Unraveling heterogeneous microviscosities of the 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different chain lengths

The rotational dynamics of coumarin 153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different alkyl chain lengths (alkyl = butyl, pentyl, hexyl, heptyl, octyl) ([C(n)mim][PF6], n = 4-8) to examine the alkyl chain length dependent local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (I-E/I-M) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl) propane (BPP), is also employed to study the microviscosity of [C(n)mim][PF6] as a complementary measurement. The rotational dynamics of C153 show that at a certain length of the alkyl chain there are incompact and compact domains within [C(n)mim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosities in different structural domains of [C(n)mim][PF6] with different alkyl chain lengths are investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that with an increase in the length of the alkyl chain, the microviscosity of [C(n)mim][PF6] is obviously increased first and then slightly decreased. The steady state fluorescence measurements with the microviscosity probe of BPP further prove that the microviscosity is not increased as much as expected when ionic liquids [C(n)mim][PF6] have a relatively long alkyl chain. The different heterogeneous structures of [C(n)mim][PF6] with different lengths of the alkyl chain are proposed to interpret the unusual microviscosity behaviors.