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Synthetic Metals 161 (2011) 1660– 1670

Contents lists available at ScienceDirect

Synthetic Metals

j o ur nal homep ag e: www.elsev ier .com/ locate /synmet

haracterization, liquid crystalline behavior, optical and electrochemical study ofew aliphatic–aromatic polyimide with naphthalene and perylene subunits

wa Schab-Balcerzaka,b,∗, Agnieszka Iwanc,∗∗, Marzena Grucela-Zajaca, Michal Krompieca,arzena Podgornaa, Marian Domanskib, Mariola Siwyb, Henryk Janeczekb

Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, PolandCentre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, 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 9 March 2011eceived in revised form 10 May 2011ccepted 30 May 2011vailable online 28 June 2011

eywords:aphthalene diimidesolyperyleneimidesolynaphthalimidesiquid crystals

a b s t r a c t

A new aliphatic–aromatic diamine with naphthalene diimide (DA) moiety and a polyimide with naph-thalene and perylene diimide subunits (PI) were synthesized and their thermotropic liquid crystallinebehavior was examined by differential scanning calorimetry (DSC) and polarizing optical microscopy(POM). Both compounds exhibited liquid crystalline properties. The temperatures of 5% weight loss (T5%)of the diamine and polyimide range from 350 to 373 ◦C in nitrogen. The optical properties, that is, UV–visabsorption and photoluminescence (PL) of obtained monomer and polyimide were studied in solutionand in solid state as a blend with inert poly(methyl methacrylate) (PMMA). Influence of solvent polarity,excitation wavelength and concentration on DA and PI emission was found. The investigated compoundsemitted green light in chloroform solution. Relative PL intensity of the DA and PI was investigated in NMPin relation to rhodamine-B used as a standard. The electrochemical behavior of diamine and polyimide

hermotropic compoundshotoluminescencelectrochemistryurrent–voltage characteristics

was studied (in solution) by differential pulse voltammetry (DPV). The HOMO and LUMO levels of thesecompounds were in the range of −5.44 to −6.03 eV and −3.81 eV, respectively. Energy gap (Eg) values werecalculated using cyclic voltammetry (CV) and UV–vis measurements. Introduction of a perylene moiety(PI) resulted in increase of Eg (2.22 eV) compared to DA which exhibited a Eg value of approximately1.63 eV. The electrical properties of DA and PI were investigated by current–voltage (I–V) measurementson ITO/compound/Al devices.

. Introduction

The growth of modern technology has posed a constantlyncreasing need for materials with specific properties that can per-orm well under harsh conditions [1]. Polyimides (PI), which areart of a group of high-temperature polymers constitute an impor-ant class of such materials because of their outstanding features,hat is, high thermal stability, low dielectric constant, high break-own voltage, good planarization, wear, radiation and chemical

esistance, good adhesion properties, low thermal expansion andxcellent mechanical properties. They maintain an excellent bal-nce of electrical, thermal, mechanical, physical and chemical prop-

∗ Corresponding author at: Centre of Polymer and Carbon Materials, Polishcademy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland.∗∗ Corresponding author at: Electrotechnical Institute, Division of Electrotech-ology and Materials Science, 55/61 M. Sklodowskiej-Curie Str., 50-369 Wroclaw,oland.

E-mail addresses: ewa.schab-balcerzak@us.edu.pl,schab-balcerzak@cmpw-pan.edu.pl (E. Schab-Balcerzak), a.iwan@iel.wroc.plA. Iwan).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.05.036

© 2011 Elsevier B.V. All rights reserved.

erties over a wide range of temperatures. The unique combinationof properties makes PI ideal for a variety of applications in many dif-ferent industries, that is, microelectronics, microelectromechanicalsystems, aerospace, photoelectronics, alignment layers in liquidcrystal displays, and for gas separation as membranes [2].

The interest of academic and industrial research has beendirected toward the design and synthesis of new dianhydride anddiamine monomers for producing of new polyimide materials withdesirable properties and functions useful for modern technologies.There are mainly two structural variations for aromatic polyimides,that is, five-membered and six-membered imide rings. Amongthe varieties of polyimides investigated, those with six-memberedimide rings, usually obtained from 1,4,5,8-naphthalene tetracar-boxylic dianhydride (NTDA) and 3,4,9,10-perylene tetracarboxylicdianhydride (PTCDA), have aroused substantial interest in recentyears. Polymers with six-membered imide rings, that is, poly-naphthalimides (PNIs) and polyperyleneimides (PPIs) have beenfound to demonstrate improved thermal and chemical resistance

when compared with conventional five-membered ring phthalicpolyimides [3]. Moreover, perylene and naphthalene diimides areimportant due to their photochemical behavior, photo-stabilities,

hetic M

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E. Schab-Balcerzak et al. / Synt

lectron acceptor and photoconductive properties and applica-ion as laser dyes [4]. It was found that compounds containingix-membered imide rings have prospective applications in organicopto)electronics and related fields, exemplary as materials for highfficiency solar cells [5,6], for fabrication of field effect transis-ors (FETs) [7] or light-emitting diodes (OLEDs) [8] and are highlyesirable for photonic technology [9]. The planar structure of theseolecules makes it possible to prepare fairly well-ordered thin film

n different substrates which may increase the charge transportnd luminescence of these organic films [5]. However, the majorityf polyimides based on naphthalene tetracarboxylic dianhydridend perylene tetracarboxylic dianhydrides have poor processabil-ty and low solubility in organic solvents, which contribute to ratheroor film forming properties [10]. Most of the studies are focusedn synthesis of various PNIs and PPIs for improving their solu-ility, which is a key property for many applications [3,10–13],.g. as liquid crystal alignment layers [14], luminescent materi-ls [15–20], as light-emitting electrochemical cells (LEC) [21] ors electrochromic materials [18]. It was found that electrolumines-ence intensity of a copolymer with both phthalic and naphthalicmide rings and ether linkages did not change after heating theuminescence cell to 220 ◦C [15]. Aliphatic–aromatic PNI obtainedrom NTDA and 2,2,4(2,4,4)-trimethyl-1,6-hexane diamine wasescribed as a potential candidate for light emitting materialshere red-shifted excimer-like fluorescence, thermal stability and

