Excited-state Intramolecular Double Proton Transfer Research Articles

Photochemical Mechanisms of Hydroxyquinoline Benzimidazole: Insights from Electronic Structure Calculations and Nonadiabatic Dynamics Simulations.

Excited-state intramolecular double proton transfer (ESIDPT) has received much attention because of its widespread existence in the life reactions of living organisms, and materials with this property are significant for their special luminescent properties. In this work, the complete active space self-consistent field (CASSCF) and OM2/multireference configuration interaction (OM2/MRCI) methods have been employed to study the static electronic structure calculations of the photochemistry and the possibility of ESIDPT process of hydroxyquinoline benzimidazole (HQB) molecule, along with the nonadiabatic dynamics simulations. The computational results show that the HQB molecule is relaxed to the S1-ENOL minimum after being excited to the Franck-Condon point in the S1 state. Subsequently, during the nonadiabatic deactivation process, the OH···N proton transfer and the twisting of benzimidazole occur before arriving at the single proton transfer conical intersection S1S0-KETO. Finally, the system can either return to the initial ground-state structure S0-ENOL or to the single proton transfer ground-state structure S0-KETO, both of which have almost the same probability. The dynamics simulations also show that no double proton transfer occurs. The excited-state lifetime of HQB is fitted to 1.1 ps, and only 64% of the dynamic trajectories return to the ground state within the 2.0 ps simulation time. We hope the detailed reaction mechanism of the HQB molecule will provide new insights into similar systems.

Achieving the cooperative and stepwise regulation of ESIDPT process in AFBD by introducing different electronic groups

In this paper, the effect of different electronic group substituents on the excited state intramolecular double proton transfer (ESIDPT) in AFBD is comprehensively researched by using quantum chemistry methods. Two electronic groups with comparable capabilities, including the electron-withdrawing group (-Br) and the electron-donating group (–OCH3), are chose to accomplish this study. The obtained geometric parameters, reduced density gradient and infrared spectra calculations have demonstrated that AFBD and its two derivatives experience the ESIDPT process. Subsequently, the conclusion of the potential energy surface and Bonn-Oppenheimer molecular dynamics further unveil that AFBD undergoes a cooperative ESIDPT process at ∼15 fs. Nevertheless, it should be noted that the introduction of -Br and –OCH3 transforms the proton transfer path. That is: "the cooperative ESIDPT is converted into the stepwise ESIDPT". Besides, we also confirm that the substitution of the electronic groups OCH3→H→Br can accelerate the ESIDPT process. Our work not only regulates the path of ESIDPT process, but also provides a time scale for this process, which is conducive to developing new intelligent fluorophores based on ESIDPT behavior.

Time-resolved fluorescence spectroscopy for studying stepwise excited-state intramolecular double proton transfer

Nowadays, single excited-state intramolecular proton transfer (ESIPT) is well studied and widely reported. However, excited-state multiple intramolecular proton transfer has not been thoroughly researched until the past decade. And now there is an increasing scientific interest not only in single but also in multiple ESIPT processes occurred in various molecules, because it offers great opportunities to apply such compounds in chemical sensors, molecular switches, NIR emitting devices and liquid crystals with high fluorescence. A stepwise excited-state intramolecular double proton transfer (ESIDPT) is among such multiple ESIPT processes. Similar to the single ESIPT, stepwise ESIDPT also occurs via an intramolecular hydrogen bonding way. But in the stepwise ESIDPT, two proton transfers occur sequentially in a molecule, because there is only one proton transfer site in the initial molecular structure. Then, after the first ESIPT, an additional proton transfer site appears, and the second proton transfer will occur.In this work, an emission of stepwise ESIDPT-active molecules was studied from the point of view of a time-resolved fluorescence spectroscopy. The theoretical model for emission decay was discussed on the basis of a six states model. This model consists of three pairs of ground and excited states for normal molecular form and two tautomers. It leads to three-exponential equations for emission decay functions. It was shown that a thorough analysis of fluorescence decay curves allows to estimate the total decay rate constants for each excited molecular form. The proposed model has been validated for a numerically simulated case. The experimental data have also been discussed from the point of view of the proposed model.

