Controlled Excited-State Dynamics and Enhanced Fluorescence

Article pubs.acs.org/JPCC

Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process Yuki Yamamoto,† Hayato Sakai,† Junpei Yuasa,‡,§ Yasuyuki Araki,*,∥ Takehiko Wada,∥ Tomo Sakanoue,⊥ Taishi Takenobu,*,⊥ Tsuyoshi Kawai,*,‡ and Taku Hasobe*,† †

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan § PRESTO, Japan Science and Technology Agency, Kawaguchi, 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 ‡

S Supporting Information *

ABSTRACT: Tetrasulfone[9]helicene (PTSH) was newly synthesized to improve and evaluate its fluorescence and excited-state dynamics through a single-step oxidation reaction of tetrathia[9]helicene (PTTH). In electrochemical measurements, the reduction potential of PTSH was shifted in a positive direction by approximately 1.0 V when compared to that of PTTH because of its electron-accepting sulfone units. The results of the electrochemical measurements agree with the energy levels calculated by density functional theory (DFT) methods and steady-state spectroscopic measurements. Furthermore, a significant enhancement of the absolute fluorescence quantum yield (ΦFL) was achieved. The absolute fluorescence quantum yield of PTSH attained 0.27, which is approximately 10 times larger than that of PTTH (ΦFL = 0.03). Such an enhancement of ΦFL can be successfully explained by the corresponding kinetic comparison. The reason is mainly the increased energy gap ΔEST between the lowest singlet (S1) and triplet (T1) excited states. Finally, excellent circularly polarized luminescence of PTSH was also observed. The value of the anisotropy factor gCPL was estimated to be 8.3 × 10−4 in PTSH.

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the utilization of sulfone units in helicene synthesis.35 Therefore, our concept is to oxidize thiophene rings in thiahelicenes by sulfonation to achieve conversion to thiophene S,S-dioxide-fused helicenes. In this study, to attain a significant improvement in the excited-state dynamics and fluorescence properties of the helicene skeleton through a single-step reaction, we designed and synthesized tetrasulfone[9]helicene (PTSH) through the oxidation of tetrathia[9]helicene (PTTH) as shown in Chart 1. The alkyl chains were introduced to improve the solubility for spectroscopic measurements. The process of controlling the excited-state dynamics is discussed herein.

INTRODUCTION Helicenes are nonplanar polycyclic aromatic hydrocarbons (PAHs) consisting of ortho-fused benzene and/or other aromatic rings.1,2 Because of the formation of helical skeletons, these compounds show characteristic photophysical properties such as a higher anisotropy factor (g = Δε/ε) with an increase in the number of aromatic rings.3−11 Recently, one of the major applications of helicenes is circularly polarized luminescence (CPL).12−22 However, fluorescence quantum yields of helicenes are generally extremely low because of the fast intersystem crossing (ISC).23,24 This is in sharp contrast with planar PAH derivatives.25−27 Thus, proper synthetic control of the excited dynamics is required. A significant functional improvement through a single-step reaction is one of the important challenges of the development of novel applications of helicenes, such as in electronic and optoelectronic devices. The introduction of sulfone units is one of the promising ways to improve the electrochemical and photophysical properties of these compounds.28−31 Sulfonate derivatives have been widely used as n-type light-emitting materials for organic light-emitting diodes (OLEDs)32,33 because of the simple reaction with high yields utilizing an oxidizing agent.34 The introduction of sulfone units onto the helicene skeleton is thus expected to enhance the fluorescence quantum yields. However, little attention has been focused on © 2016 American Chemical Society

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RESULTS AND DISCUSSION Synthesis. Scheme 1 summarizes the synthetic procedures for the target compound PTSH. First, thieno[3,2-b]dibenzothiophene (DBT) was synthesized by the reported synthetic method.36 Next, Pr-DBT was obtained by acylation with Weinreb amides (N-methoxy-N-methylbutanamide).37 The yield of Pr-DBT was 70%. Then, (Pr-DBT)2 was synthesized using the McMurry coupling reaction with PrReceived: February 2, 2016 Revised: March 11, 2016 Published: March 18, 2016 7421

