Synthesis and Characterization of Photoactive Methyl 4-Bromo-3-((2,6-Difluorophenyl)diazenyl) Benzoate

Article abstract of DOI:10.1007/s10870-021-00881-6

Abstract: The synthesis and structure of a novel ortho-fluoroazobenzene, methyl 4-bromo-3-((2,6-difluorophenyl)diazenyl) benzoate is described. The title molecule crystallizes in the centrocemetric space group P-1 with two rotomer molecules of the title c

Full text of DOI:10.1007/s10870-021-00881-6

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-021-00881-6
ORIGINAL PAPER
Synthesis and Characterization of Photoactive Methyl
4‑Bromo‑3‑((2,6‑Difuorophenyl)diazenyl) Benzoate
Eric D. Sylvester1 · Jason B. Benedict2
Received: 30 November 2020 / Accepted: 16 January 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
The synthesis and structure of a novel ortho-fuoroazobenzene, methyl 4-bromo-3-((2,6-difuorophenyl)diazenyl) benzoate
is described. The title molecule crystallizes in the centrocemetric space group P-1 with two rotomer molecules of the title
compound in the asymmetric unit. The position of ortho-fuoro azo rings difer between the two rotomers with one molecule
having a rotation angle of 4.4° and the other molecule having a rotation angle of 76.9° with respect to the methyl 4-bro-
mobenzoate. Due to the tight packing the pure molecule was not seen to be photoactive. However, in solution the absorption
bands in the visible region show a separation of about 20 nm as expected for o-fuoroazobenzene. A comparison to related
and previously published co-crystals of substituted azobenzenes are presented.
Graphic Abstract
The structure of a novel ortho-fuoroazobenzene, methyl 4-bromo-3-((2,6-difuorophenyl)diazenyl) benzoate reveals the
presence of two crystallographically unique rotomers in the lattice, and although the molecule is photoactive in solution, the
close-packed lattice appears to inhibit photo-induced structural reorganization in the crystalline state.
Keywords Crystal structure · Photochrome · Ortho-fuoroazobenzene · Azobenzene · Twisted-confrmation
Introduction
Photochromic solid-state materials are appealing for a wide
* Jason B. Benedict
variety of applications including separations, advanced
jbb6@bufalo.edu
sensors, drug delivery, data storage and molecular switches
1
through external control [1–7]. It has been shown that
photomechanical molecular crystals can undergo various
730 Natural Sciences Complex, Bufalo 14260-3000, USA
771 Natural Sciences Complex, Bufalo 14260-3000, USA
2
Vol.:(0123456789)
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Journal of Chemical Crystallography
mechanical motions, including twisting, bending and even
jumping, using light as the reaction mechanism, thus, con-
verting photochemical energy to mechanical energy [8–10].
The development of novel photoactive molecules that may
be incorporated into a variety of solid-state materials is cru-
cial for the advancement of next generation light responsive
smart materials [11].
be the small increase of the π* orbital; thus the change of
relative energies between the cis and trans forms of azoben-
zenes are similar.
The diference in the excitation energy of the cis isomer
relative to the trans isomer will cause a separation of the
absorbance bands of both the cis/trans isomers in the visible
light region. The addition of electron withdrawing ortho-
fuoro functional groups should lower the n-electron density
of the cis isomer, causing a decrease in the overall repulsion
of the lone pair orbitals. This should lower the HOMO rela-
tive energy resulting in a larger n to π* excitation energy for
the cis isomer.
The integration of azobenzenes into photoswitchable
materials has been of investigated due their desirable pho-
toactive properties. This class of photoswitch often exhibits
extremely fast trans to cis isomerization, on the order of
picoseconds [12], with overall excellent resistance to fatigue
[13, 14]. However, unsubstituted azobenzene based mole-
cules generally exhibit a thermal back reaction from cis to
trans without the use of light at room temperature [15–17].
The inability to perform as a bi-stable photoswitch and the
pronounced overlap of the cis/trans excitation bands in the
visible region often results in a mixture of both isomers
when the materials are irradiated. Because of this, materials
which contain azobenzenes tend to be studied under constant
irradiation of ultraviolet light to maximize the concentration
of cis isomer present. The need for constant UV irradia-
tion diminishes the potential utility of these photoswitches,
especially in biological systems [18, 19]. However, recently
it has been found that the chemical functionalization of the
phenyl ring attached to the azo group, namely ortho-fuoro
substitution, can greatly increase the stability of the cis form
of azobenzenes and increase the spectral separation of the
isomers as well [20].
