Electron-Transfer Reduction Properties and Excited-State Dynamics

Article pubs.acs.org/JPCC

Electron-Transfer Reduction Properties and Excited-State Dynamics of Benzo[ghi]peryleneimide and Coroneneimide Derivatives Koichi Ida,† Hayato Sakai,† Kei Ohkubo,‡ Yasuyuki Araki,*,§ Takehiko Wada,§ Tomo Sakanoue,∥ Taishi Takenobu,*,∥ Shunichi Fukuzumi,*,‡,⊥ and Taku Hasobe*,† †

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, Kanagawa 223-8522, Japan Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan ∥ Department of Applied Physics, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan ⊥ Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea ‡

S Supporting Information *

ABSTRACT: A series of benzo[ghi]perylene and coronene derivatives substituted by electronwithdrawing imide and ester groups were systematically synthesized considering (i) number of imide and ester groups, (ii) five- and six-membered imide groups, and (iii) the peripheral positions. The spectroscopic, electrochemical, and photophysical properties of these molecules were investigated in full detail by steady-state and time-resolved spectroscopy, cyclic voltammetry, quantum yield measurements of fluorescence and intersystem crossing, electron spin resonance (ESR), and density functional theory calculations. The synthetic introduction of proper substituents on the polycyclic aromatic hydrocarbon ring therefore enables us to successfully control the electrochemical and photophysical behaviors. The steady-state absorption and fluorescence spectra also become redshifted and broadened as compared to those of reference unsubstituted benzo[ghi]perylene and coronene. Regarding the electrochemistry, with an increase in the number of imide groups, the reduction potentials are significantly shifted to the positive direction, which indicates the large enhancement of electron-accepting properties. Then, absorption spectra of mono- and diradical anions of coronenetetraimide (Cor(Im)4), which were generated by the electrochemical reduction, extended to the near-infrared region (up to ∼1000 nm). The ESR measurements of one-electron reduced species of Cor(Im)4 demonstrate that more spin is relatively localized on the nitrogen atom in six-membered imide than that in five-membered imide. Finally, systematic comparison of quantum yields and rate constants of the excited-state dynamics also reveals that the intersystem crossing pathway was accelerated in both benzo[ghi]peryleneimide and coroneneimide derivatives, whereas the fluorescence property was dependent on the number of substituents and structural symmetry. This is in sharp contrast to the high quantum yield (ca. ∼1) of fluorescence of perylenediimides.

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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are one of the representative organic molecules among a variety of selfassembled supramolecular systems because of their pronounced structural properties for extended π-stacked assemblies.1−8 These molecules have many potential properties, such as an efficient UV/visible light absorption and high electron density, which allow their utilization in promising applications in electronic and energy conversion devices (e.g., organic fieldeffect transistors and solar cells).9−17 The planarity, core size, and electron distribution of these PAH units are supposed to play key roles in their organization and charge-transport properties. Several attempts have been made to synthesize new π-conjugated PAHs and to control their aggregate formation by using a variety of noncovalent interactions such as hydrogen bonding, π−π stacking, and solvophobic interactions.18,19 Among many benzenoid PAHs, benzo[ghi]perylene and coronene are popular © 2014 American Chemical Society

choices of p-type semiconductor materials. Coronene is the smallest homologue of benzene with 6-fold symmetry, and the six outer benzenes are fused in a planar ring.20,21 On the other hand, benzo[ghi]perylene is similar but with one fewer aromatic ring.22 Although several efforts have been made to self-assemble electron-rich aromatic cores to generate p-type semiconducting supramolecular stacked formations, to date there is a limited number of reports concerning electron-acceptor molecules, such as fullerenes,23−30 that are exploited for the design of assembled formation with electron mobility. In particular, the lowest unoccupied molecular orbital (LUMO) of C60 was energetically low-lying and triply degenerate and thus capable of accepting six electrons upon reduction.31,32 Additionally, the quantum yield of Received: January 22, 2014 Revised: March 17, 2014 Published: March 17, 2014 7710

dx.doi.org/10.1021/jp500761f | J. Phys. Chem. C 2014, 118, 7710−7720

The Journal of Physical Chemistry C

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intersystem crossing pathway ΦISC of C60 is ∼1.33 A systematic research work on the synthesis and fundamental spectroscopic and electrochemical evaluation of new n-type PAH-based molecules is essential for the future development of organic and supramolecular devices.34−36 Introduction of new electron-accepting substituents (e.g., imide groups) onto the peripheral positions of a PAH aromatic core enables us to control the electron-transfer reduction and photophysical behaviors. In particular, the diimides of the perylenetetracarboxylic acid, which are termed perylenediimides (PDIs), are of great interest in many scientific and technological fields. These red pigments show outstanding stability, high absorption properties in the visible region (ca. 400−600 nm), and fluorescence quantum yield (ca. ∼1).37,38 Moreover, PDIs are good electron acceptors widely used as electron-transport materials in artificial photosynthesis and electronic devices.19,39−51 Concerning the electrochemical properties, PDIs undergo reversible first and second one-electron reduction to yield mono- and dianions, respectively,40 and the LUMO state is not energetically degenerate because of the large difference in energy levels between LUMO and LUMO+1.52 These points are in sharp contrast with the above-mentioned trends of C60 and the related derivatives. Concerning benzo[ghi]peryleneimide and coroneneimide derivatives, a rational synthesis of these derivatives has been recently reported.53−61 When the basic chemical structures of perylene and coronene are compared, coroneneimide (or benzo[ghi]peryleneimide) derivatives have a great synthetic diversity because of a large number of reaction active sites. For example, four imide groups can be synthetically fused to a coronene core in the case of coroneneimides,53 whereas PDIs possess only two imide groups. Therefore, the spectroscopic and electrochemical properties can be readily fine-tuned by the nature and type of substituents.53,62 However, the systematic substituent effects on the electron-transfer reduction properties and excited-state dynamics (i.e., rate constants and quantum yields of fluorescence and intersystem crossing (ISC)) of benzo[ghi]peryleneimide and coroneneimide derivatives have yet to be explored. First, we synthesized benzo[ghi]peryleneimide and coroneneimide derivatives considering (i) number of electron-withdrawing units such as imide and ester groups, (ii) five- and sixmembered imide groups, and (iii) the peripheral positions as shown in Schemes 1 and 2. The electrochemical and spectroscopic properties as well as excited-state dynamics of these molecules were investigated in full detail by electro-

