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We report on femto- to nanosecond studies of the excited state intermolecular proton transfer (ESPT) reaction of trisodium 8-hydroxypyrene-1,3,6-trisulfonate (pyranine, HPTS) with the human serum albumin (HSA) protein. The formed robust 1:1 complexes (K(eq) = (2.6 ± 0.1) × 10(6) M(-1)) show both photoacid (∼430 nm) and conjugated photobase (∼500 nm) emissions of the caged HPTS in its protonated structure. The proton-transfer reactions in these complexes proceed in a large time window, spanning from 150 fs to ∼1.2 ns. The ultrafast component reflects a direct H-bond breaking and making in the robust complexes, involving the carboxylate groups of the amino acids, while the slowest one is arising from the slow dynamics of the so-called biological water. Additional time constants of the caged photoacid to give the conjugated photobase are observed, assigned to the ESPT reaction within "loose" complexes (3 to tens of picoseconds), and 130 ps and 1.2 ns due to the slow dynamics of the water molecules around the protein residues and involved in the proton transfer. The fs-ns anisotropy measurements confirm the robustness of the HPTS:HSA complexes. Our results indicate that, even though robust 1:1 complexes between HPTS and the HSA are formed, the system is heterogeneous, due to different possible interactions of the dye with the inside/outside parts of the protein. Furthermore, we find lower values of the initial anisotropy (r(0)) in the protein (0.33) and in γ-CD (0.28) in comparison with buffered aqueous solution (0.385). We propose that caging HPTS by the HSA protein and by the cyclodextrin affects the electronic redistribution in a different degree of mixing between the (1)L(a) and (1)L(b) states in the formed deprotonated form, for which the interactions of the sulfonate groups with the surroundings should play a key role.

We report on photophysical studies of lumichrome (Lc) in water at different pHs, and interacting with the human serum albumin (HSA) protein and β-cyclodextrin (β-CD) in neutral aqueous solutions. We used steady-state and picosecond time-resolved emission spectroscopy to investigate the structural changes of Lc at the ground and excited states, as well as the rotational dynamics of the complexes with HSA and β-CD. In neutral water, the predominant neutral alloxazine-type structure of Lc coexists with a small population of the anionic form. In the presence of HSA, we observed an increase in the absorption band intensity at 450 nm. This increase is due to a preferential complexation (1:1 stoichiometry, K=8600 M(-1)) of the Lc anion structures within the protein. This change is not observed when β-CD is added, in which the Lc neutral form is exclusively complexed, giving a 1:1 stoichiometry. The fluorescence lifetimes of Lc in neutral water solutions are 4.2 and 2.3 ns, assigned to anionic and neutral alloxazinic forms, respectively. Using β-CD, the lifetime of the 1:1 complexes is 0.74 ns, while in the case of HSA complexes we observed two lifetimes (0.83 and 0.14 ns), which we explained in terms of different interactions of the anions with the protein. The rotational relaxation time of free Lc in neutral water is 75 ps. For Lc:β-CD complexes this time is 0.44 ns, in full agreement with the expected value from the hydrodynamic theory. For HSA solutions, we obtained a distribution of values between ∼1 and 4.5 ns, suggesting a site heterogeneity of complexation and a different strength of binding for the involved Lc anionic forms. Our results give information about the different photorelaxation behavior of Lc within chemical and biological cavities, and might help in a better design of nanosystems for drug carriers and delivery.

