Kinetics and energetics of thermal cis-trans isomerization of a resonance-activated azobenzene in BMIM-based ionic liquids for PF6-/Tf2N- Comparison

Article abstract of DOI:10.3390/molecules22081273

BMIM PF₆ (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf₂N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF₆^- and Tf₂N^-. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF₆ and in BMIM Tf₂N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF₆^- and Tf₂N^- anions during the cis-trans isomerization.

Full text of DOI:10.3390/molecules22081273

molecules
Article
Kinetics and Energetics of Thermal Cis-Trans
Isomerization of a Resonance-Activated
Azobenzene in BMIM-Based Ionic Liquids for
−
−
PF /Tf N Comparison
6
2
Guido Angelini , Cristina Campestre 1 , Luca Scotti 2 and Carla Gasbarri 1,*
1
ID
ID
1
Department of Pharmacy, University “G. d’Annunzio” of Chieti-Pescara, via dei Vestini, 66100 Chieti, Italy;
guido.angelini@unich.it (G.A.); cristina.campestre@unich.it (C.C.)
Department of Oral Science, Nano and Biotechnology, University “G. d’Annunzio” of Chieti-Pescara,
via dei Vestini, 66100 Chieti, Italy; l.scotti@unich.it
2
*
Correspondence: carla.gasbarri@unich.it; Tel.: +39-0871-3554786
Received: 13 July 2017; Accepted: 27 July 2017; Published: 29 July 2017
Abstract: BMIM PF6 (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf N
2
(1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature
ionic liquids widely employed and investigated as reaction media. Despite the presence of the same
imidazolium ring in their structure they are different in many chemical and physical properties due to
the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene
−
−
have been used as reaction model to compare the behavior of PF6 and Tf N . Rotation is the
2
mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously
in the dark both in BMIM PF and in BMIM Tf N. The kinetic rate constants of the process have been
6
2
determined at different temperatures and the activation energies of the reaction have been calculated
according to the Arrhenius and Eyring equations. The results presented herein highlight different
−
−
solute-solvent interactions involving the PF6 and Tf N anions during the cis-trans isomerization.
2
Keywords: imidazolium ionic liquids; 4-methoxyazobenzene; rotation; Arrhenius plot; Eyring plot
1
. Introduction
Despite the existence of a large variety of photoreactive compounds and light-switchable
devices, azobenzene and its derivatives still represent relevant molecules due to their spectroscopic
properties and isomerization mechanisms. External stimuli are requested to induce the trans-cis
photoisomerization [1–3], while the thermal cis-trans conversion occurs spontaneously in the dark
owing to the fact that the trans is about 40–50 kJ/mol more stable than the cis isomer [4,5].
Rotation and inversion are the competitive mechanisms proposed for the cis-trans isomerization:
the former takes place by the formation of a dipolar transition state in which the nitrogen-nitrogen
π
-bond is heterolytically broken, while the latter involves the rehybridation of one of the nitrogen
2
atoms from sp to sp and the formation of a linear transition state in which the double bond is
retained [6]. Solvent polarity and viscosity, as well as the steric and electronic effects of a substituent,
can promote one mechanism over the other [
7
]. Typical V-shaped Hammett plots have been obtained in
different media for the cis-trans isomerization of monosubstitued azobenzenes confirming that rotation
is favored by an electron-donating group, while inversion is associated to an electron-withdrawing
substituent [8–10]. Recently, V-shaped Hammett plots have been obtained in high polar ionic liquids,
as BMIM Tf N, while linear plots have been observed in high viscous ionic liquids, as BMIM PF ,
2
6
suggesting a change of mechanism, from rotation to inversion, in the presence of electron-withdrawing
Molecules 2017, 22, 1273; doi:10.3390/molecules22081273
www.mdpi.com/journal/molecules

Molecules 2017, 22, 1273
2 of 8