chemical methods, steady-state and time-resolved spectroscopies, fluorescence quantum yield measurements, electron spin resonance (ESR) spectroscopies, and density functional theory (DFT) calculations.

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RESULTS AND DISCUSSION Synthesis. To achieve the synthesis of our target compounds, we designed synthetic schemes based on the Diels−Alder reaction or cyclization using a Pt catalyst. The representative strategy (e.g., Cor(6Im)2) is shown in Scheme 3. First, perylenediimide 2 was synthesized by maleimidation of perylene-3,4,9,10-tetracarboxylic dianhydride 1 with 4-heptylamine. Then, dibrominated perylenediimide 3 was synthesized by direct bromination of 2 at the bay region. Then, TMS acetylene perylenediimie 4 was synthesized by Sonogashira coupling reaction of 3 with trimethylsilylacetylene, followed by desilylation using tetra-n-butylammonium fluoride (TBAF) to obtain 5. Finally, Cor(6Im)2 was synthesized by PtCl2-promoted cyclization of 5. BpIm, Cor(5Im)2, Cor(Im)2(iBu)4, and Cor(Im)4 were also obtained according to reported procedures.26,27 The detailed structural data of 1H NMR measurements are shown in Figures S1−S6 of Supporting Information. Steady-State Spectroscopic Measurements. Absorption and fluorescence spectra were measured to evaluate the electronic structures of benzo[ghi]peryleneimide and coroneneimide derivatives (Figures 1 and 2). Figure 1A shows absorption spectra of benzo[ghi]peryleneimide derivatives and a reference unsubstituted Bp in CH2Cl2. The spectrum of Bp (Figure 1A, spectrum a) has characteristic strong peaks at 289, 300, 329, 346, 364, and 385 nm. The absorption peak of Bp at 406 nm is also assigned to symmetry-forbidden transitions.33 On the other hand, the peaks become broadened and red-shifted together with new peaks in the 400−500 nm region in the case of benzo[ghi]peryleneimide derivatives (spectra b and c). According to the DFT results (Tables 1 and 2), a plausible reason for the red-shifted trend is the relative lowering of levels of LUMO states by introducing electron-withdrawing groups (i.e., imide group) as compared to those of the highest occupied molecular orbital (HOMO) states, which leads to the decrease in the HOMO− LUMO gap. The molar absorption coefficients ε0−0 of BpIm (5 230 M−1 cm−1 at 475 nm) and Bp(Im)3 (49 300 M−1 cm−1 at 464 nm) calculated from the 0−0 absorption bands were much larger than that of unsubstituted Bp (600 M−1 cm−1 at 406 nm). These ε0−0 values are significantly related to the fluorescence properties (vide infra). In the fluorescence spectra (Figure 1B), a similar trend is also observed. The spectrum of Bp shows the sharp and split peaks at 399, 408, 420, 431, and 442 nm (spectrum a). On the other hand, the spectra of BpIm and Bp(Im)3 become broadened and redshifted (spectra b and c). Additionally, the emission peaks of BpIm largely become red-shifted and broadened with an increase in solvent polarity, which is considered as an usual solvent effect on the 1(π, π*) state,63 whereas the changes in Bp(Im)3 peaks are very small (Supporting Information Figure S7). To discuss this point, we employed and compared the molecular orbitals of BpIm and Bp(Im)3 calculated by DFT (Tables 1 and 2). The HOMO orbital of BpIm is relatively localized on the Bp unit, which results in lower spin densities on the imide unit, whereas the corresponding LUMO orbital of BpIm fully expands up to the imide unit. This is in sharp contrast with the trend of Bp(Im)3. Namely, both LUMO and HOMO orbitals are relatively localized on the Bp unit in Bp(Im)3. Additionally, considering the basic chemical structures, the dipole moment of

Scheme 1. Chemical Structures of Benzo[ghi]peryleneimide Derivatives in This Study

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Scheme 2. Chemical Structures of Coroneneimide Derivatives in This Study

Scheme 3. Synthetic Scheme of Cor(6Im)2a

Reagents and conditions: (a) 4-heptylamine, 150 °C, 18 h; (b) Br2, 60 °C, 4 h; (c) trimetylsilylacetylene, CuI, PdCl2(PPh3)2, NEt3, 60 °C, 12 h; (d) TBAF, 0 °C, 0.5 h; and (e) PtCl2, HCl, 85 °C, 12 h.

a

Figure 1. (A) Absorption and (B) fluorescence emission spectra of (a) Bp (dotted black line), (b) BpIm (dashed red line), and (c) Bp(Im)3 (solid blue line) in CH2Cl2. Excitation wavelengths are 454 nm for BpIm and Bp(Im)3 and 365 nm for Bp.

Figures 2A and 2B show absorption and fluorescence spectra of coroneneimide derivatives, respectively. Figures S8A and S8B of Supporting Information also show absorption and fluorescence spectra of Cor(6Im)2 and Cor(Im)2(iBu)4, respectively.

