DonorâAcceptor Complexes of Ionic Liquids and ... - ACS Publications

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Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

More than a Solvent: Donor−Acceptor Complexes of Ionic Liquids and Electron Acceptors Alexander Aster and Eric Vauthey* Department of Physical Chemistry, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland S Supporting Information *

ABSTRACT: The applicability of room-temperature ionic liquids (RTILs) as inert solvents is generally based on their electrochemical window. We herein show that this concept has its limitations if RTILs are exposed to an oxidizing environment in the presence of light. Acetonitrile solutions of RTILs with 1-methyl-3-ethylimidazolium as cation and five different anions, including thiocyanate (SCN−) and dicyanamide (DCA−), were investigated. Upon addition of organic electron acceptors to solutions of RTILs with SCN− or DCA−, charge-transfer (CT) absorption bands due to the formation of donor−acceptor complexes between the anion and the electron acceptor were observed. Time-resolved measurements from the femtosecond to the microsecond regimes were used to investigate the nature and the excited-state dynamics of these complexes upon excitation in the CT band. We show that even though the RTILs are seemingly inert according to their electrochemical properties, the dicyanamide and thiocyanate based RTILs can actively participate in photochemical reactions in oxidizing environments and therefore differ from the behavior expected for an inert solvent. This has not only important implications for the long-term stability of RTIL-based systems but can also lead to misinterpretation of photochemical studies in these solvents.

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INTRODUCTION Room temperature ionic liquids (RTILs) are considered as promising replacement for volatile solvents due to their insignificant vapor pressure, good thermal stability, and broad electrochemical window.1−4 The impact of the ionic nature of RTILs on the mechanism and dynamics of chemical processes compared to conventional neutral solvents has evolved into a flourishing research field.5−21 The interactions between charged solvent molecules and the solutes can be expected to affect the energetics and the dynamics of elementary chemical processes, especially those involving charged species such as electron or proton transfer reactions. However, before investigating the solvent/solute interactions in an RTIL, one should always question whether the stability of RTILs in some specific environments is sufficient to consider them as inert solvents. This holds especially for redox-active environments, e.g., when applied as electrolyte solvents in dyesensitized solar cells (DSSC) or as solvents for investigating electron-transfer processes. The applicability as an inert solvent is generally based on the electrochemical window of the RTIL compared to the redox potentials of the solutes.22,23 If the latter are within the electrochemical window of the solvent, the RTIL is considered as inert. However, in the presence of light, additional reaction channels have to be considered for judging about the inert character of a solvent: (1) electronic excitation increases the redox activity of the solute and can lead to reductive or oxidative quenching by the solvent;24−27 (2) the solutes may form donor−acceptor complexes (DACs) with the solvent,28−30 which are revealed by broad charge-transfer (CT) © XXXX American Chemical Society

absorption bands in the UV and visible regions. In the case of conventional solvents, both reductive/oxidative quenching and CT excitation of DACs lead to electron transfer and to the formation of ion pairs. The latter can then undergo recombination back to the parent neutral species or dissociate into free ions. If the solvent is a RTIL, these two processes result in the formation of radical pairs, which can dissociate into free radicals that may themselves recombine or react further. In the latter case, the RTIL can no longer be considered as an inert solvent as it actively participates in a photochemical reaction. Reductive and oxidative quenching by electron transfer with a RTIL was already encountered in many cases and was also used to study electron transfer in RTILs.31−33 In this case, the deviation from an inert solvent can be easily predicted using the Rehm−Weller equation, which in addition to the electrochemical window of the solvent and the redox potentials of the solutes, takes the excitation energy, E00, into account.34 In contrast, whether or not DACs are being formed and their optical excitation has an impact on the reaction of interest is a more delicate matter. This holds especially for ionic liquids, which often show optical impurities, in many cases of unknown origin, masking the possible presence of a broad CT absorption band.35−37 To the best of our knowledge, the formation of DACs involving RTILs as donors or acceptors as well as their Received: January 15, 2018 Published: February 13, 2018 A

