Excited-state Proton Transfer Reaction Research Articles

Excited State Structural Evolution of a GFP Single-Site Mutant Tracked by Tunable Femtosecond-Stimulated Raman Spectroscopy

Tracking vibrational motions during a photochemical or photophysical process has gained momentum, due to its sensitivity to the progression of reaction and change of environment. In this work, we implemented an advanced ultrafast vibrational technique, femtosecond-stimulated Raman spectroscopy (FSRS), to monitor the excited state structural evolution of an engineered green fluorescent protein (GFP) single-site mutant S205V. This mutation alters the original excited state proton transfer (ESPT) chain. By strategically tuning the Raman pump to different wavelengths (i.e., 801, 539, and 504 nm) to achieve pre-resonance with transient excited state electronic bands, the characteristic Raman modes of the excited protonated (A*) chromophore species and intermediate deprotonated (I*) species can be selectively monitored. The inhomogeneous distribution/population of A* species go through ESPT with a similar ~300 ps time constant, confirming that bridging a water molecule to protein residue T203 in the ESPT chain is the rate-limiting step. Some A* species undergo vibrational cooling through high-frequency motions on the ~190 ps time scale. At early times, a portion of the largely protonated A* species could also undergo vibrational cooling or return to the ground state with a ~80 ps time constant. On the photoproduct side, a ~1330 cm−1 delocalized motion is observed, with dispersive line shapes in both the Stokes and anti-Stokes FSRS with a pre-resonance Raman pump, which indicates strong vibronic coupling, as the mode could facilitate the I* species to reach a relatively stable state (e.g., the main fluorescent state) after conversion from A*. Our findings disentangle the contributions of various vibrational motions active during the ESPT reaction, and offer new structural dynamics insights into the fluorescence mechanisms of engineered GFPs and other analogous autofluorescent proteins.

Photophysical properties for excited-state intramolecular proton transfer (ESIPT) reaction of N-salicylidene-o-aminophenol: Experimental and DFT based approaches

Photophysical properties for excited-state intramolecular proton transfer (ESIPT) reaction of N-salicylidene-o-aminophenol (SA) Schiff base were comprehensively studied based on experimental methods combined with theoretical calculations. The results revealed that the SA was mainly presented in enol form in acidic solutions while it was predominantly existed in keto form in basic solutions. From UV-vis absorption and fluorescence emission studies, it showed that the ESIPT could effectively take place in non-polar and aprotic polar solvents. By using the CAM-B3LYP/6-311G(d,p) level of theory, it was found that the intramolecular proton transfer could preferably occur through six-membered ring transition rather than through five-membered ring transition. The dynamics of the ESIPT reactions of enol and keto tautomers were studied using TD-CAM-B3LYP with 6-311G(d,p) basis set. The potential energy curves for the intramolecular proton transfer in the ground (GSIPT) and excited state (ESIPT) exhibited that the GSIPT could occur through a low activation barrier, whereas in the case of ESIPT, the process could arise via low energy barrier.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy.

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Excited-State Proton Transfer from the Photoacid 2-Naphthol-8-sulfonate to Acetonitrile/Water Mixtures.

Steady-state and time-resolved fluorescence techniques were used to study excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to χwater ≤ 0.12, we found a slow ESPT process on the order of nanoseconds. At χwater ≈ 0.15, the RO- fluorescence band intensity is at the minimum, whereas at χwater ≈ 0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO- fluorescence band at χwater ≈ 0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH3CN/H2O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S, and this enables the ESPT process between the two sites of the molecular structure of 2N8S. In mixtures of χwater ≥ 0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the χwater in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small, 4.5 × 109 s-1. TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8-sulfonate in the excited state. The activation energy for the ESPT reaction is about 9 kcal/mol, and the RO-(S1) species is energetically above the ROH(S1) species by about 1.6 kcal/mol.

