Adsorption and Aggregation at Silica/Methanol Interfaces: The Role of Solute Structure

Abstract

Second harmonic generation (SHG) and time-resolved, total internal reflection fluorescence (TR-TIRF) spectroscopy were used to examine the adsorption, solvation, and aggregation of a coumarin solute, Coumarin 152 (C152), at the silica/methanol interface. Experiments were performed as a function of bulk C152 concentration with SHG data providing information about relative surface coverage and ground state solvation environment. TR-TIRF data measured emission lifetimes of C152 adsorbed to the silica/methanol interface. SHG spectra show strong resonance enhancement at 365 nm, a result that is blue-shifted considerably from C152's electronic excitation of ∼400 nm in bulk methanol. Given C152's solvatochromic behavior, this observation implies that C152 adsorbed to the silica/methanol interface experiences a local dielectric environment that is significantly less polar than in bulk methanol. TR-TIRF decays at sub-micromolar bulk concentrations were fit to two lifetimes: one assigned to C152 emission in bulk methanol (0.9 ns) and a longer lifetime assigned to contributions from adsorbed C152 (∼4 ns). The longer lifetime is similar to C152 in alkanes, a result that is consistent with SHG data. Isothermal data from SHG experiments show unusual behavior as bulk C152 concentration increases. Instead of approaching an asymptotic limit signifying monolayer coverage, the SHG response rises at the lowest C152 concentrations and then decreases dramatically, suggesting the onset of aggregate formation. Changes in the TR-TIRF emission behavior of C152 at higher C152 bulk concentrations support this hypothesis. These findings are interpreted in terms of C152's ability to self-associate, and the energetics of dimer formation are explored using ab initio calculations and polarizable continuum models.

Hindered Isomerization at the Silica/Aqueous Interface: Surface Polarity or Restricted Solvation?

“…A small but measurable fraction of C152's emission fits to a much longer lifetime (4.02 ns) that has been assigned previously to dimers in solution. 15 For TIR measurements carried out at the silica/aqueous interface, the fluorescence decay from a saturated C152 solution is best fit with two lifetimes: a short 0.47 ns lifetime assigned to C152 in bulk solution and a longer lifetime of 3.52 ns assigned to C152 adsorbed to the surface (Figure 3). Normalized amplitudes for the short and long lifetimes are 0.87 and 0.13, respectively.…”

“…Experiments were carried out using a locally constructed time-correlated single photon counting (TCSPC) fluorometer that has been described previously . The output of a Ti:sapphire oscillator (Chameleon, Coherent) was frequency doubled (APE Autotracker), and the 80 MHz repetition rate was attenuated to 4 MHz using an electro-optic modulator (Conoptics model 350-105).…”

“…13,14 ■ EXPERIMENTAL SECTION Experiments were carried out using a locally constructed timecorrelated single photon counting (TCSPC) fluorometer that has been described previously. 15 The output of a Ti:sapphire oscillator (Chameleon, Coherent) was frequency doubled (APE Autotracker), and the 80 MHz repetition rate was attenuated to 4 MHz using an electro-optic modulator (Conoptics model 350-105). Fluorescence emission was collected using a PicoQuant 200 time-to-amplitude converter, and the instrument response function (IRF) was measured using a nonemissive Ludox scattering solution.…”

Quantifying Solute Partitioning in Phosphatidylcholine Membranes

“…Photon emission was collected at 500 nm. Additional details about this assembly can be found in previous reports. ,, …”

“…Additional details about this assembly can be found in previous reports. 21,26,32 In a given TCSPC experiment, signal was collected until the maximum intensity at early times reached a threshold of 8000 counts. Depending on the local solvation environment of the solute (and thus radiative rate and fluorescence quantum yield), the time required for each experiment varied with temperature.…”