Experimental and theoretical comprehension of ESIPT fluorophores based on a 2-(2'-hydroxyphenyl)-3,3'-dimethylindole (HDMI) scaffold.

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. As a result, ESIPT probes are attractive candidates for ratiometric sensing of a variety of substrates. A new family of ESIPT fluorophores is described herein, inspired by the well-known 2-(2'hydroxyphenyl)benzazole (HBX) organic scaffold. The connection of 3,3'-dimethylindole (or 3H-indole) derivatives with phenol rings triggers the formation of novel 2-(2'-hydroxyphenyl)-3,3'-dimethylindole (HDMI) fluorophores, capable of stimuli-responsive ESIPT emission. This brand new family of dyes displays redshifted emission, as compared to HBX, along with an unprecedented acid/base-mediated stabilization of different rotamers, owing to supramolecular interactions with methyl groups. These compounds are therefore highly sensitive to external stimuli, such as the presence of acid or base, where protonated and deprotonated species have specific optical signatures. Moreover, a new pyridine-functionalized HDMI dye displays acid-sensitive AIE properties. The photophysical properties of all compounds have also been studied using ab initio calculations to support experiments in deciphering the nature of the various radiative transitions observed and the related excited rotameric species.

Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation.

Styrylpyrimidines with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragments were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand through theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77 K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF.

Interplay between Dual-State and Aggregation-Induced Emission with ESIPT Scaffolds Containing Triphenylamine Substituents: Experimental and Theoretical Studies.

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.

Solid-state emitters presenting a modular excited-state proton transfer (ESIPT) process: recent advances in dual-state emission and lasing applications.

This review aims at providing a broad readership of material and physical chemists, as well as those interested in ab initio calculations, about recent advances in the fields of dual solution-solid emitters and lasing applications based on organic dyes displaying an excited-state intramolecular proton transfer (ESIPT) process. ESIPT is known to be highly sensitive to the immediate environment leading to the engineering of a wide range of stimuli-responsive fluorescent dyes. With the structural diversity of ESIPT-capable fluorophores being large, many applications have been targeted over the years in the fields of optoelectronics, biology and luminescent displays. This review wishes to point out two emerging applications concerning ESIPT fluorophores, which are the answer for the quest for emitters fluorescing both in solution and in the solid state, and those capable of light amplification.

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

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.

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

This article describes the synthetic efforts towards the solubilization of organic fluorescent emitters based on a 2-(2'-hydroxybenzofuranyl)benzazole (HBBX) scaffold in aqueous media under physiological conditions (PBS, pH 7.4). These dyes are well-known to display the excited-state intramolecular proton transfer (ESIPT) process which leads to a Stokes-shifted fluorescence with enhanced photostability and strong environment dependent features. Organic dyes are hydrophobic by nature and their vectorization into aqueous media usually necessitates amphiphilic polymers. In this study, we show that the incorporation of one or two sulfobetaine fragments, a highly biocompatible zwitterionic unit leads to the vectorization in buffer solution at pH 7.4 while keeping a reasonable ESIPT fluorescence emission. The photophysical properties of all dyes were studied in multiple solvents and showed that, depending on structure and environment, different excited-state species are observed: normal or tautomeric species, as well as a competitive anionic fluorescent derivative. This study shows that it is not only possible to solubilize fluorescent ESIPT dyes in water using sulfobetaine(s) but also that the optical properties can be finely tuned depending on small structural inputs.

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

The excited-state intramolecular proton transfer chromophores were regarded as good materials for laser action generation due to their inherent four-level photocycle. The excitation-dependent properties of these compounds enable light amplification from two distinct forms: both enol and keto, making it possible to obtain dual fluorescence emission. Herein, we report that a third option is possible for the first time stimulated emission was realized with a deprotonated ESIPT molecule based on a novel rigidified 2-(2'-hydroxyphenyl)benzothiazole derivative, triggering the possibility to fabricate real-time tunable active material. Through the rational engineering of the ratio of each emissive species, a red-green-blue device was fabricated with the possibility of white light generation. The degenerated two-wave mixing setup was applied to construct a continuously tunable distributed feedback laser.

Excited-State Intramolecular Proton Transfer Dyes with Dual-State Emission Properties: Concept, Examples and Applications.

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.

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

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.

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

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.

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

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.

Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies.

A series of five excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2'-hydroxyphenyl) benzoxazole (HBO) scaffold, functionalized with a mono-or bis-(trialkylsilyl) acetylene extended spacer are presented. Investigation of their photophysical properties in solution and in the solid-state in different matrix, along with ab initio calculations gave useful insights into their optical behavior. Random lasing studies were conducted on a series of PMMA doped thin films, showing the presence of stimulated emission above the threshold of pumping energy density (ρth ≈ 0.5-2.6 mJ cm-2). In this work, the similarity of four level laser systems is discussed in light of the ESIPT photocycle.

An extended excited-state intramolecular proton transfer (ESIPT) emitter for random lasing applications.

An original molecular fluorophore displaying single keto (K*) excited-state intramolecular proton transfer (ESIPT) emission is presented in this article. Substitution at the 3 and 5 positions of the phenol ring of a 2-(2'-hydroxyphenylbenzoxazole) (HBO) dye by triethylsilyl-ethynyl fragments leads to a drastic enhancement of fluorescence in the solution-state as compared to unsubstituted analogues. This intense fluorescence emission is also retained in a protic solvent like ethanol and in the solid-state as embedded in a potassium bromide pellet or as 1% doped in a poly(methyl methacrylate) (PMMA) film. The experimental optical properties were confirmed by ab initio calculations. Random lasing (RL) studies performed on this ESIPT emitter show the presence of stimulated emission occurring above the threshold level of pumping energy density (ρth ≈ 300 µJ cm-2) in the PMMA matrix. To the best of our knowledge, this study constitutes the first observation of RL based on ESIPT fluorescence.

