Fluorinated azobenzenes as supramolecular halogen-bonding building blocks

Article abstract of DOI:10.3762/bjoc.15.197

ortho-Fluoroazobenzenes are a remarkable example of bistable photoswitches, addressable by visible light. Symmetrical, highly fluorinated azobenzenes bearing an iodine substituent in para-position were shown to be suitable supramolecular building blocks both in solution and in the solid state in combination with neutral halogen bonding acceptors, such as lutidines. Therefore, we investigate the photochemistry of a series of azobenzene photoswitches. Upon introduction of iodoethynyl groups, the halogen bonding donor properties are significantly strengthened in solution. However, the bathochromic shift of the π→π* band leads to a partial overlap with the n→π* band, making it slightly more difficult to address. The introduction of iodine substituents is furthermore accompanied with a diminishing thermal half-life. A series of three azobenzenes with different halogen bonding donor properties are discussed in relation to their changing photophysical properties, rationalized by DFT calculations.

Full text of DOI:10.3762/bjoc.15.197

Fluorinated azobenzenes as supramolecular halogen-bonding
building blocks
Esther Nieland1, Oliver Weingart*2 and Bernd M. Schmidt*1
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 2013–2019.
1Institut für Organische Chemie und Makromolekulare Chemie,
Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225
Düsseldorf, Germany and 2Institut für Theoretische Chemie und
Computerchemie, Heinrich-Heine-Universität Düsseldorf,
Universitätsstraße 1, D-40225 Düsseldorf, Germany
doi:10.3762/bjoc.15.197
Received: 01 July 2019
Accepted: 16 August 2019
Published: 23 August 2019
Email:
This article is part of the thematic issue "Molecular switches".
Guest Editor: W. Szymanski
Oliver Weingart* - Oliver.Weingart@hhu.de; Bernd M. Schmidt* -
Bernd.Schmidt@hhu.de
* Corresponding author
© 2019 Nieland et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
azobenzene; DFT calculations; fluorine chemistry; halogen bonding;
photochemistry
Abstract
ortho-Fluoroazobenzenes are a remarkable example of bistable photoswitches, addressable by visible light. Symmetrical, highly
fluorinated azobenzenes bearing an iodine substituent in para-position were shown to be suitable supramolecular building blocks
both in solution and in the solid state in combination with neutral halogen bonding acceptors, such as lutidines. Therefore, we in-
vestigate the photochemistry of a series of azobenzene photoswitches. Upon introduction of iodoethynyl groups, the halogen bond-
ing donor properties are significantly strengthened in solution. However, the bathochromic shift of the π→π* band leads to a partial
overlap with the n→π* band, making it slightly more difficult to address. The introduction of iodine substituents is furthermore
accompanied with a diminishing thermal half-life. A series of three azobenzenes with different halogen bonding donor properties
are discussed in relation to their changing photophysical properties, rationalized by DFT calculations.
Introduction
The halogen bond is an attractive noncovalent interaction be- philic (halogen bond acceptor) and I2 the electrophilic (halogen
tween a polarized halogen atom (the halogen bond donor) and a bond donor) subcomponent [3-7]. Neutral halogen bonds on the
Lewis base (the halogen bond acceptor) [1,2]. A prominent ex- other hand can be generally described by R–X···Y, where R–X
ample regarding the origin of halogen bonding can be found in is the halogen bond donor, R is covalently bound to X, and Y is
inorganic solid-state chemistry. The structurally diverse group the Lewis basic halogen bond acceptor [1]. In recent years,
of polyiodides, with its rich structural chemistry is governed by halogen bonding was used to assemble molecules, leading to a
halogen bonding, where I− and I3− are considered the nucleo- variety of supramolecular architectures [8-19], as well as
2013

Beilstein J. Org. Chem. 2019, 15, 2013–2019.
