Photolariats: Synthesis, metal ion complexation and photochromism

Article abstract of DOI:10.1080/10610278.2012.678851

Photolariat development, as an extension to the family of synthetic photochromic crown ether receptors, or photocrowns, is reported. Incorporated additional chelating groups, namely two anisoles or thioanisoles, contribute in completing the metal ion coor

Full text of DOI:10.1080/10610278.2012.678851

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Photolariats: synthesis, metal ion complexation and
photochromism
Aurélien Ducrot a , Peter Verwilst a , Luca Scarpantonio a , Sébastien Goudet a , Brice
Kauffmann b , Sergey Denisov a c , Gediminas Jonusauskas c & Nathan D. McClenaghan a
a Institut des Sciences Moléculaires, University of Bordeaux/CNRS , 351 crs de la Libération,
Talence , 33405 , France
b Institut Européen de Chimie et Biologie, University of Bordeaux , 2 Rue Robert Escarpit,
Pessac , 33607 , France
c Laboratoire Ondes et Matière d'Aquitaine, University of Bordeaux/CNRS , 351 crs de la
Libération, Talence , 33405 , France
Published online: 27 Apr 2012.
To cite this article: Aurélien Ducrot , Peter Verwilst , Luca Scarpantonio , Sébastien Goudet , Brice Kauffmann , Sergey
Denisov , Gediminas Jonusauskas & Nathan D. McClenaghan (2012) Photolariats: synthesis, metal ion complexation and
photochromism, Supramolecular Chemistry, 24:7, 462-472, DOI: 10.1080/10610278.2012.678851
To link to this article: http://dx.doi.org/10.1080/10610278.2012.678851
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Supramolecular Chemistry
Vol. 24, No. 7, July 2012, 462–472
Photolariats: synthesis, metal ion complexation and photochromism
a
Aurelien Ducrot , Peter Verwilst , Luca Scarpantonio , Sebastien Goudet , Brice Kauffmann , Sergey Denisov ,
a
a
a
b
a,c
´
´
Gediminas Jonusauskasc and Nathan D. McClenaghana*
a
b
Institut des Sciences Moleculaires, University of Bordeaux/CNRS, 351 crs de la Liberation, Talence 33405, France; Institut Europeen
´
´
de Chimie et Biologie, University of Bordeaux, 2 Rue Robert Escarpit, Pessac 33607, France; Laboratoire Ondes et Matiere
´
c
`
´
d’Aquitaine, University of Bordeaux/CNRS, 351 crs de la Liberation, Talence 33405, France
(Received 30 January 2012; final version received 20 March 2012)
Photolariat development, as an extension to the family of synthetic photochromic crown ether receptors, or photocrowns, is
reported. Incorporated additional chelating groups, namely two anisoles or thioanisoles, contribute in completing the metal
ion coordination sphere with different affinities and selectivities for a range of ions. Single-crystal X-ray diffraction analysis
suggests that the thermally stable trans-form of the receptor is unsuitable for ion binding, consistent with spectrophotometric
and NMR titration results, which is largely improved in the cis-form as the basis for the photocontrolled ion
coordination/ejection. In terms of the azobenzene-containing receptor photochemistry, a photostationary state highly
enriched in the cis-form (94:6, cis-/trans-) is reached, with slow thermal return (1.1–1.4 £ 1025 s21) in the dark, which can
undergo multiple cycles without discernable photodegradation.
Keywords: photolariat; azobenzene; macrocycles; receptor; photochromism
1. Introduction
lowered binding constant. For the latter scenario, steric
effects come into play when there is a significant
geometrical change in the photoactive group between
each form of the photochrome, which varies the shape of
the receptor and hence suitability to bind a guest species.
A popular choice of photochrome is azobenzene, which
can be efficiently and reversibly switched from the
thermally stable elongated trans-form to the more compact
cis-form on UV irradiation. The return cis-to-trans
transformation can be achieved thermally or by visible
irradiation. In this way, different hosts such as cavitands
and capsules can be rendered photoactive and more
specifically the miniscule internal (yoctolitre) spaces
available for chemical encapsulation can be controlled,
which can ultimately lead to recognition, sequestration and
reactivity of small molecules, depending on the host (9, 10).
Among different manifestations of azobenzene in supra-
molecular hosts, one of the most successful implemen-
tations was the development of photocrowns where an
azobenzene unit was directly incorporated in the
macrocyclic host or in ‘butterfly’ variants (7, 11–15).
However, binding constants are typically reasonably low
and strategies to increase this parameter would be
anticipated to widen the scope of implementation of
these photoactive functional molecules. Concerning the
photoswitching and photochemistry of these macrocylic
azobenzene-containing hosts, the complexing ring imparts
differing levels of strain on the azobenzene link, which as a
A wealth of synthetic macrocyclic receptors have been
reported over the last 30–40 years, which have shown
varying affinities to a wealth of guests, notably metal ions
(1, 2). Among these receptors, stimulus-modulated species
where the binding affinity can be modified as a response to
a signal represents an expanding research area. This is due
to the scope of potential applications in biomimetic
nanotechnology as well as molecular biology, where
liberation (or uptake) of a chemical effector at a given time
is employed to ultimately allow elucidation of specific
biological pathways (3, 4). In this context, photoactive
receptors are particularly attractive as light energy can be
applied to autonomous molecules in solution with high
spatial and temporal resolution (5). Increasingly popular
two photon absorption techniques offer a further possibility
to irradiate with NIR light in the so-called therapeutic
window, allowing deeper penetration of excitation light to
activate deep lying molecules (6). Reversible photoinduced
ion release and uptake imply photochromic receptors,
where light switching between two distinct forms has a
direct influence on binding. Indeed, a wealth of
photochromes have been integrated with different recep-
tors to give access to photocontrolled binding of a range of
species (7, 8). The origin of the binding change is due to
either electronic or steric effects. In the former case, on
switching, electron density is less available to binding sites
in one form with respect to the other, with a concomitant
*Corresponding author. Email: n.mc-clenaghan@ism.u-bordeaux1.fr
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.678851
http://www.tandfonline.com

Supramolecular Chemistry
463
Figure 1. Photoswitching of prototype photolariats 1a (X ¼ O) and 1b (X ¼ S). E-isomers should weakly complex with cations, while
the Z-isomer is anticipated to be better adapted to complex.
function of size can drastically modify quantum yields,
activation energy of cis-to-trans interconversion, absorp-
tion band intensities, as well as complexation selectivities
(16–19).
q ¼ quartet, dd ¼ doublet of doublets and m ¼ multiplet.
