Synthesis and optoelectronic characterization of some triphenylamine-based compounds containing strong acceptor substituents

Journal of Luminescence 153 (2014) 5–11

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Synthesis and optoelectronic characterization of some triphenylamine-based compounds containing strong acceptor substituents Mircea Grigoras n, Teofilia Ivan, Loredana Vacareanu, Ana Maria Catargiu, Radu Tigoianu “P. Poni” Institute of Macromolecular Chemistry, Electroactive Polymers Department, 41A Gr. Ghica Voda Alley, 700487 Iasi, Romania

art ic l e i nf o

a b s t r a c t

Article history: Received 6 September 2013 Received in revised form 10 February 2014 Accepted 11 February 2014 Available online 12 March 2014

Three novel triphenylamine-based compounds containing strong electron acceptor groups have been synthesized and their comparative photophysical properties are presented. These compounds were obtained by a two-step method: (i) triphenylamine compounds with one, two and three phenylacetylene arms were synthesized by Sonogashira reaction between iodine-substituted triphenylamines and phenylacetylene, followed by (ii) post-modification of these electron-rich alkynes by addition of the strong electron acceptor, tetracyanoethylene. Characterization of all oligomers was made by FTIR, 1H-NMR, UV–vis and fluorescence spectroscopy. A batochromic shifting of the UV and photoluminescence maxima was observed with the increase of the acceptor group number. The electrochemical behavior was studied by cyclic voltammetry. The cyclic voltammograms have evidenced that triphenylamine-phenylacetylene compounds undergo only oxidation processes while compounds modified with tetracyanoethylene show both oxidation and reduction peaks associated with donor and acceptor groups, respectively. The donor– acceptor compounds coordinate metal ions (i.e., Hg2 þ and Sn2 þ ) by cyano groups resulting in the decreasing of charge transfer band intensity, and they can be used as chemosensors. & 2014 Elsevier B.V. All rights reserved.

Keywords: Triphenylamine-ethynylene compounds Tetracyanoethylene cycloaddition Photophysical properties Intramolecular charge transfer interactions

1. Introduction Arylamine-based oligomers and polymers have attracted much attention in the last years due to their unique properties that allow them to have potential applications in organic electronics, photonics and spin electronics [1–3]. Particularly, triphenylamine-based oligomers have good hole-transporting properties, high lightemitting efficiencies, photoconductivity and photorefractivity, large two-photon absorption cross sections and stabilization effect of high-spin polyradicals in organic magnets. Due to the good electron-donating nature of triphenylamine (TPA), its oligomers and polymers have been widely studied as hole-transporting materials for a number of applications, such as xerography, organic field-effect transistors, photorefractive systems, light emitting diodes, etc. [4,5]. Many triphenylamine-based dyes are used in dye-sensitized solar cells as sensitizers [6–8]. These interesting properties are associated with the presence of TPA moiety that contains the nitrogen center linked to three electron-rich phenyl groups in a propeller-like geometry [9]. As a result, many TPA-based

n

Corresponding author. Tel.: þ 40 232 217454; fax: þ 40 232 211299. E-mail address: [email protected] (M. Grigoras).

http://dx.doi.org/10.1016/j.jlumin.2014.02.032 0022-2313/& 2014 Elsevier B.V. All rights reserved.

oligomers with linear, dendrimer or star-shaped architectures have been synthesized and studied for various applications [10–20]. Since Gratzel et al. [21] have reported the first dye-sensitized organic solar cells, a considerable interest for small conjugated molecules having a donor-π-acceptor configuration has appeared. These compounds show intramolecular charge-transfer (ICT) interactions and efficient electron transfer from donor to acceptor upon photoexcitation, being promising candidates for building of the dyesensitized and bulk heterojunction (BHJ) solar cells. As donor part, triphenylamine has been widely used in combination with directly attached cyanoacetic acid [22,23], quinoxaline [24], oxadiazole [25], dicyano- and tricyanovinyl [26–29] groups. Tetracyanoethylene (TCNE) is known to be a strong organic electron acceptor. The high reactivity of TCNE toward nucleophiles or electron-rich reagents allowed introducing strong acceptor moieties into organic molecules [30]. Diederich et al. [31–33] have recently reported that a variety of electron-rich alkynes reacted with TCNE to give donor-substituted 1,1,4,4-tetracyano-1,3-butadienes in excellent yields. Furthermore, the reaction was extended to other acceptor molecules, such as 7,7,8,8tetracyanoquinodimethane (TCNQ) or using another donors [34–37], or even to obtain polymers [38]. The new class of chromophores obtained by these reactions were characterized by intense ICT interactions with a strong absorption in the visible region and promising third-order nonlinear optics [39]. The absorption maxima of these

