Theoretical insights into the excited‐state process of 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol

In this paper, we theoretically explore the motivation and behaviors of the excited‐state intramolecular proton transfer (ESIPT) reaction for a novel white organic light‐emitting diode (WOLED) material 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol (t‐MTTH). The “atoms in molecules” (AIM) method is adopted to verify the formation and existence of the hydrogen bond O? H···N. By analyzing the excited‐state hydrogen bonding behaviors via changes in the chemical bonding and infrared (IR) vibrational spectra, we confirm that the intramolecular hydrogen bond O? H···N should be getting strengthened in the first excited state in four kinds of solvents, thus revealing the tendency of ESIPT reaction. Further, the role of charge‐transfer interaction is addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. Also, the electron distribution confirms the ESIPT tendency once again. The scanned and optimized potential energy curves according to variational O? H coordinate in the solvents demonstrate that the proton transfer reaction should occur in the S₁ state, and the potential energy barriers along with ESIPT direction support this reaction. Based on the excited‐state behaviors reported in this work, the experimental spectral phenomenon has been reasonably explained.     

A detailed theoretical study on the excited‐state hydrogen‐bonding dynamics and the proton transfer mechanism for a novel white‐light fluorophore

In this work, density functional theory (DFT) and time‐dependent DFT (TDDFT) methods were used to investigate the excited‐state dynamics of the excited‐state hydrogen‐bonding variations and proton transfer mechanism for a novel white‐light fluorophore 2‐(4‐[dimethylamino]phenyl)‐7‐hyroxy‐6‐(3‐phenylpropanoyl)‐4H‐chromen‐4‐one ( 1 ). The methods we adopted could successfully reproduce the experimental electronic spectra, which shows the appropriateness of the theoretical level in this work. Using molecular electrostatic potential (MEP) as well as the reduced density gradient (RDG) versus the product of the sign of the second largest eigenvalue of the electron density Hessian matrix and electron density (sign[λ₂]ρ), we demonstrate that an intramolecular hydrogen bond O1–H2···O3 should be formed spontaneously in the S₀ state. By analyzing the chemical structures, infrared vibrational spectra, and hydrogen‐bonding energies, we confirm that O1–H2·O3 should be strengthened in the S₁ state, which reveals the possibility of an excited‐state intramolecular proton transfer (ESIPT) process. On investigating the excitation process, we find the S₀ → S₁ transition corresponding to the charge transfer, which provides the driving force for ESIPT. By constructing the potential energy curves, we show that the ESIPT reaction results in a dynamic equilibrium in the S₁ state between the forward and backward processes, which facilitates the emission of white light.     

A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system

Given the tremendous potential applications of excited state intramolecular proton transfer (ESIPT) systems, ESIPT molecules have received widespread attention. In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically study the excited state dynamical behaviors of salicyladazine (SA) molecules. Our simulated results show that the double intramolecular hydrogen bonds of SA are strengthened in the S₁ state via exploring bond distances, bond angles, and infrared (IR) vibrational spectra. Exploring the frontier molecular orbitals (MOs), we confirm that charge redistributions indeed have effects on excited state dynamical behaviors. The increased electronic densities on N atoms and the decreased electronic densities on O atoms imply that charge redistribution may trigger the ESPT process. Analyzing the constructed S₀‐state and S₁‐state potential energy surfaces (PESs), we confirm that only the excited state single proton transfer reaction can occur although SA possesses two intramolecular hydrogen bonds. In this work, we clarify the specific ESIPT mechanism, which may facilitate developing novel applications based on the SA system in future.     

Revelation of ESIPT mechanism for the novel fluorescent system HNIBT in toluene and methanol solvents: A TDDFT study

In this work, we devote to explore excited‐state intramolecular proton transfer (ESIPT) behavior for a novel fluorescent molecule naphthalimide‐based 2‐(2‐hydroxyphenyl)‐benzothiazole (HNIBT) [New J. Chem. 2019, 43, 9152.] in toluene and methanol (MeOH) solvents. Exploring weak interactions, stable HNIBT‐enol, and HNIBT‐MeOH‐enol complex can be found in S₀ state via TDDFT/B3LYP/6‐311+G(d,p) level. Given photoexcitation, intramolecular hydrogen bond O1? H2···N3 of HNIBT‐enol and HNIBT‐MeOH‐enol is dramatically enhanced, which offers impetus for facilitates ESIPT reaction. After repeated comparisons, we verify the unavailability of intermolecular hydrogen bonding effects between HNIBT‐enol and MeOH molecules. In view of excitation, HOMO (π) → LUMO (π*) transition and the changes of electronical densities indeed impulse ESIPT tendency. Via constructing potential energy curves (PECs), for both HNIBT‐enol and HNIBT‐MeOH‐enol complex, the ESIPT could only occur along with intramolecular hydrogen bond O1? H2···N3. Through comparison, the potential barrier falls from 4.124 kcal/mol (HNIBT‐enol) to 2.132 kcal/mol (HNIBT‐MeOH‐enol). Therefore, we confirm that the ESIPT of the HNIBT system happens more easily in the MeOH solvent compared with the toluene solvent.     

