Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids

Ionic liquids, being composed of ions alone, may offer alternative pathways for molecular aggregation. These pathways could be controlled by the chemical structure of the cation and the anion of the ionic liquids. Intramolecular excimer formation dynamics of a bifluorophoric probe, 1,3-bis­(1-pyrenyl)­decane [1Py(10)­1Py], where the fluorophoric pyrene moieties are separated by a long decyl chain, is investigated in seven different ionic liquids in 10–90 °C temperature range. The long alkyl separator allows for ample interaction with the solubilizing milieu prior to the formation of the excimer. The ionic liquids are composed of two sets, one having four ionic liquids of 1-butyl-3-methyl­imidazolium cation ([bmim⁺]) with different anions and the other having four ionic liquids of bis­(trifluoro­methylsulfonyl)­imide anion ([Tf₂N^–]) with different cations. The excimer-to-monomer emission intensity ratio (<i>I</i>_E/<i>I</i>_M) is found to increase with increasing temperature in sigmoidal fashion. Chemical structure of the ionic liquid controls the excimer formation efficiency, as <i>I</i>_E/<i>I</i>_M values within ionic liquids with the same viscosities are found to be significantly different. The excited-state intensity decay kinetics of 1Py(10)­1Py in ionic liquids do not adhere to a simplistic Birk’s scheme, where only one excimer conformer forms after excitation. The apparent rate constants of excimer formation (<i>k</i>_a) in highly viscous ionic liquids are an order of magnitude lower than those reported in organic solvents. In general, the higher the viscosity of the ionic liquid, the more sensitive is the <i>k</i>_a to the temperature with higher activation energy, <i>E</i>_a. The trend in <i>E</i>_a is found to be similar to that for activation energy of the viscous flow (<i>E</i>_a,η). Stokes–Einstein relationship is not followed in [bmim⁺] ionic liquids; however, with the exception of [choline]­[Tf₂N], it is found to be followed in [Tf₂N^–] ionic liquids suggesting the cyclization dynamics of 1Py(10)­1Py to be diffusion-controlled and to depend on the viscosity of the ionic liquid irrespective of the identity of the cation. The dependence of ionic liquid structure on cyclization dynamics to form intramolecular excimer is amply highlighted

Selective Quenching of 2‑Naphtholate Fluorescence by Imidazolium Ionic Liquids

The effect of addition of water-miscible ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]­[BF₄]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]­[OTf]), and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([bmpyrr]­[OTf]), on photophysical properties of 2-naphthol in water at various pHs is reported. Electronic absorbance behavior of 2-naphthol dissolved in aqueous mixtures of ILs is observed to be similar to that found in water at different pHs. The excited-state properties, however, are changed dramatically as the IL is added to the milieu. The presence of imidazolium IL results in significant quenching of the fluorescence emission from 2-naphtholate. On the contrary, pyrrolidinium IL does not quench the fluorescence from the anionic species. The quenching of 2-naphtholate fluorescence by aromatic imidazolium cations in aqueous IL mixtures is found to follow simple Stern–Volmer behavior. The aromatic imidazolium cation acts as an electron/charge acceptor during the quenching process where formation of a weakly fluorescent complex between the imidazolium cation and the excited 2-naphtholate anion possibly involving the acidic C2 proton of imidazolium is proposed. Because of the absence of such an acidic proton, the nonaromatic pyrrolidinium cation is not able to form a complex with the excited 2-naphtholate and cannot act as an electron/charge acceptor. Excited-state emission intensity decay data further corroborate this hypothesis as the intensity decay fits well to a single-exponential decay with no change in recovered lifetimes as [bmpyrr]­[OTf] is added; a double-exponential decay is required to satisfactorily fit the decay data in the presence of [bmim]­[BF₄], hinting at the presence of a weakly fluorescent complex. The uniqueness of ILs in affecting excited-state properties of the 2-naphthol system is demonstrated through comparison with NaBF₄, NaCl, and polyethylene glycol with an average molecular weight of 200, respectively, as additives

Solvatochromic Probe Response within Ionic Liquids and Their Equimolar Mixtures with Tetraethylene Glycol

