Photoisomerization of azobenzenes isolated in cryogenic matrices

Article abstract of DOI:10.1039/c6cp02583h

2,2′-Dihydroxyazobenzene (DAB), 2,2′-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. A

Full text of DOI:10.1039/c6cp02583h

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ARTICLE
Photoisomerization of Azobenzenes Isolated in Cryogenic
Matrices
Luís Duarte,a* Leonid Khriachtchev,a Rui Faustob and Igor Revab*
Received 00th January 20xx,
Accepted 00th January 20xx
2,2'-dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and
their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical
calculations. All these compounds can adopt the E and Z isomeric forms around the central CNNC moiety, which can be
enriched by several conformational and tautomeric modifications for DAB and AT. A number of the DAB and AT isomeric
forms were identified for the first time. For DAB, the E azo-enol isomer with two intramolecular six-membered quasi-rings
formed via OH···N hydrogen bonds was found after deposition. Irradiation with UV light generated a different E azo-enol
form with two intramolecular H-bonded five-membered quasi-rings. The phototransformation was shown to be reversible
and the forms could be interconverted by irradiation at different wavelengths. The isomerization between these two forms
constitutes a direct experimental observation of an E→E isomerization in azobenzene-type molecules. Further irradiations
generated a form(s) bearing both OH and NH groups. For AT, two E isomers with the CH3 groups forming five-membered
and five/six-membered quasi-rings with the azo group were observed in the as-deposited matrices. Irradiation of AT with
UV light generated a Z form that can be converted back to the E form at different irradiation wavelengths. E-AB was
deposited in a xenon matrix and both E→Z and Z→E transformations were observed (contrary to what was previously
reported in an argon matrix where only the Z→E conversion occurred). The AB photoisomerization becomes more
pronounced at elevated temperatures, thus indicating that the matrix effects responsible for hindering the AB
photoisomerization are essentially due to steric restrictions. The different photoisomerization channels observed for these
compounds are discussed in terms of a small-amplitude pedal motion of the azo group.
DOI: 10.1039/x0xx00000x
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isomerization pathways have been proposed (rotation,
inversion, concerted inversion, inversion-assisted rotation, and
pedal motion).23-32 However, at present, the mechanism of this
Introduction
The reversible isomerization of azobenzenes between the E and
Z forms (or trans and cis) with different physicochemical
properties has been successfully exploited in a wide range of
applications. In industry, they are commonly used as dyes in the
production of optical and color-changing materials such as
sunglasses, textiles, paints, and cosmetics.1, 2 In addition, the
azobenzenes are also frequently employed to give light-
induced functionality to biomolecules,3 in the development of
photoactive drug delivery systems,4, 5 molecular switches,6, 7
photoresponsive crystalline materials,8-11 and promising solar
thermal fuels.12-15
Photoisomerization of azobenzenes involves excitation in
the UV-Vis range to either the S1 or S2 state.16-19 The process
may be more complex, and the involvement of other states has
also been suggested.20-22 The mechanism of photoisomerization
of azobenzene and its derivatives has been addressed in a great
number of theoretical and experimental studies and several
process is not consensual and continues to be a challenge.
The photoisomerization of azobenzenes in condensed
phases or other confined environments has been shown to be
particularly important for their applications. For example,
Kucharski et al. synthesized azobenzene-functionalized carbon
nanotubes and showed that the enforcement of conformational
restriction and steric strain resulted in enhanced energy-
storage capabilities.15 Wöll and co-workers integrated
azobenzene molecules into metal–organic frameworks and
investigated the photoisomerization and barrier for thermal
isomerization.9, 33 Benassi et al. studied the photodynamics of
thiolated azobenzenes chemisorbed on a gold surface and
evaluated the surface effects on the photoisomerization.34
Understanding the details of photoisomerization of
azobenzenes under constraints may give valuable information
for both fundamental and applied research and contribute for
an even more successful design of photofunctional systems.
In the present work, we report on the photoisomerization of
three azobenzenes, 2,2'-dihydroxyazobenzene (DAB), 2,2'-
azotoluene (AT) and azobenzene (AB), isolated in rare-gas
matrices. All of them bear two equal substituents R, in two
equivalent-by-symmetry positions with respect to the central
N=N double bond (Scheme 1). Due to this substitution pattern,
a.Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki,
Finland. Email: luis.duarte@helsinki.fi
b.Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal.
Email: reva@qui.uc.pt
Electronic Supplementary Information (ESI) available: Structures of DAB isomeric
forms, UV-Vis spectra, relative energies, tentative assignments, and theoretical
wavenumbers and IR intensities. See DOI: 10.1039/x0xx00000x
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the highest possible symmetry of each molecule is C2h. As shown rate = 10 Hz, duration ~10 ns). In the visible range (440–600
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DOI: 10.1039/C6CP02583H
below, the C2h structures represent the most stable isomeric nm), the signal beam of the OPO was used (pulse energy of ~30–
form in each compound, and due to symmetry considerations, 40 mJ). In the UV range (230–440 nm), the frequency-doubled
a half of vibrational modes of each molecule are infrared- signal or idler beam was used (pulse energy of ~3 mJ). The
inactive. This simplifies the analysis of the vibrational spectra. selection of the irradiation wavelengths was guided by the
Furthermore, unlike in AB where R=H, the substitution pattern absorption spectra of the compounds in solution (Figure S2).
in DAB and AT makes their aromatic rings asymmetric with
respect to the internal rotation around the CN bonds. This Computational details
renders three different E isomers in DAB and AT (Scheme 1a),
which are indistinguishable in AB.