substituents [11]. Since the reaction of monosubstitued azobenzenes occurs by rotation in a polar
environment and by inversion in nonpolar media, the behavior of BMIM PF resembles that of a
6
polar solvent in comparison to BMIM Tf N in disagreement with the relative dielectric constant
2
values [12]. Generally, BMIM-based ionic liquids are characterized by ordered structural networks
in which the hydrogen atoms in position 2, 4 and 5 of the imidazolium ring and the hydrogens of
the N-alkyl groups are involved in hydrogen bonds. Moreover, one imidazolium cation is connected
by three anions and each anion is surrounded by three imidazolium cations [13]. It has been widely
demonstrated the fundamental role of the anion for the physicochemical properties of BMIM-based
ionic liquids [14
BMIM Tf N [20
–19]. In particular, many kinds of reactions have been carried out in BMIM PF and
6
–
22]. In addition to viscosity and polarity, they show different surface tension, density,
conductivity, molecular weight, thermal behavior, polarizability and dipolarity [23 29]. The aim
of this work has been the investigation of the kinetic rate constants and the energetic parameters
associated with the thermal isomerization of 4-CH O-azobenzene (MeO-AB) in BMIM PF and BMIM
2
–
3
6
−
−
Tf N to compare the effect of PF6 and Tf N and contribute to the characterization of the two
2
2
ionic liquids for their application as reaction media. The presence of the methoxy group in the
para position strongly increases the electron density in the * orbital of the MeO-AB molecule by
positive mesomeric effect leading to an highly dipolar transition state without inducing strong steric
effects which generally hinder rotation [30 31]. Although closer energetic barriers have been observed
π
,
for the cis-trans conversion of MeO-AB and unsubstitued azobenzene and despite of the inversion
represents the dominant mechanism in the case of azobenzene, the thermal isomerization of the
MeO-AB molecule proceeds only through the rotation pathway both in organic solvents and ionic
liquids [32,33] (Supplementary Materials, A–B).
2
. Results
2
.1. Thermal Cis-Trans Isomerization
After irradiation, the MeO-AB solution shows a shift in the maximum absorption band from 347 to
03 nm and the presence of a broad band at 440 nm in both the investigated ionic liquids confirming the
3
conversion of the trans into the cis isomer. In the dark, the thermal cis-trans isomerization occurs and is
carried out by the appearance of three isosbestic points at 258, 295 and 407 nm. The UV-vis spectra of
◦
the cis-trans conversion of MeO-AB in BMIM PF at 40 C are reported in Figure 1 as an example.
6
◦
Figure 1. UV-vis spectra for the thermal cis-trans isomerization of MeO-AB in BMIM PF at 40 C.
6
The first-order rate constants (kobs) measured in BMIM PF and BMIM Tf N at different
6
2
temperatures are reported in Table 1.

Molecules 2017, 22, 1273
3 of 8
Table 1. First-order rate constants for the thermal isomerization of MeO-AB in the investigated
ionic liquids.
BMIM PF
BMIM Tf N
2
10 kobs/s
6
T (K)
−
6
−1
−6
−1
1
0
kobs/s
2
88 ± 0.1
1.19 ± 0.1
6.83 ± 0.1
8.42 ± 0.1
25.5 ± 0.2
129.6 ± 0.1
1.16 ± 0.2
4.19 ± 0.7
5.61 ± 0.1
24.1 ± 0.1
61.2 ± 0.1
1
2
98 ± 0.1
3
03 ± 0.1
13 ± 0.1
23 ± 0.1
3
3
1
Reference [11].
2
.2. Arrhenius and Eyring parameters
The energetic parameters of the reaction have been estimated by applying the temperature
dependent Arrhenius and Eyring equations as described in the Experimental Section. The Arrhenius
plot shows a good correlation in both the ionic liquids. The linear fit obtained for the BMIM Tf N is
2
shown in Figure 2 as an example.
Figure 2. Arrhenius plot of MeO-AB in BMIM Tf N.
2
Similarly, linear plots have been determined by applying the Eyring equation in the investigated
ionic liquids. The Eyring plot obtained for the BMIM Tf N is reported in Figure 3 as an example.
2
Figure 3. Eyring plot of MeO-AB in BMIM Tf N.
2

Molecules 2017, 22, 1273
4 of 8
The energetic parameters of the cis-trans isomerization of MeO-AB calculated from the Arrhenius
and Eyring plots are shown in Table 2.
