New naphthalene diimide-based compounds containing triarylamine units and imine linkages: Thermal, optical and electrochemical properties

Synthetic Metals 161 (2011) 2268–2279

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New naphthalene diimide-based compounds containing triarylamine units and imine linkages: Thermal, optical and electrochemical properties Ewa Schab-Balcerzak a,b,∗ , Marzena Grucela-Zajac a , Michal Krompiec a , Henryk Janeczek b , Mariola Siwy b , Danuta Sek b a b

Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland

a r t i c l e

i n f o

Article history: Received 4 April 2011 Received in revised form 18 August 2011 Accepted 23 August 2011 Available online 29 September 2011 Keywords: Polyimides Polynaphthaleneimides Azomethines Electrochemistry Photoluminescence

a b s t r a c t Two novel poly(azomethinenaphthaleneimide)s (poly(AZ-NI)s) and azomethine-naphthalene diimide (AZ-NI) consisting of electron-donating triarylamine with imine linkages and electron-accepting naphthalene diimide moieties were prepared via condensation of N,N -bis(4-amino-2,3,5,6tetramethylphenyl)naphthalene-1,4,5,8-dicarboxyimide (DANDI) with 4-formyltriphenylamine, 4,4 -diformyltriphenylamine and 4,4 ,4 -triformyltriphenylamine. The thermal degradation kinetics of obtained compounds was studied by TGA. The activation energy (Ea ) of thermal decomposition process was estimated by the first order Coats–Redfern equation and was in the range 115.1–266.7 kJ/mol. Poly(AZ-NI)s and AZ-NI exhibited useful levels of thermal stability, their 5% weight-loss temperatures were above 350 ◦ C. Optical properties of the prepared compounds were investigated by UV–vis and photoluminescence (PL) measurements. The obtained alternating donor–acceptor compounds emitted mainly blue light. The electrochemical behavior of poly(AZ-NI)s, AZ-NI and DANDI was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). As calculated from CV, the electrochemical energy band gap (Eg ) of the linear polymer was equal to 1.15 eV, but the Eg of the branched one was lower: 1.06 eV, whereas the Eg of AZ-NI and DANDI was 1.32 and 0.47 eV, respectively. For the first time, to the best of our knowledge, polynaphthaleneimides with triphenylamine units and azomethine linkages have been described in this article. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Electronic and optoelectronic devices based on organic compounds due to their low costs, simple packaging and compatibility with a flexible substrate have recently attracted considerable attention as an alternative to conventional inorganic devices [1]. In recent years, intense research efforts have been focused on the development of new charge-transport polymers with either hole, that is, electron-donating (D) or electron transporting, that is, electron-accepting (A) properties, or with both properties in one polymer chain [2]. By modifying the donor and/or acceptor moieties, the properties of the materials can be readily changed. Although impressive progress has been done in developing electron-rich character (p-type) organic materials, the pursuit

∗ Corresponding author at: Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland; Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland. E-mail addresses: [email protected], [email protected] (E. Schab-Balcerzak). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.032

for semiconductors capable of electron transport (n-type), still lagged behind that of p-type systems [3]. Among organic n-type semiconductors, compounds consisting of electron deficient aromatic imide rings are particularly interesting. Polyimides (PIs) play a key role as materials for many applications ranging from dielectric films for the electronic industry and orientation layers liquid crystal industry, lightweight load-bearing heat-resistant composites and adhesives for the aerospace industry, to gas separation membranes due to their outstanding thermal, mechanical, electrical and chemical resistance properties [4,5]. Their outstanding properties have inspired many researchers to modify, study and utilize polyimides and diimides in the field of optoelectronics, with respect to their electron-accepting ability and possibility of creation of a A–D structure. Typical aromatic PIs consist of alternating electron acceptor diimide fragments and electron donor arylene residues of starting diamines [6]. Photoelectrical, optical and other electronic properties of PIs are determined by the electron donor–acceptor interaction in these polymers. Electron donor and acceptor moieties within a repeating unit contribute to electronic transition between the ground and excited states, which could be manipulated by the induced charge transfer (CT) from donor (D) to acceptor

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(A) under applied electric fields [7]. Variation of the chemical structure and molecular properties of A and D chain fragments permits to change these properties in wide range. Aromatic polyimides are potentially redox-active polymers, based on the capability of reversible electrochemical reduction of imides into corresponding anion radicals localized on the electron deficient rings [8]. Polyimides, taking into account the imide unit structure, are divided into two kinds, namely compounds with five- and six-membered imide rings. Among the varieties of polyimides investigated, those with six-membered imide rings have aroused substantial interest in recent years. Polymers with six-membered imide rings, that is, polynaphthaleneimides (PNIs) and polyperyleneimides (PPIs) have been found to demonstrate improved thermal and chemical resistance when compared with conventional five-membered ring phthalic polyimides [9]. PNIs and PPIs are investigated as liquid crystal alignment layers [10], luminescent materials [11–14], in light-emitting electrochemical cells (LEC) [15] or as electrochromic materials [16]. PPIs have recently emerged as a new class of n-type polymers for application in polymer solar cells [17]. On the other hand, interesting, as hole transporting materials, are compounds with triphenylamine (TPA) and/or azomethine groups. TPA is a unique molecule with useful functions such as redox activity, fluorescence, and ferromagnetism due to the high oxidazability of the nitrogen centre [18]. It is one of the most frequently used of hole transporting structures, being applied as building unit of small molecules, dendrimers, starlike compounds [19] and also polymers [20–26]. Five-membered polyimides with triphenylamine moieties have been investigated as electrochromic materials [27–29], for application in light emitting devices (OLED) [14,30], for dynamic random access memories (DRAMs) [7,31], for photovoltaic devices [6,32] and as polymeric nonlinear optical (NLO) materials [33]. However, a literature survey revealed that PNIs and PPIs containing triarylamine derivatives are rather seldom investigated. To the best of our knowledge, there are only three examples in which polymers with six-membered imide rings contain triphenylamine units [14,34,35]. Bai et al. described photoconductive properties of copolyimides obtained from 3,4,9,10-perylene tetracarboxylic dianhydride, 4,4 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4,4 -diaminotriphenylamine [34]. Yoon et al. reported PPIs synthesized from modified 4,4 -diaminotriphenylamine and for these polymers photoluminescence and electrochemical properties were studied [14]. On the other hand, Zhang et al. investigated a series of copolynphthaleneimides from 4,4 -diaminotriphenylamine and 4,4 -diaminodiphenyl ether-2,2 disulfonic acid as proton exchange membranes [35]. As it was previously mentioned, another promising hole conductors are compounds with imine linkages, namely polyazomethines (PAZs). PAZs due to the presence of C N group in their structure, which is isolelectronic with C C group, are considered as conjugated compounds with attractive electronic, optoelectronic and thermal properties which can be exploited for various applications [36]. PAs containing triphenylamine structure were synthesized from 4,4 -diaminotriphenylamine [37,38] and 4,4 diformyltriphenylamine [18,39] and their photoluminescence, electrochemical and electrical properties were studied. Inspired by the findings described above we have undertaken a preparation and investigation of new compounds, namely a model compound and polymers, which contain naphthalene diimide, triphenylamine and azomethine units in their structure. These compounds constitute a promising family of materials whose properties can be explored in various types of devices. Taking into account the literature related to the idea of preparation of compounds with both imine and six-membered diimide moieties, apart of our previous work [40], only one article reports

