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Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units Hayato Sakai, Takako Kubota, Junpei Yuasa, 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.6b01344 • Publication Date (Web): 27 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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The Journal of Physical Chemistry

Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units Hayato Sakai,*,† Takako Kubota,† Junpei Yuasa,‡,ǁ 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, Ikoma, Nara 630-0192 Japan ǁ

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

ACS Paragon Plus Environment

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ABSTRACT: A series of [5]carbohelicene derivatives substituted by electron-withdrawing maleimide and electron-donating methoxy such as maleimide-substituted [5]carbohelicene (HeliIm), and methoxy-substituted HeliIm (MeO-HeliIm) were newly designed and synthesized to examine the electrochemical properties, excited-state dynamic and circularly polarized luminescence (CPL). First, electrochemical measurements and DFT calculations of [5]carbohelicene derivatives were performed by comparing with the structural isomers: picene derivatives. Introduction of an electron-withdrawing maleimide group onto a [5]carbohelicene core contributes to the stabilized LUMO state in HeliIm as compared to that of [5]carbohelicene (Heli), whereas the energy level of HOMO state in MeO-HeliIm increases by introducing electron-donating methoxy (MeO) groups onto a HeliIm skeleton. The HOMO-LUMO gap of MeO-HeliIm is smaller than those of HeliIm and Heli, which is similar to the steady-state spectroscopic measurements. The absolute fluorescence quantum yield (ΦFL) of HeliIm (0.37) largely increased as compared to [5]carbohelicene, Heli (0.04), whereas ΦFL of MeO-HeliIm (0.22) was slightly smaller than that of HeliIm. Theses photophysical processes including intersystem

crossing

are

successfully

explained

by

the

kinetic

discussions.

Since

[5]carbohelicene derivatives show the chirality, measurements of circular dichroism (CD) and circularly polarized luminescence (CPL) were successfully performed. In particular, HeliIm and MeO-HeliIm have provide excellent circularly polarized luminescence (CPL) and the values of the anisotropy factor glum were estimated to be 2.4 × 10−3 and 2.3 × 10−3, relatively. This is the first observation of CPL in [5]carbohelicene derivatives.


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INTRODUCTION There has been recently increasing interest in polycyclic aromatic hydrocarbons (PAHs) because they have unique electronic and photphysical properties originated from the πconjugated structures.1-7 These molecules are great importance in fundamental theoretical researches as well as device applications such as field-effect transistors,3, 8-14 solar cells,15-20 lightemitting diodes.21-24 PAH derivatives have various structural isomers based on the differences in positions of condensed benzene rings. For example, pentacene (acene) and picene (phenacene) consisting of five fused benzene rings are planar structural isomers. Picene has been known to possess a zig-zag structure as well as high carrier mobility, whereas the light absorption region became blue-shifted as compared to pentacene.25-26 Thus, these isomers demonstrate the unique structural and photophysical properties induced by different arrangements of condensed benzene rings. However, little attention has been paid to the electrochemical and photophysical differences between planar and nonplanar isomers (i.e., picene and helicene).27 Helicenes are nonpolar polycyclic aromatic hydrocarbons (PAHs), which are formed by ortho-condensed benzene rings. They have been studied in various fields such as synthesis,28-30 functional supramolecular architectures,30-35 fluorescent sensors,30,

36-38

and liquid-crystalline

materials30, 39-42 because of the characteristic molecular structures. They are also well-known to demonstrate unique chiroptical properties such as circular dichroism (CD) with anisotropy factor (g-value).43-48 In particular, chiral luminescence materials and compounds indicate a possibility for circularly polarized luminescence (CPL) as new chiroptical properties, which is the differential emission of right- and left-circularly polarized light.49-64 However, the fluorescence quantum yields (ΦFL) of helicene derivatives are generally extremely low because the intersystem