DOI: 10.1021/acs.jpcc.6b01123 J. Phys. Chem. C 2016, 120, 7421−7427

Article

The Journal of Physical Chemistry C

PTTH is 3.13 eV, which is slightly larger than the corresponding optical gap (2.86 eV) (Table 2). One of the possible reasons for this difference may be the presence or absence of an interaction of PTTH with supporting electrolytes. On the other hand, the Ered1 of PTSH was determined to be −0.91 V, whereas the oxidation potential of PTSH could not be obtained because of the limited solvent-dependent electrochemical potential window (see Table 1). Thus, through the introduction of electron-accepting sulfone units onto the helicene skeleton, the reduction potentials became shifted in a positive direction by approximately 1.0 V. Theoretical calculations using density functional theory (DFT) methods at the B3LYP/6-31+G (d, p) level of theory were performed to estimate the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as shown in Table 1. The estimated LUMO levels successfully support the trends of the electrochemical results in Table 1. Briefly, the LUMO (−3.44 eV) and HOMO (−7.27 eV) levels of PTSH were significantly stabilized by approximately 1.3 and 1.7 eV, respectively, as compared to those of PTTH. Thus, we have successfully controlled the electrochemical properties of helicenes through the introduction of sulfone units onto the helicene skeletons. Steady-State Spectroscopic Measurements. To examine the electronic structures of these helicene derivatives, absorption spectra of PTTH and PTSH were measured in toluene (Figure 2A). Compared to the spectrum of PTTH, the spectrum of PTSH became largely blue-shifted and broadened. The trends of the fluorescence spectra (Figure 2B) are also similar to those of the absorption spectra. The blue-shift trends are ascribed to the introduction of the electron-accepting sulfone units, which was estimated by the above-mentioned increase in the HOMO−LUMO gap of PTSH. Moreover, the observed phosphorescence spectra of PTTH and PTSH in the range of approximately 500−750 nm (Figure 3) indicate the presence of ISC pathways (Table 2). Time-Resolved Fluorescence Lifetime Measurements and Evaluation of Excited-State Dynamics. To evaluate the excited-state dynamics of these compounds, fluorescence lifetime measurements were performed first. The measurements were carried out in a degassed toluene solution using pulsed laser light (λex = 404 nm). The fluorescence decay profiles are shown in Figure 4. These fluorescence lifetimes (τFL) were evaluated from monoexponential fits and are summarized in Table 2. The τFL of PTSH (1.96 ns) is longer than that of PTTH (0.57 ns). Next, measurements of absolute fluorescence quantum yields (ΦFL) were performed in degassed toluene, and the results are summarized in Table 2. ΦFL of PTSH (0.27) is significantly larger (approximately 10 times) than that of PTTH (0.03). Then, to further explore nonradiative processes such as the ISC pathway, quantum yields of ISC (ΦISC) were measured by 1 O2 phosphorescence experiments under the oxygen-saturated toluene solution.38,39 By utilizing the energy transfer from the triplet excited states of these helicene derivatives to O2, we can detect the 1O2 phosphorescence at approximately 1270 nm, assuming a negligible quenching process from the singlet excited states.38,39 ΦISC for each compound was evaluated from the area ratio under the respective absorbance curve. To estimate the ΦISC values, zinc tetraphenyl-porphyrin (ZnTPP) was employed as a reference compound (ΦISC = 0.88).40 The quantum yields of

Chart 1. Chemical Structures of Tetrathia[9]helicene (PTTH) and Tetrasulfone[9]helicene (PTSH)

Scheme 1. Synthetic Scheme of PTSH

DBT. Next, PTTH was obtained by oxidative photocyclization of (Pr-DBT)2 in 54% yield (two steps). Finally, PTSH was synthesized by oxidation of PTTH with mCPBA in 89% yield. It should be noted that the enantiopure samples (P)-PTSH and (M)-PTSH were synthesized from (P)-PTTH and (M)-PTTH, which were originally separated by chiral HPLC. Details on the synthetic procedures are summarized in the Experimental Section. The isolated compounds were characterized by 1H NMR, 13C NMR, and high-resolution MALDI-TOF mass spectrometry (Figures S1−S9 in the Supporting Information). Electrochemical Studies and Theoretical Calculations. The electrochemical properties of the helicene derivatives were examined by cyclic and differential pulse voltammetry (CV and DPV, respectively). The voltammograms of the helicene derivatives in acetonitrile containing 0.1 M nBu4NPF6 are shown in Figure 1 and Figures S10 and S11 in the Supporting Information. The half-wave potentials (E1/2 ) of these compounds are summarized in Table 1. The first one-electron reduction (Ered1) and oxidation (Eox1) potentials of PTTH were determined to be −1.86 and 1.27 V, respectively, versus a saturated calomel electrode (SCE). The electrochemical gap of

