Effect of linking topology on the properties of star-shaped derivatives of triazine and fluorene

Synthetic Metals 195 (2014) 266–275

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Effect of linking topology on the properties of star-shaped derivatives of triazine and fluorene Nadzeya A. Kukhta a , Jurate Simokaitiene a , Dmytro Volyniuk a , Jolita Ostrauskaite a , Juozas V. Grazulevicius a,∗ , Gytis Juska b , Vygintas Jankauskas b a b

Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT-10222, Vilnius, Lithuania

a r t i c l e

i n f o

Article history: Received 5 February 2014 Received in revised form 5 June 2014 Accepted 18 June 2014 Keywords: Dendrimers Linking topologies Solvatochromism Glass-forming Fluorescence efficiency Hole mobility

a b s t r a c t Three star-shaped molecules, having 2,4,6-triphenyl-1,3,5-triazine core and fluorene side arms linked through different linkages, were designed and synthesized. The obtained compounds were characterized by UV and fluorescence spectroscopies, differential scanning calorimetry, thermogravimetric analysis, cyclic voltammetry, time-of-flight and CELIV techniques. All the three star-shaped compounds possess high thermal stability with the temperatures of the onsets of thermal degradation around 400 ◦ C and glass formation ability with close glass transition temperatures (56–61 ◦ C). The synthesized compounds show broadband absorption with the absorption maxima of dilute solutions in the range of 350–382 nm. Dilute solutions of the fluorenyl-substituted derivatives of 2,4,6-triphenyl-1,3,5-triazine showed monomer fluorescence with fluorescence quantum yields ranging from 0.50 to 0.70. The theoretical DFT calculations showed that the geometry, optical and electrochemical properties of the synthesized star-shaped molecules depend on their linking topologies. Thus, completely flat star-shaped molecules, in which the donor and acceptor moieties are linked through the linking bridges having double and triple bonds, are characterized by smaller optical band gap and bathochromic shift compared to the derivative with twisted skeleton in which the chromophores are linked directly via single bond. The best charge-transporting properties were shown by the compound in which 2,4,6-triphenyl-1,3,5-triazine and 2-[9,9-bis(2-ethylhexyl)-9H-fluorene] moieties are linked via the linking bridges containing ethenyl linkages. Hole mobility of the amorphous layer of this compound reached 1.9 × 10−3 cm/V s at an electric field of 1.15 × 106 V/cm. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic charge-transporting materials are desirable for various optoelectronic and electronic applications including light-emitting diodes, solar cells, field-effect transistors [1,2]. In recent years white organic light emitting devices (WOLEDs) have been attracting much interest [3]. Generation of white electroluminescence in WOLEDs involves simultaneous emission of light of the three primary colors (red, green and blue) or of two complementary colors (e.g. orange and blue). A convenient method to attain white light emission from a single compound is based on chemical species that simultaneously emit blue light from molecular exited states and red-orange light from excited aggregates (excimers or electromers) formed in the solid state [4]. White

∗ Corresponding author. Tel.: +370 37 300193; fax: +370 37 300152. E-mail address: [email protected] (J.V. Grazulevicius). http://dx.doi.org/10.1016/j.synthmet.2014.06.019 0379-6779/© 2014 Elsevier B.V. All rights reserved.

electroluminescence resulting from simultaneous emission of monomers and electromers was observed with the star-shaped 1,3,5-tris[2-(9-ethylcarbazolyl-3)ethylene]benzene emitter [3]. Solution-processable blue-emitting organic glass-forming materials thin films of which can be obtained by spin coating or casting are particularly attractive. Both polymers and low-molarmass molecular materials, such as dendrimers and star-burst molecules can be used for the solution processing [5]. In contrast to polymers, molecular glasses including dendritic ones possess well-defined and monodisperse molecular structures as well as superior chemical purity, what makes them more advantageous in comparison to polymers. Molecules having 1,3,5-triazine as a core are gathering considerable interest because of their high thermal stability, interesting optical and electrochemical properties [6–9]. 1,3,5-Triazine unit possesses structural symmetry and high electron affinity, which makes it useful as electron-accepting building block for the design and synthesis of star-shaped and dendritic electroactive molecules.

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1,3,5-Triazine derivatives were reported as electron transporting hosts for the highly efficient green phosphorescent OLEDs [10]. Incorporation of a donor moiety, such as fluorene, into the molecules with electron-deficient triazine core gives an opportunity to obtain materials with interesting photophysical and photoelectrical properties and to decrease the optical band gaps. Moreover, alkylated donor moieties provide good solubility of the materials in common organic solvents, what simplifies preparation of the devices. In this paper, we present the synthesis and comparative study of the properties of the series of star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine and dialkyl fluorene, in which the chromophores are linked via the different bridges containing single, double, and triple bonds. The influence of linking topologies on the properties of organic semiconductors is of great interest. It is known, that the emission color and charge transport can be controlled via the degree of conjugation [11]. Introduction of the phenylene unit induces blue-shift in the emission spectra, while units with prolonged conjugation induce red shifts [12]. Introduction of the triple bonds leads to less pronounced red-shifting of the absorption and emission spectra than introduction of the double bond [13]. Moreover, the relative planarity of the molecule affects the color of emission as more planar materials have more effective conjugation and thus smaller band gaps [14]. Twisting of the molecular skeleton results in blue-shift in the emission and decreases charge mobility [15]. All these peculiarities encouraged us to investigate in this work structure–property relationship of the triazine derivatives.

