Modification of [5]Helicene with Dimesitylboryl: One Way To Enhance

Letter Cite This: Org. Lett. 2018, 20, 7590−7593

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Modification of [5]Helicene with Dimesitylboryl: One Way To Enhance the Fluorescence Efficiency Zheng-Hua Zhao, Meng-Yuan Zhang, Di-Hong Liu, and Cui-Hua Zhao* School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China

Org. Lett. 2018.20:7590-7593. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/19/18. For personal use only.

S Supporting Information *

ABSTRACT: The efficient synthetic route was disclosed to prepare structurally asymmetric [5]helicenes, which are substituted with either BMes2 (7B-HC) or both BMes2 and NMe2 (8B5NMe2-HC, 7B5NMe2-HC). Compared with the parent [5]helicene, these compounds show greatly enhanced fluorescence. In addition, they still retain fairly strong fluorescence in the solid state. Moreover, the complexation of 8B5NMe2-HC and 7B5NMe2-HC with fluoride can induce significant blue shift in fluorescence and the formed complexes are also highly fluorescent. significant changes in fluorescence, which might provide possibility to achieve anion-stimulated CPL properties.13 Therefore, we here disclose the synthesis, photophysical properties, as well as responses to fluoride of a new family of [5]helicenes, which contain either only one BMes2 (7B-HC) or both one BMes2 and one N,N-dimethyl amino (NMe2) groups (8B5NMe2-HC, 7B5NMe2-HC) (see Figure 1). Compared with

H

elicenes are ortho-fused polycyclic aromatic compounds,1 which have found broad applications in a variety of fields, such as nonlinear optical materials,2 molecular optoelectronic devices,3 and ligands for asymmetric organic synthesis.4 More recently, they have attracted increasing attention as the chiroptical dyes, because of their enhanced circular dichroism (CD) and circularly polarized luminescence (CPL) properties than the common chiral organic molecules.5 However, the helicenes generally suffer from the low fluorescence quantum yield (ΦF). For example, the ΦF of [5]helicene was reported to be only 0.04.6 Therefore, considerable efforts have been devoted to the design of highly emissive helicene derivatives.7,8 One plausible explanation for the poor fluorescence of helicenes is the low transition probability of the first excited state (S1), which is symmetry-forbidden.9 To improve the ΦF values of helicenes, one efficient strategy is to introduce electronaccepting substituents (e.g., maleimide, cyano groups) on the peripheral positions and further introduce electron-donating substituents (e.g., methoxy groups) to construct the “push−pull” molecules to add remarkable changes to the electronic structures of helicenes.8 However, the reported “push−pull” helicene derivatives are generally structurally symmetric and only contain moderate electron-donating methoxy groups, which is not effective to tune the emission wavelengths. Encouraged by the recent rapid progress of triarylboranes,10 we were interested in the synthesis and properties of structurally asymmetric helicenes modified with dimesitylboryl (BMes2). The BMes2 is a unique bulky electron-accepting group. In addition to the strong electron-accepting effect, BMes2 also possesses great steric bulk effect arising from two bulky Mes substituents. The steric effect of BMes2 is helpful to prevent the intermolecular interactions in the solid state and thus to achieve solid-state fluorescence,11 which is a highly pursued property for the optoelectronic devices.12 The further introduction of strong electron-donating amino was expected to induce remarkable red shift of fluorescence. Moreover, the Lewis acidity of boron center enables its complexation with Lewis bases (e.g., F−, CN−) and thus © 2018 American Chemical Society

Figure 1. Chemical Structure of the triarylborane-based [5]helicenes.

the parent [5]helicene (HC), these triarylborane-based [5]helicenes show greatly enhanced fluorescence with widely tunable emission wavelengths. In addition, the complexation of 8B5NMe2-HC and 7B5NMe2-HC with fluoride can induce significant blue shift in fluorescence and the formed complexes are also highly fluorescent. Starting from 7-bromo[5]helicene (7Br-HC), which was obtained through a benzylic(dibromo)methane coupling reaction,14 the BMes2 group was easily introduced through the borylation reaction, which was accomplished by the lithiation with n-BuLi and subsequent quenching with dimesitylfluoroborane (see Scheme S1 in the Supporting Information). To further introduce a NMe2 group, we first employed the electrophilic mononitration of 7Br-HC and subsequent transformation of nitro to NMe2 via reduction with Sn and methylation with MeI Received: October 18, 2018 Published: November 16, 2018 7590

