Photoinduced Proton Transfer Research Articles

Catalyst-free and wavelength-tuned glycosylation based on excited-state intramolecular proton transfer

The chemoselectivity of organic reactions is a fundamental topic in organic chemistry. In the long history of chemical synthesis, achieving chemoselectivity is mainly limited to thermodynamic conditions by an exogenous activation strategy. Here, we design an endogenous activation method, which can be used to control the chemoselectivity of phenol and naphthol through the photo-induced excited-state intramolecular proton transfer (ESIPT). A wavelength-tuned glycosylation is developed to showcase the penitential of this new strategy. Traditionally, an exogenous activator (electrophilic promoters) is essential to induce the cleave of a polar single bond, and this strategy has been extensively studied and used in the glycosylation chemistry, for the formation of oxocarbenium cation intermediate. In our systems, the oxocarbenium cation intermediates can be selectively formed from glycosyl donors bearing tunable chromophoric groups under mild conditions of acid-base free and redox neutrality, which enables continuous synthesis of oligosaccharides.

Solvent polarity dependent photo-induced excited state intramolecular proton transfer for triphenylamine centered bis unsymmetrical azine (TPOV): A computational study

The exquisite, regulated characteristics of triphenylamine (TPA) derivatives have served as a profound inspiration in the photochemical field. Herein, the novel TPA-centered bis unsymmetrical azine (i.e., 3-methoxysalicylaldehyde substituted 4-((E)-hydrazonomethyl)-N-(4-((E)-hydrazonomethyl)phenyl)-N-phenylaniline (TPOV)) is investigated regarding its photo-induced behaviors and excited state intramolecular proton transfer (ESIPT) mechanisms. Solvent polar-dependent hydrogen bonding interactions and charge recombination, which is induced by photoexcitation, can greatly promote the ESIPT reactions of the TPOV chemosensor. In order to unveil the detailed excited state reaction mechanism, we theoretically construct the S0-state and S1-state of potential energy surfaces (PESs). Based on the S1-state PESs for TPOV in toluene, chloroform, and acetonitrile solvents, we unveil the single ESIPT reaction of TPOV that happens along with alternative dual intramolecular hydrogen bonds (O1-H2···N3 and O4-H5···N6). The solvent polarity-regulated excited state might clear the path for the creation of innovative luminescent materials.

High-performance color and fluorescence switching of cotton fabrics via light-induced proton transfer strategy of spiropyran sulfonate molecule

Light-responsive color-switching materials have garnered persistent interest recently due to their passive response to external stimuli and manifestation of notable visual color variations. Nonetheless, their development into high-end products has been hindered by unsatisfactory overall performance, characterized by challenges such as the inability to combine color contrast and retention time, poor repeatability, and reliance on a single coloring mode. Herein, we present a negative photochromic cotton fabric (NPCF) capable of real-time dual output of optical signals (color change or fluorescence emission), featuring high color contrast and resolution, excellent reversibility, and facile multicolor printing. These attributes stem from the utilization of a unique photoinduced proton transfer (PPT) strategy exhibited by the designed and synthesized spiropyran sulfonate molecule (MC-NO2). Guided by this strategy, NPCF demonstrates significant color changes and fluorescence switching in a real-time and highly reversible manner, leveraging the remote non-contact and non-destructive nature of visible light. Moreover, direct blending of MC-NO2 with commercially available water-soluble non-stimuli-responsive dyes enables customization of various photosensitive color systems for straightforward multi-color printing, aligning well with the environmental principles of green printing. This study serves as a valuable reference for the advancement and design of diverse high-performance, switchable materials suitable for the production of high-end products.

