We present the synthesis along with structural and photophysical properties of a series of 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) and dimethylindole (HDMI) derivatives, in an effort to modulate their fluorescent properties by changing either electronic substitution or introducing planarization via ethynyl bridges. HBO and HDMI dyes are reported to undergo excited‐state intramolecular proton transfer, leading to red‐shifted emission with typically low quantum yield in solution, owing to pronounced non‐radiative deactivation channels. Introducing electron acceptors, such as benzonitrile, emerges as an effective strategy to boost the radiative transitions in these systems. In this context, this study aims at building structure–property relationships by a stepwise investigation of the influence of several parameters, that is, the nature of proton acceptor heterocycle (HBO vs. HDMI), the presence of an ethynyl moiety, and the addition of substituents (CN vs. Me). In each case, the nature of the transition(s) involved in the optical profile was also investigated by theoretical calculations.

We describe the synthesis, photophysical properties, and ab initio calculations on a series of 2-(2'-hydroxyphenyl)benzazoles (HBX), functionalized by a benzonitrile, prone to boost the fluorescent quantum yield in solution. HBX are well known to display the excited-state intramolecular proton transfer (ESIPT) process, but they typically lead to a quenched fluorescence in solution. The additional insertion of an ethynyl-triisopropylsilane substituent in the vicinity of the benzonitrile group acts as a cooperative moiety to obtain strong multiple-state emissive transitions with distinct emission wavelengths. Quantum yields of the ESIPT transition are up to 52% in solution, which are among the record values in the literature. The photophysical data can be further modulated by the nature of the heteroatom constitutive of HBX, paving the way for the development of ultrasensitive ratiometric probes.

This article describes the synthesis, along with comprehensive photophysical and ab initio characterization, of a series of 2-(2 '-hydroxyphenyl)benzoxazole (HBO) fluorophores, a family of compounds prone to undergoing an excited-state intramolecular proton transfer (ESIPT) process, functionalized with different positional isomers of quinoline or isoquinoline. We notably show that the position of the nitrogen atom at the azaheterocycle site has a key influence on both the emission profile and the photoluminescence quantum yield in solution. We also demonstrate the proton-sensitive nature of these dyes in solution, where not only does protonation trigger fluorescence enhancement, but it also acts as a transition switch between two excited states, with different emission profiles. HBO-isoquinoline displays very intense fluorescence not only in neutral and protonated dichloromethane solutions in the green-yellow region, but also in the solid state. Moreover, this dye exhibited a record Stokes shift of 11 000 cm-1.

This article describes the synthesis along with full photophysical characterization in both solution and solid-state of a series of 2-(2 '-hydroxyphenyl)benzazole linked to an aza-heterocycle (pyridine or isoquinoline) by one or two aryl rings. The impact of the length of the spacer is investigated in the light of emission wavelength, quantum yield, sensitivity to protons and possibility of transition switch upon protonation. Correlation of first-principle calculations and experimental results help decipher the excited-state dynamics, allowing to draw and discuss hypotheses concerning the nature of the excited species.

The quest for new photoinitiators (PIs) capable of efficiently absorbing multiple photons, particularly three photons, has intensified due to the multiphoton polymerization (MPP) technique. MPP enables the fabrication of next-generation photonic micro- and nanodevices with resolution beyond the diffraction limit and without shape limitation, offering considerable advantages over other methods. However, designing organic compounds acting as effective multiphoton PIs while enabling smart functionalization of manufactured structures remains challenging. In this contribution, we report using a 2-(2 '-hydroxyphenyl)benzoxazole (HBO) derivative, named HBO-NBu2, as a PI for two- and three-photon polymerization. HBO fluorophores are well-known to undergo an excited-state intramolecular proton transfer (ESIPT) process, but in this structure, the combination of a planar and linear push-pull dipolar structure, with an extensive pi-conjugation, led to a complete suppression of ESIPT, subsequently exhibiting an excellent nonlinear response. Herein, we investigate the linear photophysical properties, excited-state dynamics, and multiphoton absorption of HBO-NBu2, supported by quantum chemical calculations. Our results demonstrate that HBO-NBu2 exhibits structural rigidity, resulting in a fluorescence quantum yield of 62% in dichloromethane solution, owing to the absence of ESIPT and substantial intramolecular charge transfer (ICT) due to its push-pull nature. Moreover, this molecule shows significant two- and three-photon absorption cross-sections of 80 GM (at 800 nm) and 4 +/- 1 x 10-81 cm6 (s per photon)2 (at 1030 nm), respectively. Finally, we demonstrate a proof of concept of MPP using HBO-NBu2 as a PI, showing that even at low concentrations, it outperforms widely used PIs.

