Excited-state Intramolecular Proton Transfer Mechanism Research Articles

The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine

AbstractThe excited‐state dynamics of the excited‐state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time‐dependent density functional theory. The excited‐state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen‐bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited‐state double proton transfer (ESDPT) along the hydrogen‐bonded bridge. Importantly, the previous proposed ESDPT‐triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.

Theoretical investigation into the deciphering effects of atomic electronegativity on 2‐hydroxy‐phenyl‐tafamidis: A time‐dependent density functional theory study

AbstractIn light of the potential probe applications of 2‐hydroxy‐phenyl‐tafamidis (2HPT) and its derivatives with different atomic electronegativity (2HPT‐O, 2HPT‐S, and 2HPT‐Se), the mechanisms of excited state intramolecular proton transfer (ESIPT) of 2HPT derivatives have been investigated in this work. Via probing into the geometric parameters of hydrogen bond O‐H···N, infrared vibrational spectroscopy, and bond critical point values, we can confirm that hydrogen bonding is enhanced in the first excited state (S1). It is worth mentioning that the enhancement of hydrogen bonding in the S1 state by low atomic electronegativity 2HPT‐Se is particularly important. In light‐induced excitation, by comparing the changed behaviors of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals, it could be found that photo‐induced charge transfer happens in excited molecules, and charge redistribution facilitates ESIPT reaction. Particularly, the low atomic electronegativity could promote the ESIPT process for 2HPT system. In order to clarify the detailed ESIPT mechanism, we construct the potential energy curves, search for the transition state form, and determine the ESIPT mechanism for 2HPT fluorophores. In this work, we not only elucidated the excited state behavior of 2HPT systems but also proposed a novel mechanism for regulating ESIPT by atomic electronegativity.

Coherent Vibrational Spectrum via Time-Resolved Fluorescence for Molecular Dynamics and Identification of Emitting Species–Application to Excited-State Intramolecular Proton Transfer

Time-resolved fluorescence (TF) with high-enough resolution enables recording of a coherent vibrational spectrum (CVS). Because a CVS attained via TF (CVSF) is descended from the frequency modulation of the fluorescence spectrum, it gives the vibrational spectrum of the emitting state. Therefore, CVSF can be a powerful tool for the identification of an emitting state along with the investigation of molecular dynamics in excited states. Herein, we report CVSF of a Schiff base salicylaldehyde azine (SAA) that has two possible excited-state intramolecular proton transfer (ESIPT) sites. The ESIPT time of SAA in dichloromethane is determined to be 22 fs. Quantitative agreement between the experimental CVSF and calculated CVSF of the mono-keto isomer demonstrates that ESIPT indeed occurs in SAA only on one side. More importantly, we show that a CVSF can be utilized to identify an emitting species and its state with the help of quantum chemical calculations. Implications of the CVSF obtained by assuming impulsive excitation of vibrations are discussed in terms of the molecular mechanism of ESIPT and the generation of nuclear wave packets in the product state.

Solvent polarity‐dependent ESIPT behavior for 5‐(benzothiazole‐2‐yl)‐4‐hydroxyisophthalaldehyde fluorophore: A theoretical study

AbstractIn view of the potential applications of 5‐(benzothiazole‐2‐yl)‐4‐hydroxyisophthalaldehyde (BHI) in the rapid identification of cyanide, the mechanism of excited‐state intramolecular proton transfer (ESIPT) of BHI has been studied in this work. By exploring the geometric parameters of hydrogen bonding in aprotic solvents with different polarities, infrared (IR) vibrational spectroscopy can confirm that hydrogen bonding is enhanced in the first excited state (S1). It is worth mentioning that the enhancement of hydrogen bonding in the S1 state by polar solvents is particularly important. In light‐induced excitation, by comparing the energy gaps of the leading molecular orbitals and the charge recombination around the proton acceptor and donor in aprotic solvents of different polarities, it is further predicted that polar solvents can promote the ESIPT process of BHI. In order to clarify the detailed ESIPT mechanism, we constructed the potential energy curves, searched for the transition state (TS) form, and determined the ESIPT mechanism for BHI fluorophore. Herein, we not only elucidated the excited‐state behavior of BHI, but also proposed a mechanism for regulating ESIPT by solvent polarity.

