Temperature Effect on Dual Fluorescence of 2-(2′-Hydroxyphenyl)benzimidazole and Its Nitrogen Substituted Analogues

Screening the Optimal Probe by Expounding the ESIPT Mechanism and Photophysical Properties in Bis-HBX with Multimodal Substitutions

DFT and TD-DFT were used in this article to investigate the effects of different substitutions at multiple sites on the photophysical mechanism of bis-HBX in the gas phase. Four different substitution modes were selected, denoted as A1 (X=Me, Y=S), A2 (X=OMe, Y=S), B1 (X=Me, Y=NH), and C1 (X=Me, Y=O). The geometric parameters proved that the IHBs enhanced after photoexcitation, which was conducive to promote the ESIPT process. Combining the analysis of the PECs, it was revealed that the bis-HBX molecule underwent the ESIPT process, and the ease of the ESIPT process was in the order of A1 > A2> B1 > C1. In particular, the TICT process in A1 and B1 promoted the occurrence of the ESIPT process. In addition, the IC process was identified, particularly in C1. Meanwhile, the calculation of fluorescence lifetime and fluorescence rate further confirmed that A1 was the most effective fluorescent probe molecule. This theoretical research provides an innovative theoretical reference for regulating ESIPT reactions and optimizing fluorescent probe molecules.

Influence of Solvents and Halogenation on ESIPT of Benzimidazole Derivatives for Designing Turn-on Fluorescence Probes

This work reports a theoretical investigation of the solvent polarity as well as the halogenation of benzimidazole derivatives during excited state intramolecular proton transfer (ESIPT). It details how the environment and halogen substitution may contribute to the efficiency of ESIPT upon keto-enol tautomerism and exploits this effect to design fluorescence sensing. For this purpose, we first examine the conformational equilibrium of benzimidazole derivatives containing different halogen atoms, which results in intramolecular proton transfer, using density-functional theory (DFT) combined with the polarizable continuum model (PCM). Then we evaluate the fluorescence of the benzimidazole derivatives in different dielectric constants within time-dependent DFT (TD-DFT) approaches. Our results quantitatively allow the determination of large Stokes shifts in nonpolar solvents around 100 nm. These theoretical results are in agreement with experimental solvatochromism studies of benzimidazoles. The effect of halogenation, with fluorine, chlorine, and bromine, is less important than solvent polarization when ESIPT takes place. Thus, halogenation can be properly chosen depending on the interest of the synthesis of benzimidazole-based turn-on fluorescence in appropriate solvents.

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.

Nanoparticle and surfactant controlled switching between proton transfer and charge transfer reaction coordinates

The reaction coordinates of a molecular photo-switch 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) was tuned with a nanoparticle and surfactant. DHP undergoes excited state intramolecular proton transfer (ESIPT) and emits normal and tautomer emissions in N,N-dimethylformamide. Silver nanoparticles suppress the ESIPT and induce twisted intramolecular charge transfer (TICT). Further addition of surfactants alters the process. Interestingly, different surfactants cause different effects. Accordingly, the luminescence characteristics are altered. The anionic surfactant sodium dodecyl sulfate (SDS) restores the ESIPT process by completely detaching the molecule from the nanoparticle. The nonionic surfactant Triton X-100 (TX-100), at lower concentration, enhances the TICT emission and the ESIPT process is also observed due to the release of some fluorophore from the nanoparticle complex. But at higher concentration the fluorophores are released completely and the ESIPT process is restored. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB), at lower concentration, simply restores the ESIPT process by releasing the fluorophore. But at higher CTAB concentration, DHP enters the metalparticle-CTAB aggregate and shows enhanced ESIPT.

Boronic acid complexes with amino phenolic N,O-ligands and their use for non-covalent protein fluorescence labeling

Phenylboronic acid (PBA) forms neutral tetrahedral N,O-coordinated 6-membered cyclic complexes with stability constants reaching the values as large as 1.3 × 10⁴ M^-1 at pH 7.4 in water with amino phenolic compounds including 2-(2'-hydroxyphenyl)-1H-benzimidazole (HPBI) often used for protein probing and labeling. The crystal structures of isolated complexes demonstrate unusually high for boronate adducts degree of the tetrahedral character of the boron atom with short B-N bonds in agreement with their high solution stability. The complexation of PBA with HPBI, causes a strong enhancement of the fluorescence of the "enol" form of the ligand, increases the affinity of the dye to a protein (bovine serum albumin) and makes more pronounced the shift in emission maximum induced by the protein binding. Similar, but larger effects are observed with an amino HPBI derivative and with a stronger boronic acid benzoxaborole. Thus, the binding constant to the protein about 2 × 10⁴ M^-1 for free HPBI increases to 1.2 × 10⁶ M^-1 for the complex of 5-amino-HPBI with benzoxaborole making it suitable for an efficient non-covalent protein labeling or bioconjugation.

