Backbone-shape engineering of fused-ring electron-deficient molecular semiconductors for unipolar n-type organic transistors: synthesis, conformation changes, and structure–property correlations

Abstract

Backbone-shape engineering is one important strategy towards high-mobility molecular semiconductors. However, it has been seldom employed in modifying fused-ring electron-deficient small molecules, which can provide useful molecular structure–property correlations for developing high-performance n-type organic semiconductors. In this work, we design and synthesize a series of n-type fused-ring small molecules based on ladder-type dithienothiophen-pyrrolobenzothiadiazole (TPBT) derivatives, namely BTPOCl-c, BTPOCl-s, and BTPOCl-w. Due to the different backbone symmetry and length of TPBT derivatives, the above molecules demonstrate three shapes, i.e. C-, S-, and W-backbone shape. Intriguingly, all molecules show a slightly helical conformation between the central cores and end-groups. However, the dihedral angles in the helical conformation are different, which gradually become smaller in the trend of BTPOCl-c < BTPOCl-s < BTPOCl-w. Therefore, changing the backbone shape from C to S and then to W is conducive to enhancing the backbone planarity. Consequently, BTPOCl-w-based unipolar n-type organic transistors display the highest electron mobility among the three molecules. The above result is reasonably explained by BTPOCl-w's deepest energy levels, highest crystallinity with the shortest π–π stacking distance, and larger grain size with fewer grain boundaries. We believe that backbone-shape engineering is a promising approach for developing advanced n-type molecular semiconductors.

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1. Introduction

Because of their mechanical flexibility and solution processability, organic π-conjugated small molecules and polymers are potential semiconducting materials for light-weight, large-area, and flexible electronic devices.^1–3 The past two decades have witnessed great progress in a variety of electronic devices ranging from organic light emitting diodes,^4–6 organic photovoltaics (OPVs),^7–10 and organic thin film transistors (OTFTs)^11–17 to emerging application fields such as bioelectronics and stretchable electronics.^18–20 One of the driving forces to the above remarkable progress is the development of novel high-performance π-conjugated molecular semiconductors.

Among the organic π-conjugated molecules, dye/pigment molecules, such as perylenediimide (PDI), naphthalenediimide (NDI), isoindigo (IID), and diketopyrrolopyrrole (DPP), have been widely used as acceptors to construct n-type molecular semiconductors.^21–23 On the other hand, fused-ring electron-deficient small molecules are another important class of n-type molecular semiconductor. In 2016, Zhan and coworkers reported a planar fused-ring electron-deficient small molecule (ICC6IDT-IC) based on indacenodithiophene (IDT) for high-performance OPVs.²⁴ In 2019, Zou and coworkers further substituted the central IDT donor unit with a dithienothiophen-pyrrolobenzothiadiazole (TPBT) electron-deficient unit.²⁵ Although the photovoltaic properties of various TPBT-based small molecules are investigated thoroughly,^26–28 they have been less explored in the field of OTFTs. For the rational design of fused-ring electron-deficient small molecules, molecular design strategies of central core modifications,^29–31 substitution of electron-withdrawing end groups,^32–34 and sidechain engineering^35–38 are usually employed. Among these molecular design strategies, molecular backbone-shape engineering has been seldom used. Only a handful of p-type fused-ring molecular semiconductors have been reported, in which the molecular shape has a significant influence on the electronic structures and charge transport properties.^39,40 For example, Takimiya et al reported several naphthodithiophene (NDT)-based molecular semiconductors.³⁹ While the linear-shaped molecule NDT1 shows a higher hole mobility of 1.5 cm² V⁻¹ s⁻¹, its angular-shaped counterpart NDT3 demonstrates a lower mobility of 0.3 cm² V⁻¹ s⁻¹ (Fig. 1 top). In another instance, Michinobu et al. reported two carbazoledioxazine derivatives with angular and linear shapes. They also found that the linear-shaped small molecule Lin-CZ shows higher mobility than that of the angular-shaped one due to Lin-CZ's denser molecular packing motif (Fig. 1 top).⁴⁰ Based on the above results, we became interested in modifying TPBT-based fused-ring electron-deficient small molecules by using backbone-shape engineering and further investigate its effect on molecular conformation and charge transport properties.

