Charge-Separated Mixed Valency in an Unsymmetrical Acceptor

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Charge-Separated Mixed Valency in an Unsymmetrical Acceptor– Donor–Donor Triad Based on Diarylboryl and Triarylamine Units Keishiro Tahara, Haruya Koyama, Mamoru Fujitsuka, Ken Tokunaga, Xu Lei, Tetsuro Majima, Jun-Ichi Kikuchi, Yoshiki Ozawa, and Masaaki Abe J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00836 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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The Journal of Organic Chemistry

Unsymmetrical Acceptor– Charge-Separated Mixed Valency in an Unsymmetrical Charge-Separated Donor–Donor Triad Based on Diarylboryl and Triarylamine Units Keishiro Tahara,* † Haruya Koyama, ‡ Mamoru Fujitsuka, § Ken Tokunaga, # Xu Lei,§ Tetsuro Majima,*§ Jun-ichi Kikuchi,‡ Yoshiki Ozawa ,† and Masaaki Abe*† †

Department of Material Science and Research Center for New Functional Materials, Graduate School of Material Science, University of Hyogo, 3 -2-1, Kouto, Kamigori, Ako, Hyogo 678 -1297, Japan ‡

Graduate School of Materials Science, Nara Inst itute of Science and Technology, 8916 -5, Takayama, Ikoma, Nara 630-0192, Japan §

Institute of Scientific and Industrial Research (SANKEN), Osaka University, 8 -1, Mihogaoka, Ibaraki, Osaka 5670047, Japan #

Division of Liberal Arts, Centre for Promotion of Higher Education, Kogakuin University, 2665 -1, Nakano, Hachioji, Tokyo 192 -0015, Japan ABSTRACT: In this study, we report generation of new mixed-valence (MV) subspecies with charge -separated characters from an unsymmetrical acceptor–donor–donor (A–D–D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D–D conjugate that consisted of triarylamine (NAr 3) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IV CT) absorption derived from the bis(NAr 3)•+ moiety in the near-IR region. The charge -separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr3 to NAr2 sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into co unterionfree MV systems and their applications in molecular devices.

INTRODUCTION Mixed-valence (MV) molecular compounds have attracted significant interest in both basic research and molecular electronics.1-4 Their simple molecular structures, which consist of two redox sites linked by a π-bridge, are useful for studying intervalence charge transfer (IVCT) (i.e. intramolecular charge transfer (ICT) at zero driving force). The radical cations of bis(NAr3) derivatives, including compound 1 (Figure 1a), are some of the most intensively investigated MV compounds (where NAr3 = triarylamine)5, 6 that have deepened fundamental knowledge, including through-bond and through-space pathways6a, 6b and the effects of π-bridge structures6c-6g and topology6h on charge transfer (CT) mechanisms. Recently, responsiveness of bis(NAr3)•+ derivatives (and other MV compounds) to external stimuli such as light7 and protons8 has been examined, which has revealed new phenomena (i.e. photo-switchable mixed valency9-11 and proton-coupled mixed valency12-15). Although counterions influence CT rates and IVCT energies, their control has not received much attention. 16 Notably, protons were found to reduce the electronic coupling (HAB) in group 10 d8 MV complexes as an external charge through protonation at non-innocent ligand moieties.17 The internalization of counteranions into a diferrocenyl complex

that contained a carborane anion bridge has also been reported.18 As Lent and co-workers have highlighted the utility of zwitterionic MV compounds,19 further exploration of counterion-free MV systems 20 is important from the viewpoint of molecular quantum cellular automata (QCA),21 in which binuclear MV compounds are used as building blocks, or “cells”, to avoid a counterion-induced incorrect operation. The incorporation of boron into a π-skeleton has a dramatic impact on the photophysical and redox properties of πconjugated materials because of p-π* conjugation between the boron-centered vacant p orbital and the adjacent π-conjugated framework.22 The dimesitylboryl (BMes2) group is commonly used as an electron acceptor (A) and is often covalently connected to an electron donor (D) such as NAr3 derivatives. The resulting A–D conjugates (BAr3–NAr3), including NB (Figure 1a) and related multiple conjugates, have been intensively investigated for use in a variety of different applications.23, 24 For example, CN− and F− sensing based on changes in NAr3 → BAr3 charge transfer (CT) absorption and the emission,22, 24a-d fabrication of new device materials,24e, f and fundamental research towards understanding CT processes.24g-j Pump-probe transient absorption spectroscopy would contribute to our current understanding of the CT excited states of this series of conjugates. However, with the exception of the hexaarylben-

