Novel Germoles and Their Ladder-Type Derivatives: Modular Synthesis, Luminescence Tuning, and Electroluminescence

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE7 Dec 2022Novel Germoles and Their Ladder-Type Derivatives: Modular Synthesis, Luminescence Tuning, and Electroluminescence Xiaoshuang Xiang, Zhikuan Zhou, Xing Wu, Zhigang Ni, Lizhi Gai, Xuqiong Xiao, Liwen Xu, Zujin Zhao, Hua Lu and Zijian Guo Xiaoshuang Xiang College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Zhikuan Zhou College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Xing Wu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Zhigang Ni College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Lizhi Gai College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Xuqiong Xiao College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Liwen Xu College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 , Zujin Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Hua Lu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Material Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, and Key Laboratory of Organosilicon Material Technology of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121 and Zijian Guo State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 https://doi.org/10.31635/ccschem.022.202101625 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Considerable effort has been devoted to the design of silicon-containing π-conjugated materials for application in optoelectronic devices and fluorescent bioimaging. However, the synthesis and spectroscopic tuning of germanium (Ge)-conjugated systems are challenging because of the paucity of synthetically useful methods. Herein, we report a simple and effective method of lithium naphthalenide-induced intramolecular cyclization to construct architecturally diverse Ge-containing π-conjugated molecules, including benzogermoles and their ladder-type derivatives, with high yields of up to 92%. The photophysical properties of these molecules can be finely controlled by the introduction of electron-donating or -withdrawing substituents, and intense luminescence ranging from deep-blue to red regions in the solid state was observed. A quantitative model based on the Hammett constant against the luminescence wavelength showed a good linear correlation, allowing us to reliably predict and design luminescent materials with specific properties for applications. Notably, Ge-bridged ladder-type derivatives exhibited high photoluminescence and efficient deep-blue electroluminescence with good color purity. We believe this study will open a new avenue to organogermanium chemistry and offers greater flexibility for electronic structural tuning. Download figure Download PowerPoint Introduction Silicon-containing π-conjugated systems such as siloles have a wide range of applications in the fields of electronics, photonics, and (bio)imaging.1–4 In contrast, in the same family with Si, germanium (Ge)-based π-conjugated motifs have been far less explored although they exhibit properties that either equal or surpass those of Si analogues.5,6 Traditionally, Ge-conjugated compounds are synthesized by the reaction of dilithiated derivatives with dihalogermanes, which severely hinder the development of this important class of molecules.7–13 In recent years, the development of transition-metal-catalyzed Ge-H and Ge-C activation and diisobutylaluminum-promoted Ge-C formation has emerged as a powerful tool but with some limitations,14–19 such as the relatively poor substrate scope, multistep synthesis, and difficulty in achieving functionalization (Figures 1a and 1b). Furthermore, these methods are not suitable for the design of unsymmetrically substituted Ge-conjugated compounds bearing different substituents. Currently, no reliable strategy is yet known to synthesize Ge-based conjugated molecules with structural diversity to tailor their electronic properties. In addition, although carbon- and silicon-bridged p-phenylenevinylene has been developed in 200920 and 200321 which has found use in molecular wires and solid-state lasers. Ge-bridged p-phenylenevinylene, which may show unique photophysical properties, it has never been reported. Therefore, the exploration of an efficient synthesis method that enables the productive construction of Ge-conjugated molecules would have far-reaching implications. More