Photochromism and metal complexation of a macrocyclic styryl naphthopyran

Article abstract of DOI:10.1002/cphc.201100052

A macrocyclic benzo-15-crown-5 ether unit tethered to a photochromic naphthopyran by a styryl spacer (MEN) is shown to form a 1:1 complex with magnesium(II). The structure and dynamics of the specific host-metal interactions were investigated by PFG-NMR analysis. A combination of UV/Vis and variable temperature multi-dimensional ¹H NMR photokinetic analysis of the crown-containing styryl naphthopyran and its metal complex was used to probe the effect of metal complexation on the photochromism. Now you see it...: The behavior of the hybrid biphotochromic system based on a thermally reversible naphthopyran and an alkaline-earth cation binding benzo-15-crown-5 macrocycle linked by a photochemically bistable stilbene moiety is discussed (see picture). Copyright

Full text of DOI:10.1002/cphc.201100052

DOI: 10.1002/cphc.201100052
Photochromism and Metal Complexation of a Macrocyclic
Styryl Naphthopyran
Elena V. Tulyakova,[a] Olga A. Fedorova,[b] Jean-Claude Micheau,[c] Sergey V. Paramonov,[b, d]
Vladimir Lokshin,[d] Gaston Vermeersch†,[a] and Stephanie Delbaere*[a]
A macrocyclic benzo-15-crown-5 ether unit tethered to a pho-
tochromic naphthopyran by a styryl spacer (MEN) is shown to
form a 1:1 complex with magnesium(II). The structure and dy-
namics of the specific host–metal interactions were investigat-
ed by PFG-NMR analysis. A combination of UV/Vis and variable
temperature multi-dimensional 1H NMR photokinetic analysis
of the crown-containing styryl naphthopyran and its metal
complex was used to probe the effect of metal complexation
on the photochromism.
1. Introduction
In the last few years, naphtho-
pyrans have evolved as an inter-
esting family of photochromic
molecules because of their syn-
thetic availability, ease of modifi-
cation, durable persistency and
addressable bleaching kinetics.[1]
They are commercially used as
sun-protective
ophthalmic
lenses, optical filters and smart
windows. In addition, they
promise to be the basis of intelli-
gent materials for temporary or
permanent memories.[2,3] Di-
phenyl-naphthopyran, N, first re-
ported by Michl and Becker[4a] is
known to generate two open
isomers upon irradiation, namely
the transoid–cis, TC and the
transoid–trans, TT (Scheme 1).[4b]
When it is substituted with an
ethylenic bridge in position 8 as
in compound EN, four photo-
Scheme 1. Structures of the three generic naphthopyrans N, EN and MEN. Each specific isomer is indicated in ital-
ics.
[a] Dr. E. V. Tulyakova, Prof. G. Vermeersch, Prof. S. Delbaere
isomers (ETC, ETT, ZTC and ZTT) are expected to be found as
the result of E/Z isomerization and ring-opening.[4c] Finally, in-
troduction of a macrocycle M gives rise to the hybrid biphoto-
chromic system MEN, which allows one to combine photo-
chromic properties with metal coordination. In these condi-
tions, due to the species multiplication effects, much more ver-
satile hybrid systems can be obtained.[3f,4] For instance, a
strong stabilization of a metastable state through metal–ligand
interactions could be expected, improving its thermal durabili-
ty.[5] In other applications, the possible sequential control of
the electronic properties could find potential applications in
gated-photochromism.[4,6]
CNRS UMR 8516, Universitꢀ Lille Nord de France
3 rue du Professeur Laguesse BP83, F-59006 Lille (France)
Fax: (+33)320-964-013
E-mail: stephanie.delbaere@univ-lille2.fr
[b] Prof. Dr. O. A. Fedorova, Dr. S. V. Paramonov
A. N. Nesmeyanov Institute of Organoelement Compounds
28 Vavilova street, 119991 Moscow (Russian Federation)
[c] Dr. J.-C. Micheau
CNRS UMR 5623, Universitꢀ P. Sabatier
118 route de Narbonne, 31062 Toulouse (France)
[d] Dr. S. V. Paramonov, Dr. V. Lokshin
CNRS UPR 3118, Interdisciplinary Center of Nanoscience CINaM
13288 Marseille Cedex 9 (France)
[†] Deceased.
