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Coumarin-based Boron Complexes with Aggregation-induced Emission Panpan Zhang, Weimin Liu, Guangle Niu, Hongyan Xiao, Mengqi Wang, Jiechao Ge, Jiasheng Wu, Hongyan Zhang, Yanqing Li, and Pengfei Wang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02852 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017
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Coumarin-based Boron Complexes with Aggregation-induced Emission Panpan Zhang,†,‡ Weimin Liu,*,‡,§ Guangle Niu,‡ Hongyan Xiao, ‡ Mengqi Wang,‡ Jiechao Ge,‡,§ Jiasheng Wu,‡ Hongyan Zhang,‡ Yanqing Li,*,† and Pengfei Wang‡,§
†Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China
‡Key Laboratory of Photochemical Conversion and Optoelectronic Materials and CityU-CAS Joint Laboratory of Functional Materials and Devices, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China
§School of Future technology, University of Chinese Academy of Sciences, Beijing, 100049, P.R. China
*
[email protected];
[email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)
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Abstract Two coumarin-based boron complexes (HBN and MBN) with aggregation-induced emission were designed and synthesized. The photophysical properties of the complexes were investigated in different solvents and in the solid state. Results showed that the inhibited C=N isomerization by N,O-chelated BF2 caused the significant enhancement of fluorescence in THF. Particularly, the complexes displayed red-shifted emissions (>60 nm) in mixed solvents of CH3CN and water because of the aggregation-induced charge transfer enhancement. In solid state, the bright red emission appeared at 650 nm (620 nm), with Stokes shift of 170 nm. Cell imaging experiments indicated that the complexes have good membrane permeability and can be used as lysosome trackers. Introduction Organic fluorescent dyes have been widely used in chemosensing,1-3 fluorescent labeling,4,5 biological imaging,6,7 and organic optoelectronic devices.8 Several fluorescent dyes have been developed based on naphthalene, anthracene, pyrene, xanthene, coumarin, cyanine, oxazine, BODIPY, and their derivatives. However, these conventional fluorescent dyes occasionally cannot produce satisfactory results in practical application.9 Therefore, novel fluorescent dyes with high molar extinction coefficients, high fluorescence quantum yields (QYs), good photostability, and longer absorption and emission, are needed. Recently, rigid hybridization of two classical fluorophores has shown great potential, by which the natural advantages of each flurophore are retained while achieving high fluorescence QY, good photostability, and other desirable properties.10,11 For example, coumarin dyes have been widely investigated and applied in diverse fields because of their high QYs, biocompatibility, and large Stokes shifts.12,13 However, most coumarin derivatives exhibit absorption and emission only in the UV–visible region. Therefore, several novel fluorescent dyes based on coumarin with green to near-infrared emission have been developed by
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rigidifying with other fluorophores, such as naphthalene,14,15 pyrazolo[3,4-b]pyridine,16 xanthene,17-21 BODIPY,22 and hemicyanine15,23-25. Although the improvement of their rigid molecular structure is a facile approach to enhance the brightness of fluorophores, most fluorescent dyes with a rigid planar and large π-conjugated structure often suffer from aggregation-caused quenching (ACQ), which displays weak or quenched fluorescence at high concentrations or in the solid state.26 Another more successful approach to create emissive molecules is aggregation-induced emission (AIE),27 which exerts an effect opposite to that of ACQ. Tang et al. reported that organic molecules with AIE show non-emission in good solvents as individual molecules but become highly luminescent in poor solvents or solid state as aggregates.28,29 Boron complexes of π-conjugated chelates are known AIE-active molecules.30-32 Chujo et al. reported a series of N,O-chelated boron complexes that exhibit clear AIE characteristics.33-36 Liu et al. developed several N,N-chelated boron complexes with AIE, as well as tunable substituent-dependent emission profiles and large Stokes shift in solution.37,38 This phenomenon compelled us to explore novel AIE-active materials based on boron complexes. In our previous work, we found that the location of C=N isomerization is crucial in preventing nonradioactive decay, thus dramatically increasing the fluorescence QYs.39 Consequently, in this work we developed a new design strategy for novel boron complexes combined with interesting properties that are associated with inhibited C=N isomerization and unique AIE phenomenon. The designed boron complexes with rigidified coumarin derivatives were expected to show the following advantages: (i) large Stokes shift by the introduction of coumarin moieties; (ii) an significant enhancement in fluorescence QY because of the covalently-bridged C=N structure by N,O-chelated BF2 (Scheme 1); and (iii) aggregation-induced redshifts of emissions (>60 nm) because of charge transfer (CT) enhancement. To illustrate the importance of CT, two coumarin based boron complexes (HBN and MBN), which have different substituents in the ACS Paragon Plus Environment
