Light induced E-Z isomerization in a multi-responsive organogel: Elucidation from 1H NMR spectroscopy

Article abstract of DOI:10.1039/c5cc03609g

A multiresponsive organogel of (E)-N′-(anthracene-10-ylmethylene)-3,4,5-tris(dodecyloxy)benzohydrazide (I) showed a decrease of fluorescence intensity, decrease in mechanical strength and a change in gel morphology on irradiation with a wavelength of 365

Full text of DOI:10.1039/c5cc03609g

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-
1
>
C=O stretching peak of amide of I powder at 1644 cm has shifted molecules are connected with each other at the junction points by
-1
to 1640 cm in the xerogel state indicating intermolecular hydrogen the supramolecular interaction. On application of mechanical force
bonding between the >C=O group with –NH group. It is evident these junction points become ruptured causing the transformation
from WAXS patterns (Fig. S2) that pure I powder exhibits some to the sol state. Due to the presence of a large number of different
small peaks characterizing very low crystallinity which are absent in supramolecular interacting sites in the I molecule which also
I xerogel suggesting that gelation brings in completely amorphous operates on the fibre surface as residual force causes quick
structure. The WAXS patterns of the xerogel shows broad peaks at regeneration of network junction reforming the gel.
o
o
o
2
4
θ = 10.9 , 13.8 and 21.8 corresponding to d values at 8.0, 6.4 and
.07 Å, respectively and the last d value corresponds to the π-π
-
5
Fig. 3a displays the UV-vis spectra of I solution (10 M) and gel (0.1
w/v) in methyl cyclohexane. Two peaks at 367 and 387 nm in the
%
*
absorption spectra of the I solution corresponds to the π- π and n-
b
*
a
π transitions of I molecule. Both these peaks are red shifted to 371
and 389 nm, respectively, in the I gel. This type of red shift in the π-
a
b
c
d
-
5
Fig. 3 (a) UV-vis spectra of I solution (10 M) and gel (0.1 % w/v) in methyl
cyclohexane. (b) Fluorescence spectra of I sol (0.025% w/v) and gel (0.1%
w/v) for excitation at 370 nm.
*
π transition peak indicates π stacking between the anthracene and
benzene units during gelation which is also evident from the XRD
patterns. Also the red shift in the absorption bands may indicate
the formation of J type aggregates in the gel state where the
Fig.
2 SEM (a and b) and AFM (c and d) images of I before and after
1
9
photoirradiation respectively.
molecular planes are stacked side by side. Fig. 3b shows the
fluorescence spectra of I solution (0.025% w/v) and it’s gel (0.1%
w/v) for excitation at 370 nm. It is clearly evident from the figure
that the fluorescence intensity of the gel is ~6 times higher than
that of the solution. Also there is a 14 nm red shift of the emission
peak in the gel state than that of the solution supporting the J type
stacking distance. It is evident from the FESEM (Fig. 2a), AFM (Fig.
c) and TEM (Fig. S3) images that the I gel posses three dimensional
fibrous network morphology. The average diameter of the fibers is
5 ± 8 nm and their length is of several micrometers and the fibrils
2
4
are intertwined with each other (Fig. S4). DSC thermograms of the I
2
0
aggregate formation. The I molecule is fluorescence active due to
the presence of the anthracene moiety and in solution phase there
is a possibility of dynamic free rotation of the anthracene rotor
which causes the non-radiative annihilation of the excited state.
Consequently, the solution state of the I molecule is less
fluorescent, but in the gel state the molecules are closely packed by
H bonding and π-stacking interactions causing restriction of
gel at a concentration of 3% (w/v) (Fig. S5) exhibits a melting peak
o
at 49 C and its broad nature indicates the polydispersed nature of
1
6
the aggregates. On cooling the I gel shows an exothermic peak at
o
3
5 C indicating hysterisis in the melting and gelation temperatures
1
7
surmising first order nature of the gelation process.
