Excited-state Intramolecular Proton Transfer Research Articles

Proton transfer induced excited-state aromaticity gain for chromophores with maximal Stokes shifts.

Excited state aromaticity (ESA) offers a fascinating route for driving photophysical and photochemical processes but is challenging to harness fully due to its inherent association with unstable antiaromatic ground states. Here, we propose to circumvent this problem via the introduction of a new class of photophysical processes, the generation of ESA via an excited-state intramolecular proton transfer. We select twelve candidate molecules based on the cyclobutadiene and pentalene scaffolds and investigate their ground and excited state properties using computation. The study highlights the feasibility of proton transfer induced ESA gain and shows that it gives rise to pronounced excited-state relaxation producing Stokes shifts in excess of 2 eV. The underlying electronic structure properties are analysed in terms of the orbitals involved as well as aromaticity descriptors illustrating the pronounced changes these molecules undergo upon both excitation and proton transfer. In summary, we believe that the present work will pave the way toward a new class of chromophores with maximal Stokes shifts and excited-state relaxation.

Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls.

A combined computational and experimental study reveals that ortho-, meta- and para-aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium exchange experiments show that the ortho-isomer undergoes a facile photoprotonation at a carbon atom via excited-state intramolecular proton transfer (ESIPT). The meta-isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction via proton-coupled electron transfer (PCET). The para-isomer undergoes a water-assisted ESPT reaction. All three reactions take place in the singlet excited-state, except for the photoredox process of the meta-isomer, which involves a triplet excited-state. Computations illustrate the important role of excited-state antiaromaticity relief in these photoreactions.

Characterizing excited-state intramolecular proton transfer in 3-hydroxyflavone with ultrafast transient infrared spectroscopy.

Vibrational dynamics associated with excited-state intramolecular proton transfer in 3-hydroxyflavone and 3-hydroxy-2-(thiophen-2-yl)chromen-4-one are characterized with ultrafast transient infrared spectroscopy. The spectroscopic data reveal rapid (<100 fs) proton transfer dynamics in both species, followed by vibrational relaxation of the tautomer products within a few picoseconds. Coherent coupling of the shared proton to low-frequency modes along the hydrogen bond coordinate are also observed in the ground electronic states.

An ESIPT-active orange-emissive 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine-derived chemodosimeter for turn-on detection of fluoride ions via desilylation.

Fluoride is an essential element for oral health with an optimum concentration of 0.7-1.2 ppm in drinking water, but it is detrimental at higher concentrations, causing fluorosis, acute gastric ulcer, urolithiasis, and kidney infection, which adds immense significance to its detection in water sources. In the current study, a new chemodosimeter (HIPS-Br) is designed by protecting a 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine derivative (HIP-Br) with a fluoride recognizable tert-butyldiphenylsilane moiety and utilized for the selective detection of F- ions by an excited-state intramolecular proton transfer (ESIPT)-based fluorimetric response. The probe HIPS-Br exhibits blue fluorescence in solution, and upon the incremental addition of F- ions, it exhibits a turn-on response, exhibiting a strong orange emission at 598 nm by spontaneous cleavage of the tert-butyldiphenylsilane group to release fluorescent HIP-Br in the working solution. HIPS-Br displayed no or insignificant response towards numerous common anions, cations and small molecules, affirming its selectivity to F- ions and offered a low limit of detection (LOD) of 1.2 ppb (6.6 × 10-8 M). The real sample analysis by spiking fluorides in water and toothpaste samples showed excellent percent recoveries. The chemodosimeter was successfully utilized in the solid-phase detection of F- ions on silica-coated TLC plates and analyzed by ImageJ analysis, marking its utility in on-site quantitation purposes.

Excited-state dynamics of 3-hydroxychromone in gas phase.