discrete supermolecules [20-24]. Huber and co-workers demon- high thermal stability of the Z-isomer, now exhibiting thermal
strated the activation of a carbonyl group by halogen bonding, half-lives up to two years at room temperature [36]. Most im-
and successfully applied this concept to catalysts for Michael portant for application in supramolecular systems, it should be
addition reactions [25] and also employed neutral [26], and possible to study both states of the system on a laboratory
hypervalent iodolium derivatives as activators in a halide timescale, a key aspect in the design of our halogen bonded
abstraction reaction and as organocatalysts in Diels–Alder reac- boxes [35]. Therefore, we herein present a comprehensive in-
tions [27]. The group of Metrangolo also reported halogen vestigation of the photochemistry of highly fluorinated azoben-
bonding-promoted catalysis in water by exploiting a halogen zenes. Our efforts are supported by theoretical calculations,
bonding amino acid, which combines excellent donor proper- showing that these azobenzenes are suitable for the use as
ties with good water solubility [28], in addition to their seminal building blocks in supramolecular architectures.
contributions in the field of crystal engineering [1]. Only few
supramolecular capsules were reported so far [29], including the Results and Discussion
resorcin[4]arene capsules of Diederich and co-workers [21,23], Three different azobenzenes A1–3 were studied with regard to
triangular macrocycles assembled by self-complemented halogen bonding and photochemical properties (Scheme 1).
halogen bonding [20] and halogen bond templated, polyfluori-
nated stilbene squares used for topochemical polymerization In our experiments, tetrafluorinated A1 does not form halogen
[22]. Additionally, halonium ions [N···I+···N] were reported to bonded boxes with acceptor U1, neither in solution, nor in the
form several charged, discrete supramolecular capsules [30-33] solid state. Octafluorinated A2 forms [2 + 2] boxes in the solid
and helicates [34]. In the same line, we have demonstrated state and possibly in solution, whereas tetrafluoroiodoethynyl
recently that both E-4,4’-di(iodo)perfluoroazobenzene (A2) and A3 is as donor strong enough to reliably permit the characteri-
E-4,4’-di(iodoethynyl)perfluoroazobenzene (A3) halogen bond zation of the boxes formed in solution and in the solid state
donors can be combined with rigid u-shaped anthracene build- [35]. Electrostatic interaction plays a dominant role in halogen
ing blocks, bearing two 3,5-lutidine acceptors in 1 and 8 posi- bonding [1,12,13]. Therefore, we calculated the molecular elec-
tions, to form self-assembled boxes of 25–30 Å length in solu- trostatic potentials of the halogen bond donors A1–3 to visu-
tion and in the solid state [35].
alize their capabilities to form halogen bonded architectures
(Figure 1).
We chose azobenzene because azobenzene is one of the
simplest molecules that can undergo photoinduced isomerisa- Looking at the electrostatic surface potentials of the halogen
tion of its N=N central double bond. The photoisomerisation bond donors, one can see that A3 shows a maximum value on
reaction in n→π and π →π* excited states has been studied with the iodine atom that is most positive compared to that of A2 and
experimental [36-38] and theoretical approaches [39-44]. By A1. The evolution of the iodine potential follows our experi-
substituting azobenzenes in the ortho-positions to the N=N mental observation with iodoethynylazobenzene A3 being the
bond with electron-withdrawing fluorine substituents [45,46], strongest halogen bond donor and A1 being the weakest, within
the red-shifting of the n→π* transitions enables selective this series [48]. For potential, reversible photochemical control
addressing of both the E- and Z-isomer using visible light. of supramolecular assemblies, the halogen donors need to bind
Stabilization of the n-orbitals in the Z-isomers leads to a very both in the E- and Z-state. This is the case according to our
Scheme 1: a) Azobenzenes A1–3 employed in this study. b) U-shaped anthracene halogen bond acceptor bearing two 3,5-lutidines in 1 and 8 posi-
tions.
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Beilstein J. Org. Chem. 2019, 15, 2013–2019.
