Solid-State Emissive Triarylborane-Based [2.2]Paracyclophanes

Letter Cite This: Org. Lett. 2018, 20, 6868−6871

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Solid-State Emissive Triarylborane-Based [2.2]Paracyclophanes Displaying Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence Meng-Yuan Zhang,† Zhi-Yi Li,‡ Bo Lu,† Ying Wang,‡ Yu-Dao Ma,† and Cui-Hua Zhao*,† †

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, P. R. China

Org. Lett. 2018.20:6868-6871. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/02/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: New types of [2.2]paracyclophane derivatives, g-BNMe2-Cp and mBNMe2-Cp, in which electron-donating NMe2 and the electron-accepting BMes2 are introduced at the pseudo-gem and pseudo-meta positions, were designed and synthesized. The efficient through-space charge transfer enables the intense fluorescence with thermally activated delayed fluorescence characteristics. The quantum yields are up to 0.72 and 0.39 in cyclohexane. In addition, no significant fluorescence quenching was observed in the solid state with fluorescence quantum yields of powder up to 0.53 and 0.33. Moreover, the enantiomerically pure forms of g-BNMe2-Cp exhibit strong CPL signals with glum up to 4.24 × 10−3.

C

solid state owing to its steric bulk arising from the two Mes groups to get enough kinetic stability but also suppress selfquenching because of the large Stokes shift induced by an intramolecular charge-transfer (CT) transition, which has been well demonstrated by a series of solid-state emissive triarylboranes with intramolecular CT characteristics.9 Moreover, the introduction of electron donor and electron acceptor into two different benzene rings of [2.2]paracyclophane facilitates the well separation of HOMO and LUMO and thus the thermally activated delayed fluorescence (TADF),10 which is a highly pursued property in electroluminescence due to the outstanding ability of TADF emitters to harvest both triplet and singlet excitons.11 In spite of the numerous reported TADF materials, the examples of SOMs displaying both CPL and TADF properties are rather limited.12 Herein, we report the synthesis and properties of a new family of triarylboranebased [2.2]paracyclophane derivatives, g-BNMe2-Cp and mBNMe2-Cp, which contain both BMes2 and dimethylamino (NMe2) groups at the pseudo-gem and pseudo-meta positions, respectively (Figure 1). Both compounds display intense fluorescence with TADF characteristics. No significant fluorescence quenching was observed in the solid state. Moreover, the enantiomerically pure forms of g-BNMe2-Cp emit strong CPL signals. The two triarylborane-based [2.2]paracyclophanes were prepared via borylation of the corresponding bromide

ircularly polarized luminescence (CPL) is potentially useful in optical spintronics, optical data storage, 3D displays, as well as chirality sensing.1 To evaluate the CPL property, the photoluminescence quantum efficiency (Φlum) and the luminescence dissymmetry factor (glum) are two important parameters. So far, the CPL active materials with high glums are mainly based on chiral transition-metal complexes,2 organic helical polymers,3 and supramolecules.4 The CPL-active small organic molecules (CPL-SOMs) have recently attracted increasing attention owing to their great advantages, such as facile structural modification, finely tunable emission wavelength, and ease of manufacturing processes.5 However, it is generally difficult to achieve high Φlum together with large glum for CPL-SOMs. Another problem for the CPLSOMs is the significantly decreased photoluminescence efficiency in the solid state due to the typical aggregationcaused quenching effect,6 which greatly limits their practical applications. To obtain CPL organic dyes, [2.2]paracyclophane is one promising building block owing to its stable planar chirality.7 To suppress the emissive core unit from aggregation, the introduction of a Fréchet-type dendron is an efficient strategy.7e However, the preparation of dendrons requires a tedious synthetic route. In this context, we were interested in modification of the [2.2]paracyclophane skeleton with dimesitylboryl (BMes2) group, which is a unique bulky electron-accepting group.8 The introduction of BMes2 into a π-framework that contains an electron-donating amino group can not only prevent the intermolecular interactions in the © 2018 American Chemical Society

Received: September 18, 2018 Published: October 25, 2018 6868

DOI: 10.1021/acs.orglett.8b02995 Org. Lett. 2018, 20, 6868−6871

Letter

Organic Letters

Figure 1. Molecular and crystal structures of (a) g-BNMe2-Cp and (b) m-BNMe2-Cp.

