New anthracene-based Schiff bases: Theoretical and experimental investigations of photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.saa.2016.12.013

The new Schiff bases bearing anthracene unit were synthesized from 2-aminoanthracene and various aldehydes such as: benzaldehyde, 4-(diphenylamino)benzaldehyde, 9-phenanthrenecarboxaldehyde, 9-anthracenecarboxaldehyde, and biphenyl-4-carboxaldehyde, 2-nap

Full text of DOI:10.1016/j.saa.2016.12.013

Accepted Manuscript
New anthracene-based Schiff bases: Theoretical and experimental
investigations of photophysical and electrochemical properties
Danuta Sek, Mariola Siwy, Marzena Grucela, Grzegorz Małecki,
Elżbieta M. Nowak, Gabriela Lewinska, Jerzy Santera, Katarzyna
Laba, Mieczyslaw Lapkowski, Sonia Kotowicz, Ewa Schab-
Balcerzak
PII:
DOI:
Reference:
S1386-1425(16)30727-2
doi: 10.1016/j.saa.2016.12.013
SAA 14826
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
4 October 2016
21 November 2016
13 December 2016
Please cite this article as: Danuta Sek, Mariola Siwy, Marzena Grucela, Grzegorz Małecki,
Elżbieta M. Nowak, Gabriela Lewinska, Jerzy Santera, Katarzyna Laba, Mieczyslaw
Lapkowski, Sonia Kotowicz, Ewa Schab-Balcerzak , New anthracene-based Schiff bases:
Theoretical and experimental investigations of photophysical and electrochemical
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authors. Please check if appropriate. Saa(2016), doi: 10.1016/j.saa.2016.12.013
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ACCEPTED MANUSCRIPT
New anthracene-based Schiff bases: theoretical and experimental investigations of
photophysical and electrochemical properties
Danuta Sek1, Mariola Siwy1, Marzena Grucela1, Grzegorz Małecki2, Elżbieta M. Nowak3,
Gabriela Lewinska4, Jerzy Santera4, Katarzyna Laba1,5, Mieczyslaw Lapkowski1,5,
Sonia Kotowicz2, Ewa Schab-Balcerzak1,2 *
1Centre of Polymer and Carbon Materials, Polish Academy of Sciences,
34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland
2Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland
3Faculty of Chemical Engineering and Technology Cracow University of Technology, 24
Warszawska Str., 31-155 Krakow, Poland
4Institute of Physics, Cracow University of Technology, 1 Podchorazych Str., 30-035 Krakow,
Poland
5Silesian University of Technology, Faculty of Chemistry, 9 Strzody Str., 44-100 Gliwice,
Poland
Abstract: The new Schiff bases bearing anthracene unit were synthesized from 2-
aminoanthracene
and
various
aldehydes
such
as:
benzaldehyde,
4-
(diphenylamino)benzaldehyde, 9-phenanthrenecarboxaldehyde, 9-anthracenecarboxaldehyde,
and biphenyl-4-carboxaldehyde, 2-naphthaldehyde. Resulted azomethines were characterized
13
by IR, NMR (1H and C), elemental analysis and UV-Vis spectroscopy. The imine consists
of anthracene and biphenyl moieties exhibited liquid crystal properties and their nematic
phase showed Schlieren texture. The photoluminescence measurements carried out in solution
and in solid state as blend with PMMA revealed the ability of the imines to emission of the
blue light with quantum yield efficiency in the range of 2.18–6.03% in blend. Based on the
electrochemical experiment they showed value of energy gap (Eg) in the range of 2.5 - 2.7 eV.
Additionally, density functional theory (DFT) was applied for calculations of both electronic
structure and spectroscopic properties of synthesized Schiff bases. Moreover, the results
obtained from preliminary tests of application of the azomethines in organic photovoltaic
(OPV) devices confirmed their electron acceptor character.

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Keywords: Schiff bases, photoluminescence, electrochemistry, thermotropic liquid crystals
,
DFT calculations
1. Introduction
New conjugated organic compounds with proper optoelectronic properties which can
be easily prepared and purified are still of interest even though during past several years many
of such materials have been obtained and commercialized [1,2]. The development of organic
semiconductors, both low molecular weight compounds and polymers is due to the fact that
they can be used in plastic electronics such as organic light emitting diodes, organic field
effect transistors or organic photovoltaic devices, to mention only a few. Despite the
interesting properties of the polymers, significant advances have been reached in the field of
-conjugated low molecular compounds. Small molecules in relation to the polymers have
advantages such as well-defined structure and molecular weight without any distribution and
in most of the cases they can be easily prepared and purified. Moreover, usually better
solubility of small molecules as compared to polymeric counterparts facilitates the
applications of various types of solution processing methods for thin layers preparation.
Among various conjugated compounds, Schiff base linkers have attracted the attention of
researches. This is due to their isoelectronic character compared to their vinylene
counterparts, the inherent electron withdrawing nature, the ease of preparation and
complexation [3,4]. Schiff bases containing the azomethine linkages (N=CH) are usually
formed by the condensation reaction of a primary amine with an aldehyde with
straightforward protocols without stringent reaction conditions and water is only by-product.
Azomethines have been studied for the last two decades as materials for organic
optoelectronics [5–9]. However, the azomethines based on anthracene have not been
described in the literature from the point of view of their potential applications in
optoelectronics. Anthracene, owing to its unique chemical and electron-rich structure, low
electronic band gap and strong blue fluorescence, has been used as a building unit in many
emissive materials. Properties of several small molecules bearing anthracene structure were
lastly described [10–16].
Our research work involves the design and synthesis of new organic compounds for
potential applications in optoelectronic devices such as light emitting diodes (OLED) and
photovoltaic cells (OPV) [17–21]. Recently we reported optical and electrochemical
properties of novel Schiff bases bearing naphthalene unit together with their examination as
active layer in OPV [21]. This work reports the synthesis of a series of azomethines being

