Exhibiting Excited-state Intramolecular Proton Transfer Research Articles

Theoretical insights into the impact of hydrogen bonding interactions and proton‐transfer phenomena on the excited state of HBT‐TMS fluorophore: Solvent‐induced effects

AbstractInspired by the design of distinguished optical materials, novel organic molecules exhibiting excited state intramolecular proton‐transfer (ESIPT) characteristics have emerged as a prominent research topic. In this study, we focus on investigating the excited state dynamics of 2‐(5‐trifluoromethyl‐benzothiazol‐2‐yl)‐4,6‐bis‐trimethylsilanylethynyl‐phenol (HBT‐TMS), a novel color‐tunable multifunctional ESIPT emitter. By considering four different aprotic solvents with varying polarities, we confirm the influence of solvent polarity on photo‐induced hydrogen bonding interactions, charge redistribution, and associated ESIPT phenomena. Through comparison and quantification of the magnitudes of excited state reaction barriers in different solvents, our findings suggest that highly polar solvents facilitate the ESIPT reaction for HBT‐TMS fluorophore.

Influence of gold nanostructures on excited state intramolecular proton transfer in multidomain HTTH dye

Organic multidomain dyes exhibiting excited state intramolecular proton transfer (ESIPT) are known due to large Stokes shifts and dependence of their luminescence spectral characteristics on the properties of the environment. In this work, influence of gold nanostructures on the spectral characteristics of a “polycarbonate matrix – gold nanostructures – HTTH” system was studied using thiazole dye HTTH as an example. A hypothesis about the possibility of plasmon resonance energy transfer (PRET) between the HTTH molecules in different states, namely the ground state (enol form) and the state after proton transfer (keto form), mediated by gold nanostructures was experimentally tested. Presence of gold nanostructures in the vicinity of HTTH molecules was found to lead to the changes in the ratio of the luminescence peak intensities for the enol and keto form of these molecules. This phenomenon opens up the possibility of additional regulation of the spectral characteristics and may evidence the PRET effect in the systems containing ESIPT-exhibiting dyes and plasmonic nanostructures. The obtained results improve our understanding of the physical processes in the systems similar to the studied one and imply new practical applications of them such as fabrication of organic light-emitting diodes, sensors, super-resolution microscopy tools and ultraviolet-to-visible radiation convertors.

Polarized Fluorescence of N-Salicylideneaniline Derivatives by Direct Exchange from Photoaligned N-Benzylideneaniline Side Groups in Liquid Crystalline Polymeric Films

Abstract Photoalignment of liquid crystalline polymer with N-benzylideneaniline (NBA) side groups and subsequent in situ exchange achieves polarized fluorescence of oriented N-salicylideneaniline (SA) derivatives. In situ hydrolysis of NBA and condensation with 2-hydroxybenzaldehyde derivatives form oriented SA side groups. SA exhibits excited state intramolecular proton transfer (ESIPT), inducing polarized fluorescence with a 2.3 polarization ratio. The emissive wavelength depends on the 2-hydroxybenzaldehyde derivative. Stepwise formation of different oriented SA side groups allows for color tuning of the oriented film.

Lactim or lactam? A theoretical study on the possibility of excited-state intramolecular proton transfer in hydroxyphenyl-imidazo-isoindoles

The interest in organic molecules exhibiting Excited State Intramolecular Proton Transfer (ESIPT) has steadily increased in the last decades, as the peculiar properties of these compounds make them useful for various applications. However, designing efficient ESIPT fluorophores may result a non-trivial matter. In this work we analyze the controversial case of a family of hydroxyphenyl-imidazo-isoindoles that has been recently proposed to experience ESIPT through an unusual four-membered hydrogen-bonding cycle. We demonstrate herein that such a process is unlikely to occur in excited state, and provide a simpler rationalization to the experimental observations.

Photoactive glycoconjugates with a very large Stokes shift: synthesis, photophysics, and copper(II) and BSA sensing.

This study presents the synthesis of novel glycoconjugates by connecting benzazole and carbohydrate units with a 1,2,3-triazole linker. A simple synthetic route employing a copper(I) catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) was utilized. The synthesized compounds exhibit excited-state intramolecular proton transfer (ESIPT), resulting in longer wavelength emission with a significantly large Stokes shift (∼10 000 cm-1). These compounds show potential as chemical sensors due to their ability to detect Cu2+ ions, causing a decrease in fluorescence emission (turn-off effect). Additionally, they demonstrate strong interaction with proteins, exemplified by their interaction with bovine serum albumin (BSA) as a model protein.

