Direct Observation of the Ultrafast Evolution of Open-Shell Biradical in

Apr 25, 2017 - Division of Frontier Materials Science and Center for Promotion of Advanced Interdisciplinary Research, Graduate School of Engineering ...
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Direct Observation of the Ultrafast Evolution of Open-Shell Biradical in Photochromic Radical Dimer Yoichi Kobayashi,† Hajime Okajima,† Hikaru Sotome,‡ Takeshi Yanai,§ Katsuya Mutoh,† Yusuke Yoneda,‡ Yasuteru Shigeta,⊥ Akira Sakamoto,† Hiroshi Miyasaka,*,‡ and Jiro Abe*,† †

Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ‡ Division of Frontier Materials Science and Center for Promotion of Advanced Interdisciplinary Research, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan § Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan ⊥ Department of Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan S Supporting Information *

ABSTRACT: Delocalized biradicals have been extensively studied because of fundamental interests to singlet biradicals and several potential applications such as to two-photon absorption materials. However, many of the biradical studies only focus on the static properties of the rigid molecular structures. It is expected that the biradical properties of the delocalized biradicals are sensitive to the subtle changes of the molecular structures and their local environments. Therefore, the studies of the dynamic properties of the system will give further insight into stable radical chemistry. In this study, we directly probe the ultrafast dynamics of the delocalized biradical of a photochromic radical dimer, pentaarylbiimidazole (PABI), by time-resolved visible and infrared spectroscopies and quantum chemical calculations with the extended multistate complete active space second-order perturbation theory (XMSCASPT2). While the photogenerated transient species was considered to be a single species of the biradical, the present ultrafast spectroscopic study revealed the existence of two transient isomers differing in the contributions of biradical character. The origin of the two metastable isomers is most probably due to the substantial van der Waals interaction between the phenyl rings substituted at the imidazole rings. Unraveling the temporal evolution of the biradical contribution will stimulate to explore novel delocalized biradicals and to develop biradical-based photofunctional materials utilizing the dynamic properties.

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INTRODUCTION

On the evolution of the electronic structure, Zeng et al. have recently reported that an extended Chichibabin’s hydrocarbon shows the valence isomerization from the biradical to quinoid forms with a time scales of several hours.19 This slow valence isomerization is probably due to the large structural difference between the biradical and quinoid forms. Our groups have developed photochromic radical dimers in which the biradicallike form isomerizes to the quinoid-like form based on phenoxyl-imidazolyl radical complex (PIC)20 and hexaarylbiimidazole (HABI).21 These reports suggest that the optical properties of delocalized biradical compounds, which depend on the balance between the open-shell and closed-shell contributions, are sensitive to the subtle changes of the molecular structures and the local environments. In other words, the electronic structure of the flexible biradical systems can be rationally controlled by the structure of molecules and

Organic radicals have been extensively studied since the early 20th century because of fundamental interest in the structure and property of chemical species with unpaired electrons inside.1−4 In addition to fundamental interests, organic radicals have opened up several potential applications such as to radical polymerization initiators,5 radical batteries,6−8 and two-photon absorption materials.9−12 Meanwhile, various stable organic radicals have been synthesized by delocalizing their unpaired electrons over the molecule and by protecting them with bulky substituents. Almost all studies related to the delocalized biradicals, however, focused on the static aspects of the biradicals constructed from rigid molecular frameworks of πconjugation systems.3,4,8,13,14 On the other hand, only a few studies15−18 have been reported for the properties of πdelocalized biradicals in flexible structures where the geometrical arrangement is associated with the change of the electronic structure. © 2017 American Chemical Society

Received: February 15, 2017 Published: April 25, 2017 6382

DOI: 10.1021/jacs.7b01598 J. Am. Chem. Soc. 2017, 139, 6382−6389

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Journal of the American Chemical Society

photochromic molecules that generates two imidazolyl radicals (open-ring isomer) in a molecule by breaking the C−N bond between the imidazole rings upon UV light irradiation.23 The generated open-ring isomer was expected to be described as a single species of the resonance hybrid of a pure open-shell biradical form and a pure closed-shell quinoid form. However, the present study revealed that the photogenerated open-ring isomer of PABI has actually two metastable states most probably due to the van der Waals interaction between the phenyl rings substituted at the imidazole rings as well as the balance between the biradical and quinoid configurations (Figure 1b). While there are many reports on the ultrafast spectroscopy of the photogenerated biradical so far,24−39 almost all studies have discussed the localized biradical systems such as ketyl radicals and aryl radicals, and none of them has probed the temporal evolution of coupled radicals in real time. To the best of our knowledge, this study is the first example to probe the ultrafast formation process of the delocalized biradical from an individual radical pair.

