doi:10.1016/j.cplett.2004.07.032
Copyright © 2004 Elsevier B.V. All rights reserved.
Femtosecond studies of crown ethers: supramolecular solvation, local solvent structure and cation–π interaction
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Wenyun Lua, Weihong Qiua, Jongjoo Kima, Oseoghaghare Okobiaha, Jiaxin Hub, George W. Gokelb and Dongping Zhonga, 
, 
aDepartments of Physics, Chemistry and Biochemistry, OSU Biophysics, Chemical Physics and Biochemistry Programs, The Ohio State University, 174 West 18th Avenue, Columbus, OH 43210, USA
bDepartment of Molecular Biology and Pharmacology, School of Medicine, Washington University, 660 South Euclid Avenue, Campus Box 8103, St. Louis, MS 63110, USA
Received 11 June 2004;
revised 22 June 2004.
Available online 31 July 2004.
Abstract
We report here femtosecond studies of microsolvation, host-guest recognition, and cation–π interaction of crown ethers in organic solvents. The side-armed indole ring of the supramolecule acts as an optical probe and a π-donor. Significantly-slow solvation dynamics (e.g. 206 ps in acetonitrile) were observed, revealing ordered local solvent structures. With encapsulation of the alkali-metal cation in the macroring, solvation dynamics become much faster by one order of magnitude, reflecting significant solvent reorganization during molecular recognition. The combined data suggest a folded supramolecular structure involving intercalated solvent molecules or a sandwiched metal cation, consistent with the enhanced local polarization and the strong electrostatic interactions.
Fig. 1. Right: Molecular structure of BI18C6 and its tubular model. Left: The solid-state X-ray structure of compound BI18C6 complexed with the potassium cation K+ in CPK and tubular presentations.
Fig. 2. Steady-state fluorescence spectra of tryptophan and supramolecule BI18C6 in CH3CN and CH3OH at excitation of 290 nm. The ball–stick structures of two solvent molecules with their dipole moments are also shown. Note the different shifts in two solvents with and without addition of KI.
Fig. 3. Normalized, fs-resolved fluorescence transients of the supramolecule BI18C6 in CH3CN without and with the addition of KI in the short (left) and long (right) time ranges from more than ten gated fluorescence emissions. Note the drastic difference of transients at the long delay time without and with encapsulation of the cation K+.
Fig. 4. (a) Normalized, fs-resolved fluorescence transients of the supramolecule BI18C6 in CH3OH without and with KI from more than 10 gated fluorescence emissions. (b) Representation of normalized fs-resolved emission spectra (FRES) constructed from (a) at several times for BI18C6/KI in CH3OH. The symbol represents the experimental data and the solid line is a log-normal fit. The dashed line is the steady-state emission spectrum. (c) The fs-resolved emission maxima for the overall process (νs) and the lifetime emission (νl). In the insert, the entire evolution of emission maxima for both processes is shown to reach the stead-state emission (νss); see text.
Fig. 5. (a) Solvation correlation functions of CH3CN and CH3OH probed by the indole moiety of the supramolecule BI18C6 with and without encapsulation of the cation K+. The inset shows the correlation functions in the short time range. For clarity, the correlation function of CH3OH probed by BI18C6 is not shown in the long time range. (b) Femtosecond-resolved anisotropy for the systems studied and note the similar time scales with and without encapsulation of the cation K+.
Table 1.
Emission maxima and times from construction of fs-resolved fluorescence spectraa,b
a ν0, emission maximum at
t = 0;
νsc and
tsc, emission maximum and time when solvation ends;
νss and
tss, emission maximum and time when the emission reaches the steady state;
ν1 and
ν2, emission maxima for the two lifetime components, respectively.
b All emission maxima and times are in units of nm and ps, respectively.
Table 2.
Results obtained for solvation correlation functionsa
a All solvation correlation functions are best fitted with
. All time constants are in unit of ps.
b Previous studies using Coumarin 152 (C152) gave 0.27 ps (73%) and 1.1 ps (27%)
[27] and with C153 reported 89 fs (69%) and 630 fs (31%)
[28].
c Previous studies using C152 gave 1.2 ps (40%) and 9.6 ps (60%)
[27] and with C153 reported four components of 30 fs (10%), 280 fs (34%), 3.2 ps (30%) and 15.3 ps (26%)
[28].
Corresponding author. Fax: +1 614 292 7557
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