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Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly Polarized Luminescence and Electroluminescence Hayato Sakai, Sho Shinto, Jatish Kumar, Yasuyuki Araki, Tomo Sakanoue, Taishi Takenobu, Takehiko Wada, Tsuyoshi Kawai, and Taku Hasobe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03386 • Publication Date (Web): 19 May 2015 Downloaded from http://pubs.acs.org on May 24, 2015

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The Journal of Physical Chemistry

Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly

Polarized

Luminescence

and

Electroluminescence Hayato Sakai,*,† Sho Shinto,† Jatish Kumar,‡ Yasuyuki Araki,*,§ Tomo Sakanoue,‖ Taishi Takenobu,*,‖ Takehiko Wada,§ Tsuyoshi Kawai,*,‡ and Taku Hasobe*,† †

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,

Yokohama, Kanagawa 223-8522 (Japan) ‡

Graduate School of Materials Science Nara Institute of Science and Technology, NAIST, Ikoma,

Nara 630-0192 (Japan) §

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1

Katahira, Aoba-ku, Sendai 980-8577 (Japan) ‖

Department of Applied Physics, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555

(Japan) KEYWORDS: Helicene, Circularly Polarized Luminescence, Fluorescence Quantum Yield, Excited-State Dynamics, Chirality

ACS Paragon Plus Environment

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ABSTRACT: A new 1,2-dialkylquinoxaline-fused [7]carbohelicene ([7]Hl-NAIQx) was designed and synthesized by asymmetrically introducing two alkyl chains onto the quinoxaline unit. Direct alkylation of the quinoxaline ring of quinoxaline-fused helicene leads to discontinuity in the conjugated structure. In the single crystal analysis, the parent quinoxalinefused [7]carbohelicene ([7]Hl-Qx) was found to have a helical structure formed by two phenanthrene units and a nonplanar twisted angle between the phenanthrene and quinoxaline units. In contrast, [7]Hl-NAIQx possesses an almost planar aromatic structure between the alkylquinoxaline and phenanthrene units (torsion angle: 179°), in addition to the similar helical structure between the two phenanthrene units. The steady-state absorption, fluorescence, and circular dichroism (CD) spectra of [7]Hl-NAIQx were significantly red-shifted compared to those of [7]Hl-Qx and [7]carbohelicene ([7]Hl). These spectral changes were mainly explained by electrochemical measurements and density functional theory (DFT) calculations. Moreover, the absolute fluorescence quantum yield (FL) of [7]Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]Hl (FL = 0.02). Such a large enhancement of the fluorescence of [7]Hl-NAIQx has provided excellent circularly polarized luminescence (CPL). The value of the anisotropy factor, glum (normalized difference in emission of right-handed and left-handed circularly polarized light), was estimated to be 4.0 × 10-3. The electroluminescence of an organic light-emitting diode (OLED) utilizing [7]Hl-NAIQx was successfully observed.


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INTRODUCTION The helix is an important structural motif in biological systems, from the nanoscopic DNA double helix to microscopic viruses and seashells.1-7 Such elegance and complexity of natural helical assemblies have encouraged chemists to create well-defined and functional helical structures through synthetic and supramolecular strategies8-11 aimed at applications in chiral technologies such as asymmetric catalysis12-16 and biomimetics.17-21 Polycyclic aromatic hydrocarbons (PAHs) constitute a class of molecules that are formed by condensing benzene units. PAHs and the related molecules are of fundamental importance in various research fields of material science such as electronics and energy conversion.22-34 In particular, helicenes are a class of chiral screwed PAH derivatives comprising ortho-fused aromatic rings. Because of their characteristic structures and functionalities, they have been extensively investigated for use in many research fields and applications such as functional supramolecular architectures,35-37 fluorescent sensors,38,39 and discotic and liquid-crystalline materials.40-45 Helicenes are also known to show unique optical properties such as nonlinear optical effects and circular dichroism (CD) due to the chiral configuration originating from the helical structure,35,40,42,46-53 which demonstrates a good anisotropy factor (g-value). Thus, such a helical structure generally shows circularly polarized luminescence (CPL) properties.54-60 In addition to the above-mentioned anisotropy factor, luminescence properties (i.e., fluorescence quantum yield) are also essential for efficient CPL. Although a significant effort has been made toward helicene synthesis, only a few studies have reported high fluorescence quantum yields for helicene-like molecules with -conjugation that extends to the entire molecule.54,56,61-66 Much more efficient CPL has been observed in other materials, including