We report on steady-state and time (ns to fs regime) resolved studies of H-bonding interactions and protontransfer reaction dynamics of silica-based mesoporous material MCM-41 with an H-bond donor and acceptor guest aromatic molecule (7-hydroxyquinoline, 7HQ). We observed the ground state reaction which leads to the formation of intermediates and products of the confined molecular probe. We compare this behavior with the observed one for the dye adsorbed on the surface of silica particles, lacking the nanotubes. The result clearly shows that the formation of keto (or zwitterionic) tautomers at the ground state is enhanced by the confinement provided by the channels of MCM-41. Introduction of hydrophobic groups (by silylation of the OH groups in regular MCM-41 host) changes the ground state tautomeric equilibria and the emission behavior. A new lifetime (3.19 ns, suggested being due to a more stabilized anion of the guest) was observed in addition to the ones due to confined bound enol (0.26 ns), anion (1.5 ns), and zwitterionic (5.5 ns) structures. Both steady-state and ps-data show the importance of solvation of 7HQ structures inside MCM-41, when compared with the solid-state result. We investigated the intermolecular proton-transfer reaction dynamics in the confined structures using femtosecond-resolved emission spectroscopy, and we got the reaction times needed to produce the anion intermediates (0.3 ps) and zwitterion products (3 ps) upon electronic excitation of bound enol forms of the guest, in addition to the cooling times of the final zwitterionic form. We believe that our results might be useful for designing new nanophotonics sensors based on mesoporous materials, and open the window for further studies to better understand the chemical reactivity of silica-based nanohosts, at a short time scale.

The photodynamics of six types of indole-based squaraines (SQs) molecules deposited onto quartz substrates by using the spin-coating method have been studied by means of steady-state and time-resolved techniques. Enhanced scattering signal at the resonance wavelengths together with the broadening and shift of the absorption bands to the red side with respect to those in solution prove the formation of H-and J-aggregates in the thin film samples. Excited state deactivation dynamics were studied by the femtosecond (fs) transient absorption spectroscopy and nanosecond (ns) flash photolysis. Faster kinetics were obtained for the solid state samples as compared to those in solution. Singular value decomposition (SVD) analysis of the fs transient signal proves the presence of monomers and aggregates, enabling the separation of the kinetics for each one. For the SQ monomers, the dynamics of the singlet excited state does not depend on the pump fluence, and we thus propose that the quenching of the excited monomers is due to an energy transfer process to the aggregates. For the SQ aggregates, nonexponential fit of the experimental time profiles along with the power dependence of the transient absorption signal indicates a singlet− singlet annihilation process (γ′ ≈ 3 × 10 −15 cm 3 s −1/2 ). The combination of the long-range Forster-type mechanism and quantum delocalization is found to be the physical process that accounts for the singlet−singlet annihilation. In the flash photolysis experiments, we observed transient signals with a maximum intensity at 710 nm and lifetimes of 30 and 35 ns for SQ 41 and SQ 26, respectively. We have attributed these signals to the cis photoisomers of the aggregates deactivating to the trans photoisomers through the back photoisomerization reaction. These results provide information for a better understanding of the photodynamics of squaraines.

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.

We report on studies of salicylaldehyde azine (SAA) dissolved in dichloromethane solution and within the cages of the faujasite zeolite (NaX) using steady-state and femtosecond to nanosecond time-resolved spectroscopy. In solution, an excited-state intramolecular proton-transfer reaction takes place in less than 80 fs, leading to a keto-type tautomer. In contrast within NaX zeolite, a zwitterionic (Z) form is present both at S0 and S1 states, and a large hypsochromic shift of the stationary emission spectrum is observed. The increase in fluorescence lifetime upon encapsulation (from 54 ps to 0.2−2.8 ns) is mainly due to hindrance in twisting motion of the confined Z structure imposed by the nanocage. A significant dependence of the lifetimes on the guest concentration inside the zeolite indicates an interaction between neighboring guest molecules leading to a quenching of the fluorescence. The analysis of emission decays using stretched-exponential model suggests that the excited-state interactions between neighboring dyes play a key role in the deactivation of the trapped Z fluorophores. For the ultrafast relaxation dynamics of the SAA/NaX composite, intramolecular vibrational-energy redistribution and vibrational cooling process occur in longer times (up to 360 fs and 5 ps, respectively). Additionally, the presence of nonfluorescent twisted (n,π*) state is suggested to form in 6−10 ps. We believe that our results are important for a better understanding of the photocycle of azine-based photochromic material when interacting with nanomaterials.