3-Alkynylindoles as Building Blocks for the Synthesis of Electronically Tunable Indole-Based Push-Pull Chromophores.

In this study, two different classes of push-pull chromophores were synthesized in modest to excellent yields by formal [2+2] cycloaddition-retroelectrocyclization (CA-RE) reactions. N-Methyl indole was introduced as a new donor group to activate alkynes in the CA-RE transformations. Depending on the side groups' size and donor/acceptor characteristics, N-methyl indole-containing compounds exhibited λmax values ranging between 378 and 658 nm. The optoelectronic properties of the reported D-A-type structures were studied by UV/vis spectroscopy and computational studies. The complete regioselectivity observed in the products was elaborated by one-dimensional (1D) and two-dimensional (2D) NMR studies, and the electron donor strength order of N-alkyl indole and triazene donor groups was also established. The intramolecular charge-transfer characteristics of the target push-pull chromophores were investigated by frontier orbital depictions, electrostatic potential maps, and time-dependent density functional theory calculations. Overall, the computational and experimental results match each other. Integrating a new donor group, N-alkyl indole, into the substrates used in formal [2+2] cycloaddition-retroelectrocyclizations has significant potential to overcome the limited donor-substituted substrate scope problem of CA-RE reactions.

Introduction

A growing number of studies on the relationships between conjugated organic compounds and their electronic properties provide a better understanding of existing optoelectronic devices and support the logical design of ideal materials to fabricate next-generation ones.[1−4] Considering the applications in high-technology fields such as organic solar cells (OSCs),[5,6] organic light-emitting diodes (OLEDs),[7] organic photodetectors,[8] and organic sensors,[9] the design and synthesis of easily accessible conjugated molecules is of great importance. Almost all synthetic strategies to access target conjugated structures involve multiple cross-coupling reactions requiring expensive transition-metal catalysts.[10] There is a growing need for a synthetic approach to overcome these limitations. Short, click-type transformations are prime candidates to replace long synthetic protocols with environmentally friendly, atom/cost-economic nature.[11,12] Azide–alkyne Huisgen cycloadditions,[13] Diels–Alder reactions,[14] and alkene hydrothiolations[15] are among the most well-known and used click-type transformations in the literature. The formation of nonconjugated products in alkene hydrothiolations, explosive nature of the organic azides utilized in Huisgen cycloadditions, and high-temperature requirement encountered in a significant number of Diels–Alder reactions are still substantial issues to be resolved.[16] Formal [2+2] cycloaddition-retroelectrocyclizations (CA-RE) have lately received prominent attention as an alternative to the current click-type methods in synthesizing conjugated molecules.[17] Nonplanar push–pull chromophores obtained by this efficient and catalyst-free strategy draw significant attention with critical features such as intense intramolecular charge-transfer (ICT) bands, redox activity, good solubility in organic solvents, and thermal stability.[18] Following the first report of the CA-RE between transition metal ruthenium-substituted acetylides and electron-poor alkenes by Bruce et al. in 1981,[19] Diederich and co-workers successfully demonstrated that metal-free substrates could also participate in these transformations with their pioneer work published in 2005.[20] Interestingly, only a few studies have been reported on metal-free substrates between 1981 and 2005.[21−23] With the studies conducted between 2005 and 2021, CA-RE was applied to the synthesis of various push–pull targets, such as dendrimer-like structures,[24,25] active layer material in organic solar cells,[26,27] NLOphores,[28,29] luminescent push–pull chromophores with fluorophore-conjugated[30−32] and nonconjugated TCBDs,[33−36] polymers,[17,37,38] sensors for metal-ion detections,[39] and Aviram-Ratner-type dyads.[40] The most straightforward strategy to tune the optoelectronic properties of push–pull materials obtained via CA-RE is the variation of the structural designs by the appropriate choice of donor and acceptor groups.[17] Unfortunately, CA-RE transformations suffer from a relatively limited donor-substituted substrate scope.

Electron-deficient alkenes that have recently been utilized in CA-RE chemistry can be listed as tetracyanoethylene (TCNE),[41] 7,7′,8,8′-tetracyanoquinodimethane (TCNQ),[42] 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ),[18] tetracyanoethyleneoxide (TCNEO),[43]N,N′-dicyanoquinone diimides (DCNQIs),[44] 6,6-dicyanopentafulvenes (DCFs),[45] and 2- (dicyanomethylene)indan-1,3-dione (DCID).[46] Similarly, long-term storage limitations and instability reduce the diversity of donor-substituted alkynes used in CA-RE. Metal acetylides,[19,47] dialkylaniline,[20] ferrocene,[23] thiophene,[48] p-methoxybenzene,[48] 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY),[49] cyclopenta[b]furan-2-one,[50] metalloporphyrins,[51] carbazole-substituted alkynes,[52] ynamides,[53] tetrathiafulvalene,[17] azulene,[54] phenothiazine,[55] triazene,[56,57] and ureas[33] are donor substrates displaying sufficient reactivity in CA-RE reactions with TCNE. Surprisingly, the donor substrate scope (dialkylaniline,[42] ferrocene,[58] cyclopenta[b]furan-2-one,[59] carbazole,[60] metal acetylides,[61] azulene,[62] phenothiazine,[63] thiophene[64]) is much more limited in CA-RE reactions with TCNQ (Figure ). Herein, we hypothesized that N-alkyl indole derivatives could sufficiently activate alkynes and expand the limited substrate diversity by participating in the [2+2] CA-RE reactions. Indole donor groups offer several advantages, such as easy functionalization,[65] potential biological,[66] and nonlinear optical activities.[67] Indole motifs have also continuously been investigated as classical pharmacophores.[68] However, the use of indoles in material science is quite limited. Although indole groups are utilized as donor groups in some D–A-type push–pull systems,[69] it was surprising that N-alkyl indole derivatives have never been tested in CA-RE transformations. As reported by Anderson and co-workers,[70] terminal alkynylindoles are quite susceptible to decompositions, which could be why these species are overlooked for click-type CA-RE. Our initial assumption is that it would be possible to circumvent this limitation by increasing the molecular weight of the alkyne substrates by adding bulky substituents. Accordingly, N-alkyl indole-activated alkynes with various side groups, such as polyaromatic hydrocarbons and electron-rich and electron-poor phenyl groups, have been synthesized using Sonogashira cross-coupling reactions and tested for CA-RE with TCNE and TCNQ. The effects of different acceptor and side groups on the optoelectronic properties of indole-substituted push–pull chromophores were studied by density functional theory (DFT). The charge-transfer behavior of the target structures was further investigated by highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) representations, electrostatic potential maps, and time-dependent density functional theory (TD-DFT) calculations.

Figure 1

[2+2] Cycloaddition-retroelectrocyclizations of EDG-substituted alkynes with electron acceptors TCNE and TCNQ.

Results and Discussion

Synthesis and Characterizations

To bypass the reported issues with the stability of terminal alkynylindoles,[70] the Sonogashira cross-coupling strategy of 3-iodo-1-methyl-1H-indole (1) and a variety of alkynes 2a–i has been employed (Scheme ). 3-Iodo-1-methyl-1H-indole (1) has been accessed following the two-step protocol described in the literature.[71,72] The synthesis of 1 started from indole, which was treated with MeI for the essential protection step. Following the regioselective 3-iodination 1 was obtained in 74% yield. At the same time, alkynes substituted with electron-rich and electron-poor phenyl groups 2a–e, PAHs 2f–i, required for the synthesis of N-alkyl indole-based substrates 3a–i have also been synthesized using literature procedures.[73−78] With iodo-indole 1 and terminal alkynes 2a–i in hand, the Sonogashira cross-coupling step has been performed. While preparing disubstituted alkynes 3a, 3b, 3c, and 3e, cross-coupling reactions occurred at room temperature. On the contrary, the reactions were performed in toluene in the presence of N,N-diisopropylamine (DIPA) as a base for the synthesis of 3f, 3g, and 3h, presumably due to the low solubility of substrates in NEt3. Substrates 3d and 3i required slightly elevated temperatures for the completion of the reactions. All alkynes 3a–i were highly stable and stored under ambient conditions without any precaution for a prolonged period. These results confirm the validity of our proposal regarding the stability problems of indole-substituted terminal alkynes.