A full understanding of the photochemical pathways and
the identification of stable intermediates is critical for learning
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201100052.
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Photochromism and Metal Complexation of a Macrocyclic Styryl Naphthopyran
how to construct and manipulate these synthetic hybrid bi-
photochromic systems.[7] The purpose of this manuscript is to
analyse the complexation and the photochromic behaviour of
the macrocyclic styryl naphthopyran compound MEN and its
model compounds N and EN. The first part is dedicated to the
structural characterisation of naphthopyrans N, EN, MEN and
its metal complex with magnesium(II) cation by UV/visible and
NMR spectroscopy. The second part deals with the dynamics
of the response under irradiation of the various compounds,
looking especially at the effect of magnesium(II) cation com-
plexation on the photochromic properties of the MEN com-
pound.
2. Results and Discussion
2.1. Spectral Characterization of the Naphthopyrans N, EN,
MEN and the MEN Magnesium(II) Complex
2.1.1. Steady-State Absorption Measurements
The spectral characteristics of the model compounds N, EN,
free and complexed MEN are summarized in Table 1. In their
closed forms, the three molecules exhibit roughly the same ab-
sorption bands. However, the MEN spectrum shows a struc-
Table 1. Spectral characteristics of naphthopyrans N, EN and MEN with
its magnesium (II) complex in CH3CN at 295 K.
Figure 1. a) Normalized absorption spectra of N (a) and MEN (c), b)
spectrophotometric titration of MEN CL =1.3ꢂ10ꢁ5 m with Mg(ClO4)2
(0ꢂxM/Lꢂ80) in acetonitrile.
Compound lmax [nm]
e [mꢁ1 cmꢁ1
]
lmax [nm] open
form
N[a]
357, 343,
315
354, 340,
322
354, 340,
322
351, 337,
315
425
494
495
485
EN
4.34ꢂ104 (at
354 nm)
MEN
MgMEN
4.31ꢂ104 (at
354 nm)
4.81ꢂ104 (at
315 nm)
Scheme 2.
[a] Data extracted from ref. [8].
from UV/Vis titration by Specfit calculations are in good agree-
ment with our previously reported data for the benzo-15-
crown-5-containing styryl dyes.[5b]
tureless absorption band at 322 nm corresponding mainly to a
transition induced by the stilbene group (Figure 1a). The same
effect is also observed for EN (not shown). This band is likely
to overlap the lowest-energy naphthopyran transition peaking
at 360 nm (see the spectrum of the model compound N).[8,9]
As expected, conjugation induces a strong bathochromic shift
for the EN and MEN open forms.
The UV/Vis monitoring of the MEN host titration with mag-
nesium(II) exhibits a nice isosbestic point in the UV/Vis spectra
at 324 nm (Figure 1b). Due to its presence, it has been as-
sumed that the complexation with magnesium(II) can be de-
scribed by the simple equilibrium displayed in Scheme 2 with
the binding constant K11. The small ionic diameter of the MgII
cation (1.56 ꢁ) fits well with the size of the internal cavity of
the benzo-15-crown-5 ether unit and is expected to be insert-
ed into the cavity thus surrounded by the oxygen atoms. The
value of the binding constant (logK11 =4.5ꢀ0.1) obtained
2.1.2. Structural Analysis of the Model Styryl Naphthopyran
EN by NMR Spectroscopy
The styryl naphthopyran EN exist as E and Z isomers. Both iso-
mers were obtained by chromatographic separation. In the
1H NMR spectra, the resonances of ZN are shifted upfield
(Figure 2). Particularly, the vinyl protons H-a and H-b (3J=
12.4 Hz) are shifted to Dd=ꢁ0.60 ppm compared to the vinyl
protons in EN (3J=16.5 Hz).
Dipolar correlations in 2D-ROESY are observed between H-a
and H-2’, H-b and H-7 in EN, and between H-b and H-9, H-b
and H-7, H-7 and H-2’, 5’, 6’ in ZN, thus indicating that only
one average of several conformers resulting from free rotation
around CbꢁC8 and CaꢁC1’ is present in both cases.
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S. Delbaere et al.