Tautomeric enhancement of 2-(1H-pyrazol-5-yl)pyridine in photoinduced proton transfer by water-assisted molecules

Theoretical insight of excited state proton transfer (ESPT) in 2-(1H-pyrazol-5-yl)pyridine abbreviated as PPP interacting with water wires: PPP(H2O)n where n = 13 has been presented in both static and dynamics studies. Explicit water molecules placed around PPP have been simulated to elucidate the intermolecular hydrogen bonding interactions between them. Hydrogen bond strengthening in the excited state (S1) has been verified by shorter bond distances and redshift of IR vibrational spectra involving the proton transfer (PT) process. Furthermore, on-the-fly excited state dynamics simulations of all complexes have been performed to provide detailed information on the PT mechanism. The dynamic results show that one water molecule added to the neighboring PPP as an intermolecular hydrogen bonding bridge can promote double excited state intermolecular proton transfer up to 36% compared with its intrinsic intramolecular hydrogen bond of the PPP system. Meanwhile adding a second or third water molecule could decrease the probabilities of PT. Especially in PPP(H2O)3, a rearrangement of water molecules is observed that PT occurs via one water molecule. Hence, the models with explicit water molecules interacting with PPP as intermolecular hydrogen bonding bridge are a great representative role played by water molecules of the PT process at the molecular level.

Switching of photoinduced proton transfer from one six-membered hydrogen-bonded ring to other: a molecule of hydrazine and pH sensor.

The present study describes photophysical properties of 3-(benzo[d]oxazol-2-yl)-5-bromo-2-hydroxybenzaldehyde (HBOB) and (E)-2-(benzo[d]oxazol-2-yl)-4-bromo-6-(hydrazonomethyl)phenol (HBON) molecules with asymmetric two-way proton transfer sites. The purpose of this study is to know the direction of ESIPT out of the two-way proton transfer pathways in these molecules in both the solid and solution state. The steady state and time-resolved spectral behaviour of HBOB and HBON and a comparison of the spectral features with the two distinct control compounds 2-(benzo[d]oxazol-2-yl)-4-bromophenol (HBO) and (E)-4-bromo-2-(hydrazonomethyl)phenol (HBN) having single 6-membered hydrogen bonded network reveal that HBOB undergoes imine-amine photoisomerisation by proton transfer towards the oxazole side and HBON undergoes towards the hydrazone side with characteristic Stokes' shifted emission. Proton transfer forms with the red shifted emission of these molecules shows fast decay than the locally excited state. In the solid state, extremely high fluorescence intensity was observed, following a similar type of ESIPT pattern. Calculated ground (S0) and excited state (S1) energy barriers for the PT process obtained using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) corroborate the unidirectional excited state intramolecular proton transfer (ESIPT) process for HBOB and HBON, and the theoretical spectral features validate our experimental absorption and emission spectra well. Interestingly, the unique unidirectional ESIPT behaviour of HBOB was utilised to detect hydrazine both in solution and solid phases. On the other hand, HBON was found to be a good fluorescence pH sensor with a ratiometric color change from yellow to green in acidic and basic media.

Excited-State Proton Transfer in a Photoacid-Based Crystalline Coordination Compound: Reversible Photochromism, Near-Infrared Photothermal Conversion, and Conductivity.

By harnessing the power of coordination self-assembly, crystalline materials can act as carriers for photoacids. Unlike their solution-based counterparts, these photoacids are capable of altering the properties of the crystalline material under light and can even generate proton transfer in a solid-state environment. Due to the photoinduced proton transfer and charge transfer processes within this functional material, this crystal exhibits powerful absorption spanning the visible to near-infrared spectrum upon light irradiation. This feature enables reproducible, significant chromatic variation, near-infrared photothermal conversion, and photocontrollable conductivity for this photoresponsive material. The findings suggest that the synthesis of pyranine photoacid-based crystalline materials via coordination self-assembly can not only enhance light-harvesting efficiency but also enable excited-state proton transfer processes within solid crystalline materials, thereby maintaining and even improving the properties of photoacids.

