New thermotropic azomethine–naphthalene diimides for optoelectronic applications

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Synthetic Metals 160 (2010) 2208–2218

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Synthetic Metals

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ew thermotropic azomethine–naphthalene diimides forptoelectronic applications

wa Schab-Balcerzaka,b,∗, Agnieszka Iwanc,∗∗, Michal Krompiecb,ariola Siwya, Daniel Tapaa, Andrzej Sikorac, Marcin Palewiczc

Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34M. Curie-Sklodowska Str., 41-819 Zabrze, PolandInstitute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, PolandElectrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland

r t i c l e i n f o

rticle history:eceived 7 July 2010eceived in revised form 15 August 2010ccepted 16 August 2010vailable online 17 September 2010

eywords:zomethinesaphthalene diimidesisimideshermotropic liquid crystalslectrochemistryurrent–voltage characteristics

a b s t r a c t

A new type of thermotropic liquid crystalline compounds containing azomethine linkages andnaphthalene diimide moieties were synthesized via condensation of novel N,N′-bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene-1,4,5,8-dicarboximide with 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde and 4-octadecyloxybenzaldehyde. The structures of com-pounds are characterized by means of FTIR, NMR spectroscopy and elemental analysis; the results showan agreement with the proposed structure. Optical properties of the obtained azomethine–naphthalenediimides (AZ-NIs) in solution and in solid state as thin films on the quartz substrate were tested byUV–vis spectroscopy. The electrochemical behavior of AZ-NIs was studied by cyclic voltammetry (CV)and differential pulse voltammetry (DPV). The highest occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO) energy levels, and electrochemical (E′

g) and optical (Eg) band gapvalues were calculated using the results of electrochemical and UV/vis measurements, respectively. The

electrical properties of the azomethine–naphthalene diimides were investigated by current–voltage (I–V)measurements. I–V characteristics were performed on ITO/compound/Al, and ITO/compound:PC61BM/Aldevices in the dark and during irradiation with light (under illumination 1000 W/m2). Additionally, theazomethine–naphthalene diimides films were tested using AFM technique. The mesomorphic behavior ofthe AZ-NIs was investigated via differential scanning calorimetry (DSC) and polarizing optical microscopy(POM). First time, to the best of our knowledge, compounds with both azomethine and naphthalene

cryst

diimide units with liquid

. Introduction

During the past decades, considerable attention has beenocused on organic low and high molecular weight conjugated sys-ems called organic semiconductors both from the basic researchnd application standpoint. This is caused by the fact that theyan be exploited in organic electronic devices [1–4]. The search

or organic semiconductors caused that special attention is paid onelf-assemble materials with a high degree of intra/intermolecularrder [2]. Self-organization might supply an efficient path forharge transporting [5]. Among the self-assemble systems, low and

∗ Corresponding author at: Centre of Polymer and Carbon Materials PAS, 34 M.urie-Sklodowska Str., 41-819 Zabrze, Poland; Institute of Chemistry, University ofilesia, 9 Szkolna Str., 40-006 Katowice, Poland.∗∗ Corresponding author.

E-mail addresses: [email protected],[email protected] (E. Schab-Balcerzak), [email protected] (A. Iwan).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.08.011

alline properties were described in this article.© 2010 Elsevier B.V. All rights reserved.

high molecular weight liquid crystals (LCs) are particularly inter-esting due to their ability to form large aligned domains enablingthe formation of well-ordered thin films [2]. Thermotropic liquidcrystals (TLC) offer a variety of unique properties and have receivedgreat attention during the years due to their practical applicationsand significant efforts have been concentrated on the synthesis ofnew compounds [6]. In the past, most LCs were electrical insulatorsand were used in applications such as flat panel displays. How-ever, recent progress in LCs semiconductors significantly expensedthe possibility of potential applications for them [7]. The use ofLCs as active components in electroluminescent devices, molecu-lar wires and fibres, and photorefractive materials has been studiedin the last decades [8–11]. Furthermore, the photovoltaic prop-erties of LC compounds have been investigated [12]. Among of

the many designed and synthesized TLC compounds, rod-shapedazomethines have been of special interest [13–17].

Another type of promising LC materials are compounds withimide groups, which would be a good mesogen [18]. Aromatic imidegroups are almost planar, rigid, polar and thermostable, and thus

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hould be favorable components of liquid crystalline (LC) com-ounds regardless of whether thermotropic or lyotropic character

s taken into consideration [19]. Imides taking into considerationhe imide unit structure, are divided into two kinds, that is, com-ounds with five- and six-membered imide rings. Six-memberediimides are obtained from naphtalenetetracarboxylic dianhydrideNTDA) and perylenetetracarboxylic dianhydride (PTDA) or itserivatives. The naphthalene-1,4,5,8-tetracarboxylic diimide unit

s a highly symmetric mesogen and its L/D ratio is lower than thatf typical short mesogens [20]. A variety of liquid crystalline five-embered imides have been studied during the years include both

