The excited-state relaxation dynamics of D149, one of the metal-free substituted indoline dyes used in dye-sensitized solar cells, is studied in the whole composition range of the 1butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 )−acetonitrile binary mixture by using time-integrated absorption, emission, and time-resolved transient absorption (TA) spectroscopies. The comparative analysis of absorption and emission spectra indicates that the value of Stokes shift reduces monotonically with decreasing mixture polarity. The global analysis of timeresolved TA spectra indicates the presence of four different time components related to different processes in the excited state of the dye. Importantly, the observed timescales are highly sensitive to composition, polarity, and viscosity of the binary mixture. Increase of viscosity and decrease of polarity observed for increasing ionic liquid (IL) content in the mixture lead to overall increase in the emission lifetime (S 1 −S 0 ) of D149. At a lower IL mole fraction (X IL = 0.1), the emission lifetime shows a minimum that can be traced back to the change from the situation in which the local environment of the dye is dominated by the interactions in acetonitrile to that in which it is dominated by those in BmimBF 4 . This also is reflected in the occurrence of a minimum in relative quantum yield in the same range of X IL . The origin of the other moderately long-time component (33 ps in ACN-120 ps in BmimBF 4 ) is still debatable; however, for pure IL and all the mixtures, the composition dependence of this timescale is similar to that of the longest emission lifetime.

The inhomogeneity distribution in four imidazolium-based ionic liquids (ILs) containing the 1-butyl-3-methylimidazolium (C 4 mim) cation, coupled with tetrafluoroborate (BF 4 ), hexafluorophosphate (PF 6 ), bis(trifluoromethanesulfonyl)amide (TFSA), and trifluoromethanesulfonate (TfO) anions, was characterized using Voronoi polyhedra. For this purpose, molecular dynamic simulations have been performed on the isothermal−isobaric (NpT) ensemble. We checked the ability of the potential models to reproduce the experimental density, heat of vaporization, and transport properties (diffusion and viscosity) of these ionic liquids. The inhomogeneity distribution of ions around the ring, methyl, and butyl chain terminal hydrogen atoms of the C 4 mim cation was investigated by means of Voronoi polyhedra analysis. For this purpose, the position of the C 4 mim cation was described successively by the ring, methyl, and butyl chain terminal hydrogen atoms, while that of the anions was described by their F or O atom. We calculated the Voronoi polyhedra distributions of the volume, the density, and the asphericity parameters as well as that of the radius of the spherical intermolecular voids. We carried out the analysis in two steps. In the first step, both ions were taken into account. The calculated distributions gave information on the neighboring ions around a reference one. In the second step, to distinguish between like and oppositely charged ions and then to get information on the inhomogeneity distribution of the like ions, we repeated the same calculations on the same sample configurations and removed one of the ions and considered only the other one. Detailed analysis of these distributions has revealed that the ring hydrogen atoms are mainly solvated by the anions, while the methyl and butyl terminal H atoms are surrounded by like atoms. The extent of this inhomogeneity was assessed by calculating the cluster size distribution that shows that the dimers are the most abundant ones.

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for a mixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF6 ) ionic liquid and acetonitrile...