Structure-Directed Functional Properties of Phenothiazine Brominated Dyes: Morphology and Photophysical and Electrochemical Properties

Article abstract of DOI:10.1021/acs.cgd.6b00212

A series of nine dyes based on electron-donating phenothiazine core functionalized with various carbonyl containing electron-withdrawing moieties as primary end group and with or without bromine as a second heavy end group were designed and synthesized to generate variable supramolecular architectures with distinct thermotropic, photophysical, and electrochemical properties. The supramolecular architecture of the obtained dyes was studied in detail by single crystal X-ray diffraction and polarized light microscopy, while optoelectronic properties were investigated by UV-vis and photoluminescence spectroscopy and cyclic voltammetry measurements. The phenothiazine dyes emit light from UV to orange domain, depending on the electron-withdrawing substituent, with great quantum yield reaching up to 71% and high color purity. The introduction of the bromine as a second substituent produced a nearly 2-fold increase of the quantum yield, compared with their counterparts. It was settled that this major benefit of the bromine on the improvement of the quantum yield happened because by its presence some molecular orbital interactions were generated. An interesting and challenging achievement was evidenced in the case of the direct connection of the formyl group to the phenothiazine ring when an unexpected planarization of the dye, unknown up to now in the literature, with drastic consequences on the photophysical and electrochemical behavior was attained. It was concluded that there is a close relationship between the nature of the building blocks of the phenothiazine dyes and their ability to promote favorable properties for different optoelectronic applications.

Full text of DOI:10.1021/acs.cgd.6b00212

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Article
Structure-directed functional properties of phenothiazine brominated
dyes. Morphology, photophysical and electrochemical properties
Andrei Bejan, Sergiu Shova, Mariana Dana Damaceanu, Bogdan C Simionescu, and Luminita Marin
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00212 • Publication Date (Web): 24 May 2016
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Structure-directed functional properties of phenothiazine brominated dyes. Morphology,
photophysical and electrochemical properties
Andrei Bejan, Sergiu Shova, Mariana Dana Damaceanu, Bogdan C. Simionescu and Luminita
Marin*
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”Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania
lmarin@icmpp.ro
Abstract
A series of nine dyes based on electronꢀdonating phenothiazine core functionalized with various
carbonyl containing electronꢀwithdrawing moieties as primary end group and with or without
bromine as a second heavy end group, were designed and synthesized to generate variable
supramolecular architectures with distinct thermotropic, photophysical and electrochemical
properties. The supramolecular architecture of the obtained dyes was studied in detail by single
crystal Xꢀray diffraction and polarized light microscopy, while optoelectronic properties were
investigated by UVꢀvis and photoluminescence spectroscopy and cyclic voltammetry
measurements. The phenothiazine dyes emit light from UV to orange domain, depending on the
electronꢀwithdrawing substituent, with great quantum yield reaching up to 71 % and high color
purity. The introduction of the bromine as a second substituent produced nearly the twofold
increase of the quantum yield, compared to their counterparts. It was settled that this major
benefit of the bromine on the improvement of the quantum yield happened, due to the fact that by
its presence some molecular orbital interactions were generated. An interesting and challenging
achievement was evidenced in the case of the direct connection of the formyl group to the
phenothiazine ring when an unexpected planarization of the dye, unknown up to now in the
literature, with drastic consequences on the photophysical and electrochemical behavior was
attained. It was concluded that there is a close relationship between the nature of the building
blocks of the phenothiazine dyes and their ability to promote favorable properties for different
optoelectronic applications.
1. Introduction
Use of organic materials as active substrate in building electronic and optoelectronic
devices captured growing interest in development of highly conjugated compounds with efficient
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performances. Compared to their inorganic counterparts, organic materials bring great
advantages: are cheaper, environmentally safer and easily amenable for modulation.
Supramolecular organization capability of the small molecules enables higher charge carrier
mobility compared to polymeric semiconductors with large amorphous fractions. Hence,
developments of low molecular weight compounds and further understanding the relationship
between structure and properties are of crucial importance and still remain challenging to the
scientific community. Among organic compounds, those based on (hetero)aromatic πꢀconjugated
systems endꢀcapped with an electron donor and electronꢀacceptor giving donorꢀacceptor
chromophores proved significant broader opportunities [1].
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Phenothiazine – a fused heteroaromatic ring attracted a special attention in view of use as
active substrate in electronic and optoelectronic devices. Due to the presence of rich electron
sulfur and nitrogen atoms, phenothiazine has a strong electronꢀdonating feature and hence ability
to promote extended conjugation. Some phenothiazine based compounds proved high carrier
mobility and low emission properties being used in building photovoltaics [2ꢀ5] or field effect
transistor [6ꢀ8], while others proved high luminescence and moderate mobility giving organic
light emission diodes with good performances [9ꢀ16]. The balance of properties which guides the
material to the applications is a function of the intrinsic properties but also of the supramolecular
arrangement. Even if a plenty of phenothiazine based materials were synthesized and used in
testing for above mentioned optoelectronic devices, only for a few the supramolecular structure
was reported [17ꢀ22]. They revealed that phenothiazine ring adopts a bent structure known as
butterfly geometry, which further promotes a supramolecular architecture with large
intermolecular distances of the compounds containing it. In this way, the nonꢀradiative decay is
hindered, and thus good luminescence in solid state is preserved, making compounds containing
it promising candidates for organic light emitting diodes.
On the other hand, the efforts drained in the direction of luminescence improving of
organic compounds, recently revealed that compounds functionalized with carbonyl and bromine
moieties have an amazing increase of luminescence quantum yield even for simple rigid
chromophore units [23ꢀ25].
In this context, the present paper reports a systematic study on a large series of nine
donorꢀacceptor phenothiazine dyes functionalized with heavy bromine atom and/or electronꢀ
acceptor carbonyl groups as formyl, carboxyl and cyanoacrylic acid moieties. Six of them were
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successfully grown as single crystals. The study is related to the supramolecular packing,
photophysical and electrochemical characteristics of the obtained donorꢀacceptor dyes, with a
special concern on the understanding the structure ꢀ properties relationship and the role of
bromine on electronic structure and photophysical properties. It should be stressed that the
systematic and detailed study from morphological, optical and electrochemical point of view of a
large series of phenothiazine derivatives is reported here for the first time.