mides and diimides [21,22]. However, the literature concerningC six-membered imides in the most cases is connected with com-ounds with perylene units [23,24,7]. To the best of our knowledge,nly Yitzchaik and co-workers reported of LC naphthalene diimidesbtained from NTDA and n-hexylamine and 1-hexadecylamine25]. Additionally, it should be stressed that azomethines (AZs)ue to the presence of –HC N bond in their structure are elec-ron donors and constitute a type of hole-transporting materialsp-type organic semiconductors). At the same time, naphthaleneiimides (NDIs) and their derivatives, with compact and electroneficient cores, have demonstrated a great potential as n-typeelectron-transporting) semiconductors [26]. It was found thatompounds containing imine linkages or six-membered imideings have prospective applications in organic (opto)electronicsnd related fields, exemplary as materials for high efficiency solarells [26], for fabrication of field effect transistors (FETs) [4,27,28]r light-emitting diodes (OLEDs) [29–31] and are highly desirableor photonic technology [24]. Moreover, it should be mentionedhat naphthalene diimides have been used for the preparation oflectronically conducting materials, Langmuir–Blodgett films, �-tacked materials absorbing in the near-IR region and models forhe photosynthetic reaction center [32]. The main reason of thencreasing interest in six-membered diimides originates not onlyrom their electron acceptor properties but also from their pho-ochemical stability along with air and thermal stabilities. On thether hand, naphthalene diimides are interesting for medical appli-ations due to their demonstrated anticancer activity [33]. Weonclude that, both azomethines and naphthalene diimides findpplications in the fields of material and supramolecular science34].

It is very important to combine TLC and electrical propertiesf the organic compounds to find the practical application as liquidrystalline semiconductors which can be used to produce electronicevices by the solution process.

Inspired by the findings described above we have undertakenpreparation and investigation of a new type of LC compounds,

hat is azomethine–naphthalene diimides (abbreviated hereinafters AZ-NIs). These compounds constitute a promising family ofaterials whose properties can be explored in various types of

evices. Taking into consideration the literature survey related tohe idea of preparation of compounds containing both imine andix-membered diimide units, to the best of our knowledge, onlyne article describes polymers which repeating units consist ofentioned parts [35]. However, obtained polyazomethines contain

erylene not naphthalene units and did not show LC behavior.The objective of the present work was to synthesize new

hermotropic azomethine–naphthalene diimides, and study theirhosen photophysical properties. In this article we present thehemical (NMR, FTIR), thermal (DSC, POM), optical (UV–vis), elec-rochemical (CV and DPV), electrical (current–voltage) and AFM

haracterizations of new thermotropic liquid crystal symmetricalZ-NIs. For the preparation of novel compounds two aldehydesere selected, that is, 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-eptadecafluoroundecyloxy)benzaldehyde and 4-octadecyloxy-enzaldehyde. The fluorinated aldehyde was chosen as object of our

etals 160 (2010) 2208–2218 2209

study because of a number of reasons. Firstly, the introduction of Fatoms was found to be an effective tool to influence the electronicproperties of such semiconducting compounds as oligothiophenes[13,36]. Secondly, fluoric substituents were important for thedesign of LC materials [13,14]. It was found that with an increas-ing number of F atoms in fluorinated alkyl chains stabilizationof smectic and columnar mesophases was observed [13]. More-over, the substitution of the aromatic core with F atoms changedsuch properties as: LC phase transition temperatures, dielectricproperties, birefringence and elastic constants [13]. On the otherhand, incorporation of a fluorinated chain into the compoundenhances the thermal stability of the LC mesophases and modifiesthe mesophase morphologies in comparison with the same com-pound with aliphatic chains. Recently, synthesis, characterizationand mesomorphic properties of unsymmetrical and symmet-rical imines based on 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde were reported [14,15].

To the best of our knowledge, the compounds described hereinare the first examples of liquid crystalline compounds containingboth azomethine units and naphthalene diimide rings.

2. Experimental details

2.1. Materials

1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA),2,3,5,6-tetramethyl-1,4-phenylenediamine, N,N-dimethyl-acetamide (DMA), pyridine and toluene-4-sulphonic acid werepurchased from Aldrich Chemical Co. Imidazole, 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzal-dehyde and 4-octadecyloxybenzaldehyde were supplied fromFluka.

2.2. Synthesis of N,N′-bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene-1,4,5,8-dicarboximide(DANDI)

NTDA (0.805 g, 3 mmol), 2,3,5,6-tetramethyl-1,4-phenylenediamine (2.464 g, 15 mmol) and imidazole (1.5 g,23.25 mmol) were added to 30 ml dry pyridine and refluxed underargon atmosphere. After 6 h, this mixture was cooled to roomtemperature. The diamine precipitate was collected by filtrationand washed with water and then with hot methanol. The resultingdiamine was dried in vacuum at elevated temperature (2 h at220 ◦C and 2 h at 240 ◦C). Yield: 90%; dark green solid.

1H NMR (DMSO-d6, ı, ppm): 1.88 (s, CH3, 12H), 2.07 (s, CH3,12H), 4.59 (s, NH2, 4H), 8.75 (s, 4H). FTIR (KBr, cm−1): 3487,3403 (–NH2 stretch), 2998–2909 (C–H aliphatic), 1704, 1665 (C Oimide stretch), 1627 (–NH2 deformation), 1526 cm−1 (phenyl),1322 (C–N stretch), 768 (imide ring deformation). Anal. Calcd. for(C34H32N4O4) (560.64): C 72.84, H 5.75, N 9.99: Found C 72.27, H6.47, N 10.14. Yield: 50%, Tm = 294 ◦C.