In this article, it describes the synthesis of a series of fluorophores consisting of N-alkyl or N-aryl pyridinium groups connected at different positions of a 2-(2'-hydroxyphenyl)benzoxazole scaffold and the exploration of the photophysical properties in solution (dichloromethane) and in the solid state, as amorphous powders. All dyes display a bathochromically shifted fluorescent transition from an excited keto state, formed after excited-state intramolecular proton transfer process. A full chemical engineering study was performed by changing the nature of the substitution at the pyridinium site (alkyl or aryl), the position of the pyridinium substitution and the nature of the counterion (six examples). The nature of the radiative transitions observed in these fluorescent dyes was confirmed by Time-dependent density functional theory (TD-DFT) calculations.

We describe herein the synthesis along with full photophysical and computational studies of three series of boranil complexes (Series I-III), incorporating extended ethynyl substitution, either on the imino side (Series I), on the phenolic side (Series II), or on both sides (Series III), in order to assess the impact of the insertion of electron donors or acceptors on the fluorescence properties. A full photophysical study in solution (various solvents) demonstrated the presence of a strong intramolecular charge transfer process within these boron complexes, with fluorescence colors spanning the entire visible range. Additionally, these compounds are also emissive in the solid state, on a similar wide fluorescence range, thus providing more examples of dual solution- and solid-emissive fluorophores, based on boron complexes. Theoretical calculations on both anil ligands and boranil complexes rationalize the charge transfer nature of the excited states involved in the emissive transitions.

This letter describes the synthesis and structural, photophysical, and theoretical properties of novel fluorescent dyes derived from 2-(2'-hydroxyphenyl)benzoxazole, functionalized by tricyanofuran or α-cyano-γ-lactone groups, prone to undergo an excited-state intramolecular proton transfer (ESIPT) process. We serendipitously found that tricyanofuran can induce the formation of the corresponding lactone in basic media, leading to a highly fluorescent derivative, owing to the formation of a merocyanine in the excited state.

This article reports the synthesis, along with structural and photophysical characterization of 2-(2'-hydroxyphenyl)benzazole derivatives functionalized with various azaheterocycles (pyridine, pyrimidine, terpyridine). These compounds show dual-state emission properties, that is intense fluorescence both in solution and in the solid-state with a range of fluorescent color going from blue to orange. Moreover, the nature of their excited state can be tuned by the presence of external stimuli such as protons or metal cations. In the absence of stimuli, these dyes show emission stemming from anionic species obtained after deprotonation (D* transition), whereas upon protonation or metal chelation, ESIPT process occurs leading to a stabilized and highly emissive K* transition. With the help of extensive ab initio calculations, we confirm that external stimuli can switch the nature of the transitions, making this series of dyes attractive candidates for the development of stimuli-responsive fluorescent ratiometric probes.