Figure 1: Electrostatic potential map at different isodensity values (B3LYP/ def2/TZVP/DGZVP optimized geometries) with a) ρ = 0.0001, and
b) ρ = 0.001. For visualization, the MoleCoolQt software was used [47].
computations, as isodensity values remain almost identical upon can be found in the Supporting Information File 1, Figures S1
switching (Figure S14 in Supporting Information File 1). We and S12). By introduction of fluorine atoms ortho to the azo
therefore turned our attention to elucidate the change of photo- bond, the two n→π* of E- and Z-state become sufficiently sepa-
chemical properties upon introducing the heavy iodine to the rated to address them individually using visible light sources.
azobenzene building block, as well as the effect of the ethynyl Along with averting UV light for the photochemical reaction,
group in case of A3 (Figure 2). Vertical electronic absorption high PSS ratios can be observed, which is very desirable for ap-
spectra of the different azobenzenes were calculated at the plication in supramolecular systems [12,13,35]. Tetra- and octa-
TD-B3LYP/def2-TZVP level of theory including Grimme D3 fluorinated A1 and A2 show clear spectral separation of the
dispersion correction, using the Gaussian 16 program package n→π* bands, whereas the extended π-system of iodoethynyl A3
(see Supporting Information File 1). The azobenzenes were em- lead to a bathochromic shift of the π→π* band by 24 nm, now
bedded in a continuum using the polarizable continuum model partially overlapping with the, also broadened, n→π* band of
(PCM) for the solvent MeCN. The DGZVP all electron basis Z-A3. Apart from the photoisomerisation using light, azoben-
was used for iodine. Vertical excitation energies for the π→π* zenes A1–3 also undergo thermal back reaction, which we
and n→π* transitions of E and Z-isomers are listed in the Table studied experimentally and theoretically. To gain insight into
S9 (Supporting Information File 1).
the effect of the iodine atoms on the thermal stability we inves-
tigated A1, A2 and A3 in MeCN at elevated temperature
The computational absorption spectra are in fair agreement with (60 °C), following the works of Hecht and co-workers [45,46].
the experimental ones (Table 1) and trends are reproduced ac- The data is presented in Table 2. Additionally, we studied A3 in
cordingly (measured and calculated absorption spectra of A1 a wide range of temperatures in MeCN.
2015

Beilstein J. Org. Chem. 2019, 15, 2013–2019.
Figure 2: Top: Vertical electronic absorption spectra of a) A2 and b) A3, calculated using TD-B3LYP/def2-TZVP level of theory with Grimme D3
dispersion corrections and implicit MeCN solvent. Pink line: Z-isomer, purple line: E-isomer. Bottom: E-state of c) A2 and d) A3 (from left to right,
purple) and photostationary state (PSS, pink) after photoirradiation with λirr = 565 nm, monitored by UV–vis (MeCN, c = 10.5 and 9.3 μmol/L, respec-
tively). The inlets of c) and d) were smoothed using the Savitzky–Golay filter implemented in OriginPro to facilitate readability.
Table 1: Spectroscopic properties in MeCN for E-state and PSS after photoirradiation with λirr = 565 nm. Maxima were determined using the “Peak
Analyzer” implemented in OriginPro. Kinetic measurements were performed in MeCN at 60 °C (Supporting Information File 1).
λmax(E)
[nm]
εmax(E)
[M−1 cm−1]
λmax(PSSZ)
εmax(PSSZ)
[M−1 cm−1]
τ1/2
[h]
[nm]
A1
A2
A3
335
340
364
3.48 × 104
2.84 × 104
4.41 × 104
241
239
359
1.68 × 104
1.38 × 104
2.38 × 104
44.92
17.17
0.92
The half-lives decrease from A1 to A3, an effect that correlates The bistable character is obviously weakened upon improving
with the increase in dipole moment of the transition state (TS, the halogen bonding properties. However, most importantly, the
see Supporting Information File 1, Table S8). In the B3LYP azobenzenes still can be conveniently handled at room tempera-
computations this value is larger than for the corresponding ture with a half-life of at least a working day, allowing for
Z-isomer and leads to a stabilization of the TS in polar solvents studying both states of the systems without needs for in situ ir-
[46].
radiation (the thermal half-life of A3 at room temperature is
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Beilstein J. Org. Chem. 2019, 15, 2013–2019.