Mass spectrometry: the mass spectra were recorded by the
Centre of Studyand StructuralAnalysis Organic Moleculesat
the University of Bordeaux, France. Electrospray ionisation
(ESI) mass spectra were performed on a QStar Elite mass
spectrometer (Applied Biosystems). The instrument is
equipped with an ESI source and spectra were recorded in
the positive mode. The electrospray needle was maintained at
5000 V and operated at room temperature. Samples were
introduced by injection through a 20 mL sample loop into a
4500 mL/min flow of methanol from the LC pump. MALDI-
MS spectra were performed on a Voyager mass spectrometer
(Applied Biosystems). The instrument is equipped with a
pulsed N2 laser (337 nm) and a time-delayed extracted ion
source. Spectra were recorded in the positive-ion mode using
the reflectron and with an accelerating voltage of 20 kV. A
dithranol matrix solution was employed. GC/MS mass
spectra were performed on a Trace GC 2000 gas
chromatograph equipped with a RTX5 MS column
(length ¼ 15 m; diameter ¼ 0.25 mm; film ¼ 0.25 mm),
coupled to a Thermofinnigan Trace electron impact (EI)
mass spectrometer. Samples were measured using a source
temperature of 2008C and a 150mA ionisation current.
Electronicabsorptionspectroscopy: theelectronicabsorption
spectra were recorded on optically dilute solutions in 1 cm
pathlength quartz cells, whose temperaturewas thermostated,
using a double beam Varian Cary 5000 UV–vis-NIR
spectrophotometer. Photoirradiation systems: irradiations at
365 nm were performed with a portable Fisher Bioblock
mercury lamp (type thin layer chromatography (TLC)) with
a power of 2 £ 6 Watts. Isomerisation quantum yields were
determined using stirred standard solutions irradiating at 334
or 365 nm with a mercury–xenon lamp equipped with a
monochromator. A ferrioxalate actinometer was employed
following the Hatchard and Parker method (22). Photo-
isomerisation (trans-to-cis) of 1a and 1b was performed at
365nm while parent azobenzene isomerisation was achieved
at 334 nm because of the differences in the electronic
As part of our ongoing work in the area of
photocontrolled receptors and functional molecules and
assemblies, we report herein the synthesis, photochemistry
and complexation properties of lariat ethers, which
incorporate a photoswitching azobenzene unit, deemed
photolariats. The structural formulae of two members of
this family are shown in Figure 1. In addition to the
complexing heteroatom set of the macrocycle, two
additional complexing atoms, oxygen and sulphur (1a and
1b, respectively), are added. Para- as opposed to ortho-
substitution of the azobenzene with respect to the central
nitrogens may be anticipated to amplify the geometrical
difference between cis-form and trans-form, consequently
changing the binding properties and photochromism.
2. Experimental
2.1. Materials and methods
Commercially available starting materials were obtained
from Aldrich, AlfaAesar, Lancaster or Avocado. Compounds
2 (20) and 4 (21) were synthesised according to literature
procedures. Anhydrous absolute ethanol and dimethylforma-
mide (DMF) solvents were used as received. Dichloro-
methane and pyridine were distilled over calcium hydride
(CaH2) immediately before use. Tetrahydrofuran (THF) was
dried over sodium/benzophenone and distilled immediately
before use. Nuclear magnetic resonance (NMR): the 1H and
13C NMR spectra were recorded on a Bruker DPX200 (1H:
200 MHz) or a Bruker Avance 300 (1H: 300 MHz; 13C:
75 MHz) spectrometer. Chemical shifts (d) are expressed
relative to tetramethylsilane (TMS) using the residual signals
of the deuterated solvents (CDCl3, d6-DMSO) as an internal
reference. The coupling constants are calculated in Hertz
(Hz). For the assignment of signals, the following
abbreviations are used: s ¼ singlet, d ¼ doublet, t ¼ triplet,

464
A. Ducrot et al.
absorption spectra. The appropriate actinometer quantum
yield valuewas used in each case and correction was made for
incomplete lightabsorption (22). Allirradiation samples were
air-equilibrated.
rotating anode at the Cu Ka wavelength. The CrystalClear
(Rigaku/MSC Inc., 2006) suite was used for cell refinement
anddatareduction;SUPERFLIP(Palatinus, 2007)solvedthe
structure that has been refined using SHELXL97 (Sheldrick,
1997); The WinGX (Farrugia, 1999) software was used to
prepare material for publication. It has to be noted that it was
not possible to collect better data on these tiny and X-ray
sensitive crystals.
Single-crystal X-ray diffraction analysis: 1a: summary
of data CCDC 864623: formula: C44H50N4O8; unit cell
parameters: a ¼ 14.5845 (5), b ¼ 11.8355 (4),
c ¼ 22.7467 (8), b ¼ 97.576 (2); space group C2/c; crystal
data: (C44H50N4O8), M ¼ 762.88, T ¼ 213(2) K, mono-
clinic, space group C2/c, a ¼ 14.5845 (5), b ¼ 11.8355
2.2. Synthesis procedures
(4), c ¼ 22.7467 (8) A, b ¼ 97.576 (2), V ¼ 3892.1
3
(2)A , Dc ¼ 1.302 g cm23, Z ¼ 4, F(000) ¼ 1624, 17,692
Structural formulae of molecules 1a–9b are shown in
Figure 2.
˚
reflections measured, 2195 independent reflections
(Rint ¼ 0.049), refinement on F 2 against all reflections.
