Hydroxyphenyl Benzimidazole Research Articles

Ultrafast dual-fluorescence dynamics of 2-(2'-hydroxyphenyl) benzimidazole by femtosecond transient absorption spectroscopy

2-(2'-Hydroxyphenyl) benzimidazole (HBI) and its derivatives are widely used in fluorescent probes, luminescent materials, and other sensing materials. In this study, the time-resolved fluorescence behaviour, and excited-state dynamics of HBI and 2-phenyl-1H-benzo[d]imidazole (BI) were studied using femtosecond transient absorption (fs-TA) spectroscopy. The stimulated emission (SE) and excited-state absorption signals of the BI molecule were located at 355 and 437 nm, respectively. For HBI, these signals were located at 475 and 384 nm, respectively. The SE signals of the BI and HBI molecules in their fs-TA spectra were consistent with the experimental short-wave fluorescence at 340 and 460 nm, respectively, in the steady-state spectra. The long-wave signal of HBI at 475 nm indicates its dual fluorescence, which may be caused by proton transfer or conformational torsion after photoexcitation. The large theoretical energy barrier of 40.97 kcal/mol in first excited (S1) state of HBI excludes the latter hypothesis. Additionally, the small energy barrier (1.88 kcal/mol) in the S1 state confirmed the excited-state intramolecular proton transfer (ESIPT). This indicates that ESIPT induces dual fluorescence behaviour of the HBI molecule, which was also demonstrated by the infrared spectra, theoretical fluorescence peaks, and molecular orbitals.

Electrochemical engineering of ZIF-7 electrode using ion beam technology for better supercapacitor performance

In the study, ZIF-7 material was exposed to 500 keV copper (Cu++) ions at dosages of 1 × 1014, 1 × 1015, and 5 × 1014 ions/cm2. The ZIF-7 unirradiated reveals a polycrystalline with an intense peak at 16.231° with a crystal plane of (012). The intensity of the peak in the XRD pattern broadens as the radiation of copper ions increases. In the ZIF-7 FTIR, the CC stretching vibration of the benzene ring is visible at 1469 cm−1, and the hydroxy phenyl benzimidazole (bIm) ligand's CH bending vibration is visible at 740 cm−1. Irradiated ZIF-7 reveals CH bending at 758, 759, and 754 cm−1 from the bIm ligand and CC stretching at 1456, 1460, and 1457 cm−1. The CV plots showed redox peaks, demonstrating faradaic processes. The ZIF-7's unirradiated estimated specific capacitances at scan rates of 50, 40, 30, 20, and 10 mV/s are 156, 195, 260, 390, and 781 F/g. The estimated specific capacitance of the irradiated ZIF-7 material with copper ions of 1 × 1014 ions/cm2 is 185, 308, 462, and 925 F/g. The estimated specific capacitance for ZIF-7 with copper ions of 1 × 1015 and 5 × 1014 ions/cm2 are 187, 234, 468, 937, and 1200 F/g, and 300, 375, 500, 750, and 1200 F/g respectively. The unirradiated (ZIF-7) shows bandgap energy of 3.53 eV which is suitable for applications in gas separation, catalysis, and optoelectronics. The irradiated sample with 1 × 1014 ions/cm2, 1 × 1015 ions/cm2 and 5 × 1014 ions/cm2 revealed bandgap energy of 2.52 to 2.87 eV which is suitable energy storage application.

Investigation of Excited-State Intramolecular Proton Transfer and Structural Dynamics in Bis-Benzimidazole Derivative (BBM).

The bis-benzimidazole derivative (BBM) molecule, consisting of two 2-(2'-hydroxyphenyl) benzimidazole (HBI) halves, has been synthesized and successfully utilized as a ratiometric fluorescence sensor for the sensitive detection of Cu2+ based on enol-keto excited-state intramolecular proton transfer (ESIPT). In this study, we strategically implement femtosecond stimulated Raman spectroscopy and several time-resolved electronic spectroscopies, aided by quantum chemical calculations to investigate the detailed primary photodynamics of the BBM molecule. The results demonstrate that the ESIPT from BBM-enol* to BBM-keto* was observed in only one of the HBI halves with a time constant of 300 fs; after that, the rotation of the dihedral angle between the two HBI halves generated a planarized BBM-keto* isomer in 3 ps, leading to a dynamic redshift of BBM-keto* emission.

