Introduction

Coronene is a polycyclic aromatic hydrocarbon with electrons that are perfectly delocalized among the six peri-fused benzene rings. Coronene molecules are stacked closely in parallel with one another because of their unique flat electronic structure, which allows for excellent self-assembly and increasing electron mobility.1, 2, 3 Graphene and carbon nanotubes with excellent electron conductivity are considered to be expansions of the coronene structure. Many attempts have been made to synthesize coronene derivatives with different electrical and electronic characteristics.3, 4, 5, 6 The addition of electron-withdrawing imide groups to coronene produced an n-type semiconductor material with liquid crystalline self-assembly characteristics.7 To produce carbon derivatives of coronene, the transition metal-catalyzed coupling of bromo-substituted perylene and other aromatic moieties is popular.8, 9, 10 The fused cyclic structure of coronene is prepared through a pericyclic reaction, and the thermal coupling reaction between the perylene imides and dienophiles followed by the in situ aromatization produces the coronene structure.12, 13 Electron-deficient coronene octacarboxylic tetraimides have been developed for columnar liquid crystals.4, 5

Producing coronene derivatives via thermal synthesis is limited because of the low reactivity of electron-deficient mono-adducts. H Bock and colleagues isolated a bis-adduct using maleimide as a dienophile instead of maleic anhydride.4, 5 Many studies of coronene analogs have focused on their applications in photovoltaic devices, organic field effect transistors and liquid crystals.8, 9, 10 These applications take advantage of the electronic properties of flat molecules that allow for pi-overlap. Z Wang and K Müllen examined the extension of the aromatic core on a perylene bay.8, 9, 10 Significant optical changes were achieved by extending the length. Heterocyclic extensions, such as the N- and S-cycle, were used for their molecular recognition or optical absorption properties.8, 9, 10, 11, 12, 13 H Langhals et al.12, 13 reported a triazolinedione adduct of perylene imides, which absorbs near-infrared light. The introduction of electron-donating nitrogen to a perylene imide often involves a strong bathochromic shift.15 Despite the interest in this triazolinedione adduct, the product was formed in low yield.12, 13 This research presents a practical synthesis of the triazoline adducts of perylene imides using various reaction conditions, which is then applied for the preparation of a polyimide with near-infrared absorption.

Experimental Procedure

Materials

All reagents were purchased from Sigma-Aldrich Chemical (Yongin, Korea), and the reagent-grade solvents were dried when necessary and purified by vacuum distillation. Perylene-3,4,9,10-tetracarboxylic dianhydride and 4,4'-oxydiphthalic anhydride were purified using a conventional sublimation process. The products were purified using silica gel column chromatography (Merck, Seoul, Korea, 250–430 mesh).

Characterizations

For internal reference, the 1H-nuclear magnetic resonance (1H NMR) and 13C NMR spectra were recorded in CDCl3 using Varian Mercury 300 MHz and 75 MHz spectrometers, respectively. The Fourier-transformed infrared data were obtained with a Varian FTS 800 spectrometer (Varian, Palo Alto, CA, USA) using a KBr pellet. High-resolution mass spectrometry (HRMS) was performed using a JEOL JMS-700 mass spectrometer (Jeol, München, Germany) under fast atom bombardment (FAB) conditions. An Elementar Vario EL instrument (Hanau, Germany) was used for the elemental analysis. The ultraviolet–visible (UV–vis) spectra were measured using a Shimadzu UV-1601 UV–vis spectrometer (Shimadzu, Duisburg, Germany). The molecular weights of the polymers were estimated using a Tosoh gel permeation chromatography system (Tosoh, Tokyo, Japan). Cyclic voltammetry was performed using a Bioanalytical Systems CV-50W voltammetric analyzer (Bioanalytical Systems, West Lafayette, IN, USA) at a potential scan rate of 50 mV s−1 in a 0.1-M solution of tetrabutylammonium hexaflurorophosphate (Bu4NPF6) in dichloromethane. The thermal analyses were conducted using a Mettler Toledo TGA/SDTA 851e analyzer (Mettler Toledo, Columbus, OH, USA).