Introducing substituents R≠H permits studies of the E-E
isomerizations in DAB and AT, reactions akin to the E-E self-
The quantum chemical calculations were performed using the
Gaussian 09 program package, and the functionals (M06-2X and
B3LYP) and the basis set (cc-pVTZ) were employed as
implemented in the program.37 The equilibrium geometries for
exchange reactions in AB where the reactant and the product
the molecules in vacuum were optimized using the default
are identical. These so-called narcissistic reactions have been
Berny optimization algorithm.38 The geometry optimizations
previously reported for porphycenes.35, 36 In the present work,
were followed by harmonic frequency calculations at the same
the possibility of such reactions in AB will be tested on its
level of theory. No imaginary frequencies were found. The
analogues DAB and AT. Besides that, we discuss the effects of
harmonic vibrational frequencies above and below 2000 cm–1
different substituents and spatial confinement of matrix-
were scaled by factors of 0.95 and 0.97, respectively, to correct
isolated azobenzenes on their E-Z photoisomerization in solids
them for the basis set limitations and the anharmonicity effects.
The theoretical infrared spectra were simulated using
(Scheme 1b).
Lorentzian profiles centered at the calculated frequencies and
with the full width at half maximum (FWHM) of 2 cm−1. If not
stated otherwise, the maxima of these profiles correspond to
the calculated infrared intensities. Vertical excitation energies
of the low-energy electronic excited states were calculated
using the time-dependent density functional theory (TD-DFT).39
Results of these calculations are provided in Table S4 of the
Supplementary Information. The theoretical UV-Vis spectra
were simulated using Gaussian profiles centered at the
calculated transition wavelengths with the FWHM of 0.4 eV, as
described elsewhere.40,41 The simulated spectra are shown in
Figure S3 of the Supplementary Information.
Scheme 1. Structures of AB, AT, and DAB azobenzenes studied in this work. (a) Three
different
E
structures in AT and DAB, rendered by substituents R. (b) E-Z Results
photoisomerization of azobenzenes.
Dihydroxyazobenzene (DAB)
Geometries and energies. DAB has two hydroxyl groups
attached in the ortho position of each aromatic ring and it can
adopt the following tautomeric forms: azo-enol (DOH), keto-
hydrazone (DNH), and forms containing both the –NH and –OH
groups (NHOH). For each tautomeric form, several conformers
are possible due to the different relative arrangements of the
two aromatic rings around the NN group (E and Z). Additionally,
in the case of the DOH and NHOH tautomeric forms, different
conformers may arise due to the rotation of the hydroxyl
groups.
After taking into account all tautomeric possibilities, the
calculations predict 24 minima on the ground potential energy
surface of DAB. Eleven minima correspond to the DOH
conformers, nine minima to the NHOH forms, and the
remaining four minima to the DNH conformers. The optimized
geometries and relative energies of the DAB isomers most
relevant for the present discussion are shown in Figure 1 and
Table 1, respectively. All DAB isomers and their energies are
given in the Supporting Information (Figure S1 and Table S1).
Materials and Methods
Experimental details
Commercial AB (Fluka, purity 98%), DAB (Sigma-Aldrich, purity
97%), and AT (Sigma-Aldrich, 98%) were placed in a glass tube
connected to the vacuum chamber of the cryostat through a
shutoff valve. The compound vapors were deposited, with a
large excess of the matrix gas (argon N60 or xenon N48, Air
Liquide) onto a CsI substrate cooled using a closed-cycle helium
refrigerator (APD Cryogenics DE-202A). The deposition
temperature was 15 and 30 K for argon and xenon matrices,
respectively. The infrared spectra were recorded in the 4000–
400 cm–1 range with 0.5 cm–1 resolution using a Nicolet 6700
FTIR spectrometer equipped with DTGS and MCT-B detectors
and a Ge/KBr beam splitter. The matrices were irradiated
through a quartz window of the cryostat using tunable
narrowband (0.2 cm–1 spectral width) light provided by a
Spectra Physics Quanta-Ray MOPO-SL optical parametric
oscillator (OPO) pumped with a pulsed Nd:YAG laser (repetition
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Table 1. Relative energies (in kJ mol−1
dihydroxyazobenzene.a
)
of selected isomeric forms of 2,2’-
View Article Online
DOI: 10.1039/C6CP02583H
B3LYP
EZPVE
M06-2X
EZPVE
0.00
16.86
17.73
22.64
76.17
84.85
96.84
100.32
DAB
EGibbs
0.00
EGibbs
0.00
E-DOH1
E-DOH2
E-DOH3
E-DOH4
Z-DOH1
Z-DOH2
Z-DOH3
E-NHOH4
0.00
23.97
22.86
29.00
89.93
105.60
123.02
95.13
16.35
19.91
25.07
87.26
98.88
120.47
88.61
12.23
14.57
18.18
74.75
80.41
93.46
93.66
a

EZPVE – Relative energy including the zero-point vibrational energy
correction. EGibbs – Relative energy including a thermal correction to the
Gibbs free energy at 298.15 K. The calculations were performed with the
cc-pVTZ basis set.