6

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

materials are shifted in the 400–500 nm region of solar spectrum. In this paper, we report the synthesis of three new triphenylamine-based compounds containing strong donor and acceptor groups by postmodification of triple bonds with tetracyanoethylene. Relationship between chemical structures and optoelectronic properties of the compounds was investigated by spectral methods, such as 1H-NMR, FT-IR, cyclic voltammetry, UV–vis and fluorescence spectroscopy.

2. Experimental 2.1. Materials Triphenylamine (TPA), phenylacetylene, PPh3PdCl2, CuI, triphenylphosphine (PPh3) and tetracyanoethylene (TCNE) (all from Sigma-Aldrich) are commercial products and were used without further purification. Iodine derivatives; (4-iodophenyl)diphenylamine, bis(4-iodophenyl)phenylamine and tris(4-iodophenyl)amine were synthesized starting from triphenylamine, by iodination with KI/KIO3 at three molar ratios, as is described in the literature [40]. Solvents such as triethylamine, methylene chloride, acetic acid, and hexane were from commercial sources and used as received or dried by known methods. Carbon tetrachloride was purified by distillation at atmospheric pressure and dried on molecular sieves. 2.2. Instruments The FT-IR spectra were recorded in KBr pellets on a DIGILABFTS 2000 spectrometer. UV–vis and fluorescence measurements were carried out in solution using spectrophotometric grade solvents, on a Specord 200 spectrophotometer and Perkin-Elmer LS 55 apparatus, respectively. The fluorescence quantum yields were determined by the integrating sphere method using an FLS 980 spectrometer and CHCl3 as solvent. 1H-NMR spectra were recorded at room temperature on a Bruker Avance DRX-400 spectrometer (400 MHz) as solutions in CDCl3, and chemical shifts are reported in ppm and referenced to TMS as internal standard. The cyclic voltammograms (CV) were recorded using a Bioanalytical System, Potentiostat–Galvanostat (BAS 100B/W) system. The electrochemical cell was equipped with three electrodes: a working electrode (Pt, Φ¼ 1.6 mm), an auxiliary electrode (platinum wire), and a reference electrode (consisted of a silver wire coated with AgCl). Prior to the each experiment, Bu4NBF4 solution was deaerated by passing dry argon gas for 10 min. All measurements were performed at room temperature under argon atmosphere. Compound 1. A mixture of (PPh3)2PdCl2 (0.011 g, 0.015 mmol), CuI (0.018 g, 0.094 mmol), PPh3 (0.017 g, 0.064 mmol), phenylacetylene (1 mL) and 2 mL triethylamine were introduced a 50-mL round-bottomed flask. The mixture was stirred under dry nitrogen at 50–60 1C for 1 h. Then, a solution of (4-iodophenyl)diphenylamine (1.5 g, 4.04 mmol) and TEA (6.5 mL) was added. After 12 h of stirring at 60 1C, TEA was removed by distillation and the solid product was dissolved in ethylic ether, washed with diluted HCl, water and dried over MgSO4. The product after purification by chromatography is viscous oil that crystallized in time. Yield, 74.3%. M.p.¼ 79–80 1C. Anal. calcd for C26H19N (345.445): C, 90.4; H, 5.54; N, 4.06 and found: C, 90.08; H, 5.40; N, 4.18. FT-IR (KBr) ν, cm  1: 3435, 3033–2853 (QC–H), 2202 (–C  C–), 1588 (CQC, conjugated phenyl groups), 1488, 1316, 1277 (–CN stretching vibration), 1070, 1025, 831, 754, 689, and 497. 1H NMR (CDCl3, 400 MHz) δ, ppm: 7.52–7.48 (m, 4H), 7.41–7.20 (m, 9H), and 7.12–6.97 (m, 6H). Compound 2. A 50-mL round-bottomed flask was equipped with a reflux condenser, a magnetic stirring bar and an inlet– outlet of inert gas and flushed with nitrogen. TEA (15 mL), (PPh3)2PdCl2 (0.022 g, 0.031 mmol), CuI (0.036 g, 0.18 mmol),

