Influence of C2-Methylation of Imidazolium Based Ionic Liquids on Photoinduced Spin Dynamics of the Dissolved ZnTPP Studied by Time-Resolved EPR

Article abstract of DOI:10.1515/zpch-2016-0820

Unusual physical properties of ionic liquids (ILs) can be implemented in many different applications and are very sensitive to the structure of IL. In this work we investigate the spin dynamics of probe molecule Zn tetraphenylporphyrin (ZnTPP) dissolved in a series of ILs using time-resolved electron paramagnetic resonance (TR EPR). We compare the TR EPR characteristics in C2-methylated imidazolium-based ILs [bmmim]BF₄ and [bmmim]PF₆ and in their C2-protonated analogs [bmim]BF₄ and [bmim]PF₆ to assess the influence of C2-methylation. The corresponding TR EPR signatures are drastically different in the two types of ILs. The analysis of experimental data allows assumptions that the ZnTPP molecule is distorted in C2-methylated ILs, contrary to other organic media and C2-protonated analogs. The mobility of ZnTPP in C2-methylated ILs is smaller compared to that in C2-protonated analogs, implying different microenvironment formed around dissolved ZnTPP.

Full text of DOI:10.1515/zpch-2016-0820

Z. Phys. Chem. 2016; aop
Mikhail Yu. Ivanov, Sergey A. Prikhod’ko, Nicolay Yu. Adonin,
EInlefnlauGe.nBcageryoanfsCka2y-aManedtMhaytvlaeytiVo. Fnedoinf*Imidazolium
Based Ionic Liquids on Photoinduced Spin
Dynamics of the Dissolved ZnTPP Studied by
Time-Resolved EPR
DOI 10.1515/zpch-2016-0820
Received June 14, 2016; accepted October 11, 2016
Abstract: Unusual physical properties of ionic liquids (ILs) can be implemented
in many different applications and are very sensitive to the structure of IL. In this
work we investigate the spin dynamics of probe molecule Zn tetraphenylporphy-
rin (ZnTPP) dissolved in a series of ILs using time-resolved electron paramagnetic
resonance (TR EPR). We compare the TR EPR characteristics in C2-methylated
imidazolium-based ILs [bmmim]BF4 and [bmmim]PF6 and in their C2-protonated
analogs [bmim]BF4 and [bmim]PF6 to assess the influence of C2-methylation. The
corresponding TR EPR signatures are drastically different in the two types of ILs.
The analysis of experimental data allows assumptions that the ZnTPP molecule
is distorted in C2-methylated ILs, contrary to other organic media and C2-proto-
nated analogs. The mobility of ZnTPP in C2-methylated ILs is smaller compared
to that in C2-protonated analogs, implying different microenvironment formed
around dissolved ZnTPP.
Keywords: EPR; ionic liquids; photoexcited triplets; polarization; TR EPR.
Dedicated to: Kev Salikhov on the occasion of his 80th birthday.
*Corresponding author: Matvey V. Fedin, International Tomography Center SB RAS,
Institutskaya str. 3a, 630090, Novosibirsk, Russia, Tel.: +7 383 3301276,
e-mail: mfedin@tomo.nsc.ru; and Novosibirsk State University, 630090, Novosibirsk, Russia
Mikhail Yu. Ivanov: International Tomography Center SB RAS, 630090, Novosibirsk, Russia; and
Novosibirsk State University, 630090, Novosibirsk, Russia
Sergey A. Prikhod’ko and Nicolay Yu. Adonin: Boreskov Institute of Catalysis SB RAS, 630090,
Novosibirsk, Russia
Elena G. Bagryanskaya: Novosibirsk State University, 630090, Novosibirsk, Russia; and
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, 630090, Novosibirsk,
Russia
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1 Introduction
Ionic liquids (ILs) consist of separated cations and anions, whose recombination
into neutral molecules is sterically hindered. Such unusual structure of ILs leads
to advanced properties making them perspective as energetic materials and green
chemistry solvents [1–5]. In addition, ILs are interesting and promising media for
photolysis, redox and polymerization reactions [6, 7]. The properties of ILs are
mainly determined by interactions between cations and anions, and thus can be
tuned by adjusting the chemical structure of each component.
Of particular recent interest, it was found that 1-alkyl-3-methylimidazolium
based ILs have drastically different physicochemical properties compared to their
dimethyl-substituted analogs, in which the C2 proton is replaced by a methyl
group. Supposedly, such methyl group in C2 position of the imidazolium ring
shifts the balance between Coulombic and hydrogen bonding interactions, and
in this way strongly affects the properties of the IL. As is known from literature,
the methylation at C2 position leads to a higher melting point and higher vis-
cosity in comparison with C2-protonated ILs [8]. Such behavior was intensively
researched, and some explanations were suggested [9–13].