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zene derivatives reported by Lambert and co-workers,24j there have been no other reports of the dynamics of CT excited states being investigated using this method. In this regard, simple dyads and triads would be essential models to probe the hole and electron motion within molecular wires.25 Although synthetic strategies for realizing multiple BAr3–NAr3 conjugates that exhibit elaborate spatial arrangements of A and D sites are becoming established,26 the simple A–D–D linear arrangement has not been reported to date, partly because of the synthetic difficulties involved in unsymmetrical conjugation. In this study, we report creation of new MV subspecies with charge-separated characters by photoexcitation of an unsymmetrical A–D–D conjugate (Figure 1b). This allows the accommodation of negative charges in close proximity to the MV cationic moiety, realizing internalization of counteranions within a molecular framework. The D–D conjugate MeO-TPD 1 was chemically modified using a BMes2 group, affording the new unsymmetrical A–D–D conjugate 2. The CT properties of 2 were characterized in detail using steady-state UV-vis and luminescence spectroscopy, in addition to luminescence quantum yields and lifetimes. Furthermore, the unique chargeseparated MV (CSMV) state (denoted 2*) was successfully characterized using femtosecond transient absorption spectroscopy based on the transient IVCT absorption properties and their solvent dependence. The interplay between mobile charges and internalized charges was revealed when the ground and excites states (2•+ and 2*) were compared.

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Synthesis and characterization of the triad 2. The target triad 2 was synthesized by Pd-catalyzed cross-coupling of a NAr3 derivative containing a 4-bromobiphenyl group (4) with 4-methoxyaniline followed by a second Pd-catalyzed crosscoupling with (4-bromophenyl)dimesitylborane (Scheme 1). In the former reaction, the use of PEPPSI–IPr27 afforded secondary amine precursor 3 in good yield (Figures S1 to S3), while in the latter reaction the combination of Pd(dba)2 and BINAP produced tertiary amine 2. The new compound 2 was identified by 1H- and 13C-NMR and ESI-MS (Figures S4 to S6). The new A–D conjugate NB′ was also synthesized by the similar procedure as that for 2 (Figures S7 and S9). Scheme 1. Synthesis of 1.

Figure 2. Selected molecular orbitals for 2 and NAr3 derivatives calculated using a DFT method (isosurface values 0.02 au).

Figure 1. (a) Structures of conjugates (D = electron donor, A = electron acceptor). (b) Present approach to creating MV subspecies.

RESULTS AND DISCUSSI ON

DFT calculations predicted that the electronic interactions in the D–D framework are maintained after attachment of the BMes2 group. In the DFT-optimized structure of 2 (Table S1), the twist between the two phenyl rings in the biphenyl moiety (34.74°) was comparable with that of 1 (35.67°) (Table S2). The N···N distance in 2 (10.010 Å) is also comparable with that in 1 (10.012 Å), respectively. These indicated the small influence of the bulky BMes2 group on the bis(NAr3) structure of 2 in solution. The HOMOs of the D (MeO-TPA) and A –D (NB′) components were close in energy and well hybridized, constructing two new orbitals for A–D–D as the antibonding and bonding combination (HOMO and HOMO−1 for 2) (Figure 2). Despite the torsion in the central biphenyl bridge, these


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orbitals were delocalized between two NAr3 moieties for 2. The HOMO level of 2 increased in energy by 0.29 and 0.43 eV compared with those of MeO-TPA and NB′, respectively (Table 1). The boron-centered LUMOs remained almost unchanged in energy and distribution upon the change from the A–D conjugate NB′ to the A–D–D conjugate 2. Table 1. DFT-calculated frontier molecular orbital compositions (%). Compound 2

MO 221 (LUMO) 220 (HOMO) 119 (HOMO−1) 2•+ 220β (LUMO) 220α (HOMO) 1 162 (LUMO) 161 (HOMO) 160 (HOMO−1) 1•+ 161β (LUMO) 160β (HOMO) a X = OCH 3 (1), BMes2 (2).

E /eV −1.34 −4.54 −4.96 −6.19 −7.10 −0.49 −4.33 −4.74 −6.12 −7.02

TPA1 42 26 26 69 45 49 50 51 46 49 51

TPA2 2 73 27 54 46 50 48 52 49 47

Xa 56 1 4 1 5 0 2 2 2 3

Table 2. Electrochemical data for NAr 3 derivatives.a E 1/2 1ox E1/2 2ox ∆ E1/2 ox K cc

interactions agrees well with the above-mentioned DFT calculations of 2 and with previous reports on the parent D–D conjugate 1.5, 6 The CT characters of 2 were confirmed by UV-vis measurement in CH2Cl2 (Figure 4a). The lowest-energy absorption band for 2 at 389 nm was attributed to the CT transition from HOMO to LUMO, while the D–D conjugate did not show any corresponding CT band. The CT absorption maximum for 2 was red shifted by 994 cm-1 than for NB. This corresponds to the fact that the DFT-calculated HOMO–LUMO gap for 2 (3.20 eV) is 0.36 eV smaller than for NB (3.56 eV).

