Effect of isotope substitution on the Fermi resonance and vibrational lifetime of unnatural amino acids modified with IR probe: A 2D-IR and pump-probe study of 4-azido-L-phenyl alanine.

The infrared (IR) probe often suffers from an unexpected complex absorption profile due to the Fermi resonance and short vibrational lifetime, which restricts the application of time-resolved IR spectroscopy to investigate the site-specific structural dynamics of the protein. Researchers have found that isotope substitution to the IR probe not only removes the Fermi resonance but also extends the dynamic observation window with a prolonged vibrational lifetime. This method has been successfully applied to modify the vibrational properties of many IR probes for time-resolved spectroscopy and imaging. In this study, the effect of isotope substitution (15N) on the vibrational properties of the azide stretching band in 4-azido-L-phenylalanine has been investigated using ultrafast pump-probe and 2D-IR spectroscopy. In contrast to the earlier reports, it has been observed that the Fermi resonance remains unchanged even after isotope substitution, and there is very little change in the vibrational relaxation dynamics as well. Anharmonic frequency analysis reveals that the α-N atom of N3 is being shared between the two transitions participating in the Fermi resonance and gets affected similarly due to isotope labeling. Hence, this study unveils the specific circumstance at which the isotope labeling strategy may not be successful in eliminating the Fermi resonance band and explains the molecular origin behind it. This study also suggests definitive approaches on how to overcome the limitations related to the Fermi resonance to extend the development and application of this IR probe for biological research.

Triazole derived azo-azomethine dye as a new colorimetric anion chemosensor.

In the pursue of developing anion sensors, an efficient triazole derived azo-azomethine dye chemosensor (S) that differentially senses F‾ and AcO‾ ions has been reported. The ions recognition ability of S was investigated by colorimetric and UV-visible spectroscopic methods. Interestingly, this chemosensor molecule is virtually inactive in presence of other anions such as Cl‾, Br‾ and I‾ and HSO4‾. We have further presented a ratiometric approach to differentiate F‾ and AcO‾ ions. The reversibility of F‾ ion binding with S was established by the addition of Ca(NO3)2 to the fluoride bound S, which led to the regeneration of S. The quantum chemical calculation of energies of unbound and bound S has been employed using Density Functional Theory (DFT) to understand the interaction between chemosensor and anions. Evidence in support of fluoride-induced deprotonation of a O-H bond during the detection of F⁻ ion has been demonstrated by employing 1H NMR titration experiments.

Weak Donor-/Strong Acceptor-Linked Anthracenyl π-Conjugates as Solvato(fluoro)chromophore and AEEgens: Contrast between Nitro and Cyano Functionality.

Steady development on photophysical behaviors for a variety of organic fluorophores inspired us to generate anthracene-based fluorescent molecules with a strong acceptor and a significantly weak donor through a π-spacer. Such molecules are found to have substantial twisted conformational orientations in the solid state and enhanced apolar character because of the attachment of tolyl or mesityl group with an anthracenyl core. Upon exposure to a variety of solvents, strong solvatochromism was noticed for 4-nitro compound (84 nm red shift) in contrast to the cyano analogue (18 nm red shift). Both these probes were highly emissive in apolar solvents while nitro-analogue, in particular, could discriminate the solvents of E T(30) (a measure of microscopic solvent polarity) ranging from 31 to 37. Thus, 4-nitro compounds can be successfully employed to detect and differentiate the apolar solvents. On the contrary, the 2-nitro analogue is almost nonemissive for the same range of solvents perhaps because of favorable excited-state intramolecular proton-transfer process. The fundamental understanding of solvatochromic properties through the formation of twisted intramolecular charge-transfer (TICT) state is experimentally analyzed by synthesizing and studying the π-conjugates linked to only benzene in place of nitro or cyanobenzene, which exhibits no solvatochromism and that helped finding the possible emission, originated from the locally excited state. Moreover, the molecular structures for these compounds are determined by the single-crystal X-ray diffraction studies to examine the change in emission properties with molecular packing and alignment in the aggregated state. The measurement of dihedral angles between the substituents and anthracenyl core was helpful in finding the possible extent of electronic conjugations within the system to decipher both solvatochromism and aggregation enhanced emission (AEE)-behavior. The cyano analogue exhibited prominent AEE-behavior, whereas nitro analogues showed the aggregation-caused quenching effect. The reason behind such dissimilarity in solvatochromism and AEE-behavior between cyano- and nitro-linked anthracenyl π-conjugates are also addressed through experimental outcomes.

