Diarylamino- and Diarylboryl-Substituted DonorâAcceptor Pyrene

Article pubs.acs.org/joc

Diarylamino- and Diarylboryl-Substituted Donor−Acceptor Pyrene Derivatives: Influence of Substitution Pattern on Their Photophysical Properties Ryohei Kurata,† Akihiro Ito,*,† Masayuki Gon,‡ Kazuo Tanaka,‡ and Yoshiki Chujo‡ †

Department of Molecular Engineering, Graduate School of Engineering, and ‡Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Dianisylamino donor (D) and dimesitylboryl acceptor (A) substituents were introduced at the 1,6- and 2,7-positions of pyrene to demonstrate that the substitution patterns influence the photophysical properties. The different pictures in orbital interactions between the pyrene core and the D−A substituents led to the outcome that 1,6-substituted pyrene derivative 1 had stronger electron-donating and electronaccepting properties in conjunction with a small HOMO− LUMO gap, as compared to the 2,7-substituted derivative. For these pyrene derivatives, modest (ΦF = 0.2) to strong (ΦF = 1.0) fluorescence was detected in degassed organic solvents; 1 exhibited a typical intramolecular charge transfer (ICT) emission obeying energy-gap law, while 2 displayed a moderate inverse energy-gap law, originating from the different substitution patterns. Although theoretical calculations predicted that both 1 and 2 adopt highly twisted ICT excited states (TICT excited states) even in the gas phase, but practically, it was suggested that the observed photophysical properties could be determined by the extent of twist angle of the TICT-like excited state in accordance with the solvent polarity. Moreover, the bulky D−A substituents inhibit the intermolecular direct π−π interactions, thereby resulting in the bright and moderate solid-state emissions for 1 (ΦF = 0.76) and 2 (ΦF = 0.21), respectively.

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1-, 3-, 6-, and 8-positions,11 (iii) at the 2-, and 7-positions,12 and (ii) at the 4-, 5-, 9-, and 10-positions (K-regions).13 The substitution of electron-donating and/or electron-accepting groups at various positions of the pyrene core enables us to tune the photophysical properties of pyrene as a versatile chromophore. Appropriate choice of donor and acceptor groups substituted in the periphery of the π-bridging unit should also impact the photophysical outcomes in D−π−A molecular systems. In this regard, π-conjugated molecular systems incorporating triarylamine donor (the N center) and triarylborane acceptor (the B center) groups have received increasing interest in highly emissive materials and/or model molecules for understanding of the fundamental ICT process, and such B−π−N molecular systems often provide an effective ICT excited state.14−24 With such a background, in this paper, we focused on the B−π−N structured pyrene derivatives. It would be intriguing to examine the relationship between substitution patterns and the resulting photophysical properties. We have carried out a comparative study of 1,6- and 2,7-pyrene-based D−π−A molecules with both dianisylamino and dimesitylboryl groups (1 and 2 in Figure 1) to elucidate how the substitution pattern affects their optoelectronic properties.25

INTRODUCTION Dipolar organic chromophores with donor−π−acceptor (D−π−A) motifs, where the electron donor and acceptor moieties are suitably substituted at the periphery of the organic π-conjugated system for efficient excited-state intramolecular charge transfer (ICT), are still of interest in conjunction with the development of organic optoelectronic devices1 and of fluorescence probes for biological applications.2 The photophysical properties of such “push−pull” molecular systems depend on a choice of π-bridging units and a suitable combination of electron-donor (D) and electron-acceptor (A) substituents. Within this concept, various kinds of organic π-conjugated systems have been so far applied as π-bridging units for D−π−A chromophores.3−6 In particular, polycyclic aromatic hydrocarbon (PAH) units, such as acenes,4 rylenes,5 and so on, are recognized as important and well-studied π-bridging units. Thanks to the progress in modern synthetic methodology for the asymmetric functionalization of pyrene,6 pyrene-based “push−pull-type” chromophores have been actively pursued from the viewpoint of substantial components in organic electronics.7 Furthermore, the renewed interest in direct functionalization8 at the 2- and 7-positions of pyrene activates the investigation into the electronic structures of 2,7-functionalized pyrene derivatives.9,10 D−A substitution patterns studied so far are classified into three categories: substitution (i) at the © 2017 American Chemical Society

