Simulations of time-dependent fluorescence in nano-confined solvents

Abstract

The time-dependent fluorescence of a model diatomic molecule with a charge-transfer electronic transition in confined solvents has been simulated. The effect of confining the solvent is examined by comparing results for solutions contained within hydrophobic spherical cavities of varying size ͑radii of 10-20 Å͒. In previous work ͓J. Chem. Phys. 118, 6618 ͑2002͔͒ it was found that the solute position in the cavity critically affects the absorption and fluorescence spectra and their dependence on cavity size. Here we examine the effect of cavity size on the time-dependent fluorescence, a common experimental probe of solvent dynamics. The present results confirm a prediction that motion of the solute in the cavity after excitation can be important in the time-dependent fluorescence. The effects of solvent density are also considered. The results are discussed in the context of interpreting time-dependent fluorescence measurements of confined solvent systems.

Time-dependent fluorescence in nanoconfined solvents: Linear-response approximations and Gaussian statistics

“…1 for the 12 Å radius cavity, the AB dye molecule considered in this work sits near the wall when it is in its ground state but nearer the cavity center when it is in the electronic excited state. Hence, solute diffusion must occur after excitation and, as we have previously demonstrated for this model system, 31 can be observed in the dynamic Stokes shift. Second, as with planar solid-liquid interfaces, a nanoconfined solvent forms layers near the surface, exhibiting oscillations in the radial density that decrease in intensity moving away from the cavity wall.…”

“…Thus, this is presumably due primarily to solvent reorientation; however, we should note that there is solute diffusion, albeit limited, that occurs within the first few picoseconds and thus solvent reorientation and solute diffusion are not uncoupled. 31 The longest time scale is more than 50 ps and increases with decreasing cavity size. This is related to diffusion of the dye molecule after excitation as discussed above and in Ref.…”

“…This is related to diffusion of the dye molecule after excitation as discussed above and in Ref. 31; additional details of the connection with solute motion are provided below.…”

“…One of us has shown previously, using Monte Carlo and MD simulations, that nanoconfined solvents exhibit a number of properties that are distinct from the corresponding bulk solvent 30,31,41,42 and that these can have signatures in simulations of the time-dependent fluorescence. 31 First, the solvent polarity depends strongly on the distance from the cavity wall. The constraints placed on the solvent molecules near the wall reduce their ability to respond to the charges on a solute molecule in their midst.…”

“…30,31,35,36 The potential depends only on the radial distance of the Lennard-Jones site on the molecule from the center of the cavity. We consider a single solvent density, ρ = 0.7 g/cm 3 (the bulk density of CH 3 CN is 0.786 g/cm 3 ).…”