BpIm would be larger than that of Bp(Im)3. These suggest that the LUMO state of BpIm is relatively stabilized as compared to the HOMO state in polar solvent. Consequently, significant broadening and red-shift of BpIm were observed. 7712



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Figure 2. (A) Absorption and (B) fluorescence emission spectra of (a) Cor (dotted black line), (b) CorIm (blue line), (c) Cor(5Im)2 (red line), and (d) Cor(Im)4 (green line) in CH2Cl2. Excitation wavelength: 454 nm (Cor, 340 nm).

LUMO levels systematically become stabilized with an increase in the number of substituents. The molar absorption coefficients of coroneneimide derivatives in the longest wavelength peak (ε0−0) are also shown in Table 3. For example, ε0−0 values of Cor, CorIm, Cor(5Im)2, and Cor(6Im)2 were determined to be 240, 9 000, 16 600, and 9 500 M−1 cm−1, respectively. With an increase in the number of substituents, ε0−0 values of Cor derivatives approximately increase, whereas ε0−0 values of Cor(Im)2(iBu)4 (ε0−0, 2 800 M−1 cm−1) and Cor(Im)4 (ε0−0, 5 800 M−1 cm−1) are smaller than those of coronenediimides such as Cor(5Im)2 and Cor(6Im)2 because of the high symmetry. These trends may have an effect on the photophysical parameters (vide infra). The fluorescence spectrum of CorIm (Figure 2B, spectrum b) becomes much broader as compared to those of the other coroneneimide derivatives. The emission peaks of CorIm largely become red-shifted and broadened with an increase of solvent polarity (Supporting Information Figure S9), which is very similar to the trend observed for BpIm (vide supra). The HOMO orbital of CorIm, like BpIm, is relatively localized on the Cor unit, whereas the corresponding LUMO orbital of CorIm is delocalized up to the imide unit (Table 1). Additionally, considering the chemical structure and symmetry, the dipole moment of CorIm would be larger than those of Cor(5Im)2 and Cor(6Im)2. These points may result in broad and red-shifted spectra. Moreover, phosphorescence spectra of benzo[ghi]peryleneimide and coroneneimide derivatives were also observed in the range ca. 600−800 nm (Supporting Information Figure S10). This demonstrates the occurrence of intersystem crossing pathways (Table 3). We can therefore conclude that introduction of substituents onto Bp and Cor core units effectively enables us to tune the steady-state spectroscopic properties. Electrochemical Measurements. Electrochemical behaviors of benzo[ghi]peryleneimide and coroneneimide derivatives were investigated by cyclic voltammetry to examine the substituent effects on the reduction potentials. In CV measurements, we mainly employed CH2Cl2 as a solvent because of the solubility of these compounds. Cyclic voltammograms of benzo[ghi]peryleneimide and coroneneimide derivatives were measured in CH2Cl2 containing 0.1 M nBu4NPF6 with a sweep rate of 100 mV s−1. Typical examples are shown in Figure 3 and Supporting Information Figure S11. The successive redox couples, corresponding to the single or multiple oxidation− reduction of coroneneimide derivatives, were clearly observed in Figure 3A−D. The measured half-wave potentials E1/2 of these

Table 1. Molecular Orbitals of Coroneneimide Derivatives Calculated at the B3LYP/6-31G(d) Level of Theory

The spectroscopic properties also have behaviors similar to those of benzo[ghi]peryleneimide derivatives. In both absorption and fluorescence spectra, with an increase in the number of electronwithdrawing groups, the peaks clearly become broadened and red-shifted as compared to those of unsubstituted coronene (Cor) because of the reduction of the symmetry and/or lowering levels of LUMO states. As discussed above for Bp derivatives, the absorption peaks of Cor (385 and 410 nm) are also assigned to symmetry forbidden transitions because of the high symmetry (Figure 2A, spectrum a).64 These transitions become allowed when the symmetry is reduced.63,65 The difference in LUMO levels between Cor and Cor(Im)4 (1.62 eV) is much larger than that of HOMO levels (0.86 eV), although both the HOMO and 7713



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Table 2. Redox Potentials and Energy Levels of Benzo[ghi]peryleneimides and Coroneneimide Derivatives E11/2a

compound

−2.20 −1.33 −0.66 −2.02 −1.25 −1.11 (−1.08)b −0.95 −0.88 (−0.85)b −0.65 (−0.63)b

Bp BpIm Bp(Im)3 Cor CorIm Cor(5Im)2 Cor(6Im)2 Cor(Im)2(iBu)4 Cor(Im)4 a

E21/2a

E31/2a

−0.93

E41/2a

−1.60

−1.37 (−1.34)b −1.18 −1.08 (−1.08)b −0.96 (−0.93)b

−1.43 (−1.40)b

−1.65 (−1.63)b

HOMO (eV)c

LUMO (eV)c

−5.47 −5.54 −6.32 −5.72 −5.76 −6.04

−1.99 −2.35 −3.38 −1.71 −2.46 −2.85

−6.20 −6.49

−3.02 −3.27

−6.58

−3.33

Volts versus SCE. bMeasured in PhCN. cCalculated at the B3LYP/6-31G(d) level of theory.

