The hydrophobicity of room-temperature ionic liquids (RTILs) has been shown to have a very significant effect on the optical and structural properties of and in RTILs. The average excited state lifetime of neat RTILs has been shown to be increasing with increasing hydrophobicity of the RTILs. By employing pico-nanosecond-based fluorescence anisotropy decay, the volume of the nanoaggregates in neat RTILs have been calculated. The volume of these nanoaggregates have been shown to be decreasing with increase in hydrophobicity of the RTILs. Thus, hydrophobicity has been shown to have an important role, i.e., hydrophobicity can be used as a handle to tune the properties of RTILs as designer solvents. Moreover, the excited-state lifetime of red-emitting fluorophores, i.e., whose fluorescence emission is not perturbed by the inherent emission of RTILs, has been shown to increase with the increasing hydrophobicity of the RTILs. Highly hydrophobic RTILs have been shown to exhibit positive deviation and highly hydrophilic RTIL has been shown to exhibit negative deviation from the linear correlation between average solvation time (τs) versus viscosity/temperature (η/T).