Synergism in a probe response within a mixture hints at the presence of strong interactions involving the solvent constituents of the mixture and possibly the probe. Unusual and rare “hyperpolarity” resulting from the synergism in probe response exhibited by ionic liquid (IL) mixtures with glycol family solvents is investigated in detail for equimolar mixtures of tetraethylene glycol (TEG) with many structurally different ILs using several UV–vis absorbance and fluorescence solvatochromic probes. Thirteen different ILs, of the same cation 1-butyl-3-methylimidazolium and different anions, of the same anion bis­(trifluoromethylsulfonyl)­imide and different cations, and of C2 methyl-substituted imidazolium cations, are used to assess the structural dependence of the IL on synergism exhibited by (IL + TEG) mixture. Responses from UV–vis absorbance probes are used to obtain <i>E</i>_T [dipolarity/polarizability and/or H-bond donating (HBD) acidity] and Kamlet–Taft parameters [π* (dipolarity/polarizability), α (HBD acidity), and β (HB accepting basicity)] within (IL + TEG) mixtures. The band I-to-band III fluorescence intensity ratio of dipolarity probe pyrene along with the lowest energy fluorescence band maxima of pyrene-1-carboxaldehyde (PyCHO, a probe for the permittivity of the medium), coumarin-153 and <i>N</i>,<i>N</i>-dimethyl-6-propionyl-2-naphthylamine PRODAN (neutral photoinduced charge-transfer fluorescence probes), and 6-<i>p</i>-toluidine-2-naphthalenesulfonic acid (TNS) and l-anilinonaphthalene-8-sulfonate (ANS) (ionic photoinduced charge-transfer fluorescence probes) are used to assess whether synergism is exhibited by (IL + TEG) equimolar mixtures. Probe responses within TEG equimolar mixtures with ILs are compared to those with common organic solvents. An attempt is made to establish a correlation between the synergism observed in the probe response within an (IL + TEG) mixture and the structural features of the cation and anion of the IL, such as acidity of the protons of the cation, aromaticity of the cation, and size, shape, and coordinating ability of the anion. It is established that the solvatochromism exhibited by the probes within (IL + TEG) mixtures is due to complex coupling of several different interactions and dynamical processes involving the probe as well as IL and TEG within the mixture

Evidence of Water-in-Ionic Liquid Microemulsion Formation by Nonionic Surfactant Brij-35

Brij-35, a common and popular nonionic surfactant, is shown to form water-in-ionic liquid (w/IL) microemulsions with IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]­[PF₆]) as the bulk phase. The presence of w/[bmim]­[PF₆] microemulsions is hinted by the significantly increased solubility of water in Brij-35 solution of [bmim]­[PF₆]. The formation of w/[bmim]­[PF₆] microemulsions by Brij-35 is confirmed using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. Brij-35 forms reverse micelle-type aggregates within [bmim]­[PF₆] in the absence of added-water. These reverse micelles become w/[bmim]­[PF₆] microemulsions as the water is added to the system. As the water loading (<i>w</i>₀) is increased, the average diameter of the aggregates increases. Fourier transform infrared (FTIR) absorbance data reveal the presence of both “bound” and “free” water within the system. The “bound” water is associated with the water pools of the w/[bmim]­[PF₆] microemulsions. Excited-state proton transfer (ESPT) involving probe pyranine shows deprotonation of pyranine within the water pools of the w/[bmim]­[PF₆] microemulsions

Microviscosity Offered by Ionic Liquids and Ionic Liquid–Glycol Mixtures Is Probe Dependent

Interactions present within the solubilizing media constituted of ionic liquids (ILs) govern the outcomes of chemical processes carried out within such media by controlling the behavior of solutes dissolved therein. Fluidity afforded by IL-based media, in this context, not only reveals interactions present within the system, but it also helps decide whether the system is suitable for an application. The response of spectroscopic microviscosity probes dissolved in IL and IL-based solvents, in this regard, reveals information on both solute–solvent and solvent–solvent interactions present within the system. Interactions present within the cybotactic region of a microviscosity probe strongly depend on the functionalities present on the probe. Five different fluorescence probes representing three different classes of microviscosity reporting systems based on intramolecular excimer formation, steady-state fluorescence anisotropy, and fluorescence intensity are used to explore fluidity afforded by 11 ILs and their equimolar mixtures with tetraethylene glycol (TEG), a solvent having the ability to readily hydrogen bond, under ambient conditions. ILs are categorized into four sets: first having the same cation and different anions to assess the role of the anion, second having the same anion and different cations to assess the role of the cation, and the last two having C2–H substituted imidazolium cations with different anions to reveal the role of C2–H, if any, in H-bonding interactions involving ILs. The responses of all of the microviscosity probes within ILs and (IL + TEG) equimolar mixtures are compared with the bulk dynamic viscosities of the corresponding systems. The overall investigation reveals the lack of a trend between the probe responses representing microviscosity and the dynamic viscosities of IL and (IL + TEG) mixtures. The cybotactic region viscosity of the (IL + TEG) equimolar mixture depends on the identity of the probe; it may be higher than, lower than, or in between the microviscosities reported in both neat IL and neat TEG. Evidence of the role of C2–H on the imidazolium cation in intraspecies (within IL) or interspecies (between IL and TEG) H-bonding was not found

