Anion-tunable control of thermal Z→E isomerisation in basic azobenzene receptors

Article abstract of DOI:10.1039/c4cc07798a

Herein, we report that thermal Z→E isomerisation of simple azobenzene urea derivatives is selectively and predictably controlled by anion binding. The rate of this process depends strictly on the anion concentration and its binding affinity to the Z-isomer of the azobenzene host, i.e. increased rate constants are observed for higher anion concentration as well as for more strongly bound guests. The origin of this phenomenon is attributed to the electron density transfer from the anion to the host π-system, resulting in increased repulsion between the lone electron pairs in the NN bond.

Full text of DOI:10.1039/c4cc07798a

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Anion-tunable control of thermal Z→E isomerisation
in basic azobenzene receptors
Cite this: DOI: 10.1039/x0xx00000x
Kajetan Dąbrowa,a Patryk Niedbała,a and Janusz Jurczak*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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Herein, we report that thermal Z→E isomerisation of simple density of the N=N bond, result in considerably long-lived AB
azobenzene urea derivatives is selectively and predictably derivatives.10 Analogically, the reverse situation should facilitate
controlled by anion binding. The rate of this process depends the rate of thermal back-isomerization by increasing the
strictly on the anion concentration and its binding affinity to repulsion between lone electron pairs in the azo bond. Based on
the
constants are observed for higher anion concentration as well anionic species could be utilized here since an increase of the
as for more strongly bound guests. The origin of this electron density in the -conjugated host system is observed
phenomenon is attributed to electron density transfer from upon anion binding.12,14 Furthermore, although recognition and
the anion to the host -system, resulting in increased transport of anions is one of the most important areas of
Z
-isomer of the azobenzene host, i.e. increased rate our activity in anion coordination chemistry,11-13 we assumed that
π
π
repulsion between the lone electron pairs in the N=N bond.
supramolecular chemistry,15 so far a limited amount of work has
been done.16 Therefore, we decided to synthesize and examine
the thermodynamic and kinetic properties of para- substituted
mono- (1a) and diphenylurea (1b) AB derivatives as model hosts
Light induced control of the geometry of molecules and the
spatial arrangement of their assemblies is of fundamental
importance in material and medicinal sciences. This arises from
the fact that, contrary to other external stimuli, light possesses an
electrically neutral character and ensures that the resulting
transformation is rapid, strictly localized in space, and generally
reversible.1 The photoactive moieties that are often applied in
such transformations include diaryl- and dithienylethene,
spiropyrane, and azobenzene (AB) derivatives.1-3 Among these,
the latter appear to be the most useful owing to their synthetic
availability and robustness.4-6 In general, upon irradiation of ABs
with UV or visible light, photostationary equilibrium is established
between the near planar E-isomer and the more compact V-
shaped Z-isomer. However, when the light is off, the Z-isomer
spontaneously reequilibrates to the thermodynamically more
stable E-isomer. For some practical applications control of this
back process is desirable since this precludes the use of a
second light stimulus for regenerating the initial state of the
sample.7-9 One unusual option for this control comes from the
covalent introduction of a strong electron withdrawing fluorine
substituents to the AB ring, which, by decreasing the electron
for basic studies of their anion controlled Z→E isomerization. We
utilize the urea group as primary binding site, due to its excellent
affinity to oxo-anions,13,17,18 Preliminary quantum calculations
confirm our assumption that a slight elongation of the N=N bond
and a more negative electrostatic potential surface (EPS), which
describes increased electron density, is observed for the anion
complex than for free receptor Z-1 (a representative complex of
Z-1b with benzoate is shown in Fig. 1).
Fig. 1 Energy minimized structures (DFT/ωb97x-d/6-31G+/DMSO) of the Z-1b (a)
-
and Z-1b⊃PhCO2 (b) along with electrostatic potential surfaces (EPS) mapped
onto electron density (0.001 au); values in the legend are given in kJ·mol-1.
