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Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids Anita Yadav, and Siddharth Pandey J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08329 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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The Journal of Physical Chemistry

Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids

Anita Yadav and Siddharth Pandey*

Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016, India. *

Corresponding Author. Phone: +91-11-26596503, Fax: +91-11-26581102,

E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT:

Ionic liquids, being composed of ions alone, may offer alternate 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 (IE/IM) is found to increase with increasing temperature in sigmoidal fashion. Chemical structure of the ionic liquid controls the excimer formation efficiency as IE/IM within ionic liquids with 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 (ka) 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 the ka to the temperature as higher the activation energy, Ea. The trend in Ea is found to be similar to that for activation energy of the viscous flow (Ea,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

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dependence of ionic liquid structure on cyclization dynamics to form intramolecular excimer is amply highlighted.

INTRODUCTION Ionic liquids are the focus of extensive research due to their inherently low volatility, wide chemical diversity, fascinating physicochemical properties, and immense potential applications in multiple research areas.1-13 The appealing growth in ionic liquid research is a direct consequence of the unique composition and tailorable ionic architecture that offer several possible modes of interactions. The structural alterations in cation or anion result in widely varying physicochemical properties of ionic liquids and subsequently produce enormous number of ionic liquids that are possible. The suitable combination of functional constituent ions is responsible for their definitive properties, and this controls their features and derives immense benefits for their exploitation in a host of areas. Moreover, ionic liquids offer a solvent milieu with the possibility of uniquely different chemistry from common molecular solvents or aqueous electrolytes. Excimers are dimers with associated excited electronic states and dissociative ground states, and are characterized by structureless emission spectra.14 Formation of excimers by fluorophores is an actively followed process in modern chemistry.15-18 Intramolecular excimer formation is usually shown by molecules consisting of two identical fluorophores linked by a flexible chain.19,20 This requires close approach of the two fluorophores through internal rotation during the lifetime of the excited state.21,22 Consequently, these intramolecular excimers can offer key insights to the physicochemical properties of the solubilizing environment.23 Intramolecular excimer formation has also been utilized effectively in a variety of chemosensing applications, as a source of spontaneous ultraviolet 3



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light in excimer lamps, OLEDS, etc.24-26 The intramolecular excimer formation of various dipyrenylalkanes in organic solvents was studied by Zachariasse,27-31 Thistlethwaite,32 Martinho,33,34 Maçanita,35,36 Nishijima,37 Bright,38 and Pandey39 groups, among others, from different

points-of-view. Previous

research has shown that the excimer emission depends on chain length, chain structure, the actual fluorophore chemistry, the exact labeling site on the fluorophore and the chain, the tether

length

and

its

chemistry,

temperature,

pressure

and

the

solvent

property/composition.29,30,35-37,40,41 During an investigation of the effect of alkane chain length separating the fluorophores on intramolecular excimer formation dynamics, Thistlethwaite et al. reported the kinetics of intramolecular excimer formation of 1,3-bis(1pyrenyl)propane [1Py(3)1Py] and 1,10-bis(1-pyrenyl)decane [1Py(10)1Py] in a number of solvents using time-correlated single-photon counting measurements at 20 °C.32 They reported that 1Py(10)1Py followed simple Birks’ scheme (where only one excimer conformation is formed after excitation), whereas for 1Py(3)1Py this scheme was found to be not adhered to (two excimer conformers are proposed to exist) when dissolved in various organic solvents under ambient conditions.32 Further, they observed that the rate parameters for intramolecular excimer formation dynamics of 1Py(3)1Py is independent of the solvent property, for 1Py(10)1Py, on the contrary, the excimer formation dynamics does depend on the solvent due to the presence of longer chain separating the two pyrene fluorophoric units; interaction of the alkyl chain with the solvent molecule could alter its extension and flexibility. In 1Py(3)1Py the linking chain is sufficiently small that solvent effects on the chain are less important, whereas in 1Py(10)1Py, longer alkyl chain provides sufficient freedom and time for interaction with the solubilizing milieu, and hence, excimer with 4


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optimum configuration may form. However, Vasilescu group later reported that Birks’ kinetic model was not followed when 1Py(10)1Py was dissolved in organized media composed

of

sodium

dodecylsulfate/bovine

serum

albumin

and

sodium

dodecylsulfate/poly(ethylene oxide), respectively.42 In our previous work, we reported temperature dependent kinetics of intramolecular excimer formation of 1Py(3)1Py within ionic