Table 3. Quantum Yields and Rate Constants of Benzo[ghi]peryleneimide and Coroneneimide Derivativesa compound Bp BpIm Bp(Im)3 Cor CorIm Cor(5Im)2 Cor(6Im)2 Cor(Im)2(iBu)4 Cor(Im)4

τFL (ns) b

188 9.2 8.8 320e 10.6 9.9 11.9 11.0 21.0

ΦF

ΦISC c

0.26 0.36 0.35 0.23f 0.34 0.37 0.35 0.08 0.12

d

0.53 0.68 ± 0.17 0.62 ± 0.15 0.56f 0.65 ± 0.17 0.69 ± 0.19 0.72 ± 0.18 0.70 ± 0.18 0.74 ± 0.18

ΦIC

kF (s−1)

kISC (s−1)

kIC (s−1)

T1−S0 (eV)

S1−S0 (eV)

S1−T1 (eV)

ε0−0 (M−1 cm−1)

0.22 0.00 0.03 0.21 0.03 0.00 0.00 0.22 0.14

1.3 × 10 3.9 × 107 3.9 × 107 7.2 × 105 3.2 × 107 3.8 × 107 3.0 × 107 7.3 × 106 5.7 × 106

2.8 × 10 7.4 × 107 7.1 × 107 1.8 × 106 6.1 × 107 7.0 × 107 6.1 × 107 6.4 × 107 3.5 × 107

1.2 × 10 − 3.9 × 106 6.6 × 105 2.8 × 106 − − 2.0 × 107 6.6 × 106

2.02 2.01 2.02 2.91 2.10 2.03 2.04 2.05 2.02

3.05 2.52 2.63 2.40 2.60 2.54 2.50 2.56 2.44

1.03 0.51 0.61 0.51 0.50 0.51 0.46 0.51 0.42

600 5 200 49 300 240 9 000 16 600 9 500 2 800 5 800

6

6

6

τFL, fluorescence lifetime; ΦF, fluorescence emission quantum yield; ΦISC, intersystem crossing quantum yield; ΦIC, internal conversion quantum yield; kF, fluorescence emission rate constant; kISC, intersystem crossing rate constant; kIC, internal conversion rate constant. ΦIC = 1 − ΦF − ΦISC; kF = ΦF τFL−1; kISC = ΦISC τFL−1 kIC = ΦIC τFL−1. bReported value in EtOH.22 cReported value in EtOH.21 dReported value in PMMA.21 eReported value in EtOH.21 fReported value in EtOH.70 a

Concerning the coroneneimide derivatives, a very similar 1 of trend was observed. The first reduction potential E1/2 unsubstituted Cor is determined to be −2.02 V vs SCE. Similarly, the first reduction potentials of CorIm (−1.25 V), Cor(5Im)2 (−1.11 V), Cor(6Im)2 (−0.95 V), Cor(Im)2(iBu)4 (−0.88 V), and Cor(Im)4 (−0.65 V) were assigned. With an increase in the number of electron-withdrawing imide and ester groups, the successive positive shift of reduction potentials was observed. Namely, an increased trend of the electron-accepting property was observed. According to the DFT results (Tables 1 and 2, Supporting Information Tables S1 and S2), the LUMO orbitals of benzo[ghi]peryleneimide and coroneneimide derivatives mainly expand up to the substituted-imide units. Therefore, carbonyl groups in the imide unit may contribute to the stability of radical anion states. Additionally, the E11/2 value of Cor(6Im)2 (−0.95 V) is shifted to the positive direction as compared to that of Cor(5Im)2 (−1.11 V), which indicates that the electron-accepting property of six-membered imide-substituted coronene is superior to that of five-membered imide-substituted coronene. With regard to the Cor(Im)4, the E11/2 value of Cor(Im)4 is quite comparable to that of C60, which indicates that Cor(Im)4 (−0.65 V) is a good electron acceptor. Cor(Im)4 further exhibits the reversible and successive one-electron reduction processes producing stable multiple radical anion species up to tetraanions, Cor(Im)44− (Figure 3D), whereas only mono- and dianions were observed in CorIm and Cor(5Im)2. The LUMO of Cor(Im)4 is energetically low-lying and doubly degenerate (LUMO, 3.33 eV; LUMO+1,

Figure 3. Cyclic voltammograms of (A) CorIm, (B) Cor(5Im)2, (C) Cor(Im)2(iBu)4, and (D) Cor(Im)4 in CH2Cl2 with 0.1 M nBu4NPF6 as supporting electrolyte. Scan rate: 100 mV s−1.

compounds together with reference Bp and Cor are summarized in Table 2. The first one-electron reduction potential of unsubstituted Bp is determined to be −2.20 V against saturated calomel electrode (SCE). This value is comparable to the reported value.66 Similarly, the first one-electron reduction potentials E11/2 of BpIm and Bp(Im)3 were determined to be −1.33 and −0.66 V (vs SCE), respectively. With an increase in the number of electron-withdrawing imide groups, the successive positive shift of reduction potentials was observed.67 7714



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Figure 4. UV−vis spectral changes observed in electron-transfer reduction of Cor(Im)4 (1.0 × 10−4 M; cell length, 0.5 cm) by increasing the electrochemical voltage in CH2Cl2 at 298 K. (A) First electron-reduction and (B) second electron-reduction processes.