Amino proton donors in excited-state intramolecular proton-transfer reactions

Proton transfer involving site-specific hydrogen-bonding interactions is one of the most fundamental and important reactions in chemistry and biology. Deliberately triggering this reaction by photoexcitation enables unique and insightful mechanistic analyses. This Review describes a particularly effective method that involves exciting a photoacid containing both an amine and a basic residue and monitoring the ensuing excited-state intramolecular proton-transfer (ESIPT) reactions. Replacing a H atom on the amine with another substituent R modulates the acidity of the amine and allows for the excited-state hydrogen-bond strength to be tuned over a very broad range. In this way, one can draw empirical correlations between N−H bond distances, acidity, hydrogen-bond strength and the ESIPT kinetics and thermodynamics. For example, stronger intramolecular N−H···N hydrogen bonding leads to faster and more exergonic ESIPT. Tuning the amine and basic residues allows one to switch the ESIPT mechanism between the kinetic and thermodynamic regimes, such that molecules can generate ratiometric emission, which is suitable for white-light generation and two-colour imaging. The identity of the amine substituent R not only affects the acidity but can be differentially sensitive towards the local chemical environment. Thus, the R group transduces environmental changes into modified ESIPT rates and/or mechanisms. Such studies open new frontiers in the fundamental aspects of proton transfer in amines, as well as their largely unexplored potential applications. This Review describes excited-state intramolecular proton-transfer (ESIPT) reactions with amines as proton donors. Systematic variation of N−H bond strength, acidity and reaction rate enables ESIPT kinetics and thermodynamics to be correlated and new molecules to be designed for sensing and optoelectronics applications.

A theoretical study on excited state proton transfer in 2‐(2′‐dihydroxyphenyl) benzoxazole

AbstractThis research investigates the dynamic excited state process for a novel system 2‐(2′‐dihydroxyphenyl) benzoxazole (DHBO) for excited state proton transfer (ESPT) process based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. Because 2 intramolecular hydrogen bonds (O1―H2···N3 and O4―H5···O6) in DHBO molecules may trigger proton transfer process in the S1 state, we focus on these 2 hydrogen bonds. Our results show that only the O1―H2···N3 bond has obvious changes in both bond length and bond angle upon photoexcitation. Charge redistribution also confirms that hydrogen bond wire (O1―H2···N3) is the best way to achieve the ESPT process in the S1 state. Considering the ESPT mechanism, our theoretical potential energy curves of DHBO indicate that only the excited state single‐proton transfer process occurs via O1―H2···N3 rather than O4―H5···O6. We believe that our work not only clarifies the excited state dynamical behavior of DHBO but also promotes the investigations about ESPT reactions in intramolecular or intermolecular hydrogen bonded chemical systems.

A theoretical prediction about harnessing ESPT process for HBO derivatives

The different excited-state behaviors involved in excited-state proton transfer (ESPT) process of a series of 2-(2-hydroxyphenyl)benzoxazole (HBO) derivatives have been theoretically investigated. The primary bond lengths and bond angles were analyzed. Coupling with the infrared (IR) vibrational spectra, we confirmed that the intramolecular hydrogen bond O–H···N should be strengthened in the S1 state, which might provide the possibility for ESPT reaction, whereas introducing the fused rings may weaken the hydrogen bond in excited state. By investigating the vertical excitation process, the charge redistribution was explored. It is found that the electron-accepting –NO2 and –COOH would facilitate the ESPT reaction. With adding fused rings to HBO, less charge transfer exists in the transition process, which can reasonably explain the weakening hydrogen bond phenomenon in excited states. Via constructing the potential energy curves of both S0 and S1 states, we further confirm that electron-accepting substitutions could promote the ESPT process for HBO systems. And fused rings do inhibit ESPT reaction to a great extent. We believe this work not only elaborates the different excited-state proton transfer behaviors for a series of HBO derivatives but also presents a new harnessing ESPT process through substitutional effects.

Unraveling the Detailed Mechanism of Excited-State Proton Transfer.