6
=
Table 2. Activation energy E
∆
a
, frequency factor A, activation enthalpy
6
S for the thermal isomerization of MeO-AB in BMIM PF and BMIM Tf N.
∆H and activation entropy
=
6
2
A (s 1)
−
6
=
6
=
Ionic Liquid
Ea (kJ/mol)
∆H (kJ/mol)
∆S (J/K mol)
1
1
1
1
BMIM PF6
98.4 ± 3.1
88.6 ± 2.8
(8.93 ± 2.5) × 10
(0.13 ± 0.1) × 10
95.9 ± 3.0
86.0 ± 2.6
70.3 ± 1.9
35.2 ± 1.1
BMIM Tf N
2
3
. Discussion
Generally, temperature has no effect on the spectroscopic behavior of azobenzene and derivatives,
but strongly enhances the isomerization rate [34]. The thermal isomerization of MeO-AB becomes
faster by increasing temperature in both the ionic liquids and higher kobs values are obtained at higher
temperature. In particular, the kobs increases of about 108 times in BMIM PF and 52.5 times in BMIM
6
Tf N passing from 288 to 323 K.
2
a
The obtained E values are similar to those observed for the cis-trans conversion of azobenzene
in conventional solvents, corresponding to 84–104 kJ/mol [35]. Moreover, these data are in
agreement with the activation energy and frequency factor reported for 3-butyl-1-methyl-2-
1
1
−1
in BMIM PF and
phenylazoimidazolium, corresponding to 85
±
4 kJ/mol and (1.8
−1
in BMIM Tf N [36]. Although in BMIM PF the kinetic rate
±
1.7)
×
10
s
6
11
85
±
3 kJ/mol and (1.3
±
1.0)
×
10
s
2
6
of the MeO-AB isomerization is faster in comparison to BMIM Tf N in the investigated temperature
2
6
∆H values are higher for BMIM PF in comparison to BMIM Tf N (Table 2).
6 2
=
range (Table 1), the E
a
and
6
a
Both E and ∆H correspond to the barrier height required to activate the reagent molecules from their
=
initial state to the transition state of a reaction, and for the thermal cis-trans isomerization the initial
state consists in the solvation of the cis isomer by the reaction medium as previously described by
Asano et al. [33]. In the presence of an electron-donor monosubstitued azobenzene the barrier height of
the reaction depends on the solute-solvent interactions and tends to decrease by increasing the solvent
polarity [34]. The MeO-AB molecule is solubilized in the trans form in the ionic liquids, then the
solutions are irradiated by UV-visible light to induce the conversion from trans to cis isomer. The strong
change in the molecular geometry of the MeO-AB reduces the distance between the 4-4’positions of the
azobenzene moiety and consequently increases the dipole moment from 1.86 to 4.54 D [
8
]. The lower
6
a
E and ∆H obtained for BMIM Tf N indicate that the more polar ionic liquids provide a more efficient
=
2
stabilization of the cis MeO-AB in the ground state in comparison to BMIM PF6 [37] without changing
the energy associated to the transition state [34] as suggested by the higher frequency factor A and the
6
=
6
=
activation entropy
to the Eyring equation is referred to the redistribution of the energy of the molecule in the transition
∆S (Table 2). On the other hand, the activation entropy ∆S calculated according
6
∆S values indicate that BMIM PF offers the best
6
=
state to allow the rotation pathway. The obtained
stabilization of the dipolar transition state of the reaction in comparison to BMIM Tf N. In particular,
2
6
=
the largest
agreement with the data obtained for the investigated ionic liquids. This behavior can be attributed to
∆S implies the largest frequency factor A and reflects the faster isomerization rates [32] in
−
−
the specific properties of the PF6 in comparison to Tf N , including smaller van der Waals radius,
2
higher mobility and lower dispersion charge, involved in the solvation of the dipolar transition state
in which the formal charges generated by the resonance effect of the methoxy group are stabilized by
strictly oriented anions and cations [33].

