Solid-state fluorescence properties of small iminocoumarin derivatives and their analogues in the coumarin series

Article abstract of DOI:10.1016/j.dyepig.2016.04.039

Considering the many potential applications of fluorescent materials, dyes that exhibit emission in the solid state are in high demand, especially if they are easy to prepare and inexpensive. In this respect, exploring the behaviour of basic iminocoumarins seemed relevant. The spectroscopic properties of 3-cyano-7-diethylamino-2-iminocoumarin and a derivative bearing an ethoxycarbonyl group were therefore studied in the solid state, and a comparison was made with the corresponding coumarin. The three compounds had very close fluorescence properties in solution. In contrast, the coumarin derivative was much more emissive in the solid state than the iminocoumarins. This discrepancy was tentatively related to the crystal packing modes. A comparison was made with 7-diethylaminocoumarin to clarify the role of the cyano group, which may not be advantageous from a spectroscopic viewpoint. Using a solvent-exchange method, the three cyano compounds generated elongated microparticles, the substituted iminocoumarin derivative even led to flat microfibers. This work shows that small coumarins are valuable solid-state fluorescent dyes while small iminocoumarins should be used as building blocks to access more elaborated compounds if solid-state applications are envisaged.

Full text of DOI:10.1016/j.dyepig.2016.04.039

Accepted Manuscript
Solid-state fluorescence properties of small iminocoumarin derivatives and their
analogues in the coumarin series
Nihel Abid-Jarraya, Kacem Khemakhem, Hamida Turki-Guermazi, Souhir Abid,
Nathalie Saffon, Suzanne Fery-Forgues
PII:
S0143-7208(16)30168-1
10.1016/j.dyepig.2016.04.039
DYPI 5218
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 March 2016
Revised Date: 22 April 2016
Accepted Date: 24 April 2016
Please cite this article as: Abid-Jarraya N, Khemakhem K, Turki-Guermazi H, Abid S, Saffon N, Fery-
Forgues S, Solid-state fluorescence properties of small iminocoumarin derivatives and their analogues in
the coumarin series, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.04.039.
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Graphical abstract
40 µm
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Solid-state fluorescence properties of small iminocoumarin
derivatives and their analogues in the coumarin series
Nihel Abid-Jarraya a, Kacem Khemakhem a, Hamida Turki-Guermazi a, Souhir Abid a,
Nathalie Saffon b, Suzanne Fery-Forgues c, d
*
a Laboratory of Applied Chemistry HCGP, Faculty of Science, University of Sfax, 3038 Sfax,
Tunisia.
b Service commun RX, Institut de Chimie de Toulouse, ICT- FR2599, Université Paul Sabatier
31062 Toulouse cedex 9, France.
c CNRS; SPCMIB-UMR 5068; F31062 Toulouse, France.
d Université de Toulouse; SPCMIB-UMR 5068; F31062 Toulouse, France.
* Corresponding author. Laboratoire de Synthèse et Physico-Chimie des Molécules d’Intérêt
Biologique (SPCMIB), Université de Toulouse III, Bât.2R1, 118 route de Narbonne, 31062
Toulouse cedex 9, France. E-mail address: sff@chimie.ups-tlse.fr (S. Fery-Forgues)
ABSTRACT
Considering the many potential applications of fluorescent materials, dyes that exhibit
emission in the solid state are in high demand, especially if they are easy to prepare and
inexpensive. In this respect, exploring the behaviour of basic iminocoumarins seemed
relevant. The spectroscopic properties of 3-cyano-7-diethylamino-2-iminocoumarin and a
derivative bearing an ethoxycarbonyl group were therefore studied in the solid state, and a
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comparison was made with the corresponding coumarin. The three compounds had very close
fluorescence properties in solution. In contrast, the coumarin derivative was much more
emissive in the solid state than the iminocoumarins. This discrepancy was tentatively related
to the crystal packing modes. A comparison was made with 7-diethylaminocoumarin to
clarify the role of the cyano group, which may not be advantageous from a spectroscopic
viewpoint. Using a solvent-exchange method, the three cyano compounds generated
elongated microparticles, the substituted iminocoumarin derivative even led to flat
microfibers. This work shows that small coumarins are valuable solid-state fluorescent dyes
while small iminocoumarins should be used as building blocks to access more elaborated
compounds if solid-state applications are envisaged.
