Dual fluorescence of 2-(2'-hydroxyphenyl) benzoxazole derivatives via the branched decays from the upper excited-state

The effect of the number of conjugated C=C bonds on the ESIPT and ICT reactions of SNCN derivatives

The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Theoretical study of the photoisomerization of 1,2-bispyrazinyl-ethylene and the halogen ion salts of 1-Pyrazinyl-2-(4'-methylpyrazinyl)ethylene

CONTEXT

It has been reported that photoexcitation of azastilbene compounds like E-1,2-bispyrazinyl-ethylene (bpe) can undergo E → Z photoisomerization of its quaternary salts via the excited triplet state. However, experimentally it is possible to get low fluorescence and photoisomerisation quantum yields in a state with higher internal conversion than intersystem crossing. We modelled bpe and its methylated derivative (bpeMe), as well as its quaternary halogen salts (bpeMeX with X = F-, Cl-, Br- and I-) to study levels of fluorescence, phosphorescence and excited state potential energy surfaces (PES). Results support experimental observations of molecules where the anion of a salt is an efficient electron donor, that molecules with weak electron-donating anions like Cl- to give increased fluorescence and photoisomerization, as compared to molecules with stronger electron-donating anions like I-, which are dominated by competing electron transfer. The fluorescence of bpeMeF and bpeMeCl was found to be stronger than bpeMeBr and bpeMeI. A deep well in the triplet excited state of bpeMeI is considered responsible for the decreased photoisomerization, compared to what was experimentally observed for bpeMeCl. Uniquely, the bpeMeI molecule is characterised by near-zero splitting of the s1 and t1 excited states that can enhance charge transfer. The quaternary salt of bpeMe with stronger electron-donating Br- anion was observed to undergo fluorescence and phosphorescence at much lower energy compared to those with weak electron-donating F- and Cl- anions. This research shows how to control the excited state fluorescence, phosphorescence and isomerization of quaternary halogen salts of methyl derivatives of 1,2-bispyrazinyl-ethylene, which aids experimental design where excited state isomerization is considered.

METHODS

Geometry optimization, molecular electrostatic potential (MESP), and time dependent density functional theory (TDDFT) calculations were conducted utilizing Gaussian 16 with the B3LYP functional and the 6-31 + G(d,p) basis set. The minimum energy path (MEP) for the E to Z isomerization of the molecules was established employing the Nudged-Elastic-Band (NEB) method, implemented in Orca 4.2. Precise energies of the E → Z isomerization reaction path were determined employing CASSCF and a more accurate multireference method, NEVPT2.

Theoretical investigation on the sensing mechanism of a triphenylamine-benzofuran derived fluorescent probe for the detection of H2Sn

As one of the members of reactive sulfur species, hydrogen polysulfide (H2Sn, n > 1) plays an important role in enzyme activity and nervous system regulations, and the sensing mechanism study is of great significance for the design of novel efficient probes. Herein, we investigated the sensing mechanism of an efficient triphenylamine-benzofuran-based probe (TBF-SS) towards H2Sn using DFT method. The inherent fluorescence quenching of the probe is dominated by the twisted intramolecular charge transfer (TICT) as revealed by the torsional potential curve calculations. When the nitro fluorophenyl group is replaced by a hydroxyl group in the reaction with H2Sn, the TICT is eliminated and the excited state can return to the ground state in a radiative way, leading to strong fluorescence emission.

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.

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

A theoretical study on the excited state behavior of a series of novel triazole pyrimidine group fluorophores: ESIPT or ICT

Fluoropurine analogues are a kind of unnatural bases, which are widely used in chemistry, biological science, pharmacy and other fields. At the same time, fluoropurine analogues of aza-heterocycles play an important role in medicinals research and development. In this work, the excited state behavior of a group of newly developed fluoropurine analogues of aza-heterocycles, triazole pyrimidinyl fluorophores, was comprehensively studied. The reaction energy profiles indicate that excited state intramolecular proton transfer (ESIPT) is difficult to happen, which is further proved by fluorescent spectra results. This work proposed a new and reasonable fluorescence mechanism based on the original experiment, and found that the large Stokes shift of the triazole pyrimidine fluorophore is due to the intramolecular charge transfer (ICT) process of the excited state. Our new discovery is of great significance for the application of this group of fluorescent compounds in other fields and the regulation of fluorescence properties.

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.

Unraveling photo-induced proton transfer mechanism and proposing solvent regulation manner for the two intramolecular proton-transfer-site BH-BA fluorophore

To expound specific excited state processes of the novel excitation wavelength dependent emission BH-BA fluorophore for better subsequent applications, this wok mainly focus on exploring photo-induced hydrogen bonding geometrical changes, excited state intramolecular proton transfer (ESIPT) mechanism and related regulated behavior via solvent polarity. The differences of structural parameters, infrared (IR) vibrational spectra, core-valence bifurcation (CVB) index as well as electronic densities ρ(r) between S₀ and S₁ states related to dual hydrogen bonds (O₁-H₂···N₃ and O₄-H₅···N₆) reveal S₁-state hydrogen bonding strength facilitate ESIPT behaviors for BH-BA system. Of particular note, O₄-H₅···N₆ plays a more dominant role. Photo-induced intramolecular charge transfer (ICT) and variations of Hirshfled and NPA charges over atoms related to hydrogen bonding moieties promote the ESIPT tendency for BH-BA. Combined potential energy surfaces (PESs), transition state (TS) and intrinsic reaction coordinate (IRC) paths, we illustrate the excited state intramolecular single proton transfer (ESISPT) mechanism of BH-BA should occur along with O₄-H₅···N₆ hydrogen bonding wire, which could be adjusted by surrounding solvent polarity.

ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^OH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of L^OH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^H,OH), a monohydroxy congener of L^OH,OH. Both L^OH,OH and L^H,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for L^H,OH and two for L^OH,OH) strongly affects the luminescence mechanism and color of the emission: L^H,OH emits in the light green region, whereas L^OH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T₁ → S₀ phosphorescence and not related to ESIPT. At the same time, L^OH,OH also exhibits S₁ → S₀ fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for L^H,OH due to the absence of the hydroxy group in the imidazole moiety.

Unveiling the sensing mechanism and luminescence property of a new ESIPT-based fluorescent sensor for detecting Zn2

Recently, based on the mechanism of excited-state intramolecular proton transfer (ESIPT), a new fluorescent probe named 3-(benzo[d]thiazol-2-yl)-5-bromosalicylaldehyde-⁴N-phenyl thiosemicarbazone (BTT) was successfully synthesized [Analyst 146 (2021) 4348-4356.]. However, the importance of ESIPT processes of BTT probe and the mechanism of detecting Zn²⁺ ions have not been studied in detail. In this study, the photochemical behavior of ESIPT-chromophore and the photophysical changes of detecting Zn²⁺ ions were explained at the molecular level for the first time. The calculated spectral values were in agreement with the experiment. We not only confirmed the excited state hydrogen-bond strengthening by interaction region indicator (IRI), but also scanned the potential energy curves of BTT molecule in different electronic states, which confirmed that the hydrogen proton is easier to transfer in the first excited state. In addition, we had given the reasonable structure of the BTT-Zn²⁺ complex (L1) by comparing the binding free energies. The hole-electron distribution and interfragment charge transfer (IFCT) methods proved the excitation type of intraligand charge transfer (ILCT). Finally, the photophysical phenomenon of BTT for detecting Zn²⁺ ions is explained by calculating the electronic spectra and the energy gap (E_gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

Theoretical Investigations on the Sensing Mechanism of Phenanthroimidazole Fluorescent Probes for the Detection of Selenocysteine

The level of selenocysteine (Sec) in the human body is closely related to a variety of pathophysiological states, so it is important to study its fluorescence sensing mechanism for designing efficient fluorescent probes. Herein, we used time-dependent density functional theory to investigate the fluorescence sensing mechanism of phenanthroimidazole derivates A4 and B4 for the detection of Sec, which are proposed to be designed based on excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) mechanisms. The calculation results show that the fluorescence quenching mechanism of A4 and B4 is due to the photo-induced electron transfer (PET) process with the sulfonate group acts as the electron acceptor. Subsequently, A4 and B4 react with Sec, the sulfonate group is substituted by hydroxyl groups, PET is turned off, and significant fluorescence enhancement of the formed A3 and B3 is observed. The theoretical results suggest that the fluorescence enhancement mechanism of B3 is not based on ICT mechanism, and the charge transfer phenomenon was not observed by calculating the frontier molecular orbitals, and proved to be a local excitation mode. The reason for the fluorescence enhancement of A3 based on ESIPT is also explained by the calculated potential energy curves.

Solvent assisted excited-state deactivation pathways in isolated 2,7-diazaindole-S1-3 (S = Water and Ammonia) complexes

The role of solvent molecules in the deactivation of photo-excited 2,7-diazaindole (DAI) - (H₂O)_1-3 and DAI - (NH₃)_1-3 complexes were computationally investigated. An excited-state proton transfer (ESPT) path from the solvent to the DAI molecule was followed using the TD-DFT-D4 (B3LYP) level of theory. The computed potential energy profile of ESPT process has shown intersection between ππ* and nπ* states facilitated via relative stabilization of the nπ* state with decreasing N(7)-H_b bond length. The ESPT process, starting from the DAI-S_n (ππ*) state, crosses through a barrier ranging from 27 to 36 kJmol^-1 for water complexes and 26-30 kJmol^-1 for ammonia complexes. The energy of the excited state was rapidly decreased with a shorter N(7)-H_b bond length. Subsequently, a significant trend of finding a second intersection between the ground and the excited state was observed for all the complexes. The results firmly suggested a significant deactivation channel of excited azaindole derivatives. In the present system, two competing channels, ESPT and ESHT, were found to be energetically accessible. The energy barriers associated with the ESPT barriers for DAI-(H₂O)_1-3 complexes are similar to the ESHT barrier, depicting equal dominance of both processes. The increased basicity of the N(7) atom in the excited state resulted a facile ESPT process from the water to N(7) of the DAI molecule. However, DAI-(NH₃)_1-3 complexes show clear preference for ESHT over ESPT process owing to its higher gas-phase pK_a value making it a poor proton donor. The above systems can be used as a model to computationally and experimentally investigate the competing radiative and deactivation pathways of photo-excited solvated complexes of N-H-bearing bio-relevant molecules via proton and hydrogen transfer reactions.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

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.