Molecules 2017, 22, 1273
5 of 8
4
. Materials and Methods
4
.1. Materials and Instruments
BMIM PF (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf N (1-butyl-3-
6
2
methylimidazolium bis(trifluoromethylsulfonyl)imide) were purchased from IoLiTec and stirred under
◦
vacuum at 60 C overnight prior to use to decrease the water content below the limit detectable
by spectroscopic techniques [28
described [ ]; ethanol (99% spectroscopy grade) was purchased from Fluka and used without further
purification. The irradiation to induce the trans-cis isomerization was performed by means of a
Hg Xe arc lamp (150 W) equipped with a band-pass interference filter centered at 365.0 +2/ 0 nm
wavelength and 10.0 +2/ 2 nm bandwidth. The UV-vis spectra for the thermal cis-trans isomerization
were recorded by using a Cary 1-E spectrophotometer.
,38]. 4-CH O-azobenzene (MeO-AB) was synthetised as previously
3
8
−
−
−
4
.2. Methods
A 2.6
temperature at least for 4 days before use. An appropriate amount was transferred into a 1 cm light
path quartz cuvette containing 200 L of BMIM PF or BMIM Tf N to obtain a final concentration of
−
3
×
10 M ethanolic solution of MeO-AB was prepared and kept in the dark at room
µ
6
2
−
5
2
.6
then was irradiated for 45 min. The decreasing of the high-intensity absorption band at 347 nm (due to
the π → π* transition) and the increasing of the low-intensity band at about 440 nm (due to the n → π
transition) were used as evidence for the trans-cis photoisomerization. The thermal cis-trans conversion
×
10 M. The sample was dried under Nitrogen flow to remove the organic solvent [39,40] and
*
follows a first order decay [41
measured in the temperature range 15
maximum wavelength of the trans isomer in the dark over a period of about 24 h (Supplementary
Materials C–D) as previously described [8].
–
44] and the kinetic rate constants (kobs) were spectrophotometrically
◦
−
50 (
±
0.1) C by monitoring the absorption change at the
a
The activation energy (E ) and frequency factor (A) have been calculated from the kobs values by
the Arrhenius equation (1).
ln kobs = ln A − Ea/RT,
where R is the universal gas constant and T is the absolute temperature.
(1)
6
=
6
=
The activation enthalpy (
Eyring equation (2).
∆H ) and the activation entropy (∆S ) have been calculated by the
6
=
6
=
ln (kobs/T) = −∆H /RT + ln (k /h) + ∆S /R,
(2)
B
where k and h are the Bolzmann and Planck constants, respectively.
B
5
. Conclusions
The thermal isomerization of the investigated MeO-AB represents a model reaction to understand
the behavior of BMIM-based ionic liquids. The cis-trans conversion occurs by rotation in BMIM PF and
6
BMIM Tf N according to the electronic effect and the position of the methoxy group in the azobenzene
2
molecule. The results presented herein indicate that best solvation of the cis isomer in the ground state
−
occurs in the presence of the Tf N anions as suggested by the activation energy and the activation
2
−
enthalpy, while the reaction is faster in the presence of the PF6 anions as pointed out by the kinetic
rate constants, the frequency factor and the activation entropy. The different behavior shown by the
investigated ionic liquids can be attributed to the stronger solvation capability of the cis isomer of
MeO-AB in the ground state by BMIM Tf N and the stronger stabilization of the dipolar transition
2
state by BMIM PF6.
Supplementary Materials: Supplementary materials are available online.
Acknowledgments: This work has been supported by University “G. d
'Annunzio” of Chieti-Pescara (MIUR 2016,
6
0% Grant).

Molecules 2017, 22, 1273
6 of 8
Author Contributions: G.A. synthesized 4-methoxyazobenzene, performed the experiments and discussed the
data; C.C. and L.S. contributed reagents/materials/analysis tools; C.G. designed the project, performed the
experiments, discussed the data and wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1
.
Henzl, J.; Mehlorn, M.; Gawronski, H.; Rieder, K.H.; Morgenstern, K. Reversible cis-trans isomerization of a
single azobenzene molecule. Angew. Chem. Int. Ed. 2006, 45, 603–606. [CrossRef] [PubMed]
Turansky, R.; Konopka, M.; Doltsinis, N.L.; Stich, I.; Marx, D. Switching of functionalized azobenzene
suspended between gold tips by mechanochemical, photochemical, and opto-mechanical means. Phys. Chem.
