Article pubs.acs.org/joc
Oxygen-Bridged Diphenylnaphthylamine as a Scaffold for Full-Color Circularly Polarized Luminescent Materials Hidetaka Nishimura,† Kazuo Tanaka,‡ Yasuhiro Morisaki,§ Yoshiki Chujo,‡ Atsushi Wakamiya,*,†,∥ and Yasujiro Murata*,† †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan § Department of Applied Chemistry for Environment, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ∥ Precursory Research for Embryonic Science and Technology (PRESTO) Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡
S Supporting Information *
ABSTRACT: An oxygen-bridged diphenylnaphthylamine with a helical shape was designed and synthesized as a key scaffold for circularly polarized luminescent (CPL) materials. The introduction of electron-withdrawing groups, such as formyl and 2,2-dicyanovinyl substituents at the naphthyl moiety in this skeleton effectively decreases the LUMO level and thus allows a tuning of the band gap. The prepared model compounds exhibit intense CPL signals with a dissymmetry factor (g value) of 10−3 both in CH2Cl2 solutions and in the solid states. The emission colors of these derivatives are influenced both by the substituents as well as by solvent effects, covering the whole visible region from blue to deep red.
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INTRODUCTION Circularly polarized luminescence (CPL) refers to the differential emission of right- and left-handed circularly polarized light by chiral molecular systems,1 and has attracted substantial attention on account of numerous potential applications, such as 3D optical displays,2 CPL lasers,3 chiral recognition,4 and asymmetric photosynthesis.5 Several types of CPL-emitting organic molecules have been developed so far, and these can be classified into small organic molecules,6 polymers,7 transition metal complexes,8 and lanthanide complexes.4c,9 Among these, small organic molecules have the advantage that their photophysical properties can be potentially tuned by the structural design. Nevertheless, examples of materials, whose CPL wavelength is tunable, are still limited.6b,i,7f,i We have recently reported the synthesis of a partially oxygenbridged triphenylamine as a key skeleton for hole-transporting materials, in which three benzene rings are constrained in a quasiplanar structure by two oxygen-tethers.10 In addition to their hole-transporting ability, these compounds display intense fluorescence, thus promising potential as emissive materials. DFT calculations at the B3LYP/6-31G(d) level of theory suggested an inversion energy of 9.1 kcal/mol for the flipping of the nontethered phenyl rings in this skeleton, indicating that the two phenyl rings can easily flip in the gas phase or in solution at ambient temperature. Upon expanding the skeleton and thus increasing the steric hindrance, the ring flipping can be © 2017 American Chemical Society
suppressed to allow isolation of the corresponding helical isomers. For example, DFT calculations suggested that the replacement of one phenyl ring with a naphthyl ring in this oxygen-bridged triphenylamine skeleton increases the inversion barrier to 29.3 kcal/mol, indicating the possibility of optical resolution for this skeleton (Figure 1a). Considering the intense fluorescence of oxygen-bridged triarylamines, this skeleton can be used as a scaffold for CPL materials. Based on this notion, we have designed and synthesized oxygen-bridged diphenylnaphthylamine 1 and its derivatives 2−3, which contain electron-withdrawing groups at the naphthyl ring (Figure 1b). We anticipated that the introduction of the naphthyl ring into this asymmetric system, together with varying substituents attached should induce deflection in the HOMO and LUMO to enhance the intramolecular charge transfer character, which should enable the development of emission-color-tunable CPL systems. Synthesis of Oxygen-Bridged Diphenylnaphthylamine Derivatives. Oxygen-bridged diphenylnaphthylamine 1 was synthesized by our previously reported method, using a stepwise N-arylation followed by a 2-fold intramolecular nucleophilic aromatic cyclization (Scheme 1a).10 A Buchwald−Hartwig arylation of 2,6-difluoroaniline with o-iodoaniReceived: March 2, 2017 Published: April 21, 2017 5242
DOI: 10.1021/acs.joc.7b00511 J. Org. Chem. 2017, 82, 5242−5249
Article
The Journal of Organic Chemistry
Figure 1. (a) Molecular design for the CPL materials in this study, together with calculated ring inversion energies. (b) Chemical structures of 1−3.
Scheme 1. Synthesis of 1−3
Figure 2. UV−vis absorption spectra (solid line) and fluorescence spectra (dashed line) of 1−3 and 6 in CH2Cl2. The photograph shows the emission of 1−3.
