Excited-state double proton transfer of 7-azaindole dimers in a low-temperature organic glass

The excited-state double proton transfer of model DNA base pairs, 7-azaindole (7AI) dimers, is explored in a low-temperature organic glass of n-dodecane using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the conformations of 7AI dimers at the moment of excitation; whereas planar conformers tautomerize rapidly (<10 ps), twisted conformers undergo double proton transfer to form tautomeric dimers on the time scale of 250 ps at 8 K. The proton transfer is found to consist of two orthogonal steps: precursor-configurational optimization and intrinsic proton transfer via tunneling. The rate is almost isotope independent at cryogenic temperatures because configurational optimization is the rate-determining step of the overall proton transfer. This optimization is assisted by lattice vibrations below 150 K or by librational motions above 150 K.     

Excited-state double proton transfer dynamics of model DNA base pairs: 7-hydroxyquinoline dimers

The excited-state double proton transfer of model DNA base pairs, 7-hydroxyquinoline dimers, in benzene has been investigated using picosecond time-resolved fluorescence spectroscopy. Upon excitation, whereas singly hydrogen-bonded noncyclic dimers do not go through tautomerization within the relaxation time of 1400 ps, doubly hydrogen-bonded cyclic dimers undergo excited-state double proton transfer on the time scale of 25 ps to form tautomeric dimers, which subsequently undergo a conformational change in 180 ps to produce singly hydrogen-bonded tautomers. The rate constant of the double proton transfer reaction is temperature-independent, showing a large kinetic isotope effect of 5.2, suggesting that the rate is governed mostly by tunneling.     

Excited-state proton transfer in 7-hydroxy-4-methylcoumarin along a hydrogen-bonded water wire

TDDFT, RI-CC2, and CIS calculations have been performed for the nondissociative excited-state proton transfer (ESPT) in the S1 state of 7-hydroxy-4-methylcoumarin (7H4MC) along a H-bonded water wire of three water molecules bridging the proton donor (OH) and the proton acceptor (C[double bond]O) groups (7H4MC.(H2O)3). The observed structural reorganization in the water-wire cluster is interpreted as a proton-transfer (PT) reaction along the H2O solvent wire. The shift of electron density within the organic chromophore 7H4MC due to the optical excitation appears to be the driving force for ESPT. All the methods used show that the reaction path occurs in the 1pipi* state, and no crossing with a Rydberg-type 1pisigma* state is found. TDDFT and RI-CC2 calculations predict an exoergic reaction of the excited-state enol-to-keto transformation. The S1 potential energy curve reveals well-defined Cs minima of enol- and keto-clusters, separated by a single barrier with a height of 17-20 kcal/mol. After surmounting this barrier, spontaneous PT along the water wire is observed, leading without any further barrier to the keto structure. The TDDFT and RI-CC2 methods appear to be reliable approaches to describe the energy surfaces of ESPT. The CIS method predicts an endoergic ESPT reaction and an energy barrier, which is too high.     

Excited-state double proton transfer of 7-azaindole in water nanopools

The excited-state proton-transfer dynamics of 7-azaindole occurring in the water nanopools of reverse micelles has been investigated by measuring time-resolved fluorescence spectra and kinetics, as well as static absorption and emission spectra, with varying water content and isotope. 7-Azaindole molecules are found to exist in the bound-water regions of reverse micelles. The rate constant and the kinetic isotope effect of proton transfer are smaller than those in bulk water although both increase with the size of the water nanopool. The retardation of proton transfer in the bound regions is attributed to the increased free energy of prerequisite solvation to form a cyclically H-bonded 1:1 7-azaindole/water complex.     

Excited-state proton transfer via hydrogen-bonded acetic acid (AcOH) wire for 6-hydroxyquinoline

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogen-bonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N- group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N- group in 6HQ upon excitation to the S(1) state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogen-bonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N- group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.     