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The E-
1
hosts were readily synthesized from commercially wavelengths in comparision to the parent AB. These results are
and whereas Z-
hosts could be in line with the reported data for para-substituted ABs,22,23,25 but
available azo-precursors
2
3
1
obtained by irradiating the samples of E-
1
with UV light (Scheme explaining why any substituent in this position facilitates the
thermal isomerisation is not always clear.22 In our model system,
1).
this phenomenon can be rationalized in a simple way, in terms of
N
N
368 nm
O
extended
π-electron conjugation caused by the strong positive
R
N
N
PhNCO
R
N
Ph
NH2
N
N
N
mesomeric effect of urea groups, as pointed out by Woolley.26
ꢀT (dark)
or 410 nm
H
H
R
O
HN
This is supported by molecular orbital calculations which predict
HN
2
3
1a
1b
1a
(R=H)
E-
E-
R: H
Z-
R: PhNHCONH
1b
Z-
(R=NH2)
that for Z-
orbitals due to mixing with
In the next step, we studied the thermal kinetics of Z-
1
energy of the n orbitals is much higher than for the
orbitals of the phenylurea moiety.
in the
π
Scheme 1 Synthesis of receptors E-1 and their isomerization to Z-1.
π
1
The E- and Z-isomers of 1 exhibit distinct spectral properties and
the two isosbestic points in the UV-VIS spectra indicate that only
one process is present during both photo, as well as thermal
presence of a constant quantity of representative anionic guests,
i.e.: spherical halogens (F-, Br-), Y-shaped carboxylates (MeCO2-,
-
PhCO2 ),
and
tetrahedral
bisulfate
(HSO4-)
and
Z→
E isomerisation (Fig. 2a).19 In addition, after several cycles of
-
dihydrogenphospate (H2PO4 ), added as tetra-n-butylammonium
(TBA) salts. The results of these measurements are given in
Table 2 and they clearly demonstrate that even a small quantity
of an anion has a tremendous impact on accelerating the rate of
alternate irradiation with UV and visible light, receptors 1 still
showed very good recoverability with no photodegradation
products (Fig. 2b).
thermal Z→E isomerisation. Moreover, effect of the anion added
is simply hindered by the addition of TFA, and restored again by
the addition of Et3N.27
Tab. 2 Spectral and kinetic data for thermal Z→E isomerization of
complexes of Zꢁ1 with 10 equiv of anions at 298±0.1K[a]
ꢁ
ꢁ
ꢁ [b]
ꢁ
Host
Value
Fꢁ
15
19
6
MeCO2
PhCO2
HSO4
H2PO4
17
1a
18
31
4
10
8
ꢀλπ * (nm)
ꢁπ
k
ꢀ·10ꢁ4 (s)
3.9
3.2
37
1
4.9
24
Fig. 2 (a) Thermal isomerization of Z-1b to E-1b; (b) photoswitching between E-
1b and Z-1b isomers using UV (368 nm) and blue light (410 nm).
τ1/2 (min)
30
8 (19)[c]
4.6 (14)[c]
25 (8)[c]
6
1b
4
14
14
8
ꢀλπ * (nm)
ꢁπ
kꢀ·10ꢁ4 (s)
1/2 (min)
7.6
15
1.4
8
4.1
28
Subsequently, we determined the first-order rates of thermal
τ
isomerization for the Z-
1 hosts by monitoring the absorption
[a] Anions added as TBA salts, ꢀλπ−π* denotes the shift in maximum absorption
between free Z- and its anion complexes; [b] 200 and [c] 555 equiv of anion,
respectively, were added.
1
change in the corresponding Z- and E-isomers in darkness and
non-linear fitting of the experimental data using the Marquard
technique.20 In all cases, practically linear Arrhenius and Eyring
plots indicate that only one mechanism occurs, namely via
inversion of one of the sp-hybrydised nitrogen atoms of the azo
bond.21 Note that our measurements were conducted in a highly
polar solvent mixture, i.e. DMSO + 0.5% H2O, which is known to
significantly accelerate thermal isomerization of ABs.4,22,23
The effect of the added anions is much more pronounced for
mono-substituted Z-1a than for Z-1b; this is attributed to the
extended delocalisation of the negative charge from anion over
the second phenylurea moiety in the latter case. In addition,
those anions which most accelerate reaction rates also induced
the highest red-shift of the Z-1 host π-π* bands which is
Fortunately, the half-life of pure Z-
1 isomers was sufficiently
supported by quantum chemical DFT calculations. These effects
depend on the type of anion added and its concentration, and
large24 for subsequent studies with anions. Table 1 shows the
relevant spectral, kinetic, and thermodynamic data for these
photoswitchable hosts.
may be rationalized by the binding affinity of the Z-
1 hosts to
anions. This affinity primarily depends on two factors: anion
-
-
-
-
basicity (F- > MeCO2 > PhCO2 ~ H2PO4 > HSO4 >> Br-) and the
geometric match between the anion and binding groups of Z-
Therefore, the highest rate constant enhancement and largest
red shifts of the * band are observed for the acetate anion,
Tab. 1 Spectra, kinetic, and thermodynamic data for thermal
isomerization of azobenzene (AB) and hosts
at 298±0.1K[a]
Z→
E
1
1
.
