Excited-state Intramolecular Proton Transfer Reaction Research Articles

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement

Insights into the solvent-polarity-associated hydrogen bonding interactions and ESIPT reactions for DBOX fluorophore: a theoretical investigation

Inspired by the exquisite design of distinguished optical materials, novel organic molecules with remarkable characteristics of excited-state intramolecular proton transfer (ESIPT) have emerged as a prominent and captivating research topic. In this study, our primary focus lies in exploring the intricate dynamics of the excited state in 2-(5-(benzo[d]thiazol-2-yl)-1,3,4-oxadiazol-2-yl)-5-(diethylamino)phenol (DBOX), an extraordinary reversible molecular fluorophore that undergoes single-crystal-to-single-crystal transition. By investigating four distinct aprotic solvents with varying degrees of polarity, we unequivocally validate the significant impact of solvent polarity on photo-induced hydrogen bonding interactions, charge redistribution, and associated excited-state intramolecular proton transfer (ESIPT) phenomena. Through meticulous comparison and precise quantification of reaction barriers in diverse solvent environments, our groundbreaking findings strongly suggest that solvents with low polarities effectively facilitate the ESIPT reaction for DBOX fluorophore.

Theoretical exploring effects of solvent polarity and atomic electronegativity on excited state behaviour for BY4TP fluorophore

As widely acknowledged, the realm of novel organic molecules boasting extraordinary attributes pertaining to excited-state intramolecular proton transfer (ESIPT) has emerged as a captivating subject matter. In this context, our primary focus lies in delving into the excited-state behaviour exhibited by 2-benzoxazol-2-yl-4-triethylsilanylethynyl-phenol (BY4TP), an alluring derivative derived from 2-(2-hydroxyphenyl)benzoxazole (HBO). Relying on the examination of four distinct aprotic solvents with varying degrees of polarities, we can unequivocally affirm that solvent polarity exerts a profound influence on the intricate interplay of hydrogen bonding interactions, charge redistribution and reorganisation, as well as associated ESIPT phenomena by light. Through meticulous comparison and precise measurement of reaction barriers across diverse solvent environments, our groundbreaking findings indicate that lowly polar solvents serve as efficacious facilitators for promoting the occurrence of the ESIPT reaction in BY4TP fluorophore. By considering atomic-electronegativity-regulated hydrogen bonding effects and excited state behaviours for BY4TP-S and BY4TP-Se, we also present low atomic electronegativity with Se substation promotes ESIPT reaction. We ardently anticipate that this study will provide invaluable insights into the behaviour exhibited by BY4TP upon excitation and under the influence of solvent polarity and atomic electronegativity, while simultaneously paving new avenues for future research endeavours and applications encompassing novel HBO derivatives.

Theoretical study on the ESIPT and photophysical properties for molecules with different extension π conjugation

In a recent study, Jincheng Si and coworkers designed four candidate linker units with different extension π conjugation sizes and directions. These molecules were used for the synthesis of metal–organic framework (MOF) structures known as Olympic rings, which can detect the Cr2O72- in the aqueous phase. However, the experimental work did not provide a qualitative understanding of the excited-state dynamic mechanism. Therefore, this research focused on investigating the excited state intramolecular proton transfer (ESIPT) process and the photophysical properties of these molecules. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to conduct an in-depth analysis. The molecule structure properties were investigated by analyzing the hydrogen bond parameters and the infrared (IR) vibrational spectra. The calculated absorption and emission spectra were in good agreement with the experimental values, it was proved that the correctness of the optimized structure and the rationality of our calculation method, and we can judge their detection sensitivity by the sizes of the Stokes shift. The Hirshfeld method was used to study the enhancement mechanism of hydrogen bond interaction. Furthermore, the reaction mechanism and ESIPT reactivity of molecules with different extension π conjugation sizes and directions were explained by calculating and plotting the relaxed potential energy curve, electrostatic potentials (ESPs) and frontier molecular orbitals (FMOs). Our research found that extension π conjugation in different sizes and directions can either promote or inhibit the occurrence of the ESIPT reactions and the characteristic of intramolecular charge transfer (ICT), thereby altering the fluorescence performance. Based on our findings, we concluded that the ESIPT activity and photophysical properties of the H4BIHQC were higher than the other three molecules. This discovery enhances the potential application value of MOF as a fluorescence detection tool.

