Novel luminescent phenothiazine-based Schiff bases with tuned morphology. Synthesis, structure, photophysical and thermotropic characterization

Article abstract of DOI:10.1016/j.dyepig.2012.11.001

New Schiff base dimers based on phenothiazine heterocycle have been obtained with high yield and purity by condensation of 3-formyl-10-methyl- phenothiazine with various amines. The structural characterization was performed by elemental analysis, FTIR, 1D and 2D NMR spectroscopy and single crystal and powder wide angle X-ray diffraction. The obtained compounds have high thermal stability and some of them self-assemble into cubic mesophase and form stable molecular glass. The thermotropic study performed by polarized optical microscopy and differential scanning calorimetry revealed the possibility to control the compound morphology by thermal annealing and blending miscible components. The photophysical characterization indicates green light emission, with good absolute fluorescence quantum yield, large Stokes shift and high purity and fully saturated color. The special thermal behavior and photophysical properties appear to be significantly dependent on the special supramolecular architecture generated by the hockey-stick shape of the imino-phenothiazine mesogenic core. Collectively, the properties of these azomethines provide a starting point for new compounds with great advantages for optoelectronic applications.

Full text of DOI:10.1016/j.dyepig.2012.11.001

Dyes and Pigments 96 (2013) 686e698
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel luminescent phenothiazine-based Schiff bases with tuned morphology.
Synthesis, structure, photophysical and thermotropic characterization
,
c
Andrei Zabulica a, Mihaela Balan a, Dalila Belei b, Mitica Sava a, Bogdan C. Simionescu a
,
,
Luminita Marin a
*
a “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania
b “Alexandru Ioan Cuza” University, Department of Organic Chemistry, Iasi, Romania
c “Gheorghe Asachi” Technical University of Iasi, Iasi, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
New Schiff base dimers based on phenothiazine heterocycle have been obtained with high yield and purity
by condensation of 3-formyl-10-methyl-phenothiazine with various amines. The structural character-
ization was performed by elemental analysis, FTIR, 1D and 2D NMR spectroscopy and single crystal and
powder wide angle X-ray diffraction. The obtained compounds have high thermal stability and some of
them self-assemble into cubic mesophase and form stable molecular glass. The thermotropic study per-
formed by polarized optical microscopy and differential scanning calorimetry revealed the possibility to
control the compound morphology by thermal annealing and blending miscible components. The pho-
tophysical characterization indicates green light emission, with good absolute fluorescence quantum yield,
large Stokes shift and high purity and fully saturated color. The special thermal behavior and photophysical
properties appear to be significantly dependent on the special supramolecular architecture generated by
the hockey-stick shape of the imino-phenothiazine mesogenic core.
Received 21 September 2012
Received in revised form
29 October 2012
Accepted 1 November 2012
Available online 12 November 2012
Keywords:
Imine
Single crystal
Green luminescence
Quantum yield
Cubic mesophase
Molecular glass
Collectively, the properties of these azomethines provide a starting point for new compounds with
great advantages for optoelectronic applications.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
[13e15]. Among the reported dimers, azomethines, also named
Schiff bases or imines, are widespread due to their excellent
The thermotropic symmetric dimers formed by two identical
rigid cores linked via a single non-conjugated bridge, usually
a flexible spacer, are interesting compounds from the academic and
technological point of view and are investigated as models for
polymers and for their special liquid crystalline behavior, different
from those of the corresponding monomers. This is why the
synthesis and investigation of the dimers is the expected step in
designing new high performance materials, attracting constant
interest of the scientific community. A quick overview of the liter-
ature reveals many types of rigid units e salicylaldimine [1], cyano-
phenylene [2], azobenzene [3], oxadiazole [4], thiophene [5], indol,
pyrol, diphenyl [6], anthracene [7], pyrene [8], triphenylene [9],
indolinobenzospiropyran [10], cholosteryl [11], metalloporphirine
[12] and so on e used as rod-like, bent core or discotic mesogens in
building dimers. Besides their rich mesomorphism, all these
compounds exhibit interesting optical and/or electrical properties
properties: high thermal stability, liquid crystalline potential,
ability to form complexes, semiconducting, optical and therapeutic
activity [16,17]. Besides, the azomethine connection is easily
attainable in mild reaction conditions, with high purity and yield
[2,7], essential features for electronic and optoelectronic applica-
tions [18,19].
On the other hand, many rigid cores are designed to contain
heterocyclic units due to their ability to impart lateral and/or
longitudinal dipole combined with changes in molecular shapes
bearing a great potential for all optical signal processing, spatial
light modulation, optical information storage, organic thin film
transistors, fast switching ferroelectric materials, fluorescent
probes for the detection and analysis of biomolecules and so forth
[6]. A particularly interesting heterocycle studied for its ther-
apeutical properties but also as a promising candidate for opto-
electronic applications is phenothiazine [20,21]. Phenothiazine
behaves as a very strong electron donating unit due to the nitrogen
and sulfur as electron donating atoms. A thorough literature survey
revealed that azomethines containing phenothiazine heterocycle
received little attention [22,23]. Moreover, only few studies focused
* Corresponding author. Tel.: þ40 749142080.
E-mail address: lmarin@icmpp.ro (L. Marin).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.11.001