In recent years, 3-hydroxychromone (3-HC) and its derivatives have attracted much interest for their applications as molecular photoswitches and fluorescent probes. A clear understanding of their excited-state dynamics is essential for their applications and further development of new functional 3-HC derivatives. However, the deactivation mechanism of the photoexcited 3-HC family is still puzzling as their spectral properties are sensitive to the surrounding medium and substituents. The excited-state relaxation channels of 3-HC have been a matter of intense debate. In the current work, we thoroughly investigated the excited-state decay process of the 3-HC system in the gas phase using high-level electronic structure calculations and on-the-fly excited-state dynamic simulations intending to provide insight into the intrinsic photochemical properties of the 3-HC system. A new deactivation mechanism is proposed in the gas phase, which is different from that in solvents. The excited-state intramolecular proton transfer (ESIPT) process that occurs in solutions is not preferred in the gas phase due to the existence of a sizable energy barrier (∼0.8 eV), and thus, no dual fluorescence is found. On the contrary, the non-radiative decay process is the dominant decay channel, which is driven by photoisomerization combined with ring-puckering and ring-opening processes. The results coincide with the observations of an experiment performed in a supersonic jet by Itoh (M. Itoh, Pure Appl. Chem., 1993, 65(8), 1629-1634). The current work indicates that the solution environment plays an important role in regulating the excited-state dynamic behaviour of the 3-HC system. This study thus provides theoretical guidance for the rational design and improvement of the photochemical properties of the 3-HC system and paves the way for further investigation into its photochemical properties in complex environments.

Excited-state intramolecular proton-transfer solid-state fluorophores with aggregation-induced emission as efficient emitters for electroluminescent devices

Excited-state intramolecular proton transfer (ESIPT) materials have emerged as highly promising candidates for emitters in organic light-emitting diodes (OLEDs), attributed to their distinctive photophysical properties.

Glowing discoveries: Schiff base-cyanostilbene probes illuminating metal ions and biological entities.

Schiff bases featuring cyanostilbene units have emerged as versatile and highly effective probes for the selective detection of various metal ions as well as biologically important species. This review comprehensively highlights recent advances in the development and application of the probes, which exhibit remarkable Aggregation-Induced Emission (AIE), Twisted Intramolecular Charge Transfer (TICT), and Excited-State Intramolecular Proton Transfer (ESIPT) properties. These unique structural characteristics facilitate their potential applications in the detection of biologically important metal ions such as Zn2+, Fe3+, Cu2+, Hg2+ and Co2+ ions with high sensitivity and selectivity. Furthermore, these probes have demonstrated significant potential in the recognition of vital biological species, including arginine, hydrazine and hypochlorite (ClO-). The present review discusses the underlying detection mechanisms, emphasizing the role of the Schiff base and cyanostilbene moieties for the selective detection of particular biologically important entities. Moreover, this discussion highlights the practical applications, problems, and future directions in this fast-growing field, emphasizing the vital importance of these probes in both analytical chemistry and bioassays.

Reticular chemistry guided precise construction of zirconium-pentacarboxylate frameworks with 5-connected Zr6 clusters.

Zirconium-based metal-organic frameworks (Zr-MOFs) have been extensively studied due to their very rich structural chemistry. The combination of nearly unlimited carboxylic acid-based linkers and Zr6 clusters with multiple connectivities has led to diverse structures and specific properties of resultant Zr-MOFs. Herein, we demonstrate the successful use of reticular chemistry to construct two novel Zr-MOFs, HIAM-4040 and HIAM-4040-OH, with zfu topology. Based on a thorough structural analysis of (4,4)-connected lvt-type Zr-tetracarboxylate frameworks and a judicious linker design, we have obtained the first example of a Zr-pentacarboxylate framework featuring unprecedented 5-connected organic linkers and 5-connected Zr6 clusters. Compared with HIAM-4040, a larger Stokes shift is achieved in HIAM-4040-OH via hydroxyl group induced excited-state intramolecular proton transfer (ESIPT). HIAM-4040-OH exhibits high chemical and thermal stability and is used for HClO detection in aqueous solution with excellent sensitivity and selectivity.

Isoindoline-based fluorogenic probes bearing a self-immolative linker for the sensitive and selective detection of O-GlcNAcase activity.

O-GlcNAcase (OGA) is implicated in several important biological and disease-relevant processes. Here, we synthesized fluorogenic probes for OGA by grafting GlcNAc directly or using a self-immolative linker to the hydroxyl position of 4-hydroxylisoindoline (BHID), a typical excited-state intramolecular proton transfer (ESIPT) probe. The probe was used for a fluorogenic assay to determine the half maximal inhibitory concentration of a known OGA inhibitor and differentiate between OGA and hexosaminidase when GlcNAc is replaced by GlcNPr, where a propionyl group is used instead of an acetyl group.

