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J. Phys. Chem. B 2003, 107, 5926-5932
Solute Rotation and Solvation Dynamics in a Room-Temperature Ionic Liquid J. A. Ingram and R. S. Moog Department of Chemistry, Franklin and Marshall College, Lancaster, PennsylVania 17604
N. Ito, R. Biswas,† and M. Maroncelli* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: January 29, 2003; In Final Form: April 7, 2003
Steady-state spectra, rotation times, and time-resolved emission spectra of the probe 4-aminophthalimide (4-AP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6-]) were measured over the temperature range 298-355 K. The steady-state spectroscopy indicates that the solvation energetics of 4-AP in [bmim+][PF6-] are comparable to those of 4-AP in highly polar but aprotic solvents such as dimethylformamide and acetonitrile (π* ∼ 0.8, ENT ∼ 0.4). The rotation of 4-AP in [bmim+][PF6-] and in more conventional aprotic solvents generally conforms to the expectations of simple hydrodynamic models. Other than the fact that [bmim+][PF6-] is highly viscous, nothing distinguishes the rotation of 4-AP in this ionic liquid from its rotation in more conventional polar aprotic solvents. Time-dependent emission spectra, recorded with an instrumental response of 25 ps, indicate that solvation dynamics in [bmim+][PF6-] occur in two well-separated time regimes. Near to room temperature, the observable response takes place in the 0.1-2 ns time range. This component can be described by a stretched exponential time dependence with an exponent of 0.6-0.7, indicative of strongly nonexponential relaxation. The integral time of the observed component of solvation is proportional to the rotation time of 4-AP and to solvent viscosity, suggesting the involvement of substantial solvent rearrangement. In addition to this relatively slow component, more than half of the solvation response in [bmim+][PF6-] is faster than can be detected in these experiments, that is, takes place in 99%; 99%). Samples of 4-AP at concentrations providing optical densities of 2000 cm-1. First, the steadystate Stokes shift of 4-AP in [bmim+][PF6-] is ∼2450 cm-1 larger than in cyclohexane (Figure 1). The more sophisticated estimate of the “time-zero” spectrum46 (labeled “t ) 0” in Figure 3) also indicates a value of ∼2200 cm-1 for the total shift. The dynamic shift captured in the present experiments is only ∼1000 cm-1, meaning that less than half of the total spectral shift is
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5929
Figure 3. Examples of time-resolved emission spectra of 4-AP in [bmim+][PF6-] at 332 K. The top panel shows the spectra as reconstructed from fitted decays at various wavelengths (points) and the times indicated. The bottom panel shows the log-normal fits to these data in a frequency representation. The dashed curve labeled ss is the steady-state emission spectrum and the dashed-dot curve labeled t ) 0 is an estimate of spectrum prior to solvent relaxation (see text).
being observed here. The missing dynamics must be substantially faster than the components that are observed. For example, in the individual wavelength decays used to construct the spectra in Figure 3, there are no decay components observed with time constants less than 150 ps. In a search of the most sensitive, blue edge of the spectrum, we could detect no faster decay components, which implies that the missing dynamics must occur with time constants faster than ∼5 ps. Thus, the solvation dynamics in this ionic liquid proceeds on two highly disparate time scales, with roughly half of the dynamics being undetectably rapid, perhaps subpicosecond, and the remainder taking place on the 100 ps to 1 ns time scale. To analyze the observable portion of the dynamics, we utilize the parameters derived from log-normal fits to the spectra: the peak frequency νpk(t), the first frequency moment , and the full width of the spectrum Γ(t). These characteristics are plotted in Figure 4 and summarized in Table 2. The frequencies of the fully relaxed spectra ν(∞) increase systematically with increasing temperature, as do the frequencies of the steady-state emission spectra. The changes in ν(∞) are relatively small,