Figure 4. (a) UV-vis absorption spectra of 1 and 2 and (b) UVvis-NIR absorption spectra of 1•+ and 2•+ in CH2Cl2.

MeO-TPA 289 ─ – – NB 298 – – – NB′ 420 – – – 2 193 446 253 7.03 × 10 3 1 103 330 227 1.94 × 10 4 a Potentials in mV vs. Fc +/Fc. CH 2 Cl2 solution (1.0 mM) containing 0.10 M n Bu 4 NPF 6. b∆ E 1/2ox = E 1/2 2ox - E 1/21ox. c K c = exp(∆ E 1/2 / 25.69).

Figure 3. Cyclic voltammograms of 1 and 2 in CH2Cl2 (1.0 mM) containing 0.10 M nBu4NPF6. Scan rate: 100 mV s−1.

The effect of the incorporated BMes 2 group on the redox potentials was investigated by cyclic voltammetry in CH2Cl2 containing nBu4NPF6 as a supporting the electrolyte at 298 K (Figure 3 and Table 2). The E1/21ox value for 2 was 90 mV larger than for 1 due to the introduced electron-withdrawing BMes2 group. Compound 2 exhibited two well-defined quasireversible waves, indicating sequential oxidation of the NAr3 moieties and the formation of the one- and two-electron oxidized species 2•+ and 22+, respectively. The resulting large potential splitting (253 mV) for 2 was 122 mV larger than the difference between the E1/2 values of NB′ and MeO-TPA (131 mV), highlighting the thermodynamic stability of the MV species 2•+ with a large comproportionation constant of the order of 103 (Kc) This means that not only structural asymmetry but also electronic interactions between the NAr3 moieties contribute to the potential splitting for 2. Such electronic

Figure 5. DFT-optimized structures (top), LUMOs (isosurface values 0.02 au) (middle), and electrostatic potential maps (isosurface values 0.01 au) (bottom) of 1•+ and 2•+. Increased values of NPA charges of TPA1 (pink), TPA2 (red), and other (orange) fragments upon one-electron-oxidation from the neutral forms are also shown.

Mixed valency of 2•+ in the ground state. UV–vis–NIR measurements confirmed that 2•+ retained the organic MV characteristics of previously reported bis(NAr3)+ derivatives.5, 6 The 2•+ cation, obtained by chemical oxidation of 2 with magic blue, exhibited two new absorption bands in CH2Cl2 (Figures 4b and S10). The first band at 711 nm was assigned to the character of the NAr3+ center, according to its similarity to that of the NAr3+ derivatives reported elsewhere.28 The second band was located in the NIR region, and it was assigned to the IVCT transition between the NAr3 groups (Table S4). The IVCT band properties of 2•+ were comparable with those of



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Table 3. Absorption ( λ abs) and fluorescence emission ( λ em ) maxima, fluorescence quantum yields ( Φ F), and lifetimes (τ F) in several solvents. Excitation wavelengths correspond to λ abs.

hexane toluene CH 2 Cl2 THF

2 λ abs (nm) 394 397 396 397

ε max (10 4 cm −1M −1 ) 1.8 2.4 5.5 3.2

λ em (nm) 457 491 538 528

NB λ abs (nm) 383 383 378 386

ε max λ em (10 4 cm −1M −1) (nm) 2.5 453 4.2 493 3.1 534 2.3 525

symmetric 1•+.5 The introduced BMes2 group resulted in a slight blue shift by 350 cm−1 compared with the IVCT energy of 1•+, indicating the zero-level energy difference in 2•+. The HAB value of 2•+ was comparable with or slightly larger than that of 1•+ (Table S4). DFT calculations predicted the charge delocalization in 2+ well. Upon changing from 2 to 2•+, a 10.10°-flattening of the twist was observed between the bridging phenyl rings with a 0.021 Å-shortening of the N···N distance (Tables S1 and S2). The natural population analysis (NPA) charges in both the NAr3 moieties increased upon the change from 2 to 2•+, with slightly biased distributions of positive charges to the TPA2 side (Figure 5, top and Table S3). The comparison of 1•+ with 2•+ revealed that the introduced BMes2 group shifted the positive charges to the TPA2 side because of its electronwithdrawing properties. The charge distribution was in good agreement with the distribution of the LUMO (220β) of 2+ (Figure 5, middle and Table 1). In the electrostatic map of 2•+, the red regions representing positively charged moieties spread over two NAr3 moieties, as well as in the boron center (Figure 5, bottom).

Figure 6. (a) Fluorescence emission spectra of 2 in several solvents. (b) Lippert-Mataga plots for the solvatochromism of 2 and NB. The orientation polarizability of solvents were calculated by the equation: ∆f = (ε–1)/(2ε+1)–(n2–1)/(2n2+1), where ε and n are the dielectric constant and the refractive index.