Fig. S6 displays the rheological properties of the I gel (1.0 % (w/v)) intramolecular rotation decreasing the quenching of exitonic decay
0
at 25 C. Frequency sweep experiment of the I gel (Fig. S6a) reveals through the non-radiative paths and hence fluorescence intensity
2
1
that G'>G’’ and is independent of frequency in a wide frequency increases. Temperature dependent fluorescence spectra are used
region indicating the gel nature of the sample. Stress sweep to support the above hypothesis (Fig. S7). The fluorescence
experiments (Fig. S6b) of the I gel indicates that the G' and G'' cross intensity of the I gel decreases with increase in temperature and
each other at an oscillator stress of 4 Pa indicating the breaking of the peak position also gets blue shifted. With increasing
the gel to a sol state. Our newly designed I organogel exhibit the temperature the inception of breaking of the self assembly occurs
0
unique property of thixotropy (Fig. 1e) and for a quantitative and above ~50 C the intensity decreases sharply due to the
estimation of it we have performed time sweep experiments under breaking of network structure.
both a low strain of 0.1 % and a high strain of 100 % (Fig. S6c). Due to the presence of imine functionality in I there is a possibility
However it is noticeable that in the first, third and fifth steps the of E-Z type of isomerization on UV- irradiation that may alter the
values of G’ are almost the same which indicates that the I gel is
completely recoverable after being destroyed by mechanical force.
physical properties of the gel which is studied by different
spectroscopic techniques. The gels were irradiated with
a
For a gel to be certified as thixotropic, presence of supramolecular
interactions and driving forces for propagating in more than one
dimension is indispensable. These criteria can be fulfilled by small
wavelength of 365 nm and the physical properties of the gels were
monitored with respect to irradiation time. UV-vis spectra of the I
1
8
molecular weight gelator with negligible crystallinity. Both the I
powder and the I xerogel has prominent amorphous structure as
evident from the XRD patterns (Fig. S2) which facilitate its
thixotropic behavior. The long fibers produced by assembly of the I
gel at 0.2% (w/v) concentration (Fig. 4a) exhibits a decrease in the
absorbance values with increase in photoirradiation time. To
investigate the change in fluorescence property of the I gel with
2
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photoirradiation we have measured the fluorescence emission of
the I gel (0.6 % w/v) for irradiation with 356nm at different time
intervals (Fig. 4b). It is manifested from the figure that the
fluorescence intensity of the I gel decreases sharply with increasing
irradiation time with a concomitant blue shift of the emission
maxima. These changes in absorbance and fluorescence intensities
of the I gel upon photoirradiation may be explained as follows. I can
exist in two isomeric forms: E and Z. The E form is more stable and
normally the compound is a mixture of the two isomers with the E
form dominating. With photoirradiation with a wavelength of 365
nm the compound transforms into the Z isomer (Scheme 1). In the
Z isomer the intermolecular H bonding between the >C=O group
and –NH group becomes prohibited due to steric interference of
the anthracene moiety (Scheme 1). This disturbs the aggregation of
the I molecules which in turn prevents the π stacking phenomenon.
Hence the delocalization of the π electrons and consequently the
II(Z-isomer)
2
2
change in dipole moment value decreases. The
a
b
Scheme 1. Schematic representation of aggregation and disaggregation of
compound I (E-isomer) and II (Z-isomer) respectively.
anthracene moiety is discarded because of the absence of peak for
the m/z value at 1755 (mass of the dimer) in the MALDI-TOF spectra
(
Fig.S12) of I gel after 24 hr of UV-irradiation.
Fig. 4 (a) UV-vis spectra of I gel (0.2% w/v) with irradiation (at 365 nm) time. (b)
Variation of Fluorescence spectra of I gel (0.6% w/v) upon irradiation at 365 nm
at different time interval.