The thermodynamic parameters for the complex formation
of MEN with magnesium(II) in acetonitrile were determined by
measuring the temperature dependence of the binding con-
stants by NMR spectroscopy. The separation of host–complex
resonances, obtained by spectral deconvolution analysis, point-
ed to a sizable kinetic barrier for the exchange process and al-
lowed the determination of the values of the binding constant
(logK11 =4.3ꢀ0.2 for MgMEN at 295 K) in complete agreement
with data deduced from UV/Vis spectroscopy. The measure-
ments were repeated six times in the range 233–293 K. A
van’t Hoff plot of lnK versus Tꢁ1 (see the Supporting Informa-
tion) afforded the reaction enthalpy and entropy for the com-
plex formation (DH0 =2.9ꢀ0.1 kJmolꢁ1 and DS0 =93.1ꢀ
0.4 Jmolꢁ1 Kꢁ1). As a consequence of the cation and ligand de-
solvation, the binding process is entropically favored.
Figure 2. 1H NMR spectra of isomers a) EN and b) ZN measured at 295 K in
CD3CN.
2.1.4. NMR DOSY Analysis of the Naphthopyrans N, MEN and
the MgMEN Complex
2.1.3. Conformational Analysis of Benzo-Crown-Styryl-Naph-
thopyran MEN and its Magnesium(II) Complex
To extract further information about the structure and dynam-
ics of the host–metal interactions in solution, the translation
diffusion coefficients (D) for each compound were measured
using the pulsed-gradient spin-echo technique (PGSE) in
CD3CN at 295 K (Table 2) to determine their molecular
Due to similar retention time values, the two isomers MZN and
MEN could not be isolated by HPLC facilities. Therefore,
1H NMR spectrum of the benzo-15-crown-5-styryl-naphthopy-
ran indicates that the mixture contains 23% MZN and 77%
MEN (Figure 3). However, the MEN isomer (97% purity) could
Table 2. Diffusion coefficients and values of hydrodynamic radii calculat-
ed for EN, MEN and its magnesium(II) complex in CD3CN at 295 K.
Compound MW[a] D [nm2 sꢁ1
]
DOSY hydrodynamic radius [ꢁ] and di-
mensions[b] [diameterꢂheight]
EN
MEN
MgMEN
496.6 0.99
626.7 0.90
651.0 0.89
6.3 (19.6ꢂ5.8)
6.9 (22.2ꢂ5.8)
7.0 (22.2ꢂ5.9)
[a] Molecular weights of uncharged molecules in case of EN and MEN,
and of the cation in case of MgMEN correspond to the exact formula of
the unsolvated compounds; [b] the dimensions for oblate ellipsoid shape
were obtained by AM1 calculation.
volume.[10] The DOSY (Diffusion Order SpectroscopY) technique
is increasingly being used in conjunction with NOE to investi-
gate aggregation and encapsulation phenomena, intermolecu-
lar or interionic interactions.[11,12] For instance, it was demon-
strated that the diffusion coefficients of the various free crown
ethers were sensitive to both conformational changes and
Figure 3. 1H NMR spectra of initial mixture of a) MEN and MZN, b) MEN host
obtained after irradiation (97% purity), c) MgMEN measured at 295 K in
CD3CN.
1
changes in molecular size upon complexation.[13] The H DOSY
be obtained by photochemical methods after irradiation of the
mixture with 313 nm light in acetonitrile at 295 K (Figure 3b).
The analysis of the binding process of MEN (97% purity)
spectra were recorded first for the EN and MEN molecules and
for the MgMEN complex. The values for the free MEN host
and the MgMEN complex do not differ significantly, revealing
that the complex formed by the MEN host with magnesium(II)
at a 1:1 ratio should be of similar size as the free MEN host,
which is actually in compliance with the fact that the molecu-
lar weights of MEN and the MgMEN complex do not differ
substantially. The hydrodynamic radii rH of the spherical parti-
cles in a solvent of viscosity h could be calculated from the
translational diffusion coefficients by using the Stokes–Ein-
stein–Debye formula [Eq. (1)]:[15a,b]
1
with magnesium(II) was done through H NMR titrations. Sub-
sequently, upon successive addition of magnesium perchlorate
(0 to 1 equiv) to a solution of MEN in acetonitrile, the most
pronounced changes in the proton spectra could be observed
in the proximity of the methylene protons of the crown ether
unit located more closely to the complexed cation. The
1H NMR signals during the titration of MEN with magnesium in
ratio of 1:1 are displayed in Figure 3c.
