Anunay Samanta

Excited-state dipole moments of some Coumarin dyes from a solvatochromic method using the solvent polarity parameter, ENT

Excited-state dipole moments of a set of Coumarin dyes extensively used in laser applications have been determined for the first time, employing a solvatochromic procedure utilising the microscopic solvent polarity parameter, ENT. Our results are expected to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts is superior to that obtained using bulk solvent polarity functions. The experimentally determined dipole-moment changes are in fair agreement with recent semi-empirical computational predictions. It is shown that the discrepancies observed in some cases can be overcome using more appropriate parametrisation in the calculation. The insight provided by the present measurements into the nature of the emitting state is mentioned.

Excited-state proton-transfer dynamics of 7-hydroxyquinoline in room temperature ionic liquids

Excited-state proton transfer (ESPT) reaction of 7-hydroxyquinoline (7-HQ) mediated by methanol molecules has been studied in two room temperature ionic liquids (RTILs) using steady-state and time-resolved fluorescence measurements. While no ESPT is observable in neat RTILs, characteristic tautomer fluorescence of 7-HQ could be observed in the presence of small quantity of methanol (0.5-4.1 M). The observation of a rise time (350 ps-1.4 ns) associated with the tautomer fluorescence suggests that proton transfer in 7-HQ is indeed an excited-state phenomenon that requires considerable solvent reorganization prior to the relay of proton from the hydroxyl group to the distant ring nitrogen atom through suitably organized dimeric chain of methanol molecules. The rise time of the tautomer fluorescence, which has been found to decrease with increasing methanol concentration, is attributed to the change of viscosity of the medium upon methanol addition. While the influence of viscosity on the ESPT kinetics is evident from the data, lack of any definite correlation between the bulk viscosity and the rise time has been interpreted in terms of the microheterogeneous nature of the media that does not allow assessment of the microviscosity around 7-HQ from the bulk viscosity.

How polar are room-temperature ionic liquids?

The solvent strength and polarity of four imidazolium and pyridinium based ionic liquids, as measured using two different fluorescent probes, indicate that these liquids are more polar than acetonitrile but less polar than methanol.

Fluorescence signalling of transition metal ions by multi-component systems comprising 4-chloro-1, 8-naphthalimide as fluorophore

Abstract Fluorescence of 1a and 1b is ‘switched on’ by the quenching transition metal ions and the extent of fluorescence enhancement is found to be considerably higher than that expected on the basis of photoinduced intramolecular electron transfer in the systems. The observation has been rationalised taking into account the hydrated nature of the transition metal salts and preferential solvation of the fluorophore by the water molecules.

Microheterogeneity of some imidazolium ionic liquids as revealed by fluorescence correlation spectroscopy and lifetime studies.

The microscopic structure and dynamics of the room temperature ionic liquids (RTILs) that are responsible for some of the peculiar properties of this class of solvents continue to intrigue the researchers and stimulate new investigations. Herein, we use the fluorescence correlation spectroscopy (FCS) technique to study the diffusion of some probe molecules in RTILs, the results of which, when combined with those obtained from fluorescence lifetime studies, provide insights into the microscopic structural details of this class of novel solvents. Experiments performed with three charged and neutral probe molecules in five carefully selected 1-alkyl-3-methylimidazolium ionic liquids reveal that unlike in conventional solvents these probes exhibit a bimodal diffusion behavior in RTILs thus indicating the presence of two distinct environments. It is found that the contribution of the slow component of the diffusion increases with increasing alkyl chain length of the cation. Not only are these results supported by the biexponential decay behavior of the fluorescence intensity of the systems, but the individual values of the lifetime components and their weight allow determination of the nature of the two major environments. In essence, the results point to the potential of the two combined techniques in unraveling some of the complex features of the ionic liquids.

Fluorescence Quenching of CdS Quantum Dots by 4‐Azetidinyl‐7‐Nitrobenz‐2‐Oxa‐1,3‐Diazole: A Mechanistic Study

Fluorescence quenching of CdS quantum dots (QDs) by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady-state and time-resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge-transfer interactions. The bimolecular quenching rate constant estimated from the Stern-Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.