Azaindolizine proton cranes attached to 7-hydroxyquinoline and 3-hydroxypyridine: a comparative theoretical study.

Theoretical design of several proton cranes, based on 7-hydroxyquinoline and 3-hydroxypyridine as proton-transfer frames, has been attempted using ground and excited-state density functional theory (DFT) calculations in various environments. Imidazo[1,2-a]pyridine, pyrazolo[1,5-a]pyridine and benzimidazole were considered as proton crane units. The proton crane action requires the existence of a single enol-like form in the ground state, which under excitation goes to the end keto-like one through a series of consecutive excited-state intramolecular proton transfers (ESIPT) and twisting steps with the participation of a crane unit, resulting in a long-range intramolecular proton transfer. The results suggest that 3-hydroxypyridine is not suitable for a proton-transfer frame and 8-(imidazo[1,2-a]pyridin-2-yl)quinolin-7-ol and 8-(pyrazolo[1,5-a]pyridin-2-yl)quinolin-7-ol behave as non-conjugated proton cranes, instead of tautomeric re-arrangement in the latter, which was thought to be possible.

Photoproperties of favipiravir and its 6-substituted analogues: fluorescence controlled through halogen substitution and tautomerism.

Herein, we have showed the photophysical properties of favipiravir and its 6-substituted analogues. Also, we interpreted the origin of fluorescence of favipiravir and its 6-substituted analogues as a function of tautomerism modulation in ground and excited states. Favipiravir, the 6-fluorine derivative, showed the best photophysical profile, exhibiting a dominant emission wavelength of 430 nm, a high quantum yield (Q.Y.) of 1.0 and a long-lived state (10 ns). Its analogues also showed a maximum emission at 430 nm, but their Q.Y. values were 5-fold lower than that found for favipiravir, decreasing as a function of 6-substitution as follows: F > Cl > Br > I > H. Pyrazines bearing the least electronegative 6-substituent (X = Br, I, H) showed an extra lifetime, which was shorter (0.2-0.3 ns) and less abundant (>15%) than the main lifetime (10 ns, 85%). Further 2D excitation-emission matrix and solvent studies supported that these 3-hydroxy-2-pyrazinecarboxamides present two emissive states. The first of them (λem = 430 nm), which was the most abundant, most fluorescent and long-lived state, was characterized as "locally excited" (LE). Its fluorescence was favored with an increase of the hydrogen-donor nature of the solvent and for pyrazines having a high enolic characteristic. Thus, the high LE-fluorescence of these types of pyrazines depends on the keto-tautomerization of the ground state using a protic solvent and its feasible enol-tautomerization upon excitation. Finally, the second excited state (λem = 536 nm) was suggested as an excited-state intramolecular proton-transfer (ESIPT), and it was observed only, although discretely, for pyrazines bearing the least electronegative 6-substituent.

Excited-state dynamics of 4-hydroxyisoindoline-1,3-dione and its derivative as fluorescent probes.

Fluorescent probes have become promising tools for monitoring the concentration of peroxynitrite, which is linked to many diseases. However, despite focusing on developing numerous peroxynitrite based fluorescent probes, limited emphasis is placed on their sensing mechanism. Here, we investigated the sensing mechanism of a peroxynitrite fluorescent probe, named BHID-Bpin, with a focus on the relevant excited state dynamics. The photoexcited BHID-Bpin relaxes to its ground state via an efficient nonradiative process (∼300 ps) due to the presence of a minimum energy conical intersection between its first excited state and ground state. However, upon reacting with peroxynitrite, the Bpin moiety is cleaved from BHID-Bpin and BHID is formed. The formed BHID exhibits strong dual band fluorescence which is caused by an ultrafast excited-state intramolecular proton transfer process (∼1 ps).

Effect of structure on excited-state intramolecular proton transfer-based sensors for phosphonofluoridate G-series nerve agent vapour detection

Excited-state intramolecular proton transfer emitters have emission that is significantly red shifted relative to the absorption spectra, which enables the sensitive detection of extant hydrogen fluoride found in G-series nerve agents.