2.3. Synthesis of azomethine–naphthalene diimides (AZ-NIs)

Diamine DANDI (0.14616 g; 0.25 mmol), aldehydemolecules 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadeca-fluoroundecyloxy)benzaldehyde: 0.29112 g, 0.5 mmol; or 4-octadecyloxybenzaldehyde: 0.29112 g, 0.5 mmol and a pinch oftoluene-4-sulphonic acid were added to 5 ml DMA and heated(160 ◦C) under argon atmosphere. After 11 h, this mixture was

cooled to room temperature. The precipitate was collected by fil-tration and washed with methanol, acetone and then washed hotmethanol. The resulting precipitate was dried at 50 ◦C in vacuum.DANDI condensed with 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde resulted in compound

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Z-NI-I while DANDI condensed with 4-octadecyloxy-enzaldehyde gave compound AZ-NI-II. Yields of compounds:Z-NI-I: 48%; AZ-NI-II: 55%.

AZ-NI-I: 1H NMR (CDCl3, ı, ppm): 1.59 (t, CH2–CF2, 4H), 2.10 (d,H2, 4H), 2.15 (d, CH3–Ar, 24H), 4.15 (quart, CH2–O, 4H), 7.02 (d,H), 7.90 (d, 4H), 8.17 (d, CH N, 2H), 8.88 (s, 4H). FTIR (KBr, cm−1):949, 2878 (C–H aliphatic), 1716, 1678 (C O imide stretch), 1633CH N), 1605 (C C stretching deformations in the phenyl ring),345 (C–N stretch), 1249 (C–O–C), 770 (imide ring deformation).nal. Calcd. for (C70H50N4O6F34) (1689.11): C 49.77, H 2.98, N 3.32:ound C 48.35, H 3.17, N 3.07.

AZ-NI-II: 1H NMR (CDCl3, ı, ppm): 0.88 (t, CH3, 6H), 1.30 (quart,H2, 48H), 1.48 (quart, CH2, 8H), 1.83 (quint, CH2, 8H), 2.10 (quart,H3–Ar, 24H), 4.05 (t, CH2–O, 4H), 7.02 (d, 4H), 7.90 (d, 4H),.15 (d, CH N, 2H), 8.88 (s, 4H). FTIR (KBr, cm−1): 2920, 2850C–H aliphatic), 1716, 1677 (C O imide stretch), 1633 (CH N),605 (C C stretching deformations in the phenyl ring), 1344 (C–Ntretch), 1249 (C–O–C), 769 (imide ring deformation). Anal. Calcd.or (C84H112O6N4) (1273.81): C 79.20, H 8.86, N 4.40: Found C 78.13,

9.17, N 4.31.

.4. Characterization

Fourier transform infrared (FTIR) spectra were acquired on a60 MAGNA-IR NICOLET Spectrometer using KBr pellets. Protonuclear magnetic resonance (1H NMR) spectra were recorded on aruker AC 300 MHz spectrometer using DMSO-d6 and chloroformCDCl3) as solvents and TMS as the internal standard. Elementalnalyses were performed using Perkin Elmer Analyzer 2400. UV–visbsorption spectra were recorded in solution and solid state as filmsasted on glass using a Perkin Elmer Lambda Bio 40 UV–vis spec-rometer. Differential scanning calorimetry (DSC) was performedith a TA-DSC 2010 apparatus, under nitrogen atmosphere using

ealed aluminum pans. The transition temperatures were read athe top of the endothermic and exothermic peaks. The textures ofhe liquid crystalline phase were observed with a polarized optical

icroscope (POM) Zeiss (Opton-Axioplan) equipped with a Nikonoolpix 4500 color digital camera, and Mettler FP82 hot plate withettler FP80 temperature controller. Electrochemical measure-ents were carried out using Eco Chemie Autolab PGSTAT128n

otentiostat, using platinum wire (diam. 1 mm), platinum coil andilver wire as working, auxiliary and reference electrode, respec-ively. Potentials are referenced with respect to ferrocene (Fc),hich was used as the internal standard. Cyclic and differentialulse voltammetry experiments were conducted in a standard one-ompartment cell, in dichloromethane (Carlo Erba, HPLC grade),nder argon. 0.2 M Bu4NPF6 (Aldrich, 99%) was used as the sup-orting electrolyte.

The surface morphology investigations of the azomethine-mides were performed in air using a commercial Innova AFMystem from Veeco company. Measurements were done in Tap-ing Mode and Phase Imaging. Also Local Contrast data processingas made. Typical cantilever (about 40 N/m and <10 nm tip cur-

ature) was used. Current–voltage characteristics were detectedsing electrometer Keithley 6517B.