2. Experimental 2.1. Instrumentation NMR spectra were recorded on a Varian Inova 300 and Bruker DRX 500P spectrometers and chemical shifts are reported in parts per million relative to solvent residue peek as an internal standard. IR spectra were recorded in KBr pellets on a Perkin Elmer Spectrum GX II FT-IR System. Mass spectra were obtained by the MALDI-TOF method on Schimadzu Biothech Axima mass spectrometer. Elemental analysis was performed on the EuroEA Elemental Analyser. UV/vis and fluorescence spectra of 10−4 M solutions of the compounds were recorded in quartz cells using Perkin Elmer Lambda 35 spectrometer and Perkin Elmer LS55 fluorescence spectrometer respectively. Fluorescence quantum yields were determined using Perkin Elmer Lambda 35 spectrometer and Perkin Elmer LS55 fluorescence spectrometer by the comparative method reported by Williams et al. [16]. Thermogravimetric analysis (TGA) was performed on Metter TGA/SDTA851e/LF/1100 apparatus at a heating rate of 20 ◦ C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were done on DSC Q 100 TA Instrument at a heating rate of 10 ◦ C/min under nitrogen atmosphere. Cyclic voltammetry (CV) measurements were carried out with a glassy carbon working electrode in a three electrode cell. The measurements were performed in the dry dichloromethane solution containing 0.1 M tetrabutylammonium perchlorate (TBAPF6 ) as the electrolyte at room temperature under nitrogen atmosphere. Each measurement was calibrated with the standard ferrocene/ferrocenium (Fc/Fc+ ). Charge drift mobility measurements were performed by a xerographic time-of-flight (XTOF) [17,18] and by charge extraction by linearly increasing voltage (CELIV) [19,20] methods. The samples for the XTOF measurements were prepared by drop casting of the solutions of the synthesized compounds in tetrahydrofuran (THF) on aluminum coated glass plates with the thickness of layer ranging

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from 3 to 7 ␮m [21]. After keeping for 1 h in the saturated atmosphere of THF at room temperature, the samples were heated at 70 ◦ C for 1 h in a hot air oven. The charge carriers were generated at the layer surface by illumination with pulses of nitrogen laser (pulse duration was 1 ns, wavelength 337 nm). The transit time tt for the samples with the charge transporting material was determined by the kink on the curve of the dU/dt transient in log–log scale. The drift mobility was calculated by using the formula  = d2 /U0 tt , where d is the layer thickness, and U0 the surface potential at the moment of illumination. For the CELIV measurements sandwich-like structures ITO/TRZs/Al with the thickness of layers TRZ1, TRZ2 and TRZ3 330, 340 and 220 nm, respectively, and an active area of 7 mm2 were prepared. The thickness of the layers was measured by CELIV technique [22]. The layers from 10 mg/ml THF solution of the compounds were formed by casting method onto clean ITO coated ˚ at a glass substrate within a glove box. Al was evaporated at 15 A/s pressure below 5 × 10−5 mbar. The experimental setup consisted of a delay generator Tektronix AFG 3011 and a digital storage oscilloscope Tektronix DPO 4032. The mobility measurements were conducted in the dark box by applying a triangular voltage pulse to the samples. The charge carrier mobility was calculated 2 by formula  = 2d2 /Atmax , where A = U(t)/t is the voltage rise rate, tmax is the time for the current to reach its extraction maximum peak, d is the sample thickness. The theoretical calculations were carried out using the Gaussian 09 quantum chemical package [23]. Full geometry optimizations of the compounds in their electronic ground state were performed with DFT using the B3LYP functional consisting of Becke’s three parameter hybrid exchange functional combined with the Lee–Yang–Parr correlation functional with the 6-31G(d) basis set in vacuum. The energies of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals were obtained from single point calculations in the framework of DFT B3LYP/6-311G(d,p) approach for the CH2 Cl2 solution. Absorption spectra were simulated from the oscillator strengths of singlet transitions calculated by the TD-DFT B3LYP/6-31G(d) method in vacuum.

2.2. Materials The starting compounds i.e. 4-iodobenzonitrile, 4(bromomethyl)benzonitrile, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile, 9H-fluorene-2-carbaldehyde, 2-bromo9H-fluorene and the required chemicals, i.e. trifluoromethanesulphonic acid (CF3 SO3 H), 1-bromo-2-ethylhexane, potassium tert-butoxide (t-BuOK), triphenylphosphine (PPh3 ), Aliquat 336, tetrakis-triphenylphosphine palladium (Pd(PPh3 )4 ), bistriphenylphosphine palladium dichloride (Pd(PPh3 )2 Cl2 ), copper iodide (CuI), ethynyltrimethylsilane, tetrabutylammonium fluoride (n-Bu4 NF) solution in THF, sodium hydrosulfate (NaHSO4 ), potassium carbonate (K2 CO3 ), anhydrous sodium sulfate (Na2 SO4 ) were purchased from Sigma–Aldrich and used as received. 9,9-Bis(2-ethylhexyl)-9H-fluorene-2-carbaldehyde (3) and 2bromo-9,9-bis(2-ethylhexyl)-9H-fluorene (4) were obtained by the reported procedures [24,25]. 2,4,6-Tris[4-(bromomethyl)phenyl]1,3,5-triazine (1) (m.p.: 190–192 ◦ C, lit. m.p.: 191–193 ◦ C [26]), ({4-[bis{4-[(triphenylphosphoniumyl)methyl]phenyl}-1,3,5triazin-2-yl]phenyl}methyl)triphenylphosphoniumtribromide [27] (2) (m.p.: 279–281 ◦ C) and 2,4,6-tris(4-iodophenyl)-1,3,5triazine (8) (m.p.: 377–379 ◦ C, lit. m.p.: 378 ◦ C [28]) were also prepared according to the known procedures [20,22].