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2.1. Experimental
Materials
Iodophenol 99%, nꢀbromohexane 98%, phenothiazine 98%, cyano acetic acid 99%,
copper powder 99.9%, phosphorous oxychloride 99%, Nꢀbromosuccinimide 99% and
dimethylformamide 99.8% were purchased from Aldrich and used as received. 18ꢀCrownꢀ6 99%
from Fluka and piperidine 99.5% from Acros Organics were used without any further
purification. All the solvents of high purity used in this study were purchased from Aldrich and
used as received.
Synthesis
All the syntheses were performed with an input from literature data [29ꢀ34] and optimized
to reach the best reaction yield. Synthetic procedure and structural characterization by NMR,
FTIR and elemental analysis is provided in supplementary information section.
Crystal growth
Taking into consideration the chemical structure of the new synthesized dyes, more
specifically the presence of the aliphatic tails and the rigid aromatic core with donorꢀacceptor
character conferring polarity and ability to develop Hꢀbonds or πꢀπ interactions, conditions for
their growth as single crystals were designed. To favor the polarꢀnonpolar segregation, polar
solvents containing aliphatic parts as ethanol, acetonitrile or ethyl acetate were used. Anticipating
nonꢀcoplanar structure of the dyes and so difficult ordering, solvents with higher boiling points
along with dye size increasing have been chosen, in order to avoid fast evaporation and thus to
slowly change the concentration in the nucleation area. The compounds were dissolved into the
solvent under the oversaturation concentration, and the solution was kept in seals covered with
aluminium foils with few punched holes. This strategy worked well for 6 of the synthesized dyes,
for each large single crystal proper for Xꢀray diffraction being grown.
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Equipment
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Infrared spectra were recorded on a FTꢀIR 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 zꢀgradients. The spectra have been recorded in
DMSOꢀd6, at room temperature. The chemical shifts are reported as δ values (ppm) relative to the
residual peak of the solvent.
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Crystallographic measurements of the compounds (2), (4), (5), (6), and (9) were carried
out with an OxfordꢀDiffraction XCALIBUR E CCD diffractometer equipped with graphiteꢀ
monochromated Mo Kα radiation. Single crystals were positioned at 40 mm from the detector
and 467, 353, 248, 388, and 464 frames were measured each for 30, 25, 10, 30, and 15 s over 1°
scan width. The Xꢀray data for (7) were collected using an Agilent SuperNova dual wavelength
Xꢀray diffractometer using Cu Kα radiation. The crystal was positioned at 46 mm from the
detector and 1781 frames were measured each for 2ꢀ8 s over 1° scan width. The unit cell
determination and data integration were carried out using the CrysAlis package of Oxford
Diffraction [35]. All the structures were solved by direct methods using Olex2 [36] software with
the SHELXS structure solution program and refined by fullꢀmatrix leastꢀsquares on F² with
SHELXLꢀ97 [37]. Atomic displacements for nonꢀhydrogen atoms were refined using an
anisotropic model. Hydrogen atoms have been placed in fixed, idealized positions accounting for
the hybridization of the supporting atoms and the possible presence of hydrogen bonds in the case
of donor atoms. The molecular plots were obtained using the Olex2 program. The positional
parameters of the disordered hexyl fragment in (4) and –CH=O and Br atoms in (6) were refined
in combination with PART and SADI restrains using anisotropic/isotropic model for nonꢀH
atoms. Table 1s provides a summary of the crystallographic data together with refinement details
for studied compounds. CCDC 1414998 for (2), 1414934 for (4), 1414933 for (5), 1414156 for
(6), 1415271 for (7) and 1415052 for (9) contain the supplementary crystallographic data for this
contribution. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif
The thermotropic behavior of the dyes was investigated with an Olympus BHꢀ2 polarized
light microscope under cross polarizers equipped with a THMS 600 hot stage and a LINKAM
TP92 temperature control system.
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UV–Vis absorption and photoluminescence spectra were recorded on a Carl Zeiss Jena
SPECORD M42 spectrophotometer and a Perkin Elmer LS 55 spectrophotometer, respectively,
in solution (10ꢀ5 M), using 10 mm quartz cells. The fluorescence quantum yield (ΦF) of the
samples was measured on a FluoroMaxꢀ4 spectrofluorometer equipped with a Quantaꢀphi
integrating sphere accessory Horiba Jobin Yvon, at 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 luminescence signalꢀtoꢀnoise ratio.
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Electrochemical properties were investigated by cyclic voltammetry (CV) carried out on a
PotentiostatꢀGalvanostat (PG581, Uniscan Instruments). The electrochemical cell was equipped
with threeꢀelectrodes: a working electrode (ITOꢀcoated glass slide), an auxiliary electrode
(platinum wire) and a reference electrode (calomelꢀSCE). Tetrabutylammonium perchlorate
/dichloromethane was used as a supporting electrolyte system. The measurements were registered
for the solution dyes having the same concentration, of 1×10 − 3 M. All potentials were reported
with respect to calomel (SCE) electrode under ambient conditions, at the scan rate of 100 mV/s.
Ferrocene/ferrocenium (Fc/Fc+) redox standard was used as an external reference for calibration
(+0.45 vs SCE). The oxidation and reduction onset potentials were determined at the intercept of
the two tangents drawn at the rising current and baseline charging current of the CV curves.
3. Results and discussions
Design, synthesis, structural and supramolecular characterization
A series of nine phenothiazine dyes functionalized with bromine and/or different electronꢀ
withdrawing units containing carbonyl moiety were designed and synthesized according to the
Scheme 1. The phenothiazine has been chosen as a central core, taking into consideration its
strong electronꢀdonor character which facilitates the electron delocalization and promotes high
quantum yield [20, 38ꢀ41]. To improve more its donating ability, the phenothiazine was
substituted with Nꢀhexyloxyꢀphenylene unit [26ꢀ28, 42, 43]. The resulted phenothiazine core was
functionalized with carbonyl containing moieties having different electronꢀaccepting ability, in
order to create donorꢀacceptor (DꢀA) systems and to enhance the emission capability, as well.
Thus, the good electron delocalization and the low band gap create the premises of a good
mobility of the charge carriers. The acceptor units have different withdrawing power to give rise
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different band gaps and thus to tune the color of the emitted light. Targeting a high quantum
yield, a heavy bromine group has been introduced as a second substituent to promote the halogen
bonding probability and thus to activate efficient luminescence of the resulted pure organic
materials [26ꢀ28, 44ꢀ46].