ood solubility are required [17]. As far as PPIs are taken intoccount, most of the reports are focused on improving the pro-essability while maintaining their other valuable properties [1,21].uch polyimides are usually investigated as luminescent materi-ls for polymer light emitting diodes (PLEDs) in a wider rangehan PNIs [18–20]. It was found that the fluorescent emissionf copolymers containing perylene diimide and phthalic diimidesnits can be tuned electrochemically or switched between twoifferent redox states and reversible color changes of their filmetween red, blue and violet correspond to the neutral, radicalnion, and dianion forms of the perylene diimide moieties [18].n the other hand, in the case of optoelectronic applications theigh ordered structures in polymers are desirable. Molecular self-rganization in soft condensed phases enables creation of novelunctional materials with unique properties [22]. Self-organizationn polymers could be achieved in various ways such as hydrogenonding and �–� interactions [4] or liquid crystalline properties23]. Liquid crystals possess the ability to self-organize sponta-eously into highly ordered structures. Due to the spontaneousrdering of molecules in the liquid crystalline phase, very highlyrdered films can be obtained after the liquid crystalline films cooled to a crystalline phase. Therefore, the enhancement inharge-carrier mobility may be achieved through highly orga-ized morphology from processing in the mesophase [22,24]. Theaphthalene-1,4,5,8-tetracarboxylic imide and perylene-3,4,9,10-etracarboxylic imide units are highly symmetric mesogens andheir L/D ratio is lower than that of typical short mesogens [25].owever, as far we are aware, PNIs exhibiting LC properties haveot been reported apart of our previous work [26]. To the bestf our knowledge, there is only one example in which LC PPIsere described [27]. The time-dependent crystal-smectic transfor-ation in PPIs obtained form PTCDA and aliphatic diamines with

arious alkyl chain length ranging from C3 to C12 was investi-ated [27]. Generally, the majority of literature reports related toC compounds with naphthalene or perylene imide rings concernsiimides [28].

To the best of our knowledge, this is only one example in

hich a naphthalene diimides with perylene dimides and alkyl

hain ((CH2)12) were introduced together into polymer backbone18,29]. For the obtained polymer, xerographic electrical proper-ies [29], electrochemical properties and fluorescence [18] were

etals 161 (2011) 1660– 1670 1661

studied. However, potential LC properties of such polyimide havenot been investigated.

Recently, we described the thermal, optical (absorption inUV–vis range), electrochemical and electrical properties of sym-metrical diimide-azomethines with naphthalene moieties [30]and aromatic-aliphatic five- and six-membered thermotropicpolyimides [26]. In our previous work, polyimides were obtainedfrom two aliphatic diamines, such as: 4,4′-(butane-1,4-diylbis(oxy))bis(butane-4,1-diyl) bis(4-aminobenzoate) (PBBA470) or5,10,15,20,25,30,35,40,45,50,55,60-dodecaoxatetrahexacontane-1,64-diyl bis(4-aminobenzoate) (PBBA1200) and threedianhydrides: 1,2,4,5-benzenetetracarboxylic dianhydride(PMDA), 1,4,5,8-naphtalenetetracarboxylic dianhydride (NTDA) or3,4,9,10-perylenetetracarboxylic dianhydride (PMDA) [26]. Thesepolyimides, except the polymer obtained from PBBA1200 andPMDA, exhibited liquid crystalline properties, as confirmed by DSCand POM experiments [26].

This paper is devoted to the synthesis of a new thermotropicdiamine with naphthalene diimide moiety and a polyimide withnaphthalene and perylene diimide units and their thermal (DSC,TGA), structural (POM), optical (UV–vis, PL), electrochemical (DPV)and electrical (I–U) characterizations.

2. Experimental details

2.1. Materials

1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA) andperylene-3,4,9,10-tetracarboxylic dianhydride (PTDA), m-cresol,toluene, ethanol, diethyl ether, NMP, THF, CH3Cl, CH3CN and TEAwere purchased from Aldrich Chemical Co. and used as received.Diamine, that is, polyetheramine-Jeffamine ED-2003, was a giftfrom HUNTSMAN HOLLAND BV.

2.2. Synthesis of the diamine DA

The diamine ED-2003 (30 g; 0.15 mmol) in 20 ml mixtureethanol/toluene (1:1) was stirred until dissolved completely.Separately, NTDA (0.805 g; 3 mmol) was dissolved in 100 mlethanol/toluene (1:1) and the solution of ED-2003 diamine wasadded. The mixture was heated (97 ◦C) under argon atmosphere.After 24 h, it was cooled to room temperature. Most of the solventwas evaporated on a evaporator and then the residue was pouredonto a Petri dish to evaporate the rest of the solvent.

DA: Red-brown soft solid. Yield 90%. 1H NMR (CDCl3, ı, ppm):1.04 (m, O C CH3, 12H), 1.13 (m, CH3, 24H), 1.55 (d, N C CH3 ,6H), 3.17 (m, CH , 4H), 3.41 (m, O CH2 , 8H), 3.50 (m, O CH2 ,20H), 3.64 (m, O CH2 , 312H), 4.18 (m, O CH, 4H), 4.45 (m, N CH,2H), 5.46 (m, NH2, 4H), 8.71 (s, 4H, naphthalene). FTIR (KBr, cm−1):3554, 3379 ( NH2 stretch), 2877 (CH aliphatic), 1706, 1665 (C Oof imide), 1627 ( NH2 deformation), 1352 (C N). Anal. Calcd. for(C248H427N4O160) (5206, 34): C 57.20, H 9.14, N 1.18; Found C 56.77,H 9.55, N 1.18.

2.3. Synthesis of the polyimide PI

To DA (1.186 g; 0.5 mmol) and TEA (0.2 ml; 1.5 mmol) in 9.75 mlm-cresol PTDA (0.169 g; 0.5 mmol) was slowly added. The mixturewas heated to 180 ◦C in an argon atmosphere for 24 h, cooled toroom temperature and the resultant product PI was precipitatedinto diethyl ether. Finally, PI was dried in vacuum.