3.24 eV); thus, it is capable of accepting four electrons upon reduction. Spectroelectrochemical Characterization of Reduced Cor(Im)4. The radical anion species of Cor(Im)4 were produced by the electrochemical reduction of Cor(Im)4 by applying the negative potential. Figures 4A and 4B show absorption spectra of Cor(Im)4•− and Cor(Im)42−, respectively. In both cases, a few new broad bands with isosbestic points appear at around 500− 1000 nm corresponding to its radical anion or dianion species under the reduction process at room temperature. For example, the molar absorption coefficients of Cor(Im)4•− and Cor(Im)42− in the long wavelength region were determined to be 17 500 M−1 cm−1 (at 922 nm) and 23 700 M−1 cm−1 (at 686 nm), respectively. With regard to the radical trianion species (Cor(Im)43−), we could not detect it because the radical species was not stable under the titration process. ESR Spectra of Radical Anions of Cor(Im)4. When the dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2] is used as an electron donor, photoirradiation of a CH2Cl2 solution containing (BNA)2 and Cor(Im)4 with visible light results in the complete one-electron reduction of Cor(Im)4 to Cor(Im)4•− (Figure 5A).68 The formation of Cor(Im)4•− detected by ESR spectroscopy after the photochemical reaction of Cor(Im)4 with (BNA)2 is shown in Figure 5B; there is a characteristic broad signal of Cor(Im)4•− (g = 2.0036, ΔHmsl = 0.10 G) observed at 298 K. The computer-simulated spectrum (Figure 5C) with hyperfine coupling constants (hfc) due to nitrogen atoms (0.57 and 0.19 G) and hydrogen atoms (0.88 and 0.64 G) agrees with the observed spectrum in Figure 5B. Consequently, more electronic spins with hyperfine coupling are localized on the nitrogen atoms in the six-membered imide unit although the unpaired electron is basically delocalized on a Cor(Im)4 ring. The delocalization of the unpaired electron is depicted by the DFT calculations as shown in Table 1 (vide supra). Fluorescence Lifetime Measurement and Evaluation of Quantum Yields of Fluorescence and Intersystem Crossing. To evaluate the detailed excited-state dynamics of benzo[ghi]peryleneimide and coroneneimide derivatives, fluorescence lifetime measurements were performed. The fluorescence decays for benzo[ghi]peryleneimide and coroneneimide derivatives were examined in toluene solution using pulsed 404 nm laser light, which excited these moieties as shown in Figure 6. These fluorescence lifetimes τFL were evaluated from a monoexponential fitting for the respective compounds, and the τFL values are summarized in Table 3. For example, τFL values of BpIm (9.2 ns) and Bp(Im)3 (8.8 ns) are much shorter than that

Figure 5. (A) Reaction scheme of Cor(Im)4•−, (B) ESR spectra of radical anion of Cor(Im)4 in the photoreduction of Cor(Im)4 by (BNA)2 in CH2Cl2 at 298 K, and (C) corresponding computer simulation. The values in the inset are hyperfine coupling constants determined by the simulation (left column) with maximum slope of line widths (ΔHmsl) and calculated by DFT (right column).

of Bp (188 ns).22 In both benzo[ghi]peryleneimide and coroneneimide derivatives with electron-withdrawing groups, the fluorescence lifetimes τFL significantly decreased with an increase in the number of substituents. We also evaluated absolute fluorescence quantum yields ΦF of these compounds, and the ΦF values are summarized in Table 3. For example, ΦF values of BpIm (ΦF = 0.36) and Bp(Im)3 (ΦF = 0.35) are larger than that of Bp (ΦF = 0.26).21 In coroneneimide derivatives, the ΦF values of mono- and diimide substituted coronenes such as CorIm (ΦF = 0.36), Cor(5Im)2 (ΦF = 0.37), and Cor(6Im)2 (ΦF = 0.35), increased with increasing number of substituents, whereas ΦF values of Cor(Im)2(iBu)4 (ΦF = 0.08) and Cor(Im)4 (ΦF = 0.12) decrease. Then, quantum yields of intersystem crossing pathways ΦISC of these compounds were evaluated by 1O2 phosphorescence measurements utilizing energy transfer from the triplet excited states of these 7715



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Figure 6. (A) Fluorescence decay profiles of (a) BpIm and (b) Bp(Im)3 in toluene. (B) Fluorescence decay profiles of (a) CorIm, (b) Cor(5Im)2, and (c) Cor(Im)4 in toluene (λex = 404 nm).

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CONCLUSION Benzo[ghi]peryleneimide and coroneneimide derivatives substituted by electron-withdrawing imide and ester groups were systematically synthesized considering number of imide and ester groups, five- and six-membered imide groups, and the peripheral positions. The spectroscopic and electron-transfer reduction properties of these molecules were evaluated by cyclic voltammetry, steady-state and time-resolved spectroscopy, electron spin resonance, and DFT calculations. The steadystate absorption and fluorescence spectra became red-shifted and broadened as compared to those of unsubstituted Bp and Cor. In particular, absorption regions of benzo[ghi]peryleneimide and coroneneimide derivatives largely extended to the longer wavelength regions. The reduction potentials become systematically shifted to the positive direction with increasing number of imide and ester groups. These spectroscopic and electrochemical results also agree well with the trend observed by DFT calculations. The electron spin resonance measurements of radical anion species of coronenetetraimide demonstrated that more spin was localized on the nitrogen atom in a six-membered imide. Finally, we quantitatively estimated the quantum yields and rate constants of the excited-state dynamics. The quantum yields of intersystem crossing ΦISC increase in both benzo[ghi]peryleneimide and coroneneimide derivatives, whereas ΦF values were dependent on the number of substituents and structural symmetry. The quantum yields of internal conversion ΦIC decrease. Such unique electrochemical and photophysical properties of polycyclic aromatic hydrocarbon derivatives provide a new perspective for the construction and development of efficient electronic devices and energy conversion systems.

