Photoinduced Intramolecular Charge Transfer and S2Fluorescence in Thiophene-π-Conjugated Donor-Acceptor Systems: Experimental and TDDFT Studies

Near-Infrared II Hemicyanine Dye with Large Stokes Shift Designed by TICT Regulation for Boosting Imaging-Guided Photothermal Therapy

The serious threat that cancer poses to human health highlights the significance of early detection and effective treatment. The integration of fluorescence diagnosis and photothermal therapy in NIR-II has gained attention due to its high sensitivity, fast response, and noninvasiveness. Fluorescence, produced by the radiative relaxation process of electrons in a molecule, and photothermal, generated by the nonradiative relaxation process of electrons in a molecule, are competing photophysical processes. Hence, it is a challenge for the molecule to balance between the properties of fluorescence and photothermal. In this study, a NIR-II hemicyanine with TICT character is designed to obtain molecules with both better fluorescence and photothermal properties, utilizing positively charged pyridine salt and triphenylamine as electron acceptor and donor, respectively, and oxole as the conjugated π-bridge. HCY-995, one of the synthesized compounds, has a quantum yield of 0.09%, photothermal conversion efficiency of 54.90%, and a significant Stoke shift of 232 nm, which makes it appropriate for the integration of photothermal therapy and high-resolution imaging. This study provides new insights into the development of NIR-II molecules with fluorescent and photothermal integrated properties.

Control of the fluorescence molecule 2-(2'-hydroxyphenyl) benzothiazole derivatives by introducing electron-donating and withdrawing substituents groups

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT), we investigate the fluorescence mechanism of (E)-4-(3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyryl)-1-methylpyridin-1-ium (HBTMY) and the excited-state intramolecular proton transfer process (ESIPT) of hydroxyphenyl. Herein, we introduce two electron-donating (amino and methoxy) and two electron-withdrawing (hydrogen and cyano) groups into HBTMY to study their effects on the fluorescence and the ESIPT process. Structural parameters, infrared vibration frequency, vertical excitation and emission energies as well as frontier molecular orbitals show that the substituents have different impacts on intramolecular hydrogen bonding behavior. The result shows that the fluorescence wavelength of molecules with the amino group could reach the near-infrared area, which favors using this fluorescence in the living cell. As the ability of electron-absorbing groups increases, the forward energy barrier in the potential energy curves decreases sharply making the ESIPT process more familiar to take place. Thus, this work offers a guide for cell imaging and provides strategies to adjust and control fluorescence by introducing substituents.

The two-pronged approach of heteroatoms and substituents to achieve a synergistic regulation of the ESIPT process in amino 2-(2'-hydroxyphenyl)benzoxazole derivatives

Amino 2-(2'-hydroxyphenyl)benzazole derivatives are a class of molecules with excellent photophysical properties. Most of them can be applied as a fluorescent probe via the excited-state intramolecular proton transfer (ESIPT) process. In this work, we focus on the effects of heteroatoms (O, S) and substituents (acetylacetone, hydrogen) in the derivatives. Using DFT/TDDFT methods with the B3LYP-D3BJ functionals, the absorption and emission peaks are in good agreement with the experimental data. Results of optimized structures, infrared vibrational spectra, and reduced density gradient present the existence of the ESIPT process in the S₁ state in these molecules, it also indirectly shows that the heteroatom S is more than O, and the substituent acetylacetone is more than hydrogen has stronger hydrogen bonds. The proton transfer (PT) potential energy curves (PECs) qualitatively show that it is easier for the heteroatom S to induce ESIPT than that of O. The same for the substituent acetylacetone than that of hydrogen. Under the joint influence of the simultaneous stacking of heteroatom S and acetylacetone substituent, the energy barrier of the PT process can be effectively lowered, realizing a synergistic strategy, which can provide some guidance for the design of fluorescent materials.

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.