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The phenothiazine core has been prepared by Ullmann reaction of phenothiazine with nꢀ
hexyloxyꢀ4ꢀiodobenzene, using 18ꢀcrownꢀ6 ether as phase transfer catalyst [29], the nꢀhexyloxyꢀ
4ꢀiodobenzene being obtained via Williamson etherification of 4ꢀiodoꢀphenol with nꢀ
bromohexane [30]. The formyl substituent was grafted on the phenothiazine core via Vilsmeierꢀ
Haak reaction [31] and the obtained aldehyde was transformed into (i) cyanoacrylic derivative via
Knovenagel condensation reaction [32], and (ii) carboxyl unit via oxidation of the formyl group
[33]. The bromine derivatives were obtained using Nꢀbromosuccinimide reagent [29, 34]. All the
synthetic procedures were optimized to attain the best reaction yield. The obtained compounds
have good solubility in common organic solvents such as acetonitrile, tetrahydrofuran,
chloroform or dichloromethane, which allowed their easy characterization.
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°
Scheme 1. Phenothiazine derivatives synthesis: (a) DMF, K2CO3, 110ꢀ120 C, 8h; (b) oꢀ
°
diclorbenzene, copper powder, 18ꢀcrownꢀ6, K2CO3, 168 C, 24h; (c), (d), (f), (h) NBS, CHCl3,
°
°
CH3COOH, 0 C, 2h; (e) DMF, POCl3, C2H4Cl2, 90 C, 24h; (g) cyanoacetic acid, piperidine,
acetonitrile, 80°C, 24h; (i) THF, tꢀBuOK, 0 °C, RT, 24h; (j) NBS, DMF, 0 °C, 2h.
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The synthetic pathway was firstly confirmed by FTIR spectra by the appearance of the
main absorption bands due to the phenothiazine parent molecule (2) as: stretching of the aromatic
CꢀH bonds (3095 – 3043 cmꢀ1) and methylene and methyl units (2954 – 2850 cmꢀ1); stretching of
the C=C of the aromatic units (1607 – 1467 cmꢀ1); deformation of the aromatic rings (838 – 744
cmꢀ1); stretching of the ether unit (1245 – 1237 cmꢀ1). The newly formed moieties were evidenced
by specific group vibration bands: the C=O (1725 – 1662 cmꢀ1), CꢀBr (1167 – 1027 cmꢀ1) or C≡N
(2219 cmꢀ1). Further, the structure of the synthesized compounds has been confirmed by 1HꢀNMR
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spectra – which showed the chemical shifts of the all protons in the right integral ratio, and Cꢀ
NMR spectra (Figures 1sꢀ10s) and elemental analysis.
Apart from the above mentioned investigations, the crystal structure of the dyes under
study, except (3, 8 and 10), confirmed the targeted molecular structure and provided relevant
information about the nature of the intermolecular interactions and packing peculiarities, which
were further correlated with their properties. The results of the single crystal Xꢀray study, along
with the atom labelling scheme, are shown in Figure 1. Bond distances and angles are
summarized in Table 1S.
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Figure 1. Xꢀray molecular structure of the compounds under study
As can be seen, there are no solvate molecules in the crystals of the studied compounds.
The Xꢀray structural investigation has demonstrated that all the compounds exhibit close
molecular geometry which is in a good agreement with the values of earlier studied
phenothiazine derivatives [17ꢀ25, 31]. Thus, the mean value of the CꢀS bond lengths is 1.756 ±
0.021 Å and that of the CꢀN bond lengths within the central ring is 1.427 ± 0.039 Å. The mean
values of the CꢀSꢀC is 100.60 + 3.38°, while C1ꢀN1ꢀC12, C1ꢀN1ꢀC13 and C12ꢀN1ꢀC13 bond
angles are of 123.21 ± 1.88°, 117.60 ± 0.50° and 117.72 ± 0.98°, respectively. Similar molecular
conformations were observed for all the structures.
Analysing the planar fragments, one can be concluded that as a whole, similar
conformations are observed for all the structures in the crystal. The oxyphenyl ring is almost
perpendicular to the mean plane of the phenothiazine fragment. The dihedral angle between this
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ring and bisectral plane of the phenothiazine fragment varied in the range of 1.2°ꢀ3.7°, except the
derivative containing bromine and formyl substituents (6) for which this angle increased at 11.2°.
On the other hand, the phenothiazine core adopted different conformations in the dyes. In four of
them, namely (2), (4), (7) and (9), the phenothiazine system is folded along S1···N1 vector, with
the dihedral angles between the two benzene rings of 156.2°, 157.9°, 144.8° and 155.6°
respectively, giving the wellꢀknown butterfly conformation [17ꢀ25]. These values falls in the
range of those earlier found for Nꢀsubstituted phenothiazine derivatives [17ꢀ25]. On the contrary,
the analogue dihedral angle in the phenothiazine fragment of the (5) and (6) molecules containing
aldehyde group has values of 178.8° and 176.6° respectively, providing evidence of a planar
conformation, different from the butterfly one reported up to now for phenothiazine derivatives.
Comparing the structures of the (5) and (6) with similar ones reported in literature as having
butterfly conformation [23ꢀ25], it appeared that the determining role on planarization is played by
the phenyl substituent of the nitrogen atom of the phenothiazine. This direct connection increases
the electron donor capability of the phenothiazine [26ꢀ28, 42, 43] and consequently its ability to
conjugate with the donor formyl unit, arising planar conformers.
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The different functional groups of the compounds determined a variety of packing modes
in the crystals, once again highlighting their importance as building blocks in structural design of
the compounds for different applications. Thus, in the crystal structure of the phenothiazine core
(2), two crystallographic equivalent molecules are linked to form dimeric units through CꢀHꢁꢁꢁO
hydrogen bonding, where hexyloxyꢀphenyl oxygen atom acts as acceptor. A view of this dimeric
unit is depicted in Figure 2.
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Figure 2. View of the dimeric unit in the crystal structure (2). (Nonꢀrelevant Hꢀatoms are omitted
for clarity.) Hꢀbonds parameters: C17−HꢁꢁꢁO1 [C17−H 0.93 Å, HꢁꢁꢁO1 2.03 Å, C17ꢁꢁꢁO1(2 – x, 2 –
y, 1 – z) 2.833(7) Å, O5−HꢁꢁꢁO1 165.9°].