With these properties in mind, a novel bromine-func-
tionalized ortho-fluoroazobenzene photoswitch, methyl
4-bromo-3-((2,6-difluorophenyl)diazenyl) benzoate (1)
(Scheme 1), was synthesized and determined to be photoac-
tive. Furthermore, 1 exhibited the spectral separation char-
acteristic of o-fuoroazobenzenes, and possessed functional
groups suitable for creating light-responsive halogen-bonded
crystalline solids [26, 27]. Halogen-bonded co-crystals
have been used to monitor temperature efects on molecu-
lar pedal motion of molecular solids [28]. Additionally, 1
may be useful as an intermediate compound for additional
ortho-fuoroazobenzene photoswitches as the ester is readily
converted to the carboxylic acid to create hydrogen bonded
materials and the halogen may be used to perform coupling
reactions. Here we present the synthesis and characterization
of 1 which includes a detailed analysis of unique structural
features found in the neat crystalline solid.
It has been shown that ortho-fuoroazobenzenes, like
2,6-difuoroazobenzene, can be stable in the cis form for
up to two years in the absence of light at room temperature
[21]. Ortho-fuoroazobenzenes also have the advantageous
property of separation of the cis and trans absorbance bands
in the visible light region [21]. This means that the material
is photoswitchable using only visible light eliminating the
need for the more hazardous UV light altogether.
Experimental Details
Synthetic Procedure
All chemicals were obtained commercially from Sigma-
Aldrich and Combi-Blocks, and used as received.
These improved properties of o-fuoroazobenzenes rela-
tive to standard azobenzenes has previously been explained
using molecular orbital theory [21]. For azobenzenes, the
interaction with the lone pair orbitals on the N atoms from
the azo functional group determine the overall n (HOMO)
orbital level [22]. The trans isomer has lone pair orbitals
that interact along the N–N sigma bond greatly reducing
the energy of the n orbital level; whereas, the repulsion of
the lone pair orbitals from the azo functional group of the
cis isomer results in a higher energy HOMO [23]. Also, the
decrease of π-electron delocalization from the large dihedral
angles of the C–N=N–C bonds only found in the cis isomer
results in a larger net separation between the π and π* ener-
gies causing an increased π* orbital and a lower π orbital
[24, 25]. Therefore, the small increase of the n orbital level
in the cis form compared to the trans isomer is counteracted
Scheme 1 Synthetic route of 1
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Journal of Chemical Crystallography
1,3‑Difuoro‑2‑Nitrosobenzene
UV–Vis Spectroscopy
The synthetic procedure was performed using previously
reported methods [21]. First, 2,6-difluoroaniline was
reacted to form 1,3-difluoro-2-nitrosobenzene by par-
tially dissolving 4.76 g of Oxone (0.0155 mol, 2 eq) in
10 mL of DI water in a fask with a solution of 1 g of
2,6-difuoroaniline (0.0078 mol, 1 eq) in 10 mL DCM.
The fask was allowed to react in open air at room tem-
perature for 6 h, stirring vigorously to create emulsions
between the organic phase and aqueous phase. Next, the
organic DCM layer was separated from the aqueous layer,
and the aqueous layer was extracted further using 20 mL of
DCM, three times. The combined DCM extracts were then
washed 3 times using 10 mL of DI water. The organic sol-
vent was then removed using rotary evaporation resulting
in a brown crude solid of 1,3-difuoro-2-nitrosobenzene,
1.086 g (98%).
All UV–vis absorption spectra were recorded in absolute
darkness to prevent any room lights from interfering in the
photochemical reactions. The samples were irradiated with
a Thorlabs 365 nm LED (4.1 mW) and Thorlabs 405 nm
LED (4.1 mW). The UV–vis spectra were collected using a
Perkin Elmer Lambda 35 dual beam UV–vis spectrometer.
1H NMR Spectroscopy
Nuclear magnetic resonance spectra were recorded on a
Varian Inova-400 MHz NMR spectrometer. For the NMR
light studies, the spectra were obtained within 5 min of
irradiation.
