Dynamics of photoisomerisation and rotational relaxation of 3,3′-diethyloxadicarbocyanine iodide in room temperature ionic liquid and binary mixture of ionic liquid and water

Ionic liquids: structure and photochemical reactions

Ionic liquids are subjects of intense current interest within the physical chemistry community. A great deal of progress has been made in just the past five years toward identifying the factors that cause these salts to have low melting points and other useful properties. Supramolecular structure and organization have emerged as important and complicated topics that may be key to understanding how chemical reactions and other processes are affected by ionic liquids. New questions are posed, and an active debate is ongoing regarding the nature of nanoscale ordering in ionic liquids. The topic of reactivity in ionic liquids is still relatively unexplored; however, the results that have been obtained indicate that distributed kinetics and dynamical heterogeneity may sometimes, but not always, be influencing factors.

Photochromism of nitrobenzospiropyran in phosphonium based ionic liquids

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1',3',3'-trimethyl-6-nitrospiro[1-benzopyran-2,2'-1H-indole] (BSP) and its photo-induced merocyanine isomer (MC) were investigated in phosphonium based ILs by UV-vis absorption spectroscopy. It was found that the kinetics and thermodynamics of the BSP <--> MC equilibrium were sensitive to the nature of the anion. The MC lambda(max) shifted from 560 nm to 578 nm when in solutions of [P(1,4,4,4)][tos] and [P(6,6,6,14)][dca], respectively. The BSP isomer was highly favoured at equilibrium in the ILs studied; K(e) values observed were similar to non-polar solvents such as dichloromethane. The thermal relaxation of MC in all ILs is first order, and in comparison with aprotic polar solvents possessing comparable polarity (such as acetonitrile). Thermal relaxation rates of MC were monitored over a range of temperatures; at 293 K rates varied from 5.19 to 25.03 x 10(-4) s(-1). A non-linear relationship between K(e) and k was observed; this contradicts what is expected for BSP in molecular solvents and suggests the isomers exhibit different molecular/solvation environments. The energetics of the thermal relaxation of MC in ILs were observed; activation energies ranged from 71 to 90 kJ mol(-1) and all ILs exhibit negative activation entropies ranging between -72 and -8.2 J K(-1) mol(-1). A linear relationship between activation energy and entropy was observed.

Interaction of ionic liquid with water with variation of water content in 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6])/TX-100/water ternary microemulsions monitored by solvent and rotational relaxation of coumarin 153 and coumarin 490

The interaction of water with room temperature ionic liquid (RTIL) [bmim][PF6] has been studied in [bmim][PF6]/TX-100/water ternary microemulsions by solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 490 (C-490). The rotational relaxation and average solvation time of C-153 and C-490 gradually decrease with increase in water content of the microemulsions. The gradual increase in the size of the microemulsion with increase in w0 (w0=[water]/[surfactant]) is evident from dynamic light scattering measurements. Consequently the mobility of the water molecules also increases. In comparison to pure water the retardation of solvation time in the RTIL containing ternary microemulsions is very less. The authors have also reported the solvation time of C-490 in neat [bmim][PF6]. The solvation time of C-490 in neat [bmim][PF6] is bimodal with time constants of 400 ps and 1.10 ns.

Picosecond Time-Resolved Fluorescence Study on Solute−Solvent Interaction of 2-Aminoquinoline in Room-Temperature Ionic Liquids: Aromaticity of Imidazolium-Based Ionic Liquids

Time-resolved fluorescence spectra and fluorescence anisotropy decay of 2-aminoquinoline (2AQ) have been measured in eight room-temperature ionic liquids, including five imidazolium-based aromatic ionic liquids and three nonaromatic ionic liquids. The same experiments have also been carried out in several ordinary molecular liquids for comparison. The observed time-resolved fluorescence spectra indicate the formation of pi-pi aromatic complexes of 2AQ in some of the aromatic ionic liquids but not in the nonaromatic ionic liquids. The fluorescence anisotropy decay data show unusually slow rotational diffusion of 2AQ in the aromatic ionic liquids, suggesting the formation of solute-solvent complexes. The probe 2AQ molecule is likely to be incorporated in the possible local structure of ionic liquids, and hence the anisotropy decays only through the rotation of the whole local structure, making the apparent rotational diffusion of 2AQ slow. The rotational diffusion time decreases rapidly by adding a small amount of acetonitrile to the solution. This observation is interpreted in terms of the local structure formation in the aromatic ionic liquids and its destruction by acetonitrile. No unusual behavior upon addition of acetonitrile has been found for the nonaromatic ionic liquids. It is argued that the aromaticity of the imidazolium cation plays a key role in the local structure formation in imidazolium-based ionic liquids.

