Tunable Color of Aggregation-Induced Emission Enhancement in a Family of Hydrogen-Bonded Azines and Schiff Bases

Article abstract of DOI:10.1002/chem.201802581

A simple synthesis strategy afforded a family of hydrogen-bonded azines and Schiff-bases. Although in dilute solution these dyes are faint light emitters, all exhibit bright luminescence upon aggregation. Changing the peripheral substituents from electron donating to electron withdrawing allows emission fine-tuning in a range from bluish-green to orange-red. The crystal structures of the materials reveal that the restriction of intramolecular rotations is the main mechanism for the aggregation-induced emission enhancement properties.

Full text of DOI:10.1002/chem.201802581

A Journal of
Accepted Article
Title: Tunable Color of Aggregation Induced Emission Enhancement in
a Family of Hydrogen-Bonded Azines and Schiff Bases
Authors: Samuel Guieu, Francisco Cardona, João Rocha, and Artur
MS Silva
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Chemistry - A European Journal
FULL PAPER
DOI: 10.1002/chem.200((will be filled in by the editorial staff))
Tunable Color of Aggregation-Induced Emission Enhancement in a Family of
Hydrogen-Bonded Azines and Schiff Bases
Samuel Guieu,*,[a,b] Francisco Cardona,[a] João Rocha[b] and Artur M. S. Silva*,[a]
Abstract: A simple synthesis strategy donating to electron withdrawing allows aggregation-induced
afforded a family of hydrogen-bonded emission fine-tuning in a range from enhancement properties.
azines and Schiff-bases. Although in bluish-green to orange-red. The crystal
emission
dilute solution these dyes are faint light structure of the materials reveals the
emitters, all exhibit bright luminescence restriction of intramolecular rotations is
upon aggregation. Changing the the main mechanism for the
peripheral substituents from electron
Keywords: Azine • Schiff base •
Fluorescence • Aggregation
Induced Emission • Crystal
molecular probes for the detection of thiols in live cells.17 The study
of the solid-state organization of one benzophenone azine derivative
provided insight into the importance of polymorphs, as a change in
the crystal structure lead to a change in the emission wavelength,
induced and controlled by the application of external stimuli.18
Salicylaldehyde azines and Schiff bases are also known to undergo
excited-state intramolecular proton transfer process (ESIPT), which
gives rise to larges Stokes shifts. The ESIPT process can be facilitated
in the solid state, as the conformation is usually blocked and the
intramolecular O-H···N hydrogen bond is adequately oriented. Thus,
both ESIPT and AIEE processes can be found in salicylaldehyde
azines and Schiff bases, leading to efficient emission and large Stokes
shifts in the solid state.19 All these results make azines and Schiff-
bases promising compounds for biological imaging and luminescent
devices. But to date, such studies have not been comprehensive and,
in particular, no attempt to tune the emission wavelength has been
reported.
Here, we wish to report the synthesis and characterization of
hydrogen bonded azines and Schiff bases with tunable emission
colors. To tune the emission properties of the Schiff bases, two
strategies were adopted. The first one relies on the variation in
conjugation of the aromatic system, using hydrazine or 1,4-
phenylenediamine as linking group. The second strategy takes
advantage of the donating or withdrawing properties of the aromatic
substituents. These two approaches aim at modifying the HOMO-
LUMO energy difference and, hence, the absorption and emission
wavelengths. The results show all dyes present AIEE and ESIPT
properties, with larges Stokes shifts and good quantum yields in the
solid state.
Introduction
Aromatic imines usually absorb strongly in the visible region of the
light spectrum, making them highly colored compounds. However,
they are not known to be strong emitters, the absorbed energy being
lost as heat through vibrational relaxation. To promote the emission
process, stiffening of the backbone surrounding the imine is necessary.
Introduction of a phenol group creates a N,O complexing pocket,
which has been used to chelate transition metals such as zinc,1 copper,
nickel, platinum,2 or semi metallic elements such as boron3 or
aluminium. This has resulted in numerous luminescent complexes,
with interesting emitting properties in solution,4 and in the solid state
as crystals, amorphous powders or thin films.