Excited-State Polarizabilities: A Combined Density Functional Theory and Information-Theoretic Approach Study

Accurate and efficient determination of excited-state polarizabilities (α) is an open problem both experimentally and computationally. Following our previous work, (Phys. Chem. Chem. Phys. 2023, 25, 2131-2141), in which we employed simple ground-state (S0) density-related functions from the information-theoretic approach (ITA) to accurately and efficiently evaluate the macromolecular polarizabilities, in this work we aimed to predict the lowest excited-state (S1) polarizabilities. The philosophy is to use density-based functions to depict excited-state polarizabilities. As a proof-of-principle application, employing 2-(2'-hydroxyphenyl)benzimidazole (HBI), its substituents, and some other commonly used ESIPT (excited-state intramolecular proton transfer) fluorophores as model systems, we verified that either with S0 or S1 densities as an input, ITA quantities can be strongly correlated with the excited-state polarizabilities. When transition densities are considered, both S0 and S1 polarizabilities are in good relationships with some ITA quantities. The transferability of the linear regression model is further verified for a series of molecules with little or no similarity to those molecules in the training set. Furthermore, the excitation energies can be predicted based on multivariant linear regression equations of ITA quantities. This study also found that the nature of both the ground-state and excited-state polarizabilities of these species are due to the spatial delocalization of the electron density.

A novel HBI-based ratiometric fluorescent probe for rapid detection of trifluoroborate.

A 2-(2'-hydroxyphenyl)benzimidazole (HBI)-based ratiometric fluorescent probe, known as BTEP, was synthesized using 5-bromosalicylaldehyde as the raw material via Sonogashira coupling and condensation reaction. This probe was designed for rapid detection of boron trifluoride solutions and gases. The N and O coordination atoms in the probe undergo a boron difluoride addition with BF3, which affects the process of excited state intramolecular proton transfer (ESIPT) leading to a blue shift of fluorescence emission. Obvious changes in the fluorescence signal can be observed within 60 seconds. The introduction of an acetylene trimethylsilane fragment increases the conjugate plane and is beneficial to improving the selectivity of the probe. The I408/I479 fluorescence ratio of the probe displays a linear relationship with the concentration of BF3 in the range of 5-50 μM, with a detection of limit as low as 69.5 nM. Furthermore, the probe demonstrates specific and selective recognition of BF3 among eight common interference substances. Test strips prepared using BTEP have the capability of real-time naked-eye detection of trace BF3 gas.

Pyridyl substitution control dynamics and shape dependence of fluorescent aggregates

Nitrogen substitution on 2-(2′-hydroxyphenyl)benzimidazole (HPBI) showed impressive changes in photophysical properties. Hence, in this work, we investigated the effect of pyridyl substitution on HPBI on the kinetics and the shape of the aggregates. It was found that the presence and the position of the heteroatom strongly influence the aggregation. 2-(2'-Hydroxyphenyl)-3H-imidazo[4,5-c]pyridine (HPIP-c) forms small rods and 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) produces long needle like structures. The normal emissions of both the molecules were also enhanced along with tautomer emission. In addition, molecular dynamics simulations were performed to monitor the aggregation process. The aggregation was elucidated in term of electrostatic interaction, van der Waals interaction and hydrogen bonding.

