Synthesis and photophysical properties of novel multisubstituted benzene and naphthalene derivatives with high 2D-π-conjugation

Optical Materials 47 (2015) 118–128

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Synthesis and photophysical properties of novel multisubstituted benzene and naphthalene derivatives with high 2D-p-conjugation S. Kula a, A. Szlapa a, J.G. Malecki a, A. Maron´ a, M. Matussek a, E. Schab-Balcerzak a,b, M. Siwy b, M. Domanski b, M. Sojka c, W. Danikiewicz c, S. Krompiec a, M. Filapek a,⇑ a b c

Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Str., 41-819 Zabrze, Poland Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 27 May 2015 Received in revised form 6 July 2015 Accepted 6 July 2015 Available online 11 July 2015 Keywords: Photoluminescence Electrochemistry 2D-p-conjugated derivatives [2+1+2+1] cycloaddition Blue light emitters

a b s t r a c t A new small molecule, with D-A-D framework was prepared in good yield by using a [2+1+2+1] cycloaddition followed by the [4+2] Diels–Alders reaction. Additionally, tetra-substituted naphthalene derivatives were also prepared from in situ generated benzyne (using 2-trimethylsilylphenyl triflate and cesium fluoride). All of this compounds exhibit strong 2D-p-conjugation. The influence of this type of interactions on photophysical properties with the aid of DFT calculations was examined. The preliminary tests of application possibility of synthesized compounds in devices for optoelectronics were carried out as well. Ó 2015 Published by Elsevier B.V.

1. Introduction Thiophene-based electronic materials are still in the great interest of researchers [1]. It is a result of both the ease in the structure modifying and physicochemical properties of thiophene ring itself. The most important feature from the photovoltaic’s point of view is the strong pi-excess character of this hetero-aryl, what is crucial during donor–acceptor systems modeling. In the literature one can find multiple systems of this kind based on thiophene, in which various electron withdrawing moieties such as: naphthaleniediimide [2], thiadiazoles [3], pyrene-fused phenazine [4], pyrazole [5] and very popular nowadays diketopyrrolopyrroles [6,7] are used. In addition, quite simple substituent incorporating into molecule skeleton enables modifying HOMO, LUMO and band gap levels what affecting on photoluminescence (PL) changing the luminescence wavelength. Nowadays, especially valuable are the blue light emitters, due to the fact that by mixing with a suitable dopant it is possible to obtain OLED’s any desired color. For instance, the white light emission was achieved by mixing two ‘‘complementary lights’’ as long as the connection line of their coordinates lies across the white light region [8]. However, it ⇑ Corresponding author at: Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice, Poland. E-mail address: michal.fi[email protected] (M. Filapek). http://dx.doi.org/10.1016/j.optmat.2015.07.011 0925-3467/Ó 2015 Published by Elsevier B.V.

should be emphasized that blue light is the high energetic one (3 eV, approximately). Several challenges make it difficult to obtain amorphous materials capable of efficient emitting blue light with good stability, because so high energy can cause structural deformation or undesired chemical reaction. Motoyama et al. reports research on dithienylethene derivatives [9]. They have observed that this type of compounds after light absorption (>360 nm) or electrochemical oxidation underwent linkage through 2-position in neighboring thiophene rings. In order to increase the PL yields it is crucial to exclude every nonradiative processes [10]. One way to do this, is to stiffen the molecule skeleton by bridging elements such as: boron [11]; phosphorus [12]; sulfur [13], nitrogen [3,14] and silicon [15,16]. Another method is to synthesize sterically crowded derivatives, for instance hexaarylbenzenes [17,18]. In these molecules the rotation around aryl-benzene bond is frozen. It is the consequence of a bulky substituent presence and high aryl–aryl 2D connection (p-stacking). Tanaka et al. have proved that in 1,2-diaryl benzenes [19], communication mode between aryl groups is much higher from p–p interaction than from p-conjugation through the central benzene ring. However, there is lack of reports about the influence of this type of interactions on PL, mainly due to synthetic difficulties in 2D-p-conjugated derivatives obtaining. So far, multi-substituted benzene or naphthalene derivatives containing thiophenyls were prepared by: Stille coupling reactions [20,21] the high temperature [4+2]

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Diels–Alder reaction (with CO-extrusion) [22,23], cycloaddition cyclobutene-1,2-diones [24], cyclotrimerization [25] or via intermolecular [2+2+2] cycloaddition catalyzed by Ru(II)-complexes [26]. Recently, in the literature new synthetic route was reported i.e. [2+1+2+1] alkynes cycloaddition to b-keto ester, followed by [4+2] Diels–Alder cycloaddition. In contrast to the synthetic methods based on Stille coupling, [2+1+2+1] cycloaddition reactions allowed to obtain the 2-pyranon (as intermediate compound) which can be used both for the synthesis of multi-substituted benzene and naphthalene derivatives. In addition, the coupling reactions require strictly anaerobic and anhydrous conditions. What is in the great importance, a wide range of commercially available b-keto esters, as well as the ease in further modification of this building blocks make this strategy very interesting. In the case of Diels–Alder reactions (with CO-extrusion) difficult to obtained cyclopentadienone systems [22], and high temperatures (from 250 °C to 300 °C) during [4+2] cycloaddition [22,23] are required. On the other hand, similar cycloaddition using 2-pyranon as substrate (i.e. with CO2-extrusion) required milder conditions: 150 °C [27–29] for benzene and even room temperature for the naphthalene derivatives synthesis [27–29]. Another advantage of the [2+1+2+1] cycloaddition reaction followed by the [4+2] Diels–Alders reaction in comparison with other types of cycloaddition reaction tandem’s, is their high regioselectivity [27–29], easy functionalization of the substrates [30], as well as certain functional groups tolerance [27–29]. Tetra-substituted naphthalene derivatives can be prepared with the great success from in situ generated benzene (from 2-trimethylsilylphenyl triflate and cesium fluoride) [27,28]. Herein we reported the multi-substituted benzene and naphthalene derivatives synthesized based on this reactions tandem. The selected photophysical properties of obtained compounds were investigated with the aid of DFT calculations. The preliminary tests of application possibility of synthesized compounds in devices for optoelectronics were carried out as well.

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2.3. Typical procedure for the synthesis of multisubstituted benzenes with phenyl motif. (M1–M3) The mixture of b-keto ester (72.1 mg, 0.5 mmol) (ethyl 2-methylacetoacetate, ethyl 2-benzylacetoacetate or ethyl butyrylacetate), diphenylacetylene (213 mg, 0.6 mmol), [ReBr(CO)3(THF)]2 (10.6 mg, 0.0125 mmol), powdered MS4 Å (21.2 mg, 200 wt%-Re cat.) and toluene (1.0 mL) was vigorously stirred and heated in oil bath at 180 °C for 24 h. After that acetylene dicarboxylic acid ethylester (170 mg, 1.00 mmol) was added and heating was continued in 150 °C for the next 24 h. Then, the reaction mixture was cooled to room temperature and extracted with ethyl acetate. The organic layer was washed by water (three times), and dried over Na2SO4. Crude product was purified using column chromatography. 2.3.1. Diethyl 3-propyl-4,5-diphenylphthalate (M1) Product was purified by column chromatography (hexane/ethyl acetate, 5:1) and isolated as yellow oil with 66% yield of M1; 1 H NMR (400 MHz, CDCl3) d 7.91 (s, 1H), 7.22–7.11 (m, 6H), 7.06 –7.00 (m, 4H), 4.47 (q, J = 7.1 Hz, 2H), 4.37 (q, J = 7.1 Hz, 2H), 2.52 –2.46 (m, 2H), 1.44–1.31 (m, 8H), 0.68 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 169.69, 165.83, 145.55, 143.07, 140.76, 139.20, 138.66, 134.89, 130.17, 129.66, 129.41, 127.79, 127.68, 127.18, 127.09, 126.77, 61.65, 61.55, 33.59, 24.49, 14.58, 14.35, 14.21. EI-HRMS calc. for C27H28O4 416.1988, found 416.1982.