Zhang and colls. applied molecular dynamics simulations to study the
structure of 1-n-butyl-3-methylimidazolium and 1-ethyl-3-imidazolium ILs with
hexafluorophosphate as anion [14]. It was found that C2-methylated ILs reveal
more stable ion pair and ion cage structures in comparison with C2-protonated
analogs. Qualitatively the same phenomena were studied in Ref. [15] in [bmim]
N3 and [bmmim]N3 ILs, where C2-methylation led to a slower ion motion and sig-
nificant structural changes, particularly to a shift of the preferred anion position
relative to imidazolium ring. Despite of the great interest to the structural pecu-
liarities of C2-methylated ILs, there are only a few studies where the direct influ-
ence of IL media on the dissolved molecule was investigated with respect to the
specific role of the C2-group.
The rotational dynamics of neutral (2,5-dimethyl-1,4-dioxo-3,6-di-
phenylpyrrolo[3,4c]pyrrole) and charged (rhodamine 110) solutes was compared
in C2-methylated and C2-protonated ILs [16]. It was demonstrated that nondipolar
solute is not sensitive to the structure of cation, and the rotational diffusion obeys
classical Stokes-Einstein-Debye theory, contrary to the charged solute where a dra-
matic influence of the imidazolium ring structure takes place and faster rotation in
C2-methylated ILs was observed. The authors explained such result by increased
Coulombic interaction between cation and anion of IL, which effectively elimi-
nates the hydrogen-bonding interaction of the cation with the charged solute.
Thus, the investigation of the influence of C2-methylation in imidazolium
based ILs and understanding of the underlying mechanisms are of considerable
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TR EPR of photoexcited triplets in ionic liquidsꢂ ꢂ3
interest. A valuable approach to study the influence of IL structure on the dis-
solved molecule is provided by electron paramagnetic resonance (EPR) spectros-
copy. In this technique the paramagnetic molecule is used as a probe reflecting
its interaction with surrounding media via the EPR spectrum. A number of papers
were focused on the research of ILs and their influence on spin probes recently,
including high-field EPR spectroscopy on ILs and IL-water mixtures [8, 17–26].
In those works, as a rule, various nitroxide spin probes were used as molecular
reporters dissolved in ILs, whose mobility was monitored by continuous wave
(CW) EPR and analyzed theoretically. However, a few works were also concerned
with application of time-resolved (TR) EPR to address the unusual properties of
ILs [27, 28]. In such approach, it is most feasible to use photoexcited triplet states
as paramagnetic probes, because the strong spin-polarized signals and revers-
ibility of photoinduced reactions strongly simplify the experiments. There is
also a long history of studying the photoexcited triplet states by EPR techniques,
including naturally occurring and artificial systems relevant for photosynthesis,
studies of molecular order and dynamics of triplets in liquid crystals, implication
of triplet porphyrins as spin probes for distance measurements, etc. [29–40]. In
particular, recently we demonstrated that the sensitivity of TR EPR operating on
triplet Zn tetraphenylporphyrin (ZnTPP) to the effects of microstructuring in ILs
is higher than that for CW EPR of nitroxide spin probes [28].
In this work we apply TR EPR with ZnTPP probe to investigate the C2-meth-
ylated imidazolium-based ILs and compare the observed trends with those
found previously for their C2-protonated analogs [28]. We demonstrate high sen-
sitivity of TR EPR to the interactions between spin probe and IL. The detailed
analysis demonstrates possible distortion of ZnTPP molecule in C2-methylated
IL. In addition, longer relaxation times of spin polarization are observed in
C2-methylated ILs.
2 Experimental section
Ionic liquids 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([bmmim]BF4),
and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ([bmmim]PF6) were
synthesized in the Boreskov Institute of Catalysis SB RAS. The procedure for
obtaining selected C2-methylated ILs was the following. A 100 mL round-bot-
tomed flask fitted with magnetic stirrer bar and reflux condenser was charged
by 5.0 g (26.5 mmol) of 1,2-dimethyl-3-butylimidazolium chloride (obtained by
analogy with the literature method [41] from 1,2-dimethyl imidazole and butyl
chloride), 53 mmol of corresponding salt (NaBF4 or NaPF6) and 100 mL of acetone.