Preparation of PDIs10a

Perylene-3,4,9,10-tetracarboxylic dianhydride (10.0 mmol, 3.92 g), 10-nonadecylamine (25.0 mmol, 7.09 g) and 15 g imidazole were heated under a N2 atmosphere at 180 °C for 3 h. The reaction mixture was cooled to room temperature and dispersed in 200 ml ethanol and 500 ml of 4 M HCl and stirred overnight. The resulting red sticky solid was filtered, dissolved in chloroform, dried over MgSO4 and concentrated. The crude compound was purified using column chromatography with a chloroform eluent to produce a red solid, which was N,N′-di(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic bisimide (7.91 g, 86%). The characterizations of these PDIs are as follows:

PDI(R=C9H19). IR (KBr): 2927, 2850, 1699, 1657, 1597, 1459, 1404, 1336, 1248, 956, 851, 809, 745, 623 cm−1. 1H NMR (CDCl3, 300 MHz): 8.67 (m, 8H), 5.18 (m, 2H), 2.25 (m, 4H), 1.87 (m, 4H), 1.25 (m, 56H) 0.83 (t, 12H). HRMS (FAB+): m/z=923.6663 [MH+] (Calcd.: 923.6666).

PDI(R=C5H11). IR (KBr): 2927, 2851, 1700, 1660, 1597, 1459, 1400, 1240, 957, 851, 809, 745, 625 cm−1. 1H NMR (CDCl3, 300 MHz): 8.67 (m, 8H), 5.18 (m, 2H), 2.22 (m, 4H), 1.85 (m, 4H), 1.25 (m, 32H) 0.81 (t, 12H). HRMS (FAB+): m/z=699.4158 [MH+] (Calcd.: 699.4162).

PDI(R=2,6-diisopropyl). 1H NMR (CDCl3, 300 MHz): 8.80 (dd, 8H), 7.53 (d, 2H), 7.38 (m, 4H), 2.77 (m, 1H), 1.19 (d, 24H). HRMS (FAB+): m/z=710.3218 [MH+] (Calcd.: 711.3223).

PDI(R=CH[CH2O(C2H4O)3CH3]2). 1H NMR (CDCl3, 300 MHz): 8.61 (m, 8H), 5.72 (m, 2H), 4.19 (m, 4H), 3.98 (m, 4H), 3.78-3.40 (m, 60H). HRMS (FAB+): m/z=1123.5210 [MH+] (Calcd.: 1123.5226).

Preparation of 4-alkyl-1,2,4-triazole-3,5-diones

The synthetic procedure for these compounds was obtained from the method found in the literature,16 and their characterizations are as follows:

CPTAD

IR (KBr): 1920, 1895, 1790, 1770, 1706, 1662, 1638, 1530, 1406, 1184, 1156, 1092, 898, 747, 626, 570 cm−1. 1H NMR (CDCl3, 300 MHz): 7.55 (d, 2H), 7.46 (d, 2H). 13C NMR (CDCl3, 75 MHz): δ=157.6, 139.6, 130.4, 128.2, 125.2. LCMS: m/z=209.02 [M+] (Calcd.: 209.00).

BPTAD

IR (KBr): 1898, 1844, 1787, 1772, 1713, 1532, 1183, 1171, 1067, 1051, 897, 829, 738, 567 cm-1. 1H NMR (CDCl3, 300 MHz): 7.69 (d, 2H), 7.40 (d, 2H). 13C NMR (CDCl3, 75 MHz): δ=157.5, 133.4, 132.9, 125.4, 123.7. LCMS: m/z=252.95 [M+] (Calcd.: 252.95).

CHTAD

IR (KBr): 2935, 2854, 1840, 1770, 1530, 1456, 1423, 1390, 1258, 1185, 1138, 895, 740, 559 cm−1. 1H NMR (CDCl3, 300 MHz): δ=3.93 (m, 1H), 2.20–1.55 (m, 6H), 1.55–1.20 (m, 4H). 13C NMR (CDCl3, 75 MHz): δ=159.2, 54.6, 29.4, 25.6, 24.8. LCMS: m/z=181.05 [M+] (Calcd.: 181.09).