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supramolecular assemblies.60,67-74 The extremely low fluorescence quantum yields of helicenes are due to the rapid intersystem crossing from the excited singlet state to the triplet state. 75-77 Thus, the CPL properties of helicenes remain a major challenge.50,54,61-64,78,79 Helicenes with efficient fluorescence and CPL properties may prove useful in the synthesis of novel organic light-emitting diodes (OLEDs) since the number of OLEDs utilizing helicene derivatives is extremely limited.80-82 Quinoxalines (also known as benzopyrazines) have a significant synthetic advantage over aromatic compounds due to the convenient synthesis method between diketone and diamine derivatives. They are also known to possess a strong electron-accepting property because of the two symmetric, unsaturated nitrogen atoms. Consequently, quinoxalines are widely used as good light-emitting and electron-transporting materials due to the highly polarized imine groups. We recently reported the synthesis of quinoxaline fused-[7]carbohelicenes;37 however, the fluorescence quantum yield remained low (FL = 0.05). Kang and coworkers recently reported that the reduction of the imine unit (C=N) of quinoxalines by alkyl/aryllithiums leads to a steric change on the quinoxaline ring. 83 Such a structural change resulted from the presence of two dissimilar and asymmetric nitrogen environments in a single quinoxaline unit, one with an electron-donating (ED) sp3 site and the other with an electron-accepting (EA) sp2 site. Such a structural change is highly expected to control the optical and electronic properties. Based on the above consideration, we for the first time synthesized 1,2dialkylquinoxaline-fused [7]carbohelicene ([7]Hl-NAIQx) by introducing two alkyl chains onto a quinoxaline unit (Figure 1). Direct alkylation of the quinoxaline ring contributed to


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discontinuity in the conjugated structure of quinoxaline-fused helicene. Accordingly, quinoxaline-fused [7]carbohelicenes (i.e., [7]Hl and [7]Hl-Qx shown in Figure 1) have a typical nonplanar aromatic structure, whereas [7]Hl-NAIQx possesses a helical structure formed by two phenanthrene units and a planar aromatic structure between the alkyl-quinoxaline and phenanthrene units. With regard to the crystal packing of [7]Hl-NAIQx, one-dimensional helical columns of racemic helicene derivatives by CH- interactions are extremely unusual for the crystallography, although one-dimensional helix formation by the related heterochiral crystals has already been reported.84-87 Such a controlled structure exhibits a high fluorescence quantum yield and a relatively large g value (dissymmetric factor) of CPL in the monomeric form. Moreover, the electroluminescence of a light-emitting diode utilizing [7]Hl-NAIQx was successfully observed. The details of the synthesis, structural, and photophysical properties have been discussed here.

Figure 1. Chemical structures of quinoxaline-fused [7]carbohelicene derivatives and reference compounds in this study. [7]Hl-(NAI)2Qx was employed for only DFT calculations.


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RESULTS AND DISCUSSION Synthesis. The synthesis of quinoxaline-fused [7]carbohelicene ([7]Hl-Qx) was reported in our previous work.37 To introduce asymmetric alkyl units onto the quinoxaline skeleton, reduction of the quinoxaline imine unit was required. Therefore, in this study, nucleophilic addition adjacent to the electronegative atom (i.e., nitrogen) was applied to [7]Hl-Qx. Initially, to convert [7]Hl-Qx to [7]Hl-NAIQx, [7]Hl-Qx was reacted with n-BuLi and MeI according to the reported method.83 However, the target compound was not obtained because the reaction was highly complicated. Accordingly, we employed Na2S2O4 as the reducing agent to obtain the appropriate intermediate88 prior to the reaction with n-BuLi and MeI, as shown in Scheme 1. Concerning the conversion to 1,2-dialkyl-substituted quinoxaline, the reaction would proceed via an active intermediate of N-methylquinoxaline derivative.89-91 [7]Hl-NAIQx was obtained in a total yield of 35% from these two steps. The steric configuration of [7]Hl-NAIQx was determined by X-ray single crystal structure analysis (Figure 2). The detailed 1H and

13

C NMR data are shown in

Figures S1 and S2 in the Supporting Information (SI).

Scheme 1. Synthetic scheme of [7]Hl-NAlQx.


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Single Crystal Structures. Single crystal structures were evaluated by X-ray analysis to determine the detailed structural properties of rac-[7]Hl-NAIQx and rac-[7]Hl-Qx in the monomeric and packing states. We prepared single crystals of these compounds for X-ray diffraction analysis in accordance with the method described in the experimental section. The crystal data are summarized in SI Table S1. The single crystals were prepared by vapor diffusion at room temperature. The crystal structures of rac-[7]Hl-Qx and rac-[7]Hl-NAIQx in the monomeric forms are depicted in Figure 2. [7]HlQx has a nonplanar helical structure between the two phenanthrene units (Figure 2a). The C3−C5−C5’−C13 torsion angle between the alkyl-quinoxaline and phenanthrene units is 159°. In contrast, [7]Hl-NAIQx possesses a helical structure formed by the two phenanthrene units and a planar aromatic structure between the alkyl-quinoxaline and phenanthrene units (Figure 2b). The C39−C29−C2−C21 torsion angle between the alkyl-quinoxaline and phenanthrene units is estimated to be 179°, which is largely different with that of [7]Hl-Qx. On the other hand, the C21−C29−C2−C10 torsion angle of the two phenanthrene units in [7]Hl-NAIQx (56°) is larger than that in [7]Hl-Qx (C13−C5−C5’−C8 torsion: 44°). The difference in these torsion angles may directly affect the delocalization of -electrons in the molecules, which leads to the enhanced fluorescence properties (see below).


