Dimethyldiphenylamino-substituted carbazoles as electronically active molecular materials

Dyes and Pigments 96 (2013) 574e580

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Dimethyldiphenylamino-substituted carbazoles as electronically active molecular materials A. Tomkeviciene a, b, G. Puckyte b, J.V. Grazulevicius b, *, K. Kazlauskas a, S. Jursenas a, V. Jankauskas c _ Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania c _ Department of Solid State Electronics, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 April 2012 Received in revised form 27 September 2012 Accepted 28 September 2012 Available online 12 October 2012

Synthesis and properties of new dimethyldiphenylamino-substituted carbazoles are reported. A comparative experimental analysis of 2-, 2,7-substituted carbazole derivatives and 3-, 3,6-substituted analogues is performed. Disubstituted carbazole derivatives exhibit more than 2-fold higher glass transition temperatures (99e100
C) as compared to the monosubstituted compounds (38e40
C). Dilute solutions of the dimethyldiphenylamino-substituted carbazole compounds are found to emit in the blue region with the fluorescence quantum yield of up to 0.45. More symmetrical molecular geometry of the 2-, 2,7-substituted carbazole compounds favors closer molecular packing in the neat films thereby facilitating exciton migration and migration induced exciton quenching, which results in the significantly reduced excited state decay times (down to sub-nanoseconds) and fluorescence quantum yields (down to 0.04). The 2-, 2,7-substituted carbazole compounds are shown to demonstrate the superiority over 3-, 3,6-substituted compounds by giving rise to extended p-conjugation and expressing over one order of magnitude higher radiative decay rates. The synthesized compounds are electrochemically stable: their cyclic voltammograms show reversible oxidation behavior. The ionization energies of the synthesized compounds are in the range of 5.16e5.28 eV. Hole drift mobilities of the molecular mixtures of dimethyldiphenylamino-substituted carbazoles with bisphenol Z polycarbonate reach 105 cm2 V1 s1 at high electric fields. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Carbazole Dimethyldiphenylamino Glass-forming Thermal stability Ionization energy Charge mobility

1. Introduction Different classes of organic electronically active materials are nowadays widely studied and used as the components of optoelectronic and electronic devices [1e5]. Among electronically active materials carbazole-based compounds represent one of the most widely used and studied class of substances [6e9]. Carbazole compounds have drawn attention due to their versatility in functionalization, variety of linking topologies, good chemical and environmental stability, excellent charge transport ability etc. Nowadays carbazole-based polymers, oligomers and low-molarmass compounds are successfully utilized in organic light emitting diodes, organic solar cells, biosensors, organic-thin-film transistors and other devices [6,10e16]. It is worth mentioning that much less research has been carried out on low-molar-mass 2,7substituted carbazole derivatives as compared to 3,6-substituted derivatives. The main obstacle until recently was the lack of an * Corresponding author. Tel.: þ370 37 300193; fax: þ370 37 300152. E-mail address: [email protected] (J.V. Grazulevicius). 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.09.021

efficient synthesis procedure of the former compounds. As noted in the review [6], while the class of 3,6-substituted carbazole derivatives was found to be very interesting for electrochemical and phosphorescence applications, the class of 2,7-substituted carbazoles showed promising optical properties in the visible range. It was also recognized that the major intrinsic difference between these two families of carbazole derivatives is the effective conjugation length, being larger for 2,7-carbazole derivatives. These findings stimulated design of new 2,7-substituted carbazole derivatives with improved properties. In this paper we report on the synthesis of new carbazole derivatives having methyl-substituted amino groups at 2 or 2,7 positions of the carbazole moiety with potential application in organic optoelectronics. We chose para position of the methyl group in the diphenylamino moiety taking into account our previous study, which indicated that this position of the methoxy substituents in diphenylamino moiety attached to the 3 or 3,6 positions of the carbazole moiety offers the most evident positive effect [17]. In addition to the 2- and 2,7-substituted carbazole derivatives we have also synthesized analogous 3- and 3,6-