Figure 1. Structures of selected DOH and NHOH isomeric forms of 2,2’-
dihydroxyazobenzene calculated at the M06-2X/cc-pVTZ level of theory.42
At higher frequencies, the appearance of a broad absorption
feature in the 3200–2700 cm−1 range with a large integrated
intensity and the absence of the absorption peak around 3600
cm−1 (due to a free OH group) 43, 44 indicate that the OH groups
in DAB are involved in strong intramolecular OH···N hydrogen
bonds. Similar observations were found for another aromatic
azo compound with a strong intramolecular hydrogen bond
isolated in noble-gas matrices.45 Other characteristic
absorptions and tentative band assignments are given in Table
2 and Table S3. The experimental bands observed in argon and
xenon matrices are close to each other. It follows that the
matrix effect on the vibrational frequencies is not large and the
deviations between the calculated and experimental
frequencies are mainly due to the limitations of the theory. In
our analysis, we use mainly spectral shifts instead of absolute
band positions.
According to the calculations, most of the conformers of the
azo–enol type (DOH) are lower in energy than their tautomeric
counterparts (DNH and NHOH; see Table S1). The most stable
isomer of DAB is the planar E-DOH1 form (C2h symmetry),
bearing two six-membered quasi-rings stabilized by
intramolecular OH···N interactions. The azo-enol isomers with
two intramolecular H-bonded five-membered rings (E-DOH2)
and five/six-membered rings (E-DOH3) are found to be 16.9 and
17.7 kJ mol−1 higher in energy than E-DOH1 (M06-2X/cc-pVTZ
values). In E-DOH4, one of the six-membered rings is
maintained but one of the hydroxyl groups is rotated by 180
degrees and points away from the azo group. Its relative energy
is 22.6 kJ mol−1. The Z-DOH1, Z-DOH2, and Z-DOH3 forms have
the aromatic rings located in close geometrical proximity and
the CNNC dihedral angles of 10.1, 6.3, and 3.8 degrees,
respectively. These structures are energetically less favorable
by at least 75 kJ mol−1 (see Table 1).
The relative population of E-DOH1 at room temperature,
estimated from the calculated Gibbs free energy at 298.15 K and
the Boltzmann distribution (>98%), suggests that this form gives
the major contribution to the gas-phase equilibrium and should
be dominant in the deposited matrices. The expected
populations of the E-DOH2, E-DOH3, and E-DOH4 forms in the
gas phase are below 1%. In agreement with this estimate, only
the signature of the most stable isomer, E-DOH1, was found in
the spectra of deposited matrices.
Photochemistry. DAB isolated in cryogenic matrices was
irradiated with narrow-band light. Upon irradiation at 420 nm,
the bands of E-DOH1 decreased in intensity and a new set of
bands emerged in the spectra (Figure 3). The decrease of the
broad absorption feature in the 3200−2700 cm−1 range
indicates that the initial structure stabilized by two OH···N
hydrogen bonds is consumed. The appearance of bands at
~3500 cm−1 suggests that a new structure with the OH groups
involved in weaker intramolecular interactions is formed. By
comparing the experimental and calculated spectra, the new
set of bands was assigned to the E-DOH2 isomer (Figure 3).46 All
bands remained stable after infrared irradiation with the
spectrometer globar source for ~20 minutes. Further
irradiations at 396 nm partially consumed E-DOH2 and
regenerated E-DOH1, indicating that the conversion is
reversible.
Infrared spectra of matrix-isolated DAB. The experimental FTIR
spectra of DAB in argon (15 K) and xenon (30 K) matrices are
presented in Figure 2. The overall correspondence of the
experimental and calculated E-DOH1 spectra (M06-2X and
B3LYP levels of theory; see Figures 2 and Table S2) is quite
satisfactory in the region below 1800 cm−1.
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0.9
0.6
0.3
Ar matrix
Ar matrix
a)
*
b)
0.06
0.04
0.02
0.00
600
*
Xe matrix
Xe matrix
*
0.0
c)
E-DOH1
d)
E-DOH1
240
400
200
0
160
80
0
3600
3400
3200
3000
2800
2600
1800
1600
1400
1200
1000
800
Wavenumber (cm1)
Figure 2. Selected regions of the experimental infrared spectra of 2,2'-dihydroxyazobenzene in xenon (30 K) and argon (15 K) matrices (panels a and b) and the theoretical spectrum
of E-DOH1 calculated at the M06-2X/cc-pVTZ level of theory (panels c and d). The asterisks indicate the bands due to impurities.
Ar matrix
Ar matrix
a)
b)
0.01
0.00
0.02
0.00
Xe matrix
Xe matrix
-0.02
-0.01
c)
E-DOH2 minus E-DOH1
d)
E-DOH2 minus E-DOH1
240
120
0
300
150
0
-120
-150
3600
3400
3200
3000
2800
2600
1800
1600
1400
1200
1000
800
Wavenumber (cm1)
Figure 3. Changes in the experimental infrared spectra of 2,2'-dihydroxyazobenzene in argon (at 15 K) and xenon matrices (at 30 K) after irradiations at 420 nm for 4 and 2 min,
respectively (panels a and b). The negative bands correspond to the consumed form (assigned to E-DOH1). The positive bands correspond to the photogenerated isomer (assigned
to E-DOH2). The theoretical (M06-2X/cc-pVTZ) difference spectrum is simulated as ‘‘E-DOH2 minus E-DOH1’’ (panels c and d).