KEYWORDS
coumarin; iminocoumarin; solid-state fluorescence; photoluminescence; crystal.
1. Introduction
Fluorescent dyes that emit efficiently in the solid-state are of recent and increasing interest.
Whether used as thin layer or micro/nanoparticles, they find more and more applications in
the fields of optoelectronics, chemical sensing and biological imaging [1-5]. Such molecules
are still a few and their design is delicate. Actually, providing the molecules with a rigid,
conjugated electron system and judiciously located auxochrome groups is not enough to
ensure good solid-state emission properties. The molecular arrangement in the solid state is
also of primary importance. If this arrangement is not appropriate, emission is quenched upon
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aggregation, as frequently encountered for the great majority of fluorophores whose head-to-
head stacking leads to virtual extinction of fluorescence [6, 7].
Some families of dyes have been relatively unexplored in the solid state. This is the
case for coumarins, which paradoxically constitute the most popular class of fluorescent dyes
for use in solution and in polymers. These dyes generally emit in the blue-green region and
they have been widely used as fluorescent tracers and sensors for a very long time [7-10].
Their synthesis is quite easy, so that various substitution patterns are available, and their
spectroscopic and photophysical properties in solution have been extensively investigated.
Curiously, the solid-state fluorescence properties of coumarins and coumarin-based
fluorophores only began to attract interest less than a decade ago [11-25]. These studies have
mainly been conducted with the purpose of developing materials for organic light-emitting
diodes (OLEDs) [12-18] and non-linear optics [19]. It is now well acknowledged that many
coumarins are fluorescent in the solid state. A few authors, including us, have considered the
solid-state fluorescence properties of these dyes with respect to the molecular arrangement in
the crystals [20-23]. This approach is still in its early stages, but it is beginning to provide
some useful information for future research. To the best of our knowledge, no comparable
investigation has been undertaken for iminocoumarins that constitute a close family of dyes.
Their solid-state emission properties have hardly been reported [26-28]. The main drawback
of iminocoumarins is certainly that they are prone to hydrolysis and this may explain why
these compounds are much less known and used than coumarins. But, dyes of the two
families often display very close spectroscopic properties in solution. The advantage of
iminocoumarins is that substitution of the imino function allows easy access to elaborated
compounds. We observed that some derivatives were emissive in the solid state and we
recently showed their value to prepare fluorescent nanofibers and microparticles [26-28].
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Therefore, it seemed instructive to gain more information about these compounds and to see
whether the similarity with coumarins was also true for emission in the solid state.
The aim of the present paper was to compare the photoluminescence properties of
closely related iminocoumarin and coumarin dyes (Fig. 1). Our choice was to investigate very
simple dye molecules that could lead to cheap materials. 3-Cyano-7-diethylamino-2-
iminocoumarin (1) is the basis of various derivatives synthesized and studied in our team [26-
34]. Compound 2 was obtained by grafting an ethoxycarbonyl group on the imino function of
1 and it is a good model of substituted iminocoumarin. Compound 3 is the corresponding
coumarin analogue. For the purpose of comparison, the well-known laser dye, 7-
diethylaminocoumarin (4), was also studied in the solid state. All these compounds are
archetypes of good fluorophores in solution. According to the popular substitution pattern, the
diethylamino group in the 7-position acts as a strong electron donor group, involved in a
favorable resonance structure with the electron acceptor at the 2-position [7, 8]. In compounds
1 to 3, the cyano group was introduced at the 3-position both for synthesis conveniences and
for shifting the fluorescence spectrum to the red [29, 30]. A previous study showed that all
these compounds were indeed valuable as fluorescent markers in solution [31]. In the
following work, the photoluminescence properties were examined on powder compounds and
tentatively interpreted in the light of the crystal packing modes obtained by X-ray analysis. To
get information about the self-association properties of compounds 1 to 3, the type of
microstructures spontaneously obtained using a simple solvent-exchange process was
regarded.
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Figure 1. Chemical structures of the coumarin and iminocoumarin derivatives 1-4.
2. Experimental
2.1. Materials
Ethylacetate was from Sigma Aldrich and dimethylsulphoxide (DMSO) from Fisher
Scientific. Absolute ethanol was from VWR. High-pressure demineralized water (resistivity
18.3 MΩ.cm) was prepared with a Milli-Q apparatus (Millipore). 7-Diethylaminocoumarin
(Coumarin 110, laser grade 99%) was purchased from Sigma Aldrich and used as received.