In an endeavour to enlighten the change in electronic environment
of the molecule and to confirm its isomerization upon
photoirradiation at 365 nm wavelength we have carried out
I
1
H
transition moment integral thus decreases causing the gradual
lowering of the absorbance values with irradiation time. The
decrease in fluorescence intensity and a blue shift of the emission
maxima also occurs due to the disaggregation of the I molecules
upon photoirradiation. We have also performed rheological
NMR spectrum of the I xerogel in CDCl before (I, E-isomer) and
3
1
after (II, Z-isomer) 12 hours of photoirradiation (Fig. 5). In the H
NMR spectrum of it, signals coming in the range of δ 0.86 to 1.78
represent protons of -C11
with the 'O' atom in dodecyloxy groups [(-OCH
H
23 group. Methylene protons attached
-) protons, indicated
experiments on the
I gel (1% (w/v)) with increase in
2
as 'i'] appear at relatively downfield shifted (4.0 - 4.03 δ) region due
to strong electronic withdrawal of electronegative ‘O’ atom which
are again directly connected with aromatic ring. Signal appearing at
δ 7.18 is attributed to the (‘h’) protons. Four protons indicated by
photoirradiation time to investigate the changes in the gel strength
after photoirradiation. Frequency sweep experiments of the I gels
at different time intervals after photoirradiation are carried out (Fig.
S8) and the G’ values are plotted against irradiation time (Fig. S9).
The results clearly manifests that the G’ value hence gel strength
decreases with increasing photoirradiation time. Also it is important
to note that on photoirradiation the thixotropic property of the gel
is lost. Morphological studies are carried out on the I gel after 5
hours of irradiation to observe if there is any change in morphology
of the I gel occurs with photoirradiation. It is evident from the SEM
image (Fig. 2b) that the fibers of the I gel are disintegrating causing
lesser density of fibres and forming tiny spherical aggregates
coming from breaking of the fibres. The AFM result shown in Fig. 2d
and Fig.S10 is very interesting showing how the fibres are
disintegrating into tiny particles on irradiation. The reason for this
breaking of the fibrillar morphology of the gel is responsible for the
lower gel strength and lower fluorescence intensity. Though the
process of transformation is of molecular origin, we may consider it
as a phase change from fiber to disintegrated spherical morphology.
This spherical aggregates of Z isomers are energetically more stable
and prohibits its reversible transition to E isomer under thermal and
visible light conditions. We have used Avrami equation for this
process of changing the fluorescence intensity with irradiation time
‘
c’ and ‘d’ are found at δ (7.44 - 7.52) showing a rather complicated
multiplicity which is quite expected as both of the 'c' and ’d’ protons
should primarily show a double doublet pattern. A doublet signal at
δ (7.98 - 8.0) is assigned to ‘b’ protons. Proton ‘a’ appears as a
singlet at δ 8.47. The doublet signal at δ 8.59 may be attributed to
the protons ‘e’. The broad signal at δ 9.61 is attributed to proton ‘g’
which is attached with electronegative nitrogen atom and is
involved in H-bonding interaction here. Finally the most downfield
signal at δ 9.71 is due to proton ‘f’ which is very much deshielded
due to the coupled effect of paramagnetic anisotropic influence of
(
C=N) bond and very strong electronic withdrawal by electron
deficient imine 'N' atom. Irradiation of molecule (I) with photons of
65 nm wavelength transforms into molecule (II) by
3
photoisomerization across (C=N) bond. However in this Z-isomeric
form proton signals change their positions. This change in signal
position may be related with the change in ‘electron’ delocalization
scenario of the lone pair on (-NH-) nitrogen atom. The said nitrogen
atom lone pair is in a cross conjugated situation and therefore may
delocalize either within the amide carbonyl group or within
anthracene ring through (-N=C<) in the other side. In the trans form
-1
and we have calculated the rate constant k (0.151 min ) of E-Z
isomerization from the intercept of Fig. S11 using fluorescence
(
E-isomer) anthracene rings may remain electron rich due to the π –
2
3
stacking and hence ‘electron’ delocalization of the said lone pair
spectra (Fig. 4b). The possibility of optical dimerization of the
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takes place significantly within the amide carbonyl group. This S.M, P.C and P.B acknowledge CSIR for financial support.