PI: Red powder. Yield 40%. 1H NMR (CDCl3, ı, ppm): 1.04 (m,

O C CH3, 12H), 1.11 (m, CH3, 24H), 1.58 (d, N C CH3, 6H), 3.17(m, CH , 4H), 3.40 (m, O CH2 , 8H), 3.50 (m, O CH2 , 20H), 3.64(m, O CH2 , 312H), 4.19 (m, O CH, 4H), 4.45 (m, N CH, 2H), 8.65(s, 4H, naphthalene), 8.75 (m, 4H, perylene). FTIR (KBr, cm−1): 2871

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662 E. Schab-Balcerzak et al. / Synt

CH aliphatic); 1774, 1720, 1706, 1656 (C O of imide); 1595, 1580,508 (for perylene skeleton); Anal. Calcd. for (C113H202N2O49)2372.79): C 59.46, H 7.72, N 1.50; Found: C 60.74, H 6.77, N 1.31.

.4. Blends preparation

Polymer blends were obtained by dissolving the desired amountf compounds and PMMA in NMP to form a homogeneous solution1% vv concentration of compound in PMMA). Films cast on glassere dried in vacuum oven at 90 ◦C over 10 h.

.5. 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

Bruker AC 300 MHz spectrometer using chloroform (CDCl3) asolvents and TMS as the internal standard. Elemental analysesere performed using Perkin Elmer Analyzer 2400. Differential

canning calorimetry (DSC) was performed with a TA-DSC 2010pparatus, under nitrogen atmosphere using sealed aluminumans. The transition temperatures were read at the top of thendothermic and exothermic peaks. Thermogravimetric analysesTGA) were performed on a Perkin Elmer apparatus at a heating ratef 10◦/min under nitrogen. The textures of the liquid-crystallinehase were observed with a polarized optical microscope (POM)eiss (Opton-Axioplan) equipped with a Nikon Coolpix 4500 colorigital camera, and Mettler FP82 hot plate with Mettler FP80 tem-erature controller. Current–voltage characteristics were recordedsing Keithey 6715 electrometer. UV–vis spectra were measured

n NMP and CHCl3 solution and as blends using a Lambda Bio0 spectrometer. Photoluminescence was measured using a Var-

an Carry Eclipse fluorimeter. Electrochemical measurements werearried out using Eco Chemie Autolab PGSTAT128n potentiostat.latinum wire (diam. 1 mm), platinum coil and silver wire serveds working, auxiliary and reference electrode, respectively. Poten-ials are referenced with respect to ferrocene, which was used ashe internal standard. Differential pulse voltammetry experimentsere conducted in a standard one-compartment cell, in acetonitrile

Sigma–Aldrich HPLC gradient grade), under argon. Bu4NPF6 (Acrosrganics, 99%), recystallized from isopropanol and dried at 180 ◦C inacuum was used as the supporting electrolyte, at a concentrationf 0.1 M.

. Results and discussion

In this article, in continuation of our efforts in synthesis ofC six-membered polyimides, new thermotropic diamine (DA)ith naphthalene diimide unit and LC polyimide obtained fromentioned diamine and 3,4,9,10-perylene tetracarboxylic dian-

ydride are presented. The strategy to obtain a soluble liquidrystalline six-membered polyimide is based on the concept of aulti-block conjugated–non-conjugated polymer, where blocks of

aphthalene and perylene diimides were interspersed with non-lectroactive sp3 hybridized flexible alkoxy segments.

.1. Synthesis and characterization of the diamine and polyimide

The diamine (DA) containing naphthalene diimide and alkoxynits was synthesized by one step condensation method inthanol/toluene solution from 1,4,5,8-naphthalenetetracarboxylicianhydride (NTDA) and diamine with aliphatic and ether linkages,

hat is, polipropylooxide diamine ED-2003, as shown in Scheme 1a.

The obtained diamine was utilized for the preparation of poly-mide PI (cf. Scheme 1a). Polyimide PI was synthesized from,4,9,10-perylenetetracarboxylic dianhydride (PTDA) and DA by

etals 161 (2011) 1660– 1670

polycondensation reaction in m-cresol solution with a quite goodyield (40%). The obtained alternating polymer PI consists of naph-thalene, perylene diimide and alkoxy subunits (cf. Scheme 1a).

The assigned chemical structure of synthesized diamine andpolyimide were identified through the data from 1H NMR, IR spec-troscopy measurements and elemental analysis. In IR spectrum ofprepared diamine with naphthalene diimide unit, the anhydrideC O vibration bands at 1780 and 1768 cm−1 in NTDA disap-peared. The appearance of imide C O stretching vibration bands at1706 cm−1 and 1665 cm−1 which correspond to asymmetrical andsymmetrical stretching in the six-membered imide ring are seen(cf. Supporting information). Additionally, DA exhibited absorp-tion bands at 1352 cm−1 (C N stretching) together with absorptionband at 2877 cm−1 attributed to aliphatic groups. Moreover, in IRspectrum of diamine the characteristic peaks at 1627 cm−1 ( NH2deformation) and at 3450 cm−1 were observed.

In IR spectrum of polyimide PI the characteristic for imideabsorption bands at 1720, 1706, and 1656 cm−1 are seen (cf.Supporting information). Additionally, bands at 1595, 1580,1508 cm−1 typical for perylene skeleton were found.

The signal assignments in the 1H NMR spectra of diamine DAand polyimide PI are consistent with the proposed structure.

In the case of diamine DA, the peak at 5.46 ppm was ascribed toamine protons. In order to confirm this assumption trichloroacetylisocyanate (TAI) as in situ reagent for NMR spectroscopy ofamines was used [31]. The 1H NMR spectrum which was obtainedafter addition of TAI to diamine DA is presented in Supportinginformation. In the 1H NMR spectrum of diamine DA after addi-tion of TAI the lack of signal at 5.46 ppm was seen and additionally,the new signal at 8.01 ppm was appeared characteristic for NH pro-ton in the TAI derivative. This experiment confirms that signal at5.46 ppm was properly assigned to amine protons.

