Single-crystal UV-vis spectroscopic examination of a striking odd-even effect on structure and chromic behaviour of salicylidene alkylamines

Article abstract of DOI:10.1039/c7cc06268k

We have found that a series of N-(5-bromosalicylidene) alkylamines exhibited distinct chromic behaviour depending on the parity of their alkyl chain length. A group with an even number of carbon atoms in the alkyl chain showed photochromism, while another group with odd number showed thermochromism.

Full text of DOI:10.1039/c7cc06268k

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Single-crystal UV-Vis spectroscopic examination of a striking odd-
even effect on structure and chromic behaviour of salicylidene
alkylamines
Received 00th January 20xx,
Accepted 00th January 20xx
Hirohiko Houjou,* Hana Ikedo, Isao Yoshikawa
DOI: 10.1039/x0xx00000x
www.rsc.org/
We have found that
a
series of N-(5-bromosalicylidene) bromosalicyllidene) benzylamine (1BN) showed simultaneous
alkylamines exhibited distinct chromic behaviour depending on photo- and thermochromism, although 1BN had been
the parity of their alkyl chain length. A group with an even reported to show photochromism exclusively.8,21 This
number of carbon atoms in the alkyl chain showed discrepancy may stem from polymorphism (regrettably, the
photochromism, while another group with odd number showed crystal structures have not yet been reported). In this study,
thermochromism.
we employed single-crystal UV-Vis absorption spectroscopy
system in combination with an optical microscope equipped
with
a
temperature-controllable stage.22–25 This method
The prototropic isomerisation of salicylidene amines has been
extensively studied in relation to their thermochromic and
photochromic effects.1–5 Specifically, the colour change is
attributed to the alteration of the abundance ratio between
enol-imine (OH) and keto-enamine (NH) forms.6,7 The
expression of chromic properties depends on the molecular
structure, especially when substituents can directly influence
the electronic state of the chromophore, while the effects of
intermolecular interactions are not satisfactorily understood.8–
16 One reason for this may be the difficulty in separating these
two effects with respect to the chromic behaviour: namely, the
introduction of substituents normally causes a considerable
change in the molecular packing structure of the crystal, and
therefore, induces changes in intermolecular interactions.
Some groups are trying to give a sophisticated solution to this
allowed us to observe crystallographically pure samples, and
hence, we were able to prevent ambiguity that may arise from
the contamination of the polymorph.
The target compounds 1Cn (n = 13–18) and 1BN were
prepared in high yield by stirring 5-bromosalicylaldehyde and
the respective amines in dichloromethane, followed by
evaporating and recrystallising the product in methanol. The
compounds were readily categorised into two groups based on
appearance: 1C13, 1C15, 1C17 were yellow needles, while
1C14, 1C16, and 1C18 were colourless thin plates (Figure 1 (a)).
Namely, a distinct odd-even effect with respect to the chain
length was observed in the colour and morphology of 1Cns.
The ‘odd group’ showed intense luminescence around 520 nm
in the solid state upon UV irradiation, while the ‘even group’
did not. At first sight, 1BN obtained as yellow plates appeared
to belong to the odd group, except that it showed only minor
photoluminescence. However, 1BN demonstrated somewhat
‘amphoteric’ behaviour between that of the even and odd
groups, which will be discussed later.
problem by using co-crystallisation techniques.16
20
–
Meanwhile, we have recently found a remarkable effect of the
alkyl chain length on the chromic behaviour of N-(5-
bromosalicylidene) alkylamines (1Cn, where n is the number of
carbon atoms in the alkyl group): the even-numbered chains
induced photochromism, while the odd-numbered chains
induced thermochromism. For this series of molecules, it is
unlikely that the difference in alkyl chain length directly
influences the electronic state of the chromophore. Thus, we
can expect that a thorough examination of these compounds
will provide a clue on the long-term controversy regarding the
Br
Br
CnH2n+1
N
N
OH
1BN
OH
n (n = 13, 14, 15, 16, 17, 18)
1C
structural
criteria
that
determine
photo-
or
Scheme 1 Structures of salicylideneamines studied
thermochromism.2–5 Additionally, we have found that N-(5-
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0.8
0.6
0.4
0.2
Figure 1 Photographs of 1Cn (n = 13–18, indicated at upper row) and 1BN: (a) at
ambient temperature, (b) cooled by dipping in liquid nitrogen, (c) irradiated with
handy UV lamp (325 nm) during the cooling.