liquid

1-butyl-3-methylimidazolium

hexafluorophosphate

[bmim][PF6],

liquid

poly(ethylene glycols) (PEGs), and ([bmim][PF6] + PEG) mixtures, and observed that 1Py(3)1Py excimer dynamics is similar to those observed within the organic solvents.43 In this context, a detailed investigation of the excimer formation dynamics of 1Py(10)1Py within structurally different ionic liquids as this solubilizing media is more organized compared to that of organic solvents will offer key insights into the molecular aggregation within ionic liquids. In this paper, we report excimer formation dynamics of 1Py(10)1Py dissolved in two sets of seven different ionic liquids in the temperature range (283.15 to 363.15) K at 10 K intervals. While the first set is constituted of four ionic liquids having same 1-butyl-3methylimidazolium ([bmim+]) cation with hexafluorophosphate [PF6‒], ethylsulfate [EtSO4‒], trifluoromethysulfonate [OTf‒], and bis(trifluoromethylsulfonyl)imide [Tf2N‒] anions, ionic liquids in the second set are constituted of the same anion [Tf2N‒] with 1-methyl-1propylpiperidinium [pmpip+], N,N,N-trimethylethanolammonium [choline+], 1-ethyl-3methylimidazolium [emim+], and [bmim+] cations (Scheme 1 shows structures of the ionic liquids used). The selected ionic liquids encompass widely varying structures and physicochemical properties.

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EXPERIMENTAL Materials. 1Py(10)1Py (> 99.99%) (structure is given in Scheme 1) was obtained from Molecular Probes and was used as received. 1-Methylpyrene (1MePy, 99.9+%) was purchased from Sigma-Aldrich and it was used without further purification. Ionic liquids used are of the highest purity. [choline][Tf2N‒] (99%) was purchased from Iolitec while [bmim][PF6], [bmim][OTf], [pmpip][Tf2N], [bmim][EtSO4], [bmim][Tf2N] and [emim][Tf2N] were purchased from Covalent Associates, Inc., and they were of electrochemical grade (>99%). All ionic liquids were stored under argon in an Auto Secador desiccator cabinet. Before use, ionic liquids were rigorously dried under vacuum for at least 72 hours. Karl-Fisher titrator was used next to assess water content of an ionic liquid prior to its use. Ionic liquid was dried further till the water content became less than 100 ppm. The description of the chemicals used together with their sources is provided in Table 1.

Methods. The stock solution of 1Py(10)1Py was prepared in dichloromethane and stored in amber glass vial at 4±1 °C. The required amount of 1Py(10)1Py to prepare stock solution was weighed using Mettler−Toledo AB104−S balance with a precision of ±0.1 mg. An appropriate amount of 1Py(10)1Py solution from the stock was transferred to a glass vial. The dichloromethane was evaporated with a gentle stream of high purity nitrogen gas. Ionic liquid was added and the mixture was stirred to ensure the complete solubilization of 1Py(10)1Py to achieve the desired final concentration of 10 µM. The solutions were prepared under an argon atmosphere and sealed with parafilm to minimize the sorption of environmental moisture. All

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samples were purged with high purity N2 gas for O2 exclusion as molecular oxygen is a wellknown fluorescence quencher.17 Steady-state emission and excitation spectra were acquired on model FL 3-11, Fluorolog-3 modular spectrofluorimeter with single Czerny-Turner grating excitation and emission monochromators having 450W Xe arc lamp as the excitation source and a PMT as the detector. This spectrofluorimeter was purchased from Horiba-Jobin Yvon, Inc. The temperature was controlled with Thermo NESLAB RTE7 circulating chiller bath having stability ±0.1 °C. Spectral response from appropriate blanks was subtracted before data analysis. Since we used ultrapure ionic liquids, and pyrene and pyrene-appended compounds are known usually to have high fluorescence quantum yields,16 the contributions of blank signals at monomer and excimer decay wavelength were less than 1% of the sample signal in all cases. This is also considered to be a necessary condition for fluorescence lifetime measurement.44 All data were acquired using 1-

cm path length quartz cuvettes. The time-resolved fluorescence measurements were carried out using a Horiba-Jobin Yvon Fluorocube time-correlated single photon counting (TCSPC) fluorimeter. 1Py(10)1Py dissolved in ionic liquids at different temperatures were excited at 340 nm using a UV-pulsed NanoLED-340 source having a pulse width [OTf−] > [EtSO4−] > [PF6−] suggesting IE/IM within [bmim][Tf2N] to be the most sensitive to the temperature change whereas [bmim][PF6] to be the least close to ambient conditions. From the reported values of the dynamic viscosities (η) of these ionic liquids,46, 47 we found that the sensitivity of IE/IM on temperature for these ionic liquids close to ambient conditions follow the same trend as 1/η as η increases in the order [Tf2N−] < [OTf−] < [EtSO4−] < [PF6−] at 25 °C. Therefore, it may be inferred that for [bmim+] ionic liquids close to ambient temperature, the lower the dynamic viscosity of the ionic liquid, the more sensitive its IE/IM to the temperature. Subsequently, the dependence of IE/IM of [bmim+] ionic liquids on η over the entire temperature range 10-90 °C was assessed (a plot of IE/IM versus 1/η is presented in Fig. 3A). It is clear that the IE/IM within different [bmim+] ionic liquids having the same η are different. It is concluded that excimer formation efficiency within [bmim+] ionic liquids is specific to the identity of the ionic liquid; for same η, the IE/IM is maximum in [bmim][PF6] and minimum in [bmim][Tf2N]. Further, the increase in IE/IM with 1/ η in the temperature range 10-90 °C follows a simple two-parameter exponential growth-to-maxima expression 10


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= ∝ (1 − / )

(2)

(the fits are included in Fig. 3A as solid curves and the parameters (α and η0) with goodness-offit in terms of r2 are given in Table 2). Further data analysis reveals that for the four [bmim+] ionic liquids when η < 200 mPa.s, the IM/IE can be considered to vary linearly with η (Fig. S3). It is convenient to note that the slope of IM/IE versus η also follows the trend [Tf2N−] > [OTf−] > [EtSO4−] > [PF6−] suggesting IM/IE within [bmim][Tf2N] to be the most sensitive to the η change and [bmim][PF6] to be the least for η < 200 mPa.s.

Steady-State Fluorescence of 1Py(10)1Py within [Tf2N−] Ionic Liquids. Similar data analyses were carried out next for the four ionic liquids having [Tf2N−] anion and different cations. The IM and IE in the range 10-90 °C again, in general, show decrease and increase, respectively, with increase in temperature except for an initial increase in IM for [choline+] ionic liquid in going from 20 to 30 °C (the data for [choline][Tf2N] could not be collected at 10 °C due to the solid state of this ionic liquid, Fig. S1). The IE/IM again increases almost monotonically with increasing temperature; the variation again follows a simple threeparameter sigmoidal expression (eqn. [1]) in the investigated temperature range (Fig. 2B and Table 2). It is to be noted, however, that while IE/IM within ionic liquids with same cation and different anions are closer to each other, it is not the case within ionic liquids with same anion and different cations where the IE/IM are more spread out (Fig. 2A vs. Fig. 2B). This implies that 1Py(10)1Py IE/IM is controlled more by the ionic liquid cation than the anion; changing cation may result in significant change in the excimer formation efficiency. The slope of the IE/IM versus T data close to ambient temperature (Fig. S2) has the trend [emim+] > [choline+] > [bmim+] > [pmpip+] suggesting the [emim][Tf2N] IE/IM to be the most and [pmpip][Tf2N] to be 11



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the least sensitive to the temperature change. The η of these four ionic liquids at ambient conditions show the trend [choline+] > [pmpip+] > [bmim+] > [emim+].46, 47 If [choline][Tf2N] is not considered, the trend in the sensitivity of IE/IM on T is the same as that of IE/IM on inverse of the η closer to ambient conditions (the same was found for the four [bmim+] ionic liquids, vide supra). The lower the viscosity of the ionic liquid, the more temperature sensitive is the 1Py(10)1Py IE/IM. Among all the ionic liquids investigated, [choline][Tf2N] is the only one possessing –OH functionality on either of its ions. The presence of –OH on either of the ions is known to effect the properties of ionic liquids.48, 49 The correlation between excimer formation efficiency within the ionic liquid with the chemical structure (and the dynamic viscosity) of the ionic liquid is established nonetheless. Further data analysis was carried out between excimer formation efficiency and the dynamic viscosity for [Tf2N−] ionic liquids where IE/IM is plotted against 1/η for the four [Tf2N−] ionic liquids in the entire temperature range 10-90 °C (Fig. 3B). The IE/IM appear to increase monotonically with decrease in η. Again, for ionic liquids with same η, the IE/IM are different corroborating the finding that excimer forming efficiency is specific to ionic liquid chemical architecture. For the same η, IE/IM is maximum within [choline][Tf2N] followed by [emim][Tf2N], [bmim][Tf2N] and minimum within [pmpip][Tf2N] ionic liquid. The [choline+] is observed to facilitate the excimer formation. The IE/IM versus 1/η again fit well to a simplistic two-parameter exponential-growth-to-maxima expression (eq. [2]); the fits are shown in Fig. 3B by solid curves and recovered parameters are presented in Table 2. As observed for [bmim+] ionic liquids, for η < 200 mPa.s, IM/IE versus η can be fitted to a linear expression satisfactorily with slopes following the trend [pmpip][Tf2N] > [emim][Tf2N] > [bmim][Tf2N] > [choline][Tf2N] (Fig. S3). 12


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An important conclusion from the steady-state fluorescence data regarding the temperature dependence of excimer formation efficiency of 1Py(10)1Py within structurally different ionic liquids is not only that IE/IM is found to be the highest for the lowest viscosity ionic liquid [emim][Tf2N], its sensitivity towards temperature change is also maximum in this ionic liquid. Also, chemical structure of the ionic liquid controls the excimer formation efficiency as excimer-to-monomer intensity ratios within ionic liquids with same bulk dynamic viscosities could be significantly different.

Time-Resolved Fluorescence of 1Py(10)1Py in Ionic Liquids. In order to assess the effect of the ionic liquid structure on the dynamics of 1Py(10)1Py excimer formation, excited-state intensity decays of 1Py(10)1Py dissolved in each of the ionic liquid were collected at 376 nm (monomer decay) and 480 nm (excimer decay), respectively, using a 340 nm NanoLED for excitation at different temperatures in the range 10-90 °C. An attempt to fit the monomer and the excimer intensity decay profiles within the ionic liquids at all temperatures investigated to Birk-type framework,

14,16,29,30,32,50

where one conformer of excited monomers

form one conformer of excimers, was unsuccessful. Fit to a double exponential decay model according to Birk’s scheme predicts that the monomer intensity [ I M (t )] decays with time (t) as the sum of two exponentials and the excimer intensity [ I E (t )] decays as the difference of two exponentials:

I M (t ) = α11exp(-λ11t ) + α12 exp(-λ12 t )

(3)

I E (t ) = α 21exp(-λ21t ) + α 22 exp(-λ22 t )

(4)

with the constraints that 13



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α 21 = −1 α 22

(5)

and

λ11 = λ21 and λ12 = λ22

(6)

We found that the recovered parameters were not acceptable due to inappropriate reduced χ2, residuals, autocorrelation function and DW parameters or to the lack of adherence to constraints (5) and/or (6), or a combination of the above in all ionic liquids at each investigated temperature (data not shown). It first came as a surprise to us as Thistlethwaite group has reported 1Py(10)1Py to follow simple Birks’ scheme when dissolved in toluene, benzene, cyclohexane, and ethanol, respectively, under ambient conditions.32 However, Vasilescu, Almgren and Angelescu

found

that

for

1Py(10)1Py

dissolved

in

organized

media

of

sodium

dodecylsulfate/bovine serum albumin and sodium dodecylsulfate/poly(ethylene oxide), respectively, Birks’ kinetic model was not adhered to; both monomer and excimer fluorescence decays had to be fitted by a some of three exponentials.42 In this context, excimer formation dynamics of 1Py(10)1Py in ionic liquids is similar to organized media as opposed to organic solvents. It is to be noted that the excimer formation dynamics of 1Py(3)1Py in ionic liquids [bmim][PF6], liquid PEGs, and ([bmim][PF6] + PEG) mixtures, on the other hand, was similar to that observed in organic solvents.43 This difference is attributed to the difference in the chain length linking the fluorophores. The ten-carbon long aliphatic chain has better chances of getting its extension and flexibility altered by interacting with the solvent as opposed to the three-carbon long aliphatic chain. In case of long chain in polar solvents, alkane-solvent contacts would be unfavorable, whereas they would be favorable in relatively non-polar solvents. This is not the case for short chain as the fluorophores are already close enough. Investigation of the effect of an 14


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ionic liquid as a solvent on intramolecular excimer formation dynamics is thus more relevant for the probe 1Py(10)1Py as opposed to 1Py(3)1Py. It is clear that ionic liquids are more complex as compared to isotropic solvents as solubilizing media as far as excimer formation dynamics of 1Py(10)1Py is concerned. It is important to mention at this juncture seminal work by Liu and Guillet that outlines disadvantages of the general use of Birks’ scheme and proposes a new scheme for diffusioncontrolled excimer formation between polymer chain ends.51,

52

Unlike the Birks scheme, the

approach presented by them does not use rate expressions for excimer formation and dissociation, instead, excimer formation is described as the natural consequence of chain end diffusion. It may be a worthwhile exercise to modify the computer simulation method suggested by these authors for a short alkyl chain separated bichromophoric system dissolved in ionic liquid as the solvent media.

Excited-State Emission Intensity Decay of 1Py(10)1Py in [bmim+] Ionic Liquids. The results of the fit of the monomer emission intensity decay ( [ I M (t )] at 376 nm) to single, double, and triple exponential functions and that of the excimer emission intensity decay (

[ I E (t )] at 480 nm) to double and triple exponential functions (as fits of the excimer emission intensity decay to a single exponential decay was unacceptable), respectively, along with the recovered parameters and the goodness-of-the-fits in terms of reduced χ2 for 1Py(10)1Py dissolved in [bmim+] ionic liquids with four different anions over 10-90 °C temperature range are presented in Table S1. A careful examination of the entries for the excimer decay clearly reveals that, in general, [ I E (t )] fit satisfactorily to a double-exponential decay function (a 15



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representative fit for [pmpip][Tf2N] is shown in Fig. 4) irrespective of the identity of the ionic liquid or the temperature. Based on reduced χ2, residuals, autocorrelation functions and DW parameters, it was found that the [ I M (t )] fit best to a single exponential decay function for [bmim][PF6] and [bmim][Tf2N], and to a double exponential function for [bmim][EtSO4] and [bmim][OTf] (Table S1) at all temperatures investigated. It is to be noted that in the latter two ionic liquids, the attempt to fit the emission intensity decays (at 376 nm) of the model control compound 1-methylpyrene (where no intramolecular excimer can form) to a single exponential function also results in rather higher reduced χ2 (Table S2). In order to accommodate the slight differences in the fluorescence lifetimes due to the diversity of the local solubilizing environment of the fluorophore, monomer decay was fitted in terms of a distribution of decay components. The recovered mean decay times (τMean) and the width of the associated decay function (dτMean) along with the goodness-of-the fit in terms of χ2 are presented in Table S3. A careful examination of the data reveals that the fitting to the single exponential decay function, in most cases, is not improved significantly by the fitting to a lifetime distribution. From the single exponential decay of the [ I M (t )] , where the thermal dissociation of the excimer back to the monomer is considered negligible, the apparent rate constant of intramolecular excimer formation (ka) was obtained from the monomer decay time (τM) of 1Py(10)1Py and the fluorescence lifetime (τ0) of the model compound 1-methylpyrene which does not form excimer under these conditions:35 ka =

1

τM

−

1

(7)

τ0

It is demonstrated for numerous systems that this assumption holds good especially in solvents and media of high viscosity.35 The apparent ka thus obtained within four [bmim+] ionic liquids 16


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are listed in Table 3. The apparent ka (with error ≤ ±5%) at 303.15 K varies between 2.6×106 s‒1 to 4.5×106 s‒1 for the four [bmim+] ionic liquids. These values can be compared to the reported ka for 1Py(10)1Py of 28×106 s‒1 in toluene, 31×106 s‒1 in benzene, 54×106 s‒1 in cyclohexane, and 92×106 s‒1 in ethanol at 25 °C.32 These values in organic solvents for 1Py(10)1Py are estimated from the fit of the [ I M (t )] and [ I E (t )] to Birk’s scheme. Significantly lower apparent ka obtained within ionic liquids could be attributed, in general, to the medium effect – various electrostatic interactions present within ionic liquids result in substantially higher viscosities associated to these solvents. Although, the authors state that the bulk solvent viscosities may have only minor importance as ka is highest in ethanol, the most viscous of the four organic solvents, the viscosities of these ionic liquids as compared to the viscosities of toluene (0.59 mPa.s), benzene (0.65 mPa.s), cyclohexane (1.0 mPa.s), and ethanol (1.2 mPa.s) are significantly higher (36 to 185 mPa.s at 303.15 K).32 The medium effect is the major reason for this observation gets further support from the fact that ka for 1Py(3)1Py in PEGs and ([bmim][PF6] + PEG) mixtures of similar viscosities have similar ka values in this temperature range.43 Further, the ka of another bis-pyrenyl compound with long ester linkage, 6-(1-pyrenyl)hexyl-11(1-pyrenyl)undecanoate whose intramolecular excimer formation follows a simple Birk’s scheme within [bmim][PF6] and tetraethylene glycol (TEG) in the temperature range 10-90 °C, are only slightly higher in magnitude to those obtained for 1Py(10)1Py in ionic liquids in the same temperature range.53 The temperature dependence of the estimated apparent ka is investigated next. Fig. 5A presents plots of ln ka versus 1/T for the four [bmim+] ionic liquids. A careful examination reveals acceptable linear fit of the data thus suggesting agreement with the empirical Arrhenius law. The estimated activation energy (Ea, reported in Table 3) follows the order [bmim][PF6] > [bmim][OTf] > [bmim][EtSO4] > [bmim][Tf2N]. In general, the higher the viscosity of the 17



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[bmim+] ionic liquid, the more sensitive the ka to the temperature (and higher the Ea). Apparently, more the number of atoms in the anion of the [bmim+] ionic liquid, lower the Ea. It is convenient to note that Ea for intramolecular excimer formation by 1Py(10)1Py (30.8±0.1 kJ.mol−1) in [bmim][PF6] is similar to that by 1Py(3)1Py (~30 kJ.mol−1) and by 6-(1pyrenyl)hexyl-11(1-pyrenyl)undecanoate (~35 kJ.mol−1) in the same ionic liquid. However, Ea for 1Py(3)1Py within neat PEGs (52-59 kJ.mol−1) are significantly higher. We have noticed that the trend in Ea for four [bmim+] ionic liquids, in general, is similar to that observed for activation energy of the viscous flow (Ea,η) (order of Ea,η: [bmim][PF6] > [bmim][EtSO4] > [bmim][OTf] > [bmim][Tf2N]);46 Ea,η values being higher than the corresponding Ea values (Ea,η > Ea). This is similar to that observed for 1Py(3)1Py in [bmim][PF6] earlier, however, it is in complete contrast with the results reported for 1Py(3)1Py in neat PEGs (Ea,η < Ea). Ionic liquids, due to the presence of unique electrostatic interactions, are different from similar viscosity strongly H-bonded solvents, such as neat PEGs. These results are in-line with the suggestion that the rate of intramolecular excimer formation of bispyrenyl compounds scales better with the microviscosity of the cybotactic region than the bulk viscosity (the activation energy of the microviscous flow, Ea,ηµ, for [bmim][PF6] is 31.2 kJ.mol−1 which is similar to the Ea of intramolecular excimer formation by 1Py(10)1Py within [bmim][PF6] which is 30.8 kJ.mol−1).53 It is clear that Ea,ηµ and not Ea,η controls the cyclization of 1Py(10)1Py, interactions on the molecular level within the solvent play important role in intramolecular excimer formation. The dependence of the dynamic viscosity of the [bmim+] ionic liquid on 1Py(10)1Py apparent ka reveals interesting outcomes. The ka of different [bmim+] ionic liquids of similar viscosities are found to be significantly different. This hints at the dependence of ionic liquid 18


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structure on excimer formation dynamics of 1Py(10)1Py. For example, for similar viscosity [bmim+] ionic liquids, apparent ka appears to be the highest in [bmim][PF6]. This outcome is further reflected in the non-compliance of 1Py(10)1Py ka within [bmim+] ionic liquids to StokesEinstein relationship (i.e., at a given temperature, ka ~ 1/η does not hold). Fig. S4 shows plots of ka.η versus η at various temperatures encompassing all ionic liquids investigated at each temperature. It is clear that ka.η is not a constant. It is reported earlier that 1Py(3)1Py ka also do not obey Stokes-Einstein relationship within organic solvents15,35 as well as within PEGs and their mixtures with [bmim][PF6].43 Further in this context, plot of apparent ka versus T/η for the [bmim+] ionic liquids also reveals the absence of linearity and thus the fact that the StokesEinstein relationship is not followed (inset of Fig. 6). The deviation from Stokes-Einstein behavior by various solutes (charged or uncharged) dissolved in ionic liquids has been highlighted earlier also.54 It is noted that in ionic liquids, the liquid landscape is often heterogeneous and friction, which is a property of the solvent, becomes dependent on the identity of the solute, and more importantly, on its location. We have noticed, however, that except at very high T/η, linear behavior between apparent ka and T/η is apparent within each of the [bmim+] ionic liquids separately with slopes being highest for [bmim][PF6] and lowest for [bmim][Tf2N] (Fig. 6A). These slopes roughly correlate to the size of the anion; larger the anion, the lower the slope. It implies the coupling of the constituent of the medium with those of the reactants.35 In other words, the structure of the anion has profound effect on the cyclization dynamics of 1Py(10)1Py within ionic liquids. The linear correlation between apparent ka and T/η for individual ionic liquids clearly imply the dependence of the cyclization rate of 1Py(10)1Py on microviscosity rather than the bulk viscosity of the ionic liquid milieu – the viscosities of the cybotactic region of 1Py(10)1Py dissolved in ionic liquids with same cation but different anions 19



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having same bulk viscosities are different.

Excited-State Emission Intensity Decay of 1Py(10)1Py in [Tf2N‒] Ionic Liquids. The recovered decay times and other associated parameters for [ I M (t )] and [ I E (t )] of 1Py(10)1Py dissolved in four [Tf2N−] ionic liquids at different temperatures are presented in Table S1. From the excimer decay parameters it is apparent that in some cases [ I E (t )] fit best to a double-exponential decay model while in others a triple-exponential decay function is needed for the fit to be acceptable. On the other hand, [ I M (t )] fits best to a single exponential decay function for [bmim][Tf2N], [emim][Tf2N], and [pmpip][Tf2N], and to a double exponential decay function for [choline][Tf2N] at all temperatures investigated (Table S1). Ionic liquid [choline][Tf2N], where the cation possesses an –OH functionality, is known to have physicochemical properties that are different from imidazolium or piperidinium ionic liquids.48, 49

Again, from the single exponential decay of [ I M (t )] , apparent ka for the four [Tf2N−] ionic

liquids were obtained using eq. [7] considering dissociation of the excimer back to monomer to be negligible (Table 3). Though the magnitude of apparent ka is similar in [bmim+] and [Tf2N−] ionic liquids, the variation in apparent ka within four [Tf2N−] ionic liquids at a given temperature is more than that observed for the four [bmim+] ionic liquids (e.g., at 303.15 K, ka varies between 1.7×106 s‒1 to 6.2×106 s‒1 for the four [Tf2N−] ionic liquids). It appears that the variation in ka correlate better with the viscosities for [Tf2N−] ionic liquids as opposed to the [bmim+] ionic liquids (vide infra). The Arrhenius plots (Fig. 5B) show acceptable linear correlation between ln ka versus 1/T for each of the four [Tf2N−] ionic liquids. The estimated Ea for the [Tf2N−] ionic liquids are 20


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similar in magnitude to the [bmim+] ionic liquids (Table 3). The Ea follows the order [choline][Tf2N] > [pmpip][Tf2N] > [bmim][Tf2N] > [emim][Tf2N]. Importantly, the dynamic viscosity of these four [Tf2N−] ionic liquids also follow the same order. The higher the viscosity of the [Tf2N−] ionic liquid, the more is the minimum energy required for the cyclization of 1Py(10)1Py. It may also be inferred that ionic liquids of same anions having aromatic cations have lower Ea as compared to those constituted of non-aromatic cations. We next attempted to correlate the temperature dependence of the 1Py(10)1Py apparent ka (i.e., Ea values) on the temperature dependence of the bulk dynamic viscosity (i.e., Ea,η values) of the [Tf2N−] ionic liquids. The Ea,η follows the order [choline][Tf2N] ≈ [pmpip][Tf2N] > [bmim][Tf2N] > [emim][Tf2N] which is similar to the trend of Ea for these ionic liquids.46 Ea,η values are again found to be higher than the corresponding Ea values (Ea,η > Ea). These outcomes are similar to those observed for the four [bmim+] ionic liquids (vide supra). The outcomes of the analysis of the ka dependence on η for the four [Tf2N−] ionic liquids are different from those for the four [bmim+] ionic liquids. For ionic liquids [bmim][Tf2N], [emim][Tf2N], and [pmpip][Tf2N], apparent ka are found to be similar for similar viscosity ionic liquids irrespective of the identity of the cation [e.g., ka = 15.3 ×106 s‒1 for [bmim][Tf2N] with η = 11.22 mPa.s (at 343.15 K), ka = 15.5 ×106 s‒1 for [emim][Tf2N] with η = 11.84 mPa.s (at 333.15 K)]. It is interesting as [emim+] and [bmim+] are fairly similar in structure (and aromatic), [pmpip+] is different (it is not aromatic). Due to the presence of –OH functionality on choline+, [choline][Tf2N] ionic liquid has shown different physicochemical properties (vide supra). This has further support from the fact that ka values in [choline][Tf2N] are relatively higher as compared to other ionic liquids, though the viscosity of [choline][Tf2N] is appreciable.46 For example, among all seven ionic liquids, the maximum value of ka is observed within 21



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[choline][Tf2N] even with appreciably high viscosity. In fact, the ka of 1Py(10)1Py in [choline][Tf2N] at 80 and 90 °C (32.4×106 s−1 and 26.6×106 s−1 with viscosities 15.43 and 19.61 mPa.s, respectively) are similar to the reported ka for 1Py(10)1Py of 28×106 s‒1 and 31×106 s‒1 in low viscosity organic solvents, toluene (0.59 mPa.s) and benzene (0.69 mPa.s), respectively.32 It is proposed that due to the higher inherent dipolarity of [choline][Tf2N] (because of the presence of –OH functionality) as compared to the other ionic liquids,46 the decyl chain–ionic liquid interaction is unfavorable in this ionic liquid (as opposed to ionic liquids with no –OH functionality). This would lead to more compact decyl chain leading to closer proximity of the two pyrenyl groups. This results in increased chain flexibility with less hindrance on chain motion possibly leading to faster alignment achieved by the two pyrenyl moieties. This explanation is earlier evoked to explain faster rate of 1Py(10)1Py cyclization in ethanol as opposed to that in toluene, benzene, and cyclohexane.32 The difference in the behavior of 1Py(10)1Py cyclization within [choline][Tf2N] ionic liquid is further manifested in the context of Stokes-Einstein relationship (discussed next). The fact that within [bmim][Tf2N], [emim][Tf2N] and [pmpip][Tf2N], the apparent ka are similar for similar viscosity ionic liquids but not in [choline[Tf2N], is reflected in the plots of ka versus T/η for the four [Tf2N−] ionic liquids (Fig. 6B). A careful examination of the data presented in the plot clearly shows that, unlike that observed in [bmim+] ionic liquids, except for [choline][Tf2N], ka versus T/η for the three [Tf2N−] ionic liquids combined show acceptable linear behavior (i.e., the slopes of ka versus T/η are 0.4±0.05 mPa.K−1.s−1 within each of the [bmim][Tf2N], [emim][Tf2N], and [pmpip][Tf2N], while it is 1.4±0.08 mPa.K−1.s−1 for [choline][Tf2N]). The analysis implies that the apparent ka within the three [Tf2N−] ionic liquids follows Stokes-Einstein relationship. The cyclization rate of 1Py(10)1Py within [bmim][Tf2N], 22


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[emim][Tf2N], and [pmpip][Tf2N] is diffusion-controlled (and depends on the bulk dynamic viscosity of the ionic liquid medium irrespective of the identity of the cation). This is contrary to the outcomes in [bmim+] ionic liquids, where Stokes-Einstein relationship is not observed at all (vide supra, Fig. 6A). Clearly, within these three [Tf2N−] ionic liquids, the bulk dynamic viscosity correlates with the microviscosity of the 1Py(10)1Py cybotactic region. The anion, and not the cation, of the ionic liquid appears to dominate the cybotactic region composition around the decyl chain of 1Py(10)1Py. The dynamics of cyclization is dominated by the anion of the ionic liquid, whereas the excimer formation efficiency is controlled more by the cation of the ionic liquid. These observations can be generalized to the ionic liquids with same anion having different cations (having no –OH like functionality present) and to those with same cation having different anions.

CONCLUSIONS Intramolecular excimer formation efficiency of the model bisfluorophoric probe, 1Py(10)1Py, dissolved in ionic liquid in the temperature range 10-90 °C increases with increase in temperature indicating effect of lowering of viscosity with increasing temperature to dominate the thermal (non-radiative) deactivation processes. At a given temperature, IE/IM is more widespread for ionic liquids with same anion having different cations as opposed to ionic liquids with same cation having different anions; cation appears to control excimer formation efficiency. Given the ability to modulate the microviscosity according to the choice of ionic liquid, these results also have significance for the potential of these media in optical sensing, such as in gas detection.55 Excited-state intensity decay kinetics within ionic liquids do not adhere to a simplistic Birk’s model; this is in contrast to that observed in organic solvents albeit similar to 23



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that observed in organized media. The apparent ka are an order of magnitude lower than those estimated in common organic solvents. Both IE/IM and apparent ka for iso-viscous ionic liquids are significantly different clearly indicating the important role of the chemical structure of the ionic liquid in excimer formation dynamics; microviscosity (i.e., cybotactic region viscosity) around 1Py(10)1Py, and not the bulk dynamic viscosity of the ionic liquid gets manifested in the excimer formation dynamics. This is further supported by the fact that the activation energy obtained from the linear fit of ln ka versus 1/T scales better with the activation energy of the microviscous flow as opposed to the activation energy of the viscous flow. Except for [choline+], in all other [Tf2N‒] ionic liquids cyclization dynamics follows Stokes-Einstein relation, whereas within [bmim+] ionic liquids, it does not. It is inferred that the anion of the ionic liquid controls cyclization dynamics of the long alkyl chain to form intramolecular excimer possibly by preferentially interacting with it; cation of the ionic liquid, to a good extent, controls IE/IM by preferentially solvating the excimer due to cation – π interaction (cation of the ionic liquid and πcloud of the excited pyrenyls). The work amply highlights the features of ionic liquids as a complex solubilizing media for cyclization dynamics of long alkyl chain to form intramolecular excimer.

Acknowledgments. This work was generously funded by a grant to Siddharth Pandey from the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India [grant no. EMR/2016/005053]. Anita Yadav thanks Council of Scientific and Industrial Research (CSIR), Government of India for her fellowship.

ASSOCIATED CONTENTS 24


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Supporting Information Table S1 presents recovered intensity decay parameters for 1Py(10)1Py dissolved in investigated ionic liquids at different temperatures. Table S2 presents fluorescence lifetimes (τ0) and decay rates (kM = 1/τ0) recovered from excited state intensity decay fit to a single exponential decay function for 1-methylpyrene dissolved in investigated ionic liquids at different temperatures. Table S3 presents recovered monomer intensity decay parameters at λem = 376 nm for 1Py(10)1Py dissolved in investigated ionic liquids at different temperatures according to fitting to lifetime distribution function. Figure S1 shows variations of 1Py(10)1Py IM and IE with temperature within investigated ionic liquids. Figure S2 shows variations of 1Py(10)1Py IE/IM with temperature for investigated ionic liquids close to ambient temperature. Figure S3 shows variations of 1Py(10)1Py IM/IE with η for investigated ionic liquids only for η

Excimer Formation Dynamics of Dipyrenyldecane ... - ACS Publications

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