Temporal behavior of the singlet molecular oxygen emission in imidazolium and morpholinium ionic liquids and its implications.

The solvent effect of ionic liquids on the lifetime of singlet molecular oxygen, O2((1)Δg), was investigated by means of time-resolved near-IR emission spectroscopy. O2((1)Δg) was generated by photosensitization of methylene blue in morpholinium and imidazolium ionic liquids, both comprising various alkyl chains of different lengths. The measured time profiles of O2((1)Δg) luminescence for the (1)Δg → (3)Σg(-) transition were represented by a single-exponential decay function. The phosphorescence lifetime was found to be correlated with the alkyl chain length of the morpholinium ionic liquids. This observation was interpreted by considering efficient quenching of O2((1)Δg) through energy transfer to the high-frequency C-H stretching modes of the N-alkyl chain in the morpholinium cation. Interestingly, we found that O2((1)Δg) quenching by the C-H stretching modes of the ring of the cations was remarkably depressed. The kinetics of the O2((1)Δg) emission decay process was discussed on the basis of the heterogeneous structure of ionic liquids consisting of polar and nonpolar domains.

Does excited-state proton-transfer reaction contribute to the emission behaviour of 4-aminophthalimide in aqueous media?

4-Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent-sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time-dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited-state solvent-mediated proton-transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide-H protected derivative, N-BuAP, to prove that a solvent-assisted excited-state keto-enol transformation does not contribute to the steady-state and time-resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogen-bonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited-state proton-transfer reaction in the system, is explained by considering the difference in the influence of H(2)O and D(2)O on the nonradiative rates and ground-state exchange of the proton with the solvent.

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.

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.

Rotational dynamics of positively and negatively charged solutes in ionic liquid and viscous molecular solvent studied by time-resolved fluorescence anisotropy measurements.

Rotational dynamics of two negatively and positively charged solutes, 1-anilinonaphthalene-8-sulfonate (ANS) and ethidium bromide (EB), respectively, have been studied in an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]) and a conventional viscous solvent, glycerol, at different temperatures to obtain insight into the nature of various forces that operate in the ionic liquid. The fluorescence anisotropy of the systems decays exponentially with time in both the solvents. Under isoviscous conditions, the reorientation time of each probe molecule is found to be very similar in ionic liquid and glycerol indicating that the electrostatic forces exert negligible influence on the charged solute molecules. Analysis of the experimentally measured reorientation times of the two solutes using the Stokes-Einstein-Debye hydrodynamic theory shows that the rotational diffusion of ANS is best represented by a behavior that falls in-between the stick and slip boundary conditions, whereas EB exhibits a superstick behavior indicating its strong association with the solvent molecules. The association between the positively charged solute EB with ionic liquid and glycerol, which is also evident from a much higher value of the rotational coupling constant, appears to be mediated by hydrogen bonding interactions.