As one of the most fundamental processes, excited-state proton transfer (ESPT) plays a major role in both chemical and biological systems. In the past several decades, experimental and theoretical studies on ESPT systems have attracted considerable attention because of their tremendous potential in fluorescent probes, biological imaging, white-light-emitting materials, and organic optoelectronic materials. ESPT is related to fluorescence properties and usually occurs on an ultrafast time scale at or below 100 fs. Consequently, steady-state and femtosecond time-resolved absorption, fluorescence, and vibrational spectra have been used to explore the mechanism of ESPT. However, based on previous experimental studies, direct information, such as transition state geometries, energy barrier, and potential energy surface (PES) of the ESPT reaction, is difficult to obtain. These data are important for unravelling the detailed mechanism of ESPT reaction and can be obtained from state-of-the-art ab initio excited-state calculations. In recent years, an increasing number of experimental and theoretical studies on the detailed mechanism of ESPT systems have led to tremendous progress. This Account presents the recent advances in theoretical studies, mainly those from our group. We focus on the cases where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of excited-state double proton transfer (ESDPT), revealing the sensing mechanism of ESPT chemosensors, illustrating the effect of intermolecular hydrogen bonding on the excited-state intramolecular proton transfer (ESIPT) reaction, investigating the fluorescence quenching mechanism of ESPT systems by twisting process, and determining the size of the solute·(solvent) n cluster for the solvent-assisted ESPT reaction. Through calculation of vertical excitation energies, optimization of excited-state geometries, and construction of PES of the ESPT reactions, we provide modifications to experimentally proposed mechanisms or completely new mechanism. Our proposed new and inspirational mechanisms based on theoretical studies can successfully explain the previous experimental results; some of the mechanisms have been further confirmed by experimental studies and provided guidance for researchers to design new ESPT chemosensors. Determination of the energy barrier from an accurate PES is the key to explore the ESPT mechanism with theoretical methods. This approach becomes complicated when the charge transfer state is involved for time-dependent density functional theory (TDDFT) method and optimally tuned range-separated TDDFT provides an alternative way. To unveil the driving force of ESPT reaction, the excited-state molecular dynamics combined with the intrinsic reaction coordinate calculations can be employed. These advanced approaches should be used for further studies on ESPT systems.

Effect of Nonplanarity on Excited-State Proton Transfer and Internal Conversion in Salicylideneaniline.

Salicylideneaniline (SA) is a prototype for excited-state intramolecular proton transfer (ESIPT) reactions in nonplanar molecules. It is generally understood that the dominant photochemical pathway in this molecule is ESIPT followed by nonradiative decay due to twisting about its phenolic bond. However, the presence of a secondary internal conversion pathway resulting from frustrated proton transfer remains a matter of contention. We perform a detailed nonadiabatic dynamics simulation of SA and definitively identify the existence of both reaction pathways, thereby showing the presence of a secondary photochemical pathway and providing insight into the nature of ESIPT dynamics in molecules with nonplanar ground-state geometries.

Modulation of the Excited-State Proton Transfer Rate of d-luciferin in Mixed Reverse Micellar Systems.

The excited-state proton transfer (ESPT) rate of photo-acids in a confined medium depends on several physical parameters of the immediate environment. We introduce a new parameter in the form of charge type at the interface of reverse micellar (RM) systems to modulate the ESPT rate. We investigate the ESPT reaction of d-luciferin in mixed RM systems composed of nonionic polyoxyethylene(5)nonylphenylether (Igepal CO-520) with cationic didodecyldimethylammonium bromide (DDAB) and anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in cyclohexane (Cy) at different mole fractions of Ig (XIg) and fixed hydration. ESPT is feeble in AOT RM, whereas it is favorable in the other two RMs. Addition of Ig is observed to facilitate ESPT in AOT RM linearly, whereas in DDAB, it shows a synergistic effect. The various physical parameters of water in the mixed RM water pool have been investigated using dynamic light scattering, Fourier transform infrared, and time-resolved fluorescence spectroscopy measurements to underline the ESPT mechanism in these mixed RMs.

Zwitterions of the excited 4-([2,2′-bipyridine]-4-yl) phenol photoacid molecules: Formation and fluorescence

Based on the density functional theory and time-dependent density functional theory, we have investigated the zwitterionic structure of the excited 4-([2,2′-bipyridine]-4-yl) phenol (bpy-phenol) photoacid molecules. The zwitterion can be formed stepwisely via a proton-coupled electron transfer (PCET) reaction in a H-bonded bpy-phenol…F− complex and an intermolecular excited state proton transfer (ESPT) reaction between HF molecules and excited deprotonated anions of bpy-phenol. Supported by the results of Hirshfeld population analysis and electrostatic potential, a basic site was generated in the bpy residue due to the PCET process. With a proper F− ions concentration, the ESPT reaction can be occurred with little barrier between the HF molecules and the bpy-phenol anions, leading to the generation of zwitterions. Moreover, the zwitterion fluoresces at 550 nm, which is longer than that of bpy-phenol anions at 490 nm. Our finding not only discovers the zwitterionic process of photoacid molecules, but also pioneers a frontier field of activity study after PCET reactions.

