Spectroscopic observation of charge transfer complex formation of persistent perfluoroalkyl radical with aromatics, olefin, and ether

Article abstract of DOI:10.1016/j.jfluchem.2014.07.034

Charge transfer interaction of a persistent perfluoroalkyl radical, perfluoro-3-ethyl-2,4-dimethyl-3-pentyl (PFR-1), with benzene and methyl substituted benzenes (toluene, m-xylene, mesitylene), 1-decene, and diethyl ether was investigated by UV-vis spect

Full text of DOI:10.1016/j.jfluchem.2014.07.034

Journal of Fluorine Chemistry 167 (2014) 198–202
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Spectroscopic observation of charge transfer complex formation of
persistent perfluoroalkyl radical with aromatics, olefin, and ether
*
Taizo Ono , Kazutoku Ohta
National Institute of Advanced Industrial Science and Technology (AIST), Research Institute of Instrumentation Frontier, 2266-98, Anagahora, Moriyama,
Nagoya, Aichi 463-8560, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Charge transfer interaction of a persistent perfluoroalkyl radical, perfluoro-3-ethyl-2,4-dimethyl-
3-pentyl (PFR-1), with benzene and methyl substituted benzenes (toluene, m-xylene, mesitylene),
1-decene, and diethyl ether was investigated by UV–vis spectrophotometric measurement. It was found
Received 9 May 2014
Received in revised form 30 July 2014
Accepted 31 July 2014
that the aromatic and olefinic
p-electrons and also the unshared electron pair of ether can interact with
Available online 9 August 2014
the low-lying radical orbital of PFR-1 through CT complex formation. The Beer’s law was confirmed in the
range of 5–20 mM of PFR-1 for the aromatics and 5–40 mM for 1-decene and diethyl ether. The red shift
of lmax occurred with increasing number of methyl substituents on the benzene ring.
ß 2014 Elsevier B.V. All rights reserved.
Keywords:
Persistent perfluoroalkyl radical
Charge transfer complex
Beer’s law
UV–vis spectroscopy
1. Introduction
intermolecular interactions with the electrons of some chemicals
such as
p-electrons of the benzene derivatives or 1-decene and
The reactivity of the radical species has been well-studied,
especially the fluorinated alkyl radicals by Dolbier et al. in depth,
and reviewed [1]. However, unreactivity of the radical species has
not been a main subject, and not much concerned, because the
most important is the reactivity from the view points of various
applications such as the radical polymerization or functionaliza-
tion based on the radical chain mechanism. Since the discovery of
the persistent perfluoroalkyl radicals of perfluoro-2-methyl-3-
isopropyl-3-pentyl and perfluoro-2,4-dimethyl-3-isopropyl-3-
pentyl (PFR-1 and PFR-2 abbreviations are respectively used
hereinafter; Scheme 1), so-called Scherer radicals, they have
attracted the scientific community with its uniqueness of
semantically wrong nature of no reactivity even against 100%
fluorine gas in an extreme case of PFR-2 [2]. The study on these
persistent perfluoroalkyl radicals has been hampered with difficult
availability due to the need of manipulation of fluorine gas or an
electrochemical fluorination cell for the preparation and has
accordingly appeared sporadically in the literatures [3–10]. The
most recent ones are on the use of PFR-1 as the convenient
trifluoromethyl radical source for initiating the polymerization of
fluoro monomers [11–13]. Here we unveil some new cryptic
nature of the persistent perfluoroalkyl radicals on the unexpected
also with the unshared electron pairs of ether through charge-
transfer (CT) complexation.
2. Results and discussion
For some applicational reason, we added the persistent perfluor-
oalkyl radical solution containing PFR-1 in ca. 63% concentration into
benzene, and then wenoticed some faintcoloration occurred to give a
very pale yellow benzene solution on the fluorous phase of the vessel
bottom. The GC analysis of this pale yellow colored benzene solution
gives only benzene and PFR-1 peaks, suggesting no chemical
reactions between PFR-1 and benzene. So we expect that the color
is due to some interaction of PFR-1 with benzene and not by some
unexpected reaction productsderivedfrom PFR-1and benzene. Then,
wewondered if the coloration and concentration relationship follows
the Beer’s law or not. The UV-visual absorption spectra in the range
between 280 and 780 nm were recorded withvarious concentrations
of PFR-1 (2.50, 5.26, 10.8, 21.4, and 43.3 mM) in benzene (Fig. 1). The
Beer’s law apparently operates from 2.50 up to 21.4 mM, however,
obviously deviates from the linear relationship over 21.4 mM. This is
due to the limited solubility of PFR-1 in benzene (Fig. 2). The
maximum absorption lmax was observed at 380 nm with a molar
extinction coefficient smax of 38.3.
We then investigated the influence of the methyl substituents
introduced into the benzene ring. We chose toluene (methyl-
benzene), m-xylene (1,3-dimethylbenzene), and mesitylene
*
Corresponding author. Tel.: +81 052 736 7491; fax: +81 052 736 7224.
E-mail address: t.ono@aist.go.jp (T. Ono).
http://dx.doi.org/10.1016/j.jfluchem.2014.07.034
0022-1139/ß 2014 Elsevier B.V. All rights reserved.