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Table 2. Selected experimental wavenumbers (
ω
, in cm−1) of the E-DOH1 azo-enol isomer in argon and xenon matrices.a
Ar (15 K)
Xe (30 K)
Calculated
ω
ω
ω
I
Sym.
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Tentative Assignment
ν(CC)+δ(CH)+δ(OH)
ν(CC)+δ(OH)+δ(CH)
δ(CH)+δ(OH)+ν(CC)
δ(CH)+ν(CO)+ν(CC)
δ(OH)+δ(CH)
1625.7, 1623.6
1589.7, 1584.4
1518.0
1477.2, 1476.0
1622.7, 1622.1
1585.0, 1581.2
1511.1
1473.3, 1471.4
1364.5, 1362.6
1649.1
1602.5
1495.4
1477.8
1379.1
1322.6
1284.9
166.7
137.6
6.4
194.7
89.2
16.1
140.9
1370.2, 1368.9, 1366.9, 1364.4
1326.7, 1322.8, 1320.6, 1319.8
1282.1, 1280.7
1323.6, 1322.3, 1318.0, 1314.6
1277.8, 1277.0
ν(CC)+ν(CO)+δ(CH)
ν(CO)+ν(CC)
1249.9, 1247.8, 1245.3, 1244.3,
1240.9, 1239.2
1245.4, 1244.2, 1240.9, 1239.4,
1238.0, 1236.7
1252.0
93.6
Bu
ν(CN)+δ(CH)
1210.2, 1208.0
1151.6, 1149.3, 1147.3
1112.6, 1109.8
1036.7, 1034.7, 1032.7
943.9, 943.0, 941.6, 940.7
876.5, 876.1, 874.0, 871.5
770.0, 767.7
762.8, 762.1, 760.8
752.2, 744.5
1205.5, 1204.3
1147.5
1107.9, 1106.3
1032.1, 1030.9, 1029.8
939.8, 937.8
875.0, 874.0, 871.3, 868.9
765.5, 762.9
760.6, 758.5, 757.3
747.5, 742.9, 741.3
1200.5
1138.9
1107.3
1038.4
954.8
865.4
770.3
754.7
733.0
74.9
53.7
19.7
12.3
5.4
17.1
75.3
157.7
42.2
Bu
Bu
Bu
Bu
Au
Bu
Au
Au
Au
δ(CH)+ν(CN)
δ(CH)
δ(CH)+ν(CC)
δ(CH)+ν(CC)
γ(CH)
δ(ring)+ν(CN)
γ(CH)+γ(CN)
γ(CH)+τ(OH)
τ(OH)+γ(CH)
a The strongest components of split bands are given in bold; M06-2X/cc-pVTZ wavenumbers and infrared intensities (I, in km mol−1) are also given. The calculated
wavenumbers are scaled by 0.97. ν, stretching; δ, in-plane-bending; γ, out-of-plane bending; τ, torsion.
Upon irradiation at 323 nm, the consumption of E-DOH1 and
0.02
0.00
a)
Xe matrix
the production of E-DOH2 continued but a different set of bands
also appeared. With successive irradiation cycles between 420
nm and 323 nm it was possible to group the new bands in one
distinct set (Table S5 and Figure 4). This set indicates the
formation of at least one additional isomeric form. The bands in
the 3650-3300 cm−1 region are consistent with the OH and NH
stretching vibrations and suggest the presence of a form(s)
bearing the OH and NH groups. After comparison with the
calculated spectra, the most satisfactory agreement was found
for E-NHOH4 (Figure 1). However, this assignment is somewhat
tentative and some of the other E-NHOH forms (e.g., E-NHOH3
and E-NHOH7) cannot be completely ruled out (see Figure S1
and Table S2) because their spectra are very similar. For
prolonged irradiations, particularly at shorter wavelengths (250
-0.02
1050
E-NHOH4 minus E-DOH2
b)
700
350
0
1720
1640
1560
1480
1400
1320
Wavenumber (cm1)
nm), the appearance of bands in the region around 2145-2100 Figure 4. Changes in the experimental infrared spectrum of 2,2'-dihydroxyazobenzene in
cm−1 and at 1681 cm−1 suggests a partial decomposition of
DAB.44
a xenon matrix at 30 K induced by irradiation at 420 nm (8 min) followed by irradiation
at 323 nm (7 min; panel a). The negative absorptions correspond to E-DOH2. The positive
peaks correspond to the emerging bands of a new isomeric form(s), tentatively assigned
to E-NHOH4. The theoretical (M06-2X/cc-pVTZ) difference spectrum is simulated as ‘‘E-
NHOH4 minus E-DOH2’’ (panel b).
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Table 3. Relative energies (in kJ mol−1) of 2,2'-azotoluene isomeric forms.a
Azotoluene (AT)
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DOI: 10.1039/C6CP02583H
Geometries and energies. The AT molecule can adopt two
orientations around the N=N bond (E and Z forms). Seven
energy minima were computationally found for this compound,
four E forms and three Z forms. The optimized geometries and
relative energies of the AT isomers are shown in Figure 5 and
Table 3.