The order of multiple excited state proton transfer in ternary complex of norharmane and acetic acids

Dolores Reyman et al. found the norharmane (9H-pyrido [3,4-b] indole) (NHM) and two acetic acid molecules can form the ternary complex (NHM-2A) in component solvent of dichloromethane and acetic acid via the hydrogen bond chain (J. Lumin. 2014, 148, 64). But the specific reaction details during this process were rarely reported. In this study, we will give an insight into the reasons which promote the occurrence of this reaction as well as its reaction order. The hydrogen bond enhancing behavior in first excited state (S1) is verified through the analysis of geometric configurations, infrared spectra, frontier molecular orbitals and potential energy curves. The absorption and fluorescence spectra we calculated are well coincident with the experimental results. Meanwhile, it is obvious that the hydrogen bond intensity is gradually enhanced from N1H2⋯O3, O4H5⋯O6 to O7H8⋯N9 by analyzing the reduced density gradient (RDG) isosurface. The hydrogen bond strengthening mechanism has been confirmed in which the hydrogen bond interaction acts as driving force for excited state proton transfer (ESPT) reaction. In order to provide a reliable description of the reaction energy profiles, we compare the barrier differences obtained by m062x and B3LYP methods. We might safely draw the conclusion that the multiple ESPT is a gradual process initiated by the proton transfer of O7H8⋯N9. And we further proof the ESPT process can be completed via the NHM-2A → NHM-2AS → NHM-2AD → NHM-2AT in S1 state. Theoretical research of NHM-2A has been carried out by density functional theory (DFT) and time-dependent density functional theory (TDDFT). It is worth noting that we predicted that the fluorescence at 400 nm observed in experiment is more likely to be emitted by NHM-2AS in S1 state.

Stimuli-responsive luminescent coumarin thiazole hybrid dye: Mechanism of excited-state intramolecular double proton transfer

Recently, a novel coumarin thiazole hybrid dye with azomethine linkage is synthesized. It is revealed that the symmetrical structures 3,3'-(5,5'-((1E,1'E)-hydrazine-1,2-diylidenebis(methanylylidene))bis(2-morpholinothiazole-5,4-diyl))bis(4-hydroxy-2H-chromen-2-one) (abbreviated as CS) takes shape two intramolecular hydrogen bonds and that excited-state intramolecular proton transfer (ESIPT) takes place with thiazole nitrogen atoms [Dyes Pigments. 142, 2017, 121]. However the corresponding reaction mechanisms are lacking. This study uses density functional and time-dependent density functional theory to calculate hydrogen-bond lengths, hydrogen-bond angles, infrared vibrational spectra and Mulliken charges of relevant groups, which confirm the excited-state hydrogen-bond strengthening mechanism. Enhanced hydrogen-bond strength facilitates the ESIPT process of the CS molecule. The absorption and fluorescence emission spectra of CS in dimethyl formamide (DMF) are calculated, which are consistent with the observed results in experiment. To further illustrate mechanism of excited-state double proton transfer (ESDPT) reactions, the potential energy surfaces of CS are constructed and analyzed, which demonstrate ESDPT more likely occur stepwise rather than concertedly. The frontier molecular orbitals of CS system are also calculated and display the nature of the charge redistribution, which demonstrate that CS system exhibits light fastness because ESIPT process inhibits intramolecular charge transfer.

Cationic Effect on the Equilibria and Kinetics of the Excited-State Proton Transfer Reaction of a Photoacid in Aqueous Solutions.