Photophysical properties of 2. The CT properties of 2 were characterized by UV–vis and fluorescence spectroscopy, as well as measurements of the fluorescence quantum yields (Φ F) and lifetimes (τF). 2 is highly soluble in various organic solvents because of the introduced BMes2 group, enabling examination of the solvent dependence. The CT emission energies of 2 were slightly lower than those of NB in all of the solvents (Table 3 and Figures 6a and S11). This is consistent with the lower oxidation potential of the NAr3 moiety of 2 than that of NB. For 2, a red shift of the CT emission was observed with increasing Reichardt parameter29 ETN from hexane to CH2Cl2 in a range of 3290 cm−1. The solvatochromic shift was comparable with that of NB (3350 cm−1). These observations agreed well with the previously reported behavior of NAr3–BAr3 D–A compounds and NB. 23b, 24a, 24e, 26b In addition, neither 2 nor NB showed any solvatochromic behavior for CT

2 ΦF

τF (ns)

NB ΦF

τF (ns)

0.41 0.50 0.21 0.29

4.44 6.36 9.41 8.07

0.58 0.53 0.32 0.30

2.51 4.49 5.92 6.07

absorption, indicating that the dipole moment is weaker for the electronic ground states than for the CT excited states.23a The effect of the solvent polarity on the Stokes shift of 2 was investigated according to the method of Lippert and Mataga,30 where the difference between CT absorption and the emission energy is plotted as a function of the orientation polarizability of the solvent (∆f). In the resultant plot, the slope is equal to the ratio of 2∆µeg2 to a03, where ∆µeg and a0 are the difference between the dipole moments of the ground and excited states and the radius of the cavity in which the chromophore resides (Onsager radius),31 respectively. The obtained slope for 2 (8750 cm−1) was comparable with that for NB (8500 cm−1) (Figure 6b). From NB to 2, the molecular size increases by covalent conjugation of an additional NAr3 site, accompanied by an increase in a0. This effect is counterbalanced by an increase in ∆µeg, resulting in similar solvatochromic behavior for 2 and NB. For both 2 and NB, the ΦF values tend to decrease with increasing solvent polarity (Table 3). Combined with the abovementioned solvatochromic behavior, this tendency for 2 and NB was in agreement with the energy gap law, in which the nonradiative decay rate increases with decreasing energy gap. The lower Φ F values for 2 than for NB were also in agreement with the energy gap law, contributing to larger τF values for 2 than for NB. For 2, there was no clear correlation between ΦF and τF in the solvents, indicating that the radiative and nonradiative decay rates were quite different depending on the solvent. The difference between 2 and NB became more pronounced in the transient absorption spectra of the CT excited states as discussed below. Charge-separated mixed valency of 2 in the excited state. Although the CT emission of the NAr3–BAr3 D–A conjugates has been used in various contexts, including anion sensors, we are unaware of prior studies on the transient species, with the exception of the hexaarylbenzene derivatives reported by Lambert and co-workers.24j Thus, the transient absorption spectra of the reference compound NB were measured with excitation at 400 nm in different solvents (Figure 7, left). Within 10 ps of excitation in tetrahydrofuran (THF), a broad absorption band was observed at 763 nm with a shoulder at about 700 nm (Figure 7, left, D), which was assigned to superposition of absorption of the NAr3 radical cation (NAr3•+) and the BMes2 radical anion (BAr3•−). These radical ion species have been reported to exhibit similar absorption in the ground state.28, 32 This indicated formation of a charge-separated state for NB, where the NAr3 moiety was oxidized and the BAr3 moiety was reduced, as a result of intramolecular CT excitation of NB. The absorption of the NAr3•+ moiety red shifted with increasing solvent polarity from hexane to methanol in a range of about 1050 cm−1, reflecting the π–π* transition characteristics of NAr3•+ absorption.


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Figure 7. Transient absorption spectra of NB (left) and 2 (right) in (A) hexane, (B) toluene, (C) CH2Cl2, (D) THF, and (E) methanol upon laser flash photolysis with a 400 nm femtosecond pulse for excitation.