Theoretical Insights into Excited‐State Stepwise Double Proton Transfer Associated with Solvent Polarity for 2‐bis(benzothia‐zolyl)naphthalene‐Diol Compound

AbstractInspired by the excellent photochemical properties of 2‐(2′‐hydroxyphenyl)benzazole and its derivatives, in this work, the novel 2‐bis(benzothia‐zolyl)naphthalene‐diol (2‐BBTND) fluorophore is explored about its photo‐induced behaviors. The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 2‐BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can affirm that strong polar solvent environment promotes the excited state dynamical reaction for 2‐BBTND compound. By constructing potential energy surfaces (PESs) in S0 and S1 states, we clarify that 2‐BBTND fluorophore should undergo the stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent‐polarity‐dependent stepwise ESDPT for 2‐BBTND fluorophore.

Theoretical study based on the excited state dynamical of an oxadiazole derivative: A novel fluorescence mechanism

Excited intramolecular proton transfer (ESIPT) has been widely studied as a model system for proton transfer. In recent years, materials and biological systems containing two proton transfers have received special attention from researchers. In this work, the excited state intramolecular double-proton-transfer (ESIDPT) mechanism of a fluorescent compound based on an oxadiazole derivative, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-benzene-1,4-diol (DOX), has been comprehensively investigated through theoretical calculations. The potential energy surface curve of the reaction shows that ESIDPT can occur in the first excited state. This work proposes a new and reasonable fluorescence mechanism based on previous experiments, which has theoretical significance for the future research of DOX compounds in biomedicine and optoelectronics.

Unveiling and regulating the solvent-polarity-associated excited state intramolecular double proton transfer behavior for 1-bis(benzothiazolyl)naphthalene-diol fluorophore

Inspired by the regulatory luminescence properties of HBT derivatives, in this work, we mainly conduct a detailed theoretical exploration on the photoinduced excitation behavior of a novel di-proton-transfer type HBT derivative 1-bis(benzothiazolyl)naphthalene-diol (1-BBTND). The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 1-BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can conclude that strong polar solvent environment promotes the excited state dynamical reaction for 1-BBTND compound. By constructing potential energy surfaces (PESs) in S0 and S1 states, we clarify that 1-BBTND fluorophore should undergo a stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent-polarity-dependent stepwise ESDPT for 1-BBTND fluorophore.

Deep-red and near-infrared organic lasers based on centrosymmetric molecules with excited-state intramolecular double proton transfer activity

Deep-red and near-infrared organic lasers based on centrosymmetric molecules with excited-state intramolecular double proton transfer activity

Tunable fluorescence of Imidazo[1,2‐a]pyridine derivatives with additional proton transfer sites Harnessing excited-state intramolecular double proton transfer: Theoretical insight

Electronic properties and excited-state intramolecular proton transfers of a new designed series of doubly intramolecular hydrogen bonding molecules having imidazo[1,2-a]pyridines as proton acceptors with different types of proton donors (hydroxy-type, amino-type, and mixing-type) and their derivatives have been theoretically evaluated for fluorescent probes. Overall, all designed molecules are photoexcited at ∼420 nm and give fluorescence in the NIR region with large Stokes shifts. The fluorescence of NH- and Mixed-type molecules are notably longer than that of OH-type molecules, and those of the derivatives with a π-conjugated spacer are slightly longer than those of their analogues. Based on the kinetic and thermodynamic information, all designed molecules could not exhibit the double proton transfer (DPT) process but only single proton transfer (SPT) is preferred. Moreover, the dynamic simulations on the excited-state confirm that only SPT is allowed but not DPT in accordance with the predicted PT barriers and reaction energies, leading only NT* fluorescence but their NT* fluorescence maxima are still in the NIR region (600–900 nm) with large Stokes shifts (∼300 nm). The NIR fluorescence is caused by an additional PT unit and a π-conjugated spacer induce the ICT character evidenced by the EDD maps. Therefore, these designed molecules exhibiting NIR tautomer emission with large Stokes shifts could be potential candidates for NIR fluorescent probes with avoiding self-absorption and less photo-damage.