The weighted R-factor wR and goodness of fit GOF are
based on F 2, GOF ¼ 1.09, R[F 2 . 2s(F 2)] ¼ 0.085,
wR(F 2) ¼ 0.259. Hydrogen site location was inferred
from neighbouring sites, H-atom parameters constrained,
2195 reflections used for refinement, 254 parameters and
no restraint. The data collection was performed on a Bruker
microstar rotating anode at the Cu Ka wavelength; cell
refinement and data reduction were computed with the
Bruker suite X8 Proteum; SHELXD97 (Sheldrick, 2008)
was used to solve the crystal structure and SHELXL97
(Sheldrick, 1997) for refinement.
3: To a solution of 2 (657 mg, 2.92 mmol) in DCM
(70 mL) were added thionyl chloride (1.52 mL, 20.9 mmol)
and triethylamine (0.42 mL, 3.0 mmol) at 08C. The mixture
was refluxed for 2 h. The reaction mixture was concentrated
in vacuo and was used in coupling reactions with 6a or 6b
without further purification.
5a: A mixture of 4 (2.00 g, 8.85 mmol), thionyl
chloride (8.8 mL) and triethylamine (1.8 mL) in DCM
(300 mL) was refluxed for 3 h. The solvent and excess
thionyl chloride were removed in vacuo, yielding the
diacid chloride as a light brown solid. To a well-stirred
solution of this diacid chloride (8.85 mmol) in DCM
(150 mL), a solution of o-anisidine (2.03 mL, 18 mmol)
and triethylamine (2.47 mL, 17.7 mmol) in DCM (150 mL)
was added dropwise. The reaction mixture was stirred at
room temperature for 6 h and was then washed with water
(3 £ 100 mL) to remove the triethylammonium chloride
and dried over MgSO4. The solvent was removed in vacuo
and the residue was purified by column chromatography
(silica, pentane/ethyl acetate, 7/3, v/v). The obtained
product was further purified by recrystallisation from
5a: Summary of data CCDC 864624: crystal data:
(C24H24N2O6), M ¼ 436.45, T ¼ 213 (2) K, orthorhombic,
space group Pcca, a ¼ 16.4963 (8), b ¼ 17.9052 (9),
3
23
˚
c ¼ 7.1865 (4) A, V ¼ 2122.67 (19) A , Dc ¼ 1.366gcm
,
Z ¼ 4, F(000) ¼ 920, 10,641 reflections measured, 949
independent reflections (Rint ¼ 0.035), refinement on F 2
against all reflections. Theweighted R-factor wR and goodness
of fit GOF are based on F 2, GOF ¼ 1.33,
R[F 2 . 2s(F 2)] ¼ 0.04, wR(F 2) ¼ 0.115. Hydrogen site
location was inferred from neighbouring sites, H-atom
parameters constrained, 949 reflections used for refinement,
147 parameters and no restraint. The data collection
has been performed on a Bruker microstar rotating anode
at the Cu Ka wavelength; cell refinement and data reduction
were computed with the Bruker suite X8 Proteum;
SHELXD97 (Sheldrick, 2008) was used to solve the crystal
structure and SHELXL97 (Sheldrick, 1997) for refinement.
5b: Summary of data CCDC 864625; crystal data:
(C24H24N2O4S2), M ¼ 468.57, T ¼ 213 (2) K, triclinic,
space group P-1, a ¼ 4.702 (6), b ¼ 13.869 (5), c ¼ 18.607
1
methanol, yielding 5a (2.37 g, 61%). H NMR (CDCl3,
300 MHz, TMS): d 9.21 (s, 2H, NH); 8.43 (d, J ¼ 7.9 Hz,
2H, CHAr); 7.00 (m, 8H, CHAr); 6.59 (d, J ¼ 7.9 Hz, 2H,
CHAr); 4.72 (s, 4H, CH2); 3.46 (s, 6H, CH3) ppm. 13C
NMR (CDCl3, 75 MHz, TMS): d 165.9; 148.3; 147.2;
126.8; 124.5; 123.0; 120.9; 120.0; 114.2; 110.0; 68.5; 55.5.
GC/MS (EI): m/z 436.2 (24%, Mþ), 122.9 (100%).
5b: The sulphur analogue 5b (9.74 g, 74%) was obtained
similarly to 5a using 2-(methylthio)aniline. 1H NMR
(CDCl3, 300 MHz, TMS): d 9.54 (s, 2H, NH); 8.34 (d,
J ¼ 8.2 Hz, 2H, CHAr); 7.37 (d, J ¼ 7.8 Hz, 2H, CHAr); 7.28
(d, J ¼ 8.0 Hz, 2H, CHAr); 7.07 (d, J ¼ 7.5 Hz, 2H, CHAr);
7.02 (s, 4H, CHAr); 4,76 (s, 4H, CH2); 2,14(s, 6H, CH3) ppm.
13C NMR (75 MHz, CDCl3): d 166.6; 147.6; 137.7; 132.5;
128.7; 126.6; 125.2; 123.4;120.9; 114.9; 69.2; 18.5ppm. MS
(ESI): m/z 469.1 (M þ Hþ); 491.1 (M þ Naþ).
(8) A, a ¼ 109.908, b ¼ 90.068, g ¼ 99.338, V ¼ 1123.8
3
(2)A , Dc ¼ 1.385 g cm23, Z ¼ 2, F(000) ¼ 492, 3155
˚
reflections measured, 2340 independent reflections
(Rint ¼ 0.223), refinement on F 2 against all reflections.
