H and C chemical shift-structure effects in anhydrous β-caffeine and four caffeine-diacid cocrystals probed by solid-state NMR experiments and DFT calculations.

By using density functional theory (DFT) calculations, we refined the H atom positions in the structures of β-caffeine (C), α-oxalic acid (OA; (COOH)), α-(COOH)·2HO, β-malonic acid (MA), β-glutaric acid (GA), and I-maleic acid (ME), along with their corresponding cocrystals of 2 : 1 (2C-OA, 2C-MA) or 1 : 1 (C-GA, C-ME) stoichiometry. The corresponding C/H chemical shifts obtained by gauge including projector augmented wave (GIPAW) calculations agreed overall very well with results from magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments. Chemical-shift/structure trends of the precursors and cocrystals were examined, where good linear correlations resulted for all COO sites against the H⋯O and/or H⋯N H-bond distance, whereas a general correlation was neither found for the aliphatic/caffeine-stemming H sites nor any C chemical shift against either the intermolecular hydrogen- or tetrel-bond distance, except for the OOH sites of the 2C-OA, 2C-MA, and C-GA cocrystals, which are involved in a strong COO⋯ bond with caffeine that is responsible for the main supramolecular stabilization of the cocrystal.

Insights into cation-anion hydrogen bonding in mesogenic ionic liquids: an NMR study.

The hydrogen-bonding interaction is studied in imidazolium-based mesogenic ionic liquids in their isotropic, smectic, and solid phases and in a nanoconfined state by proton solid-state nuclear magnetic resonance (NMR). In the smectic phase, the more basic anions form stronger hydrogen bonds. A small decrease of H-bonding in the mesophase with respect to that in the isotropic phase is associated with the presence of a layered assembly with high orientational order and limited conformational freedom.

Ion conformation and orientational order in a dicationic ionic liquid crystal studied by solid-state nuclear magnetic resonance spectroscopy.

Ionic liquids crystals belong to a special class of ionic liquids that exhibit thermotropic liquid-crystalline behavior. Recently, dicationic ionic liquid crystals have been reported with a cation containing two single-charged ions covalently linked by a spacer. In ionic liquid crystals, electrostatic and hydrogen bonding interactions in ionic sublayer and van der Waals interaction in hydrophobic domains are the main forces contributing to the mesophase stabilization and determining the molecular orientational order and conformation.

NMR Spectroscopic Studies of Cation Dynamics in Symmetrically-Substituted Imidazolium-Based Ionic Liquid Crystals.

Ionic liquid crystals (ILCs) present a new class of non-molecular soft materials with a unique combination of high ionic conductivity and anisotropy of physicochemical properties. Symmetrically-substituted long-chain imidazolium-based mesogenic ionic liquids exhibiting a smectic liquid crystalline phase were investigated by solid state NMR spectroscopy and computational methods. The aim of the study was to reveal the correlation between cation size and structure, local dynamics, and orientational order in the layered mesophase.

Understanding ionic mesophase stabilization by hydration: a solid-state NMR study.

The correlation between the water contribution to hydrogen bonding within ionic sublayer, mesophase order parameter, and ion translational self-diffusion in the layered ionic liquid crystalline phase is investigated. Changes in hydrogen bonding, conformational and translational dynamics, and orientational order upon hydration were followed by solid-state NMR combined with density functional theory (DFT) analysis. We observed that the smectic mesophase of monohydrated imidazolium-based ionic liquids, which was stabilized in a wider temperature range compared to that of anhydrous materials, counterintuitively exhibited a lower orientational order of organic cations.

Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids through Various Spectroscopic Techniques.

In recent times, it has been shown that certain binary mixtures of pure ionic liquids having appropriate chemical composition can behave like a new chemical entity. However, current knowledge about the microscopic behavior of these interesting systems is rather limited. The present study is undertaken with an objective to understand the microscopic behavior in terms of intermolecular interaction, structure, and dynamics of these systems.

Evidence of homo-FRET in quantum dot-dye heterostructured assembly.

With an aim to understand the intermolecular/particle interaction and the optical properties of the inorganic-organic hybrid nanostructured materials, Förster resonance energy transfer (FRET) between negatively charged CdS quantum dots (donor) and positively charged Oxazine 170 perchlorate (acceptor) has been investigated by employing steady-state and time-resolved fluorescence spectroscopy. Investigations revealed that size-dependent changes in the FRET efficiency of different QD-dye FRET pairs occurred mainly due to the electrostatic effects. Interestingly, the present study also reveals that at a higher concentration of dye molecules, aggregation occurs on the QD surface and the quenching of dye fluorescence occurs due to homo-FRET process.

Differences in the behavior of dicationic and monocationic ionic liquids as revealed by time resolved-fluorescence, NMR and fluorescence correlation spectroscopy.

With an aim to understand the behavior in terms of the intermolecular interactions, structure and dynamics of dicationic and monocationic ionic liquids (ILs), two imidazolium-based dicationic ionic liquids (DILs), 1,8-bis-(3-methylimidazolium-1-yl)octane bis-(trifluoromethylsulfonyl)amide ([C(mim)][NTf]), 1,9-bis-(3-methylimidazolium-1-yl)nonane bis-(trifluoromethylsulfonyl)amide ([C(mim)][NTf]), and one monocationic ionic liquid (MIL), 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C(mim)][NTf]), have been investigated through combined fluorescence, electron paramagnetic resonance (EPR), NMR and fluorescence correlation spectroscopy (FCS). The DILs were synthesized by following a standard synthetic protocol and subsequently characterized by different analytical techniques. Steady state absorption, emission and EPR spectroscopic data reveal that DILs are less polar compared to MIL.