Figure 2. UV−vis absorption and fluorescence spectra of g-BNMe2Cp and m-BNMe2-Cp: red solid line, g-BNMe2-Cp in toluene; red dashed line, g-BNMe2-Cp in powder form; blue solid line, m-BNMe2Cp in toluene; blue dashed line, m-BNMe2-Cp in powder form.

precursors, g-BrNMe2-Cp and m-BrNMe2-Cp (Scheme S1 and S2). The precursor g-BrNMe2-Cp was easily synthesized by dimethyl sulfate methylation of 4-amino-13-bromo[2.2]paracyclophane.13 The synthesis of m-BrNMe2-Cp was accomplished via CuI-catalyzed monoamination,14 and subsequent dimethyl sulfate methylation starting from pseudometa-dibromo[2.2]paracyclophane, m-Br2-Cp.15 The final products g-BNMe2-Cp and m-BNMe2-Cp are stable to air and water and can be purified by flash column chromatography. Their structures were fully characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and Xray crystallography. It was noted that the signals for the two omethyl groups of mesityl are very broad in either 1H NMR or 13 C NMR spectra, especially for g-BNMe2-Cp (Supporting Information), which suggests the significant steric hindrance in these two compounds. The centroid−centroid distances between two deck benzene rings of [2.2]paracyclophane are 3.08 Å for g-BNMe2-Cp and 3.01 Å for m-BNMe2-Cp (Figure 1), which allows the efficient through-space charge-transfer transitions.16 The nitrogen center is highly pyramidalized (∑C−N−C = 345.6° for g-BNMe2-Cp, 342.1° for m-BNMe2Cp), which is in contrast to the perfect planar geometry of the boron centers. A big structure difference between these two compounds is that the nitrogen center in g-BNMe2-Cp is more coplanar with the amino-bonded benzene ring than m-BNMe2Cp. The dihedral angles between NC3 plane and the NMe2bonded benzene ring are 21.8° and 36.8°, respectively. Figure 2 shows the UV−vis absorption and fluorescence spectra, and the related data are summarized in Table 1. In toluene, g-BNMe2-Cp displays a moderately intense absorption band at 374 nm (log ε = 3.59) in addition to the intense band at 319 nm. The theoretical calculations denote that the absorption at 374 nm is assignable to the excitation to the first excited state, which corresponds to an intramolecular charge transfer transition from HOMO distributed on dimethylaminophenyl to LUMO localized on dimesitylborylphenyl (Figure 3). For the pseudo-meta isomer, m-BNMe2-Cp, it is difficult to distinguish the absorption corresponding to the CT transition to the first excited state from the absorption bands of the transition to higher excited states, which is probably ascribed to a very small oscillator strength (0.0209) of the first excited state. Regarding the emission, both compounds emit strong yellowish green fluorescence with m-BNMe2-Cp exhibiting a slightly shorter maximum wavelength than g-BNMe2-Cp (Δλ = 10 nm). The quantum yields are 0.46 and 0.34 for g-BNMe2Cp and m-BNMe2-Cp, respectively. The higher fluorescence

efficiency of g-BNMe2-Cp is consistent with its higher oscillator strength for the first excited state transition. In addition, the significant fluorescence solvatochromism was observed for both compounds in spite of trivial solvent effect on absorption (Figure S1 and Table S1). The significant fluorescence solvatochromism is consistent with the intramolecular CT characteristics of the first excited state and suggests more polarized structures in the excited state than in the ground state, which is further confirmed by the calculated dipole moment changes from the ground state to the excited state (8.87 and 10.0 D for g-BNMe2-Cp and m-BNMe2-Cp, respectively). In the nonpolar cyclohexane, the quantum yields can increase to 0.72 and 0.39 for g-BNMe2-Cp and m-BNMe2Cp, respectively. It was interesting to find that the fluorescence efficiency was sensitive to oxygen for both triarylborane-based [2.2]paracyclophanes. In oxygen-bubbled toluene, the fluorescence quantum yields decrease to 0.068 and 0.049 for g-BNMe2-Cp and m-BNMe2-Cp and the normalized fluorescence spectra are identical (Figure S3). The oxygen sensitivity of the fluorescence efficiency denotes the possibility of TADF characteristics, which was further evidenced by the time-re solved fluorescence results (Figure S4). In degassed toluene solution, the biexponential decays were obtained for both compounds. In addition to the short lifetimes (17 ns for gBNMe2-Cp and 22 ns for m-BNMe2-Cp), the fitting also gave long lifetimes of 0.38 μs for g-BNMe2-Cp and 0.22 μs for mBNMe2-Cp, which confirms they emit both prompt and delayed fluorescence. In addition, both compounds exhibit small singlet−triplet energy gaps (ΔESTs), as determined from the singlet and triplet energies that were estimated from the onset of the fluorescence and phosphorescence spectra in 2methyltetrahedronfuran (2-MeTHF) at 77K (Figure S5). The ΔESTs are 0.17 and 0.12 eV for g-BNMe2-Cp and m-BNMe2Cp, respectively. The small ΔESTs further firmly support the TADF feature of the current triarylborane-based [2.2]paracyclophanes. Combining the decay times with the fluorescence quantum yields, the rates of various processes were calculated according to the reported methods.17 The rates of reverse intersystem crossing processes (krisc) were found to be 2.84 × 106 s−1 for g-BNMe2-Cp and 4.70 × 106 s−1 for mBNMe2-Cp. Such large krisc values are characteristics of an efficient reverse intersystem crossing and in favor of the TADF properties. With regard to the photophysical properties, another notable thing is that no remarkable fluorescence quenching was 6869