well-arranged environments based on the detailed information on the geometry and electronic structure. However, the biradicals with short lifetimes inhibit X-ray crystallographic analyses, which are the most powerful tool to investigate molecular structures of biradicals. Moreover, the computational approach is also rather hard task for dealing with the coupled radicals because they are described as the resonance hybrid of the pure open-shell biradical and the pure closed-shell quinoid. It is generally undesirable to describe the wave function of resonance hybrids with a single determinant (or electronic configuration) as used in conventional density functional theory (DFT) calculations, while it should be accounted for with a quantum superposition of multiple determinants. Although the conventional experimental and theoretical methods are rather difficult to directly apply to the elucidation of the electronic and molecular structures in the flexible systems with short lifetimes, detection of the time evolution could provide the information on the structure and property of radicals in the case that the radical formation occurs very promptly after the photoexcitation.12 Very rapid reaction after the photoexcitation could keep the molecular geometry as that in the ground state and the time evolution of the electronic and vibrational spectra reflects the temporal change of electronic states and geometries. In this study, we directly observe the ultrafast dynamics of the delocalized biradical, pentaarylbiimidazole (PABI, Figure 1a), by combining time-resolved visible and infrared (IR) absorption spectroscopies and quantum chemical calculations with the multireference (or multiconfigurational) wave function theory, referred to as the extended multistate complete active space second-order perturbation (XMS-CASPT2) theory.22 PABI is one of the recently developed radical dissociation-type

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RESULTS AND DISCUSSION In the photochromic reaction of PABI, the open-ring isomer generated by UV light irradiation thermally reverts to the initial closed-ring isomer with a half-life of 2 μs in benzene at room temperature.23 The previous report only focused on the thermal back reaction of the photogenerated open-ring isomer. The open-ring isomer of PABI is described as a resonance hybrid of the open-shell biradical and closed-shell quinoid forms. The dihedral angles of the imidazole rings with respect to the phenyl ring substituted at the 2-position of the imidazole rings (Im-2-Ph dihedral angle) drastically change after the bond cleavage. PABI was synthesized as reported previously23 and the time-resolved spectroscopic measurements at the visible and IR regions were measured in benzene and deuterated dichloromethane (CD2Cl2), respectively. In many organic molecular systems, DFT calculations are powerful tools to elucidate the electronic structures and optical properties. However, technically speaking, it is not best suited to apply DFT calculations to molecular systems with a resonance hybrid electronic character because their theoretical framework is based on a single-determinant picture. The multireference wave function theory is a desirable approach to accurately describe resonate states and their excited states by constructing highly correlated many electronic wave functions. Here, we centrally use the XMS-CASPT2 theory, recently introduced as an efficient multireference approach, for our theoretical analysis. Figure 2a shows the total energies of the ground state S0 and four lowest-lying excited states S1 to S4 of PABI obtained by the XMS-CASPT2 method as a function of the reaction coordinate, which was modeled using the DFT calculations at UCAMB3LYP/def2-SVP level of the theory.40 We also conducted same calculations using M06-2X functional, which is known as a sophisticated generalized gradient approximation (GGA) functional accounting for the semilocal effects.41 We confirmed the functional dependence on the geometries of the biradicals is qualitatively minor in this study. For the formation process of the open-ring isomer, we consider the singlet states in this study because the extremely fast bond breaking process (∼140 fs, as shown later) suggests that the triplet excited state is not involved in the bond breaking process (the intersystem crossing usually takes several hundreds of picoseconds to nanoseconds).42 The details of the quantum chemical calculations

Figure 1. (a) Photochromic reaction scheme and (b) plausible detailed energy diagram of the photochromic reaction of PABI demonstrated in this study. 6383

DOI: 10.1021/jacs.7b01598 J. Am. Chem. Soc. 2017, 139, 6382−6389

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Figure 2. (a) Potential energy curves of the S0 to S4 states obtained by the combinations of DFT calculations (UCAM-B3LYP/def2-SVP level of the theory) and XMS-CASPT2 calculations, (b) the magnified potential curve of the S0 state around the open-ring isomer, and (c) optimized molecular structures of the Open 1, Open 2, and transition state of PABI. The horizontal axis of panels a and b is the distance between C11−N18 atoms. Dashed lines in panel a indicate a schematically illustrated conical intersection.

