Blue-Emitting 2-(2′-Hydroxyphenyl)benzazole Fluorophores by Modulation of Excited-State Intramolecular Proton Transfer: Spectroscopic Studies and Theoretical Calculations

Abstract

This article describes the synthesis, spectroscopic studies, and theoretical calculations of nine original fluorophores based on the 2-(2′hydroxyphenyl)benzazole (HBX) scaffold, functionalized at the 4-position of the phenol ring by ethynyl-extended aniline moieties. HBX dyes are wellknown to display an excited-state intramolecular proton transfer (ESIPT) process, owing to a strong six-membered hydrogen bond in their structure that allows for an enol/keto tautomerism after photoexcitation. Appropriate electronic substitution can perturb the ESIPT process, leading to dual fluorescence, both excited tautomers emitting at specific wavelengths. In the examples described herein, it is demonstrated that the proton transfer can be finely frustrated by a modification of the constitutive heteroring, leading to a single emission band from the excited enol or keto tautomer or a dual emission with relative intensities highly dependent on the environment. Moreover, the nature of the functionalization of the N-alkylated aniline moiety also has a significant importance on the relative excited-state stabilities of the two tautomers in solution. To shed more light on these features, quantum chemical calculations by the density functional theory are used to determine the excited-state energies and rationalize the experimental spectroscopic data.

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

“…The second band, rather broad and unstructured, located between 326 and 332 nm in toluene and 319 and 333 nm in ethanol can be attributed to the S 0 →S 1 transition characteristic of the HBO dye. Molar absorption coefficients are in the 11,400‐17,800 M −1 cm −1 range, consistent with data reported for similar compounds [21–24] . HBO‐3,5‐(tolyl) 2 dye stands out in the series with a recorded bathochromic shift of the maximum absorption wavelength (λ abs =352‐354 nm), highlighting a stronger electronic connection between the tolyl unit and the HBO core, which is also found in the simulations ( vide infra ).…”

“…Molar absorption coefficients are in the 11,400-17,800 M À 1 cm À 1 range, consistent with data reported for similar compounds. [21][22][23][24] HBO-3,5-(tolyl) 2 dye stands out in the series with a recorded bathochromic shift of the maximum absorption wavelength (λ abs = 352-354 nm), highlighting a stronger electronic connection between the tolyl unit and the HBO core, which is also found in the simulations (vide infra). HBO-5-Ant presents absorption profiles in toluene and ethanol typically observed for polyaromatic-containing fluorophores, i. e. intense vibronic bands (λ abs = 332, 367 and 387 nm in toluene) located in the UV region (Figure 4), indicating that the anthracene moiety is significantly involved in the lowest excited state(s).…”

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer (ESIPT) Emitters with Sulfobetaine Fragments

“…Protic solvents are well-known to stabilize excited enol tautomers through H-bonding interactions. 20,22 For the HBBO/HBBT derivatives 3–5 , quantum yields (QY) values between 0.01 and 0.07 are calculated, which are of the same order as those recorded for similar HBBX dyes. The insertion of a t Bu group on the benzoxazole ring as in HBO 3 does not induce any notable change in the optical properties in comparison with 4 , except for the QY value (QY = 0.01 for 3 vs. 0.06 for 4 in ethanol).…”

“…Based on literature, one can presume that the band at 480 nm could stem from excited deprotonated species D*, in this dissociative solvent leading to triple E*/D*/K* emission. 20 The replacement of the oxygen atom by sulfur atom in HBBT 5 does not lead to a significant change in absorption ( λ abs = 348–353 nm, ε = 17 600 M −1 cm −1 in ethanol). After photoexcitation, a single emission band is observed ( λ em = 405 nm), which appears to be stemming from the E*, highlighting the importance of electronic effects in the nature of the emissive species, consistent with literature.…”

“…Notably, the modulation of the heterocycle of the HBX or HBBX scaffold which corresponds to benzoxazole (HBO/HBBO), benzothiazole (HBT/HBBT) or benzimidazole (HBI/HBBI) induces drastic photophysical changes. 20…”