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
687
on polyazomethines containing this heterocycle [24,25]. Taking
into account all the above mentioned reasons, we decided to obtain
azomethine dimers containing phenothiazine heterocycle in the
rigid core, as model molecules in building new compounds
addressed to optoelectronic applications.
room temperature. The solution concentration was optimized to
obtain an absorbance around 0.055. The slit widths and detector
parameters were optimized to maximize but not saturate the
excitation Rayleigh peak, in order to obtain a good optical lumi-
nescence signal-to-noise ratio.
Films of the dimers were spin-cast with a Midas SPIN-1200D
Series spin process controller, at four different stages, starting
from 200 rpm till 700 rpm for 25 s each one. The solutions of the
dimers were prepared in DMF at 1 mg/ml concentration. The DP8,
DPE and DP10 were warmed for 1 h at 120 ꢀC because these dimers
have poor solubility in DMF. Once spin-coating is complete, the
probes were placed quickly onto a hot plate (100 ꢀC) for several
minutes to initially evaporate solvent and solidify the coating. The
samples were finally placed into a vacuum oven, overnight, at 50 ꢀC.
2. Material and methods
2.1. Reagents
4-Butoxyaniline 97%, 1,3-bis(3-aminopropyl)-1,1,3,3-tetramet-
hyldisiloxane 95%, 9,9-bis(p-aminophenyl)-fluorene 97%, N, N-
dimethylformamide (DMF) 99.5% and acetonitrile 99.9% were
purchased from SigmaeAldrich and used as received. 3-Formyl-10-
methyl-phenothiazine was synthesized according to literature [26],
and the aromatic diamines containing a flexible spacer (1,2-bis[2-(4-
aminophenoxy)ethoxy]ethane, 1,8-bis-(4-aminophenoxy) octane,
1,10-bis-(4-aminophenoxy) decane) have been synthesized in our
laboratory, too [7].
3. Experimental
Five azomethine dimers have been synthesized by classical
condensation reaction of 3-formyl-10-methyl-phenothiazine with
different diamines (Scheme 1), in a 2:1 M ratio, using dime-
thylformamide as a solvent. To avoid the formation of monoreacted
compound, about 10% excess of aldehyde was used. The reaction
mixture was refluxed for 20 h, under stirring and nitrogen purging,
then allowed to reach room temperature and filtered off. The crude
product was gently refluxed twice with methanol under stirring to
removeunreactedreagentsand DMF, filteredoff and dried overnight
in vacuum at 60 ꢀC. A model compound was also synthesized
(Scheme 1) according to the same procedure. The model compound
was obtained as single crystals by recrystallization from acetonitrile.
The structure of the synthesized azomethines has been
confirmed by two complementary methods, FTIR and 1H NMR
spectroscopies.
2.2. Equipments
Determination of carbon, hydrogen, nitrogen and sulfur content
of the compounds has been performed on a 2400 Series II CHNS
Perkin Elmer elemental analyzer.
Infrared (IR) spectra were recorded on a FTIR Bruker Vertex 70
Spectrophotometer in the transmission mode, by using KBr pellets.
The NMR spectra were obtained on a Bruker Avance DRX
400 MHz Spectrometer equipped with a 5 mm QNP direct detection
probe and z-gradients. The spectra have been recorded in DMSO-d6,
at room temperature, except for DPE for which the spectra were
recorded at 60 ꢀC. The chemical shifts are reported as
(ppm) relative to the residual peak of the solvent.
d values
3-Formyl-10-methyl-phenothiazine,
recrystallized
from
Crystallographic measurements on single crystals were carried
out with an Oxford-Diffraction XCALIBUR E CCD diffractometer
toluene, yellow single crystals.
1
H
C
using graphite-monochromated MoK
tions described in the literature [27].
a
radiation, under the condi-
9
6
2
5
16
S
O
12
11
8
7
15
3
4
13
Wide Angle X-Ray Diffraction (WAXD) was performed on
a Bruker D8 Avance diffractometer, using the Ni-filtered Cu-K
radiation (
¼ 0.1541 nm). The working conditions were 36 kV and
14
a
N10
l
CH3
30 mA. All diffractograms were investigated in the 2/40ꢀ (2 theta
degrees) range, at room temperature. The initial samples for X-ray
measurements were crystalline or powder solids, as obtained by
recrystallization. All diffractograms are reported as observed.
Thermogravimetric experiments were conducted on a Mettler
TGA/SDTA 851e device. 3e3.5 mg of each sample were heated in
Al2O3 crucibles with lids. The samples were heated from 30 to
600 ꢀC at a rate of 10 ꢀC/min, under nitrogen flow (5 mL/min).
Differential scanning calorimetry (DSC) measurements were
performed with a Pyris Diamond DSC, Perkin Elmer USA system,
under nitrogen atmosphere (nitrogen flow 20 ml/min, sample mass
3e4 mg). The transition temperatures have been read at the top of
the endothermic and exothermic peaks.
17
1H NMR (400.13 MHz, DMSO-d6, ppm)
d
¼ 9.81 (s, 1H, H15), 7.75
(d, 1H, H4), 7.61 (s, 1H, H2), 7.25 (t, 1H, H7), 7.18 (d, 1H, H9), 7.10 (d,
1H, H5), 7.04e7.01 (m, 2H, H6, 8), 3.39 (s, 3H, H17); 13C NMR
(100.16 MHz, DMSO-d6, ppm)
d
¼ 190.63 (C15), 150.38 (C11), 143.66
(C14), 130.86 (C3), 130.42 (C4), 128.05 (C7), 127.25 (C2), 126.88 (C9),
123.54 (C8), 122.38 (C12), 121.40 (C13), 115.41 (C6), 114.51 (C5),
35.65 (C17).
MP4 recrystallized from acetonitrile, yellow single crystals, 86%
yield.
18
19
1
H
9
2
20
25
27
17
S
C
15
8
7
The thermotropic behavior of the dimers and their mixtures was
studied by observing the textures with an Olympus BH-2 polarized
light microscope under cross polarizers with a THMS 600 hot stage
and LINKAM TP92 temperature control system.
UVeVis absorption and photoluminescence spectra were
recorded on a Carl Zeiss Jena SPECORD M42 spectrophotometer and
a Perkin Elmer LS 55 spectrophotometer, respectively, in solution
and film, using 10 mm quartz cells and glass plates, respectively.
The fluorescence quantum yield (FF) of the samples was measured
on a FluoroMax-4 spectrofluorometer equipped with a Quanta-phi
integrating sphere accessory Horiba Jobin Yvon, by exciting the
corresponding compound solutions at its maximum absorption, at
N
16
3
4
O
24
CH
3
28
13
12
26
22
21
14
11
N
6
5
10
CH
3
23
1H NMR (400.13 MHz, DMSO-d6, ppm)
d
¼ 8.50 (s, 1H, H15), 7.72
(d, 1H, H4), 7.67 (s, 1H, H2), 7.25e7.22 (m, 3H, H7, 18, 22), 7.18 (d, 1H,
H9), 7.04 (d, 1H, H5), 7.01e6.98 (m, 2H, H6, 8), 6.95 (d, 2H, H19, 21),
3.97 (t, 2H, H25), 3.37 (s, 3H, H23), 1.70 (m, 2H, H26), 1.44 (m, 2H,
H27), 0.94 (t, 3H, H28); 13C NMR (100.16 MHz, DMSO-d6, ppm)
d
¼ 157.15 (C20), 156.61 (C15), 147.46 (C11), 144.36 (C14), 143.98

