Terphenyl based fluorescent chemosensor for Cu and F ions employing excited state intramolecular proton transfer

A new terphenyl based bifunctional fluorescent chemosensor 3a has been synthesized, which demonstrates selective optical recognition of Cu²⁺ and F⁻ ions in two contrasting modes. The compound shows highly selective ‘On-Off’ switchable behavior toward Cu²⁺ ions and ‘On-Off-On’ behavior toward F⁻ ions among various cations and anions tested. The detection limits of chemosensor for Cu²⁺ and F⁻ ions are found to be 100 nM and 10 nM, respectively.     

Benzimidazole-based imine-linked chemosensor: chromogenic sensor for Mg2+ and fluorescent sensor for Cr3+

We synthesized an imine-linked, benzimidazole-based chemosensor that can be used for chromogenic recognition of Mg²⁺ and fluorescent recognition of Cr³⁺. The chemosensor shows sensitive, selective, and ratiometric recognition of Cr³⁺ through concurrent quenching at one wavelength and enhancement of fluorescence intensity at another wavelength. It can also be used to detect Mg²⁺ via UV–vis absorption spectroscopy. DFT calculations support these phenomena. The sensor can be used to strain microbe cells without breakage.     

Benzimidazole-based tripodal receptor: highly selective fluorescent chemosensor for iodide in aqueous solution

We synthesized a novel tripodal fluorescent receptor bearing benzimidazole motifs as recognition sites in the pods of the receptor. The recognition behavior of the receptor toward various anions was evaluated in CH3CN/H2O (9:1, v/v) solution. The receptor showed changes in fluorescent intensity only with I-, but it showed no significant changes on addition of other anions such as F-, Cl-, Br-, HSO4-, NO3-, CH3COO-, and H2PO4-.     

A physiochemical study of excited state intramolecular proton transfer process-Luminescent chemosensor by spectroscopic investigation supported by ab initio calculations

A group of novel Schiff base derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy Schiff base (SB4) has been studied using emission spectroscopy and it was detected that the two distinct ground state isomers of I and II are responsible for the emission. The comparison of the emission wavelength in hydrocarbon solvent strongly supports that trans enol form predominates over the cis enol form for Schiff base (SB4). With increasing base concentration of the solutions of hydroxy substituted Schiff bases (SB4 and SB5), two isobestic points are found which confirm the equilibrium among the trans enol form, anion and the cis enol form. The fluorescence of (SB4) quenched markedly with the gradual addition of Cu(2+) but the fluorescence properties of (SB5) was influenced by other metal ions. Therefore Schiff base (SB5) can be used as a new fluorescence sensor to detect the quantity of Cu(2+) ion in any sample solution depending on the relative intensity change. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the Schiff base derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.     

On a possible excited state proton transfer in N-formylkynurenine derivatives

It is shown that intramolecular hydrogen bonding between the

and the ortho carbonyl on the side chain of N'formylkynurenine and some parent compounds plays an important role on their spectroscopic properties. The fluorescence emissionλ_max is shifted by about 4000 cm^?1 in going from polar to non-polar solvents. This abnormally red-shifted fluorescence is attributed to an excited state proton transfer from the formamido to the ortho carbonyl of the side chain.     

Pulsed liquid lasers from proton transfer in the excited state

The application of photoinduced intramolecular proton transfer to generate stimulated radiation is reported. Tunable laser pulses are produced with a 10% efficiency using sodium salicylate or 2-(o-hydroxyphenyl)benzimidazole as the active medium. In both compounds a large population inversion results from the proton transfer taking place in the electronically excited state.     

Photo-excited intramolecular charge transfer and excited state intramolecular proton transfer behaviors for HPIBT system: A theoretical investigation

Given facile synthetic route and excellent photo stability, excited state intramolecular proton transfer (ESIPT)-active luminous materials have gained more and more attention. Here, we focus on photo-induced excitation process and the ESIPT reaction process for the novel 5-benzothiazol-2-yl-6-hydroxy-2-methyl-isoindole-1,3-dione (HPIBT) molecule. On the level of chemical geometries and infrared spectra, we verify that O─H⋯N of HPIBT should be enhanced. We find that a proton is likely to be attracted by enhanced electronic densities around N, that is, charge transfer impetus ESIPT trend. Combing potential energy curves and searching for transition state, we clarify the ultrafast ESIPT mechanism of HPIBT due to a low barrier, which legitimately explains previous experimental characteristics.     

Effect of beta-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate

The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous beta-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of beta-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of beta-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the beta-CD cavity with an association constant of 1915 M(-1) using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of beta-CD.     

