Fast Photochromic Molecules toward Realization of Photosynergetic

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Fast Photochromic Molecules toward Realization of Photosynergetic Effects Yoichi Kobayashi, Katsuya Mutoh, and Jiro Abe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01690 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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The Journal of Physical Chemistry Letters

Fast Photochromic Molecules toward Realization of Photosynergetic Effects Yoichi Kobayashi*, Katsuya Mutoh and Jiro Abe AUTHOR ADDRESS Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-101 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan AUTHOR INFORMATION Corresponding Author * [email protected].

ABSTRACT

There has been a growing interest toward the development of advanced photofunctional materials whose photoresponses involve multiple photons and/or molecules because these materials show the photoresponses which cannot be achieved by a one-photon reaction of a single chromophore. These cooperative interactions of multiple photons and/or molecules are recently termed as the “photosynergetic” effects, and the understanding and utilization of these effects are becoming important research topics. In this perspective, we overview the recent progress of the fast T-type photochromic molecules involving the stepwise two-photon

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absorption processes. While high power pulse lasers were necessary to induce conventional simultaneous and stepwise two-photon absorption processes, the stepwise two-photon absorption process with the fast photochromic compound can be initiated by extremely weak continuous wave (CW) LEDs. The basic concept and future outlook of the fast photochromism involving the stepwise two-photon absorption process will be discussed.

TOC GRAPHICS

Insert 2in×2in (5cm×5cm) graphic

KEYWORDS

photochromism,

two-photon

absorption

processes,

biradical,

quinoid,

photofunctional material

Advanced photofunctional materials which show different photophysical properties depending on the external stimuli and environments, such as photoresponses involving multiple photons and/or molecules, have received considerable attention because these materials have potentials to open up novel fundamental sciences and applications which cannot be realized by conventional photofunctional materials. These cooperative interactions of multiple photons


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and/or molecules are recently called “photosynergetic” effects, and the understanding and utilization of these effects are one of the most important research topics because these systems have potentials to realize the photoresponses beyond a one-photon reaction of a single chromophore. One of the key technologies for the advanced photofunctional materials is the twophoton absorption process (simultaneous and stepwise processes as shown later in Figure 1). The two-photon absorption process can be controlled in time and space by the excitation intensity of the incident light, and therefore, photofunctional materials with two-photon absorption processes are important not only for switch molecules but also promising for three dimensional (3D) fabrications1–3 and two-photon microscopies.4,5 One of the examples which demonstrated the photosynergetic effect is the enhanced simultaneous two-photon absorption processes of Jaggregates.6,7 However, the high power pulse lasers are necessary to induce the simultaneous two-photon absorption processes because the power threshold of the present simultaneous twophoton absorption processes is quite high. To overcome this issue, many researchers have synthesized various kinds of molecules to increase the two-photon absorption cross sections.7–9 In the meanwhile, some researchers have overcome this issue by increasing the photon-fluxdensity with a tightly focused CW light10,11 and a localized surface plasomon.12,13 While some reports have demonstrated the simultaneous two-photon absorption process by using a CW light source, the power threshold to induce the two-photon process was still too high to initiate the process by using sunlight or conventional weak CW light sources. Recently, as an alternative approach, we have applied the fast T-type photochromic molecules to the stepwise two-photon absorption process and succeeded in realizing the extremely low power threshold of the stepwise two-photon absorption process, which can be initiated even by weak CW LEDs.14 Although the spatial resolution of the stepwise two-photon absorption process is lower than that of the


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simultaneous two-photon absorption process, the extremely low power threshold of the stepwise two-photon absorption process affords many novel potential applications such as to a lowthreshold power limiting filter which protects human eyes and sensitive photodetectors. In this perspective, we overview the recent progress of the fast photochromic molecules involving the stepwise two-photon absorption processes. Firstly, we explain the backgrounds of the two-photon absorption processes and fast photochromic molecules. Secondly, we introduce recent our studies on fast photochromism involving the stepwise two-photon absorption processes. Finally, we outline the future outlook of these fast photochromic systems.

Quotes: Exploring the “photosynergetic” effect, which is a cooperative interaction of multiple photons and/or molecules, is an important strategy to realize the photoresponses beyond a onephoton reaction of a single chromophore.

Backgrounds of two-photon absorption processes. Two-photon absorption processes are divided into two processes: simultaneous and stepwise two-photon absorption processes as shown in Figure 1.


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Figure 1. Energy diagrams for (a) simultaneous and (b) stepwise two-photon absorption processes.

