Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem. B2012, 116, 13430-13438). Analysis of the measured rotational time constants of the solutes in terms of the Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that with increase in the alkyl chain length attached to the cationic component of the ionic liquids, AP shows stick to superstick behavior, PRODAN rotation lies between stick and slip boundary conditions, whereas anthracene exhibits slip to sub slip behavior. The contrasting rotational dynamics of these probe molecules is a reflection of their location in distinct environments of the ionic liquids thus demonstrating the heterogeneity of these ionic liquids. The microheterogeneity of these media, in particular, those with the long alkyl chain, is further evidence from the excitation wavelength dependence study of the rotational diffusion of the dipolar probe molecules.

The light-harvesting properties of both CdSe and CdTe nanocrystals are ideally suited for their use in quantum dot (QD)-sensitized solar cells. However, corrosion of the CdTe QD in an aqueous environment in the presence of sulfide/polysulfide electrolyte renders it unsuitable despite its better electron injection ability (compared to CdSe QD) to a large band-gap semiconductor like TiO2. In this work, we explore the stability of a CdTe QD, which we have developed exclusively for its use in ionic liquids, in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid in the presence of S2– and investigate the hole transfer process from this photoexcited QD to S2–. We not only demonstrate that an appropriate capping of the CdTe QD and use of an ionic liquid in place of the aqueous medium enhances the stability of the QD significantly in the presence of S2– but also provide evidence of hole transfer from a photoexcited QD to the sulfide salt using steady-state and time-resolved emission and ultrafast transient absorption measurements.