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amino group, were designed and synthesized. Finally, we demonstrated their application for bioimaging in HeLa cells. Scheme 1. Illustration of inhibited C=N isomerization by N,O-chelated boron complexes
Results and Discussion Design and Synthesis. While studying the photophysical properties of conformationally restricted compounds, we found that certain compounds with an unbridged C=N structure are often non-fluorescent because of the non-radiative decay of C=N isomerization. By contrast, the suppression of C=N isomerization in the excited states by the covalently-bridged C=N structure can dramatically increase their fluorescence.40-42 Thus, C=N isomerization has been widely used as a signaling mechanism for various metal ions since 2007.43 Similarly, C=N isomerization may also be inhibited by the complexation of B(III) to design bright fluorescent dyes (Scheme 1). Based on this hypothesis, the coumarin-derived Schiff base (3) was designed with a hydroxyl group at position 4 of coumarin (Scheme 2). In addition, a dialkylamino group and a nitro group were simultaneously grafted at both ends of ligand 3 to obtain long wavelength absorption. The rigid boron-bridged π-framework may intensify the emission and enhance CT. As shown in Scheme 2, ligands 3a and 3b were easily prepared by condensation of the aldehyde 2a and 2b, respectively, with 4-nitroaniline in ethanol in the presence of p-toluenesulfonic acid (pTsOH). Preparation of the aldehydes 2a or 2b starts from 3-dimethylaminophenol/8-hydroxyjulolidine in three steps according to our previous publication.18 The corresponding boron complexes HBN and MBN were synthesized when ligands 3a and 3b reacted with boron trifluoride etherate in the presence of triethylamine. The structures were further confirmed by 1HNMR, 13CNMR, and ESI-MS (Supporting Information). ACS Paragon Plus Environment
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Scheme 2. Synthetic of complexes HBN and MBN
Conditions: (a) toluene, reflux, 4 h. (b) POCl3, DMF, 60°C, reflux, 8 h. (c) 4-nitroaniline, p-toluenesulfonic acid, C2H5OH, 80°C, reflux, 3 h. (d) BF3·Et2O, NEt3, CH2Cl2, 0°C, 0.5 h. Optical properties of HBN and MBN in Tetrahydrofuran (THF). First, the photophysics of Schiff base ligands and boron complexes were investigated in THF. As shown in Figure 1a, the ligand 3a shows the longest absorption band, with the peak at 443 nm in THF. HBN displayed significantly redshifted absorption bands as compared with the ligand. This phenomenon was attributed to the extended π-conjugation skeleton produced by the embedded boron atom.44 As expected, the ligand 3a is weakly emissive in THF and its emissions were centered at 515 nm. By contrast, the boron-chelated complex HBN showed intense orange-yellow fluorescence with emission peaks at 550 nm (Figure 1b). The weaker electron-donating dimethylamine-substituted complex of MBN exhibited a 13 nm blueshift relative to HBN, with bright yellow fluorescence in THF. Similar to the observed absorption and emission profiles of HBN, the longest absorption band and emission peak of the boron complex MBN was redshifted by 36 nm and 39 nm as compared with ligand 3b (Figure 1c and 1d). Coumarin derivatives represent a unique class of fluorescent dyes with high fluorescence QYs and large Stokes shifts. In this work, the boron complexes HBN and MBN with the coumarin moiety exhibit large Stokes shifts (>70 nm) in THF. The absolute QYs of MBN and HBN as determined by the integrated ACS Paragon Plus Environment
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sphere are 0.15 and 0.49 in THF, respectively (Table 1). These data indicated that rigid hybridization of the coumarin moiety and BF2 complex retained the advantages of the coumarin moiety themselves. In addition, the Schiff base ligands 3a and 3b are non-emissive in THF; when chelated by BF2 to inhibit the C=N isomerization, their fluorescence QY can have 15-fold and 50-fold enhancement, respectively.
Figure 1. Normalized absorption (a, c) and emission spectra (b, d) of 3a/3b (black line) and HBN/MBN (red line) in THF. Table 1. Photophysical Properties of Dyes in Different Solvents. compound solvent
λab max
λem max
Stokes
ε
(nm)
(nm)
Shift
(M-1
-1
(cm )
Φf
cm-1)
3a
THF
443
515
316
47300
0.01
HBN
THF
477
550
419
53900
0.15
Acetontrile 478
565
322
57800