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condensation products of 2-aminoanthracene with various aromatic aldehydes. Their selected
properties were investigated theoretically by density functional theory (DFT) and
experimentally by using UV-vis, photoluminescence (PL) spectroscopies and cyclic
voltammetry (CV). Thermal properties of imines were tested by different scanning
calorimetry (DSC). Additionally, ability of selected Schiff bases for oxidation, protonation
and utilization as component in bulk heterojunction (BHJ) OPV cell was tested. The results
are discussed in relation to former described and mentioned above Schiff bases with
naphthalene unit [21], which can be considered as analogues of some of presented herein.
2. Experimental part
2.1. Materials
2-Aminoanthracene
96%,
4-(diphenylamino)benzaldehyde
97%,
9-phenan-
threnecarboxaldehyde 97%, 9-anthracenecarboxaldehyde 97%, biphenyl-4-carboxaldehyde
99%, 2-naphthaldehyde 98%, N,N-dimethylacetamide (DMA), N-methyl-pyrrolidone (NMP)
were obtained from Aldrich. Benzaldehyde, methanol, dichloromethane (DCHM) and
chloroform were used from POCH. Indium tin oxide (ITO) with surface resistance about 15–
20 Ω/cm2, poly(3,4-(ethylenedioxy) thiophene):poly-(styrenesulfonate) (PEDOT:PSS),
poly(3-hexylthiophene-2,5-diyl) (P3HT) and 3-buthylthiophene (P3OT) were purchased from
Aldrich Chemical Co.
2.2. Synthesis
All Schiff bases were synthesized according to the same procedure.
2-Aminoanthracene (0.5 mmol) and proper aldehyde (0.6 mmol) were dissolved in 1 ml
DMA. The reaction mixture was stirred for 1 h at room temperature and next heated at 120C
under argon atmosphere for 5.5 h. The product was precipitated in methanol. The precipitate
was filtered, washed several times with methanol and dried at 60C in vacuum.
SB-1 N-(anthracen-2-yl)-1-phenylmethanimine
1
Yield: 86%, Tm=217C. H NMR (CDCl3, , ppm): 8.66 (s, -CH=N, 1H), 8.42 (s, 2H), 8.04-
13
7.97 (m, 5H), 7,73 (s, 1H), 5.52-7.45 (m, 7H). C NMR (CDCl3, , ppm): 160.17 (CH=N).
FTIR (cm-1): 3051, 3024 (CH aromatic), 1605 (CH=N stretching), 1572 (C=C streching
deformations in phenyl ring). Elem. anal. calcd (%) for (C21H15N) (281.35): C 89.65, H 5.37,
N 4.98; found C 89.55, H 5.58, N 4.23.

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SB-2 N-(anthracen-2-yl)-1-(naphthalen-2-yl)methanimine
1
Yield: 82%, Tm=260C. H NMR (CDCl3, , ppm): 8.83 (s, -CH=N, 1H), 8.44 (d, 2H), 8.28-
13
8.24 (m, 2H), 8.07-7.90 (m, 7H), 7.80 (s, 1H), 7.59-7.54 (m, 3H), 7.48-7.47 (m, 2H). C
NMR (CDCl3, , ppm): 160.15 (CH=N). FTIR (cm-1): 3045, 3007 (CH aromatic), 1621
(CH=N stretching), 1588 (C=C stretching deformations in phenyl ring). Elem. anal. calcd (%)
for (C25H17N) (331.41): C 90.60, H 5.17, N 4.22; found C 89.22, H 5.71, N 4.25.
SB-3 N-(anthracen-2-yl)-1-(anthracen-9-yl)methanimine
1
Yield: 61%, Tm=242C. H NMR (CDCl3, , ppm): 9.90 (s, -CH=N, 1H), 8.87-8.96 (d, 2H),
8.60 (s. 1H), 8.51-8.49 (d, 2H), 8.14- 8.02 (m, 4H), 7.93 (s, 1H), 7.70-7.49 (m, 5H). 13C NMR
(CDCl3, , ppm): 159.56 (CH=N). FTIR (cm-1): 3045, 3020 (CH aromatic), 1624 (CH=N
stretching), 1600 (C=C streching deformations in phenyl ring). Elem. anal. calcd (%) for
(C29H19N) (381.47): C 91.31, H 5.02, N 3.67; found C 91.26, H 5.27, N 3.47.
SB-4 N-(anthracen-2-yl)-1-(phenanthren-9-yl)methanimine
1
Yield: 52%, Tm=241C. H NMR (CDCl3, , ppm): 9.46 (m,1H), 9.43 (s, -CH=N, 1H), 8.94-
8.93 (dd, 2H), 8.60-8.59 (dd. 2H), 8.57 (s, 1H), 8,20- 8.15 (m, 2H), 8.08-8.07 (m, 2H), 8.00(s,
13
1H), 7.83-7.78 (m, 3H), 7.74 – 7.70 (m, 2H), 7.53-7.49 (m, 2H). C NMR (CDCl3, , ppm):
161.35 (CH=N). FTIR (cm-1): 3079, 3019 (CH aromatic), 1611 (CH=N stretching), 1583
(C=C streching deformations in phenyl ring). Elem. anal. calcd (%) for (C29H19N) (381.47):
C 91.31, H 5.02, N 3.67; found C 91.20, H 5.08, N 3.59
SB-5 N-(anthracen-2-yl)-1-(biphenyl-4-yl)methanimine
1
Yield: 53%, Tm=277C. H NMR (CDCl3, , ppm): 8.71 (s, -CH=N, 1H), 8.43 (s, 2H), 8.06-
8.05 (m, 3H), 8.00 (s, 2H), 7.76-7.74 (m, 3H), 7.67 (s, 2H), 7.48-7.47 (m, 5H), 7.45 (m, 1H).
13C NMR (CDCl3, , ppm): 160.16 (CH=N). FTIR (cm-1): 3045, 3019 (CH aromatic), 1608
(CH=N stretching), 1594 (C=C streching deformations in phenyl ring). Elem. anal. calcd (%)
for (C27H19N) (357.44): C 90.72, H 5.36, N 3.92; found C 90.29, H 5.41, N 4.08.
SB-6 N-(anthracen-2-yl)-1-(N,N-diphenylamine-4-yl)methanimine
1
Yield: 59%, Tm=165C. H NMR (CDCl3, , ppm): 8.56 (s, -CH=N, 1H), 8.41 (s, 2H), 8.03-
7.98 (m. 3H), 7.84-7.82 (m, 3H), 7,49- 7.44 (m, 3H), 7.34-7.31 (m, 4H), 7.19-717(d, 4H),
7.14-7.13 (m, 4H). 13C NMR (CDCl3, , ppm): 161.30 (CH=N). FTIR (cm-1): 3051, 3028 (CH
aromatic), 1630 (CH=N stretching), 1588 (C=C streching deformations in phenyl ring).
Elem. anal. calcd (%) for (C33H24N2) (448.56): C 88.36, H 5.39, N 6.25; found C 88.02, H
5.39, N 5.87