Synthesis of a benzothiazole-based structure as a selective colorimetric-fluorogenic cyanate chemosensor

Abstract In this study, a new heterocyclic compound incorporating a benzothiazole moiety was specifically designed for the detection of cyanate anions, employing a hydrogen bonding mechanism. Through strategic integration of triazine and phenylenediamine cyclic groups into the compound’s structure, intramolecular hydrogen bonding interactions between the donor and acceptor sites were enhanced, leading to exceptional sensitivity towards cyanate anions. Utilizing the amino-type excited-state intramolecular proton transfer phenomenon, this new compound exhibited dual signals and achieved a significant Stokes’ shift via proton transfer, coupled with aggregation-induced emission properties. This unique combination resulted in visible color changes and an impressive fluorescence response, offering a promising solution for the sensitive detection of cyanate ions in critical environmental matrices. Cyanate detection at low concentrations by this as-synthesized compound (L1), accompanied by a distinct color change and a gradual fluorescence increase upon incremental cyanate addition demonstrated L1’s selectivity, as confirmed in the presence of various competing anions F−, Cl−, Br−, I−, ClO 3 − {\text{ClO}}_{3}^{-} , ClO 4 − {\text{ClO}}_{4}^{-} , NO 3 − {\text{NO}}_{3}^{-} , BrO 3 − {\text{BrO}}_{3}^{-} , CN− and CNO−. Spectrofluorometric investigations demonstrated that L1 shows significant potential as a selective cyanate anion detection candidate.

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment.

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.

Surface functionalized graphene oxide integrated 9,9-diethyl-9H-fluoren-2-amine monohybrid nanostructure: Synthesis, physicochemical, thermal and theoretical approach towards optoelectronics

The fields of energy materials, water purification, environmental protection, catalysis and biomedical technology are greatly promoted by the usage of graphene oxide (GO) and its functionalized derivatives. Among them, the application of functionalized GO in lightning has recently drawn growing attention towards optoelectronics. In the current study, designed and synthesized a 9,9-diethyl-9H-fluoren-2-amine (FA) functionalized with GO by using CDI catalyst results, FA modified graphene oxide (FA-GO) monohybrid. The structural characterization was made with the help of FTIR and Raman spectroscopy; and explored their thermal decomposition temperature (>150 °C). Further, the optical characteristics of UV–visible and photoluminescence (PL) indicate the absorption and emission of FA-GO are ∼314 nm and ∼420 nm, respectively. The calculated CIE color coordinates found to be better in thin film at deep blue emission, are x = 0.158, y = 0.023 (0.1 % PMMA matrix). The FA-GO monohybrid exhibits a striking improvement in PL caused by surface passivation and excited-state intramolecular proton transfer. In addition, density functional theory calculations are used to analyse the stability of the designed structure and its interlayer separation. The results of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are −6.357 eV and −2.286 eV respectively with the energy gap ΔE = 4.071 eV. In addition, the FMOs excel in evaluating various chemical reactivity descriptors such as electronegativity (χ = 4.3215), global hardness (η = 2.0355), softness (δ = 0.4913) and electrophilicity (ω = 4.5874). Furthermore, a high dipole moment value of 9.133 Debye of FA-GO ensures a strong intramolecular interaction. These evolutions help to improve the optical properties based on amine-functionalized GO. The outcomes, directing an approach to the development of graphene-based materials for blue lightning applications.

Vibronic structure of absorption spectrum of 2,5-di-(2-benzoxazolyl)phenol

Vibronic structure of the absorption spectrum of 2,5-di-(2-benzoxazolyl)phenol is calculated using the Franck-Condon approximation and harmonic model for vibrations. 2,5-di-(2-benzoxazolyl)phenol is an organic compound that exhibits excited-state intramolecular proton transfer. It is found that the use of normal modes and values of energy of the electronic states calculated with the B3LYP density functional provides good agreement with the experimental spectrum in terms of the position of intense bands. The largest intensity of vibronic transitions is found for deformation vibrations changing the distance between the oxygen and nitrogen atoms of the hydrogen bond, a deformation vibration of the phenol ring, stretching vibrations of the benzoxazole moieties.

