A slowing down of proton motion from HPTS to water adsorbed on the MCM-41 surface

Abstract

We report on the steady-state and femtosecond-nanosecond (fs-ns) behaviour of 8-hydroxypyrene-1,3,6-trisulfonate (pyranine, HPTS) and its interaction with mesoporous silica based materials (MCM-41) in both solid-state and dichloromethane (DCM) suspensions in the absence and presence of water. In the absence of water, HPTS forms aggregates which are characterized by a broad emission spectrum and multiexponential behavior (τsolid-state/DCM = 120 ps, 600 ps, 2.2 ns). Upon interaction with MCM41, the aggregate population is found to be lower, leading to the formation of adsorbed monomers. In the presence of water (1%), HPTS with and without MCM41 materials in DCM suspensions undergoes an excited-state intermolecular proton-transfer (ESPT) reaction in the protonated form (ROH*) producing a deprotonated species (RO(-)*). The long-time emission decays of the ROH* in different systems in the presence of water are multiexponential, and are analysed using the diffusion-assisted geminate recombination model. The obtained proton-transfer and recombination rate constants for HPTS and HPTS/MCM41 complexes in DCM suspensions in the presence of water are kPT = 13 ns(-1), krec = 7.5 Å ns(-1), and kPT = 5.4 ns(-1), krec = 2.2 Å ns(-1), respectively, The slowing down of both processes in the latter case is explained in terms of specific interactions of the dye and of the water molecules with the silica surface. The ultrafast dynamics (fs-regime) of the HPTS/MCM41 complexes in DCM suspensions, without and with water, shows two components which are assigned to intramolecular vibrational-energy relaxation (IVR) (∼120 fs vs. ∼0.8 ps), and vibrational relaxation/cooling (VC), and charge transfer (CT) processes (∼2 ps without water and ∼5 ps with water) of the adsorbed ROH*. Our results provide new knowledge on the interactions and the proton-transfer reaction dynamics of HPTS adsorbed on mesoporous materials.

Protons and Hydroxide Ions in Aqueous Systems

“…For example, the time-resolved fluorescence of the dye dubbed QCy7 is due to proton ejection to water in about 0.5 ps . The asymptotic power-law behavior shown in Figure is modified upon moving from pure water to more complex systems, such as water in reverse micelles, mesoporous surfaces, or near proteins undergoing conformational changes …”

“…For example, the time-resolved fluorescence of the dye dubbed QCy7 is due to proton ejection to water in about 0.5 ps. 114 The asymptotic power-law behavior shown in Figure 23 is modified upon moving from pure water to more complex systems, such as water in reverse micelles, 115 mesoporous surfaces, 116 or near proteins undergoing conformational changes. 117 One wonders whether spherically symmetric diffusion for a single translational coordinate (the proton−photobase separation, r) is not an oversimplified model for PT reactions in liquids.…”

Confinement Effect of Micro- and Mesoporous Materials on the Spectroscopy and Dynamics of a Stilbene Derivative Dye

“…Among the large variety of porous supports, the most studied ones are those formed by the interaction of a guest molecule with silica-based materials like zeolites, MCM-41, SBA-15, and mesoporous silica nanoparticles. These constituents provide cavities and channels which allow to tune both the spectroscopy and dynamics of trapped molecules by affecting processes like proton-transfer [2,3,4], electron-transfer [5,6,7], and energy-transfer [8,9,10]. The variety of photophysical and photochemical events showed by these composites could be used to develop smart devices such as drug nanocarriers, nanosensors, nanoOLEDs, nanolasers, energy storage nanospace, and nanophotocatalysts [1].…”

“…The ICT rate constant (10 −13 –10 −8 s) thoroughly depends on the polarity of the medium. Numerous works have demonstrated the impact of the cavity/pore size of silica-based materials on the ICT reaction dynamics of the encapsulated guests [3,16,17,18,19,20,21,22,23,24].…”

Achieving Thermally Activated Delayed Fluorescence from Benzophenone by Host–Guest Complexation

“…MSN nanostructures not only provide rigid environments to encapsulated Bzp but to some extent also inhibit solvent and dissolved oxygen diffusion around the chromophores. 15,27 The increased emission efficiency and lifetimes of a chromophore in the N 2 -gas saturation conditions compared to the air-saturated one (Figures S2 and S3) directly indicated the triplet state's Transient PL decay profiles of Bzp:MSN display a biexponential pattern with lifetime components of few nanoseconds and 40 μs (Figure 3b,c). The short lifetime component corresponds to the sum of S 1 → S 0 and S 1 → T 1 relaxations.…”