Characterizing excited-state intramolecular proton transfer in 3-hydroxyflavone with ultrafast transient infrared spectroscopy

Vibrational dynamics associated with excited-state intramolecular proton transfer in 3-hydroxyflavone and 3-hydroxy-2-(thiophen-2-yl)chromen-4-one are characterized with ultrafast transient infrared spectroscopy. The spectroscopic data reveal rapid (<100 fs) proton transfer dynamics in both species, followed by vibrational relaxation of the tautomer products within a few picoseconds. Coherent coupling of the shared proton to low-frequency modes along the hydrogen bond coordinate are also observed in the ground electronic states.

Unveiling the effects of atomic electronegativity on ESIPT behaviors for FQ-OH system: A theoretical study

In this work, we mainly focus on exploring the effects of atomic electronegativity on excited state intramolecular proton transfer (ESIPT) behavior for novel FQ-OH derivatives theoretically. Combining analyses of geometrical changes, infrared (IR) spectral variations, and bonding energies via band critical point (BCP) parameters, we clarify the excited state hydrogen bonding strength is enhancing with decrease of atomic electronegativity. In addition, photo-induced charge reorganization and different energy gap of momentous frontier molecular orbitals (MOs) further reflect intramolecular charge transfer (ICT) promotes ESIPT reaction. Low atomic electronegativity reveals excited state high kinetic dynamics and chemical activities. Via constructing potential energy curves (PECs) and searching transition state (TS), we clarify atomic electronegativity dependent ESIPT behavior for FQ-OH. Particularly, the modification of atomic electronegativity also plays critical roles in regulating UV-Vis spectra. This work not only uncovering detailed ESIPT mechanism for FQ-OH, but also presents a novel regulated mechanism via atomic electronegativity.

Theoretical Insights into the Luminescence and Sensing Mechanisms of N,N'-Bis(salicylidene)-[2-(3',4'-diaminophenyl)benzthiazole] for Copper(II)

The intramolecular proton transfer (IPT) reaction potential energy surfaces (PESs) of N,N'-bis(salicylidene)-[2-(3',4'-diaminophenyl)benzthiazole] (BTS) in the S₀ state and S₁ state are constructed. It is found that the IPT reactions in the ground state hardly take place due to the high reaction energy barrier for single-proton (6.3 kcal/mol) and double-proton transfer (14.1 kcal/mol) reactions and low backward reaction energy barriers for single-proton (1.9 kcal/mol) and double-proton transfer (1.2 kcal/mol) reactions. In comparison, an excited-state intramolecular single-proton transfer reaction is a barrierless and exothermic process, and thus, single-proton transfer tautomer T_1H contributes most to the fluorescence emission. Based on the analysis of PESs, the experimental absorption and emission spectra are reproduced well by the calculated vertical excitation energies of BTS and its photoisomerization products, and the triple fluorescence emission profile in the experiment is reassigned unequivocally. Furthermore, thermodynamic analysis of the BTS-Cu(II) complex shows that the dinuclear complex (C1) with Cu(II) coordinating with O and N atoms of the hydrogen bonds is the most thermodynamically stable structure, and the intramolecular hydrogen bonding structure in BTS is destroyed due to the chelation of Cu(II) and BTS; as a result, the IPT reaction of C1 in S₀ and S₁ states is significantly inhibited. The inhibitor of Cu(II) in the IPT reaction plays a major role in fluorescence quenching.

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.

Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenedi-imides

This work presents a systematic study of the conducting and optical properties of a family of aromatic di-imides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenedi-imide (PyDI). Quantum-chemical calculations combined with the Marcus-Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenedi-imide increases intermolecular steric interactions and hinders close intermolecular π-π stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C5-PyDI (μh = 0.004 cm2 V-1 s-1 and μe = 0.00003 cm2 V-1 s-1) than in the C5-PyDI crystal (μh = 0.16 cm2 V-1 s-1 and μe = 0.08 cm2 V-1 s-1). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C5-PyDI, corresponding to an S 1→S 0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).

