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Dyes and Pigments 96 (2013) 679e685

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

New 3,6-bis(biphenyl)diketopyrrolopyrrole dyes and pigments viaSuzukieMiyaura coupling

Riccardo Beninatto b, Giuseppe Borsato a,*, Ottorino De Lucchi a, Fabrizio Fabris a, Vittorio Lucchini b,Elisabetta Zendri b

aDipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari, Dorsoduro 2137, I-30123, Venezia, ItalybDipartimento di Scienze Ambientali, Informatica e Statistica, Università Ca’ Foscari, Dorsoduro 2137, I-30123 Venezia. Italy

a r t i c l e i n f o

Article history:Received 7 September 2012Received in revised form5 November 2012Accepted 6 November 2012Available online 20 November 2012

Keywords:DPPDiketopyrrolopyrroleSuzukieMiyaura couplingPigmentDyePhotophysical property

* Corresponding author. Tel.: þ39 041 2348537; faxE-mail address: gborsato@unive.it (G. Borsato).

0143-7208/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.dyepig.2012.11.011

a b s t r a c t

New soluble dyes and insoluble organic pigments based on the diketopyrrolopyrrole skeleton have beenprepared via SuzukieMiyaura coupling on 3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione. Boc protection of the amidic functions was advantageously used to solubilize the starting dike-topyrrolopyrrole prior to the introduction of substituents. Absorption, fluorescence emission and opticalfeatures of these new diketopyrrolopyrroles dyes are presented.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The color red in daily life is present in all objects that need urgentvisibility, such as those representing emergency or danger situa-tions. At the present time, the red pigment used in almost all paintformulations belongs to the class of the diketopyrrolopyrroles (DPP),the most common of which is known as “Ferrari Red” (C.I. PigmentRed 254). The reason for such widespread use and prominence isdue to DPP’s high mechanical and thermal stabilities, bright shadesand high color strength. The DPP has been occasionally synthesizedin the early of ‘70s [1], but only in the ‘80s the industrial synthesiswas developed by Ciba researchers via a pseudo Stobbe condensation[2]. Other different approaches have also been described by Gomp-per [3] and Langhals [4], and more recently non-symmetrical DPPswere synthesized by two-step reactions [5]. Nowadays, the scientificinterest around the DPPs molecules is focused on its optoelectronicproperties [6], in particular for the O- and N-alkylated derivatives ofDPPs [7]. In this paper we present the design and synthesis of newDPPs with different biphenyl moieties, shown in Fig. 1.

One of the main physical properties of DPP pigments is their lowsolubility (<110 mg/L in DMF solution at 25 �C) [2c]. This feature

: þ39 041 2348940.

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gives high physical and chemical stability but makes any chemicaltransformation of these compounds rather difficult. In other words,it is almost impossible to use the original DPP scaffold to preparenew organic pigments, attaching auxochromic groups and func-tionalities. To circumvent this problem, we thought to combine the“latent pigment” [8] approach with the SuzukieMiyaura cross-coupling [9] in order to produce both soluble dyes 1 and insol-uble pigments 2. The choice of this type of coupling is due to themild basic conditions and temperatures employed in this reaction.This approach allows to obtain several new DPPs with electron-donating groups (EDG) in the para position, which are difficult tosynthesize in good yield by other routes [10]. The product 1g withthe electron-withdrawing (EWG) methylsulfonyl group in paraposition was synthesized by direct oxidation of 1e.

A comprehensive study of the effect of peripheral substituentsaround the DPP core on the optical properties was carried out for allsoluble compounds 1. The knowledge of these features is essentialfor potential application of these dyes in fields such as organicphotovoltaic (OPV) [6d,11], organic field effect transistor (OFET)[12] dye-sensitized solar cells devices (DSSC) [13], solid statelaser [10a,14], and polymeric light-emitting diodes (PLED) [15].Moreover, DPPs have been used as chemosensors [16] and as dyesfor Near Infrared Spectroscopy [10b,17]. The insoluble products 2with biphenyl functionalities have the same structure of the

Fig. 1. Dyes 1 and pigments 2.