Revealing and comparing different excited-state intramolecular proton transfer processes for 3-(4-dimethylamino-phenyl)-1-(4-fluoro-2-hydroxy-phenyl)-propenone and 3-(4-dimethylamino-phenyl)-1-(4-fluoro-2-hydroxy-phenyl)-3-hydroxy-propenon

Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S₀ and S₁ states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S₁ state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications.     

A time-dependent density functional theory study on the excited state behavior of a novel T2(OH)B molecule

A novel pH-sensitive fluorescent probe T₂(OH)B was selected to theoretically investigate its excited state hydrogen bonding effects and excited state intramolecular proton transfer (ESIPT) process. First, it was verified that one intramolecular hydrogen bond is formed spontaneously in T₂(OH)B itself. Given the geometrical changes, we further confirm that the hydrogen bond should be strengthened in the first excited state. When it comes to the photoexcitation process, we present the charge redistribution around hydrogen bonding moieties facilitate the ESIPT tendency. The increased electronic densities around acceptor promote the attraction of hydrogen protons. The potential energy barrier in the constructed potential energy curves reveals that the ESIPT process of the T₂(OH)B system should be ultrafast. And comparing several nonpolar solvents, we deem solvent polarity plays little role in the ESIPT reaction. Furthermore, we also search the S₁-state transition state structure along with the ESIPT path, based on which we simulate the intrinsic reaction coordinate path. We not only confirm the ESIPT mechanism presented in this work but also clarify the ultrafast excited state process and explain previous experiment. We sincerely hope that our theoretical work could guide novel applications based on the T₂(OH)B system in future.     

The investigation of proton transfer and fluorescence‐sensing mechanisms of [2‐(2‐hydroxy‐phenyl)‐1H‐benzoimidazol‐5‐yl]‐phenyl‐methanone

Given the tremendous potential of fluorescence sensors in recent years, in this present work, we theoretically explore a novel fluorescence chemosensor [2‐(2‐Hydroxy‐phenyl)‐1H‐benzoimidazol‐5‐yl]‐phenyl‐methanone (HBPM) about its excited state behaviors and probe‐response mechanism. Using density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we explore the S₀‐state and S₁‐state hydrogen bond dynamical behaviors and confirm that the strengthening intramolecular hydrogen bond in the S₁ state may promote the excited state intramolecular proton transfer (ESIPT) reaction. In view of the photoexcitation process, we find that the charge redistribution around the hydroxyl moiety plays important roles in providing driving force for ESIPT. And the constructed potential energy curves further verify that the ESIPT process of HBPM should be ultrafast. That is the reason why the normal HBPM fluorescence cannot be detected in previous experiment. Furthermore, with the addition of fluoride anions, the exothermal deprotonation process occurs spontaneously along with the intermolecular hydrogen bond O–H?F. It reveals the uniqueness of detecting fluoride anions using HBPM molecules. As a whole, the fluoride anions inhibit the initial ESIPT process of HBPM, which results in different fluorescence behaviors. This work presents the clear ESIPT process and fluoride anion‐sensing mechanism of a novel HBPM chemosensor.     

A theoretical investigation about the excited state behavior for 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole: The water‐assisted excited state proton transfer process

In this work, based on density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods, we theoretically investigate the excited‐state process of the 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole (2HPB) system in acetonitrile and water solvents. Since acetonitrile is an aprotic solvent, it has no effect on the solvent‐assisted excited‐state proton transfer (ESPT) process. Therefore, the 2HPB molecule cannot transfer the proton in acetonitrile, which is consistent with previous experimental observation. On the other hand, 2HPB can combine one water molecule (which is a protic solvent), forming the 2HPB–H₂O complex in the S₀ state. After photoexcitation, the intermolecular hydrogen bonds O1 H2···O3 and O3 H4···N5 both get strengthened in the S₁ state, which leads to the possibility of a water‐assisted ESPT process. Further, the charge redistribution reveals the tendency of ESPT. By exploring the potential energy curves for the 2HPB–H₂O complex in water, we confirm that a stepwise double proton transfer process occurs in the S₁ state. Water‐assisted ESIPT can occur along O1 H2···O3 or O3 H4···N5 because of their similar potential barriers. Based on the stepwise ESPT mechanism, we reinterpret the absorption and fluorescence spectra mentioned in the experiments and confirm the rationality of the water‐assisted ESPT process.     

