Reversible Valence Photoisomerization between Closed-Shell

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Spectroscopy and Photochemistry; General Theory

Reversible Valence Photoisomerization between Closed-Shell Quinoidal and Open-Shell Biradical Form Ayako Tokunaga, Katsuya Mutoh, Takeshi Hasegawa, and Jiro Abe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00916 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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The Journal of Physical Chemistry Letters

Reversible Valence Photoisomerization between Closed-Shell Quinoidal and Open-Shell Biradical Form Ayako Tokunaga†, Katsuya Mutoh†, Takeshi Hasegawa‡, 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, ‡Laboratory of Chemistry for Functionalized Surfaces, Division of Environmental Chemistry, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Corresponding Author * [email protected]

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ABSTRACT. We report here a kinetic study on the thermal equilibrium process between the biradical form and the quinoidal form starting from the singlet biradical form alone. Photochromic phenoxyl-imidazolyl radical complex repeatedly generates biradical species upon UV light irradiation, and the following thermal equilibrium process responsible for the valence isomerization from the open-shell singlet biradical to the closed-shell quinoidal form is observed in the microsecond time region. The thermodynamic parameters for the equilibrium process were determined for the first time by nanosecond laser flash photolysis. We also found that the visiblelight excitation to the equilibrium state causes the valence photoisomerization from the quinoidal to the biradical form, that returns thermally to the quinoidal form.

TOC GRAPHICS

KEYWORDS. photochromism; biradical; quinoidal; valence photoisomerization; phenoxyl radical; pyrene


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Polycyclic aromatic hydrocarbons are well known to have singlet biradical character in the ground state.1-12 The strong antiferromagnetic interaction between unpaired electrons in an open-shell singlet biradical eventually produces a covalent bond and forms a closed-shell singlet (quinoidal) structure. The magnitude of the spin–spin interaction depends on the π-conjugation scaffold connecting two spin centers, and singlet biradicals can be usually described as a resonance hybrid of biradical and quinoidal structures. Molecules possessing prominent biradical character in the ground state are known as biradicaloids.11 Quinoidal molecules may exhibit biradical reactivity due to the contribution of a biradical structure. The representative biradicaloid is Chichibabin’s hydrocarbon.13,14 The electronic structure of Chichibabin’s hydrocarbon has been extensively studied and is accepted as exhibiting large biradical character in the ground state.15 On the other hand, a larger deviation from the planarity brings about the decoupling of two unpaired electrons, resulting in the separation of the energy levels of the biradical and the quinoidal state.16,17 Thus, the flexibility of the molecular framework determines whether a biradical exists as a resonance hybrid of biradical and quinoidal structures or an equilibrium between a biradical and a quinoidal form.18,19 Oda and coworkers reported a reversible photochemical and thermal isomerization between the biradical and the quinoidal form of a dibenzoannulated p-terphenoquinone derivative. They observed the transition of the equilibrium upon UV light irradiation.20 Wu and coworkers reported the irreversible thermal valence isomerization from an unstable biradical to a stable quinoidal form of newly designed tetrabenzannulated Chichibabin’s hydrocarbon.21 As is distinct from Chichibabin’s hydrocarbon, tetrabenzannulated Chichibabin’s hydrocarbon is not a planar molecule because of the large steric hindrance arising from the two anthracene units. The freshly generated biradical displayed the strong ESR signal, and decayed monoexponentially to form the stable quinoidal form with a


