Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

Spectroscopic studies of the biologically active compound 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), have been performed. Absorption studies in the UV-Vis region for FABT in polar solvents, like water or ethanol, exhibit the domination of the enol form over its keto counterpart, with a broad absorption band centered around 340 nm. In non-polar solvents such as n-heptane or heavier alkanes the 340 nm absorption band disappears and an increase of the band related to the keto form (approximately 270 nm) is observed. Fluorescence spectra (with 270 nm and 340 nm excitation energies used) show a similar dependence: for FABT in 2-propanol a peak at about 400 nm dominates over that at 330 nm while in n-heptane this relation is reversed. The solvent dependent equilibrium between the keto and enol forms is further confirmed by FTIR and Raman spectroscopies. As can be expected, this equilibrium also shows some temperature dependences. We note that the changes between the two tautomeric forms of FABT are not related to the permanent dipole moment of the solvent but rather to its dipole polarizability

Femtochemistry of orange II in solution and in chemical and biological nanocavities

In this work, we report on studies of the nature of the dynamics and hydrophobic binding in cyclodextrins and human serum albumin protein complexes with orange II. With femtosecond time resolution, we examined the proton-transfer and trans-cis isomerization reactions of the ligand in these nanocavities and in pure solvents. Because of confinement at the ground state, the orientational motion in the formed phototautomer is restricted, leading to a rich dynamics. Therefore, the emission lifetimes span a large window of tens to hundreds of picoseconds in the cavities. Possible H-bond interactions between the guest and cyclodextrin do not affect the caged dynamics. For the protein–ligand complexes, slow diffusional motion (≈630 ps) observed in the anisotropy decay indicates that the binding structure is not completely rigid, and the embedded guest is not frozen with the hydrophobic pocket. The ultrafast isomerization and decays are explained in terms of coupling motions between N–N and C–N stretching modes of the formed tautomer. We discuss the role of confinement on the trans-cis isomerization with the cavities and its relationships to frequency and time domains of nanostructure emission

Spectral and dynamical properties of a Zr-based MOF

We report on the spectra and dynamics of a Zr-naphthalene dicarboxylic acid (Zr-NDC) MOF in different diluted solvent suspensions and in a concentrated tetrahydrofuran (THF) one. In a diluted diethyl ether (DE) suspension, we observed intraparticle excimer formation between neighboring naphthalene organic linkers, leading to a red-shifted broad band in the emission spectrum and to a dynamics composed of three components τ = 650 ps, τ = 3.7 ns and τ = 13.9 ns, assigned to the excimer photoproduction, monomer and excimer lifetimes, respectively. Furthermore, both absorption and emission spectra show a blue shift in more polar solvents characterized by the solvent polarity function f(ε,n). We also observed changes in the excimer formation time (490-840 ps) probably due to a variation in the MOF structural fluctuation induced by solvent filling. The global fluorescence quantum yield of these suspensions is around 0.30 ± 0.05. At higher concentrations of the MOF particles, we observed the absorption and emission signals of aggregates having an intercrystal excimer formation in ∼5 ps in a THF suspension, ∼100 times shorter than that observed in a diluted one. Our results give the spectral and dynamical properties of a Zr-NDC MOF in solvent suspensions, opening the way to further studies of these kinds of MOFs interacting with fluorescent dyes for possible photonic applications.This work was supported by the MINECO through projects MAT2011-25472, MAT2014-52085-C2-2-P, and Consolider Ingenio 2010 (CSD2009-0050, MULTICAT) and JCCM through the project PRI-PIBIN-2011-1283. M.G. thanks the MINECO for the PhD fellowship.Peer Reviewe