Scheme 1

Synthesis of N-Methyl Indole-Activated Alkynes 3a–i

Reagents and conditions: (i) Pd(PPh3)2Cl2, CuI, Et3N, 25 °C for 3a, 3b, 3c, 3e. (ii) Pd(PPh3)2Cl2, CuI, toluene, DIPA, 25 °C, 3f, 3g, 3h. (iii) Pd(PPh3)2Cl2, CuI, toluene, DIPA, 60 °C, 3d, 3i.

After successfully preparing the stable indole-substituted alkynes 3a–i, we turned our attention to whether the N-alkyl indole group could sufficiently activate alkynes for CA-RE transformations (Scheme ). Initially, 3a was tested as a substrate in CA-RE with electron-deficient TCNE 4, and target push–pull chromophore 5a was isolated in 76% yields. At this point, it was still unclear whether the group that activates the alkyne for the reaction was N-alkyl indole or the triazene,[57,73,79] which is known to be an efficient electron donor in the literature. To confirm the electron donor role of N-alkyl indoles, 3c and 3e containing electron-withdrawing groups (phenyl and nitrophenyl) were treated with TCNE. Gratifyingly, substrates 3c and 3e also reacted smoothly with TCNE, allowing the synthesis of target products 5c (81%) and 5e (76%) respectively. This result was an indisputable proof that N-alkyl indole is a new type of donor group that can be exploited to activate alkynes used in CA-RE transformations. In the next stage, a systematic study was carried out where the indole donor group was kept fixed, and the side groups were altered. Regardless of the identity of the side groups, the target push–pull compounds 5a–i were obtained in very high yields ranging from 76 to 96%. The relatively low yield (76%) seen in compound 5e can be explained by the presence of the nitro group reducing the electron concentration on the alkyne unit. A similar situation observed in the case of compound 5a is due to the difficulties encountered in isolation.

Scheme 2

Formal [2+2] CA-RE between 3a–i and TCNE 4

After confirming the donor behavior of N-alkyl indoles, substrates 3a–i were also subjected to CA-RE with another well-known electron acceptor, TCNQ 6 (Scheme ). Unlike reactions with TCNE, some substrates (3f, 3g, and 3h) reacted with TCNQ under relatively higher temperatures. The reason for this high-temperature requirement is presumably due to steric hindrance originating from bulky naphthalene and phenanthrene groups.[80] Target push–pull compounds were obtained in moderate to excellent yields (40–96%). As previously mentioned, nitrobenzene-substituted alkyne 3e also reacted with TCNQ in a relatively low yield (40%). The slight yield differences observed in the reactions of other substrates are related to problems encountered during the isolation step.

Scheme 3

Formal [2+2] CA-RE between 3a–i and TCNQ 6

Theoretically, two possible regioisomers 7a and 7a″ would be expected to be formed during the reaction of TCNQ 6 and unsymmetrical alkyne 3a, which possess two different donor units (Scheme ). However, complete regioselectivity was observed, and only compound 7a was isolated as confirmed by two-dimensional (2D) HMBC (heteronuclear multiple quantum coherence) NMR spectroscopy (see Figure S38 in the Supporting Information (SI)).[81] ICT breaks the aromaticity of indole ring (I) while generating a new one (II), as in the case of 7a′. Therefore, the quinoidal unit prefers to be in close proximity with the strong donor as in 7a. These results demonstrate that the N-alkyl indole unit is a superior electron donor compared to the triazene moiety. The reason why N-alkyl indoles show enhanced donor ability than triazenes can be explained by the fact that the benzene ring in triazenes (requires more energy) and the pyrrole ring in N-alkyl indoles (requires less energy) lose their aromaticity during intramolecular electron transfer.

Scheme 4

Regioselectivity in the Reaction between TCNQ 6 and Alkyne 3a

UV/vis Spectroscopy

All N-alkyl indole-substituted chromophore 5a–i and 7a–i solutions were intensely colored, resulting from broad intramolecular charge-transfer absorptions in the visible region of the electromagnetic spectrum. Absorption spectra for the representatives of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives 5a, 5c, 5d, 5f, 5g, and 5h are shown in Figure (see Figure S81 in the SI for the rest of the TCBDs). TCBDs 5a–i possess two distinct low-energy absorption bands λmax,1 = 383 nm (2.08 × 104 M–1 cm–1) and λmax,2 = 442 nm (3.73 × 104 M–1 cm–1) for 5a; λmax,1 = 387 nm (1.52 × 104 M–1 cm–1) and λmax,2 = 426 nm (1.53 × 104 M–1 cm–1) for 5c; λmax,1 = 352 nm (1.29 × 104 M–1 cm–1) and λmax,2 = 427 nm (8.50 × 103 M–1 cm–1) for 5d; λmax,1 = 367 nm (1.20 × 104 M–1 cm–1) and λmax,2 = 431 nm (1.06 × 104 M–1 cm–1) for 5f; λmax,1 = 359 nm (1.83 × 104 M–1 cm–1) and λmax,2 = 395 nm (1.72 × 104 M–1 cm–1) for 5g; λmax,1 = 367 nm (1.04 × 104 M–1 cm–1) and λmax,2 = 434 nm (9.70 × 103 M–1 cm–1) for 5h. The origin of these bands is likely due to the electron transfer from donor indole to the acceptor TCBD unit. The D–A–D-type chromophore 5a showed the most bathochromically shifted ICT band (λmax,2 = 442 nm) in the TCBD series. Compounds 5f and 5h follow 5a with λmax,2 values 431 and 434 nm, respectively. The large difference observed in the λmax,2 values of 5f (32.5°, dihedral angle in between indole and dicyanovinyl units, obtained from optimized geometries, Table S19) and 5g (27.5°) with structurally similar naphthalene substituents indicates that λmax,2 values are presumably more affected by sterics than electronics. Accordingly, the observed difference in the λmax,2 values of 5f (32.5°) and 5h (30.5°) compared to other chromophores 5c (27.7°), 5d (27.9°), and 5g (27.5°) can be explained by considering the relatively large dihedral angles between donor and acceptor units. Although planarity is essential for the efficient overlap of π-orbitals and generally results in an increase in molar absorptivity and λmax values, there are examples in the literature showing exceptional advantages of nonplanar chromophores, as in this study.[82] These results demonstrate another advantage of nonplanar push–pull chromophores over planar counterparts, where the dihedral angle between donor and acceptor groups can be easily controlled by substituent modifications. Besides conformational control, the donor or acceptor nature of side groups can also be used to tune the absorption of the chromophores. While 5e with nitrobenzene side group possesses CT band at around 407 nm, 5b and 5d with methoxy and methylbenzene groups bathochromically shifted bands at 428 and 427 nm, respectively (Figures and S81 in the SI).

Figure 2

UV/vis spectra (CH2Cl2, 25 °C) of the representative chromophores 5a, 5c, 5d, 5f, 5g, and 5h.