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on polymers which repeating units consist of the aforementioned subunits [41]. However, as far we are aware, compounds containing electron-donating triphenylamine, azomethine and electron-accepting naphthalene diimide units being alternately built into model compound and polymer chains, have not been published yet. This paper is devoted to the synthesis of a new azomethinenaphthalene diimide and poly(azomethinenaphthaleneimide)s and their physical properties such as: optical (UV–vis, PL), electrochemical (CV, DPV) and thermal (DSC, TGA) were investigated. Moreover, the thermal degradation kinetics of obtained compounds was studied by thermogravimetric analysis (TGA). In this paper, Coats–Redfern method was used to determine the activation energies and pre-exponential factor for decomposition in nitrogen. 2. Experimental details 2.1. Materials 4-Formyltriphenylamine, 4,4 -diformyltriphenylamine, 4,4 ,4 triformyltriphenylamine, p-toluenesulfonic acid (PTS), N,N-dimethylacetamide (DMA), N-methyl-2-pyrrolidinone (NMP), chloroform were purchased from Aldrich Chemical Co. and used as received. N,N -bis(4-amino-2,3,5,6-tetramethylphenyl) naphthalene-1,4,5,8-dicarboxyimide (DANDI) was obtained according to the procedure described in our previous paper [40]. 2.2. Synthesis of azomethine-naphthalene diimide (AZ-NI) Diamine DANDI (0.25 mmol, 0.1402 g), monoaldehyde: 4formyltriphenylamine (0.5 mmol, 0.1409 g) and a pinch of p-toluenesulfonic acid (PTS) 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 filtration and washed with methanol, acetone and then hot methanol. The resulting precipitate was dried at 100 ◦ C in vacuum. Yield: 31%. 1 H NMR (CDCl , ı, ppm): 2.06 (s, CH , 12H), 2.15 (s, CH , 12H), 3 3 3 7.12 (d, 8H), 7.19 (d, 12H), 7.32 (d, 4H), 7.79 (s, 4H), 8.13 (s, CH N, 2H), 8.89 (s, 4H). FTIR (KBr, cm−1 ) : 2922, 2859 (C-H aliphatic), 1713, 1679 (C O imide stretch), 1631 (CH N), 769(imide ring deformation). Anal. Calcd. For (C72 H58 N6 O4 ) (1071.27): C 80.72, H 5.46, N 7.85: Found C 78.14, H 5.81, N 7.97. Tm = 422 ◦ C. 2.3. Synthesis of poly(azomethinenaphthaleneimide)s (poly(AZ-NI)s) Diamine DANDI (0.25 mmol, 0.1402 g), dialdehyde: 4,4 -diformyltriphenylamine (0.5 mmol, 0.1507 g) or DANDI (0.47 mmol, 0.2634 g), trialdehyde: 4,4 ,4 triformyltriphenylamine (0.25 mmol, 0.0822 g) and a pinch of p-toluenesulfonic acid (PTS) 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 filtration and washed with methanol, acetone and then hot methanol. The resulting precipitate was dried at 100 ◦ C in vacuum. DANDI condensed with 4,4 -diformyltriphenylamine resulted in polymer poly(AZ-NI)-I while DANDI condensed with tris(4-formylphenyl)amine gave polymer poly(AZ-NI)-II. Yields of polymers: poly(AZ-NI)-I: 71%, poly(AZ-NI)-II: 25%. Poly(AZ-NI)-I: 1 H NMR (CDCl3 , ı, ppm): 2.08 (d, CH3 , 12H), 2.15 (s, CH3 , 12H), 3.80 (s, NH2 , 2H), 7.39 (d, 4H), 7.75 (t, 2H), 7.87 (d, 4H), 8.12 (s, CH N, 2H), 8.19 (s, 3H), 8.89 (d, 4H), 9.88 (s, CHO, H). FTIR (KBr, cm−1 ) : 2922, 2855 (C-H aliphatic), 1713, 1676 (C O imide stretch), 1634 (CH N), 1583 (NH2 ), 770 (imide ring deformation). Anal. Calcd. For (C54 H43 N5 O4 )n (825.95): C 78.53, H 5.25, N 7.75: Found C 77.10, H 5.68, N 8.47.

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Poly(AZ-NI)-II: 1 H NMR (CDCl3 , ı, ppm): 2.05 (d, CH3 , 12H), 2.15 (s, CH3 , 12H), 3.88 (s, NH2 , 2H), 7.33 (d, 4H), 7.85 (t, 4H), 7.95 (d, 2H), 8.14 (s, CH N, 2H), 8.23 (s, 2H), 8.88 (s, 4H), 9.93 (s, CHO, H). FTIR (KBr, cm−1 ) : 2922, 2854 (C-H aliphatic), 1712, 1675 (C O imide stretch), 1631 (CH N), 1583 (NH2 ), 770 (imide ring deformation). Anal. Calcd. For (C90 H72 N9 O8 )n (1407.54): C 76.79, H 5.15, N 8.96: Found C 73.51, H 5.67, N 9.49. 2.4. Blend preparation Blends were obtained by dissolving the desired amount of compounds and PMMA in chloroform to form a homogeneous solution (1%, vv concentration of compound in PMMA). Films cast on glass were dried in vacuum oven at 90 ◦ C over 10 h. 2.5. Characterization Infrared (IR) spectra were acquired on a 560 MAGNA-IR NICOLET Spectrometer using KBr pellets. Proton nuclear magnetic resonance (1 H NMR) spectra were recorded on a Bruker AVANCE 600 MHz spectrometer using chloroform (CDCl3 ) as solvents and TMS as the internal standard. Elemental analyses were performed using Perkin Elmer Analyzer 2400. Thermogravimetric analysis (TGA) was done with a TGA/DSC1 Mettler-Toledo thermal analyzer with a heating rate of 10 ◦ C min−1 in a stream of nitrogen (60 cm3 min−1 ). Differential scanning calorimetry was performed with a TA-DSC 2010 apparatus (TA Instruments, Newcastle, DE, USA), under nitrogen using aluminum sample pans, at heating/cooling rate of 20 ◦ C min−1 . UV–vis absorption spectra were recorded in solution using a Perkin Elmer Lambda Bio 40 UV-VIS spectrometer. Photoluminescence spectra were measured using a Varian Carry Eclipse spectrometer. Electrochemical measurements were carried out using Eco Chemie Autolab PGSTAT128n potentiostat, using platinum wire (diam. 1 mm), platinum coil and silver wire as working, auxiliary and reference electrode, respectively. Potentials are referenced with respect to ferrocene (Fc), which was used as the internal standard. Cyclic and differential pulse voltammetry experiments were conducted in a standard one-compartment cell, in dichloromethane (Carlo Erba, HPLC grade), under argon. 0.2 M Bu4 NPF6 (Aldrich, 99%) was used as the supporting electrolyte. 3. Results and discussion In this article, we continue our efforts in synthesis of six-membered polyimides, new alternated polymers consist of electron-accepting naphthalene diimide and electron-donating triphenylamine diimine units along with model compound are presented. 3.1. Synthesis and characterization The new poly(azomethinenaphthaleneimide)s and the model compound, that is, the azomethine-naphthalene diimide were prepared from the diamine-containing naphthalene diimide unit: N,N -bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene1,4,5,8-dicarboxyimide (DANDI), described for the first time in our previous work [40] and aldehydes with triphenylamine moiety. The model compound and polymers denoted as AZ-NI and poly(AZ-NI)s, respectively, were obtained in a one step condensation reaction between DANDI and 4-formyltriphenylamine, 4,4 -diformyltriphenylamine and 4,4 ,4 -triformyltriphenylamine in DMA in the presence of a catalytic amount of PTS. The polycondensation of DANDI with 4,4 -diformyltriphenylamine and tris-(4-formylphenyl)amine gave linear and branched poly(azomethinenaphthaleneimide), respectively. The synthetic