Insight into the release mechanisms of diflunisal and salicylic acid from poly(vinyl alcohol). The role of hydrogen bonding interactions.

Diflunisal (5-(2,4-Difluorophenyl)salicylic acid, DIF), salicylic acid (SAL) derivative, which, on the one hand, is active pharmaceutical ingredient, on the other hand, belongs to the compounds exhibiting excited-state intramolecular proton transfer (ESIPT) behaviour was used to study the drug interactions with poly(vinyl alcohol) (PVA) matrix. For clarifying the nature and mechanisms of the drug-matrix interactions the salicylic acid (SAL) molecule was selected as the model active ESIPT compound, whose physicochemical properties in different media are well understood. The solute–solvent interactions (non-specific (dipole–dipole) versus specific (hydrogen bonding)) of DIF and SAL with different neat solvents were investigated using the steady-state spectroscopic technique. The solvent effect on spectral behaviours of DIF and SAL was analyzed based on the parametric solvent scales. In order to identify functional groups in the PVA matrices, determine the structure present in the studied molecule-PVA system and thus obtain information about the potential interactions between PVA and the studied molecules, the Raman spectra of pure PVA, SAL-PVA and DIF-PVA systems were measured. It has been shown that the molecular structure of the active substance entrapped in the polymer matrix affects the structure of the polymer, i.e., isotactic (SAL-PVA) versus syndiotactic (DIF-PVA) structure. The analysis of drug release kinetics revealed that the DIF is more strongly bound to PVA in comparison to SAL, which confirms conclusions drawn from the analysis of the Raman spectra i.e., the isotactic structure of SAL-PVA material results in a faster initial release process of weakly bound, located on the surface of the polymer SAL molecules.

Hydroxy-Tetraphenylimidazole Derivatives as Efficient Blue Emissive Materials for Electroluminescent Devices.

Herein, three hydroxy-tetraphenylimidazole (HPI)-based fluorophores (HPI-TPA, HPI-PCz, and HPI-CzP) are designed and synthesized by disubstituted HPI core with arylamine units of triphenylamine (TPA), phenyl carbazole (PCz), and carbazole phenyl (CzP) at 3,5-positions of the N-phenyl ring of HPI, respectively. Their photophysical properties are theoretically and experimentally examined. HPI-TPA shows a hybridized local and charge transfer (HLCT) excited state characteristic and emits deep blue color via an HLCT mechanism, while both HPI-PCz and HPI-CzP exhibit excited-state intramolecular proton transfer (ESIPT) property and display pure keto form emissions. They possess high thermal stability and are successfully fabricated as emitters in organic light-emitting diodes (OLEDs). All devices exhibit intense blue color emissions with low turn-on voltages (3.5-3.7 V). Particularly, HPI-TPA-based OLED emits light in the deep-blue region with a high maximum external quantum efficiency (EQEmax ) of 3.77% and a decent efficiency roll-off.

In silico simulation of the excited state proton transfer reaction of 2-(2-furyl)-3-hydroxychromone (FHC) in solution by empirical valence bond (EVB) method in conjunction with classical molecular dynamics

2-(2-Furyl)-3-hydroxychromone (FHC) is a dual emitter fluorescent probe exhibiting excited state intramolecular proton transfer (ESIPT) reaction in solution and inserted in biological relevant macromolecules. Its two fluorescence bands are attributed to normal form (N*) for the short wavelength and the long-wavelength band to the tautomer form (T*), the product of the ESIPT reaction. To rationalise the mechanisms that lead to the occurrence of the ESIPT reaction and predict the fluorescence spectra, the excited and ground state dynamic of this compound was followed in acetonitrile, ethanol, methanol, N-methylformamide and formamide, by the classical molecular dynamic simulation in conjunction with the empirical valence bond (EVB) model to describe bond dissociation process. A set of transferable parameters were used to fit the EVB potential energy curve to TDDFT data. Using these parameters, the occurrence time of the ESIPT reaction was predicted to be 7, 84, 113, 98, 207ps, respectively in the considered solvents, in excellent agreement with the experimentally reported values. These values are comparatively higher than those determined for the parent flavone dye (less than 100 fs), giving FHC a higher potential for sensor development. By examining structural parameters such as the distance from the proton to acceptor, the intermolecular H-bond between FHC and the solvent molecules and proton trajectories, we found that ESIPT occured in acetonitrile and alcohols as a combination of low frequency high amplitude donor-acceptor bending motion and charge redistribution in the excited state, that were modulated by the solvent H-bond donor potency. Differently in amides ESIPT occured via tunnelling mechanism due to the close energy of N* and T*. Fluorescence spectra calculated from potential energies of the EVB model along the reaction coordinates were qualitatively in agreement with the recorded spectra.