The charge-separated MV (CSMV) state of 2 (denoted 2*) was unambiguously detected by probing the transient species of 2 with a white-light continuum reaching the NIR region (Figure 7, right). After excitation of 2 for 8.0 ps at 400 nm, a broad absorption band was observed in the visible region (600−780 nm) in THF (Figure 7, right, D). This absorption was also ascribed to superposition of the absorption of the NAr3•+ and BAr3•− moieties because its similarity to the above transient absorption for NB. This result confirms that CT excitation of 2 also leads to formation of a charge-separated state. Furthermore, in the NIR region, a strong absorption band at 1425 nm was observed after excitation of 2 for 8.0 ps. This band was assigned to the IVCT band from NAr3 to NAr3•+ according to its similarity with IVCT absorption of the ground state 2•+ obtained by chemical oxidation. This assignment agreed with the fact that no IVCT band was observed upon excitation of NB. Interestingly, the absorption maximum of the transient IVCT band of 2 in THF blue shifted to 1336 nm within 16 ps of excitation. This was accompanied by a slight blue shift of absorption of the NAr3•+ moiety from 752 to 737 nm. Such a blue shift was not observed for the NB reference. This suggested that structural relaxation occurred in the excited states of 2 and could be related to the biphenyl and/or another NAr3 moiety. The solvent dependence of the transient IVCT bands of 2* was probed in terms of the energy and dynamical peak shift. Evolution of the transient IVCT band followed by a blue shift in the peak maximum was a common phenomenon in toluene, THF, and methanol (Figure 7, right, B, D, and E). The initial peak maximum within 7.0–8.0 ps of excitation slightly increased in energy with increasing solvent polarity from toluene (1465 nm) through THF (1425 nm) to methanol (1396 nm) in a range of 336 cm−1 (Figure 8). In contrast, the equilibrium peak positions were comparable for these three solvents (toluene 1336 nm, THF 1336 nm, methanol 1345 nm). Accordingly, the blue shift as a function of delay time was larger in more polar solvent. The blue shift was complete within 10 ps of excitation in the most polar solvent (methanol), while it was

complete within 16 ps in the less polar solvents (toluene and THF), suggesting involvement of the solvation dynamics in the relaxation processes.33 The slow relaxation in hexane was evidenced by the spectral change. After excitation for 7.0 ps, an IVCT band appeared at 1396 nm with a shoulder at about 1240 nm, which changed to bimodal absorption within 22 ps of excitation (Figure 7, right, A). This was suggestive of a vibrational structure, which was often observed for the IVCT bands of ground-state MV species involving a biphenyl moiety. 34 In CH2Cl2, although there was a similar tendency of evolution of the IVCT band followed by its blue shift, qualitative analysis was not performed because of the intense bleaching of 2 upon photoexcitation (Figure 7, right, C). This prevented detailed analysis of CT excited-state decay with the IVCT bands in all of the solvents, although it should be equivalent to the radiative and nonradiative decay processes.

Figure 8. Transient IVCT band energies for 2* in toluene, THF, and methanol against solvent polarity with Reichardt parameters and dynamical peak shift. Steady-state IVCT energies for 1•+ and 2•+ are also plotted. Valu es for MeO-TPD+ were adopted from ref. 5d.

The unique characteristics of the CSMV state were clarified by comparison with those of the conventional ground MV state (Figure 8). In class II MV compounds, the IVCT energies generally increase with increasing solvent polarity because of



increasing solvent reorganization energy ( λo).2,3 In a previous study, 5d 1•+ showed a relatively small blue shift from CH2Cl2 to CH3CN in a range of 1080 cm−1, reflecting the delocalized character of the class III MV compound. The introduced BMes2 group in 2 expands the range of solvents, which cannot normally be achieved because of the limited solubility of MV ions in nonpolar solvents. We have clearly shown that the equilibrium IVCT energies of 2* are far less sensitive to the solvent polarity in a wider range than the steady-state IVCT energies of the MV state of 1•+. In 2*, the mobile positive charges are not completely exposed to the surrounded solvent molecules because of the internalized negative charges at the boron center, leading to equalization of λo (Figure 9).

Figure 9. Schematic representation of diabatic (dashed line) and adiabatic (solid line) potential energy surfaces for 1•+ (left), 2•+ (middle), and 2* (right). Charge distributions of MV species are also illustrated with blue arrows representing solvent dipoles.

Another notable finding is that the increased electron density at the boron center in 2* shifts the IVCT band to about 870 cm−1 higher energy than the ground MV state of 2•+ in CH2Cl2 (Figure 8). This can be explained in terms of the increased ∆G0 in the PES of 2* compared with that of 2•+ (Figure 9). The optimized structure of the first singlet excited state (S1) of 2 was obtained by TD-DFT (Figure 10a). In the electrostatic map, the negative charges are effectively accommodated at the BAr3 moiety shown in blue in close proximity to the mobile positive charges by CS. This corresponds to the distribution of the molecular orbital 221 (Figure 10b). Importantly, the N,Bsubstituted phenyl ring acts as part of the electron-accepting BAr3 site rather than the electron-donating NAr3 site. This restructuring of the electron-donating moieties from two NAr3 sites (TPA1 and TPA2 in Figure 5) to NAr2 and NAr3 sites (DPA and TPA in Figure 1 0a) shifts the positively charged region shown in red in the electrostatic map and the molecular orbital 220 (Figure 10b) to the more electron-donating TPA side. In contrast, for 2•+, the negative charges of the counteranion have little effect on ∆G0 because the surrounded solvent molecules decrease the extent of the electrostatic interactions between 2•+ and SbCl6−. Ismagilov and Nelsen16 also reported the weak ion-pairing effect on the electron transfer rate constants for an organic MV system with nitrogen centers. In the DFT-optimized structures, as judged by the common flattening of the twist between the central biphenyl bridge upon changing from 2, the S1 state of 2 has a similar geometry