B3LYP
EZPVE
0.00
9.67
M06-2X
AT
EGibbs
0.00
9.74
EZPVE
0.00
6.80
EGibbs
0.00
3.98
18.99
17.72
43.87
46.63
56.52
E-AT1
E-AT2
E-AT3
E-AT4
Z-AT1
Z-AT2
Z-AT3
19.63
23.22
17.19
18.37
43.14
45.17
52.43
b
b
54.82
62.78
75.10
58.31
65.61
78.34
The two methyl groups in AT have net repulsive interactions
with the NN fragment. It is different from DAB, where the two
OH substituents lead to stabilizing interactions with the NN
fragment, and the most stable DAB isomer has two six-
membered quasi-rings. The most stable isomeric form of AT, E-
AT1, has C2h symmetry and the geometry, in which the methyl
groups form two five-membered quasi-rings with the azo
moiety. The second most stable form, E-AT2, is ~7 kJ mol−1
higher in energy and has Cs symmetry. As compared to E-AT1,
one of the E-AT2 CCNN dihedrals is rotated by 180 degrees and
both methyl groups are on the same side of the azo group,
forming five-membered and six-membered quasi-rings. The
higher energy E forms, E-AT3 (C2 symmetry) and E-AT4 (Ci
symmetry), have two six-membered quasi-rings. Due to the
increased steric repulsion between the –CH3 groups and the
nitrogen atoms, the aromatic rings are not in the same plane
and the CCNN dihedrals are ~27 and ~21 degrees in E-AT3 and
E-AT4, respectively. In the Z-AT forms, the aromatic rings are
located in close geometrical proximity, creating less
energetically favorable structures with relative energies ≥43 kJ
mol−1 (Table 3). The CNNC dihedral angle is 8.9, 5.7, and 2.7
degrees in Z-AT1, Z-AT2 and Z-AT3, respectively.
a
EZPVE – Relative energy including the zero-point vibrational energy
correction. EGibbs – Relative energy including a thermal correction to the Gibbs
free energy at 298.15 K. The calculations were performed with the cc-pVTZ
basis set. b Not an energy minimum at this level.
Infrared spectra of matrix-isolated AT. The experimental FTIR
spectra of AT in argon (15 K) and xenon (30 K) matrices are
presented in Figure 6. A good agreement is observed between
the strongest experimental bands and the calculations for the
E-AT1 isomeric form at the M06-2X and B3LYP levels of theory
(Figure 6 and Table S6). The strongest bands of E-AT1 are at ca.
721 and 773 cm−1 in an argon matrix (719 and 772 cm−1 in a
xenon matrix) and have the calculated counterparts at 733 and
788 cm−1 (M06-2X). These bands are assigned to the coupled CH
out-of-plane and ring-deformation modes. Other characteristic
absorptions and the tentative band assignments are given in
Table 4 and Table S7.
The relative populations of E-AT1 and E-AT2 at room
temperature, estimated from the Gibbs free energies at 298.15
K and the Boltzmann distribution, are 71% and 29% at the M06-
2X level of theory. Therefore, the E-AT2 form can contribute
non-negligibly to the gas phase equilibrium and appear in the
deposited matrices. The E-AT2 strongest bands are predicted to
be very similar to those of E-AT1 and its experimental spectral
signatures are not absolutely evident. However, a close
inspection of the spectrum reveals the presence of absorptions
at 1302, 1280, 1218, 1158, 1118 and 510 cm−1 that are
consistent with the presence of the E-AT2 form (Figure 6). Other
forms of AT are predicted to have negligible populations at
room temperature.
Figure 5. Isomers of 2,2'-azotoluene calculated at the M06-2X/cc-pVTZ level of theory.42
6 | Phys. Chem. Chem. Phys., 2016, 00, 1-10
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0.09
0.06
0.03
0.00
View Article Online
DOI: 10.1039/C6CP02583H
Ar matrix
Ar matrix
a)
a)
0.04
0.00
Xe matrix
*
-0.04
120
80
b)
Z-AT1 minus E-AT1
24 b)
E-AT1
E-AT2
40
16
8
0
-40
-80
0
810
780
750
720
690
660
1320
1280
1240
1200
1160
1120
Wavenumber (cm1)
Wavenumber (cm1)
Figure 7. Changes in the experimental infrared spectrum of 2,2'-azotoluene in
argon and xenon matrices after irradiation at 230 nm for 20 min (panel a). The
negative bands correspond to the consumed E-AT (mainly E-AT1) form. The
positive peaks correspond to the photoproduct (assigned to the Z-AT1 form). The
theoretical difference spectrum (M06-2X/cc-pVTZ) was constructed as ‘‘Z-AT1
minus E-AT1’’ (panel b).
Figure 6. Selected region of the experimental infrared spectrum of 2,2'-azotoluene in an
argon matrix at 15 K (panel a) and the calculated spectra of E-AT1 and E-AT2 at the M06-
2X/cc-pVTZ level of theory (panel b). Arrows indicate the bands of E-AT2. The asterisk
indicates a band due to traces of matrix-isolated monomeric ethanol impurity. The
calculated intensities of E-AT1 and E-AT2 are scaled by 0.71 and 0.29, respectively,
according to the expected gas-phase populations.