PPh3 (0.033 g, 0.12 mmol) and phenylacetylene (2 mL) were introduced and the mixture was stirred under dry nitrogen at 50–60 1C for 1 h. Then, a mixture of bis (4,40 -iodophenyl)amine (1.5 g, 3.01 mmol) and TEA (10 mL) was added. After 12 h of stirring at 60 1C, TEA was removed by distillation and the solid product was dissolved in ethylic ether, washed with diluted HCl, water and dried over MgSO4. After evaporation of solvent the crude product was purified by column chromatography to obtain a yellow crystalline product with 78.3% yield. M. p.¼46–47 1C. Anal. calcd for C34H23N (445.534): C, 91.65; H, 5.20; N, 3.14 and found: C, 91.48; H, 5.06; N, 3.02. FT-IR (KBr) ν, cm  1: 3437, 3050–2852 (QC–H), 2211 (–C  C–), 1590 (CQC, conjugated phenyl groups), 1506, 1439, 1280 (–CN stretching vibration), 1025, 830, 754, 687, and 500. 1H-NMR (CDCl3, 400 MHz) δ, ppm: 7.51–7.49 (d, 8H), 7.40–7.37 (d, 4H), 7.31–7.29 (m, 6H), 7.11–7.09 (t, 3H), and 7.03–7.01 (d, 2H). Compound 3. A 50-mL round-bottomed flask was equipped with a reflux condenser, a magnetic stirring bar and an inlet–outlet of inert gas and flushed with nitrogen. TEA (15 mL), (PPh3)2PdCl2 (0.033 g, 0.047 mmol), CuI (0.054 g, 0.28 mmol), PPh3 (0.051 g, 0.19 mmol) and phenylacetylene (3 mL) were introduced and the mixture was stirred under dry N2 at 50–60 1C for 1 h. Then, a mixture of tris(4-iodophenyl) amine (2.13 g, 3.4 mmol) and TEA (10 mL) was added. After 12 h of stirring at 60 1C, TEA was removed by distillation and the solid product was dissolved in ethylic ether, washed with diluted HCl, water and dried over MgSO4. After evaporation of solvent the crude product was purified by column chromatography to obtain a yellow crystalline product (yield¼65.4%). M.p.¼201–202 1C. Anal. calcd for C42H27N (545.646): C, 92.44; H, 4.99; N, 2.57 and found: C, 92.18; H, 4.78; N, 2.67. FT-IR (KBr) ν, cm  1: 3435, 3031–2937 (QC–H), 2207 (–C  C–), 1589 (CQC, conjugated phenyl groups), 1505, 1317, 1287 (–CN stretching vibration), 833, 752, 686, and 512. 1H-NMR (CDCl3, MHz) δ, ppm: 7.52–7.50 (m, 6H), 7.44–7.42 (d, 6H), 7.36–7.32 (m, 9H), and 7.07–7.05 (d, 6H). Synthesis of M1, M2, and M3. To a solution of 1 (0.43 g, 1.24 mmol) in CCl4 (15 mL) was added TCNE (0.16 g, 1.24 mmol). The mixture was stirred at reflux for 24 h in argon atmosphere. The solutions were filtered, and after evaporation of the solvent, the solid was dissolved in a minimal amount of CH2Cl2, then nhexane was added until precipitation started. Crystallization at 0 1C afforded the desired products. Compound M1 was isolated pure in 63.7% yield as dark red solid. In the case of compounds M2 and M3 they were isolated in pure state in 63.1% and 63.4% yields, respectively, as red solids. M1: M.p. ¼156–157 1C. FT-IR (KBr) ν, cm  1: 3433, 3035–2923 (QC–H), 2220 (–C  N–), 1587 (CQC, conjugated phenyl groups), 1487, 1336, 1204, 757, and 697. 1H-NMR (CDCl3, 400 MHz) δ, ppm: 7.73–7.71 (d, 2H), 7.66–7.62 (d, 2H), 7.57–7.52 (m, 3H), 7.42–7.38 (m, 4H), 7.28–7.21 (m, 6H), and 6.94–6.92 (d, 2H). ESI-MS ¼474.54. M2: M.p.¼ 109–110 1C. FT-IR (KBr) ν, cm  1: 3434, 3049–2923 (QC–H), 2219 (–C  N–), 1588 (CQC, conjugated phenyl groups), 1486, 1331, 1180, 756, 688, and 523. 1H-NMR (CDCl3, 400 MHz) δ, ppm: 7.37–7.64 (m, 4H), 7.57–7.52 (d, 8H), 7.35–7.33 (m, 6H), 7.22– 7.16 (m, 3H), and 7.01–6.99 (d, 2H). ESI-MS ¼702.75. M3: M.p.¼142–143 1C. FT-IR (KBr) ν, cm  1: 3427, 2222 (–C  N–), 1590 (CQC, conjugated phenyl groups), 1499, 1326, 1181, 835, 757, 691, and 522. 1H-NMR (CDCl3, 400 MHz) δ, ppm: 7.74–7.66 (m, 69H), 7.55–7.52 ( m, 6H), 7.36–7.35 (t, 9H), and 7.18–7.16 (d, 6H). ESI-MS¼ 930.97.