Results and Discussion
1H NMR (CDCl3, 400 MHz) δ 8.24(s, 1H), 7.66(dd,
2H), 7.53(t, 1H), 7.44(t, 2H).
Structural Commentary
1 crystallizes in the centrosymmetric space group P-1 with
two crystallographically independent molecules in the asym-
metric unit (Fig. 1). The two molecules are easily distin-
guished from one another as one molecule is nearly planar,
1∥, while the other exhibits a pronounced rotation about the
N4–C21 bond,1⊥. To our knowledge, this is the frst reported
pure azobenzene crystal structure that contains both planar
and nearly perpendicular rotational isomers within the same
lattice. Crystal data, data collection and structure refnement
details are summarized in Table 1.
Methyl 4‑Bromo‑3‑((2,6‑Difuorophenyl)diazenyl) Benzoate
(1)
To form 1, 1.30 g of methyl 3-amino-4-bromobenzoate
(0.0057 mol, 1 eq) was mixed with 0.81 g of 1,3-difuoro-
2-nitrosobenzene (0.0057 mol, 1 eq) in 20 mL of glacial
acetic acid. The reaction was heated to 80 °C for 3 days to
form a dark purple viscus solution. The solution was neu-
tralized with saturated sodium bicarbonate and extracted
with ethyl acetate and purifed by column chromatography
using 30% ethyl acetate in hexanes. The resulting 1 was an
orange/red solid, 1.25 g (62%). Then, 20 mg of 1 was dis-
solved in 5 ml of ethyl acetate and was allowed to evapo-
rate uncovered resulting in orange plate crystals suitable
for single crystal structure determination.
In order to quantify the twist present for both molecules,
the angle between the surface normal vectors of the mean
planes for the two phenyl rings was determined using Olex2
[29]. For 1∥, the angle was determined to be 4.4(1)°. In the
case of1⊥, the two rings were nearly perpendicular with an
angle between the two surface normal vectors of 76.9(1) °
(Fig. 2).
1H NMR (CDCl3, 400 MHz) δ 8.25(s, 1H), 8.02(d, 1H),
7.85(d, 1H), 7.39(m, 1H), 7.08(dd, 2H), 3.95(s, 4H).
While planar or nearly planar azobenzene structures
are common in the literature, using the CCDC database
[32], only a few neat twisted azobenzenes have been pre-
viously reported in the literature [33–36]. For example,
in the structure of dichloroazobenzene (Ref code: XOT-
CAM, Fig. 3), the twist angle was reported to be 74.9(5)
[36] which is nearly identical to 1⊥. The crystal structure
of XOTCAM does not contain the co-planar rotamer of the
dichloroazobenzene.
X‑ray Difraction
X-ray difraction data were collected on a Bruker SMART
APEX2 CCD difractometer installed at a rotating anode
source (MoKα radiation, λ = 0.71073 Å), and equipped
with an Oxford Cryosystems (Cryostream700) nitrogen
gas-fow apparatus. The data were collected by the rotation
method with a 0.5° frame-width (ω scan). Using Olex2
[29], the structure was solved with the XS structure solu-
tion program [30] using Direct Methods and refned with
the olex2.refne refnement package [31].
Supramolecular Features
In the structure of 1, extended sheets of 1∥ held together by
π···π stacking, interact with adjacent 1⊥ molecules through
C–Br···Br–C halogen bonds forming a 3-dimentional lattice
1 3

Journal of Chemical Crystallography
Fig. 1 Asymmetric unit of
1 showing the numbering
scheme. The hydrogen atoms
were removed for clarity. Atom
colors: fuorine (green), bro-
mine (maroon), oxygen (red),
nitrogen (blue), and carbon
(grey) (Color fgure online)
Table 1 Data collection and structure refnement details for 1
(Fig. 4). The molecule1⊥ forms 1-D chains with complemen-
tary molecules of1⊥ through π···π interactions of the twisted
azobenzene ring of 1⊥ (Fig. 5). Halogen bonding is catego-
rized based on whether the halogens are less than the sum of
the van der Waals radii, the angle between the bonding halo-
gens where, ∠X1 ⋯ X2−− C2 = ꢀ1 and ∠C1−− X1 ⋯ X2 = ꢀ2
(X = halogen atom) [37, 38]. Type I halogen bonding is
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C56H36Br4F8N8O8
1420.56
90
triclinic
P-1
7.6992 (11)
11.5425 (18)
15.642 (2)
83.709 (5)
85.431 (5)
77.561 (5)
1347.0 (4)
1
◦
◦
ꢀ
ꢀ
defned as, 0 ≤ ꢀ1 − ꢀ2 ≤ 15 , type II halogen bonding
ꢀ
ꢀ
◦
◦
◦
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
is defned as, ꢀ1 − ꢀ2 ≥ 30 , and,15 ≤ ꢀ1 − ꢀ2 ≤ 30 , is
defned as a quasi type I/type II halogen bond [37, 38]. Since
the halogens for 1∥ and 1⊥ are within halogen bonding dis-
tance (Br1···Br2 =3.630 Å), the halogen bonding angles were
found to be ∠C15–Br2···Br1 =169º and, ∠C1–Br1···Br2 =97º,
thus the halogen bond is described as type II.