Testing the three step excited state proton transfer model by the effect of an excess proton

In a previous work, we proposed an extended model for intermolecular excited-state proton transfer to the solvent. The model invoked an intermediate species, the contact ion-pair RO(-)...H(3)O(+), where a proton is strongly hydrogen bonded to the conjugated photabase RO(-). In this study we tested the extended model by measuring the transient absorption and emission of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) in an aqueous solution in the presence of a large concentration of mineral acids. In a neutral pH solution, the pump-probe signal consists of three time components, <1, 4, and 100 ps. The 4 ps time component, with a relative amplitude of about 0.3, was attributed to the formation of the contact ion-pair and the long 100 ps component to the dissociation of the ion-pair to a free proton and RO(-). In the presence of acid, the recombination of an excess proton competes with the geminate recombination. At a high acid concentration, the recombination process alters the time-dependent concentrations of the reactant, product and intermediate contact ion-pair. We observed that when the acid concentration increases, the amplitude of both the long and intermediate time components decreases. At about 3 M of acid, both components almost disappear. Model calculations of the acid effect on the transient HPTS signal indeed showed that the amplitude of the intermediate time component decreases as the excess proton concentration increases.     

Solvent dependent excited state spectral properties of 4-hydroxyacridine: Evidence for only water mediated excited state proton transfer process

The possibility of ground and excited state proton transfer reaction across the five member intramolecular hydrogen bonded ring in 4-hydroxyacridine (4-HA) has been investigated spectroscopically and the experimental results have been correlated with quantum chemical calculations. The difference in the emissive behaviour of 4-HA in different types of solvents is due to the presence of different species in the excited state. In non-polar solvents, the species present is non-fluorescing in nature, whereas 4-HA molecule shows normal emission from intramolecularly hydrogen bonded closed conformer in polar aprotic solvents. In polar protic solvents like MeOH, EtOH, etc. (except water), a single broad emission band is attributed to the hydrogen bonded solvated form of 4-HA. However, in case of water, fluorescence from the tautomeric form of 4-HA is observed apart from emission from the solvated form. Emission from the tautomeric form may arise due to double proton transfer via a single water molecule bonded to 4-HA. Evaluation of the potential energy surfaces by quantum chemical calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT), however, points towards the possibility of proton transfer—both intrinsic intramolecular as well as water mediated in the first excited state of 4-HA.     

Benzimidazole-based chromogenic chemosensor for the recognition of oxalic acid via counter ion displacement assay in semi-aqueous medium

An azo dye-coupled benzimidazole-based receptor 1 was synthesized and investigated as a receptor for metal ions in semi-aqueous medium. The receptor recognizes Cu²⁺ with high selectivity over other metal ions. The resultant complex 1·Cu²⁺ was found to selectively bind oxalic acid via counter ion displacement.     

Structural tuning intra- versus inter-molecular proton transfer reaction in the excited state

A series of 2-pyridyl-pyrazole derivatives 1-4 possessing five-membered ring hydrogen bonding configuration are synthesized, the structural flexibility of which is strategically tuned to be in the order of 1 > 2 > 3 > 4. This system then serves as an ideal chemical model to investigate the correlation between excited-state intramolecular proton transfer (ESIPT) reaction and molecular skeleton motion associated with hydrogen bonds. The resulting luminescence data reveal that the rate of ESIPT decreases upon increasing the structural constraint. At sufficiently low concentration where negligible dimerization is observed, ESIPT takes place in 1 and 2 but is prohibited in 3 and 4, for which high geometry constraint is imposed. The results imply that certain structural bending motions associated with hydrogen bonding angle/distance play a key role in ESIPT. This trend is also well supported by the DFT computational approach, in which the barrier associated with ESIPT is in the order of 1 < 2 < 3 < 4. Upon increasing the concentration in cyclohexane, except for 2, the rest of the title compounds undergo ground-state dimerization, from which the double proton transfer takes place in the excited state, resulting in a relatively blue shifted dimeric tautomer emission (cf. the monomer tautomer emission). The lack of dimerization in 2 is rationalized by substantial energy required to adjust the angle of hydrogen bond via twisting the propylene bridge prior to dimerization.     

A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system

Given the tremendous potential applications of excited state intramolecular proton transfer (ESIPT) systems, ESIPT molecules have received widespread attention. In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically study the excited state dynamical behaviors of salicyladazine (SA) molecules. Our simulated results show that the double intramolecular hydrogen bonds of SA are strengthened in the S₁ state via exploring bond distances, bond angles, and infrared (IR) vibrational spectra. Exploring the frontier molecular orbitals (MOs), we confirm that charge redistributions indeed have effects on excited state dynamical behaviors. The increased electronic densities on N atoms and the decreased electronic densities on O atoms imply that charge redistribution may trigger the ESPT process. Analyzing the constructed S₀‐state and S₁‐state potential energy surfaces (PESs), we confirm that only the excited state single proton transfer reaction can occur although SA possesses two intramolecular hydrogen bonds. In this work, we clarify the specific ESIPT mechanism, which may facilitate developing novel applications based on the SA system in future.     

Ratiometric chemosensor for fluorescent determination of Zn2+ in aqueous ethanol

A new ratiometric and selective fluorescent chemosensor (1) for quantification of zinc ions in aqueous ethanol has been synthesized and investigated in this work. In an environmentally friendly media of 30% (v/v) water/ethanol and 10 mM Tris-HCl neutral buffer (pH 7.03), 1 displayed selective Zn²⁺ ratiometric fluorescence response, with a dynamic working range of 1.0-8.0 μM and a detection limit of 0.5 μM Zn²⁺. The determination of Zn²⁺ in synthesized water sample was also successful.     