The peak assigned to hydrogen atoms of naphthalene unitsappeared at 8.71 and 8.65 ppm, for DA and PI, respectively. More-over, in the 1H NMR spectrum of the polyimide the signal at8.75 ppm corresponds to perylene protons. Position of both naph-thalene and perylene protons in aliphatic–aromatic polyimide PIis shifted up-field (by about 0.54 ppm and 0.13 ppm, respectively)in comparison with those found for P2B, P2A and P3B polyimidesdescribed in our former article [26]. The expected diamine andpolyimide composition was confirmed by elemental analysis (seeSection 2).

The solubility of the PI polymer and the DA diamine was quali-tatively determined by dissolution of 2.5 mg of the solid compoundin 1 ml of organic solvent at room temperature and during heat-ing of the sample to boiling temperature of the solvent. Solubilitybehavior of the obtained compounds is collected in Table 1 inSupporting information. Diamine DA and polymer PI showed dif-ferent solubility behavior in various organic solvents. Generally, thepolynaphthalimides and polyperyleneimides reported in the liter-ature [10] exhibited low solubility, even after heating. One of theadvantages of our polymer PI is full solubility in NMP, THF, CHCl3and CH3CN after heating. On the other hand, the diamine DA wassoluble in the mentioned solvents in room temperature. DiamineDA was soluble in cyclohexanone after heating, while polyimide PIwas insoluble in this solvent.

Thermal stability of DA and PI was evaluated by thermogravi-metric analysis (TGA) under a nitrogen atmosphere. The TGA dataof investigated monomer and polymer are presented in Supportinginformation. The TGA curve of DA indicates one main reactionstage, which is attributed to the decomposition of the naphthalenediimide segment. Whereas in the case of PI weight-loss processed

in two steps. The first one corresponds to loss of the naphthalenediimide at about 400 ◦C and the second step occurs due to degrada-tion of the perylene dimide units at approximately 600 ◦C. It is clearthat the thermal properties of polymers depended strongly on the

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 1660– 1670 1663

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cheme 1. (a) Synthetic route of diamine DA and polyimide PI along with chemicarevious work [26].

tructure, in this case on the microstructure of the monomer andolymer backbone. Taking into account the chemical constitutionf the polyimide and diamine studied herein, one may expect thathey should exhibit relatively inferior thermal stability due to theong propylether segments in the repeat unit. However, taking theGA results into account it was found that both DA and PI exhib-ted good thermal stability. The initial decomposition based on 5%

eight loss and the temperature at which 10% weight loss occurred,hich is usually considered the critation for assessing the ther-al stability, lay within the range of 350–373 ◦C and 362–382 ◦C,

espectively. Our previous study [26] showed that polymer P2B (cf.cheme 1b) started to decompose at a slightly higher temperature390 ◦C) than PI being the subject of this work. Moreover, taking into

ccount the amount of carbonized residue, significant differencesetween PI and P2B were found. The char yield percent at 800 ◦C

n the polymer P2B was more twice higher (40%) than in polyimideI with naphthalene and perylene sub-units (14.5%). However, the

cture of PMMA and (b) chemical structures of polyimides being the subject of our

PI possessed much longer nonaromatic fragments in comparisonwith P2B.

3.2. Liquid crystalline properties

In order to investigate the mesomorphic behavior of diamineand polyimide we performed differential scanning calorimetry(DSC) experiments under nitrogen atmosphere using differentheating and cooling rates in a temperature range from −40 ◦C toover the clearing point. The obtained results indicated that both thediamine DA and the polyimide PI exhibited liquid crystalline prop-erties. The transition temperatures and enthalpies of the diamineDA and polyimide PI determined by DSC are summarized in Table 1.

DSC studies revealed that the diamine exhibited a clearing pointtemperature higher than 40 ◦C and broad thermal range of themesophase (Table 1). DSC thermograms of the diamine DA areshown in Supporting information.

1664 E. Schab-Balcerzak et al. / Synthetic M

Table 1Transition temperatures and enthalpies of the diamine DA and polyimide PI detectedby DSC.

Code Phase transitions [◦C] (corresponding enthalpy changes) [J/g], DSC

Heating Cooling

DA 12.1, 41.8 (138)a nd36, 39.6 (164.1)b 17.3, 20.1 (94.6)b

18.1 (24.4), 33.7 (90.7), 42.5 (0.9)c 17.3, 20.2 (93.7)b

33.3 (76.2), 41.3 (10)d 22.0 (83.9), 31.7 (10.8)d

29.9 (72.9), 41.3 (30.7)e 22.0 (82.0), 31.7 (7.4)d

PI 40.7 (49.6), 249.7 (2.4)f −0.1 (34), 216.9 (4.8)35.8 (37.7), 72.4 (4.7), 245.2 (4.0)g 3.8 (37.1), 233 (5.5)34.8 (40.5), 61.3 (3.2), 240.5 (2.2)h 3.8 (37), 215.3 (4.1)

nd: not determined.a Heating 10◦/min.b Heating 2.5◦/min, cooling 2◦/min.c Heating 0.8◦/min.d Heating 5◦/min, cooling 0.3◦/min.e Heating 0.2◦/min.f ◦

iotdip

Fu

Heating and cooling 20 /min.g Heating and cooling 12◦/min.h Heating and cooling 10◦/min.

On the other hand, the polyimide PI had a temperature ofsotropisation higher than 240 ◦C and also a broad thermal rangef mesophase (Table 1). Differences in liquid crystalline proper-ies of DA and PI are clearly observed. The presence of perylene

iimide subunit in PI caused increases the value of temperature of

sotropisation in comparison with DA. DSC thermograms of theolyimide PI are shown in Fig. 1.

ig. 1. DSC traces of the polyimide PI at heating and cooling rate 20, 12 and 10◦/minnder N2 atmosphere (a) from −25 to 300 ◦C and (b) inset from 160 to 300 ◦C.

etals 161 (2011) 1660– 1670

Phase behavior of DA and PI was investigated additionally bypolarized optical microscopy (POM). Our preliminary observationssuggested that diamine DA and polyimide PI formed smectic A(SmA) mesophase with focal conic texture. Photomicrographs ofthe optical textures of mesophases obtained for DA and PI are pre-sented in Fig. 2.