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

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

Excitons in Carbonic Nanostructures

Unexpectedly bright photoluminescence emission can be observed in materials incorporating inorganic carbon when their size is reduced from macro–micro to nano. At present, there is no consensus in its understanding, and many suggested explanations are not consistent with the broad range of experimental data. In this Review, I discuss the possible role of collective excitations (excitons) generated by resonance electronic interactions among the chromophore elements within these nanoparticles. The Förster-type resonance energy transfer (FRET) mechanism of energy migration within nanoparticles operates when the composing fluorophores are the localized electronic systems interacting at a distance. Meanwhile, the resonance interactions among closely located fluorophores may lead to delocalization of the excited states over many molecules resulting in Frenkel excitons. The H-aggregate-type quantum coherence originating from strong coupling among the transition dipoles of adjacent chromophores in a co-facial stacking arrangement and exciton transport to emissive traps are the basis of the presented model. It can explain most of the hitherto known experimental observations and must stimulate the progress towards their versatile applications.

Theoretical study on excited-state intramolecular proton transfer process of cyanide group substituted 2-(2-hydroxyphenyl)benzothiazole

In this work, the influence of cyanide group substituted on the excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole (HBT) has been theoretical studied. It is found that the intramolecular hydrogen bonds of HBT and its cyanide substituted derivatives are significantly strengthened in the first excited (S₁) state, while this strengthening tends to decrease with the substitution of cyano group. The natural population analysis (NPA) is performed to interpret this phenomenon and the calculated results show that the introduction of cyano groups induces intramolecular charge transfer, leading to the decrease of electrostatic interaction of intramolecular hydrogen bond in the S₁ state compared to isolated HBT. Therefore, proton transfer of HBT and its derivatives become difficult with the introduction of cyano groups, which is consistent with the calculated potential barriers (1.95, 2.71 and 2.81 kcal/mol respectively) of the three molecules in the S₁ state. This work interprets the hindrance of cyanide group on the proton transfer of HBT and its derivatives in the S₁ state, which will bring new insight into the study of analogous molecular system.

Proton transfer triggered proton transfer: a self-assisted twin excited state intramolecular proton transfer

The double excited state intramolecular proton transfer (ESIPT) of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA) has been investigated and found to undergo a new type of proton transfer.

The dual-luminescence mechanism of the ESIPT chemosensor tetrasubstituted imidazole core compound: a TDDFT study

The dual-luminescence mechanism of the tetrasubstituted imidazole core (TIC) compound was theoretically explored by considering the excited-state intramolecular proton transfer (ESIPT) process in the present study.

Photoinduced electron transfer across an organic molecular wall: octa acid encapsulated ESIPT dyes as electron donors

Efficient photoinduced electron transfer from proton transfer dyes encapsulated within water soluble supramolecular host octa acid to electron acceptors present outside the capsule was observed in aqueous solution.

Excited-State Intramolecular Proton-Transfer Properties of Three Tris(N-Salicylideneaniline)-Based Chromophores with Extended Conjugation

The synthesis and photophysical properties of three tris(N-salicylideneaniline) (TSA) compounds containing 1,3,5-triarylbenzene, -tristyrylbenzene, and -tris(arylethynyl)benzene core units are reported. The TSA compounds underwent efficient excited-state intramolecular proton transfer (ESIPT) in solution and in solid state due to the preformed C=N⋅⋅⋅H-O hydrogen-bonded motifs of the structures. Steady-state fluorescence emission spectra of the TSA molecules revealed dual bands only in DMSO, and large Stokes shifts in other polar aprotic and protic solvents. Femtosecond transient absorption spectroscopic measurements in THF revealed lifetime values in the range of 14-16 ps for the excited-state keto-tautomer. The TSA compounds are also responsive to metal ions (Cu²⁺ and Zn²⁺ ) in DMSO, exhibit enhanced aggregate-induced emission (AIE) properties in DMSO/water mixtures, and are highly luminescent in the solid state.