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The mixture was stirred for 3 h at 50ꢀ°C. The solid was filtered and washed by
10 mL of acetone. The second portion of corresponding salt (20 mmol) was added
to the filtrate and the resulting mixture was stirred for 3 h at 50ꢀ°C. The solid was
filtered and washed by 10 mL of acetone. The filtrate was evaporated in vacuo.
The residue was solved in 30 mL of dry dichloromethane (to remove NaCl and
excess of starting salt) and filtered. The filtrate was evaporated and the product
was dried in high vacuo (10−ꢀ3 bar) at 80ꢀ°C for 6 h. Yields of products were 75ꢀ%–
85ꢀ%. The reaction of an ionic liquid sample with AgNO3 was negative, demon-
strating the absence of Cl− ions in the IL media.
Zn tetraphenylporphyrin (ZnTPP) was purchased from Sigma-Aldrich and
used as received. The excess of ZnTPP powder was dissolved in the correspond-
ing IL and then the solution was kept in an ultrasonic bath at 70ꢀ°C in the closed
Eppendorf tube for 24 h. In this way the maximum possible amount of ZnTPP was
dissolved and the rest was precipitated by centrifuging. The fraction with dis-
solved ZnTPP was then isolated and transferred into the EPR quartz tube. Then
the solution in the quartz tube was evacuated (10−ꢀ2 Torr pressure) with simulta-
neous heating at 85ꢀ°C during 3 h to reduce the amount of remaining water in IL
and to eliminate the remaining oxygen. Finally, the tube was sealed off under
vacuum.
EPR measurements were performed using a homemade TR EPR setup based
on an X-band Bruker EMX spectrometer. The temperature was controlled with
an N2-cooling system and a Lakeshore temperature controller. In all mentioned
experiments the sample was first melted in hot water (~ꢀ80ꢀ°C), shock-frozen in
liquid nitrogen and then directly transferred into the cryostat. The final tempera-
ture was equilibrated within following 20 min. A Ndꢀ:ꢀYaG laser LOTIS-TII with the
excitation wavelength of 532 nm was used. We used the unpolarized laser beam
in all experiments. The computer simulations of all spectra were performed using
EasySpin software [42]; the analysis of polarization kinetics was done using the
numerical solution of Bloch equations similar to [26].
3 Results and discussion
Time-resolved EPR is a more specific technique compared to common CW EPR. TR
EPR experiment employs laser excitation to create paramagnetic intermediates
in the sample and continuous microwave irradiation to detect the subsequent
evolution of these transient signals in time [43]. To achieve better time resolu-
tion, no phase-sensitive detection is used and zero harmonic signal is experimen-
tally observed (so-called direct detection). Depending on the spin polarization
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ꢂ5
TR EPR of photoexcited triplets in ionic liquidsꢂ
Scheme 1:ꢀChemical structures of studied ILs and ZnTPP.
mechanism, the absorptive or/and emissive signals can be detected. ZnTPP is
a well-known molecule that converts into a triplet state upon photoexcitation
(λꢀ=ꢀ532 nm) with the lifetime on the order of milliseconds depending on tempera-
ture and microenvironment [29, 30, 44–47].
In this work we aim at investigation of interactions between the photo-
excited ZnTPP molecule and ionic liquids, in particular on studying the effect
of C2-methylation in anions (Scheme 1). For this sake we perform TR EPR studies
in two C2-methylated ILs [bmmim]PF6 and [bmmim]BF4 and compare the results
with previously obtained ones for their C2-protonated analogs [bmim]PF6 and
[bmim]BF4 [28]. The glass transition/melting temperatures (Tg/Tm) of these four
ILs are known to be approximately 180/190 K for [bmim]BF4, 200/280 K for [bmim]
PF6, 205/310 K for [bmmim]BF4 and 215/313 K for [bmmim]PF6 [48–50]. Thus, in
case of C2-methylated ILs all TR EPR data in the 100–295 K temperature range
were recorded in the solid state of IL.
3.1 General comparison of TR EPR data in selected ILs
Figure 1 shows TR EPR spectra of ZnTPP in four selected ILs at 100 and 200 K. It
is evident that the presence of the methyl group in C2 position crucially affects
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Fig. 1:ꢀTR EPR spectra of ZnTPP in selected ILs at 100 and 200 K (as indicated). All spectra were
obtained by integration of TR EPR data within 1 μs after the laser flash. The spectra of C2-proto-
nated ILs were measured by us in Ref. [28].
the shape of the triplet spectrum. The spectra in C2-methylated ILs are narrower
and the canonical triplet peaks are sharper compared to C2-protonated ILs. The
zero-field splitting (ZFS) parameters of the presented spectra were determined
by simulations (more details in the next section) and are summarized in Table 1,
which also reports typical ZFS parameters of ZnTPP in various media available
from literature.