Theweighted R-factorwRand goodnessoffit GOFarebased
on F 2, GOF ¼ 0.656, R[F 2 . 2s(F 2)] ¼ 0.16,
wR(F 2) ¼ 0.36. Hydrogen site location was inferred from
neighbouring sites, H-atom parameters constrained, 5345
reflections used for refinement, 239 parameters and 17
restraints. DatacollectionwasperformedonaRigakuMM07
6a: To a solution of diamide 5a (2.06 g, 4.73 mmol) in
THF (350 mL), a solution of BH3 in THF (1 M; 47 mL,
47 mmol) was added dropwise. The mixture was refluxed
for 65 h, and the reaction was monitored by TLC
(pentane/ethyl acetate, 7/3, v/v). Water (10 mL) was

Supramolecular Chemistry
465
Figure 2. Synthesis of photolariats 1a and 1b.
added to slow down H2 evolution and solvents were
evaporated in vacuo. The resulting white solid was used
without further purification. The crude product was
dissolved in THF (200 mL), and TFA (17.8 mL, 0.24 mol)
was added dropwise. The solution was then refluxed for 1 h.
THF was evaporated, and an aqueous solution of sodium
hydroxide was added to neutralise the reaction mixture.
After extraction with DCM, the organic phase was dried
over MgSO4. The solvent was evaporated in vacuo, and the
residue was purified by chromatography (silica gel,
pentane/ethyl acetate, 8/2, v/v) to afford diamine 6a
(1.56 g, 81%) as a solid. 1H NMR (CDCl3, 300 MHz, TMS):
d 6.66 (m, 12H, CHAr); 4.78 (s, 2H, NH); 4.24 (t, 4H,
CH2ZO); 3.78 (s, 6H, CH2ZO); 3.57 (t, 4H, CH2ZN) ppm.
13C NMR (CDCl3, 75 MHz, TMS): d 149.0; 147.1; 137.9;
121.9; 121.2; 116.7; 115.3; 109.9; 109.5; 68.2; 55.3;
43.0 ppm. GC/MS (EI): m/z 408.2 (Mþ); 135.9.
J ¼ 7.5 Hz, 2H, CHAr); 6.92 (m, 4H, CHAr); 6.62–6.67 (m,
4H, CHAr); 5.37 (s, 2H, NH); 4.24 (t, J ¼ 5.7 Hz, 4H,
CH2O); 3.59 (t, J ¼ 5.9 Hz, 4H, CH2N); 2.25 (s, 6H, CH3)
ppm. 13C NMR (75 MHz, CDCl3): d 149.2; 148.0; 134.0;
129.4; 122.2; 120.7; 117.8; 115.4; 110.4; 68.2; 43.5;
18.1 ppm. MS (ESI): m/z 441.1675 (M þ Hþ); 463.1491
(M þ Naþ); calculated C24H28N2O2S2Na: 463.1484.
7a: To a solution of 3 (0.76 g, 2.92 mmol, 4 eq.) in
DCM (50 mL), triethylamine (0.4 mL) was added drop-
wise at 08C and 7a (0.27 g, 0.65 mmol, 1 eq.) was
subsequently introduced dropwise at room temperature.
This mixture was stirred for 22 h, diluted in DCM and
washed with a saturated aqueous solution of NaCl
(200 mL) and water (200 mL). The crude product was
purified by chromatography (silica gel, ethyl acetate/pen-
tane, 7/3, v/v) yielding 7a (487 mg, 87%). 1H NMR
(CDCl3, 300 MHz, TMS): d 8.16 (d, J ¼ 9.3 Hz, 4H,
CHAr); 7.35–7.23 (m, 4H, CHAr); 6.94 (m, 4H, CHAr);
6.91 (d, J ¼ 9.3 Hz, 4H, CHAr); 6.84 (m, 4H, CHAr); 4.22–
3.86 (m, 20H, CH2); 3.77 (s, 6H, CH3) ppm. 13C NMR
6b: The sulphur analogue 6b (1.96 g, 90%) was
obtained similarly to 6a using 5b. 1H NMR (CDCl3,
300 MHz, TMS): d 7.36 (d, J ¼ 8.1 Hz, 2H, CHAr); 7.16 (t,

466
A. Ducrot et al.
(CDCl3, 75 MHz, TMS): d 179.4; 170.1; 163.8; 155.1;
148.3; 141.6; 130.1; 129.5; 125.9; 121.3; 114.6; 113.5;
111.9; 69.5; 69.4; 68.4; 65.8; 65.7; 55.5; 48.4 ppm. MS
(ESI): m/z 855.4 (M þ Hþ); 877.4 (M þ Naþ).
8b: The sulphur analogue 8b (0.14 g, 18%) was
1
obtained similarly to 8a (Method B) using 7b. H NMR
(CDCl3, 300 MHz, TMS): d 8.14 (d, J ¼ 9.2 Hz, 4H,
CHAr); 7.22 (d, J ¼ 8.1 Hz, 2H, CHAr); 7.02–7.11 (m, 6H,
CHAr); 6.90 (d, J ¼ 9.6 Hz, 4H, CHAr); 6.81 (s, 4H, CHAr);
4.11 (t, J ¼ 4.5 Hz, 4H, CH2); 4.02 (t, J ¼ 6.6 Hz, 4H,
CH2); 3.76 (t, J ¼ 4.5 Hz, 4H, CH2); 3.61 (t, J ¼ 6.3 Hz,
4H, CH2); 3.51 (t, J ¼ 6.6 Hz, 4H, CH2); 3.41 (t,
J ¼ 6.0 Hz, 4H, CH2); 2.35 (s, 6H, CH3) ppm. 13C NMR
(CDCl3, 75 MHz, TMS): d 163.9; 148.7; 147.1; 141.6;
137.8; 125.8; 125.4; 124.5; 124.2; 123.8; 121.1; 114.6;
113.8; 70.0; 69.0; 68.2; 67.3; 53.9; 53.4; 14.2 ppm. HRMS
(ESI): m/z 881.2832 (M þ Naþ); calculated C44H50N4-
O10S2Na: 881.2860.