3. Results and discussion The steps for synthesis of the triphenylamine-based compounds starting from triphenylamine, are outlined in Scheme 1.

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

7

CN NC CN X CN

N N

N

M1

1 NC

X

a) N

b)

CN

CN

e)

f)

N

c)

2 X

NC

CN

CN

N

M2

X

CN NC

N

X

CN

NC

N

CN

NC

X=I

NC

X

N

CN

CN

N

CN

3

M3 CN NC

CN CN

Scheme 1. Synthetic route of compounds M1–M3. (a, b, c) – KI/KIO3, AcOH, 85 1C, 5 h; (d) – [(PPh3)2PdCl2, CuI, PPh3)], TEA, phenylacethylene, 50–60 1C, 24 h; and (e) – TCNE, CCl4.

Fig. 1. FT-IR spectra of the TPA-based compounds.

8

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

Iodination of triphenylamine with KI/KIO3, at requested molar ratio, in acetic acid, at 851C for 5 h provided compounds: (4iodophenyl)diphenylamine, bis(4-iodophenyl)phenylamine and tris(4-iodophenyl)amine [40]. The Sonogashira coupling of iodine-derivatives with phenylacetylene in high excess and the presence of catalyst [(PPh3)2dPdCl2, CuI, PPh3] using triethylamine as solvent gave compounds 1, 2 and 3, that contain triphenylamine

nuclei as core and one, two or three phenylacetylene arms, respectively. To our knowledge, only the synthesis of compound 3 was previously reported [41]. FTIR spectra of 1–3 evidence the stretching vibration of triple bond in the range 2202–2211 cm  1 (Fig. 1). Acetylene derivatives 1, 2, and 3 were modified with tetracyanoethylene (TCNE) to obtain compounds M1, M2, and M3. These compounds are readily obtained by the [2þ 2] cycloaddition

Fig. 2. UV and fluorescence spectra in CHCl3 (10–5 M) of compounds 1–3.

Fig. 3. UV–vis and fluorescence spectra of compounds M1–M3 in different solvents.