The data in Table 1 clearly show that the ZFS parameters of ZnTPP in C2-pro-
tonated ILs are close to those in other media found previously. Contrary, in case
of C2-methylated ILs one systematically observes noticeably smaller D-values
compared to other media, including C2-protonated analogs, and in general larger
E-values (especially in case of [bmmim]PF6). Since E relates to the rhombicity of
the ZFS tensor and reflects the changes of molecular structure, we suggest that
such trend arises from the distortion of originally closely planar ZnTPP, suppos-
edly due to the displacement of Zn atom out of the TPP plane and concomitant
bending of the molecule. For instance, a similar trend was found previously in
bis(o-semiquinonato)copper(II) complexes exhibiting ligand-driven out-of-plane
displacement of central copper atom [54].
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ꢂ7
TR EPR of photoexcited triplets in ionic liquidsꢂ
Table 1:ꢀThe parameters obtained from spectral simulations of TR EPR spectra of ZnTPP
obtained at τDAFꢁ=ꢁ0 ns delay after the laser flash at different temperatures. The available litera-
ture data are also given for comparison. The relative populations used in our simulations were
(Pzꢁ−ꢁPx)ꢁ:ꢁ(Pzꢁ−ꢁPy)ꢁ=ꢁ1. ZLI-1167 and E-7 exemplify trends in liquid crystals. The accuracy of D and E
values obtained here and in [26] isꢁ ꢁ10 MHz.
Solvent
ꢁ
T/Kꢁ D/MHzꢁ E/MHzꢁ
g-Valueꢁ Reference
[bmmim]PF6
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
100ꢃ
200ꢃ
100ꢃ
200ꢃ
100ꢃ
200ꢃ
100ꢃ
200ꢃ
685ꢃ
671ꢃ
812ꢃ
762ꢃ
895ꢃ
906ꢃ
910ꢃ
893ꢃ
920ꢃ
906ꢃ
894ꢃ
919ꢃ
918ꢃ
894ꢃ
924ꢃ
350ꢃ
311ꢃ
275ꢃ
254ꢃ
248ꢃ
290ꢃ
257ꢃ
188ꢃ
292ꢃ
1.999ꢃ This work
1.998ꢃ This work
1.997ꢃ This work
1.996ꢃ This work
1.998ꢃ [28]
[bmmim]BF4
[bmim]PF6
[bmim]BF4
1.998ꢃ [28]
1.996ꢃ [28]
1.996ꢃ [28]
Tolueneꢁ:ꢁchloroform (1ꢁ:ꢁ1)ꢃ 10–20ꢃ
2.0020ꢃ [37]
Toluene
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
40ꢃ
100ꢃ
110ꢃ
135ꢃ
100ꢃ
298ꢃ
284ꢃ [2.0020 2.0024 1.9968]ꢃ [51]
Tolueneꢁ:ꢁethanol (1ꢁ:ꢁ1)
Toluene
294ꢃ
291ꢃ
306ꢃ
294ꢃ
0ꢃ
–ꢃ [30]
–ꢃ [52]
–ꢃ [53]
–ꢃ [30]
–ꢃ [27]
ZLI-1167
E-7
NR4TFSI
In general, the TR EPR kinetics might reflect the spin relaxation of triplet sub-
level populations or the lifetime of paramagnetic species formed upon laser excita-
tion. Figure 2 shows the kinetic curves of ZnTPP for various ILs and temperatures.
Since the lifetime of the triplet state of ZnTPP is known to be much longer than the
kinetics shown in Figure 2 (25 ms [45] in rigid glass and possibly even higher in ILs
[5]), the observed TR EPR decays should be mainly determined by spin relaxation.
A significant lengthening of the TR EPR decay in C2-methylated ILs compared to
C2-protonated ones is observed (more details in the next section), which can be
ascribed to two factors. First, the suppressed hydrogen-bonding interactions in
C2-methylated ILs lead to an essential increase of Coulombic interactions between
anions and cations. Second, as was mentioned in the Introduction, the C2-methyl-
ated ILs have quite a pronounced tendency to form ordered microenvironments of
cations and anions, so that the dissolved spin probe becomes effectively caged and
shielded from efficient solid-state relaxation mechanisms.