TCNQ adducts 7a–i showed stronger ICT absorption bands in the near-IR region with the help of extended π-conjugation in their structure compared to TCBDs 5a–i. Similar to the absorption spectra of 5a–i, compounds 7a–i also feature two low-energy absorption bands (λmax,1 between 402 and 434 nm; 2.86–3.08 eV/λmax,2 between 612 and 658 nm; 1.88–2.03 eV). The UV/vis spectra of the selected chromophores 7a, 7c, 7d, 7f, 7g, and 7h are shown in Figure [λmax,1 = 402 nm (2.91 × 104 M–1 cm–1) and λmax,2 = 612 nm (2.84 × 104 M–1 cm–1) for 7a; λmax,1 = 434 nm (8.60 × 103 M–1 cm–1) and λmax,2 = 615 nm (1.88 × 104 M–1 cm–1) for 7c; λmax,1 = 407 nm (9.90 × 103 M–1 cm–1) and λmax,2 = 614 nm (2.32 × 104 M–1 cm–1) for 7d; λmax,1 = 423 nm (1.46 × 104 M–1 cm–1) and λmax,2 = 653 nm (1.88 × 104 M–1 cm–1) for 7f; λmax,1 = 433 nm (1.05 × 104 M–1 cm–1) and λmax,2 = 617 nm (1.95 × 104 M–1 cm–1) for 7g; λmax,1 = 422 nm (1.15 × 104 M–1 cm–1) and λmax,2 = 658 nm (1.40 × 104 M–1 cm–1) for 7h]. A similar trend in λmax values of TCBDs is also seen in TCNQ products 7f (34.2°, dihedral angle in between indole and quinoidal units, obtained from optimized geometries; Table S19) and 7h (35.4°) that possess the most red-shifted absorption bands among TCNQ products 7a (31.0°), 7c (31.3°),7d (31.4°), and 7g (31.1°). Similar to the TCNE products, this observation can be attributed to conformational distortions caused by large dihedral angles.

Figure 3

UV/vis spectra (CH2Cl2, 25 °C) of the representative chromophores 7a, 7c, 7d, 7f, 7g, and 7h.

Both TCNE and TCNQ products show positive solvatochromism (see selected two examples 5g and 7g in Figure a,b).[83] When the solvent is changed from polar (CH2Cl2) to nonpolar (n-hexane), the color of the solution of 5g changes from dark orange to pale yellow, and the ICT band shifts from 395 nm (3.14 eV) to 389 nm (3.19 eV). On the other hand, a substantial change in ICT bands of 7g is observed [from 617 nm (2.01 eV) to 549 nm (2.26 eV)] with the color change from turquoise to pale purple when the solvent is changed from polar (CH2Cl2) to nonpolar (n-hexane). The reason behind these solvatochromic behaviors of dyes 5g and 7g can simply be explained by the stabilization of the excited states more than the ground states by polar solvents. The deviation from planarity at different rates in different solvents should also not be ignored.

Figure 4

UV/vis spectra of chromophores 5g and 7g in CH2Cl2/n-hexane mixtures at 25 °C.

Computational Studies

The charge-transfer characteristics of the push–pull chromophores were further studied by time-dependent density functional theory (TD-DFT) calculations, visualizations of highest occupied molecular orbitals (HOMOs)-lowest unoccupied molecular orbitals (LUMOs), and electrostatic potential maps. Density functional theory (DFT) calculations were achieved at the B3LYP/6-31G* level of theory with CPCM solvation in CH2Cl2 using the Gaussian 09 program package.[84] Low-energy absorption bands and their corresponding oscillator strengths (see Tables S1–S18 in the SI for all of the details) were calculated using TD-DFT at the CAM-B3LYP/6-31G* level of theory on optimized geometries at the B3LYP/6-31G* level of theory with CPCM solvation in CH2Cl2. The low-energy absorption bands (see selected examples in Figures and 3) can be assigned to ICT transitions between electron-rich indole group and electron-poor cyano-rich acceptor units. In all cases, these intense bands are attributed to HOMO–LUMO transitions. Figure shows both calculated and experimental UV/vis spectra of the two representative chromophores. The overall shapes of the calculated and experimental spectra of 5a and 7a match each other well. In both cases, calculated extinction coefficients appear to be slightly overestimated. On the other hand, the calculated λmax values are somewhat lower than the experimental ones, although the results are well within the expected error range for similar chromophores.[85]

Figure 5

(a) Calculated (blue line) TD-DFT:CAM-B3LYP/6–31G* in CH2Cl2 and experimental UV/vis spectrum of 5a in CH2Cl2 (red line). (b) Calculated (blue line) TD-DFT:CAM-B3LYP/6–31G* in CH2Cl2 and experimental UV/vis spectrum of 7a in CH2Cl2 (red line).

When Figure a,b is examined in detail, it is noteworthy that the chromophores obtained by TCNQ have significantly red-shifted lower energy absorption bands compared to those obtained by TCNE. These results are in excellent agreement with the calculated band gap values for chromophores 5a–i and 7a–i (Figure ). Calculated band gap values for TCNE products range between 2.52 and 2.89 eV, while TCNQ products have lower band gap values (1.78 and 2.33 eV) compared to 5a–i. Both groups’ lowest band gap values were found in nitrobenzene-containing chromophores 5e and 7e. These results can be explained by the fact that nitrobenzene is a very powerful electron acceptor compared to other substituent groups utilized in this study.

Figure 6

Energy-level diagram of the HOMOs and LUMOs of push–pull dyes 5a–i and 7a–i estimated by DFT studies.

As another proof of ICT behavior of push–pull chromophores, frontier orbital depictions of six selected compounds are given in Table . As mentioned earlier, the lowest-energy absorption bands mainly involve HOMO–LUMO transitions. In all cases, the electron density distribution is located on the donor indole part. On the other hand, the electron density in LUMOs is mainly concentrated on electron-poor cyano-rich regions. Both HOMO and LUMO depictions highlight small but distinct overlap describing the transfer of electrons from electron-rich indole to the electron-poor cyano-rich core. Besides frontier orbital analysis, electrostatic potential maps (ESPs) were also utilized to further discuss ICT interactions. ESP visualizations give an overall idea about the charge density and polarity of the push–pull chromophores.[86] While red-colored regions show electronically the most negative locations, the blue-colored zones highlight positive areas. As expected, the blue areas are located on the electron-rich indole ring. In contrast, the red areas are located in the electron-poor but cyano-rich core regions, supporting the ICT behavior of push–pull chromophores.

Table 1

Structures, Frontier Orbital Visualizations, and Electrostatic Potential Maps [−0.03 a.u (Red) to 0.03 a.u (Blue), DFT:B3LYP/6-31G* Level of Theory] of Representative Chromophores 5a, 5f, 5h, 7a, 7f, and 7ha

The red color represents the most negative regions. On the other hand, the blue color highlights the most positive regions.

Conclusions

In this study, we prepared two new series of push–pull chromophores by formal [2+2] cycloaddition-retroelectrocyclizations. The electron-rich N-methyl indoles were utilized for the first time to activate alkynes for CA-RE transformations. With the reported synthetic approach, a significant contribution was made to improve the common limitations of the CA-RE strategy, such as limited substrate diversity and instability of substrates. Eighteen different D–A-type push–pull chromophores were isolated using two different electron-poor alkenes (TCNE and TCNQ) and nine different side groups. The wide structural diversity in this study provided insight into the structure–optical property relationships of the nonplanar push–pull chromophores. While λmax values of chromophores obtained with TCNE vary between 395 nm and 442 nm, products obtained with TCNQ have λmax values between 612 and 658 nm approaching the near-IR region. Additionally, it was confirmed that both groups of compounds show positive solvatochromism, a common property of push–pull-type compounds. The optical properties of the synthesized materials were also confirmed by computational methods. The ICT characteristics of the push–pull chromophores were clearly demonstrated using TD-DFT calculations, HOMO–LUMO visualizations, and ESPs. The observed differences in low-energy absorptions of TCNE and TCNQ adducts were also confirmed by calculated band gaps. In summary, all of these results show that indole-containing push–pull systems have a significant potential to find use in optoelectronic applications.