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2.3. Blends preparation
Blends were obtained by dissolving of the compounds and PMMA in CHCl3 and NMP
to form a homogeneous solution (concentration 1% w/w.). Blends cast on glass were dried in
vacuum oven at 90C over 2h.
2.4. Preparation of photovoltaic cells
Photovoltaic cells were fabricated on ITO-covered glass slides (15 mm x 15 mm)
which were cleaned in an ultrasonic bath using organic solvents. Next the ITO was covered
with a PEDOT:PSS thin film by spin-coating at 6000 rpm for 30 s and left for 30 min in a
vacuum heater at 120°C. Than (after 30 min) the active layer from chloroform was spun over
(at 2000 rpm for 30 s) the PEDOT:PSS thin film and left for 30 min in a vacuum heater at
60°C. Next Al contacts (thickness 100 nm) were thermal evaporated in (a high vacuum
chamber (10-6 mbar) in order to by thermal evaporation. The active surface of devices was 12
mm2. The thicknesses of active layer were measured by Woollam Spectroscopic Ellipsometer
WVASE 32.
2.5. Measurements
1H NMR and 13CNMR spectroscopy were carried out on a Avance II Ultra Shield Plus
Bruker 600 MHz Spectrometer using chloroform-d (CDCl3) as solvent and TMS as the
internal standard. FTIR spectra were recorded on a Spectrum One Spectrometer (Perkin
Elmer) with spectral resolution 0.01 nm using KBr pellets. Elemental analyses were
performed using a Vario EL III (Elementar, Germany). Differential scanning calorimetry
(DSC) was performed with a TA-DSC 2010 apparatus (TA Instruments, Newcastle, DE,
USA) accuracy 0.1C, under nitrogen using aluminium sample pans. UV-vis spectra were
registered using a Lambda Bio 40 Perkin Elmer spectrophotometer with spectral resolution
0.1 nm. The photoluminescence spectra were recorded on a VARIAN Cary Eclipse
Fluorescence Spectrophotometer with spectral resolution 2nm. Electrochemical measurements
were investigated using ATLAS 0531 Electrochemical Unit & Impedance Analyzer
potentiostat. 0.2 M Bu4NPF6 (Aldrich, 99%) in acetonitrile (Aldrich, HPLC grade) was used
as the supporting electrolyte. Measurements in solution were carried out using Pt electrode,
Pt wire and a quasi-Ag/AgCl electrode as a working electrode counter electrode and a
reference electrode, respectively. The oxidation and reduction potentials of film coated on a
ITO disk were measured using a Pt wire and a quasi-Ag/AgCl electrode as a counter electrode
and a reference electrode, respectively. Potentials were referenced with respect to ferrocene

ACCEPTED MANUSCRIPT
(Fc), which was used as the internal standard. The HOMO and LUMO levels were calculated
by assuming the absolute energy level of Fc/Fc+ as -5.1 eV to vacuum [22]. Current–voltage
(I–V) characteristics of the devices were measured using the Keithley 2400 sourcemeter. The
OPV parameters were obtained under illumination with a light intensity of 1.3 mW/cm2. The
thickness of the layers was estimated by the ellipsometic spectroscopy. This required
employment of various models and a special experimental procedure. First, the parameters
associated with the substrate, that is a glass coated with ITO, were set out using the Cauchy
model. Then the n and k parameters, as well as the thickness of the ITO (glass / ITO), were
determined by means of the Drude model with the generic oscillator describing
semiconducting properties. The Drude model was also employed to conduct separate
measurements and model calculations of Psi and Delta for the PEDOT: PSS and mixtures of
P3HT and a synthesized compound, and of P3OT and a synthesized compound, all prepared
on a glass substrate (Fig. 6S). The designated values of n and k for the above listed
compounds were next used to determine the parameters of thin layers of PEDOT: PSS on a
ITO substrate (glass / ITO / PEDOT: PSS); the thickness of the layer of PEDOT: PSS was
also determined. Finally, a layer of P3HT and a synthesized compound or, in a separate test, a
layer of P3OT and a synthesized compound, were applied on this substrate (glass / ITO /
PEDOT: PSS). The tests ended with ellipsometric measurements conducted and the thickness
of the active layer determined using dielectric functions of BHJ were calculated by applying
EMA model (for the thicknesses see Table 4).
3. Result and discussion
In the article synthesis and characterization of compounds being condensation
products of 2-aminoanthracene with various aldehydes are described from the point of view
of their thermal, spectral and electrochemical behavior. The chemical structure of obtained
compounds are presented in Fig. 1.

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OHC
N
CH
SB-1
OHC
N
CH
SB-2
CHO
N
CH
SB-3
NH2
+
OHC
N
CH
SB-4
OHC
OHC
N
CH
SB-5
N
N
CH
N
SB-6
Fig. 1. The synthetic route and chemical structures of the synthesized Schiff bases.
1
13
Instrumental technique including FTIR, H NMR, C NMR, spectroscopies and elemental
analysis were applied for the characterization of the molecular structure of the synthesized
Schiff bases. The spectral data and the elemental analysis results were found to be consisted
with the molecular structure of prepared imines (cf. Supporting information).
3.1. Thermal properties
Different scanning calorimetry (DSC) was utilized to examine thermal behavior of the
obtained Schiff bases. All the compounds exhibited well defined melting points (Tm), which
were depended on their structures. These imines melted in the in the range of 165 – 277ºC.
The highest and the lowest Tm were found for compound with biphenyl (SB-5) and
triphenylamine (SB-6) structure, respectively. The replacement of phenyl in SB-1 by
condensed aromatic rings such as naphthalene (SB-2), anthracene (SB-3) and phenanthrene
(SB-4) resulted in increase of Tm value from 217 (SB-1) to 260ºC (SB-2). It was found that
the presence of three condensed aromatic rings, that is, anthracene (SB-3 Tm=242ºC) and
phenathrene (SB-4 Tm=241ºC) unit decreases Tm about 20 ºC in relation to compound with
naphthalene moiety (SB-2). In the case of imine SB-5 after melting formed liquid crystalline
(LC) state. Fig. 2. presents DSC thermogram of SB-5 during cooling and heating scan.