Dissolved ions have a significant effect on the chemical equilibria and kinetics in aqueous solutions by changing the physical properties and hydrogen bond network of water. In this work, the ionic effects on the excited-state proton transfer (ESPT) reactions of Coumarin 183 (C183) in aqueous ionic solutions are comprehensively studied in terms of p Ka, p Ka*, activation energies, and kinetic isotope effect (KIE). The acid dissociation constants (p Ka and p Ka*) of C183 on the ground and excited states are determined by UV-visible absorption and steady-state fluorescence spectroscopy. The activation energies ( Ea) and KIE for the ESPT reaction of C183 are directly obtained by time-resolved fluorescence spectroscopy. The changes in p Ka, p Ka*, Ea, and KIE values of C183 are found to be dependent on the charge density of cations. The secondary KIE is more substantially influenced by the dissolved ions than the primary KIE. Furthermore, the ionic effects on the equilibrium (p Ka and p Ka*) and kinetic ( Ea and KIE) parameters of C183 are found to be well-explained by the free-energy reactivity relation. Our current results are very important in understanding the ionic effects on the equilibria and ESPT kinetics of photoacids in aqueous ionic solutions.

Theoretical investigations on the excited state hydrogen bonding dynamics and proton transfer behavior for novel DHDA-23-00 system

In this work, using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we theoretically explore the excited state dynamical process about a novel DHDA-23-00 system [Phys. Chem. Chem. Phys. 19 (2017) 28641-28646]. Combining electrostatic potential surface (EPS) and the reduced density gradient (RDG) methods, we can affirm the two intramolecular hydrogen bonds (O1-H2⋯O3 and O4-H5⋯O6) should be formed in the S0 state for DHDA-23-00. Exploring primary chemical structural changes (bond lengths and bond angles involved in hydrogen bonds) upon the photo-excitation, we verify these two hydrogen bonds are strengthening in the S1 state. In view of the infrared (IR) vibrational spectra, the phenomenon of strengthening O1-H2⋯O3 and O4-H5⋯O6 can be confirmed once again. Analyses about frontier molecular orbitals (MOs) in the excitation process, it can be found charge redistribution around hydroxide radical moiety could play important roles in enhancing intramolecular hydrogen bonds O1-H2⋯O3 and O4-H5⋯O6. Given this kind of charge transfer may facilitate excited state proton transfer (ESPT) process, we construct the potential energy surfaces (PESs) along with two hydrogen bonds to provide the excited state dynamical overall perspective. For DHDA-23-00, even though the charge transfer supports the ESPT process in acetonitrile solvent, excited state potential energy barriers confirm ESPT reaction does not occur for DHDA-23-00 system that is consistent with experimental phenomenon.

Excited-State Proton Transfer Mechanism of 2,6-Diazaindoles·(H2O) n ( n = 2-4) Clusters.

This paper identified a new excited-state proton transfer (ESPT) mechanism for 2,6-diazaindoles (2,6-DAI) in aqueous (H2O) solution based on time-dependent density functional theory. The calculated results show that the excited-state three proton transfer reaction cannot occur because the 2,6-DAI with two water molecules do not form hydrogen bond wires; this finding was different from those reported in previous experiments (Chung et al. J. Am. Chem. Soc. 2017, 139, 6396-6402). 2,6-DAI with three water molecules form 2,6-DAI·(H2O)3 clusters, whereas 2,6-DAI with four water molecules form 2,6-DAI·(H2O)4 cluster. These clusters participate in the ESPT reaction. To determine the ESPT mechanism of 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters, we constructed the potential energy curves of S1 and S0 states. The results confirmed the simultaneous presence of both 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters and only one proton transfer pathway. By calculating the transition states of 2,6-DAI·(H2O)3 and 2,6-DAI·(H2O)4 clusters, we found that the ESPT reaction is a consistent mechanism. Our work investigated the number of water molecules involved in the ESPT and paved the way to further study the intermolecular hydrogen bonding interactions in the biological field.

Exploring Nuclear Photorelaxation of Pyranine in Aqueous Solution: an Integrated Ab-Initio Molecular Dynamics and Time Resolved Vibrational Analysis Approach.