Experimental Section

General

Commercially available chemicals were purchased, and no additional purification has been performed. Compounds 2a,[73]2b,[75]2d,[76]2e,[77]2f,[74]2g,[74]2h,[74] and 2i(78) were prepared according to literature procedures. Solvents (dichloromethane, hexanes, and ethyl acetate) used for extraction or column chromatography procedures were distilled. Cross-coupling reactions were performed under N2 atmosphere with oven-dried glassware. Column chromatography (CC, SiO2-60 mesh) was used for the purification of target compounds. Analytical thin-layer chromatography (TLC) was carried out on aluminum sheets coated with 0.2 mm silica gel 60 F254; a UV lamp (254 or 366 nm) was utilized for the visualization. Solvents were evaporated in vacuo at 25–60 °C and 900–10 mbar. 1H and 13C{1H} nuclear magnetic resonance (NMR) spectra were obtained at 400 MHz (1H) and 100 MHz (13C{1H}), respectively. Structural assignments were made with additional information from gCOSY, gHSQC, and gHMBC experiments. Chemical shifts δ are given in parts per million (ppm) downfield from tetramethylsilane using the residual deuterated solvent signal as an internal reference (CDCl3: δH = 7.26 ppm, δC = 77.0 ppm). For 1H NMR, the resonance multiplicity is described as s (singlet), d (doublet), t (triplet), q (quartet), quint (quintet), sext (sextet), sept (septet), m (multiplet), and br. (broad). Additionally, coupling constants J are given in hertz. All spectra were recorded at room temperature. High-resolution mass spectrometry (HRMS) was carried out by the mass spectrometry 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]+.

Synthesis of 3-Alkynylindoles 3a–i

Synthesis of 3a, 3b, 3c, and 3e: Condition (i)

Iodo-indole 1 (258 mg, 1.0 mmol, 1 equiv), bis(triphenylphosphine)palladium(II) dichloride (0.030 mmol, 0.03 equiv), and copper iodide (0.030 mmol, 0.03 equiv) were added to a two-neck round-bottom flask and stirred for 30 min under inert nitrogen atmosphere. Then, triethylamine (20 mL per 1.0 mmol 1) was added into the flask via a syringe and the solution was degassed for an additional 15 min with nitrogen. Terminal alkynes 2a, 2b, 2c, and 2e (1.1 mmol, 1.1 equiv) in triethylamine (8 mL per 1.0 mmol 1) were added into the reaction medium. After stirring overnight at 25 °C, the reaction mixture was quenched with water, extracted with dichloromethane (3 × 50 mL), dried over MgSO4, and filtered. The solvent was removed under reduced pressure, and coupling products 3a, 3b, 3c, and 3e were isolated by performing column chromatography (CC) (SiO2; 9:1 n-hexane/ethyl acetate).

Compound 3a

Yield: 102 mg; yellow amorphous solid; 31%. R = 0.37 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 79–81 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ = 7.80–7.85 (m, 1H), 7.51 (quasi d, XX’part of AA’XX’-system, J = 8.9 Hz, 2H), 7.40 (quasi d, AA’part of AA’XX’-system, J = 8.9 Hz, 2H, 2H), 7.36–7.10 (m, 4H), 3.89–3.65 (m, 7H), 1.27–1.20 ppm (m, 6H); 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ = 150.6, 136.4, 132.1, 129.4, 124.0, 122.8, 121.0, 120.8, 120.5, 120.43, 120.41, 109.6, 97.6, 91.7, 82.8, 47.5, 43.0, 33.2, 13.2 ppm; IR (ATR): ν̃ = 2200 (w), 1647 (w), 1595 (m), 1326 (s), 1233 (m), 740 (s) cm–1; HRMS (ESI) m/z: [M + H]+ calcd for C21H23N4+ 331.1923; found 331.1923.

Compound 3b(87)

Yield: 144 mg; brown oil; 55%. R = 0.32 (SiO2; 9:1 n-hexane/ethyl acetate); 1H NMR (400 MHz, CDCl3, 298 K): δ = 7.81 (d, J = 7.7 Hz, 1H), 7.49 ppm (quasi d, XX’part of AA’XX’-system, J = 8.5 Hz, 2H), 7.32 (m, 3H), 7.21 (t, J = 7.3 Hz, 1H), 6.88 (quasi d, AA’part of AA’XX’-system, J = 8.5 Hz, 2H), 3.84 (s, 3H), 3.81 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 159.2, 136.4, 132.9 (2 × C), 132.0, 129.3, 122.8, 120.4, 116.7, 114.1, 109.6, 97.5, 90.9, 81.6, 55.5, 33.2 ppm.

Compound 3c(88)

Yield: 162 mg; yellow oil; 70%. R = 0.33 (SiO2; 9:1 n-hexane/ethyl acetate); 1H NMR (400 MHz, CDCl3, 298 K): δ = 7.87 (d, J = 7.7 Hz, 1H), 7.61 (dd, J = 8.1, 1.3 Hz, 2H), 7.41–7.23 (m, 7H), 3.79 ppm (s, 3H); spectral data was consistent with the literature.[88]

Compound 3e

Yield: 127 mg; yellow amorphous solid; 46%. R = 0.28 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 165–167 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ = 8.21 (quasi d, XX’part of AA’XX’-system, J = 8.9 Hz, 2H); 7.81 (d, J = 7.6 Hz, 1H), 7.65 (quasi d, AA’part of AA’XX’-system, J = 8.9 Hz, 2H), 7.42 (s, 1H), 7.40–7.30 (m, 3H), 3.85 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 146.4, 136.5, 133.5, 131.7, 131.6, 129.1, 123.8, 123.3, 121.1, 120.2, 110.0, 96.3, 90.4, 90.1, 33.4 ppm. IR (ATR): ν̃ = 2188 (w), 1536 (s), 1506 (w), 1385 (s), 1328 (s), 1169 (m) cm–1; HRMS (ESI) m/z: [M + H]+ calcd for C17H13N2O2+ 277.0977; found 277.0988.

Synthesis of 3f, 3g, and 3h: Condition (ii)

Iodo-indole 1 (258 mg, 1.0 mmol, 1 equiv), bis(triphenylphosphine)palladium(II) dichloride (0.090 mmol, 0.09 equiv), and copper iodide (0.090 mmol, 0.09 equiv) were added to a two-neck round-bottom flask and stirred for 30 min under nitrogen atmosphere. Then, toluene (6 mL per 1.0 mmol 1) and diisopropylamine (3 mL per 1.0 mmol 1) were added into the flask via a syringe and the solution was degassed for an additional 15 min with nitrogen. PAH-substituted alkynes 2f, 2g, and 2h (1.75 mmol, 1.75 equiv) in toluene (6 mL per 1 mmol 1) and diisopropylamine (3 mL per 1 mmol 1) were added into the reaction medium. After stirring overnight at 25 °C, the reaction mixture was quenched with water, extracted with dichloromethane (3 × 50 mL), dried over MgSO4, and filtered. The solvent was removed under reduced pressure, and 3f, 3g, and 3h were isolated by performing column chromatography (CC) (SiO2; 9:1 n-hexane/ethyl acetate).

Compound 3f

Yield: 231 mg; yellow amorphous solid; 82%. R = 0.26 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 128–132 °C. 1H NMR (400 MHz, CDCl3, 298 K) δ = 8.56 (d, J = 8.3 Hz, 1H), 7.93 (d, J = 7.8 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 7.81 (d, J = 8.2 Hz, 1H), 7.78 (d, J = 7.1 Hz, 1H), 7.62 (t, J = 7.6 Hz, 1H), 7.58–7.50 (m, 1H), 7.48 (d, J = 7.9 Hz, 1H), 7.45 (s, 1H), 7.39 (d, J = 7.7 Hz, 1H), 7.35–7.27 (m, 2H), 3.86 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 136.5, 133.4, 133.2, 132.5, 129.7, 129.3, 128.4, 128.0, 126.7, 126.6, 126.4, 125.5, 122.9, 122.2, 120.6, 120.4, 109.8, 97.3, 89.3, 88.4, 33.2 ppm; IR (ATR): ν̃ = 2964 (w), 2197 (w), 1510 (w), 1271 (s), 742 (w) cm–1; HRMS (ESI) m/z: [M]+ calcd for C21H15N+ 281.1204; found 281.1212.