Benzimidazole and benzothiazole fluorophores with large Stokes shift and intense sky-blue emission in aggregation as Al3+ and Pb2+ sensors

New fluorophores based on 2-(2′-hydroxyphenyl)benzimidazole (HBZ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) for metal ion sensing were designed and synthesized using a simple method. The photophysical behaviour of these fluorophores were investigated in various solvents using UV–visible and fluorescence spectra. All the heterocycles showed strong excited state intramolecular proton transfer (ESIPT) characteristics with remarkably large Stokes shift (190–252 nm). Spatial charge distribution in the frontier molecular orbitals also demonstrated the ESIPT mechanism through intramolecular charge transfer. Time resolved fluorescence measurements for these heterocycles showed two long-life decay mechanisms which may be attributed to excited state enol and keto emission. These intense sky-blue emitters also exhibited aggregation induced blue shifted emissions due to restriction of intramolecular rotation processes. Fluorescence sensing studies for metal ions revealed the good selectivity of these fluorophores towards Al3+ and Pb2+. Theoretical computations performed using density functional theory methods showed two possible geometric configurations for Al3+ binding.

THF-induced emission enhancement and reversible stimulus response of a tetraphenylethylene luminogen dependent ESIPT mechanism

A tetraphenylethene (TPE) derivative tetra(3-1H-benzo[d]imidazol-2-yl) -4-hydroxyphenyl)ethene (TBIHPE) which exhibits crystallization-induced emission enhancement (CIEE) induced by tetrahydrofuran (THF) and reversible fluorescence shifting upon HCl fuming and heating or NH3 fuming was prepared. The CIEE and reversible fluorescence shift process were controlled by imino group and tertiary amine group of 2-(2′-Hydroxyphenyl)benzimidazole (HBI) moiety, respectively. The intermolecular hydrogen bonds between THF and imino group have been observed by single-crystal X-ray structural analysis, which changed the excited-state intramolecular proton transfer (ESIPT) process of TBIHPE, and induced the CIEE phenomena. The fluorescence emission peak of TBIHPE shows blue shift upon HCl fuming stimuli in solid state. Moreover, the fluorescence emission of HCl fumed TBIHPE solid can recover by heating or fuming in NH3 vapors. Based on the results, the relationship between photophysical properties and protonation-deprotonation stimuli was discussed.

Theoretical Insights on Solvent Control of Intramolecular and Intermolecular Proton Transfer of 2-(2′-Hydroxyphenyl)benzimidazole

Excited-state proton transfer (ESPT) processes of 2-(2'-hydroxyphenyl)benzimidazole (HBI) and its complexation with protic solvents (H2O, CH3OH, and NH3) have been investigated by both static calculations and dynamics simulations using density functional theory (DFT) at B3LYP/TZVP theoretical level for ground state (S0) and time-dependent (TD)-DFT at TD-B3LYP/TZVP for excited state (S1). For static calculations, absorption and emission spectra, infrared (IR) vibrational spectra of O-H mode, frontier molecular orbitals (MOs), and potential energy curves (PECs) of proton transfer coordinate were analyzed. Simulated absorption and emission spectra show an agreement with available experimental data. The hydrogen bond strengthening in the S1 state has been proved by the changes of IR vibrational spectra and bond parameters of the hydrogen moiety with those of the S0 state. The MOs provide the visual electron density redistribution confirming the hydrogen bond strengthening mechanism. The PECs show that the proton transfer (PT) process is easier to occur in the S1 state than the S0 state. Moreover, on-the-fly dynamics simulations of all systems were carried out to provide the detailed information on time revolution. The results revealed that the excited-state intermolecular proton transfer for HBI is fast, whereas the excited-state intermolecular proton transfer for HBI with protic solvents are slower than that of HBI because the competition between intra- and intermolecular hydrogen-bonds between HBI and protic solvent. These intermolecular hydrogen-bonds hinder the formation of tautomer, hence explaining the low quantum yield found in the protic solvent experiment. Especially for HBI complexing with methanol, only ESIntraPT occurs with small probability compared to HBI with water and ammonia.