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580

substitued compounds, and thus, present a comparative study of their thermal, optical, photophysical, electrochemical, and photoelectrical properties. 2. Experimental 2.1. Instrumentation 1 H and 13C NMR spectra were recordered using Varrian Unity Inova [300 MHz (1H), 75.4 MHz (13C)] spectrometer at room temperature. All the data are given as chemical shifts in d (ppm), (CH3)4Si (TMS, 0 ppm) was used as an internal standard. The course of the reactions products were monitored by TLC Silica gel 60 F254 plates and developed with I2 or UV light. Silica gel (grade 60, 63e 200 mesh, 60  A, Fluka) was used for column chromatography. Melting points were determined using Electrothermal Mel-Temp melting point apparatus. Mass (MS) spectra were recorded on a Waters ZQ (Waters, Milford, MA). Elemental analysis was performed with an Exeter Analytical CE-440 Elemental. Infrared (IR) spectra were recorded using Perkin Elmer Spectrum GX spectrometer. The spectra of solid compounds were performed in KBr pellets. Differential scanning calorimetry (DSC) measurements were carried out using PerkineElmer DSC-7 series thermal analyzer at a heating rate 10
C/min under nitrogen flow. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851e. Absorption spectra of the dilute tetrahydrofuran (THF) solutions were recorded on UVeViseNIR spectrophotometer Lambda 950 (Perkin Elmer). Fluorescence of the dilute THF solutions and the neat films of the investigated compounds was excited by 365 nm wavelength light emitting diode (Nichia NSHU590-B) and measured using back-thinned CCD spectrophotometer PMA-11 (Hamamatsu). For these measurements, the dilute solutions of the investigated compounds were prepared by dissolving them in a spectral grade THF at 1  105 M concentration. Neat films of the investigated compounds were prepared from the 1  103 M THF solutions on the quartz substrates by drop-casting in an ambient air. Fluorescence quantum yields (Ff) of the compound solutions were estimated by comparing wavelength-integrated fluorescence intensity of the compound solutions with that of the reference. Quinine sulfate in 0.1 M H2SO4 with Ff ¼ 0.53  0.023 was used as a reference [18]. Optical densities of the reference and the sample solutions were kept below 0.05 to avoid reabsorption effects. Estimated quantum yields of the compound solutions were verified using an integrating sphere method [19]. The latter method was also employed in the evaluation of Ff of the compounds neat films. Fluorescence transients of the samples were measured using a time-correlated single photon counting system PicoHarp 300 (PicoQuant) utilizing semiconductor diode laser (repetition rate 1 MHz, pulse duration 70 ps, emission wavelength 375 nm) as an excitation source. The ionization energy (EI) of the films of the synthesized compounds was measured by the electron photoemission in air method as described elsewhere [20,21]. The samples for the measurements were prepared by dissolving materials in THF and by coating on Al plates pre-coated with w0.5 mm thick methylmethacrylate and methacrylic acid copolymer adhesive layer. The measurement error is evaluated as 0.03 eV. Hole drift mobility was measured by xerographic time of flight technique [22,23]. The samples for the charge carrier mobility measurements were prepared as described elsewhere [24]. CV measurements were carried out by a three-electrode assembly cell from Bio-Logic SAS and a micro-AUTOLAB Type III potentiostat-galvanostat. The measurements were carried out at a glassy carbon electrode in dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as electrolyte and Ag/AgNO3 as the reference electrode.

575

Oxidation potentials were obtained as an average value between red=ox each anodic and corresponding cathodic potential: E1=2 ¼ 1=2ðEpc þ Epa Þ. HOMO energy levels were estimated on the basis of the reference energy level of ferrocene (4.8 eV below the vacuum Fc ) eV below the level [25]) according to HOMO ¼ 4.8 þ (E1=2  E1=2 op vacuum level. The optical band gaps ðEg Þ were estimated from the edges of electronic absorption spectra, while LUMO values were estimated using equation LUMO ¼ HOMO  Egop .

2.2. Materials Compounds 4-bromo-20 -nitrobiphenyl [26], 2-bromocarbazole [26], 2-bromo-9-ethylcarbazole [27], 4,40 -dibromo-2nitrobiphenyl [28], 2,7-dibromocarbazole [28], 2,7-dibromo-9ethylcarbazole [12], 3-iodocarbazole [29], 3-iodo-9-ethylcarbazole [30], 3,6-diiodocarbazole [29], 3,6-diiodo-9-ethylcarbazole [31] were prepared according to the published procedures. 2-Di(4-methylphenyl)amino-9-ethylcarbazole (1). Tris-(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3, 0.2 g, 0.2 mmol) and tritert-butylphosphine ((t-Bu)3P, 0.04 g, 0.2 mmol) were dissolved under argon in 20 ml of dry toluene and stirred for 10 min at room temperature (preformation of the catalyst). Then the mixture of 2bromo-9-ethylcarbazole (1.5 g, 5.5 mmol), 4,40 -dimethyldiphenylamine (1.3 g, 6.6 mmol) and sodium tert-butoxide (3.1 g, 32.8 mmol) in 40 ml of dry toluene were added. The reaction mixture was heated at 90
C for 24 h. After recooling, the reaction mixture was diluted with ethyl acetate and the organic phase was washed with water and brine. After being dried over MgSO4 and filtered, the solvent was removed and the residue was purified by column chromatography using hexane/acetone (6/1) as an eluent. It was recrystallized from methanol. The yield of white crystals was 70% (1.5 g). Mp ¼ 153e154
C.

    MS APClþ ; 20V ; m=zð%Þ : 391 ½M þ Hþ ; 100 : Elemental analysis. Calcd. for C28H26N2 (%): C 86.12, H 6.17, N 7.17. Found (%): C 86.19, H 6.28, N 7.12. 1 H NMR (300 MHz, CDCl3, d, ppm): 1.39 (t, J ¼ 7.3 Hz, 3H, CH3), 2.39 (s, 6H, CH3), 4.24 (q, J ¼ 6.9 Hz, 2H, NCH2), 6.99 (dd, J ¼ 2.2 Hz, J ¼ 8.4 Hz, 1H, Ar), 7.10e7.16 (m, 9H, Ar), 7.23 (t, J ¼ 8.0 Hz, 1H, Ar), 7.38e7.45 (m, 2H, Ar), 7.96 (d, J ¼ 8.4 Hz, 1H, Ar), 8.04 (d, J ¼ 7.7 Hz, 1H, Ar). 13 C NMR (75.4 MHz, CDCl3, d, ppm): 14.0 (CH3), 21.1 (CH3), 37.6 (CH2), 104.0, 108.5, 116.8, 118.5, 119.1, 120.0, 121.1, 123.3, 124.3, 124.9, 130.0, 132.2, 141.3, 146.3, 147.0. IR. nmax in cm1 (KBr): (CeH Ar) 3050, 3019, (CeH) 2978, 2935, (C]C Ar) 1507, 1495, (CeN) 1226. 2,7-Di[di(4-methylphenyl)amino]-9-ethylcarbazole (2) was prepared according to the procedure similar to that described for 1. 0.1 g (0.11 mmol) of Pd2(dba)3, 0.02 g (0.11 mmol) of (t-Bu)3P, 1 g (2.8 mmol) of 2,7-dibromo-9-ethylcarbazole, 1.2 g (5.9 mmol) of 4,40 -dimethyldiphenylamine, 1.6 g (17 mmol) of sodium tertbutoxide, and 25 ml of dry toluene were used. The product was purified by column chromatography using hexane/acetone (10/1) as an eluent. The yield of yellow crystals was 62% (1.03 g). Mp ¼ 258e259
C.

