Excited-state Intramolecular Proton Transfer Dye Research Articles

Organofunctionalization of silica from rice husk ash with ESIPT dyes

Excited-state intramolecular proton transfer (ESIPT) dyes were immobilized on amorphous silica obtained from rice husk ash through surface modification methodology. The silanol groups present on the surface of silica were first functionalized with isocyanate groups to be reacted with ESIPT dyes. Scanning electron microscopy, infrared spectroscopy, thermogravimetric, nitrogen adsorption/desorption, powder X-ray diffraction, and solid-state fluorescence emission spectroscopy analysis have been carried out to characterize the materials. The synthesized silica-based fluorescent materials were used as latent fingerprint developer powders present on glass surfaces. The results showed that latent fingerprints were revealed with high contrast under ultraviolet light illumination.

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.

Two is better than one: ESIPT dyes as photoinitiators in two-photon polymerization

In this work, we present the successful application of Excited-State Intramolecular Proton Transfer (ESIPT) fluorophores as photoinitiators for microfabrication. The influence of chemical structure on such application was investigated, followed by two-photon polymerization. We have determined the quality of fabricated structures and polymerization thresholds for various dye/monomer weight ratios with a staggeringly low value of 11 nJ for one of the chromophores.

Fluorescence Lifetimes of NIR-Emitting Molecules with Excited-State Intramolecular Proton Transfer

Molecular probes based on the excited-state intramolecular proton-transfer (ESIPT) mechanism have emerged to be attractive candidates for various applications. Although the steady-state fluorescence mechanisms of these ESIPT-based probes have been reported extensively, less information is available about the fluorescence lifetime characteristics of newly developed NIR-emitting dyes. In this study, four NIR-emitting ESIPT dyes with different cyanine terminal groups were investigated to evaluate their fluorescence lifetime characteristics in a polar aprotic solvent such as CH2Cl2. By using the time-correlated single-photon counting (TCSPC) method, these ESIPT-based dyes revealed a two-component exponential decay (τ1 and τ2) in about 2-4 nanoseconds (ns). These two components could be related to the excited keto tautomers. With the aid of model compounds (5 and 6) and low-temperature fluorescence spectroscopy (at -189 ℃), this study identified the intramolecular charge transfer (ICT) as one of the major factors that influenced the τ values. The results of this study also revealed that both fluorescence lifetimes and fractional contributions of each component were significantly affected by the probe structures.

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.

Progress in Tuning Emission of the Excited-State Intramolecular Proton Transfer (ESIPT)-Based Fluorescent Probes.

In this review, we will summarize our recent progress in the design and application of novel organic sensors with emission in the near-infrared region (600–900 nm). By coupling different functional groups with excited-state intramolecular proton transfer (ESIPT) segments, new probes are developed to achieve a large Stokes shift, high sensitivity, and selectivity and to tune the emission toward the near-infrared region. The developed probes exhibit attractive optical properties for bioimaging and environmental science applications. In addition, we further discuss the photophysical properties of ESIPT dyes and how their fluorescence could be affected by structural/environmental factors, which should be considered during the development of robust ESIPT-based fluorescence probes. Their potential applications as imaging reagents are illustrated for intracellular membranes, mitochondria, lysosomes, and some biomolecules.

Dual-color fluorescent nanoparticles showing perfect color-specific photoswitching for bioimaging and super-resolution microscopy

Dual-emissive systems showing color-specific photoswitching are promising in bioimaging and super-resolution microscopy. However, their switching efficiency has been limited because a delicate manipulation of all the energy transfer crosstalks in the systems is unfeasible. Here, we report a perfect color-specific photoswitching, which is rationally designed by combining the complete off-to-on fluorescence switching capability of a fluorescent photochromic diarylethene and the frustrated energy transfer to the other fluorescent dye based on the excited-state intramolecular proton transfer (ESIPT) process. Upon alternation of UV and visible light irradiations, the system achieves 100% switching on/off of blue emission from the diarylethene while orange emission from the ESIPT dye is unchanged in the polymer film. By fabricating this system into biocompatible polymer nanoparticles, we demonstrate microscopic imaging of RAW264.7 macrophage cells with reversible blue-color specific fluorescence switching that enables super-resolution imaging with a resolution of 70 nm.