DOI: 10.1021/acs.orglett.8b03329 Org. Lett. 2018, 20, 7590−7593

Letter

Organic Letters (Scheme S2 in the Supporting Information). It was found that the nitration of 7Br-HC proceeded at two different positions, producing two regioisomers, 8-bromo-5-nitro[5]helicene (8Br5NO2-HC) and 7-bromo-5-nitro[5]helicene (7Br5NO2HC). The nitration positions were confirmed by the X-ray singlecrystal structures of corresponding NMe2-subsituted compounds, 8Br5NMe2-HC and 7Br5NMe2-HC (see Figure S1 in the Supporting Information), respectively. The borylation of 7Br5NMe2-HC occurred smoothly to afford 7-dimesitylboryl-5(N,N-dimethylamino)[5]helicene (7B5NMe2-HC). However, the borylation of 8Br5NMe2-HC to prepare 8-dimesitylboryl-5(N,N-dimethylamino)[5]helicene (8B5NMe2-HC) only gave the lithiated intermediate. Interestingly, we found that the NMe2 group could be directly introduced into 7B-HC using the similar method as described above, successfully producing two [5]helicenes modified with both BMes2 and NMe2 (see Scheme 1). The nitration of 7B-HC

Figure 2. (a) UV-vis absorption and (b) fluorescence spectra of triarylborane-based [5]helicenes and the parent HC in cyclohexane (λex = 350 nm).

HC exhibits the easily visible longest absorption at 413 nm (ε = 1100 M−1 cm−1) and intense blue fluorescence at 436 nm with a ΦF of 0.65. It was noted that the absorption and emission are significantly red-shifted with remarkably enhanced intensity, compared with the parent [5]helicene (HC), which only shows a very weak absorption at 393 nm (ε = 200 M−1 cm−1) and weak fluorescence at 403 nm (ΦF = 0.04).6 With the introduction of the NMe2 group, the absorption is further red-shifted to 430 nm for 8B5NMe2-HC and 438 nm for 7B5NMe2-HC. Along with the absorption shift, the bathochromism of fluorescence is much more significant (Δλ = 73 nm for 8B5NMe2-HC, and 118 nm for 7B5NMe2-HC, relative to 7B-HC). Notably, both compounds are fluorescent with fairly good efficiency (ΦF = 0.36 for 8B5NMe2-HC, and 0.23 for 7B5NMe2-HC). In addition, the quantum yields of these triarylborane-based [5]helicenes are independent of the excitation wavelengths. Judging from the rate constants of radiative and nonradiative decays, which are derived from fluorescence lifetimes and ΦF, the radiative decay processes of the triarylborane-based [5]helicenes become much faster than for the parent HC. Thus, the current structure modification of helicene is effective to improve the S0 → S1 and S1 → S0 transition probabilities and thus to obtain emissive helicenes with tunable fluorescence from blue at 436 nm for 7B-HC to green at 509 nm for 8B5NMe2-HC and yellow at 554 nm for 7B5NMe2-HC. In the solid state, all the triarylborane-based [5]helicenes still retain fairly strong fluorescence. Especially for 7B5NMe2-HC, the ΦF value of its powder is up to 0.38, which is even much higher than that observed in cyclohexane solution. The large steric of BMes2 and the nonplanar helical structure of helicene skeleton may play important roles to suppress the intermolecular interactions thus to achieve the intense solid-state fluorescence.16 To further shed a light on the effect of the current structural modification on the electronic structure and thus photophysical properties, theoretical calculations were performed for the three triarylborane-based [5]helicenes and the parent HC. As shown in Figure 3, the S0 → S1 transition of HC mainly consists of the mixed HOMO → LUMO (58%) and HOMO−1 → LUMO+1 (40%) transitions. All the involved molecular orbitals (MO) can spread over the entire [5]helicene framework. The MOs for each transition exhibit the same symmetry. The HOMO and LUMO are C2-symmetric and the HOMO−1 and LUMO+1 are C2antisymmetric. As a result, the S0 → S1 transition is symmetryforbidden with a very small oscillator strength (f = 0.0011). From HC to 7B-HC, the introduction of BMes2 induces great changes of LUMO while other frontier orbitals are almost not affected. In addition, the symmetry of LUMO was completely interrupted. The LUMO is mainly distributed over one-half part of [5]helicene skeleton with remarkable contribution from BMes2. Accompanied with the great change of electronic distribution, the energy level of its LUMO is significantly