3.2 Microenvironments of ZnTPP in selected ILs
In our previous study [28] we developed an approach to probe heterogeneities
in frozen C2-protonated imidazolium-based ILs using TR EPR of photoexcited
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Fig. 2:ꢀTR EPR kinetics of ZnTPP in selected ILs at 100 and 200 K. All curves are obtained by
integration of TR EPR data in the region of maximum absorption signal within 317–333 mT
range. The data on C2-protonated ILs are taken from Ref. [28]. Note that the similarity of the
kinetics in [bmim]PF6 and [bmmim]PF6 at 200 K is well reproducible, but purely accidental at
this particular temperature.
ZnTPP. The effects of microstructuring were identified by strongly time-depend-
ent shapes of TR EPR spectra and, as a counterpart, TR EPR kinetics noticeably
depending on spectral position. The observed dependences were explained by
co-existence of two ZnTPP fractions with different relaxation times and therefore
different corresponding microenvironments. In this work, we applied the same
strategy to C2-methylated imadazolium-based ILs.
Figure 3 shows the spectra of ZnTPP in selected ILs at 100, 200 and 240 K.
Each plot summarizes the spectra for certain IL and temperature at different
delays after the laser flash (τDAF). In all cases we assumed the polarization being
due to the triplet mechanism with the pattern described by relative popula-
tions (Pzꢀ−ꢀPx)ꢀ:ꢀ(Pzꢀ−ꢀPy)ꢀ=ꢀ1. The variations of Px, Py, Pz did not lead to any signifi-
cant improvement of the fit, and therefore these values were kept fixed. The ZFS
parameters D, E and the g-factor (assumed to be isotropic) were varied, resulting
in the best fits summarized in Table 1.
The spectra obtained in [bmmim]BF4 are weakly-dependent on τDAF at all tem-
peratures, whereas its C2-protonated analog [bmim]BF4 shows a clearly visible
dependence of the spectral shape on τDAF, especially at 200 K. Thus, contrary to
[bmim]BF4, the TR EPR spectra do not indicate an occurrence of different micro-
environments of ZnTPP in frozen [bmmim]BF4.
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TR EPR of photoexcited triplets in ionic liquidsꢂ ꢂ9
Fig. 3:ꢀNormalized TR EPR spectra of ZnTPP in C2-methylated and C2-protonated ILs measured
at different τDAF values (each color corresponds to a particular τDAF). Corresponding ILs and
temperatures are indicated at the plot. All spectra were obtained directly at the indicated τDAF
without integration over the time. All data on C2-protonated ILs were measured in [28] and
reprocessed here at other τDAF values for convenient comparison with C2-methylated analogs.
Simulations were performed for τDAFꢁ=ꢁ0 μs and are shown by dashed black lines.
At the same time, the TR EPR spectra of ZnTPP in [bmmim]PF6 are notably
dependent on τDAF at 200 and 240 K. But, in comparison with [bmim]PF6, the
observed tendency is just the opposite. In C2-protonated IL sharp peaks at
canonical orientations are observed at short τDAF, and the spectrum transforms
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into the more isotropic-like shape at longer τDAF. In contrast, in the C2-meth-
ylated analog the canonical peaks get sharper at longer τDAF. Such a behavior
in C2-protonated [bmim]PF6 has been explained by some fraction of ZnTPP
molecules exhibiting pronounced mobility (rotational motion) and suppos-
edly localized in micelle-like cavities formed by frozen IL. At the same time,
the opposite trend observed in C2-methylated [bmmim]PF6 can more likely be
explained by librational motions exhibited by ZnTPP. Such librations, as a rule,
lead to anisotropic relaxation resulting in sharpening of the canonical peaks at
long time delays.
Therefore, contrary to C2-protonated ILs, TR EPR spectra in their C2-methyl-
ated analogs do not indicate formation of cavities around ZnTPP, which would
likely enhance its rotational mobility. Whether different microenvironments of
ZnTPP coexist in C2-methylated ILs or not should be further investigated by ana-
lyzing the TR EPR kinetic curves.
To quantitatively analyze the TR EPR kinetics of ZnTPP in ILs we used the
numerical solution of Bloch equations to fit the experimental data, yielding the
TR EPR decays and relaxation times T1 and T2 at the output. Our previous study
[28], as well as earlier work [55], have demonstrated that the TR EPR kinetics of
TPPs is always biexponential in frozen solutions, even in common organic sol-
vents. Therefore, such kinetics should be simulated assuming two contributions
of species having different T1 and T2 (T1,1 and T2,1, T1,2 and T2,2) and weighted with
the coefficients (1–k) and k, where k refers to the slower relaxing (“long-lived”)
fraction (the second subscript 2). We assumed the transverse relaxation times
(T2,i) to be equal for simplicity, and the most informative parameter obtained from
this analysis is the relative contribution of each subsystem k. For common organic
solvents k is universally found to be ~ꢀ1/3 [28], whereas for frozen ILs it might
strongly deviate from this value, to be assigned to the effects of microstructur-
ing. The initial polarization patterns were assumed to be the same for the two
components in all cases, (Pzꢀ−ꢀPx)ꢀ:ꢀ(Pzꢀ−ꢀPy)ꢀ=ꢀ1, stemming from the triplet intersys-
tem crossing mechanism. In case of C2-methylated ILs the ZFS parameters were
assumed equal for both components, because the shape of the TR EPR spectrum
showed similar trend vs. time delay as that in common organic solvents (compare
with Figure 1 of Ref. [28]), likely to be attributed to librational motions of ZnTPP.