Received: February 9, 2017 Published: May 8, 2017 5111

DOI: 10.1021/acs.joc.7b00315 J. Org. Chem. 2017, 82, 5111−5121

Article

The Journal of Organic Chemistry

(including the vacant B 2p atomic orbital) based on the dimesitylboryl moiety lowers the energy level of LUMO, leading to a small HOMO−LUMO gap (Figure 2). On the other hand, in the 2,7-disubstituted pyrene 2, the existence of a nodal plane through the 2- and 7-positions in both the HOMO and LUMO of the pyrene core hinders the orbital interactions with two fragment MOs based each on the dianisylamino and dimesitylboryl moieties. Instead, orbital interactions of the HOMO−1 and LUMO+1 having large coefficients at the 2- and 7-positions of the pyrene core with two fragment MOs based each on the dianisylamino and dimesitylboryl moieties generate the HOMO and LUMO of 2; consequently, the inversion of the energy level ordering between the pyrene-like HOMO and HOMO−1 (and also between the pyrene-like LUMO and LUMO+1) takes place due to their orbital interactions (Figure 2). Note that the concept of the switchig of the pyrene-like LUMO and LUMO+1 has already been noticed independently by Marder and co-workers;9 moreover, such switching of the pyrene-like LUMO and LUMO+1 does not occur when the present dimesitylboryl acceptor group is replaced by the cyano acceptor group, as also pointed out by Marder’s group.12b Overall, both 1 and 2 undergo a decrease in the HOMO−LUMO gap, though the pictures of orbital interactions are completely different, and the HOMO−LUMO gap of 1 (2.71 eV) becomes narrower than that of 2 (3.13 eV), as is also estimated from the redox potentials and the absorption spectra (vide infra). More importantly, the first excited singlet states (S1) of 1 and 2 are expected to be described by a transition from the amine-localized HOMO to the boronlocalized LUMO, that is, a transition of ICT character. Synthesis. The target B−π−N-structured compounds 1 and 2 were synthesized by following the procedures shown in Scheme 1. The asymmetrically substituted intermediate compound 3 was obtained from 1,6-dibromopyrene and dianisylamine by using the palladium-catalyzed cross-coupling amination reaction26−28 in 51% yield. Lithiation of 3 with n-BuLi followed by treatment with dimesitylfluoroborane gave 1 as an orange solid in 51% yield. A similar procedure with the synthesis of 1 yielded 2 as a yellow solid in 43% yield. All of the compounds were fully characterized by NMR and elemental analysis (see the Supporting Information). X-ray Structural Analysis of 1. Single crystals of 1 (orange prism-like) suitable for X-ray crystallographic analysis were grown by slow evaporation of dilute mixed solution of CH2Cl2 and MeOH. Compound 1 crystallized in the monoclinic space group P21/n (Table S1). As shown in Figure 3, the molecular packing of 1 shows a layered structure containing a dimerlike two molecular pair (enclosed by a red broken circle in Figure 3b) as a constituent unit, in which two adjacent layers are piled up along the c axis so as to construct the so-called AB stacked layers. The interplanar separation between parallel π-faces of the dimer-like two-molecule pair was estimated to be about 7.0 Å, and thus, there exists no clear evidence of π−π interactions between the pyrene cores, owing to the bulkiness of substituted diarylamino and diarylboryl groups. Such molecular packing of 1 counts effectively in favor of the suppression of fluorescence quenching in the solid state. Turning now to the molecular structure of 1 in the crystal (Figure 4), a planar conformation was confirmed at the nitrogen and boron centers, respectively, judging from the sum of the C−N−C and C−B−C angles (360° for the nitrogen center and 359.5° for the boron center). The N1−C(4,5) and N1−C2(pyrene) bond lengths were 1.418(4), 1.425(3), and

Figure 1. Molecular structures of dianisylamino- and dimesitylborylsubstituted donor−acceptor pyrene derivatives 1 and 2, together with a schematic representation of (TD-)DFT-computed (B3LYP/6-31G*) ground (μg) and excited (μex at the Franck−Condon geometry) dipole moments.

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RESULTS AND DISCUSSION Theoretical Characterization of Pyrene Derivatives. Prior to detailed discussion on photophysical properties of pyrene derivatives 1 and 2, theoretical consideration from the viewpoint of orbital interaction provides a starting point for understanding of the substitution effect on the pyrene core. The frontier Kohn−Sham orbitals of pyrene are shown in the middle part of Figure 2, and one must pay attention to the

Figure 2. Schematic drawing of the frontier Kohn−Sham orbitals for dianisylamino- and dimesitylboryl-substituted donor−acceptor pyrene derivatives 1 and 2, together with orbital correlation diagram in comparison of the MOs for pyrene itself, at the B3LYP/6-31G* level of theory.

orbital coefficients at the 1-, 2-, 3-, 6-, 7-, and 8-positions. As has been often pointed out, the HOMO and LUMO of pyrene have large coefficients at the 1-, 3-, 6-, and 8-positions, whereas they have the nodal plane through the 2- and 7-positions. As a consequence, in the 1,6-disubstituted pyrene 1, an out-of-phase orbital interaction between the HOMO of the pyrene core and the fragment MO (including the N 2p atomic orbital) based on the dianisylamino moiety results in the raising of the energy level of the HOMO, while an in-phase orbital interaction between the LUMO of the pyrene core and the fragment MO 5112


Article

The Journal of Organic Chemistry Scheme 1. Synthesis of Compounds 1 and 2

Figure 4. ORTEP diagram of 1 (determined from X-ray structural analysis at 143 K). Hydrogen atoms are omitted for clarity; boron, nitrogen, and oxygen atoms are colored in red, blue, and gray, respectively; the thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å): N1−C2 1.442(4); N1−C4 1.418(4); N1−C5 1.425(3); B1−C7 1.587(4); B1−C9 1.578(4); B1−C10 1.579(4). Torsion angles: C1−C2−N1−C4 70.9(3)°; C3−C2−N1−C5 69.0(3)°; C6−C7−B1−C9−41.7(3)°; C8−C7−B1−C10−47.7(4)°.

Figure 3. Molecular packing in the crystal of 1 viewed down (a) the a axis and (b) the b axis in a single unit cell. Hydrogen atoms are omitted for clarity.

than the anisyl group. As shown in Figure 5a, the DFToptimized structure of 1 (B3LYP/6-31G*) was overall in accordance with the observed X-ray structure; a closer inspection, however, disclosed the relatively small optimized torsion angles of the dianisylamino and dimesitylboryl groups relative to the pyrene π-face, resulting in the slightly shorter N1−C2(pyrene) and B1−C7(pyrene) bond lengths (1.425 and 1.574 Å) than the experimental values. Since we could not obtain any information on the molecular structure of 2 in the solid state,29 we performed the DFT

1.442(4) Å, while the B1−C(9,10) and B1−C7(pyrene) bond lengths were 1.578(4), 1.579(4), and 1.587(4) Å, thus indicating no signs of quinoidal deformation of 1. The intramolecular separation between the N and B centers was 8.852(4) Å, which is in good agreement with the DFT-optimized value (vide infra). It should be noted here that the torsion angle between the NC3 plane and the pyrene π-face (ca. 70°) is larger than that between the BC3 plane and the pyrene π-face (ca. 42−48°), despite the fact that the mesityl group is bulkier 5113


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respectively, in good accordance with the optical band gaps, as described below. The observed low oxidation potentials suggest that compounds 1 and 2 are easily oxidized into radical cationic species of 1 and 2. Hence, the optical absorption spectra were recorded during stepwise electrochemical oxidation of 1 and 2 in CH2Cl2 (Figure 6). Upon oxidation of 1, several absorption bands at

Figure 5. DFT-optimized geometries of (a) 1 and (b) 2 in the ground state (S0) at the B3LYP/6-31G* level of theory. Selected bond lengths (Å): (a) N1−C2 1.425; B1−C7 1.574; (b) N1−C2 1.412; B1−C7 1.569. Torsion angles for (a): C1−C2−N1−C4 45.7°; C3−C2−N1− C5 62.3°; C6−C7−B1−C9 37.2°; C8−C7−B1−C10 37.1°; (b) C1− C2−N1−C4 = C3−C2−N1−C5 31.6°; C6−C7−B1−C9 = C8−C7− B1−C10 26.2°.