Theoretical insights into photoinduced excited‐state behaviors for the novel CHPPhl fluorophore: Effects of solvent polarity

AbstractInspired by the distinguished photochemical and photophysical properties of novel hydroxyl‐substituted tetraphenylimidazole (HPI) derivatives that could be potentially applied across various disciplines, in this work, effects of solvent polarity on excited‐state hydrogen bond effects and excited‐state intramolecular proton transfer (ESIPT) reaction of 3‐(6,9‐Diphenyl‐1H‐phenanthro[9,10‐d]imidazole‐2‐yl)‐9‐phenyl‐9H‐carbazol‐4‐ol (CHPPhl) are focused. By comparing the structural changes and infrared (IR) vibrational spectra of the E‐HBT fluorophore in polar acetone, moderate polar tetrahydrofuran and non‐polar hexane solvents, combined with the preliminary detection of hydrogen bond interaction by core–valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S1 state, which is favorable for the occurrence of ESIPT reactions. The charge recombination behavior of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimization and searching transition state (TS) form, we confirm change of surrounding solvent polarity has a regulatory effect on the ESIPT behavior for CHPPhl; that is, the lower the solvent polarity is more conducive to the ESIPT reaction.

Theoretical investigation into the effect of atomic electronegativity related chalcogen on ESIPT behaviour for the novel biphenyl-modified 2-(2’-hydroxyphenyl)benzothiazole compounds

Inspired by the remarkable photochemical and photophysical properties of novel 2-(2’-hydroxyphenyl)benzothiazole (HBT) derivatives that could be potentially applied across various disciplines, in this work, effects of atomic electronegativity of chalcogen elements on excited state hydrogen bond effects and excited state intramolecular proton transfer (ESIPT) reaction of the biphenyl-modified HBT derivatives (i.e. HBT-HH-O, HBT-HH-S and HBT-HH-Se) are focused. By comparing the structural changes and infrared (IR) vibrational spectra of the HBT-HH fluorophores in toluene solvent, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S1 state, which is favourable for the occurrence of ESIPT reactions. The charge recombination behaviour of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimisation and searching transition state (TS) form, we confirm the variations of atomic electronegativity of chalcogen have the regulatory effect on the ESIPT behaviour for HBT-HH derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.

Small is Beautiful: Electronic Origin and Synthetic Evolution of Single-Benzene Fluorophores.

ConspectusSingle-benzene fluorophores (SBFs) are small molecules that produce visible light by using only one benzene ring as the sole aromatic core. This Account centers around the chemistry of a new class of SBF that we accidentally discovered but rationally developed and refined afterward. In a failed experiment that took an unintended reaction pathway, we encountered the bright green fluorescence of ortho-diacetylphenylenediamine (o-DAPA). Despite its uninspiring look, reminiscent of textbook examples of simple benzene derivatives, this molecule had neither been synthesized nor isolated before. This discovery led to our studies on the larger DAPA family, including isomeric m-DAPA and p-DAPA. Remarkably, p-DAPA is the lightest red fluorophore, with a molecular weight of only 192. While o- and p-DAPA are emissive, m-DAPA rapidly undergoes internal conversion, facilitated by sequential proton transfer reactions in the excited state.Leveraging the synthetic utility of the amine group, we carried out straightforward single-step modifications to create a full-color SBF library from p-DAPA as the common precursor. During the course of the investigation, we made another fortuitous discovery. With increasing acidity of the N-H group, the excited-state intramolecular proton transfer reaction is promoted, opening up additional pathways for emission to occur at even longer wavelengths. Tipping the balance between the two excited-state tautomers enabled the first example of a single-benzene white-light emitter. We demonstrated the practical utility of these molecules in white light-emitting devices and live cell imaging.According to the particle-in-a-box model, it is difficult to expect a molecule with only one small aromatic ring to produce long-wavelength emission. SBFs rise to this challenge by exploiting electron donor-acceptor pairs around the benzene core, which lowers the energy of light absorption. However, this answers only half of the question. Where do the exceptionally large spectral shifts in the light emission of SBFs originate from? Chemists have long been curious about the molecular mechanisms underlying the dramatic spectral shifts observed in SBFs. Prevailing paradigms invoke the charge transfer (CT) between electron donor and acceptor groups in the excited state. However, without a large π-skeleton for effective charge separation, how could benzene support a CT-type excited state? Our experimental and theoretical studies have revealed that large excited-state antiaromaticity (ESAA) of the benzene core itself is responsible for this remarkable phenomenon. The core matters, not the periphery. With appropriate molecular design, large and extended π-conjugation is no longer a prerequisite for long-wavelength light emission.

Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer†

Comprehensive SummaryMechanochromic polymers based on non‐covalent changes have attracted much attention recently. Herein, we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process. PhMz‐NH2‐OH and PhMz=2A are designed and obtained by simple and high‐yield synthesis, and are connected into polyurethane and poly(methyl acrylate‐co‐2‐ethylhexyl acrylate), respectively. In the initial state, the PhMz‐NH2‐OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons, owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA‐2‐EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission, implying that the intramolecular hydrogen bonds matter. Furthermore, under stretching, external force can tune the emission of the PhMz=2A@PMA‐2‐EA sample from K* to E* state. Though external force can putatively still promote a bond rotation, ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen‐bond donating environment. DFT calculation confirms the force‐induced increase in dihedral angle for the transition of ESIPT‐on/off. Thus, PhMz‐NH2‐OH@PU and PhMz=2A@PMA‐2‐EA showed disparate initial ESIPT states and further different responses/sensitivity to force. This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.

Effects of substituted chalcogen atoms on excited state reaction for 1-(benzo[d]thiazol-2-yl)-6-substituted-naphthalen-2-ol derivatives

Inspired by the brilliant photophysical properties of novel organic molecules containing chalcogenide substitution, in this work, effects of atomic electronegativity of chalcogen (O, S and Se) on hydrogen bond interactions and proton transfer (PT) reaction are focused. We present the characteristic CN-BTN is the main objective to explore the influence of atomic electronegativity on the hydrogen bond interaction and excited state intramolecular proton transfer (ESIPT) behaviour by photoexcitation. By comparing the structural changes and infrared (IR) vibrational spectra of the CN-BTN derivatives (CN-BTN-O, CN-BTN-S and CN-BTN-Se) fluorophores in S0 and S1 states, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that hydrogen bond is strengthened in S1 state, which is favourable for the occurrence of ESIPT reaction. The charge recombination behaviour of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimisation and searching transition state (TS) configuration, we finally clarify the ESIPT mechanism for CN-BTN derivatives. Specially, we confirm change of atomic electronegativity has a regulatory effect on the ESIPT behaviour of CN-BTN derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.

Taking Advantage of a Luminescent ESIPT-Based Zr-MOF for Fluorochromic Detection of Multiple External Stimuli: Acid and Base Vapors, Mechanical Compression, and Temperature.

Luminescent materials responsive to external stimuli have captivated great attention owing to their potential implementation in noninvasive photonic sensors. Luminescent metal-organic frameworks (LMOFs), a type of porous crystalline material, have emerged as one of the most promising candidates for these applications. Moreover, LMOFs constructed with organic linkers that undergo excited-state intramolecular proton-transfer (ESIPT) reactions are particularly relevant since changes in the surrounding environment induce modifications in their emission properties. Herein, an ESIPT-based LMOF, UiO-66-(OH)2, has been synthesized, spectroscopically and photodynamically characterized, and tested for detecting multiple external stimuli. First, the spectroscopic and photodynamic characterization of the organic linker (2,5-dihydroxyterephthalic acid (DHT)) and the UiO-66-(OH)2 MOF demonstrates that the emission properties are mainly governed by the enol → keto tautomerization, occurring in the organic linker via the ESIPT reaction. Afterward, the UiO-66-(OH)2 MOF proves for the first time to be a promising candidate to detect vapors of acid (HCl) and base (Et3N) toxic chemicals, changes in the mechanical compression (exercised pressure), and changes in the temperature. These results shed light on the potential of ESIPT-based LMOFs to be implemented in the development of advanced optical materials and luminescent sensors.

Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues.

In this paper, an efficient synthetic route from pyrazole-chalcones to novel 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones as 3-hydroxyflavone analogues is described. The methylation of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with methyl iodide in the presence of a base yielded a compound containing a 5-methoxy group, while the analogous reaction of 5-hydroxy-2-phenyl-6-(pyridin-4-yl)pyrano[2,3-c]pyrazol-4(2H)-one led to the zwitterionic 6-(N-methylpyridinium)pyrano[2,3-c]pyrazol derivative. The treatment of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with triflic anhydride afforded a 5-trifloylsubstituted compound, which was further used in carbon-carbon bond forming Pd-catalyzed coupling reactions to yield 5-(hetero)aryl- and 5-carbo-functionalized pyrano[2,3-c]pyrazoles. The excited-state intramolecular proton transfer (ESIPT) reaction of 5-hydroxypyrano[2,3-c]pyrazoles from the 5-hydroxy moiety to the carbonyl group in polar protic, polar aprotic, and nonpolar solvents was observed, resulting in well-resolved two-band fluorescence. The structures of the novel heterocyclic compounds were confirmed by 1H-, 13C-, 15N-, and 19F-NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.