DOI: 10.1021/acs.jpcb.8b00468 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Steady-State and Transient Absorption (TA) Spectroscopy. Stationary absorption spectra were recorded on a Cary 50 spectrophotometer. The TA spectra up to 1.5 ns were recorded using a setup with a wavelength-dependent instrument-response function (IRF) of ca. 100−350 fs (fwhm) as described in detail elsewhere.46 Excitation was carried out using 400 nm pulses generated by frequency doubling part of the output of a standard 1 kHz Ti:Sapphire amplified system (Spectra Physics, Spitfire). The pump−probe setup used to record transient absorption spectra from sub-ns to μs with an IRF of 350 ps (fwhm) has been described in detail in ref 47. Excitation was performed at 355 nm using a passively Q-switched, frequency-doubled Nd:YAG laser (Teem Photonics, Powerchip NanoUV). In both setups, probing was achieved with white light pulses generated by focusing in a CaF2 plate 800 nm pulses polarized at magic angle relative to the pump pulses. All TA spectra were corrected for signals (e.g., spontaneous emission) appearing before time zero. Furthermore, the fs TA spectra were corrected for the dispersion due to the optical chirp using the optical Kerr effect.48 The sample solutions were located in a 1 mm quartz cuvette and bubbled with argon during the measurement to refresh the sample volume in the excitation spot and to remove oxygen.

impact on the inertness of the solvent have not been investigated so far. We report here on the observation of DACs upon addition of various organic electron acceptors (A) to solutions of frequently used 1-ethyl-3-methylimidazolium (EMI) RTILs in acetonitrile (ACN) (Chart 1). These DACs are formed Chart 1. Chemical Structures of the RTIL Cation and Anions and of the Electron Acceptors with Their Oxidation and Reduction Potentials vs SCE

a

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From ref 41. bFrom ref 42. cFrom ref 43. dFrom ref 44. fFrom ref 45.

RESULTS AND DISCUSSION Steady-State Absorption. The electronic absorption spectra of solutions of 0.2 M RTILs in ACN are shown with dashed lines in Figure 1B−F, whereas those of the electron acceptors in ACN are depicted in Figure 1A. In the absence of acceptor, the solutions are transparent down to at least 285 nm. Upon addition of the acceptors, the absorption spectra of the solutions with EMINTf2 (B), EMIBF4 (C), and EMITf (D) are the composites of those of the acceptors and RTILs in ACN. However, the spectra with EMIDCA (E) and EMISCN (F)

between the electron acceptor A and one of the RTIL constituents as donor. EMI-based RTILs with five different anions were selected because they allow a wide range of oxidation potentials to be covered and also because they are commonly used in energy storage and conversion.4,38−40 The electron acceptors, which are typical quenchers in photoinduced bimolecular electron-transfer studies, are characterized by different reduction potentials, which are all within the electrochemical window of the RTILs. Therefore, the latter can be expected to behave as inert solvents. The excited-state dynamics of these DACs upon CT photoexcitation were investigated in the femto to microsecond time regimes by transient electronic absorption spectroscopy. Based on these measurements, we will show that CT excitation does not only open a pathway for RTIL photodegradation, but can also lead to data misinterpretations in bimolecular electron-transfer quenching experiments.

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EXPERIMENTAL SECTION Chemicals. Acetonitrile (ACN, Roth, Rotidry, ≥99.9%), potassium thiocyanate (KSCN, Acros, ≥99%), 1-ethyl-3methylimidazolium thiocyanate (EMISCN, Iolitec, >98%), 1ethyl-3-methylimidazolium dicyanamide (EMIDCA, Iolitec, >98%), 1-ethyl-3-methylimidazolium triflate (EMITf, Iolitec, >99%), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4, Iolitec, >98%), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMINTf2, Iolitec, >99%) were used as received. Perylene (Pe, Sigma-Aldrich, sublimed grade, ≥99.5%), dicyanoethylene (DCE, Acros, 98%), dicyanobenzene (DCB, Aldrich, 98%), maleic anhydride (MA, Fluka, 99%), phthalic anhydride (PA, Aldrich, ≥99%) and pyromellitic dianhydride (PMDA, Acros, 99%) were sublimed under reduced pressure.