Compound 3g

Yield: 239 mg; dark-yellow amorphous solid; 85%. R = 0.33 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 118–122 °C. 1H NMR (400 MHz, CDCl3, 298 K) δ = 8.06 (s, 1H), 7.88 (d, J = 7.8 Hz, 1H), 7.84–7.80 (m, 3H), 7.62 (dd, J = 8.5, 1.4 Hz, 1H), 7.52–7.46 (m, 2H), 7.39–7.35 (m, 2H), 7.34–7.29 (m, 1H), 7.28–7.23 (m, 1H), 3.84 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 136.3, 133.3, 132.50, 132.46, 130.6, 129.2, 128.6, 128.0, 127.8, 127.7, 126.5, 126.3, 122.8, 121.8, 120.5, 120.3, 109.7, 97.1, 91.6, 83.9, 33.1 ppm; IR (ATR): ν̃ = 2965 (w), 22202 (w), 1510 (w), 1270 (s), 818 (w) cm–1; HRMS (ESI) m/z: [M]+ calcd for C21H15N+ 281.1204; found 281.1204.

Compound 3h

Yield: 186 mg; yellow amorphous solid; 56%. R = 0.29 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 146–150 °C; 1H NMR (400 MHz, CDCl3, 298 K) δ = 8.75–8.63 (m, 3H), 8.09 (s, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H), 7.80–7.69 (m, 2H), 7.68–7.59 (m, 2H), 7.48 (s, 1H), 7.40 (d, J = 7.7 Hz, 1H), 7.36–7.28 (m, 2H), 3.87 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 136.5, 132.6, 131.7, 131.4, 130.9, 130.3, 130.1, 129.4, 128.5, 127.3, 127.14, 127.12, 127.08, 127.0, 122.93, 122.89, 122.7, 120.9, 120.7, 120.4, 109.8, 97.3, 89.5, 88.1, 33.3 ppm. IR (ATR): ν̃ = 3050 (w), 2195 (w), 1539 (w), 1326 (s), 875 (w) cm –1; HRMS (ESI) m/z: [M]+ calcd for C25H17N+ 331.1361; found 331.1361.

Synthesis of 3d and3i: Condition (iii)

Iodo-indole 1 (258 mg, 1.0 mmol, 1 equiv), bis(triphenylphosphine)palladium(II) dichloride (0.090 mmol, 0.09 equiv), and copper iodide (0.090 mmol, 0.09 equiv) were added to a two-neck round-bottom flask and stirred for 30 min under nitrogen atmosphere. Then, toluene (6 mL per 1.0 mmol 1) and diisopropylamine (3 mL per 1.0 mmol 1) were added into the flask via a syringe, and the solution was degassed for an additional 15 min with nitrogen. Alkynes 2d and 2i (1.75 mmol, 1.75 equiv) in toluene (6 mL per 1.0 mmol 1) and diisopropylamine (3 mL per 1.0 mmol 1) were added into the reaction medium. After stirring overnight at 60 °C in an oil bath, the reaction mixture was quenched with water, extracted with dichloromethane (3 × 50 mL), dried over MgSO4, and filtered. The solvent was removed under reduced pressure and 3d, and 3i were isolated by performing column chromatography (CC) (SiO2; 9:1 n-hexane/ethyl acetate).

Compound 3d(89)

Yield: 164 mg; brown amorphous solid; 26%. R = 0.67 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 105–107 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ = 8.66 (s, 1H), 7.76 (d, J = 8.2 Hz, 2H), 7.48 (d, J = 8.2 Hz, 1H), 7.41 (t, J = 7.6 Hz, 1H), 7.35 (d, J = 7.9 Hz, 3H), 7.29–7.26 (m, 1H), 4.00 (s, 3H), 2.45 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 137.6, 136.5, 132.1, 131.3, 129.4, 129.2, 122.8, 121.4, 120.5, 120.4, 109.7, 97.5, 91.2, 82.4, 33.19, 21.61 ppm; IR (ATR): ν̃ = 2917 (m), 2203 (w), 1504 (w), 1236(m) cm–1; HRMS (ESI) m/z: [M + H]+ calcd for C18H16N+ 246.1283; found 246.1273.

Compound 3i

Yield: 169 mg; yellow amorphous solid; 55%. R = 0.54 (SiO2; 9:1 n-hexane/ethyl acetate); m.p. = 180–182 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ = 7.83 (d, J = 7.8 Hz, 1H), 7.64–7.57 (m, 6H), 7.45 (t, J = 7.4 Hz, 2H), 7.40–7.33 (m, 3H), 7.31 (d, J = 5.8 Hz, 1H), 7.22 (d, J = 7.8 Hz, 1H), 3.83 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 140.7, 140.3, 136.5, 132.4, 131.8, 129.4, 128.9, 127.6, 127.12, 127.10, 123.5, 122.9, 120.6, 120.4, 109.7, 97.3, 91.1, 84.1, 33.2 ppm; IR (ATR): ν̃ = 2199 (w), 1543 (w),1517(w), 1472 (w), 1385 (w), 841 (w) cm–1; HRMS (ESI) m/z: [M + H]+ calcd for C23H18N+ 308.1439; found 308.1432.

General Procedure for the Synthesis of 5a–i

A solution of indole-substituted alkynes 3a–i (1.0 mmol, 1 equiv) and TCNE (1.0 mmol, 1 equiv) in 1,2-dichloroethane (5 mL per 1.0 mmol 3a–i) was stirred at 25 °C until complete consumption of starting materials based on TLC analysis (approximately 24 h). Evaporation and CC (SiO2; CH2Cl2) gave target products 5a–i.

Compound 5a

Yield: 349 mg; dark-red-orange amorphous solid; 76%. R = 0.5 (SiO2; CH2Cl2); m.p. = 102–104 °C (decomposition). 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.66 (s, 1H), 7.88 (quasi d, XX’part of AA’XX’-system, J = 8.8 Hz, 2H), 7.52 (quasi d, AA’part of AA’XX’-system, J = 8.8 Hz, 2H), 7.45 (d, J = 8.2 Hz, 1H), 7.40 (d, J = 7.1 Hz, 1H), 7.35 (d, J = 8.4 Hz, 1H), 7.24–7.20 (m, 1H), 3.99 (s, 3H), 3.82 (q, J = 7.2 Hz, 4H), 1.35 (t, J = 7.2 Hz, 3H), 1.22 ppm (t, J = 7.2 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 166.9, 160.9, 156.9, 138.0, 137.2, 131.7, 127.1, 125.4, 125.0, 124.6, 121.8, 120.7, 115.3, 113.3, 112.9, 112.2, 111.5, 109.9, 81.9, 74.0, 50.0, 42.3, 34.8, 14.5, 11.3 ppm; UV/vis (CH2Cl2): λmax (ε) = 383 (2.08 × 104), 442 nm (3.73 × 104 M–1 cm–1); IR (ATR): ν̃ = 2924 (m), 2219 (m), 1595 (m), 1497 (m), 1452 (m), 1161 (m) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C27H23N8+ 459.2046; found 459.2046.

