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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 (Ϸ630 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.cyclodextrins ͉ protein ͉ H-bond ͉ twisting ͉ anisotropy F emtosecond (fs) studies of caged molecules in nanocavities provide direct information on the relationship between time and space domains of molecular relaxation (1). Therefore, simple and complex (in concept) molecular systems have been studied using cyclodextrins (CDs), proteins, micelles, pores, and zeolites as nanohosts, demonstrating the confinement effect of the hydrophobic nanocavities on the spectroscopy and dynamics of the guests (2-13). Relevant information on the ultrafast dynamics of caged wavepackets involving breaking͞making chemical bonds, and solvation has been acquired.Orange II (OII) (Fig. 1), also called acid orange 7, is a molecule that has O-H . . . N and NAN bonds and may show photoinduced intramolecular proton-transfer (IPT) and transcis isomerization reactions. It is widely used in the dyeing of textiles, food, and cosmetics and thus is found in the wastewaters of the related industries (14). It has been reported that it exists under azoenol (AZO) and ketohydrazone (HYZ) forms (Fig. 1) (15-20). In a water solution, for example, the H-atom within the O-H . . . N intramolecular H-bond is shifted to the nitrogen site, making HYZ structure the most stable one (Ϸ95%) (20). In DMSO, the HYZ population decreases to 70%, and in solid state it becomes the only populated structure (20). Recent x-ray studies of phenyl substituted 1-(arylazo)-2-naphthols showed that this kind of molecules displaying HYZ-AZO tautomerism can form intramolecular resonance-assisted H-bonds from pure N-H . . . O to pure N . . . H-O structures, depending on the phenyl derivative (21, 22). Furthermore, a recent photophysical study of similar molecules in solution has shown the occurrence of a photoinduced proton-transfer reaction in AZO to give HYZ structure with a very low emission qu...

In this contribution, we report on studies of rotational and diffusional dynamics of 7-hydroxyquinoline (7HQ) within a reverse micelle (RM) containing different amounts of water. Analyzed in terms of the wobbling-in-a-cone model, the data reveal structural and dynamical properties of the nanopool. We clearly observed three regions in the behavior of confined water molecules within the RM hosting a double proton-transfer reaction between the probe and water. This observation remarkably reproduces the change of calculated water density within this life-mimicking medium. The number of water molecules per AOT head in the transition regions changes from 2 to 5, the latter being very near to the full solvation number (6) of the RM heads. Moreover, the H-bonds breaking and making within the RM to give new structures of the probe strongly affect the environment fluidization in different extents, reflected in different relaxation times of these structures; however, they are of similar sizes. We discuss the role of RM confinement and the proton-transfer dynamics on the behavior of water and their relationships to the packing of water molecules in the studied range of concentrations.

Hydrogen bonds and related inter-and intramolecular protontransfer reactions constitute particular cases in which the energetic and dynamic properties span a wide range of values. [1][2][3][4][5] Because of its unique character (energy, directionality and quantum tunneling) in influencing the behavior of many systems, which range from materials to life sciences, the related proton (or H-atom) transfer is one of the most studied reactions. [6][7][8][9][10][11][12] A prototype system that shows an intramolecular protontransfer reaction following an electronic excitation is 3-hydroxyflavone (3HF) (Scheme 1). Besides the enol (E)form observed under different conditions, other H-bonding species can appear at the ground state (S 0 ) producing Hbonded complexes [13][14][15] or even the anionic form of 3HF.[37]Photoexcitation of E produces, in less than 50 fs, a protontransferred form with a zwitterionic character (Z) (Scheme 1). In solution (e.g., in acetonitrile or alcohol), another component, with a time constant of 5-10 ps, has been observed. [21][22][23][24][25][26][27][28][29] This component was attributed to the contribution of the normal forms, [21] the presence of disolvated intermolecular H-bonded complexes in alcohol, [25] intramolecular vibrational redistribution and relaxation, [26] or a fast equilibrium between two different excited Z structures.[28] Therefore, weak and strong intermolecular H bonds between 3HF and H-bonding solvents can be formed giving birth to different structures (Scheme 1). The rate of inter-and intramolecular proton-transfer reactions depends on the nature and strength of the intermolecular H bonds in these species. Pico-and femtosecond studies have shown the interplay between intramolecular charge-and proton-transfer reactions in a hydroxyflavone derivative and the involvement of the phenyl-moiety rotation in such an interplay. [36,39] Continuing our previous efforts in this direction, we report here on studies of 3HF in N,N-dimethyl formamide (DMF) using the femtosecond-up-conversion technique of emission. We show that, in an H-bonding acceptor solvent, anionic species of 3HF are formed within % 500 fs, while the formation of a zwitterionic (Z) structure, produced as a result of an intramolecular proton motion, takes about 5 ps due to a weaker H-bonding interaction with the solvent molecules. Results and DiscussionTo begin with the continuous-wave (CW) observation, the UV/ Vis absorption spectrum in Figure 1 A shows the formation of the anionic form (A, maximum intensity at 425 nm) of 3HF in which the proton has shifted to a DMF molecule (Scheme 1). The inset of Figure 1 A shows the UV/Vis absorption spectrum of the sample in the presence of 2 mg of (solid) NaOH, indicating the absorption band of A under these conditions. Excitation of 3HF in DMF at 340 nm shows two fluorescence bands with maxima at around 410 nm [emission of H-bonded forms, (E···S) 1,2 in Scheme 1] and 540 nm (emission of Z forms, Scheme 1) (Figure 1 B). However, when exciting at 420 nm (only A is excited), we onl...

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