We describe the synthesis, full photophysical study, and ab initio calculations of 2-(2'-hydroxyphenyl)benzazole (HBX) fluorophores substituted, at the meta position of the phenol group, by pyridine derivatives. HBX are commonly used as model dyes to study the stimuli-induced modulation of the Excited State Intramolecular Proton Transfer (ESIPT) process. The meta-substituted fluorophores reported herein, display a photophysical profile different from the previously reported ortho- and para-substituted HBO pyridine isomers. Indeed, while all dyes undergo spontaneous deprotonation in neutral conditions, leading to highly emissive anionic species; upon protonation, ortho- and para-pyridine substitution leads to resonance-stabilized keto isomers, formed after ESIPT. Protonated meta derivatives, unable to stabilize their excited structure by such electronic delocalization process, display sizable intramolecular charge transfer (ICT) processes, translating into significantly redshifted emission. In addition, all dyes present a strong emission intensity, not only in neutral and acidic solutions, but also in the solid-state. The nature of the emissive transitions was confirmed in each case by theoretical calculations combining Time-Dependent Density Functional Theory (TD-DFT) and second-order Coupled Cluster (CC2) methods. This article describes the synthesis, photophysical studies, and ab initio calculations of 2-(2'-hydroxyphenyl)benzazole (HBX) fluorophores substituted, at the meta position of the phenol group, by pyridine derivatives. As opposed to previously reported positional isomers, these meta-substituted fluorophores display sizable intramolecular charge transfer (ICT) processes, in their protonated state translating into significantly redshifted emission. The nature of the emissive transitions was confirmed by theoretical calculations combining Time-Dependent Density Functional Theory (TD-DFT) and second-order Coupled Cluster (CC2) methods. image

Excited-State Intramolecular Proton Transfer (ESIPT) emission is associated with intense single or multiple fluorescence in the solid-state, along with enhanced photostability and sensitivity to the close environment.

We detail the synthesis of a series of fluorophores containing triphenylamine derivatives along with their photophysical, electrochemical, and electronic structure properties. These compounds include molecular structures derived from imino-phenol (anil) and hydroxybenzoxazole scaffolds originating from similar salicylaldehyde derivatives and display excited-state intramolecular proton transfer. We show that depending on the nature of the π-conjugated scaffold, different photophysical processes are observed: aggregation-induced emission or dual-state emission, with a modulation of the fluorescence color and redox properties. The photophysical properties are further rationalized with the help of ab initio calculations.

The two-step synthesis, structural, and photophysical properties of a series of heteroaryl-substituted bis-anil derivatives presenting aggregation-induced emission (AIE) coupled with an excited-state intramolecular proton transfer (ESIPT) process is described. The fluorescence color of the aggregates can be fine tuned by changing the electronic nature of the peripheral substitution, leading to a wide range of emission wavelengths (from green to the near infra-red). Moreover, upon introduction of strong electron-withdrawing groups such as cyano (CN), a competition between ESIPT and deprotonation is observed leading to the emission of the anionic species at low water percentage. This observation led to the synthesis of an additional mixed AIE fluorophore, functionalized by methoxy groups on one side and cyano groups on the other side. Upon addition of water, this dye displays first anionic emission, followed by typical AIE/ESIPT red fluorescence upon formation of the aggregates. TD-DFT calculations on selected AIE dyes were performed to rationalize the nature of the emissive transitions in these derivatives.

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.

Dual-state emissive (DSE) fluorophores are organic dyes displaying fluorescence emission both in dilute and concentrated solution and in the solid-state, as amorphous, single crystal, polycrystalline samples or thin films. This comes in contrast to the vast majority of organic fluorescent dyes which typically show intense fluorescence in solution but are quenched in concentrated media and in the solid-state owing to π-stacking interactions; a well-known phenomenon called aggregation-caused quenching (ACQ). On the contrary, molecular rotors with a significant number of free rotations have been engineered to show quenched emission in solution but strong fluorescence in the aggregated-state thanks to restriction of the intramolecular motions. This is the concept of aggregation-induced emission (AIE). DSE fluorophores have been far less explored despite the fact that they are at the crossroad of ACQ and AIE phenomena and allow targeting applications both in solution (bio-conjugation, sensing, imaging) and solid-state (organic electronics, data encryption, lasing, luminescent displays). Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence is particularly suitable to engineer DSE dyes. Indeed, ESIPT fluorescence, which relies on a phototautomerism between normal and tautomeric species, is characterized by a strong emission in the solid-state along with a large Stokes' shift, an enhanced photostability and a strong sensitivity to the close environment, a feature prone to be used in bio-sensing. A drawback that needs to be overcome is their weak emission intensity in solution, owing to detrimental molecular motions in the excited-state. Several strategies have been proposed in that regard. In the past few years, a growing number of examples of DSE-ESIPT dyes have indeed emerged in the literature, enriching the database of such attractive dyes. This review aims at a brief but concise overview on the exploitation of ESIPT luminescence for the optimization of DSE dyes properties. In that perspective, a synergistic approach between organic synthesis, fluorescence spectroscopy and ab initio calculations has proven to be an efficient tool for the construction and optimization of DSE-ESIPT fluorophores.