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crossing (ISC) pathways are highly accelerated (ΦISC: ~0.9).65-67 Therefore, synthetic strategies for improvements of ΦFL values are definitely required.56, 68-71 Additionally, [5]carbohelicenes consisted by five benzene rings are generally unable to show the chirality because of the only overlapped hydrogen atoms in the terminal benzene rings as compared with carbohelicenes fused by more than six benzene rings.72-73 The CPL of carbohelicenes containing more than six benzene rings are already examined,74 whereas the CPL of [5]carbohelicene derivatives have yet to be reported, so far. By introducing bulky substituents into the terminal benzene rings, [5]carbohelicene derivatives are expected to maintain the stable chirality. To control the excited-state dynamics of PAH derivatives, one of the useful methods is to introduce new electron-accepting substituents (e.g., maleimide groups) onto the peripheral positions of an aromatic core. In particular, perylenediimides (PDIs) are one of the representative molecules because of the high fluorescence quantum yield (ΦFL: ~1).75-76 In contrast, we have recently examined the excited-state dynamics and electrochemical properites of maleimidesubstituted benzo[ghi]perylene and coronene derivatives (i.e., benzo[ghi]peryleneimide and coroneneimide).77-79 In this case, the ΦFL and ΦISC values were dependent on the symmetry of the PAH core, peripheral positions and number of the electron-withdrawing substituents. The other promising strategy for the enhanced fluorescence properties is to use “push-pull’’ molecules, which are constituted of a conjugated π-electron system asymmetrically substituted by an electron-donor unit (D) and an electron-acceptor one (A).24, 70 Herein, we demonstrate the excited-state dynamics and circularly polarized luminescence of [5]carbohelicenes derivatives substituted by maleimide units. First, we synthesized a series of nonplanar [5]carbohelicenes and planar picene derivatives substituted by electron-withdrawing maleimide (Im) and electron-donating methoxy (MeO) groups to examine the electrochemical


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and photophysical properties as shown in Chart 1. The ΦFL value of maleimide-substituted [5]carbohelicene (HeliIm) largely increased as compared to the pristine [5]carbohelicene (Heli), whereas the ΦFL value of methoxy-substituted HeliIm (MeO-HeliIm) was smaller than that of HeliIm. This is in sharp contrast with the trends in Pi derivatives. Then, we evaluated the circularly polarized luminescence (CPL) of HeliIm and MeO-HeliIm because relatively high ΦFL values were obtained. The emphasis in this work is on the observation of stable chiroptical properties in [5]carbohelicene derivatives (i.e., CD and CPL) regardless of the only overlapped hydrogen atoms in the terminal benzene rings (i.e., HeliIm). The details on the synthesis as well as structural and photophysical properties will be presented here. Chart 1. Chemical Structures of Heli and Pi Derivatives in This Study.

RESULTS AND DISCUSSION


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Synthesis and Crystal Structures of HeliIm and Pilm Derivatives. We have already established the synthetic method of poly aromatic maleimide compound from poly aromatic diester. Based on our reports,77 the syntheses of HeliIm and MeO-HeliIm were achieved by using corresponding dimethoxycarbonyl [5]carbohelicenes, HeliCO2Me80 and MeO-HeliCO2Me81 as the starting compounds by 3 steps (Scheme 1). First, HeliAH derivatives were obtained by hydrolysis of dimethoxycarbonyl [5]carbohelicenes followed by dehydration. Next, HeliIm and MeO-HeliIm were synthesized by the dehydration of corresponding HeliAH and 4-heptylamine. PiIm and MeO-PiIm were also synthesized by same scheme of HeliIm derivatives (Schemes S1 and S2 in Supporting Information (SI)). The structures of all compounds were assigned by 1H and 13C NMR and single crystal structures (Experimental Section and Figures S1-S20 in SI). The enantiomers of HeliIm and MeO-HeliIm were separated from the racemic compounds by chiral HPLC (vide infra).

Scheme 1. Synthetic Schemes of HeliIm and MeO-HeliIm.


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Figure 1. Single crystal structures of (A) (+)-(P)-HeliIm and (B) (+)-(P)-MeO-HeliIm.

Single-crystal structures were determined by X-ray analysis to evaluate the molecular structures of HeliIm and MeO-HeliIm. We prepared single crystals of these derivatives for X-ray diffraction analysis by vapor diffusion at room temperature (see: Experimental Section). The typical single crystal structures of (+)-(P)-HeliIm and (+)-(P)-MeO-HeliIm were shown in Figure 1. Additional crystal data of Heli and Pi derivatives are also summarized in Figures S13S20 in SI. The interpitch distance of HeliIm (2.92 Å) is quite similar to that of MeO-HeliIm (2.91 Å). On the other hand, by introducing bulky MeO substituents into the terminal benzene rings, the degree of steric repulsion of MeO-HeliIm clearly increases as compared to HeliIm and Heli. Moreover, it should be noted that single crystals of these derivatives continuously maintain


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the stable chirality. Accordingly, the enantiomers of HeliIm and MeO-HeliIm separated from the racemic compounds by chiral HPLC were usually stored as single crystals and we prepared the solutions using the chiral crystals of HeliIm and MeO-HeliIm just prior to the spectroscopic measurements.