Solvent-regulated fluorimetric differentiation of Al3+ and Zn2+ using a dual functional sensor based on the photo-induced electron transfer and intramolecular proton transfer mechanism

Solvent-regulated fluorimetric differentiation of Al3+ and Zn2+ using a dual functional sensor based on the photo-induced electron transfer and intramolecular proton transfer mechanism

Computational insights of excited state intramolecular proton transfer (ESIPT) based fluorescent detection and imaging of γ-glutamytranspeptidase activity

γ-Glutamytranspeptidase (GGT) is an important tumor biomarker that widely appears in the tumor cells. Therefore, accurate imaging and detection of GGT activity in live cells, serum and pathological cells grasp great importance for the diagnosis, management, and treatment of cancer. Herein, 2-(2-hydroxyl-phenyl)-6-chloro-4-(3H)-quinazolinone (HPQ) is considered as the fluorophore probe for the detection of GGT activity, which is known for the typical mechanism of excited-state intramolecular proton transfer (ESIPT). All the simulations adopted to evaluate the sensing mechanism were carried out via DFT and TDDFT calculations at CAM-B3LYP/TZVP level of theory. The emission properties of HPQ and HPQ-TD are thoroughly studied to understand the photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) process. The results reveal that the fluorescence quenching of HPQ (enol form) is assigned to the PET process, whereas the large Stokes shift in fluorescence emission of HPQ (keto form) is related with ESIPT mechanism. The obtained results are further cross validated by frontier molecular orbital (FMO) analysis, geometric analysis, and potential energy curve (PEC) scanning. Our calculations provide powerful evidence for the ESIPT based sensing mechanism of HPQ (keto-enol form) for GGT activity.

Ring Polymer Molecular Dynamics with Electronic Transitions.

Full quantum dynamics of molecules and materials is of fundamental importance, which requires a faithful description of simultaneous quantum motions of the electron and nuclei. A new scheme is developed for nonadiabatic simulations of coupled electron-nuclear quantum dynamics with electronic transitions based on the Ehrenfest theorem and ring polymer molecular dynamics. Built upon the isomorphic ring polymer Hamiltonian, time-dependent multistate electronic Schrödinger equations are solved self-consistently with approximate equation of motions for nuclei. Each bead bears a distinct electronic configuration and thus moves on a specific effective potential. This independent-bead approach provides an accurate description of the real-time electronic population and quantum nuclear trajectory, maintaining a good agreement with the exact quantum solution. Implementation of first-principles calculations enables us to simulate photoinduced proton transfer in H_{2}O-H_{2}O^{+} where we find a good agreement with experiment.

A Novel Pyridine and Julolidine Based Chemosensor for Al3+ Detection

AbstractIn this work, a pyridine and julolidine based chemosensor (L) for Al3+ with a turn‐on response in ethanol solution was designed and synthesized, whose emission intensity was enhanced by 85‐fold in the presence of Al3+ by emitting bright green fluorescence at 480 nm. Meanwhile, the recognition process was hardly affected by other common cations. Based on the fluorescence titration data, the limit of detection (LOD) for Al3+ was estimated to be as low as 2.74×10−8 M. Job's plot ascertained the coordinate stoichiometric of L to Al3+ as 1 : 1, with the association constant (Ka) of 7.24×103 M−1 depending on the Benesi‐Hildebrand equation. The combined effects of photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) were proposed as the plausible coordination mechanism of L toward Al3+. Moreover, the application ability of L for determination of Al3+ was evaluated in water samples and with test strips.

Features of photoinduced proton transfer in the presence of a polyelectrolyte

Using the example of a series of sulfo derivatives of 2-naphthol, it was shown that the charge field formed by the polyelectrolyte coil significantly changes the constants k1 and k−1 of the photoinduced proton transfer reaction, but no noticeable shift in the equilibrium constant K * was found. This observation is fundamentally different from the behavior of these substances in micellar media, where K * increases by an order of magnitude. The binding constants of the dyes with the cationic polyelectrolyte were also determined.

A Study on Mechanisms Underlying Proton Transport in Proton Pump-type Microbial Rhodopsins