Table 1. Photophysical Parameters for 1−3 and 6 in CH2Cl2a λabs [nm] (log ε) 1 2 3 6
395 461 552 355
(3.60) (4.00) (4.24) (3.81)
λem [nm] 488 580 685 399
Φb 0.86 0.44 0.09 0.19
τ [s]
kr [s−1]
25.4 6.76 1.16 3.43
× × × ×
3.4 6.5 7.8 5.5
knr [s−1] 7
10 107 107 107
5.5 8.3 7.8 2.4
× × × ×
106 107 108 108
c = 10−5 M. bAbsolute quantum yields determined by a calibrated sphere system.
a
of their absorption bands, accompanied by increased molar absorption coefficients. In the fluorescence spectra obtained from these CH2Cl2 solutions, triphenylamine 6 shows moderate emission at λem = 399 nm (Φ = 0.19, τ = 3.43 ns; Table 1), while diphenylnaphthylamine 1 exhibits an intense blueish green emission at λem = 488 nm (Φ = 0.86) with a large Stokes shift (4825 cm−1). The lifetime of this emission (τ = 25.4 ns) is relatively long compared to other π-conjugated systems. The introduction of electron-withdrawing groups into formyl derivative 2 and 2,2-dicyanovinyl derivative 3 cause a significant red-shift on their emission bands at λem = 580 nm (orange, Φ = 0.44, τ = 6.76 ns) and λem = 685 nm (deep red, Φ = 0.09, τ = 1.16 ns), respectively. These are red-shifted by 92 and 197 nm relative to that of 1, respectively, while maintaining large Stokes shifts of 4451 for 2 and 3517 cm−1 for 3, respectively. Under these conditions, radiative (kr) and nonradiative (knr) rate constants were determined; kr = 3.4 × 107 s−1 and knr = 5.5 × 106 s−1 for 1, kr = 6.5 × 107 s−1 and knr = 8.3 × 107 s−1 for 2, as well as kr = 7.8 × 107 s−1 and knr = 7.8 × 108 s−1 for 3, respectively. Chiroptical Properties in CH2Cl2. To examine the chiroptical properties of 1−3, which are expected to arise from their helical structure, preparative chiral HPLC with CHIRALPAK-IF was used to achieve the optical resolution of 1−3 in order to obtain their enantiomers in pure form (Figures S4−S6). As predicted by DFT calculations, all enantiomers of 1−3 were stable under ambient conditions, and racemization was not observed at room temperature. In order to experimentally examine the chiral stability of 1−3, monitoring
sole selectively afforded monoarylated difluoroaniline 4 in 96% yield. Subsequently, an Ullmann arylation of 4 with 1-iodo-2methoxynaphthalene11 using Cu and K2 CO 3 furnished bisarylated difluoroaniline 5 in 76% yield. After removal of the methyl groups in 5 with BBr3, treatment with K2CO3 in DMF at 120 °C yielded oxygen-bridged diphenylnaphthylamine 1 as yellow solids in 92%. Derivatives 2 and 3, which are functionalized at the 4position of the naphthyl moiety, were easily prepared by subjecting 1 to the selective Vilsmeier−Haack reaction (Scheme 1b). Accordingly, treatment of 1 with POCl3 in DMF selectively afforded monoformyl derivative 2 in 80% yield as orange solids. The reaction of 2 with malononitrile in the presence of triethylamine afforded mono-2,2-dicyanovinyl derivative 3 in 91% yield as purple solids. Compounds 1−3 were characterized by NMR spectroscopy, HRMS, and elemental analysis. Photophysical Properties in CH2Cl2. Subsequently, we examined the photophysical properties of 1−3 in CH2Cl2 (Figure 2, Table 1). In UV−vis absorption spectrum, diphenylnaphthylamine 1 shows a relatively weak absorption band at λabs = 395 nm (log ε = 3.60), which is red-shifted by 40 nm compared to that of oxygen-bridged triphenylamine 6 (λabs = 355 nm, log ε = 3.81). Furthermore, formyl derivative 2 (λabs = 461 nm, log ε = 4.00) and 2,2-dicyanovinyl derivative 3 (λabs = 552 nm, log ε = 4.24) display significant bathochromic shifts 5243
DOI: 10.1021/acs.joc.7b00511 J. Org. Chem. 2017, 82, 5242−5249
Article