Comparison of the excited-state proton transfer and single electron transfer mechanisms of the natural antioxidant Juglone and its dimer 3,3′-bijuglone

In this work, the excited-state intramolecular proton transfer (ESIPT) mechanism and antioxidant activity of Juglone (Ju) and its dimer 3,3′-bijuglone (BJu) were systematically compared and analyzed by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Via comparing the primary hydrogen bond parameters, reduced density gradient (RDG) versus sign(λ2)ρ(r) scatter plots, interaction region indicator (IRI) isosurface, topology descriptors, and infrared (IR) spectra, it can be confirmed that all the intramolecular hydrogen bonds (IHBs) of Ju and its dimer 3,3′-bijuglone (BJu) have been enhanced in the lowest excited state (SL). The intramolecular charge transfer (ICT) triggered the fluorescence quenching mechanism for Ju was confirmed based on the hole-electron analysis. Moreover, the calculated S0- and SL-stated potential energy surfaces (PESs) have proven that the excited-state intramolecular double proton transfer (ESIDPT) of BJu is stepwise rather than concerted and based on which the detailed proton-transfer (PT) process has been clarified. Compared with the first step PT behavior of BJu, the ESIPT process of Ju has barely been influenced by dimerization. Besides, the computed ionization potential (IP) has substantiated that BJu shown the better antioxidant capacity via the single electron transfer (SET) mechanism than the corresponding monomer Ju in the SL state. The relationship between the PT process and antioxidant activity of Ju and BJu has also been successfully established, which is conducive to design and synthesize highly effective PT-based antioxidants.

Excited-state intramolecular double proton transfer mechanism associated with solvent polarity for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound

Excited-state intramolecular proton transfer (ESIPT) has been well investigated in recent years, whereas we investigated the system containing double hydrogen bonds. We explore the influence of solvent polarity on the proton transfer mechanism for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound ((Oxa-OH)2). We consider three solvents with different polarities: cyclohexane, chloroform and acetonitrile. Various analytical methods indicate that hydrogen bonding is enhanced in the first excited state, which facilitates the proton transfer within the excited-state molecule. The frontier molecular orbital theory is employed to demonstrate that the system is subjected to photoexcitation to generate charge density redistribution. Three-dimensional potential energy surfaces are also constructed along the direction of hydrogen bond elongated, and their corresponding projection in the X-axis and Y-axis planes are plotted. The comparison of the potential barriers and the transition state structures of each proton transfer path reveals the excited-state intramolecular single proton transfer mechanism of the system. By comparing the potential barriers in different solvents, we also propose a mechanism that can modulate the excited-state proton transfer by changing the solvent polarity.

Theoretical Investigation of Excited-State Intramolecular Double-Proton Transfer Mechanism of Substituent Modified 1, 3-Bis (2-Pyridylimino)-4,7-Dihydroxyisoindole in Dichloromethane Solution

In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) methods were used to investigate substituent effects and excited-state intramolecular double-proton transfer (ESIDPT) in 1, 3-bis (2-pyridylimino)-4, 7-dihydroxyisoindole (BPI–OH) and its derivatives. The results of a systematic study of the substituent effects of electron-withdrawing groups (F, Cl and Br) on the adjacent sites of the benzene ring were used to regulate the photophysical properties of the molecules and the dynamics of the proton-transfer process. Geometric structure comparisons and infrared (IR) spectroscopic analysis confirmed that strengthening of the intramolecular hydrogen bond in the first excited state (S1) facilitated proton transfer. Functional analysis of the reduced density gradient confirmed these conclusions. Double-proton transfer in BPI–OH is considered to occur in two steps, i.e., BPI–OH (N) [Formula: see text] BPI–OH (T1) [Formula: see text] BPI–OH (T2), in the ground state (S0) and the S1 state. The potential-energy curves (PECs) for two-step proton transfer were scanned for both the S0 and S1 states to clarify the mechanisms and pathways of proton transfer. The stepwise path in which two protons are consecutively transferred has a low energy barrier and is more rational and favorable. This study shows that the presence or absence of coordinating groups, and the type of coordinating group, affect the hydrogen-bond strength. A coordinating group enhances hydrogen-bond formation, i.e., it promotes excited-state intramolecular proton transfer (ESIPT).