Figure 1. Cyclic voltammograms of (a) 0.5 mM PTTH and (b) 0.5 mM PTSH in acetonitrile with 0.1 M nBu4NPF6 as the supporting electrolyte. Scan rate: 0.1 V s−1. 7422


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The Journal of Physical Chemistry C Table 1. Summarized Redox Potentials and Energy Levels of PTTH and PTSH Eox2a (V)

Eox1a (V)

Ered1a (V)

Ered2a (V)

1.57

1.27

−1.86 −0.91

−2.39 −1.34

PTTH PTSH

Ered3a (V)

EHOMOb (eV)

ELUMOb (eV)

gap (eV)

−2.04

−5.54 −7.27

−2.11 −3.44

3.43 3.83

a Versus SCE, determined by cyclic voltammetry (CV), measured in acetonitrile. bVersus a vacuum, calculated at the B3LYP/6-31+G (d, p) level of theory.

Table 2. Summarized Quantum Yields and Rate Constants of PTTH and PTSH PTTH PTSH

τFLa (ns)

ΦFLb

ΦISCc

ΦICd

kFL, 107 (s−1)e

kISC, 107 (s−1)f

kIC, 107 (s−1)g

S1−S0h (eV)

T1−S0i (eV)

ΔEST (eV)

0.57 1.96

0.03 0.27

0.79 0.61

0.18 0.12

5.26 13.8

139 31.1

31.6 6.12

2.86 3.13

2.26 2.11

0.60 1.02

a

Fluorescence lifetime (τFL), excited at 404 nm. bFluorescence emission quantum yield (ΦFL), excited at 400 nm (PTTH) and 300 nm (PTSH). Intersystem crossing quantum yield (ΦISC), excited at 355 nm. dInternal conversion quantum yield (ΦIC = 1 − ΦFL − ΦISC). eFluorescence emission rate constant (kFL = ΦFLτFL−1). f Intersystem crossing rate constant (kISC = ΦISCτFL−1). gInternal conversion rate constant (kIC = ΦICτFL−1). h S1−S0 determined by UV−vis and fluorescence spectra, estimated by 0−0 absorption from steady-state absorption and fluorescence spectra in toluene. iT1−S0 estimated by phosphorescence spectra in 2:1:1 diethyl ether/toluene/ethanol (v/v/v) at 77 K. c

Figure 2. Absorption (A) and fluorescence spectra (B) of PTTH (a) and PTSH (b) in toluene. Excitation wavelengths: 390 nm (PTTH) and 350 nm (PTSH).

Figure 4. Fluorescence decay profiles of PTTH (a) and PTSH (b) in degassed toluene (λex = 404 nm).

109 s−1) is two orders greater than that of kFL (5.26 × 107 s−1). First, we performed kinetic calculations regarding the enhancement of ΦFL of PTSH. The magnitude of kFL of PTSH (1.38 × 108 s−1) is one order of magnitude greater than that of PTTH (5.26 × 107 s−1), whereas kISC of PTSH decreases to 3.11 × 108 s−1 compared to that of PTTH (1.39 × 109 s−1). Thus, the acceleration of kFL and the deceleration of kISC successfully contribute to the drastic enhancement of the ΦFL values in this system. To carefully explain the enhancement of the florescence property, the following explanation of ISC is required. It is well known that kISC is generally influenced by the energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states.41 To systematically compare the ΔEST values of these compounds, we can estimate these values from the abovementioned phosphorescence spectra (Figure 3 and Table 2). The ΔEST of PTSH (1.02 eV) is much larger than that of PTTH (0.60 eV). This may indicate that the sulfone unitsubstituted helicene with a large ΔEST relatively suppresses the corresponding ISC pathways. Additionally, phosphorescence lifetime measurements were performed. The measurements were carried out in degassed 2:1:1 diethyl ether/toluene/ ethanol (v/v/v) at 77 K (λex = 365 nm, λobs = 590 nm). The phosphorescence decay profiles are shown in Figures S12 and S13 in the Supporting Information. The phosphorescence lifetimes of PTTH and PTSH were determined to be 135 and 1.80 ms, respectively. Circular Dichroism (CD) and CPL Spectra of PTSH. To investigate the anisotropy of PTSH, we measured CD spectra