Electrochemical Properties of Heli and Pi Derivatives. The electrochemical properties of Heli and Pi derivatives were evaluated by using cyclic voltammetry and differential pulse voltammetry to discuss the substituent and ring-fused structure-dependent reduction and oxidation potentials as shown in Figure 2 and Figure S21 in SI.

Figure 2. Cyclic and differential pulse voltammograms of (A) HeliIm in CH2Cl2 and (B) MeOHeliIm in CH2Cl2 with 0.1 M nBu4NPF6 as supporting electrolyte. Scan rates: 0.1 V s-1 for CV and 0.01 V s-1 for DPV.


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In these measurements, we mainly used CH2Cl2 and DMF as solvents to widen the solventdependent electrochemical potential window. nBu4NPF6 was used as a supporting electrolyte salt. The electrochemical potentials of Heli and Pi derivatives are summarized in Table 1. It should be noted that there is no influence of racemization on the electrochemical (Figure 2) and photochemical (Figures 3 and 4) measurements in this study. The redox potentials of Heli could not be also measured because of unstable radical cation and anion species. Therefore, to support this problem, the theoretical calculations using DFT methods at the B3LYP/6-31G* level of theory was performed to estimate the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were performed (Table 1 and Figures S22-S23 in SI). First, the LUMO level of HeliIm (–2.49 eV) is stabilized by ca. 1.2 eV as compared to that of Heli (–1.29 eV), whereas the corresponding HOMO level of HeliIm (– 5.81 eV) is approximately similar to Heli (ca. –5.36 eV). Namely, introduction of an electronwithdrawing maleimide group onto a Heli core contributes to the stabilized LUMO state. Then, we experimentally observed the two successive and reversible redox couples of HeliIm, which are corresponding to the first and second one-electron reduction processes, whereas a quasireversible oxidation peak was seen. The first one-electron reduction (Ered1) in DMF and oxidation (Eox1) potentials of HeliIm in CH2Cl2 against the saturated calomel electrode (SCE) are −1.23 and 1.78 V, respectively. In the case of introduction of MeO units onto a HeliIm skeleton (MeOHeliIm), the negative shift of the oxidation potential (Eox = 1.42 V) were observed in contrast with the similar reduction potential of MeO-HeliIm (Ered1 = –1.24 V). The negative shift of oxidation potential of MeO-HeliIm was also supported by DFT methods. The HOMO level of MeO-HeliIm (–5.36 eV) increases by ca. 0.5 eV as compared to that of HeliIm (–5.81 eV),


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whereas the corresponding LUMO levels have the approximately similar value to HeliIm (ca. 2.3~2.4 eV). These electron releasing and withdrawing effects on the reduction and oxidation potentials were similarly observed in Pi derivatives in Table 1.

Table 1. Redox Potentials and HOMO-LUMO Energies of Heli and Pi Derivatives.

Compound

Eox1a

Ered1a

Ered2a

HOMOb, eV

LUMOb, eV

gap, eV

Heli

–

–

–

–5.49

–1.29

4.20

HeliIm

1.78

–1.23

–1.77

–5.81

–2.49

3.32

MeO-HeliIm

1.42

–1.24

–1.74

–5.36

–2.35

3.01

Pi

1.50c

–2.23

–

–5.49

–1.27

4.22

PiIm

1.76

–1.23

–1.81

–5.71

–2.44

3.27

MeO-PiIm

1.46

–1.24

–1.84

–5.23

–2.25

2.98

a

Volt vs. SCE in CH2Cl2. bCalculated at the B3LYP/6-31G* level of theory. cVolt vs. SCE in DMF.

Steady-State Spectroscopic Measurements of Heli and Pi Derivatives. Measurements of absorption and fluorescence spectra were performed to investigate the electric structures of Heli and Pi derivatives in toluene as shown in Figures 3, 4 and Figures S24, S25 in SI. Figure 3 indicates absorption spectra of Heli derivatives and MeO-PiIm. As compared with the spectrum of Heli (spectrum a), the spectra of HeliIm (spectrum b) and MeO-HeliIm (spectrum c) became red-shifted in the range of ca. 350 nm–500 nm region. Based on the results of electrochemical data and DFT calculations of Heli derivatives, a possible reason for red-shifted trend is lowering of the LUMO level compared to the HOMO levels because of introduction of an electronwithdrawing maleimide unit, which leads a decrease of the HOMO-LUMO gap (Table 1). In addition, estimations of molar extinction coefficients (ε0-0) from the 0−0 absorption bands were