Time‐dependent density functional theory study on the electronic excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in aqueous solution: 4AP and 4AP–(H2O)1,2 clusters

The time‐dependent density functional theory (TDDFT) method has been carried out to investigate the excited‐state hydrogen‐bonding dynamics of 4‐aminophthalimide (4AP) in hydrogen‐donating water solvent. The infrared spectra of the hydrogen‐bonded solute?solvent complexes in electronically excited state have been calculated using the TDDFT method. We have demonstrated that the intermolecular hydrogen bond C? O···H? O and N? H···O? H in the hydrogen‐bonded 4AP?(H₂O)₂ trimer are significantly strengthened in the electronically excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen‐bonding groups in different electronic states. The hydrogen bonds strengthening in the electronically excited state are confirmed because the calculated stretching vibrational modes of the hydrogen bonding C?O, amino N? H, and H? O groups are markedly red‐shifted upon photoexcitation. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electroniclly excited state of chromophores in hydrogen‐donating solvents exists in many other systems in solution. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical exploration about excited state proton transfer mechanism for a series of phenol–quinoline compounds

In the present work, three novel phenols (10a,11‐dihydro‐4bH‐indeno[1,2‐b]quinolin‐4‐ol ( 1 ), 5,6‐dihydro‐benzo[c]acridin‐1‐ol ( 2 ), and 5,5,7,7a‐tetrahydro‐4aH‐13‐aza‐benzo[3,4]cyclohepta[1,2‐b]naphthalene‐1‐ol ( 3 )) have been explored theoretically in detail. Using density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we inquire into the intramolecular hydrogen‐bonding interactions and the excited‐state intramolecular proton transfer (ESIPT) process. Exploring the steady‐state absorption and emission spectra under TDDFT/B3LYP/TZVP theoretical level in acetonitrile solvent, our calculated results demonstrate an experimental phenomenon. Based on analysis of the variations of geometrical parameters and infrared (IR) vibrational spectra, we confirm that O–H?N should be strengthened in the S₁ state. Investigating the frontier molecular orbitals (MOs) and the charge density difference (CDD) maps, it can be confirmed that the charge redistribution facilitates the tendency of the ESIPT process for 1 , 2, and 3 systems. By constructing potential energy curves, we confirm that the proton transfer should occur in the S₁ state. In particular, the ESIPT for 2 and 3 systems are nonbarrier processes in the S₁ state, which confirms that ESIPT should be exothermal spontaneously. This work explains previous experimental results and makes a reasonable assumption about the ESIPT mechanism for 1 , 2 and 3 systems. We sincerely hope our work can facilitate understanding and promoting applications about them in future.     

Theoretical insights into excited-state process for the novel 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile system

In this present work, we clarify the excited-state intramolecular proton transfer (ESIPT) mechanism for 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (BDABE) system. We present the fact that excited-state single proton transfer can occur along with one hydrogen bond, even though BDABE form consists of two intramolecular hydrogen bonds. Based on the density functional theory and time-dependent density functional theory methods, we theoretically investigate and elaborate the excited-state intramolecular dual hydrogen-bonding interactions. By simulating the electrostatic potential surface, we verify the formation of dual intramolecular hydrogen bonds for BDABE molecule in the S₀ state. Furthermore, comparing the primary bond lengths and bond angles as well as the infrared vibrational spectra, we find that the double hydrogen bonds should be strengthened in the S₁ state. When it comes to photoexcitation process, we discover the charge redistribution around hydrogen bonding moieties. The increased electronic density around proton acceptor plays the important roles in strengthening hydrogen bonds and in facilitating ESIPT reaction. In view of the possible ESIPT reaction paths (i.e., stepwise and synchronization double proton transfer) for BDABE molecule, we explored the S₀-state and S₁-state potential energy curves. This work explains experimental results and further clarifies the excited-state behaviors for BDABE system.     