Compounds 1-3 were synthesized as previously described [35]. In a first step, the
iminocoumarin (1) was obtained by condensation of 2-hydroxy-4-diethylamino-benzaldehyde
with stoichiometric amount of malononitrile and piperidine. To prepare compound 2, N-
ethoxycarbonylation of 1 was performed with ethylchloroformate. Coumarin 3 was prepared
by hydrolysis of 1 in a mixture of acetic acid and ethanol (30:70 v/v). The three compounds
were recrystallized twice in ethanol. The yields were higher than 70%. The compounds were
checked for quality by 1H and 13C NMR (attributions were made using 2D NMR), HPLC/MS,
HRMS and microanalysis.
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2.1.1. 3-Cyano-7-diethylamino-2-iminocoumarin
(3-cyano-7-diethylamino-2H-1-
benzopyran-2-ylidene) (1).
1H NMR (300 MHz, DMSO-d6): δ = 1.12 (t, J = 7.5 Hz, 6H, CH3), 3.43 (q, J = 7.5 Hz,
4H, CH2), 6.31 (s, 1H, H8), 6.59 (dd, J = 9 and 3 Hz, 1H, H6), 7.31 (d, J = 9 Hz, 1H, H5),
8.05 (s,1H, H4), 8.20 (s, 1H, NH). 13C NMR (75.4 MHz, DMSO-d6): δ = 12.30 (CH3), 44.25
(CH2), 94.17 (C3), 96.15 (C8), 106.00 (C10), 108.17 (C6), 116.67 (C≡N), 130.66 (C5),
146.35 (C4), 152.11 (C7), 153.28 (C2), 156.30 (C9). ESI+/HRMS m/z calcd for C14H16N3O:
242.1293; found: 242.1298 (MH+); elemental analysis calcd (%) for C14H15N3O: C 69.69, H
6.27, N 17.41; found: C 69.55, H 6.68, N 17.44.
2.1.2. 3-Cyano-7-diethylamino-2-(N-ethoxycarbonyl)iminocoumarin (Carbamic acid, N-(3-
cyano-7-diethylamino-2H-1-benzopyran-2-ylidene)-, ethyl ester) (2).
1H NMR (300 MHz, DMSO-d6): δ = 1.12 (t, J = 6 Hz, 6H, CH3), 1.27 (t, J = 6 Hz,
3H, CH3), 3.47 (q, J = 6 Hz, 4H, CH2), 4.20 (q, J = 6 Hz, 2H, CH2O), 6.42 (d, J = 3 Hz, 1H,
H8), 6.75 (dd, J = 9 Hz and 3 Hz, 1H, H6), 7.43 (d, J = 9 Hz, 1H, H5), 8.34 (s, 1H, H4). 13C
NMR (75.4 MHz, DMSO-d6): δ = 12.28 (CH3), 14.18 (CH3), 44.41 (CH2), 61.71 (OCH2),
91.70 (C3), 96.24 (C8), 106.39 (C10), 110.04 (C6), 115.63 (C≡N), 131.29 (C5), 148.97 (C4),
151.06 (C2), 152.95 (C7), 155.70 (C9), 158.53 (C=O). ESI+/HRMS m/z calcd for
C17H20N3O3: 314.1505; found: 314.1504 (MH+); C17H19N3O3Na: 336.1324; found: 336.1321
(MNa+); elemental analysis calcd (%) for C17H19N3O3: C 65.16, H 6.11, N 13.41; found: C
64.89, H 6.29, N 13.17.
2.1.3. 3-Cyano-7-diethylamino-2-coumarin (3-cyano-7-diethylamino-2H-1-benzopyran-2-
one) (3).
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1H NMR (500 MHz, DMSO-d6): δ = 1.15 (t, J = 6 Hz, 6H, CH3), 3.51 (q, J = 6 Hz,
4H, CH2), 6.62 (d, J = 3 Hz, 1H, H8), 6.84 (dd, J = 9 Hz and 3 Hz, 1H, H6), 7.52 (d, J = 9 Hz,
13
1H, H5), 8.55 (s, 1H, H4). C NMR (125 MHz, DMSO-d6): δ = 12.30 (CH3), 44.97 (CH2),
90.41 (C3), 96.53 (C8), 107.00 (C10), 110.35 (C6), 116.22 (C≡N), 131.35 (C5), 151.98 (C4),
153.29 (C7), 157.50 (C9), 158.51 (C2). ESI+/HRMS m/z calcd for C14H15N2O2: 243.1134;
found: 243.1143 (MH+); elemental analysis calcd (%) for C14H14N2O2: C 69.41, H 5.82, N
11.56; found: C 69.15, H 5.62, N 11.38.