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determined (Table 2). The peak corresponding to 0−0 absorption bands of Heli at 396 nm is assigned to be a symmetry-forbidden transition because of high symmetry (ε0−0: 200 M−1 cm−1).82 When the symmetry of Heli is reduced by substituents, the forbidden transition becomes allowed. Therefore, the ε0−0 values of HeliIm and MeO-HeliIm largely increase. Regarding structural isomers, the absorption region in MeO-PiIm (spectrum d) are approximately similar to MeOHeliIm (spectrum c). The similar spectral trend was thus observed in Pi derivatives (Figure S24 in SI).

Figure 3. UV-vis spectra of (a) Heli in 1,4-dioxane, (b) 20 µM HeliIm in toluene, (c) 40 µM MeO-HeliIm in toluene and (d) 40 µM MeO-PiIm in toluene. The spectrum a was reproduced from the reported result.82

We also measured fluorescence spectra of Pi and Heli derivatives in toluene at an excitation wavelength at 320 nm as show in Figure 4 and Figure S25 in SI. The fluorescence


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spectra of HeliIm derivatives became considerably red-shifted as compared to Heli. In particular, the spectrum of MeO-HeliIm was shifted by 100 nm. Pi derivatives also indicated the similar trend to Heli derivatives. The above-mentioned DFT methods and electrochemical results (Table 1) also support these results. Thus, by systematically introducing MeO and maleimide groups, the HOMO-LUMO gaps significantly decreased in both Heli and Pi derivatives.

Figure 4. Fluorescence spectra of (a) Heli in CH2Cl2, (b) 0.4 µM HeliIm in toluene (λex = 320 nm), (c) 0.4 µM MeO-HeliIm in toluene (λex = 320 nm), and (d) 0.4 µM MeO-PiIm in toluene (λex = 320 nm). The spectrum a was reproduced from the reported result.83

Measurements of Fluorescence Lifetimes and Quantum Yields of Heli and Pi Derivatives. First, fluorescence lifetime was measured to discuss the detailed excited-state dynamics of Heli and Pi derivatives. The fluorescence decays of Heli and Pi derivatives were measured using pulsed 271 or 404 nm laser light as shown in Figure 5. Fluorescence lifetimes τFL of these


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compounds were determined from mono-exponential fits and are summarized in Table 2. τFL values of HeliIm and PiIm were much shorter than those of corresponding pristine molecules (Heli: 26 ns and Pi: 57 ns). Additionally, τFL of MeO-HeliIm (8.0 ns) was approximately similar to that of HeliIm (9.8 ns), which is in contrast with the trend between PiIm (0.82 ns) and MeOPiIm (12 ns).

Figure 5. Fluorescence decay profiles of (a) Pi in toluene (λex = 271 nm), (b) PiIm in toluene (λex = 404 nm), (c) MeO-PiIm in toluene (λex = 404 nm), (d) HeiIm in toluene (λex = 404 nm) and (e) MeO-HeliIm in toluene (λex = 404 nm).

Next, we measured the absolute fluorescence quantum yields (ΦFL) of these derivatives as shown in Table 2. In helicene derivatives, the values of HeliIm (ΦFL = 0. 37) and MeO-HeliIm


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(ΦFL = 0.22) are much larger than that of of Heli (ΦFL = 0.04).66 In contrast, in the case of Pi derivatives, ΦFL of PiIm (0.10) significantly decreased, whereas ΦFL of MeO-PiIm increased up to 0.54. To discuss the detailed excited-state dynamics, we evaluated quantum yields of intersystem crossing pathways (ΦISC) of these compounds, which were evaluated by 1O2 emission measurements utilizing energy transfer from the triplet excited states of these compounds to the molecular oxygen. The ΦISC of zinc tetraphenylporphyrin (ΦISC = 0.88)84 was employed as the standard. Then, the quantum yields of internal conversion (IC) pathways (ΦIC) of these compounds were also estimated by subtracting ΦFL and ΦISC from the unity. The values of ΦFL and ΦISC were listed in Table 2. The sum of these three quantum yields for each compound is approximately ~1 considering experimental error. The ΦISC of Pi (0.36) were previously reported (see: Table 2).85 ΦISC of Heli has not been reported and was therefore assumed based on the reported ΦISC values of [4]carbohelicene66 and [6]carbohelicene.66 The ΦISC values of HeliIm (0.67) and MeO-HeliIm (0.69), which were experimentally obtained, were much smaller than that of Heli (~0.89). The ΦIC values of HeliIm (~0) and MeO-HeliIm (0.16) were accordingly calculated by subtracting the sum of ΦFL and ΦISC from the unity. In case of Pi derivatives, the ΦISC values of PiIm (0.65) and MeO-PiIm (0.31) were also determined in this measurement. Thus, introduction of a maleimide unit onto a Pi skeleton enables us to significantly increase the ΦISC value of PiIm, which is in sharp contrast with a large decrease of