2.1. Materials

2.3.2. Diethyl 3-benzyl-6-methyl-4,5-diphenylphthalate (M2) Product was purified by column chromatography (hexane/ethyl acetate, 5:1) and recrystallized from hexane/ethyl acetate (5:1) to give white crystals with 59% yield of M2; m.p. = 113.5 °C; 1H NMR (400 MHz, CDCl3) d 7.14–7.04 (m, 6H), 7.01–6.97 (m, 3H), 6.91–6.87 (m, 2H), 6.83 (d, J = 7.1 Hz, 2H), 6.79 – 6.74 (m, 2H), 4.37 (q, J = 7.1 Hz, 2H), 4.08 (q, J = 7.2 Hz, 2H), 4.00 (s, 2H), 2.12 (s, 3H), 1.39 (t, J = 7.1 Hz, 3H), 1.12 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 168.85, 168.56, 144.99, 144.85, 140.47, 139.80, 139.17, 134.81, 132.92, 132.50, 132.35, 129.96, 129.71, 128.84, 127.90, 127.73, 127.41, 126.56, 126.53, 125.63, 61.67, 61.49, 36.55, 18.52, 14.21, 13.84. ESI-HRMS calc. for C32H30O4Na [M+Na]+ 501.2042, found 501.2045.

All chemicals and starting materials were commercially available and were used without further purification. 5-Iodo-2,20 -bithiophene was prepared according to the method described in our previously published paper [31]. Solvents were distilled as per the standard methods and purged with nitrogen before use. All reactions were carried out under argon atmosphere unless otherwise indicated. Column chromatography was carried out on Merck silica gel. Thin layer chromatography (TLC) was performed on silica gel (Merck TLC Silica Gel 60).

2.3.3. Diethyl 3,6-dimethyl-4,5-diphenylphthalate (M3) Product was purified by column chromatography (hexane/ethyl acetate, 5:1) and recrystallized from hexane/ethyl acetate (5:1) to give white crystals with 71% yield of M3; m.p. = 139.5 °C; 1H NMR (400 MHz, CDCl3) d 7.16–7.04 (m, 6H), 6.88 (d, J = 6.9 Hz, 4H), 4.38 (q, J = 7.2 Hz, 4H), 2.08 (s, 6H), 1.40 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 168.93, 144.42, 139.87, 132.12, 131.86, 129.78, 127.78, 126.60, 61.66, 18.44, 14.26. ESI-HRMS calc. for C26H26O4Na [M+Na]+ 425.1729, found 425.1730.

2.2. Preparation of 1,2-Bis(2,20 -bithiophene-5-yl)acetylene (bbta) (4)

2.4. Typical procedure for the synthesis of multisubstituted benzenes with bithiophene motif. (M5–M7)

Cascade of three reactors, each containing a mixture of substrates consisting of 5-iodo-2,20 -bithiophene (15.0 g, 51.3 mmol), CuI (1.125 g, 5.9 mmol), [PdCl2(PPh3)2] (0.75 g, 1.1 mmol), acetone (500 mL), triethylamine (11.25 mL, 81 mmol) was flushed with a steady stream of acetylene (1 mol/6 h) mixed with argon (1:5 v/v) at room temperature for 6 h. After purging with acetylene was finished, the content of the reactors was left to stir for 24 h at room temperature. Then the volatile fractions from the combined mixtures were evaporated on a rotary evaporator. Crude product was purified using column chromatography (SiO2, hexane). A yellow solid was obtained with 60% yield. (Analyses agree with lit. [31].)

The mixture of b-keto ester (72.1 mg, 0.5 mmol) (ethyl 2-methylacetoacetate, ethyl 2-benzylacetoacetate or ethyl butyrylacetate), 1,2-bis(2,20 -bithiophene-5-yl)acetylene (213 mg, 0.6 mmol), [ReBr(CO)3(THF)]2 (10.6 mg, 0.0125 mmol), powdered MS4 Å (21.2 mg, 200 wt%-Re cat.) and toluene (1.0 mL) was vigorously stirred and heated in oil bath at 180 °C for 48 h. After that acetylene dicarboxylic acid ethylester (170 mg, 1.00 mmol) was added and heating was continued in 150 °C for the next 24 h two times. Then, the reaction mixture was cooled to room temperature and extracted with ethyl acetate. The organic layer was washed by water (three times), and dried over Na2SO4. Crude product was purified using column chromatography.