2.2. Apparatus
High resolution mass spectra were obtained at the ‘Service Commun de Spectrométrie de
masse de l’Institut de Chimie de Toulouse, Université de Toulouse III-Paul Sabatier’ with a
Waters XevoG2QTof used in positive electrospray mode. The 1H and 13C NMR spectra were
recorded at the ‘Service Commun de RMN de l’Institut de Chimie de Toulouse’ on a Bruker
AVANCE AC300 spectrometer operating at 300.13 and 75.48 MHz, respectively, and on a
Bruker AVANCE III HD 500 spectrometer equipped with a cryoprobe Prodigy, operating at
500.11 and 125.75 MHz. The microanalyses were obtained with a Perkin Elmer 2400 series II
elemental analyzer in the ‘Service d’Analyse Chimique du Laboratoire de Chimie de
Coordination de Toulouse’. A HPLC/MS system (AutoPurif, Waters) equipped with a UV-vis
diode array and ES mass spectrometer was used to check the purity of the compounds at the
Chemistry Platform of ITAV. UV-vis absorption spectra and steady state fluorescence spectra
on solutions were recorded with a Xenius SAFAS spectrofluorometer, using a temperature-
controlled cell set at 20°C. Solid state photoluminescence quantum yields were recorded on
the same fluorometer equipped with a BaSO4 integrating sphere. Solid samples were
deposited on a metal support and luminescence spectra were corrected. The excitation source
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was scanned in order to evaluate the reflected light for the empty sphere (La), the samples
facing the source light (Lc) and the sample out of the irradiation beam (Lb). The fluorescence
spectra were recorded with the sample facing the source light (Ec) and out from the direct
irradiation (Eb). The PM voltage was adapted to the measurement of reflected light and
emission spectra, respectively, and proper correction was applied to take into account the
voltage difference. The absolute photoluminescence quantum yield values (ΦPL) were
determined by a method based on the one developed by de Mello et al. [36], using the
formula:
ΦPL = [Ec – (1 – α) Eb] / La α
with α =1– Lc /Lb
The shape and size of the reprecipitated compounds were examined with a Zeiss Axioskop
fluorescence microscope equipped with an Andor Luca camera. The excitation wavelength
was 450-490 nm, and the emission wavelength was set at above 500 nm, using suitable filters.
Scanning electron microscopy was performed at the ‘Service Commun de Microscopie
Electronique de l’Université Paul Sabatier’ using a JEOL JSM 6700F apparatus. The powder
sample was deposited on a self-adhesive tape and metalized with platinum.
2.3. Crystallographic data
Data for compounds 1 to 4 (Table 1) were collected at low temperature (193(2) K) on
a Bruker AXS Quazar APEX II diffractometer (2 and 3), equipped with a 30 W air-cooled
microfocus source (ImS), or on a Bruker-AXS D8-Venture equipped with a CMOS Area
detector (1 and 4), using MoKα radiation (wavelength = 0.71073 Å). Phi- and omega-scans
were used. The structures were solved by direct methods (SHELXS 97) (2 and 3) or using
intrinsic phasing method (ShelXT) (1 and 4) [37, 38]. All non-hydrogen atoms were refined
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anisotropically using the least-square method on F2 [38]. These data can be obtained free of
charge via www.ccddc.cam.ac.uk/data_request/cif.