Fig. 1 Chemical structures: (top) p-type molecular semiconductors with different backbone shapes; (bottom) fused-ring electron-deficient small molecules with different backbone shapes in this study.

Herein, three TPBT-based fused-ring electron-deficient small molecules, namely BTPOCl-c, BTPOCl-s, and BTPOCl-w are designed and synthesized (Fig. 1 bottom). BTPOCl-c shows a C shape, which is also regarded as a banana shape. BTPOCl-s demonstrates an S-shape with one end-group spreading up and the other one down (similar to angular shape). BTPOCl-w exhibits a W-shape with both end-groups spreading up. All three molecules demonstrate a slightly helical conformation between the central ladder-type cores and two end-groups, which is revealed by computational density functional theory (DFT) calculations. However, the dihedral angles in the helical conformation gradually become smaller in the trend of BTPOCl-c < BTPOCl-s < BTPOCl-w, indicating that a higher planarity of the backbone conformation is achieved. This also offers the opportunity to tailor the molecular organizations and electron transport properties in OTFTs. BTPOCl-w shows the highest electron mobility among the three molecules, which is reasonably explained by the deepest LUMO energy levels and highest crystallinity with the shortest π–π stacking distance demonstrated by two dimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS) patterns and atomic force microscopy (AFM) images. These results indicated that the W-shaped derivative, BTPOCl-w, is a promising n-type molecular semiconductor for further advancing OTFT technology.

2 Results and discussion

2.1 Synthesis and characterization

The synthetic routes to the three different shaped fused-ring small molecules are shown in Scheme 1. The starting compounds 1, 4, and 7 underwent the double intramolecular Cadogan reductive cyclization in the presence of triphenylphosphine in o-dichlorobenzene at a high temperature of 180 °C, followed by the addition of 1-bromo-2-butyloctane under alkaline conditions with potassium carbonate and potassium iodide in DMF at 80 °C, affording the ladder-type fused-ring compounds 2, 5, and 8. By using the Vilsmeier–Haack reaction, the subsequent dialdehyde compounds 3, 6, and 9 can be obtained as orange solids. Finally, the three different shaped fused-ring small molecules BTPOCl-w, BTPOCl-s, and BTPOCl-c were prepared through a Knoevenagel condensation of the compounds 3, 6, and 9, respectively, with 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2Cl) in a high yield of ∼70%. The detailed procedures are summarized in the Experimental section and ESI,† with nuclear magnetic resonance (NMR) spectra and high-resolution mass spectra (MALDI-TOF). All three molecules exhibit good solubility in chloroform, dichloromethane and chlorobenzene, indicating great potential for solution-processable organic electronic devices.⁴¹ The thermogravimetry analysis (TGA) shown in Fig. S1 (ESI†) proves the good thermal stability of these electron-deficient small molecules. This is because their thermal decomposition temperatures (T_d of 5% weight loss) are all beyond 230 °C, which meets the subsequent OTFT fabrication by using thermal annealing treatment.⁴²

Scheme 1 Synthetic routes to the three different shaped fused-ring small molecules.

