showing these relations is presented in Figure 1. Such photocycle enables observation of multiple emission bands, corresponding to the simultaneous decay of the various excited species, leading in some cases to panchromatic white emissive systems.…”

Dual Solution-/Solid-State Emissive Excited-State Intramolecular Proton Transfer (ESIPT) Dyes: A Combined Experimental and Theoretical Approach

“…Enhancing molecular rigidity of the π-conjugated cores of ESIPT dyes is likely the key to increase PLQY in the solution state. Indeed, recent examples based on rigidified scaffolds derived from imidazo­[1,2- f ]­phenanthridine, 2-(2′-hydroxyphenyl)­benzimidazoles, and others have shown very promising DSSE. A tenuous balance between quasi-planarity, rigidity, and solubility is necessary to engineer ESIPT-based DSSE compounds.…”

Rational Design and Facile Synthesis of Dual‐State Emission Fluorophores: Expanding Functionality for the Sensitive Detection of Nitroaromatic Compounds

“…After cooling to room temperature, the solution was filtered. The crude products were washed with THF to afford pure compound 4 d. The characterization data, 1 H, 13 C and 19 F NMR spectra and more details can be seen in Supporting Information.…”

“…[1,[14][15][16] To circumvent the limitations of ACQ and AIE type molecules, numerous endeavors have been made to prevail DSE chromophores to improve their practicability in luminescent material fields. [17][18][19] For example, Tang's group reported a series of triphenylamine (TPA)-based DSE compounds, using conjugation-induced rigidity (CIR) strategy for twisted molecules. [3] Meng's group proposed self-isolated enhanced emission (SIEE) method through the introduction of alkyl chains into the twisted donor-acceptor (DÀ A) molecule, also successfully giving DSE materials.…”