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. 2021 Oct 19;45(5):1375–1390. doi: 10.3906/kim-2102-22

Polycyclic aromatic hydrocarbon-substituted push–pull chromophores: an investigation of optoelectronic and nonlinear optical properties using experimental and theoretical approaches

Çağatay DENGİZ 1,*
PMCID: PMC8596527  PMID: 34849054

Abstract

A series of new push–pull chromophores were synthesized in moderate to very high yields (65%–97%) by treating TCNE and TCNQ with alkynes substituted by electron-rich diethylaniline and polycyclic aromatic hydrocarbons. Some of the chromophores exhibit strong intramolecular charge-transfer bands in the near-IR region with λmax values between 695 and 749 nm. With the help of experimental and theoretical analysis, it is concluded that the trend in λ max values is affected by PAH substituents sterically, not electronically. Steric constraints led to the increased dihedral angles, reducing conjugation efficiencies. The absorption properties of push-pull compounds have been investigated in solvents possessing different polarities. All chromophores exhibited positive solvatochromism. As an additional proof of efficient charge-transfer in push–pull chromophores, quinoid character (dr) values were predicted using calculated bond lengths. Remarkably, substantial dr values (0.045–0.049) were predicted for donor diethylaniline rings in all compounds. The effects of various polycyclic aromatic hydrocarbons on optical and nonlinear optical properties were also studied by computational methods. Several parameters, such as band gaps, Mulliken electronegativity, chemical hardness and softness, dipole moments, average polarizability, first hyperpolarizability, were predicted for chromophores at the B3LYP/6-31++G(d,p) level of theory. The predicted first hyperpolarizability β(tot) values vary between 198 to 538 × 10–30 esu for the reported push–pull chromophores in this study. The highest predicted β(tot) value in this study is 537.842 × 10–30 esu, 8150 times larger than the predicted β(tot) value of benchmark NLO material urea, suggests possible utilization of these chromophores in NLO devices. The charge-transfer character of the synthesized structures was further confirmed by HOMO-LUMO depictions and electrostatic potential maps.

Keywords: polycyclic aromatic hydrocarbon, chromophore, charge-transfer, push–pull, [2+2] cycloaddition-retroelectrocyclization

1. Introduction

Donor-π-acceptor (D-π-A)-type push–pull chromophores, are well-known for their desirable features, such as tunable strong intramolecular charge-transfer bands that absorb light in a wide range of area including visible and near-IR regions, spectacular nonlinear optical properties, excellent solubility, and high thermal stabilities [1–3]. With these desired properties, push-pull chromophores have already been employed in a series of advanced applications such as photovoltaics, [4,5] light-emitting diodes, [6,7] sensors, [1,8,9] and NLO devices [1,10,11]. The successful integration of push–pull systems in high technology areas makes it necessary to design and synthesize new molecular structures with enhanced optoelectronic properties. However, synthetic strategies to access these entities are arguably limited and require multi-step protocols. The formal [2+2] cycloaddition-retroelectrocyclization (CA-RE) is one of the promising reaction candidates to circumvent these synthetic problems [1]. With its high-yielding nature and broad substrate scope, [2+2] CA-RE transformation occurs under very mild conditions without requiring a catalyst and fulfills all the requirements to be referred as a “click-type reaction” [12,13] [2+2] CA-RE transformation requires electron-rich alkynes and electron-deficient alkenes to synthesize nonplanar D-π-A systems [1]. Bruce and co-workers reported the first example of [2+2] CA-RE reactions between ruthenium-substituted acetylides and tetracyanoethylene (TCNE) in 1981 [14]. Later, metal-free substrates have also been employed in CA-RE reactions for the synthesis of structurally demanding push-pull chromophores by Diederich and co-workers [11]. The short and easy-to-perform CA-RE method has increased the variety in the design of the target push–pull chromophores. When the structures of push-pull chromophores obtained by CA-RE reactions are examined in detail, derivatization is mostly made in two main parts (donor groups and acceptor groups). The typical electron donors used in CA-RE can be listed as azulenes, [15–17] tetrathiafulvalenes, [18] methoxy groups, [19] dialkyl amines [20], ferrocenes, [21] triazenes, [22] thiophenes, [19] ureas, [8] heteroazulenes, [23] ynamines [24], and ynamides [24]. As mentioned earlier, another way to diversify the products obtained by click-type [2+2] CA-RE reactions is to change acceptor units. Similar to the donor groups, successful utilization of many acceptor units such as TCNE [21,25], 7,7,8,8-tetracyanoquinodimethane (TCNQ), [26] 6,6-dicyanopentafulvenes (DCFs),[27] N,N’-dicyanoquinonediimides (DCNQIs), [28] and 2-(dicyanomethylene)indan-1,3-dione (DCID)[29] have been demonstrated in the literature [1]. Apart from studies on donor and acceptor groups, a significant amount of research has also been conducted on π-linkers [1,20,21]. More recently, Diederich and co-workers reported Aviram–Ratner-type dyads with rigid σ-linkers obtained by CA-RE cascade [30]. Due to the nature of the [2+2] CA-RE transformations, substituents have often been used to improve solubility and stability, to increase the electron-acceptor properties, or to incorporate a second donor group into the structure. Trolez and co-workers recently reported ynamide-based push–pull systems with pyrene and perylene substituent units as near-infrared emitters [25]. Another study from the Diederich Group involved the synthesis of corannulene-based push-pull chromophores [31]. Historically, polycyclic aromatic hydrocarbons (PAHs) have been key building blocks for constructing complex π-conjugated systems [32]. Among various PAH molecules, complex push–pull systems having core PAH structures have always been very attractive targets [1,33–36]. Synthetic challenges and characterization issues associated with the solubility of the polycyclic aromatic hydrocarbons are the main barriers complicating the research into these structurally exciting molecules. In this study, we present a detailed systematic survey on structure-property relationships of naphthalene and phenanthrene-substituted push–pull chromophores. The flexibility of the CA-RE reactions allowed us to use both TCNQ and TCNE at the final stage of the synthesis to implement cyano-based acceptor groups. Optoelectronic properties of the synthesized push-pull chromophores have been investigated both experimentally (UV/vis, solvatochromism studies) and theoretically (TD-DFT studies, NLO analysis, HOMO-LUMO visualizations, electrostatic potential maps).