ACCEPTED MANUSCRIPT
(a)
(b)
at 30C (initial sample)
I-N at 262 C
N (Schlieren texture) at 261.5C
N-Cr at 243.1C
Cr at 243.1C
Fig. 2. (a) DSC curves of the SB-5 at the heating (rate 10C/min) and at cooling (rate
5C/min), under N2 atmosphere and (b) optical textures of SB-5 from POM.
Upon DSC analysis the Schiff base SB-5 during the heating scan exhibited one phase
transition at 276.5 C associated with enthalpy changes (H) 126.5 J/g. During the cooling
scan two transitions were observed, such as isotropic state to nematic (I-N) at 261.8 C (H =
-1.9 J/g) and nematic to crystal (N-Cr) at 241.1 C (H = -109.7 J/g). Thus, the SB-5 showed
monotropic LC behavior. The phase behavior of this imine was investigated by observation of
the optical textures on a polarizing optical microscope equipped with hot-stage as well.
Photomicrographs of the optical texture of mesophases obtained for SB-5 are presented in
Fig. 2b. Based on POM Schlieren texture of liquid crystal nematic mesophase was observed.

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The investigations by POM show that during a cooling scan SB-5 exhibited the full
isotropization at 277C, the nematic phase at 261.5C and crystallization at 243C (cf. Fig.
2b). This compound is similar to Schiff base repeated in our previous article Az-6, which
consists of biphenyl and naphthalene units [21]. However, the Az-6 did not showed LC
properties. It was found that the introduction of anthracene moiety in the place of naphthalene
induced the LC behavior.
3.2. Photophysical properties
Optical properties of the prepared Schiff bases were analyzed by UV-vis and
photoluminescence (PL) measurements in solvents exhibiting different dielectric constants ()
such as CHCl3 (=4.8) and NMP (=33) and also as a blend with non-emissive poly(methyl
methacrylate) (PMMA). For the blend preparation both solvents, that is, chloroform and NMP
were applied. It also should be mentioned that obtained compounds have basic nitrogen atom
that can be doped. In this work as doping agent trifluoroacetic acid (TFA) and ferric chloride
(FeCl3) were utilized for selected imines.
3.2.1 Absorption spectroscopy and DFT calculation
The electronic absorption spectra of the investigated Schiff bases recorded in both
solvents (CHCl3 and NMP) showed similar characteristics and exhibited absorption in the
range of 260-500 nm. The spectral data of azomethines are collected in Table 1.
Whereas, Fig. 3. shows the UV-vis spectra of representative azomethines. The normalized
UV-vis spectra of the obtained imines and applied substrates in NMP and chloroform solution
are presented in Fig. 1S. in Supporting information.
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
SB-3
SB-5
SB-6
SB-1
CHCl
NMP
3
300
350
400
450
500
550
600
300
350
400
450
Wavelength [nm]
500
550
600
Wavelength [nm]
(c)
(d)

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0.20
SB-1
SB-4
1.0
0.8
0.6
0.4
0.2
0.0
PMMA blend from CHCl
3
PMMA blend
NMP solution
PMMA blend from NMP
0.15
0.10
0.05
0.00
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength [nm]
Wavelength [nm]
Fig. 3. UV-vis spectra of (a) selected Schiff bases in NMP solution (concentration 1x10-5
mol/L), (b) SB-1 in NMP and chloroform solutions, (c) SB-4 in blends and in NMP solution
and (d) SB-1 in blends prepared from NMP and chloroform solutions.
Table 1. Electronic spectral data for the Schiff bases in solutions
(concentration 1x10-5 mol/L) and in blends.
Medium
NMP
Code
SB-1

max [nm]
x 104 [dm3mol-1cm-1]
5.98, 3.06, 3.15, 1.25, 1.19
264, 316, 329, 372,
391
251, 265, 319, 332,
372, 389
CHCl3
7.05, 8.06, 3.28, 3.47, 1.45,
1.37
blend from NMP
blend from CHCl3
NMP
327, 383
-
-
323, 339, 412
264, 307, 321, 337,
372, 394
5.95, 2.89, 2.61, 2.66, 1.06,
0.92
CHCl3
263, 305, 323, 337,
12.2, 5.01, 4.49, 5.00, 1.97,
1.75
SB-2
371, 391
blend from NMP
blend from CHCl3
NMP
324
-
-
327, 344, 387
266, 327, 390, 417
241, 266, 339, 372,
407
24.21, 3.06, 3.91, 4.18,
23.37, 47.57, 2.77, 3.92,
3.99
CHCl3
SB-3
SB-4
blend from NMP
blend from CHCl3
NMP
326, 405
-
-
326, 425
263, 332, 346, 375,
7.69, 3.01, 3.17, 1.49, 1.32
392
CHCl3
258, 333, 345, 390
327
346, 390
7.39, 2.63, 2.71, 1.17
-
-
33.49, 29.90, 30.73, 30.97,
11.28, 9.62
PMMA z NMP
blend from CHCl3
NMP
261, 313, 325, 341,
373, 400
CHCl3
256, 310, 324, 340,
372, 400
323, 390
326, 344, 390
266, 293, 368, 389,
408
72.08, 51.54, 48.33, 49.06,
18.12, 14.97
-
-
29.60, 15.45, 22.66, 20.35,
19.77
SB-5
SB-6
blend from NMP
blend from CHCl3
NMP

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CHCl3
260, 299, 370, 388,
408
42.76, 14.48, 21.23, 20.15,
18.99
blend from NMP
blend from CHCl3
329, 360
316, 380, 411
-
-
*to find the position the second-derivatives method was used
It was found that the spectra consist of absorption bands with maximum (max) below
300 nm being responsible for transition in aromatic rings and in the range of 320-330 nm
being attributed to the -* transition in the imine group and in the range of 370-400 nm
assigned to vibronic bands transition in anthracene moieties. Introduction of second
anthracene (SB-3) and triphenylamine (TPA) (SB-6) unit resulted in bathochromic shift of
absorption compared to the other reported Schiff bases (SB-1, SB-2, SB-4, SB-5). Nature of
the solvent used did not influence significantly on the UV-vis spectra of the compounds
detected in solutions and also in blends with PMMA (cf. Fig. 3.). Comparing the UV-vis
spectra of these Schiff bases with their naphthalene – based analogues from our previous
work, it was observed that introduction of anthracene shifted the absorption range to lower
energy.
To gain further insight into the nature of excited states involved in absorption
processes, the quantum calculations using density functional theory (DFT), with an exchange
correlation hybrid functional B3LYP and the basis 6-31G** for carbon, nitrogen and 6-31G
for hydrogen atoms have been performed for the studied Schiff bases. The calculations were
carried out with use of Gaussian09 program [23]. The ground state geometries of these
compounds were optimized in gas phase and no imaginary frequencies were found for any of
these species, computed at Ci symmetry, by vibrational analysis. Moreover calculated IR
spectra compared to experimental ones show good agreement (cf. Table S1 in Supporting
Information); the differences between calculated and experimental CH=N stretching do not
outrun 8 cm-1. Based on the optimized geometries the energy and electronic distribution of the
molecular orbitals were calculated in acetonitrile solution. The solvent effect was introduced
by PCM model [24] for an comparison with experimental electrochemical data. The highest
occupied and lowest virtual molecular orbitals are of importance for the evaluation of
molecular reactivity. As less negative is the energy of the HOMO, more susceptible is the
molecule to donate electrons and, consequently, higher is the tendency to suffer oxidation. A
similar argument can be used to interpret the tendency of a given molecule in suffer reduction,
on the basis of the energy of the LUMO. The contours of HOMOs and LUMOs are presented
in Fig. 4.