importantly, photoelectronic properties of these systems are rarely investigated, particularly their structure–property relationships, which would facilitate the future rational design for a wide range of different applications. Herein, we report a facile and universal synthetic strategy for the construction of diverse functional benzogermoles (Figure 1c). Importantly, based on this new strategy, we also realized the facile synthesis of ladder-type germoles for the first time. In addition, we demonstrate that the change of electronic properties of the substituents at the 2,3-positions can precisely modulate the luminescence of benzogermole from the deep-blue to the red region, and the Ge,Ge-bridged ladder-type molecule can afford efficient deep-blue electroluminescence (EL) with good color purity. Figure 1 | Design and development of Ge-containing π-conjugated system. (a) Heteroatom-containing π-conjugated system and the advantage of Ge element. (b) The main methods to synthesize germoles. (c) This work: a simple method to construct architecturally diverse germoles. Download figure Download PowerPoint Experimental Section Typical procedure for Zn(II)-mediated coupling with electrophiles A mixture of granular lithium (28 mg, 4.0 mmol) and naphthalene (512 mg, 4.0 mmol) in tetrahydrofuran (THF) (4 mL) was stirred at room temperature under argon for 5 h. Half of the amount of LiNaph/THF was added to a solution of (o-silylphenyl)acetylene 1 (282 mg, 1 mmol) in THF (2 mL) at 0 °C and then stirred for 5 min. A solution of zinc chloride in THF (1.2 mL, 1.2 mmol, 1.0 M) was added at −78 °C, and the resulting mixture was stirred for 0.5 h. After the cooling bath was removed, the reaction was stirred at room temperature for another 0.5 h. To this mixture were added Pd(PPh3)4 (57 mg, 0.05 mmol, 5 mol %), PPh3 (52 mg, 0.2 mmol, 20 mol %), and iodobenzene (68 μL, 0.61 mmol) successively. The resulting mixture was stirred at 60 °C for 24 h. Finally, the reaction was quenched with several drops of saturated aqueous NH4Cl solution. Further purification was performed on silica-gel columns (petroleum ether) to give targeted 2,3-aromatic subtituted benzogermole. X-ray structural determination and crystallographic data The X-ray diffraction data were collected on a Bruker SMART APEX CCD diffractometer (Bruker, Hangzhou, China) with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) using the ω − 2θ scan mode. The structure was solved by direct methods and refined on F2 by full-matrix least-squares methods using SHELX2000. X-ray crystallographic data for carbon-bridged p-phenylenevinylene ( CPV1), 3h, 3i, 3k, 3q, 5b, 5e, 9a, 12, and 15. CCDC number 2121243, 2051144–2051152 (Zip). Computational methods The geometry optimization in the ground state was optimized by the B3LYP/6-31G(d,p) based on the single-crystal structures using the Gaussian 09 program.22 Time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-31G(d,p) level of theory were performed to simulate the UV–vis spectra of the compounds. Anisotropy of the induced current density (ACID) plots were calculated by Herges's method.23 Nucleus-independent chemical shift (NICS) values were calculated using the standard gauge invariant atomic orbital (GIAO)24 method at the level of B3LYP/6-311G(d,p). All NICS values have been calculated at 1 Å above the center (NICS(1)) of each ring. Results and Discussion Design and synthesis Lithium naphthalenide (LiNaph)-induced intramolecular cyclization involves a lithio-intermediate, which can react with electrophilic groups to generate diverse structures, and thus control the electronic structure. This method is indeed useful for the synthesis of silole starting from bis(phenylethynyl)silane with 82% yield; however, it was not successfully applied to the synthesis of germole, and the reason remains unclear.25 The benzosilole was prepared in only 17% yield starting from (o-silylphenyl)acetylene using LiNaph-induced cyclization.26 In our preliminary experiments, 1 was treated with the corresponding Si-analogue under the same reaction condition at room temperature, but only trace product 3a was observed. Since the bond dissociation energy of Ge-H was lower than that of Si-H (Ge-H: 68.8 kcal/mol vs Si-H: 76.0 kcal/mol) and the calculated activation energy of transition state (TS) for Ge was lower than that of Si ( Supporting Information Figure S1), lowering the reaction temperature