Fusing Thienoisoindigo to the Conjugated Ribbons with Strong Absorption in the Second Near-Infrared Window

Open AccessCCS ChemistryCOMMUNICATION7 Nov 2022Fusing Thienoisoindigo to the Conjugated Ribbons with Strong Absorption in the Second Near-Infrared Window Yu Jiang†, Yaogang Zhang†, Yunfeng Deng, Shaoqiang Dong, Bo Li, Yuanping Yi, Zebing Zeng, Hanjiao Chen, Hao Luo and Yanhou Geng Yu Jiang† Joint School of National University of Singapore, Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207 School of Materials Science and Engineering, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 , Yaogang Zhang† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Yunfeng Deng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Joint School of National University of Singapore, Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207 School of Materials Science and Engineering, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 , Shaoqiang Dong Institute of Molecular Aggregation Science, Tianjin University, Tianjin 300072 , Bo Li State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Yuanping Yi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Zebing Zeng State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082 , Hanjiao Chen Analytical & Testing Center, Sichuan University, Chengdu 610064 , Hao Luo School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054 and Yanhou Geng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Joint School of National University of Singapore, Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207 School of Materials Science and Engineering, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 https://doi.org/10.31635/ccschem.022.202201829 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic dyes with strong absorption in the second near-infrared (NIR-II) window (1000–1700 nm) have multiple applications. However, the design and synthesis of stable NIR-II absorbing organic dyes are very challenging and constantly defy our synthetic ability. In this work, we have successfully synthesized a series of soluble and stable fused thienoisoindigo (nThIID) ribbons. The absorption maximum (λmax) of the ribbons increases from 644 nm of 1ThIID to 1252 nm of 6ThIID. Importantly, nThIIDs with n ≥ 4 all display strong absorption in the NIR-II window with molar extinction coefficients (ϵmax) greater than 105 L mol−1 cm−1 at λmax. These molecules are promising photothermal conversion dyes with photothermal conversion efficiencies of ca. 60% under 1064 nm laser irradiation. Download figure Download PowerPoint Introduction Organic dyes have been studied for more than a century, but have never lost their fascination for chemists or importance as functional materials.1–4 In recent years, organic dyes with strong near-infrared (NIR) absorption, particularly in the second NIR (NIR-II) window (1000–1700 nm), have attracted increasing attention because of their promising applications in organic electronics, bioimaging, optical sensors, and telecommunication technologies.5–14 Extending π-conjugation by fusing aromatic rings edge-to-edge is an effective strategy to push the absorption of the resulting polycyclic aromatics into the NIR region.15–19 For example, the absorption maxima (λmaxs) of [n]acenes are redshifted to 852 nm for heptacene by increasing the number of phenyl rings.20 Fused BODIPY with 8 repeating units exhibits strong NIR absorption with an λmax at 955 nm and a ϵmax of 4.46 × 105 L mol−1 cm−1 (Figure 1a).21 However, π-extended polycyclic aromatics with strong absorption in the NIR-II region are rarely reported. Extended rylene diimides,22,23 fused perylenes,24 and porphyrin tapes25 are three examples reported by Müllen, Wu, and Osuka, respectively (Figure 1a). They show λmaxs of 1007, 1750, and 2857 nm, respectively, when 8 naphthalene units, 4 perylene units, or 14 porphyrin units are fused together. On the whole, the design and synthesis of polycyclic aromatics with well-defined structure as NIR-II dyes are quite challenging because (1) the conjugation saturation behavior of the conjugated system arising through the effective conjugation length, (2) the synthetic complexity of ultra-large π-conjugation polycyclic aromatics, and (3) the poor solubility of the highly elongated polycyclic aromatics.6,15,16,18,26 Figure 1 | (a) Structures, absorption properties and optical bandgaps of the representative NIR organic dyes based on polycyclic aromatics. (b) Schematic illustration of the design strategy for nThIIDs in this work. Download figure Download PowerPoint Recently, we reported the fused isoindigos (nIIDs) with 1–6 isoindigo (IID) units (Figure 1a). With the increase of IID numbers, the λmax of the ribbon is redshifted to 967 nm for 6IID, along with an enhanced ϵmax.27 Importantly, an alkyl