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Photochromism and Metal Complexation of a Macrocyclic Styryl Naphthopyran
kBT
fT
kBT
ð1Þ
D ¼
¼
6phrH
where kB is the Bolzmann constant, T is the absolute tempera-
ture, and fT is the friction factor. To enable a fit with the real
shape of the particle in solution, all molecules were considered
as oblate ellipsoidal objects. Relative to the spherical model,
the ellipsoidal model imposes a shape factor to extract molec-
ular dimensions from the translation diffusion coefficients ob-
tained by NMR.[15b] For the sake of comparison, compound EN
was investigated in the same conditions. At room temperature
its translational diffusion coefficient about 0.99 nm2sꢁ1 corre-
sponds to a hydrodynamic radius of 6.3 ꢁ. The AM1 calculation
used to fit this value with an oblate ellipsoidal object leads to
dimensions of 19.6ꢂ5.8 ꢁ2. Furthermore, the introduction of
the bulky crown-ether substituent in MEN increases the diame-
ter up to 22.2 ꢁ. Its hydrodynamic radius is not drastically al-
tered upon binding of the magnesium(II) cation.
2.2. Photochromism
2.2.1. Investigations at 295 K by UV/Vis Spectroscopy
UV-irradiation of the trans-isomers of MEN and EN at 365 nm
in acetonitrile at ambient temperature gives rise to the forma-
tion of the thermally reversible merocyanines. The newly
formed absorption at 494–495 nm is strongly red-shifted rela-
tive to the 425 nm maximum of the corresponding ring-
opened form of the parent naphthopyran N (Figure 4a).[8a,d]
This result is in agreement with an important p-conjugation
between the naphthopyran and the stilbene moiety in the
ring-opened forms. In addition, a distinct hypsochromic shift of
the absorption band at 495 nm of the opened merocyanine
MEN form up to 15 nm was observed for the MgMEN complex
(Figure 4b).
Figure 4. a) Normalized absorption spectra of naphthopyrans MEN (c)
and N (a) obtained after 10 s irradiation at 365 nm; b) absorption spectra
of MEN (c) and its complex MgMEN (a) obtained after 10 s irradiation
at 365 nm (C=1.3ꢂ10ꢁ5 m, 295 K).
The formation of photomerocyanines occurs in the TC and TT
isomers (Scheme 1). For instance, the transoid–cis isomer can
be evidenced easily by considering the most deshielded
proton, H-2, around 8.5 ppm and the proton H-5 at 6.4 ppm
while the transoid–trans isomer is well recognized from its H-5
proton at 6.3 ppm (Figure 5a). The identification can also be
2.2.2. Identification of the Merocyanines of MEN and MgMEN
by NMR Spectroscopy
The photochemical behavior of
the macrocyclic styryl naphtho-
pyran MEN and its MgII complex
was monitored by NMR spec-
troscopy using step-wise 313 or
365 nm irradiation in CD3CN at
ambient and low temperatures.
At 295 K, only the Z/E photoiso-
merization is observed as the
photomerocyanines are not suffi-
ciently stable to be detected by
NMR spectroscopy.
On the other hand, at a lower
temperature (233 K), UV irradia-
tion with 313 or 365 nm leads to
Figure 5. 1H NMR spectra (aromatic region) measured at 233 K in CD3CN: a) mixture of MEN and MZN after 20 min
irradiation at 365 nm; b) MgMEN and MgMZN complexes after 20 min irradiation at 365 nm. Note the specific sig-
nals attributed to the isomeric forms METC, MZTC, METT and MZTT in spectrum (a) and the various complexes
both the opening of the naph-
thopyrans and the E/Z isomerisa-
tion of the ethylenic junction. MgMETT, MgMETC, MgMZTC and MgMZTT in spectrum (b).
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S. Delbaere et al.
performed on the basis of the signal intensities and their ther-
mal stability. It is well-known that the TC isomers are usually
formed in higher concentrations upon irradiation than the TT
isomers, and during the thermal relaxation process, they un-
dergo a faster reclosing.[4b] One week later, NMR spectra re-
vealed that the open forms fully disappeared and we could ob-
serve the characteristic resonances of the E and Z isomers of
initial compounds with naphthopyran in the closed configura-
tion.