Microbial rhodopsins are photoreceptive membrane proteins composed of seven transmembrane α-helical apoproteins (opsin) and a covalently bound retinal chromophore. Microbial rhodopsins exhibit a cyclic photochemical reaction referred to as photocycle when illuminated. During their photocycles, these proteins perform various functions such as ions transport and photosensing. Among the various functional types of rhodopsins found to date, we have focused on the utility of proton pump-type microbial rhodopsins as optogenetic tools for optical pH control in cells or organelles. To develop effective toolkits for this purpose, a deeper understanding of the proton-pumping mechanism in these rhodopsins may be required. In this review, we first introduce a useful experimental method for measuring rapid transient pH changes with photoinduced proton uptake/release using transparent tin oxide (SnO2) or indium-tin oxide (ITO) electrodes. In addition, we describe the unique pH-dependent behavior of the photoinduced proton transfer sequence as well as the vectoriality of proton transportation in proteorhodopsin (PR) from marine eubacteria. Through intensive ITO experiments over wide pH range, including extremely high or low pH values, in combination with photoelectric measurements using Xenopus oocytes or a thin polymer film "Lumirror," we encountered several interesting observations on photoinduced proton transfer in PR:1) proton uptake/release sequence reversal and potential proton translocation direction reversal under alkali conditions, and 2) fast proton release from D227, a secondary counterion of the protonated retinal Schiff base at acidic pH values.

A fluorescent pH switch probe for the ‘turn-on’ dual-channel discriminative detection of magnesium and zinc ions

A coumarin-based fluorescent probe is presented for the multi-channel reversible “ Turn-on ” detection of Mg 2+ ion (at 483 nm) and Zn 2+ ion (at 560 nm) as well as a base (450 nm) at different wavelengths with large Stokes shifts. • A coumarin-based probe HL is utilized for the dual channel ‘turn-on’ detection of Mg 2+ and Zn 2+ ions. • HL detects Mg 2+ and Zn 2+ ions at two different wavelengths with large Stokes shifts. • HL detects both metal ions by the inhibition of C=N bond rotation, PET and ESIPT while promoting CHEF. • Probe HL also functioned as a reversible fluorescent pH switch. • HL was utilized for the detection of Mg 2+ and Zn 2+ ions in human breast cancer cells. • Low-cost detection methods, paper-strips and polymer films, were used for the sensing of Mg 2+ and Zn 2+ ions. A coumarin-based fluorescent probe, HL (( E )-8-((benzo[d]thiazol-2-ylimino)methyl)-7-hydroxy-4-methyl-2H-chromen-2-one), is presented for the dual-channel “ Turn-on ” detection of Mg 2+ (at 483 nm) and Zn 2+ (at 560 nm) ions at two different wavelengths with large Stokes shifts. HL exhibits impressive detection limits and binding constants with both Mg 2+ and Zn 2+ ions. HL detected these ions by the inhibition of -C=N bond rotation and suppression of photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) while promoting chelation enhanced fluorescence (CHEF). For the detection of Mg 2+ and Zn 2+ ions, reversibility was achieved by using ATP. HL is not only sensitive and selective for the detection of Mg 2+ and Zn 2+ ions but also showed considerable reversible “ Turn-on-Turn-off ” fluorescent pH switch behavior. The DFT studies assisted in understanding the binding of Mg 2+ and Zn 2+ ions by the probe. HL was utilized to detect Mg 2+ and Zn 2+ ions in the human breast cancer cell line by fluorescence cell imaging studies while several low-cost detection methods were also illustrated.

Multi-parametric sensing by multi-channel molecular fluorescent probes based on excited state intramolecular proton transfer and charge transfer processes

Considering the applications of fluorescent probes and the information they provide, their brightness of fluorescence and photostability are of paramount importance. However, in the case of steady-state fluorescence spectroscopy and fluorescence microscopy, the amount of information can be increased by the application of multi-channel probes, via a multi-band fluorophore introduced in the probe molecule. In most cases, the use of such a multi-band (or multi-channel) fluorophore can also be combined with the concomitant introduction of one or several analyte receptors. Most often, the design of ratiometric probes with multi-band fluorescence emission are based on phenomena such as photoinduced intramolecular charge transfer (ICT) or excited state intramolecular proton transfer (ESIPT). Although ICT probes were up to recently the most popular, ESIPT probes and among them 3-hydroxyflavone derivatives, were shown to be the most productive. Several general problems were resolved by this family of probes, as for example the measurement of local dielectric constant, local H-bond accepting ability, water local concentration and ATP concentration in small volumes.Incorporation of such multi-channel probes into lipid membranes allowed to measure the different membrane potentials and to detect cell apoptosis. Also, it enabled to recognize and characterize the rafts formation in different lipid bilayers and peculiar features of the charged membrane interface. Such probes are also able to provide a concentration-dependent fluorescence signals upon binding of H+, Mg2+and Ba2+ions, and thus to recognize these different cations.The multi-channel probes are effective tools in the study of interactions of macromolecules such as peptides, proteins and nucleic acids. The most useful feature is that they inform simultaneously about several physical parameters, in this way giving a better insight in the investigated system. Thus, by comparing the reviewed probes with other modern fluorescent approaches, it can be concluded they are more informative and accurate tools.