.5. Film preparation

Films for UV–vis measurements were prepared from spin-oated compounds solutions in CHCl3. Quartz substrates were

urified using ultrasonic washer with chloroform for 1 h. Afterhat, the slides were cleaned using toluene, isopropanol and ace-one. Characteristic parameters such as speed (880 rpm) and time10 s) rotation were applied for preparation of films by spin-coatingquipment.

etals 160 (2010) 2208–2218

2.6. Device fabrication

Current–voltage measurements were performed onITO/compound/Al, and ITO/compound:PC61BM/Al. ITO–glasssubstrates were cleaned in ultrasonic washer at 15 min in ace-tone and after that in isopropanol and acetone. Thin organiclayers based on azomethine–naphthalene diimides (AZ-NIs) ormixture AZ-NIs:PC61BM in weight ratio 1:1 were dissolved atchloroform and next spread. The active layers based on AZ-NIs orAZ-NIs:PC61BM (called blend) were spin-coated onto ITO-coveredglass substrate with angular speed 900 rpm for 10 s at roomtemperature. Al electrode (about 28 cm2 area) was prepared onthe azomethine–naphthalene diimides film surface by thermalevaporation method at pressure of 5 × 10−4 Torr.

3. Results and discussion

In this work two examples of new type of thermotropic com-pounds, that is, containing imine bonds and naphthalene diimiderings, were synthesized and characterized.

3.1. Synthesis and characterization

The novel azomethine–naphthalene diimides were obtainedfrom the new diamine-containing naphthalene diimide unit(DANDI) with melting point of 294 ◦C. The diamine was preparedfrom condensation reaction of excess of 2,3,5,6-tetramethyl-1,4-phenylenediamine with 1,4,5,8-naphthalenetetracarboxylicdianhydride (NTDA) as shown in Fig. 1. While NTDA is only par-tially soluble in chloroform, synthesized DANDI was soluble in thissolvent at room temperature. The presence of four methyl groupson the phenyl rings and incorporation of the imide rings increasedthe solubility. DANDI was applied for condensation with two alde-hydes. The reaction route and chemical structure and the obtainedazomethine–naphthalene diimides are presented in Fig. 1.

4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Heptadecafluoroundecyloxy)benzaldehyde was utilizedfor preparation of compound AZ-NI-I and 4-octadecyloxybenzaldehyde for AZ-NI-II. The assigned chemicalstructure of all synthesized compounds was identified through thedata from 1H NMR, FTIR spectroscopy measurements and elemen-tal analysis. Their expected chemical constitution is confirmedby spectroscopic studies. Exemplary FTIR spectra of diamine andAZ-NI-II in the range characteristic for vibration bands of amine,carbonyl groups in dianhydride and imide rings are presented inFig. 2.

In FTIR spectra, the structure of synthesized diaminonaphtha-lene compound (DANDI) was confirmed by disappearance of theanhydride C O vibration bands at 1780 and 1768 cm−1 in NTDA andappearance of imide C O stretching vibration bands at 1704 and1665 cm−1, that is, asymmetrical and symmetrical stretch of thesix-membered imide ring (cf. Fig. 2). Moreover, characteristic –NH2stretch and deformation vibrations were clearly observed, respec-tively at the range 3482–3387 cm−1 and at 1627 cm−1 in DANDIspectrum (cf. Fig. 2a). Finally, the FTIR spectra of the compoundsobtained from the condensation reaction of DANDI with aldehydes,exhibited the imine bond (CH N) stretching vibration at 1633 cm−1

and the absorption bands at 3482–3387 cm−1 and 1627 cm−1 dis-appeared (cf. Fig. 2b). Additionally, in AZ-NIs FTIR spectra the bandsat 1605 cm−1 and at 1249 cm−1 were observed, which are ascribed

to the C C stretching deformations in the phenyl ring and etherlinkages (–C–O–C–), respectively. All compounds exhibited absorp-tion bands at around 1345 cm−1 (C–N stretch) and 770 cm−1 (imidering deformation), together with absorption bands in the rangeof 2940–2797 cm−1 due to aliphatic groups. Moreover, molecular

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mesophase to isotropic state (M1/I). On the other hand, compoundAZ-NI-II showed only two endothermic transitions. The exother-mic transition peaks appeared upon cooling (with the expectedhysteresis) describing the backwards phase transitions from the

Table 1Solubility behavior of the obtained compounds.

Code Solvents

NMP THF CHCl3 CH3CN CH2Cl2 DMSO Acetone

DANDI + ± ++ ± ± + ±AZ-NI-I ± ± ± ± ++ ± ±

ig. 1. Synthetic route and chemical structure of diamine (DANDI) and azomethineethyl ester (PC61BM).

tructure of the diamine and azomethine–naphthalene diimidesas identified from their 1H NMR spectra. 1H NMR spectrum ofANDI and AZ-NI-II is depicted in Fig. 3.

In the 1H NMR spectra of the diamine and prepared azomethine–aphthalene diimides the naphthalic protons appeared at the mostownfield. In the case of AZ-NI-I and AZ-NI-II, the azomethine pro-on signal was observed at about 8.16 ppm. The spectral data weren accordance with the expected formula.

Elemental analysis shows good agreement of the calculated andound content of carbon, nitrogen and hydrogen in the compoundssee Section 2).

The solubility of synthesized compounds was qualitativelyetermined by the dissolution of 2.5 mg of the solid in 1 ml ofrganic solvent at room temperature and under heating. Table 1ives the solubility of the azomethine–naphthalene diimides andANDI in different organic solvents.

Taking into account the data presented in Table 1, it is evidenthat the compound obtained from 4-octadecyloxybenzaldehydexhibited better solubility than the one synthesized from the fluo-inated aldehyde.