ΦISC value in the case of HeliIm. This suggests the different roles of maleimide units in Heli and Pi skeletons for the control of excited-state dyanmics. The net rate constants such as fluorescence emission kFL, intersystem crossing kISC and internal conversion kIC were listed as shown in Table 2. Before the detail discussion, it should be


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noted that kISC of Heli (~107 s–1) is one order of magnitude greater than kFL of Heli (~106 s–1). First, we perform the kinetic discussion on the enhancements of ΦFL values in HeliIm and MeO-HeliIm. kFL values of HeliIm and MeO-HeliIm (~107 s–1) are one order of magnitude greater than that of Heli (~106 s–1), whereas kISC values of HeliIm and MeO-HeliIm (~107 s–1) are approximately similar to kISC of Heli (~107 s–1). Moreover, ΦISC of HeliIm was smaller than that of Heli despite introduction of a maleimide unit (i.e., heteroatom-containing substituent). Based on the results of solvent-dependent fluorescence spectra (Figures S26-S30 in SI), an electronic configuration was assigned to the electronic transition, which is responsible for the corresponding fluorescence bands. In Heli, only the π,π* configuration transition is accessible for both S1 and T1 states because the whole π-system of Heli behave as a chromophore. According to the theory of ElSayed’s selection rule,86 ISC between 1(π,π*) and 3(π,π*) is a spin-forbidden transition. However, the vibration derived from out-of-plane evokes the mixing between singlet and triplet states. Actually, Heli demonstrates the presence in an out-of-plane vibrational mode of aromatic C-H (Figure S31 in SI).87 Hence, this trend is coincident with the large ΦISC of Heli (~0.89). In contrast, in the infrared (IR) spectral measurements of HeliIm and MeO-HeliIm, we observed mainly signals corresponding to the C-H in-plane vibration (Figures S32 and S33 in SI), which results in the deceleration of ISC process via out-of-plane mode. In addition, in the IR spectrum of MeOHeliIm, the signals derived from the vibration of methyl group were observed (Figure S33 in SI), which leads to the increase of ΦIC (ΦIC of MeO-HeliIm: 0.16). This would be probably the reason for the decrease of ΦFL in MeO-HeliIm relative to that in HeliIm. On the other hand, in the case of Pi derivatives, kFL values of PiIm and MeO-PiIm (~107 s–1) are roughly one order of magnitude greater than that of Pi (~106 s–1). In contrast, kISC of PiIm (~108 s–1) is much larger than those of Pi (~106 s–1) and MeO-PiIm (~107 s–1), which indicates that a maleimide unit contributes to


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significantly accelerate the spin-orbit coupling. The possible reasons for the enhanced ΦFL value of MeO-PiIm would be attributable to the ‘push-pull’ characters by introduction of methoxy (push) and maleimide (pull) units. Considering the basic theory of photochemistry,86 kFL is highly associated to the molar absorption coefficients. We accordingly estimated the molar absorbance coefficients ε0−0 from the 0−0 absorption band (Table 2). The ε0−0 values of Heli and Pi derivatives are much larger than those of pristine Heli and Pi. These are coincident with the trends of kFL. To further discuss the intersystem crossing pathways, typically, the energy gap between the singlet excited state (S1) and the triplet excited state (T1) affects kISC.86 Thus, the energy gaps (ΔES-T) of Heli and Pi derivatives were estimated by measuring the phosphorescence spectra (See: SI Figures S34 and S35) as shown in Table 2. In Heli derivatives, the differences of ΔES-T in all systems are relatively small, which leads to the similar kISC values. On the other hand, the large ΔES-T value of Pi relative to MeO-PiIm was identical with relatively small kISC of Pi. By further comparison between Pi and PiIm, the similar values of ΔES-T do not agree with an increase of ΦISC. In this case, the possible reason for enhancement of ΦISC is the spin−orbit coupling caused by carbonyl group of maleimide unit.77, 86 Thus, we have successfully controlled the excited-state dynamics of structural isomers between Heli and Pi derivatives.