The carboxyl units of the (7) and (9) dyes proved to be appropriate fragments to play the
role of protonꢀdonor and protonꢀacceptor for a classical OꢀH···O hydrogen bonding. Thus, in
their crystals, the carboxylic groups are involved into formation of a stable cyclic hydrogen
bonded system. Consequently, the main crystal packing motif is characterized by a dimeric
centrosymmetric associate, as shown in Figures 3a and 3b, respectively. In the crystal, the
dimeric units are further extended through short CꢀH···O or CꢀH···N intermolecular contacts to
form supramolecular ribbons.
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a)
b)
Figure 3. a) View of the dimeric associate in the crystal structure of (7). Hꢀbonds parameters:
O3−HꢁꢁꢁO2 [O3−H 0.82 Å, HꢁꢁꢁO2 1.90 Å, O3ꢁꢁꢁO2(2 – x, –y, 2 – z) 2.612(2) Å, O3−HꢁꢁꢁO2
145.3°]; C8−HꢁꢁꢁN2 [C8−H 0.93 Å, HꢁꢁꢁN2 2.60 Å, C8ꢁꢁꢁN2(x – 1, y, z) 3.451(3) Å, C8−HꢁꢁꢁN2
152.8°]. b) A view of a supramolecular ribbon in the crystal structure (9). Hꢀbonds parameters:
O3−HꢁꢁꢁO2 [O3−H 0.82 Å, HꢁꢁꢁO2 1.79 Å, O3ꢁꢁꢁO2(1 – x, 1 – y, 1 – z) 2.604(2) Å, O3−HꢁꢁꢁO2
171.8°]; C8−HꢁꢁꢁO2 [C8−H 0.93 Å, HꢁꢁꢁO2(1 – x, 1 – y, 2 – z) 2.53 Å, C8ꢁꢁꢁO2 3.442(3) Å,
C8−HꢁꢁꢁO2 167.0°]( Nonꢀrelevant Hꢀatoms are omitted for clarity.)
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The analysis of the crystal structure of the formyl containing compound (5) has shown
that the packing of the neutral molecules was driven by the system of C−HꢁꢁꢁO intermolecular
hydrogen bonding resulting into a threeꢀdimensional supramolecular network. The centroidꢀtoꢀ
centroid distances of 4.052 Å between benzene rings of centrosymmetrically related
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phenothiazine moieties evidenced that weak πꢀπ stacking interactions within the network were
also present. The view of packing diagram in the crystal structure (5) is depicted in Figure 4.
Figure 4. View of the 3D supramolecular architecture in the crystal structure (5). Hꢀbonds
parameters: C8−HꢁꢁꢁO2 [C8−H 0.93 Å, HꢁꢁꢁO2 2.41 Å, C8ꢁꢁꢁO2(–x, –0.5 – y, 0.5 – z) 3.316(3) Å,
C8−HꢁꢁꢁO2 165.8°]; C17−HꢁꢁꢁO1 [C17−H 0.93 Å, HꢁꢁꢁO1 2.51 Å, C17ꢁꢁꢁO1(1 –x, –y, 2 – z)
3.413(3) Å, C17−HꢁꢁꢁO1 164.7°]; C15−HꢁꢁꢁO2 [C15−H 0.93 Å, HꢁꢁꢁO2 2.57 Å, C15ꢁꢁꢁO2(–x, –0.5
+ y, 1.5 – z) 3.368(3) Å, C15−HꢁꢁꢁO2 144.3°].
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By introduction of the bromine substituent, the crystal structure of the formyl containing
5
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molecule (6) changed, the
motif. In the crystal structure (6), the neutral molecules are associated to form a linear array due
to the system of stacking, as shown in Figure 5. The Cg1···Cg1(–x, –y, 2 – z) and
Cg1···Cg2(–x, 1 – y, 2 – z) distances are of 3.632 Å and 3.751 Å, respectively, signature of
strong stacking interactions. The C9ꢀBr1 distance (1.817(6) Å) is only slight smaller
πꢀπ stacking becoming the driving force responsible for the packing
πꢀπ
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πꢀπ
compared to the average CꢀBr distance (1.88ꢀ1.93 Å), or compared to the CꢀBr distance in the
compound (4) (C4ꢀBr1: 1.908(6) Å; C9ꢀBr2: 1.895(6) Å), indicating a weak conjugation with the
phenothiazine core due to bromine electronic structure (Figure 1) [47ꢀ49].
Figure 5. πꢀπ stacking interactions in the crystal structure of (6). Hydrogen atoms are omitted for
clarity.
It is important to note, that the planar conformation observed for the phenothiazine
fragment of the (5) and (6) molecules appeared as a consequence of the direct connection of the
electronꢀacceptor formyl with electronꢀdonating phenothiazine. This particular donorꢀacceptor
system has produced a strong electron delocalization, affording the planarization of the
phenothiazine core and further πꢀπ stacking interactions which were absent in the phenothiazine
derivatives reported up to now [17ꢀ25]. This is of further interest in choosing building blocks for
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the material design addressed to applications which claim good high charge carrier mobility.
Analyzing the case of the derivative containing bromine (6) compared with its counterpart (5),
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one could be observed stronger πꢀπ interactions on a hand (the centroidꢀtoꢀcentroid distances of
3.632 Å and 3.751 Å in the (6) compared to 4.052 Å in the (5)) and lower planarization effect of
the phenothiazine core on the other hand (the dihedral angle value in the phenothiazine fragment
is 176.6° in the (6) compared to 178.8° in the (5)), indicating the bromine with an own
contribution in the selfꢀassembling process.
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In the crystal structure of the compound containing two bromine substituents (4) the πꢀπ
stacking interaction Cg1···Cg1(2 ꢀx, 2 ꢀy, 1 – z) of 3.820 Å in combination with the C15ꢀ
H···Cg1(2 ꢀx, 2 ꢀy, 2 – z) contacts of 2.887 Å resulted into the formation of tetrameric clusters as
the main crystal packing units (Figure 6). It appeared that the introduction of the bromine
substituent favors the
good charge transport.