688
A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
S
CH O
S
CH N R1
H2N R1
- H2O
N
N
CH3
CH3
- 2 H2O H2N R NH2
S
CH N R
N
HC
S
N
N
CH3
CH3
R1: -Ph-(CH2)3-CH3 (MP4);
Scheme 1. Synthesis of the dimers and the model compound.
(C17), 130.90 (C3), 128.68 (C4), 127.90 (C7), 126.82 (C9), 125.95 (C2),
122.98 (C8),122.29 (C12),122.26 (C18, 22),121.44 (C13),114.98 (C6),
114.86 (C19, 21), 114.51 (C5), 67.30 (C25), 35.41 (C23), 30.75 (C26)
18.71 (C27), 13.67 (C28).
DP10, washed with methanol, yellow greenish powder, 90%
yield.
FTIR (KBr, cmꢁ1): 3054 (¼CeH stretch of the aromatic rings),
2932e2851 (CeH stretch of the aliphatic chains), 1618 (eC]Ne
stretch), 1600, 1505, 1467 (C]C ring stretch), 1240 (CeOeC
stretch), 835 (stretch of 1,4 e phenylene ring).
FTIR (KBr, cmꢁ1): 3057 (¼CeH stretch of the aromatic rings),
2955e2866 (CeH stretch of the aliphatic chains), 1619 (eC]Ne
stretch), 1599, 1504, 1463 (C]C stretch of aromatic ring), 1244
(CeOeC stretch), 838 (stretch of 1, 4 e phenylene ring).
Elemental analysis calc. for C24H24N2OS (388.5): C 74.19; H 6.23;
N 7.21; S 8.25. Found: C 73.81; H 6.53; N 7.44; S 8.19.
Elemental analysis calc. for C50H50N4O2S2 (803.11): C 74.78; H
6.28; N 6.98; S 7.98. Found: C 74.62; H 6.31; N 7.13; S 7.91.
DPE, washed with methanol, yellow greenish powder, 88%
yield.
Single crystal: C24H24 N2 O S; space group P 21; cell dimensions:
1H NMR (400.13 MHz, DMSO-d6, 60 ꢀC, ppm)
d
¼ 8.47 (s, 1H,
a ¼ 8.2485(5) b ¼ 8.0466(10) c ¼ 15.3900(11); cell angles:
H15), 7.71 (d, 1H, H4), 7.65 (s, 1H, H2), 7.24e7.15 (m, 3H, H7, H18,
H22), 7.16 (d, 1H, H9), 7.03e6.96 (m, 5H, H5, 6, 8, 19, 21), 4.12 (t, 2H,
H25), 3.78 (t, 2H, H26), 3.65 (s, 2H, H28), 3.82 (s, 3H, H23).
FTIR (KBr, cmꢁ1): 3056 (¼CeH stretch of the aromatic rings),
2961e2864 (CeH stretch of the aliphatic chains), 1619 (eC]Ne
stretch), 1600, 1503, 1467 (C]C stretch of aromatic ring), 1245
(CeOeC stretch), 833 (stretch of 1,4 e phenylene ring).
Elemental analysis calc. for C46H42N4O4S2 (779.00): C 70.93; H
5.43; N 7.19; S 8.23. Found: C 70.58; H 5.51; N 7.27; S 8.11.
DPF, washed with methanol, deep yellow powder, 94%
yield.
a
¼ 90.00(2)
b
¼ 90.243(5)
g
¼ 90.00; V ¼ 1021.46; Z ¼ 2, Z0 ¼ 0;
R ¼ 4.76%.
DP8, washed with methanol, fine green needles crystals, 92%
yield.
FTIR (KBr, cmꢁ1): 3054 (¼CeH stretch of the aromatic rings),
2933e2861 (CeH stretch of the aliphatic chains), 1618 (eC]Ne
stretch), 1600, 1504, 1466 (C]C stretch in aromatic rings), 1241
(CeOeC stretch), 834 (stretch of 1,4 e phenylene ring).
Elemental analysis calc. for C48H46N4O2S2 (775.06): C 74.39; H
5.98; N 7.23; S 8.27. Found: C 73.92; H 6.37; N 7.51; S 8.14.