    MS APClþ ; 20V ; m=zð%Þ : 586 ½M þ Hþ ; 100 : Elemental analysis. Calcd. for C42H39N3 (%): C 86.12, H 6.71, N 7.17. Found (%): C 85.99, H 6.81, N 6.98. 1 H NMR (300 MHz, CDCl3, d, ppm): 1.27 (t, J ¼ 7.3 Hz, 3H, CH3), 2.37 (s, 12H, CH3), 4.10 (q, J ¼ 6.9 Hz, 2H, NCH2), 6.90e7.29 (m, 20H, Ar), 7.84 (d, J ¼ 8.3 Hz, 2H, Ar).

576

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13 C NMR (75.4 MHz, CDCl3, d, ppm): 14.0 (CH3), 21.1 (CH3), 37.3 (CH2), 104.4, 117.1, 120.4, 120.5, 124.0, 124.1, 130.0, 131.9, 141.6, 146.2. IR. nmax in cm1 (KBr): (CeH Ar) 3054, 3021, (CeH) 2979, 2937, n (C]C Ar) 1506, 1457, (CeN) 1272. 3-Di(4-methylphenyl)amino-9-ethylcarbazole (1A) was obtained by an improved Ullmann coupling reaction. 1 g (3.1 mmol) of 3iodocarbazole, 0.74 g (3.7 mmol) of 4,40 -dimethyldiphenylamine, 3.5 g (24.9 mmol) of powdered anhydrous potassium carbonate, 0.79 g (12.5 mmol) of copper powder, and 0.11 g (0.4 mmol) of 18crown-6 were refluxed in o-dichlorobenzene (10 ml) under argon for 24 h. Then copper and inorganic salts were removed by filtration of the hot reaction mixture. The solvent was removed under reduced pressure and the crude product was purified by column chromatography using hexane/acetone (10/1) as an eluent. The yield of yellowish powder was 80% (0.97 g).

    MS APClþ ; 20V ; m=zð%Þ : 391 ½M þ Hþ ; 100 : Elemental analysis. Calcd. for C28H26N2 (%): C 86.12, H 6.17, N 7.17. Found (%): C 86.01, H 6.42, N 7.01. 1 H NMR (300 MHz, CDCl3, d, ppm): 1.51 (t, J ¼ 7.3 Hz, 3H, CH3), 2.37 (s, 6H, CH3), 4.40 (q, J ¼ 6.9 Hz, 2H, NCH2), 7.05e7.12 (m, 8H, Ar), 7.23 (t, J ¼ 8.0 Hz, 1H, Ar), 7.31e7.53 (m, 4H, Ar), 7.95 (s, 1H, Ar), 8.00 (d, J ¼ 8.0 Hz, 1H, Ar). 13 C NMR (75.4 MHz, CDCl3, d, ppm): 14.2 (CH3), 21.0 (CH3), 37.9 (CH2), 108.8, 109.4, 118.6, 118.9, 120.9, 123.0, 124.0, 125.5, 126.0, 129.9, 131.1, 137.2, 140.3, 140.7, 146.8. IR. nmax in cm1 (KBr): (CeH Ar) 3049, 3022, (CeH) 2976, 2920, n (C]C Ar) 1507, 1482, (CeN) 1226. 3,6-Di[di(4-methylphenyl)amino]-9-ethylcarbazole (2A) was prepared according to the procedure similar to that described for 1A. 0.5 g (1.1 mmol) of 3,6-diiodo-9-ethylcarbazole, 0.48 g (2.5 mmol) of 4,40 -dimethyldiphenylamine, 1.2 g (8.8 mmol) of potassium carbonate, 0.28 g (4.4 mmol) of copper powder, 0.2 g (0.7 mmol) of 18-crown-6, and 10 ml of o-dichlorobenzene were used. The product was purified by column chromatography using hexane/acetone (10/1) as an eluent. The yield of yellow crystals was 64% (0.42 g). Mp ¼ 223e224
C.

    MS APClþ ; 20V ; m=zð%Þ : 586 ½M þ Hþ ; 100 : Elemental analysis. Calcd. for C42H39N3 (%): C 86.12, H 6.71, N 7.17. Found (%): C 86.16, H 6.80, N 7.21. 1 H NMR (300 MHz, CDCl3, d, ppm): 1.51 (t, J ¼ 7.3 Hz, 3H, CH3), 2.34 (s, 12H, CH3), 4.37 (q, J ¼ 6.8 Hz, 2H, NCH2), 6.99e7.08 (m, 16H, Ar), 7.28-7.36 (m, 4H, Ar), 7.76 (s, 2H, Ar).