Photophysics of 1-azacarbazole dimers: a reappraisal

A systematic study of 1-azacarbazole (1AZC) dissolved in 2-methylbutane (2MB) at gradually decreasing temperatures from room temperature to 77 K revealed the chromophore to exhibit four fluorescence emissions: a structured fluorescence in the UV region that is due to the 1-azacarbazole monomer, a structureless emission centered at 500 nm and assigned to the centrosymmetric dimer formed by double hydrogen bonding, an also structureless emission centered at ca. 400 nm and due to a noncentrosymmetric doubly hydrogen bonded dimer, and a fourth, structured emission at 357 and 375 nm due to a card-pack dimer. Evidence obtained from dilute solutions of 1-azacarbazole is for the first time assigned to a card-pack dimer, consistent with the photophysical behavior of carbazole in the same medium. Previously established photophysical evidence for such an interesting compound, which has been used as a model for studying light-induced double proton transfer mutational mechanisms, is completed or discussed here. The evidence obtained in this work reveals that 1AZC at a 10-4 M solution in 2MB does not exhibit doubly hydrogen bonded centrosymmetric dimer emission as the temperature decreases from room temperature up to 113 K (with a corresponding exponential increase of the solvent viscosity). At this temperature and below, however, the doubly hydrogen bonded centrosymmetric dimer emission appears. This evidence and others implemented in this work contradict the assumption of Waluk et al. that the appearance of the doubly hydrogen bonded centrosymmetric dimer is hindered by an increased viscosity of the medium.     

Excited-state proton transfer kinetics of carbazole

Rate constants for the excited-state proton transfer reaction of carbazole in aqueous alkaline solution have been determined using picosecond single photon counting. Fluorescence decay measurements show that the back reaction is slow compared to the fluorescence decay time and therefore equilibrium is not attained in the excited state. The validity of a pK value for the lowest excited state determined from steady-state fluorescence measurements assuming equilibrium is discussed. It is concluded that the thermodynamic pK^* value for carbazole is 10.98.     

Excited-state proton transfer in chiral environments. 1. Chiral solvents

Excited-state proton transfer from the "super" photoacid 5,8-dicyano-2-naphthol to 2-butanol is faster in the enantiopure solvent than in its racemic form. The difference observed is discussed in terms of long-range order in homo- and heterodimers of 2-butanol.     

Excited-state proton transfer reactions of 10-hydroxycamptothecin

Time-resolved and steady-state emission characterization of 10-hydroxycamptothecin reveals a rich but less complex proton-transfer behavior than its parent hydroxyquinoline. The electronic effect of the additional electron-withdrawing ring makes the excited-state both less basic and more acidic than the parent and adds to the class of high-acidity excited-state proton donors in photochemistry and photobiology.     

Excited-state intramolecular proton transfer in 2,4,5-triarylimidazole molecules

Excited-state intramolecular proton transfer (ESIPT) in the 2,4,5-triarylimidazole molecules was studied by spectral-luminescent technique. For 4,5-diphenyl-(2-hydroxyphenyl)imidazoles, the ESIPT occurs in both liquid and glassy matrices at 77 K. For 4,5-diphenyl-(2-hydroxynaphthyl)imidazole, the ESIPT requires rotation of molecular fragments and is not observed at 77 K.     

Excited-state intramolecular proton transfer in hydroxyoxime-based chemical sensors

The electronic structure of a series of β-hydroxy-oximes, with different aromatic cores (naphthalene, pyrene, coumarin, pyridine) between the oxime and the hydroxyl groups, has been investigated by time-dependent density functional theory (TDDFT) and of the naphthalene-based oxime, in addition, by resolution-of-identity second-order perturbative coupled cluster (RICC2) calculations with basis sets up to augmented triple-ζ quality. The particular systems have been proposed as fluorescent sensors of organophosphorus (OP) nerve agents, with enhancement of fluorescence accompanying the sensing of OP agents. It is found that the experimentally observed fluorescence quenching of the oxime sensors in their initial form can be attributed to intramolecular proton transfer upon excitation from the β-hydroxyl group to the nitrogen atom, thus forming a weakly emitting hydroxylaminoquinoid.     

Excited-state proton transfer from pyranine to acetate in methanol

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulphonate, HPTS) to acetate in methanol has been studied by steady-state and time-resolved fluorescence spectroscopy. The rate constant of direct proton transfer from pyranine to acetate (k ₁) is calculated to be ∼1 × 10⁹ M⁻¹ s⁻¹. This is slower by about two orders of magnitude than that in bulk water (8 × 10¹⁰ M⁻¹ s⁻¹) at 4 M acetate.     

Cooperativity and proton transfer in hydrogen-bonded triads.

