Solvation effect on the ESIPT mechanism of 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine

Computational insights of excited state intramolecular proton transfer (ESIPT) based fluorescent detection and imaging of γ-glutamytranspeptidase activity

γ-Glutamytranspeptidase (GGT) is an important tumor biomarker that widely appears in the tumor cells. Therefore, accurate imaging and detection of GGT activity in live cells, serum and pathological cells grasp great importance for the diagnosis, management, and treatment of cancer. Herein, 2-(2-hydroxyl-phenyl)-6-chloro-4-(3H)-quinazolinone (HPQ) is considered as the fluorophore probe for the detection of GGT activity, which is known for the typical mechanism of excited-state intramolecular proton transfer (ESIPT). All the simulations adopted to evaluate the sensing mechanism were carried out via DFT and TDDFT calculations at CAM-B3LYP/TZVP level of theory. The emission properties of HPQ and HPQ-TD are thoroughly studied to understand the photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) process. The results reveal that the fluorescence quenching of HPQ (enol form) is assigned to the PET process, whereas the large Stokes shift in fluorescence emission of HPQ (keto form) is related with ESIPT mechanism. The obtained results are further cross validated by frontier molecular orbital (FMO) analysis, geometric analysis, and potential energy curve (PEC) scanning. Our calculations provide powerful evidence for the ESIPT based sensing mechanism of HPQ (keto-enol form) for GGT activity.

Exploring the influence of intermolecular hydrogen bonding on the fluorescence properties of HQCT and HQPH fluorescent chemosensors

Recently, two trace water detection probes, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone(HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine(HQPH) have been successfully designed in the experiment. The original intramolecular proton transfer can be prevented by the water molecules, leading to fluorescence quenching. In order to investigate the fluorescence quenching mechanism, the effect of water molecules on the excited state proton transfer process will be studied in detail. In this contribution, the six models have been optimized and the related analysis have been carried out. When water molecules are involved in the proton transfer process, the energy barrier decreases significantly and the conversion of the enol structure to the keto structure is accelerated. Moreover, the intermolecular hydrogen bonding, not participating in the proton transfer process, can facilitate the proton transfer process by affecting the distribution of the electrostatic potential within the molecule, which in turn lowers the energy barrier for proton transfer.

Regulation of excited-state intramolecular proton transfer process and photophysical properties for benzoxazole isothiocyanate fluorescent dyes by changing atomic electronegativity

Excited-state intramolecular proton transfer (ESIPT) is favored by researchers because of its unique optical properties. However, there are relatively few systematic studies on the effects of changing the electronegativity of atoms on the ESIPT process and photophysical properties. Therefore, we selected a series of benzoxazole isothiocyanate fluorescent dyes (2-HOB, 2-HSB, and 2-HSeB) by theoretical methods, and systematically studied the ESIPT process and photophysical properties by changing the electronegativity of chalcogen atoms. The calculated bond angle, bond length, energy gap, and infrared spectrum analysis show that the order of the strength of intramolecular hydrogen bonding of the three molecules is 2-HOB<2-HSB<2-HSeB. Correspondingly, the magnitude of the energy barrier of the potential energy curve is 2-HOB>2-HSB>2-HSeB. In addition, the calculated electronic spectrum shows that as the atomic electronegativity decreases, the emission spectrum has a redshift. Therefore, this work will offer certain theoretical guidance for the synthesis and application of new dyes based on ESIPT properties.

The influence of intermolecular hydrogen bonds on single fluorescence mechanism of 1-hydroxy-11H-benzo [b]fluoren-11-one and 10-hydroxy-11H-benzo [b]fluoren-11-one

Solvent effects usually have an essential effect on excited-state intramolecular proton transfer (ESIPT) processes and fluorescence mechanism. This contribution presents new insights into a newly synthesized compound, namely, 10-hydroxy-11H-benzo [b]fluoren-11-one (10-HHBF), and its analogue 1-hydroxy-11H-benzo [b]fluoren-11-one (1-HHBF), which exhibit single-fluorescence properties in protic solvents (methanol, MeOH), using time-dependent density functional theory (TDDFT). The results established four schemes, namely, MeOH-1, MeOH-2, MeOH-3, and MeOH-4, for 1-HHBF and 10-HHBF in MeOH. Absorption and emission spectra showed that the 1-HHBF and 10-HHBF at the conformation MeOH-2, MeOH-3 and MeOH-4 were closer to the experimental values than those at the MeOH-1. Energy barriers indicate the possibility of the ESIPT and ESPT process in 1-HHBF and 10-HHBF under the four schemes. Moreover, reverse PT processes were easy to occur at the conformations of MeOH-2, MeOH-3, and MeOH-4 in the S₁ state. Given the single-fluorescence properties of 1-HHBF and 10-HHBF in the experiment, the conformation MeOH-1 was excluded. Therefore, our contribution proved that MeOH-2, MeOH-3, and MeOH-4 might exist in single fluorescence, and the hydrogen bond at the MeOH-2 position plays a decisive role, indicating the intermolecular hydrogen bonding interaction on the acceptor atom will have a more significant impact on the fluorescence properties of the substance.