The experimental infrared spectra of AB in argon (15 K) and
xenon (30 K) matrices are presented in Figure 8. There is a good
agreement between the experimental and calculated (M06-2X
and B3LYP levels of theory) spectra for the E-AB isomeric form
(Table S8). The complete E- and Z-AB band assignments and the
potential energy distributions are presented elsewhere.47 Due
to a large energy barrier,26 the most stable E-AB form is the only
form relevant to the gas-phase thermal equilibrium at room
temperature and, consequently, only this form is observed in
the deposited matrices (in the absence of light).47,48
The deposited (at 30 K) xenon matrix was annealed to 60 K
and irradiated at 340 nm. At this temperature the irradiation
consumes the E-AB form and produces a number of new bands
(e.g., at 701 and 759 cm−1) consistent with the Z-AB form. Before
the next irradiation at a different temperature, the produced Z-
AB form was consumed by irradiating at 420 nm. For 340 nm
irradiations at lower temperatures (50, 40, and 30 K), the E→Z
photoisomerization becomes less pronounced (Figure 9). In
different experiments, the deposited matrices were cooled
down to 20 K and 16 K and also irradiated at 340 nm. Under
these conditions, some E→Z isomerization was also observed.
Photoisomerization. AT in cryogenic matrices was irradiated
with narrow-band light. Upon irradiation at 330 nm, the bands
of E-AT1 and E-AT2 decreased in intensity and a new set of
bands emerged in the spectrum. Further irradiation at 230 nm
also consumed the initial forms and increased the same new
bands (Figure 7). By comparing the experimental and calculated
spectra, this set of new bands is assigned to the Z-AT1 form (see
Figure 5). For irradiation at 400 nm, the opposite photoprocess
(Z-AT1→ E-AT1) was observed. No indication of regeneration of
the E-AT2 form was found. Subsequent irradiation at 230 nm
recovered the Z-AT1 form. This behavior clearly indicates the
existence of a reversible phototransformation between the E-
AT1 and Z-AT1 forms.
Azobenzene (AB)
The vibrational characterization and photoisomerization of AB
in an argon matrix has been recently reported by us.47 We
observed that the irradiation of E-AB in the gas phase with UV-
Vis broadband light provided by an Hg(Xe) arc lamp led to its
partial conversion to the Z-AB isomer and trapping of both
forms in an argon matrix. However, UV-Vis irradiation could
only induce the Z→E conversion in an argon matrix, whereas the
opposite E→Z process was not achieved. In the present work,
we try to evaluate the role of the matrix in the suppression of
the E→Z photoisomerization. For this purpose, we performed a
series of irradiations of AB isolated in a xenon matrix at different
temperatures.
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View Article Online
DOI: 10.1039/C6CP02583H
Table 4. Selected experimental wavenumbers (
ω
, in cm−1) of the E-AT1 isomer in argon and xenon matrices.a
Ar (15 K)
Xe (30 K)
Calculated
Sym.
ω
ω
ω
I
Tentative Assignment
ν(CC)+δ(CH)
1602.6
1492.4, 1490.4, 1483.3
1604.6, 1599.9
1490.2, 1487.9, 1481.6,
1480.1
1631.8
1493.7
14.3
25.2
Bu
Bu
δ(CH)+ν(CC)
1464.6
1438.3
1429.6
1380.4
1304.4
1276.8
1222.5
1195.2
1152.03
1117.0
1045.6, 1042.4
1037.6
775.9, 773.1
722.9, 721.7, 720.8
1461.3
1433.5
1424.8
1376.6
1306.8, 1304.5
1274.8
1220.3
1195.5, 1193.1, 1191.6
1154.4, 1151.7, 1150.1
1116.5
1045.4, 1041.9
1035.9
1467.2
1443.4
1437.2
1378.8
1303.4
1269.5
1232.5
1192.9
1143.3
1111.8
1048.7
1043.0
788.3
27.4
14.8
15.4
3.9
16.0
7.1
28.1
8.5
22.1
3.4
Bu
Au
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Au
Au
Au
δ(CH)+δ(CH3+ν(CC)
δ(CH3)
δ(CH3)+δ(CH)
δ(CH3)+ν(C-CH3)
ν(CC)+δ(CH)+δ(CH3)
δ(CH)+ν(CC)+δ(CH3)
ν(CN)+δ(CH)
ν(C-CH3)+δ(CH)+δ(ring)
δ(CH)
δ(CH)
δ(CH)+ν(CC)
γ(CH3)
γ(CH)+γ(CN)
13.2
5.0
61.5
49.0
772.4, 720.6
721.6, 718.7
733.2
γ(CH)+γ(CH3)+τ(ring)
a
The strongest components of split bands are given in bold; M06-2X/cc-pVTZ wavenumbers and infrared intensities (I, in km mol−1) are also given.
Calculated wavenumbers are scaled by 0.97. ν, stretching; δ, in-plane-bending; γ, out-of-plane bending; τ, torsion.
