Solvent-dependent photoresponsive conductivity of azobenzene-appended ionic liquids

Article abstract of DOI:10.1039/c1cc11924a

A new class of azobenzene-based photoresponsive ionic liquids was prepared and showed reversible modulation of ionic conductivity in specific solvents under alternative UV/visible light irradiation.

Full text of DOI:10.1039/c1cc11924a

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Solvent-dependent photoresponsive conductivity of azobenzene-appended
ionic liquidsw
Shiguo Zhang, Shimin Liu, Qinghua Zhang and Youquan Deng*
Received 5th April 2011, Accepted 26th April 2011
DOI: 10.1039/c1cc11924a
A new class of azobenzene-based photoresponsive ionic liquids
was prepared and showed reversible modulation of ionic
conductivity in specific solvents under alternative UV/visible
light irradiation.
ionization mass spectrometry (see ESIw for details). Compounds
D1–D3 with the same AZO moiety exhibited almost the same
melting point (Tm) around 48 1C on differential scanning
calorimetry (DSC) recordings, while their glass transition tempera-
ture (Tg) decreased in the opposite order as the number
of methylene spacer groups between the AZO moiety and
imidazolium core (Table 1). However, compounds D3–D5
having different substituents on the AZO moiety but the same
methylene spacer showed comparable glass-transition around
ꢀ38 1C but different melting behavior. Those results indicated
that a longer methylene linkage offered more conformational
flexibility and higher packing frustrations, while substituents on
the AZO moiety highly influenced the lattice energy and thus
melting points. To make suppression of crystallization by reducing
aromatic interactions, D5 containing two methyl groups at the
ortho position of phenyl to increase the steric hindrance was
designed and synthesized. It has only a glass transition at low
temperature (ca. ꢀ38.1 1C) and thus a room temperature ionic
liquid was obtained despite its high viscosity (3671 mPaꢁs at
25 1C). All five ILs showed a strong tendency to form the
supercooled liquid state on heating above the Tg, different from
their precursor [Br]ꢀ salts (C1–C5 in Scheme S1 of ESIw) that
are liable to crystallize with their Tm range from 76 to 167 1C
(Table S1 of ESIw).
Ionic liquids (ILs) have attracted considerable attention in the
past decade because of their unique physicochemical properties.1
Incorporating ILs with photochromism/photoisomerism
functionality will give the so-called photoresponsive ILs or
photochromism/ILs systems. Recently, a few such ILs2 or
composite systems3 have been developed and exhibited attractive
features such as photoresponsive magnetic moment, photo-
responsive ionic conductivity, and structure-dependent photo-
chromism. However, further development is necessary for
in-depth investigation of their physicochemical properties and
enhancement of their performance. In addition, although much
effort has been devoted to photoresponsive ionic conductivity in
ordered matrices,4 almost no attention was directed towards that
in homogeneous solution in spite of its potential applications in
electrochemistry and other related processes in solution.
Herein, a new class of azobenzene (AZO)-based ILs (D1–D5,
see Fig. 1) and its photon-mode modulation of ionic conductivity
in specific solvents were reported. All ILs were synthesized by a
1
four-step method and confirmed by H NMR and electrospray
ILs D1–D4 have p–p* absorptions around 345 nm in ethanol,
no matter whether the para-position of the AZO moiety was
substituted with a methyl group (Table 1). However, D5 shows
strongly hypsochromic shift of the p–p* transition centered at
327 nm, due to the combined effect of electronic effects caused by
the two-methyl groups at the ortho-positions and the non-planar
Table 1 Thermal and photophysical properties of ILs (D1–D5) and
their photoresponsive conductivity in THF (25 1C)a
ILs
Tm/1C
Tg/1C
p–p*/nmb
cis, %c
Ds, %d
Ds, %e
D1
D2
D3
D4
D5
47.8
46.7
48.6
53.4
—
ꢀ23.8
ꢀ31.5
ꢀ37.8
ꢀ39.3
ꢀ38.1
343
347
347
343
327
96
98
97
96
75
6.78
6.31
5.25
5.43
3.91
10.70
8.70
7.89
8.05
5.67
Fig. 1 Photoisomerism of photoresponsive ILs.
Center for Green Chemistry and Catalysis, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000,
China. E-mail: ydeng@licp.cas.cn; Fax: +86-931-4968116;
Tel: +86-931-4968116
a
Conductivity modulation, Ds = (s0 ꢀ st)/s0, s0—initial conductivity,
st—conductivity after UV light irradiation (365 nm, 100 mW cmꢀ2). In
ethanol. Fraction of the cis isomer in THF in the PSS on UV light
b
c
w Electronic supplementary information (ESI) available: Synthesis,
characterization and conductivity measurement of ILs. See DOI:
10.1039/c1cc11924a
irradiation (calculated by the absorbance change of the band due to the
e
trans isomer according to ref. 5). d 1 ꢂ 10ꢀ4 mol Lꢀ1
.