Advances in time-resolved vibrational spectroscopy techniques provided a new stimulus for understanding the transient molecular dynamics triggered by the electronic excitation. The detailed interpretation of such time-dependent spectroscopic signals is a challenging task from both experimental and theoretical points of view. We simulated and analyzed the transient photorelaxation of the pyranine photoacid in aqueous solution, with special focus on structural parameters and low frequency skeleton modes that are possibly preparatory for the photoreaction occurring at later time, as suggested by experimental spectroscopic studies. To this aim, we adopted an accurate computational protocol that combines excited state ab initio molecular dynamics within an hybrid quantum mechanical/molecular mechanics framework and a time-resolved vibrational analysis based on the Wavelet transform. According to our results, the main nuclear relaxation on the excited potential energy surface is completed in about 500 fs, in agreement with experimental data. The rearrangement of C-C bonds occurs according to a complex vibrational dynamics, showing oscillatory patterns that are out of phase and modulated by modes below 200 cm-1. We also analyzed in both the ground and the excited state the evolution of some structural parameters involved in excited state proton transfer reaction, namely, those involving the pyranine and the water molecule hydrogen bonded to the phenolic O-H group. Both the hydrogen bond distance and the intermolecular orientation are optimized in the excited state, resulting in a tighter proton donor-acceptor couple. Indeed, we found evidence that collective low frequency skeleton modes, such as the out of plane wagging at 108 cm-1 and the deformation at 280 cm-1, are photoactivated by the ultrafast part of the relaxation and modulate the pyranine-water molecule rearrangement, favoring the preparatory step for the photoreactivity.

Excited-State Proton Transfer in 3-Cyano-7-azaindole: From Aqueous Solution to Ice.

We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (∼472 nm) emission bands. Detailed temperature-dependent studies showed that the values of activation free energy (Δ G‡) were similar between N-H and N-D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium ( Keq) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (∼450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water-ice interface property.

Theoretical Investigation of the Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Cresol Derivatives.

The novel photodeamination process of cresol derivatives 1 and 3 has been reported experimentally ( J. Org. Chem . 2015 , 80 , 10817 ). However, a full theoretical interpretation of the mechanism is still lacking. In the present study, we aim to provide insight into the factors that promote the deamination reaction through density functional theory (DFT) and time-dependent DFT methods. Calculated absorption and emission spectra are in good agreement with the experimental results. Hydrogen-bond strengthening in the excited state has been verified by analyzing relevant bond parameters and vibrational frequencies as well as frontier molecular orbitals (FMOs), implying that hydrogen-bond interaction acts as the important parameter for the excited-state intramolecular proton-transfer (ESIPT) reaction. The proton-transfer and deamination reactions have been qualitatively analyzed through Gibbs free-energy reaction profiles in different electronic states. It can be concluded that the ESIPT and photodeamination reactions occur in the excited state. To further illustrate the photodeamination mechanism, the constructed 2D potential-energy surface indicates that the photodeamination reaction is infeasible without the ESIPT reaction. This work provides the first theoretical rationale for ESIPT-induced photodeamination occurring spontaneously because of protonation of a basic nitrogen atom.

A Red-Emitting, Multidimensional Sensor for the Simultaneous Cellular Imaging of Biothiols and Phosphate Ions

The development of new fluorescent probes for cellular imaging is currently a very active field because of the large potential in understanding cell physiology, especially targeting anomalous behaviours due to disease. In particular, red-emitting dyes are keenly sought, as the light in this spectral region presents lower interferences and a deeper depth of penetration in tissues. In this work, we have synthesized a red-emitting, dual probe for the multiplexed intracellular detection of biothiols and phosphate ions. We have prepared a fluorogenic construct involving a silicon-substituted fluorescein for red emission. The fluorogenic reaction is selectively started by the presence of biothiols. In addition, the released fluorescent moiety undergoes an excited-state proton transfer reaction promoted by the presence of phosphate ions, which modulates its fluorescence lifetime, τ, with the total phosphate concentration. Therefore, in a multidimensional approach, the intracellular levels of biothiols and phosphate can be detected simultaneously using a single fluorophore and with spectral clearing of cell autofluorescence interferences. We have applied this concept to different cell lines, including photoreceptor cells, whose levels of biothiols are importantly altered by light irradiation and other oxidants.