DFT-TDDFT investigation of excited-state intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzimidazole derivatives: Effects of electron acceptor and donor groups

The excited-state intramolecular proton transfer (ESIPT) reactions of 2-(2′-hydroxyphenyl)benzimidazole (HBI) derivatives were investigated using time-dependent density functional theory (TD-DFT) method at B3LYP/TZVP theoretical level. The geometric parameters, infrared (IR) vibrational spectra, frontier molecular orbitals (MOs), Mulliken charge distribution analysis, natural bond orbital (NBO) analysis and potential energy curves were calculated to provide the direct information about the effect of electron acceptor and donor groups on the ESIPT reactions. The intramolecular hydrogen bonds are significantly strengthened and the electronic density is redistributed after a vertical excitation to S1 state. The proton transfer (PT) reactions are unlikely occur in S0 state through the analysis of potential energy curves and the Hartree-Fock energy of the stable structures. The presence of the electron acceptor NO2 group can facilitate the ESIPT reaction, while the presence of electron donor NH2 group can hinder the ESIPT reaction. In general, the electron acceptor NO2 group and electron donor NH2 group can influence the ESIPT reactions in a completely different way.

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.

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.

A CASSCF/CASPT2 insight into excited‐state intramolecular proton transfer of four imidazole derivatives

Excited-state intramolecular proton transfer (ESIPT) of four imidazole derivatives, 2-(2'-hydroxyphenyl)imidazole (HPI), 2-(2'-hydroxyphenyl)benzimidazole (HPBI), 2-(2'-hydroxyphenyl)-1H-phenanthro[9,10-d]imidazole (HPPI) and 2-(2'-hydroxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (HPPPI), were studied by the sophisticated CASSCF/CASPT2 methodology. The state-averaged SA-CASSCF method was used to optimize their geometry structures of S0 and S1 electronic states, and the CASPT2 calculations were used for the calibration of all the single-point energies, including the absorption and emission spectra. A reasonable agreement is found between the theoretical predictions and the available experimental spectral data. The forward ESIPT barriers of four target compounds gradually decrease with the increase of molecular size. On the basis of the present calculations, it is a plausible speculation that the larger the size, the faster is the ESIPT rate, and eventually, HPPPI molecule can undergo a completely barrierless ESIPT to the more stable S1 keto form. Additionally, taking HPI as a representative example, the radiationless decays connecting the S0 and S1 /S0 conical intersection structures were also studied by constructing a linearly interpolated internal coordinate (LIIC) reaction path. The qualitative analysis shows that the LIIC barrier of HPI in the keto form is remarkably lower than that of its enol-form, indicating that the former has a big advantage over the latter in the nonradiative process.

A ratiometric fluorescent probe for alkaline phosphatase via regulation of excited-state intramolecular proton transfer.

A ratiometric fluorescent probe 2-(benzimidazol-2-yl)phenyl phosphoric acid (1) for alkaline phosphatase (ALP) is designed and synthesized. The method employs the modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. Upon phosphorylated with POCl3 , HPBI shows only an emission peak at 363 nm due to the blockage of ESIPT. However, once selective enzymatic hydrolysis with alkaline phosphatase (ALP) in Tris-HCl buffer occurs, the probe 1 is returned to HPBI and the ESIPT process is switched on, which results in a decrease in the emission band at 363 nm and an increase in a new fluorescence peak around 430 nm. The fluorescence intensity ratio at 430 and 360 nm (I430/I360) increases linearly with the activity of ALP up to 0.050 U/mL and the detection limit is 0.0013 U/mL. The proposed probe shows excellent specificity toward ALP.

TD-DFT assessment of the excited state intramolecular proton transfer in hydroxyphenylbenzimidazole (HBI) dyes.

