Excited-state Proton Transfer Research Articles

Deprotonation-induced enhancement in fluorescence of 2-((2-hydroxybenzylidene)amino)phenol, a Schiff base

Excited state dynamics of neutral and anionic forms of 2-((2- hydroxybenzylidene)amino)phenol (HBAP), a Schiff base, has been elucidated using time resolved fluorescence spectroscopy. The neutral molecule exhibits keto-enol tautomerism. So, the fluorescent cis-keto excited state is formed by direct excitation as well as ultrafast excited state proton transfer (ESIPT) from the enolic excited state. It exhibits very feeble fluorescence, associated with a lifetime of tens of picosecond. The anion, however, exhibits significantly stronger fluorescence with lifetime of almost a nanosecond. The fluorescent excited state for the anion is formed by solvent relaxation around it and conformational relaxation of the molecule itself. Ion-dipole interaction in the ground state is manifested in the fluorescence properties. Comparison with very recent publications on other Schiff bases, it is established that enhancement of fluorescence, induced by deprotonation, is a general phenomenon for this class of molecules.

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.

Excited State Proton Transfers in Hybrid Compound Based on Indoline Spiropyran of the Coumarin Type and Azomethinocoumarin in the Presence of Metal Ions.

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li+, Ca2+, Zn2+ and Mg2+ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.

Not All Photoactive Yellow Proteins Are Built Alike: Surprises and Insights into Chromophore Photoisomerization, Protonation, and Thermal Reisomerization of the Photoactive Yellow Protein Isolated from Salinibacter ruber.

We demonstrate that the Halorhodospira halophila (Hhal) photoactive yellow protein (PYP) is not representative of the greater PYP family. The photodynamics of the PYP isolated from Salinibacter ruber (Srub) is characterized with a comprehensive range of spectroscopic techniques including ultrafast transient absorption, photostationary light titrations, Fourier transform infrared, and cryokinetics spectroscopies. We demonstrate that the dark-adapted pG state consists of two subpopulations differing in the protonation state of the chromophore and that both are photoactive, with the protonated species undergoing excited-state proton transfer. However, the primary I0 photoproduct observed in the Hhal PYP photocycle is absent in the Srub PYP photodynamics, which indicates that this intermediate, while important in Hhal photodynamics, is not a critical intermediate in initiating all PYP photocycles. The excited-state lifetime of Srub PYP is the longest of any PYP resolved to date (∼30 ps), which we ascribe to the more constrained chromophore binding pocket of Srub PYP and the absence of the critical Arg52 residue found in Hhal PYP. The final stage of the Srub PYP photocycle involves the slowest known thermal dark reversion of a PYP (∼40 min vs 350 ms in Hhal PYP). This property allowed the characterization of a pH-dependent equilibrium between the light-adapted pB state with a protonated cis chromophore and a newly resolved pG' intermediate with a deprotonated cis chromophore and pG-like protein conformation. This result demonstates that protein conformational changes and chromophore deprotonation precede chromophore reisomerization during the thermal recovery of the PYP photocycle.

Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy.

Proton transfer reactions are ubiquitous in chemistry, especially in aqueous solutions. We investigate photoinduced proton transfer between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and water using fast fluorescence spectroscopy and ab initio molecular dynamics simulations. Photoexcitation causes rapid proton release from the HPTS hydroxyl. Previous experiments on HPTS/water described the progress from photoexcitation to proton diffusion using kinetic equations with two time constants. The shortest time constant has been interpreted as protonated and photoexcited HPTS evolving into an "associated" state, where the proton is "shared" between the HPTS hydroxyl and an originally hydrogen bonded water. The longer time constant has been interpreted as indicating evolution to a "solvent separated" state where the shared proton undergoes long distance diffusion. In this work, we refine the previous experimental results using very pure HPTS. We then use excited state ab initio molecular dynamics to elucidate the detailed molecular mechanism of aqueous excited state proton transfer in HPTS. We find that the initial excitation results in rapid rearrangement of water, forming a strong hydrogen bonded network (a "water wire") around HPTS. HPTS then deprotonates in ≤3 ps, resulting in a proton that migrates back and forth along the wire before localizing on a single water molecule. We find a near linear relationship between the emission wavelength and proton-HPTS distance over the simulated time scale, suggesting that the emission wavelength can be used as a ruler for the proton distance. Our simulations reveal that the "associated" state corresponds to a water wire with a mobile proton and that the diffusion of the proton away from this water wire (to a generalized "solvent-separated" state) corresponds to the longest experimental time constant.