The HOMO−1, HOMO, LUMO-like orbitals for Open 1 and Open 2 are shown in Figure S13. The configuration analysis on the reference CASSCF wave functions (Figure S14) characterizes the open-ring isomer mainly as the open-shell singlet biradical configuration with a weight of about 86.3% and 84.8% for Open 1 and Open 2, respectively. The rest of the description arises primarily from the closed-shell configuration, corresponding to the quinoidal structure, with a weight of 8.6% and 9.9% for Open 1 and Open 2, respectively. Overall, the biradical character is slightly more weighted toward Open 1 relative to Open 2, while the quinoidal character is less, as summarized in Figure S15. This result suggests that Open 1 has a larger amount of open-shell biradical character than Open 2. This result is related to the larger Im-2-Ph dihedral angle of Open 1. Therefore, these calculations show that the presence of the two local minima in the open-ring isomer originated from the weak van der Waals interaction between the phenyl rings. The quinoidal electronic structure, which was observed in similar imidazole derivatives,20,21,43 is formed in the S1 state. This state is characterized as closed-shell and is higher-lying by 1.4 eV relative to the biradical S0 state. Thus, it should be naturally assumed that the barrier is rather high to activate the quinoid form in the present system. The details of the comparison between PABI and a previously reported bisimidazole derivative (BDPI-2Y)43,44 with DFT calculations are shown in the Supporting Information. To directly elucidate the properties of the biradical predicted by the quantum chemical calculation, we conducted ultrafast visible and IR transient absorption measurements of PABI. Figure 3a and b show the visible transient absorption spectra and time profiles of the transient absorbance of PABI in benzene (3.1 × 10−3 M) excited at 330 nm of a femtosecond laser pulse (∼25 fs, 200 nJ/pulse). The transient absorption dynamics until 1 ns are shown in the Supporting Information (Figure S9). At 220 fs after the excitation, a broad transient absorption band was observed over the visible region, which was probably assigned to the absorption from the S1 state to Sn states.34,39 The absorption due to the Sn ← S1 transition quickly

are shown in the Supporting Information. The photochromic reaction of PABI can be interpreted as follows. Before the C−N bond cleavage, two imidazole rings of PABI are orthogonally arranged. After the excitation of the closed-ring isomer by UV light irradiation, the energy quickly relaxes to the S1 state. The C−N bond between the imidazole rings is cleaved most probably through the conical intersection between the S0 and S1 potential curves. The two radicals just after the bond cleavage should be an individual radical pair. Then the interaction between radicals gradually increases with the decrease in the Im−2-Ph dihedral angle, and the open-ring isomer is formed. This open-ring isomer thermally reverts to the closed-ring isomer through the S0 potential curve. Figure 2b shows the magnified potential energy curve around the open-ring isomer. The figure suggests that the open-ring isomer has two metastable states, namely Open 1 (lower energy) and Open 2 (higher energy). The energy difference between the Open 1 and Open 2 is 2.26 kJ/mol for the DFT calculations without taking into account solvent effects, and it is 4.27 and 5.48 kJ/mol for the DFT-PCM (polarizable continuum model) calculations with the solvents, benzene and dichloromethane, respectively. These values are in the order of the thermal fluctuation energy at room temperature (∼2.5 kJ/mol), and therefore, this calculated result suggests that the photogenerated open-ring isomer is in the thermal equilibrium between Open 1 and Open 2. Figure 2c shows the optimized molecular structures of Open 1, Open 2, and the transition state. The main difference between the molecular structures of Open 1 and 2 is the Im-2-Ph dihedral angle, namely 31.0 and 22.5° for Open 1 and Open 2, respectively. Notably, there is a substantial van der Waals interaction between the phenyl rings substituted at the imidazole rings. Our theoretical approach reveals the transition state between Open 1 and Open 2 diminishes without use of van der Waals corrections (see Supporting Information for details). Therefore, the weak interaction between the stacked phenyl rings plays an essential role in producing the two minima of the open-ring isomer in the van der Waals-corrected potential energy curve. 6384