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2.3. Synthesis 2.3.1. 2,4,6-Tris({4-[(E)-2-[9,9-bis(2-ethylhexyl)-9H-fluoren-2yl]ethenyl]phenyl})-1,3,5-triazine (TRZ1) Potassium tert-butoxide (0.58 g, 5.21 mmol) was added to the solution of 3 (1.3 g, 3.04 mmol) and 2 (1.2 g, 0.86 mmol) in dry THF (10 ml) under argon atmosphere. After stirring for 2 h at the room temperature the reaction mixture was treated with water, extracted with ethylacetate and washed with brine. The organic phase was dried over anhydrous Na2 SO4 . After evaporation of solvents under reduced pressure the residue was purified by multiple column chromatography on silica gel using the eluent mixture of hexane and dichloromethane in a volume ratio of 10:1 and by multiple reprecipitations to methanol to afford to the yellow solid. Yield: 0.23 g (17%). 1 H NMR (500 MHz, CDCl3 , ı): 8.785 (d, 6H, J = 8.2 Hz), 8.69 (d, 3H, J = 8.0 Hz), 7.74 (d, 6H, J = 8.4 Hz), 7.70 (d, 6H, J = 7.4 Hz), 7.58–7.63 (m, 3H), 7.53–7.57 (m, 3H), 7.37–7.40 (m, 3H), 7.36–7.39 (m, 3H), 7.31–7.33 (m, 3H), 7.24–7.28 (m, 3H), 1.97–2.09 (m, 12H), 0.65–0.96 (m, 66H), 0.49–0.59 (m, 24H). 13 C NMR (125 MHz, CDCl3 , ı): 171.49, 171.10, 151.07, 150.79, 150.75, 150.71, 143.03, 141.67, 140.87, 135.38, 135.33, 135.23, 133.68, 131.43, 129.36, 128.96, 127.02, 126.80, 126.55, 126.06, 124.08, 122.05, 119.82, 119.70, 54.80, 44.68, 44.63, 44.48, 44.42, 34.59, 33.66, 28.12, 28.09, 27.05, 26.94, 22.74, 22.69, 14.12, 14.01, 10.36, 10.26. FT-IR (KBr, cm−1 ):  = 3064, 3023, 2956, 2923, 2870, 1575, 1512, 1453, 1369, 955, 821. MALDI-TOF MS (m/z): calculated for C114 H141 N3 1553.36 (M+ + H), found 1553.55. Anal. calc. (%): C 88.15, H 9.15, N 2.71; found (%): C 87.98, H 9.11, N 2.70. 2.3.2. 4-[9,9-Bis(2-ethylhexyl)-9H-fluoren-2-yl]benzonitrile (5) Compound (5) was obtained by the standard Suzuki coupling reaction. An aqueous 2 M K2 CO3 solution (4.36 ml) containing toluene (10 ml) was added to a mixture of 4 (2.35 g, 5.01 mmol), 4cyanophenylboronic acid pinacol ester (1.0 g, 4.36 mmol), Aliquot 336 (0.1 g, 0.25 mmol) and (Pd(PPh3 )4 ) (0.05 g, 0.04 mmol) under argon atmosphere. The resulting solution was kept at reflux temperature for 48 h and then cooled down to the room temperature. The reaction mixture was treated with water, extracted with ethyl acetate and washed with brine twice. The organic phase was dried over anhydrous Na2 SO4 . After evaporation of the solvents under reduced pressure the residue was purified by column chromatography on silica gel using the eluent mixture of hexane and ethyl acetate in a volume ratio of 30:1 and recrystallized from ethanol to afford white crystals. Yield: 2.2 g (42%), m.p.: 79–80 ◦ C. 1 H NMR (300 MHz, CDCl , ı): 7.72–7.81 (m, 6H), 7.55–7.61 (m, 3 2H), 7.39–7.44 (m, 1H), 7.34–7.39 (m, 1H), 7.29–7.34 (m, 1H), 1.98–2.11 (m, 4H), 0.57–0.96 (m, 22H), 0.48–0.57 (m, 8H). 13 C NMR (100 MHz, CDCl3 , ı): 151.47, 150.73, 150.66, 146.33, 142.08, 140.42, 140.39, 137.17, 137.13, 132.63, 132.60, 127.60, 127.03, 126.96, 126.20, 126.15, 124.17, 122.77, 122.67, 120.16, 120.13, 119.97, 119.14, 110.45, 55.05, 44.45, 44.41, 34.62, 34.58, 33.78, 33.68, 28.15, 28.09, 27.12, 26.91, 26.88, 22.70, 22.66, 14.00, 13.96, 10.37, 10.27, 10.24. 2.3.3. 2,4,6-Tris(4-[9,9-bis(2-ethylhexyl)-9H-fluoren-2yl]phenyl)-1,3,5-triazine (TRZ2) 0.1 M Solution of 5 (0.5 g, 1.0 mmol) in dichloromethane was drop-wise added over the period of 1 h to a vigorously stirred solution of trifluoromethanesulphonic acid (1.53 g, 0.9 ml, 10.1 mmol) in dry dichloromethane (10 ml) at 0 ◦ C under argon atmosphere. After being stirred for 72 h at ambient temperature the reaction mixture was treated with a saturated aqueous NaHCO3 solution, extracted with chloroform and washed with brine twice. The organic phase was dried over anhydrous Na2 SO4 . After evaporation of the solvents under reduced pressure the residue was purified