Photoinduced Electron Transfer Reaction in Room Temperature Ionic Liquids: A Combined Laser Flash Photolysis and Fluorescence Study

A detailed study of the photoinduced electron transfer (PET) reaction between pyrene and N,N-dimethylaniline has been made in four different room temperature ionic liquids (ILs) using steady state and time-resolved fluorescence and laser flash photolysis techniques. Unlike that in the conventional media, no exciplex emission for this well-known system could be observed in ILs. The rate constants for the PET induced quenching of the fluorescent state of pyrene, which lie between 6.9 and 37 x 107 M-1 s-1 depending on the viscosity, are found to be 2-4 times higher than the diffusion-controlled rates in ILs. The primary photoproducts of the PET process have been characterized by transient absorption spectroscopy, and the yields of the solvent-separated PET products have been determined. Even in the least viscous IL, [emim][Tf2N], the yield of the solvent-separated radical ion is estimated to be only 0.015 +/- 0.005. In more viscous ILs such as [bmim][PF6], the yield is found to be so low that absorption due to these species could not be observed. The rate constant for the escape of the ionic products from the geminate ion pair in ILs has been estimated to be nearly 2-3 orders of magnitude lower than the back electron transfer rate. However, the small fraction of the PET products, which manage to escape geminate recombination, have been found to survive much longer compared to those in less viscous conventional solvents.

Interaction of ionic liquid with water in ternary microemulsions (Triton X-100/water/1-butyl-3-methylimidazolium hexafluorophosphate) probed by solvent and rotational relaxation of coumarin 153 and coumarin 151

The interaction of ionic liquid with water in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])/Triton X-100 (TX-100)/H2O ternary microemulsions, i.e., "[bmim][PF6]-in-water" microregions of the microemulsions, has been studied by the dynamics of solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 151 (C-151). The variation of the time constants of solvent relaxation of C-153 is very small with an increase in the [bmim][PF6]/TX-100 ratio (R). The rotational relaxation time of C-153 also remains unchanged in all micremulsions of different R values. The invariance of solvation and rotational relaxation times of C-153 indicates that the position of C-153 remains unaltered with an increase in R and probably the probe is located at the interfacial region of [bmim][PF6] and TX-100 in the microemulsions. On the other hand, in the case of C-151, with an increase in R the fast component of the solvation time gradually increases and the slow component gradually decreases, although the change in solvation time is small in comparison to that of microemulsions containing common polar solvents such as water, methanol, acetonitrile, etc. The rotational relaxation time of C-151 increases with an increase in R. This indicates that with an increase in the [bmim][PF6] content the number of C-151 molecules in the core of the microemulsions gradually increases. In general, the solvent relaxation time is retarded in this room temperature ionic liquid/water-containing microemulsion compared to that of a neat solvent, although retardation is very small compared to that of the solvent relaxation time of the conventional solvent in the core of the microemulsions.

Formation of an unusual charge-transfer network from an ionic liquid

An intriguing and novel charge-transfer complex between dimethyldihydrophenazine and diethylviologen has been crystallized from an ionic liquid at room temperature, resulting in an interesting stacking motif of interrupted D***A***D type triads: efficient formation of the complex is seen within an ionic liquid and acetone, with the complex absorbing strongly across nearly the entire visible-NIR spectral region.

Dynamics of Solvent and Rotational Relaxation of Coumarin 153 in Room-Temperature Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate Confined in Brij-35 Micelles: A Picosecond Time-Resolved Fluorescence Spectroscopic Study

The dynamics of solvent and rotational relaxation of Coumarin 153 (C-153) in ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and in the ionic liquid confined in Brij-35 micellar aggregates have been investigated using steady-state and time-resolved fluorescence spectroscopy. We observed slower dynamics in the presence of micellar aggregates as compared to the pure IL. However, the slowing down in the solvation time on going from neat IL to IL-confined micelles is much smaller compared to that on going from water to water-confined micellar aggregates. The increase in solvation and rotational time in micelles is attributed to the increase in viscosity of the medium. The slow component is assumed to be dependent on the viscosity of the solution and involves large-scale rearrangement of the anions and cations while fast component is assumed to originate from the initial response of the anions during excitation. The slow component increases due to the increase in the viscosity of the medium and increase in fast component is probably due to the hydrogen bonding between the anions and polar headgroup of the surfactant. The dynamics of solvent relaxation was affected to a small extent due to the micelle formation.