τ1/2
(h)
λE
(nm)
λZ
(nm)
Host
Ea
ꢀ
H‡
TꢀS‡
ꢀ
G‡
π-π
AB
1a
321
372
392
~290
337
213.6
13.1
1.8
103
92
100
89
ꢁ7.8
ꢁ11
ꢁ10
107
101
96
which is highly basic and has geometry complementary to the
urea group18,28 (209 and 13 for Z-1a and Z-1b, respectively),
whereas the bulky and weakly basic Br- anion only influences kꢀ
marginally.In continuation, we decided to investigate the exact
1b
336
88
86
[a] λE and λZ denotes maximum absorption transition (π→π*) for the E- and Z-
isomer, respectively; values for Ea, ΔH‡, TΔS‡, and ΔG‡ are given in kJ·mol-1.
influence of the anion concentration on the thermal Z
→E
The data presented in Table 1 reveal that introducing the urea
isomerisation of Z- in detail. Assuming that the anion modifies
1
group to the AB scaffold significantly decreases the stability of Z-
the thermal rate constant according to the binding affinity of the
AB derivatives used, and that the equilibria shown in Scheme 2
1
and shift the π-π* (S2) bands of both E-1 and Z-1 toward higher
2 | J. Name., 2012, 00, 1-3
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are valid, we can conclude that the observed kinetic rate of the host concentration, and prior determination of the
constant describing Z isomerisation depends on three association constant for the E-isomer is not necessary.
parameters: namely the association constant of the Z-isomer Interestingly, Z-1a binds benzoate 4.5 times stronger than Z-1b
with the anion (Ka,Z), and two first-order rate constants for free Z- The explanation of such phenomena appears to be nontrivial,30
(k1) and its saturated complex with the corresponding anion and we plan to investigate this issue in detail in the future. On the
→
E
.
1
(k2).
other hand, as expected, higher rate constant enhancement
(k2/k1) is observed for Z-1a than for Z-1b, which corresponds with
the former‘s lower ability to diffuse electronic density (vide
supra). Notably, these findings are fully consistent with the early
work of Shinkai who has shown that thermal isomerization of
azacrown ethers is hampered by certain cations.31 Although
corresponding binding constants with Z-isomer were not
reported, we can conclude that the most strongly bound cations
with a large ionic radius (Rb+ and Cs+) caused the highest
inhibition of this process. Furthermore, to our knowledge, there
are no contraindications to using our procedure in this case.
k1
Z-H + G
E-H + G
Ka1,E
Ka,Z
Ka2,E
k2
E−HG2
E−HG
Z-HG
Scheme 2 Equilibria describing anion complexation and Z→E thermal
isomerization in solution, H – host, G – guest.
After solving the equations describing the above equilibria we
arrive at eqn (1), which affords the concentration of the complex
of Z-H with an anion at any given time, and eqn (2), which
describes the dependence of the observed first-rate constant on
Finally, for the purpose of validating this method we determined
1
the affinities of hosts
1
to benzoate anions using the H NMR
the initial concentration of the guest (assuming that [G]0
≅ [G]).
titration technique in DMSO + 0.5% H2O, which provides very
consistent results (Ka = 1130 and 230 M-1 for Z-1a and Z-1b
κt
,
[
Z − HG
]
=
[
Z − HG
0 e−
]
t
(1)
(2)
respectively). In addition, 1H NMR titrations reveal a lack of
selectivity between Z-1a and E-1a (Ka = 1290 M-1), which is in full
accordance with DFT calculations. Interestingly, for E-1b the first
k1 + k2Ka,Z
[
G 0
]
κobs ≡ fHk1+ fHGk2 =
1+ Ka,Z
[
G 0
]
binding constant (Ka,1 = 990 M-1) is 3 times higher than for Z-1b
,
And so, the parameters of interest from eqn (2) (Ka,Z and k2)
could be determined using a nonlinear regression technique with
experimentally determined rate constants for various quantities
of added guest.29 The representative results of this approach for
which could partially be attributed to repulsion between the
phenyl rings of Z-1b and the benzoate.