by multiple column chromatography on silica gel using the eluent mixture of hexane, ethyl acetate and dichloromethane in a volume ratio of 40:1:1 and by reprecipitation to methanol to afford to the white solid. Yield: 0.33 g (35%). 1 H NMR (500 MHz, CDCl3 , ı): 8.91 (dd, 6H, J = 8.4 Hz, J = 1.6 Hz), 7.87 (d, 6H, J = 8.2 Hz), 7.81 (d, 3H, J = 7.8 Hz), 7.75 (d, 3H, J = 7.5 Hz), 7.71 (t, 3H, J = 4.2 Hz), 7.69 (d, 3H, J = 7.8 Hz), 7.39–7.43 (m, 3H), 7.32–7.37 (m, 3H), 7.29 (t, 3H, J = 7.3 Hz), 1.99–2.14 (m, 12H), 0.60–0.96 (m, 66H), 0.50–0.60 (m, 24H). 13 C NMR (125 MHz, CDCl3 , ı): 171.42, 151.19, 150.74, 150.69, 150.64, 145.91, 141.39, 140.81, 140.78, 140.75, 138.61, 138.56, 138.50, 134.99, 129.47, 127.27, 126.83, 126.68, 126.16, 124.12, 122.97, 122.87, 122.77, 119.95, 119.80, 55.02, 44.50, 34.63, 33.84, 33.74, 33.71, 28.15, 28.12, 28.08, 27.12, 27.11, 26.93, 26.90, 22.72, 22.67, 13.99, 10.37, 10.34, 10.27, 10.24. FT-IR (KBr, cm−1 ):  = 3065, 3018, 2957, 2923, 2871, 1568, 1512, 1454, 1372, 813. MALDI-TOF MS (m/z): calculated for C108 H135 N3 1475.3, found 1475.6. Anal. calc. (%): C 87.93, H 9.22, N 2.85; found (%): C 87.75, H 9.20, N 2.84. 2.3.4. {2-[9,9-Bis(2-ethylhexyl)-9H-fluoren-2yl]ethynyl}trimethylsilane (6) Compound (6) was obtained by Sonogashira coupling reaction described in literature [27]. Ethynyltrimethylsilane (1.45 g, 14.9 mmol) was added to the mixture of 4 (3.5 g, 7.45 mmol), Pd(PPh3 )2 Cl2 (0.1 g, 0.14 mmol), CuI (0.017 g, 0.08 mmol) and PPh3 (0.19 g, 0.74 mmol) in dry diisopropylamine (iPrA) (20 ml) under the argon atmosphere. After stirring for 24 h at 90 ◦ C the reaction mixture was treated with water, extracted with ethyl acetate and washed with brine twice. The organic phase was dried over anhydrous Na2 SO4 . After evaporation of the solvent under reduced pressure the residue was purified by silica gel chromatography using hexane as an eluent to afford to the pale yellow liquid. Yield: 1.85 g (51%). 1 H NMR (300 MHz, CDCl3 , ı): 7.67–7.72 (m, 1H), 7.62–7.67 (m, 1H), 7.45–7.52 (m, 2H), 7.43–7.31 (m, 3H), 2.00 (dd, 4H, J = 5.4 Hz, J = 3.4 Hz), 0.71–0.95 (m, 22H), 0.53–0.55 (m, 8H), 0.31 (t, 6H, J = 2.4 Hz), 0.23 (s, 3H). 13 C NMR (100 MHz, CDCl3 , ı): 151.04, 150.69, 141.92, 140.75, 131.30, 131.16, 131.03, 127.80, 127.66, 127.51, 127.15, 127.08, 124.33, 121.18, 121.11, 121.05, 120.17, 119.59, 106.52, 93.71, 55.06, 44.97, 44.90, 44.58, 44.49, 34.81, 33.81, 33.78, 33.65, 28.29, 28.23, 28.20, 27.28, 27.23, 27.20, 27.17, 23.01, 22.93, 14.36, 14.25, 10.65, 10.62, 10.47, 0.33. 2.3.5. 9,9-Bis(2-ethylhexyl)-2-ethynyl-9H-fluorene (7) Compound (7) was obtained by the procedure described in literature [29]. The 2.5 M solution of n-Bu4 NF in THF (4.62 ml) was added drop-wise to the vigorously stirred solution of 6 (1.5 g, 3.00 mmol) in anhydrous THF (15 ml) under argon atmosphere. After stirring for 2 h at the room temperature the reaction mixture was treated with water, extracted with dichloromethane and washed with brine twice. The organic layer was dried over anhydrous Na2 SO4 . After evaporation of the solvent under reduced pressure the residue was purified by silica gel chromatography using hexane as an eluent to afford to the yellow liquid. Yield: 1.14 g (92%). The material was used for the further step without characterization. 2.3.6. 2,4,6-Tris(4-{2-[9,9-bis(2-ethylhexyl)-9H-fluoren-2yl]ethynyl}phenyl)-1,3,5-triazine (TRZ3) The mixture of 7 (1.0 g, 2.4 mmol), 8 (0.4 g, 0.58 mmol), Pd(PPh3 )2 Cl2 (0.03 g, 0.04 mmol), CuI (0.004 g, 0.02 mmol), PPh3 (0.05 g, 0.17 mmol) in dry iPrA (20 ml) was stirred at 90 ◦ C under argon atmosphere. After stirring for 24 h the reaction mixture was treated with water, extracted with chloroform and washed with brine twice. The organic layer was dried over anhydrous Na2 SO4 . After evaporation of the solvents under reduced pressure the

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269

Scheme 1. Synthetic routes to TRZ1, TRZ2, TRZ3.