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.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d’]bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d’]bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S1 state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S1 state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

Theoretical study of the excited state intramolecular double proton transfer and spectral behaviors of 7-hydroxyquinoline-8-carboxylic acid

The excited-state molecular dynamics of 7-hydroxyquinoline-8-carboxylic acid (HCA) were studied to observe its fluorescent properties and proton transfer process. The two intramolecular hydrogen bonds (O-H⋯O and O-H⋯N) of the Enol form were both strengthened after photoexcitation to the first excited (S1) state. The experimentally observed fluorescence emission (465 nm) was attributed to the theoretical Keto form (470 nm), which implies the occurrence of an excited-state intramolecular double proton transfer (ESIDPT) process. The nonadiabatic dynamics results demonstrate that a single proton transfer from the carboxylic to the nitrogen atom (process-A) occurs in 55 fs, which excludes the ESIDPT concert pathway. The potential energy surface results indicate that process-A (0.08 kcal/mol) induces the occurrence of a second proton transfer from the phenol to the oxygen atom (process-B). Compared to the stepwise ESIDPT reaction that begins with process-B (1.53 kcal/mol), process-A is energy favorable in the S1 state. Therefore, we propose a reaction path of the following: Enol in the ground state (S0) → Enol in the S1 state → proton transferred Keto in the S1 state (stepwise pathway begins with process-A) → Keto in the S0 state (fluorescence emission at 470 nm) → Enol in the S0 state (reversed proton transfer). This process confirms what was predicted by Chou (J. Phys. Chem. Lett. 2011, 2, 3063).

A two-channel mechanism for the asymmetric excited-state intramolecular double proton transfer in a hydroxyflavone derivative

The excited-state proton transfer (ESPT) mechanism of 3,7-dihydroxy-2-phenyl-6-acetyl-4H-chromen-4-one (DPACO) fluorophore was studied by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with CAM-B3LYP functional and 6–31 +G(d,p) basis set. The computation levels are reliable as evidenced by the accordance between theory and experiment. In the ground state of DPACO, two intramolecular hydrogen bonds were formed. After photo-excitation, the electron density redistributions provide the driving force for hydrogen-bonding strengthening and ESPT. The constructed potential energy surfaces (PESs) along proton transfer (PT) coordinates indicate that two PT channels exist in the excited state. Among various PT tautomers, T4*is theoretically assigned as the main fluorescence emitter, while the low fluorescence quantum yield possibly correlate with the accelerated internal conversion pathway in the PT tautomer T6*.

Effect of Microheterogeneity of Different Aqueous Binary Mixtures on the Proton Transfer Dynamics of [2,2'-Bipyridyl]-3,3'-diol: A Femtosecond Fluorescence Upconversion Study.

In this article, we have investigated the excited-state intramolecular double proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diol, BP(OH)2, in three alcohol–water binary mixtures, namely, ethanol (EtOH)–water, n-propanol (PrOH)–water, tert-butyl alcohol (TBA)–water, and dimethyl sulfoxide (DMSO)–water utilizing the femtosecond fluorescence upconversion technique. We have found that in alcohol–water binary mixtures the proton transfer (PT) pathway of BP(OH)2 is sequential and the anomalous slowdown in PT dynamics is observed in mole fraction (χ) ranges χEtOH = 0.04–0.07, χEtOH = 0.23–0.28, χPrOH = 0.17–0.30, χTBA = 0.12–0.21, and χTBA = 0.40–0.46. Our study sheds light on the involvement of water network in the PT dynamics. Reduction in water accessibility due to the involvement of water molecules in cluster formation results in hindered PT dynamics, and this retardation is more for the TBA–water binary mixture compared to that for the other two mixtures. Additionally, we have found two anomalous regions for the DMSO–water binary mixture in ranges χDMSO = 0.12–0.16 and χDMSO = 0.26–0.34. However, most interestingly, beyond χDMSO = 0.40, we do not find any growth component in the femtosecond fluorescence upconversion trace, which may be due to the change in the PT mechanism from a sequential water-mediated pathway to a concerted intramolecular pathway.

