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

Linear and Nonlinear Optical Responses of Nitrobenzofurazan-Sulfide Derivatives: DFT-QTAIM Investigation on Twisted Intramolecular Charge Transfer

In this study, we report on the green fluorescence exhibited by nitrobenzofurazan-sulfide derivatives (NBD-Si, i = 1-4). The optical responses of these studied compounds in a polar methanol solvent were simulated by the use of time-dependent density functional theory (TD-DFT) employing the Becke-3-Parameter-Lee-Yang-Parr (B3LYP) functional along with the 6-31G(d,p) basis set. The computed energy and oscillator strength (f) results complement the experimental results. The band gap was calculated as the difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Additionally, the density of states (DOS) was computed, providing a comprehensive understanding of the fundamental properties of these materials and further corroborating the experimental data. When the experimental data derived from ultraviolet/visible (UV/visible) and fluorescence spectroscopic techniques and those from simulated spectra are analyzed, the extracted values match up adequately. In addition, the NBD-sulfide compounds exhibit a large Stokes shift up to 85 nm in a polar methanol solvent. They are hypothesized to represent a novel paradigm of excited-state intramolecular charge transfer (ICT). To understand the intrinsic optical properties of NBD-Si materials, an ICT was identified, and its direction within the molecule was evaluated using the ratio of βvect and βtotal, values extracted from the computed nonlinear optical (NLO) properties. Moreover, the reduced density gradient (RDG)-based noncovalent interactions (NCIs) were employed to characterize the strength and type of NBD-Si interactions. Furthermore, noncovalent interactions were identified and categorized using the Quantum Theory of Atoms in Molecules (QTAIM) analysis. Ultimately, the combination of Hirshfeld surface analysis and DFT calculations was utilized to enhance the characterization and rationalization of these NCIs.

The impact of π-π stacking interactions on photo-physical properties of hydroxyanthraquinones

The impact of π-π stacking interactions on photo-physical properties of hydroxyanthraquinone (HA) has been investigated using the density functional (DFT) and time-dependent density functional theory (TD-DFT) calculations in the gas phase and solution media. The vertical transition is characterized with strong HOMO-LUMO transition in the complexes. The intramolecular hydrogen bond (IHB) made in the HA and π-π complexes is strengthened after S₀ → S₁ excitation, such that the proton transfers is facilitated in the first excited state. The complexes exhibit an exothermic excited state intramolecular proton transfer (ESIPT) in the solution media, which is a barrierless process for some complexes. The π-π stacking interaction affects the absorption and emission bands of HA, and provides a large Stokes shift. This indicates the desirable fluorescence properties of π-π complexes, which are cross-validated by geometries, potential energy curve scannings, electronic and vibrational spectra, and frontier molecular orbital analyses.

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.

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

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

Theoretical Study on the Atom-Substituted Quinazoline Derivatives with Faint Emission as Potential Sunscreens

Two novel compounds (HQS and HQSe) with excited-state intramolecular proton transfer (ESIPT) properties were designed based on the compound 2-(2-hydroxy-3-ethoxyphenyl)-3H-quinazolin-4-one (HQ). The parameters related to the ESIPT properties and electronic spectra of HQ and its derivatives were calculated using density functional theory and time-dependent density functional theory methods. The obtained geometric configurations, infrared vibrational spectra, and reduced density gradient scatter plots have shown that the intramolecular hydrogen bond O₁···H₁-N₁ has been weakened upon photoexcitation. Moreover, from the scanned potential energy curves, it can be found that the ESIPT processes of the three compounds have no energy barriers. It is noteworthy that HQS and HQSe can strongly absorb light in the UVA region (∼340 nm) and exhibit weak fluorescence emission in the visible light region, which comes from the keto configuration. The special optical properties of HQS and HQSe can promote their application as potential sunscreen agents.