Figure 1. Stationary electronic absorption spectra of electron acceptor solutions in ACN (A) and in the presence of 0.2 M RTIL (B−F). Dashed line: no acceptor; orange: 0.1 M DCB; purple: 0.15 M DCE; green: 0.2 M PA; red: 0.2 M MA; black: 0.05 M PMDA. B


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Time-Resolved Spectroscopy. Nanosecond TA. In general, optical excitation in the CT band of a DAC between neutral constituents leads to the formation of a contact ion pair, which can then either undergo geminate recombination or dissociate into free ions.49−52 As here D is most probably the RTIL anion, CT excitation of the RTIL−A complexes should result in a pair consisting of the neutral donor radical, D•, and the radical anion, A•−. TA measurements were performed to test the validity of this hypothesis and determine the fate of this radical pair. Figure 3 depicts ns TA spectra measured with six

reveal an additional absorption band that is not present in the spectra of the individual components. The maximum of this new band shifts to longer wavelength with increasing reduction potential of the acceptor. Furthermore, for a given acceptor, the band with the strongest electron donor EMISCN is red-shifted compared to that with EMIDCA. These features, namely the red shift with decreasing potential difference between electron donor and acceptor, are consistent with the CT absorption band of a DAC with the RTIL anion as donor. The involvement of the RTIL anion as an electron donor will be confirmed by the time-resolved measurements described below. To extract the extinction coefficient and the association constant of the DACs, the change of absorption of four RTIL-acceptor pairs upon variation of the RTIL concentration was monitored and analyzed globally assuming a 1:1 complex.35 As an example, Figure 2A shows the

Figure 3. Nanosecond transient absorption spectra measured after CT excitation at 355 nm of EMISCN (0.2 M) with DCE (A), MA (B), PMDA (C), and EMIDCA (0.2 M) with DCE (D), MA (E), and PMDA (F) in ACN.

different RTIL−A pairs in ACN after CT excitation at 355 nm. In the case of the EMISCN−DCE pair (Figure 3A), the TA spectra exhibit two broad induced absorption bands around 475 and 380 nm. The band at 475 nm can be assigned to the SCN radical dimer anion, (SCN)2•−,53 whereas that at 380 nm is characteristic of the DCE radical anion, DCE•−.54 Since the molar absorption coefficient of the DAC is about 10 times as small as those of the radicals, the ground-state bleach is not visible. The presence of (SCN)2•−, which is formed in a bimolecular reaction between SCN• and SCN−,53 is clear evidence that D is the RTIL anion. The (SCN)2•− band at 475 nm is also clearly visible in the TA spectra recorded with the EMISCN−MA and EMISCN− PMDA pairs (Figure 3B and C). The MA•− radical anion absorbs around 375 nm,54 and only the onset of its band can be observed in Figure 3B. On the other hand, the PMDA•− radical anion is responsible for the intense band at 665 nm with a shoulder at 615 nm.54 Both the rise time and the maximum intensity of the (SCN)2•− band decrease in the order DCE > MA > PMDA. Since the recombination of the radical pair and the dimerization are competing, the inverse rise time of the dimer band is the sum of the rate constants of both these processes, and the (SCN)2•− yield depends on how fast is dimerization relative to recombination. Therefore, this observed dependence can be explained by a faster recombina-

Figure 2. Absorption spectra of 0.2 M MA in ACN and increasing concentrations of EMISCN (A). Molar absorption coefficients and association constants in M−1 of DACs between EMIDCA or EMISCN and MA or DCE obtained from global analysis assuming a 1:1 association model (B).

absorption spectra of MA in ACN with increasing EMISCN concentration. The DAC spectra expressed in molar absorption coefficient obtained from the global analysis of four different D−A pairs are compared in Figure 2B. The band position correlates well with the electron donor and acceptor strength of the constituents (see Chart 1). The association constants obtained from the analysis are also given in Figure 2 and are close to 1 M−1. However, these small constants make their accurate determination as well as the estimation of the absorption coefficients difficult and add a substantial error on these quantities. C


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Figure 4. Transient absorption spectra recorded at various time delays after CT excitation at 400 nm of PMDA (0.15 M) with (A) 0.1 M EMIDCA and (B) EMISCN in ACN. (C) Early transient absorption spectra recorded with the EMIDCA-PMDA pair illustrating the spectral dynamics upon equilibration of the excited DAC. (D) Time evolution of the band area of PMDA•− between 500 and 715 nm measured with EMIDCA and EMISCN with the best multiexponential fits (black lines) and the residuals.