Compound 5b

Yield: 374 mg; dark-orange amorphous solid; 96%. R = 0.36 (SiO2; CH2Cl2); m.p. = 179–180 °C. 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.68 (s, 1H), 7.89 (quasi d, XX’part of AA’XX’-system, J = 9.1 Hz, 2H), 7.47 (d, J = 8.2 Hz, 1H), 7.40 (t, J = 7.6 Hz, 1H), 7.33 (d, J = 8.2 Hz, 1H), 7.30–7.25 (m, 1H), 7.02 (quasi d, AA’part of AA’XX’-system, J = 9.1 Hz, 2H), 4.00 (s, 3H), 3.91 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 167.0, 165.1, 160.6, 137.9, 137.4, 132.7, 125.3, 125.1, 124.7, 123.7, 120.5, 115.7, 115.2, 113.1, 112.9, 111.9, 111.6, 109.7, 82.4, 73.8, 56.0, 34.8 ppm; UV/vis (CH2Cl2): λmax (ε) = 275 (1.20 × 104), 428 nm (1.70 × 104 M–1 cm–1); IR (ATR): ν̃ = 2917 (m), 2217 (w), 1600 (w), 1496 (w), 1453 (m), 1178 (s), 757 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C24H16N5O+ 390.1355; found 390.1353.

Compound 5c

Yield: 303 mg; dark-red-orange amorphous solid; 81%. R = 0.27 (SiO2; CH2Cl2); m.p. = 178–182 οC; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.67 (s, 1H), 7.84 (d, J = 7.5 Hz, 2H), 7.66 (t, J = 7.5 Hz, 1H), 7.55 (t, J = 7.5 Hz, 2H), 7.49 (d, J = 8.2 Hz, 1H), 7.43 (t, J = 7.1 Hz, 1H), 7.38 (d, J = 8.1 Hz, 1H), 7.29 (t, J = 7.1 Hz, 1H), 4.01 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 168.1, 159.6, 137.7, 137.1, 134.4, 131.1, 129.8, 129.7, 125.0, 124.9, 124.5, 120.1, 114.8, 112.5, 111.9, 111.4, 110.9, 109.2, 86.6, 73.6, 34.6 ppm; UV/vis (CH2Cl2): λmax (ε) = 277 (2.22 × 104), 333 (1.81 × 104), 387 (1.52 × 104), 426 nm (1.53 × 104 M–1 cm–1); IR (ATR): ν̃ = 2219 (w), 1496 (w), 1316 (s), 744 (w) cm–1; HRMS (ESI-TOF) m/z: [M]+ calcd for C23H13N5+ 359.1171; found 359.1183.

Compound 5d

Yield: 388 mg; orange amorphous solid; 96%. R = 0.62 (SiO2; CH2Cl2); m.p. = 235–237 °C. 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.67 (s, 1H), 7.76 (quasi d, XX’part of AA’XX’-system, J = 7.9 Hz, 2H), 7.48 (d, J = 8.2 Hz, 1H), 7.41 (t, J = 7.1 Hz, 1H), 7.36–7.33 (m, 3H), 7.29–7.26 (m, 1H), 4.00 (s, 3H), 2.45 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 168.1, 160.3, 146.7, 138.0, 137.4, 130.8, 130.2, 128.8, 125.4, 125.2, 124.7, 120.5, 115.2, 112.9, 112.6, 111.7, 111.5, 109.6, 85.3, 73.9, 34.9, 22.1 ppm; UV/vis (CH2Cl2): λmax (ε) = 278 (1.05 × 104), 352 (1.29 × 104), 427 nm (8.50 × 103 M–1 cm–1); IR (ATR): ν̃ = 2916 (w), 2213 (w), 1497 (w), 1457 (m), 1400 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C24H16N5+ 374.1406; found 374.1410.

Compound 5e

Yield: 273 mg; yellow amorphous solid; 76%. R = 0.46 (SiO2; CH2Cl2); m.p. = 207–208 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.72 (s, 1H), 8.38 (quasi d, XX’ part of AA’XX’-system, J = 9.0 Hz, 2H), 7.96 (quasi d, AA’ part of AA’XX’-system, J = 9.0 Hz, 2H), 7.54 (d, J = 8.2 Hz, 1H), 7.47 (td, J = 6.3, 1.7 Hz, 1H), 7.37–7.28 (m, 2H), 4.04 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 166.2, 158.3, 150.6, 138.1, 137.8, 136.6, 131.2, 125.6, 125.3, 125.1, 125.0, 119.8, 114.7, 112.8, 112.2, 111.3, 110.4, 108.9, 90.9, 73.7, 35.1 ppm; UV/vis (CH2Cl2): λmax (ε) = 275 (2.16 × 104), 407 (1.48 × 104), 482 nm (3.50 × 103 M–1 cm–1); IR (ATR): ν̃ = 2916 (w), 2217 (w), 1573 (s), 1524 (w), 1498 (m), 1458 (m), 1355 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C23H13N6O2+ 405.1100; found 405.1079.

Compound 5f

Yield: 377 mg; dark-red-orange amorphous solid; 92%. R = 0.33 (SiO2; CH2Cl2); m.p. = 146–150 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.51 (s, 1H), 8.18 (d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H), 8.03–7.99 (m, 1H), 7.79 (d, J = 7.2 Hz, 1H), 7.73–7.61 (m, 2H), 7.59–7.52 (m, 3H), 7.46 (t, J = 7.6 Hz, 1H), 7.32 (t, J = 7.6 Hz, 1H), 4.03 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 167.2, 160.6, 138.2, 137.3, 135.7, 134.1, 131.9, 130.4, 129.9, 129.8, 128.9, 127.8, 125.4, 125.2, 125.1, 125.0, 124.6, 120.4, 115.2, 112.8, 112.2, 111.9, 111.2, 110.5, 91.3, 75.8, 34.9 ppm; UV/vis (CH2Cl2): λmax (ε) = 258 (1.69 × 104), 327 (1.15 × 104), 367 (1.20 × 104), 431 nm (1.06 × 104 M–1 cm–1); IR (ATR): ν̃ = 2216 (w), 1607 (m), 1502 (w), 1357 (s), 745 (w) cm–1; HRMS (ESI-TOF) m/z: [M]+ calcd for C27H15N5+ 409.1327; found 409.1328.

Compound 5g

Yield: 389 mg; dark-orange-red amorphous solid; 95%. R = 0.52 (SiO2; CH2Cl2); m.p. = 140–144 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.73 (s, 1H), 8.34 (s, 1H), 7.98 (d, J = 8.8 Hz, 1H), 7.92–7.88 (m, 3H), 7.69 (t, J = 7.6 Hz, 1H), 7.60 (dd, J = 8.0, 7.1 Hz, 1H), 7.50 (d, J = 8.2 Hz, 1H), 7.44–7.36 (m, 2H), 7.28–7.22 (m, 1H), 4.03 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 168.1, 160.1, 138.0, 137.5, 135.8, 132.61, 132.56, 130.3, 130.1, 130.0, 128.6, 128.1, 128.0, 125.2, 125.1, 124.7, 124.5, 120.4, 115.2, 113.0, 112.6, 111.7, 111.5, 109.6, 86.1, 73.7, 34.8 ppm; UV/vis (CH2Cl2): λmax (ε) = 273 (2.50 × 104), 359 (1.83 × 104), 395 (1.72 × 104 M–1 cm–1); IR (ATR): ν̃ = 2222 (w), 1506 (m), 1461 (w), 1320 (s), 747 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C27H16N5+ 410.1406; found 410.1406.

Compound 5h

Yield: 437 mg; dark-orange-red amorphous solid; 95%. R = 0.43 (SiO2; CH2Cl2); m.p. = 238–244 °C (decomposition). 1H NMR (400 MHz, DMSO-d6, 298 K); δ = 9.00–8.86 (m, 2H), 8.73 (s, 1H), 8.54 (s, 1H), 8.37 (d, J = 6.6 Hz, 1H), 8.15 (d, J = 7.3 Hz, 1H), 7.89–7.74 (m, 5H), 7.65 (d, J = 6.2 Hz, 1H), 7.50–7.30 (m, 2H), 3.93 ppm (s, 3H); 13C{1H} NMR (100 MHz, DMSO-d6, 298 K) δ = 166.3, 159.2, 139.8, 138.1, 133.8, 131.6, 130.5, 130.4, 129.7, 129.5, 128.2, 128.0, 127.8, 127.2, 125.9, 124.4, 124.1, 124.0, 123.2, 122.9, 121.8, 114.6, 113.8, 112.7, 112.2, 111.9, 110.6, 93.8, 79.2, 77.5, 34.1 ppm; UV/vis (CH2Cl2): λmax (ε) = 253 (4.55 × 104), 321 (1.37 × 104), 367 (1.04 × 104), 434 nm (9.70 × 103 M–1 cm–1); IR (ATR): ν̃ = 2202 (w), 1600 (m), 1507 (w), 1355 (s), 745 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C31H18N5+ 460.1562; found 460.1562.