The simultaneous two-photon absorption process occurs via a virtual state (Figure 1a). A high power and ultrafast pulse laser is usually necessary to induce the simultaneous two-photon absorption process because the second photon has to be absorbed almost within the absorption process of the first photon (~1 fs). In the meanwhile, the great advantage of the simultaneous two-photon absorption process is the capability of the excitation with the light which cannot be absorbed by one-photon process. This unique property attracts many researchers to develop novel simultaneous two-photon absorption materials for the applications such as to two-photon microscopies and 3D fabrications. On the other hand, the stepwise two-photon absorption process occurs via an actual excited state (Figure 1b). The power threshold of the stepwise two-photon absorption process is usually lower than that of the simultaneous process because the lifetime of the excited state as an intermediate state is much longer than that of the virtual state. For example, the stepwise twophoton absorption process via an S1 state, whose lifetime is ~nanoseconds, can be achieved by femtosecond-to-nanosecond pulse lasers without focusing the beam. The power threshold of the stepwise two-photon absorption process via a T1 state can be reduced as compared to that via the S1 state because the lifetime of the T1 state is hundreds of nanoseconds to tens of microseconds. The stepwise two-photon absorption processes via the T1 state have been extensively studied by using benzophenone,15,16 metal complexes17–19 for the applications to power-dependent singlet oxygen

generators,17–19

reverse

saturable

absorbers,20,21

and

initiators

for

radical

polymerizations.22–25 Up conversion fluorescence by using the triplet-triplet annihilation (TTA)26


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and the f-f transitions of lanthanide doped nanoparticles27–29 can be also classified as the stepwise two-photon absorption process. Notably, high efficient up conversion fluorescence has been recently demonstrated in molecular assemblies by CW lasers26,30,31 and even by weak CW incoherent light.32 Except several special cases, most of the studies on the stepwise two-photon absorption processes via the S1 and T1 states use high power pulses or CW lasers because the power thresholds of the stepwise two-photon absorption process are too high to induce the twophoton reaction by weak light sources such as CW LEDs. Recently, we have developed a novel fast photochromic molecule which involves the stepwise two-photon reaction.14 The photochromism of the fast photochromic molecule is initiated by UV light irradiation and the photogenerated transient species thermally reverts to the initial form with a time scale of sub-milliseconds to seconds. While only the photochromic reaction reversibly occurs at low excitation intensity, another photochromic reaction occurs at high excitation intensity if another photon is absorbed by the photogenerated transient species, which produces the long-lived quinoid form. While the conventional stepwise two-photon absorption processes conceptionally use the excited state as an intermediate state, this system uses the photogenerated transient species of the T-type photochromism as the intermediate state for the stepwise two-photon absorption processes. This approach greatly reduced the power threshold and expanded the potential of the stepwise two-photon absorption processes. The requirements for the photon flux density of the incident light and the lifetimes of the intermediate transient species are explained in the next section.

Power density of the incident light and lifetimes of the transient states.


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The two-photon absorption cross-section is usually used to quantitatively understand the twophoton processes. However, to understand the power threshold of the two-photon reactions more intuitively, here, we consider the photon flux density, namely, the number of photons irradiated per unit area and time. This value can be regarded as a minimum required condition for the twophoton absorption processes. Table 1 summarizes the typical energies, peak intensities, and photon flux spectral densities of different light sources: femtosecond laser pulse, nanosecond laser pulse, and CW LED.

Table 1. Energies, peak intensities, and the photon flux spectral densities of conventional light sources. The diameters of the beam spots were assumed to be 1 mm for lasers and 1 cm for LED. Ti:Sapphire laser

Nd:YAG laser

CW LED

400 nm

355 nm

365 nm

~1 µJ/pulse

~5 mJ/pulse

100 mW

~100 fs

~ 5 ns

-

Peak intensity

~1.27 GW cm−2

~127 MW cm−2

127 mW cm−2

Photon flux spectral density

0.026 photons nm−2 fs−1

2.3 photons nm−2 ps−1

2.3 photons nm−2 ms−1

Wavelength Typical Energy Duration

In the case of the femtosecond Ti:Sapphire laser, typical values of the frequency-doubled output peak intensity (wavelength: 400 nm, power: 1 µJ pulse−1, pulse duration: ~100 fs, and the diameter of the beam spot: ~1 mm) are ~1.27 GW cm−2. The photon flux density of this intensity corresponds to 0.026 photons nm−2 fs−1. If the cross-section area of a molecule, which briefly corresponds to the absorption cross-section, is assumed to be ~1 nm2 (which corresponds to the molecular absorption coefficient (ε) ~2.6×104 M−1 cm−1),33 this value indicates the number of photons irradiated to the area of a molecule per femtosecond. Since the time scale of the onephoton absorption is usually ~1 fs, this brief calculation shows that the femtosecond laser pulse