Low Amplified Spontaneous Emission Threshold and Efficient Electroluminescence from a Carbazole Derivatized Excited-State Intramolecular Proton Transfer Dye

The search for new classes of efficient electroluminescent organic laser dyes with low amplified spontaneous emission (ASE) threshold (Eth) and low optical losses is crucial toward the realization of organic injection lasers. In this work, we report in-depth studies of an organic semiconducting laser dye, benzo[d]thiazol-2-yl)-5-(9H-carbazol-9-yl)phenol (HBT-Cz), and compare the photophysical study with its parent HBT dye. While HBT is not laser-active in solution, the carbazole derivatized dye (i.e., HBT-Cz) showed a low solution ASE threshold of 21 mJ/cm2, which is comparable to a common high-performing commercial laser dye, Rhodamine 6G (Rh-6G). HBT-Cz also exhibited a low film ASE threshold of 2.4 μJ/cm2 with a low optical loss coefficient of 1.5 cm–1, which is the lowest waveguide loss coefficient reported for solution-processed organic semiconductors. Organic light-emitting diodes based on this material showed a high maximum EQE of 1.9%, which is close to the theoretical EQE limit of the emitter, an...

Mechanochemical Activation of Polymer-Embedded Photoluminescent Benzoxazole Moieties.

Despite an increasing number of studies that have investigated mechanochemical effects in polymers, the number of polymers whose fluorescence characteristics change upon exposure to mechanical stress is still limited. We here report the investigation of a mechanofluorophore based on an aliphatic ester of 2-(2'-hydroxyphenyl)benzoxazole. The free benzoxazole displays green photoluminescence, which is associated with an excited state intramolecular proton transfer (ESIPT) process, whereas aliphatic esters of this compound emit blue light. When poly(methyl acrylate) containing an esterified benzoxazole mechanophore at the center of each chain molecule was exposed to ultrasound, a significant reduction of the molecular weight and pronounced changes of the photoluminescence emission and UV-vis absorption spectra were observed. The optical changes and the fact that the time-traces for molecular weight decrease and formation of the ESIPT capable species mirror each other indicate that the mechanophore is preferentially cleaved upon sonication and that such cleavage restores the 2-(2'-hydroxyphenyl)benzoxazole motif. The concept of mechanical activation of ester-protected ESIPT dyes, and more broadly of other hydroxyl group carrying fluorophores that change their emission properties upon ester formation and cleavage, should be general and allow access to a range of other mechanofluorophores.

An NIR-emitting ESIPT dye with large stokes shift for plasma membrane of prokaryotic (E. coli) cells

A near infrared-emitting cyanine with a very large Stokes shift (Δλ ≈ 230 nm) was synthesized by coupling with excited state intramolecular proton transfer (ESIPT). Fluorescence response of the probe was examined at different pH and at different temperatures, which revealed the important role of ESIPT in the fluorescence. In comparison with commercial FM1-43 (ϕfl ≈ 0.03 in CH2Cl2), which is a popular styryl dye for membrane staining, the developed new probe 2a exhibited a significantly higher quantum yield (ϕfl ≈ 0.34 in CH2Cl2), attributing to the effective intramolecular hydrogen bonding which is present in the latter. In addition, the probe exhibited very good selectivity for imaging plasma membrane of prokaryotic cells, indicating its potential for imaging membrane of procaryotic (E. coli) cells.

ESIPT and FRET probes for monitoring nanoparticle polymer coating stability.

Coating strategies of inorganic nanoparticles (NPs) can provide properties unavailable to the NP core alone, such as targeting, specific sensing, and increased biocompatibility. Non-covalent amphiphilic NP capping polymers function via hydrophobic interactions with surface ligands and are extensively used to transfer NPs to aqueous media. For applications of coated NPs as actuators (sensors, markers, or for drug delivery) in a complex environment, such as biological systems, it is important to achieve a deep understanding of the factors affecting coating stability and behavior. We have designed a system that tests the coating stability of amphiphilic polymers through a simple fluorescent readout using either polarity sensing ESIPT (excited state intramolecular proton transfer) dyes or NP FRET (Förster resonance energy transfer). The stability of the coating was determined in response to changes in polarity, pH and ionic strength in the medium. Using the ESIPT system we observed linear changes in signal up to ∼20-25% v/v of co-solvent addition, constituting a break point. Based on such data, we propose a model for coating instability and the important adjustable parameters, such as the electrical charge distribution. FRET data provided confirmatory evidence for the model. The ESIPT dyes and FRET based methods represent new, simple tools for testing NP coating stability in complex environments.

Excited State Intramolecular Proton Transfer in Ethynyl-Extended Regioisomers of 2-(2'-Hydroxyphenyl)benzothiazole: Effects of the Position and Electronic Nature of Substituent Groups.