A density functional theory-time-dependent density functional theory investigation of photo-induced hydrogen bond and proton transfer for 2-(3,5-dichloro-2,6-dihydroxy-phenyl)-benzoxazole-6-carboxylicacid

Given the paramount importance of excited-state relaxation in the photochemical process, excited-state hydrogen bonding interactions and excited-state intramolecular proton transfer (ESIPT) are always hot topics. In this work, we theoretically explore the excited-state dynamical behaviors for a novel 2-(3,5-dichloro-2,6-dihydroxy-phenyl)-benzoxazole-6-carboxylicacid (DDPBC) system. As two intramolecular hydrogen bonds (O1 H2⋯N3 and O4 H5⋯O6) exist in the DDPBC structure, we first check if the double proton transfer form cannot be formed in the S1 state. Then, we explore the changes of geometrical parameters involved in hydrogen bonds, based on which we confirm that the dual intramolecular hydrogen bonds are strengthened on photo-excitation. The O1 H2⋯N3 hydrogen bond particularly plays a more important role in excited state. When it comes to the photo-induced excitation, we find charge transfer and electronic density redistribution around O1 H2 and N3 atom moieties. We verify the ESIPT tendency arising from the O1 H2⋯N3 hydrogen bond. In the analysis of the potential energy curves, along with O1 H2⋯N3 and O4 H5⋯O6, we demonstrate that the ESIPT reaction should occur along with O1 H2⋯N3 rather than O4 H5⋯O6. This work not only clarifies the specific ESIPT mechanism for DDPBC system but also paves the way for further novel applications based on DDPBC structure in the future.     

Different ESIPT Mechanisms for Angular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms of angular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S₁ state, which may facilitate the ESIPT process. Particularly, in toluene, the S₁‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.     

A DFT/TDDFT Investigation of Excited State Dynamical Mechanism of (E)‐1‐((2,2‐Diphenylhydrazono)methyl)naphthalen‐2‐ol

The excited‐state intramolecular proton transfer (ESIPT) mechanism of a new compound (E )‐1‐((2,2‐diphenylhydrazono)methyl)naphthalen‐2‐ol ( EDMN ) sensor, reported and synthesized by Mukherjee et al . [Sensors Actuat. B‐Chem . 2014, 202 , 1190], is investigated in detail theoretically. The calculations on primary bond lengths, bond angles, and the corresponding infrared (IR) vibrational spectra and hydrogen‐bond energy involved in intramolecular hydrogen bond between the S₀ and S₁ states confirm that the intramolecular hydrogen bond is strengthened in the S₁ state, which reveals the tendency of ESIPT reaction. The fact that the experimental absorption and emission spectra are well reproduced demonstrates the rationality and effectiveness of the time‐dependent density functional theory (TDDFT) level of theory we adopt here. Furthermore, intramolecular charge transfer based on the frontier molecular orbitals (MOs) gives indication of the ESIPT reaction. The constructed potential energy curves of both the S₀ and S₁ states while keeping the O─H distance of EDMN fixed at a series of values are used to illustrate the ESIPT process. The lower barrier of ~3.934 kcal/mol in the S₁ state potential energy curve (lower than the 8.254 kcal/mol in the S₀ state) provides the transfer mechanism.     

Unraveling solvent-dependent hydrogen bonding interaction and excited-state intramolecular proton transfer behavior for 2-(benzo[d]thiazol-2-yl)-4-(9H-carbazol-9-yl)phenol: A theoretical study

Organic chemosensors with excited-state intramolecular proton transfer (ESIPT) behavior have attracted much attention because it has great potential in a wide range of applications. Considering the paramount behavior of excited-state relaxation, in this work, we mainly focus on deciphering photo-induced hydrogen bonding effects and ESIPT mechanism for the novel 2-(benzo[d]thiazol-2-yl)-4-(9H-carbazol-9-yl)phenol (mCzOH) dye. Considering the effects of different solvents on excited-state dynamics of mCzOH flurophore, we adopt four solvents with different polarities. Analyses of fundamental structural changes, infrared (IR) vibrational spectra, and core valence partition index between S₀ and S₁ state, we confirm hydrogen bond O H···N of mCzOH should be enhanced via photoexcitation. Especially, the increase of solvent polarity could promote hydrogen bonding strengthening degree. Intramolecular charge transfer (ICT) resulting from photoexcitation qualitatively facilitates the ESIPT occurrence to a large extent. For further checking and probing into ESIPT mechanism, via constructing potential energy curves (PECs) in four solvents, we clarify the ESIPT behavior for mCzOH. Most worthy of mention is that polar solvent plays critical roles in lowering potential barrier of ESIPT reaction and in facilitating ESIPT process. We not only clarify the detailed excited-state process, but also present the solvent-polarity-dependent ESIPT mechanism for mCzOH fluorophore.     

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S₀ and S₁ states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S₁ state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S₁ state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.     