route and chemical structure of the obtained compounds are presented in Fig. 1. The formation of AZ-NI and poly(AZ-NI)s was confirmed by IR, 1 H NMR and elemental analysis. Typical IR spectrum of obtained model compounds AZ-NI which was characteristic as well as for the synthesized poly(azomethinenaphthaleneimide)s illustrates Fig. 1 in Supporting information. The IR spectra of the poly(AZ-NI)s and the azomethine-naphthalene diimide presented absorption bands due to the asymmetrical and symmetrical stretching vibrations of the carbonyl group in six-membered imide rings at 1712 and 1670 cm−1 , respectively. Moreover, all compounds exhibited absorption bands at around 1350 cm−1 (C–N stretch), at 1055 and 770 cm−1 (imide ring deformation), and imine bond (CH N) stretching vibration at 1631 cm−1 together with the bands in the range of 2940–2797 cm−1 due to aliphatic groups. Additionally, molecular structure of compounds was identified from their 1 H NMR spectra and 1 H NMR spectra of AZ-NI and poly(AZ-NI)s are depicted in Fig. 2 in Supporting information. In 1 H NMR spectra of model compounds and poly(AZ-NI)s the naphthalic protons appeared at the most downfield and azomethine proton signal was observed at about 8.12 ppm. The spectral data were in accordance with the expected formula. Moreover, the elemental analysis shows good agreement of the calculated and found content of carbon, nitrogen and the hydrogen in the model compound and polymers. However, a deficiency in carbon content of 3.28–1.43% was observed, and this is likely a result of the difficulties in burning these high-temperature compounds, which was also observed for other thermostable polymers [42]. The solubility of the synthesized compounds was qualitatively determined by the dissolution of 2.5 mg of the solid in 1 ml of organic solvent, at room temperature and under heating. Table 1 gives the solubility data of the polyimides, AZ-NI and DANDI in different organic solvents. The model compound and the linear poly(AZ-NI)-I were readily soluble in m-cresol, and even in such a low-boiling-point solvent as chloroform at room temperature. These polyimides were partially soluble in ether-type solvent such as tetrahydrofuran (THF). 3.2. Thermal stability Thermogravimetric analysis (TGA) at a heating rate of 10 ◦ C min−1 in nitrogen atmosphere was utilized to examine thermal properties together with thermal degradation kinetics of the obtained poly(azomethinenaphthaleneimide)s (poly(AZ-NI)s), azomethine-naphthalene diimide (AZ-NI) and also the diamine DANDI. The TGA thermograms of the investigated compounds are given in Fig. 2. Analysis of these curves let to estimate the mass loss associated with the processes of degradation up to 800 ◦ C. The TGA curve of all compounds except for diamine DANDI indicates one main reaction stage, which is reflected in one peak in differential weight loss curve (DTG). Taking into account the results from TGA, it was found that the obtained poly(azomethinenaphthaleneimide)s and azomethine-naphthalene diimide demonstrated good thermal stability without significant weight losses below 300 ◦ C. This implies that no thermal decomposition occurred below this temperature and that the onset decomposition temperature was as high as 350 C in the case of poly(AZ-NI)s. The thermal stability of compounds which was evaluated in terms of 5, 10% weight loss (T5 , T10 ), maximum degradation temperatures (Tmax ) and residual weight at the end of experiment are listed in Table 2. It is clear that the thermal properties of the compounds depended strongly on their structure. Taking into account the chemical constitution of the polymers and model compound used in the present study, one might expect that they should exhibit high thermal stability and the data presented in Table 2 show that these

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 2268–2279 H3 C H 2N

N H 3C

CH3

N

CH3 O

NH2

O H 3C

DANDI

COH

N

HOC

O H3 C

CH3 O

H3 C

N

CH3 O

N

C

O H 3C

N

N H 3C

COH

CH3

N

CH3 O

H N

O H3 C

COH

N

HOC

CH3

2 HOC

H

2271

N

C

CH3

AZ-NI

H 3C H N

C

CH3 O

O H 3C

N

N H 3C

CH3

N

CH3 O

H N

O H 3C

C

n

CH3

poly(AZ-NI)-I H 3C H N

C

O H 3C

N

N H 3C

H

CH3 O

CH3

N

CH3 O

H N

O H 3C

C

n

CH3

poly(AZ-NI)-II

C N

H 3C

CH3

H 3C

CH3

O

N

O

N

O

O

H3 C

CH3

H 3C

CH3 N C

H

Fig. 1. Synthetic route and chemical structure of synthesized compounds.

compounds exhibited good thermal stability. Qualitative characterization of the degradation process illustrated by the 5% weight loss temperature (T5 ), lay within the range 295–415 ◦ C. TGA showed higher thermal stability of polymers comparing to DANDI. Polymers did not loose weight up to 330 and 370 ◦ C while their monomer DANDI started to loose weight about 280 ◦ C. The first derivative of TGA versus temperature (DTG) was used to estimate the maximum decomposition temperature (Tmax ) and it was in the range of 430–445 ◦ C (cf. Table 2) for poly(AZ-NI)s. Furthermore, the residual weight at 800 ◦ C in nitrogen of polymers was in the range of 38–42%. The branched polymer (poly(AZ-NI)-II) obtained from three-functional aldehyde began to decompose at a lower temperature than the linear polymer (poly(AZ-NI)-I) prepared from the dialdehyde. However, the char yield percent at 800 ◦ C was slightly higher of this polymer than in the case of poly(AZ-NI)-I.

Table 1 Solubility behavior of the obtained compounds. Code

Solventsa NMP

THF

CHCl3

CH2 Cl2

m-Cresol

Cyclohexanone

DANDI [40] AZ-NI Poly(AZ-NI)-I Poly(AZ-NI)-II

+ ++ ± ±

± ± ± ±

++ ++ ++ ±

± ++ ± ±

+ ++ ++ ±

+ ++ ± ±

++: soluble at room temperature; +: soluble on heating; ±: partial soluble on heating. Solvents: NMP: N-metyl-2-pyrrolidone; THF: tetrahydrofuran. a The qualitative solubility was tested with 2.5 mg samples in 1 ml of solvent.