N Br

3. Results and discussion 2- and 2,7-substituted carbazole compounds 1 and 2 were synthesized as described in Scheme 1 by a palladium-catalyzed aromatic CeN coupling reaction of 2-bromo-9-ethylcarbazole or 2,7-dibromo-9-ethylcarbazole with 4,40 -dimethyldiphenylamine. For the comparison of the properties 3- and 3,6-substituted analogues of 1 and 2, i.e. compounds 1A and 2A were synthesized by Ullmann coupling reaction of 3-iodo-9-ethylcarbazole or 3,6diiodo-9-ethylcarbazole with 4,40 -dimethyldiphenylamine (Scheme 1). The chemical structures of the synthesized compounds were confirmed by 1H and 13C NMR, IR spectroscopies, mass spectrometry and elemental analysis. The glass-forming capability and thermal stability of the materials were estimated by DSC and TGA, respectively. The thermal characteristics of the carbazole derivatives 1, 2, 1A and 2A are summarized in Table 1. The values of 5% weight loss temperature (TID) of the synthesized compounds range from 321 to 411
C. Compounds 2 and 2A having two dimethyldiphenylamino groups exhibit higher 5% weight loss temperatures than their monosubstituted counterparts 1 and 1A. 2- and 2,7-substituted carbazole compounds 1 and 2 show superior thermal stability than the corresponding 3- and 3,6-substituted analogues 1A and 2A. All the synthesized compounds are capable of glass formation with glass transition temperatures (Tg) ranging from 38 to 100
C. Endothermic melting peaks were observed for the synthesized compounds, except for 1A, in the first DSC heating scans. In the second DSC heating scans, compound 2 and its 3,6-disubstituted analogue 2A revealed not only a glass transition but also crystallization and melting (Fig. 1). Meanwhile, no crystallization exotherm and melting endotherm were observed in the 2nd heating scan of compound 1. The material remained in the glassy state after melting and subsequent cooling. Compound 1A was isolated as an amorphous substance. When its sample was heated, a glass transition was observed at 39
C and no peaks due to crystallization and melting appeared. Cooling down and the following repeated heating revealed only a glass transition. The amorphous nature and low glass transition temperature of 1A can apparently be explained by the relatively loose packing of the molecules.

N

N

N Br

13 C NMR (75.4 MHz, CDCl3, d, ppm): 13.6 (CH3), 21.0 (CH3), 37.3 (CH2), 109.5, 118.8, 122.8, 123.0, 123.7, 125.3, 129.9, 131.1, 134.9, 140.2. IR. nmax in cm1 (KBr): (CeH Ar) 3053, 3022, (CeH) 2973, 2917, n (C]C Ar) 1507, 1483, (CeN) 1274.

Br I

I Pd2(dba)3, P(t-Bu)3, t-BuONa, Toluene

H

Cu, K2CO3, 18-crown-6, o-DCB

N

I

H N

N

N N

N N

N

N

N

1A 1

2 Scheme 1. Synthesis of 2(3) and 2,7(3,6)-substituted carbazole compounds.

N

N

2A

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580

577










Compound

Tg, [ C]

Tm, [ C]

Tcr, [ C]

TID, [ C]

1 2 1A 2A

40 100 38 99

151a 256 eb 223

e 168 e 145

342 411 321 373

b

3x104

Melting point was only detected during the first heating. Obtained as an amorphous materials.

Tg of the synthesized compounds is substitution pattern independent, i.e. 2- and 2,7-substituted carbazole derivatives 1 and 2 have nearly the same Tg value as the corresponding 3- and 3,6substituted counterparts 1A and 2A. However, the glass-transition temperatures of the disubstituted derivatives 2 and 2A are more than twice higher than those of the monosubstituted counterparts. Absorption and fluorescence spectra of the dilute THF solutions of compounds 1, 1A, 2 and 2A are shown in Fig. 2. The details of the photophysical properties of the compounds are summarized in Table 2. The lowest-energy absorption bands of the mono- as well as disubstituted carbazole derivatives are located at 355e405 nm, that is, shifted to longer wavelengths in respect to the absorption of single carbazole moiety (345 nm) [32] implying extended conjugation. Fluorescence spectra of all the derivatives display maxima at 400e437 nm, which again, if compared with the fluorescence maximum of carbazole moiety (365 nm), are considerably red-shifted indicating extention of electron wavefunctions from the carbazole to diphenylamino groups. Obviously, an oscillator strength of the lowest-energy absorption bands of the disubstituted carbazole compounds 2, 2A is nearly twice as large as that of the monosubstituted compounds 1, 1A as a result of a double number of diphenylamino chromophores. Note, however, that this effect is more pronounced for 2-, 2,7-substituted carbazole compounds as compared to that of 3-, 3,6-substituted analogues. In agreement with density functional theory calculations of the similar 2,7- and 3,6-disubstituted carbazole compounds [33], the absorption edge of the compounds 1A and 2A is formed by weaklyallowed S0 / S1 transition, which is forbidden for the compounds 1 and 2. As opposed to this, oscillator strength of the S0 / S2 transition for 2-, 2,7-substituted carbazole compounds is predicted to be several times larger than that for 3-, 3,6-substituted analogues. The theoretical predictions well correspond to our experimental results and also explain slightly red shifted absorption of the compounds 1A and 2A in respect to that of the compounds 1 and 2. The similar behavior was also observed in the fluorescence spectra of the mono- and disubstituted carbazole compounds. The fluorescence bands of the disubstituted compounds 2, 2A experienced

o

Tm = 256 C



2

st

1 heating

o

nd

2 heating

Tg = 100 C

o

Tcr = 168 C

0

25

50

75 100 125 150 175 200 225 250 o

Temperature, [ C] Fig. 1. DSC curves of compound 2 (heating rate 10
C min1, N2 atmosphere).