Our previous study [26] showed that polymer P2A (cf.Scheme 1b) exhibited three quite narrow liquid crystal phases M1,M2 and M3 in the range of 265–170 ◦C, while P2B exhibited onlyone mesophase. Similarly as for DA and PI, being the subjects of thiswork, a SmA mesophase with focal conic texture was observed forP2B [26]. Our present study showed that the introduction of pery-lene diimide moieties in PI does not change the kind of mesophasein comparison with P2B, which contains only naphthalene diimidesub-units.

Additionally, the influence of various heating and cooling cyclesrates on LC properties of diamine DA and polyimide PI were inves-tigated by DSC. DSC traces of the DA during various heating rate(10◦/min, 5◦/min 0.8◦/min) showed one main peak, two peaks orthree peaks, respectively. Decreasing the value of heating rate from0.8 to 0.2◦/min. did not caused significant changes in DSC traces ofDA (cf. Table 1 and Supporting information).

DSC traces of PI during heating of the sample at 20◦/min exhib-ited two peaks at 40.7 ◦C and 249.7 ◦C, while during heating of thePI at 12◦/min and 10◦/min, three peaks were found (cf. Table 1).DSC trace of the PI during cooling showed two peaks (cf. Table 1and Fig. 1).

Our previous study [26] showed that, contrary to expectations,the P3B polymer with perylene diimide units did not exhibit liq-uid crystalline properties. On the other hand, the P2A and P2Bpolyimides exhibited LC behavior. Temperature of isotropisationof PI (∼240 ◦C) was similar to the clearing point of P2B (∼236 ◦C).Polyimide P2A, having the shortest aliphatic chain (cf. Scheme 1b)exhibited a higher value of temperature of isotropisation(∼283 ◦C).

3.3. UV–vis absorption properties

The UV–vis spectra of the DA monomer and PI polymer weredetermined both in solution and in the solid state as blend withpoly(methyl methacrylate) (PMMA) and their absorption spectraldata are summarized in Table 2. UV–vis absorption spectra of theDA and PI recorded in chloroform, NMP and in solid state as blendwith PMMA are presented in Fig. 3 and in Supporting information.

In solution and in solid state diamine and polyimide exhibitedthree absorption bands (cf. Table 2). The latter two bands in PI,having a absorption maxima (�max) at 484 and 522 nm, respec-tively, are the characteristic for perylene diimide, while the peakat about 454 nm corresponds to the absorption of naphthalenediimide [33]. In DA, the �max positions were observed below 400 nmand are attributed to naphthalene diimide [33]. The solvatochromicbehavior of DA and PI in two solvents with a different dielec-tric constant (ε) and, consequently, with a different polarity, wasinvestigated. UV–vis spectra were recorded in NMP (ε = 33.0) andchloroform (ε = 4.81), the results are demonstrated in Fig. 3 andTable 2. Despite the different polarity of the solvents, the spectrashowed a �max position at almost the same wavelength. Therefore,the solvatochromic effect was not observed in the studied com-pounds. Moreover, no shift was found along with the change fromsolution to solid state, therefore no H nor J aggregates existed inthe analyzed compounds in solid state. The absorption correspond-

ing to the naphthalene unit was shifted hypsochromically in thediamine DA compared to the polyimide PI. The molar absorptioncoefficient of the polyimide PI is much higher compared to that ofdiamine (cf. Table 2).

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 1660– 1670 1665

F aminea e 300

sas

TS

ig. 2. Polarized light optical photomicrographs (cross-polarization position) of dit 282.9 ◦C, (d) at 280 ◦C, (e) at 250.1 ◦C (after cooling from the isotropic state (abov

Our previous work showed that P2A and P2B polymers in NMPolution exhibited two strong absorption peaks at 361, 381 nm and

hump at 345 nm. In the case of P3B, a similar shape of UV–vispectrum was found, however, the maximum of the band was about

able 2ummary of UV–vis experiments. For DA c = 1 × 10−4 mol/l, for PI c = 5 × 10−6 mol/l.

Code UV–vis

NMP

�max [nm] ε [l mol−1 cm−1]

DA 343; 360; 381 1690; 2139; 2181

PI 454; 484; 522 27140; 42246; 46930

a Blend with PMMA.

DA: (a) at 30 ◦C, (b) at 23.1 ◦C (after cooling from the isotropic state) and of PI: (c)◦C)) and (f) at 30 ◦C (beginning of crystallization).

100 nm bathochromically shifted in comparison with P2B (489,524 nm and a hump at 461 nm) [26]. Compared to PI, being thesubject of this work, no shift in a �max position characteristic fornaphthalene and perylene diimide (484, 522 nm) was detected.

CHCl3 In solid statea

�max [nm] ε [l mol−1 cm−1]

342; 360; 380 2263; 3269; 3532 342; 359; 380457; 489; 525 13642; 29189; 45434 452; 487; 513

1666 E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 1660– 1670

7006005004003000,0

0,1

0,2

0,3

0,4CHCl3 UV-Vis

PL e x 34 1nm

Wawelength [nm]

Abs

orba

nce

[a.u

.]

0

20

40

60

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PL intensity [a.u.]

7006506005505004504003500,00

0,05

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UV -Vis

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Abs

orba

nce

[a.u

.]

0

50

100

150

200

250CHCl3

PL e x 49 0nm

PL intensity [a.u.]

(a) (b )

Fig. 3. Absorption (straight line) and emission (dotted line) spectra of (a) diamine DA under 341 nm excitation wavelength and (b) polyimide PI under 490 nm excitationwavelength in CHCl3 solution.

Table 3Photoluminescence of DA in solution (NMP, CHCl3) and in solid state (blended with PMMA) under different excitation wavelengths.