Multifunctional fluorescence sensor as a potential theranostic agent against Alzheimer's disease

Metal ions play an important role in the pathogenesis of Alzheimer's disease (AD). Metal dyshomeostasis, β-amyloid (Aβ) accumulation and oxidative stress, etc. are related to metal ions. So, metal therapeutics has aroused increasingly more attention, especially the research of metal-involved theranostic agents. In this work, a highly selective and sensitive multifunctional fluorescence sensor 1 with a naphthol unit based on photoinduced electron transfer (PET) and excited state proton transfer (ESPT) mechanism was synthesized, and its synergistic biological effects on regulating metal dyshomeostasis, modulating Aβ accumulation and scavenging reactive oxygen species (ROS) was evaluated. The results demonstrated that 1 exhibited significant fluorescence enhancement towards Al3+ (the limit was as low as 0.01 ppm), superior chelating abilities with metal ions, even better modulation effect of Cu2+-induced Aβ1−42 accumulation than curcumin, good elimination effect of ROS, clear fluorescence image in living cells, low cytotoxic and appropriate blood brain barrier (BBB) permeability. Overall, these findings revealed that 1 could be used as a potential theranostic agent against AD for further research.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*.

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

Nonadiabatic quantum dynamics of the coherent excited state intramolecular proton transfer of 10-hydroxybenzo[h]quinoline

The photoinduced nonadiabatic dynamics of the enol-keto isomerization of 10-hydroxybenzo[h]quinoline (HBQ) are studied computationally using high-dimensional quantum dynamics. The simulations are based on a diabatic vibronic coupling Hamiltonian, which includes the two lowest pi pi ^* excited states and a npi ^* state, which has high energy in the Franck–Condon zone, but significantly stabilizes upon excited state intramolecular proton transfer. A procedure, applicable to large classes of excited state proton transfer reactions, is presented to parametrize this model using potential energies, forces and force constants, which, in this case, are obtained by time-dependent density functional theory. The wave packet calculations predict a time scale of 10–15 fs for the photoreaction, and reproduce the time constants and the coherent oscillations observed in time-resolved spectroscopic studies performed on HBQ. In contrast to the interpretation given to the most recent experiments, it is found that the reaction initiated by 1pi pi ^* longleftarrow S_0 photoexcitation proceeds essentially on a single potential energy surface, and the observed coherences bear signatures of Duschinsky mode-mixing along the reaction path. The dynamics after the 2pi pi ^* longleftarrow S_0 excitation are instead nonadiabatic, and the npi ^* state plays a major role in the relaxation process. The simulations suggest a mainly active role of the proton in the isomerization, rather than a passive migration assisted by the vibrations of the benzoquinoline backbone.Graphic

Hydration-controlled excited-state relaxation in protonated dopamine studied by cryogenic ion spectroscopy.

Ultraviolet (UV) and infrared (IR) spectra of protonated dopamine (DAH+) and its hydrated clusters DAH+(H2O)1-3 are measured by cryogenic ion spectroscopy. DAH+ monomer and hydrated clusters with up to two water molecules show a broad UV spectrum, while it turns to a sharp, well-resolved one for DAH+-(H2O)3. Excited state calculations of DAH+(H2O)3 reproduce these spectral features. The conformer-selected IR spectrum of DAH+(H2O)3 is measured by IR dip spectroscopy, and its structure is assigned with the help of quantum chemical calculations. The excited state lifetime of DAH+ is much shorter than 20ps, the cross correlation of the ps lasers, revealing a fast relaxation dynamics. The minimal energy path along the NH → π proton transfer coordinate exhibits a low energy barrier in the monomer, while this path is blocked by the high energy barrier in DAH+(H2O)3. It is concluded that the excited state proton transfer in DAH+ is inhibited by water-insertion.

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy.

Recent advances in sustainable optoelectronics including photovoltaics, light-emitting diodes, transistors, and semiconductors have been enabled by π-conjugated organic molecules. A fundamental understanding of light-matter interactions involving these materials can be realized by time-resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited-state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H-bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance.