Stimuli-responsive fluorescent dyes for electrochemically tunable multi-color-emitting devices

The preparation and characterization of two fluorescence dyes are demonstrated that can respond to the various external stimulations including solvent, acid, base and electrical potentials through changing the multi-colored fluorescent emission. The fluorescent dyes are designed as 2-(2-thiophen-3-olyl)benzothiazole derivatives consisting of di- or tri-thiophene with long alkyl chain, namely, T2-BT and T3-BT, respectively. Tn-BTs exhibit excited state intramolecular proton transfer (ESIPT) phenomenon in the organic solvent that do not involve the oxygen atom such as chloroform, dichloromethane and toluene, where the ESIPT induces red-shift of the emission spectra compared to the original emission. Tn-BTs also display clearly multi-colored emission in the cationic, neutral and anionic states upon the addition of the acid or base. Based on this feature, the white emission system was achieved with T2-BT through the addition of the 0.5 equivalent of tetrabutylammonium hydroxide. Furthermore, multi-colored electrofluorochromic (EFC) cells based on single molecules (T2-BT or T3-BT) were successfully implemented in the low electrical bias (-3 V–3 V).

Revised excited-state intramolecular proton transfer of the 3-Aminophthalimide molecule: A TDDFT study

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

Fine-tuning the photophysical properties of the five quinolin based nanophotoswitches in the gas phase, polar and nonpolar solvents: A TD-DFT approach

Fluorophores exhibiting excited state intramolecular proton transfer (ESIPT) have received increasing attention during the last decades. The swiftness of ESIPT process often avoids its inspection with only experimental methods, and theoretical explorations are often necessary to achieve a full justification of the observations and fine-tuning the ESIPT fluorescence properties. In this work, photo-switching in five novel red light emitting molecules 11hydroxy-indeno[1,2-b]quinolin-4ol (11HIQO), 4hydroxy-11H-indeno[1,2-b]quinolin-11one (4HIQO), 10H-indolo[3,2-b]quinolin-6-ol (10HIQO), benzofuro[3,2-b]quinolin-6-ol (BFQO) and benzothieno[3,2-b]quinolin-6-ol (BTQO) was comprehensively investigated using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT) at PBE0/6-311++G(2d,2p) level of theory. The interplay between π-conjugation and aromaticity was observed in all five photo-switching molecules in which are able to render both normal and keto fluorescence emissions with wavelengths longer than 610 nm and large Stokes shifts in gas phase (242–472 nm) and solvent media (187–385 nm). The normal and tautomer emissions were assigned to the relaxation of the initially excited Franck-Condon (FC) state to normal (S1-E) and tautomer (S1-K) states, respectively. The full photo-switching cycle and potential energy surfaces in the ground and excited states and structural parameters, H-bonding energy, absorption and emission wavelengths, vertical excitation and emission energies, oscillator strength, fluorescence rate constant, dipole moment, atomic charges and electron density at critical points for five new photochromic ESIPT switches were explored. The potential of these molecules as emissive switches in designing and generation of new displays and light sources in the molecular photo-devices were predicted based on their high red Stokes shifts.

Sugar‐Based Self‐Assembly of Hydrogel Nanotubes Manifesting ESIPT: Theoretical Insight and Application in Live Cell Imaging

AbstractA biopolymeric hydrogel nanotubes (BTHBC) using 3‐(benzo[d]thiazol‐2‐yl)‐2‐hydroxybenzaldehyde and chitosan have been synthesized which exhibits excited state intramolecular proton transfer (ESIPT) in both gel and solution phase. The optimized spatial structure of the monomer of BTHBC having lower energy (Energy: −1085688.464 kcal/mol) is found to be planar. FE‐SEM analysis reveals that the spherical assemblies (20–100 nm) preceded gelation. Upon aging, gelation can be observed owing to the fusion of these spherical aggregates into nanotubes (200‐500 nm). DFT study of BTHBC establishes that the migration of H atom from phenolic –OH group to the nitrogen of 2‐(2′ Hydroxyphenyl)benzothiazole (HBT) moiety is more feasible than the imine nitrogen of chitosan. The fluorescence lifetime of BTHBC is found to increase remarkably in gel phase compared to its aqueous solution. Both the cellular uptake and cell imaging efficiency suggest that the biocompatible nature of the hydrogel permits easy access of maximum number of molecules into the cells which causes enhancement of the fluorescence intensity of the cells.