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Compound
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
1
2
3
4
C14H15N3O
241.29
Triclinic
P 1
C17H19N3O3
313.35
Monoclinic
P21/n
C14H14N2O2
242.27
Triclinic
P 1
C13H15NO2
217.26
Monoclinic
P21/n
6.7900(4)
7.4020(4)
26.3968(16)
93.310(2)
90.680(2)
113.464(2)
1214.08(12)
4
5.0356(5)
20.053(2)
15.7458(17)
90
6.6270(11)
7.3007(11)
13.568(3)
99.071(8)
93.964(9)
113.597(6)
587.67(18)
2
9.8084(5)
10.2408(5)
11.5857(5)
90
b (Å)
c (Å)
α (°)
β (°)
95.434(6)
90
104.678(2)
90
γ (°)
Volume (Å3)
Z
1582.9(3)
4
1125.76(9)
4
Crystal size (mm3)
0.60 × 0.40 × 0.08
0.50 × 0.05 × 0.04 0.20 × 0.06 × 0.02 0.70 × 0.40 × 0.18
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Reflections collected/independent
Rint
21338/4592
0.0360
26632/2660
0.0893
8083/2367
36330/3450
0.0358
0.0403
Parameters/Restraints
Final R1 indice [I>2σ(I)]
wR2 [all data]
336/1
238/71
165/0
147/0
0.0993
0.0447
0.0412
0.0522
0.2644
0.1247
0.1063
0.1433
-3
0.526 and -0.401
0.178 and -0.188
0.181 and -0.190
0.314 and -0.325
Largest diff. peak and hole (e.Å )
CCDC
1459473
1459474
1459475
1459476
Table 1. Crystal data of compounds 1 to 4 and structure refinement.
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3. Results
3.1. Spectroscopic properties in solution
The optical properties of compounds 1 to 3 were investigated in a large number of solvents in
a previous work [31]. These compounds showed positive solvatochromism, which was fully
quantified. The spectra of coumarin 3 and iminocoumarin 1 were very close. The absorption
maximum was between 400 and 430 nm while emission peaked between 440 and 480 nm,
depending on the solvent. By comparison, the spectra of 2 were shifted to the red up to ~30
nm in emission, as exemplified for ethylacetate solutions in Figure 2. Compound 3 was
brightly emissive in apolar solvents (the fluorescence quantum yield ΦF being 0.87 in toluene)
and only very weakly emissive in polar solvents and in water, as it is commonly the case for
coumarin dyes. The fluorescence efficiency of iminocoumarins 1 and 2 was also decreased
with polarity, but these compounds remained emissive in solvents like dimethylsulphoxide
and ethanol (ΦF ~ 0.33 in these solvents for 1). Consequently, the iminocoumarin derivatives
appeared like attractive alternatives to coumarins for use as fluorescent tags in polar and
moderately protic solvents.
Regarding compound 4, its properties in solution and solvatochromic behavior have
been thoroughly studied in the literature [39, 40]. The spectra in ethylacetate were added to
Figure 2 to show that they were situated at shorter wavelengths than those of the cyano
analogue 3. The fluorescence quantum yield of this compound reaches 0.85 in apolar solvents
[39].
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Figure 2. Absorption (A) and fluorescence (B) spectra of compounds 1 (green lines), 2 (blue
lines), 3 (red lines) and 4 (black lines) in ethylacetate. λex = 400 nm for 1 to 3 and 370 nm for
4.
3.2. Solid-state spectroscopic properties
Microcrystalline compound 3 was sharp yellow, compound 1 was less bright and 2
appeared like a dull-looking ocher powder. Upon illumination by a hand-hold UV lamp, 3
was brightly emissive in the yellow while compounds 1 and 2 were less emissive. To quantify
this effect, the solid-state photoluminescence properties were investigated using an integrating
sphere. The three photoluminescence spectra were quite broad, and that of coumarin 3 was
slightly narrower than the others. The three spectra showed a main band peaking around 556
nm and a shoulder at long wavelengths above 610 nm that was not observed in solution. The
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solid-state emission maxima were therefore shifted to the red by ~75 nm with respect to polar
solvents. The intensity of the photoluminescence spectra was drastically varied, the quantum
yields (ΦPL) ranging from 0.13 for 3 to 0.046 and 0.028 for 1 and 2, respectively.
By comparison, compound 4 was a pale yellow powder, strongly emissive under the
UV lamp. Its photoluminescence spectrum peaked at 520 nm and its photoluminescence
quantum yield was found to be 0.26.
It must be emphasized that these solid-state characteristics only refer to the
investigated microcrystalline powders that were obtained by recrystallization in ethanol
(compounds 1 to 3) or used as received (compound 4). Most probably, different values would
have been found with single crystals as well as with powders prepared from recrystallization
in other solvents or mechanical milling, since impurities and surface defects play an essential
role in solid-state quantum yield determination [41]. The solid-state characteristics of these
compounds were gathered in Table 2, together with those of related compounds that have
been reported in the literature or acquired by us in previous works.