transient absorption. In general, the early transient absorption spectra measured upon CT excitation of DACs between neutral constituents are close to the absorption spectrum of the radical ions of the D and A subunits. In many cases, these spectral features undergo small changes on the few ps time scale, such as narrowing and shift, upon equilibration of the excited complex.56,61 TA spectra recorded at various time delays after CT excitation at 400 nm of the EMIDCA−PMDA pair in ACN are shown in Figure 4A. They reveal that the transient band seen in the ns TA spectra at 430 nm builds up within the ∼200 fs IRF of the experiment. Consequently, this band can be safely attributed to DCA• and not to (DCA)2•−. During the first few ps after excitation, small shift and narrowing of the transient bands take place upon equilibration of the excited DAC as shown in Figure 4C for the PMDA•− band and in Figure S5 for the DCA• band. To minimize the impact of the spectral dynamics, the kinetics were extracted from the area between 500 and 715 nm for PMDA•− (Figure 4D) and between 415 and 480 nm for DCA•. The time dependence of the band area could be well reproduced using a biexponential function with a rising component and a component decaying to a value larger than zero. The rise time amounts 0.7 and 0.6 ps for PMDA•− and DCA•, respectively. This component can be assigned to the equilibration of the excited DAC. This process is expected to be nonexponential,62 and therefore, these time constants should be considered as rough estimates only. The decay time amounts

tion when going to a stronger electron acceptor, i.e., when decreasing the energy gap between the radical pair and the ground state. This is fully consistent with previous investigations where the recombination dynamics of excited DACs were found to accelerate exponentially with the decreasing energy gap.55−57 Finally, the decay of the (SCN)2•− and radical anion bands follows a second-order kinetics (see Figure S3) as expected for non-geminate bulk recombination.58 The ns TA spectra recorded with the EMIDCA−A pairs (Figure 3D−F) show the radical anion bands of the acceptor as well as bands at 430 and 400 nm, which can be attributed to either the dicyanamide radical, DCA•, or to a follow-up product. Using electron-spin resonance spectroscopy, Shkrob et al. reported the observation of the DCA dimer radical anion, (DCA)2•−, upon radiolysis of a DCA-based RTIL.59,60 Furthermore, the irreversibility of the electrochemical oxidation of EMIDCA was explained by the formation of a neutral dimer, although degradation upon oxidation could not be excluded.42 With DCE and MA as acceptors, the decay of the bands at 430 and 400 nm occurs on a few tens of nanoseconds time scale and does not follow second-order kinetics. Moreover, it is accompanied by a rise of the transient absorption below 400 nm. The decay is slower with PMDA and a parallel change in the shape of the PMDA•− band can be observed. These features point to follow-up reactions, in agreement with a degradation of the samples with EMIDCA upon prolonged irradiation. Femtosecond TA. Insight into the early events after photoexcitation of the DACs was obtained using femtosecond D


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Figure 5. Time dependence of the transient absorption spectra (A) and transient spectra at selected time delays (B) measured upon 400 nm excitation of 0.2 M EMISCN and 0.2 M MA in ACN. Comparison of kinetics at 510 nm with different EMISCN concentrations (C). Time profiles of the transient absorption at 510 and 385 nm measured with the same sample (D).

to 8.5 and 8.7 ps for PMDA•− and DCA•, respectively. Assuming that geminate recombination of the radical pair is only operative at contact distance, this decay time corresponds to the lifetime of the radical pair, i.e., of the excited DAC. This lifetime, τrp, depends on the dynamics of both geminate recombination and dissociation of the pair into free radicals.63 The constant value of the band area after the decay can be attributed to the free-radical population that decays on a much longer time scale as shown by the ns TA measurements. The relative amplitude of this plateau points to a free-radical yield, Φrad, of 4%. Assuming only on-contact geminate recombination, the dissociation rate constant of the radical pair, kdis, can be estimated from the radical yield63 Φrad =