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Table 2. Summarized Quantum Yields and Photophysical Parametersa of Heli and Pi Derivatives.

kFL × 10-7, s-1

kISC × 10-7, s-1

kIC × 10-7, s-1

S1－S0, eV

T1－S0, eV

S1－T1, eV

ε0-0, M-1 cm-1

(~0.07)c

0.15

(~3.4)d

(~0.29)d

3.14e

2.46e

0.68

200f

0.67

-

4.6

6.8

-

2.76

2.03

0.73

3,600

0.22

0.69

0.16

1.9

8.6

2.0

2.53

1.99

0.54

3,380

57

0.40

0.36g

0.57

0.12

0.63

1.0

3.30h

2.49h

0.81

430

PiIm

0.82

0.10

0.65

0.25

12

81

31

2.84

2.00

0.84

8,560

MeO-PiIm

12

0.54

0.31

0.15

4.5

2.6

1.3

2.50

1.92

0.58

7,450

Compoud

τFL, ns

ΦFL

ΦISC

ΦIC

Heli

26b

0.04b

(~0.89)c

HeliIm

9.8

0.37

MeO-HeliIm

8.0

Pi

a

τFL, fluorescence lifetime; ΦFL, fluorescence emission quantum yield; ΦISC, intersystem crossing quantum yield; ΦIC, 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. ε0−0: molar extinction coefficients of 0−0 absorption bands. ΦIC =1 − ΦFL − ΦISC; kFL = ΦFL τFL−1; kISC = ΦISC τFL−1 kIC = ΦIC τFL−1. bReported value.66 cThe value was assumed based on the reported [4]carbohelicene (ΦISC = 0.86)66 and [6]carbohelicene (ΦISC = 0.91).66 dEstimated value from ΦISC of Heli: 0.89. eReported values.88 fReported values.82 gReported values.85 hReported values.89

Circular Dichroism Spectra of HeliIm Dirivatives. The enantiomers of HeliIm and MeOHeliIm from the racemic compounds were purified utilizing chiral HPLC. Based on the single crystal structures of HeliIm and MeO-HeliIm, we estimated the absolute stereostructures of these derivatives (Figure 1 and Figures S14, S15, S17 and S18 in SI). To investigate the anisotropy, CD spectra were measured as shown in Figure 6.


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Figure 6. CD spectra of (a) (+)-(P)-HeliIm in THF, (b) (−)-(M)-HeliIm in THF, (c) (+)-(P)MeO-HeliIm in THF, (d) (−)-(M)-MeO-HeliIm in THF, and (e) (−)-(M)-Heli (reproduced from the reported result43).

The single crystal structures were employed to fix the enantiomers of these compounds, which indicated the mirror image spectra. As compared to Δε value of MeO-HeliIm, HeliIm demonstrated the lower value in the range of 300–400 nm. In contrast, Δε value of MeO-HeliIm is higher than that of HeliIm in the range of ca. 400–470 nm. By introducing a maleimide unit onto a Heli core, the HOMO-LUMO gap and corresponding anisotropy became shifted to the visible region. The values of anisotropy in the absorption (gCD) region at 456 nm (HeliIm) and 475 nm (MeO-HeliIm) were determined to be ~4.8 × 10–3 and ~5.7 × 10–3, respectively. These gCD values are very comparable to the reported results of related helicene derivatives (gCD: ~10-2– 10–3).48, 90-91


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Circularly Polarized Luminescence Spectra of [5]Carboheliceneimide Derivatives. To examine the chiral nature in fluorescence, measurements of CPL spectra were performed in THF solution.

Figure 7. CPL spectra of (a) (+)-(P)-HeliIm in THF, (b) (−)-(M)-HeliIm in THF, (c) (+)-(P)MeO-HeliIm in THF, (d) (−)-(M)-MeO-HeliIm in THF. The result was an averaged spectrum based on three measurements. For example, the original three measurements of (−)-(M)-MeOHeliIm are shown in SI Figure S36.