2. Experimental section

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2.4.1. 1,2-Bis(2,20 -bithiophen-5-yl)-4,5-di(ethoxycarbonyl)-3propylbenzene (M5) Product was purified by column chromatography (toluene/ethyl acetate, 15:1) and isolated as brown oil with 49% yield of M5; 1 H NMR (400 MHz, CDCl3) d 8.10 (s, 1H), 7.22 (dd, J = 5.1, 1.1 Hz, 1H), 7.18 (dd, J = 5.1, 1.1 Hz, 1H), 7.16 (dd, J = 3.6, 1.1 Hz, 1H), 7.14 (d, J = 3.6 Hz, 1H), 7.08 (dd, J = 3.6, 1.1 Hz, 1H), 7.01 (dd, J = 5.1, 3.6 Hz, 1H), 6.98 (d, J = 3.8 Hz, 1H), 6.97 (dd, J = 5.1, 3.6 Hz, 1H), 6.83 (d, J = 3.6 Hz, 1H), 6.83 (d, J = 3.8 Hz, 1H), 4.46 (q, J = 7.2 Hz, 2H), 4.40 (q, J = 7.1 Hz, 2H), 2.63–2.57 (m, 2H), 1.58 –1.54 (m, 2H), 1.44–1.38 (m, 6H), 0.84 (t, J = 7.3 Hz, 3H).13C NMR (100 MHz, CDCl3) d 169.08, 165.43, 142.12, 140.36, 139.29, 139.05, 137.25, 137.13, 137.09, 137.06, 136.43, 134.82, 130.16, 128.93, 128.63, 128.31, 127.97, 127.93, 124.69, 124.09, 123.93, 123.78, 123.56, 61.86, 61.81, 34.06, 25.52, 14.73, 14.36, 14.20. ESI-HRMS calc. for C31H28O4S4Na [M+Na]+ 615.0768, found 615.0771. 2.4.2. 1,2-Bis(2,20 -bithiophen-5-yl)-4,5-di(ethoxycarbonyl)-3-benzyl6-methylbenzene (M6) Product was purified by column chromatography (toluene/ethyl acetate, 15:1) and isolated as brown oil with 44% yield of M6; 1 H NMR (400 MHz, CDCl3) d 7.19–7.10 (m, 5H), 7.07 (dd, J = 3.6, 1.1 Hz, 1H), 7.01 (dd, J = 3.6, 1.1 Hz, 1H), 6.98–6.91 (m, 5H), 6.84 (d, J = 3.6 Hz, 1H), 6.61 (d, J = 3.6 Hz, 1H), 6.44 (d, J = 3.6 Hz, 1H), 4.36 (q, J = 7.1 Hz, 2H), 4.17 (s, 2H), 4.05 (q, J = 7.2 Hz, 2H), 2.31 (s, 3H), 1.38 (t, J = 7.2 Hz, 3H), 1.09 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 168.32, 168.00, 140.36, 138.70, 138.62, 138.40, 138.40, 138.38, 137.86, 137.25, 137.20, 137.17, 135.27, 133.66, 133.57, 129.67, 129.24, 128.81, 128.15, 127.86, 127.81, 125.90, 124.42, 124.38, 123.74, 123.73, 123.30, 123.08, 61.91, 61.71, 36.86, 18.76, 14.22, 13.81. ESI-HRMS calc. for C36H30O4S4Na [M+Na]+ 677.0925, found 677.0917. 2.4.3. 1,2-Bis(2,20 -bithiophen-5-yl)-4,5-di(ethoxycarbonyl)-3,6dimethylbenzene (M7) Product was purified by column chromatography (toluene/ethyl acetate, 15:1) and isolated as brown oil with 56% yield of M7; 1 H NMR (400 MHz, CDCl3) d 7.17 (dd, J = 5.1, 1.1 Hz, 2H), 7.08 (dd, J = 3.6, 1.1 Hz, 2H), 6.99–6.94 (m, 4H), 6.59 (d, J = 3.6 Hz, 2H), 4.39 (q, J = 7.1 Hz, 4H), 2.27 (s, J = 9.7 Hz, 6H), 1.40 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 168.33, 138.45, 138.35, 138.15, 137.24, 134.37, 132.97, 129.11, 127.84, 124.38, 123.70, 123.29, 61.83, 18.63, 14.21. EI-HRMS calc. for C30H26O4S4 578.0714, found 578.0723. 2.5. Preparation of naphthalene derivatives 2.5.1. 1,4-Dimethyl-2,3-diphenylnaphthalene (M4) A mixture of ethyl 2-methylacetoacetate (72.1 mg, 0.500 mmol) and diphenylacetylene (107 mg, 0.600 mmol) in the presence of [ReBr(CO)3(thf)]2 (10.6 mg, 0.0125 mmol) and powdered MS4 Å (21.2 mg, 200 wt.%-Re cat.) in toluene (1.0 mL) was heated at 180 °C under argon atmosphere. After 24 h, 2-(trimethylsilyl)phenyl triflate (223.8 mg, 0.750 mmol), cesium fluoride (228 mg, 1.50 mmol), and acetonitrile (1.0 mL) were added. The reaction mixture was stirred at 25 °C for 24 h. Then, the reaction mixture was extracted with ethyl acetate. The organic layer was washed by water (three times), and dried over Na2SO4. After purification by silica gel column chromatography (hexane/methylene chloride 5:1), and recrystallized from hexane/ethyl acetate (5:1) M4 was obtained as white crystals with 72% yield; m.p. = 144.5 °C; 1H NMR (400 MHz, CDCl3) d 8.18–8.13 (m, 2H), 7.62–7.58 (m, 2H), 7.16–7.07 (m, 6H), 7.00–6.95 (m, 4H), 2.45 (s, 6H). 13C NMR (100 MHz, CDCl3) d 141.89, 139.58, 132.20, 130.57,

129.56, 127.39, 125.99, 125.90, 125.17, 16.98. EI-HRMS calc. for C24H20 308.1565, found 308.1573. 2.5.2. 2,3-Bis(2,20 -bithiophene-5-yl)-1,4-dimethylnaphthalene (M8) A mixture of ethyl 2-methylacetoacetate (72.1 mg, 0.500 mmol) and 1,2-bis(2,20 -bithiophene-5-yl)acetylene (107 mg, 0.600 mmol) in the presence of [ReBr(CO)3(thf)]2 (10.6 mg, 0.0125 mmol) and powdered MS4 Å (21.2 mg, 200 wt.%-Re cat.) in toluene (1.0 mL) was heated at 180 °C under argon atmosphere. After 48 h, 2-(trimethylsilyl)phenyl triflate (223.8 mg, 0.750 mmol), cesium fluoride (228 mg, 1.50 mmol), and acetonitrile (1.0 mL) were added. The reaction mixture was stirred at 25 °C for 48 h. Then, the reaction mixture was extracted with ethyl acetate. The organic layer was washed by water (three times), and dried over Na2SO4. After purification by silica gel column chromatography (hexane/ethyl acetate 15:1), and recrystallized from hexane/ethyl acetate (5:1) M8 was obtained as yellow crystals with 63% yield; m.p. = 177.6 °C; 1H NMR (400 MHz, CDCl3) d 8.17 – 8.11 (m, 2H), 7.65–7.60 (m, 2H), 7.16 (dd, J = 5.1, 1.0 Hz, 2H), 7.09 (dd, J = 3.6, 1.0 Hz, 2H), 6.99 (d, J = 3.6 Hz, 2H), 6.97 (dd, J = 5.1, 3.6 Hz, 2H), 6.64 (d, J = 3.6 Hz, 2H), 2.61 (s, 6H). 13C NMR (100 MHz, CDCl3) d 141.23, 137.78, 137.65, 132.97, 132.53, 131.95, 129.15, 127.82, 126.71, 125.39, 124.07, 123.43, 123.22, 17.33. EI-HRMS calc. for C28H20S4 484.0448, found 484.0446. 2.6. Film and blend preparation Films on glass substrate were prepared from chloroform solutions. The homogenous solutions were spin cast (2000 turns per 30 s) and dried in vacuum oven at 60 °C over 5 h. The thickness of the layers, measured by AFM was 160, 105 and 20 nm for M6, M5 and M8, respectively. Blend was obtained by dissolving the desired amount of compounds (M8) and poly(N-vinylcarbazole) (PVK) in chloroform to form a solution (5% vv concentration of compound in PVK). Blend cast on glass was dried in vacuum oven at 60 °C over 5 h. 2.7. Device preparation Current–voltage measurements were performed on ITO/active layer/Al device. The 2,3-bis(2,20 -bithiophene-5-yl)-1,4-dimethylna phthalene solution (5 w/v% in chloroform) was spin-cast onto ITO-coated glass substrate with angular speed 3000 turn per minute by 30 s at room temperature. Residual solvent was removed by heating the film in a vacuum at 60 °C. The thickness of the layers, measured by AFM was 25 nm. Al electrode (300 nm) was prepared on the film surface by vacuum deposition at a pressure of 1  106 Torr. 2.8. Measurements The NMR spectra were recorded on a Bruker Avance 400 MHz instrument by using CDCl3 as solvent and TMS as the internal standard. Accurate mass measurements for volatile samples were performed on Autospec Premier (Waters) double-focusing magnetic sector mass spectrometer using electron ionization (EI). Non-volatile samples have been measured on MALDI Synapt G2S HDMS (Waters) Q-TOF type mass spectrometer using electrospray ionization (ESI). In these cases measurements were done for [M+Na]+ ions because the relative abundance of [M+H]+ ions was too low. The steady-state emission and excitation spectra were measured for dichloromethane deaerated solutions (c = 1  103 mol/dm3) on the FLS-980 spectrophotometer in ambient temperature using 450W Xe arc lamp as a light source and PMT (Hamamatsu, R928P) in cooled housing as a detector. The quantum