Understanding the aggregation induced emission enhancement for a compound with excited state intramolecular proton transfer character

A few of excited state intramolecular proton transfer (ESIPT) compounds have been discovered for their aggregation induced emission enhancement (AIEE). To understand the AIEE mechanism, an ESIPT compound BTHPB (N-(4-(benzo[d]thiazol-2-yl)-3-hydroxyphenyl)benzamide) with simple structure was designed and synthesized. BTHPB showed apparent AIEE property and the emission efficiency was observed as high as 0.27 in the aggregates. On the basis of viscochromism experiments and calculations employing the linear coupling model, the restriction of the rotation between the two subunits taken place in ESIPT was considered as the main factor for the AIEE. The micro- and femtosecond transient absorption experiments offered evidence for the considerations. Additionally, we also observed a negative effect of aggregation on the fluorescence emission in the system. So the AIEE of ESIPT compound BTHPB originated from the combination effects of positive and negative factors induced by the aggregation.     

Potential energy functions for an intramolecular proton transfer reaction in the ground and excited state

We describe the development of empirical potential functions for the study of the excited state intramolecular proton transfer reaction in 1-(trifuloroacetylamino)-naphtaquinone (TFNQ). The potential is a combination of the standard CHARMM27 force field for the backbone structure of TFNQ and an empirical valence bond formalism for the proton transfer reaction. The latter is parameterized to reproduce the potential energies both in the ground and the excited state, determined at the CASPT2 level of theory. Parameters describing intermolecular interactions are fitted to reproduce molecular dipole moments computed at the CASSCF level of theory and to reproduce ab initio hydrogen bonding energies and geometries for TFNQ-water bimolecular complexes. The utility of this potential energy function was examined by computing the potentials of mean force for the proton transfer reactions in the gas phase and in water, in both electronic states. The ground state PMF exhibits little solvent effects, whereas computed potential of mean force shows a solvent stabilization of 2.5 kcal mol⁻¹ in the product state region, suggesting proton transfer is more pronounced in polar solvents, consistent with experimental findings. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Inter- and intramolecular excited state proton transfer in 2-hydroxy-9H-carzole-1-carboxylic acid

Absorption, fluorescence and fluorescence excitation spectroscopy and single photon counting time dependence spectrofluorimetry have been used to study the inter- and intramolecular excited state proton transfer (ESIPT) reactions in 2-hydroxy-9H-carbazole-1-carboxylic acid (2-HCA). Except in cyclohexane and water (pH 5) dual fluorescence is observed in rest of the solvents used. Normal Stokes shifted band seems to originate from 2-HCA-1-c and tautomer emission band from the tautomer formed by ESIPT in 2-HCA-1-c followed by structural reorganization. Both these emission band systems originate from the same ground state species. AM1 and CNDO/S-CI calculations have been carried out to establish the identity of the species. Different prototropic equilibria have been determined and discussed.     

Proton antenna effect of the gamma-cyclodextrin outer surface, measured by excited state proton transfer

The reversible proton dissociation and geminate recombination of the common photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (pyranine), either in dilute aqueous solution or when forming a complex with gamma-cyclodextrin (gamma-CD), has been studied by time-resolved fluorescence spectroscopy and supplemented by molecular modeling and dynamics simulations. We find that the dissociation rate of the proton from the excited molecule was decreased to about approximately 50% of its value in water, while the rate of recombination was doubled. These observations were evaluated by molecular modeling of the reactants at atomic resolution. The combination of the two methodologies indicates that the pyranine in the complex can assume more than one level of interaction with the solvent. The polysugar torus surrounding the pyranine perturbs the hydrogen bond in the dye's immediate vicinity and deforms the electrostatic potential inside the Coulomb cage, causing major deviations from a simple spheric symmetry. These observations can account for the special kinetic features measured for the complex. We suggest that this system can be used as a basic model for evaluating the mechanism of proton transfer in non-homogeneous systems, such as the surface of proteins or biomembranes.     

TD‐DFT study on the sensing mechanism of a fluorescent chemosensor for fluoride: Excited‐state proton transfer

An excited‐state proton transfer (ESPT) process, induced by both intermolecular and intramolecular hydrogen‐bonding interactions, is proposed to account for the fluorescence sensing mechanism of a fluoride chemosensor, phenyl‐1H‐anthra(1,2‐d)imidazole‐6,11‐dione. The time‐dependent density functional theory (TD‐DFT) method has been applied to investigate the different electronic states. The present theoretical study of this chemosensor, as well as its anion and fluoride complex, has been conducted with a view to monitoring its structural and photophysical properties. The proton of the chemosensor can shift to fluoride in the ground state but transfers from the proton donor (NH group) to a proton acceptor (neighboring carbonyl group) in the first singlet excited state. This may explain the observed red shifts in the fluorescence spectra in the relevant fluorescent sensing mechanism. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010