Mechanofluorochromism of (D-π-)2A-type azine-based fluorescent dyes

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC or h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes OUY-2, OUK-2, and OUJ-2 possessing intramolecular charge-transfer (ICT) characteristics from two (diphenylamino)carbazole–thiophene units as D (electron-donating group)–π (π-conjugated bridge) moieties to a pyridine, pyrazine, or triazine ring as A (electron-withdrawing group): grinding of the recrystallized dyes induced red or blue shifts of the fluorescent colors, that is, bathochromic or hypsochromic shifts of the fluorescence maximum wavelengths (λ^fl-solid_max). The degrees of MFC evaluated by the absolute value of differences (Δλ^fl-solid_max) in λ^fl-solid_max before and after grinding of the recrystallized dyes increased in the order of OUY-2 (+7 nm) < OUK-2 (−17 nm) < OUJ-2 (+45 nm), so that OUJ-2 exhibits obvious b-MFC, but OUK-2 exhibits h-MFC. X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) demonstrated that the recrystallized dyes were in the crystalline state but the ground dyes were in the amorphous state. When the ground solids were heated above their crystallization temperatures (T_c), the colors and fluorescent colors recovered to the original ones before grinding or converted to other ones, that is, heating the ground solids in the amorphous state induced the recrystallization to recover the original microcrystals or to form other microcrystals due to polymorph transformation. However, (D–π–)₂Ph-type fluorescent dye OTK-2 having a phenyl group as a substitute for the azine rings exhibited non-obvious MFC. Molecular orbital (MO) calculations indicated that the values of the dipole moments (μ_g) in the ground state were 4.0 debye, 1.4 debye, 3.2 debye, and 2.9 debye for OTK-2, OUY-2, OUK-2, and OUJ-2, respectively. Consequently, on the basis of experimental results and MO calculations, we have demonstrated that the MFC of the (D–π–)₂A-type azine-based fluorescent dyes is attributed to reversible switching between the crystalline state of the recrystallized dyes and the amorphous state of the ground dyes with changes in the intermolecular dipole–dipole and π–π interactions before and after grinding. Moreover, this work reveals that (D–π–)₂A fluorescent dyes possessing dipole moments of ca. 3 debye as well as moderate or intense ICT characteristics make it possible to activate the MFC.

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC of h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes: grinding of the recrystallized dyes induced bathochromic or hypsochromic shifts of the fluorescence bands.

Fine-tuning directionality of ESIPT behavior of the asymmetric two proton acceptor system via atomic electronegativity

The excited state intramolecular proton transfer (ESIPT) processes and photophysical features of 3-(benzo[d]oxazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BOHMB) and 3-(benzo[d]selenazole-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BSeHMB) molecules were investigated in detail by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The strengthened excited state hydrogen bonds (H-bond) of the title compounds are favorable to ESIPT process according to the analyses of structural parameter, infrared vibration frequency, electron density and reduced density gradient. The atomic substitution changes the intramolecular H-bond O₁-H₂…O₃ and O₁-H₂…N₄ and the fluorescence emission peaks of BOHMB-N and BSeHMB-N in normal and tautomer forms. The potential energy curves indicate that the ESIPT energy barriers of BOHMB-O, BTHMB-O and BSeHMB-O increase as the electron-withdrawing abilities of atoms (from O to S and Se) are gradually weakened. However, the ESIPT energy barriers of BOHMB-N and BTHMB-N follow the totally opposite order. For BOHMB and BSeHMB, ESIPT process prefers to occur in the direction from O-H group to the O atom.

Solvent polarity dependent ESIPT behavior for the novel flavonoid-based solvatofluorochromic chemosensors

In this work, we explore the excited-state intramolecular proton transfer (ESIPT) mechanisms and relative solvent effects for three novel 3-hydroxylflavone derivatives (i.e., HOF, SHOF, and NSHOF) in acetonitrile, dichloromethane, and toluene solvents. Through calculations, we optimize the structures of HOF, SHOF, and NSHOF. Through the analysis of a series of structural parameters related to hydrogen bonding interactions, it could be found that the hydrogen bonds of the three derivatives are all enhanced in the S₁ state, and more importantly, the excited-state hydrogen bonds of HOF are stronger than those of SHOF and NSHOF. In order to explore the effects of solvent polarity, we analyze the core-valence bifurcation (CVB) index, infrared (IR) vibration spectrum, and the potential energy curves. We find that for HOF, SHOF, and NSHOF, the strength of the excited-state hydrogen bonds increases as the solvent polarity decreases. The solvent polarity dependent ESIPT mechanisms pave the way for further designing novel flavonoid-based solvatofluorochromic probes in future.