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copy at a picosecond time scale indicates that the initially generated Open 1 equilibrates to Open 2. On the other hand, the rise kinetics are explained by the two processes. The initial rise kinetics (∼2 ps) are most probably assigned to the relaxation process from immediately after the bond breaking to Open 1 (a green arrow in Figure 1b) because the individual phenoxyl and imidazolyl radicals have weak absorptions around 700 nm.45,46 Although the XMS-CASPT2 level of theory suggests that the oscillator strengths of Open 1 and Open 2 are similar (Table S2), the spectral bandwidth gradually decreases with a time scale of picoseconds. In this case, even when the oscillator strength does not change, the rise kinetics appear around the absorption maximum. While visible transient absorption spectroscopy is a powerful tool to investigate the electronic structures of the systems, it is difficult to elucidate the detail of the molecular structure from the broad visible transient absorption bands. To investigate the molecular structures at ultrafast time scales, we conducted the time-resolved IR absorption measurements using a femtosecond laser pulse. Figure 4a and b show time-resolved IR

Figure 3. (a) Transient absorption spectra and (b) dynamics of PABI in benzene (3.1 × 10−3 M) at the visible region excited at 330 nm of a femtosecond laser pulse (∼25 fs, 200 nJ/pulse).

decays with a time constant of 140 fs (Figure S9), and other transient absorption bands appear at 470 and 725 nm within 1 ps. This transient spectrum is similar to that of the open-ring isomer found in the photochromic reaction of PABI, and therefore, this result shows that the bond cleavage process occurs with the time constant of 140 fs. The transient absorption bands at 470 and 725 nm gradually shift to 450 and 707 nm with a time scale of several picoseconds. The rise kinetics at 707 nm is analyzed with a biexponential rise function, and the time constants are 2.1 ps (69%) and 9.4 ps (31%). As will be revealed later, the faster rise component indicates the relaxation process from immediately after the bond cleavage to Open 1, while the slower component indicates the interconversion between Open 1 and Open 2. The spectral shapes after 10 ps following the excitation are almost the same as that observed for the open-ring isomer at nanosecond laser flash photolysis measurements previously.23 Although the initial fast decay with a time scale of hundreds of femtoseconds was observed in similar photochromic imidazole dimers such as hexaarylbiimidazole (HABI) and bridged imidazole dimers,34,35,39 the spectral shift and the large increase in the signal with a time scale of picoseconds have not been observed in previous systems. This spectral shift can be explained by the two metastable isomers in PABI. The multireference multistate calculations at the XMS-CASPT2 level of theory suggest that the absorption bands appear at 521 and 732 nm in Open 1 and 517 and 714 nm in Open 2. These spectral differences are well consistent with the observed spectral shift (from 470 and 725 nm to 450 and 707 nm). Therefore, the spectral shift observed by the visible spectros-

Figure 4. (a) Time-resolved IR spectra of PABI in CD2Cl2 (∼3 × 10−2 M) excited at 330 nm (∼100 fs, 3 μJ/pulse) at room temperature and (b) calculated IR absorption spectra of Open 1, Open 2, and closedring form (UCAM-B3LYP/def2-SVP level of the theory). The scaling factor (0.970) was multiplied to the calculated IR wavenumbers.

spectra of PABI in CD2Cl2 (∼3 × 10−2 M) excited at 330 nm (∼100 fs, 3 μJ/pulse) at room temperature and the calculated IR spectra of Open 1, Open 2, and closed-ring form by the broken-symmetry DFT calculations (UCAM-B3LYP-D3BJ/ def2-SVP level of the theory), respectively. The details of the quantum chemical calculations are shown in the Supporting Information. At 1 ps after the excitation, several transient absorption bands are observed, namely, the narrow bands at 1267, 1300, 1375 cm−1, a shoulder band at 1285 cm−1, and a broad band around 1350 cm−1. The broad absorption band around 1350 cm−1 quickly decays within several picoseconds. The fast decay of the broad IR absorption band is consistent with the fast rise component observed at the visible spectroscopy (2.1 ps). This time scale is slower than the 6385