may be a feasible option. After extensive investigation, we found from the 1H NMR results that the treatment of 1 with LiNaph in THF at 0 °C for 5 min produced the desired benzogermole ( 3a) smoothly upon quenching with water and 3b upon D2O quenching ( Supporting Information Figure S2), indicating the formation of the 3-lithioindene intermediate (Scheme 1). Using lithiogermaindene as the synthetic scaffold, various 2,3-substituted benzogermoles were obtained in good-to-high yields, as shown in Scheme 1. Borylation of 2 with i-PrOBpin gave 3c in 83% isolated yield. The reaction of 3-lithioindene with dimethylchlorosilane gave the 3-silyl-substituted 3d in 75% yield. Negishi cross-coupling reactions were implemented after transmetalation with zinc to obtain a broad series of 3-aryl substituted benzogermoles, including those bearing phenyl rings with electron-donating ( 3g– 3j) or -withdrawing groups ( 3k and 3l), heteroaryl groups, such as pyridyl ( 3n and 3o), quinolyl ( 3p), and thienyl ( 3q) groups in 58–92% yield. Scheme 1 | Synthesis and chemical structures of benzogermoles (reaction conditions for 3f–3q are the same as those for 3e). Download figure Download PowerPoint Generally, the molecules with D–A structures possess intramolecular charge transfer (ICT) character, which leads to facile modulation of emission wavelengths. Therefore, compounds carrying para-OMe-phenyl ( 5a) and para-NMe2-phenyl groups ( 5b– 5f) at the 2-position of benzogermole were synthesized with different electron-withdrawing or -donating groups at the 3-position. Furthermore, the pyridyl group could be methylated to form compounds 6 and 7 to further increase the ICT character (Schemes 2a and 2b). Scheme 2 | (a and b) Synthesis and chemical structures of benzogermoles. Download figure Download PowerPoint More importantly, this approach can be readily expanded to the synthesis of digerma-s-indacenes and Ge-bridged p-phenylenevinylene (Scheme 3). Diacetylene 8 was treated with LiNaph and then quenched with water or reacted with iodobenzene via Negishi coupling, which afforded 3,7-diprotonated indacenes 9a and 9b. Precursors 11 and 14 were prepared in two steps from tetrabromide 10 and 13 via lithiation with n-BuLi followed by treatment with Me2GeCl2 and subsequent reduction with LiAlH4. Upon treatment of 11 and 14 with LiNaph followed by an iodine workup,a the ladder-type Ge-bridged p-phenylenevinylenes (GePV1) 12 and (GePV2) 15 were isolated in 87% and 37% yields, respectively. All the compounds except 7 were air-stable and able to keep on the shelf under natural conditions without decomposition for at least 1 year. Thermogravimetric analysis (TGA) of the selected 3f and 12 was measured, and their decomposition temperature (Td) at 5% weight loss was 180 and 209 °C, respectively ( Supporting Information Figure S3). Scheme 3 | Synthesis and chemical structures of benzogermoles and the ladder-type molecules. (a) n-BuLi, Me2GeCl2; (b) LiAlH4. Download figure Download PowerPoint Photophysical properties The photophysical properties of the representative benzogermoles and GePVs are summarized in Figure 2 and Table 1. The details for all germaindenes are found in Supporting Information Table S1 and Figures S4–S7. All germaindenes 3a– 3q and 5a– 5f displayed broad absorption bands in the near-UV region with high molar extinction coefficients (εmax > 104 M cm−1) in THF. The photoluminescence (PL) wavelengths of these molecules could be finely regulated by changing the electronic effect of the substituents. For example, 3a had major emission peaks at 377 and 396 nm in THF and in powder, respectively, which can be readily controlled and shifted to the green region at 532 and 513 nm ( 5e) and further redshifted to 556 and 658 ( 7) nm, respectively, by tuning the electronic properties of the substituents (Figures 2a–2c). To gain insight into the electronic effect of the substituents, the plot of the luminescence wavelength in THF against the absolute value of the Hammett constant at the 2-position minus the 3-position showed a good linear correlation (R2 = 0.95) (Figure 2d). The linearity of the correlation indicates that the fine-tuning of luminescence can be achieved by selecting suitable substituents. This model provides a theoretical frame that can help in further developing emitter materials for various applications. Figure 2 | Photophysical properties of selected