"Internal and External Combined" Nonradiative Decay-Based Nanoagents for Photoacoustic Image-Guided Highly Efficient Photothermal Therapy

Rational manipulation of nonradiative decay channels is of crucial significance to improve photothermal conversion efficiency (PCE) and design photothermal agents. We first used the "internal and external combined" nonradiative decay strategy to enhance PCE. Specifically, organic IR-Y6 NPs with strong NIR absorption and high molar extinction coefficient were prepared and characterized. By means of TD-DFT calculations and fs-TA spectroscopy, the dual nonradiative decay channels composed of the free rotor (external strategy) and ultrafast dark excited states (DESs) between S0 and S1 states (internal strategy) were proved, which significantly enhanced PCE, up to 66%. IR-Y6 NPs were applied to a mice tumor model for photoacoustic image-guided photothermal therapy, showing complete tumor ablation ability and good biocompatibility for the normal organs. This work is of significance to deeply understand the nonradiation decay mechanism and rational design of high-performance PTT agents.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696-1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

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.

Tailoring Photophysical Properties of Diketopyrrolopyrrole Small Molecules with Electron-Withdrawing Moieties for Efficient Solar Steam Generation

The development of photothermal materials (PTMs) for solar steam generation (SSG) has gained tremendous attention in response to the global clean water scarcity issue. However, the investigation in employing organic small-molecule PTMs for SSG applications is rarely found due to their narrow optical absorption range to harvest solar energy and insufficient photostability for long-term use. Herein, we employ a diketopyrrolopyrrole (DPP) core unit together with electron-withdrawing (EW) endcaps and siloxane side chains to introduce stronger intramolecular charge transfer (ICT) characteristics as well as the hydrophobic character. The enhanced ICT characteristics of DPP derivatives render a broad optical absorption range, less emission, and a high nonradiative decay rate for efficient solar energy harvesting and photothermal effects. Meanwhile, the hydrophobic nature of these DPP derivatives allows the facile fabrication of novel Janus photothermal membranes for effective water vaporization and solar-to-vapor conversion efficiency. By embedding DPP derivatives to the SSG device, we showed that the solar-to-vapor efficiency can reach up to 71.8% under relatively low visible light power (∼700 W m^-2), which is, on average, 2.66 times higher than that of bulk water of similar dimension. Moreover, this report demonstrates the great potential of conjugated small molecules for photothermal applications, owing to their versatility and flexibility in structural engineering and its diminishing radiative decay properties. This may inspire more innovation and advancement in SSG applications.

Solvation effect on the ESIPT mechanism of nitrile-substituted ortho-hydroxy-2-phenyl-oxazolines

In this study, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to unveil the solvation effects on the excited-state intramolecular proton transfer (ESIPT) of nitrile-substituted ortho-hydroxy-2-phenyl-oxazoline (NOHPO) molecules in three different solvents. According to the functional analysis of the reduced density gradient, hydrogen bond in low-polar solvents is stronger compared to that in high-polar solvents, indicating that proton transfer (PT) can be influenced by the polarity of the solvent. Moreover, the geometric parameters and infrared vibration spectrum of NOHPO in different types of solvents in the S₀ and S₁ state were compared, which confirmed the above results. By analyzing electronic spectra and frontier molecular orbitals, it was found that the spectral properties were affected by different polar solvents. Molecular electrostatic potential surface calculations proved that PT took place between the H₂ atom and N₃ atom, and the natural population analysis and Hirshfeld charge reveal the charge distribution after photoexcitation. To investigate the ESIPT progress intensively, the potential energy curves of NOHPO in three types of solvents were established. The findings revealed that NOHPO could transform from enol to keto form in the S₁ state spontaneously, and ESIPT progress was promoted with the decrease in polarity.

After photoexcitation, NOHPO could transform from enol to keto form spontaneously, and ESIPT progress was promoted with the decrease in polarity.