residue was purified by multiple silica gel chromatography using hexane as eluent and by reprecipitation to methanol to afford to the yellow solid. Yield: 0.2 g (24%). 1 H NMR (500 MHz, CDCl3 , ı): 8.79 (d, 6H, J = 8.2 Hz), 7.74–7.79 (m, 6H), 7.70 (d, 6H, J = 7.8 Hz), 7.55–7.62 (m, 6H), 7.36–7.41 (m, 3H), 7.33 (t, 3H, J = 6.8 Hz), 7.29 (t, 3H, J = 7.3 Hz), 1.96–2.05 (m, 12H), 0.66–0.98 (m, 66H), 0.49–0.58 (m, 24H). 13 C NMR (125 MHz, CDCl3 , ı): 171.12, 150.89, 150.82, 150.75, 150.69, 141.99, 140.50, 140.47, 140.45, 135.43, 131.81, 130.83, 130.73, 130.63, 128.91, 127.99, 127.35, 127.24, 127.14, 127.04, 126.90, 124.12, 120.59, 120.53, 120.47, 119.99, 119.60, 93.64, 89.00, 54.90, 44.71, 44.66, 44.44, 44.37, 34.58, 33.63, 33.59, 33.48, 28.04, 26.95, 22.73, 22.67, 14.09, 13.99, 10.38, 10.37, 10.26, 10.25. FT-IR (KBr, cm−1 ):  = 3063, 2955, 2921, 2855, 2201, 1569, 1506, 1451, 1369, 815. MALDI-TOF MS (m/z): calculated for C114 H135 N3 1547.3, found 1547.9. Anal. calc. (%): C 88.49, H 8.79, N 2.72; found (%): C 88.64, H 8.80, N 2.72. 3. Results and discussion 3.1. Synthesis The synthetic routs to star-shaped derivatives of triazine and fluorene TRZ1, TRZ2, TRZ3 are shown in Scheme 1. All the synthesized molecules have 2,4,6-triphenyl-1,3,5-triazine core and fluorene side arms however the linking topology of the chromophores in these molecules is different. Compound TRZ1 in which 2,4,6-triphenyl-1,3,5-triazine and dialkyl fluorene moieties are linked via the ethenyl-containing linkage was prepared by the Wittig–Horner reaction of ylide 2 and 9,9-bis(2-ethylhexyl)9H-fluorene-2-carbaldehyde (3). TRZ2 was obtained by the electrophilic cyclization reaction of the aromatic precursor (5). The chromophores in this compound are linked via the single C–C bond. TRZ3 in which 2,4,6-triphenyl-1,3,5-triazine and dialkyl fluorene are linked via the ethynyl-containing linkage was obtained by Sonogashira cross-coupling reaction between the iodo-derivative 8 and 9,9-bis(2-ethylhexyl)-2-ethynyl-9H-fluorene (7). The compounds obtained were characterized by NMR, IR, mass spectrometries and elemental analysis. The signals in the 1 H and 13 C NMR spectra of the synthesized compounds can be assigned to the characteristic protons and the carbon atoms, respectively,

of these compounds. It is known, that the signals of the protons of the trans-vinylene groups for the 2,4,6-triphenyl-1,3,5-triazine derivatives can be observed around 7.19 ppm in the 1 H NMR spectra [30]. In the case of TRZ1 the signals of protons of the ethenyl groups can probably appear in the interval of 7.24–7.40 ppm, however they are overlapped with the signals of the protons of the fluorene moiety. The presence of the E-isomer can be proved by the evident trans-vinylene C–H vibrational peak located at 955 cm−1 in the IR spectrum [31]. The values of 93.64 and 89.00 ppm in the 13 C NMR spectrum of TRZ3 correspond to the carbon atoms of triple C C bonds linking the chromophores of this compound. All the synthesized 2,4,6-triphenyl-1,3,5-triazine derivatives exhibit the characteristic signals of carbon atoms of the triazine unit ( C N ) at ∼171 ppm in their 13 C NMR spectra. The properties of the obtained compounds were studied by UV and fluorescence spectrometries, differential scanning calorimetry, thermogravimetric analysis, cyclic voltammetry, time of flight and CELIV techniques and the effect of linking topologies on the properties was estimated. The comparison of the experimental data with those obtained by DFT calculations was performed. 3.2. Thermal properties The thermal properties of compounds TRZ1–TRZ3 were investigated by DSC and TGA. The thermal characteristics are collected in Table 1. The temperatures of the onsets of the thermal decomposition (Td ) of the compounds are rather high and comparable. They range from 399 to 402 ◦ C. This observation shows that the linking topology of the chromophores has minor effect on the thermal stability of the synthesized derivatives of 2,4,6-triphenyl-1,3,5triazine and fluorene. All the three derivatives of triazine and fluorene (TRZ1, TRZ2, TRZ3) were isolated after the synthesis and purification as amorphous solids. In the first and the following DSC heating scans they showed only glass transitions in the range of 56–61 ◦ C. The highest glass transition temperature (Tg ) of 61 ◦ C was observed for TRZ1 in which the chromophores are linked via the bridge containing double bonds. The lowest Tg (56 ◦ C) was recorded for TRZ2 in which the moieties of 2,4,6-triphenyl-1,3,5-triazine and fluorene are linked via the single C C bonds. The slight difference in glass transition

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Table 1 Optical and thermal characteristics of TRZ1, TRZ2 and TRZ3. Compound

abs a [nm]

abs b [nm]

abs c [nm]

fl a [nm]

fl b [nm]

Stokes shifta [nm]

Stokes shiftb [nm]

˚FL d

 1 e [ns]

Tg f [◦ C]

Td g [◦ C]

TRZ1 TRZ2 TRZ3

382 350 365

409 372 391

437 382 424

469 425 436

461 417 450

87 75 71

52 45 59

0.70 0.50 0.53

0.97 1.02 0.91

61 56 57

402 399 401

a b c d e f g

Dilute (10−4 M) solutions in THF. Thin solid layers. Calculated by TD-DFT B3LYP/6-31G(d). Fluorescence quantum yield determined in 10−5 M toluene solutions with a 0.5 M NaOH solution of fluorescein (10−5 M, ˚FL = 0.92) as reference. Fluorescence decay lifetimes determined in 10−5 M toluene solutions. Determined by DSC from the second heating scan. Determined by TGA.