optimization of 2. In contrast to the large torsion angles of D/A substituents in 1 in the crystalline state and the DFT-optimized structure, the small torsion angles were predicted by the DFT optimization of 2 at the B3LYP/6-31G* level of theory (Figure 5b), thus leading to the short N1−C2(pyrene) and B1−C7(pyrene) bond lengths (1.412 and 1.569 Å), as compared with the corresponding bond lengths for 1. Electrochemistry and Spectroelectrochemistry. Information about the frontier orbital energies and the stability of radical ions in solution of 1 and 2 was obtained by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2 (for oxidation) and in THF (for reduction) using n-Bu4NBF4 as electrolyte. As shown in Figure S2, both compounds displayed reversible one-electron oxidation and reduction waves. As listed in Table 1, the oxidation potential of 1 (+0.22 V) was slightly lower than that of 2 (+0.26 V), whereas the reduction potential of 1 (−2.20 V) showed a positive shift of 0.26 V compared to that of 2 (−2.46 V), indicating that 1 has a stronger electron-accepting property than 2, owing to the effective orbital interaction between the LUMO of the pyrene core and the fragment MO based on the dimesitylboryl group (Figure 2). This observation is in good correlation with the DFT-calculated HOMO and LUMO levels (Table 1). The electrochemical band gaps of 1 and 2 were thus determined to be 2.42 and 2.72 eV,

Figure 6. UV/vis/NIR optical absorption spectral change during the electrochemical oxidation from neutral to cation of (a) 1 and (b) 2 in CH2Cl2 with 0.1 M n-Bu4NBF4 at 298 K.

625, 760, and 1150 nm, having both spectral shape and pattern closely resembled to those of 1-dianisylaminopyrene radical cation,32 newly appeared. The lowest energy bands around 1150 nm can be attributed to the ICT transition from the pyrene core to the aminium radical cationic moiety.17 The higher energy bands at 625 and 760 nm correspond to a π−π* transition of the typical localized pyrene and triarylamine radical cations, respectively.33,34 In contrast, the radical cation

Table 1. Electrochemical Potentialsa by Cyclic Voltammetry (Scan Rate: 0.1 V s−1) and Differential Pulse Voltammetry at Room Temperature and Electrochemically Estimated and DFT-Computed (B3LYP/6-31G*) HOMO and LUMO Energies electrochemical compd

Eoxb (V)

Eredc (V)

1

+0.22d +0.22g +0.26d +0.25g

−2.20d −2.16g −2.46d −2.43g

2

HOMO (eV) −5.02e −5.02e −5.06e −5.05e

(−5.33f) (−5.33f) (−5.37f) (−5.36f)

DFT LUMO (eV)

HOMO (eV)

LUMO (eV)

−2.60e −2.64e −2.34e −2.37e

−4.59

−1.88

−4.71

−1.58

a

Potentials are given vs ferrocene/ferrocenium redox couple (Fc0/+). bMeasured in CH2Cl2. cMeasured in THF. dDetermined by cyclic voltammetry. Estimated assuming that the HOMO energy of ferrocene lies 4.80 eV below the vacuum level (see ref 30). fEstimated assuming that the HOMO energy of ferrocene is −5.11 eV in CH2Cl2 (see ref 31). gDetermined by differential pulse voltammetry. e

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From the low energy absorption onsets in n-hexane, the optical band gaps of 1 and 2 were estimated to be 2.34 eV (530 nm) and 2.76 eV (450 nm), respectively. These values are again in good agreement with the electrochemically estimated HOMO−LUMO gap (2.42 eV for 1 and 2.72 eV for 2). As is apparent from Figure 7, it should be noted that both 1 and 2 exhibit virtually no solvatochromism in the absorption spectra irrespective of increasing solvent polarity. This strongly suggests small dipole moments of both 1 and 2 in the ground states (Figure 1).25 The oscillator strength of the S1←S0 transition was estimated from the numerical integration of the observed lowest-energy band on the basis of eq 1,15,36 where ε is the molar extinction coefficient (in M−1 cm−1) and ν is the wavenumber (in cm−1).

of 2 exhibited only one absorption band around 750 nm originating from the localized aminium radical cationic moiety. Thus, these spectral features provide evidence that the generated charge and/or spin distribution in 1•+ is extended into the pyrene core, while that in 2•+ is localized on the dianisylamino group, suggesting a weak π−conjugation between the pyrene core and the dianisylamino group in the oxidized state of 2. Photophysical Properties. The optical absorption spectra for 1 and 2 were measured in various solvents (Figure 7).

fexp = (4.31 × 10−9)

∫ ε(ν ) d ν

(1)