.2. Mesomorphic behavior

The transition temperatures during the heating and coolingcans and associated enthalpy changes of AZ-NI-I and AZ-NI-II

hthalene diimides (AZ-NIs) and chemical structure of [6,6]-phenyl C61 butyric acid

were determined using differential scanning calorimetry (DSC)experiments and are summarized in Table 2. The obtained resultsconfirmed that both compounds exhibited liquid crystalline prop-erties. Representative DSC traces of the AZ-NIs during cooling andheating scan are presented in Fig. 4.

Upon DSC analysis the compound AZ-NI-I exhibited four enan-tiotropic transitions, such as crystal to crystal (Cr/Cr1), crystalto mesophase (Cr1/M), mesophase to mesophase (M/M1) and

AZ-NI-II ± + ++ ± ++ ± ±The qualitative solubility was tested with 2.5 mg samples in 1 ml of solvent. (++)Soluble at room temperature; (+) soluble on heating; (±) partial soluble on heating.Solvents. NMP: N-metyl-2-pyrrolidone; DMSO: dimethylsulphoxide; THF: tetrahy-drofuran.

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ipTAapcC2iat

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Fig. 2. FTIR spectra of (a) DANDI and (b) AZ-NI-II.

sotropic melt to the mesophase and finally back to the crystallinehase, and prove that these are indeed first-order phase transition.he parameters of the heating and cooling process of AZ-NI-I andZ-NI-II were similar and exhibited reversible thermal behaviors determined by DSC experiments (cf. Fig. 4). DSC thermogramsresented in Fig. 4 were obtained for AZ-NI-I at the heating andooling rates 2◦/min, while for AZ-NI-II at 2.5◦/min in nitrogen.ompound AZ-NI-I showed a liquid crystal to isotropic transition at41.9 ◦C with a �H = 1.6 J/g on the heating scan. On cooling from the

sotropic state to room temperature, an exothermic peak appearedt 240.4 ◦C with a �H = 1.0 J/g, corresponding to the transition fromhe isotropic state to the liquid crystalline phase.

Moreover, in the heating scan of AZ-NI-I, two endothermic

eaks appear: at 22.8 ◦C (2.9 J/g) and 166.4 ◦C (4.3 J/g), whichorrespond probably to melting peaks of both fluorocarbon andydrocarbon aliphatic chains. Presumably, these peaks arise fromhe incompatibility between perfluorinated part and methy-

able 2hermal parameters of AZ-NI-I and AZ-NI-II detected from DSC.

Code T [◦C] (�H [J/g])

AZ-NI-IHeatinga 22.8 (2.9), 47.7 (0.5), 72.1 (0.6), 166.4 (4.3), 186.6 (0.3),

241.9 (1.6)Coolinga 39.6, 71.2 (2.7), 156.4 (3.7), 181.2 (0.8), 240.4 (1.0)

AZ-NI-IIHeatingb 106.7 (10.3), 124.8 (20.8), 231.6 (2.8)Coolingb 94.0 (4.1), 107.9 (15.9), 200.3 (2.2)

a Heating and cooling 2◦/min.b Heating and cooling 2.5◦/min.

etals 160 (2010) 2208–2218

lene groups in the molecule. Such behavior was not observedfor AZ-NI-II, for it lacks fluorinated chains. Condensation of 4-octadecyloxybenzaldehyde with the DANDI diamine leads to athermotropic �-conjugated calamitic B–A–B triblock (AZ-NI-II),while the reaction of 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde with DANDI givesC–B–A–B–C pentablock (AZ-NI-I) liquid crystals, as presentedin Fig. 1. In our opinion, such blocks have a strong influence onthe liquid crystalline properties of the investigated compounds.The molecular structure of AZ-NI-I is composed of soft segmentscomprising the methylene (B) and perfluorinated (C) terminalchains, and a hard segment, being the azomethine–naphthalenediimide moiety (A). The lack of miscibility between the hydro-carbon aliphatic bocks (B) and the perfluorinated segments(C) might cause phase separation in AZ-NI-I. Similar observa-tions were reported earlier: phase separation in a fluorinatedthermoplastic polyurethane elastomer [37], and in fluorinatedcomb copolymers [38] were described. The phase separationin AZ-NI-I was confirmed by AFM analysis, as presented inSection 3.4 of this paper. In our previous article [15], ther-mal properties of liquid crystalline symmetrical azomethinesprepared from 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzaldehyde were reported, andsimilar conclusions concerning phase separation could be drawn.On the other hand, such behavior was not observed in non-symmetrical azomethines obtained from the same aldehyde withfluorinated chain [14].

In the DSC study, the compound AZ-NI-I exhibited two liquidcrystal phases M1, and M2 in the range 166–241 ◦C. Mesophase M1was observed in the range 166–186 ◦C (temperature range 20 ◦C),while mesophase M2 was found in the range 186–241 ◦C (temper-ature range 55 ◦C) (Table 2). On the other hand, compound AZ-NI-IIshowed a liquid crystal to isotropic transition at 231.6 ◦C with a�H = 2.8 J/g on the heating scan. Additionally, for AZ-NI-II duringheating scan one endothermic peaks appeared at 124.8 ◦C (20.8 J/g)corresponding to the temperature of melting of the compoundAZ-NI-II. Moreover, for AZ-NI-II broad peak at 106.7 ◦C (10.3 J/g)was observed probably corresponding to the melting process ofthe aliphatic part of the molecule. It was found that the AZ-NI-IIexhibited one liquid crystal phases M1 in the range 124–231 ◦C.The presence of F atoms increased slightly the isotropisation tem-perature of AZ-NI-I in comparison with AZ-NI-II.

