Aggregation-induced emission enhancement in halochalcones

Article abstract of DOI:10.1039/c6nj01387b

A family of push-pull fluorophores, consisting of a chalcone core decorated with electron-donating substituents and halogen atoms, was designed and synthesized. Luminescence studies were performed in solution, aggregate form and in the solid state. Although some compounds are only weakly fluorescent in solution, all are emissive in the solid state showing aggregation-induced emission enhancement. In the crystalline state, the halogen atoms are not involved in halogen bonds but their presence strongly influences the aggregation-induced emission properties of the fluorophores.

Full text of DOI:10.1039/c6nj01387b

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DOI: 10.1039/C6NJ01387B
LETTER
Aggregation-induced emission enhancement in
halochalcones
Cite this: DOI: 10.1039/x0xx00000x
Patricia A. A. M. Vaz,a João Rocha,b Artur M. S. Silvaa,* and Samuel Guieua,b,
*
A family of push-pull fluorophores, consisting of a chalcone core decorated with electron-
donating substituents and halogen atoms, was designed and synthesized. Luminescence studies
were performed in solution, aggregate form and in the solid state. Although some compounds
are only weakly fluorescent in solution, all are emissive in the solid state showing aggregation-
induced emission enhancement. In the crystalline state, the halogen atoms are not involved in
halogen bonds but their presence strongly influences the aggregation-induced emission
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
properties of the fluorophores
.
The development of efficient solid organic luminophores is a fluorescence efficiency in solution, because the intramolecular
burgeoning area, due to the potential applications of these torsional motions and photo-isomerization of the C=C double
materials in organic optoelectronics, OLEDs,1 organic solar bond cause radiationless relaxation of the excited molecule.17,18
cells,2 and as biological markers.3 Comparing with Yet, the introduction of electron-donating substituents on the
organometallic complexes, organic chromophores are often para positions of the phenyl rings of chalcones can turn them
easier to synthesize and to handle, being less sensitive to air or more emissive, with a large Stokes’ shift due to the push-pull
moisture. They are also easier to modify in order to tune their character of these dyes.19,20 Chalcone derivatives containing
properties for a particular application. However, organic amino-substituted phenyl groups exhibit fair quantum yields in
compounds exhibiting high emission intensity in the solid state solution, with emission wavelengths reaching the near infrared
are still difficult to design, and predicting their behavior region of the visible spectrum.18 Combining chalcone with a
remains tedious. Many luminophores that are strongly emissive pyrene ring also lead to fluorophores with good emission
in solution lose their efficiency in the aggregate form or in the properties in both solution and solid state.21
solid state. This aggregation-caused quenching (ACQ) is Here, we wish to report the design and synthesis of eight new
attributed to the formation of species, such as excimers and chalcones 1b-e and 2b-e substituted with electron-donating
exciplexes, in the condensed phase.4,5,6,7,8
groups and halogen atoms. These chromophores are based on a
Various strategies have been implemented to increase the push-pull-push system D-π-A-π-D, and the functionalization
efficiency of solid organic luminophores. An elegant approach with halogen atoms was intended to introduce halogen bonds
relies on the aggregation-induced emission enhancement with the carbonyl group to promote the luminescence of the
(AIEE) effect9 or on the aggregation-induced emission (AIE) dyes. They were found to be weakly emissive in solution and in
effect,10,11 which are the opposite of ACQ. In both cases, the solid state. Their aggregation-induced emission
chromophores which are poorly emissive in dilute solution enhancement was studied and rationalized on the basis of their
become highly fluorescent or phosphorescent in the solid state, crystalline structure.
due to restriction of their molecular motion preventing non- The halogenation of substituted benzaldehydes 3a,b and p-
radiative relaxation. This approach has also been combined methoxyacetophenone 5 was performed using N-chloro- or N-
with the formation of halogen bonds between a bromine bromo-succinimide in acetonitrile or dichloromethane at room
substituent and a carbonyl group, promoting inter-system temperature. When necessary, ammonium nitrate was added as
crossing through the heavy-atom effect, thus inducing the a catalyst. The new chalcone derivatives 1b-d and 2b-d were
phosphorescence of a purely organic solid.12
then prepared by a base-catalysed aldol condensation of the
Although some chalcone derivatives are fluorescent, most are benzaldehyde derivatives 4a-d with the appropriate
only weakly emissive and for a long time their study was acetophenones 6a-b (Scheme 1), and the compounds were
focused mainly on biological applications.13,14,15 Nevertheless, obtained in good yields (63-85%) after purification by thin
their optical behaviour has raised some interest, and they have layer chromatography over silica gel. Model compounds 1a22
found applications in linear and non-linear optics.16 The main and 2a23 were obtained following the same procedure.
limitation of using chalcones as fluorophores is their weak
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Scheme 1 Synthesis of chalcone derivatives. Reagents and
conditions: i) NBS or NCS, dichloromethane or acetonitrile,
r.t.; ii) NaOH, MeOH, reflux.