7b: The sulphur analogue 7b (680 mg, 82%) was
obtained similarly to 7a using 6b. 1H NMR (CDCl3, 200 -
MHz, TMS): d 8.10 (d, J ¼ 9.0 Hz, 4H, CHAr); 7.20–7.36
(m, 4H, CHAr); 7.05–7.17 (m, 4H, CHAr); 6.88
(d, J ¼ 9.2 Hz, 4H, CHAr); 6.82 (s, 4H, CHAr); 3.38–
4.47 (m, 20H, CH2); 2.40 (s, 6H, CH3) ppm. 13C NMR
(CDCl3, 75 MHz, TMS): d 169.8; 163.8; 148.3; 141.6;
138.6; 137.2; 129.9; 129.6; 125.8; 125.4; 124.8; 121.5;
114.6; 113.8; 69.6; 69.5; 68.3; 65.8; 47.5; 14.1 ppm. MS
(ESI): m/z 909.2 (M þ Naþ).
9a: To a suspension of 10% palladium on charcoal
(53 mg) in ethanol (15 mL), 8a (200 mg, 0.24 mmol) and
hydrazine hydrate (1.5 mL, 30.9 mmol) were added. The
mixture was refluxed for 15 h and monitored by TLC
(AcOEt/petroleum ether, 3/2, v/v). The solution was
filtered hot over a pad of Celite, and the filtrate was
evaporated in vacuo. The reaction is quantitative and
further purification proved unnecessary. 1H NMR (CDCl3,
300 MHz, TMS): d 7.08 (d, J ¼ 7.8 Hz, 2H, CHAr); 6.97 (t,
J ¼ 7.8 Hz, 2H, CHAr); 6.85 (t, J ¼ 7.8 Hz, 4H, CHAr);
6.80 (s, 4H, CHAr); 6.71 (d, J ¼ 9 Hz, 4H, CHAr); 6.59 (d,
J ¼ 9 Hz, 4H, CHAr); 4.05 (t, J ¼ 6.9 Hz, 4H, CH2); 3.95
(t, J ¼ 5.1 Hz, 4H, CH2); 3.81 (s, 6H, CH3); 3.69 (t,
J ¼ 5.1 Hz, 4H, CH2); 3.65 (m, 8H, CH2); 3.49 (t,
J ¼ 5.7 Hz, 4H, CH2) ppm. 13C NMR (CDCl3, 75 MHz,
TMS): d 153.5; 151.9; 148.7; 140.1; 138.8; 123.1; 122.0;
121.0; 120.8; 116.4; 115.8; 113.7; 111.9; 69.6; 69.5; 68.0;
67.1; 55.4; 52.8; 52.3 ppm. MS (ESI): m/z 767.4
(M þ Hþ); 789.4 (M þ Naþ).
8a: Method A. To a solution of 6a (0.20 g, 0.49 mmol) in
DMF (10 mL), Cs2CO3 (0.80 g, 2.46 mmol) and 1-[2-(2-
iodoethoxy)ethoxy]-4-nitrobenzene (23) (0.81 g, 2.40 mmol)
were added, and the reaction mixture was stirred for 20 h at
1208C, after which time another aliquot of 1-[2-(2-
iodoethoxy)ethoxy]-4-nitrobenzene (0.81 g, 2.40 mmol)
was added. This was repeated after another 20 h of reaction,
and the reaction mixture was stirred for an additional 60 h at
1208C. The reaction mixture was allowed to cool to room
temperature and was filtered over a pad of Celite. The solvent
of the filtrate was removed in vacuo, and the residue was
purified by chromatography (silica gel, pentane/ethyl acetate,
6/4, v/v)yielding8a(52mg,13%).Method B. Toasolutionof
diamide7a (1.00 g, 1.17mmol) in THF (60mL), a solution of
BH3 in THF (1 M; 12 mL, 12 mmol) was added dropwise.
The mixture was refluxed for 15 h, and the reaction was
monitored with TLC (petroleum ether/ethyl acetate, 1/4, v/v).
Some water was added to slow down H2 evolution and
solvents were evaporated in vacuo. The resulting solid was
used without further purification. The solid was dissolved in
THF (60 mL), TFA (4 mL) was added dropwise and the
mixture was refluxed for 1 h. The solvent was removed in
vacuo. The residue was dissolved in water, and the reaction
mixture was neutralised with an aqueous solution of sodium
hydroxide. After extraction with DCM, the organic phasewas
washed with water and was dried over MgSO4. The solvent
was evaporated in vacuo, and the residue was purified by
chromatography (silica gel, petroleum ether/ethyl acetate,
9b: Method A. The sulphur analogue 9b (120 mg) was
obtained analogously to 9a using 8b. Method B. To a
solution of diamide 7b (215 mg, 0.25 mmol) in THF
(20 mL), a solution of BH3 in THF (1 M; 2.4 mL,
2.4 mmol) was added dropwise. The mixture was refluxed
for 15 h, after which 20 mL MeOH was added. The
mixture was further refluxed for 1 h after which the solvent
was removed in vacuo. The resulting solid was redissolved
in 50 mL EtOH, 10% Pd/C (100 mg) and 2.5 mL hydrazine
hydrate (48.5 mmol) were added and the suspension was
refluxed for 6 h. The suspension was filtered hot over a pad
of Celite. The solvent and excess hydrazine of the filtrate
were removed in vacuo, and the residue was purified by
chromatography (silica gel, ethyl acetate/petroleum ether,
1
3/2, v/v) affording diamine 8a (0.53 g, 55%). H NMR
(CDCl3, 400 MHz, TMS): d 8.14 (d, J ¼ 9.24 Hz, 4H, CHAr);
7.07 (d, J ¼ 7.88 Hz, 2H, CHAr); 6.97 (t, J ¼ 8.0 Hz, 2H,
CHAr); 6.90 (d, J ¼ 9.28Hz,4H,CHAr);6.85(d,J ¼ 7.72 Hz,
4H, CHAr); 6.80 (m, 4H, CHAr); 4.09 (t, J ¼ 5 Hz, 4H, CH2);
4.05 (t, J ¼ 6.68 Hz, 4H, CH2); 3.82 (s, 6H, CH3); 3.76 (t,
J ¼ 4.8 Hz, 4H, CH2);3.64(t, J ¼ 5.72 Hz, 4H, CH2); 3.62 (t,
J ¼ 6.32 Hz, 4H, CH2); 3.50 (t, J ¼ 5.92 Hz, 4H, CH2) ppm.