compounds to O2, assuming that the quenching of the photoexcited singlet state was negligible in benzo[ghi]peryleneimide and coroneneimide derivatives.69 Singlet oxygen was produced for benzo[ghi]peryleneimide and coroneneimide derivatives used as a sensitizer in oxygen-saturated toluene and detected by the 1O2 phosphorescence at 1270 nm. Because the ΦISC value of Bp is reported to be ΦISC = 0.53,21 we employed Bp as the standard. The quantum yields of internal conversion (IC) pathways of these compounds ΦIC can also be calculated by subtracting ΦF and ΦISC from 1. The summarized ΦISC and ΦIC values of these compounds are shown in Table 3. It should be noted that the total value of these three quantum yields is ∼1 within the experimental error. The ΦISC values of benzo[ghi]peryleneimide and coroneneimide derivatives (up to ΦISC = 0.74) increase considering ΦISC values of Bp (ΦISC = 0.53) and Cor (ΦISC = 0.56). Finally, the net rate constants of the above three processes such as fluorescence emission kF, intersystem crossing kISC, and internal conversion kIC were determined as shown in Table 3. According to the classical theory of photochemistry,63 it is wellknown that kF is related to the extinction coefficient for absorption. Therefore, as discussed above, we estimated the molar extinction coefficients from the 0−0 absorption bands ε0−0 as shown in Table 3. For example, the ε0−0 values of benzo[ghi]peryleneimide derivatives are much larger than that of Bp, which is in agreement with the trends of kF and ΦF values. The results of coroneneimide derivatives are also similar, and ε0−0 values of CorIm, Cor(5Im)2, and Cor(6Im)2 are larger than those of Cor(Im)2(iBu)4 and Cor(Im)4 (Table 3). On the other hand, the kISC and ΦISC values of ISC pathways increased slightly. Typically, kISC is affected by the energy gap ΔEST between S1 and the triplet excited state, to which the intersystem crossing occurs.63 To systematically and simply evaluate the energy gaps of these molecules, the energy gaps (ΔEST) between S1 and T1 are estimated from spectroscopic results as shown in Table 3. However, the similar ΔEST values do not agree with the increase of ΦISC. Therefore, the main reason for enhanced ΦISC may be the spin−orbit coupling due to the introduction of additional substituents such as carbonyl groups. In any case, the high ΦISC values are in sharp contrast with fluorescence quantum yield of PDIs (ca. ∼1); see the quantum yields and rate constants of PDIs and the related compounds in Table S3 in Supporting Information.

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EXPERIMENTAL SECTION General Information. Coronene and perylene-3,4,9,10tetracarboxylic dianhydride (a starting material for Cor(6Im)2)71,72 were purchased from Tokyo Chemical Industry. Benzo[ghi]perylene was purchased from Wako Pure Chemical Industries. They were used for spectroscopic and electrochemical measurements with recrystallization from chloroform and hexane. All solvents and reagents of the best grade available were purchased from commercial suppliers and were used without further purification. Column flash chromatography was performed on silica gel (Kanto Chemical Silica gel 60 N, 40−50 μm or 100−210 μm). Preparative recycling gel permeation chromatography was performed with a high-pressure liquid chromatography apparatus (Japan Analytical Industry LC-9204) using chloroform as eluent at room temperature. We used an LC7716



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9204 apparatus equipped with a pump (JAI PI-60, flow rate 2.5 mL/min), a UV detector (JAI UV-3740), and two columns (JAIGEL 2H and 1H, 40 × 600 mm2 for each). All experiments except single-crystal X-ray diffraction measurements were performed at room temperature. 1H NMR and 13C NMR spectra were recorded on a 400 MHz spectrometer (JEOL JNM-A400, JNM-Al400, or JNM-ECX 400) using the solvent peak as the reference standard, with chemical shifts given in parts per million. CDCl3 was used as NMR solvent. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were recorded on a Bruker Autoflex Speed instrument. Sinapinic acid was used as MALDI-TOF mass matrix. The ab initio calculations were performed with Gaussian 03. The molecular geometries were optimized at the B3LYP/6-31G(d) level of theory. The molecular orbitals and vibrations were calculated at the same level. Perylenediimide 2. Perylene-3,4,9,10-tetracarboxylic dianhydride 1 (16 g, 41 mmol) was heated to reflux with 4-heptylamine (17.5 g, 152 mmol) in DMF (400 mL) overnight. After the reaction finished, methanol (600 mL) was added to the solution, an obtained suspension was filtered off, and the solution was concentrated by evaporation. The crude was purified by silica-gel column chromatography (only dichloromethane). Then, Perylenediimide 2 was boiled in butanol overnight (800 mL). Finally, the suspension was filtered off, and Perylenediimide 2 as the residue was collected and dried. Deep-red powder; yield, 21.3 g (89%). 1H NMR (400 MHz, CDCl3, δ): 8.65 (d, J = 8 Hz, 4H), 8.60 (d, J = 8 Hz, 4H), 5.24 (sept, J = 4 Hz, 4H), 2.32−2.22 (m, 4H), 1.88−1.79 (m, 4H), 1.42−1.25 (m, 8H), 0.93 (t, J = 7 Hz, 12H). Dibrominated Perylenediimide 3. Perylenediimide 2 (1.00 g, 1.70 mmol) was dissolved in dichloromethane (31 mL). Next, bromine (6.0 mL, 0.117 mol) was added to the solution and heated to reflux at 60 °C for 4 h. Then the solution was cooled to room temperature, and the excess of bromine was removed by air bubbling. After evaporation under reduced pressure, the crude was purified by silica gel column chromatography (hexane/ chloroform, 2/3). Finally, dibrominated perylenediimide 3 (isomeric ratio, 1:1) was obtained by collecting to the first band. Orange powder; yield, 1.25 g (99%). 1H NMR (400 MHz, CDCl3, δ): 9.52 (d, 8 Hz, 2H), 9.49 (d, 8 Hz, 2H), 8.92 (brs, 4H), 8.64 (brs, 4H), 5.22 (sept, J = 4 Hz, 4H), 2.26 (m, 8H), 1.81 (m, 8H), 1.31 (m, 16H), 0.92 (t, 4 Hz, 24H). 13C NMR (100 MHz, CDCl3, δ): 164.06 (br), 163.00 (br), 163.53 (br), 162.44 (br), 138.36 (br), 137.79 (br), 133.03, 132.78, 132.25, 132.64, 130.30 (br), 130.30, 129.73 (br), 129.19, 128.38, 128.06, 127.95, 127.91, 127.11, 126.30, 123.66, 123.20, 122.99, 122.60, 120.70, 54.33, 34.35, 20.06, 13.94. MALDI-TOF MS: calcd, 742.104; found, 742.610. TMS Acetylene Perylenediimide 4. Dibrominated perylenediimide 3 (750 mg, 1.01 mmol), CuI (19 mg, 0.101 mmol, 10 mol %), and Pd(PPh3)4 (70.9 mg, 0.101 mmol, 10 mol %) were dissolved in THF (75 mL). Triethylamine (75 mL) and trimethylsilylacetylene (0.70 mL, 5.04 mmol) were added to the solution, and the solution was stirred at 60 °C overnight. After the solution was cooled to room temperature, 2.0 M HClaq was added. Then the resulting mixture was extracted with dichloromethane; the organic phase was dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. The crude was purified by column chromatography (chloroform/ hexane, 1/1, v/v) to afford TMS acetylene perylenediimide 4 (isomeric ratio, 1:1). Orange powder; yield, 577 mg (89%). 1H NMR (400 MHz, CDCl3, δ): 10.21 (d, J = 8 Hz, 2H), 10.20 (d, J