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yields of fluorescence were determined by absolute method in room temperature, using the integrating sphere with solvent as a blank. The solutions of samples were first deaerated and diluted to absorbance under 0.1 to avoid inner filter effect and influences of impurities from the medium and then they were excited in the wavelengths corresponding to the excitation wavelengths of the compounds. The time-resolved measurement were prepared at optically diluted dichloromethane solutions at room temperature using the time correlated single photon counting methods on the FLS-980 spectrophotometer. Excitation wavelengths were obtained using the picosecond pulsed diode laser EPL-375 nm with 200 ns pulse period, picosecond pulsed diode EPLED-340 nm with 100 ns pulse period or picosecond pulsed diode EPLED-310 nm with 100 ns pulse period as light sources (depending on obtained excitation wavelengths) and PMT (Hamamatsu, R928P) in cooled housing was used as a detector. The system was alignment at emission wavelengths. Additionally for the analysis of a fluorescence decay, an instrument response function (IRF) was obtained. The IRF contains the information about the time response of the overall optical and electronic system. The IRF was designate using ludox solution as a standard at 375 nm, 340 nm and 310 nm, respectively. The chromaticity was estimated according to CIE 1976. Electrochemical measurements were carried out using Eco Chemie Autolab PGSTAT128n potentiostat, glassy carbon electrode (diam. 2 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 CH2Cl2 (Carlo Erba, HPLC grade), under argon. 0.2 M Bu4NPF6 (Aldrich, 99%) was used as the supporting electrolyte. Electrical measurements (current–voltage) were performed using Elektrometr Keithley 6517A. Active layers thickness was measured by atomic force microscopy (AFM) Topometrix Explorer TMX 2000. 3. Results and discussion In this article the synthesis and characterization of multi-substituted benzene and naphthalene derivatives are described from point of view of their UV–vis absorption, photoluminescence (PL) and electrochemical properties. The influence of the presence as substituent bithiophene or phenyl structure on physical synthesized compounds properties was evaluated. 3.1. Synthesis of multi-substituted benzene and naphthalene derivatives Multisubstituted benzene derivatives were obtained based on a combination of cycloaddition [2+1+2+1] and Diels–Alder reaction [4+2] [27,28]. In [2+1+2+1] cycloaddition reaction as b-keto ester we used ethyl 2-methylacetoacetate, ethyl 2-benzylacetoacetate or ethyl butyrylacetate and diphenylacetylene or 1,2-bis(2,20 -bith iophene-5-yl)acetylene as the alkyne substrate. The synthetic route and chemical structure of synthesized compounds are presented in Fig. 1. 1,2-Bis(2,20 -bithiophene-5-yl)acetylene (bbta) was obtained using [PdCl2(PPh3)2] as a catalyst precursor, copper(I) iodide as co-catalyst and triethylamine as base (for 5-iodo-2,20 -bithiophene synthesis see [31]). In contrast to the methods described in the literature [32–34] in which THF, DMF or acetonitrile were used as the solvent we chose acetone. Using of acetone increase the solubility of acetylene in the reaction medium (which mean higher concentration of this substrate) and, in turn, shortening reaction time. In the literature one can find various synthetic routes of obtaining 2-pyranone systems using as

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catalyst: [Ru3(CO)12] [35], [Ni(cod)2] [36], [(Cp⁄RhCl2)2]/Ag2CO3 [37] or DABCO [38]. However, the synthesis of 2-pyranones catalyzed by ruthenium complex [Ru3(CO)12] requires the passing carbon monoxide through the reaction mixture (20 atm CO). Reactions based on [Ni(COD)2] contain 40% mol toxic and harmful to the environment PMe3. Using DABCO requires expensive and difficult to obtain substrates [i.e. a-cyano-a,b-unsaturated ketones as activated alkenes]. Therefore, as catalyst system we used [ReBr(CO)3(THF)]2 (2–3 mol%) with the addition of powdered molecular sieves. Naphthalene derivatives were obtained from 2-pyranone (which in fact is protected diene) in [4+2] Diels– Alder reaction with in situ generated benzyne at 20 °C [27,28]. In addition, our studies have also shown that the application in this case of one-pot reaction (i.e. without isolating 2-pyranone) is not only convenient and faster, but also more efficient. We observed that [ReBr(CO)3(THF)]2 left in the reaction system increased the Diels–Alder [4+2] reaction yield [28,29]. It is worth noting that there was no rhenium catalyst system deactivation (through the formation of stable complexes with 2,20 -bithiophene-5-yl) – in many cases those catalytic systems are inactive due to strong coordination of substrate by a transition metal catalytic center [39]. There are number of stable neutral and cationic transition metals (Cr, Ru, Ir, Rh, Mn, Re, Ti, Cu, Fe, etc.) complexes with thiophene. The issue of transition metal complexation with thiophene and oligothiophenes is intensively studied in the case of HDS processes (hydrodesulfurization) [40]. Thiophene is coordinated by transition metals in many ways, but the most common are the g1-S, g1-C, and g5-coordination modes [41–43]. In addition, exchange of CO-ligand to g5-thiophene in the reaction of [Cr(CO)6] with various thiophene derivatives [Cr(CO)3(g5-substituted-thiophenes)] took place [44]. Furthermore, reaction of [RuCl2(p-cymene)]2 with tetramethylthiophene resulted in very stable [(tetramethylthiophene)RuCl2]2 complex, indicating that thiophene coordinates Ru stronger than p-cymene [45]. The above analysis indicates that strong coordination of a transition metal by thiophene and oligothiophene may interfere with some reactions involving substrates containing thiophene moiety. 3.2. Structural characterization M3, M4, M7 and M8 are symmetrical compounds, while M1, M2, M5, and M6 show asymmetry in the structure. In 1H NMR spectra signals of the naphthalene ring protons (M4 and M8) can be observed in the range of 7.58–8.18 ppm. In the case of benzene pentasubstituted derivatives M1 and M5, the singlet from benzene ring proton was observed at 7.91 ppm for M1, whereas for M5 at 8.10 ppm. In addition, for M2, M3, M4 and M8 were obtained as crystals suitable for X-ray analysis. The compound M3 crystallizes in monoclinic P21, M2 in triclinic P1, M4 in orthorhombic Pbca, and M8 in monoclinic P21/c space groups, respectively. Molecular structures and details of crystal structure determination and refinement are presented in the Supplementary material. On the other hand, signals of protons in the thiophene rings are in the range of 6.44–7.22 ppm. That is very significant shift due to the fact that thiophene protons lay at about 7.20 ppm. Such a large shift is suggesting the existence of strong p-interaction (toroidal delocalization) between ortho-substituents, similarly as reported by Tanaka [19] for o-dithienyl benzene derivatives. This hypothesis is confirmed by analysis of the crystal structure of compound M8 (see Fig. 2). Dihedral angle between bt-(central-aryl-core)-bt is as high as 55° (approximately), and distance between the ipso-carbon atoms of the thiophene rings (2.8 Å) is comparable to derivatives with high through-space interactions [19,46] (Fig. 2). It is therefore a distance so low enough to induce a dipole–dipole interactions, which affects not only the molecular structure but also on photophysical, electrochemical and PL properties.

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Fig. 1. Synthetic pathway of multisubstituted benzene and naphthalene derivatives.

Fig. 2. Dihedral angle and ORTEP drawings of M8.