kdis = kdisτrp kdis + k rec

slower recombination in agreement with the 15.2 ps component. The TA spectra in Figure 4B also show that the 475 nm band attributed to (SCN)2•− is not visible at early time but exhibits a distinct rise time. However, because of its partial overlap with the intense PMDA•− absorption, the build up of this relatively weak band is partially hidden. In contrast to PMDA•−, MA•− does not exhibit a strong absorption between 450 and 700 nm, and therefore, the EMISCN−MA pair can be used to examine the formation of (SCN)2•− in greater detail. To rule out a possible influence of the cation, the same measurements were also carried out with KSCN instead of EMISCN. The stationary and transient absorption spectra with KSCN are the same as with EMISCN as shown in Figure S6E. Directly upon excitation, the overlapping absorption bands of MA•− and SCN• can be observed below 400 nm (Figure 5A, B). This transient absorption undergoes a fast partial decay that is concurrent with the 1.2 ps rise of a broad band centered at 510 nm (Figure 5A−C). This band narrows during the first 100 ps and finally transforms into the (SCN)2•− band at 475 nm observed by ns TA spectroscopy. Several groups have also reported such a broad band, red-shifted by 20 nm relative to that of (SCN)2•− upon hole transfer from TiO2 to KSCN.68−70 This band was assigned to a weakly coupled dimeric radical anion SCN•··· SCN−, which then undergoes structural rearrangement to the stable (SCN)2•− characterized by the 475 nm absorption band. To validate this assignment, the concentrations of MA and EMISCN in ACN were changed according to the association constant in order to vary the SCN− concentration while keeping the DAC concentration unchanged. The time evolutions of the transient absorption at 510 nm measured at different EMISCN concentrations are depicted in Figure 5C. The pump intensity was held constant, and minor concentration differences of the DACs were balanced by multiplying the TA intensity with a factor derived from the steady-state

(1)

where krec is the rate constant of geminate recombination. A kdis value of 4.65 ns−1 is obtained from eq 1. This is faster than the values reported for the dissociation of geminate ion pairs in ACN, which is of the order of 1 ns−1.64−66 This difference can be explained by considering that, contrary to the radical pair considered here, ion pairs have to overcome a substantial Coulombic barrier for separation. The TA spectra recorded with the EMISCN−PDMA pair are dominated by the PMDA•− bands (Figure 4B). As shown in Figure 4D, the decay of the area of this band between 500 and 715 nm is bimodal with 1.0 and 15.2 ps components. Such decay could originate from a distribution of DACs with different mutual orientation or distance and, thus, different electronic coupling as already reported previously.67 As SCN− is a better donor than DCA−, charge recombination is less exergonic and, consequently, faster, in agreement with the 1 ps component. On the other hand, the 10% radical yield determined from the amplitude of the plateau points to a E


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the reorganized SCN• or is a transition of RP′ cannot be answered. Furthermore, the spectroscopic resemblance between the paired and the free radicals makes the dissociation of the pair into free radicals prior to dimerization difficult to resolve. Scheme 1 depicts a simplified scheme where only the essential paths are shown and other possible processes such as the recombination of RP′ are omitted. Impact on Photoinduced Electron-Transfer Studies. We now discuss the influence of these DACs when investigating bimolecular photoinduced electron-transfer reactions in RTILs or in a solvent mixture containing a RTIL. As chromophore and electron donor, we chose perylene, Pe, for its redox properties as well as the intense absorption of its S1 state and of its radical cation, Pe•+, in the visible region.64,72 Ns TA spectra recorded at different time delays after 355 nm excitation of Pe with 0.25 M DCE and 0.2 M EMIDCA in ACN are shown in Figure 6A. The early transient spectra show negative