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
689
FTIR spectra of the studied imines show an intense absorption
band in the 1619e1641 cmꢁ1 range, ascribed to the stretching of the
newly formed azomethine linkage, while the characteristic bands
of the aldehyde (1700 cmꢁ1) and amine (3345 cmꢁ1) groups are
missing, indicating the complete formation of an imine bond
during the condensation reaction [28]. The aromatic-aliphatic
imine linkage of the DPS stretch at 1641 cmꢁ1, while the wave-
length number value of the aromaticearomatic imine linkage
(MP4, DP8, DP10, DPE, DPF) is lower (1618, 1620 cmꢁ1) due to the
lower energy of the CH ¼ N bond related to the better conjugation
of the p-electrons with the phenyl ring p-electrons [29]. The other
characteristic absorption bands of the synthesized compounds are
present as well.
The 1H NMR spectra of the obtained compounds further
confirmed the targeted structures; the imine proton signal is
present at about 8.5 ppm, while the total absence of the amine
(4.5e4.9 ppm) and aldehyde (9.8 ppm) proton signals indicate that
the condensation reaction takes place according to Scheme 1 [28].
The ratios of the integrals corresponding to the azomethine vs. the
aromatic or aliphatic protons agree well with the calculated ones
for the proposed structures. The assignments of all the signals in
the 1D NMR spectra of compounds MP4, DPF and 3-formyl-
phenothiazine were performed using 2D NMR experiments e H,H-
COSY, H,C-HMQC and H,C-HMBC.
1H NMR (400.13 MHz, DMSO-d6, ppm)
d
¼ 8.44 (s, 1H, H15), 7.95
(d, 1H, H28), 7.71 (d, 1H, H4), 7.64 (s, 1H, H2), 7.48 (d, 1H, H25), 7.42
(t, 1H, H27), 7.34 (t, 1H, H26), 7.23 (t, 1H, H7), 7.17e7.11 (m, 5H, H9,
18e22), 7.03e6.97 (m, 3H, H5, 6, 8), 3.36 (H30, overlap with H2O
from solvent); 13C NMR (100.16 MHz, DMSO-d6, ppm)
d
¼ 158.96
(C15), 150.63 (C24), 150.08 (C17), 147.85 (C11), 144.28 (C14), 142.98
(C20), 139.51 (C29), 130.56 (C3), 129.02 (C4), 128.44 (C18, 22 or 19,
21), 127.94 (C7, 26), 127.72 (C27), 126.85 (C9), 126.25 (C2), 126.00
(C25), 123.08 (C8), 122.35 (C12), 121.41 (C13), 120.99 (C19, 21 or 18,
22), 120.59 (C28), 115.05 (C6), 114.54 (C5), 64.31 (C23), 35.45 (C30).
FTIR (KBr, cmꢁ1): 3056 (¼CeH stretch of the aromatic rings),
1620 (eC]Ne stretch), 1594, 1500, 1465 (C]C stretch of aromatic
ring), 840, 822 (stretch of 1,4 e phenylene ring).
4.2. Structural and supramolecular characterization
Elemental analysis calc. for C53H38N4S2 (795.00): C 80.07; H
4.82; N 7.05; S 8.07. Found: C 79.92; H 5.01; N 7.16; S 7.98.
DPS, washed with acetonitrile, light brown powder, 73% yield.
The MP4 model compound has been obtained as single crystals,
allowing its structural and supramolecular characterization.
1H NMR (400.13 MHz, DMSO-d6, ppm)
d
¼ 8.16 (s,1H, H15), 7.53e
In MP4, the phenothiazine unit adopts a butterfly shape, bended
along the S N axis; the angle formed by the neighboring S carbons is
98.5ꢀ and the angle formed by the neighboring N carbons is 118.7ꢀ,
while in 3-formyl-10-methyl-phenothiazine the angle formed by
the neighboring S carbons is 97.7ꢀ and the angle formed by the
neighboring N carbons is 118.4ꢀ (Fig. 1). The wing containing the
imine unit adopts a conformation with the C10eC8eN3eC4 torsion
angle of ꢁ13.4ꢀ and a smaller N3eC4eC7eC6 torsion angle of 9.4ꢀ,
indicating a good conjugation of the imine linkage with the
phenothiazine unit. Moreover, the wider angles in MP4 compared
to the 3-formyl-10-methyl-phenothiazine reagent, indicate
a smaller bending of phenothiazine in MP4 relative to that of the 3-
7.47 (m, 2H, H4, 2), 7.24e7.14 (m, 2H, H7, 9), 7.04e6.98 (m, 3H, H5, 6,
8), 3.48 (t, 2H, H17), 3.33(H23, overlap with H2O fromsolvent),1.64e
1.57 (m, 2H, H18), 0.52e0.45 (m, 2H, H19), 0.049 (s, 6H, H20, 21).
FTIR (KBr, cmꢁ1): 3059 (¼CeH stretch of the aromatic rings),
2952e2822 (CeH stretch of the aliphatic chains), 1641 (eC]Ne
stretch), 1600, 1576, 1466 (C]C stretch of aromatic ring), 1047
(absorbtion band for SieO), 839 (stretch of 1,4 e phenylene ring),
813 (bending band for SieO).
Elemental analysis calc. for C38H46N4S2Si2O (695.12): C 65.6; H
6.67; N 8.06; S 9.23. Found: C 65.42; H 6.71; N 8.15; S 9.21.
formyl-10-methyl-phenothiazine reagent, attributed to a better
electron delocalization. As compared to the literature data which
indicate strong non-planarity of the aromatic azomethine
pe
4. Results and discussions
a
4.1. Synthesis and characterization
compounds consisting in twisted aromatic rings related to the
CH ¼ N plane of 55ꢀ and 10ꢀ, respectively [30], the MP4 model
compound of the synthesized dimers exhibits a more coplanar
molecular conformation of the aromatic azomethine framework.
The planarity of the aromatic azomethine framework combined
with the bended phenothiazine unit gives a hockey-stick rigid core
(Fig. 1), which significantly influences the supramolecular archi-
tecture and hence the optical and thermotropic properties.
Five phenothiazine containing azomethine dimers have been
synthesized by condensation of 3-formyl-10-methyl-phenothiazine
and five different diamines. Fourof the fivediamines contain spacers
with different degree of flexibility and the last one contains fluorene
as a bulky group (Scheme 1). A model compound containing
phenothiazine and a butoxy end group has been synthesized too, in
order to verify the synthetic pathway and to better understand the
dimer properties. The structure of the studied compounds has been
confirmed by elemental analysis, FTIR,1D and 2D NMR experiments.
In the crystal, the molecules are packed forming ribbons with
ꢀ
the intermolecular distance around 8 A, extending parallel to the

690
A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
Fig. 1. The structure of the 3-formyl-10-methyl-phenothiazine (A) and MP4 molecule with the atom numbering scheme (B).
Fig. 2. The packing of MP4 molecules to generate the 3D structure (A); intermolecular distance (B).

