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Article Cite This: ACS Omega 2018, 3, 8992−9002
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Nonconjugated Fluorescent Molecular Cages of Trinuclear Fluoroborate Complexes with Salicylaldehyde-Based Schiff Base Ligands Xiaohong Zhang,†,§ Jun Shi,‡,§ Jintong Song,† Man Wang,† Xuemei Xu,† Lang Qu,† Xiangge Zhou,† and Haifeng Xiang*,† †
College of Chemistry, Sichuan University, Chengdu 610041, China Department of Cardiovascular Surgery, West China Hospital, Sichuan University, 37 Guoxue Xiang Street, Chengdu 610041, Sichuan, China
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ABSTRACT: Fluorescent organic materials are commonly πconjugated planar molecules. In the present work, however, we report a novel class of nonconjugated fluorescent molecular cages of trinuclear fluoroborate complexes (nine samples) with salicylaldehyde-based Schiff base ligands. Owing to the stress from lone pair electrons of N atom in the triethylamine bridge, these B(III) complexes exhibit unusual enantiomers with a tripodlike side-single-opening structure. They emit blue, green, and red emission with large Stokes shifts (up to 159 nm) and high fluorescence quantum yields in both solution (up to 0.24) and solid state (up to 0.25), which might contribute to their strong intramolecular hydrogen bonds and weak intermolecular and intramolecular π−π interactions. Combining their advantages of nonconjugation and biocompatibility, these flexible complexes have potential applications in living cell imaging and anion hosts. We have examined the inherent relationships between their chemical structures and emission properties and afforded a new stage for the design of nonconjugated fluorescent fluoroborate complexes.
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INTRODUCTION The development of innovative organic luminescent materials1 recently has attracted intense interest due to their wide range of applications in optical probes,2−8 organic light-emitting diodes,9−13 light-emitting electrochemical cells,14,15 and cell imaging.16−19 In general, organic luminescent materials with πconjugated planar and rigid structures are extremely susceptible to the problems of emission “aggregation-caused quenching” (ACQ)20,21 and synthesis difficulty and solubility. On the other hand, nonconjugated materials with better solubility, higher flexibility, lower cost, lower cytotoxicity, better biocompatibility, and naturally occurring22 are usually known as nonemissive materials. Until recently, there are only a limited number of fluorescent pure organic nonconjugated materials23−26 that emit ultraviolet (UV) and blue aggregationinduced emission (AIE).23,27,28 Our research group also just reported that some free ligands of nonconjugated bi- and trisalicylaldehyde-based Schiff bases (SSBs) show fine-tuned red−green−blue (RGB) AIE properties.29−31 Among the numerous classes of fluorescent materials, boron(III) complexes have perhaps the highest potential and have spectacularly risen in popularity because of their very promising advantages of relatively high molar absorption coefficients (ε), fine-tuned emission bands (λem) with high fluorescence quantum yields (Φ), and photostability. This © 2018 American Chemical Society
family of dyes includes not only well-known boron dipyrromethene32−35 but also its derivatives based on N^O,36−39 N^C,40,41 and O^O42−44 chelates. Two major drawbacks of these B(III) complexes are their problems of ACQ and self-absorption. The ACQ problem of B(III) complexes is mostly caused by intermolecular π−π stacking interactions between π-conjugated planar molecules, and the self-absorption problem results from very narrow Stokes shifts between absorption and emission spectra. Our research works continually focus on the synthesis, optical properties, and sensing applications of SSBs29−31,45−47 and their Zn(II), Cu(II), Al(III), and Pt(II) complexes.48−51 In the literature, there are many examples of fluorescent B(III) complexes with N^O-bidentate SSB ligands (Figure 1a,b).52−61 However, most of them are mononuclear or binuclear B(III) complexes (Φ up to 0.90 in solution)58 with π-conjugated planar molecular structures and often suffer the ACQ problem. Reported πconjugated fluorescent trinuclear B(III) complexes52,58 and nonconjugated fluorescent binuclear B(III) complexes60,61 are still rare. It is interesting that free ligands of π-conjugated bi-/ tri-SSBs58 and nonconjugated bi-SSBs29 are usually AIE-active Received: July 1, 2018 Accepted: July 31, 2018 Published: August 13, 2018 8992