R. Beninatto et al. / Dyes and Pigments 96 (2013) 679e685680

common Color Index P.R. 264, which is presently used in manydifferent applications, especially for metallic like finishes. Thestability of these pigments makes them suitable for food packagingand toy materials [2c]

2. Experimental section

2.1. Materials and instrumentation

All chemicals were purchased from Aldrich and were usedwithout further purification unless otherwise noted. The solventswere of analytical grade and freshly distilled before use accordingto literature procedures [18]. Tetrahydrofuran (THF) was dried overNa and benzophenone and freshly distilled before use. Toluene wasdegassed with argon in an ultrasound bath for 15 min before use inSuzukieMiyaura cross-coupling reactions.

1H NMR and 13C NMR spectra were recorded on a Varian Unity400 MHz Spectrometer. IR spectra were recorded on a Perkin ElmerSpectrumOne FT-IR Spectrometer. Melting points were determinedon a Stuart Scientific Melting Point Apparatus SMP3 and areuncorrected. UVeVis and fluorescence spectra were recorded indichloromethane or dimethylsulfoxide solutions on a Perkin ElmerLambda 40 UV/VIS Spectrometer with a Perkin Elmer PTP 6 Peltiertemperature programmer set at 25 �C and on a Perkin Elmer LS55Fluorescence Spectrometer, respectively. Fluorescence quantumyields were determined according to the literature method [19], byusing fluorescein solution in 0.1 M aqueous NaOH for compounds1a, 1b, 1d, 1e, 1f, 1g and rhodamine 101 solution in EtOH as stan-dards for product 1c, taking Ffluorescein ¼ 0.79 and Frhodamine ¼ 1.00,respectively. In order to minimize re-absorption effects, concen-trations of the analyzed samples were properly chosen to ensureabsorbance values never exceeding 0.1 at the excitation wave-length. The fluorescence quantum yield calculations were obtainedvia the main equation:

Øf ¼ IfIabs

¼ FXAS

FSAXØS

h2Xh2S

!

2.2. Synthesis of precursors 3 and 4

2.2.1. 3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3)

Compound 3 was prepared according to the literature procedure[8a], with some modifications. Sodium (0.758 g, 32.96 mmol) wasdissolved in tert-amyl alcohol (40 mL) under argon atmosphere at80 �C with a catalytic amount of anhydrous FeCl3. After the additionof 4-bromo-benzonitrile (3.00 g, 16.48 mmol) at room temperature,

the reaction mixture was heated at 95 �C and then a solution ofdi-isopropyl succinate (1.667 g, 8.24 mmol) in t-AmOH (10 mL) wasadded dropwise in 7 h. The resulting red suspension was stirred at95 �C for 17 h, cooled to 60 �C and then a solution of water (100 mL)and acetic acid (7 mL) was slowly added. The mixture was filteredand the cakewas washed with water andmethanol until the motherliquor are colorless. The productwas dried under vacuum at 60 �C for24 h to give 3 (2.138 g, yield: 58%) as dark-red pigment. EA (calcu-lated): C (48.46), H (2.26), N (6.28)/(found): C(48.23), H (2.09), N(6.32). 1H NMR ((CD3)2SO, 400MHz), d (ppm): 8.40 (4H, d, J¼ 8.1 Hz),7.82 (4H, d, J ¼ 8.1 Hz). 13C chemical shifts were not determined dueto a very low solubility of the sample. IR (KBr), cm�1: 3434, 3131,3075, 3025, 2969, 2829, 1636, 1600, 1552, 1490, 1437, 1390, 1317,1275, 1191, 1135, 1071, 1029, 1006, 833, 816, 746, 69, 620, 469.