Effect of the Alkyl Chain Length on the Rotational Dynamics of Nonpolar and Dipolar Solutes in a Series of N-Alkyl-N-Methylmorpholinium Ionic Liquids

Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem. B2012, 116, 13430-13438). Analysis of the measured rotational time constants of the solutes in terms of the Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that with increase in the alkyl chain length attached to the cationic component of the ionic liquids, AP shows stick to superstick behavior, PRODAN rotation lies between stick and slip boundary conditions, whereas anthracene exhibits slip to sub slip behavior. The contrasting rotational dynamics of these probe molecules is a reflection of their location in distinct environments of the ionic liquids thus demonstrating the heterogeneity of these ionic liquids. The microheterogeneity of these media, in particular, those with the long alkyl chain, is further evidence from the excitation wavelength dependence study of the rotational diffusion of the dipolar probe molecules.

Fluorescence response of coumarin-153 in N-alkyl-N-methylmorpholinium ionic liquids: are these media more structured than the imidazolium ionic liquids?

The fluorescence behavior of coumarin-153 (C153) has been studied in four N-alkyl-N-methylmorpholinium ionic liquids differing in the alkyl chain length attached to the N-methylmorpholinium cation as a function of the excitation wavelength and temperature to understand some of the physicochemical characteristics of these largely unexplored ionic liquids. While the polarity of the ionic liquid with the smallest alkyl chain length is found comparable to that of the commonly used imidazolium ionic liquids, the probe molecule experiences a less polar environment with increasing chain length of the alkyl group attached to the morpholinium cation. The room temperature steady-state fluorescence spectrum of C153 in these solvents is found to be dependent on the excitation wavelength, and this effect is most pronounced in long chain containing ionic liquids. A bathochromic shift of the fluorescence maximum is observed at higher temperature. The excitation wavelength and temperature dependence of the fluorescence of C153 is explained considering a domain structure of these ionic liquids. The time-resolved fluorescence anisotropy measurements indicate the microviscosity around the probe molecule to be significantly different from the bulk viscosity of the long-chain ionic liquids. The solvent reorganization dynamics, as studied by monitoring the time-dependent fluorescence Stokes shift of C153 in these ionic liquids, is found to be slow and similar to that in imidazolium ionic liquids. The time-resolved measurements under isoviscous conditions seem to provide additional support to the organized domain structure of these ionic liquids.

Free Volume Dependence of the Internal Rotation of a Molecular Rotor Probe in Room Temperature Ionic Liquids

The fluorescence efficiency of a well-known microviscosity probe, 9-(dicyanovinyl)julolidine (DCVJ), which is highly sensitive to the viscosity of the medium, has been studied in seven imidazolium ionic liquids (ILs) of varying viscosities over a temperature range of 10-60 degrees C. The microviscosities around the probe in different ILs have been estimated from the linear dependence of the logarithm of fluorescence quantum yield (log phi(f)) on the logarithm of the bulk viscosity (log eta) in various conventional solvents of different viscosities at room temperature. These microviscosities, which represent the local environments around the probe, are found to be significantly different from the directly measured bulk viscosities of these ILs. The log phi(f) vs log (eta/T) plots, which are also expected to be linear, interestingly show a bilinear behavior in more viscous ILs with a break around 28-30 degrees C. The observation of a similar break in the Arrhenius plots of the rate constant of the internal rotation in DCVJ and absence of any such break in the temperature dependence of the mobility of the ILs allow us to determine the important role of the free volume around the probe in dictating the nonradiative deactivation rate or the fluorescence efficiency of DCVJ. The break in the plots, which implies a change in the available free volume around the probe at approximately 28-30 degrees C, presumably arises from the repositioning of the probe from one environment to a different one of these microheterogeneous ILs with change of temperature.

A two-dimensional chromogenic sensor as well as fluorescence inverter: selective detection of copper(II) in aqueous medium

A new chromogenic sensor (1) has been synthesized that shows simultaneous blue shift and intensity loss of the absorption maximum as well as quenching of fluorescence, selectively, in the presence of Cu2+ ions at physiological pH.