Debashis Majhi

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, 1.64, 1.36, and 1.32 times higher than the estimated (from Stokes-Einstein-Debye theory) values for the same respective systems. This indicates that the emitting species is not the monomeric imidazolium moiety rather an associated species, and volume of the rotating fluorescing species decreases even though the length of the alkyl moiety on the anions is increased. The shift in the (1)H proton signal as well as a change in the width of the same signal upon dilution of the neat ionic liquids indicates that ionic liquids exist in the aggregated form. Further investigation through the 2D-ROESY experiment shows that interaction between imidazolium and sulfate is relatively stronger in the ethyl system than that of the longer octyl system. FCS measurements independently show that the hydrodynamic volume decreases with an increase in the anion chain length. The NMR and FCS results are consistent with the findings of the fluorescence anisotropy study.

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

The fluorescence response of coumarin 153 (C153) has been investigated in a series of 1-alkyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate room temperature ionic liquids (RTILs) with systematic variation of alkyl chain length (ethyl, butyl and hexyl) to examine the effect of the alkyl side chain length of the cationic moiety on solute and solvent relaxation dynamics. Physicochemical properties associated with the present RTILs are estimated at different temperatures. While the viscosity values increase with increasing alkyl chain length, density values decrease with increasing length of alkyl side chain. Steady state fluorescence measurements reveal that C153 experiences more nonpolar microenvironments with an increase in the alkyl chain length. Time resolved studies have demonstrated that the lengths of alkyl side chain have a noticeable role in governing the solvation dynamics in these media. It has been observed that the average solvent relaxation time estimated for these RTILs can be better correlated when both the size of the alkyl chains and the bulk viscosity of the respective RTILs are considered. Interestingly, apart from the viscosity effect, a negligible influence of alkyl chain length has been observed for rotational diffusion of C153.

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. The system is characterized by conventional analytical techniques. The photophysical properties of this supramolecular system in the absence and presence of various metal ions and nonmetal ions as additives are investigated in an acetonitrile medium. Utility of this system in an aqueous medium has also been demonstrated. The absorption and fluorescence spectrum of the molecular system consists of a broad band typical of an intramolecular charge-transfer (ICT) transition. A low quantum yield and lifetime of the NBD moiety in the present dyad indicates photoinduced electron transfer (PET) between DANS and the NBD moiety. The fluorescence intensity of the system is found to decrease in the presence of fluoride and acetate anions; however, the quenching is found to be much higher for fluoride. This quenching behavior is attributed to the enhanced PET from the anion receptor to the fluorophore moiety. The mechanistic aspect of the fluoride ion signaling behavior has also been studied by infrared (IR) and (1)H NMR experiments. The hydrogen bonding interaction between the acidic NH protons of the DPN moiety and F(-) is found to be primarily responsible for the fluoride selective signaling behavior. While investigating the cation signaling behavior, contrary to anions, significant fluorescence enhancement has been observed only in the presence of transition-metal ions. This behavior is rationalized by considering the disruption of PET communication between DANS and the NBD moiety due to transition-metal ion binding. Theoretical (density functional theory) studies are also performed for the better understanding of the receptor-analyte interaction. Interestingly, negative cooperativity in binding is observed when the interaction of this system is studied in the presence of both Zn(2+) and F(-). Fluorescence microscopy studies also revealed that the newly developed fluorescent sensor system can be employed as an imaging probe in live cells.

Easy access to new anthracenyl π-conjugates: generation of distinct AIE-active materials

Inspired by the well-established distyryl anthracenyl (DSA) π-conjugates as aggregation-induced emission active (AIE) materials and the evolution of the synthesis/optoelectronic properties of numerous anthracene-based π-conjugates, a new series of 9-or 10-(hetero)arylated anthracenyl π-conjugates were synthesized using a straightforward and metal-free protocol by starting with a unique family of organophosphonates. The fluorescence behaviors of these (hetero)arylated π-conjugates were systematically studied to determine their AIE-activities, resulting in a new class of much smaller anthracene-based AIEgens. Considerable effort was made to analyze the AIE-behaviors of selected molecules by performing single crystal X-ray diffraction, time-resolved fluorescence, and dynamic light scattering studies. Our mechanistic investigations reveal that the heteroarene(s) linked to a π-conjugated system facilitate the AIE-behaviors of these molecules. Fluorescence microscopy images of the selected compounds in both molecular and aggregated form were obtained to demonstrate their structure–property relationship. Further, a trisubstituted anthracene-based π-conjugate was synthesized and established as a new AIEgen.

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.

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. The homo-FRET process in this case has been established by exploiting steady-state fluorescence anisotropy measurements. The feasibility of aggregate formation and the homo-FRET interaction between the dye molecules has also been demonstrated through quantum mechanical calculations.