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

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.

ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^OH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of L^OH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^H,OH), a monohydroxy congener of L^OH,OH. Both L^OH,OH and L^H,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for L^H,OH and two for L^OH,OH) strongly affects the luminescence mechanism and color of the emission: L^H,OH emits in the light green region, whereas L^OH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T₁ → S₀ phosphorescence and not related to ESIPT. At the same time, L^OH,OH also exhibits S₁ → S₀ fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for L^H,OH due to the absence of the hydroxy group in the imidazole moiety.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

2-(2'-Hydroxyphenyl)benzoxazole (HBO) substituted at the 5'-position with bipyridylvinylene phenylenevinylene (compound 2) produces both normal and, via an excited-state intramolecular proton transfer (ESIPT) reaction, tautomer emissions in solvents that preserve intramolecular hydrogen bonds. The abundance of the tautomer emission from compound 2 in a poor hydrogen-bonding solvent increases in response to the application of a higher excitation energy. Based on quantum chemical calculations, the excitation-dependent dual emission is consistent with a model in which the ESIPT reaction is more favored in the S₂ than in the S₁ electronically excited state.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn²⁺ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λ_ex = 300 nm and 27% at λ_ex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S₁ state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL₂][Zn(OAc)₂]₂·2H₂O and [ZnL₂][ZnCl₂]₂·4H₂O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

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.

Excited-State Intramolecular Proton Transfer in 2-(2'-Hydroxyphenyl)pyrimidines: Synthesis, Optical Properties, and Theoretical Studies

The development of fluorescence materials with switched on/off emission has attracted great attention owing to the potential application of these materials in chemical sensing. In this work, the photophysical properties of a series of original 2-(2'-hydroxyphenyl)pyrimidines were thoroughly studied. The compounds were prepared by following well-established and straightforward methodologies and showed very little or null photoluminescence both in solution and in the solid state. This absence of emission can be explained by a fast proton transfer from the OH group to the nitrogen atoms of the pyrimidine ring to yield an excited tautomer that deactivates through a nonradiative pathway. The key role of the OH group in the emission quenching was demonstrated by the preparation of 2'-unsubstituted derivatives, all of which exhibited violet or blue luminescence. Single crystals of some compounds suitable for an X-ray diffraction analysis could be obtained, which permitted us to investigate inter- and intramolecular interactions and molecular packing structures. The protonation of the pyrimidine ring by an addition of trifluoroacetic acid inhibited the excited-state intramolecular proton transfer (ESIPT) process, causing a reversible switch on fluorescence response detectable by the naked eye. This acidochromic behavior allows 2-(2'-hydroxyphenyl)pyrimidines to be used as solid-state acid-base vapor sensors and anticounterfeiting agents. Extensive density functional theory and its time-dependent counterpart calculations at the M06-2X/6-31+G** level of theory were performed to rationalize all the experimental results and understand the impact of protonation on the different optical transitions.

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.

Insight into dual fluorescence effects induced by molecular aggregation occurring in membrane model systems containing 1,3,4-thiadiazole derivatives

This work reports on biophysical insights into the excited state intramolecular proton transfer (ESIPT) processes taking place in three 1,3,4-thiadiazole derivatives that served as model compounds, on which electronic absorption, fluorescence, Fourier-transform infrared spectroscopy (FTIR), surface plasmon resonance (SPR) and electrochemical impedance spectroscopy (EIS) studies were performed. The fluorescence spectra recorded in various solvents revealed an interesting dual fluorescence effect. In molecules in their monomeric form, the effect is associated with the ESIPT phenomenon, and may be further enhanced by aggregation-related effects, such as aggregation-induced emissions. Other spectroscopic studies on the selected molecules in a liposomal medium as a model revealed that, in a biomimetic environment, they can exist in both monomeric and aggregated forms. In both cases, however, the effects observed are closely related to the lipid's main phase transition temperature and the structure of the molecule. Introduction of specific substituents to the phenyl moiety either allows or prevents proton transfer from occurring in the excited state. The hydrophobicity changes in a lipid environment may result in an emergence of specific molecular forms and therefore either facilitate or hinder ESIPT processes. SPR and EIS confirmed the significant hydrophobicity changes in the model lipid systems, while FTIR measurements revealed a notable influence of 1,3,4-thiadiazoles on the fluidity of liposomal membranes. The results obtained clearly show that the thiadiazole derivatives are very good model molecules for studying hydrophobic-hydrophilic environments, and particularly with polymers or liposomes used as drug delivery systems.

Simple Protocol for Capturing Both Linear-Response and State-Specific Effects in Excited-State Calculations with Continuum Solvation Models

We present an effective computational protocol (cLR²) to describe both solvatochromism and fluorosolvatochromism. This protocol, which couples the polarizable continuum model to time-dependent density functional theory, simultaneously accounts for both linear-response and state-specific solvation effects. A series of test cases, including solvatochromic and fluorosolvatochromic compounds and excited-state intramolecular proton transfers, are used to highlight that cLR² is especially beneficial for modeling bright excitations possessing a significant charge-transfer character, as well as cases in which an accurate balance between states of various polarities should be restored.