Figure 1 Absorption (full lines) and emission spectra (dotted
lines) of chalcones 1a-e and 2a-e in THF solution (at ca. 1.10-5
mol L-1). Excitation at 370 nm.
The absorption and emission spectra of chalcones 1a-e and 2a-e
in THF solution are presented in Figure 1, and their
characteristics are summarized in Table 1. The chalcones with a
methoxy group on both aromatic rings, 1a-e, present a main
absorption band at ca. 336 nm, while the chalcones with a
dimethylamino group, 2a-e, have their main absorption band
slightly red-shifted to 365 nm. Being a stronger electron-
donating group, the dimethylamino group increases the
intramolecular charge transfer (ICT) and maximizes the push-
pull character of the dyes, explaining the bathochromic shift
observed between 1a-e and 2a-e. This absorption band is
attributed to the HOMO-LUMO transition and, empirically, it is
usually associated with the n→π transition attributed to the
excitation of the aromatic carbonyl group. However, recent
theoretical studies24,25 indicate that in similar compounds this
S0-S1 transition may have a π→π electronic transition character.
Indeed, the orbitals n of the carbonyl group combine with the
orbitals p of the phenyl ring affording this transition a mixed
n→π and π→π character.
Chalcones 1a-e display molar absorption coefficients in excess
of 60,000 M-1 cm-1, higher than those of chalcones 2a-e, but
they are almost non-emissive in dilute THF solution. In
contrast, chalcones 2a-e are fluorescent in solution presenting a
large Stokes’ shift of ca. 120 nm, characteristic of fluorophores
in which the electronic transition has a high ICT.
Overall, the halogen atoms present at the periphery of the
fluorophores do not influence much their optoelectronic
properties in dilute solution. The absorption and emission
characteristics of the dyes seem to depend more on the strength
of the donor substituents (methoxy vs dimethylamino) than on
the characteristics of the halogen atoms. However, when
compared with similar compounds without halogen atoms,20 it
may be concluded that the introduction of halogen atoms
slightly increased the quantum yields of the chromophores in
solution.
Table 1 Optoelectronic properties of chalcones 1a-e and 2a-e
in solution and solid state.
THF solution
Solid state
Stokes’
shift ϕf(a)
ε
Comp. λabs
λem
λem
(nm)
c)
ϕf(b) Ι/Ι0
(
(M-1cm-1)
(nm)
(nm)
(nm)
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
332
335
335
336
337
398
365
366
363
363
77 600 n.a. n.a. n.a.
68 450 n.a. n.a. n.a.
88 300 n.a. n.a. n.a.
76 800 n.a. n.a. n.a.
60 500 n.a. n.a. n.a.
464 <0.01
5
476 <0.01 29
467 <0.01 100
481 <0.01
462 <0.01
7
2
64 300 480
82 <0.01
530 <0.01 1.5
525 0.05 46
521 0.03 34
520 0.03 0.3
523 <0.01 28
44 500 488 123 0.034
40 800 488 122 0.040
30 100 487 124 0.023
29 700 488 125 0.030
(a) Excitation at 370 nm, comparison with fluorescein (quantum yield 0.90 in
water with NaOH 0.01 mol L-1).
(b) Absolute quantum yield measured using an integrating sphere.
(c) Where I is the emission intensity in THF/water (10/90) and I0 is the emission
intensity in THF solution.
Chalcones 1a-e and 2a-e (Table 1 and Figure 2) are all emissive
in the solid state. Chalcones 1a-e, substituted with methoxy
groups, emit weakly at ca. 475 nm, with low quantum yields
(<1%). Chalcones 2a-e, possessing a more electron-donating
group, emit at a red-shifted wavelength (ca. 520 nm), with
higher quantum yields of 1 to 5%. For these compounds, the
emission is red-shifted by ca. 35 nm between solution and solid
state. Like in solution, the nature of the halogen atoms does not
significantly influence the emission properties in the solid state,
the effect of the strength of the electron-donating group being
largely predominant.