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Figure 2. ORTEP diagrams of (a) rac-[7]Hl-Qx (protons omitted for clarity, ellipsoids set at 50% probability)37 and (b) rac-[7]Hl-NAIQx (protons omitted for clarity, ellipsoids set at 50% probability).

Additionally, to investigate the alkyl-substituent effects on the aggregate properties, the packing structures of rac-[7]Hl-Qx and rac-[7]Hl-NAIQx were evaluated. First, it should be noted that the single crystal structure of rac-[7]Hl demonstrates a racemic crystal structure based on the CH- interaction with a close distance of 2.8 Å (SI Figure S3), whereas rac-[7]Hl-Qx is a conglomerate with the smallest CH- interaction distance (2.9 Å), as shown in SI Figure S4. SI Figure S5 also demonstrates the single crystal structure of rac-[7]Hl-NAIQx from racemic compounds, which exhibits a right-handed helical and one-way columnar formation along the crystallographic b axis by virtue of two different intracolumn CH- interactions such as helicene-helicene and helicene-quinoxaline (SI Figures S5a, b, and c). The two different conformers of (+)-(P)-[7]Hl-NAIQx and (–)-(M)-[7]Hl-NAIQx92 are alternately stacked in the


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same column (SI Figures S5a and b).93,94 The intracolumn CH- interaction distance was estimated to be 2.9 Å (SI Figure S5c). Additionally, the one-way helical columns are arranged in the up-and-down configuration along the crystallographic a axis and the columns along the crystallographic c axis are arranged in the same configuration (SI Figure S5d). Based on the above results, we can conclude that the introduction of alkyl chains onto the [7]Hl-NAIQx unit successfully induced one-dimensional helical columns via intracolumn CH- interactions from racemic compounds.

Electrochemical Properties of [7]Carbohelicene Derivatives. The electrochemical properties of [7]carbohelicene derivatives were measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) to consider the alkyl-substituent effects on the redox potentials. The voltammograms of [7]Hl-Qx and [7]Hl-NAIQx in CH2Cl2 containing 0.1 M n-Bu4NPF6 are shown in Figure 3. The electrochemical potentials of the [7]carbohelicene derivatives and the reference [7]Hl are summarized in Table 1. It should be emphasized that the background current is relatively high because of the low [7]Hl-NAIQx concentration resulting from its limited solubility (0.1 mM) in CH2Cl2. The first one-electron reduction (Ered1) and oxidation (Eox) potentials of [7]Hl-Qx in CH2Cl2 against the saturated calomel electrode (SCE) are −1.34 V and 1.47 V, respectively. The Eox value of [7]Hl-Qx is similar to the value reported for [7]Hl (Eox: 1.42 V), whereas the Ered1 value of [7]Hl-Qx is significantly shifted in the positive direction by ~0.7 V compared to the value reported for [7]Hl (Ered1: –2.02 V).37 In contrast with those of [7]Hl-Qx, quasi-reversible oxidation and reduction peaks are found in the CV for [7]Hl-NAIQx (Figure 3). The first oxidation and reduction


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potentials of [7]Hl-NAIQx (Eox: 0.95 V and Ered1: –1.74 V) are shifted in the negative direction by 0.5 V and 0.4 V, respectively. Accordingly, the HOMO-LUMO gap of [7]Hl-NAIQx is slightly smaller than that of [7]Hl-Qx. This agrees well with the spectroscopic behaviors (see below).

x10

18 6

-6

9

15 3 12 0

6 -6 3

-6

9 -3

x10

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3 A

-9 0 -12 -3 -15 -6

3 A

-18 -9

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E vs. SCE, V Figure 3. Cyclic and differential pulse voltammograms of 0.1 mM [7]Hl-NAIQx in CH2Cl2 with 0.1 M n-Bu4NPF6 as supporting electrolyte. Scan rate: 0.1 V/s for CV and 0.01 V/s for DPV.

To gain an insight into the electronic structures, we carried out DFT calculations using the Gaussian suite of programs at the B3LYP level of theory and the 6-31G* basis set (Table 1 and Figure 4). First, we compared the HOMO and LUMO energy levels of [7]Hl (Table 1 and Figures 4a and b) and [7]Hl-Qx (Table 1 and Figures 4c and d). The LUMO state of [7]Hl-Qx (–


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2.26 eV) becomes largely stabilized compared to that of [7]Hl (–1.49 eV), whereas both the HOMO levels are very similar (~–5.4 eV). Regarding the molecular orbitals, the HOMO of [7]Hl-Qx is relatively localized on the helicene unit, whereas the corresponding LUMO state becomes significantly delocalized up to the quinoxaline unit. This is probably the reason for the more stabilized energy of the LUMO state in [7]Hl-Qx. In contrast with [7]Hl-Qx, the HOMO level of [7]Hl-NAIQx is higher (–4.98 eV) than that of [7]Hl (–5.36 eV), whereas the LUMO level (–1.81 eV) becomes stabilized relative to that of [7]Hl (–1.49 eV) (Figures 4e and f). Therefore, the introduction of alkyl substituents induces the delocalization of the LUMO state in the planar structure between the alkyl-quinoxaline and phenanthrene units, which indicates that the LUMO does not sufficiently extend up to the other phenanthrene unit (side view in Figure 4f). This is in sharp contrast with that of [7]Hl-Qx. The results of the DFT calculations agree with the trend of the electrochemical measurements shown in Table 1.