However, in case of C2-protonated ILs the two components have different ZFS,
therefore decomposition of the TR EPR spectra/kinetics is more complicated; it
was described in detail in our previous work [28] (here we only use these previ-
ously obtained values). Note that the coexistence of several triplet species with
different properties was found previously in other systems as well [56].
Figure 4 shows the temperature dependence of the longitudinal relaxation
times T1,1 and T1,2 and the relative amount of the slow relaxing fraction of ZnTPP
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TR EPR of photoexcited triplets in ionic liquidsꢂ
ꢂ11
Fig. 4:ꢀ(a, b) T1,1 and T1,2 times obtained by simulation of TR EPR decays, and (c) the slow-relax-
ing fraction k. The data on C2-protonated ILs is taken from Ref. [28].
(k) in all selected ILs. One observes that, in general, the magnitudes of all para-
meters and all trends are similar in the two types of ILs. At the same time, in most
cases T1,1 and T1,2 values are higher in C2-methylated ILs, resulting in longer TR
EPR decays observed, being most obvious at 100 K (see also Figure 2).
Special attention should be given to the temperature dependence of the
parameter k. This parameter in C2-protonated ILs could have been reliably meas-
ured only at Tꢀ<ꢀ200 K due to the melting/softening of such ILs at Tꢀ>ꢀ200 K [28].
In C2-methylated ILs, however, k can be followed up to nearly room temperature,
and demonstrates monotonic decrease (Figure 4c). Assuming that two micro-
environments of ZnTPP coexist at low temperatures in C2-methylated ILs similar
to C2-protonated analogs, this means that the “slow-relaxing” phase gradually
disappears with temperature, and close to the melting point all C2-methylated IL
represents homogeneous ordering.
Thus, we observe qualitatively different manifestations of two microenviron-
ments of ZnTPP in C2-protonated and C2-methylated imidazolium-based ILs. In
case of C2-protonated [bmim]BF4 and [bmim]PF6 we observed manifestations of
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two microenvironments simultaneously in spectra and kinetics of TR EPR. The
spectra changed their shape vs. time becoming more isotropic-like at long time
delays; at the same time, the kinetics were biexponential, with contributions from
ZnTPP in each environment. In case of C2-methylated [bmmim]BF4 and [bmmim]
PF6 we did not observe any characteristic changes in the TR EPR spectra assigna-
ble to the two microenvironments; however, the TR EPR kinetics did demonstrate
biexponential behavior characteristic of two microenvironments. In addition, the
results of Section 3.1 show that ZnTPP is likely to be distorted in C2-methylated ILs,
implying that interaction between ZnTPP and cations/anions of IL is strong (the
latter potentially can be verified by optical spectroscopy methods in the future).
All these trends can be explained by different timescales and amplitudes
of ZnTPP motion in these two types of ILs. In C2-protonated ILs, evidently,
noticeable rotation of ZnTPP on the microsecond scale is observed, resulting in
isotropic-like shapes developed at long time delays. In C2-methylated ILs, most
likely the motion of ZnTPP is characterized by small-angle librations. In case of
[bmmim]PF6 the amplitude of librations is higher, because the spectral shapes
do depend on τDAF. In case of [bmmim]BF4 such librations should have smaller
amplitude, so that the dependence of the spectrum on τDAF is negligible. Note that
the latter conclusion is additionally supported by the temperature dependence
of longitudinal relaxation times in both ILs (Figures 2 and 4). If at 100 K TR EPR
decays are comparable for both ILs, at 200 K the kinetics in [bmmim]PF6 decays
much faster compared to that in [bmmim]BF4 implying more efficient relaxation
mechamisms, plausibly due to stronger librations of ZnTPP in [bmmim]PF6.
4 Conclusions
In this work we applied TR EPR to investigate experimentally the influence of
C2-methylation in methyl-imidazolium ILs on the spin dynamics of the dissolved
photoexcited triplet ZnTPP. Physical properties of ILs are known to be dramati-
cally dependent on the chemical structure of cationic and anionic components.