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can supply more than two photons during the absorption process if the laser spot is more focused. On the other hand, in the case of the conventional CW UV LED (wavelength: 365 nm, power: ~100 mW, diameter of the beam spot: ~1 cm), the photon flux density is calculated to be 2.3 photons nm−2 ms−1. This value is obviously not sufficient to induce the simultaneous two-photon absorption process. Moreover, this simple calculation shows that the photon flux density of the CW LED is extremely low as compared to those of pulse lasers. The power threshold of the stepwise two-photon absorption process depends on the lifetimes of the intermediate states. To induce the stepwise two-photon absorption process by using conventional CW LEDs, the lifetime of the photogenerated excited state should be longer than the millisecond time scales. Figure 2a shows the typical time scales of the relaxation processes from the photogenerated transient states.


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Figure 2. (a) Typical time scales of lifetimes of photogenerated transient states. (b) Schematics of stepwise two-photon absorption processes via the S1 state, T1 state, and the colored isomer of Ttype photochromic reactions.

The excited states of some lanthanide compounds have longer lifetimes than milliseconds due to the f-f transition nature. However, the lifetimes of the excited states of typical organic molecules are usually less than tens of microseconds at room temperature under air condition. This time scale is not enough to induce the stepwise two-photon reaction by CW LEDs. Therefore, the different strategy is required to realize the stepwise two-photon absorption process by CW LEDs. In conventional stepwise two-photon absorption processes, the electronic excited states such as the S1 and T1 states are used as the intermediate states because these photophysical properties are reversible and easily accessible by light irradiations. We expect that the intermediate state can be replaced with other transient states as long as the transient states can be reversibly generated by light irradiation. Thus, we proposed to use the thermodynamically-unstable transient species (colored isomer) generated during the fast photochromic reactions. The stepwise two-photon absorption processes are schematically summarized in Figure 2b. As compared to the excited states, the lifetimes of the colored isomer are the microseconds to hundreds of milliseconds time scales. Therefore, the photogenerated colored isomer has more chance to absorb photons, which indicates the power threshold of the two-photon absorption process is greatly reduced. If the colored isomer induces another photochemical reaction by the additional photon absorption, this system can be an efficient two-photon induced photochemical reaction system. For examples, in the context of this definition, photoactivatable fluorophores based on thermally-reversible photochromic compounds can be classified as stepwise two-


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photon induced photochemical reaction systems.34–36 The power threshold of the two-photon reaction can be controlled by tuning the lifetime of the colored isomer.

Quotes: Application of the thermodynamically-unstable transient species of the T-type photochromic molecules to the stepwise two-photon absorption process greatly reduces the power threshold the of the two-photon absorption process.

If the lifetime of the transient species is too long, for examples, more than seconds, the stepwise two-photon process of the system cannot be controlled by the excitation intensity because the prolonged light irradiation induces the two-photon reaction even when the excitation intensity is low. These systems could be also useful such as in fluorescent imaging, where the photoactivated signal can be accumulated by prolonged irradiation of the UV light.37,38 However, these kinds of reactions are usually classified as the stepwise photochemical reactions. Multiphotochromic systems, which have multiple photochromic units in a molecule, have been extensively studied by using various photochromic units.39–44 However, the photogenerated isomers of these systems are so stable that there is no power threshold of the incident light to induce the stepwise reactions. In those cases, the stepwise photochromic reactions have to be controlled by the total irradiation time. Therefore, the time scale of the stepwise two-photon reactions with the fast photochromic molecules is a niche time scale between conventional two-photon absorption processes and stepwise photochemical reactions.

Fast radical dissociation-type photochromic molecules.


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While there are many reports on the photochromic compounds,43,45,46 there are few photochromic compounds whose photochromic reaction are in the order of milliseconds to hundreds of milliseconds time scales.47,48 Bridged imidazole dimers are one of the fast photochromic molecules whose photochromic reactions take sub-milliseconds to hundreds of milliseconds. The molecular framework of the bridged imidazole dimer is based on hexaarylbiimidazole (HABI), which was a radical-dissociation-type photochromic molecule developed by Hayashi and Maeda in 1960s (Figure 3a).49–51

Figure 3. Photochromic reaction schemes of (a) HABI, (b) [2.2]PC bridged imidazole dimer, and (c) pictures representing the time profiles of the color change of [2.2]PC bridged imidazole dimer in benzene upon CW UV light irradiation (365 nm).52

HABI cleaves the C–N bond between the imidazole rings by UV light irradiation and generates an imidazolyl radical pair. The imidazolyl radicals are stable, reddish purple color, and diffuse in