Figure 3. Photoluminescence spectra of the powder compounds 1 (green line), 2 (blue line), 3
(red line) and 4 (black line). λex = 440 nm for 1, 2 and 3; λex = 370 nm for 4.
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Compound X
R3
R4
R5
R6
R7
λem
Molecular arrangement Interplanar
distance (Å)
Reference
ΦPL
(nm)
NH
CN
CN
CN
H
H
H
H
H
H
H
H
H
H
H
H
H
NEt2 554
NEt2 558
NEt2 556
NEt2 520
0.046 Head-to-tail dimer
0.028 Slipped parallel
0.13 Head-to-tail dimer
3.39
3.46
3.44
This work
This work
This work
This work
1
2
3
4
N-C(O)OEt
O
O
0.26 Isolated
dimer
head-to-tail 3.57
N-C(O)OEt
N-C(O)OEt
N-C(O)OEt
NH=C(CN)2
CN
CN
CN
H
H
H
H
H
OMe H
486
0.13 Slipped parallel
0.041 nd
3.39
nd
[27]
[27]
[27]
[28]
5
6
7
8
H
OMe 468
OMe 508
NEt2 730
OMe H
0.34 nd
nd
C4H3S H
H
H
0.008 Head-to-tail dimer
Maximal π-π overlap
0.13 nd
3.39
NH=C(CN)2
NH=C(CN)2
C4H3S H
C4H3S H
H
H
OMe H
616
nd
[28]
[28]
9
H
OMe 624
0.13 Slipped
dimer
head-to-tail 3.37
10
no π-π overlap
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N-C(O)- C14H29 CN
H
H
H
H
H
NEt2 535
NEt2 432
0.042 Slipped
head-to-tail 3.39
monomer, nd
[26]
[21]
11
12
dimer
O
H
Me
0.29
Isolated
Slipped parallel,
no π-π overlap
O
O
H
H
CF3
H
H
H
H
NEt2 522
NEt2 512
0.40 Isolated
dimer
head-to-tail 3.529
[21]
[21]
13
14
C2F5
0.30 Isolated
dimer
head-to-tail 3.458
Table 2. Solid-state emission maximum (λem), photoluminescence quantum yield (ΦPL) and molecular arrangement in the crystal for various
coumarin and iminocoumarin derivatives. The numbering of the substituents refers to their position on the molecules (see Fig. 1). nd = not
determined.
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3.2. Crystal packing mode
To clarify the spectroscopic behavior of our compounds in the solid state, it seemed necessary
to investigate the molecular arrangement, and more particularly the stacking of the aromatic
rings, which drastically influences the photoluminescence process. Single crystals amenable
for X-ray analysis were obtained from ethanol solutions of compounds 1 to 3, and methanol
solution of 4, by slow evaporation of the solvents. The molecular arrangement was different
according to the compounds, but in every case, the flat coumarin heterocycles were involved
in π-stacking interaction (Fig. 4).
For compounds 1, 3 and 4, overlying molecules packed in antiparallel pairs where
they were distant by 3.39, 3.44, and 3.58 Å, respectively. The dimers of iminocoumarin 1
were displayed according to a herringbone fashion. The angle between the planes was 24.45°.
Eight short contacts were distinguished per molecule of 1. The oxygen and nitrogen atoms of
the heterocycle were involved in intense interactions with adjacent molecules. All the
molecules of coumarin 3 were aligned in parallel planes. Each of them established nine CH/O
short contacts with its neighbours. In particular, short contacts took place between the
coumarin carbonyl groups and one hydrogen atom of the diethylamino groups of overlapping
molecules. Coumarin 4 was organized as isolated dimers that formed between them an angle
of 43.26°. Each molecule of 4 was involved in 10 short contacts.
In compound 2, stacked molecules were separated by 3.46 Å and arranged as slipped
parallel, so that the overlap was quite reduced compared to the other compounds. The O and
N atoms of the iminocoumarin heterocycle were not involved in short contacts. In contrast,
the O atoms of the ethoxycarbonyl group were involved in short contacts with three other
molecules. The role of the carbonyl group in the structuration of the molecular network has
already been noticed for a closely related long-chain derivative of 2 [26]. The cyano group
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was also involved in a short contact with a neighbouring molecule. As a result, each molecule
of 2 established 17 short contacts with its neighbours.