ACCEPTED MANUSCRIPT
In order to more accurately describe the structure of the frontier molecular orbitals, the
contribution of a group (anthracene, -HC=N-, phenanthrene and others aromatic groups) to a
molecular orbital was calculated using Mülliken population analysis. GaussSum 3.0 [25]
program was used to calculate group contributions to the molecular orbitals and to prepare the
partial density of states (DOS) spectra. The DOS spectra were created by convoluting the
molecular orbital information with Gaussian curves of unit height and FWHM of 0.3 eV. The
DOS spectra were presented on the Fig. 2S in Supporting information. It was found that the
HOMO orbitals of the SB-2, SB-1, SB-5 and SB-4 compounds are mainly localized on the
anthracene moiety. In the case of SB-3 and SB-6, HOMO is localized on the conjugated
double bonds system of whole molecule comprising the aromatic rings and double bonds of -
CH=N- groups.
HOMO
LUMO
SB-1 -5.26 eV
SB-2 -5.24 eV
-1.97 eV
-2.05 eV
SB-3 -5.12 eV
SB-4 -5.22 eV
-2.33 eV
-2.06 eV

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SB-5 -5.24 eV
-2.03 eV
SB-6 -4.99 eV
-1.92 eV
Fig. 4. HOMO and LUMO contours of the Schiff bases.
LUMOs are localized over the single bonds in the molecules but the participation of
anthracene moiety is predominant in SB-2, SB-1, SB-5 and SB-6. Whereas LUMO of SB-3 is
mainly localized on the anthracene-9-yl fragment of the molecule and in the case of SB-4
anthracene and phenanthrene fragments have a similar contribution to the LUMO (cf. Fig. 2S,
Fig. 4 and Table 4S.).
3.2.2. Photoluminescence and DFT calculations
The representative PL spectra in solutions are depicted in Fig. 5. and PL data are
collected in Table 2.
Table 2. PL maxima emission bands of the Schiff bases
in solutions and blends (ex = 330 nm).
em [nm]
Code
in solution
CHCl3
462
463
443
442
465
497
in blends
from CHCl3 from NMP
NMP
408
373
407
428
408
-
465
466
465
463
465
497
401
397
400
402
398
421
SB-1
SB-2
SB-3
SB-4
SB-5
SB-6

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(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
NMP
ex=360 nm
CHCl3
ex 360 nm
SB-1
SB-2
SB-3
SB-4
SB-5
SB-6
0.8
0.6
0.4
0.2
0.0
SB-1
SB-2
SB-3
SB-4
SB-5
SB-6
400
450
500
550
600
650
700
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
(b)
1.2
SB-1
ex320nm
PMMA blend from CHCl3
PMMA blend from NMP
1.0
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
Wavelength [nm]
(c)
Fig. 5. PL spectra of prepared Schiff bases (a) in NMP chloroform solution
(concentration = 1x10-5 mol/L), (b) in blends with SB-1 prepared from CHCl3 and NMP
solutions and (c) the quantum yields (f) of the compounds in chloroform solution
(ex = 360 nm) and in blends with PMMA (ex = 420 nm).
Excitation wavelength (ex) influenced on emission intensity, however, the highest intensities
were observed at ex in the range of 320-360 nm in dependence on the compound structure.
Maximum of emission band (em) of the Schiff bases in chloroform solution were observed at
lower energy region than in NMP and the differences depended on the aldehyde structure

ACCEPTED MANUSCRIPT
used for condensation with 2-aminoanthracene. The effect of the aldehyde structure residue in
the compounds on emission band was more significantly seen in chloroform solution than in
NMP. Slightly higher emission intensity for all the compounds was observed in CHCl3
solution than in NMP (in arbitrary units). In all cases (in NMP and CHCl3), the fluorescence
emission spectra compared to the absorption spectra were not found to be mirror image (Fig.
1S and cf. Fig. 5). This differences occurred because electronic excitation greatly altered the
nuclear geometry, hence the spacing of the vibrational energy levels of the excited states was
different to that of the ground state. The emission of SB-1, SB-3 and SB-4 in CHCl3 solution
exhibited very well resolved vibronic structures. This fine structure appears to be a
manifestation of enhanced vibronic coupling, which clearly suggests rigidification of the
excited state molecular geometry of the compounds and its better stabilization by solvation. In
contrast, emission spectra of SB-5 was an entirely unstructured. The bathochromic shift (i.e.
lower energy) of emission maximum was observed in sequence SB-2, SB-1 ≈ SB3 ≈ SB-4,
SB-5, SB-6. Emission spectra of azomethines were also registered in solid state as blend with
PMMA. Dispersed luminescent compound in nonemissive polymer matrix let to eliminate the
self-quenching of fluorescence due to the dilution effect [26,27]. Blends involving
luminescent compound and inert polymer as matrices are often used in polymer light emitting
diodes as a tool to increase the efficiency of the devices [26,28]. Fig. 3d presents the
representative PL spectra in blend obtained from both solutions. It is interesting and
unexpected that emission bands of the blends depended on the solvent used for the blends
preparation. It is necessary to stress that the blends were vacuum dried to remove the solvent.
For the most of investigated Schiff bases em of the blends prepared from CHCl3 solution was
red shifted in comparison with the blends obtained from NMP solution, similarly to the
phenomenon observed in the case of PL detected in solution. For the blends prepared from
CHCl3 solution the quantum yield (f) of PL was estimated using the integrating sphere. The
blends showed f in the range of 2.18–6.03% higher than in solution (cf. Fig. 5c). The Schiff
bases prepared from 2-aminoanthracene compare with their analogues obtained from 2-
naphtylamine [21] showed lower f in the case of compound with phenyl (SB-1), and
naphthalene (SB-2) and second anthracene (SB-3) ring and higher in the case of imines with
phenanthrene structure (SB-4).
The azomethines in chloroform and NMP solutions as well as blends, obtained from
these solvents, excited at 330 nm reveal photoluminescence (cf. Table 2). In order to
determine the nature of the excitation, the electronic spectra of the compounds were