Figure 3. Phosphorescence spectra of PTTH (a) and PTSH (b) in N2purged frozen ETE (2:1:1 diethyl ether/toluene/ethanol, v/v/v, 77 K). Excitation wavelengths: 390 nm (PTTH) and 350 nm (PTSH).

the internal conversion pathways (ΦIC) of helicenes were also calculated by subtracting the above-mentioned ΦFL and ΦISC values from 1. The summarized ΦISC and ΦIC values are shown in Table 2. It should be emphasized that the sum of the quantum yields of these three processes is close to 1, within the experimental error, because they were evaluated by different measurement methods. ΦISC of PTSH is 0.61, which is much smaller than that of PTTH (0.79) due to significant enhancements of the corresponding ΦFL values. Finally, the net rate constants fluorescence emission (kFL), intersystem crossing (kISC), and internal conversion (kIC) were determined (Table 2). Before the detailed discussion, it should be emphasized that, for PTTH, the magnitude of kISC (1.39 × 7423



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as shown in Figure 5A. Intense mirror-image Cotton effects were clearly observed within the experimental error. To

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CONCLUSIONS

In this work, fluorescent tetrasulfone[9]helicene (PTSH) was successfully synthesized through a single-step reaction from tetrathia[9]helicene (PTTH). The introduction of sulfone units onto the helicene skeleton contributes to the highly fluorescent characteristic of PTSH when compared to the fluorescence of the parent thiahelicene. In particular, ΦFL of PTSH attains 0.27. This value is significantly larger than that of PTTH (ΦFL = 0.03). Such a significant enhancement is successfully explained by the acceleration of kFL and the deceleration of kISC. Further, the ΔEST value of PTSH (1.02 eV) is much larger than that of PTTH (0.60 eV). Circularly polarized luminescence was successfully observed. The gCPL of PTSH was estimated to be 8.3 × 10−4. We believe that such a strategy for the enhancement of fluorescence properties provides a new perspective for further development of luminescent materials.

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EXPERIMENTAL SECTION General Methods. All high-grade solvents and reagents were purchased from commercial suppliers such as Tokyo Chemical Industry, Nacalai Tesque, and Wako Pure Chemical Industries. All commercial reagents were used without further purification. Column flash chromatography was performed on silica gel (Fuji Silysia Chemical Ltd. PSQ-60B or PSQ-100B). Racemic PTTH was separated using hexane as the eluent at room temperature on a DAICEL CHIRALPAK IA column. 1H NMR and 13C NMR spectra were recorded on a 400 MHz spectrometer (JEOL ECX-400, AL-400, or ALPHA-400) using the solvent peak as the reference standard, with chemical shifts given in parts per million (ppm). CDCl3 and DMSO-d6 were used as the NMR solvent. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were recorded on a Bruker Ultraflex instrument. The DFT and TDDFT calculations were performed using Gaussian 09 at the B3LYP/6-31+G (d, p) level of theory. DBT and N-methoxy-Nmethylbutanamide were synthesized using the reported synthetic method.36,37 Synthesis of Pr-DBT. DBT (2.00 g, 8.33 mmol) was dissolved in dry THF (150 mL). Then, a 1.6 M solution of nBuLi in hexane (5.70 mL, 9.16 mmol) was added dropwise to the mixture at 0 °C under a nitrogen atmosphere. The solution was stirred for 1 h at 0 °C, and then N-methoxy-Nmethylbutanamide (1.10 g, 8.33 mmol) was added dropwise. The reaction mixture was stirred for 1 h at 0 °C and was then allowed to warm to room temperature. Next, the reaction was quenched with the addition of 10 mL of water, and the organic layer was extracted with toluene, washed with water, dried over anhydrous Na2SO4, and evaporated to afford a yellow-brown oil. The crude product was purified by column chromatography on silica gel eluting with 10:1 hexane/ethyl acetate (v/v). PrDBT (1.81 g, 70% yield) was obtained as a yellow solid. 1H NMR (400 MHz, CDCl3): δ 8.82 (1H, s), 8.57 (1H, d, J = 7.9 Hz), 8.00−7.94 (3H, m), 7.63 (1H, dd, J = 7.6, 7.6 Hz), 7.55 (1H, dd, J = 7.6, 7.6 Hz), 3.16−3.14 (2H, m), 1.93−1.90 (2H, m), 1.11 ppm (3H, t, J = 7.4 Hz). 13C NMR (100 MHz, CDCl3): δ 194.4, 144.4, 140.6, 139.8, 136.9, 135.1, 134.0, 130.5, 126.2, 125.3, 124.8, 123.6, 123.1, 122.1, 121.0, 41.4, 18.0, 13.9. High-resolution MALDI-TOF: m/z calcd 311.0565 [M + H]+, found 310.8643 (see Figures S1−S3). Rf = 0.34 (10:1 hexane/ ethyl acetate, v/v). Mp: 56 °C. Synthesis of (Pr-DBT)2. Zinc dust (211 mg, 3.22 mmol) was added to dry THF (200 mL) at 0 °C under an atmosphere