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For all chalcones 1a-e and 2a-e, AIEE tests were performed in 2b,c, which have X1 = Cl, than for the compounds which have
THF/water mixed solvents (Figures 3, 4, and SI). Because all X1 = Br (Figure 4). In contrast, the nature of X2 (Cl or Br) does
compounds exhibited a similar behaviour only the experiment not have a noticeable influence on the I/I0 value. VIinewteArretisclteinOgnllinye,
DOI: 10.1039/C6NJ01387B
conducted for 2b is presented, as an example. For all the I/I0 value does not seem to be influenced by the nature of
compounds, the emission intensity was low until the the donor groups, NMe2 and OMe. The larger I/I0 values are
compounds started to precipitate at 60% water. As the water obtained when X1 = Cl, independently of the type of substituent
content in the solvent mixture increased from 60% to 90% the R.
amount of solid suspended in the solution increased and the
emission improved. As depicted in Table 1, the intensity
increased by a factor up to 100 between the solution and the
aggregate form.
Figure
4 Aggregation-induced emission enhancement of
Chalcones 1a-e and 2a-e in THF-Water.
Crystals suitable for single crystal X-ray diffraction were
obtained for compounds 2b and 2d, which crystallized in the
monoclinic centrosymmetric space group P21/n, with Z=8, with
bond lengths and angles in the normal range.26 The asymmetric
units of 2b and 2d consist of two rotamers (Figures 5a and 5b),
which arrange in a dimer through three C-H…O hydrogen
bonds. In 2b, the two molecules present a different dihedral
angle for the enone part, 21.1º and 10.5º. 2d presents similar
features, with dihedral angles of 21.6º and 10.4º for the enone
part of each molecules of the asymmetric unit. For 2b and 2d,
the bond lengths confirmed the localization of the double bonds
of the enone. The dihedral angle between the two phenyl rings
of the chalcone derivatives is between 33º and 42º for 2b and
2d, resulting in a bent conformation, probably disrupting the
conjugation of the backbone.
Figure 2 Photoluminescence spectra in solution (ca. 10-5 mol L-
in THF, solid lines) and solid state (dotted lines) and
photographs of the same samples under a UV lamp. The
excitation wavelength was 365 nm.
1
In the crystal packing, the dimers are held together through a
combination of C-H…O hydrogen bonds, C-H…π hydrogen
bonds, C-H…Cl and C-H…Br (for 2d) hydrogen bonds, and
some close contact between the aromatic rings (Figures 5c and
5d). Noticeably, there are no halogen-halogen contacts, and no
halogen bonds. The nitrogen atom is not involved in
supramolecular interactions. Comparing the structures of 2b
and 2d, it can be seen that the nature of the halogen, Br or Cl,
does not influence much the organization of the molecules in
the crystal.
Overall, these intermolecular interactions efficiently block the
torsional vibrations of the phenyl rings and of the enone
moiety, and extended π - π interactions are not present. These
two features are important to maximize the emission of the
solid through AIEE.
Figure 3 Aggregation induced emission enhancement of
compound 2b in THF-water.
As shown by the AIEE test (Table 1), the place of the halogen
atom is an important feature determining the I/I0 value. The I/I0
value increased more significantly for compounds 1b,c and
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Chemistry Research Unit (EXPL/QEQ-SUP/1041/2013) and
the Portuguese National NMR Network (RNRMN). SG also
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thanks
the
FCT
for
a
post-doctoral
grant
DOI: 10.1039/C6NJ01387B
(SFRH/BPD/70702/2010).
Notes and references
a
University of Aveiro, QOPNA, Department of Chemistry, 3810-193
Aveiro, Portugal. E-mail: artur.silva@ua.pt; sguieu@ua.pt
b
University of Aveiro, CICECO, Department of Chemistry, 3810-193
Aveiro, Portugal. E-mail: rocha@ua.pt
†
Electronic Supplementary Information (ESI) available: [Synthetic
procedures, NMR spectra, absorption and emission spectra, AIEE tests
and crystal structures]. See DOI: 10.1039/b000000x/
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synthesized and their structures fully characterized, including,
for two of them, the crystal structure. The optoelectronic
properties of these materials have been studied in solution, in
aggregate form and in the solid state. The compounds bearing a
dimethylamino substituent present a strong push-pull character
and fair quantum yields in solution and in the solid state. All
compounds exhibit AIEE, their photoluminescence intensity
increasing upon aggregation in mixed solvent up to a hundred
fold. This property has been rationalized on the basis of the
crystal structures obtained for two chromophores. The presence
of halogen atoms, usually quenching the emission through the
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these chromophores in solution. In the solid state, the presence
of halogen atoms enhances the emission, probably by changing
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FCT/MEC and when appropriate co-financed by FEDER under
the PT2020 Partnership Agreement. Thanks are due to the
University of Aveiro and the Portuguese Fundação para a
Ciência e a Tecnologia (FCT) for funding the Organic
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Graphical abtract:
Decorating the periphery of chalcones with halogen atoms increased their aggregation-
induced emission enhancement properties, even if their optoelectronic properties are
dominated by the other substituents.