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calculated with use of CIS(D) method [29,30] with chloroform as solvent in the polarizable
continuum model (PCM). The calculated UV-vis spectra presents Fig. 4S in Supplementary
materials and the calculated electronic transitions corresponding to the excitation wavelength
are collected in Table 3S. The assignment of the calculated orbital excitations was based on
an overview of the composition of the HOMOs and LUMOs. The composition of the selected
HOMOs and LUMOs are given in Table 4S. In general, in the case of the Schiff bases, except
SB-6, the excitations connected with the emission processes have →* character with
contribution of n→* transitions due to the fact that the HOMO is mainly localized on the
anthracene part. In the case of SB-6 the triphenylamine orbitals play an important role in the
HOMO and therefore the n→* transitions have no place. Hypsochromic shift of the
maximum emission wavelength (cf. Table 2 and Fig. 5a) by changing the solvent from
chloroform (=4.81) to NMP (=33) confirms the participation of n→* transition in the
excitation process.
3.2.3. Doping investigations
In this work ferric chloride (FeCl3) and TFA as oxidizing and protonating agent,
respectively, were used to study their influence on optical properties of the selected Schiff
base, that is, SB-1 in dichloromethane solution. Ferric chloride is an oxidant of choice for
chemical oxidation studies because it is spectroscopically transparent in the spectral window
used for the absorbance measurements. It also is a one-electron oxidant. It was found that
addition of increasing amount of FeCl3 to the azomethine in solution induced color changes as
can be seen in Fig. 6a and surprisingly slightly influences on UV-vis spectra (cf. Fig. 6b). The
color changes were not observed under electrochemical oxidation, which was irreversible (cf.
section 3.4.).
(a)
virgin,
+1FeCl3,
+2FeCl3,
+3FeCl3,
+4FeCl3,
+5FeCl3,

ACCEPTED MANUSCRIPT
(b)
(c)
25
20
15
10
5
ex=330nm
CH2Cl2
SB-1
1.0
0.8
0.6
0.4
0.2
0.0
400
300
200
100
0
SB-1+2FeCl3
SB-1+3FeCl3
SB-1+4FeCl3
SB-1+5FeCl3
SB-1
SB-1+1FeCl
3
3
3
SB-1+2FeCl
SB-1+3FeCl
0
250
300
350
400
450
500
550
600
350
400
450
500
550
600
650
Wavelength [nm]
Wavelength [nm]
Fig. 6. (a) Oxidizing effect of FeCl3 on color changes of SB-1, (b) UV-vis spectra of the SB-1
after oxidizing with FeCl3, (c) PL spectra of the SB-1 in dichloromethane after addition of
FeCl3 (ex=330nm) (see left and right scale).
The addition of hydrazine hydrate reduced the oxidized intermediate and the original color of
the virgin sample returned. The reversible cycles of chemical oxidation/neutralisation were
possible without any noticeable color deterioration of the neutral or oxidized states. This
confirm the robustness of the azomethine bond. Hydrolysis and decomposition of the imine
would easily be seen by the disappearance of the visible color [31]. In order to indicated the
chemically oxidized intermediate additional investigations should be carried out which are out
of the scope of the present work. Our aim was confirmation of the stability of imine bond.
Investigations of influence of oxidation on PL were also carried out. When the mole ratio of
the azomethine to FeCl3 was 1:1 emission band broaden to lower energy and PL intensity
increase, which may suggest formation of the chemically oxidized intermediate. When 2 eq.
of the oxidant was added, that means that mole ratio of the azomethine to FeCl3 was 1:2, two
emission bands were seen at 474 nm and 554 nm. The first one comes probably from the
emission of neutral form and the second one is due to PL of the chemically oxidized
intermediate. After increasing amount of FeCl3 to the ratio of 1:3 only one emission band was
observed at 554 nm coming from oxidized form PL. Further adding of the oxidant (mole ratio
1:4, 1:5, 1:6) did not change the position and intensity of the emission band. After additional
of 1 eq. of FeCl3 the emission intensity increased about 30 times (in arbitrary units) in
comparison with the intensity of the virgin sample. However the further increasing of FeCl3
amount caused decreasing of the emission intensities. In Fig. 6c PL spectra (ex=330 nm) of
SB-1 after oxidation with FeCl3 are depicted.

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As was mentioned the imine linkages can be also protonated. In this study TFA was
used as protonating agent of the SB-1, as an example. After adding of TFA in the mole ratio
of the azomethine and acid equal 1:1 in dichloromethane (con. 10-5M) solution, in UV-vis
spectrum significant decrease of absorption in the range of 320-330 nm, responsible for -
CH=N- group absorption was observed as it is seen in Fig. 7a. Increasing of the amount of the
acid (mole ratio 1:3, 1:5, 1:10) caused further decrease of this band and another broad band
with low intensity centered at 453 nm appeared due to conjugate acid formation by proton
addition. The isosbastic point was seen at 414 nm which confirms the coexistence of discrete
chromophores. Probably at the same time the color of the solution changed as it can be seen in
Fig. 7b. Similar to oxidation, protonation caused increase of PL intensity. When mole ratio of
the azomethine to the acid was 1:1 emission intensity (ex=330 nm) increased about 50 times
in arbitrary units with 6 nm hypsochromic shift of the band in comparison with the virgin
sample.
(a)
2.0
SB-1
SB-1:TFA 1:1
SB-1:TFA 1:3
SB-1:TFA 1:5
SB-1:TFA 1:10
329
1.5
1.0
0.5
0.0
336
390
371
355
414
453
300
400
500
600
700
Wavelenght [nm]
(b)
virgin
+1TFA
+3TFA
+5TFA
+10TFA