2. Experimental section

2.1. General information

All reagents were purchased as reagent grade and used without further purification. Compounds 2 [37], 3 [37], 6 [38], 7 [38], 10 [39], and 11 [39] were prepared according to literature procedures. Solvents for extraction or column chromatography were distilled before usage. Reactions under exclusion of air or water were performed in oven-dried glassware and under argon or N2 atmosphere. Column chromatography (CC) was carried out using SiO2-60 mesh. Analytical thin-layer chromatography (TLC) was performed on aluminum sheets or glass plates coated with 0.2 mm silica gel 60 F254; visualization with a UV lamp (254 or 366 nm). Evaporation in vacuo was performed at 25–60 °C and 900–10 mbar. Reported yields refer to spectroscopically and chromatographically pure compounds that were dried under a high vacuum (0.1–0.05 mbar) before analytical characterization. 1H and 13 C nuclear magnetic resonance (NMR) spectra were recorded at 400 MHz (1H) and 100 MHz ( 13 C), respectively. Chemical shifts d are reported in ppm downfield from tetramethylsilane using the residual deuterated solvent signal as an internal reference (CDCl3: dH = 7.26 ppm, dC = 77.0 ppm). For 1H NMR, coupling constants J are given in Hz, and the resonance multiplicity is described as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). All spectra were recorded at 298 K. High-resolution mass spectrometry (HR-MS) was performed by the MS-service of the Central Laboratory at Middle East Technical University, Turkey. Masses are reported in m/z units as the molecule ion as [M + H]+.

2.2. General procedure A: synthesis of TMS-protected alkynes 2, 6, and 10

In a 25 mL round bottom flask aryl bromide (1.00 mmol, 1 equiv.), bis (triphenylphosphine) palladium (II) chloride (0.09 mmol, 0.09 equiv.) and copper iodide (0.09 mmol, 0.09 equiv.) were added. The flask was flushed with nitrogen for 30 min, toluene (6 mL) and diisopropylamine (3 mL) were added via syringe into the flask and flushed with nitrogen for an additional 15 min, followed by the addition of trimethylsilylacetylene (3.00 mmol, 3 equiv.). After stirring overnight at 60 °C, the solvents were removed under reduced pressure, target TMS-protected alkynes 2, 6, and 10 were isolated in 78%–92% yields by performing column chromatography (CC) (SiO2; c-hexane).

Compound 2: colorless oil; 175mg, 78%; CC: (SiO2; c-hexane); R f = 0.33 (SiO2; c-hexane); 1H NMR (400 MHz, CDCl3, 298 K): δ = 0.29 (s, 9 H), 7.44–7.54 (m, 3 H), 7.74–7.84 (m, 3 H), 8.00 ppm (s, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 133.02, 132.99, 132.1, 128.7, 128.0, 127.92, 127.87, 126.8, 126.6, 120.5, 105.6, 94.7, 0.17 ppm. Spectral data was consistent with literature [37].

Compound 6: colorless oil; 202 mg, 90%; CC: (SiO2; c-hexane); R f = 0.40 (SiO2; c-hexane); 1H NMR (400 MHz, CDCl3, 298 K): δ = 0.33 (s, 9 H), 7.41 (t, J = 7.7 Hz, 1 H), 7.52 (t, J = 7.5 Hz, 1 H), 7.58 (t, J = 7.6 Hz, 1 H), 7.70 (d, J = 7.1 Hz, 1 H), 7.83 (t, J = 7.5 Hz, 2 H), 8.33 ppm (d, J = 8.2 Hz, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 133.5, 133.2, 131.0, 129.1, 128.4, 127.0, 126.5, 126.3, 125.3, 120.9, 103.2, 99.6, 0.26 ppm. Spectral data was consistent with literature [38].

Compound 10: pale yellow oil; 253 mg, 92%; CC: (SiO2; c-hexane); R f = 0.37 (SiO2; c-hexane); 1H NMR (400 MHz, CDCl3, 298 K): δ = 0.41 (s, 9 H), 7.56–7.63 (m, 1 H), 7.64–7.77 (m, 3 H), 7.85 (d, J = 7.9 Hz, 1 H), 8.06 (s, 1 H), 8.44–8.54 (m, 1 H), 8.60–8.74 ppm (m, 2 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 132.6, 131.24, 131.22, 130.5, 130.1, 128.7, 127.7, 127.21, 127.17, 127.08, 127.05, 122.9, 122.7, 119.6, 103.4, 99.3, 0.28 ppm. Spectral data was consistent with literature [39].

2.3. General procedure B: synthesis of terminal alkynes 3, 7, and 11 via TMS-deprotection

TMS-protected alkynes (1.00 mmol, 1 equiv.) were dissolved in methanol (10 mL) and THF (10 mL) mixture. Then, potassium carbonate (5.00 mmol, 5 equiv.) was added to this solution. After filtration, evaporation, and column chromatography (CC) (SiO2; c-hexanes), terminal alkynes 3, 7, and 11 were obtained in 73%–95% yields.

Compound 3: grey solid; 131 mg, 86%; CC: (SiO2; c-hexane); R f = 0.59 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 3.16 (s, 1 H), 7.45–7.60 (m, 3 H), 7.75–7.90 (m, 3 H), 8.04 ppm (s, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 133.1, 132.9, 132.4, 128.7, 128.2, 127.91, 127.90, 127.0, 126.8, 119.5, 84.1, 77.6 ppm. Spectral data was consistent with literature [37].