Insights into the photophysical properties of 2-(2′-hydroxyphenyl) benzazoles derivatives: Application of ESIPT mechanism on UV absorbers

In this present work, four novel molecules (BPN, BPNS, BPS, and BPSN), possessing excited-state intramolecular proton transfer (ESIPT) characteristics, were designed to quantify the impacts of substituent effects on their photophysical properties. By exploring the primary geometrical parameters concerning hydrogen bonds, it should be noticed that the intramolecular hydrogen bonds (IHBs) of the studied molecules have been strengthened at S1 state. Infrared vibrational spectra analysis illustrates that adding electron-donating group thiophene to the proton donor side can weaken the IHBs in comparison to the electron-withdrawing group pyridine. Through investigating the absorption and fluorescence spectra, it can be clearly found that the maximum absorption peaks of the studied molecules are all located in the UVA region, and their regions of fluorescence peaks are harmless to human skin. Furthermore, considering the light intensity factor, it can be concluded that BPNS is the most potential to be used as UV absorbers in the studied molecules. This work investigates the effects of the positions and types of substituent groups on photophysical properties of 2-(2′-hydroxyphenyl) benzazoles derivatives, which can help design and exploit novel UV absorbers.

Theoretical investigation on the fluorescence properties and ESIPT mechanism of the Al3+ ion sensor 1-((2-hydroxynaphthalen-1‐yl)methylene)urea(OCN)

A new action mechanism for the fluorescent detection on the Al3+ ion of the sensitive 1-((2-hydroxynaphthalen-1-yl)methylene)urea(OCN) is theoretically studied. The extensive theoretical calculations on the OCN and the isomer structure OCN-T are performed. The emission and absorption spectra consistent with the experiment value. The absorption spectra peaks (362 nm and 326 nm) of OCN and OCN-T molecules are attributed to the experimentally observed absorption spectra at 356 nm and 314 nm, respectively. The calculated fluorescence value of the OCN-AL structure is 460 nm, while the OCN-T-AL structure has no fluorescence. These results better explain that OCN and its isomers OCN-T are involved in the absorption, and the detection spectrum signal is emitted from the OCN-AL complex. The OCN and OCN-T molecules are obvious hydrogen bonding systems. The excited state intramolecular proton transfer photochemical behaviors and detecting Al3+ ion photophysical changes were explained for the first time at the molecular level. As driving force of excited state intramolecular proton transfer (ESIPT) reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals. To further elucidate the proton transfer reactive paths, the scanned the potential energy curves (PECs) of OCN and OCN-T chemosensor in different electronic states are plotted. This work proposes a reasonable explanation for the detection mechanism of the OCN sensor.

A novel precipitating-fluorochrome-based fluorescent probe for monitoring carbon monoxide during drug-induced liver injury

Carbon monoxide (CO), as one of significant gas transmitter, is closely associated with a variety of physiological and pathological processes. Although plenty of fluorescent probes have been prepared for detecting CO, most of them suffer from water-soluble fluorophores and short emission wavelength, which tends to diffuse and is limited to apply in vivo. Herein, a novel water-soluble fluorescent probe (HPQ-MQ-CO) is prepared to detect CO by releasing a precipitating fluorochrome (HPQ-MQ-OH), which is developed by introducing the 1-ethyl-2-methylquinoline group into HPQ to obtain long emission wavelength and good diffusion resistant ability. Allyl formate, as the identification unit of CO, has good water solubility and quenches the fluorescence of HPQ-MQ-CO. When the probe reacts with CO and Pd2+, an long-emission and solid-state fluorescence signal at 650 nm can be observed, which is based on excited-state intramolecular proton transfer (ESIPT) mechanism. When the concentration of CO is raised to 100.0 μM, the fluorescence is increased 29 times, indicating the sensitivity of the probe. Moreover, this probe shows prominent selectivity for CO compared with other interfering species. Given these advantages, HPQ-MQ-CO can be used for CO detection in HepG2 cells and zebrafish by in-situ and long-term fluorescence imaging. In addition, this probe can monitor the up-regulation of CO in HepG2 cells and zebrafish during drug-induced liver injury (DILI).