πꢀπ stacking interaction of the phenothiazine dyes, creating premises for a
Figure 6. Tetramer supramolecular unit in the crystal structure (4)
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Thermotropic behavior
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All the obtained dyes exhibited strong birefringence under polarized light microscopy
(POM), according to their crystalline nature (Figure 7a, b, c). They melted during the heating
and, as expected, the melting point increased once the electronꢀwithdrawing character of the
substituent increased resulting in a more extended molecular conjugation which induced longer
rigid structures and consequently weaker mobility. The introduction of the bromine as a second
substituent increased more the melting point compared to the counterparts without bromine, in
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agreement with the morphological conclusions which indicated stronger πꢀπ stacking interaction.
On the other hand, the crystallization occurred (i) during the cooling only for the compounds (6)
and (9), (ii) slowly from the glassy state at room temperature for the compounds (2), (3), (4) and
(5) (iii) while the compounds (7), (8) and (10) freeze as amorphous glasses and didn’t crystallize
even after 6 months (Figure 7 d, e, f). The crystallization behavior suggested difficulties of
ordering from molten state, probably because of the highly disorder introduced by the aliphatic
chain and noncoplanar arrangement of the oxyphenyl unit to the phenothiazine fragment. On the
other hand, the ordering from the glassy state led to continuous crystalline films, without cracks,
an important advantage for real world applications [50].
b) (6), 1H, RT
c) (8), 1H, RT
a) (5), 1H, RT
d) (5), 1C, RT
e) (6), 1C, 120 oC
f) (9), 1C, 197 oC
Figure 7. POM images (magnification 200x, C: cooling; H: heating)
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Photophysical properties
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8
The UVꢀvis absorption behavior brings information related to the conjugation degree of
the molecules, the absorption capability and allows the estimation of the optical band gap
between the HOMO and LUMO levels. As can be seen in Figure 8, all the compounds have an
absorption maximum around 260 nm corresponding to the spin allowed πꢀπ* benzenoid
transitions of the local aromatic units and a broad absorption band with the maximum between
322 and 343 nm attributed to the localized conjugated skeleton formed by the phenothiazine
donor (D) – acceptor (A) framework. In addition, the dyes containing formyl or cyanoacrylic acid
unit exhibited a supplementary absorption band with maximum around 400 and 450 nm,
respectively, attributed to the intramolecular charge transfer (ICT) from the central electronꢀ
donor phenothiazine to the strong electronꢀacceptor formyl or cyanoacrylic moiety, respectively
[30]. This band appeared only as a weak shoulder for the other dyes, related to the weaker
electronꢀacceptor character of the bromine or dimer formation between carboxylic units [51, 52].
The superior absorbance and the stronger bathocromic shifting of the ICT band of the
cyanoacrylic dye is associated with (i) the higher electronꢀacceptor capability of the cyanoacrylic
moieties compared to the other electronꢀacceptor units, and (ii) the presence of the vinyl bond as
a πꢀbridge that allows the charge separation [53].
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0.8
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0.1
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0.00
4
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300
350
400
Wavelength (nm)
450
300
400
Wavelength (nm)
500
A
250
300
350
400
450
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 8. UVꢀvis absorption spectra, in THF solutions (10ꢀ5M) (the inset represents the
magnification of the absorption domain over 300 nm)
Concerning the other absorption bands, one can be observed that the formyl containing
dyes exhibited the most important bathocromic shift attributed to the strongest electron
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delocalization originating from the planar structure and πꢀπ interactions as was proved by the Xꢀ
ray diffraction. The bromine containing dye (3) also showed a slight bathocromic shift compared
to the parent molecule (2), indicating that the electronꢀwithdrawing effect of the bromine was
prevalent over its mesomeric electronꢀdonating effect. In the case of the dyes containing bromine
as the second substituent (4, 6, 8, 10), no significant effect upon absorption position was
observed, according with the weak conjugation between the bromine atom and the phenothiazine
core. The absorption parameters attributed to the localized conjugated skeleton formed by donorꢀ
acceptor framework are given in Table 1.
Table 1. The absorption parameters attributed to the localized conjugated chromophore
Code
λmax
2
322/
404
3
329/
372
4
338/
365
5
342/
460
6
342/
467
7
326/
530
8
326/
531
9
333/
433
10
335/
441
/
λ
onset nm)
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3.85/ 3.76/
3.06 3.33
3.66/
3.39
3.62/
2.69
3.62/
2.65
3.82/
2.33
3.82/
2.33
3.65/
2.86
3.67/
2.81
Eg(eV)
λ
max/λonset: wavelength of absorption maximum / wavelength of absorption edge; Eg: energy band gap calculated for
the absorption maximum of the peak and for the absorption edge.
The photoluminescence spectra were registered by exciting dye solutions with light of
different wavenumber corresponding to the absorption maxima of each dye. All the dyes were
luminescent even under day light, the emission color shifted from UV to orange along with the
increment of the electronꢀacceptor capability of the substituent (Figure 9). Thus, the color of the
emited light was blue for the phenothiazine core (2), and the bromine (3 and 4) and carboxylic
derivatives (9 and 10); green for the formyl derivatives (5 and 6); and orange for the cyanoacrylic
derivatives (7 and 8). Since the emission wavelength is controlled by the conjugation length, the
dramatic red shifting of the emission maxima (around 200 nm) is explained by the considerable
differences between the electronꢀwithdrawing strengths of the acceptor groups.
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500
400
300
200
100
0
10000
8000
6000
4000
2000
0
12
10
8
5
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9
2
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2
0
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500
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500
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600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
c)
b)
a)
d)
Figure 9. Photoluminescence spectra of the studied dye solutions (THF, 10ꢀ5M) (a, b, c) and their
luminescence under illumination with UV light (d)
Comparing the emission maxima of the bromine containing compounds (4, 6, 8, 10) with
their counterparts without bromine (3, 5, 7, 9), a slight blue shift trend of the emission maxima
was observed (Table 2s). This feature was also evident when compared the phenothiazine core
(2) with bromine containing dye (3). Considering the weak electronꢀwithdrawing effect of the
bromine this blue shifting was attributed to the weakening of the electronꢀdonating ability of the
phenothiazine, as was evidenced for other bromine derivatives [54].