PH modulated luminescent switching and discriminative detection of amino acid based on metal-organic framework

The detection of glutamic (Glu) or aspartic (Asp) acids is vital for human nutrition and diagnosis of disease. Herein, the dht ligand containing hydroxy group (-OH) is used to design and synthesize a 2D luminescent [Cd2(idc)(dht)(H2O)4] (1); H2idc = 4,5-imidazoledicarboxylic acid and H2dht = 2,5-dihydroxyterephthalic acid for sensing amino acids. The compound 1 can discriminatively detect Asp and Glu among other amino acids through blue-shifted emission (yellow → green). The dual sensing mechanism may be attributed to the intermolecular excited-state proton transfer between MOF and water to produce keto form along with the subsequent switching of keto form to enol form by protonation causing the increased band gap energy. This material can serve several benefits in terms of high selectivity, fast response (30s), good reproducibility and low LOD value of 11.34 μM which is less than the harmful concentration of Glu for human health (>400 μM). In addition, 1 shows the broad range detection of Glu covering in safe and unsafe levels. For on-site detection of Glu, MOF-based paper is devised and can be applied through color-scanning application in smartphone. Besides, this sensor can serve to detect Glu in real samples with good recovery.

Marcus inversion is observed for excited state proton transfer in the adiabatic limit using naphthol based photoacids

Marcus theory is applied on ESPT reactions in the adiabatic limit, using cyano-naphthol derivatives. A systematic variation of the reaction exergonicity is achieved by changing the proton donor, solvent (acts as base) and the temperature. Marcus inverted regions are observed for the proton recombination steps when the solvent is changed and temperature variation was used to vary the reaction exergonicity.

Observation of Excited State Proton Transfer between the Titania Surface and Dye Molecule by Time-Resolved Fluorescence Spectroscopy

Observation of Excited State Proton Transfer between the Titania Surface and Dye Molecule by Time-Resolved Fluorescence Spectroscopy

Near-IR two-photon absorption photostabilizers for polymers intensified by molecular assembly in mixed organic solvents/H2O

This study reported on a variety of target phenanthridine dyes carrying hydroxy groups that were employed as near-IR two-photon absorption (TPA) photostabilizers for polymers. It was demonstrated that the target phenanthridine dyes underwent reversible excited state proton transfer (ESPT) in strong polar protic and aprotic solvents, such as ethanol, which was more remarkable under near-IR two-photon excitation. It was determined that the target dyes processed regular molecular stacking in mixed organic solvents/H2O solvents (such as ethanol/H2O, 90% water volume). The orderly molecular aggregation of the target phenanthridine dyes intensified the reversible ESPT emission under one-photon and near-IR two-photon excitation, respectively. Hence, the absorbed energy was consumed by the reversible ESPT process of the target dyes. As a consequence, the target dyes were evaluated as near-IR TPA photostabilizers for polymers. In contrast, the reference phenanthridine dyes lacking of hydroxy groups did not exhibit ESPT or photostability under light irradiation.

Design of Large Stokes Shift Fluorescent Proteins Based on Excited State Proton Transfer of an Engineered Photobase.

The incredible potential for fluorescent proteins to revolutionize biology has inspired the development of a variety of design strategies to address an equally broad range of photophysical characteristics, depending on potential applications. Of these, fluorescent proteins that simultaneously exhibit high quantum yield, red-shifted emission, and wide separation between excitation and emission wavelengths (Large Stokes Shift, LSS) are rare. The pursuit of LSS systems has led to the formation of a complex, obtained from the marriage of a rationally engineered protein (human cellular retinol binding protein II, hCRBPII) and different fluorogenic molecules, capable of supporting photobase activity. The large increase in basicity upon photoexcitation leads to protonation of the fluorophore in the excited state, dramatically red-shifting its emission, leading to an LSS protein/fluorophore complex. Essential for selective photobase activity is the intimate involvement of the target protein structure and sequence that enables Excited State Proton Transfer (ESPT). The potential power and usefulness of the strategy was demonstrated in live cell imaging of human cell lines.

Indications for an intermolecular photo-induced excited-state proton transfer of p-nitrophenol in water

Nitrophenols are well-known model systems to investigate photoinduced ultrafast excited-state channel branching. For example, photoexcitation of o-nitrophenol leads to stimulated emission within the first few hundreds of femtoseconds superimposed by an intramolecular proton transfer as well as intersystem crossing and vibrational relaxation. Here, we report on photo-induced processes after excitation of the first spectroscopically bright excited state of p-nitrophenol in aqueous solution at pH values ranging from 1 to 10. After excitation at wavelengths around 317 nm, transient absorption spectra were recorded between 350 and 700 nm for delay times of up to several hundreds of picoseconds. While many of the above-mentioned relaxation pathways still occur, the studies provided also strong indications for an intermolecular proton transfer forming p-nitrophenolate. The corresponding spectral features are observable at pH 7 and below on a hundreds of picosecond timescale.

