Limit guest mobility

The diffusion, or percolation,41 of a molecule within the channels of CyD or FAU inclusion compounds is expected to be much less than that in conventional solvents. Often, the decay rate (k0) of a carbene in a nonviscous organic solvent is diffusion limited, cf. kd(MeOH) = 1.20 x 1010M-1 s-1 25°C.67 Therefore, limited guest mobility not only will hamper intermolecular reactions but it should also prolong the lifetime (Y) of the carbene reaction intermediate, if intramolecular modes of decay are not accessible. 

To further characterize the mobility of the IRE loop, time-resolved isotropic fluorescence emission decay components of the IRE RNAs were determined as a function of temperature. Some details of the measurements and data assessment will be necessary here to appreciate both the utility of the information and caveats about its literal interpretation. Considering first the TCSPC instrument itself, some uncertainty in the measurements arise from its intrinsic parameters. With 300 nm incident light, the IRF of the photomultiplier tube ranged from 190 to 276 ps full-width at half-height (FWHH). The width of the IRF and the time resolution (32.5 ps/channel) limit the short components that can be reliably extracted from the fit, and certainly those <200 ps will have large errors on their amplitudes and lifetimes. Fluorescence emission decay components as short as 9—20 ps (Larsen et al., 2001) and 30—70 ps (Guest el al., 1991) (and much shorter by Wan et al., 2000) have been measured for 2AP in a stacked conformation, but in our instrument, a fit to such a short lifetime would be inaccurate. 

The phenomenon may be ascribed to the limited mobility of the naphthyl moiety when involved together with a guest molecule in the chiral Y CD cavity. 

The phenomenon observed here, which is common to most polymerization reactions, demonstrates that polymers can interfere sterically with processes involving movement of parts of the guest molecules (fluorescent probes). As the polymer glassy state is approached, the relative free volume diminishes sharply, and the medium viscosity increases rapidly mobility becomes restricted and the deactivation rate of the probe becomes controlled by the microscopic free volume provided by the polymer. This accounts for the abrupt increase in fluorescence until the limiting conversion is reached, at which point fluorescence levels off. 

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