Although the organic dyes based on excited state intramolecular proton transfer (ESIPT) mechanism have attracted significant attention, the structure-property relationship of ESIPT dyes needs to be further exploited. In this paper, three series of ethynyl-extended regioisomers of 2-(2'-hydroxyphenyl)benzothiazole (HBT), at the 3'-, 4'- and 6-positions, respectively, have been synthesized. Changes in the absorption and emission spectra were correlated with the position and electronic nature of the substituent groups. Although 4'- and 6-substituted HBT derivatives exhibited absorption bands at longer wavelengths, the keto-emission of 3'-substituted HBT derivatives was found at a substantially longer wavelength. The gradual red-shifted fluorescence emission was found for 3'-substituted HBT derivatives where the electron-donating nature of substituent group increased, which was opposite to what was observed for 4'- and 6-substituted HBT derivatives. The results derived from the theoretical calculations were in conformity with the experimental observations. Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.

Iron Oxide Nanoparticles Labeled with an Excited-State Intramolecular Proton Transfer Dye.

Excited-state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica-coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.

Fluorescence ratiometric sensing of polyols by phenylboronic acid complexes with ligands exhibiting excited-state intramolecular proton transfer in aqueous micellar media

2-Phenyl-3-hydroxy-4(1H)-quinolone possessing dual fluorescence due to excited-state intramolecular proton transfer (ESIPT) forms stable complex with phenylboronic acid with blue shifted emission maximum in micellar medium of a cationic surfactant even though the compound lacks required for complexation with boronic acids cis-diol structure. No complexation is observed in the presence of neutral or anionic surfactants. Titrations of this complex with polyols including sugars and nucleotides at pH 8 displace free quinolone showing ratiometric response, which allows determination of polyols with detection limits 0.05–1mM and unusually wide linear dynamic ranges. Another ESIPT dye 2-(2′-hydroxyphenyl)−1H-benzimidazole also lacking cis-diol structure forms equally stable complex with phenylboronic acid and allows ratiometric determination of polyols with similar characteristics. The results of this study demonstrate that blocking ESIPT of signaling molecule by complexation of the receptor with the proton donor group eliminates the low energy emission from tautomeric form but strongly enhances the high energy emission typical for “normal” form of signaling molecule creating a possibility of ratiometric sensing.

A highly selective ESIPT-based fluorescent probe for cysteine sensing and its bioimaging application in living cells

A novel fluorescent probe, viz. PI-Cys, was developed on the basis of the excited state intramolecular proton transfer (ESIPT) mechanism for the detection of cysteine (Cys). It consists of an new ESIPT dye as the fluorophore, linked via a hydroxyl moiety to an acrylate group. The acrylate group acts as an ESIPT blocking agent as well as a functional recognition moiety. In the presence of Cys, the reaction of Cys with PI-Cys results in cleavage of the acrylate moiety from PI-Cys, thereby unleashing the ESIPT process and leading to about 20-fold enhancement of fluorescence. The probe exhibits high sensitivity and selectivity toward Cys over homocysteine and glutathione. Finally, the probe was successfully applied for fluorescence bioimaging of Cys in living cancer cells.

Association of α-Synuclein with Lipid Vesicles. Stopped-Flow Kinetics of Concerted Binding and Conformational Change

Alpha-synuclein (AS), a 140aa intrinsically disordered protein, self-associates into oligomeric forms and aggregates into amyloid fibrils in Parkinson's disease. Certain mutations affect these processes and accelerate disease pathogenesis. The physiological roles of AS are a matter of speculation. Membrane binding is undoubtedly involved and the protein acquires α-helical structure in the process (1).We have studied the thermodynamics and kinetics of AS-membrane association utilizing vesicles (SUVs) of differing composition. Functionally neutral single cysteine mutants of AS were labeled with a polarity sensitive excited-state intramolecular proton transfer (ESIPT) probe (MFE). Double cysteine mutants were labeled with a FRET pair (Alexa Fluor488, Alexa Fluor568) at a series of selected positions in the primary sequence. Kinetic studies were conducted by stopped-flow, using 5-20 nM protein concentrations and increasing levels of SUVs (generally 20-200 µM) Signal changes indicative of membrane association were observed: increased intensity and shape change of dual band ESIPT emission, and altered FRET with the Alexa dyes. The analysis revealed a two-step reaction sequence in the time range <10 s. We attribute the first step to binding, and from the dependence on lipid concentration determined the second order rate constants and corresponding spectroscopic parameters. The second concentration independent step (1-10 s range) presumably arises from conformational changes in the protein (α-helix formation) and its accommodation to or perturbation of the lipid microenvironment (ESIPT dye).Accompanying thermodynamic measurements led to estimates of dissociation constants as a function of membrane composition, charge, and shape (SUVs, LUVs). A new experimental protocol (slopes), implemented in a microplate reader, circumvented technical problems usually manifested in titrations of protein with lipid.[1] Pfefferkorn C M, Jiang, Lee J C (2012). Biophysics of α-synuclein membrane interactions. Biochim Biophys Acta1818:162-171.