Chem. Phys. 2010, 12, 13922–13932. [CrossRef] [PubMed]
2
.
3.
4.
5.
Diau, E.W.G. A new trans to cis photoisomerization mechanism of azobenzene on the S (n,
Chem. A 2004, 108, 950–956. [CrossRef]
Rau, H.; Lueddecke, E. On the rotation-inversion controversy on photoisomerization of azobenzenes.
Experimental proof of inversion. J. Am. Chem. Soc. 1982, 104, 1616–1620. [CrossRef]
Dias, A.R.; Minas de Piedade, M.E.; Martinho Simoes, J.A.; Simoni, J.A.; Teixeira, C.; Diogo, H.P.;
Meng-Yan, Y.; Pilcher, G. Enthalpies of formation of cis-azobenzene and trans-azobenzene. J. Chem. Termodyn.
π
*) surface. J. Phys.
1
1
992, 24, 439–447. [CrossRef]
6.
7.
8.
Cimiraglia, R.; Asano, T.; Hofmann, H.J. Mechanism of thermal Z/E isomerization of aromatic azo
compounds. Relation between rotation and inversion states. Gazz. Chim. Ital. 1996, 126, 679–684.
Bandara, H.M.D.; Burdette, S.C. Photoisomerization in different classes of azobenzene. Chem. Soc. Rev. 2012
1, 1809–1825. [CrossRef] [PubMed]
,
4
De Maria, P.; Fontana, A.; Gasbarri, C.; Siani, G.; Zanirato, P. Kinetics of the Z-E isomerization of
monosubstituted azobenzenes in polar organic and aqueous micellar solvents. Arkivoc 2009, 8, 16–29.
[CrossRef]
9
1
1
.
Talaty, E.R.; Fargo, J.C. Thermal cis-trans-isomerization of substituted azobenzenes: a correction of the
literature. Chem. Comm. 1967, 2, 65–66. [CrossRef]
0. Herkstroeter, W. Mechanism of syn-anti isomerization of azomethine dyes. J. Am. Chem. Soc. 1973, 95,
686–8691. [CrossRef]
1. Angelini, G.; Canilho, N.; Emo, M.; Kingsley, M.; Gasbarri, C. Role of solvent and effect of substituent on
8
azobenzene isomerization by using room-temperature ionic liquids as reaction media. J. Org. Chem. 2015, 80,
7
430–7434. [CrossRef] [PubMed]
1
2. Angelini, G.; Chiappe, C.; De Maria, P.; Fontana, A.; Gasparrini, F.; Pieraccini, D.; Pierini, M.; Siani, G.
Determination of the polarities of some ionic liquids using 2-nitrocyclohexanone as the probe. J. Org. Chem.
2
005, 70, 8193–8196. [CrossRef] [PubMed]
3. Dupont, J. On the solid, liquid and solution structural organization of imidazolium ionic liquids. Braz. Chem. Soc.
004, 15, 341–350. [CrossRef]
4. Anthony, J.L.; Anderson, J.L.; Maginn, E.J.; Brennecke, J.F. Anion effects on gas solubility in ionic liquids.
J. Phys. Chem. B 2005, 109, 6366–6374. [CrossRef] [PubMed]
1
1
1
2
5. Stolte, S.; Arning, J.; Bottin-Weber, U.; Matzke, M.; Stock, F.; Thiele, K.; Uerdingen, M.; Welz-Biermann, U.;
Jastorff, B.; Ranke, J. Anion effects on the cytotoxicity of ionic liquids. Green Chem. 2006, 8, 621–629.