Compound 7: pale yellow solid; 111 mg, 73%; CC: (SiO2; c-hexane); R f = 0.55 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 3.49 (s, 1 H), 7.44 (t, J = 7.7 Hz, 1 H), 7.54 (t, J = 6.9 Hz, 1 H), 7.60 (t, J = 6.9 Hz, 1 H), 7.75 (d, J = 7.1 Hz, 1 H), 7.87 (d, J = 8.3 Hz, 2 H), 8.37 ppm (d, J = 8.3 Hz, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 133.6, 133.2, 131.4, 129.4, 128.4, 127.1, 126.6, 126.2, 125.2, 119.9, 82.1, 81.9 ppm. Spectral data was consistent with literature [38].

Compound 11: grey solid; 192 mg, 95%; CC: (SiO2; c-hexane); R f = 0.56 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 3.48 (s, 1 H), 7.58–7.64 (m, 1 H), 7.65–7.74 (m, 3 H), 7.86 (d, J = 7.9 Hz, 1 H), 8.07 (s, 1 H), 8.42–8.52 (m, 1 H), 8.64–8.74 ppm (m, 2 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 132.8, 131.0, 130.9, 130.4, 129.9, 128.5, 127.6, 127.0, 126.9, 126.7, 122.7, 122.5, 118.4, 81.8, 81.5 ppm (15 out of 16 signals expected). Spectral data was consistent with literature [39].

2.4. General procedure C: synthesis of diethylaniline-substituted alkynes 4, 8, and 12

In a 25 mL round bottom flask N,N-diethyl-4-iodoaniline (1.00 mmol, 1 equiv.), bis (triphenylphosphine) palladium (II) chloride (0.09 mmol, 0.09 equiv.) and copper iodide (0.09 mmol, 0.09 equiv.) were added. The flask was flushed with nitrogen for 30 min, toluene (6 mL) and diisopropylamine (3 mL) were added via syringe into the flask and flushed with nitrogen for an additional 15 min, followed by the addition of terminal alkyne (2.00 mmol, 2 equiv.). After stirring overnight at 60 °C, the solvents were removed under reduced pressure, target alkynes 4, 8, and 12 were isolated in 64%–90% yields by performing column chromatography (CC) (SiO2; DCM/c-hexane 1:4).

Compound 4: pale yellow solid; 213 mg, 71%; CC: (SiO2; DCM/c-hexane 1:4); m.p. 133–135 °C; R f = 0.28 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.19 (t, J = 7.1 Hz, 6 H), 3.39 (q, J = 7.1 Hz, 4 H), 6.64 (d, J = 8.8 Hz, 2 H), 7.40–7.52 (m, 4 H), 7.57 (dd, J = 8.4, 1.3 Hz, 1 H), 7.75–7.85 (m, 3 H), 8.01 ppm (s, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 147.7, 133.3, 133.2, 132.5, 130.7, 128.7, 127.92, 127.85, 127.8, 126.5, 126.3, 121.8, 111.3, 109.0, 91.5, 87.7, 44.5, 12.7 ppm; HRMS: m/z calcd for C22H22N+: 300.1752; found: 300.1754 [M + H]+.

Compound 8: pale yellow solid; 270 mg, 90%; CC: (SiO2; DCM/c-hexane 1:4); m.p. 81–83 °C; R f = 0.42 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.20 (t, J = 7.1 Hz, 6 H), 3.40 (q, J = 7.1 Hz, 4 H), 6.67 (d, J = 8.7 Hz, 2 H), 7.45 (t, J = 7.7 Hz, 1 H), 7.48–7.56 (m, 3 H), 7.59 (t, J = 7.6 Hz, 1 H), 7.72 (d, J = 7.1 Hz, 1 H), 7.79 (d, J = 7.1 Hz, 1 H), 7.86 (d, J = 8.1 Hz, 1 H), 8.48 ppm (d, J = 8.3 Hz, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 147.8, 133.39, 133.37, 133.2, 129.7, 128.3, 127.9, 126.62, 126.57, 126.4, 125.5, 122.1, 111.4, 109.1, 96.1, 85.4, 44.5, 12.7 ppm; HRMS: m/z calcd for C22H22N+: 300.1752; found: 300.1744 [M + H]+.

Compound 12: pale yellow solid; 225 mg, 64%; CC: (SiO2; DCM/c-hexane 1:4); m.p. 138–140 °C; R f = 0.25 (SiO2; DCM/c-hexane 1:4); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.21 (t, J = 7.1 Hz, 6 H), 3.41 (q, J = 7.1 Hz, 4 H), 6.68 (d, J = 8.9 Hz, 2 H), 7.54 (d, J = 8.9 Hz, 2 H), 7.56–7.74 (m, 4 H), 7.80–7.90 (m, 1 H), 8.05 (s, 1 H), 8.55–8.75 ppm (m, 3 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 147.8, 133.2, 131.7, 131.5, 130.8, 130.3, 130.1, 128.5, 127.3, 127.1, 127.04, 126.98, 122.8, 122.7, 120.8, 111.4, 109.0, 95.8, 85.6, 44.5, 12.7 ppm (21 out of 22 signals expected); HRMS: m/z calcd. for C26H24N+: 350.1909; found: 350.1909 [M + H]+.

2.5. General procedure D: synthesis of 13, 15, and 17

A solution of the diethylaniline-substituted alkyne (0.15 mmol, 1 equiv.) and TCNQ (0.15 mmol, 1 equiv.) in 1,2-dichloroethane (20 mL) was stirred at 25 °C until complete consumption of starting material. After evaporation of the solvent, target chromophores 13, 15, and 17 were isolated in 65%–83% yields by performing column chromatography (CC) (SiO2; DCM).