Compound 5i

Yield: 414 mg; dark-red-orange amorphous solid; 95%. R = 0.68 (SiO2; CH2Cl2); m.p. = 240–241.5 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.71 (s, 1H), 7.94 (d, J = 8.6 Hz, 2H), 7.76 (d, J = 8.6 Hz, 2H), 7.62 (dd, J = 8.4, 1.5 Hz, 2H), 7.55–7.42 (m, 5H), 7.40 (d, J = 7.5 Hz, 1H), 7.30 (t, J = 7.1 Hz, 1H), 4.02 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 167.7, 160.1, 147.7, 138.8, 138.0, 137.5, 130.7, 130.0, 129.32, 129.26, 128.5, 127.4, 125.3, 125.2, 124.8, 120.5, 115.2, 112.9, 112.5, 111.7, 111.5, 109.6, 85.7, 73.8, 34.9 ppm; UV/vis (CH2Cl2): λmax (ε) = 276 (2.24 × 104), 378 nm (3.87 × 104 M–1 cm–1); IR (ATR): ν̃ = 2917 (m), 2217 (w), 1601 (m), 1502 (w), 1456 (m), 1399 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C29H18N5+ 436.1562; found 436.1558.

General Procedure for the Synthesis of 7a–i

A solution of indole-substituted alkynes 3a–i (1 mmol, 1 equiv) and TCNQ (1.5 mmol, 1.5 equiv) in 1,2-dichloroethane (5 mL per 1.0 mmol 3a–i) was stirred at 25 °C (for 7a–d, and 7i) or 60 °C (for 7e–h) in an oil bath until complete consumption of starting material based on TLC analysis (approximately 24 h). Evaporation and CC (SiO2; CH2Cl2) gave target products 7a–i.

Compound 7a

Yield: 487 mg; dark-green amorphous solid; 91%. R = 0.30 (SiO2; CH2Cl2); m.p. = 149–152 °C (decomposition); 1H NMR (400 MHz, CDCl3, 298 K) δ = 7.78 (quasi d, XX’part of AA’XX’-system, J = 8.8 Hz, 2H), 7.68 (dd, J = 9.6, 1.9 Hz, 1H), 7.55 (d, J = 8.1 Hz, 1H), 7.46 (quasi d, AA’part of AA’XX’-system, J = 8.8 Hz, 2H), 7.42–7.35 (m, 3H), 7.31–7.23 (m, 2H), 7.14 (dd, J = 9.5, 1.9 Hz, 1H), 7.01 (dd, J = 9.5, 1.9 Hz, 1H), 3.91 (s, 3H), 3.80 (q, J = 7.1 Hz, 4H), 1.34 (t, J = 7.1 Hz, 3H), 1.21 ppm (t, J = 7.1 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 170.3, 155.8, 154.4, 146.7, 138.1, 136.4, 135.4, 133.9, 131.5, 131.3, 129.9, 126.2, 124.9, 124.54, 124.45, 123.2, 121.2, 120.2, 114.58, 114.55, 114.4, 113.7, 112.8, 110.7, 82.7, 71.9, 49.5, 41.8, 33.9, 14.1, 10.9 ppm; UV/vis (CH2Cl2): λmax (ε) = 402 (2.91 × 104), 612 nm (2.84 × 104 M–1 cm–1); IR (ATR): ν̃ = 2200 (w), 1595 (m), 1457 (m), 1304 (s), 1506 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C33H27N8+ 535.2359; found 535.2358.

Compound 7b

Yield: 307 mg; dark-blue amorphous solid; 66%. R = 0.20 (SiO2; CH2Cl2); m.p. = 151–153 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 7.79 (d, J = 9.0 Hz, 2H), 7.67 (dd, J = 9.6, 1.8 Hz, 1H), 7.53 (d, J = 8.1 Hz, 1H), 7.45–7.37 (m, 3H), 7.29 (dd, J = 9.4, 1.5 Hz, 2H), 7.14 (dd, J = 9.6, 1.9 Hz, 1H), 7.00–6.90 (m, 3H), 3.92 (s, 3H), 3.87 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 170.5, 164.4, 154.6, 146.6, 138.4, 136.7, 135.7, 134.1, 132.5, 131.8, 126.8, 126.5, 125.3, 124.91, 124.86, 123.6, 120.4, 115.4, 114.8, 114.74, 114.68, 113.9, 112.9, 111.1, 83.3, 72.5, 55.9, 34.3 ppm; UV/vis (CH2Cl2): λmax (ε) = 341 (2.55 × 104), 409 (1.57 × 104), 610 nm (3.25 × 104 M–1 cm–1); IR (ATR): ν̃ = 2917 (m), 2204 (w), 1598 (m), 1507 (w), 1437 (m) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C30H20N5O+ 466.1668; found 466.1669.

Compound 7c

Yield: 362 mg; dark-blue amorphous solid; 83%. R = 0.12 (SiO2; CH2Cl2); m.p. = 183–187 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 7.74 (d, J = 7.5 Hz, 2H), 7.66 (dd, J = 9.6, 1.9 Hz, 1H), 7.59–7.52 (m, 2H), 7.48 (t, J = 7.9 Hz, 2H), 7.44–7.37 (m, 3H), 7.32–7.26 (m, 2H), 7.15 (dd, J = 9.6, 1.8 Hz, 1H), 7.00 (dd, J = 9.6, 1.8 Hz, 1H), 3.92 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 171.4, 154.1, 145.4, 138.0, 136.3, 135.4, 134.3, 133.60, 133.55, 131.9, 129.6, 126.2, 125.2, 124.8, 124.6, 123.3, 120.0, 114.3, 114.2, 112.8, 111.9, 110.9, 86.8, 72.8, 33.9 ppm (25 out of 27 expected signals observed); UV/vis (CH2Cl2): λmax (ε) = 283 (1.28 × 104), 325 (1.17 × 104), 434 (8.60 × 103), 615 nm (1.88 × 104 M–1 cm–1); IR (ATR): ν̃ = 2205 (w), 1603 (m) 1510 (w), 1251 (s), 756 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C29H18N5+ 436.1562; found 436.1563.

Compound 7d

Yield: 423 mg; dark-blue amorphous solid; 94%. R = 0.37 (SiO2; CH2Cl2); m.p. = 230–232 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 7.69–7.60 (m, 3H); 7.53 (d, J = 8.0 Hz, 1H), 7.43–7.37 (m, 3H), 7.31–7.26 (m, 4H), 7.14 (dd, J = 9.6, 1.4 Hz, 1H), 6.96 (dd, J = 9.5, 1.8 Hz, 1H), 3.92 (s, 3H), 2.40 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 171.5, 154.5, 146.1, 145.6, 138.4, 136.5, 135.7, 134.0, 132.1, 132.0, 130.7, 130.0, 126.6, 125.5, 125.0, 124.9, 123.6, 120.4, 114.71, 114.68, 114.6, 113.4, 112.5, 111.1, 85.8, 73.1 34.2, 22.0 ppm; UV/vis (CH2Cl2): λmax (ε) = 328 (1.54 × 104), 407 (9.90 × 103), 432 (9.70 × 103), 614 nm (2.32 × 104 M–1 cm–1); IR (ATR): ν̃ = 2916 (m), 2200 (w), 1601 (m), 1508 (w), 1432 (m) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C30H20N5+ 450.1719; found 450.1710.