DOI: 10.1021/jacs.7b01598 J. Am. Chem. Soc. 2017, 139, 6382−6389

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Journal of the American Chemical Society decay of the S1 state (∼140 fs) but faster than the slow rise component observed at the visible spectroscopy (9.4 ps). Therefore, the absorption band most probably reflects the transient structures between the S1 state and Open 1 on the potential curve (green arrow process in Figure 1b) and the vibrational cooling process. After the decay of the broad absorption band at 1360 cm−1, the transient absorption spectrum at 5 ps is very similar to those of Open 1 (red curve) and Open 2 (blue curve) obtained by the DFT calculations as shown in Figure 4. The transient absorption bands gradually shift to the higher wavenumber with a time scale of several picoseconds with an isosbestic point at 1287 cm−1. The isosbestic point shows that this process is not due to the vibrational cooling but due to a reaction between two states. The transient difference absorption spectra obtained by subtracting the transient absorption spectrum at 1 ps more clearly shows the higher frequency shift with a time scale of picoseconds (Figure S10). These spectral shifts can be explained by the spectral evolution caused by the interconversion from Open 1 to Open 2 by the DFT calculations (Figure 4b). The time profile of the transient absorbance at 1276 cm−1 is fitted with the single exponential decay function and the time constant is 12 ps, which is consistent with the time constant obtained from the visible spectroscopy (9.4 ps). These results measured by visible and IR spectroscopies suggest that the equilibrium between Open 1 and Open 2 is more weighted toward Open 2, while the quantum chemical calculations suggest that the energy of Open 1 is very slightly lower than that of Open 2 (by 5.48 kJ/mol in dichloromethane). This deviation may be explained by the error caused by the use of the geometries determined by DFT, which is generally insufficient for describing antiferromaginetic raical states with high quantitative accuracy. Both visible and IR transient absorption measurements suggest that Open 1 is formed after the bond cleavage and then Open 1 equilibrates with Open 2 at picosecond time scales. To get an alternative evidence for the dynamic structural changes between two isomers, tert-butyl group-substituted PABI (tBuPABI, Figure 1a) was synthesized. The bulky substituents of tBu-PABI restrict the change of the Im−2-Ph dihedral angle and van der Waals interaction between the phenyl rings. Therefore, the rise kinetics of the transient absorption spectra should slow down in tBu-PABI. The photochromic properties of tBu-PABI in benzene were investigated by the nanosecond laser flash photolysis (Figure S8). The absorption spectra of the closed- and open-ring isomers of tBu-PABI are very similar to those of PABI, and the photogenerated open-ring isomer of tBu-PABI thermally reverts to the initial closed-ring isomer with a half-life of 1.7 μs. Figure 5a shows the transient absorption spectra of tBu-PABI in benzene (3.0 × 10−3 M) at the visible region excited at 330 nm of a femtosecond laser pulse (200 nJ/pulse). Transient absorption spectra in subpicoseconds to hundreds of picoseconds are very similar to those of PABI although the absorption maxima slightly shift to the longer wavelength. Namely, after the generation of the broad absorption assigned to the Sn ← S1 transition, two absorption bands appear at 490 and 750 nm within 1 ps. These absorption bands shift to the shorter wavelength, and the band at 750 nm increases with a time scale of picoseconds to tens of picoseconds. The shape of the spectra does not change after 100 ps following the excitation, and the spectra after 100 ps can be assigned to the

Figure 5. (a) Transient absorption spectra of tBu-PABI in benzene (3.0 × 10−3 M) at the visible region excited at 330 nm of a femtosecond laser pulse (200 nJ/pulse) and transient absorption dynamics of PABI and tBu-PABI in benzene probed at 707 and 730 nm, respectively.

