2-((4-(2-Ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile

Abstract

New small acceptor–donor (A–D) molecules have been recently investigated as a component of organic solar cells. In this research, 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile was prepared from 4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole in a two-step process via Vilsmeier–Haack formylation and Knoevenagel reaction with malononitrile. The structures of newly synthesized compounds were established by means of elemental analysis, high-resolution mass spectrometry, ¹H, ¹³C NMR, IR and UV–Vis spectroscopy, as well as mass spectrometry.

1. Introduction

The development of the components of organic photovoltaic devices (solar cells (OSCs) and light-emitting diodes (OLEDs)) has led to a significant increase in the structural complexities of organic compounds so that their physicochemical and special properties correspond to the necessary data [1,2]. Over the past decades, the small molecules used in these photovoltaic devices have evolved from the simplest ones with the structure D-A, D-π-A, D-A-D or A-D-A to more complex ones containing several different donor and acceptor moieties in various combinations (for example, D-A1-π-A2, A-π-D-π-A, etc.) [3]. The complication of the structure of organic compounds has led to difficulties in their synthesis, high costs of starting compounds, and, ultimately, the understanding that these compounds cannot be used in final devices [4]. This is all the more surprising because one of the advantages of using organic substances has always been their cheapness [5,6]. Recently, small molecules of the acceptor–donor type (A–D) have become more actively studied to create single-component organic solar cells with a homojunction (HOSC) based on small donor–acceptor molecules [7,8]. As a rule, triphenylamine [7,8] or phenothiazine [9] groups were studied as a donor component, while dicyanovinyl moiety was most often used as an acceptor [7,8,9]. Herein, we report the synthesis of 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile as a new small molecule and study its photophysical properties.

2. Results and Discussion

The target 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile 1 was synthesized in two steps. The treatment of N-(2-ethylhexyl) 1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole 2 with the Vilsmeier–Haack reagent prepared in situ from POCl₃ and DMF at 85 °C for 24 h resulted in the formation of 7-carbaldehyde derivative 3 (Scheme 1) as for other N-substituted 1,2,3,3a,4,8b-hexahydrocyclopenta[b]indoles [10,11]. The resulting aldehyde 3, in turn, was introduced in the Knoevenagel condensation reaction with malononitrile by refluxing the reaction mixture in toluene for 4 h. The total yield of 1 for the two steps was 55%.

The structures of 4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole-7-carbaldehyde 3 and 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile 1 were confirmed by means of elemental analysis, high-resolution mass spectrometry, ¹H, ¹³C NMR, IR and UV–Vis spectroscopy, as well as mass spectrometry. Based on the ¹³C NMR data, compounds 1 and 3 were found to exist as mixtures of two diastereomers due to the stereocenter in the 2-ethylhexyl group (see Materials and Methods and Supplementary Materials).

We measured the optical absorption spectra for compound 1 in DMSO and compared it with the spectra of compounds containing the dicyanomethylene group [12], a similar donor framework and a rhodanine acceptor fragment (Figure 1) [13]. It was found that chromophore 1 intensely absorbs visible light, with a maximum intensity at a wavelength of 430 nm (ε = 97,028 M⁻¹⋅cm⁻¹). For this chromophore, an emission spectrum was also recorded; the emission band turned out to be a mirror absorption band, and its maximum was 510 nm (Stokes shift = 2038 cm^–1).

Compared to the most studied analogue, 2-(4-(dimethylamino)-benzylidene)-malononitrile 4, chromophore 1 has a longer wavelength absorption maximum with a slightly higher extinction coefficient and, at the same time, a shorter wavelength emission band and, consequently, a smaller Stokes shift. This can be explained by the greater rigidity of the amino group and the absence of rotational degrees of freedom of this fragment in compound 1, in comparison with analogue 4. If we compare compound 1 with compounds containing a similar donor framework and a rhodanine acceptor fragment (5,6), compound 1 exhibits a shift, in the absorption band, to the blue region due to the lower electron-withdrawing ability of the dicyanomethylidene fragment, and the Stokes shift is slightly smaller but, at the same time, it is comparable (

Table 1. Photophysical parameters obtained for 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile 1 and similar chromophores 4–6: absorption maximum wavelength λ_abs, maximum molar extinction coefficient ε, emission maximum wavelength λ_em and Stokes shift ∆ν.

Compound	λabs nm	εmax mol × 1−1 × cm−1	λem nm	Stokes Shift ∆ν cm−1
1	462	97,028	510	2038
4	447	81,008	557	4418
5	519	–	592	2376
6	517	–	590	2393

).

3. Materials and Methods

4-(2-Ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole 2 was prepared according to the published method [14]. The solvents and reagents were purchased from commercial sources and used as received. Elemental analysis was performed on a 2400 Elemental Analyzer (Perkin Elmer Inc., Waltham, MA, USA). ¹H and ¹³C NMR spectra were taken with a Bruker AM-300 machine (Bruker AXS Handheld Inc., Kennewick, WA, USA) (at frequencies of 300 and 75 MHz) in CDCl₃ solution, with TMS as the standard. J values are given in Hz. MS spectrum (EI, 70 eV) was obtained with a Finnigan MAT INCOS 50 instrument (Hazlet, NJ, USA). IR spectrum was measured with a Bruker “Alpha-T” instrument in KBr pellet. High-resolution MS spectrum was measured on a Bruker micrOTOF II instrument (Bruker Daltonik Gmbh, Bremen, Germany) using electrospray ionization (ESI). Solution UV–visible absorption spectra were recorded using an Agilent Cary 60 UV–Vis spectrophotometer (Agilent Technologies, Inc. Headquarters, Santa Clara, CA, USA) in standard 10 mm photometric quartz cells in HPLC-grade DMSO at a concentration of 5 × 10⁻⁶ M. Luminescence spectra were recorded using an Agilent Cary Eclipse (Agilent Technologies, Inc. Headquarters, Santa Clara, CA, USA) in HPLC-grade DMSO at a concentration of 10⁻⁶ M.

