Comparative studies of structural, thermal, optical, and electrochemical properties of azines with different end groups with their azomethine analogues toward application in (opto)electronics

Article abstract of DOI:10.1021/jp407623u

Two series of azines and their azomethine analogues were prepared via condensation reaction of benzaldehyde, 2-hydroxybenzaldehyde, 4-pyridinecarboxaldehyde, 2-thiophenecarboxaldehyde, and 4-(diphenylamino) benzaldehyde with hydrazine monohydrate and 1,4-phenylenediamine, respectively. The structures of given compounds were characterized by FTIR, ¹H NMR, and ¹³C NMR spectroscopy as well as elemental analysis. Optical, electrochemical, and thermal properties of all compounds were investigated by means of differential scanning calorimetry (DSC), UV-vis spectroscopy, stationary and time-resolved photoluminescence spectroscopy, and cycling voltammetry (CV). Additionally, the electronic properties, that is, orbital energies and resulting energy gap were calculated theoretically by density functional theory (DFT). Influence of chemical structure of the compounds on their properties was analyzed.

Full text of DOI:10.1021/jp407623u

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Article
Comparative Studies of Structural, Thermal, Optical and Electrochemical
Properties of Azines with Different End Groups with Their
Azomethine Analogues Towards Application in (Opto)Electronics
Danuta Sek, Mariola Siwy, Katarzyna Bijak, Marzena Grucela-Zajac, J. G. Malecki,
Karolina Smolarek, Lukasz Bujak, Prof. Sebastian Mackowski, and Ewa Schab-Balcerzak
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp407623u • Publication Date (Web): 19 Aug 2013
Downloaded from http://pubs.acs.org on August 28, 2013
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Comparative Studies of Structural, Thermal, Optical and Electrochemical Properties of
Azines with Different End Groups with Their Azomethine Analogues Towards
Application in (Opto)Electronics
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Danuta Sek1, Mariola Siwy1, Katarzyna Bijak2, Marzena GrucelaꢀZajac2, Grzegorz Malecki2,
Karolina Smolarek3, Lukasz Bujak3, Sebastian Mackowski3, Ewa SchabꢀBalcerzak*1,2
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
3Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus
University, 5 Grudziadzka Str., 87-100 Torun, Poland
Abstract: Two series of azines and their azomethine analogues were prepared via
condensation reaction of benzaldehyde, 2ꢀhydroxybenzaldehyde, 4ꢀpyridinecarboxaldehyde,
2ꢀthiophenecarboxaldehyde,
and
4ꢀ(diphenylamino)benzaldehyde
with
hydrazine
monohydrate and 1,4ꢀphenylendiamine, respectively. The structures of given compounds
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were characterized by FTIR, HNMR, and 13C NMR spectroscopy as well as elemental
analysis. Optical, electrochemical, and thermal properties of all compounds were investigated
by means of differential scanning calorimetry (DSC), UV–vis spectroscopy, stationary and
timeꢀresolved photoluminescence spectroscopy, and cycling voltammetry (CV).
*Corresponding author: at Institute of Chemistry, University of Silesia, Katowice, Poland, Centre of Polymer
and Carbon Materials, Polish Academy of Sciences, Zabrze, Poland
Telephone number: +48 32 359 1642
E-mail addresses: ewa.schabꢀbalcerzak@us.edu.pl, eschabꢀbalcerzak@cmpwꢀpan.edu.pl
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Additionally, the electronic properties, that is, orbital energies and resulting energy gap were
calculated theoretically by density functional theory (DFT). Influence of chemical structure of
the compounds on their properties was analyzed.
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Keywords: diimines, imines, luminescence, HOMO and LUMO energy levels, DFT
calculation
1. Introduction
Monodispersed πꢀconjugated compounds with well defined structures have drawn
attention since many years as materials for a variety of electronic and optoelectronic devices
such as light emitting diodes, photovoltaic cells and many others1,2. Among them, azines, that
is, NꢀN linked diimines (RꢀCH=NꢀN=CHꢀR) and azomethines having imine linkages (–
CH=Nꢀ) and being easily synthesized, seem to be very interesting compounds. Additionally,
their properties can be tuned by protonation with Lewis acids and complexation with Bronsted
acids. Azines (diimines) are 2,3ꢀdiaza analogues of 1,3ꢀbutadiene and comprise a class of
compounds that has been receiving increasing attention in recent years because of their utility
in a number of interesting reactions and applications3 Azines have been utilized in bond
formation reaction, polymerization, in the design of liquid crystals, and the synthesis of
heterocyclic compounds4,5 Due to the presence of imine groups they are potential ligands and
have been used as ligands in coordination chemistry6,7. Azines are also considered as
nonlinear optical materials8. Moreover, they can exhibit antibacterial, antifungal, antitumor
activity4,9. It was found that some of azines exhibit aggregationꢀinduced emission
enhancement (AIEE) characteristics, which is promising for applications in optical sensors,
light emitting diodes, photovoltaic cells, and photoemitters10. It should be mentioned that only
limited number of organic compounds have been reported do display AIEE characteristics. On
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the other hand azomethines are interesting alternatives to vinylene linkages and they are
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isoelectronic to their carbon counterparts and can be exploited for various applications11ꢀ16
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They as well exhibit high chemical, oxidative and reductive resistance and interesting
photophysical properties17,18
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In this work we prepare and investigate two series of azines and azomethines and
comprehensively compare their selected properties. This research is the continuation of our
efforts in the search of organic materials both polymers and low molecular weight compounds
and for applications in organic (opto)electronics19ꢀ24. In our previous work heterocyclic azine
with thiophene rings19 and azomethine with TPA units20 were described. Here, we report on
synthesis of two series of symmetrical azines and azomethines in reaction of hydrazine or 1,4ꢀ
phenylenediamine with various aldehydes containing phenyl or hydroxyphenyl, or pyridine,
or thiophene or triphenylamine (TPA) units and compare their spectral and electrochemical
properties. While most of these compounds have been descried in literature, no comparative
research focused on their optical and electrochemical properties has been carried out. To our
best knowledge only one work describes the studies of photoluminescence and
electrochemical properties of azomethines prepared from 1,4ꢀphenylene diamine and
benzaldehyde and 2ꢀthiophenecarboxaldehyde11. Short literature review below is devoted to
the azines and their azomethine analogues investigated in this work. Lee et al.25 described
synthesis of azines via condensation of adduct 1:1 hydrazine and carbon dioxide with
salicylaldehyde, benzaldehyde and others. Pomar et al.26 used triisopropylsilylhydrazine for
synthesis of various azines. Shah et al27 obtained azines via thermal transformation of aryl
semicarbazones at 260°C. Also in papers28ꢀ30 different methods of azines synthesis were
reported. Crystalline structure of salicylazine was investigated by Mijanuddin et al.31. Third –
order nonlinear behavior of azines synthesized from thiophenaldehyde and its bromoꢀ
substituted derivatives were investigated by Ghazzali et al.32. Lin et al.33 described crystalline
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structures of thiophene and methylthiophenazines azines complexed with Fe2(CO)9.