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half-life of 495 min at 298 K. Therefore, there are two distinct valence isomers in tetrabenzannulated Chichibabin’s hydrocarbon, i.e. the biradical form and the quinoidal form. They also determined the thermodynamic parameters for this decay process. The Gibbs free energy barrier, ∆G‡, was determined as 95.0 ± 2.5 kJ mol–1 at 298 K. This large activation energy barrier was ascribed to severe steric hindrance during the transition from an orthogonal biradical to a butterfly-like quinoidal form. Though this valence isomerization is a one-way chemical reaction, it is worth noting that this was the first report for the dynamic behavior of the valence isomerization from a biradical to a quinoidal form. As stated above, the chemistry of singlet biradical has been explored mainly by controlling the molecular planarity or the π-conjugation scaffold connecting two spin centers. Though the applications of open-shell singlet biradicals in nonlinear optics22-24, organic electronics25, and spintronics26 have been extensively investigated, many fundamental issues such as the equilibrium between a biradical and a quinoidal form, and the photochemical responses remain to be answered. Herein, we report the first detailed analyses of the kinetics of the biradical−quinoidal equilibrium and an unprecedented photoresponse of the equilibrium, leading to break fresh ground for the photochemistry of singlet biradical. We found the photoexcitation at the equilibrium state induces the transition from the quinoidal to the singlet biradical form. Scheme 1. Photochromic Reaction Scheme of Py-RPIC.


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We previously reported a photochromic molecule, Py-RPIC, which shows the sequential photochromic behavior with drastic spectral evolutions from submicroseconds to milliseconds due to the thermal equilibrium process from the biradical to the quinoidal form.27 UV light irradiation to the colorless closed form (CF) of Py-RPIC causes the photochemical bond breaking of the C–N bond affording the colored short-lived biradical (BR) which is rapidly equilibrated with the closed-shell quinoidal form (Q) (Scheme 1). The combined study of transient visible absorption spectroscopy, time-resolved Fourier transform infrared (TR-FTIR) spectroscopy, and the DFT calculation revealed that BR does not return to CF at all with a timescale of microseconds but equilibrates with Q with a half-life of ∼200 ns at 298 K. The decay of the absorption band of the C–O stretching vibrational mode, 1582 cm−1, of the phenoxyl radical and the rise of the absorption band of the C=O stretching vibrational mode, 1630 cm−1, of the carbonyl group indicates that the double-bond character of the C–O bond of the colored isomer increases in the time scale of hundreds of nanoseconds. That is, it indicates that the photogenerated BR thermally isomerizes to Q in this time scale. Figure 1a shows transient absorption spectra recorded in the microsecond time scale excited with a 355 nm nanosecond laser pulse under an air atmosphere. The photogenerated BR gradually converted to Q with an isosbestic point at 430 nm and reached the equilibrium, and then the equilibrated BR and Q monoexponentially reverts to CF following first-order kinetics with a half-life of 3.9 ms at 298 K (Figure S2a). We performed the measurements of time-profiles of transient absorbance in the temperature ranges between 283 and 323 K and obtained the activation parameters for the ringclosing reaction to form CF by using the Eyring equation (Figure S2b, Table S1). The ∆H‡, ∆S‡ and ∆G‡ for the thermal back reaction are 52.6 kJ mol−1, −24.9 JK−1 mol−1, and 60.0 kJ mol−1,


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respectively, at 298 K. The fast and slow decays are not affected by the presence of molecular oxygen, indicating that the triplet sate is not involved in the photochromic reaction. These results suggest that the activation energy barrier for the valence isomerization to form Q is smaller than that for the radical coupling reaction to reform CF. That’s why the photogenerated BR relaxes to the energetically close Q and reaches the equilibrium between BR and Q at ∼3.0 µs at 298 K by the kinetic controlled reaction. On the other hand, the equilibrated BR and Q return to CF with a time scale of milliseconds by the thermodynamic controlled reaction.28 The large difference in the time scales for the kinetic controlled reaction and the thermodynamic controlled reaction make this photochromic reaction the sequential one involving the equilibrium between BR and Q. To our knowledge, this is the first observation of the thermal equilibrium process between a biradical form and a quinoidal form starting from almost a pure singlet biradical form. The reason why the thermodynamic study on the biradical−quinoidal equilibrium has not been reported lies in the difficulties to observe the dynamic behavior in conventional biradical systems. The unprecedented characteristic of Py-RPIC is that the biradical form can be repeatedly generated upon UV light irradiation and the following thermal equilibrium process can be also observed in the microsecond time region. Large difference in the time scales for the fast equilibration and the slow back reaction to CF justifies the independent kinetic analysis in each time scales. In order to extract the pure absorption spectra for BR and Q, the transient absorption spectra were reduced by factor analysis using a singular value decomposition (SVD) algorithm and then globally fit to the two-state equilibrium model (eq 1) using the Olis GlobalWorks software. 29-31


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(1)

The calculated pure spectra corresponding to BR and Q are presented in Figure 1b. The rate constants, kA and kB, obtained from the global analysis were determined as 2.90 × 106 s−1 and 0.76 × 106 s−1, respectively, at 298 K. Thus, the equilibrium constant, KC = kA/kB, for the biradical−quinoidal equilibrium was determined as 3.8.