Haloarene-Linked Unsymmetrically Substituted Triarylethenes: Small AIEgens To Detect Nitroaromatics and Volatile Organic Compounds.

Unsymmetrically substituted triarylalkenes as aggregation-induced emission-active fluorogens (AIEgens) are sporadically explored by different researchers. In this Article, naphthalene, biphenyl, and haloarene-linked new triarylethenes are conveniently synthesized and presented as unsymmetrically substituted extensive π-conjugates to continue the discovery of small molecules as new AIEgens. Moreover, fluorophores attached to haloarenes are noteworthy, but such compounds are barely investigated as AIEgens. The possible mechanism underlying this AIE-behavior has also been addressed by the support of experimental/theoretical outcomes. Moreover, two of these small AIEgens are fruitfully employed for rapid sensing of nitroaromatic compounds (NACs) where picric acid (PA, as a model explosive) showed a strong quenching efficiency with the detection limit of 39 or 48 ppb along with other nitroaromatics such as p-nitrotoluene and p-nitrophenol. This quenching could be visualized by the naked eye under a UV (365 nm) lamp and performed almost in an aqueous medium. Such alkenes are also proved to exhibit very clean on/off fluorescence switching properties for polar volatile organic compounds (VOCs).

Understanding the Microscopic Behavior of the Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt through Time-Resolved Fluorescence, Nuclear Magnetic Resonance (NMR), and Electron Paramagnetic Resonance (EPR) Studies.

The present study has been undertaken with an aim to find out the suitability of a binary solvent system, comprising an anionic liquid, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OHEMIM][NTf2]) and ethylene glycol (EG), toward lithium ion battery applications. For this purpose, the behavior in terms of structure, intermolecular interaction, and dynamics of several solvent systems, [OHEMIM][NTf2], [OHEMIM][NTf2]-LiNTf2(lithium bis(trifluorimethylsulfonyl)imide), [OHEMIM][NTf2]-EG, and [OHEMIM][NTf2]-EG-LiNTf2, is investigated by carrying out steady state and time-resolved fluorescence, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) measurements. Both steady-state fluorescence and EPR studies have pointed out that the micropolarities of [OHEMIM][NTf2]-EG-LiNTf2 are close to those of neat RTIL. However, studies on rotational dynamics have revealed that the structural organization of [OHEMIM][NTf2]-LiNTf2 is significantly influenced upon addition of EG. Interestingly, the average solvation time is observed to be relatively faster in [OHEMIM][NTf2]-EG-LiNTf2 than those in other solvent systems. Since average solvation time and conductivity are inversely related to each other, the present observation indicates that the introduction of EG is helpful in increasing the electrical conductivity of [OHEMIM][NTf2]-EG-LiNTf2. Translational diffusion coefficient measurements in [OHEMIM][NTf2]-EG-LiNTf2 and [OHEMIM][NTf2]-LiNTf2 through NMR spectroscopy have also indicated the suitability of [OHEMIM][NTf2]-EG-LiNTf2 as a potential electrolytic medium for battery applications.

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. In the present study, two chemically distinguishable solutes with structural similarity in their fluorophore unit are employed so that the difference in the rotational behavior of the solutes, if any, can be correlated with the solute-solvent interactions, and not with other factors like the size of the solutes etc. The steady state absorption and emission data indicate that HNBD and ANBD exist in different microenvironments in both ILs. The rotational diffusion data for both the solutes have been analyzed in light of hydrodynamic and quasi hydrodynamic theories. Interestingly, the rotational dynamics of ANBD is observed to be relatively fast and shows slip hydrodynamics, whereas HNBD exhibits hindered rotation with superstick behavior. The hindered rotation of HNBD as compared to that of ANBD in both ILs has been explained by considering the strong hydrogen bonding interactions between the two N-H groups of HNBD and sulfate anions of ILs. The faster rotation of ANBD with the increasing anion chain length has been explained by invoking quasi hydrodynamic theory. The presence of a specific hydrogen bonding interaction between solute (HNBD) and solvent (ILs) molecules has been confirmed by (1)H-NMR experiments. FCS measurements have independently demonstrated the relatively strong association of HNBD with the present ILs as compared to that of ANBD.