open-ring isomer as was observed at the nanosecond laser flash photolysis measurements. Figure 5b shows time profiles of the transient absorbance of PABI and tBu-PABI in benzene probed at 707 and 730 nm, respectively. The dynamics clearly show that the rise of the tBuPABI is slower than that of PABI due to the bulky substituents. This result indicates that the rise component is due to the rotation of two imidazole rings. The rise kinetics of tBu-PABI at 730 nm are fitted with biexponential functions and the time constants of the rise kinetics are 1.7 ps (32%) and 11 ps (68%) for tBu-PABI, while those of PABI at 707 nm are 2.1 ps (69%) and 9.4 ps (31%) ps. The increase in the statistical weight of the slower rise component of tBu-PABI is most probably due to the presence of the tert-butyl groups, which might somehow increase the yield for Open 2 at the thermal equilibrium. Figure 6a shows the time-resolved IR spectra of tBu-PABI in CD2Cl2 (∼3 × 10−2 M) excited at 330 nm (∼100 fs, 3 μJ/ pulse) at room temperature. The time-resolved IR spectra of tBu-PABI also show the higher wavenumber shift as was observed for the IR absorption spectra of PABI. The transient difference absorption spectra obtained by subtracting the transient absorption spectrum at 1 ps is subtracted and is shown in the Supporting Information (Figure S10). These difference spectra also indicate the interconversion from Open 1 to Open 2. Figure 6b shows the normalized time profiles of the IR signals of PABI and tBu-PABI probed at 1276 cm−1, which are assigned to the stretching vibrational mode of the C− N bond in the imidazole ring. The deceleration of the rise kinetics of tBu-PABI as compared to those of PABI is more 6386

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Kanto Chemical Co., Inc. and ACROS Organics and were used without further purification. 2-(1,3-Dioxolan-2-yl)benzaldehyde (S1). This was synthesized according to the literature procedure.23 1,2-Bis(4-tert-butyl)phenylethane-1,2-dione (S2). This was synthesized according to the literature procedure.47 1,2-Bis{4,5-di(4-tert-butyl)phenyl-1H-imidazol-2-yl}benzene (S3). S1 (54 mg, 0.304 mmol), S2 (95 mg, 0.30 mmol), and ammonium acetate (131 mg, 1.70 mmol) were stirred at 110 °C in CHCl3 (2 mL) in a sealed tube for 14 h (Scheme 1). Then acetic acid (0.25 mL) and

Scheme 1. Synthetic Scheme of tBu-PABI

1,2-bis(4-tert-butyl)phenylethane-1,2-dione (30 mg, 0.093 mmol) were added, and the reaction mixture was stirred at 110 °C in a sealed tube for 22 h. The reaction mixture was cooled to room temperature and neutralized with aqueous NH3. The reaction mixture was extracted with CH2Cl2, and the organic layer was washed with water and passed through phase separator paper. After the solvent was removed, the crude mixture was purified by silica gel column chromatography (ethyl acetate/hexane = 1:3) to give S3 as pale yellow solid. The solid was further purified by recrystallization from CH2Cl2/hexane (24 mg, yield: 10%). 1H NMR (400 MHz, DMSO-d6): δ 13.80 (s, 2H), 8.13 (m, 2H), 7.58 (m, 2H), 7.48−7.30 (m, 16H), 1.31 (s, 36H). 13C NMR (100 MHz, CDCl3): δ 150.15, 146.36, 132.60, 130.42, 130.36, 127.73, 126.97, 126.77, 125.50, 34.78, 31.60. HRMS (ESI-TOF) calculated for C52H58N4 [M + H]+: 739.4734, found: 739.4747. 2,3,4′,5′-Tetrakis(4-tert-butylphenyl)spiro[imidazo[2,1-a]isoindole-5,2′-imidazole] (t-Bu-PABI). All manipulations were carried out with the exclusion of light. Under nitrogen, a solution of potassium ferricyanide (281 mg, 0.853 mmol) and KOH (180 mg, 3.21 mmol) in water (6 mL) was added to a solution of S3 (15 mg, 0.020 mmol) in benzene (3 mL). The mixture was vigorously stirred at 60 °C for 2 h. The organic layer was washed with water and passed through phase separator paper. After the solvent was removed, the crude mixture was purified by silica gel column chromatography (ethyl acetate/CH2Cl2 = 1:40) to give 2,3,4′,5′-tetrakis(4-tert-butylphenyl)spiro[imidazo[2,1a]isoindole-5,2′-imidazole] (t-Bu-PABI) as yellow solid (9 mg, yield: 60%). 1H NMR (400 MHz, DMSO-d6) δ: 7.95 (d, J = 5.0 Hz, 1H), 7.61 (t, J = 5.0 Hz, 1H), 7.50 (d, J = 5.2 Hz, 2H), 7.43 (d, J = 5.2 Hz, 4H), 7.39 (t, J = 5.0 Hz, 1H), 7.28 (d, J = 5.2 Hz, 2H), 7.23 (m, 5H), 7.18 (d, J = 5.2 Hz, 2H), 7.12 (d, J = 5.2 Hz, 2H), 1.29 (s, 18H), 1.24 (s, 9H), 1.09 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 170.29, 154.98, 151.84, 150.94, 149.15, 142.14, 139.94, 132.01, 131.08, 130.28, 129.22, 128.03, 127.19, 127.04, 126.21, 125.09, 125.03, 125.00, 121.59, 121.00, 110.90, 35.03, 34.50, 34.45, 31.32, 31.15, 31.08. HRMS (ESI-TOF) calculated for C52H56N4 [M + H]+: 737.4578, found: 737.4560. Femtosecond Visible Transient Absorption Measurements. Transient absorption spectra in the visible region were measured using a home-built setup based on dual noncollinear optical parametric amplifiers (NOPAs, TOPAS-white, Light Conversion). The pulsed light source was a Ti:sapphire regenerative amplifier (Spitfire, SpectraPhysics, 802 nm, 1 W, 1 kHz, 100 fs) seeded by a Ti:sapphire oscillator (Tsunami, Spectra-Physics, 802 nm, 820 mW, 80 MHz, 100 fs). The output of the Ti:sapphire amplifier was equally divided into two portions. The first one was used for driving a NOPA, which is tuned to 660 nm. Its output was then frequency-doubled with a 50-μm βbarium borate (BBO) crystal. The generated second harmonics at 330 nm were temporally compressed using a prism compressor and used for the excitation of the sample. The second portion of the amplifier was introduced into the other NOPA and converted into infrared pulses at 950 nm. This 950 nm pulse was focused into a 2 mm CaF2 plate after passing through a delay stage to generate femtosecond