An unusual deprotonation trend in 2-(2'-hydroxyphenyl)pyridoimidazoles

The anion sensitivity and the deprotonation nature of the nitrogenous analogues of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) are investigated in a polar aprotic medium. It is observed that the substitution of pyridyl nitrogen enhances the anion sensitivity. However, despite the enhanced sensitivity of the nitrogenous analogues the deprotonation of these molecules in the presence of strong anions is less favored as compared to HPBI. This anomalous trend observed for the nitrogenous analogs is discussed and explained using theoretical calculations and experimental findings. It is also found that the sensitivity towards anions and the formation of anions also depend on the position of the pyridyl nitrogen.

Aggregation induced enhanced emission of 2-(2'-hydroxyphenyl)benzimidazole

In this study, the aggregation induced emission enhancement (AIEE) of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) is reported. To investigate the AIEE process of HPBI, absorption/fluorescence spectroscopy, fluorescence imaging and field emission scanning electron microscopy were employed. A comparative study with 2-phenylbenzimidazole (PBI) divulges the significance of the hydroxyl group in the AIEE process. Further, molecular dynamics simulations have been carried out with explicit solvent molecules to follow the aggregation process of HPBI with time. The obtained molecular dynamics simulation results not only predicted the formation of aggregates but also provided detailed insight and information on the molecular interactions. The cellular studies showed aggregates yield higher fluorescence in the visible region inside HeLa cells in comparison to monomeric compounds which failed to exhibit any visible fluorescence inside the cell. The obtained aggregates were further found to be biocompatible and therefore can be used for bio-imaging applications.

Tuning ESIPT fluorophores into dual emitters

Using first-principle approaches, we show how ESIPT can be controlled by fine-tuning of substituents, hence leading to new dual emitters.

Dyes undergoing excited-state intramolecular proton transfer (ESIPT) are known to present large Stokes shifts as a result of the important geometrical reorganisation following photon absorption. When the ESIPT process is not quantitative, one can obtain dual emitters characterised by two distinct fluorescence bands, observed due to emissions from both the canonical and ESIPT isomers. However, dual emission generally requires to maintain a very specific balance, as the relative excited-state free energies of the two tautomers have to be within a narrow window to observe the phenomenon. Consequently, simple chemical intuition is insufficient to optimise dual emission. In the present contribution, we investigate, with the help of quantum-mechanical tools and more precisely, time-dependent density functional theory (TD-DFT) and algebraic diagrammatic construction (ADC), a wide panel of possible ESIPT/dual emitters with various substituents. The selected protocol is first shown to be very robust on a series of structures with known experimental behaviour, and next is applied to novel derivatives with various substituents located at different positions. This work encompasses the largest chemical library of potential ESIPT compounds studied to date. We pinpoint the most promising combinations for building dual emitters, highlight unexpected combination effects and rationalise the impact of the different auxochromes.

Exclusive excited state intramolecular proton transfer from a 2-(2′-hydroxyphenyl)benzimidazole derivative

The existence of trans-enol was made unviable by crafting a steric hindrance that stops the normal emission of bis-HPBI.

Fluoride-driven 'turn on' ESPT in the binding with a novel benzimidazole-based sensor

A novel fluorescence sensor (BIP) bearing NH and OH subunits displayed a highly selective and sensitive recognition property for fluoride over other anions. Fluoride-driven ESPT, poorly used in anion recognition and sensing, was suggested to be responsible for the fluorescence enhancement with a blue shift of 35 nm in the emission spectrum.

White emitters by tuning the excited-state intramolecular proton-transfer fluorescence emission in 2-(2'-hydroxybenzofuran)benzoxazole dyes

The synthesis, structural, and photophysical properties of a new series of original dyes based on 2-(2'-hydroxybenzofuran)benzoxazole (HBBO) is reported. Upon photoexcitation, these dyes exhibit intense dual fluorescence with contribution from the enol (E*) and the keto (K*) emission, with K* being formed through excited-state intramolecular proton transfer (ESIPT). We show that the ratio of emission intensity E*/K* can be fine-tuned by judiciously decorating the molecular core with electron-donating or -attracting substituents. Push-pull dyes 9 and 10 functionalized by a strong donor (nNBu2 ) and a strong acceptor group (CF3 and CN, respectively) exhibit intense dual emission, particularly in apolar solvents such as cyclohexane in which the maximum wavelength of the two bands is the more strongly separated. Moreover, all dyes exhibit strong solid-state dual emission in a KBr matrix and polymer films with enhanced quantum yields reaching up to 54 %. A wise selection of substituents led to white emission both in solution and in the solid state. Finally, these experimental results were analyzed by time-dependent density functional theory (TD-DFT) calculations, which confirm that, on the one hand, only E* and K* emission are present (no rotamer) and, on the other hand, the relative free energies of the two tautomers in the excited state guide the ratio of the E*/K* emission intensities.