Code �exc. [nm] �PLmax [nm]

In NMP (ε = 33.00)a In CHCl3 (ε = 4.81)a Blend

0–0 0–1 0–2 0–0 0–1 0–2 0–0 0–1

DA c = 1 × 10−3 mol/l341 408 515 589 489 526 – 470 –360 491 518 587 433 481 526 470 505490 584 nd nd 536 nd nd 535 –c = 1 × 10−4 mol/l341 407 513 586 491 – – –360 489 516 583 431 480 518 –c = 1 × 10−5 mol/l341 414 481 586 478 – – –360 485 524 582 433 475 520 –

n

3

s(ap

dtptiP

PdD

TP

d: not determined.a ε Dielectric constant.

.4. Photoluminescence properties

In our work, four strategies are reported for tuning the emis-ion color of DA and PI: (i) changing the excitation wavelength,ii) changing the concentration, (iii) changing the solvent polaritynd (iv) blending a light emitting compound with a nonemissiveolymer (PMMA).

The main chain flexible spacer in the PI polyimide and the DAiamine acts as an inert barrier that isolates the emissive cen-er [34], which is important in the case of PPIs. The majority oferylene-containing polymers exhibit strong fluorescence in solu-ion whereas only weak emission is observed in the solid state. Thiss due to the high content of perylene chromophore which leads toL quenching through aggregation [19].

The emission spectra of diamine DA (Table 3) and polyimideI (Table 4) were recorded in NMP and chloroform solution withifferent excitation wavelengths, that is, at 341, 360 and 490 nm forA and 341, 490 and 521 nm for PI. On the other hand, the recording

able 4hotoluminescence of PI in solution (NMP, CHCl3) and in solid state (blended with PMMA

Code �exc. [nm] �PLmax [nm]

In NMPa (ε = 33.00)b

0–0 0–1

PI 341 536 577

490 537 577

521 536 577

a c = 5.5 × 10−6 mol/l.b ε Dielectric constant.

the PL spectra of DA and PI in NMP and chloroform solutions, that is,solvents that differed in polarity led to investigation of the solventeffect on PL properties.

Fig. 3 and figures presented in Supporting information showthe absorption and emission spectra of the DA and PI in NMP andchloroform solution under different excitation wavelengths.

Photoluminescence properties of diamine DA and polyimidePI were investigated under excitation in maximum of absorptionbands (see Tables 3 and 4).

The emission spectra were recorded (Tables 3 and 4) for thediamine DA and polymer PI in NMP and chloroform solutions byexciting at both the naphthalene absorption �max as well as at theperylene absorption �max. PI spectra in both solvents under 490and 521 nm excitation wavelength show spectral overlap of the

naphthalene emission with that of the perylene absorption(Fig. 3 and Supporting information). On the contrary, absorptionand emission spectra of DA in NMP and chloroform solu-tion under 341, 360 and 490 nm excitation wavelength do not

) under different excitation wavelengths.

In CHCl3a (ε = 4.81)b Blend

0–0 0–1 0–0 0–1

534 575 527 561534 575 527 568534 575 527 568

E. Schab-Balcerzak et al. / Synthetic M

8007006005004000

50

100

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L in

tesi

ty [

a.u.

]

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(a)

(b)

(c)

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.u.]

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.u.]

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NMP ex 52 1 nm NMP ex 49 0 nm CHC l

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CHC l3 ex 490 nm

N

O

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NMP

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F −3

(s

oi

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ig. 4. PL under different excitation wavelengths: (a) DA in NMP (c = 1 × 10 mol/l),b) DA in chloroform solution (c = 1 × 10−3 mol/l) and (c) PI in NMP and chloroformolution (c = 5.5 × 10−6 mol/l).

verlap with the absorption spectra (Fig. 3 and Supportingnformation).

.4.1. Effect of excitation wavelength on PLThe emission maxima for 1 × 10−3 mol/l of DA and for

.5 × 10−6 mol/l of PI in NMP and chloroform solution underifferent excitation wavelengths are presented in Fig. 4 andables 3 and 4.

First, along with an increase of excitation wavelength (from 490o 521 nm), an increase of the relative intensity of the photolu-

inescence of PI in NMP and chloroform solution was observedFig. 4c). For diamine DA, depending on the kind of solvent, a dif-

etals 161 (2011) 1660– 1670 1667

ferent behavior was found (cf. Fig. 4a and b). In NMP solution, alongwith increase of excitation wavelength (from 341 to 490 nm) adecrease of the relative intensity of the photoluminescence of DAwas noted (Fig. 4a). An opposite behavior was observed for DA inchloroform solution (Fig. 4b).

No changes in the PL maximum of polyimide PI along withincrease the excitation wavelength in both solvents were found(Table 4 and Fig. 4c). For diamine DA, along with an increase ofexcitation wavelength the photoluminescence spectra showed adifferent behavior (Table 3 and Fig. 4a and b). In NMP and chlo-roform solution the emission spectrum of DA under excitationwavelength 360 nm exhibited a similar behavior. Differences in PLof DA were found under 341 and 490 nm excitation wavelength(Fig. 4a and b). The emission spectra of the polyimide PI havingnaphthalene and perylene units in the main chain are independentof the excitation wavelength while PL spectra of diamine DA hav-ing naphthalene unit and NH2 groups are more sensitive to theexcitation wavelength.

3.4.2. Effect of concentration on PLEffect of concentration on photoluminescence in NMP and

chloroform solution is exemplified by DA and shows significantspectral changes in the emission band and PL intensity withincrease of the diamine concentration (Table 3, and Supportinginformation). Concentration of DA in solution was 1 × 10−3 mol/land 1 × 10−4 mol/l.

With the decrease of DA concentration, the PL spectra showchanges in PL maximum and PL intensity (cf. Table 3, andSupporting information). With the decrease of DA concentrationfrom 1 × 10−3 mol/l to 1 × 10−5 mol/l a decrease of the relativeemission intensity in both solvents was observed (Supportinginformation). The lowest PL intensity of DA at 1 × 10−5 mol/l wasdetected. PL data of DA in dependence on the concentration in NMPand chloroform solution reveal a small modulation in the positionof the maximum of the emission bands (cf. Table 3 and Supportinginformation).

The first peak in PL spectra reflects the 0–0 transition, second the0–1 transition and the third the 0–2 transition of DA. In the emissionspectra of DA under excitation at 360 nm along with increase theconcentration three kinds of transitions (0–0, 0–1 and 0–2) weredetected (Table 3). A similar behavior for DA in NMP solution underexcitation 341 nm was observed (Table 3).