Aggregation may also stiffen the backbone of Schiff bases of the salen
type, turning them from non-emissive in solution to luminescent in
the solid state. This phenomenon is the opposite of ‘Aggregation
Caused Quenching’ usually observed with organic chromophores,
and is known as ‘Aggregation-Induced Emission Enhancement’
(AIEE),5-8 which is different from the formation of J-aggregates9,10 or
the observation of phosphorescence instead of fluorescence.11,12 This
approach has been successfully applied to the synthesis of
luminescent square tubes,13 and to the detection of small amounts of
hydrazine in water.14 Salicylaldehyde azine derivatives, with AIEE
properties, have also been used for cellular imaging15,16 and as
[a]
[b]
Dr. S. Guieu, Dr. F. Cardona, Prof. Dr. A. M. S. Silva,
QOPNA, Department of Chemistry, University of Aveiro, 3810-193
Aveiro, Portugal. E-mail: sguieu@ua.pt ; artur.silva@ua.pt
https://sites.google.com/site/artursilvaua/
Results and Discussion
Dr. S. Guieu, Prof. Dr. J. Rocha,
CICECO, Department of Chemistry, University of Aveiro, 3810-193
Aveiro, Portugal. E-mail: rocha@ua.pt
All compounds were obtained following a straightforward strategy
(Scheme 1). The corresponding aldehyde or ketone was condensed
with hydrazine in a two to one ratio, in refluxing methanol, and the
product was collected by filtration or purified over silica gel flash
Supporting information for this article is available on the WWW under
http://www.chemeurj.org/ or from the author.
1
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10.1002/chem.201802581
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column chromatography. When 1,4-phenylenediamine was used
instead of hydrazine, the double condensation was readily obtained
with the aldehyde derivatives. The double condensation with the
ketones requested the use of longer reaction time or more
concentrated solutions to obtain the product, noticing that the mono
imine was also obtained in some cases. Compounds were
characterized by NMR spectroscopy and HRMS (see Supporting
Information).
1a-7a and 1b-6b are almost non-emissive in dilute solution but
exhibit bright emission in solid state, which can be ascribed to the
formation of J-aggregates or to Aggregation-Induced Emission
Enhancement. To decide which phenomenon is responsible for this
behaviour, the emission spectra of solutions and suspensions of 1a-
6b in THF-water mixtures were recorded (Figure 3 and SI). All
compounds present the same behavior: the solutions are almost non-
emissive, but when the ratio of water reaches 60-70%, a precipitate
forms and the suspension is brightly emissive.
Figure 1. Normalized absorption spectra of 1a-7a in THF solution.
Scheme 1. Synthesis of the fluorophores. Reagents and conditions: i) hydrazine hydrate,
methanol, reflux; ii) 1,4-phenylenediamine, methanol, reflux.
The photophysical properties were first studied in dilute THF solution.
The dyes present an absorption maximum between 351 and 391 nm,
with molar extinction coefficients from 16,000 to 110,000 M-1 cm-1
(Table 1, Figure 1 and SI). There is no clear trend in the absorption
properties but the introduction of electron donating or withdrawing
substituents, and aromatic rings, red shifts the absorption maximum,
and increases the absorption coefficient. All dyes emit faintly upon
excitation at 400 nm (Figure 2 and SI), with quantum yields <0.1% in
solution. Stokes shifts are large, ca. 9,000 cm-1, indicating an ESIPT
process.
Figure 2. Normalized emission spectra of 1a-7a in THF solution. Excitation at 400 nm.
The spectra of the non-emissive dyes are not shown (3a, 7a).
Table 1. Absorption and emission properties in THF solution and in the solid state.
THF solution
Solid state
The solid-state emission spectra were also recorded (Figure 4 and SI).
1a-6b emit at similar wavelengths in the solid state and in solution,
but the quantum yields are much higher in the former (Table 1). The
absence of noteworthy shift between solution and solid-state emission
rules out J-aggregates formation as the reason for the observed
emission enhancement.Fehler! Textmarke nicht definiert. The restriction of
intramolecular rotation (RIR) is, thus, the likely mechanism for such
emission enhancement.
In the solid state, the emission maxima range from 523 nm to 620 nm,
depending on the fluorophore substituents, and on the presence or
absence of a phenyl ring between the two nitrogen atoms. Concerning
the substituents, the addition of electron donating groups (OMe) at
the periphery of the dyes induces an emission blue-shift, whereas the
addition of electron withdrawing groups (Br) causes a red-shift. The
change between hydrazine and 1,4-phenylenediamine linker induces
 (M-1 cm-1) [a]
f (%) [b]
< 0.1
< 0.1
n.a.
< 0.1
< 0.1
< 0.1
n.a.
n.a.
n.a.
n.a.
< 0.1
< 0.1
< 0.1
f (%) [c]
40
54
4
10
13
11
19
20
1

Dye
λ
abs (nm)
λ
em (nm)
λ
em (nm)
1a
2a
3a
4a
5a
6a
7a
1b
2b
3b
4b
5b
6b
357
364
391
373
377
374
360
370
341
368
384
351
382
16 300
22 000
40 900
34 600
86 100
99 800
57 200
110 000
45 400
21 800
44 800
19 200
52 500
560
565
n.a.