In conclusion, we obtained two photoswitchable AB derivatives
1
which readily undergo photo and thermal isomerization. E and Z
isomers possess different spatial arrangements of binding
pockets, which affect their binding properties. In our studies, the
more compact Z-1b bound anions more weakly and in a 1:1
fashion, whereas the corresponding E-1b bound them more
strongly and in a 1:2 mode. It seems evident that varying the
anions and their concentration can be used for precise control of
complexes of Z-1 with a benzoate anion are shown in Figure 3.
the thermal Z→E isomerization rates and these changes could
be easily hindered by the simple addition of an acid, and restored
again by the addition of the base. This approach would seem to
be important for biological applications, as has been shown in
several reports devoted to AB photoswitches.32-34 Finally, we
show that kinetic data could be used as a simple and accurate
method for determining both the association constant for the Z-
isomer with an ionic guest and the maximum rate constant
change for the thermal Z
→
E isomerisation. The influence of
, on
variously substituted azobenzene core receptors, based on
1
anion recognition and E-Z isomerization is currently under study
in our laboratory.
Fig.
3 Change in the observed first-order rate constant of thermal Z→E
isomerization of receptors Z-1a (red diamonds) and Z-1b (blue diamonds) upon
addition of TBA benzoate determined under isothermal conditions (298.0±0.1 K)
by UV-Vis spectroscopy, errors estimated to be less than 5%.
This work was supported by the Foundation for Polish Science
(VENTURES/2010-5/1) co-financed by the EU European
Regional Development Fund. We would also like to acknowledge
Prof. J. Waluk, Dr. F. Ulatowski and J. M. Granda for fruitful
discussions.
Evaluation of the data indicates that the dependence between
guest added and observed rate constant is accurately described
by non-linear fitting using Eq. 2 and provides precise values for
Notes and references
both association constants Ka for Z-1 and rates of back-
a
Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka
isomerization for the saturated anionic complex. Unlike routinely
44/52, Warsaw 01ꢁ224 (Poland), Eꢁmail: janusz.jurczak@icho.edu.pl.
1
used titration under H NMR control, this method is independent
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DOI: 10.1039/C4CC07798A
†
Electronic Supplementary Information (ESI) available: Synthesis and 27. Control experiments with H3PO4 and Et3N reveal that pH has
characterization of hosts
photoisomerisation, and binding studies; Cartesian coordinates and
visualization of MOs of the compounds studied, see
DOI: 10.1039/c000000x/
1
,
details of quantum calculations,
virtually no effect on the thermal ZꢁE isomerisation of Zꢁ1, which is
common for pushꢁpull ABs (see A. M. Sanchez and R. H. Rossi, J.
Org. Chem., 1995, 60, 2974ꢁ2976).
28. B. P. Hay,T. K. Firman and B. A. Moyer, J. Am. Chem. Soc., 2005,
127, 1810ꢁ1819.
1. H. Dürr and H. BouasꢁLaurent, Photochromism: Molecules and 29. C. Bohne, Chem. Soc. Rev., 2014, 43, 4037ꢁ4050.
Systems: Molecules and Systems, Elsevier Science, 2003.
2. W. Szymański,J. M. Beierle,H. A. V. Kistemaker,W. A. Velema and
B. L. Feringa, Chem. Rev., 2013, 113, 6114ꢁ6178.
30. The reports available in the literature on photoswitchable anion
receptors are inconclusive in this regard, see ref 16.
31. S. Shinkai,T. Nakaji,T. Ogawa,K. Shigematsu and O. Manabe, J. Am.
Chem. Soc., 1981, 103, 111ꢁ115.
3. C. Brieke,F. Rohrbach,A. Gottschalk,G. Mayer and A. Heckel,
Angew. Chem., Int. Ed., 2012, 51, 8446ꢁ8476.
32. M. R. Banghart,M. Volgraf and D. Trauner, Biochemistry, 2006, 45
,
4. H. M. D. Bandara and S. C. Burdette, Chem. Soc. Rev., 2012, 41
,
15129ꢁ15141.