OCTAD

IR (KBr): 2940, 2850, 1840, 1775, 1530, 1423, 1390, 1258, 1240, 1186, 1140, 895, 560 cm−1. 1H NMR (CDCl3, 300 MHz): δ=3.64 (m, 2H), 1.75–1.40 (m, 2H), 1.28–1.00 (m, 10H), 0.81 (t, 3H). LCMS: m/z=211.15 [M+] (Calcd.: 211.13).

Preparation of N-fused coronene octacarboxylic tetraimide: general coupling procedure

4-(p-Chlorophenyl)-1,2,4-triazole-3,5-dione (CPTAD; 15.0 mmol, 3.14 g) and p-chloranil (7.50 mmol, 1.84 g) were added to a solution of perylene diimide (PDI; 1.50 mmol, 1.38 g) in dried benzene (15 ml). The mixture was heated at 75 °C under a N2 atmosphere for 7 days. The resulting mixture was concentrated, re-dissolved in n-hexane and filtered through sintered glass. The filtrate was evaporated to dryness and roughly purified using column chromatography, with chloroform and acetone/chloroform (1/10, v/v) as eluents. An isolated dark blue product was precipitated into methanol to give pure FCTI -Cl (1.70 g, 85%). The characterization of this compound is as follows: IR (KBr): 3106, 2955, 2854, 1776, 1731, 1703, 1661, 1603, 1572, 1495, 1369, 1300, 1232, 1174, 829, 806, 774, 731 cm−1. 1H NMR (CDCl3, 300 MHz): δ=8.87 (m, 4H), 7.65 (d, 4H), 7.49 (d, 4H), 4.94 (m, 2H), 2.11 (m, 4H), 1.83 (m, 4H), 1.25 (m, 56H), 0.83 (t, 12H). 13C NMR (CDCl3, 75 MHz): δ=161.8, 157.4, 143.0, 135.5, 133.3, 129.9, 128.3, 127.9, 125.6, 124.0, 123.4, 116.3, 55.9, 32.2, 29.9, 29.8, 29.6, 27.3, 22.9, 14.4. UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=703 (15 200), 773 nm (18 800). HRMS (FAB+): m/z=1337.6331 [MH+] (Calcd.: 1337.6337).

Other bis-adducts were similarly prepared, and their characterizations are as follows:

FCTI-Br

IR (KBr): 3106, 2955, 1776, 1727, 1704, 1603, 1464, 1408, 1170, 734, 698, 664, 626 cm−1. 1H NMR (CDCl3, 300 MHz): 8.85 (m, 4H), 7.60 (m, 8H), 4.93 (m, 2H), 2.11 (m, 4H), 1.83 (m, 4H), 1.25 (m, 56H) 0.83 (t, 12H). 13C NMR (CDCl3, 75 MHz): δ=161.9, 158.8, 142.9, 133.4, 132.9, 128.7, 128.2, 125.7, 124.0, 123.6, 123.4, 116.3, 55.8, 32.2, 29.9, 29.8, 29.6, 27.3, 22.9, 14.4. UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=703 (15 300), 773 nm (18 800). HRMS (FAB+): m/z=1425.5320 [MH+] (Calcd.: 1425.5327).

FCTI-Cy

IR (KBr): 3100, 2955, 2925, 2854, 1777, 1722, 1706, 1606, 1505, 1455, 1407, 1349, 735, 504 cm−1. 1H NMR (CDCl3, 300 MHz): 8.80 (m, 4H), 5.00 (m, 2H), 4.14 (m, 2H) 2.32–2.01 (m, 8H), 2.00–1.69 (m, 14H), 1.25 (m, 62H) 0.86 (t, 12H). 13C NMR (CDCl3, 75 MHz): δ=161.9, 158.7, 144.1, 133.5, 125.9, 124.0, 123.2, 116.4, 55.5, 53.4, 32.2, 29.9, 29.8, 29.6, 29.1, 27.2, 26.0, 25.2, 22.9, 14.4. UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=719 (19 400), 792 nm (17 900). HRMS (FAB+): m/z=1281.8052 [MH+] (Calcd.: 1281.8055).