Compound 13: A dark-green solid; 327 mg, 65%; CC: (SiO2; DCM); m.p. 165–167 °C; R f = 0.27 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.23 (t, J = 7.1 Hz, 6 H), 3.45 (q, J = 7.1 Hz , 4 H), 6.69 (d, J = 9.1 Hz , 2 H), 7.03 (dd, J = 9.5, 1.9 Hz, 1 H), 7.14 (dd, J = 9.5, 1.9 Hz, 1 H), 7.25–7.35 (m, 3 H), 7.58 (t, J = 7.7 Hz , 2 H), 7.64 (t, J = 7.0 Hz , 1 H), 7.72 (dd, J = 8.7, 1.9 Hz, 1 H), 7.84–7.95 (m, 3 H), 8.16 (s, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 172.9, 154.3, 152.1, 151.3, 135.9, 135.3, 135.1, 134.5, 132.7, 132.3, 131.8, 131.5, 129.8, 129.7, 128.1, 127.8, 125.3, 124.9, 124.7, 123.4, 115.2, 115.0, 113.4, 113.3, 112.6, 112.5, 87.3, 70.9, 45.1, 12.7 ppm; HRMS: m/z calcd for C34H26N5 +: 504.2188; found: 504.2192 [M + H]+.

Compound 15: A dark-green solid; 418 mg, 83%; CC: (SiO2; DCM); m.p. 174–176 °C; R f = 0.23 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.24 (t, J = 7.1 Hz, 6 H), 3.46 (q, J = 7.1 Hz, 4 H), 6.67 (d, J = 9.1 Hz, 2 H), 7.13 (d, J = 10.7 Hz, 1 H), 7.17–7.35 (m, 4 H), 7.40 (d, J = 9.5 Hz, 1 H), 7.54 (t, J = 7.7 Hz, 1 H), 7.59–7.66 (m, 2 H), 7.69 (t, J = 7.7 Hz, 1 H), 7.96 (d, J = 8.0 Hz, 1 H), 8.04 ppm (d, J = 8.3 Hz, 2 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 172.4, 153.9, 153.2, 151.3, 136.8, 135.5, 134.4, 133.92, 133.90, 133.8, 130.4, 129.7, 129.6, 128.6, 127.3, 125.2, 125.1, 124.5, 124.0, 115.1, 113.1, 112.4, 112.2, 91.9, 71.1, 45.0, 12.7 ppm (27 out of 30 signals expected); HRMS: m/z calcd for C34H26N5 +: 504.2188; found: 504.2188 [M + H]+.

Compound 17: A dark-green solid; 449 mg, 81%; CC: (SiO2; DCM); m.p. 191–193 °C; R f = 0.20 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.20 (t, J = 7.1 Hz, 6 H), 3.42 (q, J = 7.1 Hz, 4 H), 6.64 (d, J = 9.1 Hz, 2 H), 7.13 (d, J = 10.4 Hz, 1 H), 7.22–7.36 (m, 4 H), 7.40 (dd, J = 9.5, 1.6 Hz, 1 H), 7.62–7.84 (m, 4 H), 7.87 (s, 1 H), 7.92 (d, J = 7.9 Hz, 1 H), 8.09 (d, J = 8.1 Hz, 1 H), 8.68 (d, J = 8.3 Hz, 1 H), 8.76 ppm (d, J = 7.8 Hz, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 172.3, 153.9, 153.0, 151.2, 136.8, 135.6, 134.4, 134.3, 133.4, 132.9, 132.1, 131.0, 130.3, 130.0, 128.2, 128.0, 127.82, 127.80, 125.5, 125.4, 125.1, 124.7, 124.1, 123.0, 115.0, 114.9, 113.2, 112.5, 112.2, 92.1, 72.0, 45.1, 12.7 ppm (33 out of 34 signals expected); HRMS: m/z calcd for C38H28N5 +: 554.2345; found: 554.2344 [M + H]+.

2.6. General procedure E: synthesis of 14, 16, and 18

A solution of the diethylaniline-substituted alkyne (0.15 mmol, 1 equiv.) and TCNE (0.15 mmol, 1 equiv.) in 1,2-dichloroethane (20 mL) was stirred at 25 oC until complete consumption of starting material. After evaporation of the solvent, target chromophores 14, 16, and 18 were isolated in 95%–97% yields by performing column chromatography (CC) (SiO2; DCM).

Compound 14: A dark-purple solid; 415 mg, 97%; CC: (SiO2; DCM); m.p. 196–198 °C; R f = 0.27 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.26 (t, J = 7.1 Hz, 6 H), 3.49 (q, J = 7.1 Hz, 4 H), 6.71 (d, J = 9.5 Hz, 2 H), 7.60 (t, J = 7.5 Hz, 1 H), 7.68 (t, J = 7.5 Hz, 1 H), 7.80–8.00 (m, 6 H), 8.23 (s, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 169.3, 163.0, 152.8, 135.7, 133.1, 132.7, 132.0, 130.1, 129.93, 129.92, 129.4, 128.9, 128.1, 127.9, 124.3, 117.7, 114.8, 113.9, 112.6, 112.2, 111.7, 86.6, 45.3, 12.6 ppm; HRMS: m/z calcd for C28H22N5 +: 428.1875; found: 428.1875 [M + H]+.

Compound 16: A dark-purple solid; 415 mg, 97%; CC: (SiO2; DCM); m.p. 204–206 °C; R f = 0.30 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.25 (t, J = 7.1 Hz, 6 H), 3.48 (q, J = 7.1 Hz, 4 H), 6.71 (d, J = 9.4 Hz, 2 H), 7.50–7.62 (m, 2 H), 7.65 (t, J = 8.0 Hz, 1 H), 7.70–7.86 (m, 3 H), 7.98 (d, J = 8.0 Hz, 1 H), 8.07 (d, J = 8.7 Hz, 1 H), 8.10 ppm (d, J = 8.0 Hz, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 168.5, 164.1, 152.7, 134.8, 134.0, 133.0, 130.4, 129.9, 129.7, 129.3, 128.6, 127.6, 124.92, 124.85, 118.4, 114.8, 114.0, 112.1, 111.8, 111.5, 91.9, 75.1, 45.1, 12.6 ppm; HRMS: m/z calcd for C28H22N5 +: 428.1875; found: 428.1876 [M + H]+.