A turn-off fluorescence sensor for cyanide detection which in turn inhibit 2-way ESIPT investigated by experimental and theoretical study

A ligand based on 3-((quinoline-7-yl) methyeleneamino) phenazine-2-ol) (HAPQA) has been designed and synthesized for the selective detection of cyanide ions. The ligand HAPQA exhibits excited-state intramolecular proton transfer (ESIPT) with a long wavelength emission fluorescent (575nm) which gradually quenches with the addition of cyanide ions. The photophysical properties of the probe as well as its selectivity towards CN− ions are explored using UV–visible, emission spectroscopy and time-resolved fluorescence spectroscopic studies. The ligand exhibits strong H-bonding-mediated inhibition of 2-way excited-state intramolecular proton transfer (ESIPT) in HAPQA selectively with cyanide ions over all other ions. The deep orange colour fluorescence of the ligand in acetonirile turns into bright green coloured solution in presence of cyanide ions with a detection limit of 0.60μM. DFT calculations has been done to study the ground and excited state energy optimized structure and proposed mechanism which is in harmony with the experimental findings and the ESIPT process observed in ligand HAPQA. Thus, the receptor can be used as a Colorimetric and fluorescent sensor for the determination of CN− ion.

Nanomolar Cu2+Detection in Water Based on Disassembly of AIEgen: Applications in Blood Serum, Cell Imaging and Complex Logic Circuits

AbstractHere in, we report self‐assembled aggregates of AIEgen 1, which exhibits excited state intramolecular proton transfer (ESIPT) coupled aggregation induced emission (AIE) with intense green fluorescence (λem=526 nm) in aqueous solution as well as in solid state. AIEgen 1 behaves as disassembly based fluorescent probe for selective and sensitive detection of Cu2+ in drinking water with lowest limit of detection of 0.261 ppb (4.13 nM) (much lower than recommended by EPA). AIEgen 1 finds applications for the detection of Cu2+ in blood serum, environmental (tap, pond and industrial waste water) samples. Probably the interaction of Cu2+ ions at Schiff‐base unit of AIEgen 1 via metal chelation disrupts the self‐assembled aggregates and results in disassembly of the nanorods into smaller spherical nanostructures as also evidenced in DLS, SEM and TEM studies. Incubation of HeLa cells with AIEgen 1 results in fluorescence ‘switch‐on’ showing cell internalization of AIEgen 1 with low cytotoxicity and thus establishes the potential of AIEgen 1 in cell imaging. The reversibility influorescence / absorbance of 1‐Cu2+ complex with EDTA helps in elaboration of various reconfigurable and superposed logic gates such as NOT (at F425, A380), INH (at A430), IMP1 (at A380) and IMP2 (at F526). By observing the output switching in absorbance and fluorescence channels on addition of Cu2+ (In1) and EDTA (In2) as inputs, ‘set‐ reset’ flip flop memory element and complementary IMP/INH logic circuits have been elaborated.

Spectroscopic evaluation of a novel multi-element sensitive fluorescent probe derived from 2-(2′-phenylbenzamide)benzimidazole: Selective discrimination of Al3 + and Cd2 + from their congeners

A novel fluorescent chemosensor 2 exhibiting excited-state intramolecular proton transfer (ESIPT) process has been synthesized by condensation approach, among which 2 could be used as a multielement sensitive fluorescent probe for both Al3+ and Cd2+. The emission wavelength of 2 underwent blue shift of 78nm upon binding with these ions. The cation-induced inhibition of ESIPT phenomenon was responsible for these fluorescence changes. Moreover, the absorption spectra of 2 showed changes with F− and AcO− ions also. TD-DFT studies well displayed the blue shift in the emission wavelength of 2 upon binding with Al3+ ions.