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to that of 2+ (Table S2), resulting in little contribution of HAB and λin to the difference in the IVCT energies of 2* and 2•+. This is in line with a previous report on binuclear Pt(II) biscatecholato complexes with a common biphenyl bridge.34b

Figure 10. (a) Optimized structure (top) and electrostatic potential maps (isosurface values 0.01 au) (bottom) of the S1 state of 2, with increased values of NPA charges of DPA (pink), TPA (red), and BMes2Ph (blue) fragments upon one-electron-excitation from 2. The original values of NPA charges of fragments are also shown in parentheses for clear indication of the CSMV state. (b) Key molecular orbital contributions and energy diagram for the S1 state of 2, which was mainly dominated with a coefficient of 0.696 by a singly excited configuration: 220 (HOMO) → 221 (LUMO) in the ground state.

There are many reports on the dynamics of MV compounds initiated by optical IVCT excitation followed by ultrafast back electron transfer, 5d, 35 especially for inorganic MV complexes. 36 However, generation of MV states by the photoinduced electron transfer process is extremely rare.37 In an organic naphthabranched triad containing two NAr3 (D) sites and a naphthalene diimide (A) site, 37a upon excitation of the cyclometalated Ir complex bridge, the formed triplet excited state converted to the excited CS state within 12 ps in CH2Cl2, where one NAr3 was oxidized and the naphthalene diimide was reduced. However, the IVCT band between D and D •+ could not be investigated in detail because the absorption was relatively low in energy and appeared outside the experimental region. Although dyads composed of a perylene diimide (A) site and carbazole (D) containing two NAr3 sites were also reported, the focus was on their two-photon-absorbing with inattention to the NIR absorptions of the radical cation moiety in the excited CS state.37b In an inorganic linear triad composed of two triruthenium cluster (A) sites and a zinc porphyrin (D) site, 37c photoinduced electron transfer from the singlet excited state of D to A produced the excited CS state, although the IVCT band between A and A − was not detectable because of the ultrafast back electron transfer. This work represents the first detailed characterization of the CSMV states based on the transient IVCT absorption properties, their solvent dependence, and comparison with those of the ground MV states. This was achieved by selection of the parent 1•+ cation that exhibits a strong IVCT band within the wavelength region of the experimental setup. Selection of the BMes2 group was also advanta-


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geous to detect the CSMV state in terms of the longer lifetimes (nanosecond order) of the CT excited states of the purely organic NAr3–BAr3 D–A conjugates 23,24,26 compared with those of the above inorganic linear triad. 37c In the context of molecular QCA, several related techniques for MV complexes have been reported such as construction of assemblies as a prototype of QCA wires, visualization of charge distributions by scanning tunneling microscopy, 38 and external charge-induced charge redistributions.39 To respond to potential disruption of QCA operations by counterions, Lu and Lent19 proposed internalization of counterions leading to the coexistence with opposite charges, which is called “selfdoping”, and theoretically designed zwitterionic MV boron clusters bearing two allyl cations. In relation to Lent’s proposal of self-doping,19 the present findings for 2* provide a new way to generate the counterion-free MV state by introducing photo-responsiveness to a MV precursor and succeeding optical excitation. We are unaware of prior studies that realized self-doping of MV precursors in the context of molecular QCA. Although our investigation was performed with a MV state containing two redox sites, in principle, this approach should apply to exploration of zwitterionic QCA cell candidates containing four redox sites with two mobile charges, which can be accumulated by two-photon excitation.40 Furthermore, although 1•+ was selected here for complete characterization of the IVCT bands in the CSMV state, control of the charge delocalization would be achievable by regulating the π-bridged lengths between the NAr3 sites, as well as other redox sites. The spatial arrangements of the mobile and opposite charges would also be achieved by purposefully introducing the above sites via chemical modification. The present case of unsymmetrical linear conjugation provides basic insight into the effects of the increased electron density at an electron-accepting site on mobile positive charge distributions.