Table 3. Extracted apparent photochemical rate constants (h) under 313
and 365 nm irradiation at 295 and 233 K and thermal rate constants of re-
laxation (k) at 233 K.
EN and ZN
MEN and MZN
MgMEN and
MgMZN
lirr =313 nm
T=295 K
hEN!ZN
hZN!EN
2.22ꢂ10ꢁ5
1.31ꢂ10ꢁ4
1.43ꢂ10ꢁ5
6.56ꢂ10ꢁ4
8.8ꢂ10ꢁ5
6.1ꢂ10ꢁ4
lirr =365 nm
T=295 K
hEN!ZN
hZN!EN
2.28ꢂ10ꢁ4
1.74ꢂ10ꢁ4
3.95ꢂ10ꢁ5
4.09ꢂ10ꢁ4
3.38ꢂ10ꢁ4
1.5ꢂ10ꢁ3
Consequently, irradiation of the free host MEN as well as of
the magnesium(II) complex MgMEN at 233 K resulted in the
formation of the four expected photomerocyanines, TC and TT
with the E and Z isomeric forms of the ethylenic function
(Figure 5).
hEN!ZN
hZN!EN
hEN!EO
hZN!ZO
hEO!EN
hZO!ZN
2.92ꢂ10ꢁ4
3.51ꢂ10ꢁ4
2.22ꢂ10ꢁ3
2.02ꢂ10ꢁ4
5.99ꢂ10ꢁ3
1.05ꢂ10ꢁ3
6.09ꢂ10ꢁ4
2.14ꢂ10ꢁ3
4.23ꢂ10ꢁ3
1.77ꢂ10ꢁ3
6.55ꢂ10ꢁ4
5.54ꢂ10ꢁ5
7.69ꢂ10ꢁ4
1.52ꢂ10ꢁ3
2.26ꢂ10ꢁ3
3.15ꢂ10ꢁ3
6.30ꢂ10ꢁ4
8.89ꢂ10ꢁ4
lirr =313 nm
T=233 K
2.2.3. Kinetic Analysis of the Photochemical and Thermal
Processes
hEN!ZN
hZN!EN
hEN!EO
hZN!ZO
hEO!EN*
hZO!ZN*
7.11ꢂ10ꢁ4
6.72ꢂ10ꢁ4
5.38ꢂ10ꢁ3
4.75ꢂ10ꢁ4
3.46ꢂ10ꢁ3
9.60ꢂ10ꢁ4
2.66ꢂ10ꢁ5
4.01ꢂ10ꢁ4
4.24ꢂ10ꢁ3
1.73ꢂ10ꢁ3
3.95ꢂ10ꢁ4
1.29ꢂ10ꢁ3
1.65ꢂ10ꢁ3
3.92ꢂ10ꢁ3
2.71ꢂ10ꢁ3
4.19ꢂ10ꢁ3
4.52ꢂ10ꢁ4
8.56ꢂ10ꢁ4
lirr =365 nm
T=233 K
1
The peak-intensity measurements in H NMR spectra recorded
at regular time intervals during irradiation with 313 and
365 nm, and during the thermal evolution at 233 K allowed us
to plot the time evolution of concentrations. To simplify, the
concentrations of open photomerocyanines with the same E/Z
isomeric forms were merged such that ZTC+ZTT=ZO and
ETC+ETT=EO. Therefore, the system can be described by a
four-species model system as displayed in Scheme 3. This
T=233 K
kEO!EN
kZO!ZN
1.81ꢂ10ꢁ4
1.70ꢂ10ꢁ4
1.42ꢂ10ꢁ4
1.38ꢂ10ꢁ4
2.55ꢂ10ꢁ4
1.43ꢂ10ꢁ4
Several remarkable features are gathered in the Table 3. The
consequences of the magnesium(II) cation addition to the ini-
tial host ligand can be estimated by comparing the apparent
photochemical rate constants in the MEN and MgMEN col-
umns. In all the cases, the apparent photochemical rate con-
stant of the E!Z isomerisation is increased in the complexed
host ligand. Moreover, addition of magnesium(II) also induces
a higher rate of the reverse Z!E isomerisation under 365 nm.