Probing the ESIPT process in 2-amino-1,4-naphthoquinone: thermodynamics properties, solvent effect and chemometric analysis

The developing of fluorescent probes for disease diagnosis is a very important task, which favors precision in the diagnosis and success in the treatment. Recently, amino-naphthoquinone derivatives showed to be efficient fluorescent probes for disease diagnosis. Those compounds exhibit excited-state intramolecular proton transfer (ESIPT), which is the main mechanism responsible for their use as fluorescent probes. The understanding of the ESIPT mechanism for naphthoquinones is an important way of developing more efficient and selective fluorescent probes. In this work, the ESIPT process for ANQ was performed at the TD-DFT/CAM-B3LYP/DGTZVP and DFT/B3LYP/DGTZVP level for the electronic and geometric studies. These parameters were selected for the PCA analysis. The solvent effect was investigated by using PCM and IEF-PCM in chloroform, water and methanol. 2-Amino-1,4-naphthoquinone (ANQ) showed blue emission for fluorescence, having keto–keto* absorption at 4.50 eV and the enol–enol* decay at 2.75 eV. The solvent effect was evaluated, and the ESIPT process of ANQ was favorable in nonpolar and polar solvents. Furthermore, the thermodynamics properties showed that the ESIPT is favorable with a proton transfer equilibrium constant of ~105.

Confinement effect on the photophysics of ESIPT fluorophores

The photophysics of three benzothiazole derivatives that exhibit excited state intramolecular proton transfer (ESIPT) were investigated in a confined medium afforded by the water soluble supramolecular host octaacid (OA).

The effect of hydrogen bond strength on emission properties in 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridines

Abstract The correlation between the structures and molecular properties on the one hand and the fluorescence properties on the other for a set of 2-(2′-hydroxyphenyl)imidazo[1,2- a ]pyridines exhibiting excited-state intramolecular proton transfer (ESIPT) was analyzed via comparison of the optical data and computational results. Ab initio calculations were used to characterize the electronic transitions of 2-(2′-hydroxyphenyl)imidazo[1,2- a ]pyridine and its three derivatives both in non-polar and polar solvents by using DFT and TDDFT methods and the IEFPCM model. The calculations involving geometry optimization in the excited state allow to precisely predict the emission wavelength from the excited keto state. We constructed and studied the correlations between the positions of the emission maximum for the compounds considered and the hydrogen bond energies values for ρ ( r ) and ▽ 2 ρ ( r ) at the hydrogen bond critical point and the aromaticity of chosen structural fragments. It was found that the fluorescence properties of ESIPT molecules could be manipulated by directed modification of their chemical structure, most importantly by the introduction of electron-withdrawing functionalities into their proton donor moieties. The electron-withdrawing CN group hypsochromically shifts the emission from the keto excited state by ∼60 nm and strengthens at the same time the intramolecular hydrogen bond.

Photophysics of a proton transfer phototautomer within biological confinement of a protein: Spectroscopic and molecular docking studies

The present work demonstrates the effect of biological confinement on the photophysics of a proton transfer phototautomer viz., 2-hydroxy-1-naphthaldehyde (HN21). HN21 is a potential candidate exhibiting excited-state intramolecular proton transfer (ESIPT) reaction and thereby generating the phototautomer (i.e., proton transferred keto form) in the excited-state. The ESIPT photophysics of the probe (HN21) is found to be remarkably modified within the confined bio-environment of a model transport protein Bovine Serum Albumin (BSA). Such considerable modification of the ESIPT photophysics of the probe has been exploited to determine the probe–protein binding strength (binding constant, K(±10%)=1.23×104M−1). The probe–protein binding process is found to be thermodynamically feasible (ΔG=−24.25kJmol−1). The present work also delves into evaluation of the probable binding location of the probe (HN21) within the biomacromolecular assembly of the protein by blind docking simulation technique, which reveals that HN21 favorably binds to the hydrophobic subdomain IIIA of BSA. Circular dichroism (CD) spectroscopy delineates the effect of probe binding on the protein secondary structure in terms of decrease of α-helical content of BSA with increasing probe concentration. Apart from this, excitation–emission matrix fluorescence technique is found to hint at the effect on protein tertiary structure upon binding to the probe. The modulated dynamics of the proton transfer phototautomer of HN21 within the biological confinement is investigated in this context by time-resolved fluorescence decay measurements. The present work also accentuates the mutually corroborating data found from experimental and computational studies.

ESIPT induced AIEE active material for recognition of 2-thiobarbituric acid

New carbazole based compound 3 is designed and synthesized which exhibits excited state intramolecular proton transfer (ESIPT) phenomenon and induces the aggregation induced emission enhancement (AIEE) behaviour in H2O:THF (60:40). The aggregates of 3 in H2O:THF (60:40) has been utilized for recognition of 2-thiobarbituric acid. A comparative study is accomplished using barbituric acid, urea, thiophene, carbon disulphide, and thiourea. It was observed that among all the analytes tested, aggregates of 3 recognizes 2-thiobarbituric acid with highest efficacy. The role of ESIPT in induction of AIEE is justified by comparative studies with a reference compounds 5 and 6 under similar conditions.