Interrogating Confined Proton-Transfer Reaction Dynamics within Mesoporous Nanotubes

Abstract

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.

Enhanced carbon dioxide conversion at ambient conditions via a pore enrichment effect

“…With the narrow or crowded pores relative to the molecule size, the molecular structures of the adsorbate are more easily affected by the confined environment, so the confinement effect is the dominant factor. When the pore size increases or the adsorbate size decreases, we believe that the confinement effect should gradually weaken because the adsorption space is large enough for the adsorbate under the circumstances 43 , and the pore enrichment effect caused by the molecular aggregation turn into the most important factor. For the CO 2 adsorption in the M-Salen-COFs designed by us, the enrichment just changes the concentration of reactants around the catalytic centre, not the obviously electronic structure of reactant.…”

“…To obtain stable frameworks, we optimize the geometries for each COF through annealing method. The classical molecular dynamics package LAMMPS 43 is used with the universal force field (UFF) 44 . A periodic unit cell is employed.…”

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

“…Consequently, upon electronic excitation of E, an intramolecular electronic charge redistribution occurs in E* that leads to the formation of the keto (K*) or zwitterion (Z*) tautomer. When these species return to S 0 , they can be stabilized in this or other tautomeric forms (e.g., rotated K), or return to the E species. − The mechanism and rate constant of the ESIPT reactions depend on the PESs of the E* and K* tautomers in the first electronically first excited single state (S 1 ). , The presence or absence of barriers for both processes in S 0 and S 1 opens the possibility of controlling the related spectroscopy and dynamics. ,− Thus, for a PES without an energy barrier between the wells of E* and K* species, the ESIPT reaction is ultrafast (fs regime) and irreversible. , When the PES has a barrier in the conversion of E* to K*, the reaction may be reversible or irreversible but will be slower (ps–ns regime) than in the barrierless situation. ,− An accurate description of the reaction dynamics at the PESs of ESIPT processes in gas phase requires at least two reaction coordinates: (i) the proton motion within the corresponding IHB and (ii) the vibrational or torsional (angle) mode involving the distance between the partners; the angle between the involved moieties in the transfer could be important in the global reaction dynamics and subsequent processes. ,,, In the presence of an energy barrier in the PESs, the proton motion may occur via tunneling, which adds another (quantum) dimension to the reaction. In solution, the PESs might be complex, involving a solvent coordinate where its polarity and H-bonding could play an important role in shaping the spectroscopy and dynamics.…”

Structural Characterization of Proteins Adsorbed at Nanoporous Materials

“…[60][61][62] The mobility and diffusivity of a small solute confined inside mesoporous silica pores are slower than those in a bulk solution. [63][64][65][66][67][68][69] The slow dynamics of the solute has been explained by the interactions between the solute and the inner pore surface, intermolecular hydrogen bonding network of the solvent molecules, and the solvation shell around the solute. The properties of the solvent and solute molecules confined inside Fig.…”