= 8 Hz, 2H), 8.82 (m, 4H), 8.65 (m, 4H), 5.24 (sept, J = 4 Hz, 4H), 2.30 (m, 8H), 1.82 (m, 8H), 1.31 (m, 16H), 0.94 (t, J = 4 Hz, 24H), 0.39 (s, 36H). 13C NMR (100 MHz, CDCl3, δ): 164.73 (br), 164.45 (br), 163.82 (br), 163.54 (br), 138.89 (br), 138.33 (br), 136.80, 135.11, 134.50, 134.28, 133.65, 131.18 (br), 130.62 (br), 129.75, 128.25, 128.13, 127.90, 127.77, 127.71, 127.62, 124.02, 123.57, 122.77, 122.31, 120.68, 120.12, 106.66, 106.36, 106.12, 106.05, 54.58, 34.92, 34.84, 34.76, 20.52, 20.47, 14.38, 14.46, 0.11. MALDI-TOF MS: calcd, 778.362; found, 779.097. Diacetylene Perylenediimide 5. TMS acetylene perylenediimide 4 (250 mg, 0.321 mmol) was dissolved in CH2Cl2 (9 mL). After the solution was cooled to 0 °C, 1.0 M tetrabutylammoniumfluoride (TBAF) in THF (1.20 mL, 1.20 mmol) was added dropwise to the solution. After the solution was stirred for 15 min, water was added. Then the resulting mixture was extracted with dichloromethane; the organic phase was dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. Finally, diacetylene perylenediimide 5 was obtained as a red solid. Red powder; yield, 200 mg (99%). 1H NMR (400 MHz, CDCl3, δ): 10.05 (d, J = 8 Hz, 2H), 10.04 (d, J = 8 Hz, 2H), 8.85 (m, 4H), 5.23 (sept, J = 4 Hz, 4H), 3.82 (s, 4H), 2.27 (m, 8H), 1.82 (m, 8H), 1.33 (m, 16H), 0.92 (t, J = 4 Hz, 24H). 13C NMR (100 MHz, CDCl3, δ): 164.19 (br), 163.50 (br), 162.95 (br), 162.55 (br), 138.59, 135.57, 133.68, 132.89, 131.96, 130.91, 130.27, 129.53, 128.42, 128.06, 127.76, 127.51, 124.46, 123.94 (br), 122.43, 122.10 (br), 119.32, 118.85, 86.57, 86.36, 84.32, 84.26, 54.37, 54.26, 54.15, 52.08, 34.40, 30.92, 29.68, 25.07, 20.17, 20.07, 13.97, 13.55. MALDI-TOF MS: calcd, 634.283; found, 634.891. Cor(6Im)2. Diacetylene perylenediimide 5 (100 mg, 0.148 mmol) was dissolved in toluene (5 mL). PtCl2 (7.8 mg, 20 mol %) and 1.0 M HCl (0.03 mL, 20 mol %) were added to the solution, and the solution was stirred at 90 °C for 16 h. After the solution was cooled to room temperature, the resulting mixture was concentrated under reduced pressure. The crude was purified by column chromatography (only dichloromethane) to afford Cor(6Im)2. Red powder; yield, 12 mg (12%). 1H NMR (400 MHz, CDCl3, δ): 9.60 (s, 4H), 8.77 (s, 4 H), 5.55 (sept, J = 4 Hz, 4H), 2.55 (m, 4H), 2.08 (m, 4H), 1.58 (m, 8H), 1.09 (t, J = 8 Hz, 12H). 13C NMR (100 MHz, CDCl3, δ): 165.38 (br), 130.86 (br), 130.09 (br), 129.59, 129.03, 123.17, 123.12, 120.40, 54.72, 34.92, 20.45, 14.24. MALDI-TOF MS: calcd, 634.283; found, 634.934. Bp(Im)3. Benzo[ghi]perylene hexaethyl ester (1.35 g, 1.9 mmol) was heated to reflux overnight with KOH (7 g) in ethanol (30 mL). The mixture was then poured into 10% aqueous hydrochloric acid (100 mL); the precipitated solid was collected by filtration, washed with water, and air-dried, yielding 1.1 g of a dark orange solid (crude trianhydride). The solid was crushed and mixed with imidazole (10 g); 4-heptylamine (0.82 g, 7.1 mmol) was added, and the mixture was heated at 180 °C for 5 h. Ethanol (40 mL) was then added, and the mixture was poured into 10% aqueous hydrochloric acid (200 mL); the product was extracted with CH2Cl2 (3 × 100 mL). The combined extracts were concentrated, and the product was purified by column chromatography eluting with CH2Cl2 and precipitation upon cooling the hot solution in butanol. Yield: 2.0 g (1.7 mmol, 91%) as an orange-yellow waxy solid. 1H NMR (400 MHz, CDCl3, δ): 10.53 (s, 2H), 9.45 (d, J = 8 Hz, 2H), 9.19 (d, J = 8 Hz, 2H), 5.36 (m, 2H), 4.53 (m, 1H), 2.42−2.26 (m, 6H), 1.94−1.78 (m, 6H), 1.45−1.32 (m, 12H), 1.01−0.94 (m, 18H). 13C NMR (100 MHz, CDCl3, δ): 168.15, 163.60 (br), 162.71 (br), 132.17, 129.73 (br), 7717