Unveiling the influence of atomic electronegativity on the double ESIPT processes of uralenol: A theoretical study

In this work, the effects of atomic electronegativity (O, S, and Se atoms) on the competitive double excited-state intramolecular proton transfer (ESIPT) reactions and photophysical characteristics of uralenol (URA) were systematically explored by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated hydrogen bond parameters, infrared (IR) vibrational spectra, reduced density gradient (RDG) scatter plots, interaction region indicator (IRI) isosurface and topology parameters have confirmed the six-membered intramolecular hydrogen bond (IHB) O4H5^…O3 is the stronger one in all the three studied compounds. Subsequently, frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population analysis essentially uncover that the electron redistribution has induced the ESIPT process. Besides, the constructed potential energy curves (PECs) have indicated that the ESIPT process prefers to occur along the O4H5^…O3 rather than the O1H2^…O3 and the proton-transfer energy barrier is gradually decreased with the weakening of atomic electronegativity from URA to URA-S and URA-Se. In a conclusion, the attenuating of atomic electronegativity has enhanced the IHBs of URA and thereby promoting the ESIPT reaction, which is helpful for further developing novel fluorophores based on ESIPT behavior in the future.

Enhancing fluorescence of benzimidazole derivative via solvent-regulated ESIPT and TICT process: A TDDFT study

In this work, we use density functional theory and time dependent density functional theory to explore the ESIPT and TICT process of 6-(1H-Benzoimidazol-2-yl)-2,3-dimethoxy-phenol (BIDOP) in cyclohexane (CHX) and tetrahydrofuran (THF) solvent, respectively. It reveals that ESIPT process of BIDOP can occur in both CHX and THF solvent at the first excited state with similar reaction barrier. Remarkably, compared to barrierless from keto (K*) to TICT state of BIDOP in THF solvent, the reaction barrier between K* and TICT state is up to 20.28 kcal/mol for in CHX that TICT process is inhibited in CHX solvent. The absence of nonradiative decay TICT state of BIDOP in CHX solvent induces higher fluorescence in CHX compared to in THF solvent. These findings indicate that CHX solvent can effectively enhance fluorescence of BIDOP. Our study highlights a convenient approach for enhancing fluorescence and is significant for photophysics and photobiology field.

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.

Sensing mechanism of fluorogenic urea with fluoride in solvent media: A new fluorescence quenching mechanism

The interaction of 1-Phenyl-3-(pyren-1-yl) urea (LH) and fluoride anion (F^-) with a unique ON¹-OFF-ON² fluorescent response has been investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The hydrogen-bonding dynamics and photophysical properties of the complex LH-F, as well as its isolated receptor LH and anion form L-H₁, have been studied in detail. We demonstrate that the intermolecular hydrogen bond (N-H…F) of the complex LH-F is greatly enhanced in the electronically excited state. The nonradiative deactivation via electron transfer and internal conversion rather than excited-state intramolecular proton transfer (ESIPT) can be facilitated by the excited state hydrogen bond strengthening. The results have been cross-validated by molecular structure, electronic spectra, frontier molecular orbitals, and infrared spectra as well as hydrogen bond binding energy. These results indicate that the current calculations completely reproduce the experimental results and provide compelling evidence for the sensing mechanism of LH for F^-.

Substituent effect on ESIPT and hydrogen bond mechanism of N-(8-Quinolyl) salicylaldimine: A detailed theoretical exploration

The effects of substituent on excited-state intramolecular proton transfer (ESIPT) and hydrogen bonding of N-(8-Quinolyl) salicylaldimine (QS) have been studied by theoretical calculation with DFT and TDDFT. The representative electron-withdrawing nitryl and electron-donating methoxyl were selected to analyze the effects on geometries, intramolecular hydrogen bond interaction, absorption/fluorescence spectra, and the ESIPT process. The configurations of the three molecules (QS, QS-OMe and QS-NO₂) were optimized in the ground and excited states. The structure parameters, infrared spectra, hydrogen bond interactions, frontier molecular orbitals, absorption/fluorescence spectra, and potential curves have cross-validated the current results. The results show that the introduction of substituent results in a bathochromic-shift of the absorption and fluorescence spectra with large Stokes shift, and is more beneficial to the ESIPT process. The current work will be beneficial to the improvement of ESIPT properties and deepen understanding of the mechanism of ESIPT process.