2.2. Optical and electrochemical properties

The effect of backbone shape is observable in the UV-vis-NIR absorption profiles of the three small molecules. Fig. 2a and b show the UV-vis-NIR absorption spectra of the three different shaped fused-ring small molecules in dilute chloroform solution and thin films with detailed optical data summarized in Table 1. In chloroform solution, all three compounds exhibit similar vibronic shoulders with strong absorption from 600–800 nm. In detail, the maximum absorption peak is slightly red-shifted from 697 nm for BTPOCl-w to 708 nm for BTPOCl-s while the molecule BTPOCl-c shows an even more red-shifted maximum absorption peak at 714 nm. In the thin-film state, the same trend of gradual bathochromic shift is also observed. Interestingly, compared to their absorption in dilute solution, BTPOCl-w demonstrates an obviously enhanced vibration shoulder peak (0–1) compared to those of BTPOCl-c and BTPOCl-s, indicating much enhanced intermolecular π–π interactions in the solid state.⁴³ The above results imply that backbone shape can alter the absorption profile, which can effectively reflect the different molecular packing behavior of the three molecules. Next, electrochemical cyclic voltammetry (CV) was utilized to estimate the energy levels of these molecules in thin films. A platinum disk working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode were employed. The three molecule-based films were drop-cast from chloroform solutions on a Pt working electrode (2 mm in diameter). The 0.1 M tetra-n-butylammonium hexafluorophosphate (nBu₄NPF₆) in acetonitrile was used as the supporting electrolyte solution and thoroughly purged with argon before all CV measurements. The corresponding cyclic voltammograms are plotted in Fig. 2c. The estimated highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels are listed in Table 1 and displayed in Fig. 2d. Compared to the HOMO/LUMO energy levels of the C-shaped molecule BTPOCl-c (−5.71 eV/−3.69 eV), changing the backbone shape from C to S leads to the downshift of the HOMO and LUMO levels (e.g.BTPOCl-s, E_HOMO = −5.77 eV/E_LUMO = −3.76 eV). Moreover, this trend is strengthened by altering the backbone shape from S to W. Thus, BTPOCl-w shows the deepest E_HOMO of −5.85 eV/E_LUMO of −3.82 eV among the three molecules. This result is probably due to the stronger electron-withdrawing ability of the W-shaped molecule than those of C- and S-shaped molecules. The E^ele_g values of BTPOCl-c, BTPOCl-s, and BTPOCl-w were determined to be 2.02 eV, 1.99 eV, and 2.03 eV, respectively. This trend is consistent with the UV-vis-NIR results (Table 1). Overall, it implies that controlling the shape of small molecule-based organic semiconductors could facilely tune the absorption and energy levels, which might provide an efficient way to optimize the optoelectronic properties of molecular semiconductors. In this study, the W-shaped small molecule BTPOCl-w shows the deepest LUMO level, which is beneficial for efficient electron carrier transport.

Fig. 2 (a) and (b) Normalized absorption spectra of BTPOCl-c, BTPOCl-s, and BTPOCl-w (a) in dilute chloroform solution and (b) as thin films. (c) The cyclic voltammograms plots of BTPOCl-c, BTPOCl-s, and BTPOCl-w. For calibration, the redox potential of ferrocene (FeCp₂) was measured under the same conditions. (d) The diagram of the HOMO/LUMO energy levels of BTPOCl-c, BTPOCl-s, and BTPOCl-w.

Table 1 Summary of the optical and electronic properties

Compounds	λ sol max (nm)	λ film max (nm)	λ onset (nm)	E ^optg ^d (eV)	HOMO^e (eV)	LUMO^e (eV)	E ^eleg ^f (eV)
a In chloroform solution. b In neat films. c Obtained from the onset wavelength of the absorption of films. d Estimated with the equation: E^optg = 1240/λonset. e Measured by the cyclic voltammetry (CV) method. f Obtained with the equation: E^eleg = ELUMO − EHOMO.
BTPOCl-c	714	781	828	1.50	−5.71	−3.69	2.02
BTPOCl-s	708	775	831	1.49	−5.77	−3.76	1.99
BTPOCl-w	697	756	820	1.51	−5.85	−3.82	2.03

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2.3 Conformation control via backbone-shape engineering