temperatures of the derivatives can be attributed to the difference in self-organization of molecules, which results from variety in geometry. Thus, aromatic triazine and fluorene moieties of TRZ1 and TRZ3 are conjugated through unsaturated groups. Accordingly, TRZ1 and TRZ3 have planar aromatic skeleton (Fig. 1). In TRZ2 fluorene and triazine moieties are linked through the single bond. The chromophores in this compound are twisted by the dihedral angle of 38◦ (obtained as the result of the geometry optimization) and conformationally are more labile than in conjugated molecules TRZ1 and TRZ3, which apparently makes the intramolecular interaction weaker [15,32]. 3.3. Optical and photophysical properties UV and fluorescence spectra of TRZ1–TRZ3 are shown in Fig. 2. The wavelengths of absorption and emission maxima and the Stokes shifts are collected in Table 1. Two absorption bands around 250–290 and 350–380 nm were observed both for the solutions and the films of all the synthesized derivatives. These bands can be associated with ␲–␲* transition in triphenyltriazine and fluorene chromophores. The absorption and emission spectra of the compounds with extended conjugation, i.e. TRZ1 and TRZ3, are red-shifted with respect to those of TRZ2, in which donor and acceptor units are

linked through the single bond. Optimization of the molecular structures shows that TRZ1 and TRZ3 have planar structure of ␲-electron systems, while in the molecules of TRZ2 fluorene moieties are twisted with respect of 2,4,6-triphenyl-1,3,5-triazine core (Fig. 1) which results in the reduced conjugation. Absorption spectra of the neat films of the investigated compounds are broader and red-shifted by 22–27 nm as compared with the corresponding spectra of the solutions. This observation can be explained by the enhanced intermolecular interactions in the solid state. It can be noted that the long-wave absorption bands both of solutions and thin solid films consist of two components. They are expressed more clearly in the spectra of TRZ1 and TRZ3. Theoretical absorption spectra are red-shifted relatively to the spectra recorded from dilute solutions of the star-shaped molecules (Table 1, Fig. 2). Comparison of the distribution of electron density in HOMO and LUMO orbitals (Fig. 1) shows that HOMO–LUMO transition in these molecules has a typical charge transfer nature from the donor moieties to the acceptor ones. It can be also noted that in this transition the phenyl rings and double or triple bonds are involved. The frontier orbitals of TRZ1–TRZ3 are shown in Fig. 1. The LUMO has high distribution density on the electron-accepting 2,4,6-triphenyl-1,3,5-triazine core, whereas the HOMOs are dominated by the electron-rich fluorene moieties and by unsaturated conjunctions. Such donor–acceptor (D–A) architecture can lead to

Fig. 1. (a) The view on the DFT B3LYP/6-31G(d) optimal structures of TRZ1, TRZ2 and TRZ3; (b and c) DFT B3LYP/6-31G(d) frontier orbitals of TRZ1, TRZ2 and TRZ3.

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Fig. 2. Absorption and fluorescence (ex = 310 nm) spectra of dilute (10−4 M) solutions in THF (thin solid lines), thin layer films (dashed lines) and calculated spectra (dot lines) of (a) TRZ1, (b) TRZ2, (c) TRZ3.

the separated electron density distribution between the HOMO and LUMO. This separation can provide the efficient hole- and electrontransporting properties. Fluorescence spectra of the dilute THF solutions and thin films do not depend on the excitation wavelength. This observation allows to suggest that the excitation energy can efficiently be transferred from the electron-donating fluorene moieties to the electron-accepting triazine core. The Stokes shifts recorded for the dilute THF solutions of TRZ1, TRZ2 and TRZ3 range from 71 nm to 87 nm, while those observed for the thin films are in the range of 45–59 nm. The relatively large Stokes shifts of triazine-based star-shaped molecules in polar solvent (THF) indicates the stabilization of the intramolecular charge-transfer excited state. In order to investigate the effect of solvatochromism on the excited state properties of the triazine derivatives absorption and fluorescence spectra of 10−4 M toluene, THF and dimethylsulfoxide (DMSO) solutions were compared (Figs. 2 and 3). Solvatochromism is caused by differential solvatation of ground state and first excited state of the light-absorbing molecule [33]. All the triazine derivatives TRZ1–TRZ3 exhibit positive solvatochromism, characterized

271

Fig. 3. Absorption and fluorescence spectra (ex = 310 nm) of the dilute (10−4 M) toluene and dimethylsulfoxide solutions of (a) TRZ1, (b) TRZ2 and (c) TRZ3.

by the batochromic shift of the emission spectra with the increase of solvent polarity. Thus, the fluorescence spectra of the solutions of the investigated compounds in polar DMSO (ε = 46.7) are red-shifted by 30–48 nm comparing to the spectra of the solutions in less polar THF (ε = 7.6), which are, in turn, red-shifted by 29–42 nm relatively to the spectra of the solutions in non-polar toluene (ε = 2.3). This observation can be explained by the stabilization of the first excited state, so called Franck–Condon excited state, compared to the ground state with the increase of the solvent polarity. In turn, the absorption spectra exhibit negligible solvent dependence. These phenomena can be explained by the remarkable changes of the dipole moments of the molecules upon photonic excitation relatively to the dipole moments of ground states, as it was previously shown in the work of Wang [34], indicating on the charge-transfer character of the lowest excited states. The fluorescence spectra of the solutions of all the three triazine derivatives in nonpolar toluene exhibit several low energy (0.11–0.14 eV) sequence vibronic bands. This indicates that molecules are not associated in toluene solution. Fluorescence quantum yields (˚FL ) of the synthesized starshaped compounds were measured for the toluene solutions with a 0.5 NaOH solution of fluorescein (10−5 M, ˚FL = 0.92) as a reference.