The phase transitions were additionally analyzed based on theirenthalpy and entropy values visualized in Fig. 5.

The melting process shows the highest enthalpy values. Theenthalpy values decrease from 20.8 J/g for the AZ-NI-II to the min-imal value 2.9 J/g for the AZ-NI-I. The parameters of the heatingand cooling process are similar (cf. Table 2). It is known that allobserved phase transitions can be quantitatively analyzed based ontheir entropy (�S) values. However, this value is not widely investi-gated in the papers [39]. Entropy changes (�S) of phase transitionswere calculated using the following formula:

�S = �H

T

where enthalpy �H and temperature T of phase transitions weretaken from the DSC calorimetry. All measurements were performedusing the identical calorimetric setup.

In our case the highest value of entropy exhibited AZ-NI-II(Fig. 5).

Phase behaviors of the both compounds were investigated addi-tionally by observation of the optical textures on a polarizingoptical microscope equipped with hot-stage. Photomicrographs ofthe optical textures of mesophases obtained for the AZ-NI-I andAZ-NI-II are presented in Fig. 6.

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of DAN

awhawtc

iiCIitt[at

Fig. 3. 1H NMR spectra

The differences in the temperature of isotropisation andlso kind of the mesophases of the investigated compoundsere detected. This behavior indicates role of the alde-yde structure in creating their mesomorphic properties ofzomethine–naphthalene diimides. It was found that compoundith fluorine substituents exhibited more enantiotropic transi-

ions than the azomethine–naphthalene diimide with aliphatichains.

Generally, the types of liquid crystal phases can be preliminar-ly concluded based on the DSC and POM analysis. Our preliminarynvestigations suggested that both compounds exhibited smectic

mesophase (SmC) with Schlieren texture. Moreover, AZ-NI-exhibited smectic A phase (SmA). The tentative mesophases

dentifications and the sequence of phase transitions related to

he both compounds are based on the identification of tex-ures appearing in two reference textbooks for liquid crystals40,41] and on repeated POM and DSC experiments. However,dditional experiments are necessary to confirm our supposi-ions.

DI (a) and AZ-NI-II (b).

3.3. Optical and electrochemical properties

The optical properties of the studied azomethine–naphthalenediimides were analyzed by UV–vis absorption spectroscopy. TheUV–vis absorption spectra were recorded both in chloroform solu-tion and in solid state as thin films spin-coated on quartz substrate.The range of UV–vis measurements was limited by the trans-parency of the used solvent and the substrate. The representativeabsorption spectra of DANDI, AZ-NIs in solution and in AZ-NI-II filmare depicted in Fig. 7.

Absorption spectra of the studied AZ-NIs showed similar char-acteristics, i.e., the intense band with the maximum (�max) locatedin at 275 nm and a structured band at lower energies with threemaxima at 343, 360 and 379 nm. The bands above 340 nm are char-

acteristic for naphthalene imides and are attributed to the �–�*transition in the naphthalene tetracarboxcylic diimide conjugatedcore [42]. The positions of the absorption band at higher energyregion of AZ-NIs compounds compared to that of diamine DANDIdid not change (cf. Fig. 7 insert). It suggests that polar properties

Page 7

2214 E. Schab-Balcerzak et al. / Synthetic M

F ◦

aa

oStbbafiieo

tvo

r

ig. 4. DSC traces of: (a) AZ-NI-I obtained on the heating and cooling scan at 2 /minnd (b) AZ-NI-II obtained on the heating and cooling scan at 2.5◦/min, under N2

tmosphere.

f DANDI are not affected by the condensation with aldehydes.mall hipsochromic shift of the transition below 300 nm betweenhe diamine and the AZ-NIs was observed. The absorption bandeing responsible for �–�* transition in the imine group is coveredy absorption of the naphthalimide units. No differences betweenbsorption properties of AZ-NI-I and AZ-NI-II in solution and inlm were found. It indicates that the conformations of compounds

n solution and in the films are the same. AZ-NIs exhibited opticalnergy band gap values (Eg) at about 3 eV calculated from the onsetf UV–vis absorption band.

Electrochemical properties of AZ-NIs in solution were inves-

igated by cyclic voltammetry (CV) and differential pulseoltammetry (DPV). Fig. 8 presents the obtained voltammogramsf investigated azomethine–naphthalene diimides.