The Journal of Organic Chemistry study was conducted on the racemization in toluene at 100 °C for 1 and 80 °C for 2 and 3. Every hour, the changes in the ratio of each fraction of the enantiomer were monitored using HPLC with CHIRALPAK-IF (Figure S7). According to these results, the ring inversion energies were determined to be 29.0 kcal/mol for 1, 27.6 kcal/mol for 2, and 27.1 kcal/mol for 3, respectively. These are quantitatively in good agreement with the values predicted by DFT calculations (B3LYP/6-31G(d)); 29.3 kcal/mol for 1, 27.8 kcal/mol for 2, and 26.4 kcal/mol for 3, respectively (Figures S1−S3). With the pure enantiomers of 1−3 in hand, the chiroptical properties of 1−3 were investigated by circular dichroism (CD) and CPL spectroscopy in CH2Cl2 solutions (Figure 3 and Table S1). CD spectroscopy of the (P)- and (M)-helices of 1− 3 revealed clear Cotton effects for the corresponding absorption bands, i.e., they are mirror images of each other. Specifically, the first eluted fractions of 1−3 (fraction 1) exhibited a (−) Cotton effect, while the second eluted fraction (fraction 2) exhibited a (+) Cotton effect at 400−600 nm (Figure 3). On the basis of a comparison with the results of the TD-DFT calculations, a (P)-helix was assigned to the first fraction, and an (M)-helix to the second fraction of enantiomerically pure 1−3. In the CPL spectra, 1−3 exhibit substantial signals, owing to their helicity in addition to their strong emissive properties. In general, the degree of CD and CPL intensity is determined by the dissymmetry factor (g), which is defined as gabs = 2(εL − εR)/(εL + εR) for CD and as gem = 2(IL − IR)/(IL + IR) for CPL, respectively. Therein, εL, εR, IL, and IR refer to the absorption and emission intensities for left- and right-handed light, respectively. By measuring the CD signals for each of the pure enantiomers of 1−3 in CH2Cl2, gabs values of 5.6 × 10−3 for 1, 2.1 × 10−3 for 2, and 0.9 × 10−3 for 3 were obtained, and the CPL signals delivered gem values of 4.7 × 10−3 for 1, 1.4 × 10−3 for 2, and 0.9 × 10−3 for 3, respectively. Compared to previously reported small organic molecules,6 these values are relatively high, suggesting the utility of oxygen-bridged diphenylnaphthylamine as a scaffold for CPL materials. Electronic Structures. To gain deeper insight into the electronic structures, as well as the photophysical and chiroptical properties of oxygen-bridged diphenylnaphthylamines, DFT and TD-DFT calculations were carried out on 1−3 at the B3LYP/6-31G(d) level of theory, and the results are summarized in Figure. 4a. The Kohn−Sham (KS)-HOMO of 1 (−4.79 eV) is delocalized over the entire skeleton, whereas the KS-LUMO (−1.25 eV) of 1 is mainly located on the naphthalene ring. Accordingly, the electron-withdrawing groups attached to the naphthyl moieties in 2 and 3 further delocalize the KS-LUMOs, and thus effectively decrease the energy levels in formyl derivative 2 (KS-LUMO: − 2.10 eV) and 2,2dicyanovinyl derivative 3 (KS-LUMO: − 2.88 eV). In contrast, the KS-HOMO levels (2: − 5.09 eV; 3: − 5.34 eV) decrease moderately, which leads to narrower HOMO−LUMO gaps in 2 and 3 relative to that of 1. These differences in the electronic structure were confirmed by cyclic voltammetry measurements (Figure S8). Upon introduction of electron-withdrawing groups, the reduction potentials show significant shifts in positive direction (Epc = −2.92 for 1, − 2.06 for 2, and −1.66 V vs Fc/Fc+ for 3), whereas the shifts of the oxidation potentials are only moderate (E1/2 = +0.36 for 1, + 0.50 for 2, and +0.50 V vs Fc/Fc+ for 3). The TD-DFT calculations on the S0 ground states indicate that the longest absorption for 1−3 should be assigned to
Figure 3. UV−vis absorption (solid) and fluorescence (dashed) spectra (top), and CD and CPL spectra (bottom) for (a) 1, (b) 2, and (c) 3 in CH2Cl2. The red and blue bars show the calculated CD bands (CAM-B3LYP/6-31G(d)) for the (P)- and (M)-helices, respectively. The transition energies have been calibrated using a factor of 0.88. Photographs show the emission of 1−3.