Scheme 1. Synthesis of Triarylborane-Based [5]Helicenes

also occurs at two positions. The two regioisomerically nitrated products are inseparable through flash column chromatography, because of their very close polarity. Fortunately, the two regioisomers can be separated upon NO2 reduction to NH2. One final product of this synthetic route shows completely same NMR spectra as 7B5NMe2-HC prepared, as described in Scheme S2. Another final product was identified as 8B5NMe2HC, in which the NMe2 position was determined through 1 H−1H COSY, 1H−1H NOESY spectra of the corresponding precursor 8B5NH 2−HC (Figure S2 in the Supporting Information). The four triplets in 1H NMR preclude the location of amino at the two terminal benzene rings. In 1H−1H NOESY spectrum, the crosspeak that corresponds to the coupling of two singlet signals indicates the spatial proximity of the corresponding two protons, which are adjacent to boryl and amino, respectively. The current synthetic routes to prepare the triarylborane-based [5]helicenes can avoid utilization of photocyclodehydrogenation,15 which has been the typical synthetic method for helicenes but suffers from the low reaction yield and high dilution and limits the structure diversity. Moreover, it was proved that BMes2 has the similar directing effect as bromo on the electrophilic substitution reaction, which has never been uncovered, despite the vast utilization of BMes2 for triarylboranebased functional materials. The ultraviolet-visible light (UV-vis) absorption and fluorescence spectra of triarylborane-based [5]helicene derivatives are shown in Figure 2, and the related data are summarized in Table 1. In cyclohexane, the BMes2-subsituted [5]helicene 7B7591

DOI: 10.1021/acs.orglett.8b03329 Org. Lett. 2018, 20, 7590−7593

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Organic Letters Table 1. Photophysical Properties Data of Triarylborane-Based [5]Helicenes and the Parent HC 7B-HC 8B5NMe2-HC 7B5NMe2-HC HC

λabsa,b (nm)

εa (M−1 cm−1)

413 430 438 393

1.1 × 10 2.8 × 103 2.3 × 103 2.0 × 102 3

λema (nm) 436 509 554 403

ΦFa,c 0.65 0.36 0.23 0.04e

λem(powder) (nm) 446 534 569 −

ΦF(powder)d 0.14 0.19 0.38 −

τ (ns)

kr (s−1)

knr (s−1)

9.1 7.0 8.8 26e

7.1 × 10 5.1 × 107 2.6 × 107 1.5 × 106 7

3.8 × 107 9.1 × 107 8.8 × 107 3.6 × 107

In degassed cyclohexane at room temperature. bOnly the longest absorption maximum wavelengths are given. cCalculated using fluorescein as a standard. dAbsolute quantum yields were determined using an integrating sphere. eReported values in 1,4-dioxane.6 a

Figure 3. Kohn−Sham energy levels, pictorial drawing of frontier orbitals, and transitions of triarylborane-based [5]helicenes and the parent HC at the S0 geometries, calculated at PBE0/6-31G(d).