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.

Fluorescence of methylated derivatives of hydroxyphenylimidazopyridine. Resolution of strongly overlapping spectra and a new ESIPT dye showing very efficient radiationless deactivation

The ground- and excited-state behaviour of the isomeric species 2-(2'-methoxyphenyl)imidazo[4,5-b]pyridine (1-OMe) and 2-(2'-hydroxyphenyl)-4-methylimidazo[4,5-b]pyridine (1-NMe) in neutral and acid media has been studied by UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy. The new dye 1-NMe is non-fluorescent in neutral media except in trifluoroethanol, where it shows a very weak fluorescence. 1-NMe also exhibits highly solvent-dependent fluorescence intensity in acidic media. We propose that the neutral species experiences a fast excited-state intramolecular proton transfer (ESIPT), relaxing afterwards by intramolecular twisting associated with internal charge transfer (TICT) and subsequent very fast internal conversion of the proton-transferred TICT structure. The behaviour of 1-NMe in acidic media is explained by the existence of a ground-state tautomeric equilibrium between species with intramolecular hydrogen bonds N-HOH and NHO. The first type of tautomers dissociates at the hydroxyl group in water and ethanol, but fluoresces in acetonitrile and trifluoroethanol due to the inability of these solvents to accept the proton. The second type of tautomers is non-emissive due to fast radiationless deactivation through an ESIPT-TICT process. The fluorescence of 1-OMe was investigated in neutral and acidic media, demonstrating the photobasic character of the pyridine nitrogen. A ground-state equilibrium between pyridinium and imidazolium cations was found for this species, showing overlapping absorption and fluorescence spectra. We devised a method to resolve the spectra by applying principal component global analysis to a series of excitation spectra taken at different emission wavelengths, which allowed estimation of the equilibrium constant between the cations.

Design and Synthesis of Trithiophene-Bound Excited-State Intramolecular Proton Transfer Dye: Enhancement on the Performance of Bulk Heterojunction Solar Cells

In an aim to harvest UV-near-visible (360-440 nm) photons as well as to increase the morphology in the bulk heterojunction solar cells, we report herein the strategic design, synthesis, and characterization of a novel excited-state intramolecular proton-transfer dye, 3-hydroxy-2-(5-(5-(5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)-4H-chromen-4-one (FT), which bears two key functional groups, namely 3-hydroxychromone chromophore and trithiophene backbone and is then exploited into the blends of regioregular poly(3-hexylthiophene) (RR-P3HT) and phenyl-C(61)-butyric acid methyl ester (PCBM). FT acts as an excellent UV-near visible absorber, which then undergoes excited-state intramolecular proton transfer, giving rise to an orange-red proton-transfer emission that was reabsorbed by P3HT via a Forster type of energy transfer. Introduction of FT to P3HT/PCBM blend films also improves the morphology of phase separated structure, in particular, enhances the interaction of P3HT chains and the hole mobility. In this work, under the optimized condition of P3HT: PCBM:FT of 15:9:2 in weight ratio, the best performance of the device B-FT2 revealed consistent enhancements in the efficiency (eta) 4.28% and short-circuit current (J(sc)) 12.53 mAcm(-2), which are higher than that (3.68% and 10.28 mAcm(-2)) of the best performance of the control device B (P3HT:PCBM 15:9 in weight ratio) by 16 and 22%, respectively.

All-optical polymeric interferometeric wavelength converter comprising an excited state intramolecular proton transfer dye

We designed and demonstrated an all-optical wavelength converter using a polymeric Mach–Zehnder interferometer (MZI) comprised of an excited state intramolecular proton transfer (ESIPT) dye, 2,2′-{oxybis[4-(4-methoxyphenyl)quinoline-6,2-diyl]}bis(5-methoxyphenol) (MQ). This MZI wavelength converter is composed of the MQ dye-doped polymeric waveguide and a thick light blocking metal film. A feature of this device is that one arm of the MZI can be irradiated by 355 nm pulses (signal beam), while the other arm was not, thus allowing a differential phase shift in the submicrosecond time scale. Because of the refractive index change of the ESIPT dye in one arm of interferometer upon irradiation with the signal beam, phase modulation of the continuous-wave probe light propagating in the irradiated arm of the MZI takes place, leading to the intensity modulation at the output defined by the signal beam, resulting in an all-optical wavelength converter, that is, the conversion of the signal modulation to output signal modulation of the probe light of the MZI. The characteristics of the wavelength converter are well described by a simple kinetic model.