To proper compare the dyes emission ability, the fluorescence spectra were registered on
solutions of similar molar concentration and their intensity was calibrated using the Raman
scatter peak of water [55]. As can be seen in Figure 9a, b. c, the emission intensity increased as
the electronꢀwithdrawing ability of the acceptor group increased, following the same trend as the
red shifting of the emission color, suggesting that conjugation strength influences the emission
intensity, too. Most probably, the explanation for this behavior is given by the reduced amount of
nonꢀradiative decay via rotational moving and conformational change, as the conjugation strength
increases [56].
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The increased luminescence is also reflected in the quantum yield values. As can be seen
in Table 2, the absolute quantum yield in acetonitrile solution was low for the phenothiazine core
(2), and monoꢀbromine derivative (3), but progressively increased as the electronꢀacceptor ability
of the substituent moiety became higher, reaching up to 71%. Remarkably, the introduction of the
bromine as second substituent enhances the quantum yield more than twice. Taking into
consideration the weak influence of the bromine upon the absorption, its drastic influence on
emission could be attributed to its ability to promote singletꢀtriplet conversion by enhancing spinꢀ
orbit coupling between the excited state electrons and massive nucleus of bromine atoms, which
effective busted the quantum yield [44ꢀ46]. Another important observation was the good
agreement between the luminescence intensity expressed in Raman units and the absolute
quantum yield, once again confirming the reliability of calibration using the Raman scatter peak
of water [55].
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Figure 10. The chromaticity diagram of the dyes
To proper appreciate the chromatic perception of the human eye on the light color of the
dye emission, their chromaticity diagram was registered according to available CIEꢀ1931
standard (Figure 10). The chromaticity coordinates from the dye molecules showed the
photoluminescence emission colors spanned between blue and yellow visible regions. The (5),
(7) and (8) dyes were located on the peripheral zone of the green and yellow region, respectively,
indicating emission of pure and fully saturated color, while the (10) dye was located in the blue
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region, close to the peripheral zone corresponding to a high color purity of 93 %. The
chromaticity coordinates of emission and color purity degree of the studied dyes are summarized
in Table 2.
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Table 2. Photophysical characteristics of the studied dyes
Code
Stokes
shift
η1%
X2
2
122
3
4
5
6
7
8
9
129
10
126
103 106
148
154
280
272
3.5
3.1 8.5
0.23 0.25
0.18 0.27
24.5
0.37
0.64
100
66.4
0.30
0.54
59
35.5
0.42
0.59
100
71.0
0.41
0.60
100
29.3
0.16
0.29
66
7.8
0.28
0.26
26
0.16
0.33
93
Y3
PC4(%)
CL5
52
37
purple blue Greenꢀ greenꢀ
bluish
greenꢀ
yellow yellow green
greenꢀ
yellowish yellowish green green bluish bluish
1 absolute quantum yield determined by exciting with light wavenumber corresponding to the absorption maxima of
2, 3
4
the localized conjugated skeleton formed by donorꢀacceptor framework;
chromaticity coordinates; PC: color
purity of emitted light; 5 CL: the color of emitted light
The Stokes shift values were comprised between 49 and 280 nm, resulting in an
insignificant overlapping between the absorption and emission profiles, meaning that losses of
the emitted photons by reꢀabsorption phenomena were not possible. This is an important feature
for multichannel imaging in biochemical applications [57].
Electrochemical behavior
Cyclic voltammetry (CV) of the studied compounds was performed in order to investigate
their redox behavior and to further understand their electronic structure. A special concern on the
influence of building blocks upon the electronic structure of the dyes and their electrochemical
properties was followed, thus allowing assessing their potential as optoelectronic materials. The
electrochemical data of the synthesized phenothiazine dyes are collected in Table 3.
Table 3. Electrochemical data of phenothiazine derivatives 2ꢀ10
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red
Eonsetox ꢀ oxidation onset potential; Eonset ꢀ reduction onset potential, Epa ꢀ anodic peak potential; Epc ꢀ cathodic peak
potential; EHOMO ꢀ the HOMO energy level, ELUMO ꢀ the LUMO energy level, Eg ꢀ energy gap measured from
electrochemical data according to the Eonset
All the phenothiazine dyes electrochemically oxidize, indicating their pꢀtype doping
capability and hole transport characteristics. The parent phenothiazine molecule (2), revealed two
oxidation peaks at the anodic peak potentials Epa1 = 0.814 V and Epa2 = 1.634 V vs. SCE,
respectively, and one reduction peak at the cathodic peak potential Epc = 0.640 V vs. SCE. The
two oxidation peaks were related to the formation of radical cation and dication species. The first
oxidation of the phenothiazine led to the formation of the radical cation species at the nitrogen
center, which further converted to the dication species by oxidation at the free electron pair of the
sulfur atom [58, 59]. The anodic scan evidenced that the first phenothiazineꢀcentered oxidation
potential could be fineꢀtuned using suitable electronꢀacceptor substituents, the anodic peak
potential (Epa) values ranging from 0.814 to 1.068 V vs. SCE. A closer inspection of the
electrochemical behavior of the monoꢀsubstituted phenothiazine dyes (3), (5), (7) and (9)
revealed the first reversible oneꢀelectron oxidation was shifted to higher values (Epa1 = 0.953,
1.016, 0.964 and 0.966 V vs. SCE, respectively) relative to that of the phenothiazine parent
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molecule (2) (Epa1 = 0.814 V vs. SCE) as a consequence of the electronꢀwithdrawing effect of the
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substituents (Figure 11).
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Figure 11. CV diagrams of phenothiazine (2) and its electronꢀwithdrawing moieties
monosubstituted analogous, (3), (5), (7) and (9), in the positive region
As a matter of fact, the electronꢀwithdrawing substituents have an active contribution in
the formation of a pushꢀpull system, by accepting the electrons from the electronꢀdonating
phenothiazine. Hence, the electron density on phenothiazine core should decrease once the
electronꢀacceptor character of the substituents increases, and oxidation should become more
difficult. Among the used substituents, cyanoacrylic acid moiety has the strongest electronꢀ
acceptor character and bromine the lowest one. Therefore it was expected as the cyanoacrylic
acidꢀsubstituted phenothiazine (7) to be the most difficult oxidized. Instead, the formylꢀ
substituted phenothiazine dye (5) exhibited the most anodically shifted peak (Figure 11). In close
correlation with its Xꢀray molecular structure presented in figure 1 and UVꢀVis absorption
behavior mirrored in figure 8, such a feature was attributed to the better strength electronic
conjugation of the dye (5) (in which the phenothiazine donor was directly connected with the
formyl acceptor) that prevailed over the electronꢀwithdrawing character. On the other hand, the
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ability of the carboxylic acid in (9) to promote Hꢀbonding (see the crystal motif) and to polarize
is a direct function of the electronic properties and bonding order of the atoms that make up the
COOH moiety. Thus, the electronic density is "pulled" from the hydroxyl hydrogen through the
conjugated carboxyl group and, consequently, this group could exert a lower electronꢀaccepting
effect in (9) compared to the aldehyde unit in (5), explaining its easier oxidation. From all
investigated substituents only the formyl and cyanoacrylic groups hampered the reversibility of
the first monoꢀelectronic oxidation process of the phenothiazine, probably due to the low stability
of the intermediate radical cation – affected by the electronꢀwithdrawing mesomeric effect.