1 ꢂ 10ꢀ3 mol Lꢀ1
.
c
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Table 2 Photoresponsive conductivity of D1 in various molecular
solvents (25 1C)
1 ꢂ 10ꢀ4 mol Lꢀ1b 1 ꢂ 10ꢀ3 mol Lꢀ1b
ET(30)a/
c
c
std
s0
std
83.0
Solvent
kcal molꢀ1
s0
Methanol
Ethanol
Acetonitrile
Acetone
55.4
51.9
45.6
42.2
17.23
5.42
16.53
17.82
3.41
17.20
5.38
16.45
17.82
3.42
83.2
39.8
128.3
134.0
26.0
39.4
128.0
134.0
26.0
Cyclohexanone 39.8
Chloroform
Diethyl ether
Dichloromethane 40.7
Tetrahydrofuran 37.4
39.1
34.5
0.02
0.10
2.81
1.800
0.35
0.02
0.09
2.68
1.679
0.32
0.08
0.08
0.36
11.98
8.04
0.36
11.28
7.18
Ethyl acetate
38.1
1.753
1.630
a
Polarity scale of solvents denoted by solvatochromic Reichardt’s dye
b
Fig. 2 UV-induced absorbance changes of D1 and D5 in ethanol and
THF (5 ꢂ 10ꢀ5 mol Lꢀ1, 3 mW cmꢀ2, time interval = 10 s).
30, data from ref. 7. Time of UV light (100 mW cmꢀ2) irradiation
for obtaining stable and minimal conductivity is ca. 75 and 200 s for
1 ꢂ 10ꢀ4 and 1 ꢂ 10ꢀ3 mol Lꢀ1, respectively. mS cmꢀ1, initial
c
structure caused by the steric hindrance. All ILs give distinct
photoisomerism in solution. For example, upon UV light irradia-
tion D5 in ethanol (Fig. 2) shows an obvious decrease in the strong
p–p* transition (327 nm) with a concomitant increase in the weak
n–p* transition (434 nm). Isosbestic points observed at 298 and
406 nm are indicative of a conversion between only two species,
i.e., the trans and cis forms. The absorption spectra and photo-
stationary state (PSS) for the same ILs in different solvents
are almost identical (Fig. 2), suggesting that photoisomerism is
structure-dependent rather than solvent-dependent. There is no
doubt that the spectral feature (4250 nm) of ILs surely originated
from the AZO moiety since neither the [BMIm]+ moiety (1-butyl-
3-methylimidazolium) nor [NTf2]– (bis(trifluoromethanesulfonyl)-
amide) has absorption in this spectral region and all ILs exhibit
spectra similar to their nonionic precursor B (Scheme S1 and
Fig. S1 of ESIw), thus no strong electronic communications
between AZO and imidazolium moieties happened because of
the intermediate methylene spacer. According to the relative
conductivity. mS cmꢀ1, conductivity after UV light irradiation.
d
and an ionic moiety is indispensable for photoresponsive
conductivity.
Conductivity of ILs in all three solvents without exception
shows obvious decrease upon UV light irradiation and subsequent
recovery to the initial state after visible light irradiation. Photo-
responsive conductivity of ILs D1–D5 in THF (10ꢀ4 mol Lꢀ1) is
given in Table 1. Most of them exhibit high modulation (5%–7%),
except D5 that has the smallest change of 3.91%, probably
because of its lowest cis content (ca. 75%, Table 1) in the PSS.
The conductivity modulation was interestingly found to be
depressed from D1 to D3, indicating that the prolonged length
of the spacer between AZO and imidazolium moieties weakens the
effect of photoresponsive conductivity. D1 in THF with large
modulation is investigated as a typical example. As shown in
Fig. 3, its conductivity decreases rapidly in 25 s under UV light
irradiation and recovers by visible light irradiation within
6 minutes. The modulation can be repeated over several cycles
(inset in Fig. 3). Both the time evolutions of absorbance and
conductivity can be fitted by mono- or bi- exponential function.
With identical light intensity and under either UV or visible
irradiation, time for obtaining stable conductivity is obviously
longer than that for PSS of absorbance. For example, time for
conductivity is ca. 75 s and 350 s for UV and visible light
irradiation, respectively, while that for absorbance is ca. 25 s
and 270 s, respectively. Moreover, increasing UV light intensity
l
energy of the lowest lying 1(n–p*) and (p–p*) states, the ILs’
photoresponse can be attributed to an azobenzene type with a
1
low-lying (n–p*) state,6 thus possessing a long-lived cis-isomer
relative to the methyl orange-based ILs,2b which can be attributed
1
l
to an aminoazobenzene type where (n–p*) and (p–p*) are at
comparable energies and its thermal cis to trans isomerization
occurs too fast at ambient temperature.