610
603
545
n.a.
n.a.
n.a.
n.a.
595
590
565
541
568
620
595
620
550
523
587
525
604
610
568
550
2
3
7
3
[a] Recorded at concentration of ca. 10-5 mol L-1. [b] Determined by comparison with
fluorescein,20 quantum yield 95% in aqueous NaOH 0.1 mol L-1. [c] Determined using an
integrating sphere, with excitation at 300 nm.
2
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10.1002/chem.201802581
Chemistry - A European Journal
a shift in both absorption and emission maxima. Red shifts are
observed for the compounds bearing an aldimine group CH=N (1a→
1b, 4a→4b, and 6a→6b), and blue shifts are observed for the
compounds bearing a ketimine group CR=N (2a→2b, 3a→3b, and
5a→5b). To better understand the structure-properties relationship,
the crystalline structure of some compounds was studied.
distances between the adjacent dipoles are too large to allow a strong
interaction, thus the exciton splitting is close to zero.23 This is
confirmed by the absorption and emission spectra: there is almost no
shift in the maxima between the solution and solid-state emission
spectra.
Figure 5. Crystal structure of 1a, a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
The molecules of 2a also arrange in a herring-bone fashion24 (Figure
6), but the molecules stack in a staircase fashion with close contact.
The angle between the dipole (represented by the O···O axis) and the
line of molecular centers in the staircase alignment is 54.72º, which
Figure 3. Emission spectra in THF:water solutions or suspensions of
according to the exciton model gives a nearly zero band
dihydroxybenzalazine 1a (ca. 10-3 mol L-1, λex = 400 nm), and photograph of the
Textmarke nicht definiert.
splitting.Fehler!
The distance between adjacent
solutions/suspensions under UV light (365 nm).
molecules is too long to allow a strong interaction of the dipoles, in
accord with the absence of shift observed between the solution and
solid-state emission spectra.
Figure 6. Crystal structure of 2a. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
Figure 4. Normalized solid-state emission spectra of 1a-7a (excitation at 370 nm).
In the crystal of 3a (Figure 7), the two phenol rings are coplanar but
the phenyl rings are perpendicular to the imine bond. The molecules
arrange in planes, into which the molecules are cross-linked through
C-H···O hydrogen bond between an aromatic C-H and a hydroxyl
group. There are no π-π interactions. The dipoles are too far away
from each other to allow a significant interaction, and thus there is no
exciton splitting. In solution, the phenol ring sees its motion restricted
by the intramolecular O-H···N hydrogen bond, but the other phenyl
ring is free to rotate. Similarly to the other compounds, the rotation
around the N-N single bond is not restricted in solution, and as a
consequence, the preferred relaxation pathway is through torsional
vibrations and the molecule is non-emissive in solution. In the
crystalline state, the torsional vibrations of the N-N single bond are
restricted, and the phenyl ring is efficiently blocked by two
intermolecular C-H···O hydrogen bonds, thus promoting
fluorescence.
Single crystals suitable for X-ray diffraction were grown for
compounds 1a, 2a, 3a, 5a, 1b, 2b, and 3b (see SI for the CIF files).
Bond lengths and angles are in normal range.21 As expected, all
compounds present strong intra-molecular O-H···N hydrogen bonds,
that are also present in solution, as witnessed by the NMR spectra
exhibiting a OH proton resonance between 11 and 15 ppm.
Compound 1a can adopt several crystal forms, but in this study only
one polymorph was obtained, so the discussion is based on this form.
The obtained crystal structure of 1a is shown in figure 5.22 The two
phenyl rings are coplanar with the two imino groups and, as a result,
the π system is fully conjugated. The molecules pack in a herring-
bone fashion. In the layers, the molecules are linked by C-H···C
contacts between an aromatic C-H and the C-C=N carbon atoms. The
layers stack without significant interactions (no contact shorter than
the sum of the Van der Waals radii). There are no π-π interactions.
This organization could be described as an H or J-aggregate, but the
3
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10.1002/chem.201802581
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In contrast to compound 1b, compound 2b is not planar (Figure 10).
The steric hindrance created by the methyl substituents forces the
central phenyl rings to be close to perpendicular to the imine plane
(dihedral angle of 80.38º), which in turn is co-planar with the phenol
ring (dihedral angle of 2.00º). This torsion breaks the conjugation in
the chromophore, and explains why its emission maximum is blue-
shifted relatively to compound 1b. It also prevents any π-π
interactions, and forces the dipole to stay far from each other.
