11 research outputs found
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-bisÂ(1-pyrenyl)Âdecane
[1Py(10)Â1Py], where the fluorophoric pyrene moieties are separated
by a long decyl chain, is investigated in seven different ionic liquids
in 10â90 °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-methylÂimidazolium
cation ([bmim<sup>+</sup>]) with different anions and the other having
four ionic liquids of bisÂ(trifluoroÂmethylsulfonyl)Âimide
anion ([Tf<sub>2</sub>N<sup>â</sup>]) with different cations.
The excimer-to-monomer emission intensity ratio (<i>I</i><sub>E</sub>/<i>I</i><sub>M</sub>) 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><sub>E</sub>/<i>I</i><sub>M</sub> values within
ionic liquids with the same viscosities are found to be significantly
different. The excited-state intensity decay kinetics of 1Py(10)Â1Py
in ionic liquids do not adhere to a simplistic Birkâs scheme,
where only one excimer conformer forms after excitation. The apparent
rate constants of excimer formation (<i>k</i><sub>a</sub>) 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><sub>a</sub> to the temperature with higher activation energy, <i>E</i><sub>a</sub>. The trend in <i>E</i><sub>a</sub> is found to be similar to that for activation energy of the viscous
flow (<i>E</i><sub>a,Ρ</sub>). StokesâEinstein
relationship is not followed in [bmim<sup>+</sup>] ionic liquids;
however, with the exception of [choline]Â[Tf<sub>2</sub>N], it is found
to be followed in [Tf<sub>2</sub>N<sup>â</sup>] ionic liquids
suggesting the cyclization dynamics of 1Py(10)Â1Py 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 2âNaphtholate Fluorescence by Imidazolium Ionic Liquids
The effect of addition of water-miscible ionic liquids
(ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]Â[BF<sub>4</sub>]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate
([bmim]Â[OTf]), and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate
([bmpyrr]Â[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 SternâVolmer
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]Â[OTf] is added;
a double-exponential decay is required to satisfactorily fit the decay
data in the presence of [bmim]Â[BF<sub>4</sub>], 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 NaBF<sub>4</sub>, 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 âhyperpolarityâ
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 UVâvis absorbance
and fluorescence solvatochromic probes. Thirteen different ILs, of
the same cation 1-butyl-3-methylimidazolium and different anions,
of the same anion bisÂ(trifluoromethylsulfonyl)Âimide 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 UVâvis absorbance probes
are used to obtain <i>E</i><sub>T</sub> [dipolarity/polarizability
and/or H-bond donating (HBD) acidity] and KamletâTaft parameters
[Ď* (dipolarity/polarizability), Îą (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]Â[PF<sub>6</sub>]) as the bulk phase. The
presence of w/[bmim]Â[PF<sub>6</sub>] microemulsions is hinted by the
significantly increased solubility of water in Brij-35 solution of
[bmim]Â[PF<sub>6</sub>]. The formation of w/[bmim]Â[PF<sub>6</sub>]
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]Â[PF<sub>6</sub>] in the absence of added-water. These reverse micelles become w/[bmim]Â[PF<sub>6</sub>] microemulsions as the water is added to the system. As the
water loading (<i>w</i><sub>0</sub>) is increased, the average
diameter of the aggregates increases. Fourier transform infrared (FTIR)
absorbance data reveal the presence of both âboundâ
and âfreeâ water within the system. The âboundâ
water is associated with the water pools of the w/[bmim]Â[PF<sub>6</sub>] microemulsions. Excited-state proton transfer (ESPT) involving
probe pyranine shows deprotonation of pyranine within the water pools
of the w/[bmim]Â[PF<sub>6</sub>] microemulsions
Microviscosity Offered by Ionic Liquids and Ionic LiquidâGlycol 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 soluteâsolvent and solventâsolvent
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 C2âH substituted
imidazolium cations with different anions to reveal the role of C2âH,
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 C2âH
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
bisÂ(trifluoromethylsulfonyl)Âimide (LiTf<sub>2</sub>N)-added ionic
liquid 1-ethyl-3-methylimidazolium bisÂ(trifluoromethylsulfonyl)Âimide
([emim]Â[Tf<sub>2</sub>N]) system up to a LiTf<sub>2</sub>N mole fraction
(<i>x</i><sub>LiTf<sub>2</sub>N</sub>) of 0.40 is investigated
using a fluorophoreâquencher pair of pyreneânitromethane
in the 298.15â358.15 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><sub>LiTf<sub>2</sub>N</sub>, and the temperature. Pyrene lifetimes decrease with
increasing temperature at a given <i>x</i><sub>LiTf<sub>2</sub>N</sub> with lifetime becoming more sensitive to temperature
at higher LiTf<sub>2</sub>N concentration. The pyreneânitromethane
fluorophoreâquencher pair follows a simplistic SternâVolmer
formulation, indicating the quenching to be purely dynamic in nature
affording dynamic quenching constants (<i>K</i><sub>D</sub>) in the process. <i>K</i><sub>D</sub> along with the estimated
bimolecular quenching rate constant (<i>k</i><sub>q</sub>) within LiTf<sub>2</sub>N-added [emim]Â[Tf<sub>2</sub>N] first increases
with increasing LiTf<sub>2</sub>N until <i>x</i><sub>LiTf<sub>2</sub>N</sub> âź 0.10, decreasing monotonically thereafter
until <i>x</i><sub>LiTf<sub>2</sub>N</sub> = 0.40. The decrease
in <i>K</i><sub>D</sub> and <i>k</i><sub>q</sub> with increasing <i>x</i><sub>LiTf<sub>2</sub>N</sub> is
attributed to the exponentially increased dynamic viscosity with increasing <i>x</i><sub>LiTf<sub>2</sub>N</sub> of the ([emim]Â[Tf<sub>2</sub>N] + LiTf<sub>2</sub>N) system. The initial increase in <i>K</i><sub>D</sub> and <i>k</i><sub>q</sub> is controlled by
the structural changes within the system as LiTf<sub>2</sub>N is added
to [emim]Â[Tf<sub>2</sub>N]. It is proposed that the presence of [LiÂ(Tf<sub>2</sub>N)<sub>2</sub>]<sup>â</sup> 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 StokesâEinstein formulation is not followed
by the ([emim]Â[Tf<sub>2</sub>N] + LiTf<sub>2</sub>N) system in general,
it is found to be obeyed at fixed <i>x</i><sub>LiTf<sub>2</sub>N</sub>. 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, 5,5â˛,6,6â˛-tetraÂchloro-1,1â˛-diÂethyl-3,3â˛-diÂ(4-sulfoÂbutyl)-benzÂimidaÂzole
carboÂcyanine (TDBC) and 5,5â˛-diÂchloro-3,3â˛-diÂ(3-sulfoÂpropyl)-9-methyl-benzoÂthiaÂcarbo
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 Stokesâ 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 cetylÂtriÂmethylÂamÂmonium
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 doÂdecylÂsulfate
(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 1âButyl-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)Âpropane [1Py(3)Â1Py] is investigated within mixtures
of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
([bmim]Â[PF<sub>6</sub>]) with PEGs of average molecular weight (MW)
200 (PEG200), average MW 400 (PEG400), number-average MW <i>M</i><sub>n</sub> 570â630 (PEG600), and number-average MW <i>M</i><sub>n</sub> 950â1050 (PEG1000) over the complete
composition range at a 10° interval in the temperature range
10â90 °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]Â[PF<sub>6</sub>], 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]Â[PF<sub>6</sub>] +
PEG) mixtures, PEG as opposed to [bmim]Â[PF<sub>6</sub>] 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]Â[PF<sub>6</sub>] +
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 âNHâ(CO)â
and/or âSâSâ 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+ (âź10â5 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, 5,5â˛,6,6â˛-tetrachloro-1,1â˛-diethyl-3,3â˛-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ⼠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 absorbanceî¸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