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N.A. Kukhta et al. / Synthetic Metals 195 (2014) 266–275 Table 3 The calculated data for TRZ1, TRZ2 and TRZ3.

Fig. 4. Fluorescence decay curve of the dilute toluene solution (10−5 M) of TRZ1 (ex = 340 nm).

The ˚FL values are given in Table 1. The synthesized compounds exhibited moderate fluorescence quantum efficiencies in the range of 0.50–0.70. The highest value of 0.70 was observed for TRZ1, in which the chromophores are linked via the double bond. This result can be attributed to the fact that TRZ1 has a completely flat aromatic part (geometry optimization by DFT B3LYP/6-31G(d) method) (Fig. 1). The lowest value of ˚FL was observed for TRZ2 (˚FL = 0.50) in which the chromophores are linked via the single bonds. Thus, fluorescence efficiency of TRZ1 is higher than that of TRZ2 due to the presence of extended conjugation via the double bonds of the bridge linking the chromophores of TRZ1. In turn, TRZ3, in which donor and acceptor units are linked through the linking bridge containing triple bond, performs the fluorescence quantum yield of 0.53. Lower value of fluorescence quantum yield of TRZ3 compared to that of TRZ1 can be explained by the stronger electronegativity of triple bond comparing to double bond [32]. In order to investigate the behavior of the star-shaped molecules in the excited state, fluorescence decay curves were recorded of their 10−5 M toluene solutions. Fluorescence decay curves of the dilute solutions of TRZ1, TRZ2 and TRZ3 can be adequately described by the single exponential functions with 2 not exceeding 1.26 (Fig. 4). This observation shows that the emission of the solutions of all three compounds is of monomeric character. Fluorescence lifetime values are given in Table 1. They are rather short and range from 0.91 to 1.02 ns. 3.4. Electrochemical properties In order to investigate the redox properties of the compounds synthesized cyclic voltammetry (CV) measurements with a glassy carbon working electrode in a three electrode cell were performed. The measurements were carried out in the dry dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as the electrolyte at the room temperature under argon atmosphere. CV curves of TRZ1, TRZ2 and TRZ3 are shown at Fig. 5. Electrochemical characteristics of the compounds are collected in Table 2. Table 2 Electrochemical characteristics of TRZ1, TRZ2 and TRZ3. opt

Compound

onset Eox [eV]

IP [eV]

Eg

TRZ1 TRZ2 TRZ3

0.74 1.30 1.23

5.54 6.10 6.03

2.86 3.75 3.03

[eV]

EA [eV] −2.68 −2.35 −3.00

Compound

EHOMO [eV]

ELUMO [eV]

Egcalc [eV]

IPcalc [eV]

EAcalc [eV]

TRZ1 TRZ2 TRZ3

−5.62 −5.93 −5.83

−2.55 −2.38 −2.63

3.07 3.55 3.20

6.06 6.38 6.23

−2.99 −2.83 −3.03

All the compounds under consideration undergo irreversible oxidation. The solid state ionization potential energy was estimated from the onset oxidation potential by using the relationship onset ) [35], where the potential is related to that of IPss = −(4.8 + Eox ferrocenium/ferrocene. The IP values are relatively high and range opt from 5.54 to 6.10 eV. The optical band gap Eg values were determined from the onset absorption wavelength. The electron affinity EA values were obtained by subtraction of the optical band gap opt from the IP using the crude approximation EAss = IPss − Eg [35]. They were found to be −2.68, −2.35, −3.00 eV for TRZ1, TRZ2 and TRZ3, respectively. The optical band gaps of TRZ1 and TRZ3 are smaller, than that of TRZ2. These observations are consistent with the geometry optimization data. The values of EHOMO ,  ELUMO , Egcalc = EHOMO − ELUMO , vertical electron affinities (EAcalc ) and ionization potentials (IPcalc ), calculated by the DFT B3LYP/6311G(d,p) approach for the CH2 Cl2 solution (Table 3), correlate with the experimentally determined values of ionization potentials and electron affinities (Table 2). The vertical IPcalc was calculated as the difference between the full energy of the cation at neutral geometry and the neutral molecule at neutral geometry. The electron affinity was calculated as EAcalc = IPcalc − Egcalc . 3.5. Charge-transporting properties Charge-transporting properties of the derivatives of 2,4,6triphenyl-1,3,5-triazine and fluorene were estimated by XTOF and CELIV techniques. XTOF is the non-destructive technique characterized by the absence of the top contact influence. XTOF is known as the technique allowing to obtain charge carrier mobility at electric fields as high as 5 × 105 V/cm [12,13]. Since the synthesized materials are promising for the two-electrode device application, we used the CELIV technique for additional investigation of the materials based on diode structure. In air only the ability to transport holes was detected for the layers of TRZ1–TRZ3. Electron-transporting properties were not detected for these compounds apparently due to the relatively high LUMO levels. The layers of the compounds TRZ1–TRZ3 are characterized by dispersive hole transport. Typical XTOF transients recorded for the layer of TRZ1 at 25 ◦ C are shown in Fig. 6(a). Fig. 7 shows the electric field dependencies of hole mobilities for the solution-processed layers of TRZ1–TRZ3 deduced from the sequences of XTOF measurements. The functional dependence of hole mobility on electric field is fitted as  = 0 ·exp(˛·E1/2 ), where 0 is zero field mobility, ˛ is the field dependence parameter, and E is the electric field. Fig. 7 shows the fitting results. The zero field hole mobilities for the layers of TRZ1, TRZ2 and TRZ3 are (8.8 ± 2.3 × 10−6 ), (2.3 ± 0.2 × 10−4 ), and (1.2 ± 0.4 × 10−8 ) cm2 /V respectively, and the field dependence parameters obtained are (4.9 ± 0.3 × 10−3 ), (2 ± 0.2 × 10−3 ), (9.2 ± 0.3 × 10−3 ) cm/V, respectively. The obtained values are rather different, what is apparently due to different packing of the molecules in the solution-casted thin films. Hole drift mobilities in the layers of TRZ1 and TRZ2 well exceed 10−4 cm2 /V s and approach 10−3 cm2 /V s at high electric fields (>1 × 106 V/cm). Fig. 6(b) presents the dark CELIV signal for TRZ1. The equilibrium charge carriers accumulated on the blocking contact at applied linearly decreasing negative voltage and transported through the layer of the TRZ1 at applied linearly increasing positive voltage (U) of the pulse duration (t) (Fig. 6b). The characteristic extraction