0.0
400
500
600
Wavelength (nm)
A distinct odd-even effect was observed for the chromic
behaviour of the Schiff bases examined. When a small portion
(ca. 10 mg) of the crystalline samples in test tubes was dipped
in liquid nitrogen, the odd-group samples exhibited apparent
discolouration, whereas the even-group samples remained
colourless (Figure 1(b)). In addition, the even-group samples
showed no colour change upon heating until reaching their
melting points. When the test tubes were irradiated with a UV
lamp during cooling by liquid nitrogen, the even-group
samples turned a reddish orange while the odd-group samples
showed intensified luminescence but no recognizable colour
change (Figure 1(c)). Among the even-group samples, only
Figure 3 Temperature-dependent UV-Vis absorption spectra measured for the
single crystal of 1C14 under microscope (selected). Dashed lines indicate
measurement ca. 1 sec after UV irradiation with a Hg lamp. For full version, see
Figure S3. The colour code (red = 30°C, violet = –90°C) is the same as in Figure 2.
As representatives of the odd- and even-group samples, 1C15
and 1C14 were examined with microscopic UV-Vis absorption
spectroscopy. Figure 2 shows the temperature-dependent
spectra measured for the single crystal of 1C15. In the range
from –75 °C to 30 °C, the absorbance around 416 nm increases
1C14 showed photochromism at ambient temperature while in proportion to the temperature (Figure S2). Below –75 °C,
the others reacted to UV irradiation only at low temperatures.
Interestingly, 1BN showed discolouration during cooling with
liquid nitrogen and photochromism at ambient temperature.
Similar phenomena regarding the coexistence of photo- and
thermochromism have been so far reported.17,18,26 We
observed an apparent hysteresis with respect to the
photochromism: cooling the sample under continuous UV
irradiation from ambient to liquid nitrogen temperatures
results in reddish-orange colouration whereas irradiation of
the sample after cooling does not lead to obvious colouration
(Figure S1).
the spectral profile is slightly changed, and the intensity of the
absorption band was slightly weakened as the temperature
was lowered. At each temperature, UV irradiation did not
influence the spectral profile (Figure 2, inset). Figure 3 shows
the temperature-dependent spectra measured for the single
crystal of 1C14. Without UV irradiation, the spectra showed no
notable absorption band in the visible region, so it was not
possible to discern whether the slight changes in the baseline
are meaningful. Upon UV irradiation, the spectra drastically
changed to reveal an intense band over the 400–550 nm
region. This new band disappeared within a few seconds when
the UV source was removed. As temperature decreased, the
band gently intensified and its vibronic structure became
slightly sharpened. The rate of colouration and decolouration
did not noticeably change with temperature.
Figure 4 shows the absorption spectra measured for the single
crystal of 1BN under conditions similar to 1C15 and 1C14. The
spectra at 15 °C showed a moderate absorption band around
425 nm, together with a weak, structured edge toward 550
nm. When the sample was cooled to –75°C, the band around
425 nm was considerably diminished, while the low-energy
edge was almost unchanged. Upon UV irradiation, the
absorbance drastically increased, especially in the range from
450 to 550 nm. The difference spectra (Figure 4, inset) is quite
similar to that observed for 1C14 after UV irradiation. The
spectral profile reverted after the UV irradiation was stopped.
In contrast to 1C14, the UV-induced spectral change became
small as temperature decreased. Also, the colouration and
decolouration processes were both decelerated at low
temperatures. These properties seemed to be a cause of the
apparent hysteresis in the photochromic behaviour of 1BN
(Figure S1).
0.8
0.6
0.4
0.2
0.0
0.3
0.2
0.1
0.0
T (°C)
30
15
0
-15
-30
-45
-60
-75
-90
-105
-120
-135
400 500 600
Wavelength (nm)
400
500
600
Wavelength (nm)
Figure 2 Temperature-dependent UV-Vis absorption spectra measured for a
single crystal of 1C15 under microscope. Inset: the difference between spectra
measured with and without UV irradiation. The colours of the plots correspond
to varying temperatures (from –135 to 30 °C with 15 °C increment).
2 | J. Name., 2012, 00, 1-3
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Unfortunately, the structures of the even-group samples have
not been clarified due to difficulty in preparing crystals
suitable for the analysis. Comparing the powder diffraction
patterns of the even-group samples revealed a common
feature: there were a series of equally-spaced peaks in a
0.8
0.6
0.4
0.2
0.0
0.2
0.1
0.0
region 3–15° of 2θ, suggesting a lamellar structure with a long
period (d001 = 46.1 Å, 51.3 Å, 55.2 Å, for n =14, 16, 18,
400
500
600
respectively) (Figure S7). The overall diffraction patterns were
Wavelength (nm)
entirely distinct from those of the odd-group samples (d001
=
35.7 Å, 38.9 Å, and 42.4 Å, for n = 13, 15, and 17, respectively,
Figure S8), indicating that the crystal structures were also
classified into two groups depending on the parity of the alkyl
chain. The profiles of infrared spectra were also classified into
the even and odd groups, and the positions of some major
peaks clearly shifted alternately with respect to the chain
length (Figure S9). These results suggest that the difference in
molecular environment induced the change in the molecular
framework, probably due to enol–keto tautomerisation.