Figure 4. Crystal packing mode of 3-cyano-7-diethylamino-2-iminocoumarin (1), 3-cyano-7-
diethylamino-2-(N-ethoxycarbonyl)iminocoumarin (2), 3-cyano-7-diethylamino-2-coumarin
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(3) and 7-diethylaminocoumarin (4) showing the arrangement of overlapping molecules. The
overlap between aromatic systems is indicated in pink colour.
3.3. Preparation of microparticles via the reprecipitation method
Organic micro- and nanoparticles may be prepared using various techniques such as
milling [42], homogenization methods [43] and precipitation methods [44], as well as a
combination thereof. The two first methods are top down technologies, i.e. starting from bulk
materials to produce small particles. They are industrially relevant, but milling is a slow
process that generates residues and homogenization methods use big energy-consuming
devices. Precipitation methods are bottom-up processes, starting from dissolved molecules.
They consist in rapidly injecting an organic solution, where the organic compound is initially
dissolved, in a non-solvent medium, leading to nanoparticle nucleation and further growth. In
the popular ‘reprecipitation method’, an aqueous medium is used as antisolvent [45]. It
seemed instructive to see what type of particle can be obtained using this simple and cheap
method, which relies on the self-association properties of the compounds.
To do so, a small volume of dimethylsulphoxide solutions of the three compounds at 3
× 10-2 M was injected in water at room temperature so that the dye concentration in the
reprecipitation medium was 6 × 10-4 M. The mixture was left under stirring in the dark at
room temperature for 2 h, and then it was observed by fluorescence microscopy. The three
turbid suspensions contained elongated fluorescent microparticles. Iminocoumarin 1 gave
rod-like microcrystals that were more than 100 µm long and 2.5 µm wide (Fig. 5a). The tips
were much brighter than the body and fractures allowed light to escape, revealing a clear
wave-guiding effect [46]. Substituted iminocoumarin 2 gave tiny fibrils, while coumarin 3
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gave a mixture of elongated and flat microcrystals (Figs. 5b and c). These particles did not
measure more than 30 µm in length and tended to form sort of a ball. Reprecipitation was
repeated at low dye concentration (3 × 10-5 M in the reprecipitation medium). In this case,
coumarin 3 and iminocoumarin 1 produced very small crystals that were hardly visible with
the fluorescence microscope, resulting in a fluorescent background. In contrast, compound 2
still gave the same type of tiny fibers. For the three compounds, the particles obtained at low
dye concentration were filtered, dried and observed by scanning electron microscopy (SEM).
Agglomerates of inhomogeneous flat microparticles were observed for 1 and 3, as illustrated
for 1 in Fig. 6a. For 2, SEM revealed the presence of flat microfibers with regular gauge,
measuring in average 5 µm long and 500 nm wide (Fig. 6b).
Figure 5. Fluorescence microscopy image of microparticles obtained by reprecipitation of
compounds 1 (a), 2 (b) and 3 (c) at 6 × 10-4 M in water with 2% DMSO. Excitation 450-490
nm, emission > 500 nm.
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Figure 6. Scanning electron microscopy image of the microparticles obtained by
reprecipitation of compounds 1 (a) and 2 (b) at 3 × 10-5 M in water with 0.1 % DMSO.
4. Discussion
This work confirms that very simple coumarin are photoluminescent. This is also the case for
iminocoumarin derivatives, but with extremely different efficiencies. To explain these
differences, it is tempting to look for the relationship between the photoluminescence
efficiency and intermolecular interactions that take place in the solid state. Simple observable
inputs (i.e. relative orientation of stacked molecules, extent of overlap between the aromatic
systems, intermolecular distance, nature and number of short contacts) may be considered.
Most of our molecules packed in antiparallel pairs, as it seems to be frequently the
case for small coumarin and iminocoumarin derivatives substituted by a diethylamino group
at the 7-position [21, 26, 28, 47]. The first evidence is that these antiparallel dimers lead to
strongly emissive materials when they are isolated, that is when the angle between them is
significant, as for compound 4. This result is in line with the conclusions of Park et al. who
reported that, in the coumarin series, isolated dimers are well photoluminescent (e.g.
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compounds 12, 13 and 14) [21]. Most probably, this arrangement confines the excitation
energy in a given dimer and limits the formation of excitons.