Figure 5. CD (A) and CPL (B) spectra of PTSH in THF (the P form (a) and M form (b)).

determine the helical structures of these derivatives, timedependent density functional theory (TDDFT) calculations were performed using the Gaussian suite of programs at the B3LYP level of theory and the 6-31+G(d, p) basis set (see Figure S14 in the Supporting Information). The anisotropy factors (gCD) of these molecules were calculated, and gCD of PTSH was estimated to be 4.7 × 10−3. To further investigate the structural information of the excited states of the helicene derivatives, we measured the CPL spectra of PTSH with high ΦFL values. CPL is the differential emission of right-handed and left-handed circularly polarized light by chiral luminescence systems. The extent of CPL is given by the anisotropy factor gCPL = 2(IL − IR)/(IL + IR).16 It should be noted that IL and IR are the intensities of the lefthanded and right-handed CPL spectra, respectively. The CPL spectrum of PTSH was successfully detected as shown in Figure 5B. The obtained CPL spectra are actually mirror images. The gCPL of PTSH is estimated to be 8.3 × 10−4. Among the related small organic compounds, this value is similar to the previously reported compound in a monomeric state.12,15 7424


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Spectroscopic Measurements. UV−vis absorption spectra were recorded on a PerkinElmer (Lamda 750) UV−vis− NIR spectrophotometer. Fluorescence and phosphorescence emission spectra were recorded on a Parkin Elmer (LS-55) spectrofluorophotometer. The absolute fluorescence quantum yields were measured by a Hamamatsu Photonics C9920-02 system equipped with an integrating sphere and a red-sensitive multichannel photodetector (PMA-12; excitation wavelength 300 nm for PTSH, 400 nm for PTTH). Fluorescence lifetimes were measured on a HORIBA Scientific time-correlated singlephoton counting system (FluoroCube) with the laser light (DeltaDiode, laser diode head, 404 nm, pulse width 50 ps) as the excitation source. Phosphorescence lifetimes were recorded on a Horiba Jobin Yvon FluoroMax Plus spectrofluorometer. 1 O 2 Phosphorescence Measurements. The 1 O 2 * emission spectrum was measured using the following setup. A sample solution in toluene was bubbled with O2 for 20 min before the measurements. The solution was irradiated with a 355 nm continuous wave laser (EKSPLA 2210A, 1 kHz operation, 6 mW). The emission in the NIR region from the sample solution was chopped at 84 Hz, then introduced to the monochromator (Shimadzu, SPG-120IR), and detected by a photodiode (New Focus, model 2153). The diode signal was accumulated by the digital lock-in amplifier (NF Electronic Instruments LI5640) operated in the chopper frequency and then transferred to a personal computer. The computer controlled the monochromator to obtain the emission spectrum. ΦISC in toluene was measured following the general method with zinc tetraphenyl-porphyrin (ZnTPP) employed as a reference compound (ΦISC = 0.88).40