The CPL spectra of these derivatives are coincident with the corresponding CD spectra at the longest wavelength (400-470 nm) as shown in Figure 7 and Figure S36 in SI. The CPL degree of HeliIm derivatives were estimated by the luminescence dissymmetric factor (glum), which is defined as glum = 2(IL− IR)/(IL + IR), where IL and IR are the luminescence intensities of left and right circularly polarized light. The glum values were finally determined to be ~2.4 × 10–3 and ~2.3


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× 10–3, respectively. This is the first example of CPL signals of [5]helicene derivatives. As compared to the reported CPL results of carbohelicenes and their derivatives containing more than six benzene rings (glum: ~10-2–10–3),56, 68, 71 HeliIm and MeO-HeliIm demonstrated the similar glum values. These results indicate that [5]carbohelicene derivative are good candidates as chiroptical molecules. To further improve the glum values, the helical supramolecular oraganization may be a feasible opption.52

CONCLUSION In conclusion, we have demonstrated the excited-state dynamics and circularly polarized luminescence of [5]carbohelicene derivatives substituted by maleimide units. A series of nonplanar [5]carbohelicenes and planar picene derivatives, which are substituted by electronwithdrawing imide and electron-donating methoxy group, were successfully synthesized. The electrochemical and photophysical properties of Heli and Pi derivatives were successfully examined together with DFT calculations. For example, ΦFL of HeliIm attains 0.37, which is larger than that of the pristine Heli (ΦFL = 0.04). In contrast, ΦFL of MeO-HeliIm (ΦFL = 0.22) was smaller than that of HeliIm. This is in sharp contrast with the trends in Pi derivatives. Then, in addition to the circular dichroism (CD) spectra, we evaluated the circularly polarized luminescence (CPL) spectra of HeliIm and MeO-HeliIm because the chiral structures and relatively high ΦFL values were obtained. The glum values of HeliIm and MeO-HeliIm were estimated to be ~2.4 × 10–3 and ~2.3 × 10–3, respectively. Actually, this is the first CPL observation in [5]helicene derivatives. Such a synthetic strategy for improved fluorescence and CPL properties provide a new way for further developments of luminescent materials.


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Experimental Section HeliIm. HeliCO2Me (500 mg, 1.27 mmol) was added to the mixture solution (1,4dioxane/EtOH=1/1, v/v) (500 mL) under a nitrogen atmosphere. The mixture solution was refluxed at 110 °C for 6 h. After cooling to room temperature, acetic acid (150 mL) was added to the mixture and stirred at room temperature for 24 h. After adding water to the mixture, HeliAH was extracted with CHCl3. The collected organic phase was dried over anhydrous MgSO4 and evaporated, and HeliAH as the crude was obtained. Then, HeliAH (442 mg) and 4-heptylamine (0.42 mL, 2.79 mmol) was dissolved in dry DMF (11.5 mL) and refluxed for 5 h. After cooling to room temperature, HeliIm was extracted by ethyl acetate, dried over anhydrous MgSO4 and evaporated. Finally, the crude was purified by chromatography on silica gel eluting with hexane/ethyl acetate (15/1, v/v), and HeliIm (yield: 39% (3 steps), 203 mg) as a yellow solid was obtained. 1H NMR (CDCl3) δ: 9.10 (d, J = 8.8 Hz, 2H), 8.40 (d, J = 8.5 Hz, 2H), 8.08 (d, J = 8.8 Hz, 2H), 7.98 (d, J = 8.5 Hz, 2H), 7.58 (dd, J = 7.4, 3.7 Hz, 2H), 7.27 (dd, J = 7.4, 3.7 Hz, 7H), 4.39-4.31 (m, 1H), 2.22-2.17 (m, 2H), 1.78-1.69 (m, 2H), 1.39-1.33 (m, 4H), 0.93 (t, J = 7.3 Hz, 6H). 13C NMR (CDCl3) δ: 170.12 (s), 133.16 (s), 131.55 (s), 130.55 (s), 129.95 (s), 129.48 (s), 128.05 (s), 127.89 (s), 126.59 (s), 125.82 (s), 125.13 (s), 121.46 (s), 51.57 (s), 34.83 (s), 34.76 (s), 19.97 (s), 13.80 (s). High resolution MALDI-TOF MS: m/z calcd: 445.204 [M] found: 445.387. MeO-HeliIm. MeO-HeliCO2Me (500 mg, 1.10 mmol) was added to the mixture solution (1,4-dioxane/EtOH=1/1, v/v) (500 mL) under a nitrogen atmosphere. The mixture solution was refluxed at 110 °C for 6 h. After cooling to room temperature, acetic acid (150 mL) was added to