benzogermole and GePVs. (a) PL spectra of 2-substituted benenzogermoles 3a, 5a–5b, 9a, and GePVs in powder form. (b) PL spectra of 2,3-substituted benzogermoles 3g–3i and 5c–5f and 7 in powder form. (c) Photographs of the powders under 365 nm light irradiation. (d) Luminescence wavelength versus the absolute value of Hammett constant at 2-position minus the 3-position, (e) PL spectra of 5c in THF/water mixtures with different water fractions (fw). Concentration: 10 mM. Inset: Fluorescence photos of the corresponding luminogens in THF/water mixtures (fw = 0 and 95 vol %) under 365 nm light irradiation. Download figure Download PowerPoint Table 1 | Photophysical Properties of Selected Benzogermoles and the Ladder-Type Molecules in THF and in the Powder Form In THF In Powder Form λabs (nm) λem (nm) SSa (cm−1) ΦFb τfc (ns) λem (nm) FWHMemd (nm) Φ Fb τfc (ns) kre (108)/s knrf (108)/s 3a 328 377 3962 0.18 0.79 396 35 0.18 1.10 1.64 7.45 3e 297 421 9917 0.02 —g 419 64 0.17 1.42 1.20 5.85 3g 325 398 5643 0.003 —g 395 57 0.16 0.76 2.11 11.05 3i 302 465 11600 0.01 — 437 62 0.29 1.00 2.90 7.10 3n 316 432 8497 0.006 — 461 76 0.53 5.90 0.90 0.80 5a 338 394 4205 0.47 2.30 414 59 0.25 2.04 1.23 3.68 5b 366 445 4850 0.76 3.74 458 49 0.40 2.83 1.41 2.12 5c 366 489 6872 0.02 — 464 58 0.73 3.78 1.93 0.71 5d 370 499 6987 0.03 — 477 75 0.60 3.90 1.54 1.03 5e 376 532 7799 0.01 — 513 66 0.40 7.20 0.56 0.83 5f 371 513 7461 0.005 — 485 67 0.61 4.40 1.39 0.89 9a 389,408 430,454 2451 0.53 1.85 477 49 0.38 1.36 2.79 4.56 12 350 405 3880 0.73 5.52 425 44 0.62 4.21 1.47 0.90 15 411,435 453,480 913 0.50 2.51 521 48 0.35 1.50 2.33 4.33 aSS, Stokes shift. bFluorescence quantum yield determined using the absolute method. cFluorescence lifetime. dFull width half maximum (FWHM). eRadiative decay rate kr = ΦF/τ. fNonradiative decay rate knr = (1-ΦF)/τ. gToo low to be detected. 2-Substituted benzogermoles 3a, 5a, and 5b had moderate to high PL quantum yields (ΦF) of 0.18 ( 3a), 0.47( 5a), and 0.76 ( 5b) in THF, and 0.18 ( 3a), 0.25( 5a), and 0.40 ( 5b) in powder, respectively. Digerma-s-indacene 9a had an intense blue luminescence with ΦF of 0.53 and 0.38 in THF and in powder, respectively. GePVs 12 and 15 showed blue and green luminescence with ΦF of 0.73 and 0.50 in THF and 0.62 and 0.35 in powder, respectively. Both types of the ladder-type molecules 12 and 15 could serve as good purity emitter materials because of their narrow full width at half maximum (FWHM) values of 44 and 48 nm (Table 1). The excited-state decay profiles can be described with a single-exponential fit in the nanosecond range. Generally, 2,3-aryl-substituted benzogermoles exhibit very low ΦF values in THF due to the rotation of aryl substituents in solution, the nonradiative decay of the excited state is predominant, leading to the faint luminescence. However, emission in the solid state is effective. For example, benzogermoles 5c– 5f, which contained the electron-donating groups at the para-position of the 2-substituent, exhibited high luminescence (ΦF = 0.40–0.73). This is a typical aggregation-induced emission (AIE) process, molecules are nonluminescent in a dilute solution because their excited species are nonradiatively annihilated by the intramolecular rotation of the phenyl rotors at 2,3-positions. The intramolecular rotation is restricted in the solid state due to the physical constraint involved.27–29 Notably, the PL wavelength of benzogermoles in the deep-blue and blue regions was precisely modulated for every 10 nm (Table 1), which highlights its excellent luminescence control. To validate the AIE characteristics, the PL behaviors of 5c– 5f in the aggregated state were investigated, as shown in Figure 2e and Supporting Information Figures S8 and S9 and Table S2. Taking 5c as an example, and its PL intensity increases slowly when the water fraction (fw) in THF/water is low, the PL intensity is boosted with the increase of fw beyond 80 vol % (Figure 2e). The ΦF value is enhanced 26-fold from pure THF solution (0.02) to an aqueous medium (0.52), indicating its AIE nature. Crystal analysis To understand the effects of the structure and packing modes on the emission properties, X-ray crystal structures of the six benzogermoles were obtained, and the crystal packings were analyzed. The structures revealed that the Ge-core is highly planar, and the 2-substituent exhibits better π-conjugation with the core