Crystalline structure of complexes of azines bearing pyridine moieties with different silver
salts were investigated by Kennedy et al6 and by Vatsadze et al34. Azines from
triphenylaminoaldehyde and 4ꢀ(diethyamino)benzaldehyde were found by Fu et al35 to be
very effective for Hg2+ ion detection. One of the azines presented in our work, that is, the one
prepared from salicylaldedyde and hydrazine, was investigated as AIEE compound10.
The main goal of this work is the comparison of properties of symmetrical azines with
their azomethine analogues considering their structural (NMR, IR), thermal (DSC), optical
(UVꢀvis, PL), and electrochemical (CV) properties and discussion in relation to the published
literature data as well. Additionally, the electronic properties of obtained compounds are
described theoretically by density functional theory (DFT).
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2. Experimental part
2.1. Materials
4ꢀ(Diphenylamino)benzaldehyde, 4ꢀpyridinecarboxaldehyde, 2ꢀthiophenecarboxaldehyde, 2ꢀ
hydroxybenzaldehyde, hydrazine monohydrate (98%), trichloroacetic acid and N,Nꢀ
dimethylacetamide (DMA) were obtained from Aldrich. Ethanol (anhydrous), benzaldehyde
and acetic acid (99.5%) were supplied from POCH and 1,4ꢀphenylendiamine was obtained
from Fluka. Azine AZꢀ4 and azomethine AZꢀ5 were described in our previous work19 and20.
2.2. Synthesis of azines and azomethines
2.2.1. Synthesis of azines AZ-1 and AZ-3.
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Into aldehyde: benzaldehyde (1.0612 g, 10 mmol) or 4ꢀpyridinecarboxaldehyde (1.0711 g, 10
mmol) dissolved in 5 ml ethanol the solution of hydrazine (0.2043g, 4 mmol) in 3 ml ethanol
was added dropwise. After few seconds yellow product started to precipitate and 2 ml ethanol
was added. The reaction mixture was stirred for 2 h at room temperature. The precipitate was
filtrated, washed several times with ethanol and dried at 50°C in vacuum for 2 days.
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AZ-1: Yellow powder. Yield: (0.32 g) 48%. 1H NMR (CDCl3, δ, ppm): 8.67 (s, ꢀCH=N, 2H),
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7.85 (d, 4H), 7.45 (m, 6H). FTIR (cmꢀ1): 1624 (CH=N stretching), 3000 (CH aromatic). C
NMR (CDCl3), (ppm), δ: 162.05 (CH=N), 134.13, 131.21, 128.80. Elem. anal. calcd (%) for
(C14H12N2) (208.26): C 80.74, H 5.81, N 13.45; found C 80.90, H 6.08, N 13.28.
DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 95 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 72°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 96 °C
Tm=95°C in ref.27
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AZ-3: Yellow powder. Yield: (0.76 g) 76%. H NMR (CDCl3, δ, ppm): 8.76 (d, 4H), 8.57 (s,
ꢀCH=N, 2H), 7.70 (d, 4H). 13C NMR (CDCl3), (ppm), δ: 160.43 (CH=N), 150.58, 122.10.
FTIR (cmꢀ1): 1628 (CH=N stretching), 3028 (CH aromatic). Elem. anal. calcd (%) for
(C12H10N4) (210.23): C 68.56 H 4.79 N 26.64; found C 68.66, H 4.88, N 26.82.
DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 187 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 164°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 187 °C
2.2.2. Synthesis of azine AZ-2.
Into salicylaldehyde (1.2212 g, 10 mmol) dissolved in 5 ml ethanol the solution of hydrazine
(0.3037 g, 5 mmol) in 4 ml ethanol was added dropwise. The reaction mixture was stirred for
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24 h at room temperature. The precipitate was filtrated, washed several times with ethanol
and dried at 50°C in vacuum for 2 days.
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AZ-2: Yellow powder. Yield: 75%. 1H NMR (DMSO, δ, ppm): 11.12 (s, ꢀOH, 2H), 9.01 (s, ꢀ
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CH=N, 2H), 7.70 (d, 2H), 7.40 (t, 2H), 6.98 (m, 4H). C NMR (CDCl3), (ppm), δ: 162.78
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(CH=N), 159.82, 133.44, 132.55, 119.72, 117.16. FTIR (cmꢀ1): 1624 (CH=N stretching), 3043
(CH aromatic).
DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 220 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 156°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 220 °C
Tm=216 °C in ref.36
2.2.3. Synthesis of azine AZꢀ5
4ꢀ(Diphenylamino)benzaldehyde (0.5467 g, 2 mmol) was dissolved in 10 ml ethanol at 60(C
and then solution of 1 mmol of hydrazine (0.0511 g, 1 mmol) in 1 ml ethanol was added
dropwise. The solution was heated to 80(C. After few minutes yellow product started to
precipitate. The reaction mixture was stirred for 4 h at 80(C. The precipitate was filtrated,
washed several times with ethanol (cold and hot) and dried at 50(C in vacuum for 1 day.