absorption shown in Figure S2. If the fast rising absorption at 510 nm were originating from SCN•···SCN−, either the rise of the signal should accelerate according a pseudo-first-order kinetics or the maximum band intensity should increase if static dimerization were occurring. However, both the kinetics and the amplitude are independent of the EMISCN concentration as depicted in Figure 5C. Nevertheless, this figure shows that the decay of the 510 nm signal slows as the EMISCN concentration is increased. This effect is due to the larger amplitude and faster build-up of the (SCN)2•− band. The higher intensity of the (SCN)2•− band upon increasing EMISCN concentration is clearly visible in Figure S6A−C. It is further supported by measurements in pure EMISCN as illustrated in Figure S6D showing the same rise time at 510 nm but a much faster evolution to the (SCN)2•− band at 475 nm due to the higher concentration of SCN−. Consequently, the observation that the intensity and build-up of the 475 nm band only but not of the 510 nm band depend on the SCN− concentration strongly suggests that the 510 nm band is not related to a dimeric radical anion. Additionally, the formation of a 2:1 DAC cannot account for the above observations. The coexistence of both 1:1 and 2:1 DACs between SCN− and methyl viologen with similar CT absorption has been suggested by Ebbesen et al.71 However, the relative concentration of the 2:1 DACs should strongly depend on that of SCN−. Consequently, if the 510 nm band were due to (SCN)2•− in the excited 2:1 DAC, its intensity should increase with EMISCN concentration. To account for the ensemble of observations, we propose the photocycle depicted in Scheme 1 where the rise of the 510 nm Scheme 1. Simplified Representation of the Photocycle of (SCN−−A) DAC upon CT Excitation

Figure 6. (A) Ns transient absorption spectra measured at different time delays after 355 nm excitation of Pe with 0.25 M DCE and 0.2 M EMIDCA in ACN. (B) Time evolution of the ground-state bleach and the Pe radical cation bands at different concentrations of RTIL.

band is associated with structural relaxation of the excited DAC, from RP to RP′. The bimodal decay of the excited DAC population (Figure 4D) could be due to a decrease of the electronic coupling during this rearrangement. Therefore, recombination is initially fast and slows down upon rearrangement to RP′. This idea is supported by recent observations by Castner and co-workers that small rotational and translational displacements between donor and acceptor suffice to significantly change the electronic coupling.21 This could be an alternative explanation to the above-mentioned distribution of geometries. Dimerization occurs upon diffusional encounter between the reorganized pair and the bulk SCN−, accounting for the EMISCN concentration dependence at long time scales. Whether the band at 510 nm arises from a local transition of

bands below 500 nm due to the bleach of the S1 ← S0 transition (ground-state bleach, GSB) and to the S1 → S0 stimulated emission (SE), as well as a positive band around 690 nm originating from Sn ← S1 excited-state absorption (ESA). Both SE and ESA bands exhibit a rapid decay due to the electrontransfer quenching of Pe in the S1 state by DCE as depicted in Scheme 2 (ET) and Scheme 3A. This decay is accompanied by the concurrent rise of a band at 540 nm originating from the D5 ← D1 transition of Pe•+. As shown in Figure 6B, the Pe•+ band as well as the GSB exhibit a fast decay on the sub-nanosecond time scale, which can be attributed to the geminate recombination of the (Pe•+DCE•−) ion pair, in agreement with previous measurements in ACN.73 F


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optical window than most conventional solvents, it is easier to overlook broad CT bands which can resemble the tail of the ionic liquid absorption. (3) The additional ion formation via RTIL-based DACs could overlap with the intrinsic ion dynamics of the chromophore-quencher pair and, if not carefully taken into account, may lead to erroneous conclusions. Photodegradation. In addition to its impact on fundamental studies in RTILs, the formation of DACs has also consequences for their applications. All six RTIL−A pairs with EMIDCA and EMISCN undergo photodegradation after a few hours, yielding dark red and yellow solutions, respectively. The photoinduced formation of radicals described here possibly leads to polymerization, which in turn results in strong absorption in the visible, as observed in electrochemical studies of SCN−.74 Such photoprocess might be at the origin of the fast degradation of DSSCs using EMIDCA and EMISCN as solvents for the I3/I− redox couple.75

Scheme 2. Electrochemical Potential Scheme Illustrating the Electrochemical Window of EMIDCA, the Oxidation of the Chromophore in the Ground and Excited State, the Reduction of the Acceptor, and the Energetically Feasible Charge-Transfer Processes: Excited-State Electron Transfer Quenching of Pe by DCE (ET) and Hole Transfer (HT) from DCA• Generated upon CT Excitation of the (DCA−− DCE) DAC to Pe in the Ground State