chain can be attached to each nitrogen atom, endowing nIIDs with good solubility. Compared with IID, ThIID, in which the outer phenyl rings of IID are replaced by thiophene units, has better planarity and lower aromaticity, encouraging the quinoidal resonance structure.28–31 These features of ThIID promote achieving a low bandgap of the conjugated molecule. In this report, our stepwise syntheses resulted in a series of soluble and stable fused ThIID ribbons (nThIIDs, n = 1–6, n represents the number of ThIID), as shown in Figure 1b. The termini of the nThIIDs were capped with phenyl rings to ensure stability. The photophysical and electrochemical properties of the resulting nThIIDs were characterized in detail. All nThIIDs stabilize both multiple cations and anions. Their optical bandgaps decrease upon increasing the number of the ThIID unit. The 4ThIID, 5ThIID, and 6ThIID all show intense NIR-II absorption with λmaxs of 1038, 1149, and 1252 nm, respectively, representing a new class of NIR-II organic dyes. Results and Discussion The synthesis of nThIIDs is outlined in Scheme 1. 3,6-Bis(N-Boc)thieno[3,2-b]thiophene ( 1) was prepared according to previous reports.32 The synthesis of benzothieno[3,2-b]pyrrol-2-one ( 6), benzothieno[3,2-b]pyrrole-2,3-dione ( 7), and thieno[3,2-b]thiophene bis-oxindole ( 9) is depicted in the Supporting Information. p-Toluenesulfonic acid catalyzed aldol condensation of 6 and 7 gave 1ThIID in a yield of 57%. Thieno[3,2-b]thiophene bis-isatin ( 4), the key intermediate for the construction of the nThIID ribbons (n ≥ 2), was previously synthesized from 3 in a very low yield (2%).32 We developed a modified route to 4 that enabled the subsequent synthesis of nThIIDs. Compound 1 was converted to 3 via an alkylation–deprotection reaction sequence in a total yield of 79%. Subsequently, compound 4 was obtained in an acceptable yield of 50% from 3 in the presence of oxalyl chloride and triethylamine (Et3N) with CH2Cl2 as the solvent. The careful control of the ratio of oxalyl chloride and Et3N is essential in this step. Et3N acts as a Lewis base to remove the in situ generated HCl and thereby facilitates the acylation reaction. However, the complete neutralization of HCl is unwanted because neutralization can shut down the ring-closing reaction. Subsequently, the condensation of 4 and 6 in a ratio of 1∶1 afforded 2ThIID and 8 in yields of 16% and 49%, respectively. A reductive dimerization of 8 in the presence of hexaethylphosphorous triamide (PNEt2)3 furnished 3ThIID (49%), and the aldol condensation of 8 and 9 gave 4ThIID (63%). Compound 5, which is the key intermediate for the synthesis of 5ThIID and 6ThIID, was synthesized by the dimerization of 4 in a yield of 29%. The condensation of 5 and 6 in a ratio of 1∶1 generated 10, which was then dimerized to afford 5ThIID in a yield of 58%. 6ThIID was obtained in a yield of 61% by the condensation of 9 and 10. Scheme 1 | Synthetic routes to nThIIDs. Download figure Download PowerPoint The structures of nThIIDs were verified by 1H NMR spectroscopy, high-resolution matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) and elemental analysis. 1ThIID and 2ThIID showed well-resolved 1H NMR spectra in CDCl3 at room temperature, and the peak integrations were consistence with their structures ( Supporting Information Figures S1 and S2). In contrast, 3ThIID showed broad peaks and 4ThIID–6ThIID displayed no signals in the aromatic region at room temperature ( Supporting Information Figures S3–S6). The same phenomena were observed when the spectra were measured in deuterated o-dichlorobenzene (o-DCB) at 110 °C ( Supporting Information Figures S3–S6), excluding the reason of aggregation. We ascribed this observation to the open-shell ground state character of the molecules (vide infra). As shown in Figures 2a–2d, 3ThIID–6ThIID all show the expected molecular ion peaks, and the isotopic distributions are in good agreement with the calculated spectra. The purity of 3ThIID–6ThIID was also confirmed by high-performance liquid chromatography ( Supporting Information Figures S7–S10). nThIIDs are soluble in common organic solvents such as CH2Cl2, CHCl3, and o-DCB. They all have good stability as indicated by a photo-stability test. For example, the absorption spectrum of 6ThIID in CCl4 remained almost unchanged under ambient air and light conditions after 15 days ( Supporting Information Figure S11). Figure 2 | High-resolution MALDI-TOF mass spectra of (a) 3ThIID, (b) 4ThIID, (c) 5ThIID, and (d) 6ThIID. exptl, experimental; calcd, calculated. Download figure Download PowerPoint Figure 3a shows the UV–vis–NIR absorption spectra of nThIIDs in CCl4. The related data are summarized in Supporting Information Table S1. All nThIIDs show a weak high energy band and a strong low energy