Both compounds exhibited two electrochemically reversibleeduction processes characteristic for arylenediimides [26,28], at

Fig. 5. Influence of the aldehyde structure on the entha

etals 160 (2010) 2208–2218

very similar potentials, that is, the first at ca. −0.97 V and the sec-ond at ca. −1.46 V. However, their oxidation behavior was different:AZ-NI-II has one quasi reversible peak at 0.64 V, while the fluori-nated AZ-NI-I exhibits two apparently irreversible peaks at 0.49 and0.71 V. These oxidation peaks may be attributed to the diarylim-ine moieties, which are identical in both molecules. This markeddiscrepancy in oxidation behavior of AZ-NI-I and AZ-NI-II maystem from a different supramolecular organization (as evidencedby thermal studies of the mesomorphic behavior) and differences inthe mode of adsorption on the electrode between the fluorinatedand the non-fluorinated AZ-NI. Determination of the first reduc-tion and oxidation potentials of AZ-NIs allowed the calculation ofHOMO (highest occupied molecular orbital), LUMO (lowest occu-pied molecular orbital) and energy gap values. The HOMO levelsof compounds were obtained as E1/2 that were calculated as theaverage between Epa (anodic peak) and Epc (cathodic peak). Allthe electronic parameters from electrochemical measurements aresummarized in Table 3.

It is noteworthy that the electrochemical (E′g) and optical

(Eg) band gaps differ strongly; a similar behavior (discrepancy inHOMO levels derived from spectroscopy and electrochemistry) wasdescribed by Gawrys et al. for a series of thienyl derivatives ofnaphthalenediimides [4] and can be attributed to the fact the low-est transition observed in the UV–vis measurement (naphthalenediimide �–�*) does not involve the orbital from which an electronis removed during oxidation (the oxidation occurs on the diarylim-ine moiety).

3.4. AFM analysis

In AFM experiments the influences of the chemical structureof the azomethine–naphthalene diimides on the surface morphol-ogy of the materials were investigated. Films on the glass substratewere obtained by dissolving at room temperature the compound inchloroform to form a homogenous solution. Residual solvent wasremoved by heating the film. Fig. 3 shows AFM images obtained forAZ-NIs.

The films showed a very interesting morphology, characteristicfor systems capable of forming organized supramolecular struc-tures in linear compounds with different types of sub-units. AFMimages of the AZ-NI-I and AZ-NI-II are presented on a typical, pla-nar view of the topography as well as Inclination transformation,which was used to provide a better view of fine structures and fea-tures. Moreover, a Phase Image was placed, which shows mapsof viscoelastic forces, useful for imaging inhomogeneous mate-rials (cf. Fig. 9). Surface of AZ-NI-I sample reveled two kinds of

interesting ordered structures. Large field scans (25 �m × 25 �m)shows neuron-like features – large, round objects (1–2 �m in diam-eter), surrounded with oblong (2–5 �m in length) ones. Small areaPhase Image scans (1 �m × 1 �m) presented ordered, 20–30 nmwide and few hundreds nm long lamellar structures. On the other

lpy and entropy of phase transition of the AZ-NIs.

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E. Schab-Balcerzak et al. / Synthetic Metals 160 (2010) 2208–2218 2215

obtained for: (a) AZ-NI-I (188, 205 ◦C) and (b) AZ-NI-II (133, 195 ◦C).

hih

dreap

ul

3

IFIiid

vtcDio3doow4

sIs

Fig. 6. Photomicrographs of the optical textures of mesophases

and, AZ-NI-II reveals a homogenous, grainy (about 300–500 nmn diameter) surface, with homogenously distributed holes andills.

When the topography is measured, one can extract certain dataescribing the properties of the surface. A typical parameter isoughness (Ra, Rms). However, it is useful to use another param-ter such as skew (the unbalance of height distribution maximum)nd kurtosis (the peak’s width on height distribution). Mentionedarameters for two investigated surface are presented in Table 4.

Presented results of the research proved the AFM techniques areseful and powerful tool for micro- and nanoscale diagnostic of the

iquid crystal surfaces.

.5. Current–voltage characteristics

Two kinds of devices with the following architecture:TO/compound/Al and ITO/compound:PC61BM/Al were fabricated.irst, current–voltage (I–V) measurements of AZ-NI-I and AZ-NI-I performed on ITO/compound/Al devices in the dark and underllumination with light (halogen lamp, about 1000 W/m2) werenvestigated. Current–voltage curves of investigated devices in theark and during irradiation with light are shown in Fig. 10.

It can be seen that current increases with the increase of appliedoltage, which confirms the semiconducting properties of inves-igated compounds. From the obtained results it was possible toalculate a luminal voltage for illuminated and dark measurements.ifferences between devices ITO/compound/Al with and without

llumination were observed (cf. Fig. 9a). For example, the turn-n voltage of the device ITO/AZ-NI-I/Al was observed at about.0 V at room temperature without illumination, while for thisevice under 1000 W/m2 illumination it was about 1.8 V. On thether hand, the turn-on voltage of the device ITO/AZ-NI-II/Al wasbserved at about 2.3 V at room temperature under 1000 W/m2,hile for the same device, but without illumination, it was about

.0 V.Differences were found also with the change of the device

tructure. Current–voltage curves of ITO/AZ-NI-I:PC61BM/Al andTO/AZ-NI-II:PC61BM/Al in the dark and under illumination arehown in Fig. 10b. It can be seen that for the devices ITO/AZ-

Fig. 7. UV–vis absorption spectra of DANDI, AZ-NIs in chloroform solution (a) andAZ-NI-I in film deposited on quartz substrate and in solution.