ACCEPTED MANUSCRIPT
(c)
1200
SB-1
SB-1:TFA 1:1
463
1000
SB-1:TFA 1:3
SB-1:TFA 1:5
SB-1:TFA 1:10
800
507
600
518
400
200
0
350
400
450
500
550
600
650
700
Wavelenght [nm]
Fig. 7. (a) UV-vis spectra of SB-1 in dichloromethane after adding of trifluoroacetic acid
(TFA), (b) and (c) PL spectra of SB-1 after adding of trifluoroacetic acid (ex=330nm).
When the mole ratio of the azomethine SB-1 to TFA was 1:5 two overlapped emission bands
at 464 nm and 507 nm were seen. Increasing amount of the acid (mole ratio 1:10) caused shift
the emission band to 518 nm even though the band is broad and emission at 464 nm is also
seen. Similar to the oxidation with FeCl3, increasing amount of acid over mole ratio 1:1,
caused decrease of the PL intensities. In Fig. 7c the photoluminescence spectra of the
azomethine SB-1 are depicted.
3.3. Electrochemical properties
Electrochemical properties of synthesized Schiff bases in MeCN solution (except SB-3
because of insufficient solubility) and as thin films deposited on ITO substrates were
investigated. In Table 3. electrochemical data are presented, while exemplary CV
voltammogram are presented in Fig. 8.
In CV voltammogram of all Schiff bases dissolved in MeCN two oxidation peak were
observed (Figure 8a). For all compounds, except SB-6, potential of first irreversible oxidation
process was found at 0.74 V, and it is mostly contributed by anthracene moiety. Second
quazi-reversible oxidation potential was found at around 1.1 V. For compound SB-6 both
oxidation potentials were negatively shifted to 0.51 V and 0.95 V respectively. Significantly
lower oxidation potentials of SB-6 are associated with the presence of triphenylamine unit.
Further investigations of SB-6 with different scan rate, showed that in this case, three
oxidation processes occur. In between two firstly observed oxidation peaks, new poorly
formed oxidation peak, at 0.73 V appears while decreasing scan rate (Figure 8b).
Measurements performed for thin films on ITO showed only one oxidation process (except

ACCEPTED MANUSCRIPT
SB-1). In CV voltammogram of SB-1 two oxidation peaks were detected, respectively 0.31 V
and 0.81 V. Only Schiff base SB-6, containing triphenylamine units exhibited quasi-reversible
oxidation process, with Eox onset at 0,10 V, which is connected to creation of cation radical in
triphenylamine unit [32–34]. In a solid state, cation radical dimerizes and the color change is
observed from yellow to red. For other compounds oxidation was found to be irreversible.
Moreover, onset of oxidation potential measured for films on ITO was usually lower then
Eox,onset found for the same compound in MeCN solution. All tested compounds, both in
solution and as film on ITO, showed one irreversible reduction process with Ered onset in the
range -2.05 V to -1.75 V, which is associated with a reduction of imine group.
Table 3. Electrochemical data for investigated Schiff bases.
In MeCN
Eox onset
[V]
0.65
0.66
On ITO
Ered onset Eox onset
In MeCN
Ered onset EHOMO
Symbol
ELUMO
[eV]
Eg
[V]
[V]
[V]
[eV]
[eV]
2,70
2,61
-1.92*
2,61
2,69
2,50
-2.05
-1.95
-
-1.96
-2.04
-2.05
0.14
0.65
0.17
0.33
0.68
0.10
-1.97
-1.99
-1.75
-1.96
-2.04
-1.94
-5,75
-5,76
-4.97*
-5,75
-5,75
-5,55
-3,05
-3,15
-3.35*
-3,14
-3,06
-3,05
SB-1
SB-2
SB-3
SB-4
SB-5
SB-6
-
0.65
0.65
0.45
*calculated from CV measurements on ITO
(a)
(b)
3.0
20 mV/s
10 mV/s
5 mV/s
4
3
2.5
2.0
1.5
2
1.0
0.5
1
0.0
0
-0.5
-1.0
-1
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
+
+
Potential [V vs Fc/Fc ]
Potential [V vs Fc/Fc ]
(c)

ACCEPTED MANUSCRIPT
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
+
Potential [V vs Fc/Fc ]
Fig. 8. CV voltammogram of (a) SB-1 in MeCN solution (scan rate 20 mV/s), (b) SB-6 in
MeCN solution with different scan rate and (c) SB-4 film coated on a ITO disk (scan rate 50
mV/s) (electrolyte 0.1M Bu4NPF6 in MeCN).
The HOMO and LUMO energy levels (or rather, ionization potentials and electron
affinities, assuming that IP of ferrocene equals -5.1 eV is [22]) and Eg of the azomethines
were estimated from the electrochemical oxidation and reduction onset potentials measured in
MeCN solution. LUMO energy was similar for all investigated compounds and it was in the
range -3.15 eV to -3.06 eV. Azomethine SB-6 exhibited the highest HOMO energy (-5.55 eV)
and the lowest energy band gap (2.5 eV).
The theoretically obtained energies of HOMO are similar to experimental data (cf. Fig.
4 and Table 3); the highest difference (0.53 eV) is observed for SB-4 compound. The virtual
orbitals are generally more difficult to describe theoretically than the occupied ones and the
errors in the LUMO eigenvalues are considerable larger. Therefore the calculated LUMO
values are usually much higher in energy than those estimated experimentally. Compare the
estimated the HOMO and LUMO energies of these Schiff bases with previously presented
azomethines [21] it should be stressed that replacement of naphthalene unit with anthracene
one raised the value of LUMO energy level (about 0.7 eV) and triggered increase in the
energy band gap values in the range of 0.92 – 1.65 eV.
3.4. Preliminary photovoltaic activity testing
The Schiff bases (SB-1, SB-3, SB-4, SB-5 and SB-6) were tested as component of
active layer in bulk heterojunction (BHJ) photovoltaic (PV) cells. Two kinds of solar cells
were prepared and characterized based on active layers consisting of blends of P3HT or P3OT
and imines (weight ratio 1:1 or 1:2). Additionally, ellipsometric measurements of active
layers were carried out and refractive index for the standard wavelength 633 nm, is as
follows: n=1.60 SB-1:P3HT (1:1), n=1.47 SB-1:P3HT (1:2), n=1.45 SB-5:P3HT (1:1), n=1.45