Sensing mechanism of fluorescent sensor to Cu2+ based on inhibiting ultra-fast intramolecular proton transfer process

A novel and efficient chemosensor 1 for detecting Cu²⁺ has recently been developed. However, the photophysical properties of chemosensor 1 and its response mechanism to Cu²⁺ are still unclear. Herein, the density functional theory and the time-dependent density functional theory approaches are implemented to investigate the excited state behavior of chemosensor 1 and its sensing mechanism for Cu²⁺ is revealed. Through constructing the potential energy curve with the dihedral angle of hydroxide radical as a variable, the irreversibility of the adjustment of the hydrogen proton direction is determined. This feature provides a favorable geometric configuration condition for the formation of intramolecular hydrogen bond. Moreover, the reduced density gradient analysis and topological analysis are performed to visualize the hydrogen bond strength, it is found that the hydrogen bond is enhanced in first singlet excited state (S₁) compared with that in ground state (S₀). The chemosensor 1 has only a low potential barrier in the S₁ state, indicating that it could undergo an ultra-fast excited state intramolecular proton transfer (ESIPT) process. Furthermore, the reaction sites of chemosensor 1 and Cu²⁺ is theoretically predicted by the electrostatic potential analysis and the coordination mode of 1 + Cu²⁺-H⁺ is confirmed. Thus, we verify that the deprotonation inhibits the ESIPT behavior and leads to fluorescence quenching to achieve the recognition of chemosensor 1 to Cu²⁺. In addition, the binding energy of Cu²⁺ with chemosensor 1 is greater than that of Mg²⁺ and Zn²⁺, the high selectivity of chemosensor 1 to Cu²⁺ is illustrated. Our investigation clarifies the sensing mechanism of chemosensor 1 to Cu²⁺ based on inhibiting ultra-fast ESIPT process, which provides a theoretical basis for the development of new metal ion sensors.

Excited state electronic structures and photochemistry of different oxidation states of 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)

The molecular structures of 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), were calculated by using time-dependent density functional theory (TDDFT) model with M062X method with 6-311G (d, p) basis set. In this work, the ABTS were theoretically investigated from the geometric structure, the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), the energy level gap ΔE_HOMO-LUMO of the molecular ground state, excited stated properties and the electronic absorption spectra of different oxidation states. We studied the energy levels of LUMO and HOMO of ABTS in different oxidation states. Frontier molecular orbital analysis can provide insight into the nature of excited states. ABTS was synthesized from N-ethylamine by total synthesis. Then, we measured the UV-Vis spectra of ABTS before and after being oxidized by K₂S₂O₈. The calculated electronic structures and photochemical properties of different oxidation state of ABTS were in accordance with the experimental result. This work demonstrates the relationship between the electronic structures and photochemistry of different oxidation states ABTS hence paves the way for the rationally synthesis and deepen understanding of the photophysical properties of ABTS materials.

Unpaired Electron-Induced Wide-Range Light Absorption within Zn (or Cu) MOFs Containing Electron-Withdrawing Ligands: A Theoretical and Experimental Study

In photocatalysis, it is of general interest to understand and design wide-range light-absorbing inorganic/organic hybrid materials with an excellent photo-induced intramolecular charge-transfer (ICT) effect. To verify the role of unpaired electrons in enhancing ICT within electron-withdrawing ligand-based metal-organic frameworks (MOFs), the molecular structure, density of states (DOS), and electronic structure of strong electron-deficient pyridine-diketopyrrolopyrrole (P-DPP)-based Zn (or Cu) MOFs were calculated in Gaussian package to validate the unpaired electron ICT. The electron spin resonance technique has detected the unpaired electrons for the coordination systems containing Zn-O or Cu-O clusters and P-DPP ligand on photoexcitation. The estimated band gaps from the DOS calculation for P-DPP-Cu and P-DPP-Zn are 1.4 and 2.4 eV, respectively, showing a good agreement with the experimental UV-vis optical spectra. The partial DOS, dipole moment, and frontier orbital analysis prove that the ICT should happen from Zn-O or Cu-O clusters to P-DPP ligands. This research may contribute to a comprehensive understanding of electron-withdrawing ligand-induced ICT within MOFs and shed light on the design of light-absorbing MOFs with excellent ICT or conductivity.