[CrossRef]
1
1
1
6. Latif, M.A.M.; Micaêlo, N.; Rahman, M.B.A. Solvation free energies in [bmim]-based ionic liquids: Anion
effect toward solvation of amino acid side chain analogues. Chem. Phys. Lett. 2014, 615, 69–74. [CrossRef]
7. Dos Santos, D.J.V.A.; Cordeiro, M.N.D.S. Effect of replacing [NTf ] by [PF ] anion on the [BMIM]-[NTf2]
2
6
ionic liquid confined by gold. Mol. Simul. 2015, 41, 455–462. [CrossRef]
8. Gasbarri, C.; Croce, F.; Meschini, I.; Bowen, C.H.; Marinelli, L.; Di Stefano, A.; Angelini, G. Single-walled
carbon nanotubes in highly viscous media: A comparison between the dispersive agents [BMIM][BF ], L121,
4
and Triton X-100. Chem. Eur. J. 2016, 22, 546–549. [CrossRef] [PubMed]
1
9. Al-Aqmar, D.M.; Abdelkader, H.I.; Abou Kana, M.T.H. Imidazolium based ionic liquids as novel benign
media for liquid-dye laser systems. J. Mol. Liq. 2017, 231, 370–378. [CrossRef]

Molecules 2017, 22, 1273
7 of 8
2
2
2
0. Hubbard, C.D.; Illner, P.; van Eldik, R. Understanding chemical reaction mechanisms in ionic liquids:
Successes and challenges. Chem. Soc. Rev. 2011, 40, 272–290. [CrossRef] [PubMed]
1. Chiappe, C.; Pomelli, C.S. Computational studies on organic reactivity in ionic liquids. Phys. Chem. Chem. Phys.
2013, 15, 412–423. [CrossRef] [PubMed]
2. Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de Lauth-Viguerie, N.; Mingotaud, C. Micelles in
ionic liquids: Aggregation behavior of alkyl poly(ethyleneglycol)-ethers in 1-butyl-3-methyl-imidazolium
type ionic liquids. Chem. Phys. Chem. 2006, 7, 99–101. [CrossRef] [PubMed]
2
3. Angelini, G.; Gasbarri, C. Polymeric aggregates in ionic liquids: the green future of the delivery systems.
Curr. Drug Targets 2015, 16, 1606–1611. [CrossRef] [PubMed]
2
4. Ab Rani, M.A.; Brant, A.; Crowhurst, L.; Dolan, A.; Lui, M.; Hassan, N.H.; Hallett, J.P.; Hunt, P.A.;
Niedermeyer, H.; Perez-Arlandis, J.M.; et al. Understanding the polarity of ionic liquids. Phys. Chem.
Chem. Phys. 2011, 13, 16831–16840. [CrossRef] [PubMed]
2
5. Ue, M. Mobility and ionic association of lithium and quaternary ammonium salts in propylene carbonate
and γ-butyrolactone. J. Electrochem. Soc. 1994, 141, 3336–3342. [CrossRef]
2
6. Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, A.B.H.; Watanabe, M. Physicochemical properties and structures
of room temperature ionic liquids. 1. Variation of anionic species. J. Phys. Chem. B 2004, 108, 16593–16600.
[CrossRef]
2
2
2
3
3
3
7. Schade, A.; Behme, N.; Spange, S. Dipolarity versus polarizability and acidity versus basicity of ionic liquids
as a function of their molecular structures. Chem. Eur. J. 2014, 20, 2232–2243. [CrossRef] [PubMed]
8. Angelini, G.; Gasbarri, C. Solvent scales comparison by using
α-nitrocyclohexanone as probe in ionic liquids,
organic solvents and CH CN/CHCl mixtures. Tetrahedron 2017, 73, 3036–3039. [CrossRef]
3
3
9. Gasbarri, C.; Angelini, G. Polarizability over dipolarity for the spectroscopic behavior of azobenzenes in
room-temperature ionic liquids and organic solvents. J. Mol. Liq. 2017, 229, 185–188. [CrossRef]
0. Blevins, A.A.; Blanchard, G.J. Effect of positional substitution on the optical response of symmetrically
disubstituted azobenzene derivatives. J. Phys. Chem. B 2004, 108, 4962–4968. [CrossRef]
1. Whitten, D.G.; Wildes, P.D.; Pacifici, J.G.; Irick, G., Jr. Solvent and substituent on the thermal isomerization
of substituted azobenzenes. Flash spectroscopic study. J. Am. Chem. Soc. 1971, 93, 2004–2008. [CrossRef]
2. Doki c´ , J.; Gothe, M.; Wirth, J.; Peters, M.V.; Schwarz, J.; Hecht, S.; Saalfrank, P. Quantum chemical
investigation of thermal cis-to-trans isomerization of azobenzene derivatives: Substituent effects, solvent
effects, and comparison to experimental data. J. Phys. Chem. A 2009, 113, 6763–6773. [CrossRef] [PubMed]
3. Baba, K.; Ono, H.; Itoh, E.; Itoh, S.; Noda, K.; Usui, T.; Ishihara, K.; Inamo, M.; Takagi, H.D.; Asano, T. Kinetic
3
study of thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4'-nitroazobenzene
in ionic liquids [1-R-3-methylimidazolium bis(trifluoromethylsulfonyl) imide with R = butyl, pentyl, and
hexyl]. Chem. Eur. J. 2006, 12, 5328–5333. [CrossRef] [PubMed]
3
4. Joshi, N.K.; Fuyuki, M.; Wada, A. Polarity controlled reaction path and kinetics of thermal cis-to-trans
isomerization of 4-aminoazobenzene. J. Phys. Chem. B 2014, 118, 1891–1899. [CrossRef] [PubMed]
5. Asano, T.; Okada, T.; Shinkai, S.; Shigematsu, K.; Kusano, Y.; Manabe, O. Temperature and pressure
dependences of thermal cis-to-trans isomerization of azobenzenes which evidence an inversion mechanism.