An Excited State Intramolecular Proton Transfer-Based Fluorescent Probe with a Large Stokes Shift for the Turn-on Detection of Cysteine: A Detailed Theoretical Exploration

DFT and TDDFT calculations are adopted to study the sensing mechanism of a turn-on-type cysteine fluorescent probe (2-(1-phenyl-imidazo[1,5-α]pyridine-3-yl)phenyl acrylate, denoted as MZC-AC). The photoinduced electron transfer (PET) process of MZC-AC and the excited state intramolecular proton transfer (ESIPT) process of MZC have been investigated in detail. We demonstrate that the fluorescence quenching of MZC-AC is ascribed to the PET mechanism and the large Stokes shift fluorescence emission of MZC is the result of the ESIPT mechanism. The results have been cross-validated by geometries, frontier molecular orbital analysis, and potential energy curve scanning. As a result, our calculations completely reproduce the experimental results and give powerful evidence for the sensing mechanism of MZC-AC for cysteine.

Theoretical Study on the Sensing Mechanism of Novel Hydrazine Sensor TAPHP and Its ESIPT and ICT Processes

The photophysical and photochemical properties of the novel hydrazine sensor TAPHP and the TAPDP generated by the cyclization reaction of TAPHP with hydrazine are investigated using the density functional theory and time-dependent density functional theory. The results show that both the excited-state intramolecular proton transfer and intramolecular charge transfer can occur for TAPHP and TAPDP. Analysis of bond parameters and infrared vibrational spectra indicate that hydrogen bonds are enhanced in the first excited state, which is beneficial to excited-state intramolecular proton transfer. The strength of hydrogen bonds is also visualized by using the independent gradient model and topological analysis. The core-valence bifurcation index and bond critical point parameters are further employed to measure hydrogen bonds. The reaction path of proton transfer is obtained through the potential energy curves. The excitation of TAPHP and TAPDP is attributed to the charge transfer excitation, which is determined by the characteristics of the hole-electron distribution. The reaction site and product configuration are verified by atomic charge and ¹H-NMR spectra. The negative free energy difference indicates that the reaction between TAPHP and hydrazine can proceed spontaneously. In addition, the absorption and fluorescence spectra agree well with the experimental results, confirming that TAPHP is an excellent sensor of hydrazine.

The mechanism of the excited-state proton transfer of Salicylaldehyde azine and 2,2'-[1,4-Phenylenebis{(E)- nitrilomethylidyne}] bisphenol: Via single or double proton transfer

The Salicylaldehyde azine (H₂SA) and 2,2'-[1,4-Phenylenebis{(E)-nitrilomethylidyne}] bisphenol (H₂SPA) with double proton transfer characteristics were synthesized recently (Phys. Chem. Chem. Phys., 2018, 20, 23,762). However, the detailed theoretical interpretation of proton transfer (PT) mechanism is inadequate. In the present work, density functional theory (DFT) and time-density functional theory (TDDFT) are employed to study the proton transfer mechanism of H₂SA and H₂SPA in detail. Bond parameters, infrared (IR) spectra and frontier molecular orbitals (FMOs) calculated by PBE0/TZVP method indicate the strength of hydrogen bond is enhanced in S₁ state, which can be visualized by the reduced density gradient (RDG) analysis. The potential energy surfaces (PESs) of H₂SA and H₂SPA are also constructed. The small barriers indicate that both the single proton transfer and double proton transfer of H₂SA and H₂SPA are more likely to occur in the S₁ state. In addition, the properties of H₂SA and H₂SPA after chelation with Li⁺ have also been theoretically characterized. According to the calculated fluorescence spectra of compounds (H₂SA-Li⁺ and H₂SPA-Li⁺), it was found that only the planar structure of H₂SA-Li⁺ can form metallogel, which verified the experimental results.