DOI: 10.1021/acs.orglett.8b02995 Org. Lett. 2018, 20, 6868−6871

Letter

Organic Letters Table 1. Photophysical Properties of g-BNMe2-Cp and m-BNMe2-Cp g-BNMe2-Cp m-BNMe2-Cp

λabsa,b (nm)

log εa

λema (nm)

ΦFa,c

λem(powder) (nm)

ΦF(powder)d

τpa,e (ns)

τda,e (μs)

374 327

3.59 4.13

531 521

0.46 (0.068) 0.34 (0.049)

525 487

0.53 0.33

17 (99.65%) 22 (99.66%)

0.38 (0.35%) 0.22 (0.34%)

a In degassed toluene at room temperature. bOnly the longest absorption maximum wavelengths are given. cCalculated using fluorescein as a standard. Values in parentheses are for oxygen-bubbled toluene solutions. dAbsolute quantum yields were determined using an integrating sphere. e Amplitudes of lifetimes are given in the parentheses.

Figure 4. CD (4.0 × 10−5 M) and CPL (1.0 × 10−5 M) spectra of gBNMe2-Cp in toluene. Figure 3. Kohn−Sham energy levels, pictorial drawing of HOMOs and LUMOs, and transitions of g-BNMe2-Cp and m-BNMe2-Cp, calculated at the PBE0/6-31G(d) level of theory.

dissymmetric factors of fluorescence in solution are comparable to other CPL-active [2.2]paracyclophanes.7 In summary, we have disclosed new types of [2.2]paracyclophane derivatives, g-BNMe2-Cp and m-BNMe2-Cp, which are modified with the electron-donating NMe2 and electron-withdrawing BMes2, simultaneously. These compounds can show intense fluorescence with good quantum yields and TADF characteristics. In addition, no significant fluorescence quenching was observed in the solid state, which ensures the strong fluorescence in the solid state. Moreover, the enantiomerically pure forms of g-BNMe2-Cp exhibit strong CPL signals with glum up to 4.24 × 10−3. Therefore, we here have demonstrated a very rare example and molecule design for the organic fluorophores that exhibit three highly pursued properties, TADF, intense solid state fluorescence, and CPL at the same time. Further investigation on the practical applications of g-BNMe2-Cp utilizing its intriguing properties is under way in our group.

observed for both g-BNMe2-Cp and m-BNMe2-Cp. Consequently, they are also very emissive in the solid state. The quantum yields of powders are 0.53 and 0.33, which are very close to those of toluene solution. Again, the pseudo-gemsubstituted molecule shows higher fluorescence efficiency in the solid state. The fluorescence spectra of powders are even slightly more blue-shifted than those of the toluene solutions. Compared with the recently reported TADF-emissive [2.2]paracyclophanes suffering from severe solid-state fluorescence quenching,10 our molecular design is effective to suppress fluorescence quenching and thus to achieve intense solid-state fluorescence. Encouraged by the intense fluorescence of g-BNMe2-Cp both in solution and in the solid state, we prepared its enantiomerically pure forms to explore the chiroptical properties. The enantiomers of g-BNMe2-Cp were prepared starting from enantiopure pseudo-gem-substituted bromo amines, g-BrNH2-Cp, which were obtained through optical resolution of its racemic form with (R)-(−)-10-camphorsulfonyl chloride according to the literature.18 Both enantiomers have an enantiomeric excess (ee) of >99% (Figure S6). Intense solid-state fluorescence was also observed for the enantiopure g-BNMe2-Cp (ΦF = 0.68 for Rp-isomer; 0.59 for Sp-isomer). This compound has a very high specific rotation ([α]25 = +509 for Rp-isomer; −557 for Sp-isomer in toluene). In addition, mirror-image Cotton effects with large Δε were observed in the CD spectra (Figure 4). The Rp enantiomer displays a small positive signal around 393 nm and a large positive signal around 313 nm. The dissymmetry factor of absorbance (gabs) at the longest wavelength (393 nm) was determined to be +1.44 × 10−3. More interestingly, intense mirror-image CPL signals were also observed. The Rp enantiomer exhibits positive CPL that is peaked at 529 nm. The glum at the emission maximum was determined to be +4.24 × 10−3. It was noted that glum is much higher than gabs although they have the same sign, suggesting that the great structure differences between the ground state and excited state exist. In addition, the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02995. Preparation, photophysical properties, calculation details, NMR spectra of g-BNMe2-Cp and m-BNMe2-Cp (PDF) Accession Codes

CCDC 1866269 and 1866859 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 6870

DOI: 10.1021/acs.orglett.8b02995 Org. Lett. 2018, 20, 6868−6871

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Organic Letters ORCID

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Ying Wang: 0000-0001-6638-8026 Cui-Hua Zhao: 0000-0002-4077-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We are gratefully acknowledge funding from the National Natural Science Foundation of China (Grant No. 21572120 and 21272141).

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DOI: 10.1021/acs.orglett.8b02995 Org. Lett. 2018, 20, 6868−6871