J. Am. Chem. Soc. 1981, 103, 5161–5165. [CrossRef]
3
3
3
3
3
4
6. Asaka, T.; Akai, N.; Kawai, A.; Shibuya, K. Photochromism of 3-butyl-1-methyl-2-phenylazoimidazolium in
room temperature ionic liquids. J. Photochem. Photobiol. A: Chem. 2010, 209, 12–18. [CrossRef]
7. Crecca, C.R.; Roitberg, A.E. Theoretical study of the isomerization mechanism of azobenzene and
disubstituted azobenzene derivatives. J. Phys. Chem. A 2006, 110, 8188–8203. [CrossRef] [PubMed]
8. Tran, C.D.; De Paoli Lacerda, S.H.; Oliveira, D. Absorption of water by room-temperature ionic liquids:
Effect of anions on concentration and state of water. Appl. Spectrosc. 2003, 57, 152–157. [CrossRef] [PubMed]
9. Gasbarri, C.; Angelini, G. Spectroscopic investigation of fluorinated phenols as pH-sensitive probes in mixed
liposomal systems. RSC Adv. 2014, 4, 17840–17845. [CrossRef]
0. Gasbarri, C.; Guernelli, S.; Boncompagni, S.; Angelini, G.; Siani, G.; De Maria, P.; Fontana, A. Fine-tuning of
POPC liposomal leakage by the use of
0, 202–210. [CrossRef] [PubMed]
β-cyclodextrin and several hydrophobic guests. J. Liposome Res. 2010,
2
4
1. Gasbarri, C.; Angelini, G.; Fontana, A.; De Maria, P.; Siani, G.; Giannicchi, I.; Della Cort, A. Kinetics of
demetallation of a zinc-salophen complex into liposomes. Biochim. Biophys. Acta Biomembr. 2012, 1818,
7
47–752. [CrossRef] [PubMed]

Molecules 2017, 22, 1273
8 of 8
4
2. Angelini, G.; Pisani, M.; Mobbili, G.; Marini, M.; Gasbarri, C. Neutral liposomes containing crown ether-lipids
as potential DNA vectors. Biochim. Biophys. Acta Biomembr. 2013, 1828, 2506–2512. [CrossRef] [PubMed]
3. Velluto, D.; Gasbarri, C.; Angelini, G.; Fontana, A. Use of simple kinetic and reaction-order measurements for
the evaluation of the mechanism of surfactant-liposome interactions. J. Phys. Chem. B 2011, 115, 8130–8137.
4
[CrossRef] [PubMed]
4
4. De Maria, P.; Fontana, A.; Siani, G.; D’Aurizio, E.; Cerichelli, G.; Chiarini, M.; Angelini, G.; Gasbarri, C.
Synthesis and aggregation behaviour of a new sultaine surfactant. Colloids Surf. B 2011, 87, 73–78. [CrossRef]
[PubMed]
Sample Availability: Samples of 4-methoxyazobenzene are available from the authors.
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).