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

of TCNE to the strong donor activated triple bond followed by cyclo-reversion of adduct under mild conditions leading to a new class of non-planar, charge transfer (CT) chromophores. Due to the high efficiency and chemical orthogonality toward many other reactions, it is classified as a new “click chemistry reaction” [42]. The reactivity largely depends on the electron-donating groups substituted by the alkyne moiety, and triphenylamine strong donor was found to afford the desired donor–acceptor structures in a quantitative yield at room temperature [34]. Some small nonplanar donor–acceptor molecules prepared by these reactions were demonstrated to show excellent second-order and thirdorder nonlinear optical properties [39]. All new compounds were isolated as deep red solids and found to be stable under ambient conditions. Identities and structures of the compounds were confirmed by 1H-NMR and FT-IR spectroscopy, and mass spectroscopy. In the FTIR spectra (Fig. 1) the stretching vibrations of the internal alkynes disappeared and were replaced by the stretching vibration of the cyano groups at 2220 cm  1 (M1), 2219 cm  1 (M2) and 2222 cm  1 (M3). The 1H-NMR spectra of all compounds have proved their structure (Section 2). Introduction of the electron-acceptor groups at the periphery of triphenylamine-based derivatives leads to internal charge-transfer complexes, evidenced by strong colors and new UV–vis absorptions.

9

364 nm for 2 and 368 nm for 3. It was noticed that the values of fluorescence quantum yields of the sample increase from 1 (18.29%) to 2 (21.06%) and 3 (43.39%) with increasing of conjugated arms number. The fluorescence quantum yield for compounds M1–M3 (excited at λICT) is very low (  1%) due to the efficient electron transfer from donor to acceptor upon photoexcitation. The emission spectra show a bathocromic shift in the order: 1 (407 nm), 2 (408 nm), and 3 (412 nm), and this may be also explained by increasing of the conjugation length. A dependence of fluorescence maxima of compounds upon excitation wavelengths was not observed.

3.1. Photophysical properties The photophysical properties of all compounds were examined by UV–vis and fluorescence spectroscopy in diluted CHCl3 solution (10–5 M). Normalized UV–vis absorption and photoluminescence (PL) spectra of compounds 1–3 are shown in Fig. 2. All absorption spectra have two absorption regions with maxima at 250–330 nm and 330–400 nm with or without any vibronic features. The first absorption is assigned to π–π* transition in aromatic rings while the second maximum is due to π–π* absorption in whole conjugated molecule. The three new compounds have a triphenylamine central group and one, two or three arms of phenylacetylene type. As a conclusion, with increasing of arms number the absorption maximum is red-shifted from 352 nm (1) to 364 nm (2) and 368 nm (3), and this behavior could be explained by participation of nitrogen central atom to conjugation, the conjugation path between the three branches being uninterrupted. Diluted solutions of compounds in CHCl3 are characterized by a yellowishgreen fluorescence. The fluorescence quantum yields were measured for compounds 1–3 using the absolute method in which the ratio of the number of absorbed photons of a sample and the number of consequently emitted photons is directly measured. The high-diluted CHCl3 solutions were excited at 353 nm for 1,

Fig. 4. Relationship between solvent polarity parameter and λmax of the intramolecular charge transfer band.

Fig. 5. UV–vis spectral changes of M3 (1.87  10–6 M) in DMF at 20 1C after the stepwise addition of Hg2 þ (from 1  10–4 M to 8  10–4 M).

Table 1 Summary of UV–vis and fluorescence spectra of M1–M3 in several solvents. Solvent

Toluene

CHCl3

CH2Cl2

DMF

ET(30) (kcal/mol)

33.9

39.1

40.7

43.2

EN T

0.099

0.259

0.309

0.386

M1 Abs/nm Em/nma

330, 474 389

276, 330, 494 407

266, 360, 490 410

292, 476 439

M2 Abs/nm Em/nma

330, 492 372

276, 330, 502 376

276, 328, 500 376

330, 478 440

M3 Abs/nm Em/nma

344, 502 382

276, 334, 511 410

266, 334, 509 412

286, 338, 496 440

a

Emission peaks are obtained at λex ¼ 330 nm.