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3.3. Thermal degradation kinetics

100 90

Weight [%]

80 70 60 50

poly(AZ-NI)-II poly(AZ-NI)-I

40 30 20

DANDI

10

AZ-NI 100

200

300

400

500

600

700

800

o

Temperature [ C] Fig. 2. Thermogravimetric curves of investigated polymers poly(AZ-NI)s, model compound AZ-NI and diamine DANDI.

Table 2 Thermal behavior data of the diamine DANDI, azomethine-naphthalene diimide and poly(azomethinenaphthaleneimide)s. Code

T5% [◦ C]a

T10% [◦ C]a

Tmax [◦ C]b

Char yield [%]c

DANDI

295

310

18

AZ-NI Poly(AZ-NI)-I Poly(AZ-NI)-II

415 390 350

425 410 395

325 (I step), 460 (II step) 445 445 430

a b c

14 37 42

T5% , T10% : temperatures at 5, 10, 25% weight loss, respectively. Temperature of maximum decomposition rate. Residual weight when heated to 800 ◦ C in nitrogen.

Additionally, to evaluate the thermal properties, that is, glass (Tg ) or melting (Tm ) transitions of obtained polymers DSC analysis was utilized. However, no thermal transitions (Tg or Tm ) were detected in the DSC runs (first and second heating) up to 300 ◦ C in the case of polymers. The reason might be the rigid structure of poly(azomethinenaphthaleneimide)s, which notably reduces the chain mobility [43]. It seems to be interesting to compare thermal stability of our polymers with similar published systems. However, to the best of our knowledge, PNIs containing in their structure TPA moiety was reported only in one article [35]. As was mentioned in Section 1 Zhang et al. investigated a series of copolynphthaleneimides from 4,4 -binaphthyl-1,1 ,8,8 tetracarboxylic dianhydrides, 4,4 -diaminotriphenylamine and 4,4 -diaminodiphenyl ether-2,2 -disulfonic acid [35]. All sulfonated copolyimides exhibited a three-step degradation pattern, with initial weight loss around 100 ◦ C, the second around 280–300 ◦ C and the third step about 570 ◦ C which was assigned to the degradation of polymer main chain. On the other hand polyperyleneimides (PPIs) with modified TPA moiety exhibited higher thermal stabilities (determined in air) than polymers described in this work, it is expected because of perylenediimide units are more thermally stable than naphthalene units [14].

Table 3 Activation energy (Ea ) and pre-exponential factor (A) of thermal decomposition of synthesized compounds calculated by Coats–Redfern method. Code

Ea [kJ/mol]

A [1/min]

R2

DANDI AZ-NI Poly(AZ-NI)-I Poly(AZ-NI)-II

128.20a 323.72b 266.72 184.71 115.08

9.27 × 109 a 2.75 × 1022 b 6.32 × 1018 4.05 × 1012 2.16 × 107

0.9997a 0.9957b 0.9958 0.9964 0.9989

a b

First step of thermal decomposition. Second step of thermal decomposition.

Thermogravimetric analysis (TGA) is the most widely used technique to evaluate the pyrolysis behavior of polymeric materials because of its simplicity and the useful information afforded from a simple TGA thermogram [44]. Many kinetic methods applicable to TGA data exist to characterize the thermal degradation of polyimides such as Kissinger, Ozawa, Horowitz–Metzger, Coats–Redfern, MacCallum–Tanner and van Krevelen [45]. However, mathematical verification of the methodology showed that the MacCallum–Tanner and Coats–Redfern methods are more precise than the other ones [45]. Recently, Coats–Redfern method is reported as the one which offers the most precise results [46]. It should be noted that activation energy (Ea ) values calculated from TGA measurements are actually composite values for various processes occurring simultaneously in a given temperature range [47]. To simplify the kinetic equations, each of the methods makes certain assumptions, and as a result certain processes and factors are overemphasized while others are neglected. Hence, the reaction orders and Ea values as derived directly from TGA curves seldom have any precise mechanistic significance [47]. To determine thermal degradation kinetics of prepared poly(AZNI)s, AZ-NI and DANDI from TGA data, the Coats–Redfen method was applied [10–12]. This method, as reviewed by Johnson and Gallagher [12] is an integral method that assumes various reaction orders and compares the linearity in each case to select the correct order. This method assumes [47]: (i) that only one reaction mechanism operates at a time, (ii) that the calculated activation energy value is for this mechanism and (iii) that rate of degradation can be expressed by the following basic rate equation: d˛ = k(1 − ˛)n dt

(1)

where n is the reaction order and ˛ is degree of conversion. The derivation of the Coats–Redfen method is outlined in the original papers [10,11]. The final operative equations are given below:



log

1 − (1 − ˛)1−n T 2 (1 − n)



= log



AR 2RT 1− E ˇE



−

E 2.303RT

/ 1 for n = (2)

log

 −log(1 − ˛)  T2

= log



AR 2RT 1− E ˇE



−

E 2.303RT

for n = 1 (3)

where ˇ is the heating rate (K min−1 ), A is the pre-exponential factor (min−1 ), R is the gas constant (8.314 J mol−1 K−1 ). The degree of conversion ˛ is defined as the ratio of actual weight loss to total weight loss and can be calculated from equation: ˛=

W0 − Wt W0 − Wf

(4)

where W0 is the initial mass of sample, Wt is the mass of the sample at temperature t and Wf is the final mass at a temperature at which the mass loss is approximately unchanged. Solid decomposition reactions may take place by one of a number of elementary mechanisms, as well as combinations of these mechanisms. Thus, it is difficult to find a meaning for the reaction order of polyimide thermal decomposition [48]. In this work, the reaction order of degradation was assumed to be one, which was confirmed by carried out calculations of correlation coefficient. The equation (3) for the first order kinetics was applied. Plots of log[−log(1 − ˛)/T2 ] versus 1/T obtained for DANDI, AZ-NI and poly(AZ-NI)s using TGA data, are presented in Fig. 3.

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 2268–2279

2

1,0

DANDI AZ-NI poly(AZ-NI)-I poly(AZ-NI)-II

0,9 0,8

Absorbance [a.u.]

‐6,1

log[‐log(1‐ α) / T ]

a

- DANDI (II step) - AZ-NI - poly(AZ-NI)-I - poly(AZ-NI)-II

‐5,9

2273

‐6,3 ‐6,5 ‐6,7 ‐6,9

0,7 0,6 0,5 0,4 0,3 0,2

‐7,1

0,1 1,39

1,44

1,49 ‐3

(1/T) / 10 K

0,0

1,54

300

‐1

Fig. 3. Plots for determination of Ea and A of DANDI, AZ-NI and poly(AZ-NI)s thermal decomposition.