T. Ono, K. Ohta / Journal of Fluorine Chemistry 167 (2014) 198–202
199
F3C
Rf
CF
F3C
CF3
.
CF
CF
CF3
CF3
PFR-1: Rf = F
PFR-2: Rf = CF3
Scheme 1. The structures of the persistent perfluoroalkyl radicals.
(1,3,5-trimethylbenzene) as representatives. As shown in Fig. 3,
the coloration becomes red-shifted with more methyl substitu-
ents in the benzene ring. The UV–vis absorption spectra were
measured in the same manner to see if the Beer’s law works or not.
All benzene derivatives are subjected to the Beer’s law in the same
concentration range from ca. 2 mM to ca. 20 mM, and somehow
saturation occurred in all cases near ca. 40 mM concentration,
again due to the solubility limit of PFR-1 (Fig. 4). The red shift
occurred with successive methyl substitutions. Thus, the values of
Fig. 2. The plot of the absorbances under various concentrations of PFR-1 (2.50, 5.26,
10.8, 21.4, 43.3 mM) in benzene at the lmax (380 nm). The good linearity between
2.5 and 21.4 mM, follows the Beer’ law (correlation coefficient is 0.996). The molar
extinction coefficient was 38.3.
the
for toluene, m-xylene, and mesitylene, respectively. The corre-
sponding molar extinction coefficients values for those were
37.9, 26.8, and 23.8, respectively. Both phenomena may be in line
with the increasing -electron densities of the benzene rings
lmax changed from 380 nm of benzene to 406, 438 and 477 nm
s
p
caused by methyl substituents although further theoretical study
is needed. The blank UV–vis measurement was carried out for the
perfluorohexane solution of PFR-1 at the concentration of 40 mM
for the same window of 280–780 nm (Fig. 5). It was confirmed that
PFR-1 itself had no absorption at the range of 280–780 nm (only
the shoulder was seen outside the range 280–780 nm). Therefore,
the UV–vis absorption measured in the above were proved to be
all based on the CT mechanisms between PFR-1 and the benzene
derivatives.
interactions which was subjected to Beer’s law in the same
concentration range from ca. 2 mM to ca. 20 mM, and further to ca.
40 mM concentration in contrast with the benzene derivatives,
suggesting higher solubility of PFR-1 in 1-decene (Fig. 7). The
values of lmax and smax were 344 nm and 12.5 Abs molÀ1 cmÀ1
,
respectively.
We next concerned the possibility of CT interaction with the
unshared electron pair of the ether. A very pale yellow color
developed when PFR-1 was dissolved into diethyl ether. The Beer’s
law was satisfied in the whole range of concentrations 2.58, 5.67,
11.8, 22.1 and 43.5 mM like the 1-decene case due to the high
We then investigated on the
p electorons of the olefins which
are less dative than the above benzene derivatives for the CT
interaction with PFR-1. We chose 1-decene as a representative for
this purpose. The coloration was invisible in this case as is seen in
the picture (Fig. 6). However, the UV–vis absorption spectra of the
1-decene solution of PFR-1 measured under various concentra-
tions supported the existence of the same kind of intermolecular
solubility of PFR-1 in diethyl ether (Fig. 8). The values of lmax and
smax were 332 nm and 15.4, respectively.
It is not unusual to see such coloration by the combination of
electron donors and acceptors through the CT complex formation.
The well known in fluorine chemistry is the CT complex formation
between hexafluorobenzene and benzene or N,N-dialkyl anilines
[14,15]. However, there is no precedent, in the best of our
Fig. 3. Coloration of the methylated benzene solutions of PFR-1 by charge-transfer
interactions. Red shift occurred with increasing number of methyl substituents in
benzene ring.
Fig. 1. UV–vis spectra of the PFR-1 solution in benzene under various
concentrations.