Caught in the act: real-time observation of the solvent response that promotes excited-state proton transfer in pyranine.

Photo-induced excited-state proton transfer (ESPT) reactions are of central importance in many biological and chemical processes. Identifying mechanistic details of the solvent reorganizations that facilitate proton transfer however, is challenging for current experimental and theoretical approaches. Using optical pump THz probe (OPTP) spectroscopy and molecular dynamics simulations, we were able to elucidate the ultrafast changes in the solvation environment for three derivatives of pyranine: the photoacid HPTS, the methoxy derivative MPTS, and the photobase OPTS. Experimentally, we find damped oscillations in the THz signal at short times and our simulations enable their assignment to vibrational energy transfer beatings between the photoexcited chromophore and nearby solvent molecules. The simulations of HPTS reveal strikingly efficient sub-ps energy transfer into a particular solvent mode, that is active near 4 THz, and which can provide the requisite energy required for solvent reorganization promoting proton transfer. Similar oscillations are present in the THz signal for all three derivatives, however the signal is damped rapidly for HPTS (within 0.4 ps) and more slowly for MPTS (within 1.4 ps) and OPTS (within 2.0 ps). For HPTS, we also characterize an additional phonon-like propagation of the proton into the bulk with a 140 ps period and an 83 ps damping time. Thermalization of the solvent occurs on a time scale exceeding 120 ps.

A novel 2,6-bis(benzoxazolyl)phenol macrocyclic chemosensor with enhanced fluorophore properties by photoinduced intramolecular proton transfer.

Macrocyclic ligand L, in which a 2,6-bis(2-benzoxazolyl)phenol (bis-HBO) group is incorporated in triethylenetetramine, was designed and synthesized with the aim of creating a chemosensor with high selectivity and specificity for metal cations in an aqueous environment. The availability of several proton acceptors and donors, and amine and phenol hydroxy groups, respectively, affects the keto-enol equilibrium in both the ground and excited states, and the ligand properties show dependence on the pH of the solution. L is fluorescent in the visible range, through an excited-state intramolecular proton transfer (ESIPT) mechanism. The results of an exhaustive characterization of L by spectroscopic techniques and DFT calculations, as well as of its Zn(II), Cd(II) and Pb(II) complexes, show promising properties of L as a ratiometric metal cation chemosensor, since metal coordination prevents the ESIPT and gives rise to a peculiar displacement of the fluorescence emission from green to blue with Zn(II) and Cd(II), while with Pb(II) the fluorescence is quenched.

Unraveling photo-induced proton transfer mechanism and proposing solvent regulation manner for the two intramolecular proton-transfer-site BH-BA fluorophore

To expound specific excited state processes of the novel excitation wavelength dependent emission BH-BA fluorophore for better subsequent applications, this wok mainly focus on exploring photo-induced hydrogen bonding geometrical changes, excited state intramolecular proton transfer (ESIPT) mechanism and related regulated behavior via solvent polarity. The differences of structural parameters, infrared (IR) vibrational spectra, core-valence bifurcation (CVB) index as well as electronic densities ρ(r) between S0 and S1 states related to dual hydrogen bonds (O1-H2···N3 and O4-H5···N6) reveal S1-state hydrogen bonding strength facilitate ESIPT behaviors for BH-BA system. Of particular note, O4-H5···N6 plays a more dominant role. Photo-induced intramolecular charge transfer (ICT) and variations of Hirshfled and NPA charges over atoms related to hydrogen bonding moieties promote the ESIPT tendency for BH-BA. Combined potential energy surfaces (PESs), transition state (TS) and intrinsic reaction coordinate (IRC) paths, we illustrate the excited state intramolecular single proton transfer (ESISPT) mechanism of BH-BA should occur along with O4-H5···N6 hydrogen bonding wire, which could be adjusted by surrounding solvent polarity.