Effects of substitution and conjugation on photophysical properties of ESIPT-based fluorophores with the core of 4-aminophthalimide

4-Aminophthalimide is a highly fluorescent signaling unit with excellent photophysical properties and wide application foregrounds. Based on this, a range of theoretical investigations are conducted on the fluorescent probe (E)-5-((2-hydroxybenzylidene) amino) isoindoline-1, 3-dione (HID) with the core of 4-aminophthalimide using density functional theory (DFT) and time-containing density functional theory (TD-DFT) methods in this paper. The optimized configurations, vertical excitation and emission energies, electronic characteristics and excited-state intramolecular proton transfer (ESIPT) behaviors of the probe HID are discussed in detail. Furthermore, to enhance the luminescent properties of HID, five novel compounds have been designed based on the structure of HID by introducing amino, methoxy and naphthalene groups (–NH2, –OMe and C10H8). Our work thoroughly explores how the property and position of substituents and conjugation affect photophysical characteristics and ESIPT processes. We find that the ESIPT dynamics can be modulated by the substitution and conjugation effects. Specifically, the introduction of amino and methoxy groups at the ortho-position and the introduction of the naphthalene group promote the ESIPT behavior of HID1, whereas the introduction of amino and methoxy groups at the meta-position exhibits the contrary impact. Therefore, we boldly infer that the introduction of electron-donating groups in the ortho-position and the introduction of the conjugated group make the ESIPT process more effortless to occur, whereas the introduction of substituents with opposing natures in the meta-position makes the ESIPT process more difficult to occur. In addition, the ionization potentials (IP), electron affinities (EA) and reorganization energies (λh and λe) of molecules are calculated to assess their potential as luminescent materials. Our work not only reveals the luminescence and ESIPT mechanism of the probe HID1, but also proposes to modulate the ESIPT process through the substitution and conjugation effects. In particular, the designed molecules have better photoelectric properties as a result of their red-shifted absorption and fluorescence spectra, smaller energy gaps, larger transferred charges and greater charge transferred distances, which offers some valuable ideas for the experimental development of more efficient organic luminescent materials with ESIPT properties.

A novel “AIE+ESIPT” mechanism-based fluorescent probe for visual alternating recognition of HSO3−/H2O2 and its HSO3− detection in food samples

Maintaining redox homeostasis in organisms plays a crucial role in human life and health, and changes in the levels of SO2 and H2O2, as typical representative substances of reductants and oxidants, directly affects intracellular redox homeostasis. Hence, it is necessary to manufacture fluorescent probes that can detect HSO3− and H2O2. In this work, a benzopyrylium salt containing benzothiazole derived fluorescent probe HBTO-H with dual properties of aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) has been developed. The probe displays rapid and reversible test of HSO3− and H2O2 in DMF/Tris (v:v = 2:8, pH = 7.3, Tris 10 mM) mixed solvent with high selectivity, good anti-interference ability and a wide applicable pH range (6–11). The detection limits of HBTO-H for HSO3− and H2O2 are 0.187 μM and 0.152 μM, respectively. The response mechanisms of HBTO-H toward HSO3− and H2O2 have been demonstrated to be Michael addition reaction and its retro-reaction based on 1H NMR and HRMS analyses. In addition, HBTO-H has been successfully applied to check HSO3− in sugar and wine specimens. Moreover, HBTO-H was proved to be mitochondria-targetable and can alternately detect exogenous HSO3− and H2O2 in MCF-7 cells, which is desired to be applied to trace HSO3− in organisms in the future.

Modeling of absorption spectrum of 2,5-bis(2-benzoxazolyl)hydroquinone

Vibronic structure of the absorption band of 2,5-bis(2-benzoxazolyl)hydroquinone is calculated for the transitions to the first excited state using the Franck-Condon approximation and harmonic model for vibrations. 2,5-bis(2-benzoxazolyl)hydroquinone is an organic compound that undergoes the excited state intramolecular proton transfer. It is found that the use of normal modes and values of energy of the electronic states calculated with the ωB97X-D3 density functional provides good agreement with the experimental spectrum, overestimating energy of the 0-0 transition by 0.13 eV. The largest intensity of vibronic transitions is demonstrated by deformation vibrations changing the distance between the oxygen and nitrogen atoms of the hydrogen bonds, a deformation vibration of the oxazole rings, deformation and stretching vibrations of the hydroquinone moiety.