1x104

1 Φ f =0.26

1

0

b

2A

4x104

0 1

2A Φ f =0.11

3x104 2x104

1

1A Φ f =0.12

1A

2x104

ε (M-1cm-1)

a

a

4x104




2

2 Φ f =0.45

Norm. Fluorescence Intensity (arb. units)

Table 1 Thermal characteristics of compounds 1, 2 and 1A, 2A.

1x104 0

300

400

500

0

Wavelength (nm) Fig. 2. Absorption (solid line) and fluorescence (dashed line) spectra of dilute THF solutions (105 M) of a) 2- and 3-monosubstituted carbazole compounds 1 and 1A, and b) 2,7- and 3,6-disubstituted carbazole compounds 2 and 2A. Fluorescence quantum yields indicated.

red shift in respect to those of the monosubstituted counterparts 1, 1A. The fluorescence quantum yields of 3- and 3,6-substituted derivatives (1A and 2A) were rather low and close (Ff ¼ 0.11e 0.12), whereas fluorescence quantum yields of 2- and 2,7substituted compounds were estimated to be somewhat higher, i.e. 0.26 for the monosubstituted carbazole compound 1 and 0.45 for the disubstituted compound 2. Interestingly, irrespective of the substitution pattern, the disubstituted carbazole derivatives demonstrated twice as small Stokes shift (w0.2 eV) as compared to that of the monosubstituted derivatives (0.39 eV). Fluorescence spectra of the thin films of compounds 1, 1A, 2 and 2A are depicted in Fig. 3. Essentially, the spectral shapes of the compounds are found to be very similar to those observed for dilute solutions. However, a closer inspection of the spectrum of compound 2 allows to detect additional emission band in the longwavelength tail (>500 nm) of the spectrum. As compared to the solution spectra, the maxima of the fluorescence spectra of the films are slightly red-shifted by ca. 4e10 nm as a result of the intermolecular interaction in the solid state, which also causes lowering (in most cases) of the fluorescence quantum yields. The solid state enables excitation migration over molecules via hopping process by permitting excitons to reach nonradiative decay sites (lattice defects, distortions etc), and thus, degrade their radiative properties [8,34]. The drop of Ff in the films of compounds 1A and 2A is very small (up to 3%), whereas for compounds 1 and 2 the drop of Ff is much larger (up to 41%). A similar Ff behavior was also observed for 2,7- and 3,6-substituted carbazole trimers [33]. The largest Ff drop of 41% experiencing disubstituted carbazole compound 2 also possesses the distinct long-wavelength emission tail as discussed above, which most likely originates from nonemissive excimer-like states. The most elongated and symmetrical molecular geometry of the 2,7-substituted carbazole derivative 2 obviously facilitates tight arrangement of the molecules in the solid film, which enhances excitonic effects. The latter result precludes utilization of compound 2 as a blue emitter in the neat form, however, it does not rule out the alternative possibility of the compound usage as a blue dopant.

578

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Table 2 Photophysical properties of 105 M THF solutions and thin films of 2(3)- and 2,7(3,6)-substituted carbazole compounds 1(1A) and 2(2A), respectively. Compd.

Thin film

Dilute solution

labs, [nm]

ε, [M1 cm1]

lmax , [nm] f

Ff

s, [ns]

kr, [ 108 s1]

knr, [ 108 s1]

Stokes shift, [eV]

lmax , [nm] f

Ff

s,a [ns]

1

268, 317, 355

31400, 20300, 20500

400

0.26

1.4

1.86

5.29

0.39

410

0.17

1A

305, 375

32600, 2200

425

0.12

5.6

0.21

1.57

0.39

429

0.09

2

276, 311, 384

38500, 28700, 40700

407

0.45

1.4

3.21

3.93

0.18

412

0.04

2A

308, 361, 405

46500, 7100, 2000

437

0.11

7.0

0.16

1.27

0.22

444

0.12

0.4 (52%) 1.2 (26%) 7.2 (22%) 0.26 (6%) 2.6 (21%) 6.8 (73%) 0.1 (51%) 0.8 (9%) 6.4 (40%) 1.2 (39%) 6.6 (61%)

a

Fractional intensities of each fluorescence decay component are indicated in brackets.

Fig. 4 illustrates fluorescence transients of the dilute THF solutions of 2- and 3-monosubstituted carbazole compounds 1 and 1A, and 2,7- and 3,6-disubstituted carbazole compounds 2 and 2A. The transients follow single exponential decay profile with the decay time constants (s) of 1.4 ns for compounds 1 and 2, and 5.6 and 7.0 ns for compounds 1A and 2A, respectively. To reveal the contributions of the competing radiative and nonradiative excitedstate relaxation mechanisms, the radiative and nonradiative relaxation rates, kr and knr, respectively, were evaluated for all the studied compounds (see Table 2). As expected from the modest fluorescence quantum yields (Ff < 0.5) of the compounds, the nonradiative decay unambiguously dominates in excited-state relaxation processes, and it changes only slightly with the number of diphenylamino substituents (compare 1 to 2 and 1A to 2A). However, the flexible dimethyldiphenylamino substituents at 2-, 2,7- positions, in contrast to the more rigid carbazolyl substituents [33], enhance knr for ca 3 times as compared to that observed for 3-, 3,6-substituted derivatives (cf. knr of 1 with that of 1A and of 2 with that of 2A). The most important result is the tremendous

a

1

Norm. Fluorescence Intensity (arb. units)

enhancement of the radiative relaxation rate, which occurs upon substitution pattern modification. In the case of monosubstituted compounds, change of the substitution pattern from 3- to 2(compounds 1A and 1) results in nearly 10-fold increase of kr, whereas in the case of disubstituted derivatives, alteration of the substitution pattern from 3,6- to 2,7- (compounds 2A and 2) boosts kr up to 20 times. Note that the increase of kr perfectly correlates with the increase of extinction coefficient (ε) of the lowest-energy absorption bands, thus again confirming importance of the radiative processes upon variation of the substitution pattern. The remarkable enhancement of radiative relaxation rate for 2-, 2,7substituted carbazole compounds caused by the 2e4-fold increased fluorescence quantum yield and 4e5-fold shortened excited-state relaxation time demonstrates an advantage of this