DOI: 10.1021/acsomega.8b01504 ACS Omega 2018, 3, 8992−9002
ACS Omega
Article
computation, which predicts one absorption peak at 336 nm. The lower energy absorption is contributed by S0 → S1 (353 nm, oscillator strength f OSC = 0.0386, the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO), 74%; HOMO − 1 → LUMO, 18%; HOMO → LUMO + 1, 6%) and S0 → S2 (347 nm, f OSC = 0.0350, HOMO − 1 → LUMO, 50%; HOMO → LUMO + 1, 27%; HOMO − 1 → LUMO + 1, 11%; HOMO → LUMO, 5%). The energy-level and orbital isosurface diagrams of B (Figure 2) reveal that the HOMO and HOMO − 1 of B are composed primarily of not only two π-conjugated units of iminomethylphenol but also the nonconjugated triethylamine bridge, which might contribute to the electron-donating nature of triethylamine. On the contrary, its LUMO and LUMO + 1 are mainly made up of the π-functions of iminomethylphenol units rather than the triethylamine bridge. Therefore, the lower energy absorption can mainly be assigned to the intraligand π → π* transition involving molecular orbitals essentially localized on the iminomethylphenol units and intramolecular charge transfer (ICT, n → π*) transition from electron-donating trimethylamine (lone pair electrons of N atom) to the πconjugated iminomethylphenol units. The difluoroborate units have little contribution to the lower energy absorption. For free ligand of tri-SSB,31 however, the lower energy absorption was mostly contributed by n → π* transition from electrondonating trimethylamine to the π-conjugated iminomethylphenol units. These computational data are consistent with the fact that B (λabs = 346 nm in MeCN) has a red-shifted absorption spectrum compared with the tri-SSB ligand (λabs = 314 nm in MeCN). The absorption spectra of all of the B(III) complexes in pure organic solvent of CH2Cl2 (2.0 × 10−5 mol dm−3) are given in Figure 3. The introduction of steric hindrance −Me (3-Me, λabs = 355 nm in MeCN) or −t-butyl (3-t-Bu, λabs = 353 nm) (Figure 3 and Table 1) substituents to the simplest B (λabs = 347 nm) has little effect on absorption spectra. However, except the presence of electron-accepting −F (3-F-B, λabs = 353 nm), the presence of electron-accepting −Cl (3,5-Cl-B, λabs = 369 nm), electron-donating −OMe (3-OMe-B, λabs = 374 nm; 5-OMe-B, λabs = 388 nm), −NEt2 (4-NEt2-B, λabs = 375 and 347 nm), or π-extended system (Naph-B, λabs = 370 and 332 nm) induces obvious red shifts in absorption spectra. It is obvious that the UV absorption behavior of 4-NEt2-B solution is unique, which might be contributed by the strong electron-donating property of NEt2 groups. As our previous report,31 free ligands of tri-SSBs showed very weak blue fluorescence in the dilute organic solvents because their intramolecular rotations (IRs) of C−N and C−C single bonds in the central triethylamine bridge provide a possible way to nonradiatively annihilate their excited states and result in the absence of fluorescence consequently. However, it is unexpected that most of the B(III) complexes exhibit strong fluorescence in both solid and pure organic solvent of toluene, CH2Cl2, or acetone (Table 1 and Figures 4 and 5). As an example, B emits strong blue fluorescence (λem = 455−461 nm, Φ = 0.061−0.083) with large Stokes shifts (up to 110 nm) in low polar solvent of toluene, CH2Cl2, or acetone. The absence of fluorescence in the solution of free triSSB ligand contributed to the nature of n → π* transition and IRs;31 on the other hand, the transition of B originate from not only n → π* but also π → π* transition (Figure 2), which would help achieve strong fluorescence in both solution and solid state (see the later discussion). In addition, the B(III)
Figure 1. Reported fluorescent B(III) complexes (a) with πconjugated and nonconjugated N^O-bidentate SSB ligands. Trinuclear fluoroborate complexes (b) in this work.
materials, but their B(III) complexes are ACQ-active materials (Figure 1a).58,61 The free ligands of nonconjugated trimethylamine-linking tri-SSBs are AIE-active materials as well (Figure 1b),31 and we wonder if their trinuclear B(III) complexes are AIE- or ACQ-active materials. Herein, we demonstrate a novel class of trinuclear fluoroborate complexes with nonconjugated trimethylamine-linking tri-SSB ligands (nine samples, Figure 1b) that exhibit unusual enantiomers with a tripodlike sidesingle-opening structure and emit strong RGB fluorescence in both solution and solid state with large Stokes shifts.