Effects of π-expansion, an additional hydroxyl group, and substitution on the excited state single and double proton transfer of 2-hydroxybenzaldehyde and its relative compounds: TD-DFT static and dynamic study

The effects of π-expansion, an extra hydroxyl group, and substituents on the photophysical properties, the excited state single proton transfer and the double proton transfer of 2-hydroxybenzaldehyde and its relatives have been theoretically investigated using TD-DFT.

Elaborating a new excited state intramolecular proton transfer (ESPT) mechanism for a new π-conjugated dye 2, 2′-((5-(2-(4-methoxyphenyl)ethenyl)-benzene-1,1-diyl)-bis-(nitrilomethylylidene)-diphenol)

In this work, the excited state dynamical behavior of a novel π-conjugated dye 2,2′-((5-(2-(4-methoxyphenyl)ethenyl)-benzene-1,1-diyl)-bis-(nitrilomethylylidene)-diphenol) (C1) has been investigated. Two intramolecular hydrogen bonds of C1 are tested to pre-existing in the ground state via AIM and reduced density gradient. Using a time-dependent density functional theory (TDDFT) method, it has been substantiated that the intramolecular hydrogen bonds of C1 should be strengthened in the S1 state via analyzing fundamental bond length, bond angles, and corresponding infrared vibrational modes. The most obvious variation of these two hydrogen bonds is the O4–H5···N6 bond, which might play important roles in excited state behavior for the C1 system. Furthermore, based on electronic excitation, charge transfer could occur. Just due to this kind of charge re-distribution, two hydrogen bonds should be tighter in the first excited state, which is consistent with the variation of hydrogen bond lengths. Thus, the phenomenon of charge transfer is reasonable evidence for confirming the occurrence of the excited state proton transfer (ESPT) process in the S1 state. Our theoretically constructed potential energy surfaces of C1 show that excited state single proton transfer should occur along with the O4–H5···N6 hydrogen bond rather than the O1–H2···N3 bond. We not only clarify the ESIPT mechanism for C1 but put forward new affiliation and explain a previous experiment successfully.

The mechanism of ratiometric fluoride sensing and the ESIPT process for 2,6-dibenzothiazolylphenol and its derivative

In this work, we explore the excited state intramolecular proton transfer (ESIPT) process and the relevant fluoride-sensing mechanism of two novel chemical systems, 2,6-dibenzothiazolylphenol (26DB) and bis-2,6-dibenzothiazolylphenol (Bis-26DB).

Ingenious modification of molecular structure effectively regulates excited-state intramolecular proton and charge transfer: a theoretical study based on 3-hydroxyflavone

Harnessing ingenious modification of molecular structure to regulate excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics holds great promise in fluorescence sensing and imaging. Based on the 3-hydroxyflavone (3HF) molecule, 2-(2-benzo[b]furanyl)-3-hydroxychromone (3HB) and 2-(6-diethylamino-benzo[b]furan-2-yl)-3-hydroxychromone (3HBN) were designed by the extension of the furan heterocycle and the introduction of a diethylamino group. The analysis of important hydrogen bond length, frontier molecular orbitals, infrared spectra, and potential curves have cross-validated our results. The results indicate that proper site furan heterocycle extension and diethylamino donor group substitution not only shift the absorption and emission spectra to the red but also effectively modulate the excited-state dynamic behaviors. Strengthened ICT characteristics from 3HF to 3HB and to 3HBN make the occurrence of ESIPT increasingly difficult due to the higher energy barriers, which indicates that the ESIPT and ICT processes are competitive mechanisms. We envision that our work would open new windows for improving molecular properties and developing more fluorescent probes and organic radiation scintillators.

Harnessing ingenious modification of molecular structure to regulate excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics holds great promise in fluorescence sensing and imaging.