Unveiling the effects of atomic electronegativity on the ESIPT mechanism and luminescence property of new coumarin benzothiazole Fluorophore: A TD-DFT exploration

The excited-state intramolecular proton transfer (ESIPT) mechanism, photophysical properties of 8-(benzo[D] thiazole-2-yl)-7-hydroxy-2H-benzopyran-2-one (L-HKS) and the effect of O/Se atomic substitution on L-HKS have been studied in detail based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The S atom in the thiazole ring of L-HKS has been replaced by O/Se atom (denoted to L-HKO/L-HKSe) to analyze the effects of atomic electronegativity on the intramolecular H-bond, absorption/emission spectrum and ESIPT process. Through the analysis of series of calculated results, it can be found that the intramolecular H-bonds at normal form and tautomer form are enhanced and weakened in the S1 state, respectively, which is favorable to ESIPT process. The potential energy curves revealed that the ESIPT process is much easier to occur gradually from L-HKO to L-HKS and L-HKSe, as the electron-withdrawing ability of atom (from O to S and Se) is weakened. The atomic substitution also has an effect on the photophysical properties. From L-HKO to L-HKS, the emission peak at tautomer form red-shifts 70 nm. The energy gaps of the three compounds follow the decreased order of L-HKO (4.866 eV) > L-HKS (4.753 eV) > L-HKSe (4.371 eV) with the weakened electron-withdrawing ability of atom (from O to S and Se), which leads to the gradual red-shift of the absorption spectra from L-HKO to L-HKS and L-HKSe.

Bifunctional fluorescent probe for detecting and imaging hydrazine hydrate both in vitro and in vivo

Hydrazine (N2H4), an important alkaline agent with reducing activity, is widely used in agriculture and chemical industry. Although N2H4 is not produced in the human body, inhalation of trace N2H4 from the environment can cause serious harm to organisms. Therefore, it is important to accurately detect trace N2H4 for ensuring safety. Compared with the traditional N2H4 probe, dual-channel detection can provide better imaging resolution. The design of conventional dual-channel probes is based on the excited-state intramolecular proton transfer (ESIPT), which is limited by the choice of matrix. Developing novel probe that does not depend on the ESIPT mechanism and can provide two-channel detection is needed. Herein, a novel bifunctional fluorescent probe PTFD was designed and synthesized. Compared with the monofunctional fluorescent probe PTD, PTFD can achieve dual-channel detected N2H4 with a rapid response, excellent selectivity, and high sensitivity (detection limit of 0.08 µM). Importantly, PTD and PTFD can be applied for imaging of N2H4 in HeLa cells and zebrafish because of their good optical properties and low toxicity.

Comparison of the excited-state proton transfer and single electron transfer mechanisms of the natural antioxidant Juglone and its dimer 3,3′-bijuglone

In this work, the excited-state intramolecular proton transfer (ESIPT) mechanism and antioxidant activity of Juglone (Ju) and its dimer 3,3′-bijuglone (BJu) were systematically compared and analyzed by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Via comparing the primary hydrogen bond parameters, reduced density gradient (RDG) versus sign(λ2)ρ(r) scatter plots, interaction region indicator (IRI) isosurface, topology descriptors, and infrared (IR) spectra, it can be confirmed that all the intramolecular hydrogen bonds (IHBs) of Ju and its dimer 3,3′-bijuglone (BJu) have been enhanced in the lowest excited state (SL). The intramolecular charge transfer (ICT) triggered the fluorescence quenching mechanism for Ju was confirmed based on the hole-electron analysis. Moreover, the calculated S0- and SL-stated potential energy surfaces (PESs) have proven that the excited-state intramolecular double proton transfer (ESIDPT) of BJu is stepwise rather than concerted and based on which the detailed proton-transfer (PT) process has been clarified. Compared with the first step PT behavior of BJu, the ESIPT process of Ju has barely been influenced by dimerization. Besides, the computed ionization potential (IP) has substantiated that BJu shown the better antioxidant capacity via the single electron transfer (SET) mechanism than the corresponding monomer Ju in the SL state. The relationship between the PT process and antioxidant activity of Ju and BJu has also been successfully established, which is conducive to design and synthesize highly effective PT-based antioxidants.