Compound 18: A dark-purple solid; 454 mg, 95%; CC: (SiO2; DCM); m.p.157–159 °C; R f = 0.33 (SiO2; CHCl3); 1H NMR (400 MHz, CDCl3, 298 K): δ = 1.24 (t, J = 7.1 Hz, 6 H), 3.47 (q, J = 7.1 Hz, 4 H), 6.71 (d, J = 9.3 Hz, 2 H), 7.67 (t, J = 7.5 Hz, 1 H), 7.75–7.85 (m, 5 H), 7.90, (s, 1 H), 7.94 (d, J = 7.9 Hz, 1 H), 8.12–8.18 (m, 1 H), 8.71 (d, J = 8.4 Hz, 1 H), 8.75–8.85 (m, 1 H); 13 C NMR (100 MHz, CDCl3, 298 K): δ = 168.5, 164.3, 152.7, 133.3, 132.9, 132.6, 131.1, 130.7, 130.5, 130.1, 129.7, 128.5, 127.88, 127.87, 127.1, 126.0, 124.0, 123.0, 119.0, 114.9, 114.0, 112.1, 111.9, 111.6, 92.2, 75.9, 45.2, 12.7 ppm; HRMS: m/z calcd for C32H24N5 +: 478.2032; found: 478.2032 [M + H]+.

3. Results and discussion

3.1. Synthesis of donor-substituted substrates

It was the initial goal of the project to prepare PAH-substituted alkynes by considering several key points such as solubility of the substrates, sufficient donor-activation for the subsequent CA-RE transformations, and commercial availability of the reagents. The reaction sequence to access substrates 4, 8, and 12, required for the synthesis of target push–pull structures initiated with the conversion of the aryl bromides 1, 5, and 9 to TMS-protected alkynes 2, 6, and 10 via Sonogashira cross-coupling reactions (Scheme 1). All three compounds 2, 6, and 10 were successfully synthesized according to the literature procedure reported by Ouyang and co-workers [39]. Upon subsequent deprotection using K2CO3 in MeOH/THF mixture, aryl alkynes 3, 7, and 11 were obtained in 86, 73, and 93 yields, respectively [39]. The final Sonogashira cross-coupling of PAH-substituted alkynes 3, 7, and 11 and N,N-diethyl-4-iodoaniline provided target donor-substituted substrates 4, 8, and 12 in 71, 90, and 64 yields, respectively. The reported synthesis could also be carried out between aryl halides 1, 5, 9, and N,N-diethyl-4-ethynylaniline. However, the stability of all three PAH-substituted alkynes under ambient conditions, in contrast to N,N-diethyl-4-ethynylaniline, guided us to follow the proposed synthetic strategy.

Scheme 1.

Scheme 1

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PAH (naphthene and phenanthrene) and donor group (diethylaniline)-substituted acetylenes 4, 8, and 12 were treated with strong acceptors TCNE/TCNQ for the final [2+2] CA-RE step under ambient conditions, which successfully afforded target push-pull chromophores in high yields. Even with the bulky TCNQ, reactions proceeded smoothly at room temperature (Scheme 2). No significant difference was observed regarding the effect of substituent positions on isolated yields. The yields were slightly lower in CA-RE transformations of TCNQ compared to that of TCNE, mainly due to solubility and purification difficulties that were encountered during column chromatography.

Scheme 2.

Scheme 2

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Although there are two possible regioisomers 13 and 19 that can be formed theoretically during the reaction of TCNQ and alkyne 4, the reaction was fully regioselective and afforded only 13 (Scheme 3). The reason behind this well-studied selectivity can easily be deducted from resonance structures of 13 and 19. Intramolecular charge transfer breaks the aromaticity of the diethylaniline ring (I) while forming a new one (II), as in the case of 13 [1,26].

Scheme 3.

Scheme 3

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3.2. UV/vis absorption spectra

The formal [2+2] CA-RE of 4, 8, 12 with TCNE/TCNQ provided dark green and dark orange chromophores, respectively. The UV/vis absorption data was in full agreement with this observation (Figure 1a and b). While dark orange-colored compounds 14, 16, 18 possess intramolecular charge transfer (ICT) bands with λmax values between 408 and 475 nm, ICT bands of dark-green chromophores 13, 15, 17 are significantly shifted to longer wavelengths and are located between 695 and 749 nm. This substantial difference in λmax values can be explained by the extended π-conjugation pathways in compounds 13, 15, 17, which decreases HOMO-LUMO band gaps [40]. The introduction of different PAH (phenanthrene, 1-naphthyl, and 2-naphthyl) substituents in 14, 16, 18 leads to important differences in λmax values. 1-naphthyl and phenanthrene-substituted chromophores 16 and 18 display a similar ICT bands at 408 and 411 nm, respectively (ε = 18600 M–1 cm–1 for 16; ε = 22,800 M–1 cm–1 for 18). On the other hand, 2-naphthyl-substituted product 14 showed a bathochromically shifted band (λmax = 474 nm, ε = 22,300 M–1 cm–1). The opposite trend was observed for the chromophores 13, 15, 17. Compounds 15 and 17 have ICT bands at 739 and 749 nm, respectively (ε = 28,700 M–1 cm–1 for 15; ε = 21,500 M–1 cm–1 for 17). 2-naphthyl-substituted product 13 exhibited hypsochromically shifted ICT band (λmax = 695 nm, ε = 26200 M–1 cm–1). As it will be discussed in more detail in the theoretical section, the trend in λmax values is affected by PAH substituents sterically, not electronically. The steric restrictions from the substitution pattern of the PAHs 15 and 17 lead to a substantial deconjugation, as indicated by the large dihedral angle between the acceptor and the donor groups (15: 33 ° and 17: 34 °). The dihedral angle in chromophore 13 was relatively smaller (28 °) compared to that of 15 and 17. The large dihedral angle between the diethyl aniline donor and the cyano-based acceptor moiety prevents efficient linear π-conjugation in 15 and 17, which leads to a bathochromic shift [41,42]. A similar but completely opposite trend was observed for the compounds 14 (18 o), 16 (31 o), and 18 (33.2 o). Despite the hypsochromic shift seen in λmax values, chromophores 16 and 18 also possess CT bands at around 474 nm with relatively smaller extinction coefficients compared to that of 14. The decrease in these extinction coefficients can simply be explained by the observed deviation from planarity due to increase in dihedral angles of compounds 16 and 18.