Ab initio MP2/6-311+G(3df,2pd) and MP2/aug-cc-pVTZ calculations have been carried out to investigate the structures and properties of AHXHYH(3) (A=F, Cl; X=F, Cl; Y=N, P) hydrogen-bonded complexes. Significant cooperative effects are observed in the XHYH3 dyads in the triads due to the presence of the polar near-neighbor AH. These effects are greater when the polar partner is HF, which is a better proton donor than HCl. Structural changes, red shifts of proton-donor stretching frequencies, nonadditive interaction energies, and electron density redistributions unambiguously demonstrate that the X--HY hydrogen bond (HB) is stronger in the triads than in the corresponding dyads, while the X--H bond of the proton donor becomes weaker. Even more pronounced cooperative effects are observed in the AHXH dyads due to the presence of the YH3 partner. These effects are weaker in complexes having PH3 rather than NH3 as the proton acceptor, since NH3 is a stronger base. Cooperativity also enhances the proton-donating ability of the YH3 moiety, with the result that all complexes except FHFHPH3 are cyclic. Cooperativity, together with the ease of breaking the Cl--H bond in ClHClHNH3 and FHClHNH3, leads to proton transfer (PT), so that these two complexes are better described as approaching hydrogen-bonded ClHCl- x +HNH3 and FHCl- x +HNH3 ion pairs.     

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones

Nonradiative decay pathways associated with vibronically coupled S₁(ππ*)–S₂(nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck–Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S₁(ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S₂(nπ*) within a few femtoseconds while the other decays to S₁ minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S₂(nπ*), competing with the traditional pathway occurring on S₁(ππ*).     

Excited-state proton transfer in methanol-doped ice in the presence of KF

Steady-state and time-resolved emission techniques were employed to study the photoprotolytic cycle of an excited photoacid in ice in the presence of a low concentration of a weak base-like F(-). In previous studies we found that the photoprotolytic cycle in methanol-doped ice (1% mol fraction) is too slow to be observed at temperatures below 190 K. In this study we found that at temperatures below 240 K an additional proton-transfer process occurs in ice doped with 10 mM KF. We attributed this reaction to the creation of a mobile L-defect by F(-) ions. We used a diffusion-assisted reaction model, based on the Debye-Smoluchowski equation, to account for the direct reaction of the L-defect with the excited photoacid at temperatures below T < 240 K. Below 160 K the spectroscopic properties as well as the photoprotolytic cycle change dramatically. We propose that below 160 K the sample enters a new phase. The excited-state proton-transfer (ESPT) process was observed and followed down to a liquid nitrogen temperature of approximately 78 K. In the low-temperature phase the ESPT rate is almost twice as much as at 180 K and the temperature dependence of the rate is very small. The kinetic isotope effect of the ESPT at the low-temperature phase is small of about 1.3.     

Large chiral recognition in hydrogen-bonded complexes and proton transfer in pyrrolo[2,3-b]pyrrole dimers as model compounds

The chiral recognition in the formation of hydrogen-bonded (HB) dimers of 1,6a-dihydropyrrolo[2,3-b]pyrrole derivatives as well as in their proton-transfer processes have been studied by means of ab initio calculations. The heterochiral dimers are in general the most stable ones, but amphiprotic substituents that are able to form attactive interactions with twin groups revert this tendency. Energy differences up to 4.0 kcal/mol have been found favoring the homo- or heterochiral complexes. Two possible proton-transfer processes have been studied, the concerted one and the nonconcerted one. The compresion of the systems in the transition structures produce an increase in the energetic differences when compared to the corresponding minima complexes. A Steiner-Limbach relationship has been found for the geometrical properties of the HB in the minima and transition states calculated here. The electron density and its Laplacian at the bond critical point have been found to correlate with the HB distance.     

Excited-state proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.     

Excited-state intramolecular proton transfer distinguishes microenvironments in single- and double-stranded DNA

Herein, the efficient interaction of an environment-sensitive fluorophore that undergoes excited-state intramolecular proton transfer (ESIPT) with DNA has been realized by conjugation of a 3-hydroxychromone (3HC) with polycationic spermine. On binding to a double-stranded DNA (dsDNA), the ratio of the two emission bands of the 3HC conjugates changes up to 16-fold, so that emission of the ESIPT product increases dramatically. This suggests an efficient screening of the 3HC fluorophore from the water molecules in the DNA complex, which is probably realized by its intercalation into dsDNA. In sharp contrast, the 3HC conjugates show only moderate changes in the dual emission on binding to a single-stranded DNA (ssDNA), indicating a much higher fluorophore exposure to water at the binding site. Thus, the 3-hydroxychromone fluorophore being conjugated to spermine discriminates the binding of this polycation to dsDNA from that to ssDNA. Consequently, ESIPT-based dyes are promising for monitoring the interaction of polycationic molecules with DNA and probing the microenvironment of their DNA binding sites.