Stokes Shift Dynamics in Ionic Liquids: Temperature Dependence

Dynamic emission Stokes shift and liquid-like dielectric solvation of band edge carriers in lead-halide perovskites

Lead-halide perovskites have emerged as promising materials for photovoltaic and optoelectronic applications. Their significantly anharmonic lattice motion, in contrast to conventional harmonic semiconductors, presents a conceptual challenge in understanding the genesis of their exceptional optoelectronic properties. Here we report a strongly temperature dependent luminescence Stokes shift in the electronic spectra of both hybrid and inorganic lead-bromide perovskite single crystals. This behavior stands in stark contrast to that exhibited by more conventional crystalline semiconductors. We correlate the electronic spectra with the anti-Stokes and Stokes Raman vibrational spectra. Dielectric solvation theories, originally developed for excited molecules dissolved in polar liquids, reproduce our experimental observations. Our approach, which invokes a classical Debye-like relaxation process, captures the dielectric response originating from the incipient anharmonicity of the LO phonon at about 20 meV (160 cm-1) in the lead-bromide framework. We reconcile this liquid-like model incorporating thermally-activated dielectric solvation with more standard solid-state theories of the emission Stokes shift in crystalline semiconductors.

Solvation, rotational relaxation and fluorescence correlation spectroscopic study on ionic liquid-in-oil microemulsions containing triple-chain surface active ionic liquids (SAILs)

In this article, solvation dynamics and rotational relaxation approaches have been applied to explore the microheterogeneity of surface active ionic liquid (SAIL) containing microemulsions, i.e. [P₁₃][Tf₂N] or [N₃₁₁₁][Tf₂N]/[BHD][AOT]/[IPM].

Understanding Structure-Property Correlation in Monocationic and Dicationic Ionic Liquids through Combined Fluorescence and Pulsed-Field Gradient (PFG) and Relaxation NMR Experiments

Steady state, time-resolved fluorescence and NMR experiments are carried out to gain deeper insights into the structure-property correlation in structurally similar monocationic and dicationic room-temperature ionic liquids (RTILs). The excitation wavelength dependent fluorescence response of fluorophore in 1-methy-3-propyllimidazolium bis(trifluoromethylsulfonyl)amide [C3MIm][NTf2] is found to be different from that of 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide [C6(MIm)2][NTf2]2 and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [C6MIm][NTf2]. The outcomes of the present solvent dynamics study in [C3MIm][NTf2] when compared with those in [C6(MIm)2][NTf2]2 and in [C6MIm][NTf2] from our previous studies (Phys. Chem. Chem. Phys. 2014, 16, 12918-12928) indicate the involvement of dipolar rotation of imidazolium cation during solvation. To correlate the findings of solvation dynamics study with the dipolar rotation of the imidazolium ring, pulsed-field gradient (PFG)-NMR technique for translational diffusion coefficient measurement and (1)H as well as (19)F spin-lattice relaxation measurements are employed. NMR investigation reveals that an ultrafast component of solvation can be related to the dipolar rotation of imidazolium cation; hence, the role of dipolar rotation of cations in governing the dynamics of solvation in ILs cannot be ignored. Analysis of the rotational relaxation dynamics data by the Stokes-Einstein-Debye hydrodynamic theory unveils distinctive features of solute-solvent interaction in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2.

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid

We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888 ][NTf2 ]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888 ][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

Investigation of the influence of alkyl side chain length on the fluorescence response of C153 in a series of room temperature ionic liquids

Variation of average solvation time with the product of temperature averaged viscosity and the radius of the cation of different room temperature ionic liquids (RTILs) with varying cationic chain length.

Molecular dynamics investigation of the vibrational spectroscopy of isolated water in an ionic liquid

Experimental studies examining the structure and dynamics of water in ionic liquids (ILs) have revealed local ion rearrangements that occur an order of magnitude faster than complete randomization of the liquid structure. Simulations of an isolated water molecule embedded in 1-butyl-3-methyl imidazolium hexafluorophosphate, [bmim][PF6], were performed to shed insight into the nature of these coupled water-ion dynamics. The theoretical calculations of the spectral diffusion dynamics and the infrared absorption spectra of the OD stretch of isolated HOD in [bmim][PF6] agree well with experiment. The infrared absorption line shape of the OD stretch is narrower and blue-shifted in the IL compared to those in aqueous solution. Decomposition of the OD frequency time correlation function revealed that translational motions of the anions dominate the spectral diffusion dynamics.

Fluorescence response of a dipolar organic solute in a dicationic ionic liquid (IL): is the behavior of dicationic IL different from that of usual monocationic IL?

The solvation dynamics of coumarin 153 are compared in di and in mono-cationic-based ionic liquids for the first time.

Conductivity and Solvation Dynamics in Ionic Liquids

It was shown recently that a simple dielectric continuum model predicts the integral solvation time of a dipolar solute ⟨τsolv⟩ to be inversely proportional to the electrical conductivity σ0 of an ionic solvent or solution. In this Letter, we provide a more general derivation of this connection and show that available data on coumarin 153 (C153) in ionic liquids generally support this prediction. The relationship between solvation time and conductivity can be expressed by ln(⟨τsolv⟩/ps) = 4.37 - 0.92 ln (σ0/S m(-1)) in 34 common ionic liquids.