To figure out the backbone-shape effects on the molecular conformation of these molecules, density-functional theory (DFT) calculation was conducted at the B3LYP/6-31G(d) level. Methyl groups were reasonably used for N-substituents and alkyl side groups to shorten the calculation time without scarifying the accuracy.⁴⁴Fig. 3a–c summarizes the top view and side view of the optimized molecular conformations of the three molecules. The torsion angles between the central ladder-type cores and the end-groups are calculated to be 1.9° and 1.5° for BTPOCl-c, 2.1° and 1.5° for BTPOCl-c, and 1.1° and 1.9° for BTPOCl-c. Thus, it reveals that all three molecules have a relative planar conformation because of the S⋯O and H⋯O intramolecular noncovalent interactions.^45–47 Interestingly, all three molecules demonstrate a slightly helical conformation between the central ladder-type cores and two IC-2Cl end-groups (see the side view in Fig. 3a–c). However, the dihedral angles in the helical conformation are different, which are calculated to be 9.2° for BTPOCl-c, 5.5° for BTPOCl-s, and 2.3° for BTPOCl-w. This suggests that altering the backbone shape from C to S and then to W is conducive to enhancing planarity.⁴⁸ As illustrated in Fig. 3d–f, the calculated HOMO/LUMO values are gradually downshifted with altering the backbone shape from C to S and then to W, which is consistent with the CV results. While the HOMO orbitals of all three molecules are mainly located on the ladder-type central cores, the distribution pattern of the LUMO orbitals is different for the three molecules. For the C-shaped BTPOCl-c, it mainly localizes on the ladder-type central core. However, BTPOCl-s shows relatively more distribution at one of the end groups (IC-2Cl). Excitingly, BTPOCl-w's LUMO orbitals widely delocalize on the whole backbone, indicating the most effective LUMO orbital overlapping during the solid state packing. Thus, high charge carrier mobility can be expected for BTPOCl-w.^49–51 Furthermore, the electrostatic potential (ESP) distributions of the three small molecules were computed with the corresponding images shown in Fig. 3g–i. The positive ESP values distribute on the most conjugated surfaces of the three molecules (green to blue colored regions), indicating their electron-deficient characteristics. Changing the backbone shape from C to S and then to W, the positive areas (especially the blue colored regions) of ESP distribution are gradually increased, implying that the electron-withdrawing properties of the three molecules are gradually strengthened. In addition, the ESP distributions of BTPOCl-s present an asymmetric ESP distribution owing to its asymmetric chemical structure, which might be beneficial to enhance intermolecular electrostatic interactions.⁵² Overall, the backbone shape has a large influence on the molecular conformation and corresponding electronic structures and energy levels. Among the three molecules, BTPOCl-w demonstrates the most electron-deficient properties with a very planar backbone conformation, which can facilitate the electron transport in the OTFT devices.⁵³

Fig. 3 (a)–(c) DFT optimized geometry of the three small molecules BTPOCl-c (a), BTPOCl-s (b), and BTPOCl-w (c). (d)–(f) The calculated HOMO/LUMO orbitals and energy levels of the three small molecules BTPOCl-c (d), BTPOCl-s (e), and BTPOCl-w (f). (g)–(i) The electrostatic potential (ESP) distributions of the three small molecules BTPOCl-c (g), BTPOCl-s (h), and BTPOCl-w (i).

2.4 Unipolar n-type thin-film transistors

Since the three molecules have different shapes and conformations, they should have an influence on the charge-carrier transporting properties (Fig. 4a). Thus, top-gate bottom-contact (TG/BC) OTFTs were fabricated to investigate this effect. The devices were fabricated by spin-coating the small-molecule solutions (5 g L⁻¹ in CHCl₃) onto a gold patterned glass substrate to form the organic semiconducting layer. After thermal annealing at 90 °C for 30 min, the dielectric layer was fabricated with a spin-coating process of a 1500 rpm 60 μL polymethyl methacrylate (PMMA) butylacetate solution and annealed at 90 °C for 30 minutes. Finally, a 100 nm Ag layer was deposited on the dielectric layer with a thermal evaporation process as the gate electrode by using a shadow mask. The detailed fabrication procedure is described in the ESI.† As shown in Fig. 4b and Table 2, one could find that the charge-transporting performance is dramatically improved upon altering the backbone shape from C to S and then to W. The C-shape molecule BTPOCl-c exhibits a unipolar electron mobility (μ_e) of 0.01 cm² V⁻¹ s⁻¹ and on–off ratio (I_on/I_off) of ∼10³ (Fig. 4c). Changing the C-shaped banana conformation to the asymmetric S conformation leads to a twice growth of μ_e up to 0.02 cm² V⁻¹ s⁻¹ (Fig. 4e and Table 2). This enhancement partially resulted from the asymmetric ESP distribution owing to its asymmetric chemical structure, which can strengthen the intermolecular electrostatic interactions. Interestingly, in the case of BTPOCl-w in which the conformation is further changed to W shape, a five-fold increase in the electron charge transport is observed. Thus, BTPOCl-w displays μ_e of 0.05 cm² V⁻¹ s⁻¹ with improved I_on/I_off of ∼10⁴ (Fig. 4g and Table 2). The significantly enhanced μ_e is due to BTPOCl-w's low-lying LUMO level and very planar backbone conformation, which can facilitate the electron transport in the OTFT devices. Overall, it is remarkable that improving the OTFT device performance can be achieved through small molecules’ backbone-shape engineering, which is correlated to the changes of their HOMO/LUMO energy levels and solid state packing behaviors.