AZꢀ5: Yellow powder. Yield: (0.44 g) 81%. 1H NMR (CDCl3, (, ppm): 8.57 (s, ꢀCH=N, 2H),
7.65 (d, 4H), 7.29 (t, 8H), 7.14 (d, 8H), 7.08 (t, 4H), 7.05 (d, 4H). 13C NMR (CDCl3), (ppm),
(: 160.88 (CH=N), 150.43, 147.00, 129.46, 125.39, 123.94, 122.70. FTIR (cmꢀ1): 1616
(CH=N stretching), 3034 (CH aromatic). Elem. anal. calcd. (%) for (C38H30N4) (542.67): C
84.10, H 5.57, N 10.32; found C 83.01 H 5.72 N 10.09.
DSC: I heating run (sample after synthesis heating rate 20 °C/min) 242°C
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 207°C
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II heating run (after rapid cooling and cooling rate 15 °C/min) Tg 77 °C, “cold” crystallization
(max exothermic peak) 126, 197 Tm 242 °C
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2.2.4. Synthesis of azomethine AM-1
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Into 1,4ꢀphenylendiamine (0.5405 g, 5 mmol) dissolved in 4 ml ethanol the solution of
benzaldehyde (1.0612 g, 10 mmol) in 6 ml ethanol was added. The reaction mixture was
stirred for 2 h at 80°C and cooled to room temperature. The precipitate was filtrated, washed
several times with ethanol and dried at 50°C in vacuum for 2 days.
AM-1: Yellow powder. Yield: (0.86 g) 63%. 1H NMR (CDCl3, δ, ppm): 8.52 (s, ꢀCH=N, 2H),
7.92 (dd, 4H), 7.49 (d, 6H), 7.29 (s, 4H). 13C NMR (CDCl3), (ppm), δ: 159.79 (CH=N),
149.95, 136.22, 131.35, 128.78, 121.82. FTIR (cmꢀ1): 1615 (CH=N stretching), 3027 (CH
aromatic). Elem. anal. calcd (%) for (C20H16N2) (284.35): C 84.48, H 5.67, N 10.21; found C
85.16, H 5.86, N 9.89.
DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 142 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 130°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 142 °C
2.2.5. Synthesis of azomethine AM-2
Salicylaldehyde (0.2442 g, 2 mmol), 1,4ꢀphenylendiamine (0.1081 g, 1 mmol) and a pinch of
trichloroacetic acid were added to 5 ml DMA and heated (160°C) under argon atmosphere.
After 17 h the mixture was cooled to room temperature. The precipitate was collected by
filtration, washed with hot methanol and dried.
AM-2: Orange powder. Yield: (0.10g) 33%. 1H NMR (DMSO, δ, ppm): 13.07 (s, ꢀOH, 2H),
9.03 (s, ꢀCH=N, 2H), 7.68 (d, 2H), 7.55 (s, 4H), 7.42 (t, 2H), 6.99 (m, 4H). 13C NMR
(CDCl3), (ppm), δ: 163.16 (CH=N), 161.18, 147.17, 133.30, 132.33, 122.26, 119.16, 117.31
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FTIR (cmꢀ1): 1615 (CH=N stretching), 3053 (CH aromatic).
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DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 211 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 180°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 210 °C
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2.2.6. Synthesis of azomethine AM-3
Into 4ꢀpyridinecarboxaldehyde (1.0711 g, 10 mmol) dissolved in 2.5 ml ethanol the solution
of 1,4ꢀphenylendiamine (0.5405 g, 5 mmol) in 5.5 ml ethanol was added. The reaction
mixture was stirred for 2 h at 80°C, then cooled to room temperature and kept in the fridge
overnight. The precipitate was filtrated, washed several times with ethanol and dried at 50°C
in vacuum for 2 days.
AM-3: Yellow powder. Yield: (0.39 g) 30%. 1H NMR (CDCl3, δ, ppm): 8.77 (d, 4H), 8.52 (s,
ꢀCH=N, 2H), 7.77 (d, 4H), 7.34 (s, 4H). 13C NMR (CDCl3), (ppm), δ: 157.64 (CH=N),
150.62, 149.66, 142.67, 122.15. FTIR (cmꢀ1): 1633 (CH=N stretching), 3031 (CH aromatic).
Elem. anal. calcd (%) for (C18H14N4) (286.33): C 75.50 H 4.93 N 19.57; found C 75.18, H
5.14, N 19.19.
DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 189 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 147°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 188 °C
2.2.7. Synthesis of azomethine AM-4
Into 2ꢀthiophenecarboxaldehyde (1.1215g, 10 mmol) dissolved in 3 ml ethanol the solution of
1,4ꢀphenylendiamine (0,5407g, 5 mmol) in 7 ml ethanol was added dropwise and then 2 drops
of acetic acid were added. The reaction mixture was stirred for 3 h at 80°C and the precipitate
was filtrated, washed several times with ethanol and dried at 40°C in vacuum.
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AM-4: Yellow powder. Yield: (1.23 g) 84%. H NMR (CDCl3, δ, ppm): 8.61 (s, ꢀCH=N,
2H,), 7.51 (d, 2H), 7.49 (d, 2H), 7.27 (s, 4H), 7.14 (t, 2H). 13C NMR (CDCl3), (ppm), δ:
152.31 (CH=N), 149.30, 142.88, 132.12, 130.28, 127.74, 121.89. FTIR (cmꢀ1): 1608 (CH=N
stretching), 3068 (CH aromatic). Elem. anal. calcd (%) for (C16H12N2S2) (296.41): C 65.28 H
3.42 N 9.51; found C 65.27, H 4.01, N 9.61.
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DSC: I heating run (sample after synthesis, heating rate 20 °C/min) Tm 175 °C.
cooling run (cooling rate 15 °C/min) crystallization (max exothermic peak) 166°C
II heating run (after rapid cooling and cooling rate 15 °C/min) Tm 176 °C
Tm=150°C in ref.32 and 148ꢀ149 °C in ref.33
2.3. Blend preparation
Blends were obtained by dissolving the desired amount of compounds and PMMA in NMP to
form a homogeneous solution (1% vv concentration of compound in PMMA). Films cast on
glass were dried in vacuum oven at 90ºC over 10 h.