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SB-5:P3OT (1:1), n=1.68 SB-3:P3HT (1:1), n=1.72 SB-3:P3HT (1:2) and n=1.42 SB-
3:P3OT (1:2).
The photovoltaic experiments were done for non-encapsulated devices. The fabricated
BHJ PV cells consist of electrodes: aluminum acts as cathode (collecting electrons) and ITO
applied as the anode for the purposes of collecting holes, PEDOT:PSS used as an hole
transporting material (HTM) and anode buffer layer and active layers. At the beginning of
investigations the donor properties of Schiff bases in OPV were examined. However, in the
case of devices with PCBM:imine active layer the photovoltaic effect was not observed. In
next step, the cells with azomethines as acceptor component of active layers were contracted.
As donor in devices two poly(3-alkylthiophene)s were tested, that is, regioregular P3HT and
P3OT. When comparing HOMO and LUMO levels of Schiff bases and poly(3-
alkylthiophene)s it is not possible to foresee unequivocally which pair of the compounds will
have good photovoltaic properties. Fig. 9. displays the representative dark and illuminated
current density versus voltage (J–V) characteristic for a device based on SB-3. Fig.5S in
Supporting information shows J-V characteristic for others imines. Whereas the photovoltaic
parameters: an open-circuit voltage (Voc), a current density (Jsc), a fill factor (FF) and power
conversion efficiency (PCE) are gathered in Table 4.
Table 4. Characteristics of the devices in the configurations:
ITO/PEDOT:PSS/P3HT:imine/Al and ITO/PEDOT:PSS/P3OT:imine/Al,
under light illumination (1.3 mW/cm2).
Active layer
Thickness
[nm]
97.4
118
99.3
100
Weight
ratio
1:1
1:2
1:1
1:1
1:2
1:2
1:2
1:2
1:1
1:1
1:1
Jsc
Voc
[V]
FF
PCE
[%]
0.15
0.23
0.09
0.25
0.39
0.15
0.11
0.12
0.11
0.25
0.19
0.12
[μA/cm2]
14.5
24.4
10.4
20.6
36.5
16.1
16.4
15.1
26.1
30.7
25.7
15.3
0.610
0.580
0.505
0.672
0.654
0.530
0.364
0.459
0.188
0.431
0.454
0.451
0.220
0.209
0.233
0.239
0.214
0.227
0.236
0.225
0.284
0.251
0.217
0.223
SB-1:P3HT
SB-1:P3OT
SB-3:P3HT
106
91.4
98.3
93.6
102
98.2
101
SB-3:P3OT
SB-4:P3HT[35]
SB-4:P3OT
SB-5:P3HT
SB-5:P3OT
SB-6:P3HT[35]
SB-6:P3OT
104
1:1
Considering the data presented in Table 4 can be concluded that the increase of imine ratio in
active layer with P3HT from 1:1 to 1:2 improve value of the current density, which may
improve the power conversion efficiency. The increase of current density in device with lager
imine ratio is probably due to the lager interfaces between electron donor (P3HT) and

ACCEPTED MANUSCRIPT
acceptor (imine) interface and better change transfer to the electrodes as was observed for
device based on P3HT and PCBM [36]. Considering the effect of poly(3-alkylthiophene) on
device performance, higher value of both Jsc and open-circuit voltage was observed for P3HT
given higher value of PCE, except for SB-5. In the case of device with imine bearing biphenyl
structure (SB-5) the higher Jsc and PCE showed the cell with P3OT. Thus, this imine is more
compatible with P3OT in photovoltaic structures. The best PCE about 0.39 % exhibited
device with Schiff base consists of two anthracene units (SB-3). It can be explained, for
example by more suitable position of orbital energy level of SB-3 (cf. Fig.7a). The value of
PCE is significantly lower compare to BHJ PV cell based on P3HT and typical acceptor
PCBM. However, the acceptor activity of investigated Schiff bases was demonstrated. In our
former work [21] we presented the PV properties of devices based on azomethine Az-3 which
is an analogue of SB-3. Compare the results, can be said that higher PCE value was obtained
for device with active layer consists of SB-3 and P3HT in relation to cell bearing Az-3 and
P3HT. In the case of Az-3 the increase of imine content also effected on improvement of
PCE. However, contrary to SB-3 better PV parameters exhibited device with Az-3 and P3OT
compare to P3HT. It can be explained by different chemical structure of SB-3 and Az-3,
which determines their HOMO and LUMO energy levels. Thus, LUMO and HOMO of SB-3
can be more compatible with orbitals energy of P3HT contrary to Az-3.
(a)
0
-5
-10
-15
-20
-25
dark
-30
SB-3:P3OT 1:2
-35
-40
SB-3:P3HT 1:1
SB-3:P3HT 1:2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage [V]

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(b)
-3,00 eV
-3,35 eV
-3,55 eV
-4,20 eV
-4,80 eV
-4,90 eV
-5,00 eV -4,97 eV
-5,25 eV
Fig. 9. (a) Simple energy level diagram of the HOMO, LUMO levels of components of OPV
cell and (b) the dark and illuminated current density versus voltage characteristics for devices
based on SB-3.
4. Conclusions
Anthracene-based Schiff bases containing in their structure phenyl, naphthalene,
biphenyl, anthracene, phenanthrene and triphenlyamine were synthesized and characterized.
The study revealed pronounced effects of chemical structure on the optical and redox
properties of investigated compounds. The absorption of imine with two anthracene units was
ranging up to 550 nm. Introduction of second anthracene unit resulted in shift of absorption to
lower energy region in comparison with others imines. All Schiff bases emitted blue light in
solution and blended with PMMA. Doping of compound with acid and FeCl3 changed both
their absorption in UV–vis range and PL properties, that is increased intensity of emitted
light. The hydrolytic and chemical oxidative resistance on exposure to acid and FeCl3 of
selected imine was demonstrated. The reducing and raising the HOMO and LUMO energy
values, respectively, together with introduction of second phenyl ring (SB-2 and SB-5)
compare with SB-1 was seen. The lowest energy band gap 2.5 eV exhibited azomethine with
TPA unit. The presented Schiff bases, based on photovoltaic tests, can be treated as n-type
semiconductor with HOMO level between -5.55 and -5.76 eV. In summary, compound with
two anthracene structures connected by imine bond due to their electronic properties and
electron acceptor character seems to be the most perspective as material for optoelectronic
applications.

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Acknowledgement
The research was co-financed by the National Research and Development Center (NCBiR)
under Grant ORGANOMET No: PBS2/A5/40/2014. The calculations have been carried out in
Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl)
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Graphical abstract

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Highlights
> Novel Schiff bases with anthracene unit were prepared. > The theoretical and experimental
studies of their photophysical properties were carried out. > They exhibited low
electrochemically estimated energy gap. > They emitted blue light as blend with PMMA with
quantum yield efficiency in the range of 2.97- 6.03 %. > The photovoltaic properties of
selected imine as active layer in organic solar cells were tested. > The highest conversion
efficiency was 0.39 %, which confirmed its electron acceptor character. >