Solvent effect on the excited-state intramolecular double proton transfer of 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to study the solvatochromic effect and the excited-state intramolecular double proton transfer (ESIDPT) of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPI-OH) in different kinds of solvents. The hydrogen bonding parameters and IR spectra reveal that in the excited state, the strength of excited hydrogen bond increase with the decrease of solvent polarity. Furthermore, the reduction density gradient (RDG) analysis confirms the corresponding conclusion. Frontier molecular orbitals (FMOs) are analyzed, illuminating that the smaller the polarity of solvent, the smaller the energy gap between the HOMO and LUMO. The structures of BPI-OH (N) (normal), BPI-OH (T₁) (single), and BPI-OH (T₂) (double) were optimized. Previous reports found the double protons in BPI-OH molecule are transferred step-by-step process BPI-OH(N)→BPI-OH(T₁)→BPI-OH(T₂) in the ground state (S₀) and the first excited singlet state (S₁). Here, the potential energy curves of O₁-H₂ and O₄-H₅ in the S₀ and S₁ states were scanned in four kinds of solvents, respectively. It was found that in S₁ state, BPI-OH(N)→BPI-OH(T₁) was more prone to proton transfer than BPI-OH(T₁)→BPI-OH(T₂). In addition, by comparing the reaction energy barriers of the four kinds of solvents, it can be found that ESIPT is difficult to occur with the increase of solvent polarity. Meanwhile, it was also studied that MeOH as an explicit solvent was more likely to promote the ESIPT process than other implicit solvents.

Investigating the Influence of Electronic Effects of Functional Groups on the Fluorescence Mechanism of Probes in Water Samples

This study investigates the fluorescence quenching mechanism of formaldehyde detection probe Naph1 and its contrast probe Naph3 in water samples and discussed the effect of the electron-donating group and electron-withdrawing group on fluorescence characteristics based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We optimized the structures of the four probes Naph1, Naph1-S, Naph3, and Naph3-S (Scheme 1) and calculated the absorption and emission spectra, which were in good agreement with the experiment. Frontier molecular orbitals (FMOs) were used to analyze the charge arrangement in the excited state. To investigate the intramolecular proton transfer (ESIPT) phenomenon, a potential energy curve was constructed. The amount of fragment charge transfer was analyzed by the IFCT method, and then it was determined whether there was an intramolecular charge transfer (ICT) process. It was found that there was an ICT process in Naph3. The electronic effect of the functional groups did not determine the ICT characteristics and the fluorescence characteristics of the substance. Furthermore, the spin-orbit coupling  (SOC) constant based on the intersystem crossing (ISC) was supplemented, which showed that the fluorescence quenching of Naph1 and Naph3 was caused by the ISC and the corresponding quenching of Naph3-S was caused by charge transfer (CT) in the excited state.

Theoretical investigations on forward-backward ESIPT processes of three fluorophores deriving from 2-(2'-hydroxyphenyl)thiazole

The photophysical properties and excited-state intramolecular proton transfer (ESIPT) processes for 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (1), 2-(2'-hydroxyphenyl)-4-phenylthiazole (2), 2-(2'-hydroxyphenyl)-4-hydroxymethyl-thiazole (3) were studied at the TD-B3PW91/6-31 + G(d, p)/IEFPCM level. The structures of 1-3 were fully optimized and the corresponding structural parameters, infrared spectra and electron densities in the ground (S₀) and the first excited (S₁) states were analyzed. The calculated absorption and fluorescence wavelengths of 1-3 reproduced the experimental data. The potential energy curves of the S₀ and S₁ states were built and the ESIPT processes were clarified. Our results showed that the intramolecular H-bonds of 3 and 2 in the S₁ state were the strongest and the weakest, respectively, and then the ESIPT potential barriers of 3 and 2 were the lowest and highest, respectively. Among the three phenol-thiazole type probes, the compound 2 with phenyl ring group at the 4 position of the thiazole ring had the larger π-conjugation, and had the higher ESIPT potential barrier at the same time. The corresponding compound 1 and 3 with CH₂Cl and CH₂OH had the lower ESIPT barrier.

Selective Synthesis and Photoluminescence Study of Pyrazolopyridopyridazine Diones and N-Aminopyrazolopyrrolopyridine Diones

The newly designed luminol structures of pyrazolopyridopyridazine diones and N-aminopyrazolopyrrolopyridine diones were synthesized from versatile 1,3-diaryfuropyrazolopyridine-6,8-diones, 1,3-diarylpyrazolopyrrolopyridine-6,8-diones, or 1,3-diaryl-7-methylpyrazolopyrrolopyridine-6,8-diones with hydrazine monohydrate. Photoluminescent and solvatofluorism properties containing UV–Vis absorption, emission spectra, and quantum yield (Φ_f) study of pyrazolopyridopyridazine diones and N-aminopyrazolopyrrolopyridine diones were also studied. Generally, most of pyrazolopyrrolopyridine-6,8-diones 6 exhibited the significant fluorescence intensity and the substituent effect when compared with N-aminopyrazolopyrrolopyridine diones, particularly for 6c and 6j with a m-chloro group. Additionally, the fluorescence intensity of 6j was significantly promoted due to the suitable conjugation conformation. Based on the quantum yield (Φ_f) study, the value of compound 6j (0.140) with planar structural skeletal was similar to that of standard luminol (1, 0.175).