A DFT/TDDFT Study on Excited State Process of a Novel Probe 4′-Fluoroflavonol

A novel fluorescent probe 4′-fluoroflavonol (4F) was reported by Serdiuk et al. (RSC Adv 6:42532, 2016) in a previous paper. Spectroscopic studies on excited-state proton transfer (ESPT) of 4F was mentioned, while the mechanism of ESPT for 4F isdeficiency. In this present work, based on the time dependent density functional theory (TDDFT), we investigated the excited-state intramolecular proton transfer (ESIPT) mechanism of 4F theoretically. The primary bond lengths, bond angles and the infrared (IR) vibrational spectra involved in the formation of hydrogen bonds vertified the intramolecular hydrogen bond was strengthened, which manifests the tendency of excited state proton transfer. According to the results of calculated potential energy curves along O–H coordinate, an about 13.18 kcal/mol barrier has been found in the S₀ state. However, a barrier of 3.29 kcal/mol was found in the S₁ state, which demonstrates that the proton transfer process is more likely to occur in the excited state. In other words, the proton transfer was facilitated by photoexcitation. Particularly, the study about ESIPT mechanism of 4F should be helpful for further understanding property of fisetin.     

Time-Dependent Density Functional Theory Study on Hydrogen and Dihydrogen Bonding in Electronically Excited State of 2-Pyridone–Borane–Trimethylamine Cluster

In this work, the intermolecular dihydrogen and hydrogen bonding interactions in electronically excited states of a 2-pyridone (2PY)–borane–trimethylamine (BTMA) cluster have been theoretically studied using time-dependent density functional theory method. Our computational results show that the S₁ state of 2PY–BTMA cluster is a locally excited state, in which only 2PY moiety is electronically excited. The theoretical infrared (IR) spectra of the 2PY–BTMA cluster demonstrate that the N–H stretching vibrational mode is slightly blue-shifted upon the electronic excitation. Moreover, the computed IR spectrum of the 2PY–BTMA cluster exhibits no carbonyl character due to the extension of the C=O bond length in the S₁ state. However, the N–H bond is shortened slightly upon photoexcitation. At the same time, the H···H and H···O distances are obviously lengthened in the S₁ sate by comparison with those in ground state. In addition, the electron density of the carbonyl oxygen is diminished due to the electronic excitation. Consequently, the proton acceptor ability of carbonyl oxygen is decreased in the electronic excited state. As a result, it is demonstrated that the intermolecular dihydrogen and hydrogen bonds are significantly weakened in the electronically excited state.     

Exploring and elaborating the excited state mechanism of a novel AIE material 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl)benzothiazole

A novel aggregation-induced emission material 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl)benzothiazole (2-CHBT) has been investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Via reduced density gradient (RDG) versus sign(λ₂)ρ analyses, we firstly verify the formation of intramolecular hydrogen bond in 2-CHBT system. Analyzing the primary geometrical parameters in 2-CHBT and comparing the changes about infrared (IR) vibrational spectra, we confirm that the hydrogen bond O-H···N is strengthened in the S₁ state upon excitation. Exploring photo-excitation process, the frontier molecular orbitals (MOs) and charge density difference (CDD) analyses have been performed using TDDFT/B3LYP/TZVP theoretical level, based on which we verify the charge transfer phenomenon referring to the S₀?→?S₁ transition. And the CDD around the hydrogen bond moiety provides the tendency of ESIPT for 2-CHBT molecule. Comparing the energy gap between HOMO and LUMO orbitals in cyclohexane, toluene, chloroform, and DMSO solvents, we predict the ESIPT reaction might be more active in non-polar solvents. In addition, constructing potential energy curves and searching transition state (TS) structures, we further clarify the ESIPT mechanism and verify that non-polar solvents might facilitate the ESIPT process. Our simulated results reappear experimental spectral results and successfully explain experimental phenomenon.     

Total Syntheses of the Isomeric Aglain Natural Products Foveoglin A and Perviridisin B: Selective Excited‐State Intramolecular Proton‐Transfer Photocycloaddition

Selective excited‐state intramolecular proton‐transfer (ESIPT) photocycloaddition of 3‐hydroxyflavones with trans , trans ‐1,4‐diphenyl‐1,3‐butadiene is described. Using this methodology, total syntheses of the natural products (±)‐foveoglin A and (±)‐perviridisin B were accomplished. Enantioselective ESIPT photocycloaddition using TADDOLs as chiral hydrogen‐bonding additives provided access to (+)‐foveoglin A. Mechanistic studies have revealed the possibility for a photoinduced electron‐transfer (PET) pathway.