When bromine atom was introduced as the second substituent, the phenothiazine dyes (4),
(6), (8) and (10) became harder to be oxidized, indicating a decreased electron density on the
phenothiazine core compared to the monoꢀsubstituted analogues (Figure 12). This is in agreement
with the predominance of the negative inductive effect of the bromine atom over its positive
mesomeric effect, the bromine atom exerting an electronꢀwithdrawing capability, as was
confirmed by Xꢀray crystal structure of (4) and (6) and photophysical data of all Brꢀsubstituted
phenothiazine derivatives. However, its capability to accept electrons appears to be minimal in
(4), when the first substituent is a bromine atom also.
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-5
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Figure 12. Diagrams of representative monoꢀ and diꢀsubstituted phenothiazine dyes, obtained by
current sweep in the positive direction
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The broad irreversible oxidation wave observed in the potential range from 1.619 to 1.777
V vs SCE corresponds to the formation of the highly reactive phenothiazine dications, and it is no
longer present for the compound (9). The irreversibility in the oxidation of the dications is most
likely due to a following chemical reaction with trace water or the solvent [60]. On the reverse
scanning, one reduction process assigned to the reduction of the phenothiazine radical cations
occurred at Ered from 0.677 to 0.787 V vs. SCE in the case of monosubstituted derivatives (Figure
11). The presence of the bromine as a second substituent caused the appearance of two closely
spaced reduction waves attributed to two phenothiazine radical cations, at the peak potentials
presented in Table 3 (Figure 12). As the two closely spaced reduction waves were oneꢀelectron
waves, appeared reasonable to assume that the two identical phenothiazine molecules in the
disubstituted derivatives (4, 6, 8 and 10) were not completely electronically decoupled, so that
these two waves represented individual one electron reduction of each phenothiazine radical
cation at a slightly different potential. The overlapping reduction waves indicated that the two
phenothiazine molecules had some molecular orbital interactions the most probably due to the
ability of bromine to promote selfꢀassembling through πꢀπ stacking, as Xꢀray data of (4) and (6)
indicated; if the two phenothiazine units should be completely out of plane or orthogonal to one
another, a single oneꢀelectron reduction wave should be observed like in the case of
monosubstituted counterparts, for such a behavior the bromine atom being responsible.
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Scanning in the negative direction to ꢀ2 V vs SCE, all the phenothiazine dyes exhibited
reduction waves, in accord with the electron deficient nature of the electronꢀacceptor units,
indicating their nꢀtype dopability and electron transport characteristics (Table 3). The most
interesting feature was the large reduction peak centered at ꢀ1.39 V vs. SCE, without a
corresponding anodic wave for the parent molecule (2) (Figure 13). It appears that the Nꢀ
substitution of the phenothiazine with hexyloxyꢀphenyl moiety make this molecule suitable for
electrochemical reduction (it can accept electrons). A close look to the crystal structure of (2)
shows that two crystallographic equivalent molecules are linked to form dimeric units through Cꢀ
HꢁꢁꢁO hydrogen bonding, were the O atom of hexyloxyꢀpheny unit acts as a proton acceptor. In
the light of this finding, it appears that the electron density on O became enough lower as to
accept one electron and promote the nꢀdoping process of (2). In the electronꢀwithdrawing
monsubstituted molecules (3, 5, 7 and 9), the reduction peak potential vary by 0.42 V, between
ꢀ1.27 and ꢀ1.69 V vs. SCE, a decreasing being observed as much as the electron delocalization
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progressively increased due to the higher electronꢀaccepting capability. When the first bromine
substituent was grafted on the phenothiazine core (3), this trend was followed, but it exerted a
small influence on the reduction potential. When the bromine was placed as a second substituent
(4, 6, 8, 10) it facilitated the reduction of the molecule dyes, being accomplished by the
anodically shift of the reduction potential (Figure 13). Such a feature accounts for bromine ability
to increase the strength of the pushꢀpull system.
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Figure 13. CV diagrams of phenothiazine (2) and its Brꢀsubstituted analogous in the negative
region
The electronꢀwithdrawing substituents produced peculiar reduction waves. The formylꢀ
substituted phenothiazine (5) displayed two reduction waves (Table 3), the one peaking at ꢀ1.55
vs. SCE being attributed to the reduction of the aldehyde group, while (7) and (9) showed only
one reduction peak at ꢀ1.69 and ꢀ1.42 V, respectively, being attributed to the formation of radical
anions of the electronꢀwithdrawing cyanoacrylic and carboxylic units, respectively (Figure 14). A
further oxidation to neutral species of the formed radical anions was observed for both (7) and (9)
molecules. A similar trend was noticed for their Brꢀsubstituted counterparts, (8) and (9), but at
slightly more positive potential values (Table 3).
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Figure 14. CV diagrams of the phenothiazine derivatives (5), (7), and (9) in the negative region
As a general conclusion, the electrochemical behavior of the compounds under study
suggests that pꢀ and nꢀtype doping to conducting state of these phenothiazine materials is
feasible. These systems can therefore be used as electrochemically amphoteric pushꢀpull
chromophores that can accept or release electrons at relatively low voltages. For applications in
organic lightꢀemitting diodes (OLEDs) or organic fieldꢀeffect transistors as functional holeꢀ and
electronꢀ transport layer this is a beneficial property. Additionally, the highly fluorescent
phenothiazine derivatives 5 – 9 could also serve as redox switchable emitters in OLED devices.