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through an aqueous filter at low temperature. The ESR spectrum in room temperature was measured under nonsaturating microwave power conditions using a JEOL X-band spectrometer (JES-RE1XE) with an attached variable temperature apparatus. The magnitude of modulation was chosen to optimize the resolution and the signal-to-noise ratio of the observed spectra when the maximum slope line width of the ESR signals was unchanged with a larger modulation magnitude. The g values were calibrated with an Mn2+ marker.

129.09 (br), 126.99, 126.72, 126.65, 124.78 (br), 124.03, 123.10, 122.76 (br), 122.41, 54.63, 52.38, 34.62, 34.46, 20.24, 20.07, 14.06, 13.82. MALDI-TOF MS: calcd, 777.378; found, 777.979. CorIm. Benzo[ghi]perylene (150 mg, 0.543 mmol), maleic acid anhydride (2.13 g, 21.7 mmol), and p-chloranil (280 mg, 1.14 mmol) were added to a vessel and heated at 220 °C for 6 h. After the mixture was cooled at room temperature, 4 mL of pxylene was added, and the mixture was heated at 115 °C for 2 h. The mixture was filtered and washed with chloroform/ ethylacetate (2/1). This yielded 255 mg of a black solid. The obtained black solid, 1.03 mL (6.89 mmol) of 4- heptylamine, and 4 mL of DMF were added to a vessel and heated at 150 °C for 12 h. After the mixture was cooled at room temperature, the mixture was filtered and washed with methanol. The crude solid was separated by column chromatography with chloroform/ toluene (1/1) to afford 0.178 g of CorIm (0.380 mmol, 70%, 2 steps). 1H NMR (400 MHz, CDCl3, δ): 9.16 (d, J = 8 Hz, 2H), 8.24 (d, J = 8 Hz, 2 H), 8.21 (d, J = 8 Hz, 2H), 8.12 (d, J = 8 Hz, 2H), 8.07 (d, J = 8 Hz, 2H), 4.57 (m, 1H), 2.44 (m, 2H), 2.02 (m, 2 H), 1.63 (m, 4H), 1.15 (t, J = 4 Hz, 6H). 13C NMR (100 MHz, CDCl3, δ): 171.00, 128.75, 128.35, 127.88, 126.15, 125.99, 124.32, 124.11, 122.14, 122.06, 120.67, 120.59, 51.67, 35.09, 20.28, 14.08. MALDI-TOF MS: calcd, 467.189; found, 467.600. Spectroscopic Measurements. UV/vis absorption spectra were recorded on Perkin Elmer (Lambda 750) UV−vis−NIR spectrophotometer. Fluorescence and phosphorescence emission spectra were recorded on a Perkin Elmer (LS-55) spectrofluorophotometer. Fluorescence lifetimes were measured on a Horiba Scientific time-correlated single-photon counting system (FluoroCube) with laser light (DeltaDiode, laser diode head, 404 nm) as an excitation source. The absolute fluorescence quantum yields were measured by a Hamamatsu Photonics C9920-02 system equipped with an integrating sphere and a redsensitive multichannel photodetector (PMA-12; excitation wavelength, 300 nm). Electrochemical Measurements. Cyclic voltammograms were recorded on an IviumStat 20 V/2.5 A potentiostat by 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. Ferrocene−ferrocenium redox couple was used as an internal standard. All solutions were purged prior to electrochemical and spectral measurements by using nitrogen gas. 1 O2 Phosphorescence Measurements. A 150 W xenon arc lamp was used as the monitor light source. The 1O2* emission spectrum was measured by the following setup: Sample solution in toluene was bubbled by O2 20 min before measurement. 355 nm laser pulse irradiated (EKSPLA 2210A, 1 kHz operation, 6 mW) the solution. 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 a digital lock-in amplifier (NF Electronic instruments LI5640), which was operated in the chopper frequency, and then transferred to the PC. The PC controlled the monochromator to obtain the emission spectrum. The ΦISC value in solution was measured following a general method with benzo[ghi]perylene (ΦISC = 0.53) as the standard. ESR Measurements. A quartz ESR tube (internal diameter, 4.5 mm) containing a CH2Cl2 solution of Cor(Im)4 was irradiated in the cavity of the ESR spectrometer with the focused light of a 1000 W high-pressure Hg lamp (Ushio-USH1005D)

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ASSOCIATED CONTENT

* Supporting Information S

Details of 1H NMR spectra, cyclic voltammograms, solventdependent fluorescence spectra, phosphorescence spectra, and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +81-45-566-1806. Fax: +81-45-566-1697. *E-mail: [email protected]. Phone: +81-22-217-5610. Fax: +81-22-217-5609. *E-mail: [email protected]. Phone: +81-3-5286-2981. Fax: +81-3-5286-2981. *E-mail: [email protected]. Phone: +81-66879-7368. Fax: +81-6-6879-7370. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (23108721 & 23681025 to T.H., 23750014 to K.O., and 20108010 to S.F.) and the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools from MEXT, Japan, Mitsubishi Foundation, and KOSEF/MEST through WCU project (R31-2008-000-100100), Korea.

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REFERENCES

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