Probing the microscopic structural organization of neat ionic liquids (ILs) and ionic liquid-based gels through resonance energy transfer (RET) studies.

With the aim to understand the role of the ionic constituents of ionic liquids (ILs) in their structural organization, resonance energy transfer (RET) studies between ionic liquids (donor) and rhodamine 6G (acceptor) have been investigated. RET studies have been exploited for the present investigation due to the fact that the said process is extremely sensitive to the distance, and a change in the donor-acceptor distance due to a change in the structural organization can be probed. Basically, steady state and time-resolved fluorescence measurements have been carried out in two different sets of ILs, where in one set (1-ethyl-3-methyl imidazolium alkyl sulfate) the alkyl side chain length on the anionic moiety is systematically varied and in the other set variation is done on the cation (aromatic and nonaromatic).

Linking Diffusion-Viscosity Decoupling and Jump Dynamics in a Hydroxyl-Functionalized Ionic Liquid: Realization of Microheterogeneous Nature of the Medium.

Analysis of time-resolved fluorescence anisotropy data in light of the Stokes-Einstein-Debye hydrodynamic description reveals significant decoupling of rotational motion of the solute and the viscosity of the medium for a hydroxyl-functionalized ionic liquid (IL). This behavior and NMR experiments indicate that the hydroxyl-functionalized IL is more heterogeneous than other structurally similar ILs. Considering that recent theoretical investigations have demonstrated that the jump dynamics and hydrogen-bond fluctuations are closely related in viscous media, in such a case the hydrodynamic description can provide inconsistent results, and the present inapplicability of the hydrodynamics description in explaining solute rotation in a viscous hydroxyl-functionalized IL perhaps provides experimental support to the role of orientational jumps and hydrogen bond formation in that event.

Probing the interactions of structurally similar but chemically distinguishable organic solutes with 1-ethyl-3-methylimidazolium alkyl sulfate (alkyl = ethyl, hexyl and octyl) ionic liquids through fluorescence, NMR and fluorescence correlation spectroscopy (FCS) studies.

The rotational diffusion of two organic solutes, 4-(azitidine-1-yl)-7-nitrobenzo[1,2,5]oxadi-azole (ANBD) and 7-nitrobenzo[1,2,5]oxadiazole-4-amine (HNBD), has been examined in 1-ethyl-3-methylimidazolium alkyl sulfate (alkyl = ethyl, hexyl and octyl) ionic liquids with the aim of understanding intermolecular interactions among solute and solvent molecules. Solute-solvent interaction has also been investigated by employing fluorescence, NMR and fluorescence correlation spectroscopy (FCS). The ionic liquids (ILs) having a fixed cationic moiety are chosen to monitor the roles of the alkyl chain length as well as the solute-anion (sulfate) interactions in the rotational diffusion of the solutes in the given ILs.

Probing the Aggregation Behavior of Neat Imidazolium-Based Alkyl Sulfate (Alkyl = Ethyl, Butyl, Hexyl, and Octyl) Ionic Liquids through Time Resolved Florescence Anisotropy and NMR and Fluorescence Correlation Spectroscopy Study.

Aggregation behavior of a series of neat 1-ethyl 3-methylimidazolium alkyl sulfate (alkyl = ethyl, butyl, hexyl, and octyl) ionic liquids has been investigated through combined time-resolved fluorescence spectroscopy, 1-D and 2-D NMR spectroscopy, and fluorescence correlation spectroscopy (FCS). Interestingly, experimentally measured rotational relaxation times (τr) for ethyl, butyl, hexyl and octyl systems are measured to be 2.25, 1.

Analyte interactions with a new ditopic dansylamide-nitrobenzoxadiazole dyad: a combined photophysical, NMR, and theoretical (DFT) study.

We report herein the synthesis and photophysical studies on a new multicomponent chemosensor dyad comprising two fluorescing units, dansylamide (DANS) and nitrobenzoxadiazole (NBD). The system has been developed to investigate receptor-analyte binding interactions in the presence of both cations and anions in a single molecular system. A dimethyl amino (in the DANS unit) group is used as a receptor for cations, and acidic hydrogens of sulfonamide and the NBD group are used as receptors for anions.

Probing the aggregation behavior of 4-aminophthalimide and 4-(N,N-dimethyl) amino-N-methylphthalimide: a combined photophysical, crystallographic, microscopic and theoretical (DFT) study.

Two fluorescent molecules, 4-aminophthalimide (AP) and 4-(N,N-dimethyl)amino-N-methylphthalimide (DMP) have been used as the building blocks to fabricate fluorescent organic nano particles. DMP, the analogue of AP, has been synthesized by substituting all the amine hydrogens of AP with methyl groups to get an idea about the effect of intermolecular hydrogen bonding interactions (N-H···) on the aggregation behavior of these molecules. All the systems have been characterized by field emission scanning electron microscopy (FESEM).