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T. Ono, K. Ohta / Journal of Fluorine Chemistry 167 (2014) 198–202
Fig. 4. UV–vis absorption specta of PFR-1 solutions in various methylated benzene derivatives, (1) toluene, (2) m-xylene, and (3) mesitylene under various concentrations. The
absorbance vs concentration plot was made at each max (1a) 406 nm for toluene, (2a) 438 nm for m-xylene, (3a) 477 nm for mesitylene. The Beer’s law operates at 3–20 mM,
l
but not at ca. 45 mM due to the solubility limit of PFR-1 in each benzene derivative (correlation coefficients are 0.9977, 0.9993, and 0.9998, respectively. The corresponding
molar extinction coefficients are 37.9, 26.8, 23.8, respectively.
knowledge, for the CT complex formation between alkyl radical
and benzene, olefin and ether.
Undoubtedly, a main reason for the persistency of PFR-1 comes
from the steric hindrance caused by 5 trifluoromethyl groups
surrounding the radical center, by extruding ups and downs from
the nodal plane passing through three carbons next to the radical
center. As is well known, the size of a trifluoromethyl group is
between the sizes of iso-propyl and tert-butyl groups in the
hydrocarbon system [16]. These trifluoromethyl armor prevent
any reagents from attacking the radical center. For example, a
biradical oxygen molecule has no access to the radical center and
even the smallest reagent gas, a hydrogen molecule, is also unable
to access to the radical center due to the presence of this armor,
thus there is no reactivity to PFR-1 [17]. With this sterically rigid
armor in mind, the molecular orbital interactions between the
radical’s SOMO and benzene’s HOMO seem to be difficult. We
wonder if some CT interaction may occur through the rim of the
PFR-1 molecule, but at first we investigated that CT interaction
may occur between
p-electrons of the benzenes and the PFR-1
precursor olefin, hexafluoropropene trimers (perfluoro-4-methyl-
3-isopropyl-2-pentene and perfluoro-2,4-dimethyl-3-ethyl-2-
pentene), because these perfluoroolefins has a low-lying C55C
double bond orbital embedded inside the trifluoromethyl armor
with almost the same molecular frameworks. Thus, we examined
the UV–vis spectrum of the toluene solution of a hexafluoropro-
pene trimer mixture (ca. 40 mM). The shoulder absorbance near
280 nm due to the
p–p* transition of the C55C double bond
appeared, but there was no CT-based UV–vis absorption (Fig. 9).
Therefore, it is very unlikely that the CT interaction of PFR-1 with
the solvents studied above may occur through the direct HOMO-
SOMO interactions. At present moment, all the discussion given
here is qualitative and not rigid, so that the in-depth theoretical
treatment for what mechanism operates for the present CT-complex
system is necessary for the future study.
Fig. 5. A UV–vis spectrum of the PFR-1 solution (ca. 40 mM) in perfluorohexane
showed no absorption at the range of 280–780 nm.