Turn on the luminescence channel by the suppression of conical intersection process for the sulfur-containing hydrogen-bonded molecules

The sulfur-containing hydrogen bonds exhibit notable potential in gene and protein engineering. As an important research direction of sulfur-containing hydrogen bonding dynamics, the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton has garnered significant interest. Recently, Chou et al. synthesized a series of sulfur-containing H-bonded molecules, inclusive of 3-thiolflavone (3 TF), its trifluoromethyl derivative (3FTF), and its diethylamino derivative (3NTF), thereby inaugurating a new chapter in the ESIPT reaction of the thiol proton. Nevertheless, many open issues cannot be answered solely by experimental means and call for a detailed theoretical investigation, especially the relationship between the molecular excited-state properties and different luminescence properties of the three molecules. Herein, we validate that the molecules undergo the ESIPT reaction of the thiol proton and identify the time scale through the time-dependent density functional theory (TDDFT) calculation. Moreover, employing the spin-flip TDDFT approach, the analysis of the non-radiative transition process reveals that difference in Gibbs free energy of activation (ΔG⧧) for the transition from Frank-Condon point to minimum energy conical intersection (MECI) point primarily accounts for the different luminescence properties of the three molecules. Besides, we also reveal the reason for the abnormal blue-shifted fluorescence of the 3NTF in the aggregated state by the ONIOM (TDDFT: UFF) strategy. These findings are favorable to understanding the luminescence mechanism of sulfur-containing H-bonded molecules and provide a reference for the synthesis and design of the luminescent materials based on the molecules with thiol proton transfer properties.

Polycrystalline monophenyl solid emitters with amplified spontaneous emission

There are many reports of small molecules exhibiting amplification spontaneous emission laser properties, but there are few reports of single benzene rings exhibiting laser properties while achieving high solid state emission from polycrystalline states. This work takes two very simple organic molecules and constructs an efficient single benzene ring solid state emitter. The crystal laser properties are based on aggregation‐induced emissions (AIE) generated through an excited‐state intramolecular proton transfer reaction (ESIPT).

Regulating the ESIPT processes and photophysical properties of indole derivates by introducing the appropriate functional groups

In this work, the substitution effect of different electron-donating and electron-withdrawing groups on the electron spectra and excited state intramolecular proton transfer (ESIPT) properties of the chromophore 2-(2-hydroxy-5-methylphenyl)benzo[d]oxazole-5-sulfonic acid (BOM-1) has been studied via the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. All the compounds exhibit the dual-fluorescence emission, and the fluorescence spectra of the derivatives show redshift emission compared with that of the original molecule, resulting in the larger Stokes shift as expected. The results were cross-verified by the molecular geometry, frontier molecular orbitals, infrared spectroscopy and other auxiliary calculations. These results show that the electron-withdrawing group greatly promote the occurrence of the forward ESIPT process and increase the Stokes shift. What is more special is that the Stokes shift and the energy barrier of the forward ESIPT are significantly enlarged by introducing both electron-withdrawing and electron-donating groups. The energy barrier, absorption and fluorescence spectra of luminescent molecules can be adjusted effectively by introducing appropriate functional groups. In addition, the emission spectra from the ultraviolet region (359 nm) to the orange region (601 nm) are obtained. This work would contribute to the understanding of the mechanism of ESIPT reactions and improves the luminescence properties of ESIPT molecules.

Effectively controlling luminescent property and the ESIPT behavior of styryl dyes by atomic substituent

In view of the unique luminescent characteristics, excited state intramolecular proton transfer (ESIPT) materials have attracted lots of attention. Considering the influence of different atomic substituents on the photophysical properties and ESIPT reaction, we systematically studied the molecular and electronic properties of styryl dyes with O, S and Se substituents by the quantum chemistry methods. The results indicated that the ESIPT energy barrier of styryl dyes reduced with atomic substituents (O < S < Se). And potential energy curves (PECs) showed that the ultra-fast ESIPT reaction of styryl dyes could be controlled and regulated by atomic substituents. In addition, the styryl dyes had a large stokes shift, which reproduced the double fluorescence properties. It meaned that the enol and keto structures of three styryl dyes interconvert by ESIPT channel under the photoexcitation. Therefore, the special optical properties of styryl dyes could improve their application as potential biological imaging.

Molecular Principles of the Excited-State Intramolecular Thiol Proton Transfers in 3-thiolflavone Derivatives.