In accordance with that, we have observed noticeably different TR EPR manifes-
tations in C2-protonated and C2-methylated analogs. We preliminary assign these
differences to different mobility of ZnTPP in the two types of ILs. On the one hand,
unlike in C2-protonated ILs, no definite conclusions on types of microenviron-
ments formed around ZnTPP in C2-methylated ILs can now be done. On the other,
the shapes of the TR EPR spectra observed in C2-methylated ILs indicate possi-
ble distortion of the ZnTPP molecule compared to that in other media, including
C2-protonated analogs of the same ILs. These observations need further investi-
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TR EPR of photoexcited triplets in ionic liquidsꢂ
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gation and systematization, especially via increasing the number of studied ILs
in order to derive more general trends, and will be the topic of our future studies.
Acknowledgement: This work was supported by the Russian Science Foundation
(No. 14-13-00826).
References
1. T. Welton, Chem. Rev. 99 (1999) 2071.
2. J. P. Hallett, T. Welton, Chem. Rev. 111 (2011) 3508.
3. J. Dupont, R. F. de Souza, P. A. Suarez, Chem. Rev. 102 (2002) 3667.
4. R. D. Rogers, G. A. Voth, Acc. Chem. Res. 40 (2007) 1077.
5. M. Alvaro, B. Ferrer, H. Garcia, M. Narayana, Chem. Phys. Lett. 362 (2002) 435.
6. V. I. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615.
7. A. Shkrob, T. W. Marin, R. A. Crowell, J. F. Wishart, J. Phys. Chem. A 117 (2013) 5742.
8. R. Stoesser, W. Herrmann, A. Zehl, A. Laschewsky, V. Strehmel, Z. Phys. Chem. 220 (2006)
1309.
9. T. Endo, M. Imanari, H. Seki, K. Nishikawa, J. Phys. Chem. A 115 (2011) 2999.
10. P. Bonhote, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg. Chem. 35
(1996) 1168.
11. J. McLean, M. J. Muldoon, C. M. Gordon, I. R. Dunkin, Chem. Commun. (2002) 1880.
12. O. O. Okoturo, T. J. VanderNoot, J. Electroanal. Chem. 568 (2004) 167.
13. E. I. Izgorodina, R. Maganti, V. Armel, P. M. Dean, J. M. Pringle, K. R. Seddon,
D. R. MacFarlane, J. Phys. Chem. B 115 (2011) 14688.
14. Y. Zhang, E. J. Maginn, Phys. Chem. Chem. Phys. 14 (2012) 12157.
15. J. B. Hooper, O. N. Starovoytov, O. Borodin, D. Bedrov, G. D. Smith, J. Chem. Phys. 136 (2012)
194506.
16. S. R. Prabhu, G. B. Dutt, J. Phys. Chem. B 118 (2014) 9420.
17. B. Y. Mladenova, N. A. Chumakova, V. I. Pergushov, A. I. Kokorin, G. Grampp, D. R. Kattnig,
J. Phys. Chem. B 116 (2012) 12295.
18. M. A. M. NoeI, R. D. Allendoerfer, R. A. Osteryoung, J. Phys. Chem. 96 (1992) 2391.
19. A. Kawai, T. Hidemori, K. Shibuya, Chem. Lett. 33 (2004) 1464.
20. R. G. Evans, A. J. Wain, C. Hardacre, R. G. Compton, ChemPhysChem 6 (2005) 1035.
21. V. Strehmel, A. Laschewsky, R. Stoesser, A. Zehl, W. Herrmann, J. Phys. Org. Chem. 19
(2006) 318.
22. V. I. Pergushov, N. A. Chumakova, M. Ya. Mel’nikov, G. Grampp, A. I. Kokorin, Dokl. Phys.
Chem. 425 (2009) 69.
23. V. Strehmel, H. Rexhausen, P. Strauch, Phys. Chem. Chem. Phys. 12 (2010) 1933.
24. V. Strehmel, ChemPhysChem 13 (2012) 1649.
25. Y. Akdogan, J. Heller, H. Zimmermann, D. Hinderberger, Phys. Chem. Chem. Phys. 12 (2010)
7874.
26. D. R. Kattnig, Y. Akdogan, C. Bauer, D. Hinderberger, Z. Phys. Chem. 226 (2012) 1363.
27. A. Kawai, T. Hidemori, K. Shibuya, Mol. Phys. 104 (2006) 1573.
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Download Date | 12/7/16 11:34 AM

ꢀꢀꢀꢁ
ꢀM.Yu. Ivanov et al.