with a smaller torsion angle than the 3-substituents ( Supporting Information Figure S10). The Hirshfeld surface analysis and fingerprint plots30 of 5e exhibited two significant intermolecular CH···π interactions with distances of ∼2.8 Å located at the 2,3-substituents (Figures 3d–3f), which was conducive to the restriction of the intramolecular motion (RIM) of both substituents and led to the efficient enhancement of the luminescence. In the case of 3q, no red spots were on the Hirshfeld which the of interactions (Figures This that the intermolecular interactions are and the packing luminescence in the solid state (ΦF = In addition, the effect of the further the luminescence. The Hirshfeld surface analysis of 3k, 3h, and 3i showed with the luminescence ( Supporting Information Figure results further the for the AIE using the Hirshfeld The ΦF of the ( was that of which could be due to the facile formation of the intermolecular and in 3n than in because of the of the group at the 3-position in Figure 3 | Crystal packing and Hirshfeld surface (a) structure. (b) Hirshfeld (b) fingerprint plots of (d) structure. (e) Hirshfeld fingerprint plots of Crystal structure and representative structural data of 15 (GePV2) with at are for of 15 (GePV2) in Hirshfeld surface with the range of to are shown red on the Download figure Download PowerPoint We also the of 12 and 15 (Figures 3g–3i and Supporting Information Figures and The of the in 15 were and the bond angle was and the bond of Å was to the of The conjugated core was from (Figure and the packing structure the very with an of The CH···π was also at a of Å the In addition, Å) were the two groups, by the red in the Hirshfeld and the in the of its fingerprint 12 a packing in which the the is the of the carbon-bridged is Å ( Supporting Information Figure and theoretical calculations Furthermore, we the properties by carrying in The was determined to be on the substituents (Table 2 and Supporting Information Figure and Table For example, 3a displayed at the benzogermoles such as 3i, and 9a exhibited a for 3i and for and the for 3i and for 12 and 15 displayed with for 12 and 0.40 for and the for 12 and for Table 2 | Properties and of the Selected Benzogermoles and GePVs 3a 3i 5f 9a 12 0.79 15 vs in with M determined from the of the absorption spectra in level determined from the level determined from the and calculated at the B3LYP/6-31G(d,p) level of The molecular orbital and molecular orbital energy obtained by were with those determined from the calculated The of the and showed all the core and 2-substituent, with some density at the ( Supporting Information Figures and Therefore, both 2,3-substituents to the Ge were the major to the electronic was on the Ge which the lowering of the with the the were with each the less for Ge with Si, as shown in Figure the electronic of the at the is with the of Si > Ge > This of effective results from the less efficient orbital the orbital and the orbital of the Ge which has a quantum number and distances Ge and the ring. with ladder-type which showed significant luminescence quenching from solution to solid Ge-bridged 12 its luminescence in the solid The can be by the crystal packing the the in was Å) than the Å) in ( Supporting Information Figure This indicates that the atomic and bond the intermolecular NICS calculations (Figure and calculations (Figure were The an value of is the corresponding rings in and are and To the of Ge in the excited the coupling the S1 state and excited were calculated at the level ( Supporting Information Figure For the molecules, very were indicating which was further by (ΦF = of in solution. In the case of 3f and its the value in the of benzogermole 3f is than those of the indicating that benzogermole Therefore, the effect and its in the excited of Ge on the molecular and the further Figure | (a) energy level and the of Si, and Ge-bridged ladder-type molecules using the B3LYP/6-31G(d,p) The are shown at an value of The red the electronic at (b) values calculated at the (c) Download figure Download PowerPoint luminescent materials are for devices However, Ge-containing are developed for application in 12 deep-blue luminescence in the solid a preliminary study on its was in using 12 as an A was with the of as the and were as and respectively. The of the was as low as of which the

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