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RESULTS AND DISCUSSION Synthesis and Characterization. The free tri-SSB ligands were reasonably easy to be synthesized by the condensation of primary triethylamine with 3 equiv of salicylaldehyde precursor in ethanol under refluxing condition.31 The B(III) complexes were prepared by using BF3−OEt2 and tri-SSB ligands in toluene at 60 °C with excellent yields (70−88%).61 Most of the B(III) complexes have a bad solubility in petroleum ether, hexane, and water but a good solubility in CH3CN, CH2Cl2, dimethylformamide, and dimethyl sulfoxide (DMSO). All of the complexes are stable under air. For most of the complexes, good-quality single crystals could be obtained by the method of slow solvent diffusion/evaporation (CH2Cl2/hexane). The B(III) complexes are not new,62 but we report our systematic studies of the inherent relationships between their chemical structures and fluorescent properties and the potential application in ion sensing. Photophysical Properties. Table 1 lists the UV−visible absorption and fluorescence data of all synthesized B(III) complexes at room temperature. To gain insight into the nature of the excited states and transitions, density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations were also carried out for B with the Gaussian 09 program package (B3LYP 6-31G(d,p)). The computational absorption spectrum is virtually identical to the experimental absorption spectrum (Figure 2). The lower energy absorption band of B (λabs = 347 nm in CH2Cl2) is reproduced well by the 8993
DOI: 10.1021/acsomega.8b01504 ACS Omega 2018, 3, 8992−9002
ACS Omega
Article
Table 1. Photophysical Data of the B(III) Complexes at Room Temperature medium B
3-F-B
3-Me-B
3-OMe-B
3-t-Bu-B
3,5-Cl-B
4-NEt2-B
5-OMe-B
Naph-B
toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN DMSO solid toluene CH2Cl2 ethanol acetone MeCN
λabs/nm (ε/dm3 mol−1cm−1) 280(1.78 266(4.98 263(3.75 343(1.25 262(4.89 270(4.26
× × × × × ×
104); 104); 104); 104) 104); 104);
346(1.28 × 104) 347(1.61 × 104) 347(1.23 × 104)
283(2.15 265(4.74 269(5.08 355(1.05 265(5.22 275(5.36
× × × × × ×
104); 104); 104); 104) 104); 104);
353(9.85 × 103) 353(1.08 × 104) 349(1.14 × 104)
283(3.11 266(6.38 271(4.87 358(1.26 274(4.69 277(6.02
× × × × × ×
104); 104); 104); 104) 104); 104);
358(1.33 × 104) 355(1.46 × 104) 355(1.25 × 104)
291(1.25 277(5.34 277(7.70 358(2.35 278(7.66 287(7.43
× × × × × ×
104); 104); 104); 104) 104); 104);
379(6.00 × 103) 374(1.74 × 104) 368(1.45 × 104)
279(5.73 270(4.86 269(3.35 353(1.26 270(3.93 279(4.75
× × × × × ×
104); 104); 104); 104) 104); 104);
353(1.29 × 104) 353(1.43 × 104) 356(1.05 × 104)
289(1.25 270(3.15 270(3.66 373(1.10 270(3.62 275(3.46
× × × × × ×
104); 104); 104); 104) 104); 104);
373(1.11 × 104) 369(1.05 × 104) 365(1.26 × 104)
280(7.80 273(2.00 269(4.05 345(5.05 270(6.78 277(1.04
× × × × × ×
103); 104); 103); 104) 103); 104);
346(8.02 × 104) 347(7.50 × 104) 347(4.57 × 104)
281(2.29 269(3.50 271(3.00 385(1.18 273(3.05 275(2.98
× × × × × ×
104); 104); 104); 104) 104); 104);
391(1.23 × 104) 388(1.38 × 104) 383(1.18 × 104)
330(1.94 332(2.70 328(2.68 340(1.91 328(2.65
× × × × ×
104); 104); 104); 104); 104);
372(1.63 370(2.21 370(2.24 373(2.14 371(2.39
346(1.37 × 104) 345(1.72 × 104)
344(1.34 × 104) 345(1.59 × 104)
353(1.32 × 104) 353(1.76 × 104)
366(1.58 × 104) 366(2.26 × 104)
352(1.28 × 104) 352(1.96 × 104)
363(1.40 × 104) 360(1.89 × 104)
350(7.57 × 104) 355(8.01 × 104)
384(1.29 × 104) 380(1.57 × 104) × × × × ×
104) 104) 104) 104) 104)
8994
λem/nm
Stokes shift/nm
Φ
455 459 461 461 457 458 489 476 470 475 475 467 479 518 475 477 479 474 479 476 513 512 517 519 519 525 521 532 464 470 469 464 477 472 461 492 488 489 490 490 489 513 461 482 494 464 494 494 499 520 530 522 521 530 527 579 456 455 459 455 450
109 112 114 118 111 113
0.061 0.064 0.032 0.083 0.012 0.001 0.071 0.037 0.034 0.014 0.035 0.003 0.002 0.053 0.10 0.074 0.058 0.077 0.033 0.006 0.064 0.015 0.013 0.015 0.014 0.008 0.007 0.030 0.15 0.13 0.099 0.14 0.052 0.008 0.16 0.033 0.03 0.005 0.031 0.001