FCTI-Oc

IR (KBr): 3100, 2955, 2920, 1770, 1730, 1706, 1611, 1500, 1454, 1411, 1350, 1120, 741, 511 cm−1. 1H NMR (CDCl3, 300 MHz): 8.80 (m, 4H), 5.00 (m, 2H), 3.82 (m, 2H) 2.11 (m, 4H), 1.83 (m, 4H), 1.62 (m, 4H), 1.25 (m, 76H) 0.80 (m, 18H). UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=719 (19 300), 792 nm (18 000). HRMS (FAB+): m/z=1341.8981 [MH+] (Calcd.: 1341.8994).

FCTI-11

IR (KBr): 3106, 2958, 2854, 1776, 1731, 1703, 1660, 1572, 1495, 1371, 1300, 1233, 1174, 825, 805, 774, 731 cm−1. 1H NMR (CDCl3, 300 MHz): δ=8.87 (m, 4H), 7.65 (d, 4H), 7.49 (d, 4H), 4.94 (m, 2H), 2.10 (m, 4H), 1.83 (m, 4H), 1.25 (m, 24H), 0.80 (t, 12H). 13C NMR (CDCl3, 75 MHz): δ=161.8, 157.3, 143.0, 135.3, 132.3, 129.8, 128.3, 127.9, 125.5, 124.1, 123.2, 116.2, 55.0, 32.0, 29.6, 27.3, 22.9, 14.0. UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=703 (15 000), 775 nm (18 300). HRMS (FAB+): m/z=1113.3835 [MH+] (Calcd.: 1113.3823).

DPTI-Cl

IR (KBr): 3105, 2966, 2958, 2850, 1777, 1730, 1728, 1703, 1662, 1572, 1495, 1371, 1174, 801, 729 cm−1. 1H NMR (CDCl3, 300 MHz): δ=9.21 (s, 4H), 7.59 (s, 6H), 7.60–7.44 (m, 2H), 7.38–7.19 (m, 4H), 2.62 (m, 2H), 1.16 (d, 24H). UV–vis (CHCl3): λmax (ɛ, l mol−1 cm−1)=700 (13 000), 771 nm (15 300). HRMS (FAB+): m/z=1125.2888 [MH+] (Calcd.: 1125.2894).

Preparation of a diamine monomer

2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (3.0 mmol, 1.1 g), n-octyl alcohol (8.0 mmol, 1.3 ml) and triphenylphosphine (6.1 mmol, 1.6 g) were dissolved in anhydrous tetrahydrofuran (15 ml) under a N2 atmosphere, and diethyl azodicarboxylate (1.6 mmol, 1.0 ml) was added dropwise over 30 min at 10 °C. After stirring for 10 h, the mixture was concentrated and mixed with diethyl ether (40 ml) to remove the insoluble phosphine oxide. The resulting ether solution was concentrated and purified using column chromatography with ethyl acetate and n-hexane (1/4, v/v) as the eluents to produce 2,2-bis(3-amino-4-n-octyloxyphenyl)hexafluoropropane (620 mg, 35%). The characterization of this compound is as follows: 1H NMR (CDCl3, 300 MHz): 6.91–6.62 (m, 6H), 4.05 (t, J=6.3Hz, 4H), 3.8 (s, 4H), 1.88 (m, 4H), 1.80–1.25 (m, 20H), 0.99 (t, J=6.9Hz, 6H). 13C NMR (CDCl3, 75 MHz): δ=147.1, 135.9, 126.1, 124.5 (q), 120.7, 117.2, 110.3, 68.4, 63.9 (m), 32.2, 29.6, 29.5, 29.5, 26.4, 22.9, 14.3. LCMS: m/z=590.30 [M+] (Calcd.: 590.33).