Figure 1.

Figure 1

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UV/vis spectra (CH2Cl2, 25 oC) of chromophores a) 14, 16, 18 and b) 13, 15, 17.

All push-pull systems 13–18 feature strong CT bands that are assigned to intramolecular charge transfer from the donor diethylaniline group to acceptor polycyano units accessed by the CA–RE of TCNE and TCNQ. To further support this claim, protonation-reneutralization experiments were conducted by using CF3COOH (TFA) and NEt3 (Figure 2a and b). Treatment of selected chromophores with TFA resulted in the disappearance of the CT bands via protonation of the diethylaniline unit. Upon treatment with NEt3, re-neutralization occurred, and CT bands were successfully recovered. With these experiments, the CT nature of the low energy bands has been confirmed [40,43,44].

Figure 2.

Figure 2

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UV/vis spectrum of selected chromophores a) 14 and b) 13. The charge transfer band disappeared when acidifying the solution with trifluoroacetic acid and reappeared when neutralizing the solution with triethylamine.

Figure 3.

Figure 3

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UV/vis spectra of chromophore 14 in CH2Cl2/hexane mixtures at 25 oC

Figure 4.

Figure 4

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UV/vis spectra of chromophore 13 in CH2Cl2/hexane mixtures at 25 oC.

All push-pull systems 13–18 feature strong CT bands that are assigned to intramolecular charge transfer from the donor diethylaniline group to acceptor polycyano units accessed by the CA–RE of TCNE and TCNQ. To further support this claim, protonation-reneutralization experiments were conducted by using CF3COOH (TFA) and NEt3 (Figure 2a and b). Treatment of selected chromophores with TFA resulted in the disappearance of the CT bands via protonation of the diethylaniline unit. Upon treatment with NEt3, re-neutralization occurred, and CT bands were successfully recovered. With these experiments, the CT nature of the low energy bands has been confirmed [40,43,44].

3.3. Theoretical studies

All calculations (DFT and TD-DFT) were performed using Gaussian 09 program package [45]. The effect of different functionals and basis sets on chromophore 14 has been evaluated. Since optimization results were obtained for B3LYP/6-31G(d), B3LYP/6-31++G(d,p), and CAM-B3LYP/6-31G(d), CAM-B3LYP/6-31++G(d,p) were very similar; further optimizations were obtained at the functional using basis set B3LYP/6-31G(d). As a solvation model, a conductor-like polarizable continuum model (CPCM) in CH2Cl2 was utilized. Initially, the highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) and electrostatic potential (ESP) map analyses were used to explain ICT behavior in push-pull systems 13–18. As can be seen in Table 1, HOMO is mainly located on the diethylaniline region of the molecules in all chromophores 13–18. On the other hand, LUMO covers the area where cyano groups are present. Together with the partial overlap of HOMO and LUMO frontier orbitals, CT from diethylaniline groups to cyano groups can be claimed for the designed push-pull systems. In addition to FMO analysis, electrostatic potential maps were also analyzed to further evaluate CT interactions in push–pull chromophores. ESP representations describe the total charge density and the molecular polarity in push–pull systems [46]. Red and blue color codes have been used to identify the most negative and positive areas, respectively. As expected, red regions were located around cyano groups; on the other hand, blue regions were identified around the donor diethylaniline donor group.

Table 1.

Molecular structures, HOMO and LUMO depictions, and ESP maps of 13–18. While the most negative areas are represented by red, the most positive areas are represented by blue color. ESP is mapped over the range –0.03 a.u (red). to 0.03 a.u (blue). B3LYP/6-31G(d) level of theory was used for DFT calculations.

13 14 15 16 17 18
Molecular Structure graphic file with name turkjchem-45-1375-fig_t1_11.jpg graphic file with name turkjchem-45-1375-fig_t1_12.jpg graphic file with name turkjchem-45-1375-fig_t1_13.jpg graphic file with name turkjchem-45-1375-fig_t1_14.jpg graphic file with name turkjchem-45-1375-fig_t1_15.jpg graphic file with name turkjchem-45-1375-fig_t1_16.jpg
HOMO graphic file with name turkjchem-45-1375-fig_t1_21.jpg graphic file with name turkjchem-45-1375-fig_t1_22.jpg graphic file with name turkjchem-45-1375-fig_t1_23.jpg graphic file with name turkjchem-45-1375-fig_t1_24.jpg graphic file with name turkjchem-45-1375-fig_t1_25.jpg graphic file with name turkjchem-45-1375-fig_t1_26.jpg
LUMO graphic file with name turkjchem-45-1375-fig_t1_31.jpg graphic file with name turkjchem-45-1375-fig_t1_32.jpg graphic file with name turkjchem-45-1375-fig_t1_33.jpg graphic file with name turkjchem-45-1375-fig_t1_34.jpg graphic file with name turkjchem-45-1375-fig_t1_35.jpg graphic file with name turkjchem-45-1375-fig_t1_36.jpg
ESP graphic file with name turkjchem-45-1375-fig_t1_41.jpg graphic file with name turkjchem-45-1375-fig_t1_42.jpg graphic file with name turkjchem-45-1375-fig_t1_43.jpg graphic file with name turkjchem-45-1375-fig_t1_44.jpg graphic file with name turkjchem-45-1375-fig_t1_45.jpg graphic file with name turkjchem-45-1375-fig_t1_46.jpg
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Table 2.

Calculated quinoidal character values (δr) and total average atomic charges (δ) by ESP fitting on donor, acceptor and PAH parts of push–pull chromophores 13–18.