Fig. 6. Cyclic voltammograms of compounds 1–3 in CH2Cl2. All solutions contained Bu4NBF4 (0.1 M) as the supporting electrolyte, scan rate is 50 mV/s.

10

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

Normalized UV–vis absorption and photoluminescence (PL) spectra of compounds M1–M3 are shown in Fig. 3. Absorption and emission spectra were recorded in various organic solvents: CHCl3, CH2Cl2, DMF and toluene and the results are summarized in Table 1. All compounds present a first absorption band in the 270– 430 nm region followed by a second transition at longer wavelengths. Based on triphenylamine spectrum as reference, the first absorption band is assigned to a π–π* transition in aromatic ring while the second band (absent for triphenylamine) can be assigned to internal charge transitions (ICT) between triphenylamine and acceptors groups. It can be observed that with increasing of cyano group number the absorption maximum assigned to internal charge transfer complex is red-shifted from M1 to M3. Thus, in chloroform solution λmax ICT is shifted from 494 nm (M1) to 502 nm (M2) and 511 nm (M3). Evidently, the ICT interactions are more significant for the trisubstituted compound M3 than for the mono (M1) and disubstituted (M2) because of the increased π-conjugation and enhanced electron accepting ability of the tricyano moiety. For

the emission spectra in various organic solvents, the compounds M1–M3 display similar behaviors. The emission peak is red-shifted at higher wavelength (bathocromic effect) in the order: M1–M3, showing that there is obvious π–π* delocalization and the significant increase of the effective conjugation length with the increase of the cyano moiety number in structure molecules. The UV–vis spectra of M1–M3 were registered in several organic solvents and it was observed that the λmax of the charge transfer band decreases with increase of solvent polarity (negative solvatochromism) (Fig. 4). The correlation is not respected for toluene, probably due to the donor character of the aromatic solvent and its competitive participation in donor–acceptor complex formation. The solvent polarity was appreciated from the empirical parameters of solvent polarity Et(30) and normalized EN T value [43] (Table 1). The emission spectra (excitation wavelength was for all compounds, λex ¼330 nm) are red-shifted with solvent polarity (normal solvatochromism). This behavior is related to a better stabilization of the charge-separated excited state in polar solvents

Fig. 7. Cyclic voltammograms of compounds M1–M3 in CH2Cl2. All solutions contained Bu4NBF4 (0.1 M) as the supporting electrolyte, scan rate is 50 mV/s.

M. Grigoras et al. / Journal of Luminescence 153 (2014) 5–11

than in less polar solvents. The fluorescence is based on emission from the twisted intramolecular charge transfer states resulted from UV light excitation [44]. The presence of metal ions modifies the optical properties of M1–M3 compounds and this is based on the fact that the cyano groups are able to form coordination complex with metal ions. Therefore, the absorption and emission spectra are changed and some peaks are shifted or their intensities modified. For example, the influence of Hg2 þ ions on the absorption and emission spectra of compound M3 is presented. By adding Hg2 þ ions in concentration ranging from 1  10–4 M to 8  10–4 M, to the M3 solution (1.87  10–6 M) in DMF, the UV–vis absorption curves were slightly changed (Fig. 5). The intensity of peaks 286 and 338 nm decreases while the intensity of the ICT band is also diminished and blueshifted at 487 nm due to the coordination of metal ions through cyano groups. The same behavior was observed for Sn2 þ metal ions. Therefore these compounds can be used for colorimetric detection of Hg2 þ and Sn2 þ metal ions. 3.2. Electrochemical experiments Cyclic voltammetry is a suitable method that gives information about redox stability of the compounds and position of HOMO and LUMO levels, important parameters for photovoltaic applications. Electrochemical properties of all compounds were studied in a three-electrode electrochemical cell in deaerated dichloromethane solution containing Bu4NBF4 (0.1 M) as electrolyte at room temperature. The potentials were measured against Ag/AgCl as reference electrode. Cyclic voltammograms (CV) of 1, 2 and 3 show two oxidation peaks, the first one being very distinct and situated at 1.150 V, 1.137 V and 1.130 V, respectively, vs. Ag/AgCl, while the second anodic peak is broader (Fig. 6). These peaks are assigned to the oxidation of the triphenylamine groups to the cation-radicals and the subsequent oxidation to dications. As it is expected the first oxidation potential has decreased with increasing of phenylacetylene arms number and the donor strength. Fig. 7 shows typical CV curves of M1–M3, evidencing oxidation and reduction peaks assigned to the aromatic amino donor and TCNE-based acceptor moieties. Therefore, these compounds are ambipolar. The first oxidation peak is clearly evidenced in all CVs and it is assigned for oxidation of amine to amine cation while the second oxidation step represents the subsequent oxidation of amine cation to dication. On the negative potential region these compounds show multiple quasi reversible reduction steps centered on the –CQC(CN)2 groups indicating formation of stable anions (M  ) and dianions (M2 ). The tetracyanobutadiene moiety can accommodate two electrons in the reduction step according to Diederich et al. [31]. As expected, increasing the number of cyano groups in the acceptor moieties leads to enhancement of the acceptor strength and thus the anodic shift of the first reduction potential from 0.818 V (M1) to 0.755 V (M2) and  0.709 V (M3) respectively, is observed.