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 well-known 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.

We describe the synthesis and photophysical properties in both solution and in the solid-state of dual-state emitters undergoing an Excited-State Intramolecular Proton Transfer (ESIPT) process. These fluorophores, built around the 2-(2′-hydroxyphenyl)benzoxazole (HBO) scaffold incorporate a N-methylpyridinium unit, at the 3 or 5 position of the phenol ring, prone to reduce the vibrational relaxation in the excited-state thanks to resonance effects and consequently allowing increasing the solution-state quantum yield (QY). In order to further optimize their optical properties, an additional ethynyl-extended triisopropylsilyl (TIPS) spacer was introduced with the aim to combine molecular rigidification and π-delocalization, as cooperative effects to enhance dual-state emission (DSE) properties. Molecular engineering studies and comparison with unsubstituted compounds reveal the significant impact of the position of the ethynyl-TIPS moiety on the photophysical properties, especially the fluorescence color. As several phenomena can compete with the ESIPT process, all dyes were modelled using ab initio calculations to rationalize the nature of the excited-states.

Excited-state intramolecular proton transfer (ESIPT) dyes typically show strong solid-state emission, but faint fluorescence intensity is observed in the solution state owing to detrimental molecular motions. This article investigates the influence of direct (hetero)arylation on the optical properties of 2-(2'-hydroxyphenyl)benzoxazole ESIPT emitters. The synthesis of two series of ESIPT emitters bearing substituted neutral or charged aryl, thiophene, or pyridine rings is reported herein along with full photophysical studies in solution and solid states, demonstrating the dual solution-/solid-state emission behavior. Depending on the nature of substitution, several excited-state dynamics are observed: quantitative or partially frustrated ESIPT process or deprotonation of the excited species. Protonation studies revealed that pyridine substitution triggered a strong increase of quantum yield in the solution state for the protonated species owing to favorable quinoidal stabilization. These attractive features led to the development of a second series of dyes with alkyl or aryl pyridinium moieties showing strong tunable solution/solid fluorescence intensity. For each series, ab initio calculations helped rationalize and ascertain their behavior in the excited state and the nature of the emission observed by the experimental results.

The search for simple, low-cost, versatile, easily accessible, stimuli-responsive, highly emissive molecular fluorophores emitting both in solution and in the solid-state has prompted us to investigate the optical properties of a series of synthetically accessible salicylaldehyde derivatives possessing a π-conjugated moiety at their 4-position. These dyes are mainly known as synthetic intermediates but can also display sizeable Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence owing to the presence of a 6-membered H-bonded ring in their structure. The photophysical properties of these compounds have been studied in solution (multiple solvents) and in the solid-state, as doped in PMMA films, KBr pellets or as powders leading to the observation of a pronounced fluorosolvatochromism. Emission wavelengths in the range 400–654 nm, along with photoluminescence quantum yields up to 76 % were recorded. Modification of the spacer (ethynyl, vinyl or direct connection) involved the π-delocalization triggers major differences in terms of maximum emission wavelength and fluorescence quantum yields in the various media studied. All photophysical observations are rationalized by first-principle calculations.