Compound (δr) (Å) δ donor (eV) δ acceptor (eV) δ PAH (eV)
13 Ring I = 0.049, Ring II = 0.072 0.487 –0.614 0.124
14 Ring III = 0.048 0.388 –0.573 0.188
15 Ring I = 0.048, Ring II = 0.071 0.463 –0.476 0.012
16 Ring III =0.045 0.362 –0.456 0.094
17 Ring I = 0.047, Ring II = 0.071 0.494 –0.434 –0.071
18 Ring III = 0.045 0.360 –0.452 0.094
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The energy level diagrams of push–pull dyes are depicted in Figure 5. The estimated frontier orbital energy levels showed that there is a substantial increase in the HOMO–LUMO band-gap of TCNE adducts 14, 16, and 18 compared to TCNQ adducts 13, 15, and 17 due to the shortened π-conjugation pathway in the former case. The theoretical band gaps for push–pull dyes are 1.87, 2.57, 1.72, 2.30, 1.73, 2.28 eV, respectively. The theoretically obtained band gap values are slightly higher compared to the optical band gaps 1.44, 2.28, 1.31, 1.82, 1.31 1.77 eV [47]. No significant improvement in band gaps were obtained from the TD-DFT band gaps 2.01, 2.77, 1.88, 2.52, 1.89, 2.51 eV for compounds 13, 14, 15, 16, 17, and 18 respectively. However, the trend in theoretical band-gap values can also be seen in experimental band-gap values for both TCNE and TCNQ adducts. E red,1 values were reported for the benchmark cyano acceptors, such as TCNE (–0.32 V), and TCNQ (–0.25 V). A key property of this class of compounds is their electron-accepting power increasing from TCNE to TCNQ. It is also well-known that CA-RE products from TCNQ possess smaller band-gap compared to the products from TCNE. This can also be explained by the increased conjugation length in between donor and acceptor cyano groups in TCNQ products [1].

Figure 5.

Figure 5

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Energy level diagram of the frontier orbitals of push–pull dyes estimated by DFT calculations.

Figure 6 shows both theoretical and experimental UV/vis spectra for the selected compounds 13 and 14. Time-dependent density theory calculations (CAM-B3LYP/6-31G(d)) were applied on optimized push-pull structures of 13–18 with CPCM solvation in CH2Cl2. In order to match the theoretical UV/vis spectra with the corresponding theoretical spectra, wavelengths were red-shifted by 0.2 eV and 0.6 eV for compounds 13 and 14, respectively. Scaling of extinction coefficients was also required (scaled by 2.29 for 13 and by 1.87 for 14) since these values are slightly overestimated by TD-DFT calculations. However, the overall shapes of the theoretical spectra are relatively well-estimated (see Figures 6a and b). Although the reason for the overestimation in molar absorption coefficients in TD-DFT calculations is still unclear, the choice of basis set and solvation method may be responsible from a slight inaccuracy in dihedral angles that may facilitate donor-acceptor conjugation efficiency and increase the molar absorption coefficients. All push–pull chromophores 13–18 possess charge-transfer bands originated from HOMO to LUMO transitions. The observed error in the calculated transitions are in the expected range for similar push–pull systems reported in the literature [48].

Figure 6.

Figure 6

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a) Calculated (red-shifted by 0.6 eV, scaled by 1.87, red line) TD-DFT:CAM-B3LYP/6–31G(d) level of theory in CH2Cl2 and experimental UV/vis spectrum of 14 in CH2Cl2 (blue line). b) Calculated (red-shifted by 0.2 eV, scaled by 2.29, red line) TD-DFT:CAM-B3LYP/6–31G(d) level of theory in CH2Cl2 and experimental UV/vis spectrum of 13 in CH2Cl2 (blue line).

Quinoid character values (dr) are commonly used to predict the amount of charge transferred in push-pull systems [11,19]. If there is no charge-transfer, dr value equals 0 and represent a perfect benzene structure. On the other hand, fully quinoidal structures possess dr values in between 0.08–0.1. Bond lengths from optimized geometries of compounds 13–18 have been used for the dr value calculations (Table 2). A substantial quinoid character values (dr = 0.045–0.049) were predicted for diethylaniline donor groups in all chromophores. A slight increase in dr values of TCNQ adducts was observed compared to TCNE adducts. The predicted quinoid characters were comparable to earlier reports on CA-RE products [11,19]. Inspired by the literature on strong D-A systems [49,50] these results were further confirmed by total average atomic charges (δ) by ESP fitting on donor, acceptor and PAH parts of push–pull chromophores. Atomic charges were calculated based on the ESP fitting scheme of Merz–Singh–Kollman (MK) [51]. All chromophores were divided into three parts: diethylaniline donor, PAH substituents, and cyano-based acceptor groups (Table 2). Accordingly, all chromophores possess charge-transfer from donor to acceptor units. The total average charge differences between donor and acceptor units are significantly higher in quinone based structures 13, 15, and 17. The most enhanced charge-transfer capacity were predicted for compound 13 with δ donor = 0.487 eV and δ acceptor = –0.614 eV. This result is fully consistent with the highest quinoidal character value calculated for chromophore 13 (dr = 0.049). In summary, chromophores obtained by [2+2] CA–RE of TCNQ are better chromophores in terms of charge-transfer interactions compared to chromophores obtained by [2+2] CA–RE of TCNE. Additionally, chromophores-substituted with 2-naphthyl group enhance donor-acceptor properties compared to chromophores substituted with phenanthrene and 1-naphthyl. Presumably, deviation from planarity is the main reason behind this observation due to bulky phenantrene and geometrically constrained 1-naphthyl substitutents.

Table 3.

The electric dipole moment μ (D), EHOMO, ELUMO, ∆E (EHOMO–ELUMO), electronegativity (χ), global chemical hardness (η), global softness (σ), average polarizability [α(tot)], first hyperpolarizability [β(tot)] at the CAM-B3LYP/6-31++G(d,p) level of theory in CH2Cl2.