candidates for bulk-heterojonction and dye-sensitized solar cells. The coordination of metal ions (Hg2 þ ) by cyano groups is accompanied by modification of optical properties and show that these CT complexes can be used as chemosensors.

Acknowledgments The authors thank to the Romanian National Authority for Scientific Research (UEFISCDI) for financial support (Grant PN-IIID-PCE-2011-3-0274, Contract 148/2011).

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

4. Conclusion A series of electron-rich triphenylamine-based derivatives have been synthesized by Sonogashira coupling of iodine derivatives of triphenylamine with phenylacetylene. Photophysical studies indicate that compounds show a bathocromic shift of UV and emission maxima with number of phenylacetylene arms. The postmodification of these compounds with tetracyanoethylene led to intramolecular charge transfer complexes with red-shifted of the absorption and emission spectra. All compounds have board absorption spectra responses over the solar spectrum being good

11

[34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

A. Iwan, D. Sek, Prog. Polym. Sci. 36 (2011) 1277. M. Thelakkat, Macromol. Mater. Eng. 287 (2002) 442. R. Lui, Y. Li, Q. Xiao, J. Chang, H. Zhu, J. Lumin. 132 (2012) 191. M. Thelakkat, J. Hagen, D. Haarer, H.W. Schmidt, Synth. Met. 102 (1999) 1125. Y. Shirota, J. Mater. Chem. 10 (2000) 1. Y. Shirota, J. Mater. Chem. 15 (2005) 75. F. Zhang, Y. Luo, J. Song, X. Guo, W. Liu, C. Ma, Y. Huang, M. Ge, Z. Bo, Q. Meng, Dyes Pigments 81 (2009) 224. Z. Ci, X. Yu, M. Bao, C. Wang, T. Ma, Dyes Pigments 96 (2013) 619. J. Jia, Y. Zhang, P. Xue, P. Zhang, X. Zhao, B. Liu, R. Lu, Dyes Pigments 96 (2013) 407. C. Lambert, W. Gaschler, E. Schmalzlin, K. Meerholz, C. Brauchle, J. Chem. Soc. Perkin Trans. 2 (1999) 577. S.J. Chung, K.S. Kim, T.C. Lin, G.S. He, J. Swiatkiewicz, P.N. Prasad, J. Phys. Chem. B 103 (1999) 10741. J.M. Lupton, I.D.W. Samuel, R. Beavington, P.L. Burn, H. Bassler, Adv. Mater. 13 (2001) 258. K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.W. Ko, Chem. Mater. 14 (2002) 1354. C. Ego, A.C. Grimsdale, F. Uckert, Y.G. Srdanov, K. Mullen, Adv. Mater. 14 (2002) 809. Q. Fang, B. Xu, B. Jiang, H. Fu, W. Zhu, X. Jiang, Z. Zhang, Synth. Met. 155 (2005) 206. T.H. Li, K.L. Tong, S.K. So, L.M. Leung, Synth. Met. 155 (2005) 116. M. Sonntag, K. Kreger, D. Hanft, P. Strohriegl, S. Setayesh, D. Leeuw, Chem. Mater. 