13C NMR (CDCl3, 75 MHz, TMS): d 163.8; 153.5; 148.7;
141.4; 138.7; 125.7; 123.2; 122.0; 121.0; 120.7; 114.5; 113.6;
111.9; 69.8; 68.9; 68.1; 67.2; 55.4; 52.8; 52.4 ppm. MS
(MALDI): m/z 826.2 (Mþ), 849.3 (M þ Naþ).
1
4/1, v/v), yielding 9b (140 mg, 75%). H NMR (CDCl3,
300 MHz, TMS): d 7.26 (d, J ¼ 8.0 Hz, 2H, CHAr); 7.11
(m, 6H, CHAr); 6.85 (s, 4H, CHAr); 6.75 (d, J ¼ 8.8 Hz,
4H, CHAr); 6.63 (d, J ¼ 8.8 Hz, 4H, CHAr); 4.05 (t,
J ¼ 6.7 Hz, 4H, CH2); 3.99 (t, J ¼ 5.0 Hz, 4H, CH2); 3.73
(t, J ¼ 5.0 Hz, 4H, CH2); 3.63 (t, J ¼ 6.3 Hz, 4H, CH2);
3.56 (t, J ¼ 6.57 Hz, 4H, CH2); 3.44 (t, J ¼ 6.1 Hz, 4H,
CH2); 2.39 (s, 6H, CH3) ppm. 13C NMR (CDCl3, 75 MHz,

Supramolecular Chemistry
467
TMS): d 152.0; 148.8; 147.3; 140.1; 137.8; 125.2; 124.6;
124.3; 123.9; 121.1; 116.4; 115.9; 113.9; 69.8; 69.6; 68.1;
67.3; 53.8; 53.4; 14.3 ppm. HRMS (ESI): m/z 823.3531
(M þ Naþ); calculated C44H56N4O6S2Na: 823.3533.
1a: A pyridine solution (60 mL) of dianiline 9a (66 mg,
0.086 mmol) and CuCl (65 mg, 0.66 mmol) was stirred in
the dark under air for 20 h at room temperature, and the
solvent was then evaporated. The residue was purified by
chromatography (silica gel, petroleum ether/ethyl acetate
1:1, v/v) affording azolariat 1a (23 mg, 35%). 1H
NMR (CDCl3, 300 MHz, TMS): d 7.75 (d, J ¼ 9 Hz, 4H,
CHAr); 7.12 (d, J ¼ 9 Hz, 4H, CHAr); 6.93 (m, 4H, CHAr);
6.67–6.85 (m, 8H, CHAr); 4.37 (t, J ¼ 3.9 Hz, 4H, CH2);
3.81 (t, J ¼ 8.4 Hz, 4H, CH2); 3.78 (s, 6H, CH3ZO); 3.70
(t, J ¼ 4.2 Hz, 4H, CH2); 3.39 (t, J ¼ 6 Hz, 4H, CH2); 3.28
(t, J ¼ 7.5 Hz, 4H, CH2); 3.14 (t, J ¼ 5.7 Hz, 4H, CH2)
ppm. 13C NMR (CDCl3, 75 MHz, TMS): d 161.1; 153.9;
148.5; 147.3; 138.6; 124.2; 123.4; 122.2; 120.8; 116.4;
114.7; 113.3; 112.0; 71.0; 69.7; 68.2; 66.3; 55.5; 52.9;
29.8 ppm. HRMS (ESI): m/z 763.3711 (M þ Hþ);
785.3527 (M þ Naþ); calculated C44H51N4O8: 763.3701.
1b: The sulphur analogue 1b (32 mg, 21%) was
temperatures (8a could be isolated in 13% yield from 6a
after heating at 1208C for 100 h). Therefore, an alternative
route via the secondary amides 7a and 7b was followed.
The secondary amines 6a and 6b were reacted with acid
chloride 3, yielding the corresponding amides 7a and 7b in
good yields. Subsequently, the amides were reduced by
borane in THF. In the case of the reduction in 7b, this
reaction proved ineffective. It was observed that in this case
partial oxidation of the nitrobenzene moieties occurred.
Although it is known from literature reports that
nitrobenzenes can be directly reduced to the corresponding
symmetrical azobenzenes by LiAlH4 (24), it was, in light
of the decomposition caused by this reagent in a previous
step, decided to perform the azobenzene formation in two
steps via the aniline intermediates. The reduction in the
nitrobenzenes 8a and 8b to give anilines 9a and 9b
proceeded quantitatively in both cases (25).
Inspired by a literature report of a one-pot borane
reduction in a secondary amide and a subsequent Pd/C-
catalysed hydrogenation (26), the direct multi-stage one-
pot reduction in amide 7b to aniline 9b was performed.
The reaction proceeded in an improved overall yield of
75%. Finally, the oxidation in anilines 9a and 9b to the
corresponding azobenzenes 1a and 1b was achieved using
a copper catalyst, prepared in situ from pyridine and CuCl
(27). In this reaction, the thermodynamically favoured
trans-isomers (E) are exclusively formed.
1
obtained analogously to 1a using 9b. H NMR (CDCl3,
300 MHz, TMS): d 7.77 (d, J ¼ 8.7 Hz, 4H, CHAr); 7.12
(d, J ¼ 9 Hz, 4H, CHAr); 7.0–7.10 (m, 8H, CHAr); 6.68–
6.79 (m, 4H, CHAr); 4.37 (t, J ¼ 3.9 Hz, 4H, CH2); 3.82 (t,
J ¼ 7.5 Hz, 4H, CH2); 3.70 (t, J ¼ 3.9 Hz, 4H, CH2); 3.33
(t, J ¼ 6.3 Hz, 4H, CH2); 3.16 (t, J ¼ 8.1 Hz, 4H, CH2);
2.99 (t, J ¼ 6 Hz, 4H, CH2); 2.33 (s, 6H, CH3) ppm. 13C
NMR (CDCl3, 75 MHz, TMS): d 161.1; 148.4; 147.3;
147.0; 138.3; 125.5; 124.5; 124.1; 124.0; 123.7; 120.7;
116.3; 113.4; 70.8; 69.5; 68.1; 66.3; 54.0; 54.0; 14.2 ppm.