The effect is, however, almost negligible at an irradiation wave-
length of 313 nm. On the other hand, the effect of the pres-
ence of a free macrocycle can be seen by comparing the EN
and MEN columns. Under 365 nm irradiation, there is a clear-
cut decrease in the E!Z photochemical rate, while this effect
is not significant under 313 nm. On other hand, the general
trend for the reverse Z!E isomerisation is to increase in the
presence of the free macrocyclic unit. It is also interesting to
consider the apparent rate constants obtained at 233 K in
order to compare the reactivity of each isomer (E and Z)
toward ring opening as opposed to the E vs Z photoisomeriza-
tion, under 313 and 365 nm irradiation. The relative reactivity
is obtained by dividing the two apparent photochemical rates
(see Table 4). For instance, under 313 nm irradiation for EN, we
have hEN!ZN/hEN!EO =2.22eꢁ3/2.92eꢁ4=7.6ꢃ8.
Scheme 3. Four-species system showing E vs Z photoisomerisation, photo-
chemical ring opening vs ring closure and thermal relaxation. E- and
Z-(TC+TT) are merged into E-O and Z-O open isomers, respectively.
The results show that this ratio, which characterizes the inti-
mate reactivity of the excited state is sensitive both to the
structure and wavelength of irradiation. In EN and MEN, it ap-
pears that whatever the irradiation wavelength is, the ring
opening is much more efficient in the case of the E isomer.
The tendency mentioned above is less pronounced for the
magnesium(II) complex of host macrocyclic ligand.
system was analyzed quantitatively by means of our self-devel-
oped kinetic-curve-fitting Sa software.[17] The estimated appar-
[18]
ij
ent photochemical rate constants (h ) of the two isomeriza-
tion processes E vs Z and of the ring opening vs ring closure,
and the thermal rate constants, kij, of the relaxation monitored
at 233 K are reported in Table 3.
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Photochromism and Metal Complexation of a Macrocyclic Styryl Naphthopyran
stimulations according to two main pathways: the electrocyclic
Table 4. Relative photochemical reactivities (ring-opening vs E,Z isomeri-
ring opening of the naphthopyran entity through cleavage of
the C(sp3)ꢁO bond and the photoreversible E,Z-isomerization.
At 295 K, the ring closure from the photomerocyanine to the
naphthopyran moiety is sufficiently rapid to be without any
effect on the E/Z isomerization. On the other hand, at 233 K,
this ring closure reaction is slow and its contribution to the
whole relaxation dynamics under irradiation can easily be sub-
tracted. No [2p+2p]-cycloaddition of the MEN stilbene unit
was observed.
sation) of the various isomers at 233 K under 313 and 365 nm irradiation.
Relative Reactivity
EN
MEN
MgMEN
EN!EO vs EN!ZN (313 nm)
EN!EO vs EN!ZN (365 nm)
ZN!ZO vs ZN!EN (313 nm)
ZN!ZO vs ZN!EN (365 nm)
8
8
0.6
0.7
7
3
1.6
2
160
0.8
4
1
NMR photokinetic analysis of the photochromic behavior
using numerical methods showed that in all cases, the quan-
tum yield of the Z!E isomerization is higher than the reverse
photochemical back reaction E!Z. At lower temperatures, in
EN and MEN, the ring opening is much more efficient than the
E!Z isomerization. The opposite effect was observed in the
case of the ZN and MZN isomers. On the other hand, no clear-
cut effect can be seen in MgMEN when the macrocycle is Mg-
filled.
From the recorded [E]:[Z] photosteady-state concentration
ratio (from NMR spectral analysis) and the measured values of
the molar extinction coefficients (from absorption spectra anal-
ysis), the quantum yield ratios for the two reversible processes
FZ/FE can be estimated from FZ/FE (at lirr)=(eE/eZ) (at lirr)ꢂ
([E]/[Z]) (final at lirr, Table 5).
The knowledge of the photocoloration, photoisomerization
and thermal bleaching parameters obtained for the hybrid bi-
photochromic molecule MEN could be helpful for the con-
struction of more elaborate light and metal-sensitive systems
exhibiting well-established gated-photochromism and photo-
modulation effects.