Figure 1. (a) Transient absorption spectra of Py-RPIC in toluene (5.7 × 10−5 M) at 298 K in the microsecond time scale excited with a 355 nm nanosecond laser pulse (5.5 mJ/pulse), and (b) resolved pure spectra of the biradical and the quinoidal form obtained by the global analysis. Inset is the associated pure fraction profiles for the biradical and the quinoidal form determined by the global analysis. We investigated the temperature dependence of the biradical−quinoidal equilibrium to determine the activation parameters, ∆H‡, ∆S‡ and ∆G‡, for the forward and the reverse reaction, and the Gibbs free energy difference, ∆G0, between BR and Q. Each transient absorption spectra measured at different temperatures is analyzed by the same procedures described above (Figure S3, S4). The activation parameters for the forward and the reverse reaction were determined from the Eyring analyses of the rate constants obtained at different temperatures (Figure S5,


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Table S3). The ∆H‡, ∆S‡ and ∆G‡ at 298 K for the forward reaction are 29.3 kJ mol−1, −22.9 JK−1 mol−1, and 36.1 kJ mol−1, respectively, and those for the reverse reaction are 34.7 kJ mol−1, −15.3 JK−1 mol−1, and 39.3 kJ mol−1, respectively. By subtracting the ∆G‡ of the forward reaction from that of the reverse reaction, the ∆G0 is determined as 3.18 kJ mol−1. Thus, we have succeeded to reveal the thermodynamic parameters for the biradical−quinoidal equilibrium for the first time. We also investigated the photoresponsivity of the biradical−quinoidal equilibrium by employing double pulse nanosecond laser flash photolysis. The measurements were carried out at 243 K for the toluene solution to allow obtaining high S/N ratio spectra by decreasing the reaction rate. The biradical−quinoidal equilibrium was established at ∼40 µs after UV light excitation at 243 K, whereas the system reached the equilibrium at ∼3.0 µs at 298 K. Figure 2a shows the transient absorption spectra obtained by double pulse laser flash photolysis under an air atmosphere.

Figure 2. (a) Transient absorption spectra of Py-RPIC in toluene (4.9 × 10−5 M) at 243 K obtained by double pulse laser excitation with 355 and 430 nm (λex1 = 355 nm, pulse width = 5 ns, pulse energy = 3.5 mJ; λex2 = 430 nm, pulse width = 5 ns, pulse energy = 8.0 mJ, time delay = 40 µs), and (b) time profiles of the ∆Absorbance at 400 and 520 nm. The fluorescence signals just after the laser excitation were removed.


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The time delay between the first laser pulse, λex1 = 355 nm, and the second laser pulse, λex2 = 430 nm, was set to 40 µs. The wavelength of the second laser pulse was set to avoid the photochromic reaction of CF. As shown in Figure S6, the photochromic reaction of CF can be induced with a 430 nm nanosecond laser pulse alone. However, the ∆Absorbance upon the 430 nm light irradiation was small enough not to affect the double pulse nanosecond laser flash photolysis. The absorption spectra at 3.5 µs and 40.6 µs are superposed upon each other. That is, Q is transformed to BR by the irradiation with the second laser pulse. The system evolves to recover the equilibrium by the thermal relaxation from BR to Q in the dark. This valence photoisomerization from Q to BR become easier to understand by plotting the ∆Absorbance as a function of time. Figure 2b shows the time profiles of the ∆Absorbance at 400 and 520 nm. The magnitudes of the ∆Absorbance at 400 and 520 nm provide easy measures for the fraction changes of Q and BR, respectively. As shown in Figure 2b, the concentration of BR increases by exciting with the 430 nm laser pulse while that of Q decreases. These photoresponses are not affected by the presence of molecular oxygen (Figure S9), excluding the participation of a triplet state in the valence photoisomerization.