Taking into account Fig. 3 it can be seen that the plots are linear suggesting that the decomposition of investigated compounds follows first order kinetics [50]. The activation energies were calculated from the slope and the intercept gives A values. The calculated slope is: −16.907, −13.93, −9.6468 and −6.0107 for DANDI (second step of decomposition), AZ-NI, poly(AZ-NI)-I and poly(AZ-NI)-II, respectively. The calculated kinetic parameters: activation energies (Ea ) and pre-exponential factors (A) are collected in Table 3. One can notice from Table 3 that the correlation coefficient (R2 ) of the plotted is in the range 0.9958–0.9997, approximately equal to 1 and confirm that the assumed reaction order as one is correct. The determined Ea values lay in the range 115.08–323.72 kJ/mol. Considering the influence of polymer structure on Ea value it was found that branched poly(AZ-NI)-II exhibited lower activation energy of thermal decomposition than linear poly(AZ-NI)-I. Moreover, the pre-exponential factor for the decomposition is the lowest in the case of poly(AZ-NI)-II. The main difference between these two polymers is a greater amount of azomethine linkages in poly(AZ-NI)-II, therefore it is likely that during heating the imine (−HC N−) linkage is the first breaking unit [41]. It can be concluded that the decomposition process proceeds easily in poly(AZ-NI)-II. It is interesting to compare our results with published data. However, to the best of our knowledge, the activation energy of decomposition of six-membered polyimides was reported in only one article [50]. Ea values of thermal decomposition of polyimides obtained from 3,4,9,10-perylenetetracarboxylic acid anhydride and aromatic diamines with hydrazo group and ether linkages calculated by Horowitz–Metzger method were in the range of 87.03–95.28 kJ/mol [50]. It was found that Ea of polyimides with five-membered rings determined using FreemanCarroll method ranged from 131.41 to 307.13 kJ/mol [51]. The Ea of decomposition process of polyesterimides calculated also by Ozawa method lays in the range 131.6–244.4 kJ/mol [44]. Vora et al. obtained Ea by Coats–Redfern model of fluoropolyimides ranged from 27.1 to 32.6 kcal/mol (113.3–136.27 kJ/mol) [49]. Chang et al. compared Ea determined by different methods (Van Krevelen, Horowitz–Metzger, Coats–Redfern, MacCallum–Tanner) of copolymers obtained from 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride and oxydianiline and ␣,␻-aminopropyl polydimethylsiloxane oligomers [48]. It was found that Ea value calculated by the Coats–Redfern method was smaller in comparison with others. On the other hand, Chen et al. found that Ea obtained by Ozawa, KAS or Coats–Redfern methods were close to each other (205–264 kJ/mol) for polyimides with ether and hexafluoroisopropylidene groups [52].

350

400

450

500

Wavelength [nm]

b

0,6

poly(AZ-NI)-I

NMP CHCl3 Blend

0,5

Absorbance [a.u.]

‐7,3 1,34

0,4

0,3

0,2

0,1

0,0 300

350

400

450

500

Wavelength [nm] Fig. 4. Optical absorption spectra of studied compounds in chloroform solution (a) and AZ-NI in NMP, chloroform solution (concentration 1 × 10−5 mol/l) and in blend (b).

The activation energy of pyrolysis for the synthesized compounds is high, demonstrating the high thermal stability of the polymers and the model compound. 3.4. Optical properties The optical, that is, absorption and emission, properties of the studied poly(azomethinenaphthaleneimide)s, azomethinenaphthalene diimide and diamine DANDI were analyzed by UV–vis and photoluminescence (PL) spectroscopies in solution and in the solid state as blends with poly(methyl methacrylate) (PMMA). The solvatochromic behavior of the polymers and the model compound in two solvents, namely NMP and chloroform, with a different dipole moment and consequently with a different polarity, was studied. The effect of excitation wavelength and concentration on PL properties and was also examined. 3.4.1. Ultraviolet–visible investigations The representative UV–vis absorption spectra of DANDI, AZ-NI and poly(AZ-NI)s in solutions and in solid state as blend with PMMA are depicted in Fig. 4. Electronic absorption spectra of the studied DANDI, AZ-NI and poly(AZ-NI)s showed similar characteristics, i.e. the weak band with the maximum (max ) located around 295 nm and a structured band at lower energies with three maxima at about 341 (1 ), 362 (2 ) and

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Table 4 UV–vis absorption characterization of the diamine DANDI, azomethine-naphthalene diimide and poly(azomethinenaphthaleneimide)s (conc. = 1 × 10−5 mol/l). Code

UV–vis NMP (ε = 33.00)a

DANDI

AZ-NI

Poly(AZ-NI)-I

Poly(AZ-NI)-II

a b

CHCl3 (ε = 4.81)a

In solid stateb

max [nm] (eV)

ε [l mol−1 cm−1 ]

max [nm] (eV)

ε [l mol−1 cm−1 ]

max [nm] (eV)

294 (4.22) 363 (3.42) 380 (3.26) 295 (4.20) 324 (3.83) 342 (3.63) 360 (3.44) 381 (3.25) 342 (3.63) 362 (3.43) 383 (3.24) 289 (4.29) 341 (3.64) 362 (3.43) 381 (3.25)

24,170 53,420 48,570 23,020 20,020 27,940 41,900 45,740 48,880 67,810 90,620 29,660 46,180 74,210 87,080

299 (4.15) 61 (3.43) 380 (3.26) 291 (4.26) 323 (3.84) 342 (3.63) 359 (3.45) 380 (3.26) 342 (3.63) 361 (3.43) 381 (3.25) 289 (4.29) 341(3.64) 361(3.43) 380(3.26)

19,130 51,740 54,520 14,500 13,280 21,910 36,370 44,000 45,900 62,870 93,480 19,100 37,190 65,060 82,700

361 (3.43)

340 (3.65); 358 (3.46); 378 (3.28)

341 (3.64); 360 (3.44); 381 (3.25) 341 (3.64); 361 (3.43); 379 (3.27)

ε: dielectric constant. Blend with PMMA.

381 nm (3 ), typical for naphthalene diimides, which are attributed to the ␲–␲* transition in the naphthalene tetracarboxylic diimide conjugated core [53]. The absorption intensities of the three bands are declining in the order of 3 –2 –1 in agreement with literature data except for DANDI in NMP solution. In the case of the polymers the band (4.22 eV) at the highest energy region is much less structured compared to model compound and diamine DANDI. Absorption spectral data of studied compounds are summarized in Table 4. Considering the data from Table 4, the position of the absorption band is almost the same for AZ-NI and poly(AZ-NI)s. Similarly as it was observed in our previous work [40] the absorption band being responsible for ␲–␲* transition in the imine group is covered by absorption of the naphthalene imide units. Moreover, the absorption originating from the TPA units is in the same range as that of the naphthalene imide moieties and consists of two bands with max around 340 and 380 nm. In the solid state, as blends with PMMA, the investigated compounds exhibited similar absorption properties as in solution, taking into account max position and shape of the absorption bands, except for DANDI (cf. Fig. 4b and Table 4). In the latter case, an absorption band with one maximum was observed in the blend. No shift was found along with the change from solution to solid state, therefore no H nor J aggregates existed in the analyzed model compound and polymers in solid state. Additionally, UV–vis spectra were recorded in NMP solution to study the solvatochromic effect. Fig. 4b presents the UV–vis absorption spectra of the exemplary AZ-NI in NMP (ε = 33.0) and chloroform (ε = 4.81) solution, that is, in solvents differing in polarity (cf. Table 4). Despite the different polarity of the applied solvents, the spectra showed a max position at almost the same wavelength. Therefore, the solvatochromic effect was not observed in the studied compounds. In the more polar solvent (i.e. NMP) the values of molar extinction coefficients calculated at max (εmax ) were higher. Only in the case of 3 (3.26 eV) of DANDI and poly(AZ-NI)-I an opposite tendency was found, that is, the value of εmax was higher in chloroform solution. Condensation of DANDI with the aldehydes caused a decrease of molar absorption coefficients of bands characteristic for ␲–␲* transition in the imide group (cf. Table 4). The polymers, compared to AZ-NI, showed higher absorbances at max . opt The optical energy band gap (Eg ) was estimated using followopt