Dearomatization of Benzenoid Arenes Triggered by Triplet Excited State Intramolecular Proton Transfer.

The detailed mechanism of photoinduced dearomatization of benzenoid arenes is investigated using both the high-level ab initio method and density functional theory. The results suggest that the optically allowed singlet excited state (S2) can quickly decay to the lowest triplet excited state (T1) through a barrierless internal conversion and intersystem crossing. Importantly, we find a triplet excited state intramolecular proton transfer (T-ESIPT) pathway to produce a diradical triplet intermediate (3MO-H), which can trigger the subsequent [4 + 2] dearomatization reaction. Furthermore, the diastereoselectivity of the reaction was illustrated by the rotation of the O-H group of 3MO-H, which could be effectively modulated by the solvent effect (arising from the strength of the intermolecular hydrogen bond) and the substituted effect (arising from the strength of the electron-donation group). This photochemical mechanism can explain well the experimental observations, and the novel T-ESIPT process can open a new door in studying the photoinduced proton transfer reactions.

Can Brønsted Photobases Act as Lewis Photobases?

Dative bonding or Lewis acid-base chemistry underpins a large number of chemical phenomena in a variety of fields, such as catalysis, metal-ligand interactions, and surface chemistry. Developing light-controlled Lewis acid-base interactions could offer a new way of controlling and understanding such phenomena. Photoinduced proton transfer, that is, excited-state Brønsted acidity and basicity, has been extensively studied and applied. Here, in direct analogy to excited-state Brønsted basicity, we show that exciting a photobasic molecule with light generates a thermodynamic drive for the transfer of a Lewis acid from a donor to a photobasic molecule. We have used the archetypal BF3 as our Lewis acid and our photoactive Lewis bases are a family of quinolines, which are known Brønsted photobases as well. We have constructed the experimental Förster cycle for this system and have verified it computationally to demonstrate that a significant drive (0.2-0.7 eV) exists for the transfer of BF3 to a photoexcited quinoline. The magnitude of this drive is similar to those reported for Brønsted photobasicity in quinolines. Computational results from TDDFT and energy decomposition analysis show that the origin of such an effect is similar to the Brønsted photoactivity of these molecules, in that they follow the Hammett parameter of substituent groups. These results suggest that photobases may be capable of controlling the chemical phenomena beyond proton transfer and may open opportunities for a new handle in photocatalysis.

Time-Resolved Infrared Spectroscopy with Multivariate Analysis on Photoinduced Proton Transfer in Aromatic Urea-Acetate Anion Complexes.

1-Anthracen-2-yl-3-phenylurea (2PUA) is an aromatic urea compound, which forms a hydrogen-bonded complex with an acetate anion (AcO-), 2PUA-AcO- complex. We investigated the photoinduced reaction of the 2PUA-AcO- complex in dimethyl sulfoxide (DMSO) by nanosecond time-resolved infrared (TR-IR) spectroscopy. TR-IR spectra obtained after the photoexcitation of 2PUA with the equal concentration of AcO- were consistently explained by a photoinduced proton transfer model. The spectral and temporal profiles of the TR-IR spectra largely depended on concentration conditions of 2PUA and AcO-. Under the condition where excessive amounts of AcO- existed, the TR-IR spectra contained an unexpected signal whose amplitude was related to the concentration of free AcO- in the solution. Using singular value decomposition analysis of the concentration-dependent TR-IR spectra, we extracted the spectral component that reflects the photoinduced reaction of the 2PUA-AcO- complex. The extracted spectrum resembled the TR-IR spectrum obtained under the equal concentration condition, indicating that the same proton transfer occurs during the photoinduced reaction of the 2PUA-AcO- complex irrespective of the concentration conditions. Comparing the steady-state and transient IR spectra of the 2PUA with AcO- in DMSO with density functional theory calculations suggests that both 2PUA-AcO- complex and tautomer species interact with solvent DMSO molecules in their electronic ground states to a large extent.