Preparation of a polyimide (red-polyimide)

Perylene-3,4,9,10-tetracarboxylic dianhydride (0.10 mmol, 39 mg) and 2,2-bis(3-amino-4-n-octyloxyphenyl)hexafluoropropane (1.0 mmol, 591 mg) were placed in a round-bottom flask equipped with a refluxing condenser and charged with anhydrous m-cresol (4 ml) under a N2 atmosphere. The mixture was heated for 4 h at 200 °C after adding isoquinoline (110 mg). 4,4'-oxydiphthalic anhydride (0.90 mmol, 279 mg) was added to the reaction mixture and further stirred for 24 h at 200 °C. The cooled mixture was precipitated into methanol, filtered and dried in a vacuum oven (70 °C, 12 h) to produce red-polyimide (0.71 g, 89%). The characterization of this compound is as follows: 1H NMR (CDCl3, 300 MHz): 8.75 (br, perylene Ar), 7.98 (d), 7.60–7.25 (m), 7.05 (d), 3.99 (m), 1.8–1.55 (m), 1.55–1.00 (m), 0.90 (t). See Supplementary Data for the detailed NMR spectrum. Gel permeation chromatography (tetrahydrofuran, polystyrene standard): number average molecular weight (Mn)=8.3 kg mol−1, Mw=14.1 kg mol−1. UV–vis (CHCl3): λmax=458, 490, 526 nm. Anal. Calcd. for (C55H49F6N2O6)0.1 (C47H47F6N2O7)0.9: C, 65.74; H, 5.35; N, 3.21; O, 12.64. Found: C, 65.37; H, 5.71; N, 3.05; O, 12.81.

Preparation of green-polyimide

Red-polyimide (0.50 g), CPTAD (0.24 g) and p-chloranyl (0.14 mg) were placed in a round-bottom flask equipped with a refluxing condenser, and anhydrous benzene (8 ml) was added under a N2 atmosphere. The red solution was heated for 6 days at 75 °C and a dark green color was observed. The cooled mixture was poured into methanol, filtered and dried in a vacuum oven (70 °C, 12H) to produce green-polyimide (0.54 g). The characterization of this compound is as follows: 1H NMR (CDCl3, 300 MHz): 9.25 (br, perylene Ar), 8.00 (d), 7.60–7.20 (m), 7.05 (d), 3.99 (m), 1.8–1.55 (m), 1.55–1.00 (m), 0.90 (t). (The detailed NMR spectrum is provided in the Supplementary Data.) Gel permission chromatography (tetrahydrofuran, polystyrene standard): Mn=8.5 kg mol−1, Mw=14.3 kg mol−1. UV–vis (CHCl3): λmax=650, 709, 782 nm. Anal. Calcd. for (C71H53Cl2F6N8O10)0.1(C47H47F6N2O7)0.9: C, 64.79; H, 5.25; N, 3.98; O, 12.76. Found: C, 64.61; H, 5.47; N, 3.69; O, 12.93.

Solar cell fabrication and measurement

A bulk heterojunction solar cell was fabricated by first dissolving poly(3-hexylthiophene) and a bis-adduct (1/1 or 1/2, wt/wt) in a co-solvent of chloroform and toluene (1/1, v/v). This solution was spin-coated onto indium tin oxide-covered glass substrates that were previously coated with 40 nm of PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)). After drying the coated indium tin oxide, an Al top electrode (150 nm) was deposited in vacuo onto the organic layer using a shadow mask via thermal evaporation (base pressure<1.5 × 10−8 Pa). The pixel size, as defined by the overlap of the indium tin oxide substrate and the Al back electrode, was 4 mm2. Thermal annealing was performed at 80 °C (10 min) under a dry Ar atmosphere. The current–voltage of the solar cell was measured under simulated solar light illumination at an intensity of 100 mW cm−2 (AM 1.5) provided by a solar simulator (Orel 300 W). The electrical data were recorded using a Keithley 236 source-measure unit. The photocurrent action spectra were obtained by illuminating the samples with light from a 300 W Xenon lamp that was dispersed by a single-grating monochromator (DM151i, DongWoo Optron, Gwangju, South Korea). The film thickness was measured using a KLA Tencor Alpha-step IQ surface profilometer (KLA Tencor, Milpitas, CA, USA).