Table 1. Redox Potentials and Energy Levels of [7]Carbohelicene Derivatives and [7]Hl Helicene [7]Hl-Qx

Eoxa 1.47b

Ered1a –1.34b

Ered2a –1.78b

EHOMOe [eV] –5.47

ELUMOe [eV] –2.26

[7]Hl-NAIQx

0. 95 (0.96)c

–1.74 (–1.78)c

–

–4.98

–1.81

[7]Hl

1.42d

–2.02d

–

–5.36

–1.49

a d

V vs SCE. bReported value in CH2Cl2.37 cDetermined by differential pulse voltammetry (DPV). Reported value in MeCN.95 eCalculated by DFT method with B3LYP/6-31G*.


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Helicene

HOMO

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LUMO

(a)

(b)

(c)

(d)

[7]Hl

Top view

Top view

Side view

Side view

[7]Hl-Qx

(f)

(e) Top view

Top view

Side view

Side view

[7]Hl-NAIQx

Figure 4. Molecular orbitals of [7]carbohelicene derivatives (B3LYP/6-31G* level). (a) HOMO and (b) LUMO of [7]Hl, (c) HOMO and (d) LUMO of [7]Hl-Qx, and (e) HOMO and (f) LUMO of [7]Hl-NAIQx.


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To further investigate the alkyl-substituent effect on the optimized structures, DFT calculations

were

also

performed

on

symmetrical

1,2,3,4-tetraalkyl-quinoxaline-fused

[7]carbohelicene ([7]Hl-(NAI)2Qx) (SI Figure S6). The energy levels of the HOMO (–4.68 eV) and LUMO (–1.48 eV) of [7]Hl-(NAI)2Qx are higher than those of [7]Hl-NAIQx because of the additional electron-donating units. Moreover, the optimized structure of [7]Hl-(NAI)2Qx demonstrates the nonplanar aromatic structure between the alkyl-quinoxaline and phenanthrene units, which is in sharp contrast with that of [7]Hl-NAIQx. Thus, our strategy of asymmetrical introduction of an alkyl substituent onto the quinoxaline unit successfully contributes to the structural change, as compared to [7]Hl-Qx and [7]Hl-(NAI)2Qx.

Steady-State Absorption Spectra of [7]Carbohelicene Derivatives. The steady-state absorption spectra of [7]carbohelicene derivatives are measured in THF solution, as shown in Figure 5. The spectra of [7]Hl-Qx (spectrum a) and [7]Hl-NAIQx (spectrum b) are noticeably red-shifted compared to that of [7]Hl (spectrum c).96 In particular, the spectrum of [7]Hl-NAIQx extends to around 500 nm and is red-shifted by ~100 nm relative to that of [7]Hl. As discussed above (Table 1), the introduction of a quinoxaline unit stabilizes both the HOMO (–5.47 eV) and LUMO (–2.26 eV) levels of [7]Hl-NAIQx, compared to those of [7]Hl. A possible reason for the red-shifted spectrum of [7]Hl-Qx is the relatively lower LUMO level and the similar HOMO level to that of [7]Hl, which leads to a lower HOMO-LUMO gap. In contrast, the HOMO level of [7]Hl-NAIQx has a higher value (–4.98 eV) than that of [7]Hl (–5.36 eV), whereas the LUMO level (–1.81 eV) is stabilized relative to that of [7]Hl (–1.49 eV).


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Consequently, the red-shift of [7]Hl-NAIQx is attributable to the smaller HOMO-LUMO gap. Moreover, we evaluated the molar extinction coefficients from the 0-0 absorption bands (0-0) (Table 2). The absorption peak of [7]Hl (0-0: 474 M-1 cm-1) at 410 nm is assigned to be a symmetry-forbidden transition.37 However, these transitions become allowed when the symmetry is reduced. Therefore, the 0-0 values of quinoxaline-fused compounds largely increase. Another reason for the red-shift may be the intramolecular charge transfer (ICT) because of the formation of an unsymmetrical structure by alkylation (see below).83

12 (a) (b) (c)

-1

10 8

-4

-1

 × 10 , M cm

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6 4 2 0

300

400

500

Wavelength, nm Figure 5. UV-vis spectra of (a) [7]Hl-Qx in THF, (b) [7]Hl-NAIQx in THF and (c) [7]Hl in CH2Cl2 (reproduced from the reported result96).