Do organic solutes experience specific interactions with ionic liquids?

In an attempt to understand the nature of interactions between organic solutes and room temperature ionic liquids, temperature-dependent rotational relaxation of two structurally similar nondipolar solutes--2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)--has been examined in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6(-)]). Even with the ionic liquid, where the cation and the anion are strongly associated, the solute DPP experiences specific interactions, which is evident from its reorientation times that are 50%-60% longer in relation to DMDPP. It has been noticed that the reorientation times of both the solutes are faster in [bmim+][PF6(-)] than in glycerol, which is also a strongly associated solvent and whose viscosity is similar to the ionic liquid. This observation has been explained by taking into consideration the relative sizes of the solvents. By comparing the ratios of the reorientation times of DPP to DMDPP, in [bmim+][PF6(-)] and glycerol, it has been deduced that the strengths of the interaction between DPP-[bmim+][PF6(-)] and DPP-glycerol are the same.

Fluorescence Studies in a Pyrrolidinium Ionic Liquid: Polarity of the Medium and Solvation Dynamics

While the imidazolium ionic liquids have been studied for some time, little is known about the pyrrolidinium ionic liquids. In this work, steady-state and picosecond time-resolved fluorescence behavior of three electron donor-acceptor molecules, coumarin-153 (C153), 4-aminophthalimide (AP), and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), has been studied in a pyrrolidinium ionic liquid, N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, abbreviated here as [bmpy][Tf2N]. The steady-state fluorescence data of the systems suggest that the microenvironment around these probe molecules, which is measured in terms of the solvent polarity parameter, E(T)(30), is similar to that in 1-decanol and that the polarity of this ionic liquid is comparable to that of the imidazolium ionic liquids. All three systems exhibit wavelength-dependent fluorescence decay behavior, and the time-resolved fluorescence spectra show a progressive shift of the fluorescence maximum toward the longer wavelength with time. This behavior is attributed to solvent-mediated relaxation of the fluorescent state of these systems. The dynamics of solvation, which is studied from the time-dependent shift of the fluorescence spectra, suggests that approximately 45% of the relaxation is too rapid to be measured in the present setup having a time resolution of 25 ps. The remaining observable components of the dynamics consist of a short component of 115-440 ps (with smaller amplitude) and a long component of 610-1395 ps (with higher amplitude). The average solvation time is consistent with the viscosity of this ionic liquid. The dynamics of solvation is dependent on the probe molecule, and nearly 2-fold variation of the solvation time depending on the probe molecule could be observed. No correlation of the solvation time with the probe molecule could, however, be observed.

On the Optical Properties of the Imidazolium Ionic Liquids

Room-temperature ionic liquids, particularly those based on substituted imidazolium cations, are currently being extensively studied for a variety of applications. Herein, we explore the suitability of several imidazolium salts in optical applications by carefully examining the electronic absorption and fluorescence behavior of these substances, generally believed to be transparent in most of the UV region and fully transparent in the visible region. It is shown that all imidazolium ionic liquids are characterized by significant absorption in the entire UV region and a long absorption tail that extends into the visible region. These absorption characteristics are attributed to the imidazolium moiety and its various associated structures. When excited in the UV or early part of the visible region, these liquids exhibit fluorescence, which covers a large part of the visible region and shows dramatic excitation wavelength dependence. The excitation wavelength dependent shift of the fluorescence maximum has been rationalized taking into consideration the existence of the various associated structures of the ionic liquids and the inefficiency of the excitation energy-transfer process between them. The results imply that these liquids may have serious drawbacks in some of the optical studies.

Dynamics of Solvation and Rotational Relaxation of Coumarin 153 in Ionic Liquid Confined Nanometer-Sized Microemulsions

The effects of confinement of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate on solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in Triton X-100/cyclohexane microemulsions have been explored using steady-state and picosecond time-resolved emission spectroscopy. The steady-state and rotational relaxation data indicate that C-153 molecules are incorporated in the core of the microemulsions. The average rotational relaxation time increases with increase in w ([bmim][BF(4)]/[TX-100]) values. The solvent relaxation in the core of the microemulsion occurs on two different time scales and is almost insensitive to the increase in w values. The solvent relaxation is retarded in the pool of the microemulsions compared to the neat solvent. Though, the retardation is very small compared to several-fold retardation of the solvation time of the conventional solvent inside the pool of the microemulsions.