On the Fine-Tuning of the Excited-State Intramolecular Proton Transfer (ESIPT) Process in 2-(2'-Hydroxybenzofuran)benzazole (HBBX) Dyes.

Herein, a full investigation of the optical properties and first-principles calculations of a large series of original 2-(2'-hydroxybenzofuran)benzazole (HBBX) dyes is described. The electronic substitution on the π-conjugated core of the fluorophores and the nature of the heteroatom (O, S, N) was varied extensively to assess the necessary parameters to trigger a partial frustration of the excited-state intramolecular proton transfer (ESIPT) process, which results in the emission of both tautomers, that is, enol and keto (E* and K*). The optical properties, studied in solution and in the solid state, revealed the appearance of either an intense single K* or a dual E*/K* emission; a feature that is highly dependent on the electronic substitution (donating or accepting), the heteroelement, and the close environment. Subtle modifications of these parameters allowed the establishment of structure-property relationships that were successfully rationalized by first-principles calculations. In particular, the E*/K* emission intensity ratio was shown to be directly related to the free energies of the two emissive tautomers in the excited state.

Polyanils and Polyboranils: Synthesis, Optical Properties, and Aggregation-Induced Emission.

A first series of polyanils were synthesized by a simple condensation between either isomers of phenylenediamine derivatives or 1,3,5-benzenetriamine and 4-(diethylamino)salicylaldehyde, while a second series resulted from the condensation between 4,6-dihydroxyisophthalaldehyde or 2,5-dihydroxyterephthalaldehyde and differently substituted anilines. All these polyanils showed good chelating abilities toward trivalent boron fragments such as BF2 or BPh2 to yield the corresponding boranils. The optical properties of these novel fluorophores have been studied both in solution and in the solid-state and show emission wavelengths covering the entire visible spectrum and near-infrared (NIR), depending on molecular structure, substitution, and environment. While faintly fluorescent in solution in their molecular state, some polyanils show typical aggregation-induced emission (AIE) behavior upon addition of increasing amounts of water in THF solution, leading to a sizable enhancement of fluorescence intensity.

Highly fluorescent extended 2-(2'-hydroxyphenyl)benzazole dyes: synthesis, optical properties and first-principle calculations.

The investigation of the optical properties of extended 2-(2-hydroxyphenyl)benzazole dyes showed a complete frustration of the excited-state intramolecular proton transfer (ESIPT) process leading to a novel family of highly fluorescent fluorophores. In the case of a benzothiazole ring, restoration of ESIPT can be observed in acidic medium leading to ratiometric sensing. These experimental results have been rationalised by first-principle calculations.

Substituent and solvent effects on the excited state deactivation channels in anils and boranils.

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

White emitters by tuning the excited-state intramolecular proton-transfer fluorescence emission in 2-(2'-hydroxybenzofuran)benzoxazole dyes.

The synthesis, structural, and photophysical properties of a new series of original dyes based on 2-(2'-hydroxybenzofuran)benzoxazole (HBBO) is reported. Upon photoexcitation, these dyes exhibit intense dual fluorescence with contribution from the enol (E*) and the keto (K*) emission, with K* being formed through excited-state intramolecular proton transfer (ESIPT). We show that the ratio of emission intensity E*/K* can be fine-tuned by judiciously decorating the molecular core with electron-donating or -attracting substituents. Push-pull dyes 9 and 10 functionalized by a strong donor (nNBu2 ) and a strong acceptor group (CF3 and CN, respectively) exhibit intense dual emission, particularly in apolar solvents such as cyclohexane in which the maximum wavelength of the two bands is the more strongly separated. Moreover, all dyes exhibit strong solid-state dual emission in a KBr matrix and polymer films with enhanced quantum yields reaching up to 54 %. A wise selection of substituents led to white emission both in solution and in the solid state. Finally, these experimental results were analyzed by time-dependent density functional theory (TD-DFT) calculations, which confirm that, on the one hand, only E* and K* emission are present (no rotamer) and, on the other hand, the relative free energies of the two tautomers in the excited state guide the ratio of the E*/K* emission intensities.

Luminescent materials: locking π-conjugated and heterocyclic ligands with boron(III).

Multidisciplinary research on novel organic luminescent dyes is propelled by potential applications in plastic electronics and biomedical sciences. The construction of sophisticated fluorescent dyes around a tetrahedral boron(III) center is a particular approach that has fueled the creativity of chemists. Success in this enterprise has been readily achieved with simple synthetic protocols, the products of which display unusual spectroscopic behavior. This account is a critical review of recent advances in the field of boron(III) complexes (excluding BODIPYs and acetylacetonate boron complexes) involving species displaying similar coordination features, and we outline their potential development in several disciplines.

Modeling optical signatures and excited-state reactivities of substituted hydroxyphenylbenzoxazole (HBO) ESIPT dyes.

The potential energy surfaces of dyes displaying strong excited-state intramolecular proton transfer (ESIPT) are investigated with the help of ab initio tools. It allows us to rationalize the interplay between the excited-state transition free energies and the observed optical signatures.