Femtosecond molecular dynamics of tautomerization in model base pairs

Hydrogen bonds commonly lend robustness and directionality to molecular recognition processes and supramolecular structures. In particular, the two or three hydrogen bonds in Watson–Crick base pairs bind the double-stranded DNA helix and determine the complementarity of the pairing. Watson and Crick pointed out, however, that the possible tautomers of base pairs, in which hydrogen atoms become attached to the donor atom of the hydrogen bond, might disturb the genetic code, as the tautomer is capable of pairing with different partners. But the dynamics of hydrogen bonds in general, and of this tautomerization process in particular, are not well understood. Here we report observations of the femtosecond dynamics of tautomerization in model base pairs (7-azaindole dimers) containing two hydrogen bonds. Because of the femtosecond resolution of proton motions, we are able to examine the cooperativity of formation of the tautomer (in which the protons on each base are shifted sequentially to the other base), and to determine the characteristic timescales of the motions in a solvent-free environment. We find that the first step occurs on a timescale of a few hundred femtoseconds, whereas the second step, to form the full tautomer, is much slower, taking place within several picoseconds; the timescales are changed significantly by replacing hydrogen with deuterium. These results establish the molecular basis of the dynamics and the role of quantum tunnelling

Femtochemistry in Nanocavities: Reactions in Cyclodextrins

We present our first studies of the femtosecond dynamics of reactions in confined nanocavities in water solutions. Intramolecular proton transfer and isomerization dynamics of a hydrogen-bonded molecule (HPMO) in liquid solutions and encapsulated in the cavity formed by β-cyclodextrin (diameter ∼8 Å) is studied using the technique of fluorescence up-conversion. Our results suggest that the proton transfer in aprotic solvents occurs in much less than 300 fs while upon encapsulation this initial step is slowed to the subpicosecond time scale. Furthermore, in these aprotic solvents, HPMO undergoes a picosecond twisting motion around the interaromatic single bond, which is noticeably inhibited when the molecule is inside the nanocavity. Such studies of condensed-phase femtochemistry in nanocavities offer several promising extensions

Femtochemistry in Nanocavities: Reactions in Cyclodextrins

We present our first studies of the femtosecond dynamics of reactions in confined nanocavities in water solutions. Intramolecular proton transfer and isomerization dynamics of a hydrogen-bonded molecule (HPMO) in liquid solutions and encapsulated in the cavity formed by β-cyclodextrin (diameter ∼8 Å) is studied using the technique of fluorescence up-conversion. Our results suggest that the proton transfer in aprotic solvents occurs in much less than 300 fs while upon encapsulation this initial step is slowed to the subpicosecond time scale. Furthermore, in these aprotic solvents, HPMO undergoes a picosecond twisting motion around the interaromatic single bond, which is noticeably inhibited when the molecule is inside the nanocavity. Such studies of condensed-phase femtochemistry in nanocavities offer several promising extensions

Photodynamics of Zr-based MOFs: Effect of explosive nitroaromatics

The present work describes the spectroscopic and photodynamics of two different Zr mixed-linkers MOFs (Zr-NDC/Tz and Zr-NDC/CN) and their interaction with nitroaromatics. Both MOFs exhibit comparable spectroscopic behaviours, with a broad emission band mainly due to the naphthalene excimers within their three-dimensional structure. Flash photolysis experiments show a slow radiative electron-hole (e-h) recombination, reflected as a large negative absorption band. The interaction with the selected nitroaromatic compounds produces a static fluorescence quenching of Zr-NDC/Tz. Interestingly, the addition of trinitrophenol (TNP) induces the formation of a charge-transfer complex, helped by intermolecular H-bonds formation, as shown by the steady-state and ps-time-resolved emission experiments. Remarkably, the (e-h) recombination is strongly affected due to the inhibition of the ligand-to-cluster charge transfer process within the MOF. The quenching constants for the nitroaromatics lacking -OH groups are in the order of 10 M, while it is two orders of magnitude higher for the TNP (1.8 × 10 M). Both MOFs are highly selective toward TNP. We also demonstrate the possibility to recycle these MOFs without significant loses in their ability to detect TNP. Our findings give the clues to understand the fluorescence quenching mechanism of new Zr-based MOFs in presence of explosive-like molecules, opening the way to improve these nanomaterials as highly selective sensor of nitroaromatics.This work was supported by the JCCM and MINECO through projects: MAT2011-25472, MAT2014-52085-C2-2-P, MAT2014- 57646-P and PEII-2014-003-P. M. Gutiérrez thanks the MINECO for the PhD FellowshipPeer Reviewe