3.3. DFT calculations We performed the quantum theoretical calculations using density functional theory (DFT), with an exchange correlation hybrid functional B3LYP and the basis 6-31G(2d,p) for carbon, oxygen, 6-311G(3d,p) for sulfur and 6-31G for hydrogen atoms. The calculations were carried out with use of Gaussian 09 program [47]. The geometries of compounds M2, M3, M4, M8 and simulated geometries of M1, M5, M6, M7 were optimized in vacuum and no imaginary frequencies were found for any of these species, computed at Ci symmetry, by vibrational analysis. Based on the optimized geometries the energy and electronic distribution of molecular frontier orbitals were calculated in acetonitrile. The solvent effect was introduced by PCM (with dielectric constant e = 35.688) for a comparison with experimental electrochemical potentials. The frontier orbitals are of extreme importance for the evaluation of molecular reactivity. As much is negative the energy of the Highest Occupied Molecular Orbital (HOMO), more susceptible is the molecule to donate electrons and, consequently, higher is the tendency to suffer oxidation. A similar argument can be used to interpret the tendency of a given molecule in suffer reduction, on

the basis of the energy of the Lowest Unoccupied Molecular Orbital (LUMO). The contours of HOMOs and LUMOs are presented in Table 2 in Supplementary materials and in general, for all these molecules, the HOMO orbitals were concentrated in the aromatic parts of these molecules (phenyl rings and/or bithiophene), indicating that the oxidation process would have to happen primarily in this region. LUMOs are localized on nonbonding pz orbitals with considerable contribution of ester moiety in M1–M3 and M5–M7 molecules. The HOMOs energies of compounds M5–M7 are higher than M1–M3 ones and LUMOs are have lower energies. Therefore the bithiophene substituent exerts more stabilizing effect than phenyl ring. The same effect is visible in the case of compounds M4 and M8 where energy of HOMO–LUMO gap of M8 is much lower than in M4. 3.4. Photophysical properties As one can see at the UV–vis spectra of M1–M3 compounds absorbance band (attributed to p–p⁄ phenyl group transitions) is located between 280 and 320 nm, with absorption maxima at

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M1 vs M5

M2 vs M6

M3 vs M7

M4 vs M8

Fig. 3. Comparison of R-bt and R-Ph compounds (normalized excitation and emission spectra).

about 290 nm (see Fig. 3). In the case of M4 in absorption spectrum 3 maxima can be seen [282, 293 and 305 nm]. On the other hand, in the case of analogous bithiophenyl compounds the most intensive bands lie always in the area of 280–400 nm (Fig. 3) originated from the p–p⁄ transition of bithiophenyl groups. The lowest energy transition was recorded for M8 which implies a longer conjugation path. The emission spectra of the compounds were investigated in dichloromethane solution at ambient temperature. The solutions of the compounds were exited in the range of 321–419 nm and emitted light in the range of 382–509 nm. The only exception was the M4 solution which emission maximum occurs at 351 nm. The emission properties of the studied compounds are summarized in Table 1 [48]. Fig. 4 (see supp. inf. section 5) presents the excitation and emission spectra of the multisubstituted benzenes with phenyl (R-Ph) and bithiophene (R-bt) motifs. The maxima, both excitation and emission, of R-bt compounds show bathochromic shift compared with R-Ph analogues. Moreover the energies of excitations, especially in the case of R-bt compounds, are much lower than lowest energy absorption maxima. So, the emission active transitions have the n ? p⁄ character. The highest quantum efficiencies were determined for M6 and M7 bithiophene analogues equal to 41% and 23%, respectively. Although the unsymmetrical M6 is characterized by almost two time higher value of U than symmetrical M7. However the molecules differ from each other not only by symmetry but also by substituents. Therefore the differences in quantum

efficiency and Stock shift observed for these compounds are rather the result of differences in the molecular structures (different substituents, different number of aromatic rings) than the symmetry or asymmetry of the molecule. By comparison of the spectroscopic data for appropriate pairs of the compounds i.e. M1 and M5, M2 and M6, M3 and M7 one can see that lifetimes for R-bt are longer than for R-Ph. However, in the case of the M4 and M8 is completely opposite situation. For the selected compounds, which exhibited the highest PL quantum yield in solution (M5, M6 and M8), the ability to light emission in solid state as thin film on glass substrate was tested. UV–vis absorption spectra of prepared films are depicted in Fig. 2S (see supp. inf. section 5) The UV–vis spectra of compounds in solution were compared with the spectra obtained in film. In the case of M5 and M6 the curves showed the same shape. The absorption maximum of all compounds was red-shifted in the range of 17–33 nm compared to that in solution. Additionally, the absorption band of compounds M6 and M8 is broadened to lower energy region in contrary to M5 for which the absorption range is similar in solution and in solid state. All compounds both in CH2Cl2 solution and as film emitted light from the green region (Fig. 5a and supp. inf. section 5). However, the kem of compounds in solid state was slightly bathochromically shifted (4 nm for M6 and 8–12 nm for M8) in relation to solution. M5 as film exhibited very weak emission without kem structured. Additionally, the effect of excitation wavelength (kex ) on PL of compounds in films was evaluated. As can be seen from Fig. 4a a

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Table 1 Photophysical properties of M1–M8 compounds. Code

M1

M2

M3

M4

M5

M6

M7

M8

Ered1 (onset, CV) E1/2red1a Ered1 (onset, DPV) Eox (onset, CV) Eox (onset, DPV) Eg (CV)b HOMO (CV)c LUMO (CV)d HOMO (DFT) LUMO (DFT) Eg (DFT) Eg (OPTe) kexc (nm) kem (nm) s (ps)f 102 Uemg DEexc-em (cm1)

2.28 2.41 2.08 1.54 1.65 3.82 6.64 2.82 6.33 1.79 4.53 3.93 324 418 937 14.48 6941

2.43 – 2.46 1.48 1.41 3.91 6.58 2.67 6.47 1.63 4.84 4.05 396 455; 480 539 10.25 3274

2.56 2.6 2.59 1.58 1.55 4.14 6.68 2.54 6.66 1.15 5.51 4.04 326 382 867 4.69 4497

1.20 1.24 1.16 1.1 1.16 2.3 6.2 3.9 5.70 1.10 4.60 3.76 277;321 351 5485 11.45 2663

2.48 – 2.36 0.71 0.78 3.19 5.81 2.62 5.66 2.08 3.58 3.09 393 465 1263 9.55 3940

2.59 – 2.47 1.07 1.08 3.67 6.18 2.51 5.65 1.84 3.81 3.41 408 509 964 41.23 4863

2.44 – 2.5 0.56 0.45 3.00 5.66 2.66 5.65 1.82 3.83 3.36 419 495 1351 22.89 3664

1.25 1.39 1.19 0.39 0.36 1.64 5.49 3.85 5.60 1.42 4.18 3.40 394 455; 480 526 8.44 3403

Measured in CH2Cl2 solution, GC (reduction) and Pt (oxidation) as working electrode. a E1/2red – formal redox potential calculated as (Eox + Ered)/2 from cyclic voltammetry. b Eg = Eox (onset)  Ered (onset). c HOMO = 5.1  Eox. d LUMO = 5.1  Ered. e Calc. from: Eg = 1240/kabs [48]. f Estimated as average values of decay constants obtained after bi- or triexponential fitting of measured decay curves. g Absolute quantum yields obtained using integrating sphere in optically diluted dichloromethane solutions at 298 K.

Fig. 4. UV–vis spectra of compounds with bithiophene units (a) measured for dichloromethane deaerated solutions (c = 1  103 mol/dm3) and (b) in film on glass substrate; fluorescence of compounds M6 (c) and M8 (d) in solid state under irradiation with light at 365 nm.

lacks of kex impact on kem position was observed. On the other hand, kex influenced on PL intensity. For the naphthalene derivative with bithiophene rings blend with PVK was prepared with 5%

content of M8. PVK emitted light with kem about 400 nm in solid state as film on glass substrate (Fig. 4b). Blending two luminescent compounds together may produce emission which kem is

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4

2

0 500

PVK λ= 340nm M8 λ =390nm M8+PVK λ =340nm

1200 1000 800 600 400

Intensity PL [a.u.]

Intensity PL [a.u.]

Intensity [a.u.]