restricts any marked effect of electron withdrawing carbonyl group
Notes and references
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3
Fig. 5 H NMR spectrum of I (E-isomer) and II (Z-isomer) in CDCl .
within the aromatic phenyl ring. In contrast, in Z-isomer due to
unfavourable orientations of anthracene moieties, π-stacking can’t
occur effectively and therefore ‘electron’ delocalization favourably
occurs towards the anthracene ring. Also the disposition of the lone
pair of the said nitrogen atom is in closer proximity to the
6
7
8
A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
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anthracene ring. These two reasons may cause
a greater
delocalization of the lone pair to the anthracene moiety than that
of the E isomer. Therefore electron withdrawing effect of carbonyl
group on the phenyl ring splits the (-O-CH
2
-) group signals (at 3-4 δ)
of dodecyloxy groups. The H NMR spectrum shows that (-O-CH -)
1
9
1
1
1
J. H. Kim, M. Seo, Y. J. Kim and S. Y. Kim, Langmuir, 2009, 25,
1761.
2
signals in case of Z-isomer is split in two signals. The ‘j’ protons
present in more deshielded environment comes at higher δ value
0 Q. Chen, D. Zhang, G. Zhang and D. Zhu, Langmuir, 2009, 25
1436.
1 J. B. Beck and S. J. Rowan, J. Am. Chem. Soc., 2003, 125
3922.
,
1
(3.81 δ) than ‘i’ protons at 3.50 δ showing as expected (1:2) signal
,
area ratio. Electron withdrawing effect of carbonyl group also shifts
the position of ortho protons (h) to downfield direction at 8.6 δ,
compared to the E-isomer. Now, in the Z form due to increased
steric hindrance, H-bonding with the (-NH-) hydrogen atom is not
1
2 K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T.
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bonding also should place lesser amount of (+ve) charge on the
nitrogen atom of (C=N) of any individual molecule and this is
reflected by the relative upfield shift of the (-CH=N-) proton ‘f’ (8.95
δ). Proton ‘e’ and ‘b’ appears at δ (8.10 – 8.09) and δ (8.03 – 8.01)
respectively showing doublet nature. Proton ‘c’ appears at δ (7.63 –
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,
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1
the signal at δ (7.57 – 7.54) represents a complicated pattern with
somewhat increased signal area than that can be expected for ‘d’
protons. Therefore, this signal may represent a combined signal
5
1
2
8
arising from the merger of signals corresponding to protons ‘d’ and 17 C. Daniel, C. Dammer, J. M. Guenet, Polymer, 1994, 35, 4243.
‘
a’ as both of these two sets of protons are present in comparably 18 A. Dawn, T. Shiraki, H. Ichikawa, A. Takada, Y. Takahashi, Y.
electron shielded environment considering electronic delocalization
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moiety. So it is evident from the H NMR spectra that the loss of
aromaticity of anthracene is not occurring here ruling out the
probability of photodimerisation of anthracene.
So we have successfully tailored an organic super-gelator with
thermo-responsive, mechano-responsive and photo-responsive
behaviour and established that the molecule undergoes a E-Z type
of isomerization across (C=N) bond upon UV-irradiation at 365 nm
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,
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,
vary significantly upon UV irradiation and it may find use for sensing
and release applications.
13126.
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Light induced E-Z Isomerization along imine bond in a multiresponsive organogel of
anthracene attached 3,4,5-tris(dodecyloxy) benzohydrazide gelator altering morphology,
1
fluorescence and mechanical properties is elucidated from H NMR spectra.
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