2.2.2. Di-tert-butyl 3,6-bis(4-bromophenyl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (4)

Compound 4was prepared according to the literature procedure[8a]. A solution of 3 (0.748 g, 1.68 mmol) and a catalytic amount of4-(dimethylamino)pyridine (DMAP) in dry THF (120 mL) was stir-red at room temperature under argon atmosphere. After 15 min,di-tert-butyl dicarbonate (Boc2O) (1.50 mL, 1.54 mmol) was added.The green yellow fluorescent solution was stirred at 35 �C over-night, and then the solvent and excess of Boc2O were removedunder reduced pressure at 45 �C to give 4 (1.083 g, yield: 99%) asyellow solid. EA (calculated): C (52.03), H (4.05), N (4.33)/(found):C(51.89), H (3.87), N (4.28). 1H NMR (CDCl3, 400 MHz), d (ppm):7.64-7.59 (8H, m), 1.44 (18H, s). 13C NMR (CDCl3, 100 MHz), d (ppm):159.2,148.1,145.4,131.9, 130.0,127.1,126.4,112.7, 85.9, 27.7. IR (KBr),cm�1: 3434, 2985, 2929, 1753, 1714, 1625, 1583, 1482, 1474, 1457,1398, 1370, 1325, 1289, 1261, 1211, 1144, 1104, 1057, 1006, 892, 875,847, 822, 800, 760, 735, 713, 685, 646, 615, 489. MW(C28H26Br2N2O6): 646.32 g/mol. MS (ESI)m/z: 669 (MþþNaþ, 100%).

2.3. General procedure for SuzukieMiyaura cross-couplingderivatives (1a-f)

To a suspension of degassed toluene (30.0 mL) and K2CO3(9.2 mL of a 2 M solution) were added, under argon atmosphere, 4(0.200 g, 0.31 mmol), boronic acid 5 (0.93 mmol, 3 equiv.) anda catalytic amount (1% m/m) of Pd(PPh3)4. The resulting mixturewas heated at 90 �C overnight, then cooled down to roomtemperature, and solvents were removed under reduced pressureat 80 �C. The products 1 was purified from the crude reaction byprecipitation with acetonitrile, and the solid was washed withwater. All new compounds 1 were shown to be homogeneous bytheir TLC examination with different polarity elution mixtures (seeSupporting Information e Part II).

R. Beninatto et al. / Dyes and Pigments 96 (2013) 679e685 681

2.3.1. Di-tert-butyl 3,6-bis(4’-methoxybiphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1a)

It was prepared according to general procedure,1awas obtainedas a brilliant red solid (216 mg, yield: 99%). Mp: 190e192 �C (withdecomposition). EA (calculated): C (71.98), H (5.75), N (4.00)/(found): C (71.76), H (5.62), N (4.06). 1H NMR (CDCl3, 400 MHz),d (ppm): 7.83 (4H, d, J ¼ 8.4 Hz), 7.68 (4H, d, J ¼ 8.4 Hz), 7.60 (4H, d,J ¼ 8.8 Hz), 7.00 (4H, d, J ¼ 8.8 Hz), 3.87 (6H, s), 1.46 (18H, s). 13CNMR (CDCl3, 100 MHz), d (ppm): 160.1, 159.8, 148.5, 145.9, 144.1,143.1,136.2,132.5,129.3,128.4,126.5,114.6,112.3, 85.4, 55.5, 27.8. IR(KBr), cm�1: 3400, 3193, 2974, 2834, 1765, 1742, 1706, 1600, 1496,1476, 1448, 1370, 1283, 1250, 1205, 1177, 1144, 1060, 1037, 998, 878,822, 797, 772, 704, 671, 601. MW (C42H40N2O8): 700.78 g/mol. MS(ESI) m/z: 701 (Mþ, 100%).