T. Ono, K. Ohta / Journal of Fluorine Chemistry 167 (2014) 198–202
201
Fig. 6. The photo of the PFR-1 solution (43.4 mM) in 1-decene and the UV–vis spectra measured under various concentrations (3.1, 5.1, 11.1, 21.8, and 43.4 mM).
Fig. 7. The plot of the absorbances under various concentrations of PFR-1 (3.1, 5.1,
11.1, 21.8, and 43.4 mM) in 1-decende at the max (344 nm). The good linearity in
Fig. 8. The plot of the absorbances under various concentrations of PFR-1 (2.58, 5.67,
11.8, 22.1, and 43.5 mM) in diethyl ether at the max (332 nm). The good linearity in
l
l
the whole range shows the Beer’ law is satisfied (correlation coefficient is 0.9996).
The molar extinction coefficient was 12.5.
the whole range shows the Beer’ law is satisfied (regression coefficient is 0.9993).
The molar extinction coefficient was 15.4.
3. Conclusions
The charge transfer (CT) interactions between the persistent
perfluoroalkyl radical, perfluoro-2-methyl-3-isopropyl-3-pentyl
(PFR-1), and
p-electrons of aromatics were confirmed by the
coloration in the visible light region. The UV–vis absorption spectra
showed the Beer’s law operated in the range of ca. 3–20 mM for
benzene, toluene, m-xylene, and mesitylene, and the red shift of
l
max occurred with increasing numbers of the methyl substituents,
thus, 380, 406, 438 and 477 nm in this order. The spectroscopic
saturation of CT-band absorption seen in ca. 40 mM of PFR-1 in
these benzene derivatives is very likely to the solubility limit of the
PFR-1 in these aromatic solvents. The same kind of CT interactions
of PFR-1 with
p-electrons of 1-decene and also with the unshared
electron pair of diethyl ether was observed. Due to the higher
solubility of PFR-1 in 1-decene and diethyl ether, the Beer’s law
worked at higher concentration (up to ca. 40 mM of PFR-1).
4. Experimental
Hexafluoropropene trimers are obtained as a mixture of
perfluoro-4-methyl-3-isopropyl-2-pentene and perfluoro-2,4-di-
methyl-3-ethyl-2-pentene with almost 1–2 molar ratio from
Fig. 9. UV–vis spectrum of the perfluorohexane solution of hexafluoropropene
trimers containing perfluoro-4-methyl-3-isopropyl-2-pentene and perfluoro-2,4-
dimethyl-3-ethyl-2-pentene with 1:2 ratio in 40 mM concentration.

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Daikin Industries Co. Ltd. 1-Decene and diethyl ether were distilled
prior to use. PFR-1 was prepared by direct fluorination of the
hexafluoropropene trimers mixture at room temperature by
introducing undiluted fluorine gas into the bottom of the long
column of the mixture placed in a long glass reaction tube under
vigorous stirring. The rate of the gas introduction was regulated so
as to be totally absorbed and F2 bubbles does not reach the surface
of the reaction mixture. Under this reaction conditions, all fluorine
gas was utilized to the full for the formation of PFR-1 without any
loss. The fluorination was sustained until all the starting trimers
are consumed. The resultant solution was analyzed by GC to be 63%
of PFR-1 and 37% of perfuoro-2,4-dimethyl-3-ethylpentane (area %
by GC). The UV–vis measurement was carried out on the sample
solutions obtained by vigorously shaking the above 63% PFR-1
solution with each solvent at the concentration from ca. 3 to
40 mM based on the assumption that the added PFR-1 is all
dissolved into the solvent. However, in the benzene and benzene
derivatives cases, fluorous phases remained at the bottom of the
solutions to some extent in the ca. 40 mM concentrations and thus,
the real concentration is less than the expected values for these
cases. The UV–vis measurement was carried out over the range
from 280 to 780 nm by using Shimadzu UV–VIS–NIR Spectrometer
UV-3600 with a quartz tube having a 1-cm light path.
Acknowledgement
The fluorine gas was kindly gifted from Daikin Industries Co. Ltd.
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