2.4. Measurements
Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker AC 400
MHz spectrometer using chloroformꢀd (CDCl3) or DMSOꢀd6 as solvent and TMS as the
internal standard. FTIR spectra were recorded on a BIOꢀRAD FTS 40 A Spectrometer using
KBr pellets. Differential scanning calorimetry (DSC) was performed with a TAꢀDSC 2010
apparatus (TA Instruments, Newcastle, DE, USA), under nitrogen using aluminium sample
pans. Elemental analyses were performed using Perkin Elmer Analyzer 2400. UVꢀvis
absorption spectra were recorded using a Lambda Bio 40 Perkin Elmer spectrophotometer.
The PL spectra were obtained on a VARIAN Cary Eclipse Fluorescence Spectrophotometer.
Fluorescence lifetime measurements on blends were made using timeꢀcorrelated single photon
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counting technique (Becker & Hickl, SPCꢀ150). The samples were excited with 30 ps pulsed
laser at λex=405 nm with the repetition rate of 20 MHz. Fluorescence was filtered using
FGL515 longpass filter combined with a bandꢀpass filter 550/40 and detected with a fast
avalanche photodiode (idQuantique id100ꢀ50). We measured 30 decays for different places
across every sample. Electrochemical measurements were carried out using Jaissle IMP 83 PC
TꢀBC potentiostat. Cyclic voltammetry experiments were conducted in a standard
onecompartment cell, in dichloromethan (Fluka, HPLC grade). 0.1M tetrabutylammonium
hexafluorophosphate (Bu4NPF6 Aldrich, 99%) was used as the supporting electrolyte. The
oxidation and reduction potentials of film coated on a ITO disk were measured using a Pt wire
and a Ag/AgCl electrode as a counter electrode and a quasi reference electrode, respectively.
Potentials were referenced with respect to ferrocene (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 ꢀ4.82 eV to vacuum. Theoretical DFT calculations were carried out using
Gaussian09 package37.
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3. Results and discussion
In this article we describe our onꢀgoing efforts in the synthesis of materials for
potential optoelectronic applications and the conjugated low molecular weight compounds
bearing in structure imine linkages, that is, azines and their azomethine analogues.
3.1. Synthesis and characterization
The azines (abbreviated in text as AZ) and azomethines (abbreviated in text as AM)
were prepared in a condensation reaction of various aldehydes such as: 4ꢀ
diphenylaminobenzaldehyde, 4ꢀpyridinecarboxaldehyde, 2ꢀthiophenecarboxaldehyde, 2ꢀ
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hydroxybenzaldehyde and benzaldehyde and hydrazine and 1,4ꢀphenylenediamine,
respectively. The chemical structures of the obtained compounds are presented in Fig. 1.
[Fig.1.]
9
It should be noticed that the data concerning AZꢀ4 and AMꢀ5 have been described in our
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previous works and here are repeated for comparison with AMꢀ4 and AZꢀ5, respectively19,20
.
1
Instrumental techniques including FTIR, H NMR and 13C NMR spectroscopies and
elemental analysis were applied for the characterization of the molecular structure of these
compounds. The spectral data of all the compounds were found to be consistent with their
molecular structures. The absence of the residual amino and aldehyde groups together with
the appearance of a band typical for imine bonds was confirmed by NMR and FTIR spectra.
The chemical shifts of the imine protons and carbons and IR absorption band of imine unit are
presented in Table 1.
[Table 1]
1
In the H NMR spectra of the investigated compounds the imine proton signal was observed
in the range of 8.40–9.03 ppm. It can be seen that in the azines the signal of hydrogen in
1
CH=N group in H NMR spectra was shifted to lower field values in comparison with the
azomethines synthesized from the same aldehydes. The biggest differences in the chemical
shifts were observed in the compounds obtained from thiophene aldehyde AZ-4 and AM-4
(0.18 ppm) and from triphenylamine aldehyde AZ-5 and AM-5 (0.17 ppm) and AZ-1 and
AM-1 (0.15 ppm). This seems to confirm the better conjugation in azines in comparison with
their azomethine analogues. The only reverse behavior was detected in the compounds
synthesized from –OH substituted aldehyde i.e. salicylaldehyde. However, in this case the
1
difference in CH=N group proton signal in H NMR was very small. Moreover, in these
compounds, that is, AZ-2 and AM-2, the signal of the CH=N group proton was significantly
downfield shifted in comparison with the other AZ and AM investigated in this work. It
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indicates the highest degree of conjugation of πꢀelectrons with phenyl ring πꢀelectrons in both
AZ-2 and AM-2 compared with other compounds probably due to better planarization as a
consequence of possibility for intramolecular hydrogen bond formation between hydroxylic
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group and nitrogen31. In the C NMR spectra of the AZ and AM, the signals in the range of
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152.31–163.16 ppm, present in the spectra of all compounds, confirmed the existence of
carbon atoms in the imine group. Considering the signal of carbon in CH=N group in 13C
NMR in relation to chemical structure of obtained compounds the same direction of the shift
was found as for the signal of hydrogen atom in imine group in 1H NMR. In FTIR spectra of
investigated compounds the band characteristic for the HC=Nꢀ stretching vibration was
detected in each case in the spectral range of 1608–1633 cmꢀ1 (cf. Table 1). From the
literature data it follows that a free, isolated C=N group absorbs at about 1660 cmꢀ1 and this
band is associated with the stretching vibrations of this group38. The frequency of this band is
strongly reduced with conjugation of the C=N group with phenyl ring due to diminishing of
the energy of the C=N bond and delocalization of nitrogen pair into imine double bond.