Theoretical insights into effects of solvent polarity on excited‐state N–H proton transfer behavior for a new fluorophore of 3‐tosylamino‐N‐cyclohexylphthalimide

AbstractCompared with O–H···O‐type system, the N–H···O‐type system with excited‐state intramolecular proton transfer (ESIPT) provides a better research object for exploring kinetics and thermodynamics because the existence of amino group can be replaced by various groups. In this work, the ESIPT mechanism of a novel amino type proton donor (3TsAPI) has been studied, and its effects of solvent polarity have been investigated. We optimize the structures of 3TsAPI using density functional theory (DFT)/time‐dependent density functional theory (TDDFT) methods, and electronic spectra data are consistent with the experiment. Through the analyses of structural parameters and spectra related to hydrogen bonding, it is found hydrogen bonding is enhanced in S1 state, and analyses of bond critical point (BCP) parameters and core‐valence bifurcation (CVB) indexes show excited‐state hydrogen bonding is stronger in solvents with weaker polarity. We construct the potential energy curves and find polarity of solvents can regulate the potential energy barrier of ESIPT process and then further elucidate the ESIPT mechanism of 3TsAPI.

Is it possible to switch ESIPT-channel of hydroxyanthraquinones with the strategy of modifying electronic groups?

In this work, to unveil the substituent effect on the excited-state intramolecular proton transfer (ESIPT) reaction of 1,8-Dihydroxyanthraquinone (DHAQ), four derivatives DHAQ-Me, DHAQ-NH2, DHAQ-COOH, and DHAQ-CN were considered using quantum chemistry methods in the acetonitrile (ACN) phase. The excited-state intramolecular hydrogen bonds (IHBs) (O1-H2⋯O3 and O5-H4⋯O3) strengthening mechanism was confirmed by computing primary geometric parameters, infrared (IR) spectra and reduced density gradient (RDG) scatter plots. The strength relationship of the dual IHBs within all the investigated compounds was established based on the calculated binding energies. Charge-recombination triggered ESIPT mechanism has been demonstrated by the analysis of frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population. Besides, the hole-electron analysis indicates that a remarkable intramolecular charge transfer (ICT) behavior has induced the fluorescence intensity to decrease significantly for DHAQ-NH2 in K1 form. DHAQ derivatives with electron-withdrawing groups substituted (DHAQ-COOH and DHAQ-CN) have exhibited a larger Stokes shift, which is a desirable property for manufacturing luminescent materials. From the located transition state (TS) structures and the calculated potential energy curves (PECs), it is noticeable that the dominant channel for ESIPT reactions is always the O1-H2⋯O3 hydrogen bonding wire, which is irrelevant to the types of substituents introduced, although O5-H4⋯O3 is the stronger one for DHAQ-Me and DHAQ-NH2. This theoretical research may well deepen the comprehension of the relationship between IHB and ESIPT behavior and thereby provide valuable guidance to synthesize efficacious ESIPT-based organic chromophores experimentally.

An ESIPT-based NIR-emitting ratiometric fluorescent probe for monitoring hydrogen peroxide in living cells and zebrafish

Hydrogen peroxide (H2O2) is closely related to a variety of human diseases. It is a challenge to dynamically monitor biological H2O2 activity. Hence, we design and prepare a NIR-emitting ratiometric fluorescent probe (HBQ-B) for detecting H2O2 according to the excited-state intramolecular proton transfer (ESIPT) mechanism using the benzyl boronic pinacol ester group as a recognition unit. HBQ-B displayed specificity toward H2O2 over other bioactive analytes. Meanwhile, HBQ-B exhibited a good linear relationship between the fluorescence ratio (I656/I538) changes and the concentrations of H2O2 (0–10 μM). HBQ-B also showed a large Stokes shift (234 nm) and a marked detection limit (40.2 nM). HBQ-B was utilized to image the intracellular H2O2 in living MCF-7 cells, HeLa cells and macrophages (RAW 264.7 cells), respectively. In addition, HBQ-B was used to monitor the dynamics of H2O2 level changes during zebrafish development.