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
691
Table 1
a axes. The molecular ribbons are arranged in anti-parallel direc-
ꢀ
The d-spacing/A of the phenothiazine compounds.
ꢀ
tions with the inter-ribbons distance around 3.8 A, forming
ꢀ
molecular layers (Fig. 2). The inter-layers distance is about 15.4 A.
Code d-spacing/Å (corresponding 2 angle)
q
This arrangement of the molecules inside the layers suggests the
forming of a lateral and a longitudinal dipole of the phenyl-imine-
phenothiazine mesogenic core. Bond lengths and angles are given
in supplementary information.
MP4 15.01 (5.89); 7.8 (11.53); 5.15 (17.4); 3.91 (23.16); 3.17 (29.04)
DP8 34.1 (2.59); 19.9 (4.43); 16.7 (5.29); 11.6 (7.61); 10 (8.83); 8.7 (10.19);
8.32 (10.67); 4.59 (19.6); 3.75 (24.26)
DP10 42.25 (2.09); 21.19 (4.17); 12.21 (7.25); 8.61 (10.31); 7.4 (12.02); 5 (17.8);
3.76 (24.14)
To have an insight upon the 3D architecture of the studied
dimers, their wide angle X-ray diffraction (WAXD) has been per-
formed and the obtained diffractograms have been compared to the
WAXD pattern of the MP4 model compound.
DPE 31.21 (2.83); 15.65 (5.65); 10.46 (8.47); 7.87 (11.29); 5.11 (17.52); 3.95
(22.92)
DPF 9.62 (9.21); 8.61 (10.31); 7.78 (11.41); 5.96 (14.96); 4.79 (18.76); 4.35
(20.72); 3.88 (23.4)
Bold signifies the d-spacing corresponding to the intermolecular, inter-ribons and
inter-layer distances.
The WAXD diffractogram of the MP4 crystalline sample shows
intense reflections in the medium and wide angle regions (Fig. 3).
The corresponding distances have been calculated using the Bragg
law, and their values are enclosed in Table 1. The distances found by
the two different X-ray diffraction methods gave close values; the
most intense WAXD reflections correspond to the intermolecular,
inter-ribbons and inter-layer distances measured by Single Crystal
crystallography.
Due to the structure and X-ray pattern similarities between the
MP4 model compound and the dimers containing flexible spacers
(DP8, DP10, DPE), a similar 3D structure is proposed.
4.3. Photophysical properties
As compared to the model compound, the WAXD pattern of
the dimers is characterized by lower intensity peaks, situated in
positions close to those of MP4 model compound. The compounds
containing an aliphatic spacer show many reflections at smaller
angles, indicating different inter-layer distances. This is explained
by two concurrent factors: the position of the aliphatic spacer
between the two rigid end units which hinder its free rotational
movement to adopt the lowest energy trans conformations, and,
on the other hand, the low solubility and high self-assembling
The dimers containing polymethylene and triethoxy spacers
(DP8, DP10, DPE) show a similar absorption trace to the MP4 model
compound, consisting in a broad absorption pattern with three
maxima around 288, 324 and 384 nm, which indicate three
different chromophores (Fig. 4).
Taking into consideration the structural characteristics of the
MP4 model compound (see structural characterization) and liter-
ature data, the 288 nm (4.3 eV) peak has been ascribed to the
pep*
potential that led to
a fast crystallization and forced the
electronic transition into the phenylene ring [31]; the 324 nm
(3.8 eV) peak corresponds to the chromophoric unit formed by
conjugation of the imine linkage with the phenothiazine ring-
system; the broad maximum around 384 nm (3.2 eV) has been
ascribed to the chromophoric unit formed by an extended conju-
gation of the imine linkage with both phenylene and phenothiazine
units. This attribution is sustained by the absorption spectrum of
the siloxane spacer containing dimer, which presents only one
maximum around 308 nm (4.02 eV) due to the chromophoric unit
formed by conjugation of the imine bond with the phenothiazine
ring; the other two peaks are missing due to the absence of the
phenylene ring. The fluorene containing dimer presents an
absorption pattern similar to the flexible spacer containing
compounds, due to the similar conjugation possibilities; the
broader peak around 290 nm is due to the fluorene ring. All
aliphatic spacer to adopt various proportion of gauche conformers.
These peaks in the low angle region are missing into the WAXD
pattern of the fluorene containing dimer, due to the absence of
the flexible unit which favors the layering. The d-spacing values
calculated for the reflections in the wide angle region are close to
the intermolecular and inter-ribbon distances of the model
compound.
DP8
DPE
DP10
DPF
MP4
DPS
300
350
400
450
500
Wavelength (nm)
Fig. 3. X-ray diffraction patterns of the phenothiazine compounds (a) MP4, (b) DP8, (c)
DP10, (d) DPE, (e) DPF.
Fig. 4. UVeVis spectra of the phenothiazine containing imines.