The expected composition of the chosen investigated compounds was confirmed by
elemental analysis as well. The results of the elemental analysis were compared with the
calculated values of carbon, hydrogen, and nitrogen contents and show rather good agreement
between calculated and experimentally determined values.
Differential scanning calorimetry (DSC) in nitrogen atmosphere was utilized to
examine thermal properties of the obtained azomethines and azines. It is clear that thermal
properties of the compounds depended strongly on their chemical structure and different
behavior of imines and azines during DSC heating and cooling runs was observed. DSC
thermogram of most azines and azomethines, that is, with phenyl (AZ-1, AM-1),
hydroxyphenyl (AZ-2, AM-2), pyridine (AZ-3, AM-3) and thiophene units (AZ-4, AM-4)
during first heating run (heating rate 20 °C/min) showed one endotherm indicating melting of
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the sample. During cooling (at heating rate 15 °C/min and rapid cooling) only one exothermic
peak in DSC thermograms was observed confirming crystallization of the compounds. The
second heating scan revealed only the melting point. A different behavior was found in the
case of the azine and azomethine containing the TPA moiety (AZ-5 and AM-5). DSC
thermograms of AZ-5 are presented in Fig. 2.
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[Fig.2]
The DSC thermogram of AZ-5 features endothermic peak confirming melting of the sample
at 242°C during first heating run. After rapid cooling, during second heating run, glass
transition at 77°C, “cold” crystallization (exothermic peak) at 126 and 197°C, and melting
endotherm at 242 °C were detected. The observed double coldꢀcrystallization exotherms in
samples of low crystallinity could be attrbited to three various phenomena: (i) to the
superposition of the melting of crystals, formed by the annealing pretreatments, and the coldꢀ
crystallizations occurring during DSC heating39, (ii) the competition between the
crystallization from the nuclei formed during cooling, and that from spontaneous nucleation40
and (iii) coꢀexistence of two kinds of amorphous regions41. In the case of AM-3 during first
heating run “cold” crystallization at 145 °C, and melting point at 195°C was found, as
reported in our previous work20. On the other hand, after rapid cooling, the second heating
scan revealed only glass transition at 83 °C. Thus, AZ-5 and AM-5 exhibit behavior typical
for molecular glasses consisting of melting temperature (Tm) of the crystalline sample and
when isotropic liquid was cooled down, a glass was spontaneously formed via a supercooled
liquid. Next, when the amorphous glass sample was again heated, a glassꢀtransition
phenomenon was observed and on further heating above Tg, the Tm could be detected18,42
.
Molecular glasses may form uniform, transparent amorphous thin films by vapor deposition
and spinꢀcoating methods contrary to low molar mass compounds with strong tendency for
crystallization. Films from crystalline materials could be prepared only by using polymeric
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binders. Azines prepared from the same aldehydes as azomethines exhibited comparable
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values of Tm as expected for AZ-1 and AM-1.
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3.2. Optical properties
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The optical properties of the obtained compounds were analyzed by UVꢀvis and PL
spectroscopies. The solvatochromic behavior of the azines and azomethines in various
solvents (chloroform, THF, NMP, DMSO) with different dipole moments and consequently
with a different polarity was studied. Optical properties of the compound in the solid state as a
blend with nonemissive poly(methyl methacrylate) (PMMA) were investigated as well.
Dispersed luminescent compound in nonemissive polymer matrix provide a way to eliminate
the selfꢀquenching of fluorescence due to the dilution effect43,44. Polymer blends involving
luminescent compound and inert polymers as matrices are often used in polymer light
emitting diodes as a tool to increase the efficiency of the devices43,45. Additionally, the UVꢀ
vis and PL properties of the compounds before and after protonation with HCl were tested.
3.2.1. Ultraviolet-Visible Investigations
The spectroscopic data and exemplary UVꢀvis spectra of investigated compounds are
presented in Table 1S in Supporting Information and Fig. 3, respectively.
[Fig.3]
In most cases absorption spectra of azines and azomethines showed similar characteristics in
all solvents, that is, they exhibit two absorption bands, one in lower energy region with
maximum (λmax) in the range of 270ꢀ297 nm, and second with λmax located between 355 and
407 nm. Two azines containing phthalic and pyridine rings (AZꢀ1 and AZꢀ3) exhibited one
absorption band with λmax around 300 nm (cf. Fig. 2a). Taking into account the UVꢀvis
spectral data it was found that the λmax position shows no clear dependence on the dielectric
constant of the solvent. Absorption bands in CHCl3 (ε=4.81) and DMSO (ε=46.70) were
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hypsochromically or batochromically shifted in the range of 7 nm, depending on the
compound structure. Fig. 3c, d displays the influence of the solvents on absorption bands of
AZ-1 – AM-1 and AZ-3. The influence of the dielectric constant of the solvent on molar
absorption coefficient was more pronounced (cf. Fig. 3e and Table 1S). Absorption bands at
the longest wavelength were found for the compounds with triphenylamine units i.e. AZ-5
and AM-5. The compounds from salicylaldehyde AZ-2 and AM-2 absorbed the light at about
50 nm shorter wavelengths. The azomethines absorbed at longer wavelengths in comparison
with the azines synthesized from the same aldehydes, except the ones with triphenylamine
moieties.
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The influence of protonation on absorption properties in UVꢀvis range was also
investigated. From theoretical point of view protonation of nitrogen atom in CH=N unit
should lead to bathochromic shift of UVꢀvis absorption band because involving free electron
pair of nitrogen atom with hydrogen bonding formation enables twist of aldehyde aromatic
ring, which enhances conjugation of π electrons of CH=N and aromatic ring. In this study the
azines and azomethines were protonated with HCl in NMP solution using equimolar amount
of HCl and CH=N group and the results are presented in Table 2. We find that only pairs AZ-
1 – AM-1, AZ-3 – AM-3 and AZ-4 – AM-4 follow the theoretical prediction, in these cases
protonation causes bathochromic shift of electronic absorption bands and the differences in
absorption wavelengths of protonated and unprotonated samples were in the range of 6ꢀ37
nm. For other compounds hypsochromic shift of the absorption band after protonation was
observed; the biggest effect, amounting to a 64 nm shift, was found for AZ-5 and AM-2.