6

1400

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

200 0 400

600

500

600

Wavelength [nm]

Wavelength [nm]

(a)

(b)

Fig. 5. PL spectra of (a) M8 and (b) comparison of emission of PVK, M8 in film and blend.

intermediate between those for two components, and with higher efficiency than either [49]. In the PL spectra of blend domination of emission from PVK was seen. Thus, M8 does not play the role of emitter and can be considered as semiconductor. Blend emitted light with the highest intensity compare to M8 as film and pure PVK (cf. Fig. 5b).

Electrochemical properties of the studied polymers were investigated in CH2Cl2 solution by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The electrochemical oxidation and reduction onset potentials were used for estimation of the HOMO and LUMO energies (or rather, ionization potentials and electron affinities) of the materials (assuming that IP of ferrocene equals 5.1 eV [1]). The calculated HOMO, LUMO levels together with electrochemical energy band gap (Eg) are presented in Table 1. Firstly, the electrochemical properties during reduction process were investigated. All of the tested diesters (M1–M3 and M5–M7) undergoes reduction laying between 2.28 V (M1), and 2.59 V (M6) (so, at very similar potentials), while naphthalene derivatives (M4 and M8) undergoes reduction much easier i.e. at 1.20 and 1.25, respectively (see Fig. 6). It can therefore be assumed that the negative charge is located mainly on naphthalene core (for M4 and M8) and ester moiety (for all others compounds). This hypothesis is also confirmed by DFT calculations (see Section 3.5). On the other hand, in the case of oxidation, phenyl substituted derivatives compared to the corresponding bithiophenyl one are significantly different, what was expected (see Fig. 7). Without a doubt during oxidation of M5–M8 the cation-radical (located almost exclusively at bithiophenyl) is being formed. All of bithiophenyl compounds are (potentially) monomers due to having two terminal electro-polymerisable substituent [50,51]. However, for M6, M7 and M8 polymerization does not occur. In our opinion this phenomena can be ascribed to the through-space hole delocalization over the o-bithiophene units via a toroidal fashion as for some hexaarybenzenes [17,52], or o-dithienyl benzene derivatives [19]. This interaction stabilizes oxidated forms and preventing subsequent dimerization via terminal thienyls. In addition M8 is easiest to oxidize – i.e. at 0.39. It is therefore the potential too low for separated bithienylene substituents oxidation [53,54]. In this case, therefore, there is a strong intramolecular interactions lowering potential. Only in case of M5

dI [µA]

M1 M2 M3 M4 M5 M6 M7 M8

20

0 -3

-2

-1

0

1

2

+

E (vs. Fc/Fc ) [V] Fig. 6. DPV of M1–M8 in 0.2 mM in 0.1 M Bu4NPF6 in CH2Cl2 (potential step = 2.5 mV/s).

I [µA]

3.5. Electrochemical properties

40

140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -0.4 -0.2

M1 M2 M3 M4 M5 M6 M7 M8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

+

E (vs. Fc/Fc ) [V] Fig. 7. Oxidation of obtained compounds on a platinum electrode; sweep rate m = 100 mV/s, 0.2 mM in 0.1 M Bu4NPF6 in CH2Cl2.

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50

I [µA]

I [µA]

75

40 20

25

0

0

-20

-25

-40

0,0

0,2

0,4

1st scan 2nd scan 5th scan 10th scan 15th scan 20th scan

60

1st scan 2nd scan 3rd scan 4th scan 5th scan

0,6

0,8

1,0

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

+

E (vs. Fc/Fc ) [V]

+

E (vs. Fc/Fc ) [V]

(a)

(b)

Fig. 8. Oxidation of M5 (a) and poly-M5 film (b) in monomer free solution on a platinum electrode; sweep rate m = 100 mV/s, 0.2 mM in 0.1 M Bu4NPF6 in CH2Cl2. The arrow shows the shifting of the peak maximum (ascribed to oxidation of poly-M5) during subsequent scans.

1.0

the polymerization occurred above 0.78 V (Fig. 8a) giving polymer stable during several p-doping/dedoping cycles (Fig. 8b). During this process we do not observed the decomposition of polymer film.

UV–vis spectroelectrochemistry is extremely useful tool during donor–acceptor systems investigations. This measurements allowed not only to determine precisely frontier orbitals levels, the energy band gap, but also it can be used to examine the stability of electrochemically generated radical forms what is crucial in organic electronics research. There were not changes in the UV–vis spectrum observed during reduction. Therefore, this is confirmation of electrochemical and DFT results – LUMO is located exclusively on moieties which do not absorbs at UV–vis spectrum range i.e. at diester or naphthalene units. Surprisingly, in the case of M7, M6, M8 dimerization through bitiophenyl units does not occur – in this situation growing of the p–p⁄ quaterthiophene absorption band at a slightly lower energy (100 nm, approximately) would arise, as we reported previously [55]. The only exception was observed in M5 – at potential slightly higher than EOX ErCi process take place. A broad absorption band covering the entire spectrum of measurable area is formed (Fig. 9). Without a doubt it is the polaronic band [56,57] which means that in this case, the electropolimerization occurs. 3.7. Current–voltage characteristics In order to investigate the usefulness of selected compounds (M6, M5, and M8) as materials for organic electronics a preliminary study of current–voltage characteristic were performed. The electrical behavior of single layer devices with the following architecture: ITO/compound/Al was measured. It should be mentioned that these characteristics were obtained under ordinary laboratory conditions in non-optimized test devices. Fig. 10 shows current density–voltage (J–V) characteristics of the fabricated devices. The typical exponential J–V behavior is seen in the range of forward bias, which is defined here as positive bias on the bottom ITO electrode. Thus, layer of ITO was used as an anode. The shape of J–V characteristics confirms semiconducting nature of investigated compounds. The devices demonstrated low turn-on voltages, that

0V 0.7 V 0.8 V 0.9 V 1.05 V 1.45 V

0.8

Absorbance [a.u.]

3.6. Spectroelectrochemical measurements

0.9

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

300

400

500

600

700

800

900

1000

1100

Wavelength [nm] Fig. 9. UV–Vis spectra recorded in situ during the anodic oxidation of M5, measurement was done on ITO electrode in 0.1 M Bu4NBF4 solution in CH2Cl2.

is, of 1.2, 1.4 and 1.8 V for M8, M5 and M6, respectively. Taking into account the work functions for ITO (4.8 eV) and for Al cathode (4.3 eV) and HOMO and LUMO levels of tested compounds estimated based on CV measurements, the most promising seem to be M8 due to the highest HOMO level of the tested compounds. However, in all cases there is a high charge injection barrier at ITO/compound interface. Charge injection barrier may be reduced by incorporation of poly(3,4-(ethylenedioxy)thiophene):poly-(styr enesulfonate) (PEDOT:PSS) layer with workfunction (5.2 eV) on the ITO contact. On the other hand, considering the highest current density reached under the lowest voltage, the most promising seems to be M6 (cf. Fig. 10b). The current density–voltage characteristic was also performed for blend PVK with M8 (cf. Fig. 10c). The device showed turn-on voltage of 1.4 V, which is lower than the pure PVK (4.3 V). The formed diode ITO/PVK:M8 blend (25 nm)/Al showed blue electroluminescence. The photos with EL response of the device is displayed in Fig. 10f. The performance of prepared devices may be improved also by utilization of additional layers for diode construction such as Alq3 as electron transport layer and LiF as electron injection layer. The tested compound M8, based on results reported in Section 3.4 may not be treated as emitter, but rather as dopant improving fabricated devices performance. Thus, other emitter, better than PVK should be applied.