Surface Hovering of Probe on Cyclodextrin Nanoparticle that is Otherwise Sequestered in Isolated Cyclodextrin Cavity

AbstractExperimental evidence of hovering of probe on the surface of β‐cyclodextrin nanoparticles (β‐CDNPs) is given in the present work. A special double proton transfer (DPT) probe was used for this purpose. 2,2′‐bipyridine‐3,3′‐diol (BP(OH)2) shows excited state intramolecular DPT (ESIDPT) in different solvents. In water, this process takes place through two mechanisms: (i) normal intramolecular proton transfer and (ii) unconventional DPT through “water‐wire” formation between the prototropic centers. The second mechanism is favoured at low availability of water around the probe. This concept was exploited in the present case to propose possible existence of BP(OH)2 on the surface of the β‐CDNPs due to uncertainty in occupying the host cavity as all are similar and cluttered together. The probe would otherwise is completely sequestered within isolated CD nanocavity in aqueous medium. Spectroscopy has been used as a tool to establish the proposition.

Insight into the excited-state double proton transfer mechanisms of doxorubicin in acetonitrile solvent

Doxorubicin (DXR) is theoretically investigated with an aim to explore the excited-state intramolecular double proton transfer (ESIDPT) mechanism regarding stepwise versus synchronous double proton transfer.

Modulation of the Excited-State Dynamics of 2,2'-Bipyridine-3,3'-diol in Crown Ethers: A Possible Way To Control the Morphology of a Glycine Fibril through Fluorescence Lifetime Imaging Microscopy.

In this article, we have investigated the modulation of excited-state intramolecular double proton transfer (ESIDPT) dynamics of 2,2'-bipyridine-3,3'-diol (BP(OH)2) in two crown ethers (CEs), namely, 18-Crown-6 (18C6) and 15-Crown-5 (15C5). From steady-state UV-visible measurements, we have shown that there is no significant interaction between the dienol tautomeric form of BP(OH)2 and two CEs. However, in the presence of CEs, an additional emission band (∼415 nm) is generated along with the diketo tautomer band (∼465 nm). In time-resolved analysis, we have observed the generation of ∼260 ps rise component in the presence of 18C6. Therefore, by combining the results of steady-state and time-resolved emissions, we have proposed that the water-assisted ESIDPT route of BP(OH)2 generates a hydronium ion (H3O+) in the excited state. 18C6 binds nicely to this H3O+ ion. As a result, retarded ESIDPT dynamics is observed in 18C6. However, as 15C5 cannot bind H3O+ properly, no rise component is found. With the addition of potassium chloride (KCl), the contribution of the rise component decreases due to unavailability of free 18C6 cavity to capture the H3O+ ion generated in the excited state. Addition of calcium chloride (CaCl2) leads to complete removal of the rise component due to the inhibition of the water-assisted ESIDPT route. From wavelength-dependent behavior, we have observed that the rise component is present only at 465 nm in 18C6. We have also shown that the fibrillar morphology of glycine can be successfully probed through fluorescence lifetime imaging microscopy using BP(OH)2 as an imaging agent. Modulation of fibrillar morphology has been found in the presence of two CEs. The interaction of glycine fiber with CEs can be explained by lifetime distribution analysis.

Electrostatic Embedding To Model the Impact of Environment on Photophysical Properties of Molecular Crystals: A Self-Consistent Charge Adjustment Procedure.

A case study of 1,8-dihydroxy-2-napthaldehyde (DHNA)-exhibiting an excited-state intramolecular double proton transfer resulting in photophysical properties sensitive to the surrounding environment-has been used to assess the performance of electrostatic embedding approaches designed to accurately recover the effects of a bulk crystalline environment on calculated photophysical properties. The first approach, based on time-dependent density functional theory (TD-DFT) applied in a QM/QM' scheme, makes use of a background point charge distribution which can accurately reproduce the exact ground-state Ewald potential of the bulk crystal. The second approach seeks to "optimize" these charges in a self-consistent manner in order to reproduce the electrostatic field produced by the environment at the excited state. Using these two approaches, both absorption and emission properties of molecular crystals, such as the position and the relative shift in the emission bands in the solid state with respect to solution, can be accurately reproduced. More generally, the results obtained show how these computationally affordable approaches can be used to predict the excited-state behavior of molecules in condensed phases, thus allowing their employment to predict or design new molecular materials with enhanced photophysical properties.