As a result, the value of f exp was calculated to be 0.22 and 0.30 for 1 and 2, respectively. Time-dependent density functional method (TD-DFT) calculations (B3LYP/6-31G*) of 1 gave 535 nm for the excitation wavelength corresponding to the S1←S0 transition (the computed oscillator strength: f TD−DFT = 0.33). This lowest energy excitation is described mainly by the HOMO−LUMO transition. Considering the amino-localized HOMO and the boryl-localized LUMO (Figure 2), this transition is of the ICT character. The same excitation wavelength was calculated at 397 nm at the CAMB3LYP/6-31G* level of theory ( f TD−DFT = 0.67). The B3LYP/ 6-31G* calculations underestimate the experimental lowest energy absorption maximum by 0.37 eV, while the CAMB3LYP/6-31G* calculations overestimate it by 0.43 eV. Furthermore, the CAM-B3LYP/6-31G* computations overestimate the experimental oscillator strength of the S1←S0 transition ( fexp = 0.22). Judging from the calculated transition energies and the corresponding oscillator strengths (Figure S3 and Table S2), it can be safely said that the B3LYP functional reproduces more correctly the observed spectrum of 1 than the CAM-B3LYP functional. More noteworthy is that the excitation wavelength corresponding to the S1←S0 transition of 2 was calculated to be 443 nm (f TD−DFT = 0.34) at the B3LYP/ 6-31G* level of theory, showing a good accordance with the experimental values. More importantly, the CAM-B3LYP/ 6-31G* calculations provided the incorrect order of the first and second excited states: the first excited ICT state and the second excited state were inverted, thus resulting in the forbidden S1←S0 transition for 2 (Figure S3). However, it should be noted that B3LYP and CAM-B3LYP functionals have both advantages and disadvantages in dealing with the ICT excited and locally excited (LE) states;37 generally, CAMB3LYP is suited for estimation of the ICT excited states, while it is improper for estimation of the LE states in TD-DFT calculations,38 and for instance, the ordering of excited states in pyrene itself was correctly reproduced using CAM-B3LYP functional.35 In contrast with the weak solvatochromism in absorption spectra (Figure 7), both 1 and 2 displayed a pronounced bathochromic shift in emission spectra (excitation wavelength: 370 nm) with increasing solvent polarity from n-hexane to CH2Cl2, resulting in a bathochromic shift of 99 and 103 nm, respectively, as has been reported for B−π−N molecular systems14−24 (Figure 8 and Table 2). Such a positive bathochromic shift in emission spectra, as opposed to negligible solvatochromism in absorption spectra, is indicative of an

Figure 7. Optical absorption spectra of (a) 1 and (b) 2 in various solvents; n-hexane (blue), toluene (red), chloroform (green), THF (purple), CH2Cl2 (light blue), and acetone (orange). The observed spectra in acetone are shown only for the available absorption window (>320 nm).

Compound 1 showed a structureless absorption band at 460 nm corresponding to the S1←S0 excitation (εmax = 18500 M−1cm−1 in n-hexane). Similarly, a structureless absorption band corresponding to the S1←S0 excitation of 2 was also observed at 400 nm (εmax = 26200 M−1 cm−1 in n-hexane). In addition, compound 2 showed an absorption band with a vibrational progression corresponding to the S2←S0 excitation at 340 nm (εmax = 107000 M−1 cm−1 in n-hexane), which is characteristic of the S2←S0 excitation of pyrene itself, called “pyrene-like” transition,35 supported by the fact that there exists no orbital interaction between the HOMO of pyrene core and the fragment MO (including N 2p atomic orbital) based on the dianisylamino group, thus leaving the HOMO of pyrene itself as the pyrene-localized HOMO−1 in 2 (Figure 2). In the case of 1, orbital mixing between the frontier MOs of pyrene core, dianisylamino, and dimesitylboryl groups is explicit in the next lowest energy bands with a complicated spectral shape below 400 nm. 5115


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As shown in Figure S5, the Lippert−Mataga plot showed a linear relationship with the solvent polarity, and the slope, which is proportional to (Δμ)2, was estimated as 11500 cm−1 for 1 and 16800 cm−1 for 2. Assuming that the Onsager radius (a) takes almost the same value for both 1 and 2, Δμ for 2 is considered to be about 21% larger than that for 1. This difference can be simply attributed to the longer intramolecular separation between the N and B centers (8.897 Å for 1 and 10.086 Å for 2 at the B3LYP/6-31G*) when the effective ICT is expected for both 1 and 2. Considering that the DFTcalculated Δμ (= μexFC − μg) of 26 D for 2 is about 45% larger than 18 D for 1 (Figure 1), it was suggested that a decrease in the excited-state dipole moment due to the structural relaxation from the FC state should be taken into account for the realistic estimation of Δμ. The estimated slope for 1 was similar to the values obtained for the oligotriarylamine−triarylborane molecular systems recently reported by Wenger and co-workers.17 They deduced that an increase in dipole moment change upon ICT excitation due to an increase in molecular size is compensated by an increase in Onsager radius, thus leading to a similar solvatochromism regardless of the molecular size. The present compounds may fall under the same category. To gain the insight into the structural relaxation from the FC states of 1 and 2, TD-DFT optimizations of the first excited singlet states (S1) of 1 and 2 were carried out at B3LYP/ 6-31G* level of theory. As shown in Figure 9, a common trend was seen in the optimized excited-state geometries for both 1 and 2: (i) the dianisylamino group twists further out of conjugation with the pyrene π-bridging unit than in the ground state, increasing the torsion angle between the NC3 and pyrene planes from 54° to 83° for 1 and from 32° to 80° for 2; (ii) the N−Cpyrene bond lengthens by 0.040 Å for 1 and 0.053 Å for 2, in conjunction with increasing torsion angle between the NC3 and pyrene planes; (iii) the dimesitylboryl group becomes more coplanar with the pyrene π-bridging unit than in the ground state, decreasing the torsion angle between BC3 and pyrene planes from 37° to 25° for 1 and from 26° to 17° for 2; (iv) the B−Cpyrene bond shortens by 0.032 Å for 1 and 0.021 Å for 2, in conjunction with decreasing torsion angle between the BC3 and pyrene planes. As a result, for both 1 and 2, the lowest singly occupied molecular orbital (L-SOMO) is clearly localized on the twisted dianisyl groups, while the highest singly occupied molecular orbital (H-SOMO) is delocalized over the pyrene π-bridging unit and the dimesitylboryl group, forming a so-called twisted intramolecular charge transfer (TICT) excited state (Figure 10).42 Such TICT excited states of 1 and 2 are predicted to be relaxed from the corresponding FC states by 0.41 and 0.38 eV, respectively. The TD-DFT-estimated emission wavelengths of 794 nm ( f = 0.03) for 1 and 618 nm ( f = 0.06) for 2 were, however, longer than the emission maxima observed in various solutions. Note again that the CAM-B3LYP/6-31G* calculations gave the incorrect order of the first and second excited states for 2, and moreover, the computed emission wavelength of 470 nm ( f = 0.70) for 1 was shorter than the emission maximum observed in n-hexane. Turning now to the evaluation of the observed fluorescence quantum yields (ΦF) for 1 and 2 and their dependence on the solvent polarity (Table 2), compound 1 showed the highest quantum yield of 0.71 in n-hexane with the lowest polarity. With increasing solvent polarity, however, the quantum yield gradually decreased, and a rapid drop was seen in acetone (ΦF = ∼0.03). Such a solvent polarity dependency of ΦF is typically found in the lower energy emission from the ICT