Theoretical insights into excited-state hydrogen bonding effects and intramolecular proton transfer (ESIPT) mechanism for BTS system

In this work, N,N'-bis(salicylidene)-(2-(3',4'-diaminophenyl)benzothiazole) (named as "BTS") system was studied about its excited-state intramolecular proton transfer (ESIPT) process. The analyses about reduced density gradient (RDG) reveal the formation of two intramolecular hydrogen bonds in BTS system. Bond lengths and angles, infrared (IR) vibrations as well as frontier molecular orbitals (MOs) using TDDFT method indicate that the strength of hydrogen bond should be enhanced in the S1 state. Particularly, hydrogen bond O1-H2···N3 undergoes larger variations compared with O4-H5···N6, which infers that hydrogen bond O1-H2···N3 may play a decisive role in the ESIPT process of BTS system. Given the two hydrogen bonds of BTS molecule, two types of potential energy curves have been constructed, which confirms that only single proton transfer process occurs due to lower energy barrier along with O1-H2···N3 rather than O4-H5···N6. This work not only presents a reasonable explanation for previous experiment, but also clarifies the specific ESIPT mechanism for BTS system.

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.

Excited state intramolecular proton transfer (ESIPT) luminescence mechanism for 4-N,N-diethylamino-3-hydroxyflavone in propylene carbonate, acetonitrile and the mixed solvents

In this work, density functional theory (DFT) and time density functional theory (TDDFT) methods were employed to investigate the nature of the double fluorescence emission of DEAHF in these three solvents. We analyzed the geometric structures, vibrational frequencies, frontier molecular orbitals (MOs), molecular electrostatic potential surface (MEPS), calculated absorption and fluorescence spectra and the potential-energy curves for DEAHF. All the results show that the intramolecular hydrogen bond of DEAHF is strengthened from S₀ to S₁ and the electron density redistribution occurs between the proton acceptor and donor, which can facilitate ESIPT. Moreover, the geometric structures, absorption and emission spectra, MEPS and potential-energy curve of DEAHF are identical. It reveals theoretically that ACN and PC can maintain the polarity of the solvent with 1:1 mixing, which is consistent with the experimental results.

Structure–reactivity relationship of probes based on the H2S-mediated reductive cleavage of the CC bond

The structure–reactivity relationship of H₂S-mediated reductive cleavage of CC bond was studied and utilized to develop probes for detecting H₂S.

Solvent-sensitive circularly polarized luminescent compounds bearing a 9,9′-spirobi[fluorene] skeleton

Chiral 9,9′-spirobi[fluorene] derivatives with a donor–π–acceptor system were prepared and found to exhibit solvent-sensitive circularly polarized luminescence.

Proton transfer reaction confined within carbon nanotubes: Density functional theory and quantitative structure–property relationship analysis

In this work, we focus our attention on chemical reactions confined within carbon nanotubes. As a result of the confinement within carbon nanotubes, novel physical and chemical properties are found for the confined materials. We consider the feasibility of proton transfer inside carbon nanotubes. To do that, we have chosen formamide as the simplest real model for exhibiting the tautomerization in DNA. We have used the quantitative structure–property relationship method, based on geometry optimization and quantum chemical structural descriptors, to illustrate the potential of using the confined space inside carbon nanotubes, which will provide comprehensive information about carbon nanotubes. All calculations have been carried out using density functional theory quantum calculations with the B3LYP functional. The geometries optimized by the Gaussian program were transferred to the computer software DRAGON to calculate pertinent descriptors that could be used in the quantitative structure–property relationship model.