60 K
50 K
40 K
30 K
a)
0.02
0.00
0.6
0.4
0.2
Ar matrix
a)
-0.02
Xe matrix
Z-AB minus E-AB
b)
80
40
0
0.0
90
b)
E-AB
-40
-80
60
30
0
850
800
750
700
650
600
Wavenumber (cm1)
850
800
750
700
650
600
Figure 9. Changes in the experimental infrared spectrum of azobenzene in a xenon
matrix after irradiation at 340 nm for 15 min at 30, 40, 50, and 60 K (panel a). The
negative bands correspond to the consumed isomer (E-AB). The positive bands
correspond to the photogenerated isomer (Z-AB). Each 340 nm irradiation was preceded
by consumption of the Z form by irradiating at 420 nm for 2 min. The calculated
difference spectrum (M06-2X/cc-pVTZ) is constructed as ‘‘Z-AB minus E-AB’’ (panel b).
Wavenumber (cm1)
Figure 8. Selected region of the experimental infrared spectra of azobenzene isolated in
xenon (30 K) and argon (15 K) matrices (panel a) and the calculated spectrum of E-AB at
the M06-2X/cc-pVTZ level of theory (panel b).
8 | Phys. Chem. Chem. Phys., 2016, 00, 1-10
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Remarks on the photoisomerization process
They have found that once the system is excited VtioewtAhrteiclbe rOinglihnet
1
DOI: 10.1039/C6CP02583H
S2( ππ*) state, the proton transfer from the azo-enol form to
the keto-hydrazone form is barrierless. Along the 1ππ*
relaxation path, the dark S1(1nπ*) state can be efficiently
populated, but the proton transfer is inhibited due to a barrier
of about 42 kJ mol−1.50
One of the most striking findings of the present work is the
isomerization (or the lack of it) observed for these azobenzenes.
For DAB, no E to Z photoisomerization was found in argon and
xenon matrices. The most stable form observed in the matrices
after deposition, E-DOH1, has two strong intramolecular
hydrogen bonds that enforce the planar structure. The presence
of this structural restriction may affect the channels of the
isomerization. In fact, Bandara et al. studied 2,2'-
diaminoazobenzene derivatives with several intramolecular
hydrogen bonds in diethylether/ethanol solutions frozen at 77
K and reported minimal to none E→Z photoisomerization.49 The
photoisomerization was observed when the formation of the
hydrogen bonds was prevented by alkylation of the –NH groups.
In our case, the E→Z isomerization may be also suppressed by a
matrix-induced barrier resulting from the reorganization of the
matrix material in order to accommodate the bent Z form (that
requires a larger “3D-cage” when compared to the “2D-cage”
sufficient to accommodate the planar E form).
The E-DOH2 form is similarly locked in a planar configuration
by two intramolecular hydrogen bonds (Figure 1) and can be
formed from E-DOH1 by three possible mechanisms: (i) pedal
motion of the azo group, (ii) inversion around the CNN angles,
and (iii) rotation of both phenol groups around the CN bonds. In
the confined matrix environment, movements involving a
smaller reorganization of the matrix material, such as small-
amplitude pedal motion of the azo group, seem to be more
plausible. The non-observation of E-DOH3 among the
photoproducts indicates that rotation of the phenol groups
does not occur and mechanism (iii) should not be operational.
However, without further theoretical calculations involving the
excited state dynamics of both forms in condensed phases, the
pathway (ii) cannot be completely ruled out.
The E-DOH1→E-DOH2 isomerization, constitutes a direct
experimental observation of an E→E isomerization in
azobenzene-type molecules. For non-substituted azobenzene
(AB), this process cannot be detected in our experiments. It
becomes possible for DAB, due to the introduction of two OH
substituents at the 2 and 2' positions of the aromatic rings. The
present observation also indirectly supports that similar E→E
isomerizations, representing self-exchange reactions, may
occur in azobenzene. These self-exchange reactions have been
observed previously for porphycenes.35 In porphycenes, those
reactions were detected by methods of polarized spectroscopy,
due to the change in the transition moment of the system.
Similarly, the self-exchange reactions in azobenzenes may be
detectable experimentally by other methods using the
methodology based on polarized spectroscopy or the analyses
of anisotropy data.36
The observation of a NHOH-type form(s) of DAB (Figure 4)
resulting from its tautomerization is in agreement with a recent
theoretical work on 2-hydroxyazobenzene. Guang et al. have
employed the complete active space self-consistent field
(CASSCF) and its multistate second-order perturbation (MS-
CASPT2) methods to explore the photochemical mechanism of
2-hydroxyazobenzene in the two lowest excited singlet states.
In the case of AT, the E→Z and Z→E photoisomerization
occurred in argon and xenon matrices. This molecule has two
bulky methyl substituents and requires a larger trapping space
than the other studied azobenzenes. It is possible that this
difference in the trapping morphology decreases the spatial
limitations enforced by the matrices on AT in the process of its
photoisomerization. Furthermore, the methyl groups lead to a
relative stabilization of azotoluene Z forms (the relative
energies between the most stable E and Z forms of DAB, AB, and
AT are 76.17, 52.03 and 43.14 kJ mol−1, respectively), which may
play a role in lowering the barrier for the isomerization process.