band, which are designated as bands I and III. 4ThIID, 5ThIID, and 6ThIID also display an additional band (band II) between the bands I and III. Increasing the number of ThIID leads to a gradual red shift (λmax from 435 nm to 621 nm) of the band I. The λmax values of band III are 644, 824, 929, 1038, 1149, and 1252 nm for 1ThIID, 2ThIID, 3ThIID, 4ThIID, 5ThIID, and 6ThIID, respectively. Clearly, the λmax of the band III shows an obvious red shift, >600 nm, from 1ThIID to 6ThIID. The band II becomes pronounced from 4ThIID to 6ThIID. In comparison to our previously reported 6IID, the band III of 6ThIID had a bathochromic shift of 285 nm. As the π-conjugation extended, the ϵmax of nThIIDs progressively increased, except for 6ThIID, which showed a smaller ϵmax relative to 5ThIID. The decreased ϵmax of 6ThIID may be due to its large diradical character.33,34 Nevertheless, 6ThIID also displayed a ϵmax value as large as 3.21 × 105 L mol−1 cm−1. Of particular note, 6ThIID showed a high NIR-II absorptivity across the region of 1000–1450 nm with the ϵ > 1.0 × 105 L mol−1 cm−1. This is highly desirable for NIR-II bioimaging and photodetector applications.11,35 An obvious kink point was observed in the plot of 1/λmax against the reciprocal of ThIID numbers (1/N) (Figure 3b), consistent with the emergence of diradical character from 3ThIID. From the solution to film state, nThIIDs showed a red-shifted absorption spectrum, and the Δλmax of band III is decreased significantly with increases in the number of ThIID units ( Supporting Information Figure S12 and Table S1). The Δλmax for 1ThIID and 2ThIID exceeds 100 nm, while only a 28 nm red shift is observed for 6ThIID. Figure 3 | (a) UV–vis–NIR absorption spectra of nThIIDs in CCl4. (b) Plot of the 1/λmax against the reciprocal of ThIID numbers (1/N). (c) Cyclic voltammograms of nThIIDs in CH2Cl2 with a scan rate of 100 mV s−1. (d) Transition energy alignments. (e) NTO analyses of band II for 6ThIID. SCE, a saturated calomel electrode. Download figure Download PowerPoint To gain insight into the electronic structures of nThIIDs, density functional theory (DFT) and time-dependent DFT calculations (TD-DFT) were conducted (see Supporting Information for more computational details). All nThIIDs display a planar geometry ( Supporting Information Figure S13), and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) delocalize over the entire conjugated backbones ( Supporting Information Figures S14–S18). A closed-shell ground state was found for 1ThIID and 2ThIID, while 3ThIID, 4ThIID, 5ThIID, and 6ThIID showed open-shell ground state character, with the diradical character (y0) values of 0.001, 0.098, 0.285, and 0.481, respectively. The open-shell ground state character of nThIID was also confirmed by electron paramagnetic resonance (EPR) spectra in CH2Cl2 ( Supporting Information Figure S19). Almost no signals were observed for 3ThIID, consistent with its very small y0. 4ThIID, 5ThIID, and 6ThIID all exhibited evident EPR signals at g = 2.0043, and the spin concentration was determined to be 10.0% for 4ThIID, 15.3% for 5ThIID, and 18.9% for 6ThIID using 2,2-diphenyl-1-picrylhydrazyl as standard,36 indicating the enhanced open-shell character along with the increase of the conjugation length. The obvious open-shell ground state character for 4ThIID, 5ThIID, and 6ThIID are likely derived from their small optical bandgap,24,37 which could increase the admixing of the frontier molecular orbitals in the ground state. A similar phenomenon has been observed in other low bandgap molecules.24 In addition, the high tendency of the ThIID unit to form a quinoidal structure may also contribute to the appearance of open-shell character. Variable-temperature EPR (VT-EPR) of 5ThIID and 6ThIID in the solid state was recorded to estimate their singlet–triplet energy gap (ΔES–T). By fitting the VT-EPR data with the Bleaney–Bowers equation, the ΔES–T was estimated as −2.50 kcal mol−1 for 5ThIID and −1.95 kcal mol−1 for 6ThIID ( Supporting Information Figure S20). Figure 3d shows the energy levels and allowed electronic transitions of band II and band III for nThIIDs from TD-DFT calculations. The corresponding oscillator strengths (f) and calculated absorption spectra are displayed in Supporting Information Table S1 and Figure S21; the calculated results for band I are also shown in the SI ( Supporting Information Table S1). The band III of 1ThIID and 2ThIID is mainly assigned to the ground state (GS)→first excited state (ES1) transition, while that of 3ThIID, 4ThIID, 5ThIID, and 6ThIID corresponds to the GS→ES3 transition (Figure 3d). The natural transition orbitals (NTO) analyses of the corresponding excited state indicate that the band III is mainly assigned to the local excitation (LE) of the whole molecular skeleton ( Supporting Information Figures S22–S26). The band II corresponds to the transitions of GS→ES6 (4ThIID) and GS→ES7 (5ThIID and 6ThIID) (Figure 3d), which are the hybridization of LE and charge transfer (CT) transitions, as revealed by the NTO analyses. The proportion of CT components gradually increases with the enhancement of the diradical characters, which are 72%, 76%, and 80% (the sum of α and β spin orbitals) for 4ThIID, 5ThIID, and 6ThIID, respectively (Figure 3e and Supporting Information Figures S27 and S28). The redox properties of the nThIIDs were studied in CH2Cl2 by cyclic voltammetry (Figure 3c). All nThIIDs exhibited reversible or quasi-reversible redox waves both in the negative and positive potential regions. The number of reversible reduction and oxidation waves increased from 1ThIID to 6ThIID, meaning that long nThIIDs can stabilize both multiple cationic and anionic states. With the π-conjugation extension, the potential differences between the adjacent redox waves decreased from 1ThIID to 5ThIID and became hard to distinguish for 6ThIID ( Supporting Information Figure S29). This phenomenon is often observed in long conjugated oligomers.38 From the onsets of the first reduction and oxidation potentials, the HOMO and LUMO energy levels of 1ThIID, 2ThIID, 3ThIID, 4ThIID, 5ThIID, and 6ThIID are determined to be −5.23/−3.51, −4.86/−3.73, −4.67/−3.86, −4.56/−.3.99, −4.53/−4.03, and −4.53/−4.05 eV, respectively. Clearly, the reduction of the LUMO and elevation of the HOMO both contribute to the narrowing of the bandgap. All compounds also show quasi-reversible or reversible redox behaviors in the film state, as shown in Supporting Information Figure S30. Like other IID-containing conjugated molecules,39–41nThIIDs are also non-emissive, indicating an efficient nonradiative transition from excited states to ground states. Then photothermal conversion properties of 4ThIID, 5ThIID, and 6ThIID were investigated in o-DCB (4 μmol L−1) under laser radiation at λ = 1064 nm (0.4 W cm−2). As shown in Figure 4a, the solution temperatures increased rapidly from 21.2 °C to 41.8 °C for 4ThIID, to 38.0 °C for 5ThIID, and to 38.4 °C for 6ThIID. As a control, the pure solvent showed almost no temperature change under the same condition. Photothermal conversion efficiencies (η) of 4ThIID, 5ThIID, and 6ThIID were calculated to be 58.7%, 59.6%, and 63.0%, respectively (Figure 4b and Supporting Information Figure S31), according to the reported method.42 The photothermal conversion properties of 4ThIID–6ThIID nanoparticles were also investigated at 1064 nm ( Supporting Information Figure S32), and the η values for 4ThIID, 5ThIID, and 6ThIID nanoparticles were 46.6%, 45.9%, and 47.7%, respectively. Figure 4 | (a) The temperature variations of 4ThIID, 5ThIID, and 6ThIID solutions in o-DCB (4 μmol L−1) and pure solvent under the laser irradiation at λ = 1064 nm (0.40 W cm−2). (b) Linear fitting of time versus −ln θ obtained from the cooling period of 6ThIID solution in o-DCB. θ is defined as the ratio of ΔT to ΔTmax. Download figure Download PowerPoint Conclusion A series of soluble and stable fused ThIID ribbons were synthesized using aldol condensation and reductive dimerization as the key reactions. The synthetic approach enables the preparation of ThIID ribbons with well-defined lengths. With increasing ThIID number, the main absorption bands of the ribbons are rapidly redshifted to the NIR-II region along with ϵmax above 105 L mol−1 cm−1. All the ribbons can undergo multi-electron electrochemical oxidation and reduction. 4ThIID, 5ThIID, and 6ThIID also show promising photothermal conversion properties under 1064 nm laser irradiation. This work offers a new polycyclic aromatic system with strong NIR-II absorption that may have potential applications in NIR-II bioimaging, photodetection and so on. Supporting Information Supporting Information is available and includes synthesis and characterization of new compounds ( Supporting Information Schemes S1 and S2, Figures S1–S10, and Figures S33–S46), additional DFT calculation data ( Supporting Information Figures S13–S18, Figures S21–S28, and Tables S1 and S2), and photothermal conversion data ( Supporting Information Figures S31 and S32). Conflict of Interest There is no conflict of interest to report. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 22005211 and 52121002). Prof. Yonghao Zheng is acknowledged for his help on the analysis of VT-EPR results. References 1. Würthner F.; Saha-Moeller C. R.; Fimmel B.; Ogi S.; Leowanawat P.; Schmidt D.Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials.Chem. Rev.2016, 116, 962–1052. Google Scholar 2. Liang M.; Chen J.Arylamine Organic Dyes for Dye-Sensitized Solar Cells.Chem. Soc. 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