The excited-state intramolecular proton transfer (ESIPT) effect has attracted considerable attention due to its potential applications in photoluminescent materials. However, only a few theoretical reports have investigated the ESIPT process involving sulfur-hydrogen bonds. Herein, we systematically investigated the ESIPT effect of three 3-thiolflavone derivatives containing sulfur-hydrogen bonds with M06-2X functional combined Def2-TZVP basis set. The intramolecular sulfur-hydrogen bonds were confirmed in the ground and excited states via analyzing the bond lengths, interaction energies, and infrared vibrational spectra. Besides, we demonstrated that the electron-withdrawing group led to a more stable tautomer compared to the electron-donating group. Conversely, the electron-donating group played a crucial role in reducing the energy barrier of the ESIPT reaction due to the strengthening of the hydrogen bond in the excited state. Interestingly, the substituent group can determine the excited-state electronic properties of keto tautomers. Specifically, the electron-withdrawing group caused significant improvement in the nπ* transition configuration, significantly reducing the radiation rate. The electron-donating group increased the proportion of the ππ* transition configuration, thus, the keto tautomer had bright emission. We expect these findings to open new avenues for designing potential luminescent materials.

Dynamics simulation of excited-state proton transfer reactions of 8-hydroxyquinoline with water clusters: A TD-DFT study

Detailed pictures of the excited-state proton transfer reactions of 8-hydroxyquinoline (8HQ) and its small clusters of water have been systematically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods at B3LYP/TZVP and TD-B3LYP/TZVP level of theory, respectively. Intramolecular hydrogen bond (intraHB) and intermolecular hydrogen bond (interHB) interactions between 8HQ and water cluster become stronger in the first excited state (S1), confirmed by hydrogen bond distances and topological analysis. In addition, TD-B3LYP dynamics simulations have been elucidated and revealed that excited-state intramolecular proton transfer (ESIntraPT) and multiply excited-state intermolecular proton transfer (ESInterPT) reactions of these complexes can take place in the ultrafast time scale of femtoseconds. Increasing the number of water molecules may give rise to a barrier energy that is anti-correlated to the probability of tautomer species from phototautomerization, especially in 8HQ-W1 and 8HQ-W1+1. Moreover, water molecules initiate multiple proton transfer occurring through two- and three-step ESInterPT namely excited-state double proton transfer (ESDPT) and excited-state triple proton transfer (ESTPT) in which the interHBs are formed by rearrangement of water molecules of 8HQ-W2. This circumstance takes a longer time (up to 470 fs) compared to that of ESIntraPT. Thus, the simulated results from this study could provide insight into ESIntraPT and ESInterPT of phototautomerization and the significant photodynamics which could not be found from the experimental aspects of 8HQ and its solvent clusters.

Photogrammetry of Ultrafast Excited-State Intramolecular Proton Transfer Pathways in the Fungal Pigment Draconin Red.

Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of -COH rocking and -C=C, -C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck-Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique "W"-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump-probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.

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

AbstractOrganic 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.

Solvent conditions effect on the excited state intramolecular proton transfer mechanism and photophysical property of 1′-hydroxy-2′-acetonaphthone: A DFT/TD-DFT analysis

Recently, a fluorescence probe 1′-hydroxy-2′-acetonaphthone (HAN) detecting of metal ions is synthesized and characterized experimentally (Malini J, J. Photochem. Photobiol. A Chem., 2021, 418: 113431.). However, comprehensive research on its excited state intramolecular proton transfer (ESIPT) mechanism and photophysical property is required. In the present work, we select four solvents with different dielectric constant to investigate the effect on ESIPT process and photophysical property of HAN molecule. By means of the density functional theory (DFT) and time-density functional theory (TDDFT) methods, all theoretical calculations are accomplished. Through insights into the hydrogen bond geometrical parameters, infrared (IR) vibrational spectra, molecular electrostatic potential (MEP) surfaces, intramolecular hydrogen bond (IHB) strengthening mechanism can be verified, which providing powerful contribution to the occurrence of ESIPT reaction. Based on the analysis of frontier molecular orbitals (FMOs) and Mulliken’s charges and hole–electron, the intramolecular charge transfer (ICT) character is explained. Potential energy curves (PECs) reveal the proton transfer reaction take place more favorably in the S1 state under all solvent conditions. In addition, the energy barriers of QS molecule have ascended higher and higher from toluene (TOL) to sulfoxide (DMSO). The ESIPT behavior is preferred to proceed in the solvent with high dielectric constant instead of solvent with low dielectric constant.