Figure 3. (a) Temporal changes of the transient absorbance at 520 nm of the toluene solution of Py-RPIC after the double laser pulse excitation by changing the time delay at 243 K. (4.2×10−5 M; λex1 = 355 nm, pulse width = 5 ns, pulse energy = 3.5 mJ; λex2 = 430 nm, pulse width = 5 ns,


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pulse energy = 6.6 mJ, delay time; 500 ns–40 µs). The gray-colored curve was obtained by the single laser pulse excitation with a 355 nm laser pulse. (b) Black solid line: the rise profile of the fraction of Q obtained by the global analysis of the transient absorption spectra measured by the single laser pulse excitation (λex = 355 nm, pulse width = 5 ns, pulse energy = 5.5 mJ) at 243 K. Green circles: the ∆∆Absorbance at 520 nm as a function of the delay time. The valence photoisomerization was also confirmed by measuring the ∆Absorbance at 520 nm by changing the time delay between the first and second excitation pulses in the double pulse nanosecond laser flash photolysis measurement (Figure 3a). We define ∆∆Absorbance = ∆Absorbance430 − ∆Absorbance355 as the difference between the ∆Absorbance at 520 nm after the first laser excitation at 355 nm (∆Absorbance355) and that after the second laser excitation at 430 nm (∆Absorbance430). The ∆∆Absorbance corresponds to the population of BR generated by the excitation with the 430 nm laser pulse. The variation in ∆∆Absorbance as a function of the time delay between the two laser pulses was in good agreement with the rise profile of the fraction of Q formed with a 355 nm nanosecond laser pulse alone (Figure 3b).

The

photoexcitation of BR with a 450 or 500 nm nanosecond laser pulse after the excitation with the first 355 nm laser pulse did not show changes in the ∆∆Absorbance at all (Figure S8), indicating that BR does not show valence photoisomerization to yield Q. On the other hand, if the excitation wavelength of the second laser pulse is shorter than 430 nm, not only the valence photoisomerization of Q but also the photochromic reaction of CF affording BR would be occurred. It is thought that the photochemical reaction channel leading to the formation of BR exists at a higher excited singlet state of Q because neither 450 nor 500 nm light could induce the valence photoisomerization.


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In conclusion, we have analysed the sequential photochromic behavior of a photochromic phenoxyl-imidazolyl radical complex, Py-RPIC. The photochromic reaction of Py-RPIC includes the following three processes; i) the photochemical bond breaking of the C–N bond affording the open-shell biradical form, ii) the thermal equilibrium process by the valence isomerization to form the closed-shell quinoidal form, and iii) the thermal back reaction to reproduce the closed form with keeping the equilibrium between the biradical form and the quinoidal form. The unique photochromic response of Py-RPIC makes it possible to investigate the kinetic study on the thermal equilibrium process between the biradical form and the quinoidal form. The activation parameters for the forward and the reverse reaction of the biradical−quinoidal equilibrium were determined for the first time by nanosecond laser flash photolysis. Moreover, we have also found that the visible-light excitation to the equilibrium state results in the valence photoisomerization from the quinoidal to the biradical form. The biradical−quinoidal equilibrium and the valence photoisomerization are of special interest not only for the singlet biradical chemistry but also for their applications in future photoswitching materials.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Transient absorption spectra, global analyses, and Eyring analyses (PDF) AUTHOR INFORMATION ORCID


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Katsuya Mutoh: 0000-0002-9778-8329 Takeshi Hasegawa: 0000-0001-5574-9869 Jiro Abe: 0000-0002-0237-815X Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported in part by JSPS KAKENHI Grant Number JP26107010 in Scientific Research on Innovative Areas "Photosynergetics". 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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Scheme 1 25x8mm (600 x 600 DPI)



Figure 1 35x15mm (300 x 300 DPI)


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Reversible Valence Photoisomerization between Closed-Shell

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