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Steady-State Fluorescence Spectra of [7]Carbohelicenes. The steady-state fluorescence spectra of [7]carbohelicene derivatives were also measured, as shown in Figure 6A. The emission maxima of [7]Hl-Qx (spectrum a) and [7]Hl-NAIQx (spectrum b) are significantly red-shifted in comparison with that of [7]Hl (spectrum c).97 In particular, the spectrum of [7]Hl-NAIQx significantly extends beyond the 700 nm region and is red-shifted by ~100 nm relative to that of [7]Hl. These results are indicated by the photographs of the luminescent colors of [7]Hl-Qx (green) and [7]Hl-NAIQx (yellow) in THF (Figure 6B). The red-shifted trends of [7]Hl-Qx and [7]Hl-NAIQx relative to [7]Hl are also explained by the above-mentioned DFT calculations and electrochemical results. Additionally, as stated above, such a red-shifted trend may be attributable to the typical ICT emission. SI Figures S7 and S8 show the UV-vis and fluorescence spectra of [7]Hl-Qx and [7]Hl-NAIQx in various solvents, respectively. [7]Hl-NAIQx demonstrates a significant positive solvatochromic shift, depending on the solvent polarity (SI Figure S8b), which indicates intramolecular donor-acceptor interactions.83,98 Consequently, the introduction of the alkylated quinoxaline unit successfully contributes to changes in the spectral features and luminescent colors.


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Figure 6. (A) Fluorescence spectra of (a) 2.0 M [7]Hl-Qx in THF; ex = 330 nm, (b) 2.0 M [7]Hl-NAIQx in THF; ex = 330 nm and (c) [7]Hl in 1,4-dioxane (reproduced from the reported result.97). (B) Photographs of of [7]Hl-Qx and [7]Hl-NAIQx (10 M) in THF under white light and 365 nm UV light illumination.

Quantum Yields and Fluorescence Lifetimes of [7]Carbohelicenes. In order to examine the detailed excited-state dynamics of quinoxaline-fused [7]carbohelicene derivatives, fluorescence lifetime measurements were first performed. The fluorescence decays for [7]Hl-Qx (decay a) and [7]Hl-NAIQx (decay b) were performed using pulsed 404 nm laser light in toluene, which excited these moieties, as shown in Figure 7. The fluorescence lifetimes (FL) of these compounds, as shown in Table 2, were determined from a monoexponential fitting.


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5

10

4

10 Counts

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3

10

2

10

(a) (b)

1

10

0

10

-5

0

5

10

15

Time, ns Figure 7. Fluorescence decay profiles of (a) [7]Hl-Qx and (b) [7]Hl-NAIQx in toluene. Excitation wavelength: 404 nm (pulse width: 100 ps).

The FL values of [7]Hl-Qx

and [7]Hl-NAIQx are estimated to be 1.66 ns and 4.00 ns,

respectively, which are smaller than that of the reference [7]Hl (13.8 ns). We then examined the absolute fluorescence quantum yields (FL) of the quinoxaline-fused [7]carbohelicene derivatives, which are summarized in Table 2. As compared with the FL values of [7]Hl (FL = 0.02), those of [7]Hl-Qx (FL = 0.05) and [7]Hl-NAIQx (FL = 0.25) are significantly greater. In particular, the FL value of [7]Hl-NAIQx is more than 10 times larger than that of [7]Hl. This value is quite comparable to previous reports on helicene-like molecules.55,61,62 Next, the quantum yields of the ISC pathways of these compounds (ISC) were evaluated by the


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measurement of 1O2 phosphorescence utilizing energy transfer from the excited triplet states of the helicene derivatives to O2. A negligible quenching contribution from the singlet-excited state of helicene derivatives was assumed.26,99-101 Singlet oxygen produced using [7]carbohelicene derivatives as sensitizers in oxygen-saturated toluene, was observed by 1O2 phosphorescence at 1270 nm. Since the ISC value of fullerene (C60) in solution was reported to be ISC = 0.96,102 C60 was used as the standard. Additionally, the ISC of [7]Hl has not been reported and was assumed based on the reported ISC values of [6]helicene, [8]helicene, and [9]helicene (Table 2). The quantum yields of the internal conversion (IC) pathways of helicenes (IC) can also be estimated by subtracting FL andISC from 1. The summarized ISC and IC values are shown in Table 2.

Table 2. Photophysical Parameters of [7]Carbohelicene Derivatives.

Helicene

FLa [ns]

0-0

FLc

ISCd

IC

kFL × 10-7 [s-1]

kISC × 10-7 [s-1]

kIC × 10-7 [s-1]

S1–S0 [eV]

T1–S0 [eV]

S1–T1 [eV]

[M-1 cm-1]

[7]Hl-Qx

1.66

0.05

0.96

0.00

3.01

57.8

–

2.62

1.93

0.69

14,500

[7]Hl-NAIQx

4.00

0.25

0.62

0.13

6.25

15.5

3.25

2.42

1.56

0.86

4,700

[7]Hl

13.8b

0.02b

(~0.9)e

–

0.145b

(~6.5)f

–

2.79g

2.12h

0.67h

474i

FL: fluorescence lifetime,FL: fluorescence emission quantum yield,ISC: intersystem crossing quantum yield,IC: internal conversion quantum yield,kFL: fluorescence emission rate constant,

kISC: intersystem crossing rate constant, kIC: internal conversion rate constant. S1–S0: determined by UV-vis and fluorescence spectra. T1–S0: determined by phosphorescence spectra. 0-0: molar extinction coefficients of 0-0 absorption bands. IC = 1–FL–ISC, kFL = FL FL–1, kISC = ISC FL–1, kIC = IC FL–1. aExcited at 404 nm. bReported values in 1,4-dioxane.75,97,103 cExcited at 330 nm. dExcited at 450 nm. eThe value was assumed based on the reported [6]carbohelicene (ISC = 0.91),75 [8]carbohelicene (ISC = 0.92) 75 and [9]carbohelicene (ISC = 0.91).75 fEstimated value from ISC of [7]Hl: 0.9. gReported value.103 hReported value.75 iReported value.37