2.3.2. Di-tert-butyl 3,6-bis(30,40-dimethoxybiphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1b)

It was prepared according to general procedure, 1b was obtainedas a reddish solid (198 mg, yield: 84%). Mp: 184e186 �C (withdecomposition). EA (calculated): C (69.46), H (5.83), N (3.68)/(found):C (69.31), H (5.98), N (3.64). 1H NMR (CDCl3, 400 MHz), d (ppm): 7.84(4H, d, J¼ 8.5 Hz), 7.68 (4H, d, J¼ 8.5 Hz), 7.22 (2H, dd, J¼ 8.4Hz), 7.16(2H, s), 6.98 (2H, d, J ¼ 8.4 Hz), 3.98 (6H, s), 3.95 (6H, s), 1.48 (18H, s).13C NMR (CDCl3, 100 MHz), d (ppm): 159.7, 149.6, 148.6, 145.9, 144.3,140.2, 133.0, 132.0, 129.2, 126.7, 119.9, 112.4, 111.7, 110.5, 85.5, 56.2,27.8. IR (KBr), cm�1: 3422, 3215, 2974, 2929, 2834, 1751, 1700, 1613,1597, 1524, 1502, 1462, 1398, 1367, 1331, 1289, 1275, 1253, 1219, 1174,1144, 1060, 1026, 881, 833, 802, 766, 741, 704, 651, 618. MW(C44H44N2O10): 760.83 g/mol. MS (ESI) m/z: 762 (Mþ þ Hþ, 100%).

2.3.3. Di-tert-butyl 3,6-bis(4’-(dimethylamino)biphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1c)

It was prepared according to general procedure,1cwas obtainedas a dark violet solid (224 mg, yield: 99%). Mp: 188e190 �C (withdecomposition). EA (calculated): C (72.71), H (6.38), N (7.71)/(found): C (72.75), H (6.29), N (7.66). 1H NMR (CDCl3, 400 MHz),d (ppm): 7.81 (4H, d, J ¼ 8.5 Hz), 7.67 (4H, d, J ¼ 8.5 Hz), 7.58 (4H, d,J ¼ 8.8 Hz), 6.80 (4H, d, J ¼ 8.8 Hz), 3.03 (12H, s), 1.46 (18H, s). 13CNMR (CDCl3, 100 MHz), d (ppm): 159.9, 150.7, 148.7, 145.8, 144.4,129.3, 128.0, 127.6, 125.8, 122.6, 112.8, 112.0, 89.5, 85.3, 40.6, 27.7. IR(KBr), cm�1: 3437, 3019, 2952, 2829, 1597, 1586, 1485, 1460, 1437,1412, 1345, 1325, 1295, 1208, 1163, 1088, 1071, 1048, 1018, 984, 970,872, 808, 744, 707, 654, 573, 537. MW (C44H46N4O6): 726.86 g/mol.MS (ESI) m/z: 727 (Mþ, 21%), 364 (Mþ/2 þ Hþ, 79%).

2.3.4. Di-tert-butyl 1,4-dioxo-3,6-bis(4-styrylphenyl)pyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1d)

It was prepared according to general procedure,1dwas obtainedas deep red solid (138 mg, yield: 65%). Mp: 167e169 �C

Scheme 1. Synthesis of c

(with decomposition). EA (calculated): C (76.28), H (5.82), N(4.04)/(found): C (76.04), H (5.69), N (4.01). 1H NMR (CDCl3,400 MHz), d (ppm): 7.79 (4H, d, J ¼ 8.4 Hz), 7.62 (4H, d, J ¼ 8.4 Hz),7.55 (4H, d, J ¼ 7.3 Hz), 7.39 (4H, t, J¼ 7.5 Hz), 7.30 (2H, t, J¼ 7.3 Hz),7.18 (2H, d, J ¼ 16.4 Hz), 7.14 (2H, d, J ¼ 16.4 Hz), 1.46 (18H, s). 13CNMR (CDCl3, 100 MHz), d (ppm): 187.6, 182.5, 159.7, 152.1, 148.6,141.0, 137.2, 129.3, 129.0, 127.5, 127.0, 126.5, 123.4, 112.6, 85.5, 46.6,27.9. IR (KBr), cm�1: 3422, 3025, 2980, 2924, 1734, 1717, 1611, 1600,1544,1507,1490,1448,1393,1370,1337,1278,1255,1202,1141,1046,1006, 973, 895, 869, 830, 819, 774, 752, 696, 665, 640, 623, 587, 520.MW (C44H40N2O6): 692.80 g/mol. MS (ESI) m/z: 693 (Mþ, 100%).