When 1 × 10−3 mol/l concentration of DA in chloroform solutionwas used at 341 nm excitation the emission spectra were resolvedinto two peaks 489 and 526 nm, which can be attributed to the 0–0,and 0–1 transitions, respectively. At a lower concentration of DA inchloroform (1 × 10−4 mol/l and 1 × 10−5 mol/l) one PL maximumat about 490 nm was observed, which can be attributed to the 0–0transition.

3.4.3. Effect of kind of solvent on PLSolvent polarity and polarizability affect the energy levels of

excited states in the organic compounds [35]. Therefore, both thespectral shape and the position of fluorescence band of the com-pounds should, in principle, be affected. A substantial red shift wasobserved with an increase in solvent polarity and consequently inthe dielectric constant (Tables 3 and 4). This behavior is clearly seenunder 490 nm excitation wavelength. In such a polar aprotic solventas NMP, the maximum of emission was 48 nm bathochromically-shifted in comparison with PL maximum in nonpolar chloroform

solution. This behavior suggested that solvatochromism existed inthe photoluminescence spectra of DA. On the other hand, for theinvestigated polyimide PI with naphthalene and perylene units noinfluence of the kind of solvent was found. This different behavior

1 hetic Metals 161 (2011) 1660– 1670

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70000

80000

90000

100000

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PL in

tens

ity [C

PS]

Wavelength [nm]

ex 341 nm ex 360 nm ex 490 nm

(a)

500 550 600 6500

100000

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300000

400000

500000

600000

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PS]

Wavelength [nm]

ex 341 nm ex 490 nm ex 521 nm

(b)

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668 E. Schab-Balcerzak et al. / Synt

uggested that probably intermolecular interactions in solution ofiamine and polyimide are different.

.4.4. Relative PL intensityRelative PL intensity of the DA and PI was estimated in NMP solu-

ion in relation to rhodamine-B used as a standard. The emissionpectra of rhodamine-B were recorded under the same experi-ental conditions as for investigated diamine and polyimide, that

s, the same concentration (5.5 × 10−6 mol/l) and excitation wave-ength. In the case of DA and PI the emission band at the lowernergy region was taken into account (for DA �em = 589 nm, for PIem = 577 nm). Relative photoluminescence intensity of the DA (forem = 589 nm) was found about 50% upon excitation at 341 nm inelation to emission intensity of rhodamine-B under the same exci-ation wavelength, concentration and solution. For PI polyimide,elative PL intensity was found in the range of 17–157% in relationo the standard (cf. Supporting information). Polyimide showed theowest PL intensity (17%) under excitation equal 341 nm, whereas,nder excitation 490 and 521 nm exhibited higher intensity in com-arison with rhodamine-B.

.4.5. PL in solid stateAdditionally, we have studied the emission spectra of free trans-

arent foils obtained by casting NMP solution of diamine DA andolyimide PI with a nonemissive polymer, PMMA, under differentxcitation wavelengths (cf. Tables 3 and 4). Concentration of PI (DA)n PMMA blend was 1%.

With the increase of excitation wavelength, the PL spectra ofA and PI blends show a red shift. For diamine DA in solid state

under 341 and 360 nm) peaks at 470 and 535 nm reflect the 0–0ransition, while the one at 505 nm corresponds to the 0–1 transi-ion. Along with an increase in excitation wavelength to 490 nm, aathochromic shift of about 65 nm was observed (Table 3). On thether hand, the emission spectra of PI in solid state under differ-nt excitation wavelength were similar and were resolved into twoeaks 527 and 568 nm, which can be attributed to the 0–0 and 0–1ransition, respectively (cf. Table 4).

Emission spectra of DA and PI in solution are red shifted inomparison with PL spectra in solid state. PL of DA and PI in solidtate (blended with PMMA) under different excitation wavelengthscasted from NMP solution) are presented in Fig. 5.

.5. Electrochemical properties

Because of the high solubility of DA and PI in typical solventssed in electrochemical investigations, it was impossible to mea-ure the electrochemical characteristics of a thin film. Therefore,he electrochemical properties of DA and PI were studied in ace-onitrile solution, using differential pulse voltammetry (Fig. 6 andable 5) [36,37].

The first reduction peak of both materials appears at −0.99 Vnd is attributed to first one-electron reduction of naphtal-nediimide (and, in the case of PI, perylenediimide as well)

oieties [38,39]. For PI, a second reduction peak was found

t −1.10 V, corresponding to the second reduction peak of theerylenediimide (E1/2 is found at about −1.18 V for N,N′-diarylerivatives [39] and −1.27 for N,N′-di-sec-butylperylenediimide

able 5edox potentials, HOMO and LUMO energies and electrochemical band gaps (Eg) of DA an

Code EHOMO [eV] ELUMO [eV] Eg [eV] Ered1 [V]

DA −5.44 −3.81 1.63 −0.99

PI −6.03 −3.81 2.22 −0.99

red, Eox: reduction and oxidation peak potentials determined by DPV; EHOMO = −4.8 −1.19 Ered − 4.7851; E′

g = E′LUMO − E′

HOMO.

Fig. 5. PL in solid state (blended with PMMA) under different excitation wave-lengths: (a) DA and (b) PI (cast from NMP solution).

[38]). The second reduction peak of the naphtalenediimide moi-eties (which should be located at about −1.4 to −1.5 V vs. Fc)was not detected, because the potential window was limited bythe reduction wave of water absorbed by the polyether blockspresent in the structure of DA and PI. The presence of water waslikely to be responsible for the shift of the second reduction peakof PI to less negative potentials. DA is oxidized at 0.64 V, due tothe presence of amino groups (the same value of oxidation peakpotential was obtained by us for another diamino derivative ofnaphthalenediimide [40]), the electrochemical Eg is 1.63 eV. Theoxidation peak potential of PI equals to 1.23 V (a value similar tothat obtained by Bard group for N,N′-diarylperylenediimides [39]).The electrochemical band gap of PI is 2.22 eV, comparable to thatof N,N′-diarylperylenediimides (2.1–2.2 eV) [39], and similar to theoptical Eg of various N,N′-disubstituted perylenediimides (2.3 eV)[41,42], but much smaller than the Eg of naphthalenediimides

(3.2 eV) [43]. Optical energy band gap of PI (Eopt.

g = 2.2 eV) was cal-culated additionally from absorption spectrum using the equationEopt.

g = 1240/� and is in agreement with the value determined fromDPV measurements. On the other hand, the optical energy band

d PI.