Dyes undergoing excited state intramolecular proton transfer (ESIPT) received increasing attention during the last decades. If their unusual large Stokes shifts and sometimes dual-fluorescence signatures have paved the way toward new applications, the rapidity of ESIPT often prevents its investigation with sole experimental approaches, and theoretical simulations are often welcome, if necessary, to obtain a full rationalization of the observations. In the present paper, we evaluate both the absorption and the fluorescence spectra of, respectively, the enol and keto forms of a series of hydroxyphenylbenzimidazole (HBI) using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT). Optical spectra were obtained accounting for both vibronic and environmental effects. The aim of this work is therefore not to evaluate the radiationless pathway going through the twisted ESIPT structures, though excited-state reaction paths between enol and keto forms have been rationalized. First we have compared three dyes differing by the strength of the donor groups, and we have quantified the impact of the flexible butyl chain substituting the imidazole side. In accordance with experiments, we show that the presence of a dialkylamino auxochrome allows to tune the excited-state potential energy surface leading to a weaker tendency to ESIPT. This trend is rationalized in terms of both structural and electronic effects. Next, larger hydroxyphenyl-phenanthroimidazole (HPI) were considered to assess the impact of a stronger π-delocalization. 0-0 energies and vibrationally resolved spectra of the corresponding fluoroborate derivatives were studied as well. The dialkylamino auxochrome significantly decreases the 0-0 energies due to the presence of an important charge transfer character, while the addition of a BODIPY moiety induces a change of the emission signature now localized on the BODIPY side rather than on the NBO core.

Excited state proton transfer of 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues in bovine serum albumin.

The interaction of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (HPIP-c) with BSA was explored. Upon interaction with BSA both normal and tautomer emissions are significantly enhanced. However, the fluorescence ratios of the normal band to the tautomer band of HPBI and HPIP-b decrease, but that of HPIP-c increases. From the tautomer emission, the stoichiometry and association constants were determined. HPBI exists as cis- and trans-enolic and zwitterionic forms, whereas HPIP-b and HPIP-c are present as monoanions in addition to cis- and trans-enols. The study shows that different conformers of all three molecules bind at different binding sites of BSA.

Photoinduced Hydrogen Atom Transfer in 2-(2′-Hydroxyphenyl)benzimidazole Substituted with Anthracene

Abstract Photoinduced intramolecular hydrogen atom transfer was observed in anthracene-substituted 2-(2′-hydroxyphenyl)benzimidazole (HBI) derivatives, depending on substitution position.

The nonlinear optical properties of HBI in different solvents

2-(2'-Hydroxyphenyl) benzimidazole (HBI) is one kind of organic molecules featuring excited state proton transfer (ESPT). The nonlinear optical properties of 2-(2'-hydroxyphenyl) benzimidazole (HBI) in different polar solvents were investigated by the Z-scan technique. The experimental results show that the nonlinear refractive indices and TPA coefficient of HBI increase with the enhancement of solvent polarity. The optical nonlinearity obviously sensitive to the solvent polarity allows them to be potentially applicable for the amplitude manipulation of the optical nonlinearity.

Optical nonlinearity of HBI in different solvents

Abstract 2-(2'-Hydroxyphenyl) benzimidazole (HBI) is one kind of organic molecules featuring excited-state proton transfer (ESPT). The nonlinear optical properties of 2-(2'-hydroxyphenyl) benzimidazole (HBI) in different polar solvents were investigated by means of Z-scan technique under the excitation of the 1064 nm picoseconds laser pulse. The experimental results show that the nonlinear refractive indices decrease with the enhancement of the polarity of the solvent. The nonlinear refractive indices sensitive to the solvent polarity allow them to be widely used for the optoelectronic devices.

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

The effects of temperature on the dual fluorescence of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues, viz., 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c), were investigated in solvents of different polarity and hydrogen bonding capability. Absorption, steady-state, and time-resolved emission spectroscopic techniques were employed for the experimental study. Density functional theoretical calculations were performed to find the relative population of the conformers. The calculations predict that, with increase in temperature, the population of trans-enol increases, while that of cis-enol decreases. At all temperatures, the population ratio of cis-enol to trans-enol increases in the order HPIP-c < HPIP-b < HPBI. Except for HPBI in methanol and ethylene glycol, the fluorescence of both emissions decreases with an increase in temperature and is more pronounced in the tautomer band than in the normal band. The data are analyzed using the Arrhenius and van't Hoff equations. The change in the fluorescence with temperature is governed by (i) the change in the relative population of conformers and (ii) the increase in non-radiative decay from the excited states. The increase in non-radiative decay from the normal emission competes with the increase in the relative population of trans-enol with a rise in temperature.