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.

Substituent effect on ESIPT and hydrogen bond mechanism of N-(8-Quinolyl) salicylaldimine: A detailed theoretical exploration

The effects of substituent on excited-state intramolecular proton transfer (ESIPT) and hydrogen bonding of N-(8-Quinolyl) salicylaldimine (QS) have been studied by theoretical calculation with DFT and TDDFT. The representative electron-withdrawing nitryl and electron-donating methoxyl were selected to analyze the effects on geometries, intramolecular hydrogen bond interaction, absorption/fluorescence spectra, and the ESIPT process. The configurations of the three molecules (QS, QS-OMe and QS-NO₂) were optimized in the ground and excited states. The structure parameters, infrared spectra, hydrogen bond interactions, frontier molecular orbitals, absorption/fluorescence spectra, and potential curves have cross-validated the current results. The results show that the introduction of substituent results in a bathochromic-shift of the absorption and fluorescence spectra with large Stokes shift, and is more beneficial to the ESIPT process. The current work will be beneficial to the improvement of ESIPT properties and deepen understanding of the mechanism of ESIPT process.

Emission color-tunable oxazol(in)yl-substituted excited-state intramolecular proton transfer (ESIPT)-based luminophores

Oxazolinyl- and arylchalcogenazolyl-substituted hydroxyfluorenes exhibiting excited-state intramolecular proton transfer (ESIPT) are described as potent and highly modular luminophores. Emission color tuning was achieved by varying the π-expansion and the insertion of different chalcogen atoms.

ESIPT-Related Origin of Dual Fluorescence in the Selected Model 1,3,4-Thiadiazole Derivatives

In our previous work, we discussed the emergence of the dual fluorescence phenomenon in selected compounds from the group of 1,3,4-thiadiazoles. The results obtained in a number of experimental studies, supported by [TD]DFT calculations, clearly indicated that the phenomenon of dual fluorescence stemmed from an overlap of several factors, including the correct conformation of the analyzed molecule and, very significantly in this context, aggregation effects. Where those two conditions were met, we could observe the phenomenon of intermolecular charge transfer (CT) and the emergence of electronic states responsible for long wave emissions. However, in light of the new studies presented in this paper, we were able, for the first time, to provide a specific theory for the effect of dual fluorescence observed in the analyzed group of 1,3,4-thiadiazoles. We present the results of spectroscopic measurements conducted for two selected analogues from the 1,3,4-thiadiazole group, both in polar and non-polar solvents, which clearly evidence (as we have already suspected in the past, albeit have not shown in publications to date) the possibility of processes related to emission from the tautomer formed in the process of excited state intramolecular proton transfer, which is responsible for the long-wavelength emissions observed in the selected analogues. The presented results obtained with the use of UV-Vis, fluorescence (stationary and time-resolved), FTIR, and Raman spectroscopy, as well as from calculations of dipole moment changes between the ground and excited state with the use of two derivatives with different structures of the resorcylic system, corroborated our standing hypothesis. At the same time, they excluded the presence of ground state keto forms of the analyzed analogues unless necessitated by the structure of the molecule itself. In this case, aggregation factors enhance the observed effects related to the dual fluorescence of the analyzed compounds (by way of AIE-aggregated induced emissions).

The origin of the longer wavelength emission in 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole and its analogue 2-phenylamino-5-(2-hydroxybenzono)-1,3,4-thiadiazole† ‡

In aqueous solution, 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT) was found to emit dual emission and the longer wavelength emission was assigned to the combination of aggregation and conformational change. In a number of molecules that possess an intramolecular hydrogen bond between the proton donor and the acceptor, the longer wavelength emission is often observed due to the emission from the tautomer formed by excited state intramolecular proton transfer (ESIPT). Therefore, an analogue of FABT, 2-phenylamino-5-(2-hydroxybenzono)-1,3,4-thiadiazole (PHBT), was synthesized to determine the origin of the longer wavelength emission. The luminescence of PHBT and its methoxy derivatives was studied and compared with that of FABT. Theoretical calculations were also performed on both FABT and PHBT. Based on the experimental and theoretical investigations, the nonexistence of the keto tautomer in the ground state and the origin of the longer wavelength emission are divulged.

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.

Low-Lying Excited States of hqxcH and Zn-hqxc Complex: Toward Understanding Intramolecular Proton Transfer Emission