Fig. 4 (a) Top-gate/bottom-contact device configuration used in this study. (b) The electron-mobility comparison profile of the three molecules. (c), (e) and (g) Typical n-type transfer characteristics of the transistors for (c) BTPOCl-c; (e) BTPOCl-s; (g) BTPOCl-w. (d), (f), and (h) Typical n-type output characteristics of the transistors for (d) BTPOCl-c; (f) BTPOCl-s; (h) BTPOCl-w.

Table 2 Summary of small-molecule-based OTFT performance

Compounds	μ e,max (cm^2 V^−1 s^−1)^a	μ e,avg (cm^2 V^−1 s^−1)^b	V th (V)	I on/Ioff
a The maximum values of the electron mobilities. b The average values calculated from at least 10 devices.
BTPOCl-c	0.01	0.009	24.78	10^3
BTPOCl-s	0.02	0.01	29.17	10^4
BTPOCl-w	0.05	0.03	30.74	10^4

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2.5 Characterization of molecular organization and film morphologies

To further investigate the influence of the different molecular conformations on the solid-state packing organization and correlate it with the different OTFT performances, the two-dimensional (2D) grazing incidence wide-angle X-ray scattering (2D-GIWAXS) patterns of the studied three molecules were measured.⁵⁴ As shown in Fig. 5a–j, all three molecule-based thin films demonstrate multiple diffraction rings, indicating isotropic packing features. However, the out-of-plane (OOP) and in-plane (IP) π–π stacking (010) diffraction peaks of BTPOCl-c are not obvious, suggesting its very weak crystallinity (Fig. 5b and c). Changing the C-shaped banana conformation to the asymmetric S conformation leads to higher intensity and shorter distance of the π–π stacking (3.47 vs. 3.55 Å), indicating that BTPOCl-s has higher crystallinity than that of BTPOCl-c (Fig. 5j and k), which is due to the strengthened intermolecular electrostatic interactions. Intriguingly, the 2D patterns of BTPOCl-w exhibit much stronger diffraction peaks in both the OOP and IP directions, indicating that BTPOCl-w owns the best crystallinity (Fig. 5g, h, and i). Meanwhile, the OOP diffraction peak (010) of BTPOCl-w shows the smallest FWHM (0.145 Å⁻¹) and highest crystalline coherence length (L_c) of 43.3 Å among the three molecules, further indicating its higher thin-film crystallinity (Table 3). Combined with the shortest π–π stacking distance of 3.41 Å, BTPOCl-w demonstrates the largest number of π–π stacking layers of 23.5 according to Scherrer analysis (Table 3). Therefore, it reveals that BTPOCl-w has an excellent long-range order. As shown in the proposed molecular packing models (Fig. 5j–l), changing the backbone conformation from C to S and then to W, the molecules tend to pack more densely and compactly with increased crystallinity and shorter π–π stacking distance, which results from the more planar backbone and increased intermolecular interactions. In addition, atomic force microscopy (AFM) images of BTPOCl-c and BTPOCl-s display smaller grain size, while BTPOCl-w shows larger grain size and fewer grain boundaries (Fig. 6). Overall, with the combination of the highest crystallinity with the shortest π–π stacking distance and larger grain size with fewer grain boundaries, BTPOCl-w demonstrates the highest electron mobility among the three small molecules.