lowered (ΔE = 0.52 eV). More notably, the S0 → S1 transition of 7B-HC is dominated by the HOMO → LUMO transition. The oscillator strength increases by a factor of ∼30 and the calculated absorption wavelength shifts bathochromically by 36 nm. With further introduction of NMe2 in 8B5NMe2-HC and 7B5NMe2HC, the HOMO energy levels are elevated remarkably with the extra electronic contribution from NMe2, while the LUMOs remain essentially unchanged. Comparatively, the 7B5NMe2HC has a higher HOMO energy level than 8B5NMe2-HC. Therefore, it is reasonable that the longest absorption wavelength changes in the order of 7B-HC < 8B5NMe2-HC < 7B5NMe2HC. In the time-dependent density functional theory (TD-DFT) optimized S1 state (Figure S9), the electronic distributions are very similar to those of S0 state. The energy levels of the frontier orbitals and the calculated transition energies change in the same trend as in S0 state. It is notable that the difference of HOMO energy level between 8B5NMe2-HC and 7B5NMe2-HC increases to 0.18 eV from 0.09 eV in S0 state. The higher HOMO energy level of 7B5NMe2-HC and thus longer absorption and emission wavelength might be ascribed to that the relative position BMes2 and NMe2 in 7B5NMe2-HC is more suitable to stabilize the CT state judging from the corresponding resonance structure (see Figure S7 in the Supporting Information), which would lead to more efficient conjugation between NMe2 and [5]helicene skeleton, as evidenced by the higher coplanarity between NC3 with the attached benzene ring and the shorter N−C bond length, particularly in the S1 state (see Figure S8 and Table S2 in the Supporting Information). Again, the oscillator strengths of S1 → S0 deactivation of triarylboranebased [5]helicenes are more than 30 times higher than the parent HC. Hence, the theoretical calculations clearly support that the current structure modification is effective for the tuning of the electronic structure of [5]helicenes, providing an efficient way not only to improve the transition probabilities of the S1 but also

to tune the HOMO−LUMO energy gaps and thus emission wavelengths. The theoretical calculations also suggest a charge transfer characteristics for the S1 state of 8B5NMe2-HC or 7B5NMe2-HC, which is consistent with their great fluorescence solvatochromism (see Figures S4 and S5 in the Supporting Information). Another thing that attracts our interest is the fluorescence changes of 8B5NMe2-HC and 7B5NMe2-HC under the stimulation of fluoride anions considering their CT characteristics (see Figure S6 in the Supporting Information). In THF solution, 8B5NMe2-HC and 7B5NMe2-HC emit orange fluorescence at 553 nm and red fluorescence at 601 nm, respectively. Although the quantum yields are lower than those in cyclohexane due to the solvatochromism effect (ΦF = 0.21 for 8B5NMe2-HC, 0.09 for 7B5NMe2-HC), they are still much higher than the parent HC. In the presence of excess fluoride, the fluorescence is significantly blue-shifted (Δλ = 92 nm for 8B5NMe2-HC, 121 nm for 7B5NMe2-HC). Again, the emission of 7B5NMe2-HC is longer than 8B5NMe2-HC (Δλ = 19 nm). Notably, the fluorescence intensity turns much stronger with significantly enhanced efficiency. The quantum yields are up to 0.42 for 8B5NMe2-HC and 0.33 for 7B5NMe2-HC. The remarkable fluorescence shift induced by addition of fluoride ions and fairly good quantum yields, irrespective of the regioisomer upon addition of fluoride might suggest the possibility to achieve fluoride-responsive CPL for chiral helicenes that are modified with both amino and boryl groups. In summary, we have built up the efficient synthetic route to prepare a new family of triarylborane-based [5]helicenes, which are modified with either BMes2 (7B-HC) or both BMes2 and NMe2 (8B5NMe2-HC and 7B5NMe2-HC) with asymmetric structure. The introduction of BMes2 into the parent [5]helicene is effective to enhance the transition probability of S1 and shift the emission to longer wavelength. The emission wavelengths of 7592

DOI: 10.1021/acs.orglett.8b03329 Org. Lett. 2018, 20, 7590−7593

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Organic Letters

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8B5NMe2-HC and 7B5NMe2-HC are further red-shifted by incorporation of NMe2. In addition, the NMe2 position exhibits great influence on the emission. Thus, it is possible to obtain emissive [5]helicene derivatives with widely tunable emission wavelengths. Moreover, addition of fluoride to 8B5NMe2-HC and 7B5NMe2-HC induces significant blue shift of fluorescence, providing the possibility to achieve fluoride-responsive CPL. Although the current [5]helicene derivatives still suffer from racemization, our current results will undoubtedly lay the ground basis for the design of emissive chiral helicenes. The preparation of chiral triarylborane-based [n]helicenes with larger helicene skeleton is underway in our group.

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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.8b03329. Synthesis, photophysical properties, calculation details, and NMR spectra of triarylborane-based [5]helicenes (PDF) Accession Codes

CCDC 1866712, 1866719, and 1866757 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]. ORCID

Cui-Hua Zhao: 0000-0002-4077-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21572120 and 21272141). REFERENCES

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DOI: 10.1021/acs.orglett.8b03329 Org. Lett. 2018, 20, 7590−7593