2.3.5. Di-tert-butyl 3,6-bis(4’-(methylthio)biphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1e)

It was prepared according to general procedure,1ewas obtainedas brownish solid (225 mg, yield: 99%). Mp: 190e192 �C (withdecomposition). EA (calculated): C (68.83), H (5.50), N (3.82)/(found): C (68.70), H (5.38), N (3.87). 1H NMR (CDCl3, 400 MHz),d (ppm): 7.84 (4H, d, J ¼ 8.5 Hz), 7.69 (4H, d, J ¼ 8.5 Hz), 7.58 (4H, d,J ¼ 8.5 Hz), 7.34 (4H, d, J ¼ 8.5 Hz), 2.54 (6H, s), 1.46 (18H, s). 13CNMR (CDCl3, 100 MHz), d (ppm): 162.2, 159.7, 157.7, 148.5, 145.9,143.8, 139.2, 136.7, 129.3, 127.6, 126.7, 112.5, 109.6, 85.5, 27.8, 15.8. IR(KBr), cm�1: 3434, 2980, 2918, 1737, 1714, 1619, 1591, 1482, 1454,1429, 1393, 1365, 1337, 1317, 1286, 1272, 1255, 1208, 1141, 1096,1060, 1001, 967, 892, 872, 844, 813, 777, 758, 727, 707, 671, 651, 492.MW (C42H40N2O6S2): 732.91 g/mol. MS (ESI) m/z: 733 (Mþ, 100%).

2.3.6. Di-tert-butyl 3,6-bis(4’-tert-butylbiphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1f)

It was prepared according to general procedure,1fwas obtainedas a deep red solid (147 mg, yield: 63%). Mp: 189e191 �C (withdecomposition). EA (calculated): C (76.57), H (6.96), N (3.72)/(found): C (76.39), H (6.90), N (3.65). 1H NMR (CDCl3, 400 MHz),d (ppm): 7.84 (4H, d, J ¼ 8.6 Hz), 7.72 (4H, d, J ¼ 8.6 Hz), 7.60 (4H, d,J ¼ 8.6 Hz), 7.50 (4H, d, J ¼ 8.6 Hz), 1.45 (18H, s), 1.37 (18H, s). 13CNMR (CDCl3, 100 MHz), d (ppm): 159.8, 151.6, 148.5, 146.0, 144.3,141.8, 137.2, 129.2, 127.0, 126.9, 126.1, 112.4, 85.4, 34.8, 31.5, 27.7. IR(KBr), cm�1: 3434, 2963, 2868, 1748, 1714, 1616, 1605, 1496, 1474,1457,1393,1367,1334,1289,1278,1255,1205,1146,1110,1057,1001,897, 872, 822, 772, 758, 732, 699, 570. MW (C48H52N2O6):752.94 g/mol. MS (ESI) m/z: 754 (Mþ þ Hþ, 100%).

2.4. Oxidation of compound 1e to methylsulfonyl 1g

2.4.1. Di-tert-butyl 3,6-bis(4’-(methylsulfonyl)biphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (1g)

To a suspension of 1e (101 mg, 0.14 mmol) and Na2CO3 (73 mg,0.69 mmol) in CH2Cl2 (25 mL), a solution of 3-chloroperbenzoicacid 70% (mCPBA) (136 mg, 0.55 mmol) in CH2Cl2 (25 mL) was

ompounds 1 and 2.

Table 2Substituent effects on the absorption (lmax) and fluorescence spectra (Fmax) of thesynthesized dyes 1 determined in CH2Cl2.