Compound 7e

Yield: 192 mg; navy blue amorphous solid; 40%. R = 0.38 (SiO2; CH2Cl2); m.p. = 158–159.5 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.27 (quasi d, XX’ part of AA’XX’-system, J = 8.9 Hz, 2H), 7.85 (quasi d, AA’part of AA’XX’-system, J = 8.9 Hz, 2H), 7.63 (dd, J = 9.6, 1.9 Hz, 1H), 7.49 (d, J = 8.0 Hz, 1H), 7.43–7.38 (m, 3H), 7.35–7.30 (m, 2H), 7.27–7.20 (m, 1H), 7.03 (dd, J = 9.6, 1.9 Hz, 1H), 3.92 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 169.1, 154.0, 150.0, 143.4, 140.1, 138.4, 136.4, 135.5, 133.3, 133.2, 130.8, 126.31, 126.29, 126.0, 125.2, 124.8, 123.7, 119.9, 114.3, 114.2, 113.7, 112.4, 111.6, 111.4, 90.3, 74.9, 34.3 ppm; UV/vis (CH2Cl2): λmax (ε) = 284 (2.33 × 104), 328 (1.60 × 104), 628 nm (2.40 × 104 M–1 cm–1); IR (ATR): ν̃ = 2201 (w), 1600 (m), 1507 (w), 1343 (s) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C29H17N6O2+: 481.1413; found 481.1427.

Compound 7f

Yield: 461 mg; dark-green amorphous solid; 95%. R = 0.27 (SiO2; CH2Cl2); m.p. = 193–195 °C. 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.05 (dd, J = 8.1, 2.8 Hz, 2H); 7.97–7.92 (m, 1H), 7.75 (d, J = 7.2 Hz, 1H), 7.65–7.51 (m, 5H), 7.44–7.36 (m, 3H), 7.30–7.27 (m, 1H), 7.20 (dd, J = 9.6, 1.8 Hz, 1H), 7.14 (dd, J = 9.6, 1.6 Hz, 1H), 7.04 (dd, J = 9.6, 1.6 Hz, 1H), 3.90 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 171.0, 154.0, 146.6, 138.2, 137.0, 136.5, 134.4, 134.2, 134.1, 133.9, 133.7, 130.7, 129.9, 129.8, 128.7, 127.4, 127.0, 125.7, 125.2, 124.9, 124.0, 123.4, 120.2, 115.6, 114.6, 114.5, 113.3, 112.2, 111.2, 91.6, 73.7, 34.2 ppm. (32 out of 33 signals expected); UV/vis (CH2Cl2): λmax (ε) = 343 (1.25 × 104), 423 (1.46 × 104), 653 nm (1.88 × 104 M–1 cm–1); IR (ATR): ν̃ = 2228 (w), 1600 (m), 1504 (w), 1332 (s), 754 (w) cm–1; HRMS (ESI-TOF) m/z: [M]+ calcd for C33H19N5+ 485.1640; found 485.1639.

Compound 7g

Yield: 388 mg; dark-turquoise amorphous solid; 80%. R = 0.38 (SiO2; CH2Cl2); m.p. = 175–179 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.26 (s, 1H), 7.91 (d, J = 8.7 Hz, 1H), 7.86 (d, J = 8.7 Hz, 2H), 7.79 (dd, J = 8.7, 1.7 Hz, 1H), 7.71 (dd, J = 9.6, 1.7 Hz, 1H), 7.64 (t, J = 6.9 Hz, 1H), 7.62–7.54(m, 2H), 7.45 (s, 1H), 7.43–7.35 (m, 2H), 7.32–7.25 (m, 2H), 7.14 (dd, J = 9.5, 1.7 Hz, 1H), 7.05 (dd, J = 9.5, 1.7 Hz, 1H), 3.91 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 171.6, 154.4, 146.0, 138.4, 136.7, 135.7, 135.4, 134.0, 132.7, 132.3, 132.03, 131.96, 130.0, 129.9, 129.8, 128.1, 127.9, 126.5, 125.5, 125.1, 124.9, 124.8, 123.6, 120.3, 114.7, 114.6, 113.5, 112.5, 111.2, 86.7, 72.9, 34.3 ppm (32 out of 33 signals expected); UV/vis (CH2Cl2): λmax (ε) = 276 (2.00 × 104), 338 (1.80 × 104), 433 (1.05 × 104), 617 nm (1.95 × 104 M–1 cm–1); IR (ATR): ν̃ = 2202 (w), 1600 (m), 1430 (w), 1337 (s), 740 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C33H20N5+ 486.1719; found 486.1719.

Compound 7h

Yield: 504 mg; dark-green amorphous solid; 94%. R = 0.26 (SiO2; CH2Cl2); m.p. = 234–238 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 8.76 (d, J = 8.4 Hz, 1H), 8.68 (d, J = 8.4 Hz, 1H), 8.13 (d, J = 8.2 Hz, 1H), 8.02 (s, 1H), 7.92 (d, J = 7.9 Hz, 1H), 7.80 (t, J = 7.7 Hz, 1H), 7.75 (t, J = 7.7 Hz, 1H), 7.71–7.63 (m, 2H), 7.61 (d, J = 8.0 Hz, 1H), 7.54 (dd, J = 9.5, 1.7 Hz, 1H), 7.42–7.34 (m, 3H), 7.31–7.24 (m, 2H), 7.22 (dd, J = 9.5, 1.6 Hz, 1H), 7.08–7.04 (m, 1H), 3.88 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ = 170.9, 153.9, 146.4, 138.2, 136.8, 136.5, 134.8, 133.7, 133.23, 133.18, 132.2, 131.1, 130.4, 130.2, 129.9, 128.3, 128.0, 127.94, 127.85, 127.1, 125.8, 125.4, 125.1, 124.9, 124.2, 123.5, 123.0, 120.1, 115.6, 114.5, 114.4, 113.3, 112.3, 111.2, 91.7, 74.1, 34.2 ppm; UV/vis (CH2Cl2): λmax (ε) = 345 (1.11 × 104), 422 (1.15 × 104), 658 nm (1.40 × 104 M–1 cm–1); IR (ATR): ν̃ = 2203 (w), 1598 (m), 1505 (w), 1345 (s), 738 (w) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C37H22N5+ 536.1875; found 536.1874.

Compound 7i

Yield: 491 mg; dark-blue amorphous solid; 96%. R = 0.55 (SiO2; CH2Cl2); m.p. = 220–221 °C; 1H NMR (400 MHz, CDCl3, 298 K); δ = 7.85 (d, J = 8.6 Hz, 2H); 7.71–7.66 (m, 3H), 7.60–7.56 (m, 3H), 7.48–7.38 (m, 6H), 7.33–7.27 (m, 2H), 7.18 (dd, J = 9.6, 1.9 Hz, 1H), 7.02 (dd, J = 9.6, 1.9 Hz, 1H), 3.93 ppm (s, 3H); 13C{1H} NMR (100 MHz, CDCl3, 298 K); δ = 171.0, 154.5, 150.9, 146.8, 145.9, 138.8, 138.4, 136.7, 135.7, 133.9, 133.2, 132.1, 131.3, 130.6, 129.3, 129.1, 128.3, 127.3, 126.5, 125.5, 125.1, 125.0, 123.6, 120.4, 114.6, 113.4, 112.5, 111.7, 111.2, 86.1, 72.9, 34.3 ppm; UV/vis (CH2Cl2): λmax (ε) = 340 (2.00 × 104), 402 (2.10 × 104), 618 nm (2.49 × 104 M–1 cm–1); IR (ATR): ν̃ = 2918 (m), 2198 (w), 1600 (m), 1508 (w), 1456 (m) cm–1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C35H22N5+ 512.1875; found 512.1883.