ing equation: Eg = hc/offset , where h is the Planck constant, c is the light velocity, and offset is the absorption edge wavelength opt of the optical absorption spectra. The values of Eg were found to be 3.10 eV for AZ-NI, 3.02 eV for poly(AZ-NI)-I and 2.95 eV and

opt

for poly(AZ-NI)-II. Eg calculated for PPIs with TPA derivatives described in literature, was in the range of 2.61–2.95 eV [14]. Mentioned polymers exhibited one absorption band in the UV–vis range with max = 382 and 339 nm. 3.4.2. Photoluminescence properties Many factors can influence the photoluminescence (PL) properties of organic compounds mainly arising directly from chemical structure as well as from experimental conditions. In this work, the influence of four factors on the emission spectra was considered: changing the excitation wavelength, changing the kind of solvent, changing concentrations and blending the light-emitting compound with a nonemissive polymer (PMMA). The emission spectra of diamine DANDI, AZ-NI and poly(AZ-NI)s were recorded with a different excitation wavelength in solutions at two concentrations: 1 × 10−4 and 1 × 10−5 mol/l. On the other hand, acquisition of the PL spectra of studied compounds in NMP and chloroform solutions, that is, solvents that differed in polarity, allowed to analyze the solvent effect on PL properties. The investigated model compound and polymers consist of three different kinds of chromophores: naphthalene diimides, azomethine linkages and triphenylamine units and their photoluminescence might be attributed to these moieties. In the first step of investigation, chloroform solutions of the studied compounds at concentration of 1 × 10−4 mol/l were excited with different wavelengths to establish conditions that provide the best fluorescence spectrum, that is, with the highest relative luminescence intensity. The emission spectra were recorded under excitation wavelengths in the maximum of the absorption band, i.e. 340, 360 and 380 nm. Additionally, the excitation wavelengths were chosen on the basis of the fluorescence excitation spectra taken in chloroform which were not identical to the absorption spectra, especially for DANDI and the model compound AZ-NI. The influence of the excitation wavelength on PL properties, that is, on the position of emission band maximum (em ) and intensity of emitted light is presented in Fig. 5, while the emission spectral data of studied compounds in solutions are summarized in Table 5. The investigated compounds under different excitation wavelengths (ex ) exhibited, in most cases, a single emission band, with the maximum (em ) located in the blue region and also in the green range. Changes in both the em position and its intensity with an increase of ex from 318 to 480 nm were observed. A bathochromic shift of em position along with longer ex was detected. In the case of diamine DANDI, the emission spectra showed the same

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110 AZ-NI

DANDI

ex 318 ex 330 ex 363 ex 380 ex 408 ex 420 ex 450 ex 480

PL intensity [a.u.]

35 30 25

ex 318 ex 330 ex 342 ex 360 ex 381 ex 408 ex 420 ex 450 ex 480

100 90

PL intensity [a.u.]

40

2275

20 15

80 70 60 50 40 30

10

20 5

10

0 350

0 400

450

500

550

600

650

700

400

450

500

70 poly(AZ-NI)-I

600

650

80 poly(AZ-NI)-II

ex 318 ex 330 ex 342 ex 362 ex 381 ex 408 ex 420 ex 450 ex 480

700

50 40

ex 318 ex 330 ex 341 ex 362 ex 381 ex 408 ex 420 ex 450 ex 480

70

PL intensity [a.u.]

60

PL intensity [a.u.]

550

Wavelenght [nm]

Wawelenght [nm]

30 20

60 50 40 30 20

10

10

0 350

0 400

450

500

550

600

650

400

450

Wavelenght [nm]

500

550

600

650

Wavelenght [nm]

Fig. 5. The emission spectra of the DANDI (a), AZ-NI (b), poly(AZ-NI)-I (c) and poly(AZ-NI)-II (d) under various excitation wavelengths in chloroform solution at conc. 1 × 10−4 mol/l.

maximum under excitation at 318 and 330 nm (em = 485 nm), 360 and 380 nm (em = 495 nm), and under ex = 408 and 420 nm at (em = 492 nm). The emission spectra of the diamine under excitation at 318 and 330 nm exhibited a red shift (44 nm) in comparison with emission under ex = 480 nm. Furthermore, under excitation at 480 nm two emission maxima were observed and the lowest relative intensity of emitted light was found. The highest PL intensity for DANDI was observed under ex = 408 nm. The condensation

of DANDI with aldehyde with triphenylamine units did not significantly influence on emission properties such as position of em (AZ-NI compared to diamine). However, under excitation at 318 nm and 330 nm a much weaker emission was observed for AZ-NI compared to DANDI. The em maxima of the model compound are slightly hipsochromic shifted in relation to DANDI under excitation at 420, 450 and 480 nm. Contrary to the diamine, AZ-NI under excitation wavelength from higher energy region resulted in much

Table 5 Photoluminescence of investigated compounds in solution (NMP, CHCl3 ) in different concentrations and in under different excitation wavelengths. Code

PL max [nm] Solvent

DANDI

NMP CHCl3

AZ-NI

NMP CHCl3

Poly(AZ-NI)-I

NMP CHCl3

Poly(AZ-NI)-II

NMP CHCl3

ca

c1 c2 c1 c2 c1 c2 c1 c2 c1 c2 c1 c2 c1 c2 c1 c2

ex [nm] 318

330

340

360

380

408

420

450

480

484 428 485 485 455 400 486 445 478 472 462 460 469 459 455 484

484 428 487 488 455 416 452 445 478 472 462 460 469 459 455 445

nm nm nm nm nm nm 458 nm nm nm 481 nm nm nm 465 nm

nm nm 495 nm nm nm 457 nm nm nm 481 nm nm nm 465 nm

nm nm 495 nm nm nm 457 nm nm nm 481 nm nm nm 465 nm

494 488 492 491 464; 512 464 502 501 478 472 462 460 481 465 465 460

494 480 492 480; 501 517 479; 512 503 503 479 477 463 461 495 479 496 494

514 516 512 515 520 517 503 503 517 522; 570 550 520 518 517 519 518

530 530 529; 561 530 521 524 523 525 533; 558 533; 578 539; 562 527; 562 535 5,33 537 536

Bold data indicates the most intense luminescence of compound. a Concentration: c1 = 10−4 [mol/L]; c2 = 10−5 [mol/L]; nm: not measured.