Compoundno.

lmax (nm)

absorptionε (L∙mol�1∙cm�1) Fmax (nm)

emissionStokesshift (nm)

F

1a 468 17,400 539 71 0.3891b 472 18,500 545 73 0.3331c 518 15,400 642 124 0.0051d 479 17,400 554 75 0.1721e 469 28,700 544 75 0.3881f 463 34,200 531 68 0.4821g 451 23,300 528 77 0.484

R. Beninatto et al. / Dyes and Pigments 96 (2013) 679e685682

added dropwise over 15 min at room temperature. After 20 min,the crude reaction was washed twice with water, the organic layerdried over MgSO4 and the solvent was removed under reducedpressure. The product was purified by washing the crude withacetonitrile and water to give 1g afford as a yellow solid (55 mg,yield: 50%). Mp: 182e184 �C (with decomposition). EA (calculated):C (63.30), H (5.06), N (3.52)/(found): C (63.08), H (4.92), N (3.55). 1HNMR (CDCl3, 400MHz), d (ppm): 8.05 (4H, d, J¼ 8.5 Hz), 7.89 (4H, d,J ¼ 8.5 Hz), 7.83 (4H, d, J ¼ 8.5 Hz), 7.75 (4H, d, J ¼ 8.5 Hz), 3.11 (6H,s), 1.49 (18H, s). 13C NMR (CDCl3, 100 MHz), d (ppm): 159.4, 148.4,145.8, 145.4, 142.2, 140.2, 129.5, 128.5, 128.3, 128.2, 127.5, 113.0,109.6, 85.8, 44.8, 27.8. IR (KBr), cm�1: 3428, 2980, 2924, 1745, 1703,1613,1594, 1516, 1488,1393, 1367, 1339,1306,1261, 1213,1149, 1110,1065,1004, 956, 948, 875, 830, 777, 744, 721, 676, 556, 539, 517, 492,447. MW (C42H40N2O10S2): 796.90 g/mol. MS (ESI) m/z: 797(Mþ, 100%).

2.5. General procedure for deprotection of DPP 1

Reagents 1 (60mg) was dissolved in the smallest volume of pureacetone and immediately removed under reduced pressure, inorder to obtain a thin film on the surface of a one neck round flask.The compound was brought to 180 �C without stirring for 20 min,then cooled to room temperature and connected to a high vacuumpump for 30 min. All products 2 are obtained in quantitative yields.The great insolubility of 2 made the NMR characterization of thesesubstrates not possible. Only the elemental analysis and IR are re-ported. Melting points of 2 are higher than 350 �C.

2.5.1. 3,6-bis(4’-methoxybiphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2a)

It was prepared according to general procedure, 2awas obtainedas dark-red pigment. EA (calculated): C (76.78), H (4.83), N (5.60)/(found): C (76.53), H (4.68), N (5.64). IR (KBr), cm�1: 3422, 3126,1641, 1600, 1499, 1437, 1418, 1250, 1197, 1177, 1144, 1032, 998, 850,825, 797, 746, 710, 632, 595.

Table 1Boronic acids used in SuzukieMiyaura cross-coupling.

Products 1 Boronic acid 5 Yield (%)

1a 99

1b 84

1c 99

1d 65

1e 98

1f 63

1h e

2.5.2. 3,6-bis(30,40-dimethoxybiphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2b)

It was prepared according to general procedure, 2b as fine deep-red pigment. EA (calculated): C (72.84), H (5.03), N (5.00)/(found): C(72.93), H (4.91), N (5.06). IR (KBr), cm�1: 3406, 1639, 1602, 1502,1404, 1253, 1219, 1200, 1141, 1051, 1023, 1006, 878, 830, 813, 763,707, 612.

2.5.3. 3,6-bis(4’-(dimethylamino)biphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2c)

It was prepared according to general procedure, 2cwas obtainedas fine deep-red pigment. EA (calculated): C (77.54), H (5.74), N(10.64)/(found): C (77.31), H (5.61), N (10.48). IR (KBr), cm�1: 3400,1636,1588,1538,1502, 1437, 1401, 1365,1295,1197, 1166,1141, 1060,1043, 1006, 945, 847, 811, 769, 746, 699, 632, 581.