400
500
600
Wavelength (nm)
Figure 4 Temperature-dependent UV-Vis absorption spectra measured for the
single crystal of 1BN under microscope (solid lines). Dashed lines indicate
measurement ca. 1 sec after UV irradiation with a Hg lamp. Inset: the difference
between spectra measured with and without UV irradiation. The colour code
(orange = 15°C, dark blue = 75°C) is the same as in Figure 2.
The above results are reminiscent of findings in our previous
study that revealed
a noticeable spectroscopic change
The observed difference in chromic behaviour between the
even- and odd-group samples can be attributed to their crystal
structures because they have the same chromophore structure
and hence the same electronic state when isolated. Indeed,
the solution UV-Vis absorption spectra of 1Cn (n = 13–18) were
nearly identical (Figure S4). Single crystal X-ray structure
analyses were successfully conducted for odd-group samples
and 1BN. Figure 5 shows the molecular packing structure in
the crystal of 1C15. The salicylidene rings were obliquely
stacked along b-axis to form a herringbone-like wall, and the
alkyl chains were interdigitated among the walls. The
salicylidene rings were surrounded by the alkyl chains and
have no remarkable interatomic contact with molecules in the
proximity except for close contacts (3.10 Å and 3.18 Å at 93 K)
between the bromine atom and certain alkyl hydrogen atoms
(Figure 5, red circle). It may be worth noting that the Br–H
contacts were elongated to 3.17 Å and 3.24 Å, respectively, as
temperature was raised to 253 K. We commonly observed
similar features among the odd-group samples (Figure S5),
suggesting that this type of packing is possible only for alkyl
chains in the odd group. In the crystal structure of 1BN, two
crystallographically inequivalent molecular units have similar
non-covalent contacts between Br and H atoms (Figure S6).
The roles of similar interatomic contacts in the formation of
supramolecular architectures have been noted by Garcia et
al.27
associated with polymorphic change observed for a series of a
naphthylmethylidene alkylamines.28 In that work, the
difference in molecular arrangement (alternate or bunting
shape with respect to the hydrogen bonding network) was
clearly correlated with the profiles of absorption spectra and
patterns of infrared spectra. Those results suggested that the
length of alkyl chain strikingly influences the molecular
arrangement in the crystal, and hence the electronic state of
the chromophore, through the change in the relative stability
of the tautomers. However, to the best of our knowledge, it
was the first observation of such a clear odd-even effect not
only on the spectroscopic properties but also on the chromic
behaviour of salicylidene alkylamines. We suggest that a deep
insight into alkylamine Schiff bases would shed a light on the
mechanism of photo- or thermochromism in the molecular
system. It should be noted that the previously reported works
utilised spectroscopic methods for polycrystalline films, rigid
glasses, powder diluted with inorganic salts, and so on. Our
present attempts provide quantitative data directly relatable
to the crystal structures using single crystal microscopic UV-Vis
absorption spectroscopy.
During the past few decades, there has been progress toward
understanding the photochromism and thermochromism of
salicylaldehyde Schiff bases. 29 Originally, research focused
on salicylideneanilines, which are reported to show
thermochromism when the phenyl ring is virtually co-planar to
the salicylidene ring, but otherwise show photochromism.1
However, this empirical rule has been broken by a number of
exceptions so far.12,16,18 It is also widely accepted that a less
densely-packed structure tends to induce photochromism.2
Salicylidene benzylamines were investigated as analogues of
salicylideneanilines to explore the effects of the additional
methylene spacer between the imine nitrogen and the phenyl
ring. The present results, however, imply that the chromic
behaviour of benzylamine Schiff bases should be understood
as analogues of alkylamine Schiff bases. A comprehensive
investigation on these materials is currently in progress.
36
–
Figure 5 The molecular packing structure of 1C15 as viewed along the
crystallographic b-axis.
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Acknowledgements
A part of this work (X-ray single crystallographic structure
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Nano Characterization, University of Tokyo, and was supported
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DOI: 10.1039/C7CC06268K
A simple salicylideneamine exhibits either photochromism or thermochromism
depending on the number of its alkyl carbons.