692
A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
Table 2
compounds exhibit a similar absorption edge around 450 nm
(2.75 eV). Compared to other reported Schiff bases [16], the
absorption of studied dimers around 384 nm indicates a very good
conjugation, confirmed by Single Crystals diffraction as well.
The new dimers display daylight fluorescence characterized by
remarkable large Stokes shifts determined by absorption/emission
spectroscopy.
Photophysical characteristics of the studied azomethines.
Egb/eV Stokes
shiftc/nm
141
Fsol Ffilm 1931 CIE x; yg
a
d
f
Code lmax abs/nm
lmax
a
em./nm
MP4 288, 322, 384 525 (502) 3.22
DP8 288, 326, 382 526 (504) 3.24
DP10 288, 326, 382 526 (504) 3.24
DPE 288, 322, 384 526 (502) 3.22
DPF 384 (392)
DPS 308
0.125 0.08 0.398; 0.598
0.126 0.08 0.404; 0.588
0.125 0.08 0.424; 0.627
0.127 0.09 0.405; 0.610
0.375 0.17 0.468; 0.547
144
144
142
151
176
The luminescence properties of the studied dimers, in both
solution and film, have been explored by excitation at their
absorption maximum wavelengths. All compounds emit green light
(around 526 nm), except for DPS, which emits greenish light
(484 nm) (Fig. 5A), reflecting the influence of the conjugation
length upon emission wavelength. The emission spectra are char-
acterized by a sharp emission band, the position and intensity of
which does not significantly depend on the wavelength of exciting
light (Fig. 5B). This indicates that the main fluorophore is the one
needing the smallest energy amount for electronic transition and
thus having the most extended conjugation, which is the unit
formed by conjugation of the phenothiazine moiety with the
phenylene ring through the imine linkage. The blue shifting of the
lmax of DPS emitted light sustains once more this attribution; the
emission wavelength being controlled by conjugation, in the
absence of the phenylene ring the conjugation length is less
extended and the DPS emits greenish light. By the contrary, the lmax
of the emitted light of the DPF fluorene containing dimer is red
shifted by 10 nm due to the supplementary stabilization of the DPF
535 (526) 3.22
484 (483) 4.02
ee
ee
ee
a
In DMF, values in parenthesis are for thin film samples.
Spectroscopically determined energy gap taken from the absorption maxima
b
(Eg ¼ 1240/lmax).
c
Stokes shift between the absorption and emission of the chromophores.
Absolute fluorescence quantum yield in solution, at room temperature.
Not available.
Absolute fluorescence quantum yield in film, at room temperature.
Chromaticity coordinates [39].
d
e
f
g
quinoid structure as a consequence of the electron-donor effect of
fluorene [32].
The fluorescence spectra of the solid films obtained by casting
10ꢁ5 w/v solution on glass, registered in similar conditions as in the
case of solutions, show the same optical parameters in terms of
emission wavelength and intensity, suggesting little fluorescence
quenching. Taking into account the structure of the model
compound, this can be explained by the long intermolecular
distances which hinder the non-radiative decay by intermolecular
energy transfer, under molecular aggregation conditions [33]. The
photophysical parameters are given in Table 2.
A
DP10
The fluorescence quantum yield (Ffl) of the studied azome-
thines, in solution and solid-state, was measured using an inte-
grating sphere [34]. The azomethine linkage is known to quench
the fluorescence of even the most intrinsically fluorescent fluo-
rophores [35,36]. However, as seen in Table 2, the studied
phenothiazine-based azomethine compounds show good absolute
Ffl values, comparable to polyfluorene vinylene, used in emitting
devices [37]. The dimers show good Ffl, even in thin films, sug-
gesting that there is little self-quenching. Remarkably, the highest
Ffl in solution and solid-state, was obtained for the dimer con-
taining phenothiazine and fluorene chromophores, the two fluo-
rophores do not seem to quench each other.
DP8
DPS
DPF
MP4
DPE
The Stokes shift values are comprised between 141 and 176 nm,
resulting in a negligible overlap between the absorption and
emission spectra of the studied azomethines, preventing the re-
absorption of the emitted light (Fig. 6) [38]. This large Stokes shift
allows unique advantages for detection of emitted light fluores-
cence in biological applications and is very important in order to
avoid undesired loses in the optoelectronic devices.
In the color perception study, one of the first mathematically
defined color spaces is the CIE-1931 XYZ color space [39]. The
chromatic perception of human eyes to a specific optical spectrum
is usually characterized by a chromaticity diagram. According to the
available CIE standard, the chromaticity coordinates of the studied
compounds have been calculated (Table 2) and they indicated that
MP4 and DP8 are situated in the yellow green region, while DPF is
in the greenish yellow region. They are on the periphery zone, i.e.
they emit high purity and fully saturated color (Fig. 7).
300
350
400
450
500
550
600
650
Wavelength (nm)
B
288 nm
326 nm
382 nm
4.4. Thermal properties
The thermal properties of the studied dimers have been moni-
tored by thermogravimetric analysis (ATG), differential scanning
calorimetry (DSC) and polarized optical microscopy (POM).
All compounds present good stability at high temperature; the
onset of decomposition was registered around 400 ꢀC, except for
400
450
500
550
600
650
700
Wavelength(nm)
Fig. 5. Emission spectra of (A) studied compounds, excited at their maximum of
minimum energy absorption, (B) DP8, excited at all maxima of absorption.

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
693
Stokes shift
100
90
80
70
60
DP8
DPS
DPF
50
40
30
0
100
200
300
400
500
600
300
350
400
450
500
550
600
650
o
Temperature ( C)
Wavelength (nm)
Fig. 8. Thermal stability of studied dimers evidenced by thermogravimetric graphs.
Fig. 6. Stokes shift of DP8.
the compound containing the siloxane spacer (DPS) whose
decomposition started around 260 ꢀC. The decomposition process
ended around 500 ꢀC and the residual chair percent ranged
between 35 and 52. These thermal parameters indicate a good
thermal stability, which makes these compounds suitable for
applications requiring prolonged and repetitive exposure to high
temperature or high-intensity lasers. Some representative ther-
mogravimetrical curves are given in Fig. 8.
To establish the thermodynamic behavior of the studied
compounds and their morphological stability, respectively, two
complementary measurement techniques, differential scanning
calorimetry (DSC) and polarized optical microscopy (POM), have
been applied with a heating/cooling rate of 5oC/min. Thus, in the
DSC thermogram, the MP4 model compound shows a sharp left
tailed endothermic peak in the first heating scan, corresponding to
melting. A broad exothermic peak is present in the first cooling
scan, corresponding to the slow and incomplete crystallization. An
exothermic and an endothermic peak appear in the second heating
scan, corresponding to the cold crystallization and melting,
respectively (Fig. 9A).
Fig. 9. A) DSC curves of (a) first heating, (b) first cooling, and (c) second heating of
MP4, and first heating of (d) DP8 and (e) DP10. (B) DSC thermogram of DPE (a) first
heating, (b) first cooling, (c) second heating.
Fig. 7. The chromaticity diagram.