2.5.4. 3,6-bis(4-styrylphenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2d)

It was prepared according to general procedure, 2d was ob-tained as fine deep-red pigment. EA (calculated): C (82.91), H(4.91), N (5.69)/(found): C (82.99), H (4.84), N (5.77). IR (KBr), cm�1:3434, 3131, 3025, 2840, 1636, 1597, 1546, 1513, 1490, 1448, 1406,1331, 1194, 1141, 1068, 1034, 962, 951, 864, 822, 800, 755, 688, 660,626, 573, 525, 436.

2.5.5. 3,6-bis(4’-(methylthio)biphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2e)

It was prepared according to general procedure, 2ewas obtainedas fine deep-red pigment. EA (calculated): C (72.15), H (4.54), N(5.26)/(found): C (71.93), H (4.42), N (5.17). IR (KBr), cm�1: 3428,3120, 2974, 2840,1639,1602, 1516,1482,1437,1406, 1317,1197,1141,1093, 1040, 998, 850, 813, 758, 744, 721, 674, 632, 520, 433.

2.5.6. 3,6-bis(4’-tert-butylbiphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2f)

It was prepared according to general procedure, 2fwas obtainedas a fine deep-red pigment. EA (calculated): C (82.58), H (6.57), N(5.07)/(found): C (82.39), H (6.35), N (5.00). IR (KBr), cm�1: 3434,3137, 2957, 2901, 2862, 1639, 1602, 1499, 1446, 1409, 1384, 1362,

Table 3Substituent effects on the absorption (lmax) and fluorescence spectra (Fmax) of thesynthesized pigments 2 determined in DMSO. *The ε values of 2a and 2e could notbe determined because of their low solubility.

Compoundno.

lmax (nm)

absorptionε (L∙mol�1∙cm�1)* Fmax (nm)

emissionStokesshift (nm)

2a 541 / 554 132b 543 15,300 557 142c 566 31,200 629 632d 564 35,800 574 102e 544 / 558 142f 536 47,600 550 142g 540 46,400 557 17

Fig. 2. (a) UVeVis spectra of all BOC-protected products 1; (b) UVeVis spectra of alldeprotected products 2.

R. Beninatto et al. / Dyes and Pigments 96 (2013) 679e685 683

1323, 1267, 1194, 1141, 1110, 1043, 1001, 853, 819, 755, 682, 626, 567,514, 467.

2.5.7. 3,6-bis(4’-(methylsulfonyl)biphenyl-4-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2g)

It was prepared according to general procedure, 2g was ob-tained as a fine deep-red pigment. EA (calculated): C (64.41), H(4.05), N (4.69)/(found): C (64.53), H (3.96), N (4.71). IR (KBr), cm�1:3434, 3131, 3014, 2924, 2845, 1641, 1605, 1572, 1521, 1488, 1446,1409, 1300, 1200, 1146, 1090, 1040, 1001, 965, 822, 780, 744, 674,629, 562, 545, 514, 483.

3. Results and discussion

3.1. Design and synthesis of products 1 and 2

The synthetic path to access DPP’s 1 and 2 is shown in Scheme 1.Initially the approach involves the derivation of 3with Bocmoieties[8b], in order to solubilize the starting material in common solvents

Fig. 3. Vials with the solution of dyes 1 in dichlorom

used in organic synthesis. This type of reaction affords 4 in quan-titative yield. Furthermore the Boc residue, established as a basiccondition protecting group [20], is suitable for SuzukieMiyauracross-coupling conditions on compound 4. The boronic acids usedto obtain the products 1 are summarized in Table 1. Six soluble dyes1a-f and six insoluble pigments 2a-f bearing electron-donatinggroups (EDG) were obtained via direct cross-coupling from thecorresponding boronic acids, followed by the thermal deprotectionof the BOC groups. 2d was already synthesized trough anotherroute, and its spectral properties are consistent with our data [21].