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600 M8 M5 M6

M6 M5 M8

500

Current density [μA/cm2]

Current Density [μA/cm2]

500 400 300 200

400 300 200 100

100

0

0

0

2

4

6

8

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

Voltage [V]

Electric field intensity [MV/cm]

(a)

(b)

1,8

2,0

2,2

Current density [μA/cm2]

4000

3000

2000

1000

0 0

2

4

6

Voltage [V]

(c)

(d)

(e)

(f)

Fig. 10. (a) Current density–voltage (J–V) characteristics of ITO/compound/Al with organic layer thickness around 210, 80 and 40 nm for M6, M5 and M8, respectively; (b) current density–electric field intensity characteristics of prepared devices; (c) J–V plot for ITO/PVK:M8 blend (25 nm)/Al; (d) AFM image of blend layer; architecture (e) and (f) photos of fabricated devices ITO/PVK:M8 blend/Al under applied voltage.

4. Conclusions

Acknowledgements

In conclusion, a new small molecule, with D-A-D framework was prepared in good yield by using a [2+1+2+1] cycloaddition followed by the [4+2] Diels–Alders reaction. This methodology can be used for the preparation of various derivatives with other central fragments. Despite having unprotected terminal bithiophene moieties this type of molecules can be repeatedly p-doped. This phenomena was ascribed to the through-space hole delocalization over the o-bithiophene units via a toroidal fashion, what was proven by (spectro)electrochemical measurements and DFT calculations. Discussed diarylbenzenes exhibited efficient green/blue light emission with Uem in solution up to 42%. In fabricated devices (with ITO/PVK: compound blend/Al architecture) M5, M6 and M8 act as dopant improving performance.

This work was supported by NCN Grant No. DEC-011/01/B/ST5/06309 and NCBiR Grant No. PBS2/A5/40/2014. Sławomir Kula acknowledges a scholarship from the Forszt project co-financed by the European Social. Agata Szłapa and Marek Matussek acknowledges a scholarship from the DoktoRIS project co-financed by the European Social. Calculations have been carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.optmat.2015.07. 011.

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References [1] P. Bujak, I. Kulszewicz-Bajer, M. Zagorska, V. Maurel, I. Wielgus, A. Pron, Chem. Soc. Rev. 42 (2013) 8895–8999, http://dx.doi.org/10.1039/c3cs60257e. [2] H. Jiang, Macromol. Rapid Commun. 31 (2010) 2007–2034, http://dx.doi.org/ 10.1002/marc.201000040. [3] R. Grykien, B. Luszczynska, I. Glowacki, E. Kurach, R. Rybakiewicz, K. Kotwica, M. Zagorska, A. Pron, P. Tassini, M.G. Maglione, A. De, G. Del Mauro, T. Fasolino, R. Rega, G. Pandolfi, C. Minarini, S. Aprano, Opt. Mater. 37 (2014) 193–199, http://dx.doi.org/10.1016/j.optmat.2014.05.023. [4] Q. Fan, Y. Liu, M. Xiao, H. Tan, Y. Wang, W. Su, D. Yu, R. Yang, W. Zhu, Org. Electron. 15 (2014) 3375–3383, http://dx.doi.org/10.1016/j.orgel.2014. 09.019. [5] V. Ramkumar, P. Kannan, Opt. Mater. 46 (2015) 314–323. [6] L. Huo, J. Hou, H.-Y. Chen, S. Zhang, Y. Jiang, T.L. Chen, Y. Yang, Macromolecules 42 (2009) 6564–6571, http://dx.doi.org/10.1021/ma9012972. [7] M. Chandrasekharam, M.A. Reddy, K. Ganesh, G.D. Sharma, S.P. Singh, J.L. Rao, Org. Electron. 15 (2014) 2116–2125, http://dx.doi.org/10.1016/ j.orgel.2014.05.033. [8] Z. Gao, Z. Wang, T. Shan, Y. Liu, F. Shen, Y. Pan, H. Zhang, X. He, P. Lu, B. Yang, Y. Ma, Org. Electron. 15 (2014) 2667–2676, http://dx.doi.org/10.1016/ j.orgel.2014.07.019. [9] K. Motoyama, H. Li, T. Koike, M. Hatakeyama, S. Yokojima, S. Nakamura, M. Akita, Dalton Trans. 40 (2011) 10643–10657, http://dx.doi.org/10.1039/ C1DT10727E. [10] Y. Liu, X. Sun, Y. Wang, Z. Wu, Synth. Met. 198 (2014) 67–75, http://dx.doi.org/ 10.1016/j.synthmet.2014.09.031. [11] A. Wakamiya, K. Mishima, K. Ekawa, S. Yamaguchi, Chem. Commun. (2008) 579–581, http://dx.doi.org/10.1039/B716107G. [12] K. Geramita, J. McBee, T.D. Tilley, J. Org. Chem. 74 (2009) 820–829, http:// dx.doi.org/10.1021/jo802171t. [13] S. Barlow, S.A. Odom, K. Lancaster, Y.A. Getmanenko, R. Mason, V. Coropceanu, J.-L. Brédas, S.R. Marder, J. Phys. Chem. B 114 (2010) 14397–14407, http:// dx.doi.org/10.1021/jp100774r. [14] Z. Wang, M. Liang, L. Wang, Y. Hao, C. Wang, Z. Sun, S. Xue, Chem. Commun. 49 (2013) 5748–5750, http://dx.doi.org/10.1039/C3CC42121J. [15] R. Grisorio, G.P. Suranna, P. Mastrorilli, G. Allegretta, A. Loiudice, A. Rizzo, G. Gigli, K. Manoli, M. Magliulo, L. Torsi, J. Polym. Sci.: Polym. Chem. 51 (2013) 4860–4872, http://dx.doi.org/10.1002/pola.26914. [16] J. Liu, X. Sun, Z. Li, B. Jin, G. Lai, H. Li, C. Wang, Y. Shen, J. Hua, J. Photochem. Photobiol. A: Chem. 294 (2014) 54–61, http://dx.doi.org/10.1016/ j.jphotochem.2014.07.021. [17] Y. Zou, J. Zou, T. Ye, H. Li, C. Yang, H. Wu, D. Ma, J. Qin, Y. Cao, Adv. Funct. Mater. 23 (2013) 1781–1788, http://dx.doi.org/10.1002/adfm.201202286. [18] L.-C. Wang, J. Sun, Z.-T. Huanga, Q.-Y. Zheng, Cryst. Eng. Comm. 15 (2013) 8511–8521, http://dx.doi.org/10.1039/C3CE40856F. [19] Y. Tanaka, T. Koike, M. Akita, Chem. Commun. 46 (2010) 4529–4531, http:// dx.doi.org/10.1039/C0CC00128G. [20] D.M. Cho, S.R. Parkin, M.D. Watson, Org. Lett. 7 (2005) 1067–1068, http:// dx.doi.org/10.1021/ol050019c. [21] T. Muto, T. Temma, M. Kimura, K. Hanabusa, H. Shirai, J. Org. Chem. 66 (2001) 6109–6115, http://dx.doi.org/10.1021/jo010384r. [22] D.J. Gregg, C.M.A. Ollagnier, C.M. Fitchett, S.M. Draper, Chem. Eur. J. 12 (2006) 3043–3052, http://dx.doi.org/10.1002/chem.200501289. [23] C.J. Martin, B. Gil, S.D. Perera, S.M. Draper, Eur. J. Org. Chem. (2011) 3491– 3499, http://dx.doi.org/10.1002/ejoc.201100332. [24] V. Nair, A.N. Pillai, P.B. Beneesh, E. Suresh, Org. Lett. 7 (2005) 4625–4628, http://dx.doi.org/10.1021/ol051723w. [25] S. Li, H. Qu, L. Zhou, K. Kanno, Q. Guo, B. Shen, T. Takahashi, Org. Lett. 11 (2009) 3318–3321, http://dx.doi.org/10.1021/ol901153p. [26] C.-Y. Wu, Y.-C. Lin, P.-T. Chou, Y. Wang, Y.-H. Liu, Dalton Trans. 40 (2011) 3748–3753, http://dx.doi.org/10.1039/C0DT01502D. [27] Y. Kuninobu, H. Takata, A. Kawata, K. Takai, Org. Lett. 10 (2008) 3133–3135, http://dx.doi.org/10.1021/ol801226t. [28] Y. Kuninobu, M. Nishi, A. Kawata, H. Takata, Y. Hanatani, S.Y. Salprima, A. Iwai, K. Takai, J. Org. Chem. 75 (2010) 334–341, http://dx.doi.org/10.1021/ jo902072q. [29] Y. Kuninobu, K. Takai, Chem. Rev. 111 (2011) 1938–1953, http://dx.doi.org/ 10.1021/cr100241u. [30] M.G. Ferlin, G. Chiarelotto, V. Gasparotto, L.D. Via, V. Pezzi, L. Barzon, G. Palù, I. Castagliuolo, J. Med. Chem. 48 (2005) 3417–3427, http://dx.doi.org/10.1021/ jm049387x.