694
A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
Interestingly enough, in POM, during the heating scans with
The dimers containing polymethylene spacer (DP8 and DP10)
have a thermotropic behavior quite similar to that exhibited by the
MP4 model compound. In the DSC thermogram, they show two
close endothermic peaks in the first heating scan (Fig. 9A), an
exothermic peak in the first cooling scan and a left tailed endo-
thermic peak in the second heating scan. In POM, between the two
endothermic peaks, in the first heating scan, the crystalline sample
melts to a fine texture (Fig. 10c) which transforms in an optically
isotropic liquid that further nucleates in a clear mosaic texture,
1oC/min, the crystalline sample melted in a mosaic texture
(Fig. 10a), then the sample became optically isotropic, which state
nucleates in birefringent straight-edged squares, rhombic, hexa-
gons and rectangles (Fig. 10b). The textures appeared in a narrow
temperature interval (2 ꢀC), corresponding to the endothermic tail.
Based on its typical behavior, the second texture has been ascribed
to a three-dimensional thermotropic cubic mesophase [40]. In
cooling scans, the crystallization occurs in the form of spherulites.
Fig. 10. POM microphotographs of the MP4 and DP8 compounds (H: heating; C: cooling).

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
695
ascribed to a cubic mesophase, as well (Fig. 10d). By cooling from
the isotropic state, the crystallization occurs in the form of a fine
granular texture. Under special conditions e slowly cooling the
sample before the complete isotropization e a mosaic texture
appears when cooling the sample by 1ꢀC/min (Fig. 10e) or a marble
texture occurs when cooling the sample by 5 ꢀC/min (Fig. 10f). Both
textures lack fluidity. This behavior suggests the ordering difficul-
ties of the molecules in the absence of nucleation centers, because
of the non-coplanar structure of the phenothiazine ring-system
which leads to weak intermolecular forces and thus high inter-
molecular distances (see Structural and supramolecular character-
ization). The self-assembling in the tridimensional cubic
mesophase is driven by the special supramolecular architecture
generated by the hockey-stick shape of the phenothiazine-imine-
phenylene core, which consists in long intermolecular distances
but short inter-ribbon and inter-layer distances. Due to this archi-
tecture, when increasing temperature the intra-ribbons ordering is
lost before losing the supramolecular ordering. As a consequence,
in the short temperature range of coexistence of both intra-ribbon
disordering and supramolecular ordering, the cubic mesophase
appears.
second heating scan (Fig. 9B). By POM, corresponding to the short
temperature range between the two DSC endothermic peaks, a fine
viscous texture appears which exhibits fine crystals in isotropic
liquid, under mechanical stress. This behavior is attributed to
a dimensional polydispersity of the crystallites and to the coexis-
tence of various order degrees which lead to different melting
temperatures. By cooling from the isotropic state, the DPE sample
freezes, giving an amorphous glass and in the second heating scan
a high temperature cold crystallization before melting occurs
(Fig. 11a). This behavior is reproducible in many heating/cooling
scans in DSC and POM as well. Under slow cooling rates, a viscous
fine granular texture is induced in a short temperature range,
indicating a monotropic mesophase (Fig. 11b). Depending on the
thermal treatment, an amorphous or mesomorphic glass is ob-
tained as continuous films, without defects. The amorphous glass
remains morphologically stable at room temperature, even after
two month, due to the high cold crystallization point.
The fluorene containing dimer exhibits two broad endohermic
peaks in DSC thermograms, corresponding (in POM) to the
appearance of a fine viscous texture under mechanical stress
(Fig. 11c) and its slow isotropization, respectively. This type of fine
viscous texture, named plastic mesophase, is often exhibited by
stiff-chain thermotropic polymers without flexible spacers, hardly
ordering in molten state [2,28,41e43]. In the next cooling/heating
cycles, the sample exhibits only high glass transition; the sample
The dimer containing a triethoxy spacer behaves as a molecular
glass [19]. In the DSC thermogram, the DPE sample exhibits two
endothermic peaks in the first heating scan, an inflexion of the
cooling trace, an exothermic and an endothermic peak in the
Fig. 11. POM microphotographs of the DPE and DPF compounds and DP10 þ DPF mixture (H: heating; C: cooling).