Synthesis of 4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole-7-carbaldehyde 3 (Supplementary Materials).

POCl₃ (2.53 mL, 27.42 mmol) was introduced dropwise into dry DMF (15 mL) at 5 °C, and the reaction mixture was stirred at room temperature for 30 min. Then, a solution of 4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole 2 (630 mg, 2.32 mmol) in dichloroethane (15 mL) (preliminarily distilled over P₂O₅) was added, and the reaction mixture was kept for 24 h at 85 °C in an argon atmosphere, and then cooled, poured into cold water (50 mL) and extracted with chloroform (3 × 35 mL). The combined organic extracts were washed with brine, dried over MgSO₄ and evaporated. The residue was purified by column chromatography on silica gel (Silica gel Merck 60, eluent hexane/ethyl acetate, 10:1, v/v). Yield 414 mg (60%), pale-yellow oil. R_f = 0.26 (Hexane:ethyl acetate, 10:1, v/v). ¹H NMR (300 MHz, CDCl₃): 9.54 (s, 1H), 7.54–7.41 (m, 2H), 6.21 (d, J = 8.6, 1H), 4.36–4.24 (m, 1H), 3.78–3.63 (m, 1H), 3.24–3.12 (m, 1H), 3.08–2.96 (m, 1H), 2.08–1.57 (m, 9H), 0.97–0.78 (m, 6H). ¹³C NMR (75 MHz, CDCl₃, signals of the second diastereomer are given in brackets): 189.4, 157.6, 134.8, 134.1, 126.0, 124.4, 103.4, 69.3 (68.9), 49.0 (48.8), 44.7, 37.7 (37.6), 35.3, 32.6 (32.4), 31.1 (30.9), 28.9 (28.8), 24.4, 24.3 (24.1), 23.2, 14.1, 10.9 (10.8). HRMS-ESI (m/z): calcd for (C₂₀H₂₉NO) 300.2321, found m/z 300.2322. MS (EI, 70 eV), m/z (I, %): 299 (27), 200 (100), 172 (4), 158 (12), 143 (4), 130 (8), 57 (3), 41 (8), 29 (9), 18 (25). IR, ν, cm⁻¹: 2957, 2929, 2860, 2729, 1672, 1602, 1511, 1455, 1327, 1229, 1163, 1100, 801, 734. UV–Vis spectrum, λ_max: 358 nm (ε = 18,323 M^-1cm⁻¹). Anal. calcd for C₂₀H₂₉NO (300.2321): C, 80.22; H, 9.76; N, 4.68. Found: C, 80.24; H, 9.73; N, 4.72%.

Synthesis of 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile 1 (Supplementary Materials).

A mixture of aldehyde 3 (142 mg, 0.48 mmol), malononitrile (47 mg, 0.712 mmol), NH₄OAc (55 mg, 0.712 mmol) and AcOH (1 mL) in toluene (40 mL) was refluxed with molecular sieves 4 Å (3 g) for 4 h. The reaction mixture was diluted with water (50 mL) and extracted with ethyl acetate (3 × 30 mL). The combined organic extracts were washed with a 1% sodium carbonate solution (3 × 50 mL), dried over MgSO₄ and evaporated under reduced pressure. The residue was purified by column chromatography on silica gel (Silica gel Merck 60, eluent dichloromethane/methanol, 10:1, v/v). Yield 92 mg, (55 %), dark-red oil. R_f = 0.53 (CHCl₃). ¹H NMR (300 MHz, CDCl₃): 7.67 (1H, s), 7.43 (d, J = 8.6, 1H), 7.29 (s, 1H), 6.22 (d, J = 8.6, 1H), 4.42–4.31 (m, 1H), 3.78–3.66 (m, 1H), 3.27–3.16 (m, 1H), 3.12–3.01 (m, 1H), 2.06–1.60 (m, 6H), 1.43–1.18 (m, 9H), 0.94–0.79 (m, 6H). ¹³C NMR (75 MHz, CDCl₃, signals of the second diastereomer are given in brackets): 157.7, 157.39, 157.36, 136.8, 135.1, 126.1, 120.2, 116.8, 115.8, 104.4, 69.4 (69.0), 48.6 (48.4), 44.5, 37.7 (37.5), 35.4, 32.1 (32.3), 31.0 (30.8), 28.9 (28.8), 24.3, 24.13 (24.07), 23.1, 14.1, 10.8 (10.7). HRMS-ESI (m/z): calcd for (C₂₃H₂₉N₃) 348.2434, found m/z 348.2434. MS (EI, 70eV), m/z (I, %): 347 (17), 248 (100), 232 (11), 220 (15), 206 (67), 192 (8), 182 (11), 165 (7), 152 (10), 140 (12), 127 (4), 115 (3), 57 (8), 41 (17), 29 (17). IR, ν, cm⁻¹: 2957, 2925, 2857, 2389, 2214, 1727, 1616, 1499, 1328, 1178, 1104, 805, 604. UV–Vis spectrum, λ_max: 462 nm (ε = 97,028 M⁻¹cm⁻¹). Emission spectrum, λ_max: 510 nm. Anal. calcd for C₂₃H₂₉N₃: C, 79.50; H, 8.41; N, 12.09. Found: C, 79.54; H, 8.37; N, 12.11%.