Results and Discussion

Depending on the starting diene and dienophile, thermal cycloaddition is a good method for synthesizing fused cycles. Coronene can be obtained from the ring expansion of the double cycloaddition reaction between perylene and an alkyne.4, 5 This reaction is often limited to a perylene structure. Electron-withdrawing substituents on perylene require harsh reaction conditions for cycloaddition. For example, PDI shows very low conversion with malonic anhydride at 200 °C, and the successive reactions of the resulting mono-adduct does not occur for a bis-adduct.4, 5 A more reactive dienophile is needed for the formation of the bis-adduct. Soluble PDI has been derived from perylene tetracarboxylic dianhydride with a long alkyl chain swallowtail or a ring-distortion substituent, such as 2,6-diisopropyl phenyl.17, 18 10-Aminononadecane reacted with perylene tetracarboxylic dianhydride to produce a PDI with two C19-alkyl groups. No adduct was detected when PDI was treated with propynyl benzoate in 1,2-dichlorobenzene under reflux. 1,2,4-Triazoline-3,5-dione was considered a strong dienophile, which produced a perylene adduct containing N-heterocycle. H Langhals et al.12, 13 reported the cycloaddition of PDI and 4-phenyl-1,2,4-triazole-3,5-dione, albeit in low yield. They improved the yield by using a 10-fold excess of dienophile and chloranyl, and isolated the mono-adduct and bis-adduct in 49% and 20% yield, respectively. The addition of chloranyl led to the oxidative re-aromatization of the adducts and shifted the reversible cycloaddition. The bis-adduct has an interesting structure and produces an N-heterocyclic analog of coronene, which would be generated through a ring-opening reaction,19 as shown in Figure 1. Many attempts have been made to prepare the bis-adduct in a practical synthetic yield. The thermal cycloaddition of PDI and CPTAD was examined at different temperatures and quantities of oxidant. The results are summarized in Table 1. The reaction was performed under refluxing xylene for the favorable thermal aromatization of a nonaromatic intermediate. The reaction did not progress and the PDI was recovered. This was attributed to the thermal instability of CPTAD, which begins to decompose at temperatures lower than 80 °C, according to the thermogravimetric analysis (Supplementary Material). Using an excess of CPTAD compensated for the thermal cycloaddition reaction rate at lower temperatures. The cycloaddition required a long reaction time and was optimized at 75 °C. The reaction occurred without an additional oxidant in entry 1, where the initial adduct could be oxidized by the excess CPTAD. This was feasible because the CPTAD was prepared from the oxidation of a corresponding diazine and was reduced through hydrogenation.16 CPTAD is unstable in the presence of moisture and heat and decomposes during the long cycloaddition period. Owing to these CPTAD-consumption pathways, an excess of CPTAD is essential for a better yield. The reaction yield increased until a fivefold excess of chloranyl was used. Chloranyl represses the loss of CPTAD as a competitive oxidant as well as by shifting the cycloaddition. The bis-adduct FCTI-Cl was isolated in 85% yield when the reaction was performed with 5- and 10-fold excesses of chloranyl and CPTAD, respectively, for 7 days, as shown in entry 7 (Table 1). A mono-adduct12, 13 could be detected in small quantities during the reaction but disappeared by the end. Nitrogen substituents at the bay site of the mono-adduct provided electron density to the electron-deficient PDI and enhanced the reactivity of the mono-adduct, which led to the formation of a bis-adduct. Under the optimized conditions, the PDI coupling reaction occurred with various triazolediones dienophiles to afford each bis-adduct in high yield. Table 2 lists the synthetic results for a range of adducts. To some extent, the reaction yield correlates with the solubility of the PDI derivative. The less soluble PDI with 6-undecyl resulted in a lower yield than the PDI with nonadecyl, as shown in entry 5 in Table 2. EOTI-Cl and DPTI -Cl were isolated in yields of 78% and 74% yield, respectively, along with a small quantity of starting PDI and known materials. This might be due to the lower solubility of the initial nonaromatic intermediate, which may undergo retro-cyclization or decomposition in an unexpected manner. The polymerization of DPTI-Cl was attempted through an aryl–aryl coupling reaction. Most of the PDI-Cl was recovered when the DPTI-Cl was heated with an excess of copper in dimethylformamide under reflux. The palladium-catalyzed polymerization of DPTI-Br or DPTI-Cl failed to generate polymers under Stille’s coupling conditions.20 The focus of the present study then shifted to a polymer reaction in which a perylene-involved polymer reacted with CPTAD. A copolyimide was developed from perylene tetracarboxylic dianhydride and a diamine.21, 22 For the soluble polyimide, the diamine was alkylated with an n-octyl group, and the perylene content was controlled using 4,4'-oxydiphthalic anhydride. The soluble red-polyimide contains 10 mol% perylene and a Mn of 8.3 kg mol−1, as shown in Figure 2. When the red-polyimide was heated with CPTAD and chloranyl in benzene at 75 °C, the solution color became dark green after 6 days and the green-polyimide was isolated. The perylene content of the polyimide was determined by 1H NMR spectroscopy. When the eight perylene protons (Ha) were compared with two specific protons (Hb) from the 4,4'-oxydiphthalic anhydride unit in red-polyimide, the peak integration gave an ∼4:9 ratio, indicating that there was 10 mol% of the perylene unit. The mole percentage was retained in the green-polyimide, with a HA and HB ratio of ∼2:9. The cycloaddition of CPTAD apparently occurred on the perylene unit of red-polyimide. The chemical shifts of the aromatic protons of the polyimides were compared before and after the cycloaddition. A singlet peak for the red-polyimide at 8.7 p.p.m. was assigned to a perylene ring; this peak moved to 9.2 p.p.m. for the green-polyimide after cycloaddition. The relative peak integration of the perylene protons was reduced by 50% in the green-polyimide, as expected. The UV–vis absorption spectra of the polyimides explained the evident color change from the cycloaddition on a perylene unit (Figure 3). The characteristic absorption of the PDI below 550 nm disappeared and new absorption was observed at 600–850 nm after cycloaddition, yielding a bis-adduct FCTI-Cl. Similar absorption bands were observed in the spectrum for the green-polyimide, indicating the cycloaddition onto a perylene unit of the polyimide. In addition, a mono-adduct that appeared during cycloaddition showed a transient absorption range between the PDI and the bis-adduct. Table 3 lists the optical characterization of the prepared N-fused coronene derivatives. The optical band gap, which was calculated from the absorption edge of the film spectrum, was 1.3 eV. A low band gap is required for semiconductors,8, 9, 10 near-infrared applications12, 13 and solar cells.21, 22