The overlap of the aromatic systems was varied in the order 2 < 3 < 1 < 4. By
considering only this parameter, compound 4 could be expected to be the least emissive and
compound 2 the most emissive in the solid state, which is contrary to observation. In contrast,
the intermolecular distance in antiparallel pairs was actually increased with the
photoluminescence efficiency, and seems to be a more relevant parameter.
The molecules studied are not involved in hydrogen bonds, which have been reported
to decrease the photoluminescence efficiency [21]. But, they all are involved in many short
contacts. The large number of short contacts in 2 is probably detrimental to
photoluminescence.
Obviously, a qualitative reasoning based on these simple observable inputs gives a
tendency, but it cannot account for the complexity of the intermolecular interactions involved.
Even for very simple cases like these ones, elaborated DFT calculations would necessary to
fully report on the efficiency of emission in the solid state.
It would be premature to conclude from this work that coumarin dyes emit much
better in the solid state than iminocoumarins and their derivatives. This is probably the case
for small molecules, because small iminocoumarins tend to give a closely packed crystalline
network. However, a careful examination of Table 2 shows that proper substitutions lead to
photoluminescent iminocoumarin compounds that compare well with coumarins. However, in
the iminocoumarin series, it can be necessary to drift from classical substitution patterns. For
instance, it seems that the NEt2 group does not promote photoluminescence. By comparing 2
with analogue 6, it can be seen that the quantum yield was hardly changed when the NEt2
group at the 7-position was replaced by a methoxy group that is a weak electron donor.
Substitution by a methoxy group at the 6-position (compound 5) or by two methoxy groups at
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the 5 and 7-position (compound 7) is much more advantageous [27]. The advantage of a
methoxy group with respect to a diethylamino group was also shown for the elaborated
iminocoumarin derivatives 8 to 10 [28].
Finally, the comparison of 3 with 7-diethylaminocoumarin 4 allows the influence of
the cyano group at the 3-position to be better understood. This group, the introduction of
which is justified by synthesis conveniences [29, 30], allows absorption and fluorescence
spectra to be shifted to long wavelengths in solutions, although this is achieved at the expense
of the fluorescence quantum yield [29]. Qualitatively, a similar effect takes place in the solid
state: the emission spectra were shifted to the red and the photoluminescence quantum yield
was decreased by half when passing from 4 to 3. In these molecules, the presence of the
cyano group at the 3-position is therefore not an advantage for solid-state emission. In
addition, it can be noticed that substitution by a small electron donor or withdrawing group at
the 4-position leads to compounds 12, 13 and 14 that emit better than 3 in the solid state.
Let us now take a closer look at the self-association properties. The formation of
elongated particles via the reprecipitation method was already noticed for several coumarin
and iminocoumarin derivatives [24-28]. It is a natural trend of these heterocycles that is
confirmed here for compounds 1 to 3. In the present case, the N-ethoxycarbonyl group makes
compound 2 more hydrophobic than 1 and 3, so that flat fibers were obtained, even at low dye
concentration in the reprecipitation medium. The regular gauge of these fibers could be an
advantage, but the photophysical properties are not sufficient for applications in the field of
materials.
5. Conclusion
This work confirmed that small coumarin derivatives may have valuable photoluminescence
properties. In contrast, the small iminocoumarin analogues were not very attractive in the
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solid state from a spectroscopic viewpoint. Consequently, both families of dyes showed a
very different behaviour in the solid state, contrary to what occurs in solution. However,
iminocoumarins lend themselves well to various chemical modifications. Our experience
showed that proper substitution leads to dyes that are strongly emissive in the solid state,
including at rather long wavelengths [27, 28]. The natural tendency of these compounds to
give elongated microcrystals can be strengthened, thus leading to very long microfibers [26].
Finally, the chemical stability of iminocoumarins can also be markedly improved by
substitution [28]. In conclusion, this type of small coumarins can be used as they are to
access cheap and robust solid-state emitting materials, while iminocoumarins should rather be
seen as starting compounds to access more elaborated dyes.
Acknowledgments
The authors acknowledge the French Ministry of Foreign and European Affairs and the
Ministry of Higher Education, Scientific research and Technology in Tunisia for their
financial support (PHC-Utique program, managed by CMCU, action intégrée n° 11G 1204).
We are very grateful to EuroNanoMed ERA-NET JTC2011 (FONDIAG project) for financial
support. We would also like to thank Ms. Marine Soulié for LC/MS measurements and Dr.
Nadine Leygues for her help with NMR spectra.
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