1809ꢁ1825.
33. P. Gorostiza,M. Volgraf,R. Numano,S. Szobota,D. Trauner and E. Y.
Isacoff, Proc. Natl. Acad. Sci. USA, 2007, 104, 10865ꢁ10870.
34. A. A. Beharry,L. Wong,V. Tropepe and G. A. Woolley, Angew.
Chem., Int. Ed., 2011, 50, 1325ꢁ1327.
5. A. A. Beharry and G. A. Woolley, Chem. Soc. Rev., 2011, 40, 4422ꢁ
4437.
6. E. Merino, Chem. Soc. Rev., 2011, 40, 3835ꢁ3853.
7. J. GarciaꢁAmorós,M. DíazꢁLobo,S. Nonell and D. Velasco, Angew.
Chem., Int. Ed., 2012, 51, 12820ꢁ12823.
8. J. A. Ihalainen,J. Bredenbeck,R. Pfister,J. Helbing,L. Chi,I. H. M.
van Stokkum,G. A. Woolley and P. Hamm, Proc. Natl. Acad. Sci.
USA, 2007, 104, 5383ꢁ5388.
9. J.ꢁH. Ha and S. N. Loh, Chem. Eur. J., 2012, 18, 7984ꢁ7999.
10. D. Bléger,J. Schwarz,A. M. Brouwer and S. Hecht, J. Am. Chem.
Soc., 2012, 134, 20597ꢁ20600.
11. J. M. Granda and J. Jurczak, Chem. Eur. J., 2014, 20, 12368ꢁ12372.
12. K. Dąbrowa,M. Pawlak,P. Duszewski and J. Jurczak, Org. Lett.
,
,
2012, 14, 6298ꢁ6301.
13. P. Dydio,D. Lichosyt and J. Jurczak, Chem. Soc. Rev., 2011, 40
2971ꢁ2985.
14. D. E. Gomez,L. Fabbrizzi,M. Licchelli and E. Monzani, Org. Biomol.
Chem., 2005, , 1495ꢁ1500.
3
15. P. A. Gale,N. Busschaert,C. J. E. Haynes,L. E. Karagiannidis and I.
L. Kirby, Chem. Soc. Rev., 2014, 43, 205ꢁ241.
16. For recent example, see: K.ꢁS. Jeong,Y. R. Choi,G. C. Kim,H.ꢁG.
Jeon,J. Park and W. Namkung, Chem. Commun., 2014,
doi:10.1039/C4CC07560A, for the recent review, see: S. Lee and A.
H. Flood, J. Phys. Org. Chem., 2013, 26, 79ꢁ86.
17. V. Amendola,L. Fabbrizzi and L. Mosca, Chem. Soc. Rev., 2010, 39
,
3889ꢁ3915.
18. A.ꢁF. Li,J.ꢁH. Wang,F. Wang and Y.ꢁB. Jiang, Chem. Soc. Rev.
2010, 39, 3729ꢁ3745.
,
19. In all UVꢁVis experiments we used 5•10ꢁ5 M concentration of the
host 1 and DMSO+0.5% water as a solvent mixture.
20. L. M. Schwartz and R. I. Gelb, Anal. Chem., 1978, 50, 1592ꢁ1594.
21. T. Asano and T. Okada, J. Org. Chem., 1986, 51, 4454ꢁ4458.
22. J. Dokic,M. Gothe,J. Wirth,M. V. Peters,J. Schwarz,S. Hecht and P.
Saalfrank, J. Phys. Chem. A, 2009, 113, 6763ꢁ6773.
23. N. Nishimura,T. Sueyoshi,H. Yamanaka,E. Imai,S. Yamamoto and S.
Hasegawa, Bull. Chem. Soc. Jpn., 1976, 49, 1381ꢁ1387.
24. In comparison, structurally similar ureaꢁbased host reported in ref 26
backꢁisomerize in the matter of seconds in the polar environment.
25. A. A. Blevins and G. J. Blanchard, J. Phys. Chem. B., 2004, 108
,
4962ꢁ4968.
26. O. Sadovski,A. A. Beharry,F. Zhang and G. A. Woolley, Angew.
Chem., Int. Ed., 2009, 48, 1484ꢁ1486.
4 | J. Name., 2012, 00, 1-3
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