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Here, it should be noted that the sum of the above three quantum yields is close to unity within the experimental error. The FL value of [7]Hl-NAIQx (FL = 0.25) is significantly greater than the FL values of [7]Hl-Qx (FL = 0.05) and [7]Hl (FL = 0.02), whereas the corresponding ISC value is lower. Finally, the net rate constants of fluorescence emission (kFL), intersystem crossing (kISC), and internal conversion (kIC) were calculated as shown in Table 2. The emphasis is on the dramatically higher kFL value of [7]Hl-NAIQx compared to that of [7]Hl. In particular, the kFL values of [7]Hl-NAIQx (6.25 × 107 s-1) and [7]Hl-Qx (3.01 × 107 s-1) are one order of magnitude greater than that of [7]Hl (1.45 × 106 s-1). According to the established theory of photochemistry,104 it is well known that kFL depends on the molar absorption coefficients (ε). We therefore estimated the ε values from the 0-0 absorption bands (ε0-0), as shown in Table 2. The ε0-0 values of [7]Hl-Qx and [7]Hl-NAIQx are much larger than that of [7]Hl, which agrees with the trends of the kFL values. On the other hand, we considered the rate constant, kISC, and the quantum yield, ISC, of the ISC pathways. The kISC values of [7]Hl-Qx and [7]Hl-NAIQx are 5.78 × 108 s–1 and 1.55 × 108 s–1, respectively. As is the case with kFL, these values are larger than that of [7]Hl (kISC = ~6.5 × 107 s–1). Typically, kISC is affected by energy gaps between S1 and the triplet-excited state. To determine the energy gaps of these molecules, the energy gaps between S1 and T1 (S1T1) were evaluated from the corresponding phosphorescence spectra, as shown in Table 2 (see: SI Figure S9). The similar S1-T1 values (~0.7–0.8 eV) probably do not affect kISC. Concerning the superior fluorescence properties of [7]Hl-NAIQx, the following two important points emerge from the above discussions as reasons. The first is the ICT emission based on the asymmetrical dialkylquinoxaline (i.e., ED-EA pair unit).83 The other is the planar aromatic structure between the alkyl-quinoxaline and phenanthrene units caused by alkylation, which may decelerate the


19


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ISC pathway compared to the nonplanar structure. The excited state of [7]Hl-NAIQx is largely dependent on the planar configuration but not the helicene unit (i.e., the two phenanthrene units) considering the molecular orbital of the LUMO state (Figure 4f). Schmidt and co-workers also reported that the spin-orbit coupling is sensitive to the degree of nonplanarity.105 Thus, our strategies successfully contribute to the enhanced FL value of [7]Hl-NAIQx

Circular Dichroism (CD) Spectra of Quinoxaline-Fused [7]Carbohelicenes. The enantiomer was separated from the racemic compound by using chiral HPLC. Then, to evaluate the anisotropy, we measured the CD spectra, as shown in Figure 8. To fix the helical structures of [7]Hl-NAIQx, we employed TDDFT calculations obtained by the Gaussian suite of programs at the B3LYP level of theory and the 6-31G* basis set (SI Figures S10 and S11). The (+) and (–) enantiomers of these compounds demonstrate the mirror-image spectra within the experimental error. In the [7]Hl-Qx derivatives [i.e., [7]Hl-Qx (spectra a and b) and [7]HlNAIQx (spectra c and d)], lower  values are observed in the range 250–300 nm compared to that of [7]Hl (spectrum e), whereas  values are higher than that of [7]Hl in the 400–500 nm region. Introduction of the quinoxaline group onto the helicene unit contributes to the modulation of the HOMO-LUMO gap and anisotropy in both UV and visible regions. The anisotropy in the absorption (gCD) region was calculated at 435 nm and the values are 4.0 × 10–4 and 1.3 × 10–3 for [7]Hl-Qx and [7]Hl-NAIQx, respectively.


20

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Figure 8. CD spectra of (a) (+)-(P)-[7]Hl-Qx in THF, (b) (–)-(M)-[7]Hl-Qx in THF, (c) (+)-(P)[7]Hl-NAIQx in THF, (d) (–)-(M)-[7]Hl-NAIQx in THF and (e) (+)-(P)-[7]Hl in CHCl3 (reproduced from the reported result49).