of nitrogen. Then, TiCl4 (0.21 mL, 1.94 mmol) was added dropwise to the mixture at 0 °C under a nitrogen atmosphere. The mixture was stirred for 1 h at 0 °C, and then the mixture was refluxed for 2 h. Next, dry pyridine (0.13 mL, 1.61 mmol) was added, and the solution was refluxed for 15 min. After 15 min, Pr-DBT (500 mg, 1.61 mmol) in dry THF (30 mL) was added by cannulation. The reaction mixture was refluxed for 18 h. Then, the reaction was quenched with the addition of 10 mL of water, and the organic layer was extracted with toluene, washed with brine, dried over anhydrous Na2SO4, and evaporated to afford a yellow-brown oil. The crude product was purified by column chromatography on silica gel eluting with 5:1 hexane/ethyl acetate (v/v). The product was used in the next step without further purification. Synthesis of PTTH. Iodine (172 mg, 0.680 mmol) was added to a solution of (Pr-DBT)2 (400 mg, 0.680 mmol) in dry toluene (500 mL) under a nitrogen atmosphere. The reaction mixture was irradiated using a 400 W high-pressure Hg lamp at room temperature for 5 h. Next, the toluene layer was washed with aqueous sodium thiosulfate (Na2S2O3), water, and brine, dried over anhydrous Na2SO4, and evaporated to afford a dark yellow solid. The crude product was purified by column chromatography on silica gel eluting with 5:1 hexane/ethyl acetate (v/v). PTTH (245 mg, 54% yield, two steps) was obtained as a yellow solid. 1H NMR (400 MHz, CDCl3): δ 7.99 (2H, d, J = 8.3 Hz), 7.73 (2H, d, J = 8.3 Hz), 7.48 (2H, d, J = 8.1 Hz), 6.98 (1H, dd, J = 6.1, 2.2 Hz), 6.95 (1H, dd, J = 6.1, 2.2 Hz), 6.13 (2H, dd, J = 8.1, 6.5 Hz), 6.10 (2H, d, J = 6.5 Hz), 3.29−3.25 (4H, m), 2.03−2.00 (4H, m), 1.26 ppm (6H, t, J = 7.3 Hz). 13C NMR (100 MHz, CDCl3): δ 138.5, 138.4, 135.8, 135.7, 133.9, 132.6, 132.0, 131.6, 128.0, 125.1, 122.6, 121.0, 120.3, 120.2, 119.3, 34.7, 23.5, 14.8 ppm. High-resolution MALDI-TOF: m/z calcd 588.1075 [M + 2H]+, found 587.8748 (Figures S4−S6). Rf = 0.60 (5:1 hexane/ethyl acetate, v/v). Mp: 219 °C. Synthesis of PTSH. m-CPBA (contains ca. 30% water, 252 mg, 1.02 mmol) was added to a stirred solution of PTTH (50 mg, 0.0853 mmol) in dry CH2Cl2 (50 mL) at room temperature. The mixture was stirred for 24 h. Then, the organic layer was washed with brine, dried over anhydrous Na2SO4, and evaporated to afford a white solid. The crude product was purified by column chromatography on silica gel eluting with CH2Cl2/ethyl acetate (5:1, v/v), PTSH (54 mg, 89% yield) was obtained as a white solid. 1H NMR (400 MHz, CDCl3): δ 8.00 (2H, d, J = 7.8 Hz), 7.78 (2H, d, J = 7.3 Hz), 7.72 (2H, d, J = 7.8 Hz), 7.41 (2H, dd, J = 7.8, 7.8 Hz), 6.89 (2H, dd, J = 7.3, 7.8 Hz), 6.66 (2H, d, J = 7.8 Hz), 3.27−3.10 (4H, m), 1.93−1.89 (4H, m), 1.23 (6H, t, J = 7.3 Hz). 13C NMR (100 MHz, DMSO-d6): δ 142.8, 142.1, 142.0, 141.4, 135.8, 131.9, 131.0, 128.8, 127.2, 127.1, 126.2, 124.2, 123.3, 123.0, 122.5, 29.5, 23.7, 13.9. High-resolution MALDI-TOF: m/z calcd 717.0748 [M + 3H]+, found 717.2609 (see Figures S7−S9). Rf = 0.64 (5:1 CH2Cl2/ethyl acetate, v/v). Mp: >300 °C. Electrochemical Measurements. Cyclic voltammograms were recorded on an Iviumstat 20 V/2.5 A potentiostat using a three-electrode system. A platinum electrode was used as the working electrode. A platinum wire served as the counter electrode, and a saturated calomel electrode was used as the reference electrode. A ferrocene/ferrocenium redox couple was used as an internal standard. All solutions were purged using nitrogen gas prior to electrochemical measurements.

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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.jpcc.6b01123. Details of 1H NMR spectra, 13C NMR spectra, MALDITOF mass spectra, cyclic voltammograms, phosphorescence lifetimes, and simulated CD spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was partially supported by Grants-in-Aid for Scientific Research (26286017, 26620159, 15H01003 “πSystem Figuration”, and 15H01094 “Photosynergetics” to T.H., 25410167 to Y.A., 26102012 “π-System Figuration” to T.T., and 25886012 to H.S.). This work was performed under the Cooperative Research Program of the Network Joint Research Centre for Materials and Devices.

(1) Shen, Y.; Chen, C. F. Helicenes: Synthesis and Applications. Chem. Rev. 2012, 112, 1463−1535.

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