14ꢀ
28. M. Yu. Ivanov, S. L. Veber, S. A. Prikhod’ko, N. Yu. Adonin, E. G. Bagryanskaya, M. V. Fedin,
J. Phys. Chem. B 119 (2015) 13440.
29. H. Levanon, Chem. Phys. Lett. 90 (1982) 465.
30. O. Gonen, H. Levanon, J. Phys. Chem. 89 (1985) 1637.
31. J. Fessmann, N. Rösch, E. Ohmes, G. Kothe, Chem. Phys. Lett. 152 (1988) 491.
32. V. Hamacher, J. Wrachtrup, B. von Maltzan, M. Plato, K. Möbius, Appl. Magn. Reson. 4
(1993) 297.
33. P. Jaegermann, M. Plato, B. von Maltzan, K. Möbius, Mol. Phys. 78 (1993) 1057.
34. K. Ishii, J. Fujisawa, Y. Ohba, S. Yamauchi, J. Am. Chem. Soc. 118 (1996) 13079.
35. D. Carbonera, M. Di Valentin, C. Corvaja, G. Agostini, G. Giacometti, P. A. Liddell, D. Kuciaus-
kas, A. L. Moore, T. A. Moore, Devens Gust, J. Am. Chem. Soc. 120 (1998) 4398.
36. M. Kiefer, S. M. Kast, M. R. Wasielewski, K. Laukenmann, G. Kothe, J. Am. Chem. Soc. 121
(1999) 188.
37. K. Ishii, S. Abiko, N. Kobayashi, Inorg. Chem. 39 (2000) 468.
38. Y. Kobori, S. Yamauchi, K. Akiyama, S. Tero-Kubota, H. Imahori, S. Fukuzumi, J. R. Norris,
PNAS 102 (2005) 10017.
39. M. Di Valentin, M. Albertini, E. Zurlo, M. Gobbo, D. Carbonera, J. Am. Chem. Soc. 136 (2014)
6582.
40. C. Hintze, D. Bücker, S. D. Köhler, G. Jeschke, M. Drescher, J. Phys. Chem. Lett. 7 (2016)
2204.
41. J. Dupont, C. S. Consorti, P. A. Z. Suarez, R. F. de Souza, Org. Synth. 79 (2002) 236.
42. S. Stoll, A. Schweiger, J. Magn. Reson. 178 (2006) 42.
43. M. D. E. Forbes, L. E. Jarocha, S. Y. Sim, V. F. Tarasov, Adv. Phys. Org. Chem. 47 (2013) 1.
44. M. Crouch, C. H. Langford, J. Photoch. Photobio. A 52 (1990) 55.
45. Y. Morishima, K. Saegusa, M. Kamachi, J. Phys. Chem. 99 (1995) 4512.
46. A. Sukhanov, K. B. Konov, K. M. Salikhov, V. K. Voronkova, E. A. Mikhalitsyna, V. S. Tyurin,
Appl. Magn. Reson. 46 (2015) 1199.
47. J. Schlüpmann, K. M. Salikhov, M. Plato, P. Jaegermann, F. Lendzian, K. Möbius, Appl. Magn.
Reson. 2 (1991) 117.
48. C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, J. F. Brennecke, J. Chem. Eng.
Data 49 (2004) 954.
49. S. Zhang, N. Sun, X. He, X. Lu, X. Zhang, J. Phys. Chem. Ref. Data 35 (2006) 1475.
50. P. Kölle, R. Dronskowski, Eur. J. Inorg. Chem. (2004) 2313.
51. S. Yamauchi, M. Tanabe, K. Takahashi, S. Islam, H. Matsuoka, Y. Ohba, Appl. Magn. Reson.
37 (2010) 317.
52. O. I. Gnezdiiov, A. E. Mambetov, A. A. Obynochny, K. M. Salikhov, Appl. Magn. Reson. 25
(2003) 157.
53. A. Regev, T. Galili, H. Levanon, J. Phys. Chem. 100 (1996) 18502.
54. S. L. Veber, M. V. Fedin, S. V. Fokin, R. Z. Sagdeev, V. I. Ovcharenko, E. G. Bagryanskaya,
Appl. Magn. Reson. 37 (2010) 693.
55. V. Rozenshtein, A. Berg, H. Levanon, U. Krueger, D. Stehlik, Yu. Kandrashkin, A. van der Est,
Isr. J. Chem. 43 (2003) 373.
56. P. A. Lane, L. S. Swanson, Q.-X. Ni, J. Shinar, Phys. Rev. Lett. 68 (1992) 887.
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