Circularly Polarized Luminescence (CPL) Spectra of Quinoxaline-Fused [7]Carbohelicene Derivatives. To understand the chiral nature in fluorescence, CPL measurements of these compounds in THF solution were performed. No appreciable CPL is observed from the [7]Hl-Qx solution (SI Figure S12). In contrast, (+)-(P)-[7]Hl-NAIQx (spectrum a) and (–)-(M)-[7]Hl-NAIQx (spectrum b) solutions exhibit CPL profiles with negative and positive signs, respectively (Figure 9). The signs of CPL are consistent with those of the corresponding CD spectra at the longest wavelength (around 400–500 nm).106 The degree of CPL is given by the luminescence dissymmetric factor (glum), which is defined as glum = 2(IL – IR)/(IL + IR), where IL and IR are the


21


luminescence intensities of left and right circularly polarized light, respectively. In contrast with the negligible CPL properties of [7]Hl-Qx, the glum value for [7]Hl-NAIQx is estimated to be 4.0 × 10–3. This value is comparable to those reported in the monomer state of small organic molecules.54,61-63

0.8 (a) (b)

0.6 0.4 0.2 IL-IR

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0.0 -0.2 -0.4 -0.6 -0.8 450

500

550 600 650 Wavelength, nm

700

Figure 9. CPL spectra of (a) (+)-(P)-[7]Hl-NAIQx and (b) (–)-(M)-[7]Hl-NAIQx in THF.

The asymmetric factor of CD and CPL is determined by a scalar product of electric (e) and magnetic (m) transition dipole moments. According to the recent result by Mori and Inoue et al,49 e and m in the calbo[n]helicene systems are parallel to the -orbital directions and helix axes, respectively. They basically state that the helix axis components relevant to m increase


22

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and the relative angles between e and m decrease with increasing number of benzene rings (n), which is the reason for the large asymmetric factor of [n]calbohelicene systems in the large number of n. Considering the above points, [7]Hl-Qx seems to possess the expanded -orbital perpendicular to the magnetic dipole moment (m), which may result in the low CPL signal.107 In contrast, in the case of [7]HI-NAIQx, the significant enhancement of fluorescence quantum yield (ΦFL) probably contributes to the successful observation of the CPL signal.

Fabrication and Electroluminescence of an Organic Light-Emitting Diode (OLED) Utilizing [7]Hl-NAIQx. To examine the electroluminescence property of [7]Hl-NAIQx, we fabricated an OLED doped with [7]Hl-NAIQx. As shown in Figure 6, [7]Hl-NAIQx has a broad absorption in the spectral range varying from UV to ~500 nm region. Poly(9,9-dioctylfluorene) (PFO) has a fluorescence spectrum of 400–500 nm. Therefore, we chose PFO as the host material for [7]Hl-NAIQx, considering the overlapping spectral region.108 The fabricated OLED was a bilayer structure with an

indium

tin

oxide

(ITO)

anode,

a

hole

transport

layer

of

poly(3,4-ethylene

dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), an electron transport and emissive layer of PFO/[7]Hl-NAIQx, and a calcium/aluminum cathode (denoted as ITO/PEDOT:PSS/PFO:[7]Hl-NAIQx (95:5)/Ca/Al). The detailed fabrication procedure is described in the experimental section in SI. The J-V-L characteristics and corresponding electroluminescence spectrum of an OLED are shown in Figures 10A and 10B. Although the device operation voltage is relatively high, we observe a stable electroluminescence spectrum (Figure 10B), which agrees well with the corresponding steady-state fluorescence spectrum (spectrum b in Figure 6).


23


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Figure 10. (A) Current–luminance–voltage (J-L-V) characteristics and (B) electroluminescence (EL) spectrum of ITO/PEDOT:PSS/PFO: [7]Hl-NAIQx-(95:5)/Ca/Al).


Page 24 of 36

an

24

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However, the fluorescence intensity gradually decreases with the increase in operation time because of the oxidation of the Ca electrode. Nevertheless, we can conclude that [7]Hl-NAIQx is stable enough for OLED applications since no change in the spectral shape was observed during operation. Further studies are underway to observe the circular dichroism in the electroluminescence spectrum.

CONCLUSION In this work, we designed and synthesized 1,2-dialkylquinoxaline-fused [7]carbohelicene ([7]Hl-NAIQx). This compound was obtained by a single-step synthesis utilizing alkylation of the parent quinoxaline-fused [7]carbohelicene ([7]Hl-Qx). Direct alkylation on the quinoxaline unit converts the quinoxaline-fused helicene to a discontinuous conjugated structure. In the single crystal analysis of [7]Hl-NAIQx, a significant structural change by alkylation was observed, as compared to the parent [7]Hl-Qx. In [7]HI-NAIQx, one of the phenanthrene units is almost in the same plane as the quinoxaline. Besides, the dihedral angle is so larger than in [7]Hl-Qx, that one may wonder whether the interesting properties of helicens are entirely maintained. However, the red-shifted spectral features and smaller electrochemical HOMOLUMO gap of [7]HI-NAIQx relative to those of pristine phenanthrene, [7]Hl and [7]Hl-Qx may indicate that [7]HI-NAIQx is basically a helicene-like molecule with -conjugation. In particular, the absolute fluorescence quantum yield (FL) of [7]Hl-NAIQx is 0.25, which is more than 10 times larger than that of [7]carbohelicene (FL = 0.02). The two plausible reasons for the enhanced FL are (i) intramolecular charge transfer (ICT) emission, and (ii) structural change


