CHEMISTRY: Probing Free Volume

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Science  27 Feb 2004:
Vol. 303, Issue 5662, pp. 1259c
DOI: 10.1126/science.303.5662.1259c

Molecular motion in liquids and glasses is often explained in terms of the extent of free volumes that can exist between the molecules. Vallée et al. present single-molecule spectroscopic data that characterize local changes in a free volume space in polymer films. The dye molecule, tetraphenoxy-perylenetetracarboxyl diimide, can exist in two conformations: one in which the core of the molecule lies flat and one in which the core is twisted. The flat conformer has a shorter fluorescence lifetime and a larger van der Waals volume (greater by 0.2 cubic nm). Fluorescence lifetime distributions were measured for this dye in thin films of two polymers of the same family: poly(methyl methacrylate) (PMMA) and poly(n-butyl methacrylate) (PnBMA). The smaller side chains of PMMA result in a lower free volume, and indeed, only longer lifetime traces of the twisted dye conformer were seen. However, for PnBMA, about 30% of the dye molecules could find holes large enough to produce the larger flat conformer. Thus holes as large as 0.2 nm3 can form in PnBMA but not in PMMA. — PDS

J. Am. Chem. Soc. 10.1021/ja031599g (2004).

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