Figure 1
figure 1

The cycloaddition of perylene diimide (PDI) and several dienophiles for N-fused coronene analogs.

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Table 1 The optimized conditions for the formation of a bis-adduct of perylene diimide
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Table 2 The synthetic results for the bis-adduct of perylene diimide with various substituents
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Figure 2
figure 2

The cycloaddition of perylene-containing polyimide and the 1H-nuclear magnetic resonance (1H NMR; CDCl3) analysis. A full color version of this figure is available at Polymer Journal online.

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Figure 3
figure 3

The ultraviolet–visible (UV–vis) absorption spectra of perylene diimide (PDI), mono- and bis-adducts, red-polyimide and green-polyimide. A full color version of this figure is available at Polymer Journal online.

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Table 3 The optical properties of N-fused coronenes and polymers
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The cyclic voltammetry analysis of FCTI-Cl determined that the lowest unoccupied molecular orbital and the highest occupied molecular orbital levels were 3.75 and 5.38 eV, respectively. The lowest unoccupied molecular orbital level is similar to that of phenyl-C61-butyric acid methyl ester (PCBM), which is a popular acceptor material in an organic solar cell. Table 4 summarizes the performance of the solar cells that used FCTI-Cl as an acceptor. A bulk heterojunction solar cell was fabricated with a composite of poly(3-hexylthiophene) and FCTI-Cl. The current densities (Jsc) of the devices were very low and therefore showed low conversion efficiencies. The efficiency was slightly enhanced with thermal passivation but was still low when compared with the known devices using poly(3-hexylthiophene)/PCBM.21, 22

Table 4 The results of a solar cell using bis-adducts as an acceptor material
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In conclusion, N-fused coronene octacarboxylic tetraimide derivatives were synthesized by thermal cycloaddition with high yields. The successive cycloaddition of 1,2,4-triazoline-3,5-dione occurred at both bay sites of PDI and primarily yielded a bis-adduct under the optimized reaction conditions. The reaction was performed on the perylene unit of perylene-containing polyimide and produced a polyimide with the characteristic near-infrared absorption.