Excited state intramolecular proton transfer (ESIPT) has been a topic of interest due to its potential to lead to multiple emissions. Although many organic molecules showing ESIPT emission are already known, studies on metal complexes showing ESIPT and their related theoretical understandings are very limited. In this study, we focus on [Zn(hqxc)₂(DMSO)₂] (Zn-hqxc: hqxc = 3-hydroxy-2-quinoxalinecarboxylate, DMSO = dimethyl sulfoxide), which shows ESIPT emission in the solid state, even though the hqxcH ligand does not show ESIPT emission. To gain insights into the role of the zinc atom and the emission mechanisms, we examined excited states of free hqxcH and the Zn-hqxc complex using time-dependent density functional theory calculations. From the results, it was shown that the zinc atom triggers a structural change of the hqxcH ligand from the lactam form (3,4-dihydro-3-oxo-2-quinoxalinecarboxylic acid) to the enol form (3-hydroxy-2-quinoxalinecarboxylic acid), where the latter form has several stable excited states. Several stable geometries were found for singlet and triplet excited states, suggesting that emissions for the Zn-hqxc complex can be both phosphorescence and fluorescence caused by the enol-enol, keto-keto, and keto-enol forms of the two hqcx ligands in the complex. We found that the photophysical properties of the Zn-hqxc complex are dominated by the ligand due to the filled d¹⁰ of Zn(II). The presented results suggest that, to design new ESIPT metal complexes, one possible approach is to combine a metal atom showing ligand centered emission and a ligand that has separate ESIPT and coordination sites.

Excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2′-hydroxybenzofuranyl)benzoxazole (HBBO) scaffold functionalised with oligo(ethylene glycol) (OEG) chains

This article describes the multi-step synthesis of 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) derivatives functionalised with one to three oligo(ethylene glycol) (OEG) chains with the goal to allow a good vectorization in aqueous media.

A theoretical elucidation of the mechanism of tuneable fluorescence in a full-colour emissive ESIPT dye

We reinvestigate with ab initio tools the origin of the diverse colours in a complex multi-ESIPT dye, and we propose a new assignment for the blue fluorescence.

Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole

Excitation-dependent multiple fluorescence of a 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivative (1) is described. Compound 1 contains the structure of a charge-transfer (CT) 4-hydroxyphenylvinylenebipy fluorophore and an excited-state intramolecular proton transfer capable (ESIPT-capable) HBO component that intersect at the hydroxyphenyl moiety. Therefore, both CT and ESIPT pathways, while spatially mostly separated, are available to the excited state of 1. The ESIPT process offers two emissive isomeric structures (enol and keto) of 1 in the excited state, while the susceptibility of 1 to a base adds another option to tune the composite emission color. In addition to the ground-state acid-base equilibrium that can be harnessed for the control of emission color by excitation energy, compound 1 exhibits excitation-dependent emission that is attributed to solvent-affected ground-state structural changes. Therefore, depending on the medium and excitation wavelength, the emission from the enol, keto, and anion forms could occur simultaneously, which are in the color ranges of blue, green, and orange/red, respectively. A composite color of white with CIE coordinates of (0.33, 0.33) can be materialized through judicious choices of medium and excitation wavelength.

Excited state intramolecular proton transfer in julolidine derivatives: an ab initio study

We have studied, using ab initio tools, a series of recently prepared fluorescent julolidine derivatives, undergoing Excited State Intramolecular Proton Transfer (ESIPT). We show that the computed free energy change in the excited state (ΔGES) can be used to predict the preference for enol, keto, or dual emission. Indeed, two julolidine molecules experimentally show dual emission, consistent with our finding of a small ΔGES. In agreement with experimental outcomes the complexation between the ESIPT centre and BF2 increases the rigidity of the fluorophore and greatly facilitates emission at energies close to the original enol (E*) fluorescence band. The protonation of the imino group also suppresses ESIPT and sole E* emission is obtained. We disclose that chemical substitution can significantly tune the radiationless deactivation of the enol related to the C[double bond, length as m-dash]N bond rotation of the ESIPT centre. While there is a significant barrier for the experimentally studied compounds we have found a strong correlation between the barrier height and the electron donating strength of the phenyl substituent. Strong donors such as amines facilitate the barrierless non-radiative decay from E* back to the ground state, while weak electron donors make the barrier sufficiently high to allow ESIPT. Strong electron accepting groups such as -NO2 further increase this barrier. This work therefore illustrates the fine interplay necessary to design dual emitters.

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

An original ESIPT emitter showing random lasing properties is presented in this article.

Neurodevelopmental Disorders and Environmental Toxicants: Epigenetics as an Underlying Mechanism

The increasing prevalence of neurodevelopmental disorders, especially autism spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD), calls for more research into the identification of etiologic and risk factors. The Developmental Origin of Health and Disease (DOHaD) hypothesizes that the environment during fetal and childhood development affects the risk for many chronic diseases in later stages of life, including neurodevelopmental disorders. Epigenetics, a term describing mechanisms that cause changes in the chromosome state without affecting DNA sequences, is suggested to be the underlying mechanism, according to the DOHaD hypothesis. Moreover, many neurodevelopmental disorders are also related to epigenetic abnormalities. Experimental and epidemiological studies suggest that exposure to prenatal environmental toxicants is associated with neurodevelopmental disorders. In addition, there is also evidence that environmental toxicants can result in epigenetic alterations, notably DNA methylation. In this review, we first focus on the relationship between neurodevelopmental disorders and environmental toxicants, in particular maternal smoking, plastic-derived chemicals (bisphenol A and phthalates), persistent organic pollutants, and heavy metals. We then review studies showing the epigenetic effects of those environmental factors in humans that may affect normal neurodevelopment.