Table 2: Activation process parameters for the E→Z isomerisation in MeCN at 60 °C, B3LYP, def-TZVP basis for C, H, N, F, DGZVP all electron
basis for iodine. Grimme D3 dispersion correction was applied. Values were computed using the KistHelp program [49] employing classical transition
state theory and including the effects of Wigner-tunnelling (Supporting Information File 1).
ΔU
[kJ mol−1]
ΔG
[kJ mol−1]
ΔH
[kJ mol−1]
ΔS
[J mol−1]
kZ-E
[s−1]
τ1/2
[h]
τ1/2 exp.
[h]
A1
A2
A3
124.10
118.86
113.13
114.32
108.73
99.89
119.06
114.49
108.69
14.22
17.28
26.42
9.46 × 10−6
7.06 × 10−6
1.70 × 10−6
20.35
2.73
0.11
44.92
17.17
0.92
14.98 hours in MeCN, see the Supporting Information File 1, anthracene U1 units, connected by C–H···F contacts. The
Figures S6–S8).
azobenzenes A2 interact by lamellar 2D π-stacking, anthracene
U1 interact predominantly by C–H···π interactions as both the
In addition to that, slow evaporation of an equimolar solution of solubilizing mesitylene group and the two perpendicular luti-
U1 and A2 in benzene furnished red-orange single crystals suit- dine acceptors effectively prevent stacking of the anthracenes
able for X-ray analysis of a [2 + 2] halogen-bonded box, body (Supporting Information File 1, Figure S17). This was
U1···A2, over the course of a few days in quantitative yield. also the key to being able to lower the temperature to charac-
Single-crystal analysis confirms the formation of a U1···A2 box terize formation in the 1H NMR, where the solubility of the
in the solid state (Figure 3).
assemblies in benzene was improved by adding a solubilizing
mesitylene group to the halogen bonding acceptor U1 to avoid
The U1···A2 box has a principal length of approximately 25 Å precipitation of box A2···U1 during previous titration experi-
(anthracene–anthrance distance) and a height of 5 Å (distance ments [35].
between the ipso-carbons of the lutidines). The lutidine acceptor
units are curved inwards (with N···I–C angles of 165 and 172°) Conclusion
and show N···I distances of 2.78 and 2.82 Å to the azobenzene The performed calculations show that both E- and Z-isomer are
donors. As observed for the other boxes assembled by halogen equally able to undergo halogen bonding. By improving the
bonding reported by us [35], parts containing fluorinated strength of halogen bonding going from tetrafluorinated A1 to
azobenzenes A2 are segregated from the perhydrogenated octafluorinated A2 to A3, especially by introducing the
Figure 3: a) Space-filling model of U1···A2. The kinked alignment of both the lutidine units of U1 and the azobenzenes A2 can be seen. b) Part of the
X-ray crystal structure showing the halogen bonding azobenzene A2 in detail. Selected bond lengths: N3–I1 2.7810(2), I2–N4 2.816(2) Å.
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Acknowledgements
We acknowledge the Fonds der Chemischen Industrie for a ma-
terial cost allowance grant (B.M.S) and the Strategic Research
Fund of Heinrich Heine University (F-2018/1460-4). E.N. is
supported by a Deutschlandstipendium. We thank Y.
Garmshausen for his advices on azobenzene photochemistry
and C. Czekelius for sharing analytical equipment. Computa-
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ORCID® iDs
Esther Nieland - https://orcid.org/0000-0001-5213-6303
Oliver Weingart - https://orcid.org/0000-0001-6033-3702
Bernd M. Schmidt - https://orcid.org/0000-0003-3622-8106
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