[31] S. Krompiec, M. Filapek, I. Grudzka, S. Kula, A. Słodek, Ł. Skórka, W. Danikiewicz, P. Ledwon, M. Lapkowski, Synth. Met. 165 (2013) 7–16, http:// dx.doi.org/10.1016/j.synthmet.2012.12.030. [32] M. Pal, N.G. Kundlu, J. Chem. Soc., Perkin Trans. 1 (1996) 449–451, http:// dx.doi.org/10.1039/P19960000449. [33] P. Chuentragool, K. Vongnam, P. Rashatasakhon, M. Sukwattanasinitt, S. Wacharasindhu, Tetrahedron 67 (2011) 8177–8182, http://dx.doi.org/ 10.1016/j.tet.2011.08.042. [34] W. Zhang, H. Wu, Z. Liu, P. Zhong, L. Zhang, X. Huang, J. Cheng, Chem. Commun. 46 (2006) 4826–4828, http://dx.doi.org/10.1039/B607809E. [35] T. Fukuyama, Y. Higashibeppu, R. Yamaura, I. Ryu, Org. Lett. 9 (2007) 587–589, http://dx.doi.org/10.1021/ol062807n. [36] Y. Kajita, T. Kurahashi, S. Matsubara, J. Am. Chem. Soc. 130 (2008) 17226– 17227, http://dx.doi.org/10.1021/ja806569h. [37] S. Mochida, K. Hirano, T. Satoh, M. Miura, J. Org. Chem. 74 (2009) 6295–6298, http://dx.doi.org/10.1021/jo901077r. [38] W. Liu, G. Zhao, Org. Biomol. Chem. 12 (2014) 832–835, http://dx.doi.org/ 10.1039/C3OB41763H. [39] S. Krompiec, N. Kuz´nik, M. Krompiec, R. Penczek, J. Mrzigod, A. Tórz, J. Mol. Catal. A: Chem. 253 (2006) 132–146, http://dx.doi.org/10.1016/ j.molcata.2006.03.015. [40] M. Brorson, J.D. King, K. Kiriakidou, F. Prestopino, E. Nordlander, Metal Cluster in Chemistry, Wiley-VCH, Weinheim, 1999, pp. 741–781. [41] M. Landman, T. Waldbach, H. Görls, S. Lotz, J. Organomet. Chem. 678 (2003) 5– 14, http://dx.doi.org/10.1016/S0022-328X(03)00383-8. [42] J.R. Lockemeyer, T.B. Rauchfuss, A.L. Rheingold, S.R. Wilson, J. Am. Chem. Soc. 111 (1989) 8828–8834, http://dx.doi.org/10.1021/ja00206a010. [43] Md.N. Uddin, M.A. Mottalib, N. Begum, S. Ghosh, A.K. Raha, D.T. Haworth, S.V. Lindeman, T.A. Siddiquee, D.W. Bennett, G. Hogarth, E. Nordlander, S.E. Kabir, Organometallics 28 (2009) 1514–1523, http://dx.doi.org/10.1021/om801075p. [44] M.J. Sanger, R.J. Angelici, Organometallics 13 (1994) 1821–1831, http:// dx.doi.org/10.1021/om00017a045. [45] E.A. Ganja, T.B. Rauchfuss, C.L. Stern, Organometallics 10 (1991) 270–275, http://dx.doi.org/10.1021/om00047a060. [46] C. Rios, R. Salcedo, Molecules 19 (2014) 3274–3296, http://dx.doi.org/ 10.3390/molecules19033274. [47] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian Inc, Gaussian 09, Revision A.1, Wallingford CT, 2009. [48] M. Lapkowski, P. Data, S. Golba, J. Soloducho, A. Nowakowska-Oleksy, Opt. Mater. 33 (2011) 1445–1448, http://dx.doi.org/10.1016/j.optmat.2011.02.018. [49] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Chem. Rev. 109 (2009) 897–1091, http://dx.doi.org/10.1021/cr000013v. [50] S. Tarkuc, E. Sahmetlioglu, C. Tanyeli, I.M. Akhmedov, L. Toppare, Opt. Mater. 30 (2008) 1489–1494, http://dx.doi.org/10.1016/j.optmat.2007.08.009. [51] M. Krompiec, I. Grudzka, M. Filapek, Ł. Skorka, S. Krompiec, M. Łapkowski, M. Kania, W. Danikiewicz, Electrochim. Acta 56 (2011) 8108–8114, http:// dx.doi.org/10.1016/j.electacta.2011.05.132. [52] S.V. Rosokha, I.S. Neretin, D. Sun, J.K. Kochi, J. Am. Chem. Soc. 128 (2006) 9394– 9407, http://dx.doi.org/10.1021/ja060393n. [53] J. Heinze, B.A. Frontana-Uribe, S. Ludwigs, Chem. Rev. 110 (2010) 4724–4771, http://dx.doi.org/10.1021/cr900226k. [54] A. Jarczyk-Jedryka, K. Bijak, D. Sek, M. Siwy, M. Filapek, G. Malecki, S. Kula, G. Lewinska, E.M. Nowak, J. Sanetra, H. Janeczek, K. Smolarek, S. Mackowski, E. Schab-Balcerzak, Opt. Mater. 39 (2015) 58–68, http://dx.doi.org/10.1016/ j.optmat.2014.10.065. [55] A. Slodek, M. Filapek, G. Szafraniec, I. Grudzka, W.A. Pisarski, J.G. Malecki, L. Zur, M. Grela, W. Danikiewicz, S. Krompiec, Eur. J. Org. Chem. (2014) 5256– 5264, http://dx.doi.org/10.1002/ejoc.201402241. [56] K. Olech, J. Sołoducho, K. Laba, P. Data, M. Lapkowski, Sz. Roszak, Electrochim. Acta 141 (2014) 349–356, http://dx.doi.org/10.1016/j.electacta.2014.07.084. [57] P. Data, P. Zassowski, M. Lapkowski, W. Domagala, S. Krompiec, T. Flak, M. Penkala, A. Swist, J. Soloducho, W. Danikiewicz, Electrochim. Acta 122 (2014) 118–129, http://dx.doi.org/10.1016/j.electacta.2013.11.167.