Figure 8. Normalized emission spectra of (a) 1 and (b) 2 in various solvents; n-hexane (blue), toluene (red), chloroform (green), THF (purple), CH2Cl2 (light blue), acetone (orange), and in the solid state (black). The spectrum for 1 in acetone is omitted due to the very weak emission.

efficient ICT from the N center to the B center through pyrene π-bridging unit, that is, a large excited-state dipole moment as compared with the ground-state one, which is also supported by the DFT-calculated dipole moments, 21.13 and 30.18 D, respectively, for the S1 excited states at the Franck−Condon (FC) geometries of 1 and 2 (Figure 1). It is apparent from the observed emission spectra (Figure 8) that no excimer emission at longer wavelength39 was observed probably because of the bulky dianisylamino and dimesitylboryl groups of these molecules. In addition, the dual emission, which is often seen in D−π−A molecules,16 was not observed in all of the solvents used. To estimate experimental dipole moment changes between the ground and excited states, the Stokes shifts (Δν) for 1 and 2 were plotted as a function of solvent polarity parameters (Δf), according to the Lippert−Mataga method (eq 2),40 where h is the Planck constant, c the speed of light, a the radius of the solvent cavity formed around the molecule considered (Onsager radii),41 μex and μg (and μex − μg = Δμ) the excitedand ground-state dipole moments, and Δνvac the extrapolation of Δν to the gas phase, respectively. In addition, Δf is defined by the dielectric constant εs and the refractive index n for each solvent (eq 3). Δν =

Δf =

2Δf hca

3

(μex − μg )2 + Δν vac

εs − 1 n2 − 1 − 2 2εs + 1 2n + 1

(2)

(3) 5116


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The Journal of Organic Chemistry Table 2. Photophysical Properties of Compounds 1 and 2 1

solvent

λabsa (nm)

n-hexane

463

534

toluene

469

CHCl3

2

Δν (cm−1)

knr (107 s−1)

τF (ns)

2871

0.71 (1.00)

6.2

8.7

11.5

4.7

575

3931

0.70 (0.85)

8.2

11.7

8.5

3.7

461

601

5053

0.57 (0.67)

8.5

14.9

6.7

5.1

THF

465

626

5531

0.19 (_)d

3.2

16.8

5.9

25.3

CH2Cl2

461

633

5894

0.21 (_)d

3.9

18.6

5.4

20.3

acetone

459

∼658

∼6588

0.026 (_)d

0.7

27.0

4.3

14.4

18.9

5.3

1.7

0.9

12.1

588

0.76

τ0 (ns)

kr (107 s−1)

ΦFc

solid

λemb (nm)

139

n-hexane

398

458

3292

0.07 (0.22)

7.7

toluene

402

488

4384

0.16 (0.27)

8.7

54.4

1.8

9.7

CHCl3

399

523

5943

0.20 (0.23)

6.5

32.5

3.1

12.3

THF

399

550

6881

0.34 (0.44)

6.1

17.9

5.6

10.8

CH2Cl2

397

561

7364

0.36 (0.42)

6.5

18.1

5.5

9.9

acetone

395

600

8649

0.15 (0.21)

3.2

21.3

4.7

26.6

14.4

68.6

1.5

5.5

solid

491

0.21

110

Absorption maximum at the lowest energy band measured at 1 × 10−4 M. bEmission maximum upon photoexcitation at λ = 370 nm measured at 1 × 10−5 M. cAbsolute fluorescence quantum yield determined with a calibrated integrating sphere system; the value in parentheses were measured in degassed solvent. dSignificant difference was not observed in degassed solvent.

a

Figure 9. TD-DFT-optimized geometries of (a) 1 and (b) 2 in the first excited state (S1) at B3LYP/6-31G* level of theory. Selected bond lengths in Å: (a) N1−C2 1.465; B1−C7 1.542; (b) N1−C2 1.465; B1−C7 1.548. Torsion angles for (a) C1−C2−N1−C4 81.0°; C3−C2−N1−C5 84.1°; C6−C7−B1−C9 25.2°; C8−C7−B1−C10 24.7°; (b) C1−C2−N1−C4 = C3−C2−N1−C5 79.5°; C6−C7−B1− C9 = C8−C7−B1−C10 16.9°.

excited state and can be explained by the so-called energy-gap law,43 in which nonradiative decay becomes favorable due to a smaller energy separation between the ground and excited states. As is apparent from the TD-DFT-optimized geometry

Figure 10. Frontier Kohn−Sham orbitals of the TD-DFT-optimized S1 states of (a) 1 and (b) 2 at the B3LYP/6-31G* level of theory; L-SOMO and H-SOMO stand for the lowest and highest singly occupied molecular orbitals. 5117


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especially in n-hexane solution of 2 (τ0 = 110 ns). The τ0 value for 2 decreases with increasing solvent polarity from 110 to 21 ns, whereas the corresponding value for 1 gradually increases with increasing solvent polarity from 9 to 27 ns, indicating again the weak conjugation between the donor and acceptor substituents and the pyrene π-bridge in 2. Such a long excitedstate lifetime often causes quenching of fluorescence by oxygen.35 As shown in Table 2, the quantum yield was remeasured in degassed n-hexane solution of 1 and, consequently, reached unity, without changing the spectral shape. It is notable that an improvement of quantum yield of 2 in degassed solution was observed in all the solvents used, while that of 1 was gradually diminished on going from n-hexane to CHCl3, and no improvement was confirmed in polar solvents like THF, CH2Cl2, and acetone, in good correlation with the solvent polarity dependence of the nonradiative decay rates of 1 and 2. In the solid state, compound 1 exhibited bright orangecolored emission (λem = 588 nm) with an excellent quantum yield of 0.76, while compound 2 was green-emissive (λem = 491 nm), still retaining a moderate quantum yield of 0.21 (Figure 8 and Table 2). As is apparent especially from the molecular packing in the crystal of 1 (Figure 3), the bulky D/A substituents hinder the effective π−π interactions between the pyrene cores, resulting in the solid-state photoluminescence for both compounds without excimer-like emission even in the solid state. It is noteworthy that the emission maxima of 1 and 2 in the solid state are present between the emission maxima observed in toluene and CHCl3, though the solid state often shows red shift in emission relative to the solution state, which is explained by the emission originating from the excimer formation. When comparing the X-ray and DFT-optimized structures, especially, for 1, the molecular structures in the solid state are most likely to adopt a twisted conformation, to some extent, similar to those for the TICT states of 1 (Figures 4 and 10a); however, the restricted relaxation from the FC state in the rigid medium does not lead to the more red-shifted emission observed in the polar solution state. Interestingly, in 2-MeTHF glass at 77 K, where the relaxed conformational twisting is also restricted, the emission spectra for both molecules were almost the same as the broad spectra without vibronic fine structure (Figure S6). These observations indicate that the observed emission of 1 and 2 arises from their TICT-like excited states determined by the perturbation of polar environments.