Synthesis, photophysical and electrochemical properties of pyridine, pyrazine and triazine-based (D-π-)2A fluorescent dyes

The donor-acceptor-π-conjugated (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole thiophene units as D (electron-donating group)-π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group, A) have been designed and synthesized. The photophysical and electrochemical properties of the three dyes were investigated by photoabsorption and fluorescence spectroscopy, Lippert-Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λ_max,abs) and the fluorescence maximum (λ_max,fl) for the intramolecular charge-transfer characteristic band of the (D-π-)₂A fluorescent dyes show bathochromic shifts in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption bands of the (D-π-)₂A fluorescent dyes are nearly independent of solvent polarity, while the fluorescence bands showed bathochromic shifts with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert-Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δμ (= μ_e - μ_g) value, which is the difference in the dipole moment of the dye between the excited (μ_e) and the ground (μ_g) states, increases in the order of OUY-2 < OUK-2 < OUJ-2. Therefore, the fact explains our findings that OUJ-2 shows large bathochromic shifts of the fluorescence maxima in polar solvents, as well as the Stokes shift values of OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λ_max,abs and λ_max,fl and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of the azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

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.

Intramolecular charge transfer and solvation dynamics of push-pull dyes with different π-conjugated linkers

The solvation-dependent excited state dynamics of two push-pull fluorophores with donor-π-acceptor (D-π-A) structures were investigated using steady-state and ultrafast transient absorption (TA) spectroscopy, backed by theoretical calculations. Identical D and A groups were present in both dyes, which differed only in the structure of their central π-conjugated linkers. Dye 1 features a p-phenylenediethynyl linker, while dye 2 contains a 2,5-diethynylthiophene linker. From the steady-state spectra, no appreciable shifts in absorption bands were observed, whereas large red-shifts in emission were seen with increasing solvent polarity, which indicated that the excited states were more polar than the ground state. Theoretical calculations support charge transfer from the triphenylamine (TPA) donor to the pentafluorosulfanyl (SF₅) acceptor viaπ-conjugated linkers to form an intramolecular charge transfer (ICT) state. TA spectra revealed that a solvation-stabilized conformationally relaxed intramolecular charge transfer (ICT') state was formed in polar solvents, but only an ICT state was observed in nonpolar solvent. The SE band was quenched within 1 ps in high-polarity solvent, which corresponds to the low fluorescence quantum yield. It can be concluded that the dye with the p-phenylenediethynyl π-linker (i.e., dye 1) exhibits a larger degree of ICT than the thiophene analogue (i.e., dye 2). These findings demonstrate how solvation can fine-tune the photophysical properties of push-pull dyes, and this study highlights the importance of π-conjugated linkers in the excited state ICT process.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

Excited state hydrogen bond and proton transfer mechanism for (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine: A theoretical investigation

A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ₂)ρ, we firstly verify the double intramolecular hydrogen bonds (O1H2⋯N3 and O4H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S₀-state and S₁-state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future.

A theoretical study of the ESIPT mechanism of 3-hydroxyflavone derivatives: solvation effect and the importance of TICT for its dual fluorescence properties

As the dielectric constant decreases, the ESIPT reaction occurs more easily and TICT is good at emitting double fluorescence.

Density Functional Theory Applied to Excited State Intramolecular Proton Transfer in Imidazole-, Oxazole-, and Thiazole-Based Systems

Excited state intramolecular proton transfer (ESIPT) is a photoinduced process strongly associated to hydrogen bonding within a molecular framework. In this manuscript, we computed potential energy data using Time Dependent Density Functional Theory (TDDFT) for triphenyl-substituted heterocycles, which evidenced an energetically favorable proton transfer on the excited state (i.e., ESIPT) but not on the ground state. Moreover, we describe how changes on heterocyclic functionalities, based on imidazole, oxazole, and thiazole systems, affect the ESIPT process that converts an enolic species to a ketonic one through photon-induced proton transfer. Structural and photophysical data were obtained theoretically by means of density functional theory (DFT) calculations and contrasted for the three heterocyclics. Different functionals were used, but B3LYP was the one that adequately predicted absorption data. It was observed that the intramolecular hydrogen bond is strengthened in the excited state, supporting the occurrence of ESIPT. Finally, it was observed that, with the formation of the excited state, there is a decrease in electronic density at the oxygen atom that acts as proton donor, while there is a substantial increase in the corresponding density at the nitrogen atom that serves as proton acceptor, thus, indicating that proton transfer is indeed favored after photon absorption.

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.