Schweighauser et al. studied the rates of the Z→E isomerization
of several alkyl-substituted azobenzene derivatives in different
solvents by temperature-controlled UV-Vis spectroscopy and
51
DFT calculations. They proposed that the isomerization was
influenced by the attractive London dispersion interactions
stabilizing the Z isomers. The non-observation of E-AT3 in our
experiments (analogous to the E-DOH2 form in DAB) can also be
explained by the effect of the methyl substituents. In E-AT3, the
steric repulsion between the –CH3 groups and the nitrogen
atoms results in a distorted non-planar geometry and the E→E
reaction channel (AT1→AT3) can become less favorable
compared to that leading to the E→Z photoisomerization.
For AB, the observation of the E→Z and Z→E isomerizations
in a xenon matrix, in contrast to an argon matrix where only
Z→E was observed,47 suggests that the matrix affects the
photoisomerization process. It appears interesting to compare
the present observation in matrices with the results of the
recent studies. Böckmann et al. have performed nonadiabatic
QM/MM molecular dynamics simulations to investigate the
photoisomerization of azobenzene in the gas and liquid
phases.24, 52 They have found that the Z→E photoisomerization
is only slightly affected by the liquid bulk environment
compared to the gas phase while the E→Z process is strongly
hindered in the liquid phase. The simulations have revealed that
the photoisomerization mechanism is initially dominated by a
small-amplitude pedal motion of the central CNNC moiety that
competes in the S1 state with the movement of the aromatic
rings toward planarity.53 However, due to the initial orientation
of the aromatic rings in both forms, these two competing
movements only favor the Z→E process. In addition, they have
also found that the vertical excitation brings the Z-AB to a
steeper Franck–Condon (FC) region compared to E-AB,
providing a large driving force for isomerization. For E-AB, the
FC region is relatively flat and the resulting forces are smaller,
thus, making the E→Z photoisomerization more sensitive to
environmental effects.
Despite the obvious differences between AB in the liquid
phase and in solid matrices, the theoretical results discussed
above are helpful for the rationalization of our experimental
observations. The matrix materials used (argon and xenon)
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Journal Name
usually adopt a face centered cubic (fcc) crystal structure where constraints imposed by the solid rigid medium. TVoiewreAcrtaicllle, Oonnlinlye
DOI: 10.1039/C6CP02583H
small molecules can be trapped in interstitial or substitutional the Z→E process was observed in the previous studies in an
sites.54 In the case of larger molecules, like the ones studied in argon matrix.47
this work, multiple substitutional sites (resulting from
Our results provide valuable experimental insights on the
“removal” of several matrix atoms) usually occur. As the atomic photoisomerization process of different azobenzenes in
radii increase from argon to xenon, the size of the possible condensed phase environments. We present a rationalization of
trapping sites also increases, and it seems reasonable to expect our findings in terms of the pedal motion of the azo group.
that steric effects imposed by the environment on the However, a totally comprehensive interpretation of the matrix
azobenzenes are reduced in xenon matrices. This seems to be effects and
a
full explanation of the different
the case for AB where both E→Z and Z→E processes were photoisomerization channels observed for the compounds
observed in a xenon matrix whereas in an argon matrix only the studied here is a challenge for computational chemistry and
Z→E process occurred. The increase of the AB future experiments.
photoisomerization efficiency at elevated temperatures is
consistent with this picture.
Acknowledgements
This work was supported by Project KUMURA of the Academy
of Finland (No. 1277993). The CSC-IT Center for Science is
Conclusions
Three
structurally
related
azobenzenes,
2,2'- thanked for computational resources. I. R. acknowledges the
dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT), and Portuguese “Fundação para a Ciência e a Tecnologia” (FCT), for
azobenzene (AB), were investigated by matrix-isolation infrared the Investigador FCT grant. The research leading to these results
spectroscopy and theoretical calculations at the M06-2X (and has also received support from LASERLAB-EUROPE, grant
B3LYP)/cc-pVTZ level. The spectral signatures of DAB and AT in agreement n° 284464, EC's Seventh Framework Programme.
cryogenic matrices and their light-induced transformations The Coimbra Chemistry Centre (CQC) is supported by FCT
have been reported for the first time.
through the project UI0313/QUI/2013, co-funded by UE. The
For DAB, among the possible isomeric forms, only the azo– authors thank the Advanced Computing Laboratory at
enol form E-DOH1 with two OH groups involved in strong University of Coimbra for providing computing resources that
intramolecular hydrogen bonds is found in the as-deposited have contributed to the present study.
argon and xenon matrices. UV irradiation at 420 nm generates
a
different azo–enol isomeric form of DAB with two
Notes and references
intramolecular H-bonded five-membered rings (E-DOH2). The
phototransformation is reversible and irradiating at 396 nm
recovers the E-DOH1 isomer, probably by a pedal motion. The
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ARTICLE
Journal Name
53.
Other calculations also suggest the existence of pedal
View Article Online
motion. See, for example, references 24,27 and 29. Pedal
motion has also been reported for azobenzene crystals: J.
Harada and K. Ogawa, Chem. Soc. Rev., 2009, 38, 2244-
2252.
Despite several studies on the subject, the exact
morphology of solid cryogenic matrices is unknown.
Additionally, due to the nature of the deposition process
(which involves rapid cooling from the gas phase), the
structure of the formed matrix is unlikely to be perfect and
additional trapping sites resulting from lattice
imperfections are also possible.
DOI: 10.1039/C6CP02583H
54.
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