for the S1 state of 1 (Figure 10), the sudden drop in quantum yield in acetone is likely to be due to the TICT excited state, often observed in polar solvents, resulting in a relatively low radiative rate constant, as compared with nonradiative rate constant (vide infra).18,42 In contrast, compound 2, possessing stronger dipolar character in the excited state than 1 (Figure 1), exhibited moderate fluorescence quantum yields in the range of 0.07−0.36 for all six solvents used: ΦF = 0.15 even in acetone, in spite of a quite large Stokes shift of over 8000 cm−1. To address the origin of the solvent polarity dependence of the observed fluorescence quantum yields for 1 and 2, fluorescence lifetime (τF) was measured in a series of solvents with different polarity (Table 2). All of the fluorescence decay profiles were well fitted by the single exponential functions, suggesting the radiative decay from a single specific excited state. In addition, the radiative and nonradiative decay rate constants (kr and knr) were calculated from the ΦF and τF values. In the case of 1, the kr value gradually decreases, while the knr value rapidly increases with increasing solvent polarity, leading to a very low ΦF value in acetone. This trend is again explainable by the energy-gap law, in which the ICT excited state relaxed by the surrounding polar solvent molecules facilitates the fast nonfluorescent decay. In particular, both the small kr value and the large Stokes shift of ca. 6600 cm−1 suggests the formation of the TICT excited state of 1 in acetone solution, as shown in Figure 10a. In contrast, an unusual solvent dependence was observed for 2: interestingly, the kr value gradually increased with increasing solvent polarity, while the knr value remained roughly unchanged in all the solvents except for acetone, thus providing a moderate inverse energy-gap law behavior, as has been reported for some B−π−N molecular systems,18a,19 and even in acetone, moderately high quantum yield of 0.15 is still realized in comparison with the almost quenched emission for 1. The observed difference in photophysical behavior between 1 and 2 can be ascribed to the difference in conjugation with the pyrene π-bridging unit due to the D−A substitution patterns, as has already been discussed on the orbital interaction in 1,6- and 2,7-substituitions (Figure 2). The molecular design based on the substitution patterns on the pyrene π-bridging unit has also been pointed out by Marder and co-workers.12b,35 In the case of 1, 1,6-substituted pyrene π-bridge facilitates the effective conjugation between the donor and acceptor groups. However, 2,7-substituted pyrene π-bridge for 2 mediates moderately between the donor and acceptor groups. As a consequence, such a moderate conjugation in compound 2 probably results in the TICT-like excited states with a weakly charge-separated character,16,21b,44 leading to a longer excited-state lifetime due to the prevention of fast ICT process. With increasing solvent polarity, both the twisting and charge-separation in the TICT-like excited state gradually increases by the reorganization of the surrounding polar solvent molecules. This observation is in contrast to the case of 2-cyano-7-(N,N-diethylamino)pyrene, which does not form the TICT excited state even in polar solvents.12b Note that a shoulder due to the vibrational progression of the pyrene core was detected in the emission spectrum of 2 only in n-hexane, suggesting a locally excited (LE) state character16 in the fluorescence state (Figure 8). Pyrene itself, as has been already known, has a long fluorescence lifetime (τF = 354 ns in degassed toluene).35 Such a conspicuous feature was also confirmed for 1,6- and 2,7-substituted pyrenes 1 and 2. As seen in Table 2, this was revealed by the long pure-radiative lifetime (τ0), calculated from τF/ΦF,

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CONCLUSION Dianisylamino- and dimesitylboryl-substituted pyrene derivatives 1 and 2 have been synthesized as D−π−A chromophores with different substitution patterns. From the DFT consideration, amino-group substitution raises the energy level of the HOMO, and boryl-group substitution lowers the energy level of the LUMO for both derivatives 1 and 2; however, the modes of orbital interactions between the pyrene core and the D−A substituents are completely different, depending on whether orbital coefficients at the substituted positions on the pyrene π-bridge are zero or not. Such small HOMO−LUMO gaps were confirmed by the electrochemical and optical absorption spectroscopic studies. It was demonstrated that the different substitution patterns led to remarkable differences in the photophysical properties of the pyrene derivatives: 1 exhibited a typical intramolecular charge transfer (ICT) emission with the positive solvatochromic behavior obeying the energy-gap law, while 2 displayed a moderate inverse energy-gap law, though the Stokes shift 5118