CONCLUSION We have shown that the covalent introduction of an additional A site to a D–D conjugate is successful for diversifying MV states through internalization of opposite charges within a molecular framework, leading to a unique counterion-free CSMV state. The characteristics of the CSMV state were clarified by detailed analysis of transient IVCT absorptions using femtosecond transient absorption spectroscopy for the first time. In the A–D–D triad 2, increase in the electron density at the A site by photo-excitation was accompanied with restructuring of the central D site from NAr3 to NAr2 sites. This biased positive charge distributions to the D site at the end, whose degree was larger for the CSMV state 2* than that for conventional MV state 2•+ resulted from the usual substituent effects of the electron-withdrawing BMes2 group. On the other hand, the equilibrium IVCT energies of 2* are far less sensitive to the solvent polarity than those of conventional ground MV state 1•+, revealing the unique characteristics of the CSMV state. Conjugation with the BMes2 site would be applicable to a wide range of MV cations, although selection of the redox components requires consideration of the lifetimes of the excited CS states. We believe that these findings are related to molecular QCA techniques. In line with preceding theoretical studies,19 we realized self-doping of a binuclear precursor to generate a half-cell by optical excitation, providing a chemical option toward molecular expression of QCA.

EXPERIMENTAL SECTION

Materials and general measurements. All solvents and chemicals used in the syntheses were of reagent grade and were used without further purification. Magic blue (tris(4bromophenyl)aminium hexachloridoantimonate) was purn chased from Sigma-Aldrich. Bu4NPF6 (tetra-nbutylammonium hexafluorophosphate) was purchased from Tokyo Kasei Kogyo (TCI) and recrystallized from methanol before use. (4-bromophenyl)dimesitylborane,23b 4bromophenyl-4',4''-dimethoxytriphenylamine (4),41 and NB23b were synthesized according to previously reported procedures. The UV-vis-NIR absorption spectra were measured on a JASCO V-670 spectrometer at room temperature. The fluorescence spectra were measured on a Horiba FluoroMax 4P spectrometer. The 1H- and 13C{1H}-NMR spectra were recorded using JEOL JNM-ECP400 and JNM-ECA600 spectrometers installed at the Nara Institute of Science and Technology; tetramethylsilane (TMS) was used as an internal standard (0 ppm) for 1H- and 13C-NMR analysis. The ESI-MS and EI-MS were obtained using JEOL JMS-T100CS and JMS-700 MStation spectrometers, respectively. The femtosecond transient absorption spectra were measured by the pump-and-probe method using a regeneratively amplified titanium sapphire laser as reported previously. 42 In the present study, the sample was excited using a 400 nm laser pulse that was generated using a type I BBO crystal. The fluorescence decay profiles were measured by the single photon counting method using a streakscope.43 The ultrashort laser pulse was generated by a Ti:sapphire laser. For excitation of the sample, the output of the Ti:sapphire laser was converted to the second harmonic oscillation (400 nm) using a type I BBO crystal. Synthesis. N,N,N'-Tris(4-methoxyphenyl)benzidine (3). [1,3-Bis(2,6-Diisopropylphenyl)imidazol-2-ylidene](3chloropyridyl)palladium(II) dichloride (PEPSITM-IPr) (7.0 mg, 6.5 µmol) was dissolved in 7.0 mL of dry toluene under N 2 atmosphere. After being stirred for 10 min at room temperature, 4 (200 mg, 0.434 mmol), 4-methoxyaniline (133 mg, 1.09 mmol), and sodium tert-butoxide (50 mg, 0.52 mmol) were added to the solution. The mixture was degassed by freeze-pump-thaw cycles and stirred at 120°C overnight. After being cooled to room temperature, the mixture was filtered through a Celite bed. The filtrate was diluted with 50 mL of toluene and washed with brine. After drying over anhydrous MgSO4, the organic phase was concentrated to dryness. The target product was purified by column chromatography on silica gel using ethyl acetate/n-hexane (1:4). Yield: 183 mg (84%). 1H-NMR (600 MHz, DMSO-d6, ppm): δ = 3.71 (s, 3H), 3.74 (s, 6H), 6.80-6.84 (m, 2H), 6.86-6.89 (m, 2H), 6.89-6.93 (m, 4H), 6.94-6.97 (m, 2H), 7.00-7.04 (m, 4H), 7.04-7.08 (m, 2H), 7.39-7.44 (m, 4H), 7.96 (s, 1H). 13C{1H}-NMR (125 MHz, DMSO-d6, ppm): δ = 55.12, 55.14, 114.5, 114.8, 115.0, 120.2, 120.3, 126.2, 126.6, 129.8, 132.3, 135.9, 140.2, 143.88, 146.7, 153.7, 155.4. Anal. Calc. for C33H30N2O3: C, 78.86; H, 6.02; N, 5.57. Found: C, 78.58; H, 5.82; 5.44%. HR-EI-MS (m/z). Calc. for C33H30N2O3 ([M]+): 502.2256. Found: 502.2258. N-(4-Dimesitylboryl)phenyl-N,N',N'-tris(4methoxyphenyl)benzidine (2). Bis(dibenzylideneacetone)palladium(0) (Pd(dba)2) (17 mg, 30 µmol) and (S)-(-)- 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) (19 mg, 30 µmol) were dissolved in 10 mL of dry toluene under N2 atmosphere. After being stirred for 10 min at