π−π* transitions from the HOMO to the LUMO. Reflecting the narrow HOMO−LUMO gaps in 2 and 3, the wavelengths for the longest absorption of 2 (407 nm, f = 0.1994) and 3 (472 nm, f = 0.4444) are red-shifted compared to that of 1 (358 nm, 5244
DOI: 10.1021/acs.joc.7b00511 J. Org. Chem. 2017, 82, 5242−5249
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The Journal of Organic Chemistry
compared to those in the S0 ground state (absorption). Such small oscillator strength values in the S1 state may explain the rather long lifetimes due to the small radiative rate constants (∼107 s). Nevertheless, 1 exhibits an intense emission band with a high quantum yield (λem = 488 nm, Φ = 0.86, τ = 25.4 ns), which is based on a well-suppressed nonradiative rate constant (∼10 6 s), although the reason behind this phenomenon is not clear yet. The chiroptical g values for CD and CPL could also be estimated by TD-DFT calculations at the TD-CAM-B3LYP/631G(d) level of theory on the S0 ground and S1 excited states of the optimized structures (Table 2). The g values were evaluated Table 2. Theoretical Chiroptical Parameters for 1−3 1 2 3 a
state
λ [nm]
D [10−36esu2cm2]
R [10−40esu2cm2]
S0 S1 S0 S1 S0 S1
358 440 407 495 472 545
6.37 5.47 17.3 12.1 44.6 28.9
105 72.9 117 80.2 130 90.8
|g| [10−3] 6.6 5.3 2.7 2.7 1.2 1.3
(5.6)a (4.7)a (2.1)a (1.4)a (0.9)a (0.9)a
Experimental g values obtained for CH2Cl2 solutions.
according to the following equation: g = 4R/D, where D and R denominate the dipole and rotatory strength, respectively.14 D is defined by the oscillator strength (f) and the corresponding excitation energy (ν/cm−1) as follows (eq 1):
D=
3he 2 f 8π 2νmec
(1) −34
−1
where h is the Planck constant (6.626 × 10 J s ), e the electron charge (4.80 × 10−10 esu), me the electron mass (9.11 × 10−31 kg), and c the speed of light (3.00 × 1010 cm s−1). R was evaluated directly by TD-DFT calculations, and the results for 1−3 are summarized in Table 2. The obtained TD-DFTderived gabs and gem values for 1−3 are quantitatively in good agreement with the experimental values, supporting the validity of the present theoretical calculations. These results thus demonstrate the utility of this method for the prediction of CD and CPL parameters for newly synthesized compounds. Solvent Dependency in Fluorescence. As previously discussed, the theoretical calculations suggest that an ICT character is induced in the excited states. Hence, we examined the solvent effect on the photophysical properties of 1−3. The data are summarized in Supporting Information. The corresponding Lippert−Mataga plots15 show that the differences in the dipole moment from S0 to S1 (Δμ) are 8.8 for 1, 10.0 for 2, and 11.4 D for 3 (Figure S36), which demonstrates the induced ICT character especially in the excited state S1. As a consequence of such an induced ICT character, 1−3 exhibit significant red-shifts of their emission bands with increasing solvent polarity (Figure 5). In cyclohexane, the emission of 1− 3 is blue (λem = 459 nm), light green (520 nm), and red (608 nm), respectively. In polar solvents, such as CH3CN or DMSO, the emission of 1−3 is green (λem = 500 nm in DMSO), red (600 nm in DMSO), and deep red (706 nm in CH3CN), respectively. Interestingly, these emissions cover the whole visible region, thus displaying full-color emission (Figure 5). The fluorescence quantum yields of these compounds are varied in these solvents (Figure 5). The maximum quantum yields were obtained for 1 (ΦF = 0.86) in CH2Cl2 and for 2 (ΦF
Figure 4. Pictorial presentation of the frontier orbitals, a plot of the Kohn−Sham HOMO and LUMO energy levels, and the optical transition with oscillator strength for the optimized structures of (a) the ground (S0) states (TD-CAM-B3LYP/6-31G(d)//B3LYP/631G(d)) and (b) the excited (S1) states (TD-CAM-B3LYP/631G(d)//TD-B3LYP/6-31G(d)) of 1−3.