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.

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). Photophysical behavior of these well characterized systems has been investigated in molecular as well as aggregated forms. Interestingly, while the AP-aggregates exhibit a blue-shifted absorption band (as compared to AP in its molecular form), DMP-aggregates exhibit a red-shifted absorption band (as compared to DMP in its molecular form). These absorption data indicate the formation of H and J aggregates for AP and DMP, respectively. The intermolecular interactions that are responsible for the molecular self assembly of AP and DMP are studied by using X-ray crystallography. X-ray analysis demonstrates the presence of strong intermolecular hydrogen bonding interactions in AP, but only weak interactions (C-H···O, C-H···π) in the case of DMP. X-ray analysis also demonstrates that varying the nature of intermolecular interactions leads to different modes of aggregation. Theoretical studies (DFT and TD-DFT) have been carried out to investigate how different modes of aggregation lead to changes in the optical (UV-VIS spectra) properties of these systems.

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 and rotational relaxation dynamics of coumarin 153 have been investigated in a dicationic ionic liquid (IL), 1,6-bis-(3-methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), for the first time to provide a comprehensive and a quantitative understanding of the nature of the ionic fluid and its influence on the average solvation and rotational relaxation time. On several occasions, the photophysical data obtained in the present dicationic IL have also been compared with the monocationic imidazolium-based ionic liquid so as to find out the difference between their behaviors. The dicationic ionic liquid has been synthesized via a two step process and subsequently characterized by conventional spectroscopic methods. Steady state absorption and fluorescence measurements reveal that the polarity of the ionic liquid is close to that of dichloromethane. Steady state excitation wavelength dependent fluorescence measurement indicates the micro-heterogeneous nature of the ionic liquid. However, the steady state excitation wavelength dependent fluorescence response is found to be similar for both the dicationic and a structurally similar monocationic ionic liquid. In the time-resolved fluorescence studies, contrary to the monocationic imidazolium-based ionic liquid, no missing ultra-fast component of solvation has been observed in the present dicationic IL. Excitation wavelength dependence of the average solvation and rotation times also indicates the micro-heterogeneous nature of these media. When viscosity dependence (η) of the measured average solvation 〈τs〉 and rotation 〈τr〉 times are verified by the relation: 〈τx〉 ∝ (η/T)(p) (where 'x' is solvation or rotation, p is the exponent and T is the temperature), the fractional dependence of both average solvation and rotational times with the medium viscosity have been observed. The recent findings (J. Chem. Phys., 2012, 136, 174503; Chem. Phys. Lett., 2011, 517, 180; ChemPhysChem, 2012, 13, 2761) and the outcome of the present study suggest that the observed viscosity-diffusion (η-D) decoupling for the present dicationic ionic liquid is due to the dynamic heterogeneity of the medium.

Toward understanding solute-solvent interaction in room-temperature mono- and dicationic ionic liquids: a combined fluorescence spectroscopy and mass spectrometry analysis.

Rotational relaxation dynamics of nonpolar perylene, dipolar coumarin 153, and a negatively charged probe, sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS), have been investigated in a dicationic ionic liquid, 1,6-bis-(3-methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), and a structurally similar monocationic ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), to have a comprehensive and a quantitative understanding on the solute-solvent interaction in these media. Analysis of the rotational relaxation dynamics data by Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that perylene rotation is found to be the fastest compared to the other two probes and shows slip to sub-slip behavior, coumarin 153 rotation lies between the stick and slip boundary, and MPTS shows a superstick behavior in [C6MIm][NTf2]. Interestingly, MPTS exhibits a normal SED hydrodynamics in dicationic [C6(MIm)2][NTf2]2, in spite of the fact that dicationic ionic liquid contains two cationic sites bearing acidic hydrogen (C2-H) which may be available to form stronger interaction with the negatively charged MPTS. The difference in the rotational diffusion behavior of these three probes is a reflection of their location in different distinct environments of these ILs. Superstick behavior of MPTS in monocationic IL has been attributed to its specific hydrogen bonding interaction with the corresponding imidazolium cation. The relatively faster rotational behavior of MPTS in dicationic IL has been explained by resorting to mass spectrometry. Mass spectral analysis demonstrates that positively charged (imidazolium) sites in dicationic IL are strongly associated with negatively charged bis-(trifluoromethylsulfonyl)amide anion (NTf2(-)), which in turn makes it difficult for imidazolim cation to have stronger hydrogen bonding interaction with bulkier negatively charged molecule MPTS.