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argon atmosphere. The mixture was stirred at −78 °C for 30 min. Then a solution of dimesitylfluoroborane (192 mg, 0.72 mmol) in 2 mL of anhydrous THF was added dropwise, and the mixture was stirred at room temperature for 18 h. After evaporation of the solvent, the crude product was chromatographed on silica gel (toluene as eluent) and further purified by HPLC to give 1 as an orange solid (182 mg, 51%): mp >300 °C; 1H NMR (400 MHz, CD2Cl2) δ = 1.93 (s, 12H), 2.31 (s, 6H), 3.74 (s, 6H), 6.76 (d, J = 9.03 Hz, 4H), 6.82 (s, 4H), 6.94 (d, J = 9.03 Hz, 4H), 7.70 (d, J = 8.05 Hz, 1H), 7.80 (d, J = 9.03 Hz, 1H), 7.89 (d, J = 9.27 Hz, 1H), 7.89 (d, J = 7.81 Hz, 1H), 8.01 (d, J = 7.81 Hz, 1H), 8.07 (d, J = 8.05 Hz, 1H), 8.08 (d, J = 9.03 Hz, 1H), 8.22 (d, J = 9.27 Hz, 1H); 13C NMR (100 MHz, CD2Cl2) δ = 21.4, 23.3, 55.8, 114.8, 123.9, 124.7, 124.9, 125.2, 126.6, 126.8, 126.9, 127.0, 127.5, 127.6, 127.7, 128.7, 128.8, 133.3, 133.7, 135.1, 139.5, 140.8, 142.8, 143.3, 144.2, 145.0, 155.1Anal. Calcd for C48H44BNO2: C, 85.07; H, 6.54; N, 2.07. Found: C, 84.79; H, 6.72; N, 2.00. 2-Bromo-7-(di-4-anisylamino)pyrene (4). A mixture of 2,7-dibromopyrene (364 mg, 1.01 mmol), 4,4′-dimethoxydiphenylamine (216 mg, 0.94 mmol), Pd(dba)2 (18 mg, 0.03 mmol), Xantphos (34 mg, 0.06 mmol), and NaO-t-Bu (150 mg, 1.56 mmol) in toluene (10 mL) was stirred under an argon atmosphere at 110 °C for 22 h. After evacuation of the solvent, the residue was dissolved in CH2Cl2 and washed with brine. The organic layer was separated and dried over Na2SO4. After evaporation of the solvent, the crude product was chromatographed on silica gel (n-hexane/CH2Cl2, 1:1, v/v, as eluent) to give 4 as a yellow solid (132 mg, 28%): mp 169−170 °C; 1H NMR (400 MHz, CDCl3) δ = 3.88 (s, 6H), 6.87 (d, J = 9.03 Hz, 4H), 7.15 (d, J = 9.03 Hz, 4H), 7.71 (s, 2H), 7.78 (d, J = 9.03 Hz, 2H), 7.83 (d, J = 9.03 Hz, 2H), 8.16 (s, 2H); 13C NMR (100 MHz, CDCl3) δ = 55.5, 114.9, 117.8, 118.6, 119.6, 123.3, 126.5, 126.6, 127.1, 128.0, 131.8, 131.8, 141.1, 147.4, 156.0. Anal. Calcd for C30H22NO2Br: C, 70.87; H, 4.36; N, 2.76. Found: C, 70.73; H, 4.46; N, 2.71. 2-(Dimesitylboryl)-7-(di-4-anisylamino)pyrene (2). To a solution of 4 (152 mg, 0.3 mml) in 6 mL of anhydrous THF was added 2 M n-BuLi in cyclohexane (0.2 mL, 0.4 mmol) at −78 °C under an argon atmosphere. The mixture was stirred at −78 °C for 30 min. Then a solution of dimesitylfluoroborane (96 mg, 0.36 mmol) in 2 mL of anhydrous THF was added dropwise, and the mixture was stirred at room temperature for 20 h. After evaporation of the solvent, the crude product was chromatographed on silica gel (n-hexane/toluene, 1:2, v/v, as eluent) and further purified by HPLC to give 2 as a yellow solid (91 mg, 43%): mp >300 °C; 1H NMR (400 MHz, CD2Cl2) δ = 1.99 (s, 12H), 2.34 (s, 6H), 3.80 (s, 6H), 6.87 (s, 4H), 6.88 (d, J = 9.03 Hz, 4H), 7.16 (d, J = 9.03 Hz, 4H), 7.65 (s, 2H), 7.72 (d, J = 9.03 Hz, 2H), 7.86 (d, J = 9.03 Hz, 2H), 8.19 (s, 2H); 13C NMR (100 MHz, CD2Cl2) δ = 21.4, 23.7, 55.8, 115.1, 116.9, 119.9, 126.8, 127.1, 127.2, 128.5, 128.8, 130.0, 133.2, 133.4, 139.0, 141.1, 141.3, 142.3, 142.5, 148.3, 156.6. Anal. Calcd for C48H44BNO2: C, 85.07; H, 6.54; N, 2.07. Found: C, 84.92; H, 6.64; N, 2.09. X-ray Crystal Structure Analysis. Crystallographic data for 1 were collected on a commercial CCD diffractometer at 143 K using graphite-monochromated Mo Kα radiation (λ = 0.71071 Å) from a rotating anode operating at 50 kV and 40 mA. The structures were solved by direct method (SIR9245) and refined by full-matrix leastsquares procedures on F2 (SHELXL-9746). The non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were located on the calculated positions and refined as a riding model. All calculations were performed using the CrystalStructure crystallographic software package (version 4.0; Rigaku Corp.), except for the refinement, which was performed using SHELXL-97. CCDC-1529361 contains the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. Quantum Chemical Calculations. Quantum chemical calculations were performed with using a hybrid Hartree−Fock/density functional theory method (B3LYP47 and CAM-B3LYP48). Full geometrical optimizations for 1 and 2 were carried out, and furthermore, their local minimum structures were confirmed by performing subsequent frequency analyses. Excitation energies and the geometrical

increased with increasing solvent polarity. Interestingly, the long fluorescence lifetime characteristic for pyrene itself was reflected in the observed pure-radiative lifetimes (τ0) for 1 and 2, and moreover, 2 had much longer pure-radiative lifetime than 1, indicating a weak π-conjugation between the donor- and acceptor-substituents and the pyrene π-bridge in 2. In addition, the improvement of the fluorescence quantum yield in the degassed solvents was confirmed for both 1 and 2, owing to the suppression of the fluorescence quenching by oxygen. These observations can be rationalized by the fact that 2,7-pyrenylene is considered to be a moderately effective π-bridge, whereas 1,6-pyrenylene is an effective π-bridge for the molecular design of D−π−A molecular systems. Considering the TD-DFT-optimized highly twisted geometries in the S1 states for 1 and 2 in the gas phase, the excited states of the present pyrene derivatives can be anticipated to relax from the Franck−Condon geometries into a TICT-like excited state, which is likely to be achieved in the excited state of 1 in polar medium like acetone. In the range of nonpolar to medium polar environments, the fluorescence states could be determined by the extent of twist angle of TICT-like excited states. Finally, along with the disclosure of the X-ray structure of 1, it is noteworthy that both 1 and 2 exhibited solid-state fluorescence, owing to the fact that steric hindrance of the bulky dianisylamino and dimesitylboryl substituents hinders the concentration quenching in the solid state. In particular, 1,6-substituted pyrene derivative 1 showed bright orangecolored emission at 588 nm with an excellent quantum yield of 0.76. Overall, the combination of diarylamino-electron-donating and diarylboryl-electron-accepting groups is considered to be promising for the design of highly emitting PAH molecular systems, as has recently been exemplified by the case of hexaperi-hexabenzocoronene.23