17 (2005) 3031. J. Li, C. Ma, J. Tang, C.S. Lee, S. Lee, Chem. Mater. 17 (2005) 615. X. Wang, P. Yang, G. Xu, W. Jiang, T. Yang, Synth. Met. 155 (2005) 464. P. Wei, X. Bi, Z. Wu, Z. Xu, Org. Lett. 7 (2005) 3199. B. O'Regan, M. Gratzel, Nature 353 (1991) 737. F. Zhang, Y.H. Luo, J.S. Song, X.Z. Guo, W.L. Liu, C.P. Ma, Y. Huang, M.F. Ge, Z. Bo, W.B. Meng, Dyes Pigments 81 (2009) 224. Y. Yue, J. Kang, M. Yu, Dyes Pigments 83 (2009) 72. D.W. Chang, H.J. Lee, J.H. Kim, S.Y. Park, S.M. Park, L. Dai, J.B. Baek, Org. Lett. 13 (2011) 3880. Z. He, C.W. Kan, C.L. Ho, W.Y. Wong, C.H. Chui, K.L. Tong, S.K. So, T.H. Lee, L. M. Leung, Z. Lin, Dyes Pigments 88 (2011) 333. P. Leriche, P. Frere, A. Cravino, O. Aleveque, J. Roncali, J. Org. Chem. 72 (2007) 8332. P. Quin, J. Wiberg, E.A. Gibson, M. Linder, L. Li, T. Brinck, A. Hagfeldt, B. Albinsson, L. Sun, J. Phys. Chem. C 114 (2010) 4738. X. Sun, Y. Liu, X. Xu, C. Yang, G. Yu, S. Chen, Z. Zhao, W. Qiu, Y. Li, D. Zhu, J. Phys. Chem. B 19 (2005) 10786. E. Ripaud, T. Rousseau, P. Leriche, J. Roncali, Adv. Energy Mater. 1 (2011) 540. A.J. Fatiadi, Synthesis (1987) 749. M. Kivala, C. Boudon, J.P. Gisselbrecht, P. Seiler, M. Gross, F. Diederich, Angew. Chem. Int. Ed. 46 (2007) 6357. S. Kato, M. Kivala, W.B. Schweizer, C. Boudon, J.P. Gisselbrecht, F. Diederich, Chem. – Eur. J. 15 (2009) 8687. M. Yamada, P. Rivera-Fuentas, W.B. Schweizer, F. Diedrerich, Angew. Chem. Int. Ed. 49 (2010) 3532. Y. Li, K. Tsuboi, T. Michinobu, Macromolecules 43 (2010) 5277. A. Lelliege, P. Blanchard, T. Rousseau, J. Roncali, Org. Lett. 13 (2011) 3098. T. Shoji, J. Higashi, S. Ito, T. Okujima, M. Yasunami, N. Morita, Chem. – Eur. J. 17 (2011) 5116. X. Tang, W. Liu, J. Wu, C.S. Lee, J. You, P. Wang, J. Org. Chem. 75 (2010) 7273. Y. Washino, T. Michinobu, Macromol. Rapid Commun. 32 (2011) 644. B. Esembeson, M.L. Scimeca, T. Michinobu, F. Diederich, I. Biaggio, Adv. Mater. 20 (2008) 4584. Y. Kuwabara, H. Ogawa, H. Inada, Y. Shirota, Adv. Mater. 6 (1994) 677. E. Gagnon, T. Maris, J.D. Wuest, Org. Lett. 12 (2010) 404. M. Kivala, F. Diederich, Acc. Chem. Res. 42 (2009) 235. C. Reichardt, Solvents and Solvent effects in Organic Chemistry, 3rd ed., Wiley-VCH, Weinheim, 2003. Y. Hirata, T. Okada, T. Nomoto, J. Phys. Chem. A 102 (1998) 6585.