13 14 15 16 17 18
μ (D) 27.9240 19.8620 27.8594 19.9083 26.5140 20.3709
EHOMO (eV) –5.38 –5.72 –5.41 –5.70 –5.41 –5.69
ELUMO (eV) –3.51 –3.15 –3.69 –3.40 –3.68 –3.41
∆E (eV) 1.87 2.57 1.72 2.30 1.73 2.28
χ (eV) 4.445 4.435 4.550 4.550 4.545 4.550
η (eV) 0.935 1.285 0.860 1.150 0.865 1.140
σ (eV–) 1.0695 0.7782 1.1627 0.8695 1.1560 0.8771
α(tot) (×10–24 esu) 135.891 85.746 136.768 83.060 148.505 94.295
β(tot) (×10–30 esu) 451.403 198.435 536.872 228.638 537.842 239.935
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The successful utilization of organic molecules in nonlinear optics motivates researchers to develop rational design strategies [52,53]. The fast and inexpensive theoretical calculations compared to experimental measurements provide a great advantage for the design of NLOphores with tailor-made properties. The most common NLOphore structures are generally containing D-π-A-type molecular frameworks [54,55]. Strong charge-transfer interactions in push-pull systems 13–18 motivated us to evaluate NLO properties using computational tools. To predict the NLO properties, DFT calculations have been applied on compounds 13–18 using basis set CAM-B3LYP/6-31++G(d,p) level of theory. The electronegativity (χ), global chemical hardness (η), global softness (σ), electric dipole moment μ (D), average polarizability [α(tot)], and first hyperpolarizability [β(tot)] values were calculated according to the equations (1–6) shown below.

β = [(βxxx + βxyy + βxzz)2 + (βyyy + βxxy + βyzz)2 + (βzzz + βxxz + βyyz)2]1/2 (1)

α = 1/3 (αxx+ αyy + αzz) (2)

μ = [(μx)2 + (μy)2 +( μz)2]1/2 (3)

χ = –1/2 (EHOMO + ELUMO) (4)

η = –1/2 (EHOMO–ELUMO) (5)

σ = 1/ η (6)

Overall results are summarized in Table 3. Molecular geometry is crucial as it dictates the NLO properties of the compounds. There are several strategies to modulate NLO responses, such as changing solvent choice, altering donor-acceptor strength, or π-linker length. In this study, we mainly focused on the effect of PAH rings on average polarizability [α(tot)] and first hyperpolarizability [β(tot)] values. When we compare TCNE and TCNQ adducts, the lowest [α(tot)] and [β(tot)] values were predicted for chromophores 13 and 14, which possess naphthalene groups substituted at two positions. For the rest of the compounds, differences in [α(tot)] and [β(tot)] values are not very significant. Compounds with the bulky phenanthrene or naphthene group substituted at one position exhibited almost similar NLO responses. This prediction can be supported by the calculated and optically measured band gaps for 13 and 14. The HOMO-LUMO energy gap for 13 is significantly larger compared to 15 and 17. A similar trend can also be seen in TCNQ adducts. We found a general trend by which β(tot) increases with the size of the spacer between donor and acceptor groups as can be seen by higher β(tot) values in TCNQ products compared to those of TCNE products. We presume that smaller band-gap results in more efficient charge transfer and, as a result, larger NLO responses, as can be seen in Table 2. Optimized structures also displayed significant deviation from planarity in the case of compounds 15 (33 o), 17 (34 o), 16 (31 o), 18 (33.2 o) compared to 13 (28 o), and 14 (18 o). Accordingly, we have shown that substituent groups can play an important role in modulating NLO properties of push-pull type chromophores, although they are mainly utilized as solubilizing groups or side groups to improve the physical properties of chromophores. The highest predicted β(tot) value in this study is 537.842 × 10–30 esu for chromophore 17. That value is 8150 times larger than the benchmark NLO material urea, β(tot) value of 0.066 × 10–30 esu, calculated at the CAM-B3LYP/6-31++G(d,p) [22]. In the final part, the chemical properties of chromophores will be discussed using equations (4–6). The results are very promising when compared with the literature. For example, push-pull system, p-nitroaniline, possesses β(tot) value of 9.2 × 10–30 esu [56]. Similarly, [60] fullerene-fused dihydrocarboline derivative is calculated to have β (tot) value of 54 x 10–30 esu [57]. In another study, β(tot) values of 21 – 286 × 10–30 esu are reported for push-pull 1,3-thiazolium-5-thiolates [58].

Koopmans’ theorem states that HOMO and LUMO energies are related to ionization potential and electron affinity, respectively. Accordingly, the Mulliken electronegativity (χ) can be estimated by equation 4. Besides electronegativity, chemical hardness (η) is another term to be used for chemical behavior predictions of materials. Global hardness is directly related to the HOMO-LUMO gap and can be defined as the resistance of an atom to charge-transfer. Equations 4 and 5 show that larger HOMO-LUMO gaps are required to improve hardness values. Compounds 14, 16, 18 are predicted to have higher global hardness values with their larger band gaps compared to 13, 15, and 17. An opposite trend can be seen in global softness (σ), values as expected from Equation 6. In summary, compounds 13, 15, and 17 are expected to be more reactive compared to 14, 16, and 18.

4. Conclusions

In this study, 6 new polycyclic aromatic hydrocarbon-substituted push–pull chromophores have been synthesized using click-type [2+2] cycloaddition-retroelectrocyclization. The synthetic approach worked smoothly under ambient laboratory conditions without requiring heat or catalyst. Optoelectronic properties of the highly-colored chromophores were investigated by using both experimental (UV/vis spectroscopy) and computational methods. Solvatochromism and pH studies were performed for all 6 chromophores. Computational studies (TD-DFT, electrostatic potential maps, HOMO-LUMO orbital depictions) were further confirmed that all chromophores undergo intramolecular charge transfer. The HOMO-LUMO energy gap of dye 13 is found to have the largest energy gap resulting in worse charge-transfer properties as compared to 15 and 17. A similar trend was also observed for chromophores 14, 16, and 18. PAH-substituted push-pull chromophores are predicted to have significant NLO properties, and these properties can simply be tuned by changing substituent PAH structures. The present study provides valuable knowledge for the design and synthesis of new NLOphore systems in the near future.

Acknowledgment

This work has been supported by the Middle East Technical University Scientific Research Projects Coordination Unit (Project No: AGEP-103-2019-10098).

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