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the mixture and stirred at room temperature for 24 h. After adding water to the mixture, MeOHeliAH was extracted with CHCl3. The collected organic phase was dried over anhydrous MgSO4 and evaporated, and MeO-HeliAH as the crude was obtained. Then, MeO-HeliAH (450 mg) and 4-heptylamine (0.36 mL, 2.2 mmol) was dissolved in dry DMF (11.5 mL) and refluxed for 5 h. After cooling to room temperature, MeO-HeliIm was extracted by ethyl acetate, dried over anhydrous Na2SO4 and evaporated. Finally, the crude was purified by chromatography on silica gel eluting with hexane/ethyl acetate (10/1, v/v), and HeliIm (yield: 41% (3 steps), 228 mg) as a yellow solid was obtained.1H NMR (CDCl3) δ: 9.02 (d, J = 8.8 Hz, 2H), 8.03 (d, J = 8.8 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H), 7.69 (d, J = 2.5 Hz, 2H), 7.24 (dd, J = 8.8, 2.5 Hz, 2H), 4.39-4.31 (m, 1H), 2.22-2.17 (m, 2H), 1.39-1.34 (m, 4H), 0.93 (t, J = 7.3 Hz, 6H). 13C NMR (CDCl3) δ: 170.21 (s), 156.80 (s), 131.04 (s), 130.66 (s), 129.75 (s), 129.32 (s), 128.28 (s), 126.85 (s), 125.99 (s), 119.42 (s), 110.23 (s), 54.95 (s), 51.55 (s), 34.84 (s), 34.77 (s), 19.98 (s), 13.80 (s). High resolution MALDI-TOF MS: m/z calcd: 505.225 [M] found: 505.399. Preparation and Analyses of Single-Crystal Structures. Single crystals of rac-HeliIm for packing formation analysis were prepared by vapor diffusion of toluene into a MeOH solution and rac-MeO-HeliIm for packing formation analysis were prepared by vapor diffusion of EtOAc into a MeOH solution at room temperature. Analyses of crystal structures were performed on a Rigaku R-AXIS RAPID diffractometer with graphite monochromated Mo Kα radiation. The structures were solved by direct methods (SHELXS-97). Computation of molecular graphics was performed with ORTEP-3 for Windows. Additional crystallographic information is available in the Supporting Information.

ASSOCIATED CONTENT


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

13

C

NMR spectra, tabulated crystallographic data, single-crystal structures of rac-HeliIm, (+)-(P)HeliIm, (−)-(M)-HeliIm, rac-MeO-HeliIm, (+)-(P)-MeO-HeliIm, (−)-(M)-MeO-HeliIm, PiIm, MeO-PiIm, illustrated molecular orbitals of Heli, HeliIm, MeO-HeliIm, Pi, PiIm and MeO-PiIm, steadystate, UV−vis, fluorescence spectra, cyclic and differential pulse voltammograms of Pi, PiIm and MeO-PiIm, phosphorescence emission spectra of Heli, HeliIm, MeO-HeliIm, Pi, PiIm and MeO-PiIm, UV−vis and fluorescence spectra of Heli, HeliIm, MeO-HeliIm, Pi, PiIm and MeO-PiIm in various solvents, simulated IR spectra of Heli, IR spectra and simulated IR spectra of HeliIm and MeO-HeliIm, Displacement vectors of molecular vibrations calculated in B3LYP/6-31G* level of Heli, HeliIm and MeO-HeliIm. CCDC-1451077 (rac-HeliIm), CCDC1451103 ((+)-(P)-HeliIm), CCDC-1451078 ((−)-(M)-HeliIm), CCDC-1451079 (rac-MeOHeliIm), CCDC-1451080 ((+)-(P)-MeO-HeliIm), CCDC-1451102 ((−)-(M)-MeO-HeliIm), CCDC-1451081 (PiIm) and CCDC-1451083 (MeO-PiIm) 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.

AUTHOR INFORMATION Corresponding Author *T. Hasobe. E-mail: [email protected]. Phone: +81 (45) 566 1806 *T. Kawai. E-mail: [email protected]. Phone: +81 (743) 72 6170, *Y. Araki. E-mail: [email protected]. Phone: +81-22-217-5610, *T. Takenobu. E-mail: [email protected]. Phone: +81 (3) 5286 2981


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research (Nos. 26286017, 26620159, 15H01003 “π-System Figuration”, and 15H01094 “Photosynergetics” to T.H., No. 25410167 to Y.A., No. 26102012 π-System Figuration" to T.T. and No. 25886012 to H.S.). This work was performed under the Cooperative Research Program of "Network Joint Research Centre for Materials and Devices". We are grateful to Mr. Shohei Katao (NAIST) for singlecrystal X-ray diffraction analysis.

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