Figure 6. (a) Time-resolved IR spectra of tBu-PABI in CD2Cl2 (∼3 × 10−2 M) excited at 330 nm (∼100 fs, 3 μJ/pulse) at room temperature and (b) transient IR absorption dynamics of PABI and tBu-PABI in CD2Cl2 probed at 1276 cm−1.

pronounced in IR spectroscopy than that in visible spectroscopy. These dynamics are fitted with the single exponential functions and these time constants are 12 and 20 ps for PABI and tBu-PABI, respectively. Both results of the transient visible and IR absorption measurements show that the bond cleavage process of PABI first generates Open 1, and then Open 1 interconverts to Open 2 at about 10 ps in PABI.

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CONCLUSION We directly observed the ultrafast formation dynamics of the biradical of the photochromic PABI by combining the transient visible and IR absorption spectroscopies and the XMS-CASPT2 calculations. The DFT and XMS-CASPT2 calculations predicted the existence of the two metastable states in the photogenerated transient species of PABI, Open 1 and Open 2. The origin of the existence of the two metastable species is the substantial van der Waals interaction between the phenyl rings substituted the imidazole rings. Femtosecond visible and IR absorption spectroscopies detect the equilibrium process between the two states whose biradical contributions are different with the time constant of about 10 ps. Revealing the dynamic process of the biradical will stimulate researchers to explore further flexible and delocalized biradical systems.

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EXPERIMENTAL SECTION

Synthesis. All reactions were monitored by thin-layer chromatography carried out on 0.2 mm E. Merck silica gel plates (60F-254). Column chromatography was performed on silica gel (Wakogel C300). 1H NMR spectra were recorded at 400 MHz on a Bruker AVANCE III 400 NanoBay. DMSO-d6 and CDCl3 were used as deuterated solvent. ESI−TOF−MS spectra were recorded on a Bruker micrOTOF II-AGA1. All glassware was washed with distilled water and dried. Unless otherwise noted, all reagents and reaction solvents were purchased from TCI, Wako Co. Ltd., Aldrich Chemical Co., Inc., 6387