Electronic structure
To gain an insight upon the electronic structure of the dyes, the HOMO and LUMO
energy levels were calculated based on the assumption that the oxidation process corresponds to
the electron removal from HOMO orbitals (ionization potential, IP), and the reduction process
corresponds to the electron addition to the LUMO level (electron affinity, EA). Thus, the energy
levels of HOMO and LUMO orbitals of the investigated phenothiazine dyes can be estimated vs.
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zero vacuum level from the onset of the oxidation (Eoxonset) and reduction (Eredonset) potential vs
SCE, respectively. The external ferrocene/ferrocenium (Fc/Fc+) redox standard Eonset is 0.45 V
versus SCE. Assuming that the HOMO energy for the Fc/Fc+ standard is 4.80 eV with respect to
the zero vacuum level [61], the HOMO and LUMO energy levels and the electrochemical energy
band gap (Eg) (in eV) were calculated according to the following equations:
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ELUMO = EA = Eredonset + 4.35; EHOMO = IP = Eoxonset + 4.35; Eg = EHOMO − ELUMO
The values of the HOMO and LUMO energy levels and the energy gap characteristics for
the phenothiazine dyes (2ꢀ10) calculated from electrochemical data are summarized in Table 3.
The EA values varied between 3.43 and 3.76 eV, the highest one being obtained for the parent
molecule (2), which does not contain any electronꢀacceptor substituent on the phenothiazine core.
An evident reduction of the LUMO energy (in the absolute value), was noticed for the
phenothiazine derivatives containing cyanoacrylic acid substituent (7) and (8), attributed to the
increased delocalization of the electrons between donor and acceptor units. Thus, the LUMO
level appeared to be mainly located at the electronꢀwithdrawing cyanoacrylic moiety through the
πꢀspacer, giving the strongest donorꢀacceptor strength. In perfect agreement with foregoing
observations, the Br atom decreased the LUMO level to some extent, thus proving ones more its
“pull” effect on the phenothiazine core. On the other hand, the IP values ranged between 4.99 and
5.24 eV, a magnification being accomplished as the electronꢀacceptor capability of the
substituent increased, as previously discussed. Using the HOMO energy level of (2) as a
reference, it could be observed that the electronꢀacceptor substituents guided the HOMO energy
level to values comparable with well recognized semiconducting polymers as poly(pꢀ
phenylenevinylene) (5.11) or poly(pꢀphenylene) (5.42 eV) [62].
The band structure of the (1ꢀ10) dyes calculated from electrochemical data is depicted in
Figure l5. As can be seen, the resulted values for the energy band gap of the phenothiazine dyes
(2ꢀ9) vary between 1.23 and 1.77 eV. An important disparity was ascertained between the energy
band gap values measured by UVꢀVis absorption and electrochemical techniques. There is no
contradiction between these values, since they refer to distinct processes. The UVꢀVis absorption
data provide an estimation of the energy difference of the HOMOꢀLUMO orbitals while the
obtained optical band gap is consistent with the energy required to form a tightlyꢀbound exciton.
The redox potentials measured by cyclic voltammetry afford the absolute energetic positions of
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the HOMO and LUMO levels and the electrochemical or transport band gap depicts the energy
required to form free charge carriers [63].
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Figure l5. Band diagrams of the phenothiazine dyes 2ꢀ10 as evaluated from electrochemical data
The energy barriers between the lightꢀemitting materials and electrodes can be assessed
on the basis of the work functions of electrodes and EA and IP of the respective material. Taking
into consideration the work function of ITO anode (4.8 eV) and aluminium cathode (4.3 eV), the
hole and electron injection barriers, respectively, between the emitting compounds and electrodes
can be calculated as ꢂEh = IP ꢀ 4.8 (eV) and ꢂEe = 4.3 ꢀ EA (eV). Moreover, the difference
between the electron and hole injection barriers which give information about the balance in the
two injections, can be calculated as (ꢂEe ꢀ ꢂEh). The barrier energy values are comprised in
Table 4.
Table 4. The barrier energies of phenothiazine derivatives 2 ꢀ 10
ꢀEh ꢀ barrier energy for hole injection from ITO to the dye; ꢀEe ꢀ barrier energy for the electron injection from Al to
the dye; ꢀEe ꢀ ꢀEh ꢀ the difference of barrier energy between electron and hole injection to the dye.
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Both hole and electron injection barriers had small values indicating an easier injection of
both charge carriers. Lower difference between hole and electron injection barriers (ꢂEe ꢀ ꢂEh)
means improved injection balance of electrons and holes from the cathode and anode,
respectively. It was evident that introduction of the bromine increased the hole injection barrier in
a similar manner as all the other electronꢀacceptor units do. Meanwhile, the bromine exerted a
slightly influence on the electron injection barrier, except the case of the cyanoacrylate
derivatives (7, 8) when it considerably increased due to the higher electron delocalization
encountered in these systems between the donor and acceptor units through the vinyl πꢀ bridge.
However, the barrier energy differences (ꢀEe ꢀ ꢀEh) of these phenothiazine dyes were very low,
comparable to that of the semiconducting light emitting polymers used as electronꢀ and hole
transporting lightꢀemitting layers in OLED devices [64].
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Conclusions
A series of nine dyes built by functionalization of a donor phenothiazine core with heavy
bromine and/or carbonyl containing acceptors have been successfully synthesized and their
crystallographic, thermotropic, photophysical and electrochemical peculiarities were investigated
in detail.
The crystallographic study demonstrated that phenothiazine substituents play a decisive
role on the morphological peculiarities, leading to different packing motifs as dimers, tetramer
clusters, supramolecular ribbons or even linear arrays with unusual phenothiazine πꢀπ stacking. It
was evidenced that the direct connection of the formyl group to the phenothiazine unit lead to a
high planarization and consequently high electron delocalization and πꢀπ stacking interactions,
indicating this combination of building blocks as a potential strategy in designing compounds for
applications which claim high charge carrier mobility. The introduction of the bromine as second
substituent appeared to favor even stronger πꢀπ stacking interactions.
The direct connection of the acceptor units to the phenothiazine donor led to a lower band
gap, while the presence of a π bridge facilitated the charge separation resulting in a strong
intramolecular charge transfer and huge Stokes shift of about 280 nm, which account for
multichannel imaging applications.
The use of acceptor substituents with various electronꢀwithdrawing strength produced
drastic shifting of the color of emitted light from UV to orange.
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