1A film Φf =0.09 1 film Φ f =0.17 0 1

b 2A film Φf =0.12 2 film Φf =0.04

0

400

500

600

Wavelength (nm) Fig. 3. Fluorescence spectra of the neat films of a) 2- and 3-monosubstituted carbazole compounds 1 and 1A, and b) 2,7- and 3,6-disubstituted carbazole compounds 2 and 2A. Fluorescence quantum yields indicated.

Fig. 4. Fluorescence transients of dilute THF solutions (105 M) of a) 2(3)-monosubstituted carbazole compounds 1(1A) and b) 2,7(3,6)-disubstituted carbazole compounds 2(2A) measured at fluorescence band maxima. Lines indicate singleexponential fits of the experimental data.

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580

substitution pattern against 3-, 3,6-substitution pattern. As revealed by the comparative experimental and theoretical analysis of the similar carbazole derivatives, i.e. 2,7- and 3,6-substituted carbazole trimers [33], the 2,7-substitution pattern facilitates extension of the p-conjugation over both side-substituents via central carbazole moiety, whereas the 3,6-substitution pattern restricts the p-conjugation within only one side substituent and the central carbazole group. This substantial feature implies twice as large transition dipole moment in the 2,7-substituted carbazole compounds and originates from the more elongated and more symmetrical shape of the molecules. As opposed to the monoexponential fluorescence transients of the compound solutions, fluorescence decay profiles of the compound thin films (not shown) exhibit highly non-exponential behavior. This behavior accompanied by the red shifted fluorescence bands in the solid films is a signature of dispersive exciton hopping through the localized states in the disordered media [35] and can be attributed to the spectral diffusion phenomenon [8]. The fluorescence decay profiles were fitted by using two- or threecomponent exponential decay models to disclose the main contributing component (see Table 2). All the compounds feature fast (sub-nanosecond) initial excited state decay in the condensed phase, which is mainly caused by the excitation migration facilitating excitation quenching at nonradiative decay sites. However, it is worth mentioning that the major decay components (with fractional intensities >50%) of the 3- and 3,6-substituted compounds 1A and 2A are 6.8 ns and 6.6 ns, respectively, which are similar to those measured in the compound solutions (5.6 ns and 7.0 ns). Conversely, the dominant excited state decay components of the 2and 2,7-substituted carbazole compounds 1 and 2 are 0.4 ns and 0.1 ns, respectively, which are up to an order of magnitude faster than the decay time constants of the compound solutions. The substantial difference of the dominant decay components again points out more favorable molecular packing in the films of 2-, 2,7substituted carbazole compounds 1 and 2, which enhances exciton migration and migration induced exciton quenching at nonradiative decay centers. To elucidate the energetic conditions for energy and electron transfer in dilute solutions, the HOMO/LUMO values were estimated by performing cyclic voltammetry (CV) measurements. The cyclic voltammograms of the solutions of all the synthesized compounds in dichloromethane show reversible oxidation behavior. Fig. 5 illustrates CV curves of the 2-substituted carbazole compound 1 and of its 3-substituted analogue 1A for the comparison. The CV curve of the compound 1 is positively shifted as compared to that of the compound 1A. In contrast to the

579

Table 3 HOMO, LUMO, EI, band gap energies, and electrochemical characteristics of 1, 2 and 1A, 2A.a Compound

E1/2, [V]

Egop b, [eV]

EIc, [eV]

EHOMOd, [eV]

ELUMOe, [eV]

1 2 1A 2A

0.44 0.25 0.35 0.27

3.20 3.06 3.00 2.89

5.28 5.16 5.28 5.17

5.00 4.81 4.91 4.83

1.80 1.75 1.91 1.94

a The CV measurements were carried out at a glassy carbon electrode in dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as electrolyte and Ag/AgNO3 as the reference electrode. Each measurement was calibrated with ferrocene (Fc). b The optical band gaps Egopt estimated from the edges of electronic absorption spectra. c Ionization energy was measured by the photoemission in air method from films. d Fc Þ, when E Fc ¼ 0.24 V. EHOMO ¼ 4:8 þ ðE1=2  E1=2 1=2 e ELUMO ¼ EHOMO  Egopt .