696
A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
Table 4
gives a stable amorphous glass at room temperature. By cooling the
sample from the plastic mesophase, a birefringent glass, without
defects has been obtained.
The siloxane containing dimer shows only one glass transition in
heating and cooling scans. The thermodynamic behavior is
summarized in Table 3.
In conclusion, the thermodynamic behavior studied by DSC and
POM indicates a versatile behavior of the phenothiazine containing
dimers, depending on the single non-conjugated bridge nature and
thermal treatment, suggesting the possibility of tailoring the
morphology of the samples.
Thermotropic behavior of physical mixtures.
Code
DSC transitions/oC
(J/g)
Observations
MP4 þ DP8 1H: Cr1 139 (42.3) I1; Many heating/cooling scans lead to an
Cr2 196 (33.7) I2
1C: I1 þ I2 185 (ꢁ42.4)
Cr2; 115 (ꢁ13.1) Cr1
homogeneous crystalline phase
4C: I 158 (ꢁ56) Cr
MP4 þ DP10 1H: Cr1 138 (30) I1;
Many heating/cooling scans lead to an
homogeneous crystalline phase
Cr2 192 (59) I2
1C: I1 þ I2 159 (ꢁ61.2)
Cr2; I1 88 (ꢁ2.6) Cr1
4C: I 134 (ꢁ69.6) Cr
DP8 þ DP10 1H: Cr 200 (115.2) I
1C: I 167 (ꢁ76.8) Cr
The mixture behaves as a single
crystalline compound
4.5. Physical mixtures
2H: Cr 200 (76.2) I
The obtaining of polycrystalline films is one of limiting factors of
mass production of organic electronics. Uncontrollable grain
growth, their randomness in orientation and grain boundary effect
are the main factors hindering the formation of uniform films.
Therefore, obtaining highly uniform organic films with acceptable
optic and/or electrical performances from cost-effective thermal
processing is one of the key challenges for commercializing large
scale organic optoelectronic devices [18,19].
The versatile thermotropic behavior of the studied phenothia-
zine containing imines and on the other hand the importance of
stable, controlled morphology for optic and optoelectronic appli-
cations inspired us to investigate the possibility of tuning
morphology, by obtaining mixtures of compounds containing the
same rigid core. Taking into account the thermotropic behavior of
the studied dimers, two kinds of binary mixtures have been
produced: crystallineecrystalline (MP4 þ DP8; MP4 þ DP10;
DP8 þ DP10) and crystallineeamorphous (MP4 þ DPF; DP8 þ DPF;
DP10 þ DPF; DP8 þ DPS). The blending of the samples has been
realized in 1:1 M ratio. Their thermal behavior was monitored by
DSC and POM measurements (Table 4).
MP4 þ DPF 1H: Cr1 137 (29.2) I1; An amorphous glass has been obtained
Cr2 195 (5.4) I2
1C: I 84 G
after the mixing of the two components
2H: G 84 I
DP8 þ DPF 1H: Cr1 198 (60.4) I1; Behavior typical for molecular glass
Cr2 239 (9.2) I2
1C: I 85 G
2H: G 85 I 132 (ꢁ38.8)
Cr 193 (19.2) I
DP10 þ DPF 1H: Cr1 197 (53) I1;
Cr2 240 (11.4) I2
Behavior typical for molecular glass
1C: I 89 G
2H: G 86 I 134 (ꢁ26.2)
Cr, 193 (29.5) I
DP8 þ DPS 1H: Cr1 205 (13.3) I
1C: I 141 (ꢁ27.7) Cr
4H: Cr 204 (11) I
The two components maintain their own
behavior, even after four heating/cooling
cycles
mixture shows phase separation with two well delimited melting
points and two crystallizations corresponding to the two compo-
nents, in the first heating/cooling scan. But their delimitation
decreases when they reach the fourth heating/cooling scan; the
occurrence of only one melting/crystallization process indicates
a homogeneous crystalline mixture. When the mixed compounds
have close melting/crystallization points (DP8 þ DP10), they
behave as a single crystalline compound even from the first heat-
ing/cooling scan.
The mixture of crystalline compounds leads to a crystalline
blend. When the mixed compounds exhibit large differences
between their melting points (MP4 þ DP8; MP4 þ DP10), the
Table 3
Thermotropic behavior of studied azomethines.
Interestingly enough, crystallineeamorphous blends lead to
amorphous glasses. When using amorphous DPF, a typical behavior
of molecular glass exhibiting a high glass transition in the cooling
Code DSC transitions/oC
(J/g)
Observations
MP4 1H: Cr 140 (114.8) I The endothermic peak is left tailed; corresponding
1C: I 93 (ꢁ64.4) Cr1 to the temperature range of the endothermic tail
2H: Cr1 101 (ꢁ37.3) a cubic mesophase is evidenced by POM, in special
Cr2 140 I
conditions
DP8 1H: Cr 215 (19.7) Cub A cubic mesophase is formed during the heating
224 (83.4) I
cycle
1C: I 188 (ꢁ89) Cr
2H: Cr 221 Cub 224
(89.9) I
3H
2H
DP10 1H: Cr 205 (28.6) Cub A cubic mesophase is formed during the heating
216 (93) I
cycle
1C: I 173 (ꢁ85.3) Cr
2H: Cr 216 (95.8) I
DPE 1H: Cr1 179 Cr2 181 Typical behavior of molecular glass, reproducible
1H
1C
(87.4) I
on many heating/cooling scans
1C: I 58 G
2H: G 55 I 95 (ꢁ41.7)
Cr 177 (57.6) I
DPF 1H: Cr1 217 (13) PM An amorphous glass has been obtained by cooling
265 (42.3) I
1C: I 142 G
2H: G 141 I
from isotropic state
A mesomorphic glass has been obtained by
cooling from plastic mesophase state
An amorphous glass has been obtained at low
temperature
DPS 1C: I 14 G
2H: G 14 I
0
50
100
150
200
250
o
Temperature ( C)
The transition temperatures have been read at the top of DSC peaks. Cr: crystalline;
I: isotropic; Cub: cubic mesophase; PM: plastic mesophase; G: glassy state.
Fig. 12. DSC thermogram of DP10 þ DPF mixture.

A. Zabulica et al. / Dyes and Pigments 96 (2013) 686e698
697
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been detected after mixing the components in the molten state.
This behavior is reproducible in many heating/cooling scans. As an
example, Fig. 11d illustrates the cold crystallization of the
DP10 þ DPF mixture under POM, while Fig. 12 shows its DSC
thermogram. On the other hand, when using amorphous DPS, the
two components kept their own behavior even after four heating/
cooling scans. The different behavior of the two kinds of
crystallineeamorphous mixtures is explained by the different
mixing and ordering capacity of the two amorphous compounds.
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Acknowledgments
The financial support of the European Social Fund e “Cristofor I.
Simionescu” Postdoctoral Fellowship Programme (ID POSDRU/89/
1.5/S/55216), Sectoral Operational Programme Human Resources
Development 2007e2013 is acknowledged.
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