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 2268–2279

a

AZ-NI

NMP CHCl3

120

PL intensity [a.u.]

smaller emission intensity compared to lower energy ex . AZ-NI exhibited the biggest bathochromic shift of em equal to 37 nm under different ex and highest emission intensity under excitation at 450 nm. The polymers under excitations: 318, 330, 408 and 420 nm emitted light with em shifted hipsochromically in comparison to AZ-NI. Whereas under excitation at 450 and 480 nm, a bathochromic shift was observed in the spectra of the polymers in relation to the model compound. For poly(AZ-NI)s the maximum relative emission intensity was attained under excitation at 318 and 330 nm. The emission spectra of poly(AZ-NI)-1 and poly(AZ-NI)-II under ex 318 and 330 nm showed a red shift of em equal to 77 and 82 nm, respectively, compared to em position at ex = 480 nm, respectively. Summarizing, the fluorescence spectrum with the highest emission was obtained when the chloroform solution of: DANDI was excited at 408 nm, AZ-NI at 450 nm, poly(AZ-NI)-I at 318 nm and poly(AZ-NI)-II at 330 nm. In Table 5 bold data indicate the band with the highest relative emission intensity. PL spectra of investigated compounds under excitation wavelength which resulted in the highest emission are presented in Fig. 3 in Supporting information. It was found that the model compound emitted green light with significantly higher intensity than blue light. On the other hand, both polymers and DANDI emitted blue light with much higher intensity than the green light. The em position of the branched polymer poly(AZ-NI)-II is shifted hipsochromically compared to the linear polymer poly(AZ-NI)-I. The influence of solution polarity on emission of investigated compounds was studied. Solvent polarity and polarizability affect the energy levels of excited states in the organic compounds [54]. Therefore, both the spectral shape and the position of fluorescence band of the compounds should, in principle, be affected. Exemplary emission spectra of AZ-NI and poly(AZ-NI)-II in NMP and chloroform solution are shown in Fig. 6. A substantial red shift was observed with an increase in solvent polarity (and, consequently, in the dielectric constant) in the case of the polymers under excitation at 318, 330 and 408 nm (cf. Table 5). In such a polar solvent as NMP, the maximum of emission was 14 or 16 nm bathochromically shifted in comparison with PL maximum in less polar chloroform solution. Taking into account the emission band with the highest intensity, it was found that in the case of DANDI and AZ-NI the emission intensity decreased as the medium changed from NMP to chloroform (cf. Fig. 6a). It is expected that the relative PL intensity will increase as we change from a less polar to a highly polar solvent [55]. By increasing the dielectric constant (ε) of the solvent, the shielding between molecules increases, and consequently, PL self-quenching between neighboring molecules diminishes [55]. However, unexpectedly, in the case of poly(AZ-NI)s, an opposite tendency was found, that is, higher emission intensity in CHCl3 than in NMP solution (cf. Fig. 6b). Compare PL properties of our polymers with literature data, it was found that PPIs with TPA derivatives emitted light with em = 493 nm and 570 nm in NMP solution [14]. Additionally, the effect of concentration on photoluminescence in NMP and chloroform solution was examined. In most cases, spectral changes in the emission band and PL intensity with decrease of the compound concentration were found. Taking into account the influence of concentration on the emission band with the highest intensity (ex = 408 for DANDI, 450 nm for AZ-NI, 318 nm for poly(AZ-NI)-I and 330 nm poly(AZNI)-I), it was observed that a decrease of polymer concentration blue shifted the em position slightly (cf. Table 5). Considering the PL intensity with increasing solution concentration, not the same tendency was found for all investigated compounds, that is, lower emission was observed with less concentrated solution in the case of DANDI, AZ-NI and poly(AZ-NI)-I in chloroform, while the branched polymer emitted light with higher intensity in the

100

80

60

40

20

0 450

500

550

600

650

700

Wavelenght [nm]

b

90

poly(AZ-NI)-II

NMP CHCl3

80 70

PL intensity [a.u.]

2276

60 50 40 30 20 10 0 400

450

500

550

600

650

Wavelenght [nm] Fig. 6. The emission spectra in chloroform and NMP solution (conc. 1 × 10−4 mol/l) of the AZ-NI under exc = 450 nm (a) and of poly(AZ-NI)-II under exc = 330 nm (b).

solution with lower concentration compared to the more concentrated solution. In investigations of optical properties, that is, absorption and emission in UV–vis range, the Stokes shifts could be analyzed. The Stokes shift is generated by electronic or geometrical structure relaxation of the photoexcited molecule [56], which is induced by the intramolecular charge transfer process [5]. This information suggests significant conformational differences between an absorbing ground state (S0 ) and the emitting excited state (S1 ) [57]. It is important from a practical point of view of potential applications, if the Stokes shift is too small, then the emitted light will be self-absorbed and the luminescence efficiency will decrease in devices [58]. Calculated Stokes shifts are collected in Table 6 and additionally, typical PL and absorption spectrum of exemplary AZNI is presented in Fig. 4 in Supporting information. The UV–vis absorption spectra of the investigated compounds are not overlapped with their emission spectra. The emission maxima are strongly red shifted from their optical absorption maxima. Considering the emission band with the highest intensity, it was found that: (i) the highest value of Stokes shift was obtained for AZ-NI about 7000 cm−1 , (ii) higher Stokes shift was calculated in NMP than in chloroform solution. The calculated Stokes shift values for the investigated compounds indicated significant differences in the energy loss, which occurred during the transition from S0 to S1 . Additionally, the fluorescence characteristics of obtained compounds in solid state as blend with PMMA were studied. The blends

E. Schab-Balcerzak et al. / Synthetic Metals 161 (2011) 2268–2279

2277

Table 6 The calculated Stokes shifts for investigated compounds. Code

Solvent

DANDI

NMP CHCl3 NMP CHCl3 NMP CHCl3 NMP CHCl3

AZ-NI Poly(AZ-NI)-I Poly(AZ-NI)-I

Stokes shifta [cm−1 ] ex = 318 nm

ex = 330 nm

ex = 408 nm

ex = 450 nm

2951 5697 1247 3844 4923 4508 4460 3844

5655 5782 4269 4192 5189 4602 4925 4338

5824 5949 4695 6356 4923 4508 4741 4577

6861 6785 7016 6435 6767 8065 6942 7048

Bold data indicate the  calculated for the most intense luminescence of compound. a Stokes shifts calculated according to the equation  = (1/abs − 1/em ) × 107 (cm−1 ).