triphenylamino substituted derivatives of oxadiazole which showed anodic electropolymerization [36], the repeated scans for the synthesized compounds revealed no any signs of electropolymerization. This was apparently due to the presence of methyl groups in the para positions of the diphenylamino moieties. We assume that diphenylamino substituent is able to prevent electrochemical coupling by stabilization of the radical cations of 1A thus preventing it from electrodimerization. Taking HOMO energy level (4.8 eV) of the ferrocene/ferrocenium redox system as a reference [37], HOMO energies for the synthesized compounds were calculated. The obtained values are given in Table 3. Note that EHOMO values may not represent absolute solid-state or gas-phase ionization energies, however they can be used to compare the different compounds. The EHOMO energies of the monosubstituted carbazole compounds 1 and 1A are somewhat lower than those of the disubstituted compounds 2 and 2A. Interestingly, the EHOMO are only weakly affected by the positions of dimethyldiphenylamino groups. EHOMO values together with optical band gap energies ðEgop Þ were used to calculate the LUMO energy levels. The LUMO energies were found to be dependent on the linking topology of the substituents. The compounds 1 and 2 having dimethyldiphenylamino groups at 2 and 2,7 positions of the carbazole ring, respectively, showed slightly higher LUMO energy levels than the corresponding compounds 1A and 2A with the substituents at 3 and 3,6 positions. When considering the use of an organic materials for holetransporting applications it is important to have an understanding of their solid state ionization energies (EI). The EI were measured by the electron photoemission in air method. The results obtained are summarized in Table 3. Usually the photoemission

-3

10

-4

2

I, [μA]

μ, [cm /Vs]

1x10

-5

1x10

-6

10

Al+[1+(PC-Z), 1:1] MC+[2+(PC-Z), 1:1] Al+[1A+(PC-Z), 1:1] MC+[2A+(PC-Z), 1:1]

-7

10

-8

10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

+

0

200

400

600 1/2

800

1000

1200

1400

1600

1/2

E , [V/cm]

E vs Ag/Ag , [V] Fig. 5. Cyclic voltammograms of 1 (dashed line) and 1A (solid line) in argon-purged dichloromethane solution (scan rate of 50 mV s1).

Fig. 6. Electric field dependencies of the hole drift mobilities (m) in charge transport layers of the synthesized compounds 1, 1A, 2 and 2A doped in PC-Z (mass proportion 1:1).

580

A. Tomkeviciene et al. / Dyes and Pigments 96 (2013) 574e580

Table 4 Hole mobility data for the molecular mixtures of compounds 1, 2 and 1A, 2A with PC-Z. Layer composition

da, [mm]

mob, [cm2 V1 s1]

1 þ PC-Z, 1:1 2 þ PC-Z, 1:1 1A þ PC-Z, 1:1 2A þ PC-Z, 1:1

5.4 7.0 4.5 9.0

3.7 4.7 1.4 9.0

a b c d

   

108 107 106 107

mc, [cm2 V1 s1] 1.1 6.0 3.8 8.0

   

105 106 105 106

ad, [cm1/2 V1/2] 0.0071 0.0032 0.0042 0.0027

Layer thickness. Mobility value at zero field strength. Mobility value at 6.4  105 V cm1 field strength. The PooleeFrenkel parameter.

experiments are carried out in vacuum, since the sample surface oxidation and gas adsorption distort measurement results. In our case, the investigated materials were stable in respect to oxidation and the measurements could be carried out in air. The measured EI of the disubstituted carbazole compounds 2 and 2A were found to be lower than those of the corresponding monosubstituted compounds 1 and 1A. In agreement with the CV results, this observation indicates that EI values, in close similarity to EHOMO values, are affected by the number of dimethyldiphenylamino groups, and relatively unaffected by the positions of these groups. Charge transport properties of the synthesized dimethyldiphenylamino-substituted carbazole compounds 1, 2 and 1A, 2A were studied by the xerographic time-of-flight technique. Fig. 6 shows electric field dependencies of hole drift mobilities (m) of the films of the molecular mixtures of 1, 2 and 1A, 2A with bisphenol Z polycarbonate (PC-Z). At room temperature, m showed linear dependencies vs the square root of the electric field. Such dependencies of charge mobility on electrical field are characteristic for many organic charge-transporting materials and are predicted by the BasslereBorsenberger model [38]. The solid solutions of 1, 2 and 1A, 2A in PC-Z demonstrated hole drift mobility values in the range from 6.0 ✕ 106 cm2 V1 s1 to 3.8 ✕ 105 at an electric field of 6.4 ✕ 105 V cm1 at room temperature (Table 4). The hole drift mobility values are rather similar and weakly dependent on the positions of dimethyldiphenylamino groups. The solid solutions of the monosubstituted carbazole compounds 1 and 1A in bisphenol Z polycarbonate exhibited slightly higher hole drift mobility values than those of the disubstituted compounds 2 and 2A. This observation can apparently be explained by the more favorable packing of the monosubstituted compounds in PC-Z. These hole drift mobilities are rather high keeping in mind that charge mobilities in pure amorphous molecular materials are usually by 1e2 orders of magnitude higher than those observed in molecularly doped polymers [39]. 4. Conclusions We have synthesized new dimethyldiphenylamino-substituted carbazole compounds. The glass transition temperatures of the synthesized compounds range from 38 to 100
C. The advantage of 2-, 2,7-substitution pattern of the carbazole compounds against 3-, 3,6-substitution pattern is demonstrated via 10 times enhanced radiative decay rates resulting in 2e4-fold increased fluorescence quantum yield and 4e5-fold shortened excited-state decay time. Meanwhile in the solid state, 2-, 2,7-substituted carbazole derivatives feature more favorable molecule arrangement for the enhanced exciton migration and migration induced exciton quenching, which results in the significantly reduced excited state decay times (down to sub-nanoseconds) and fluorescence quantum yields (down to 0.04) as compared to those of the 3-, 3,6-