Page 9

2216 E. Schab-Balcerzak et al. / Synthetic Metals 160 (2010) 2208–2218

V) of

Nwcfis

F(

Fig. 8. Cyclic voltammograms (CV) and differential pulse voltammograms (DP

I-I:PC61BM/Al and ITO/AZ-NI-II:PC61BM/Al at room temperatureithout and with illumination the current increased quicker in

omparison with the devices without PC61BM. This behavior con-rmed the influence of the azomethine–naphthalene diimidestructure on the obtained I–V characteristics. The differences

ig. 9. AFM images of the azomethine–naphthalene diimides (from the top): AZ-NI-I andcentral) and the Phase Imaging (right) is presented.

AZ-NI-I (a and c) and AZ-NI-II (b and d). The peak at 0 V (on b and d) is Fc/Fc+.

found in the I–V results confirm the difference in planarity ofthe AZ-NIs compounds structure and different conformations ofthe compounds in film. For both type of devices, that is, withand without PC61BM, a small photo-current effect was found (cf.Fig. 10).

AZ-NI-II, respectively. Topography (left) processed with Inclination transformation

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E. Schab-Balcerzak et al. / Synthetic Metals 160 (2010) 2208–2218 2217

Table 3HOMO–LUMO energy levels, electrochemical (E′

g) and optical band gap (Eg) values and redox properties of azomethine–naphthalene diimides.

Code HOMOa [eV] LUMOb [eV] LUMO′b [eV] Eg [eV] E′g [eV] Ered1 [V] vs. Fc Ered2 [V] vs. Fc Eox1 [V] vs. Fc Eox2 [V] vs. Fc

AZ-NI-I −5.29 −2.29 −3.84 3.00 1.47 −0.96 −1.46 0.49c 0.71c

AZ-NI-II −5.44 −2.37 −3.83 3.07 1.61 −0.97 −1.42 0.64 –

a HOMO levels are calculated from Eox1, assuming that Fc/Fc+ is −4.80 eV relative to vacuum [28].b LUMO levels were estimated from the HOMO levels and energy gaps, LUMO′ is calculated from Ered1, reduction and oxidation potentials are calculated as formal half-wave

potentials E1/2 (i.e. the average between Epa and Epc).c Oxidation peak potential (irreversible peak).

Fig. 10. Current–voltage curves of ITO/AZ-NI-I/Al and ITO/AZ-NI-II/Al (a) and ITO/AZ-NI-I:PC61BM/Al and ITO/AZ-NI-II:PC61BM/Al devices (b) in the dark and under illumi-nation of 1000 W/m2, and scheme of devices investigated in this work.

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2218 E. Schab-Balcerzak et al. / Synthetic M

Table 4The surface parameters of AZ-NI-I and AZ-NI-II.

Code Surface statisticsa

Ra [nm] Rms [nm] Skew Kurtosis Surface area ratio

AZ-NI-I 70.1 113.0 3.7 22.4 1.056

4

acap

1

2

3

4

5

6

oao

A

m

[[

[[

[

[

[[

[[[[

[

[[

[[[

[

[[

[[

[

[[[

[

[[[40] D. Demus, L. Richter, Textures of Liquid Crystals, 1st ed., Verlag Chemie, Leipzig,

AZ-NI-II 39.5 51.6 0.6 2.5 1.064

a Values calculated for scanning field 25 �m × 25 �m (625 �m2 scan area).

. Conclusions

In summary, we prepared two symmetricalzomethine–naphthalene diimides by condensation of diamine-ontaining six-membered diimide rings with aromatic-aliphaticldehydes. The following conclusions can be drawn from theresent work:

. Two new symmetrical azomethine–naphthalene diimides con-taining mesogenic unit have been synthesized and character-ized. The molecular structures were identified by FTIR, NMR, andelemental analysis, and the results were in accordance with theexpected molecular formula.

. New aromatic diamine with naphthalene unit was synthesizedand characterized by FTIR, NMR, UV–vis and elemental analysis.

. Both azomethine–naphthalene diimides showed the smecticmesomorphism. The mesophases of the AZ-NI-I have been pre-liminarily designated to be a SmC and SmA phases, whilecompound AZ-NI-II has probably smectic C phase.

. The compounds depend on the structure exhibited mesophasesover a narrow or wide temperature range.

. The electrochemical and optical band gaps calculated from elec-trochemical measurements (E′

g) differed strongly from the oneobtained from the optical absorption edge (Eg). The fluorinatedazomethine–naphthalene diimide exhibited smaller value of E′

g(1.47 eV) than compound without F atoms (1.61 eV). Both com-pounds exhibited two electrochemically reversible reductionprocesses at similar potentials, that is, the first one was estimatedat −0.97 V and the second at −1.46 V.

. Preliminary investigations of the current–voltage characteristicsfor devices such as ITO/AZ-NIs/Al, and ITO/AZ-NIs:PC61BM/Al inthe dark and during irradiation with light confirmed their semi-conductive properties of the organic thin film.

In conclusion, the results could lead to further devel-pments of new family compounds, that is, thermotropiczomethine–naphthalene diimides which could guide the designf optoelectronic devices.

cknowledgments

The authors thank Dr. H. Janeczek for DSC and POM measure-ents. M.K. is grateful for the financial support of the Foundation

[

[

etals 160 (2010) 2208–2218

for Polish Science (START scholarship) and for a scholarship fromthe UPGOW project co-financed by the European Social Fund.

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