HRMS (ESI): m/z 817.3059 (M þ Naþ); calculated
C44H50N4O6NaS2: 817.3064 (M þ Naþ).
Single-crystal X-ray diffraction data were also
sought for the photolariats and some intermediates
(see Figure 3(a)–(c) and Experimental section). The
crystal structure of 1a-E (Figure 3a) shows a C2-symmetry
axis. The trans-azobenzene moiety is almost planar,
inducing a stretched conformation of the macrocycle.
Both methoxybenzene groups are repulsed to the outside.
˚
The width of the cavity is around 3.9 A, and the stretched
conformation leads us to conclude that complexation of a
3. Results and discussion
3.1. Synthesis and single-crystal X-ray diffraction
structures
Photolariats 1a and 1b were synthesised via a multi-step
approach from a common precursor (4), as detailed in the
preceding section and is schematised in Figure 2.
The diacid 4 was activated by thionyl chloride and was
then reacted with an excess of either o-anisidine or
(methylthio)aniline, yielding the corresponding amides 5a
and 5b in moderate yields. Subsequently, the amides were
reduced to secondary amines. The application of LiAlH4 as
a reducing agent leads to decomposition, whereas NaBH4
was not capable of reducing the amides to any considerate
extent. However, theapplication of borane inTHF, followed
by acid treatment to break the NZB adduct bond, led to the
efficient reduction in the amides of 5a and 5b to the
corresponding amines 6a and 6b in good to excellent yields.
The direct alkylation of the secondary amines was
discovered to be low yielding and slow even at elevated
Figure 3. Single-crystal X-ray structure of 1a-E (a), 5a (b) and
5b (c). Sulphur atoms are represented as spheres.

468
A. Ducrot et al.
cation inside this cavity should be disfavoured. On the other
hand, the lesselongated cis-form may be anticipated to have
a less drastic effect on the binding site. Indeed, molecular
modelling suggests that 1a-Z adopts a more globular shape,
which is conducive with complexation. (Due to the low
energy barrier for single CZC bond rotation, one definitive
structural orientation is not shown.)
Figure 4(a),(b) show the sequential trans-to-cis and cis-to-
trans photoisomerisation during six cycles. Each time the
same photostationary state is obtained, with only a slight
difference between 1a and 1b. Both 1a and 1b are thus
photoswitchable and fully reversible over several cycles.
Trans-to-cis photoisomerisations of 1a-E and 1b-E
(Figure 5(a),(c)) are almost quantitative according to
1
Additionally, intermediates 5a and 5b were crystal-
lised, and the structures obtained from single crystals
grown in methanol are shown in Figure 3(b),(c),
respectively. Their X-ray structures are very different,
despite the similarity of their compositions. In 5a, the
intramolecular hydrogen bonds formed (implicating the
methoxy oxygen and the relatively acidic amide proton)
facilitates folding of the molecule, which tends to a helical
structure. In contrast in 5b, intramolecular hydrogen bonds
are less favoured and only intermolecular interactions
were identified. This underlies the difference between
oxygen and sulphur atoms as hydrogen bond acceptors.
recorded H NMR spectra, see spectra of photoproducts
1a-Z and 1b-Z (Figure 5(b),(d), respectively), where
characteristic signals of trans-forms almost fully dis-
appear. On integrating peak areas, a yield of 94% for 1a
and 1b is calculated, which is somewhat higher than
related previously reported azobenzenophanes (72%) (11).
Quantum yields of trans-to-cis photoisomerisations of 1a
(0.66) and 1b (0.65) are three times higher than that of
azobenzene (circa 0.2) on irradiating the analogous
absorption band (29). This difference may arise from
reduced strain on the tether between extremities of the
azobenzene on passing to the cis-form from the trans-form
(30). Indeed, the mechanism of azobenzene photoisome-
risation, which is still a source of contention, may be
anticipated to receive contributions from non-sterically
demanding in-plane inversion of the angle of the azo-
nitrogen atoms or an out-of-plane rotation about the
N ¼ N double bond (rotation mechanism) (8, 19, 31–35).
Additionally, photoswitching does not affect the chemical
shift/chemical environment of the methoxy group.
3.2. Photo- and thermal isomerisation
As trans-conformations of azobenzenes in general are
thermodynamically more stable than cis-conformations, it is
anticipated that at equilibrium in the dark the population of
photolariats will consist of 100% trans-form (1-E). Figure 4
shows this to be the case for both 1a (Figure 4(a), solid line)
and 1b (Figure 4(b), solid line). This trans-form is
characterised by an absorption band around 240 nm
attributed to p–p* transitions localised on the phenyl
groups, a relatively intense band around 360 nm due to
symmetry-allowed p–p* transitions, which are delocalised
through the molecule including the two nitrogen atoms, and
a weak band around 450 nm originating from symmetry-
forbidden n–p* transitions occurring at the central nitrogen
atoms (28). On irradiation with UV light (365 nm) the
azobenzeneunitwassuccessfullyisomerisedtothecis-form,
where the symmetry-allowed p–p* transitions band gets
weaker and undergoes a hypsochromic shift. The band
ascribed to the forbidden n–p* transition also increases
slightly (Figure 4(a),(b), dotted line). The photostationary
state consists almost exclusively of the cis-form, as judged
from changes in the 350–400 nm spectral region, which was
verified by NMR (vide infra).
Thermal cis- to trans-isomerisations of 1a and 1b were
followed spectrophotometrically at 303, 313 and 323 K.