substituted analogues. The ionization energies and HOMO energy levels of these compounds are affected by the number of dimethyldiphenylamino groups and unaffected by the linking topology of these groups. Hole drift mobilities in the films of the molecular mixtures of the synthesized compounds with bisphenol Z polycarbonate reach 105 cm2 V1 s1 at high electric fields. Acknowledgements A. Tomkeviciene acknowledges European Union Structural Funds project ”Postdoctoral Fellowship Implementation in Lithuania” for funding her postdoctoral fellowship. We thank Habil. Dr. V. Gaidelis for the help in ionization potential measurements. References [1] Pron A, Gawrys P, Zagorska M, Djurado D, Demadrille R. Chem Soc Rev 2010; 39:2577. [2] Shirota Y, Kageyama H. Chem Rev 2007;107:953. [3] Sanyin Q, Tian H. Chem Commun 2012;48:3039. [4] Huang JH, Su JH, Li X, Lam MK, Fung KM, Fan HH, et al. J Mater Chem 2011;21: 2957. [5] Tong QX, Chan MY, Lai SL, Ng TW, Wang PF, Lee CS, et al. Dyes Pigments 2010; 86:233. [6] Morin JF, Leclerc M, Ades D, Siove A. Macromol Rapid Commun 2005;26:761. [7] Blouin N, Leclerc M. Acc Chem Res 2008;41:1110. [8] Karpicz R, Puzinas S, Krotkus S, Kazlauskas K, Jursenas S, Grazulevicius JV, et al. J Chem Phys 2011;134:204508. [9] Grigalevicius S. Synt Met 2006;156:1. [10] Thomas KRJ, Lin JT, Tao YT, Ko CW. J Am Chem Soc 2001;123:9404. [11] Niu YH, Huang J, Cao Y. Adv Mater 2003;15:807. [12] Shen JY, Yang XL, Huang TH, Lin JT, Ke TH, Chen LY, et al. Adv Funct Mater 2007;17:983. [13] Krotkus S, Kazlauskas K, Miasojedovas A, Gruodis A, Tomkeviciene A, Grazulevicius JV, et al. J Phys Chem C 2012;116:7561. [14] Drolet N, Morin JF, Leclerc N, Wakim S, Tao Y, Leclerc M. Adv Funct Mater 2005;15:1671. [15] Li JL, Grimsdale AC. Chem Soc Rev 2010;39:2399. [16] Chang CC, Wu JY, Chien CW, Wu WS, Liu H, Kang CC, et al. Anal Chem 2003; 75:6177. [17] Sakalyte A, Simokaitiene J, Tomkeviciene A, Keruckas J, Buika G, Grazulevicius JV, et al. J Phys Chem C 2011;115:4856. [18] Adams MJ, Highfield JG, Kirkbright GF. Anal Chem 1977;49:1850. [19] Mello JC, Wittmann HF, Friend RH. Adv Mater 1997;9:230. [20] Malinauskas T, Daskeviciene M, Kazlauskas K, Su HC, Grazulevicius JV, Jursenas S, et al. Tetrahedron 2011;67:1852. [21] Miyamoto E, Yamaguchi Y, Yokoyama M. Electrophotography 1989;28:364. [22] Montrimas E, Gaidelis V, Pazera A. Lith J Phys 1996;6:569. [23] Gudeika D, Lygaitis R, Mimaite V, Grazulevicius JV, Jankauskas V, Lapkowski M, et al. Dyes Pigments 2011;91:13. [24] Matoliukstyte A, Grazulevicius JV, Jankauskas V. Mol Cryst Liq Cryst 2007; 466:85. [25] Thelakkat M, Schmidt HW. Adv Mater 1998;10:219. [26] Lux M, Strohriegl P, Hoecker H. Macromol Chem 1987;188:811. [27] Limburg WW, Yanus JF, Williams DJ, Goedde AO, Pearson JM. J Polym Sci Polym Chem Ed 1975;13:1133. [28] Dierschke F, Grimsdale AC, Mullen K. Synthesis 2003:2470. [29] Tucker SH. J Chem Soc 1926;1:548. [30] Grigalevicius S, Tsai MH, Grazulevicius JV, Wu CC. J Photochem Photobiol A: Chem 2005;174:125. [31] Rodriguez-Parada JM, Percec K. Macromolecules 1986;19:55. [32] Simokaitiene J, Grigalevicius S, Grazulevicius JV, Rutkaite R, Kazlauskas K, Jursenas S, et al. J Optoelectron Adv Mater 2006;8:876. [33] Tomkeviciene A, Grazulevicius JV, Kazlauskas K, Gruodis A, Jursenas S, Ke TH, et al. J Phys Chem C 2011;115:4887. [34] Sivamurugan V, Kazlauskas K, Jursenas S, Gruodis A, Simokaitiene J, Grazulevicius JV, et al. J Phys Chem B 2010;114:1782. [35] Kersting R, Mollay B, Rusch M, Wenisch J, Leising G, Kauffmann HF. J Chem Phys 1997;106:2850. [36] Kamtekar KT, Wang C, Bettington S, Batsanov AS, Perepichka IF, Bryce MR, et al. J Mater Chem 2006;16:3823. [37] Thelakkat M, Ostrauskaite J, Leopold A, Bausinger R, Haarer D. Chem Phys 2002;285:133. [38] Borsenberger PM, Weiss DS. Organic photoreceptors for xerography. New York: Dekker; 1998. p. 768. [39] Grigalevicius S, Blazys G, Ostrauskaite J, Grazulevicius JV, Gaidelis V, Jankauskas V. J Photochem Photobiol A: Chem 2003;154:161.