f = 0.0837), and are accompanied by a larger oscillator strength. These results are qualitatively consistent with the observed absorption spectra of 1−3. The rather small oscillator strength obtained for the longest absorption in 1−3 suggests a nonnegligible contribution of the intramolecular charge transfer (ICT) transition character induced by the deflection in the HOMO and LUMO. In order to discuss the emission properties, theoretical calculations were also conducted on the S1 excited states of 1− 3. The structures of the S1 excited states were optimized at the TD-B3LYP/6-31G(d) level of theory, and the emission properties were evaluated by single-point calculations at the TD-CAM-B3LYP/6-31G(d) level of theory for the optimized structures of the S1 states (Figure 4b).6d,13 Similarly to the S0 states, the deflection in the KS-HOMO and KS-LUMO on the S1 excited states is observed for 1−3, corroborating that an ICT character should be induced in the excited states. The more significant deflection of the KS-HOMO and KS-LUMO in the S1 excited state leads to smaller oscillator strength values (emission) for 1 (440 nm, f = 0.0585), 2 (495 nm, f = 0.1149), and 3 (545 nm, f = 0.2488), thus increasing the ICT character 5245
DOI: 10.1021/acs.joc.7b00511 J. Org. Chem. 2017, 82, 5242−5249
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The Journal of Organic Chemistry
Figure 5. Photographs of emission and emission wavelengths and quantum yields for (a) 1, (b) 2, and (c) 3 in a variety of solvent.
= 0.96) and 3 (ΦF = 0.63) in cyclohexane. The pure enantiomers of 2 and 3 in cyclohexane exhibit comparable gabs and gem values to those in CH2Cl2 in their CD and CPL spectra, respectively (gabs: 1.9 × 10−3 for 2, and 1.1 × 10−3 for 3, gem: 1.7 × 10−3 for 2, and 0.9 × 10−3 for 3, Figures S31−S32). Single Crystal X-ray Structure. Single crystal X-ray structural analyses were carried out on racemic crystals (rac-1 and rac-2) as well as enantiomerically pure crystals (ena-1 and ena-2) in order to determine any potential difference in the molecular structure and packing motifs that could affect the solid-state properties (Figure 6). Due to the lack of heavy elements in the compounds used in this study, the absolute structures for the enantiomerically pure crystals could not be unambiguously determined, as they are usually obtained by a Flack parameter with anomalous X-ray scattering. Thus, the absolute structures for each of the enantiomers of 1−3 were determined on the basis of a comparison between the TD-DFT calculations and the CD spectra in solution (vide supra). The crystal structure analyses confirmed that all molecules of 1 and 2 in both racemic and enantiomerically pure crystals have helical structures with dihedral angles (∠C2−C1−C14−C15) of 72.6 and 73.1° for rac-1, 75.7° for ena-1, 72.8° for rac-2, and 76.5° for ena-2, which are in good agreement with those of the optimized structures from the DFT calculations (1: 72.3°, 2: 73.4°). The small differences in the dihedral angles could be attributed to packing forces. In the packing structure, a onedimensional columnar structure with slipped π-stacking was observed for both racemic and enantiomerically pure crystals. The rac-1 crystals contain two independent molecules per unit cell, which form a pair in a columnar stacking structure. In the case of rac-2 crystals, (P)- and (M)-helices of 2 form independent columnar structures, which are aligned alternately. In the packing structure of enantiomerically pure crystals, by adopting a slipped π-stacking in these columnar structures, the cofacial distances for the central benzene rings (ena-1: 3.44 Å; ena-2: 3.33 Å) are shorter than those in the racemic crystals (rac-1: 3.68 Å; rac-2: 3.62 Å). Fluorescence Properties in the Solid State. Subsequently, we examined the fluorescence properties of 1−3 in the solid state for both the racemic and enantiomerically pure crystals. On account of the differences in packing motifs,
Figure 6. X-ray crystal structures: ORTEP drawings (50% probability for thermal ellipsoids) and packing structures for (a) rac-1, (b) ena-1, (c) rac-2, and (d) ena-2. The P- and M-helices in the racemic crystals are shown in red and blue, respectively.
crystals of 1−3 exhibit different emission behavior depending on the racemic mixtures or the enantiomerically pure compounds (Table 3, Figures S19, S21, and S23). Whereas both crystals of rac-1 and ena-1 show similar blueish green emission (rac-1: λem = 480 nm; ena-1: 482 nm), the quantum yield of ena-1 (Φ = 0.55) is significantly higher than that of rac1 (Φ = 0.35). In case of 2 and 3, the differences in the Table 3. Photophysical and Chiroptical Parameters for 1−3 in the Solid State state rac-1b ena-1c rac-2b ena-2c rac-3b ena-3c
crystal crystal nanoparticled crystal crystal nanoparticled crystal crystal nanoparticled
λabs [nm]
401
469
572
λem [nm]
Φa
τ [ns]
480 482 487 572 585 583 735 706 669
0.35 0.55 0.41 0.09 0.14 0.13 0.09 0.02