The curves tracing the kinetics of changing absorption
were fit using Equation (1) (16)
A ¼ ðA0 2 A Þe2kT þ A
ð1Þ
1
1
Here, A is the measured absorbance, A0 is the initial
absorbance, A1 is the absorbance when the thermal
isomerisation (100% of trans-form) and k is the rate
constant at temperature T. Activation energies (Ea) were
then determined from the Arrhenius equation. Thus,
similar Ea values of 99 and 101 kJ.mol–1 for the thermal
cis-to-trans isomerisation of 1a and 1b were noted,
respectively. These values are slightly higher than that
measured for parent azobenzene measured under identical
conditions (90 kJ.mol21), pointing to an influence of the
macrocyclic tether which may disfavour the rotation
mechanism of isomerisation (vide supra).
On irradiating the cis-form with visible light (450nm),
the photochemical reversion primarily to the trans-form is
effected; however, the photostationary state that is attained
after a few minutes of irradiation is not exclusively trans
(see Figure 4(a),(b), dashed line). For 1a and 1b, the
photostationary state is estimated to consist of 80% and 74%
of the trans-form, respectively. The azobenzene can recover
completely its original form by thermal isomerisation in the
dark, showing that no photodegradation had occurred.
The reversibility of the photoreactions was
tested through several irradiation cycles. The insets in
3.3. Complexation
As the single-crystal X-ray structure showed (Figure 3(a)),
1-E is less geometrically suitable than 1-Z to form ion
inclusion complexes. 1H NMR experiments (see Figure 6)
show the formation of metal ligand complexes, in this case
with sodium, and offer a means to measure the difference
of binding strength between the trans- and cis-forms.
Indeed, the singlet resonance at 3.74 ppm, attributed to the

Supramolecular Chemistry
469
Figure 4. (a) Electronic absorption spectra of 1a-E (solid line), 1a-Z (dotted line) and the photostationary state after photoisomerisation
Z to E (dashed line), (b) electronic absorption spectra of 1b-E (solid line), 1b-Z (dotted line) and the photostationary state after
photoisomerisation Z to E (dashed lines). Insets: Isomerisation cycles: absorption at 365 nm on irradiating alternatively with UV and
visible light.
methoxy group protons, is shifted further by Naþ in the
cis-conformation (Dd ¼ 0.47 ppm cf. Dd ¼ 0.18 ppm).
Equally, some differences in chemical shifts for CH2 and
aromatic proton resonances can be noted.
Indeed, the presence of potential axial binders O (1a) or S
(1b) is anticipated to affect the stability constants and
1
selectivities of 1a and 1b towards different ions. In H
NMR titrations, several samples with constant concen-
tration of photolariat and variable concentration of cation
were analysed. For each signal, the variation in the
chemical shift was plotted versus the concentration of
cations, for example, Figure 6(e) shows the titration of 1a-
Titrations of photolariats with a range of Groups I and
II metal cations (as triflate salts), and selected transition
1
metals were performed using H NMR spectroscopy or
spectrophotometry in CD3CN or CH3CN, respectively.

470
A. Ducrot et al.
(d)
(c)
(b)
(a)
7.5
7.0
6.5
6.0
5.5
5.0
ppm
4.5
4.0
3.5
3.0
2.5
Figure 5. 1H NMR spectra of photolariats (2 mM). (a) 1a-E, (b) 1a-Z, (c) 1b-E, (d) 1b- Z. 1a-E and 1b-E spectra were recorded before
any irradiation on samples stored in the dark. 1a-Z and 1b-Z spectra were recorded after irradiation at 365 nm.
(e)
(d)
(c)
(b)
(a)
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
ppm
1
Figure 6. Complexation-induced changes on H NMR spectra of photolariat 1a in MeCN (2 mM). (a) 1a-E only, (b) 1a-E with 20
equivalents of sodium triflate, (c) 1a-Z only, (d) 1a-Z with 20 equivalents of sodium triflate, (e) 1H NMR chemical shift variation in the
CH3O-group protons versus number of sodium per ligand. Experimental variations in the chemical shift (n), fitted curve with calculated
values (2).

Supramolecular Chemistry
471
Table 1. Photoisomerisation quantum yields, activation energies and binding constants of photolariats 1a and 1b.
cLog Ka
aFl
bEa (kJ.mol21
)
Naþ
Kþ
Ca2þ
Cu2þ
Zn2þ
Tb3þ
Agþ
1a-E
1a-Z
1b-E
1b-Z
0.66
1.28d
2.69d
1.28d
2.03d
0d
4.36e
4.41e
4.78e
4.89e
4.46e
4.67e
4.81e
4.58e
5.24e
5.30e
99
1.7d
0.65
4.62e
3.94e
101
Notes: (a) Quantum yields of trans-to-cis photoisomerisation (Fl at 365 nm); (b) kinetic constants of thermal cis-to-trans isomerisation; (c) logarithm of binding constants of 1a
and 1b with different ions; (d) binding constants determined from 1H NMR titrations; (e) binding constants established from spectrophotometric titrations.
`
Aquitaine; University of Bordeaux I and Ministere de la
Recherche et de l’Enseignement Superieur (A.D.; S.D.) is
gratefully acknowledged.
Z with sodium (36). Following the spectrophotometric
titrations, binding constants were extracted from a Hill
plot. All binding constants are shown in Table 1.
´
These studies corroborate that cis-conformations are
indeed better adapted for metal ligand complex formation,
and in general complexes with Groups I and II metal ions
are weaker than transition metal complexes. However, a
larger differentiation of binding constants between trans-
and cis-forms is noted with the former cases, which is
conducive with a higher proportion of ejected ion after
isomerisation. Following this rationale, the best candidates
for photoejection are sodium and calcium with 1a. On the
other hand, binding constants of larger magnitude are
obtained with sodium.
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Incorporation of additional chelating groups in the
coordination sphere of azobenzene-containing crown
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photocontrolled binding sites. While the binding constants
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Financial support from the European Research Council under the
European Community’s Seventh Framework Programme
´
(FP7/2008-2013) ERC grant agreement no. 208702; Region

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