Ered2 [V] Eox1 [V] E′LUMO [eV] E′

HOMO [eV] E′g [eV]

– 0.64 −3.60 −5.50 1.90−1.10 1.23 −3.60 −6.32 2.72

Eox; ELUMO = −4.8 − Ered; Eg = ELUMO − EHOMO; E′HOMO = −1.4 Eox − 4.650; E′

LUMO =

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 1660– 1670 1669

F ne) anF id line

gdogoepacha[aooiE[(

l

aTai

Fc

ig. 6. (a) Differential pulse voltammograms of acetonitrile solutions of PI (solid lic. (b) HOMO–LUMO energy levels and electrochemical band gaps of DA and PI (sol

ap of DA was definitively higher (Eopt.g = 3.1 eV) than the value

etermined from DPV measurements (not surprisingly, because thexidation potential of DA is related to the oxidation of the aminoroups and not the diimide moiety) and similar to band gaps ofther naphtalenediimides [43]. Frontier orbital energies (or rather,lectron affinity EA and ionization potential IP) have been com-uted from the electrochemical data (assuming EHOMO = −4.8 − Eox

nd ELUMO = −4.8 − Ered) and are given in Table 5. This method ofalculation of EA and IP (and also Eg) is still widely applied, butas been criticized for its lack of relevance to experimental EAsnd IPs determined by photoelectron spectroscopy. D’Andrade et al.36] and Djurovich et al. [37] compared the IPs and EAs (from UPSnd IPES measurements) with redox potentials (measured in DMFr acetonitrile solutions), for a range of compounds relevant torganic electronics. For IPs (hereafter denoted E′

HOMO), a follow-ng fit has been found: E′

HOMO = −1.4 Eox − 4.6 [36], while for theAs (E′

LUMO) the slope is closer to unity: E′LUMO = −1.19Ered − 4.78

37]. Furthermore, the relation between the ‘transport energy gap’E′

g = EA − IP = E′LUMO − E′

HOMO) and the optical band gap is as fol-

ows: E′g = 1.39 Eopt.

g = −0.46 [37].Therefore it seemed interesting to convert our electrochemical

nd optical results to the absolute scale using these equations (cf.

able 5). The transport band gap of PI calculated from DPV is 2.72 eVnd is similar to the E′

g calculated from Eopt.g (2.60 eV) and the exper-

mental E′g determined for N,N′-disubstituted perylenediimides (ca.

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

1E-4

Cu

rre

nt [µ

A]

Voltage [V]

PI

DA

ig. 7. Rectification behavior of ITO/compound/Al devices, plotted semilogarithmi-ally.

d DA (dashed line): reduction (down) and oxidation (up), the peak at 0 V is due to – calculated as −4.8 – E1/2, dashed line – calculated from fits to UPS/IPES).

2.65 eV) [37]. Moreover, the “corrected” E′HOMO value (−6.32 eV) is

much closer to the UPS result (−6.6 eV ± 0.2 eV) than the “ferrocenescale” EHOMO (−6.03 eV).

3.6. Current–voltage measurement

Devices with the following architecture ITO/DA/Al and ITO/PI/Alwere constructed and investigated. Indium-tin oxide (ITO)-coatedglass was applied as the bottom electrode and was coated with DAor PI, which served as the active layer. Aluminum was thermallydeposited in vacuum (∼10−6 Torr) as the top electrode. The elec-trical behavior of devices measured under forward bias follows acharacteristic pattern shown in Supporting information.

Differences between device with DA or PI used as an activelayer were observed. For example, the turn-on voltage of the deviceITO/DA/Al was observed at about 6 V, while for device ITO/PI/Althe current started to increase exponentially with voltage at about1.5 V. Moreover, the current varied by more than 100% of magni-tude between devices. Additionally, reverse bias behavior of thedevices was studied and is presented in Fig. 7.

As shown in Fig. 7, devices ITO/compound/Al exhibited differentreverse bias behavior in the range of −6 to 12 V. In the case of devicewith PI used as an active layer a rectifying effect was observed [44].

4. Conclusion

A new diamine DA containing the naphtalenediimide moietyand an alternating conjugated–non-conjugated polyimide PI inwhich naphthalene and perylene diimide blocks are interspersedwith oligopropyloether segments have been synthesized. Bothcompounds exhibit liquid crystalline properties and produce asmectic A mesophase. Moreover, the diamine and the polyimideshowed good thermal stabilities (T5% > 340 ◦C). The optical, that is,UV–vis absorption and emission, properties of the polyimide as wellas the diamine were extensively examined in solution and in solidstate. The influence of excitation wavelength, concentration andkind of solvent on PL properties was investigated. Diamine DA andpolyimide PI can be used as green emitters. Relative photolumi-nescence intensity of the DA and PI investigated in NMP solutionwere found to be about 50% and in the range of 17–157% dependingon the excitation wavelength, respectively, as compared with theemission of rhodamine-B used as a standard. The perylene moietiesin PI was less effective to reduce the electrochemical energy band

gap than the amino groups in DA (electrochemical Eg PI = 2.22 eV, Eg

DA = 1.63 eV). The investigated compounds exhibited low electro-chemical band gap being promising for optoelectronic applications.Preliminary investigations of the electrical characteristics (I–V) of

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he diamine DA and polyimide PI for device ITO/compound/Al con-rmed their semiconductor nature.

cknowledgements

The authors would like to thank to Huntsman Corporationor polyetheramine – Jeffamine ED 2003. M.K. acknowledges thenancial support of the Foundation for Polish Science (START schol-rship, 2009–2010).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.synthmet.2011.05.036.

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