Fig. 5 (a)–(c) 2D GIWAXS patterns and the related 1D profiles in the out-of-plane and in-plane direction based on BTPOCl-c. (d)–(f) 2D GIWAXS patterns and the related 1D profiles in the out-of-plane and in-plane direction based on BTPOCl-s. (g)–(i) 2D GIWAXS patterns and the related 1D profiles in the out-of-plane and in-plane direction based on BTPOCl-w. (j)–(l) Proposed molecular packing models of (j) BTPOCl-c, (k) BTPOCl-s, and (l) BTPOCl-w.

Table 3 Crystallographic parameters of small molecules in the out-of-plane direction

Compounds	q z [Å^−1]	d L–L [Å]	q xy [Å^−1]	d π–π [Å]	FWHM [Å^−1]	L c [Å]	L c/d
BTPOCl-c	0.396	15.9	1.771	3.55	0.461	13.6	3.8
BTPOCl-s	0.376	16.7	1.809	3.47	0.341	18.4	5.3
BTPOCl-w	0.391	16.1	1.839	3.41	0.145	43.3	23.5

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Fig. 6 Tapping-mode atomic force microscope (AFM) topography (a)–(c) and the corresponding phase (d)–(f) images of (a) and (d) BTPOCl-c, (b) and (e) BTPOCl-s, and (c) and (f) BTPOCl-w.

3. Conclusions

In summary, backbone-shape engineering is employed to tune the molecular conformation and electron transport properties of the fused-ring electron-deficient small molecules. Thus, three TPBT-based fused-ring electron-deficient small molecules with C-, S-, and W-shaped backbones, namely BTPOCl-c, BTPOCl-s, and BTPOCl-w are designed and synthesized. All three molecules demonstrate a slightly helical conformation between the central ladder-type cores and two IC-2Cl end-groups. However, the dihedral angles in the helical conformation vary significantly, whose value gradually becomes smaller in the trend of BTPOCl-c < BTPOCl-s < BTPOCl-w. It thus suggests that altering the backbone shape from C to S and then to W is conducive to enhancing the planarity of the backbone conformation. This also offers the opportunity to tailor the UV-vis-NIR absorption profiles, HOMO/LUMO energy levels, molecular packing organizations, and charge carrier transporting properties in OTFTs. As a result, BTPOCl-w shows the highest electron mobility among the three molecules, which is reasonably explained by the deepest HOMO/LUMO energy levels, highest crystallinity with shortest π–π stacking distance, and larger grain size with fewer grain boundaries. We believe that backbone-shape engineering will guide the way to developing more promising fused-ring electron-deficient small molecules for high performance OTFTs and OPVs.

4. Experimental details

4.1 Chemical synthesis and characterization

All commercially available solvents, reagents, and chemicals were purchased from Aldrich, TCI and several other reagent companies and used as received without further purification unless otherwise specified. All reactions and manipulations were carried out with the use of a standard inert atmosphere and Schlenk techniques. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were measured on a Varian Mercury Plus-400 spectrometer. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, and m = multiplet. Deuterated chloroform was used as the solvent. The MALDI-TOF mass spectra were measured by a Bruker autoflex maX MALDI-TOF mass spectrometer. Synthesis of BTPOCl-c was adapted from the published literature.⁵⁵ The synthetic procedures of the intermediates are described in the ESI.† Synthesis of BTPOCl-w and BTPOCl-s is described as follows.