γ (°)
Volume (Å3)
Z
ρcalc (g/cm3)
1.751
3.08
703.6
0.1×0.02×0.01
Mo Kα (λ=0.71073)
μ (mm−1
F(000)
)
Database Survey
Crystal size (mm3)
Radiation
A search of the Cambridge Structural Database (version
5.41 updated through August 2020) using orthro-fuoro-
azobenzene returned 77 hits when the protons were not
specifed. Similar molecules containing halogen-halogen
interactions between halogen substituted azobenzenes have
been reported, e.g. AZ07 (Fig. 6a) [39]. The structure of
AZ07 pack using C–Br···π interactions, π···π stacking of the
aromatic rings, and more notable type II halogen bonding
C–Br···Br–C, forming a herringbone shaped pattern. What
gives rise to the distinctive herringbone shape is the type
II halogen bond (Br1···Br1 =3.526 Å, ∠C1–Br1···Br1 =169°).
2Θ range for data collection (°) 2.62 to 56.32
Index ranges
− 10≤h≤10, − 15≤k≤15, −
20≤l≤20
28,109
Refections collected
Independent refections
Data/restraints/parameters
Goodness-of-ft on F2
Final R indexes [I≥2σ (I)]
Final R indexes [all data]
Largest dif. peak/hole (e Å−3
6593 [Rint =0.0774, Rsigma =0.0761]
6593/0/381
1.032
R1=0.0450, wR2=0.0879
R1=0.0802, wR2=0.1001
0.89/− 0.93
)
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Journal of Chemical Crystallography
Fig. 2 Diagram illustrating the
two diferent rotamers present
in 1. Atom colors: fuorine
(green), bromine (maroon),
oxygen (red), nitrogen (blue),
and carbon (grey) (Color fgure
online)
Fig. 3 Crystal structure of
the reported XOTCAM [36]
emphasizing the twisted
dihedral angle. Atom colors:
fuorine (green), oxygen (red),
nitrogen (blue), carbon (grey),
and hydrogen (white) (Color
fgure online)
Fig. 4 Diagram illustrating a the halogen bonding network of 1
viewed down [100] and b extended sheets interactions of1∥ molecules
viewed down [010]; 1⊥ interactions were omitted for clarity. Atom
colors: fuorine (yellow), bromine (brown), oxygen (red), nitrogen
(blue), carbon (grey), and hydrogen (white) (Color fgure online)
The compound AZ07 has similar halogen con-
tacts as the 1⊥/1∥ reported (Fig. 6b). The contacts of
1∥ with 1⊥ molecules are C–Br···Br–C type II halo-
gen bonds (Br1···Br2 = 3.630 Å, ∠C15–Br2···Br1 = 169°,
∠C1–Br1···Br2 = 97°). However, 1 is unique due to the non-
planar nature of 1⊥, causing the overall structure to have
3-dimensionality not found in the extended sheets of the
planar AZ07.
Photoisomerization
The photochromism of 1 in solution was monitored using
UV–vis absorption spectroscopy. The following three spec-
tra of a 23 μM solution of 1 in MeOH were obtained after
dissolving the compound, irradiating the solution with
365 nm light for 1 h, and fnally after irradiating the solu-
tion with 405 nm light for 1 h (Fig. 7). As expected for
1 3

Journal of Chemical Crystallography
isolated and readily determined for both isomers. The spec-
trum collected after irradiating the sample using 405 nm of
light for 1 h indicated that the solution contained 90% trans
isomer and 10% cis. After irradiating the sample for 1 h
using 365 nm of light, the relative concentration of the cis
isomer increased resulting in a solution containing a 70% to
30% ratio of trans and cis isomers, respectively.