Table 5. Analysis of the E vs Z photoisomerisation at 295 K, under 313
and 365 nm irradiation.
EN:ZN
MEN:MZN
MgMEN:MgMZN
[E]:[Z] initial
5:95
77:23
97:3
90:10
49000
22000
2.23
25000
5400
4.63
76:24
86:14
82:18
48000
21500
2.23
15500
6700
2.31
[E]:[Z] (final at 313 nm)
[E]:[Z] (final at 365 nm)
eE (313 nm)
eZ (313 nm)
eE/eZ (313 nm)
eE (365 nm)
86:14
39:61
49000
22500
2.18
25500
5700
4.47
Experimental Section
eZ (365 nm)
General Information: The styryl naphthopyrans EN and MEN were
synthesized by the Wittig method as based on an earlier report.[9]
The MEN isomer (97% purity) isomer was obtained photochemical-
ly after irradiation of the MZN and MEN isomeric mixtures at
313 nm in CH3CN and 295 K. Solvents and reagents were obtained
from commercial suppliers and were used without further purifica-
tion.
eE/eZ (365 nm)
FZ/FE (313 nm)
FZ/FE (365 nm)
ꢃ15
ꢃ70
ꢃ15
ꢃ3
ꢃ40
ꢃ10
These ratios are always higher than one, thus indicating that
Z!E photoisomerisation is always favored with respect to the
E!Z reverse reaction. Comparison between MEN and MgMEN
shows that, both at 313 and 365 nm irradiation, the quantum
yields ratio FZ/FE is reduced significantly by about a factor of
4 after metal cation addition. Moreover, this ratio is within the
same order of magnitude for EN and MgMEN. The effect of
the macrocyclic ring’s existence is annulled when it is MgII-
filled.
UV/Vis Absorption: Measurements were performed according to
the procedure described in ref. [5b]. The binding constant and the
stoichiometry were obtained from the titration curve by fitting
these changes using the nonlinear regression analysis program
SPECFIT32. Samples (1ꢂ10ꢁ5 m in CH3CN) were irradiated in quartz
cells (10 mm) at 295 K in an in-house-constructed apparatus at-
tached to the UV/Vis spectrometer. The kinetic parameters of the
ring closure reaction were determined at each temperature by
monitoring the disappearance of the colored form at the wave-
length of maximum absorbance after having removed the irradiat-
ing source.
3. Conclusions
NMR Experiments: All NMR experiments were recorded on a Bruker
Avance DRX 500 MHz spectrometer using CD3CN as internal refer-
ence in a TXI Bruker 5 mm probe.[20] Data acquisition and process-
ing were performed with Topspin 2.1 software (Bruker). Aliquots of
a metal perchlorate solution (0.01–0.1m in CD3CN) were added to
In summary, we investigated a hybrid biphotochromic mole-
cule MEN based on a thermally reversible naphthopyran and
an alkaline-earth cation binding benzo-15-crown-5 macrocycle
linked by a photochemically bistable stilbene moiety. This
ligand forms a 1:1 complex with magnesium(II) cation. Addi-
tionally, the thermodynamic parameters of the complexation,
the diffusion coefficients and the hydrodynamic radius of the
various free and complexed species have been determined.
MEN hybrid biphotochromic naphthopyran gives rise to a
complex reaction network and reacts upon photochemical
1
a MEN solution (0.8–1.0 mm in CD3CN) and H NMR spectra were
recorded after each addition. The quantities of the different species
were calculated from the integrated areas of the NMR signals. The
translational diffusion coefficients were determined by the PGSE-
1
NMR technique by monitoring the H signal. This method was first
introduced by Stejskal and Tanner, and in our experiments, the
BPP-STE-LED sequence combining constant time, stimulated echo,
ChemPhysChem 2011, 12, 1294 – 1301
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www.chemphyschem.org
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Acknowledgements
This work was supported by the Centre National de la Recherche
Scientifique (CNRS) and Russian RFBR programs (09-03-00283,
10-03-93106). The 500 MHz NMR facilities were funded by the
Rꢀgion Nord-Pas de Calais (France), the Ministꢁre de la Jeunesse
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Keywords: alkaline earth metals
·
nmr spectroscopy
·
photochromism
chemistry
·
structure elucidation supramolecular
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Received: January 21, 2011
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