Probing the microscopic aspects of 1-butyl-3-methylimidazolium trifluoroacetate ionic liquid and its mixture with water and methanol: a photophysical and theoretical (DFT) study.

Considering the potential of mixed ionic liquid-cosolvent systems in wide range of applications, photophysical and theoretical studies on an industrially important ionic liquid, 1-butyl-3-methylimidazolium trifluoroacetate (BMIMTFA), and also its mixture with water and methanol have been investigated. Two organic dipolar solutes coumarin 153 (C153) and 2-aminonitrofluorene (ANF) have been used as the probe molecule for the present study. Steady-state absorption and emission spectral behavior of C153 has not been significantly influenced by both the cosolvents. However, excitation wavelength dependent measurements with ANF in the BMIMTFA-water and BMIMTFA-methanol show entirely different photophysical response. For BMIMTFA-methanol system the average solvation and rotational time is found to be less than that in BMIMTFA-water system. Quite interestingly, time-resolved fluorescence anisotropy measurements reveal two different solute-solvent coupling constant (C(obs)) even if same mole fraction of water and methanol is used for the mixed solvent systems. Theoretical calculations also reveal stronger intermolecular interaction between IL and methanol than that between IL and water. The present combined photophysical and theoretical calculations seem to suggest different microscopic structural organization in the two binary systems.

Diffusion-viscosity decoupling in solute rotation and solvent relaxation of coumarin153 in ionic liquids containing fluoroalkylphosphate (FAP) anion: a thermophysical and photophysical study.

Steady state and time-resolved fluorescence behavior of coumarin153 (C153) has been investigated in two ionic liquids (ILs), namely 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MOEMPL][FAP]) and 1-(2-methoxyethyl)-1-methylmorpholinium tris(pentafluoroethyl)trifluorophosphate ([MOEMMO][FAP]) in order to find out the viscosity-diffusion decoupling during solvation and rotational relaxation of C153. Thermophysical studies have also been carried out to understand the physicochemical properties of the media. At 293 K, the measured viscosity of [MOEMMO][FAP] is 8 times higher than that of [MOEMPL][FAP]. The data obtained from steady state and time-resolved fluorescence measurements show the deviation of average solvation and rotation times from conventional hydrodynamics. The decoupling of solute and solvent dynamics from medium viscosity is manifested through fractional viscosity dependence (η) of the measured average solvation (<τ(s)>) and rotation (<τ(r)>) times: <τ(x)> [proportionality] (η/T)(p) (x denotes solvation or rotation and T is the temperature) covering the p value 0.69 < p < 0.85 for solvent relaxation and 0.48 < p < 1.10 for solute rotation. The excitation wavelength dependent fluorescence studies have been performed to correlate the experimental findings with the heterogeneity of the medium. While the excitation wavelength dependent time-resolved fluorescence studies of coumarin153 reveal a very similar variation of average solvation time with a change in excitation wavelengths for both the ionic liquids, the steady state excitation wavelength dependent measurements of 2-amino-7-nitrofluorene (ANF) show a higher (630 cm(-1)) shift of the fluorescence maximum for highly viscous ionic liquid as compared to that (430 cm(-1)) of another much less viscous ionic liquid. The recent theoretical (Chem. Phys. Lett.2011, 517, 180) and experimental (J. Chem. Phys.2012, 136, 174503) findings and the outcome of the excitation wavelength dependent fluorescence measurements in the present case seem to suggest that both static and dynamic heterogeneity may play an important role in the observed viscosity-diffusion (d-η) decoupling for highly viscous ionic liquid.