Synthesis of BTPOCl-w. To a solution of compound 5 (22.0 mg, 0.0239 mmol) and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2Cl) (37.8 mg, 0.144 mmol) in dry CHCl₃ (10 mL) was added pyridine (1 mL) under N₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, and then the mixture was poured into CH₃OH (100 mL) and filtered, and the residue left in the filter paper was dissolved by CHCl₃. After removing the solvent, the residue was purified using column chromatography on silica gel (eluent: n-hexane: CH₂Cl₂ = 1 : 1, v/v), yielding a dark purple solid (23.0 mg, 68%).¹H NMR (400 MHz, CDCl₃, ppm): δ = 9.24 (s, 2H), 8.75 (s, 2H), 7.94 (s, 2H), 4.59–4.58 (m, 4H), 4.43(t, 4H, J = 6.0 Hz), 2.12–2.00 (m, 6H), 1.60–1.41 (m, 14H), 1.00–0.66 (m, 48H); ¹³C NMR (100 MHz, CDCl₃, ppm): δ = 185.8, 158.7, 153.2, 147.6, 139.6, 139.4, 139.0, 137.5, 137.4, 136.2, 135.9, 134.8, 128.9, 127.0, 125.2, 120.5, 115.0, 114.9, 114.8, 79.5, 69.4, 54.3, 39.3, 32.0, 31.8, 30.4, 30.1, 29.6, 25.9, 23.0, 22.9, 22.7, 14.3, 14.2, 14.1; MALDI-TOF MS: calcd for C₇₆H₈₂Cl₄N₈O₄S₃, 1408.4379; found, 1409.0007.

Synthesis of BTPOCl-s. To a solution of compound 6 (56.0 mg, 0.0576 mmol) and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2Cl) (84.6 mg, 0.321 mmol) in dry CHCl₃ (10 mL) was added pyridine (1 mL) under N₂. The mixture was refluxed for 16 hours and then allowed to cool to room temperature, and then the mixture was poured into CH₃OH (100 mL) and filtered, and the residue left on the filter paper was dissolved by CHCl₃. After removing the solvent, the residue was purified using column chromatography on silica gel (eluent: n-hexane: CH₂Cl₂ = 1 : 1, v/v), yielding a dark purple solid (52.0 mg, 63%).¹H NMR (400 MHz, CDCl₃, ppm): δ = 9.38 (s, 1H), 9.29 (s, 1H), 8.81 (s, 1H), 8.76 (s, 1H), 7.99 (s, 1H), 7.92 (s, 1H), 4.81(t, 2H, J = 6.0 Hz), 4.76–4.75 (m, 2H), 4.59–4.58 (m, 2H), 4.47 (t, 2H, J = 8.0 Hz), 2.15–2.02 (m, 6H), 1.63–1.30 (m, 26H), 1.06–0.67 (m, 46H); ¹³C NMR (100 MHz, CDCl₃, ppm): δ = 186.7, 186.1, 162.6, 158.9, 158.3, 153.7, 147.6, 147.6, 139.5, 139.2, 139.1, 139.0, 138.7, 138.6, 138.4, 137.4, 137.2, 137.1, 136.2, 136.1, 135.1, 133.9, 131.9, 128.8, 128.2, 127.0, 126.8, 125.2, 120.6, 120.2, 118.0, 114.8, 114.6, 113.7, 79.5, 74.5, 69.0, 68.1, 55.6, 55.2, 39.7, 39.2, 32.1, 31.9, 31.8, 31.7, 30.8, 30.4, 30.1, 29.9, 29.8, 29.7, 29.7, 29.6, 29.6, 29.5, 26.0, 25.8, 23.1, 23.1, 23.0, 22.9, 22.9, 22.7, 14.3, 14.3, 14.3, 14.2, 14.0, 14.0; MALDI-TOF MS: calcd for C₈₃H₉₂Cl₄N₈O₄S₄, 1534.4849; found, 1535.0177.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 52203216 and 22375051), and the Natural Science Foundation of Shanghai (21ZR1406900). Y. C. acknowledges the financial support by the Postdoctoral Research Foundation of China (2022M720756 and YJ20210249). The authors thank beamline 14B1 of Shanghai Synchrotron Radiation Facility for providing the beam time and assistance during experiments.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qm01189e

‡ These authors contributed equally.

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