To assess the degree to which the pure molecular crys-
tal was photochromic, the crystal of 1 which contains
only the trans isomer was irradiated with 365 nm of light
for a period of 1 h. A second crystal structure was then
obtained after irradiation. Unfortunately, although some-
what expected, there was no change in the difraction pat-
tern after irradiation. Subsequent analysis revealed no
new regions of electron density in the solved structure.
Fig. 5 Diagram illustrating the π···π interactions of 1⊥ molecules
viewed down [001]; 1∥ interactions were omitted for clarity. Atom
colors: fuorine (yellow), bromine (brown), oxygen (red), nitrogen
(blue), carbon (grey), and hydrogen (white) (Color fgure online)
o-fuoroazobenzenes, the absorption maxima of the cis and
trans isomers in the visible region show a separation of
about 20 nm.
1
The photoisomerization was also monitored using H-
1
NMR spectroscopy. Two H-NMR spectra were obtained
after irradiating the sample using the same irradiation condi-
tions mentioned above (Fig. 7). Although both spectra col-
lected showed the presence of both cis and trans isomers,
the bulk solution after irradiation with 405 nm light for 1
h indicated the majority of the mixture was the trans iso-
mer (Fig. 8a). This can be seen by comparing the signals at
3.95 ppm (a), 7.08 ppm (b), 7.39 ppm (c) and, 7.85 ppm (d)
with the signals of the cis isomer at 3.82 ppm (a′), 6.83 ppm
(b′), 7.12 ppm (c′), and 7.75 ppm (d′). After irradiating the
sample with 365 nm of light for 1 h, the increase in the
concentration of the cis isomer was observed (Fig. 8b). The
integrated intensities of the peaks located at 3.95 ppm (a)
and 3.82 ppm (a′), representing the methyl ester for the trans
and cis isomers, respectively were then used to calculated
relative fractions of the two isomers in the solutions. The
methyl ester peaks were selected due to them being well
Fig. 7 UV–vis spectra of a 23 μM solution of 1 in MeOH (triangle,
red line), after irradiating with 365 nm for 1 h (square, blue line) and,
405 nm for 1 h (circle, green line); inset is an expansion of the visible
absorption region (Color fgure online)
Fig. 6 Crystal structure highlighting the halogen···halogen interactions of a AZ07 [39] and b 1. Atom colors: fuorine (green), bromine (maroon),
oxygen (red), nitrogen (blue), carbon (grey), and hydrogen (white) (Color fgure online)
1 3

Journal of Chemical Crystallography
Fig. 8 1H-NMR (400 MHz,
CDCl3) spectrum of collected 1
after a 1 h of irradiation using
405 nm light and b after 1 h of
irradiation using 365 nm light
As is often the case for azobenzene-based systems, the
tight crystal packing prevent any large structural changes
needed for isomerization [40, 41] Using a probe radius of
1.2 Å and approximate grid spacing of 0.7 Å in Mercury,
resulted in zero calculated residual void space within the
crystal lattice 1 (Fig. 9). This is consistent with previ-
ously reported non-photochromic neat azobenzene crystals
which exhibit tightly packed crystalline lattices.
Conclusion
The successful synthesis and characterization of a novel
bromine-functionalized ortho-fuoroazobenzene photos-
witch will be of interest to researchers seeking to design
light-responsive halogen-bonded materials. The crys-
tal structure of 1 confrms the chemical structure of the
1 3

Journal of Chemical Crystallography
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isomer. Unfortunately, as with many azobenzenes, this
one included, the pure molecular crystals do not exhibit
photoisomerization. Although the crystal of 1 is not pho-
toactive, the synthesized molecule in solution exhibits the
same band separation in the visible light region between
the cis and trans isomers expected with o-fuoroazoben-
zenes. As photoisomerization was observed in solution,
it seems entirely possible that multi-component crystals
containing 1 could likewise exhibit photoactivity. There-
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