Azines with phenylene (AZ-1) and pyridine (AZ-3) moieties exhibited higher bathochromic
shift of absorption bands than the azomethine analogues (AZ-1 and AZ-3). It is necessary to
point out that in AZ-3 and AM-3 there are four nitrogen atoms, which can be protonated i.e.
in CH=N groups and in the pyridine rings. Therefore, the electronic spectra were detected in
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NMP using 2 moles and 4 moles of HCl on 1 mole of the compound as well. When the ratio
of AZ-3 or AM-3 to HCl was 1:4, the value of λmax was found at the same wavelength. Thus,
along with increasing the amount of protonation agent shows no effect on the λmax. In the case
of these compounds, probably, mainly interaction of HCl with nitrogen present in pyridine
ring takes place. Similar as it was observed in compounds with imine linkages prepared from
3,8ꢀdiaminoꢀ6ꢀphenylphenanthridine in which mainly nitrogen in phenanthridine structure
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was protonated46,47
.
Electronic spectra were also detected for blends of the azines and azomethimes in
polymethylmethacrylate (PMMA). In most cases absorption bands in solid state were
bathochromically shifted in comparison with respective solutions.
3.2.2. Photoluminescence
Emission spectra of the azines and azomethines were investigated in CHCl3, THF,
NMP and DMSO and in solid state and using various excitation wavelengths (λex). The
corresponding data is collected in Table 2S in Supporting Information. It was found that both
the choice of the solvents and the excitation wavelength influenced the emission spectrum and
also its intensity. Three of the investigated compounds (without AZ-4 and AM-5), that is,
AZ-2, AM-2 and AZ-5 emitted light in all considered solvents. In chloroform solution, the
emission of azines and azomethines with phenyl (AZ-1, AM-1), pyridine (AMZ-3, AM-3)
and imine with thiophene (AM-4) rings was very week. Moreover, AZ-1, AZ-3, AM-3, and
AM-4 practically exhibit no emission in DMSO solution. It was observed that heteroatomic
azomethine with thiophene units (AM-4) exhibited photoluminescence only in NMP solution.
As an example, Fig. 4a. shows influence of the solvent on photoluminescence spectra for AZ-
5 under λex=340 nm excitation, whereas Fig.1S in Supporting Information presents
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photographs of the compounds in CHCl3, NMP solution, and in blends irradiated with 366 nm
light .
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[Fig.4]
9
As it can be seen in Fig. 4, the choice of the solvent influences not only the intensity and
maximum of emission wavelength (λem), but also the spectrum shape. The observed
broadening of the emission spectrum can be explained as a result of increase of
conformational disorder48. It was found that photoluminescence intensity depended also on
the excitation wavelength. As an example, the relationship between photoluminescence
intensities and λem and excitation wavelengths in chloroform solution are illustrated in Fig.
4b. for AZ-5. For all the compounds an increase of excitation wavelength caused a shift of
emission band towards lower energies. However, in the most cases, the maximum emission
band with the highest intensity in different solvents was observed at different excitation
wavelengths (cf. Table 2S). It is obvious that the compound structure plays important role in
determining the PL properties. Fig. 4c presents λem of investigated compounds in NMP
solution under the same λex equal to 360 nm. When considering the photoluminescence in
NMP solvent, it was found that for the azines, with the exception of AZ-2, the maximum
emission was observed under excitation at 330ꢀ350 nm, while for the azometines the highest
intensity was observed under excitation at 420ꢀ450 nm. The azomethines emitted light with
maximum intensity at longer wavelengths than the azines, the maximum emission ranges
from 517 nm (AM-4) to 497 nm (AM-3) in NMP solution. For azines in the same solvent, the
λem position was hypsochromically shifted and appears between 477 nm (AZ-5) and 400 nm
(AZ-3). The compounds synthesized from salicylaldehyde exhibited the reverse behavior. In
this case, the azine AZ-2 emitted light with the highest intensity at longer wavelength (539
nm), while AM-2 emission was hypsochromically shifted to 403 nm. Bathochromic shift of
the λem in NMP of azomethine and azine was detected for the pair AM-4 ꢀ AZ-4 being equal
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to 111 nm, for AM-3 – MZ-3 : 97 nm, and for AM-1ꢀAZ-1 : 82 nm. For the pair AM-2 –
AZ-2 the maximum emission band for the azomethine was hypsochromically shifted by 136
nm in comparison with the azine. It was found that azomethine with thiophene rings (AM-4)
exhibited emission with the lowest intensity. The AM-1 and AM-4 described in literature
emitted light with λem at 410 and 416 nm in acetonitrile, respectively.
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Additionally, in investigations of optical properties, the Stokes shifts estimated as a
difference between the 0–0 peaks of absorption and PL spectra, were analyzed. From a
practical point of view, large Stokes shifts are desirable in order to avoid selfꢀabsorption of
emitted light that reduce luminescence efficiency in devices49. The calculated Stokes shift
values of the studied compounds are collected in Table 4.
[Table 4]
The Stokes shift was calculated for the most intense emission band in given solution. The
highest value of Stokes shift was observed for azine with pyridine rings (AZ-3).