In order to achieve an exhaustive study on the effects of thesubstituent in para position on the optical properties of the DPPcore, the series 1 and 2were completed with the introduction of anEWG. Unfortunately, the direct introduction with the SuzukieMiyaura coupling of an EWG, such as the nitro group (e.g. boronicacid 5h, with slight changes of the original literature procedures:Pd2(dba)3 with different ligands such as t-Bu3PþBF4, JohnPhos,Cy-JohnPhos, XPhos, PhCy2P and K3PO4 as base) [22], proved notpossible. In this case, the only product detected in the crude reac-tion mixture was the 4,40-dinitrobiphenyl compound, derived bythe homo-coupling of 1h as previously reported in similar cases[23]. However the electron withdrawing sulfonyl group wasintroduced in good yield via oxidation of 1e with meta-chlor-operbenzoic acid under basic conditions.

3.2. Photophysical characterization of compounds 1 and 2

The effect of substituent on photophysical properties for solublecompounds 1 are summarized in Table 2. All measurements wereobtained in solution of 1 in CH2Cl2. The lmax for all products 1 arebathochromically shifted in respect to the maximum of 424 nm[5a] of DPP 2,5-bis-(t-butoxycarbonyl) derivative, and the largesteffect was obtained for the product 1c. The observed Stokes shiftsfor dyes 1 were on average in the range of 70e80 nm, exceptcompound 1c, that shows a Stokes shift equal to 124 nm. This datafor 1c indicated its smaller difference of energy between thelowest vibrational level of the excited state S1 and vibrationallevels of the ground state comparing to other dyes. In Table 2 arealso shown the fluorescence quantum yield (F). In this case, thehigher values of quantum yield were obtained for substrate 1g,with the sulfonyl electron withdrawing group, and for 1f, with thetert-butyl group, that prevent the pep interactions betweendifferent molecules [24].

The photophysical properties of new pigments 2 in DMSOsolution are presented in Table 3. Relative to the Boc protectedmolecules 1, the lmax of all of products 2 are bathochromicallyshifted in respect to the maximum of 496 nm of the P.R. 255. Also inthis case, the largest difference between 3 and series of products 2was observed for the molecule 2cwith the electron donating groupdimethylamino, and the same compound gives a value of Stokesshift equal to 63 nm. For 2a and 2e the molar extinction coefficientswere not calculated, due to the severe insolubility of thesepigments in the solvent. Finally in Fig. 2 are shown the UVeVisSpectra of protected compounds 1 and corresponding pigments 2.

In Fig. 3 are shown the vials of resulting solutions of the dyes 1in dichloromethane, lighted with visible (A) and UV light (B).

ethane, lighted with visible (A) and UV light (B).

R. Beninatto et al. / Dyes and Pigments 96 (2013) 679e685684

4. Conclusions

The synthetic protocol reported herein allows for the homolo-gation/functionalization of the para position of the aromatic ringsin the DPP scaffold via SuzukieMiyaura cross-coupling, as shownwith the preparation of seven different DPP-based soluble dyes 1.Such dyes, after simple thermal removal of t-BOC residues, aretransformed into a new class of insoluble DPP pigments 2 in highyields and purity. All dyes 1 showed a bathochromic shift comparedto the original DPP core, regardless of the nature of substituent. Thehigh Stokes shifts measured make the fluorophore emission bath-ochromically shifted, so an easy discrimination of the fluorescencesignal from Rayleigh scattering is possible and the signal-to-noiseratio is increased [25]. These results are important for the poten-tial application of these compounds as molecular probes in imagingtechniques.

The Stokes shifts of all compounds were higher than the typicalvalue required, indeed, in the case of laser dyes applications(50 nm), and a good mirror-type shape of the absorption andluminescence bands has been also detected (see Supporting Infor-mation e Part II).

The changing of the emission maxima of the reagent 1e(544 nm) compared to the product 1g (528 nm), during theoxidation process, after appropriate modifications to increasesolubility, makes this substrate useful as chemosensor for thedetection of S-oxidizing agents. We are currently investigating sucha possibility.

Acknowledgments

This workwas funded byMIURwithin the PRIN national project.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://dx.doi.org/10.1016/j.dyepig.2012.11.011

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