DOI: 10.1021/jacs.7b01598 J. Am. Chem. Soc. 2017, 139, 6382−6389

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Journal of the American Chemical Society white light continuum for the probe pulse. The probe pulse was divided into signal and reference pulses. The signal pulse was guided into the sample and then the both pulses were detected using a pair of multichannel photodiode array (PMA-10, Hamamatsu). For tBu-PABI in benzene solution, the output of a collinear optical parametric amplifier (TOPAS-Prime, Light Conversion) at 1180 nm was used for generation of white light continuum. The polarization of the excitation pulse was set to the magic angle with respect to that of the probe pulse. The excitation pulse was characterized by self-diffraction frequency-resolved optical gating and the pulse width was estimated to be 25−30 fs. The chirping of the white light continuum was evaluated by an optical Kerr gate signal of a sapphire plate and used for the corrections of the spectra. The fwhm of the cross-correlation between the pump and probe pulses was about 70 fs. The typical excitation power was 200 nJ/pulse at the sample position. During the measurement, the sample solution was circulated with a homemade rotation cell with 200-μm optical length. Steady-state absorption spectra were recorded before and after the transient absorption measurement to examine photodegradation of the sample, and no permanent change in absorbance was observed. Picosecond Time-Resolved IR Absorption Measurements. The experimental setup is described as follows. The fundamental output from a Ti:sapphire regenerative amplifier (Solstice Ace, Spectra Physics, 800 nm, 6 W, 1 kHz, ∼100 fs) was divided into two beams. Both were used to excite two optical parametric amplifiers (OPAs) and generate pump and probe pulses. The probe IR pulse was obtained by difference-frequency generation between the signal and idler waves from one OPA (TOPAS-C, Light Conversion). The spectral bandwidth was ∼170 cm−1 in fwhm. The probe beam was divided into two paths by a ZnSe half mirror. One portion of the beam was focused on and transmitted through the sample; it was used as a sample. The other was used as a reference. Both were introduced into a 19 cm spectrograph (TRIAX190, HORIBA JOBIN YVON) with a slightly different height offset. The dispersed two beams were simultaneously detected by a 2 × 64-channel liquid-nitrogen-cooled HgCdTe detector array and integrated by 128 box-car integrators (IR12−128, InfraRed Associates). The normalized IR signals were obtained by dividing the sample intensities by the reference intensities. The pump UV pulse (330 nm) was obtained by forth harmonic generation of the signal wave from the other OPA (TOPAS-prime, Light Conversion). The pump beam was modulated at half the repetition rate of the probe beam (500 Hz) by a mechanical chopper. The modulated pump beam was passed through a delay stage and focused on the sample noncollinearly against the probe beam. The normalized IR signals with and without the pump pulses were separately accumulated by the computer. The pump-induced infrared absorption was obtained by dividing the pump-on signal by the pumpoff signal. The cross-correlation time between the pump and probe pulses, which was determined by the rise of a transient infrared absorption of photoexcited silicon due to free carriers, was ∼0.5 ps. The spectral resolution of the setup was ∼3 cm−1. Since the spectral coverage of the IR setup was 90 cm−1, two spectral regions (1250− 1320 cm−1 and 1320−1400 cm−1) were measured separately. The energy of the probe pulse at the sample position was less than 0.3 μJ, and that of the pump pulse was less than 3 μJ. PABI and tBu-PABI in CD2Cl2 solution were filled in a CaF2 cell (optical path length; 200 μm). To prevent photodegradation of the samples, the sample stage was moved right and left during the experiment (velocity 1 mm/s). We confirmed that the color of the samples and their IR spectra did not change even after 10 h experiment.

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absorption measurements, quantum chemical calculations (PDF) Side-view animation of DFT-scanned molecular structures (AVI) Top-view animation of DFT-scanned molecular structures (AVI) DFT-scanned geometries of PABI (XYZ)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yoichi Kobayashi: 0000-0003-3339-3755 Jiro Abe: 0000-0002-0237-815X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by JSPS KAKENHI Grant No. JP26107010 in Scientific Research on Innovative Areas “Photosynergetics” and JSPS KAKENHI Grant Nos. JP15K17846 and JP16H04101 for Y.K. and T.Y., respectively. Financial assistance for this research was also provided by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013−2017.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01598. 1

H and 13C NMR, HR-ESI−TOF−MS spectra, HPLC chromatograms, steady-state absorption spectra, transient 6388

DOI: 10.1021/jacs.7b01598 J. Am. Chem. Soc. 2017, 139, 6382−6389

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