room temperature, 3 (200 mg, 0.398 mmol), (4bromophenyl)dimesitylborane (161 mg, 0.398 mmol), and sodium tert-butoxide (46 mg, 0.48 mmol) were added to the solution. The mixture was degassed by freeze-pump-thaw cycles and stirred at 120°C overnight. After being cooled to room temperature, the mixture was filtered through a Celite bed. The filtrate was diluted with 50 mL of toluene and washed with brine. After drying over anhydrous magnesium sulfate, the organic phase was concentrated to dryness. The target product 2 was purified by silica gel column chromatography using toluene/n-hexane (4:1) and recrystallized from ethyl acetate and methanol. Yield: 106 mg (32%). 1H-NMR (600 MHz, DMSO-d6, ppm): δ = 1.97 (s, 12H), 2.23 (s, 6H), 3.74 (s, 6H), 3.75 (s, 3H), 6.76-6.82 (m, 8H), 6.90-6.94, 6.957.00, 7.02-7.06, 7.11-7.16 (m, 14H), 7.19-7.23 (m, 2H), 7.437.46 (m, 2H,), 7.53-7.57 (m, 2H). 13C{1H}-NMR (125 MHz, DMSO-d6, ppm): δ = 20.7, 22.9, 55.1, 55.2, 114.9, 115.2, 117.4, 119.3, 125.3, 126.7, 126.9, 126.9, 127. 9, 128.3, 130.8, 133.3, 135.6, 137.4, 138.3, 138.3, 139.7, 139.9, 141.3, 144.7, 147.6, 151.3, 155.7, 156.8. UV-vis (CH2Cl2): [λmax/nm (εmax)], 396 (4.10 × 104). HR-ESI-MS (m/z). Calc. for C57H55BN2O3 ([M]+): 825.4342. Found: 825.4348. 4-Dimesitylboryl-4'-methoxytriphenylamine (NB′). Bis(dibenzylideneacetone)palladium(0) (Pd(dba)2) (5.0 mg, 7.4 µmol) and (S)-(-)- 2,2'-bis(diphenylphosphino)-1,1'binaphthyl (BINAP) (5.0 mg, 7.4 µmol) were dissolved in 10 mL of dry toluene under N 2 atmosphere. After being stirred for 10 min at room temperature, 4-methoxydiphenylamine (200 mg, 1.00 mmol), (4-bromophenyl)dimesitylborane (405 mg, 1.00 mmol), and sodium tert-butoxide (108 mg, 1.20 mmol) were added to the solution. The mixture was degassed by freeze-pump-thaw cycles and stirred at 120°C overnight. After being cooled to room temperature, the mixture was filtered through a Celite bed. The filtrate was diluted with 50 mL of toluene and washed with brine. After drying over anhydrous magnesium sulfate, the organic phase was concentrated to dryness. The target product was purified by column chromatography on silica gel using toluene/n-hexane (3:2) and recrystallized from ethyl acetate and methanol. Yield: 46 mg (9%). 1 H-NMR (600 MHz, DMSO-d6, ppm): δ = 1.97 (s, 12H), 2.23 (s, 6H), 3.75 (s, 3H), 6.75-6.72 (m, 2H), 6.78 (s, 4H), 6.946.98 (m, 2H), 7.10-7.15 (m, 5H), 7.18-7.22 (m, 2H), 7.32-7.37 (m, 2H). 13C{1H}-NMR (125 MHz, DMSO-d6, ppm): δ = 20.7, 22.9, 55.2, 115.2, 117.1, 124.3, 125.3, 127.9, 128.3, 128.8, 129.6, 137.3, 138.3, 138.4, 139.7, 141.4, 146.0, 151.5, 156.8. HR-ESI-MS (m/z). Calc. for C37H38BNO ([M]+): 523.3046. Found: 523.3050. DFT calculation data. The DFT calculations were carried out using the Gaussian09 program package.44 The restricted or unrestricted three-parameterized Becke–Lee–Yang–Parr (B3LYP or UB3LYP ) hybrid exchange-correlation functional45 was used with the 6-31G(d) basis set46. The stability of the optimized gas-phase structure was confirmed by calculating the molecular vibrational frequencies, in which no imaginary frequencies were observed.

ASSOCIATED CONTENT Supporting Information 1 13 1 Copies of H and C{ H}NMR spectra, HR-ESI-MS and HR-EIMS spectra, DFT calculation data, and UV-vis(-NIR) and luminescence spectral data.

Page 8 of 12

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI JP16H06514 in Coordination Asymmetry and 18K04890, and "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

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Charge-Separated Mixed Valency in an Unsymmetrical Acceptor

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