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CONCLUSIONS A series of RTILs composed of the EMI cation and five frequently used anions were spectroscopically screened for the formation of DACs with five electron acceptors of differing reduction potential. Such complexes, characterized by a broad absorption band in the UV/vis region, were observed with the RTILs with the lowest oxidation potentials, EMISCN and EMIDCA. On the basis of transient absorption measurements, the CT absorption band could be assigned to DACs between the RTIL anions and the corresponding acceptors. These measurements reveal that CT excitation of these DACs leads to the formation of a radical pair that can dissociate into free radicals, which in turn can undergo further reactions. Such processes were observed using electron acceptors with a reduction potential well within the electrochemical window of the RTILs, a criterion that is often used to assess the inertness of a solvent. This study reveals that the formation of DACs narrows down the applicable electrochemical window in the presence of light. The upper limit of this window is no longer determined by the reduction potential of EMI+ but depends on the reduction potential of the acceptor that affects the CT transition energy of the complex (Scheme S1). Our study also demonstrates that the presence of such DACs can strongly complicate photochemical investigations in RTILs, even though neither reductive nor oxidative quenching of the excited chromophore is taking place. Indeed, CT excitation of the DAC leads to the oxidation of the RTIL anion giving a neutral radical, which can in turn oxidize the chromophore in the ground state. In terms of frontier MOs, the hole left in the HOMO of the DAC upon photoexcitation is transferred to the HOMO of the chromophore (Scheme S1). This parasitic process takes place as soon as the oxidation potential of the RTIL anion is higher than that of the chromophore. This is generally the case in a RTIL selected to avoid reductive quenching of the chromophore. We showed that occurrence of this unwanted process can lead to misinterpretations in electron-transfer studies as well as to a photodegradation of the solvent. In general, the presence of such complexes should be checked when using RTILs because they can be more than a solvent.

Scheme 3. Schematic Representation of the Bimolecular Photoinduced Electron Transfer between Pe and DCE (A). Parallel Excitation of a RTIL−DCE DAC Leads to an Additional Path for the Formation of Pe•+ from Pe in the Ground State (B)

This decay is not complete, and residual GSB and Pe•+ absorption attributed to free ions can be observed. In the absence of RTIL, this residual signal decreases on the microsecond time scale due to nongeminate diffusional recombination of Pe•+ and DCE•−. However, in the presence of EMIDCA as cosolvent, both GSB and Pe•+ bands increase again on the hundreds of ns time scale, although the excited Pe population has already entirely decayed. This indicates that this late formation of Pe•+ occurs from Pe in the electronic ground state. As depicted in Figure 6B, this increase accelerates substantially with increasing RTIL concentration. Moreover, if EMIDCA is replaced by a RTIL which does not show a CT band, the slow formation of Pe•+ is not observed as illustrated in Figure S4. Furthermore, this process could not be detected in the absence of DCE in the EMIDCA/ACN mixture, ruling out a direct oxidative quenching of the chromophore by EMIDCA. The time-resolved data indicate that this late Pe•+ formation is due to a hole transfer from DCA•, (DCA)2•− or an unknown degradation product, DCA•′, resulting from the CT excitation of the (DCA−−DCE) DAC, to Pe in the ground state, as depicted in Scheme 2 (see also Scheme 3B). In principle, such a process is not limited to Pe but could occur with most aromatic chromophores used in bimolecular photoinduced electrontransfer studies. As shown in Scheme 2, this unwanted followup reaction is operative with any chromophore with a smaller oxidation potential than that of the RTIL anion or, in terms of frontier molecular orbitals (MO), when the HOMO of the chromophore is above that of the RTIL anion (Scheme S1). Such a parasitic process is problematic for several reasons: (1) The molar absorption coefficients of the oxidized RTIL anions are usually very low compared to those of the ions of commonly used chromophores and may therefore be invisible in TA measurements. (2) Since pure RTILs have a narrower

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b00468. G


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Literature spectra of the radicals: additional stationary and transient absorption spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +123 (0)123 4445556. Fax: +123 (0)123 4445557. ORCID

Eric Vauthey: 0000-0002-9580-9683 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Fonds National Suisse de la Recherche Scientifique (Project No. 200020-165890) as well as the University of Geneva for financial support.

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REFERENCES

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