Electroabsorption spectroscopy of electronic transition for room-temperature ionic liquid molecules dispersed in a polymer matrix

- We measure electric field effects on absorption spectra of ionic liquids.
- Changes in dipole moment and polarizability on photoexcitation are determined.
- Pyridinium ions show reorientation along electric fields in polymer matrix.
- Lower limit of electric dipole moment in the ground state is evaluated.

Electroabsorption spectroscopy of electronic transitions in the room-temperature ionic liquids (RTILs) 1-butylpyridinium bis(trifluoromethylsulfonyl)imide ([C4py][NTf2]) and 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C4mpy][NTf2]) dispersed in poly(methyl methacrylate) (PMMA) is reported. [C4py][NTf2] shows a π → π∗ transition of the cation [C4py]+ with an absorption maximum at 258 nm. The changes in the dipole moment and the electronic polarizability between the excited and ground states are determined by analysis of the Stark effect in the absorption band. A field-induced reorientation effect of the pyridinium chromophore is observed at room temperature. This observation implies that the cation [C4py]+ is not rigidly fixed to the polymer matrix, which is consistent with the results of previous studies on hybrid materials of RTILs and PMMA. The lower limit of the dipole moment in the ground state is derived from analysis of the field-induced reorientation effect, and the results for [C4mpy][NTf2] are essentially the same as those for [C4py][NTf2].