A novel mechanism of intramolecular proton transfer in the excited state of 3-hydroxy-4H-benzochromone derivatives: A new explanation at the theoretical level

The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of a series of 2-(1H-1,3-benzodiazol-2-yl)-3-hydroxy-4H-benzo[h]chromen-4-one (HBC4H) derivatives are explored theoretically. From these, a new explanation of ESIPT mechanism of 3-hydroxy-4H-benzochromone derivatives has been proposed. Firstly, three isomers are optimized in toluene solvent. Through the analysis of the hydrogen bond parameters and infrared vibration spectrum, it is found that the hydrogen bonds of HBC4H should be strengthened in the S1 state. In order to explain the mechanism of ESIPT, the Boltzmann distribution and torsion forces of the three isomers are deliberated, and the searched transition state (TS) structures are depicted. It is discovered that the potential barriers are all lower than 5.00 kcal/mol in the S1 state, and HBC4H-Ia and HBC4H-Ic may coexist in the S0 state, and more significantly, among the three isomers, HBC4H-Ia may be the most favorable form of ESIPT.

Excited-state intramolecular double proton transfer mechanism associated with solvent polarity for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound

Excited-state intramolecular proton transfer (ESIPT) has been well investigated in recent years, whereas we investigated the system containing double hydrogen bonds. We explore the influence of solvent polarity on the proton transfer mechanism for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound ((Oxa-OH)2). We consider three solvents with different polarities: cyclohexane, chloroform and acetonitrile. Various analytical methods indicate that hydrogen bonding is enhanced in the first excited state, which facilitates the proton transfer within the excited-state molecule. The frontier molecular orbital theory is employed to demonstrate that the system is subjected to photoexcitation to generate charge density redistribution. Three-dimensional potential energy surfaces are also constructed along the direction of hydrogen bond elongated, and their corresponding projection in the X-axis and Y-axis planes are plotted. The comparison of the potential barriers and the transition state structures of each proton transfer path reveals the excited-state intramolecular single proton transfer mechanism of the system. By comparing the potential barriers in different solvents, we also propose a mechanism that can modulate the excited-state proton transfer by changing the solvent polarity.

Substituent effect induced the distinctive ESIPT reaction and photophysical property of N-salycilidene-5-chloroaminopyridine

Two novel molecules (SCLPY-MeO and SCLPY-CN) are designed based on the N-salycilidene-5-chloroaminopyridine (SCLPY) via introducing the methoxyl (–OCH3) and cyano (-CN) groups at the meta-position of the chlorine atom, respectively, to explore the effect of the substitution of electron-withdrawing and donating groups on the excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of SCLPY, employing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The significant calculated structural parameters associated with the intramolecular hydrogen bonds (IHBs) have demonstrated that the IHBs intensity is enhanced in the excited state and SCLPY is endowed with the strongest IHB. The partial quenching of the short-wavelength fluorescence in SCLPY-CN has been satisfactorily explained by calculating the transition electric dipole moment and the long-wavelength fluorescence of that exhibits the largest Stokes shifts relative to the other molecules. Moreover, introducing the –OCH3 group has evidently enhanced the luminescent intensity of SCLPY. On account of the potential energy curves (PECs), scanning with gradually elongating the O1-H2 bond, it can be known that the forward proton transfer process is more readily to occur in the S1 state due to the relatively lower energy barriers compared with the ground state. Moreover, SCLPY is provided with the lowest energy barrier, implying that the ESIPT behavior is most likely inclined to proceed in SCLPY. Therefore, it is well illustrated that the introductions of –OCH3 and –CN groups indeed have an impact on the ESIPT process and photophysical properties of SCLPY.