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273

Fig. 5. Cyclovoltammperometric (CV) curves of TRZ1, TRZ2 and TRZ3 measured in dilute CH2 Cl2 solutions.

time tmax is seen in the current transients as maximum (Fig. 6b). From the data of the dark CELIV signals the charge carrier mobili2 , where A = U(t)/t is ties were calculated by formula  = 2d2 /Atmax the voltage rise rate, d is the sample thickness. The values of charge carrier mobility for the synthesized star-shaped molecules, derived from the dark CELIV experiments were obtained at electric field

E1/2 = (U·tmax /t·d)1/2 (Fig. 7). The hole mobility values obtained by the dark CELIV technique are in good agreement with the results deduced from XTOF. Despite the similarities of the structures of TRZ1–TRZ3, the hole mobility values and the field dependencies exhibited differences in the accessible electric field range. The hole mobility values are lower and the field dependence is stronger for TRZ3 (Fig. 7). At high electric fields the hole mobility values of TRZ3 approach those of TRZ1 and TRZ2. The hole mobilities of TRZ1–TRZ3 are strongly field-dependent (Fig. 7). Many recent studies of chargetransporting properties of organic disordered materials were described by a formalism based on disorder, due to the theory of Borsenberger et al. [36]. The formalism is premised on the argument that charge transport occurs by hopping through a manifold of localized states that are distributed in energy. From the point of view of this formalism the field dependence of charge mobility is caused by the difference between the energetic disorder  and the positional disorder dependent terms. The presence of different linking bridges in TRZ1, TRZ2 and TRZ3 containing single, double, and triple bonds apparently affects energetic disorder  and leads to the differences in hole mobility values and the field dependences. Free volume effects due to the different bridges containing single, double, and triple bonds in TRZ1–TRZ3 might also be a possible reason of the difference in charge-transporting properties.

Fig. 6. XTOF transients in the log–log scale (a) and dark CELIV signals (b) for the TRZ1.

Fig. 7. Electric field dependencies of the XTOF and CELIV hole mobilities for the layers of TRZ1, TRZ2 and TRZ3.

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4. Conclusions Star-shaped triazine-based molecules with fluorene side arms, linked through the linkages containing double, single, and triple bonds, were designed and synthesized by Wittig–Horner reaction, electrophilic cyclization reaction and Sonogashira cross-coupling, respectively, at the main steps. The synthesized compounds show high thermal stability with the temperatures of the onsets of thermal degradation up to 402 ◦ C. The nature of linking topology has minor influence on the thermal stability of the triazine derivatives. They are capable of glass formation with the glass transition temperatures being in the range of 56–61 ◦ C. The highest value of the glass transition temperature was exhibited by the molecule in which the chromophores are linked through double bond. Fluorescence spectra of dialkyl fluorene and 2,4,6-triphenyl1,3,5-triazine based compounds significantly depend on the solvent polarity, indicating on the charge-transfer character of the lowest excited states, with the more pronounced character for the compounds with extended conjugation. The dilute solutions of the synthesized compounds are characterized by the singleexponential fluorescence decay curves and by rather small values of fluorescence lifetimes (0.91–1.02 ns). They exhibit relatively high fluorescence quantum yield values (0.50–0.70). The highest fluorescence efficiency was exhibited by the material in which the chromophores are linked through double bond due to the most efficient conjugation. The synthesized star-shaped molecules undergo irreversible oxidation as confirmed by cyclic voltammetry. They are characterized by relatively small values of the optical band gaps in the range of 2.86–3.75 eV. DFT calculations revealed flat geometry for the molecules with double and triple bond bridges, and twisted geometry for the donor and acceptor moieties linked by single bond. Charge-transporting properties of the derivatives of 2,4,6triphenyl-1,3,5-triazine and fluorene were estimated by XTOF and CELIV techniques. They are characterized by dispersive hole transport with the values of hole mobility 1.9 × 10−3 , 1.6 × 10−3 and 4.4 × 10−4 cm2 /V s respectively at electric field of 1.15 × 106 V/cm. The best charge-transporting properties were shown by the compound in which 2,4,6-triphenyl-1,3,5-triazine and 2-[9,9bis(2-ethylhexyl)-9H-fluorene] moieties are linked via ethenyl linkage. The data obtained by XTOF method are in good agreement with those recorded by CELIV method.

Acknowledgments This research was funded by the European Social Fund Grand no. VP1-3.1-SMM-07-K-02-005 under the Global Grant measure. Dr. Andrzej Swinarew (Department of Polymers and Materials Technology, University of Silesia) is thanked for the recording of mass spectra by the MALDI-TOF method and Dr. Tadas Malinauskas (Department of Organic Chemistry, Kaunas University of Technology) is thanked for recording fluorescence decay curves.

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