Benzothiazole-pyridone and benzothiazole-pyrazole clubbed emissive azo dyes and dyeing application on polyester fabric: UPF, biological, photophysical and fastness properties with correlative computational assessments

Positional isomers of benzothiazole-pyridone and benzothiazole-pyrazole containing disperse azo dyes are reported. These heterocyclic azo dyes are decorated with 'separate ESIPT core' and show emission in seven solvents of different polarity. After application on polyester fabric, "very good to excellent" light and washing fastness properties were observed. Thermal stability of 'dyed fabric' was analysed by sublimation fastness test- and found 'very good to excellent' ratings at 210 °C. Ultraviolet Protection Factor (UPF) analysis of four 'dyed fabric' indicates the blocking 96-97% of UV radiation. Dyes were found effective on gram positive and negative bacteria by agar diffusion method and all the 'dyed fabrics' also showed more than 92% or 94% reduction of S. aureus or K. pneumoniae respectively by 'AATCC 100' method. Structures of the dyes were optimized using Density Functional Theory (DFT) to deduce stable tautomeric form. Calculated HOMO-LUMO gap is then compared with antibacterial activities. Electrophilicity index and lightfastness property were also compared and found to have very good correlation.

A new interpretation of the ESIPT mechanism of 2-(benzimidazol-2-yl)-3-hydroxychromone derivatives

The present study demonstrates the excited-state intramolecular proton transfer (ESIPT) mechanism of 2-(benzimidazol-2-yl)-3-hydroxychromone (DH3B2) is based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. We find that DH3B2-C is the main conformation to occur ESIPT. Moreover, we get the different results of DH3B2 for the ESIPT mechanisms in comparison with the previous reports. We have optimized three isomers (DH3B2-A, DH3B2-B and DH3B2-C), and calculated absorption and fluorescence spectra, which agree well with the experimental data. Furthermore, we prove the hydrogen bond is enhanced in the S₁ state by comparing infrared vibrational spectra, the relevant bond length and bond angle. In our calculations, the results of the three levels of calculations (CAM-B3LYP/TZVP, B3LYP/TZVP and PBEPBE/TZVP) indicate that DH3B2-C is the most stable conformation, by compared the single point energy of three isomers. By constructed the potential energy surfaces (PESs), we find the converted relationship among the three isomers; DH3B2-C is the main conformation in which DH3B2 exists. Furthermore, combination with reduced density gradient (RDG) function, the hydrogen bond of DH3B2-C is stronger than that of DH3B2-A and DH3B2-B, which proves that DH3B2-C form is the most favorable form for ESIPT among the three isomers. Meanwhile, we have further investigated the ESIPT mechanisms of DH3B2, via constructing the potential energy curves (PECs). These results have shown that DH3B2-C is easier to ESIPT occur than DH3B2-A and DH3B2-B. Therefore, the proton receptors of the ESIPT are mainly the benzimidazole nitrogen atoms.

Effect of number and different types of proton donors on excited-state intramolecular single and double proton transfer in bipyridine derivatives: theoretical insights

Intra-HBs are strengthened upon photoexcitation, confirmed by red-shift in vibrational mode and topology analysis. Number and type of donors result in difference in photophysical properties. Occurrence of ESIPT depends on barrier and reaction energy.

The novel excited state intramolecular proton transfer broken by intermolecular hydrogen bonds in HOF system

2-(4-(Dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4Hchromen-4-one (HOF) was synthesized in experiment (Wang et al., Sensor. Actuat. B-Chem. 277 (2018) 484), and its photophysical and photochemical properties was reported. However the corresponding full theoretical interpretation of mechanisms is inadequate. In the present research, the intermolecular hydrogen bond structure of HOF-methanol complex (HOF-2M) was found, and mechanism of alcohols monitoring of HOF was deeply studied using the density functional theory (DFT) and time-dependent density functional theory (TDDFT). The enhancing mechanism of the excited state hydrogen bond is verified by analyzing the hydrogen bond parameters, infrared spectra and frontier molecular orbitals. Importantly, the reduced density gradient visual analysis and topological quantificational analysis confirm that the intramolecular hydrogen bond of HOF is broken by strong intermolecular hydrogen bonds of HOF-2M using the Atoms-In-Molecule theory. The obtained absorption and emission spectra are found to agree well with the experimental results and the complete quenched keto-emission in methanol and ethanol solvents provide a suitable sensing mechanism for detecting alcohols. The reaction path of the excited state intramolecular proton transfer for HOF is explained in detail through the constructed potential energy curves.

Revelation solvent effects: excited state hydrogen bond and proton transfer of 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol

ESIPT reaction of an MMT molecule is gradually inhibited with increasing solvent polarity.