Rotating the C-N Bond in a Coumarin-Pyridine-Based Sensor for Pattern Recognition of Versatile Metal Ions.

A cross-reactive sensor array is powerful for high-throughput discrimination of various kinds of metal ions. However, the construction of a multicomponent sensor array is always time-consuming and cost-ineffective. Herein, a practical four-component X1-based sensor array (X1SA) was obtained by simply dissolving a single dye molecule X1 in respective solvents such as methanol, ethanol, dimethyl sulfoxide, and acetonitrile. In this design, X1 exhibits strong solvatochromic fluorescence properties via an excited-state intramolecular proton transfer and intramolecular charge transfer combined mechanism. Moreover, rotation of the C-N bond between the pyridine and coumarin units in X1 enabled it to coordinate with metal ions through different binding modes, which acted as an additional dimension of the sensor array. Inspired by this C-N bond rotation strategy, X1SA was determined to be powerful in discriminating 20 kinds of metal ions in both phosphate-buffered saline and 5% serum media in a range of 0.1-100 μM. In addition, the sensor array was also successfully applied in differentiating similar and mixed metal ions such as Fe3+/Fe2+, Cd2+/Hg2+, and Sn2+/Pb2+ in serum samples, which is meaningful for investigating the biological roles of iron and early diagnosis of related metal poisoning accidents.

Tactfully unveiling the effect of solvent polarity on the ESIPT mechanism and photophysical property of the 3-hydroxylflavone derivative

In this contribution, the solvent effects on the excited-state intramolecular proton transfer (ESIPT) and photophysical properties of 2-(4-(diphenylamine)phenyl)-3-hydroxy-4H-chromen-4-one (3HF-OH, Dyes Pigm. 2021, 184, 108865) in the dimethylsulfoxide (DMSO), acetonitrile (ACN), dichloromethane (DCM) and cyclohexane (CYH) phases have been comprehensively explored by using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The obtained bond lengths, bond angles and infrared (IR) vibration analysis related to the intramolecular hydrogen bond (IHB) reveal that the IHB intensity of 3HF-OH is weakened as the solvent polarity increased. Besides, the ESIPT process changes from the endothermic to the exothermic with the enlargement of solvent polarity, and the reaction barrier increases gradually. It is worth noting that the molecular configuration torsion of 3HF-OH is gradually intensified with the decline of solvent polarity, which aggravates the twisted intramolecular charge transfer (TICT) state and thereby partially attenuates the short-wavelength fluorescence of 3HF-OH in the CYH solvent. In addition to these, the structural torsion has restrained the occurrence of the ESIPT behavior by means of elevating the energy barrier. This theoretical research would provide valuable guidance for regulating and controlling the photophysical behavior of compounds via the strategy of changing solvent polarity.

Strategy to Design a Flexible and Macromolecular Sensor to Bind Cd2+ Ions: A Complete Photophysical Analysis and Bio-Imaging Study.

A novel triazole-bridged coumarin–benzimidazole-conjugated fluorescence sensor (4) has been developed for selective detection of Cd2+ over other competitive metal ions. The sensor exhibited quick “turn-on” responses upon interaction with a very low level of Cd2+ (14 nM). The photophysical changes in the complexation of Cd2+ with sensor 4 have been explained through the excited-state intramolecular proton transfer mechanism. The involvement of benzimidazole and triazole moieties in Cd2+ binding was confirmed by different spectroscopic techniques such as UV–vis, Fourier transform infrared, nuclear magnetic resonance, and ESI mass. The diameter of the circular shape of the sensor decreased upon complexation with Cd2+, which was confirmed by field-emission scanning electron microscopy. Furthermore, the quantum chemical (density functional theory) calculation supported the mechanism of interactions and the mode of binding of 4 toward Cd2+. The sensor was more effective for finding Cd2+ in two living cells, C6 (rat glial cell) and Hep G2 (human liver cell).

Solvent Effect on Excited-State Intramolecular Proton-Coupled Charge Transfer Reaction in Two Seven-Membered Ring Pyrrole-Indole Hydrogen Bond Systems

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g-PPDBI and e-PPDBI, by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.