PL em [nm]

 [cm−1 ]

DANDI AZ-NI Poly(AZ-NI)-I Poly(AZ-NI)-II

469s ; 518 (ex = 408 nm) 510 (ex = 450 nm) 425; 537 (ex = 318 nm) 428; 531 (ex = 330 nm)

8396 6847 2117; 7625 3021; 7631

s: shoulder.

were excited with a wavelength which gave the highest emission intensity in solution. The maxima of emission in blends containing about 1% of DANDI, AZ-NI and polymers are presented in Table 7 and Fig. 7 shows the exemplary PL spectrum of poly(AZ-NI)-II in solution and in blend. The PL spectra of the compound in blends changed, compared with their spectra in solution. In comparison with the solution, the emission of blends was more intense (Fig. 7) because of the dilution effect. The PL spectrum of AZ-NI in blend showed one emission band with em position red shifted in relation to solution. Whereas, polymers exhibited emission with two maxima and in the case of the branched polymer the maximum at lower energy region is better structured than in linear polymer. The emission band of blends became broader relative to those of solutions. 3.5. Electrochemical properties Electrochemical properties of the studied compounds were investigated in solution (DANDI and AZ-NI) and thin film (AZ-NI and both polymers) by cyclic voltammetry and differential pulse voltammetry (cf. Fig. 8). Table 8 shows the electrochemical parameters obtained. The materials exhibited two reversible reduction peaks characteristic for naphthalene diimides [59,60] as well as a partially reversible for AZ-NI and irreversible for poly(AZ-NI)-I and poly(AZNI)-II oxidation of the imine and triphenylamine moieties [60,61]. This finding contrasts those of Pron et al. and Rybakiewicz et al. on the behavior of arylenediimide-triphenylamine conjugates: in their case the oxidation processes were reversible with onsets in the range of 0.37–0.50 V versus Fc [62,63]. Hence, it is the imine bond that is responsible for the irreversibility of the oxidation of the compounds described herein. As one can see on the reduction DPV curves in Fig. 8, there is also a shoulder on the right-hand slope of the second reduction peak of the diimide, at about −1.34 V (for AZ-NI and poly(AZ-NI)-II). The reduction of imine moieties is expected to require much lower potentials (in the range of −2.0 to −2.5 V versus Fc [64,65]), therefore it seems impossible that the peak at −1.34 V can be caused by imine reduction. UV–vis spectroelectrochemical measurements for thin films of poly(AZ-NI)-I and poly(AZ-NI)-II drop cast on ITO confirm that the first reduction is located on the diimide: the positions of the characteristic bands (cf. Fig. 9, poly(AZ-NI)-I: 473, 605, 694 and 772 nm, poly(AZ-NI)-II: 475,

100

poly(AZ-NI)-I

CHCl3 Blend

80

PL intensity [a.u.]

Code

a

60

40

20

0 350

400

450

500

550

600

Wavelenght [nm]

b

160

poly(AZ-NI)-II

CHCl3 Blend

140 120

PL intensity [a.u.]

Table 7 PL data for the investigated compounds in solid state as blend with PMMA.

100 80 60 40 20 0 350

400

450

500

550

600

650

Wavelenght [nm] Fig. 7. The emission spectra of polymers in chloroform solution (conc. 1 × 10−4 mol/l) and in blend with PMMA (1% conc.) (a) poly(AZ-NI)-I (exc = 318 nm) and (b) poly(AZ-NI)-II (exc = 330 nm).

606, 695 and 774 nm) are almost identical to those reported for other naphthalene diimide anion radicals [66–68]. Further reduction of the films resulted in destruction and complete dissolution of the material, we believe that this might be attributed to the peak at −1.34 V on the DPV (see above). Attempts to record the UV–vis spectra of oxidized films were also unsuccessful: the films dissolved in the electrolyte, presumably because of secondary reactions (including hydrolysis) of the oxidized imine moieties [64]. Electrochemical band gaps (Eg ) were calculated from the oxidation onset potential and first reduction onset potential. The results

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Table 8 Electrochemical data for DANDI, AZ-NI, poly(AZ-NI)-I and poly(AZ-NI)-II. DANDIs E1/2r1 (CV) Er1 (DPV) E1/2r2 (CV) Er2 (DPV) Er (onset, CV) Er (onset, DPV) Eox (onset, CV) Eox (onset, DPV) Ep,ox (CV) Ep,ox (DPV) Eg (CV) Eg (DPV)

−0.984 −0.98 −1.445 −1.42 −0.88 −0.77 −0.41 −0.35 −0.21; 0.29; 0.65 −0.21; 0.24; 0.63 0.47 0.36

AZ-NIs −0.984 – −1.432 – −0.86 0.46 – 0.640 – 1.32

AZ-NIf

Poly(AZ-NI)-If

Poly(AZ-NI)-IIf

−0.945 −0.98 −1.407 −1.42 −0.81 −0.79 0.50 0.48 0.628 0.65; 0.97 1.31 1.27

−0.949 −1.00 −1.403 −1.44 −0.84 −0.76 0.29 0.26 0.834 0.85 1.15 1.02

−0.984 −1.00 −1.418 −1.46 −0.76 −0.72 0.30 0.23 1.134 0.98 1.06 0.95

s: measured in CH2 Cl2 solution; f: measured in thin film immersed in CH3 CN, E1/2 (CV): formal redox potential calculated as (Eox + Ered )/2 from cyclic voltammetry, E(DPV): formal redox potential estimated as difference between the peak potential and peak potential of ferrocene, Ep,ox : oxidation peak potential, Eg = Eox (onset) − Er (onset).

Fig. 9. Normalized UV–vis spectra of thin films of electrochemically reduced (at −1.2 V) poly(AZ-NI)-I and poly(AZ-NI)-II.

values strongly differ compared with each other. Nevertheless, it should be stressed out, that in bisimides N-substituted with triaryl amines the experimentally detected absorption band of the lowest energy does not correspond to the HOMO to LUMO transition but to a transition of a higher energy. This is caused by the fact that the oscillator strength for the HOMO to LUMO transition in these compounds is zero, as shown by DFT calculations [62,63]. Therefore direct comparison of the optical and electrochemical is not appropriate in this case.

4. Conclusions

Fig. 8. Cyclic voltammograms (left) and differential pulse voltammograms (right) of thin films of AZ-NI, poly(AZ-NI)-I and poly(AZ-NI)-II on Pt electrode. The peak at 0.0 V is Fc/Fc+ (internal standard).

from DPV measurements are slightly lower than those from CV, because of the higher sensitivity of this technique. The electrochemical Eg of AZ-NI is 1.31 eV (both in the solid state and in solution) according to CV and 1.27 in solid-state DPV. The band gap of the linear polymer poly(AZ-NI)-I is 1.15 (CV) or 1.02 (DPV), while that of the branched poly(AZ-NI)-II is 0.07–0.09 eV lower (CV: 1.06, DPV: 0.95). The obtained values of Eg are much smaller that in the case of PPIs with TPA described in literature, that is, 2.74 and 1.96 eV [14]. As can be seen from Section 3.4.1. Ultraviolet–vis investigations, the electrochemical and optical energy band gap

In the present study, two novel alternating polynaphthaleneimides and a naphthalene diimide, consisting of imine and triphenylamine units were synthesized. The results of TGA analyses showed that the polymers and model compound have enough resistance against thermal decomposition. The initial decomposition temperature of polymers (T5 ) was above 350 ◦ C. The carbine reside of these polymers was observed between 32 and 47% at 800 ◦ C. Poly(azomethinenaphthaleneimide)s, azomethine-naphthalene diimide and naphthalene diimide exhibited photoluminescence properties and emitted blue light with higher intensity than green except for the model compound. The electrochemical behavior of the novel polymers, model compound and monomer (namely the diamine) were examined and compared. The electrochemical energy band gap calculated from DPV measurements of DANDI was 0.36 eV, of the model compound 1.27 eV and of the polymers was 1.02 and 0.95 eV, respectively. These

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