Under UV/visible light irradiation, no detectable change
of conductivity of D1 was observed in acetone, chloroform,
diethyl ether and cyclohexanone, and only detectable response
(o1%) in methanol, ethanol and acetonitrile. However, a
relatively large and reversible change does exist in dichloro-
methane, ethyl acetate and THF (Table 2). Before we could
attribute this result to photo-induced change, some control
experiments need to be conducted. First, no obvious temperature
fluctuation with irradiation (ꢃ0.1 1C) was observed in the
experiments since both of the UV and visible light source are
cold light-emitting diodes (LEDs), and the conductivity was
unchanged without irradiation. Second, control experiments
also showed that UV/vis light irradiation is inert to the cases
of neat THF, THF solution of [BMIm][NTf2], THF solution
of nonionic AZO compound B, and even the mixture of B
and [BMIm][NTf2]. This corroborates that the change of
conductivity is really photo- but not temperature-induced
and a covalent bond between a photo sensor (AZO moiety)
Fig. 3 Time evolutions of the absorbance (A) and conductivity (B) of
D1 in THF (1 ꢂ 10ꢀ4 mol Lꢀ1) upon UV (100 mW cmꢀ2) and visible
light (30 mW cmꢀ2) irradiation (25 1C, data of absorbance at 346 nm
are from Fig. S2 of ESI.w The line is the best-fit curve with mono- or
bi- exponential function). Inset is the reversible modulation of the
conductivity over several cycles (switching time: 75 s for UV and 350 s
for visible light irradiation).
c
6642 Chem. Commun., 2011, 47, 6641–6643
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was found to shorten significantly the response time for obtaining
stable conductivity, while had no obvious effect on modulation
(Table S2 of ESIw).
2.57–3.75 ꢂ 10ꢀ3 and 5.12–7.44 ꢂ 10ꢀ4 mol Lꢀ1 (Table S3
of ESIw), respectively, indicating that aggregation occurred in
both solvents and is less feasible in EtOH. Moreover,
the relatively low conductivity of ILs in less polar solvents
(chloroform and diethyl ether, in Table 1) suggested the popula-
tion of the intimate ion-pairs. The above results confirmed that
solvent with moderately low polarity is sufficient to facilitate wide
modulation. However, the zero modulation in acetone and
cyclohexanone with high conductivity is not clear at this current
stage, which may be related to the unchanged aggregation state
of ion clusters under photoirradiation caused by the carbonyl
group-induced specific solvent–solute interactions.
The photoresponsive conductivity observed here cannot be
explained simply by the photoinduced structural change of the
AZO moiety since photoisomerism occurred in all solvents
with comparable magnitude (Fig. 2). Recent experiments
evidenced that a substantial fraction of ions in ILs is associated
in the form of ion pairs or ion aggregates,1f,8 wherein the size
and the relative proportion of the formed aggregates or
clusters decrease with increasing solvent polarity and IL
concentration.9 In specific solvents such as THF, ILs D1–D5
could exist both as free ions and ion clusters of variable
aggregation. Considering that ionic conductivity (s) correlates
significantly with the number of carriers (n), the charge of
carriers (q), and the mobility of carriers (m) (eqn (1)),4c s of the
ILs solution could be related to the ion aggregates and can be
In summary, a new class of azobenzene-based photoresponsive
ILs, including a room temperature IL, D5, was obtained
through rational design. All ILs show distinct and reversible
solvent-dependent photoresponsive conductivity, which could
be related to photoisomerism-induced reversible change of
aggregation behavior of ionic clusters. The result demonstrates
the possibility of in-depth investigation of the aggregation
behavior of ILs.
written as follows:
X
þ
þ
þ
þÞ
s ¼
niqimi ¼ nðC ÞmðC Þ þ nðC A ÞmðC A
2
2
i
ð1Þ
This work was financially supported by the National Natural
Science Foundation of China (No. 20533080, 21002107).
þ
þ
2
þ nðC A ÞmðC A Þ þ . . .
3
3
2
UV light irradiation of ILs in THF first results in conformational
change of the AZO moiety (trans to cis), subsequently the photo-
isomerism changes the size and the relative proportion of ion
clusters, where free ions or small clusters decrease but large
clusters possessing less effective charge and limited mobility
increase, leading to a decrease in conductivity. Thus the
solvent-dependent photoresponsive conductivity is likely attri-
buted to the reversible change in the aggregation state of ion
clusters induced by photoisomerism, as shown in Fig. 4A.
Under UV/vis light irradiation, the prolonged response time in
conductivity as compared to that in absorbance (Fig. 3) could
be related to the additional process, i.e., photoisomerism-
induced change in ion clusters. Concentrated solution in
THF facilitated wide modulation (Table 1) further supporting
this hypothesis, since aggregation of the IL components is
more prevalent at the higher concentrations.9 In contrast, the
small or zero modulation in either high or low polar solvents
may be originated from its less aggregation or intimate ion-pairs.9
For example, like in the case of ILs in water,10 all ILs exhibit
typical concentration-dependent conductivity of two linear
fragments in ethanol and THF (Fig. 4B and Fig. S3 of ESIw)
with the critical aggregate concentration (CAC) values of
Notes and references
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Fig. 4 (A) Cartoon illustrating the plausible mechanism for photo-
responsive conductivity of ILs in specific solvents (C-cation, A-anion).
(B) Concentration-dependent conductivity of D1 in THF and ethanol
(25 1C).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6641–6643 6643