Florescence Quenching within Lithium Salt-Added Ionic Liquid

Salt-added ionic liquid media have emerged as a versatile alternative to the conventional electrolytes in several applications. A lithium bis­(trifluoromethylsulfonyl)­imide (LiTf₂N)-added ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([emim]­[Tf₂N]) system up to a LiTf₂N mole fraction (<i>x</i>_LiTf₂N) of 0.40 is investigated using a fluorophore–quencher pair of pyrene–nitromethane in the 298.15–358.15 K temperature range. Excited-state intensity decay of pyrene fits best to a single-exponential decay function irrespective of the concentration of nitromethane, <i>x</i>_LiTf₂N, and the temperature. Pyrene lifetimes decrease with increasing temperature at a given <i>x</i>_LiTf₂N with lifetime becoming more sensitive to temperature at higher LiTf₂N concentration. The pyrene–nitromethane fluorophore–quencher pair follows a simplistic Stern–Volmer formulation, indicating the quenching to be purely dynamic in nature affording dynamic quenching constants (<i>K</i>_D) in the process. <i>K</i>_D along with the estimated bimolecular quenching rate constant (<i>k</i>_q) within LiTf₂N-added [emim]­[Tf₂N] first increases with increasing LiTf₂N until <i>x</i>_LiTf₂N ∼ 0.10, decreasing monotonically thereafter until <i>x</i>_LiTf₂N = 0.40. The decrease in <i>K</i>_D and <i>k</i>_q with increasing <i>x</i>_LiTf₂N is attributed to the exponentially increased dynamic viscosity with increasing <i>x</i>_LiTf₂N of the ([emim]­[Tf₂N] + LiTf₂N) system. The initial increase in <i>K</i>_D and <i>k</i>_q is controlled by the structural changes within the system as LiTf₂N is added to [emim]­[Tf₂N]. It is proposed that the presence of [Li­(Tf₂N)₂]⁻ anionic clusters stabilizes the partial positive charge that develops on excited pyrene during the electron/charge transfer to nitromethane during the quenching process. While the Stokes–Einstein formulation is not followed by the ([emim]­[Tf₂N] + LiTf₂N) system in general, it is found to be obeyed at fixed <i>x</i>_LiTf₂N. The role of structural changes within the system beyond viscosity increase on the quenching process is amply highlighted

Aggregation of Carbocyanine Dyes in Choline Chloride-Based Deep Eutectic Solvents in the Presence of an Aqueous Base

Deep eutectic solvents (DESs) have shown potential as novel media to support molecular aggregation. The self-aggregation behavior of two common and popular carbocyanine dyes, 5,5′,6,6′-tetra­chloro-1,1′-di­ethyl-3,3′-di­(4-sulfo­butyl)-benz­imida­zole carbo­cyanine (TDBC) and 5,5′-di­chloro-3,3′-di­(3-sulfo­propyl)-9-methyl-benzo­thia­carbo cyanine (DMTC), is investigated within DES-based systems under ambient conditions. Although TDBC is known to form J-aggregates in basic aqueous solution, DMTC forms H-aggregates under similar conditions. The DESs used, glyceline and reline, are composed of salt choline chloride and two vastly different H-bond donors, glycerol and urea, respectively, in 1:2 mol ratios. Both DESs in the presence of base are found to support J-aggregates of TDBC. These fluorescent J-aggregates are characterized by small Stokes’ shifts and subnanosecond fluorescence lifetimes. Under similar conditions, DMTC forms fluorescent H-aggregates along with J-aggregates within the two DES-based systems. The addition of cationic surfactant cetyl­tri­methyl­am­monium bromide (CTAB) below its critical micelle concentration (cmc) to a TDBC solution of aqueous base-added glyceline shows the prominent presence of J-aggregates, and increasing the CTAB concentration to above cmc results in the disruption of J-aggregates and the formation of unprecedented H-aggregates. DMTC exclusively forms H-aggregates within a CTAB solution of aqueous base-added glyceline irrespective of the surfactant concentration. Anionic surfactant, sodium do­decyl­sulfate (SDS), present below its cmc within aqueous base-added DESs supports J-aggregation by TDBC; for similar SDS addition, DMTC forms H-aggregates within the glyceline-based system whereas both H- and J-aggregates exist within the reline-based system. A comparison of the carbocyanine dye behavior in various aqueous base-added DES systems to that in aqueous basic media reveals contrasting aggregation tendencies and/or efficiencies. Surfactants as additives are demonstrated to control and modulate carbocyanine dye self-aggregation within DES-based media. The unique nature of DESs as alternate media toward affecting cyanine dye aggregation is highlighted

Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1-pyrenyl)propane within 1‑Butyl-3-methylimidazolium Hexafluorophosphate and Its Polyethylene Glycol Mixtures

Mixtures of ionic liquid with polyethylene glycol (PEG) have shown interesting features as solubilizing media. Intramolecular excimer formation dynamics of 1,3-bis-(1-pyrenyl)­propane [1Py(3)­1Py] is investigated within mixtures of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]­[PF₆]) with PEGs of average molecular weight (MW) 200 (PEG200), average MW 400 (PEG400), number-average MW <i>M</i>_n 570–630 (PEG600), and number-average MW <i>M</i>_n 950–1050 (PEG1000) over the complete composition range at a 10° interval in the temperature range 10–90 °C. Irrespective of the composition of the medium and the temperature, excited-state intensity decay of the excimer fluorescence best fits to a three-exponential decay function, suggesting the presence of one excited-state monomer and two kinetically distinguishable excimers where both excimers are populated simultaneously by the excited monomer with no interconversion between the two excimers. In neat PEGs for temperatures ≤ 50 °C, intensity decay data of monomer fluorescence best fits to a single-exponential decay function, which implies the dissociation of both excimers back to the monomer to be insignificant. As the temperature is increased, the fits become closer to a double-exponential decay function, implying dissociation of one of the excimers to become significant. In neat [bmim]­[PF₆], while a double-exponential decay function is required to fit the monomer excited-state intensity decay data at lower temperatures, three exponentials are required to satisfactorily fit the data at higher temperatures, suggesting both excimers significantly dissociate back to the monomer at higher temperatures within the ionic liquid. Within long-chain PEG-containing ([bmim]­[PF₆] + PEG) mixtures, PEG as opposed to [bmim]­[PF₆] controls the excimer formation dynamics by supposedly wrapping around the excimer, thus hindering dissociation back to the monomer. The overall rate constant of the excimer formation within ([bmim]­[PF₆] + PEG) mixtures is found to scale better with the microviscosity rather than the bulk viscosity of the mixtures

Highly Efficient Intramolecular Excimer Formation in a Disulfide-Linked Dipyrenyl Compound: Proton Recognition and Fluidity Assessment

Intramolecular excimer formation has been utilized extensively in chemical sciences, especially to probe solvation within complex media as well as to assess physicochemical properties of the solubilizing milieu. Pyrene has been employed extensively as a fluorescence probe for this purpose due to its favorable multidimensional fluorescence properties. Termini-capped dipyrenyl scaffolds possessing various functionalities comprise the majority of such compounds. A new both end-tagged dipyrenyl compound DTP is designed and synthesized, which exhibits significantly high intramolecular excimer formation efficiency in polar solvents. The presence of a −NH–(CO)– and/or −S–S– functionality on the chain linking the two pyrenyl groups facilitates intramolecular excimer formation. Excited-state emission intensity decay reveals that the excimer formation exclusively takes place in the excited state with only one excimer conformation. The rate constant of excimer formation is found to be higher for DTP as compared to a similar compound with an alkyl backbone. The dependence of the excimer formation on the solvent (protic versus aprotic) as well as on temperature reveals further insights into the excimer formation process. The excimer formation by DTP is found to be highly sensitive to the presence of H+: the relative excimer formation efficiency decreases drastically in the presence of a small amount of H+ (∼10–5 M). Further, the recognition of protons by DTP via intramolecular excimer formation is also observed to be highly selective in nature. Based on the observation that both the excimer formation efficiency and kinetics depend on the viscosity of the solubilizing milieu, fluidity assessment of the (dimethyl sulfoxide + acetonitrile) mixture was carried out using DTP. Further, DTP is found to be an effective probe for the assessment of the amount of water in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide as well as in a deep eutectic solvent composed of choline chloride and urea in a 1:2 mol ratio. Highly efficient and rapid intramolecular excimer formation not only establishes DTP as a useful and versatile probe but also offers strategic pathways for designing effective excimer-forming compounds

Contrasting Behavior of Classical Salts versus Ionic Liquids toward Aqueous Phase J-Aggregate Dissociation of a Cyanine Dye

The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ≥ 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (<5 wt %), the added IL acts similarly to the inorganic salts, reducing the overall absorbance of TDBC in the solution most likely due to cation exchange causing TDBC precipitation. Addition of a molecular solvent, ethanol, at 15 wt % results in an initial increase in monomer absorbance, albeit to a much lesser extent than for the corresponding molar fraction of IL, which then decreases over time with recovery of J-aggregate absorbancequite opposite the time-dependent behavior seen for TDBC in PB at pH 12.0 with >5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated