Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1-pyrenyl

Sep 30, 2015 - Mixtures of ionic liquid with polyethylene glycol (PEG) have shown interesting features as solubilizing media. Intramolecular excimer f...
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Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1pyrenyl)propane within 1‑Butyl-3-methylimidazolium Hexafluorophosphate and Its Polyethylene Glycol Mixtures Anita Yadav, Narayanan D. Kurur, and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India S Supporting Information *

ABSTRACT: 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 Mn 570−630 (PEG600), and number-average MW Mn 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 threeexponential 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.

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INTRODUCTION Ionic liquids have shown potential as solvents with their unique properties and promising nonvolatile behavior.1−5 Unlike conventional molecular solvents used in chemistry, ionic liquids are composed entirely of ions which might lead to unusual and sometimes favorable results in chemical processes conducted in their presence. Although important applications of ionic liquids in synthesis, catalytic, polymerization, separation, and extraction processes have been demonstrated, certain limitations still remain.6−10 Two striking and crucial constraints are the limited solubility of a rather large number of common solutes in them and their high viscosity.11−14 One approach adopted by a number of researchers to mitigate the former problem is to look at cosolvent-modified ionic liquid systems. This also provides a way to tune their physicochemical properties.15−25 The environmentally friendly physicochemical properties, such as low vapor pressure, high chemical stability, and nontoxicity of polyethylene glycols (PEGs), make them particularly relevant as cosolvents in many industrial, pharmaceutical, and biomedical applications.26−29 In recent investigations, PEG-modified ionic liquid systems have shown certain unusual physicochemical behavior.17−19 For example, © XXXX American Chemical Society

the dipolarity/polarizability and the H-bond-donating (HBD) acidity of mixtures of PEGs of different average molecular weight (MW) with ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([brim][PF6]) were found to be significantly higher (hyperpolarity) than those of pure [bmim][PF6] or PEG.17,18 Similarly, the dynamic viscosity of certain ([bmim][PF6] + PEG) mixtures mixture was found to be higher (hyper viscosity) than that of either of the components also exhibited rare “hyperviscosity”.30 These interesting and unusual properties of ([bmim][PF6] + PEG) mixtures may afford unconventional and hard-to-predict results in important chemical reactions and processes. Excimer (excited dimer) formation by fluorophores is an actively pursued process in modern chemistry research.31−35 Intramolecular excimer formation is usually shown by molecules consisting of two identical fluorophores, especially polycyclic aromatic hydrocarbons, linked by a flexible chain.36,37 This requires close approach of the two fluorophores through Received: July 28, 2015 Revised: September 29, 2015

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DOI: 10.1021/acs.jpcb.5b07282 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B internal rotation during the lifetime of the excited state38,39 unlike during intermolecular excimer formation where the two fluorophores diffuse toward each other. Consequently, these excited bifluorophoric molecules (intramolecular excimers) can provide key insights to the microfluidity of the solubilizing environment (i.e., the cybotactic region).40 Intramolecular excimer formation has also been utilized effectively in a variety of chemosensing applications mostly based on ratiometric measurements involving wavelengths of the monomer and excimer emissions, respectively. 41 Emission of excimer molecules is used as a source of spontaneous ultraviolet light in excimer lamps.42 Recently, a blue light source in fluorescent organic light emitting diodes (OLEDs) based on intramolecular excimer emission was designed.43 In biophysics, intramolecular excimer formation is utilized to determine the distance between biomolecules.44 The intramolecular excimer formation of various dipyrenylalkanes in organic solvents was studied, among others, by the Zachariasse,45−49 Thistlethwaite,50 Martinho,51,52 Maçanita,53,54 Nishijima,55 Bright,56 and Pandey57 groups from different points of view. A number of these studies focused on the fluorescence decay behavior, especially the number of exponentials observed and the relative magnitudes of their amplitudes.47,48,53,54 It is clear from these investigations that the excimer emission depends most importantly on the solvent property/composition.47,48,53−55,58,59 The chain length, chain structure, actual fluorophore chemistry, exact labeling site on the fluorophore and the chain, tether length and its chemistry, temperature, and pressure also play a part. Among the dipyrenylalkanes, 1,3-bis-(1-pyrenyl)propane, [1Py(3)1Py], is perhaps the most investigated as the intramolecular excimer formation dynamics of 1Py(3)1Py is shown to be highly sensitive to the temperature and the solubilizing milieu.45−50 While the analysis of excited-state intensity decay data is found to be more complicated for 1Py(3)1Py, for probe 1,3-bis-(2pyrenyl)propane [2Py(3)2Py] with just a slight change in the labeling of the fluorophore site, interestingly the decay kinetics follows a simple biexponential model (the so-called Birk’s scheme).32,33,60 The intriguing features of ionic liquid [bmim][PF6] and its PEG mixtures as solubilizing milieu may exert a profound effect on excimer formation dynamics of 1Py(3)1Py. In this paper, we report results of our investigation of excimer formation dynamics of 1Py(3)1Py dissolved in ionic liquid [bmim][PF6] and its mixture with PEGs varying in molecular weights over the complete composition range in the temperature interval from 10 to 90 °C.

required amount of 1Py(3)1Py to prepare stock solution was weighed using a Mettler-Toledo AB104-S balance with a precision of ±0.1 mg. The ([bmim][PF6] + PEG) mixtures were prepared by mass using a Denver Instrument balance having a precision of ±0.1 mg. PEG1000 is solid at ambient conditions with a melting point of ∼38 °C. Therefore, the solutions for PEG1000 were prepared by melting it in a water bath at ∼60 °C and then mixing it with appropriate aliquots of [bmim][PF6]. This system was allowed to equilibrate to the desired temperature before any data acquisition. An appropriate amount of 1Py(3)1Py solution from the stock was transferred to a glass vial. The ethanol was evaporated with a gentle stream of high-purity nitrogen gas. Ionic liquid [bmim][PF6], PEG, or appropriate ([bmim][PF6] + PEG) mixture was added, and the mixture was stirred to ensure the complete solubilization of the probe to achieve the desired final concentration of 1 μM of the probe 1Py(3)1Py. The solutions were prepared under an argon atmosphere and sealed with parafilm to minimize the sorption of environmental moisture. All samples were purged with highpurity N2 gas for O2 exclusion as molecular oxygen is a wellknown fluorescence quencher.34 Steady-state emission and excitation spectra were acquired on model a FL 3-11, Fluorolog-3 modular spectrofluorimeter with single Czerny-Turner grating excitation and emission monochromators having a 450 W 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 a Thermo NESLAB RTE7 circulating chiller bath having stability ±0.01 °C. Spectral response from appropriate blanks was subtracted before data analysis. Since we used ultrapure [bmim][PF6] and pyrene and pyreneappended compounds are known usually to have high fluorescence quantum yields,33 the blank signal was 99.99%) was obtained from Molecular Probes and used as received. 1-Methylpyrene (1MePy, 99.9+%) was purchased from Sigma-Aldrich, and it was used without further purification. PEG200, PEG400, PEG600, and PEG1000 were obtained in highest purity from Sigma-Aldrich and stored under dried conditions. Ionic liquid [bmim][PF6] was obtained from Merck (ultrapure, halide content < 100 ppm, water content PEG600 > PEG200 > PEG1000 versus order of Ea,η: PEG400 > PEG200 > PEG600 > PEG1000), the Ea,η values being lower than the corresponding Ea values (Ea > Ea,η for PEGs). In complete contrast, within [bmim][PF6], while Ea,η are reported to be higher than those within PEGs,30 H

DOI: 10.1021/acs.jpcb.5b07282 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B surprisingly Ea,η is found to be higher than Ea (Ea < Ea,η for [bmim][PF6]). It was reported earlier that the rate of intramolecular excimer formation of bispyrenyl compounds scales better with the microviscosity of the milieu rather than the bulk viscosity of the system.71 As proposed earlier, we believe this to be the case here as it is supported by the fact that the activation energy of the microviscous flow, Ea,ημ, for [bmim][PF6] (31.2 kJ mol−1)71 is similar to the Ea of intramolecular excimer formation by 1Py(3)1Py within [bmim][PF6] (∼30 kJ mol−1). While for most systems53,55,58,59 it is conventionally observed that Ea > Ea,η (this is also the case within neat PEGs), we were surprised by the fact that Ea < Ea,η within [bmim][PF6]. It is clear, however, that as reported earlier Ea,ημ and not Ea,η controls the cyclization. For 1Py(3)1Py, the rotational dynamics and energetics of the chain, which are most affected by the microscopic properties (i.e., the interactions present on the molecular level), play an important role in intramolecular excimer formation. One of the interesting features of this investigation is the relation of 1Py(3)1Py intramolecular excimer formation dynamics with the viscosity of the mediumfor different isoviscous media, the fitting protocols for excited-state intensity decay along with the recovered parameters are different (vide supra). This also transforms to significantly different values of ka for systems having very similar viscosities. For example, estimated ka (with error ≤ 5%) are 0.91 × 106 versus 1.55 × 106 s−1 for PEG200 at 10 °C (viscosity = 116.02 cP) versus PEG400 at 20 °C (viscosity = 115.95 cP), 2.19 × 106 versus 6.33 × 106 s−1 for PEG200 at 20 °C (viscosity = 63.96 cP) versus PEG600 at 40 °C (viscosity = 64.48 cP), 6.69 × 106 versus 10.74 × 106 s−1 for PEG400 at 40 °C (viscosity = 44.40 cP) versus PEG600 at 50 °C (viscosity = 43.82 cP), 3.39 × 106 versus 8.52 × 106 s−1 for PEG400 at 30 °C (viscosity = 69.62 cP) versus PEG1000 at 50 °C (viscosity = 70.89 cP), and 2.92 × 106 versus 4.59 × 106 s−1 for PEG600 at 30 °C (viscosity = 101.68 cP) versus PEG1000 at 40 °C (viscosity = 107.83 cP). Careful examination of the aforementioned analysis clearly indicates that for the isoviscous PEG systems, the higher the average MW of the PEG, the higher the value of ka. It appears the longer polymer chains of PEG help the kinetics of 1Py(3)1Py intramolecular excimer formation. Clearly, the wrapping of polymer chains not only helps to reduce the dissociation of the excimer back to the monomer but also expedites the process of excimer formationthe longer the chains, faster the rate of excimer formation. It was revealed earlier that 1Py(3)1Py intramolecular excimer formation rate constants do not obey the Stokes−Einstein relationship (i.e., at a given temperature, ka ≈ 1/η does not hold).31,53 We found that the Stokes−Einstein relationship is not obeyed within PEGs and their mixtures with [bmim][PF6] as well. Figure S1 shows a representative plot of ka·η versus η at 30 °C encompassing all systems investigated at this temperature. It is clear that ka·η is not a constant. Similar observations are recorded at other temperatures also (data not shown). Apart from the inherent complexity associated with ([bmim][PF6] + PEG) mixtures as solubilizing media (presence of hyperviscosity and hypermicroviscosity),30 the fact that two different 1Py(3)1Py excimer conformations exist within these system also contributes to the observation that the Stokes− Einstein relationship does not hold within these systems. Plots of ln ka versus ln η for 1Py(3)1Py at 30, 40, and 50 °C (encompassing neat PEGs only) are shown in Figure 8 (slopes obtained from linear regression analysis are presented in the

Figure 8. Plot of ln ka vs ln η for 1Py(3)1Py for neat PEGs and neat [bmim][PF6] at 30, 40, and 50 °C. Slopes are presented in the inset.

inset). From fair-to-good linearity of the plots, it appears that ka is proportional to η−α with α < 1.0 within neat PEGs. The coefficient α is found to decrease with increasing temperature (inset Figure 8). Similar outcomes were reported earlier for intramolecular excimer formation by another dipyrenyl compound, 1,5-bis(1-pyrenylcarboxy)pentane, dissolved in nalkane solvents at −30, 25, and 75 °C and are attributed to the decreased coupling as the temperature is increased between the excimer-forming molecule and the solubilizing milieu as the polarizability of the medium (or the refractive index) decreases with increasing temperature.53 It is interesting to mention here that due perhaps to the inherent complexity (including reported hyperviscosity and hypermicroviscosity) associated with the ([bmim][PF6] + PEG) mixtures, the ln ka versus ln η for 1Py(3)1Py within the mixtures does not show acceptable linear behavior (e.g., a representative analysis affords R2 = 0.7045 at 30 °C when neat PEGs along with all mixtures of ([bmim][PF6] + PEG) are considered). It appears that while it is relatively easy to interpret 1Py(3)1Py intramolecular excimer formation rate constants in the context of the Stokes−Einstein relationship within neat PEGs, it is not so for ([bmim][PF6] + PEG) mixtures. This amply highlights the inherent complexities, as far as interactions within the milieu are concerned, associated with ionic liquid mixtures of PEGs. Viscosity variations of ionic liquid containing binary mixtures, in general, are known to show complicated trends and can be modeled only after modifying the existing theories.72 Composition Dependence of Rate Constants of 1Py(3)1Py Intramolecular Excimer Formation within ([bmim][PF6] + PEG) Mixtures. The decay times recovered from the fits of the excited-state intensity decay of the monomer emission to a single-exponential function for ([bmim][PF6] + PEG) mixtures as a function of ([bmim][PF6] mole fraction at different temperatures are listed in Table S4. Careful examination of the decay times reveals that irrespective of the temperature and the identity of the PEG constituting the mixture, in general, the decay time values show maxima at x[bmim][PF6] in the range 0.5−0.7. Interestingly, this is similar to the variation in microviscosities of the ([bmim][PF6] + PEG) mixtures as reported by 1Py(3)1Py.30 Therefore, the recovered decay time depends on the microviscosity afforded by the ([bmim][PF6] + PEG) mixtures. Subsequently, the recovered decay time values within ([bmim][PF6] + PEG) mixtures I

DOI: 10.1021/acs.jpcb.5b07282 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B appear to depend on the H-bonding network present within the medium involving C2−H of the imidazolium cation and oxygen of the termini hydroxyl and/or the ethoxy units of the PEG (H bonding involving the PF6 anion of the ionic liquid and the termini hydrogen of the PEG hydroxyl group may also contribute) as proposed earlier;17−19,30 the more the H-bonded network, the longer the decay time. Further, the H-bond donating acidity (α)17 and the dipolarity/polarizability (as revealed by the ENT and π* values17 along with pyrene and pyrene-1-carboxaldehyde fluorescence responses19) of the ([bmim][PF6] + PEG) mixtures also show maximum values in the region slightly rich in [bmim][PF6] (closer to the equimolar composition).17−19 Compositions of the mixtures possessing relatively higher dipolarity/polarizability appear to afford longer recovered times. Therefore, it may be concluded that the recovered decay time values correlate well with the dipolarity/polarizability and the H-bond-donating acidity of the ([bmim][PF6] + PEG) mixtures. The overall rate constant (ka) for intramolecular excimer formation by 1Py(3)1Py within ([bmim][PF6] + PEG) mixtures was estimated for the systems at temperatures where [IM(t)] showed best fit to a single-exponential decay function (ka are presented in Table 3). Careful examination of the ka values reveals an interesting outcome. For all ([bmim][PF6] + PEG) mixtures at a given temperature, as the mole fraction of [bmim][PF6] is increased, ka first decreases, attains a minimum value, and then increases again. For [bmim][PF6] mixtures with PEGs investigated, this minimum in ka usually appear at x[bmim][PF6] ≈ 0.6. This was surprising at first; however, we realized that ([bmim][PF6] + PEG) mixtures show unusual hyperviscosity as well as hypermicroviscosity, where at certain compositions dynamic viscosities and/or microviscosities of the mixtures are found to be higher than the corresponding values in both neat [bmim][PF6] and neat PEG.30 It is found that the microviscosities of ([bmim][PF6] + PEG) mixtures, as manifested through the 1Py(3)1Py monomer-to-intramolecular excimer steady-state emission intensity ratio, demonstrate this hyper effect where the hypermicroviscosities are found to have the maximum magnitude around x[bmim][PF6] ≈ 0.6 (the maximum magnitude of the hyperviscosities, on the other hand, are closer to x[bmim][PF6] ≈ 0.8). It appears the ka for 1Py(3)1Py intramolecular excimer formation within ([bmim][PF6] + PEG) mixtures scale with the microviscosity of the mixture. This is in accord with the report that the kinetic parameters of intramolecular excimer formation by another bispyrenyl probe, 6-(1-pyrenyl)hexyl-11(1-pyrenyl)undecanoate, also scale better with the “microviscosity” of the ([bmim][PF6] + tetraethylene glycol) mixtures.71

temperatures and only one of the excimers starts to dissociate back to monomer appreciably as the temperature is increased in the range 10−90 °C. It is proposed that PEGs wrap around the excimer(s), thus hindering the dissociation back to the monomer. As a medium, [bmim][PF6] at higher temperatures is more similar to the organic solvents where dissociation of both excimers back to the monomer happens significantly. The wrapping around is absent within ionic liquid as the media. Long-chain PEGs are able to exert the wrapping around of the excimer(s) within ([bmim][PF6] + PEG) mixtures, thus controlling the intramolecular excimer formation dynamics. The excimer formation dynamics as well as the overall rate constants of excimer formation are found to be different for systems with very similar viscosities. The Stokes−Einstein relation does not hold within these systems due partly to the complexities associated with these media and to the presence of two excimer conformers. The overall rate constant of excimer formation scales better with the microviscosity (and not the bulk viscosity) of the ([bmim][PF6] + PEG) mixtures. The excimer formation dynamics of this probe amply reasserts the inherent complexities associated with ionic liquids, PEGs, and their mixtures.

CONCLUSIONS Intramolecular excimer formation dynamics of 1Py(3)1Py dissolved in ionic liquid [bmim][PF6], liquid PEGs, and ([bmim][PF6] + PEG) mixtures conforms to the DMD scheme. According to this scheme, one excited-state monomer (1 M*) and two kinetically distinguishable excimers (1D1* and 1 D2*), where both excimers are populated simultaneously by the excited monomer with no interconversion between the two excimers, are present irrespective of the mixture composition and the temperature within the range 10−90 °C. This is similar to the observation within the organic solvents in this temperature range. While both excimers readily dissociate back to the monomer within organic solvents, in contrast, this dissociation back is hindered in neat PEGs at lower

ACKNOWLEDGMENTS This work was generously funded by a grant to Siddharth Pandey from the Council of Scientific and Industrial Research (CSIR), Government of India [grant no. 01(2767)/13/EMRII]. A.Y. also thanks CSIR for her fellowship. Help in data collection and analysis from Dr. Shruti Trivedi and Dr. Rewa Rai, respectively, is deeply appreciated and acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07282. Recovered intensity decay parameters for 1Py(3)1Py dissolved in neat PEGs and neat [bmim][PF6] and ([bmim][PF6] + PEG) mixtures at different temperatures; fluorescence lifetimes (τ0) and decay rates (kM = 1/τ0) recovered from excited-state intensity decay fit a single-exponential decay function for 1-methylpyrene dissolved in [bmim][PF6], PEGs; and their mixtures at different temperatures; decay times recovered from fit of the excited-state intensity decay of monomer to a singleexponential function for [bmim][PF6], PEGs, and their mixtures at different temperature; kaη versus η for 1Py(3)1Py within [bmim][PF6], PEGs, and their mixtures at 30 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-11-26596503. Fax: +91-11-26581102. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.jpcb.5b07282 J. Phys. Chem. B XXXX, XXX, XXX−XXX