Next step of PL characterization of azines and azomethines concerns their emission
properties in solid state as a blend with PMMA. PL spectra of blends were recorded under λex
which gave the highest emission in NMP solution. Remarkably, we find that significant
differences in photoluminescence of the azines and the azomethimes in NMP solution
(considering position of λem with the highest intensity) were less pronounced in blends of the
compounds with PMMA. Even though the azomethines in blend emitted at longer
wavelengths (except for AMꢀ2) than the azines, the shift of the bands was in the range of 16ꢀ
85 nm, while in the case NMP solution was in the range of 73ꢀ136 nm. In the case of the
compounds from salicylaldehyde, the emission band of AZ-2 was bathochromically shifted
by 57 nm in comparison with that of AM-2, i.e. the reversed behavior was found than in the
other pairs. However, all compounds in blends with PMMA emitted blue light (cf. Fig. 4c). It
should be mentioned that emission of AZ-5 blend was not registered because of too high PL
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intensity. The position of the emission band of blends was bathochromically shifted in
relation to the compound in NMP solution except for AZ-4 and AM-4 (cf. Fig.5a).
[Fig. 5]
9
As it was mentioned, the presence of nitrogen atom, that is, the atom with a free
electron pair, in imine units, gives an opportunity for modification of the optical properties
due to protonation with HCl. Protonation of the compounds was carried out at room
temperature in NMP solution. Fig. 5b shows the influence of protonation with HCl on
photoluminescence intensity for AZ-1. In most cases, upon protonation, we observed an
increase of photoluminescence intensity, except for the AZ-5 where protonation resulted in
reduced PL intensity and hypsochromical shift of the emission band by 27 nm.
Additionally, photoluminescence lifetime measurements were performed on blends of
synthesized azines (AZ1-AZ-3, AZ-5) and azomethines (AM-1ꢀAM-4) under λex=405 nm. In
Fig. 6 we show examples of the measured fluorescence transients obtained for the pair AZ-2
and AM-2.
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[Fig. 6]
Red points correspond to transients fitted with threeꢀexponential decay characteristics, while
transients displayed in blue were obtained for the same sample but at different places across
it. The first observation derived from the timeꢀresolved experiments concerns the uniformity
of the blends. In all cases the distribution and variation of measured transients were minimal,
resulting in decay time changes less than 10%. Such a small variation indicates excellent
uniformity of the structure and homogeneous molecular ordering in the blends. These
observations are important for applying these materials as possible light emitting devices. In
order to quantitatively analyze measured transients, we approximated them with threeꢀ
exponential decay model, with the exception of AM-4, where twoꢀexponential model yielded
highꢀquality fit. Obtained decay constants are summarized in Table 5.
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[Table 5]
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The fastest decays are obtained for AZ-2 and AM-2 pair of all the studied molecules, while
AZ-3 and AM-3 pair exhibit the longest transient behavior. In the first case the longest decay
constants are lower than 5 ns, while in all other samples they are around 7 ns. From the results
included in Table 5 it is also apparent that in general the decay constants determined for
azines are shorter than those obtained for azomethimes.
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3.3. DFT calculations
We performed the quantum theoretical calculations using density functional theory
(DFT), with an exchange correlation hybrid functional B3LYP and the basis 6ꢀ31G(2d,p) for
carbon, nitrogen, oxygen, 6ꢀ311G(3d,p) for sulfur and 6ꢀ31G for hydrogen atoms. The
simulated geometries of these compounds were optimized in vacuum and no imaginary
frequencies were found for any of these species, computed at Ci symmetry, by vibrational
analysis. Moreover, comparison of calculated and experimentally measured IR spectra show
good agreement; the differences between calculated and experimental CH=N stretching band
is less than 25 cmꢀ1 (cf. Table1). Exemplary calculated and experimental IR spectra of AZ-2
and AM-2 are shown in Supporting Information, Fig. 1S. Based on the optimized geometries,
the energy and electronic distribution of molecular frontier orbitals were calculated in
acetonitrile. The solvent effect was introduced by PCM model. The frontier orbitals are of
extreme importance for the evaluation of molecular reactivity. is the higher is the negative the
energy of the Highest Occupied Molecular Orbital (HOMO), the 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 to suffer reduction,
on the basis of the energy of the Lowest Unoccupied Molecular Orbital (LUMO). The
contours of HOMOs and LUMOs of exemplary compounds (AZ-1, AM-1, AZ-3, AM-3, AZ-
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5 and AM-5) are depicted in Fig.7, whereas data obtained for AZ-2, AM-2, AZ-4 and AM-4
are included in Supporting Information, Fig. 2S.
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[Fig. 7]
9
In general, for all these molecules, the HOMO orbitals were concentrated in the aromatic
rings and in its neighborhood (ꢀCH=Nꢀ), indicating that the oxidation process should happen
primarily in this region. This corresponds to the calculations of electronic density, which
indicate that this region has the highest density of negative charge (cf. Fig. 8).
[Fig. 8]
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LUMOs are localized on nonbonding pz orbitals excluding double bonds and aromatic parts of
the molecules. The calculated frontier orbital energy levels are shown in Table 5.
[Table 5]
The calculated HOMO and LUMO levels of AM-5 were ꢀ5.35 and ꢀ2.19 eV, respectively.
Taking into account the character of molecular orbitals, we can draw several conclusions.
The HOMOs energies of azine compounds (AZ) are lower than azomethine ones (AM). This
can be explained by the fact that a phenyl group is incorporated between =NꢀN= group and
expected πꢀπ conjugation effect leads to a rise in HOMO energy. Moreover, the oxidation
process involves the removal of an electron from HOMO, thus azomethine compounds with
higher energy levels would be more susceptible to donate electrons.
The calculation results indicated that the introduction of a hydroxyl substituent to the
phenyl ring decreased the HOMO energy (AM-1 vs AM-2), which is consistent with the
experimental data. Similarly, replacement of phenyl ring in AM-1 compound by pyridine or
thiophene results in a decrease of the HOMO energy.
3.4. Electrochemical properties
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Cyclic pulse voltammetry (CV) is a useful and valuable technique to study redox
behavior, electrochemical stability, and to get information about HOMO and LUMO energy
values of hole and electron transport materials and to examine the barriers for charge
injection. The electrochemical behavior of the azines (AZ-1-AZ-3, and AZ-5) and
azomethines (AM-1-AM-4) was studied by CV for samples deposited as films on ITO
substrates. All tested compounds exhibited electrochemical behavior. The obtained
electrochemical data are listed in Table 6 and the exemplary cyclic voltammograms of AM-4
are shown in Fig. 9.