Structural Origin and Vibrational Fingerprints of the Ultrafast Excited State Proton Transfer of the Pyranine-Acetate Complex in Aqueous Solution.

The excited state proton transfer (ESPT) reaction from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) to an acetate molecule has been investigated in explicit aqueous solution via excited state ab initio molecular dynamics simulations based on hybrid quantum/molecular mechanics (QM/MM) potentials. In all the trajectories, the direct proton transfer has been observed in the excited state within 1 ps. We find that the initial structural configuration extracted from the ground state distribution strongly affects the ESPT kinetics. Indeed, the relative orientation of the proton donor-acceptor pair and the presence of a water molecule hydrogen bonded to the phenolic acid group of the pyranine are the key factors to facilitate the ESPT. Furthermore, we analyze the vibrational fingerprints of the ESPT reaction, reproducing the blue shift of the acetate CO stretching (COac), from 1666 to 1763 cm-1 testifying the transformation of acetate to acetic acid. Finally, our findings suggest that the acetate CC stretching (CCac) is also sensitive to the progress of the ESPT reaction. The CCac stretching is indeed ruled by the two vibrational modes (928 and 1426 cm-1), that in the excited state are alternately activated when the proton is shared or bound to the donor/acceptor, respectively.

In silico simulation of the excited state proton transfer reaction of 2-(2-furyl)-3-hydroxychromone (FHC) in solution by empirical valence bond (EVB) method in conjunction with classical molecular dynamics

2-(2-Furyl)-3-hydroxychromone (FHC) is a dual emitter fluorescent probe exhibiting excited state intramolecular proton transfer (ESIPT) reaction in solution and inserted in biological relevant macromolecules. Its two fluorescence bands are attributed to normal form (N*) for the short wavelength and the long-wavelength band to the tautomer form (T*), the product of the ESIPT reaction. To rationalise the mechanisms that lead to the occurrence of the ESIPT reaction and predict the fluorescence spectra, the excited and ground state dynamic of this compound was followed in acetonitrile, ethanol, methanol, N-methylformamide and formamide, by the classical molecular dynamic simulation in conjunction with the empirical valence bond (EVB) model to describe bond dissociation process. A set of transferable parameters were used to fit the EVB potential energy curve to TDDFT data. Using these parameters, the occurrence time of the ESIPT reaction was predicted to be 7, 84, 113, 98, 207ps, respectively in the considered solvents, in excellent agreement with the experimentally reported values. These values are comparatively higher than those determined for the parent flavone dye (less than 100 fs), giving FHC a higher potential for sensor development. By examining structural parameters such as the distance from the proton to acceptor, the intermolecular H-bond between FHC and the solvent molecules and proton trajectories, we found that ESIPT occured in acetonitrile and alcohols as a combination of low frequency high amplitude donor-acceptor bending motion and charge redistribution in the excited state, that were modulated by the solvent H-bond donor potency. Differently in amides ESIPT occured via tunnelling mechanism due to the close energy of N* and T*. Fluorescence spectra calculated from potential energies of the EVB model along the reaction coordinates were qualitatively in agreement with the recorded spectra.

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH2) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S0 and S1 states were scanned with varying O–H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH2 can undergo the ESDPT process in the S1 state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.

Effects of azole rings with different chalcogen atoms on ESIPT behavior for benzochalcogenazolyl-substituted hydroxyfluorenes

ESIPT behavior has attracted a lot of eyes of researchers in recent years because of its unique optical properties. Due to its large Stokes shift and double emission fluorescence, white light can be generated in the fluorophore based on the excited state intramolecular proton transfer (ESIPT) principle. The excited state proton transfer behavior of hydroxylated benzoxazole (BO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) have been investigated in heptane, chloroform and DMF solvents. By comparing the infrared vibration spectra and the variation of bond parameters from the S0 to S1 states, and analyzing the frontier molecular orbitals, the influence of hydrogen bond dynamics, the solvent polarity, charge redistribution and the effects of different proton acceptors on proton transfer were observed. The only structural difference among the three substituted hydroxyfluorenes is the heteroatom in the azole ring (oxygen, sulfur and selenium, respectively). We have scanned the potential energy curve of the ESIPT process, and compared the potential barrier, it is found that the heavier chalcogen atoms are more favorable for proton transfer. At the same time, the potential application of changing heteroatoms in the azole ring by walking down the chalcogenic group in crystal luminescence color regulation is also discussed.