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EXPERIMENTAL SECTION

Synthesis. All of the purchased reagents were of standard quality and used without further purification. All of the purchased solvents were purified, dried, and degassed by standard procedures. Column chromatography was performed with silica gel (neutral (pH 7.0 ± 0.5) spherical grain 40−100 μm in diameter). 1H and 13C NMR spectra were measured by a commercial 400 MHz FT-NMR spectrometer. Chemical shifts of NMR spectra were determined relative to internal tetramethylsilane (TMS) standard (δ) and were given in parts per million (ppm). 1-Bromo-6-(di-4-anisylamino)pyrene (3). A mixture of 1,6-dibromopyrene (539 mg, 1.50 mmol), 4,4′-dimethoxydiphenylamine (312 mg, 1.36 mmol), Pd(dba)2 (21 mg, 0.04 mmol), Xantphos (52 mg, 0.09 mmol), and NaO-t-Bu (215 mg, 2.24 mmol) in toluene (15 mL) was stirred under an argon atmosphere at 110 °C for 20 h. The reaction mixture was washed with brine. The organic layer was separated and dried over Na2SO4. After evaporation of the solvent, the crude product was chromatographed on silica gel (n-hexane/CH2Cl2, 3:2, v/v, as eluent) to give 3 as a yellow solid (350 mg, 51%): mp 195−196 °C; 1H NMR (400 MHz, CD2Cl2) δ = 3.74 (s, 6H), 6.76 (d, J = 9.27 Hz, 4H), 6.94 (d, J = 9.27 Hz, 4H), 7.76 (d, J = 8.29 Hz, 1H), 7.86 (d, J = 9.27 Hz, 1H), 7.93 (d, J = 8.29 Hz, 1H), 8.13 (d, J = 9.27 Hz, 1H), 8.17 (d, J = 8.29 Hz, 1H), 8.20 (d, J = 8.29 Hz, 1H), 8.20 (d, J = 9.27 Hz, 1H), 8.36 (d, J = 9.27 Hz, 1H); 13C NMR (100 MHz, CD2Cl2) δ = 55.8, 114.8, 119.9, 124.1, 124.3, 125.2, 125.6, 126.1, 126.3, 126.7, 127.1, 127.5, 127.6, 128.8, 129.1, 130.1, 130.6, 130.9, 143.2, 143.3, 155.2. Anal. Calcd for C30H22NO2Br: C, 70.87; H, 4.36; N, 2.76. Found: C, 70.77; H, 4.53; N, 2.71. 1-(Dimesitylboryl)-6-(di-4-anisylamino)pyrene (1). To a solution of 3 (268 mg, 0.53 mml) in 8 mL of anhydrous THF was added 2 M n-BuLi in cyclohexane (0.3 mL, 0.6 mmol) at −78 °C under an 5119



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parameters at the S1 states for 1 and 2 were determined on the basis of the TD-DFT49 with the same functionals. All of the computations employed the 6-31G* basis set.50 All of these computational approaches are implemented in the Gaussian 09 package of ab initio MO calculations.51 Electrochemical Measurements. The redox properties were evaluated by CV and DPV in CH2Cl2 (for oxidation and in THF (for reduction) solution at 298 K with 0.1 M tetra-n-butylammonium tetrafluoroborate (n-Bu4NBF4) as supporting electrolyte (scan rate 100 mV s−1) using a commercial electrochemical analyzer. A threeelectrode assembly was used, which was equipped with a platinum disk (2 mm2) and a platinum wire as the working and counter electrodes, respectively, and Ag/0.01 M AgNO3 (acetonitrile) was used as the reference electrode. The redox potential were referenced against a ferrocene/ferrocenium (Fc0/+) redox potential measured in the same electrolytic solution. Spectroelectrochemical Measurements. Spectroelectrochemical measurements were carried out with a custom-made optically transparent thin-layer electrochemical cell (light pass length = 1 mm) equipped with a platinum mesh and a platinum coil as the working and counter electrodes, respectively, and Ag/0.01 M AgNO3 (acetonitrile) was used as the reference electrode. The potential was applied with a commercial electrochemical analyzer. Photophysical Measurements. UV/vis−NIR absorption spectra were obtained with a commercial optical absorption spectrometer. Fluorescence spectra were recorded on a commercial absolute photoluminescence quantum yield measurement system. The fluorescence lifetime measurement was performed on a spectrofluorometer system; excitation was carried out using a UV diode laser (375 nm).

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b00315. X-ray crystallographic data of compound 1 (CIF) H and 13C NMR spectra of compounds 1−4 and electrochemical, optical spectroscopic, and DFT computed data (PDF) 1

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Akihiro Ito: 0000-0002-8698-0032 Masayuki Gon: 0000-0002-5540-5908 Kazuo Tanaka: 0000-0001-6571-7086 Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JSPS KAKENHI Grant No. JP15H00734). We are grateful to Dr. Tetsuaki Fujihara and Prof. Dr. Yasushi Tsuji (Kyoto University) for X-ray structural analysis of compound 1 and Dr. Takashi Matsumoto (Rigaku Corp.) for X-ray structural analysis of compound 2. Elemental analyses were performed by the Center for Organic Elemental Microanalysis, Kyoto University. Numerical calculations were partly performed at the Supercomputer System of Kyoto University (Japan) and the Research Center for Computational Science in Okazaki (Japan). 5120


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