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[Table 6] [Fig. 9]
All of the azines demonstrated only irreversible one or two step reduction processes (except
for AZ-4 and AZ-5), whereas the azomethines showed both oxidation and reduction process,
however, also irreversible. For AZ-4 the reduction was a quasiꢀreversible in nature and this
azine also exhibited irreversible oxidation19. The HOMO and LUMO energy levels of the
compounds were estimated from the electrochemical oxidation and reduction onset potentials,
assuming the ionization potential (IP) of ferrocene to be equal to ꢀ4.8 eV, according to wellꢀ
established equations50
EHOMO = ꢀ 4.8 ꢀ Eox(onset)
ELUMO = ꢀ 4.8 – Ered(onset)
In the case when no reduction was observed, the LUMO level was estimated from the HOMO
energy level and the onset of absorption (UVꢀvis) taking into account the optical band gap
(Egopt) using the equation
opt
ELUMO= EHOMO + Eg
The LUMO level estimated by this method is not an accurate way of measuring the electron
affinity, however, allows for selection of suitable combinations with electron acceptors in
devices51.
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The lowest oxidation potentials (0.23 and 0.26 V vs. Fc (peak onset)) were recorded
for AM-2 and AM-3, respectively (cf. Table 6). Considering the Eox (onset) it can be
observed that the imines with hydroxyphenyl (AM-2) and pyridine (AM-3) rings were more
readily oxidized than azomethine with phenyl (AM-1) and thiophene (AM-4) units.
Comparing the ability for oxidation of azine (AZ-4) and their azomethine analogue (AM-4), it
was found that azomethine was easier to oxidize. Taking into account the chemical structure
of azines and their ability for reduction, it was observed that azines with pyridine (AZ-3) and
thiophene (AZ-4) units were easier to reduce than azines with phenyl (AZ-1) and
hydroxyphenyl (AZ-2). HOMO energy level of azines was in the range of ꢀ5.05ꢀ ꢀ5.59 eV.
Azine AZ-4 exhibited significantly lower value of the HOMO energy level (ꢀ6.12 eV)19 than
its azomethine analogue (AM-4) (ꢀ5.59 eV). Azines with phenyl (AZ-1) and hydroxyphenyl
(AZ-2) units showed lower value of LUMO level than their imine analogues (AM-1 and AM-
2) contrary to AZ-3 and AZ-4, for which LUMO values were higher than azomethines with
pyridine (AM-3) and thiophene rings. The HOMO and LUMO levels of AM-4, estimated
based on CV measurements, correspond to the previously published data11. Comparing the
energy gap (Eg) values of azines and azomethines obtained using both electrochemical and
optical methods it can be concluded that imines are more conjugated than their azine
analogue. The theoretically predicted HOMO energies are in good agreement with
experimental data in the case of AZ-5, AM-5 and AM-1 compounds and much higher for
AM-2 and AM-3. On the other hand, in all cases the DFT calculated LUMO levels are
compatible with the CV data, except for AM-1. Generally, the theoretically estimated LUMO
energy levels are lower than obtained from CV measurements (except for AZ-4). The
differences may be related to various effects of conformations of compounds in films and in
solvents, particularly that the calculated and experimental IR spectra correspond to each other
(cf. Table 1).
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Symmetrical azines and their azomethine analogues containing in their structure
phenyl, hydroxyphenyl, TPA, and heteroatomic rings such as pyridine and thiophene were
synthesized and characterized. The study revealed clearly pronounced effects of chemical
structure on the optical and redox properties of both types of investigated compounds.
The conclusions drawn from the study are as follows:
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• azine and azomethine with TPA units form molecular glasses and show a glass
transition at 77 and 83°C, respectively. Azines prepared from the same aldehydes as
azomethines exhibited comparable values of melting point expect for compounds with
phenyl rings. In this case higher Tm was observed for azomethine;
• UV–vis absorption band responsible for imine structure in the case of the azines was
hypsochromicaly shifted in relation to azomethines obtained from the same aldehydes;
•
all compounds emitted blue light in NMP solution and blended with PMMA.
Heteroatomic imine with thiophene moieties emitted light only in NMP solution and
with the lowest intensity as compared to the other compounds. Compounds with TPA
units exhibited the highest PL intensity. Comparing PL properties of azines and their
azomethine analogues it can be concluded that azomethines emitted light with the
highest intensity with λem bathochromically shifted in relation to λem of azines (except
for the compounds with hydroxyphenyl structure). The longest photoluminescence
lifetime in blend was detected for compounds with pyridine ring, that is, AZ-3 and
AM-3 pair. The blends reveal very good uniformity as measured using timeꢀresolved
fluorescence. In most cases, the protonation with HCl causes significant increase of PL
intensity;
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•
electrochemical studies revealed that azines undergo irreversible reduction process,
whereas, their azomethine analogues exhibit also the ability to oxidation, however,
irreversible in nature. In the case of azine with thiophene units, both reduction and
oxidation processes were observed. The presence of hydroxyphenyl and pyridine units
in azomethines leads to higher HOMO levels. On the other hand, the value of LUMO
level of these imines is lower than for other azomethines, which results in decreasing
electrochemical energy gap. The investigated azines exhibit optical energy gaps in the
range of 3.14–3.78 eV, therefore larger than corresponding azomethines (Eg= 2.72 –
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CV
3.02eV). Electrochemically evaluated Eg was lower than Egopt and was in the range
of 2.09 – 2.47 eV.
Acknowledgements
K. Bijak and M. GrucelaꢀZajac are grateful for the financial support of the DoktoRIS project
and M. GrucelaꢀZajac is grateful for the financial support of the UPGOW coꢀfinanced by the
European Union within the European Social Found. Authors thank Dr. H. Janeczek for DSC
experiments.
Supporting Information Available
This information is available free of charge via the Internet at http://pubs.acs.org.
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