25


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Page 26 of 36

during the planar formation between the alkyl-quinoxaline and phenanthrene units, which decelerates the ISC pathway. We have also evaluated the CPL property of [7]Hl-NAIQx because of the efficient FL value. The value of the anisotropy factor, gCPL, was calculated to be 4.0 × 103

. Finally, the electroluminescence of an OLED utilizing [7]Hl-NAIQx as the emissive layer was

also observed. Such a systematic design, synthesis, and fluorescence property of quinoxalineutilized nonplanar molecular systems may open a new possibility for future development of efficient optoelectronic applications.

EXPERIMENTAL SECTION [7]Hl-NAIQx. [7]Hl-Qx (34.2 mg, 0.072 mmol) was dissolved in EtOH (200 mL). Then, the solution was stirred at 78 oC for 1 h. Next, after adding 20 mM aqueous sodium dithionite (300 mL) to the parent solution, it was stirred at 78 ºC for 16 h. After cooling to room temperature, the resulting solid was collected by filtration, washed several times with distilled water, and dried in vacuo at 60 oC for 1 h. Next, the solid (313.2 mg) was dissolved in 1,2-dimethoxyethane (20 mL). 2.6 M n-BuLi in hexane (0.90 mL, 2.3 mmol) was added dropwise over 5 min to the solution at 78 oC. After adding MeI (5.0 mL, 80 mmol) to the solution, the solution was stirred at room temperature for 13 h. After completion of the reaction, distilled water was added, and then the organic layer was extracted with ethyl acetate, washed with brine, dried over anhydrous MgSO 4, and evaporated. The crude product was purified over silica gel column using hexane/ethyl acetate (15:1, v/v). Finally, the product was purified by gel permeation chromatography, and [7]Hl-NAIQx (12.3 mg, 0.022 mmol, 31%) was obtained as an orange solid. 1H-NMR (400 MHz, CDCl3):  (ppm): 8.34 (d, J = 8.3 Hz, 1H), 7.96 (d, J = 8.3 Hz, 1H), 7.90 (d, J = 8.3 Hz, 1H), 7.81 (d, J = 8.3 Hz, 1H), 7.65 (d, J = 8.8 Hz, 1H), 7.50 (d, J = 8.3 Hz, 1H), 7.45 (d, J = 8.8 Hz,


26

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1H), 7.40 (dd, J = 7.3, 3.7 Hz, 1H), 7.32 (d, J = 8.3 Hz, 2H), 7.21-7.15 (m, 2H), 7.00-6.97 (m, 2H), 6.88-6.85 (m, 1H), 6.80-6.77 (m, 2H), 6.48-6.43 (m, 2H), 6.26-6.23 (m, 1H), 3.37 (s, 3H), 2.14 (d, J = 8.3 Hz, 2H), 1.31-1.27 (m, 2H), 1.01-0.95 (m, 2H), 0.56 (t, J = 7.3 Hz, 3H);

13

C-

NMR (100 MHz, CDCl3):  (ppm): 161.2, 132.3, 132.1, 129.6, 128.9, 128.8, 127.8, 127.6, 126.9, 126.4, 126.3, 125.9, 125.8, 125.5, 123.9, 122.8, 118.2, 66.1, 37.5, 36.0, 27.8, 22.2, 13.7; MALDI-TOF mass 554 ([M+2H]2+). (See: SI Figures S1 and S2) Preparation and Analyses of Single Crystal Structures. Single crystals of rac-[7]Hl-Qx and rac-[7]Hl-NAIQx for packing formation analysis were prepared by vapor diffusion of CHCl3 into a MeOH solution at room temperature. Analyses of crystal structures were performed on a Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo K radiation. The structures were solved by direct methods (SHELXS-97). CCDC-982754 and CCDC-982758 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif. Computation of molecular graphics was performed with ORTEP-3 for Windows.

ASSOCIATED CONTENT Supporting Information. Experimental section, detailed synthetic details and procedures, 1H and

13

C NMR, single crystal structures of rac-[7]Hl, rac-[7]Hl-Qx and rac-[7]Hl-NAIQx,

molecular orbitals of [7]Hl-(NAI)2Qx, steady-state UV-vis an fluorescence spectra of [7]Hl-Qx and [7]Hl-NAIQx, phosphorescence emission spectra, simulated CD spectra and CPL spectra of P-[7]Hl-Qx. This material is available free of charge via the Internet at http://pubs.acs.org.


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Page 28 of 36

Corresponding Author *E-mail: [email protected] (T.H.), [email protected] (T.K.) [email protected] (Y.A.), [email protected] (T.T.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research (Nos. 26286017 & 26620159 & 15H01003 & 15H01094 to T.H. and No. 25410167 to Y.A. and No. 25886012 to H.S.) and Mitsubishi Foundation. We are grateful to Mr. Shohei Katao (NAIST) for the single crystal X-ray diffraction analyses. This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices."

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