Figure 7. Crystal structure of 3a. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
Compound 5a, bearing bromo substituents, also arranges in a herring-
bone fashion in the crystalline state (Figure 8). In the staircase
organization along the b axis, the molecules stack through two
Br···C=N contacts and one C···C contact between the C-OH, leading
to parallel displaced π-π interactions. The angle between the dipole
(represented by the O···O axis) and the line of molecular centers in
the staircase alignment is 32.18º, which according to the exciton
model categorizes the interaction as J-aggregates. The shift in the
emission maxima in solution and in the solid state is small (ca. 20
nm), but the quantum yield is much higher in the solid state. Therefore,
an additional stiffening of the structure in the solid state is the main
reason for the observed increase in emission efficiency.
Figure 10. Crystal structure of 2b. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
Compound 3b crystallizes with two independent molecules in the
asymmetric unit (Figure 11), arranged in a herring-bone fashion and
cross linked through C-H···C and C-H···O interactions. The imine
bond is coplanar with the phenol ring, and at a dihedral angle of
57.82º (or 59.87) with the central ring, and of 71.76º (or 69.62) with
the other phenyl. These torsion angles prevent a stacking of the
molecules and keep the dipoles far away from each other. These
torsion angles also break the conjugation of the backbone, and explain
why the emission is blue-shifted relatively to compound 3a.
Figure 8. Crystal structure of 5a. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue; Br, brown.
The structure of 1b in the solid state confirmed the planarity of the
compound, and the co-planarity of the three phenyl rings (Figure 9).
The compounds stack on top of each other along the b axis, at a
distance of ca. 3.29 Å, the main axis of the molecules at an angle of
46.25º with the line of molecular centers. If the dipole of the molecule
is viewed along the O-H···N axis, it is at an angle of 57.43º to the line
of the molecular centers. This value does not allow the classification
into J or H-aggregates, as it is close to the limit between the two.
Figure 11. Crystal structure of 3b. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
In short, consideration of the crystal structures shows that the
aggregates cannot be described as J or H-aggregates. The distances
between the molecules are too large, preventing a significant
interaction between the dipoles. Moreover, the emission wavelength
does not change significantly between solution and solid state. The
increase in the emission intensity observed when the molecules
aggregate is therefore due to the restriction of molecular motion.
It has to be noticed that, in the solid state, the quantum yields of the
compounds with 1,4-phenylenediamine linker are much lower than
those with hydrazine linker. If the restriction of torsional vibrations
around the N-N single bond is the main cause for the AIEE effect
Figure 9. Crystal structure of 1b. a) asymmetric unit, b) packing. Hydrogen bonds and
short contacts indicated as black dashed lines. Ellipsoids are drawn at the 50% level,
hydrogen are shown as sphere of arbitrary radius of 0.30 Å. C, grey; H, white, O, red; N,
blue.
4
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Conclusion
[6]
[7]
[8]
A family of hydrogen-bonded azines and Schiff bases has been
prepared and characterized. Although their emission intensity in
dilute solution is very low, it increases dramatically in the solid-state.
The study of the material’s crystal structures demonstrated their
Aggregation-Induced Emission Enhancement properties. By
changing the substituents from electron donating to electron
withdrawing, and by increasing or decreasing the conjugation of the
aromatic backbone, the emission color could be tuned from blue-
green to orange-red.
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Experimental Details: All procedures and characterizations are included in the Supporting
Information. CCDC-1843629-1843633 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK. Fax: C44 1223 336 033. E-mail:
deposit@ccdc.cam.ac.uk).
Acknowledgements
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Thanks are due to University of Aveiro, FCT/MEC for the financial support to the
QOPNA research Unit (FCT UID/QUI/00062/2013) and CICECO–Aveiro Institute of
Materials (POCI-01-0145-FEDER-007679; FCT–UID/CTM/50011/2013), through
national funds and where applicable co-financed by the FEDER, within the PT2020
Partnership Agreement, and also to the Portuguese NMR Network. S. Guieu also thanks
the FCT for his post-doctoral grant (SFRH/BPD/70702/2010), the pAge project: Life-
long protein aggregation (Centro-01-0145-FEDER-000003) and acknowledges the
financial support from the European Union Framework Programme for Research and
Innovation HORIZON 2020, under the TEAMING Grant agreement No 739572 - The
Discoveries CTR.
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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5
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Chemistry - A European Journal
Entry for the Table of Contents (Please choose one layout only)
Layout 1:
Hydrogen bonded azines and Schiff
bases are non-emissive in dilute
A rainbow of AIEEgens

solutions
but
exhibit
bright
luminescence upon aggregation, in a
range from bluish-green to orange-red.
Samuel Guieu,* Francisco
Cardona, João Rocha, Artur M. S.
Silva* ……...…… Page – Page
Tunable Color of Aggregation-
Induced Emission Enhancement
in a Family of Hydrogen-Bonded
Azines and Schiff Bases
6
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