Theoretical investigation on ESIPT mechanism for a novel Sal-3,4-benzophen system

In this work, we theoretically investigate the properties of excited state process for a novel salicylidene sal-3,4-benzophen (Sal-3,4-B) system, which contains two intramolecular hydrogen bonds (O1-H2[Formula: see text]N3 and O4-H5[Formula: see text]N6). Based on the density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we find these two hydrogen bonds should be strengthened in the S1 state, while the O4-H5[Formula: see text]N6 one could be largely affected upon the excitation process. Analyses about infrared (IR) vibrational spectra about hydrogen bond moieties also confirm this viewpoint. Frontier molecular orbitals (MOs) depict the nature of electronic excited state and support the excited state intramolecular proton transfer (ESIPT) reaction.Two kinds of stepwise potential energy curves of Sal-3,4-B in the S1 state demonstrate that only one proton could be transferred. Also based on constructing potential energy curves, the synergetic situation could be eliminated. Due to the specific ESIPT mechanism for Sal-3,4-B, we successfully explain the previous experiment and provide a reasonable attribution to the second emission peak of experiment.

An analogy study on ESIPT reaction for 3BHC sensor between polar DMF and nonpolar toluene

A comparison about excited state intramolecular proton transfer (ESIPT) mechanism of a new sensor 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde (3BHC) in polar solvent dimethylformamide (DMF) and nonpolar solvent toluene have been investigated within the framework of the time-dependent density functional theory (TD-DFT) method. The reproduced previous experimental absorption and emission spectra via our calculations reveals the reasonability of the DFT and TD-DFT theoretical level. The staple bond lengths, bond angles, and corresponding infrared vibrational spectra demonstrate that the intramolecular hydrogen bond of 3BHC should be strengthened in both polar DMF and nonpolar toluene. Two kinds of ESIPT mechanisms for different solvents have been put forward; there is a low potential barrier in the ESIPT process in the DMF solvent, whereas there is almost a nonbarrier for the ESIPT process in the toluene solvent. Hence, we could conclude that the ESIPT process of 3BHC sensor is more likely to occur in the nonpolar solvent upon the photoexcitation, based on which, the excited state behavior of 3BHC could be controlled.

Charge-transport properties of 4-(1,2,2-tri-phenyl-vinyl)-aniline salicylaldehyde hydrazone: tight-packing induced molecular 'hardening'

Based on first-principles calculations, the relationship between molecular packing and charge-transport parameters has been investigated and analysed in detail. It is found that the crystal packing forces in the flexible organic molecule 4-(1,2,2-triphenylvinyl)-aniline salicylaldehyde hydrazone (A) can apparently overcome the dynamic intramolecular rotations and the intramolecular steric repulsion, effectively enhancing the molecular rigidity and decreasing the internal reorganization energy. The conducting properties of A have also been simulated within the framework of hopping models, and the calculation results show that the intrinsic electron mobility in A is much higher than the corresponding intrinsic hole mobility. These theoretical investigations provide guidance for the efficient and targeted control of the molecular packing and charge-transport properties of organic small-molecule semiconductors and conjugated polymeric materials.

A theoretical study about the excited state intermolecular proton transfer mechanisms for 2-phenylimidazo[4,5-b]pyridine in methanol solvent

Within the framework of DFT and TDDFT methods, we have investigated the novel system 2-phenylimidazo[4,5-b]pyridine (PIP) with respect to the dynamical behavior of its excited state in methanol (MeOH) solvents.

Elaborating the excited-state proton transfer behaviors for novel 3H-MC and P2H-CH

We have explained the ESPT mechanism and shown the excited state dynamical overall perspective for 3H-MC and P2H-CH.

Density functional theoretical investigation of intramolecular proton transfer mechanisms in the derivatives of 3-hydroxychromone

The ground and excited state intramolecular proton transfer mechanisms of 3-HTCA and 3-HTC-DiCN have been investigated systematically.