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the medium. The imidazolyl radicals thermally recombine to revert to the initial HABI and the time scale of the thermal back reaction is seconds to minutes depending on the diffusion of the radicals in the medium. In 2008, our group developed a bridged imidazole dimer, where two imidazole rings are constrained by a molecular linker, to restrict the diffusion of the radical.53 The molecular linker accelerates the rate of the thermal back reaction of HABI. By replacing the molecular linkers from naphthalene53–55 to [2.2]paracyclophane ([2.2]PC)52,56–68 (Figure 3b), 1,1bi-2-naphthol,69 biphenyl,70 and so on,71 the rate of the thermal back reactions can be tuned from the sub-microseconds to hundreds of milliseconds time scales. For example, the [2.2]PC bridged imidazole dimer in benzene solution instantaneously changes its solution color from colorless to blue upon UV light irradiation, and the coloration spot instantaneously follows the spot of UV light irradiation as shown in Figure 3c.52 While the triphenylimidazolyl radical has absorption bands at 390 and 560 nm, the biradical species of the [2.2]PC bridged imidazole dimer has an additional broad absorption at the longer wavelength. The broad absorption is assigned to the through-space radical-radical interaction due to the proximity of two radicals. This through-space interaction of the radicals makes the solution color blue, which is different from the color of the triphenylimidazolyl radicals (reddish purple). The thermal back reactions of the radical species of almost all reported bridged imidazole dimers follow the first-order decay kinetics, while conventional HABIs show the bimolecular recombination. These photochromic radical dimers are robust even after repeated irradiations of high power UV laser pulses more than 10,000 times. As a first example, we introduce the fast photochromic reaction induced by the stepwise twophoton reaction of the zinc tetraphenylporphyrin-substituted bridged imidazole dimer. This system demonstrated the efficient inter-chromophore electron transfer at the higher excited state to induce the fast photochromic reaction beyond the Kasha’s rule. Although the power threshold


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of this two-photon absorption process is still high, this example gives useful information to develop novel photofunctional materials utilizing the higher excited states.

Stepwise two-photon induced fast photochromism via the higher excited state.72 In the case of the diarylethene derivatives, it was reported that the stepwise two-photon absorption processes dramatically enhance the efficiency of the photochromic reactions from the closed to open forms tens-to-thousand times as compared to the one-photon process.73,74 The stepwise two-photon induced photochromic reactions have been extensively studied in diarylethene and flugide derivatives.73–80 However, these reports were limited to the stepwise two-photon reaction in a single chromophore. To extend the stepwise two-photon induced photochromic reaction to bi-chromophore and multi-chromophore systems,81–84 we recently developed a zinc tetraphenylporphyrin (ZnTPP)-substituted bridged imidazole dimer (ZnTPPImD) (Figure 4a).72


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Figure 4 (a) Photochromic reaction scheme of ZnTPP-ImD. (b, c) Transient absorption spectra of ZnTPP-ImD in degassed benzene (4.6 × 10−5 M) excited with a 532 nm picosecond laser pulse at (b) low (0.06 mJ mm−2) and (c) high (1.2 mJ mm−2) pump power. (d) Difference spectrum obtained from the transient absorption spectra excited at low and high pump intensities at 300 ps. The transient spectrum of ZnTPP-ImD at 2 ms time delay excited with a nanosecond laser pulse is also shown as a red line. (e) Pump power dependence of the logarithmic transient absorption signal of ZnTPP-ImD in degassed benzene (4.6 × 10−5 M) excited at 532 nm and probed at 770 nm. The slope of the straight line is 2.72


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Porphyrins have large absorption bands at visible regions called the Soret and Q bands, and therefore, the ZnTPP moiety can be an effective light harvesting unit to induce the stepwise twophoton absorption process. We have examined the excitation intensity dependences of the transient absorption spectra from the picoseconds to milliseconds time scales and the picosecond emission decay measurements to reveal the mechanism of the photochromic reaction. Figures 4b and c show the picosecond transient absorption spectra at different excitation intensity. At low excitation intensity, the observed broad transient absorption spectra are assigned to the ZnTPP moiety. That is, the absorption at the picosecond time scale is assigned to the absorption from the S1 state and that at the nanosecond time scale is assigned to the absorption from the T1 state. On the other hand, at high excitation intensity, the different transient absorption spectrum was observed. The normalized transient absorption spectrum excited at high excitation intensity was subtracted by the normalized spectrum excited at low excitation intensity. The subtracted transient absorption spectrum is very similar to the transient absorption spectrum of ZnTPP-ImD at 2 ms and that of [2.2]PC bridged imidazole dimer (Figure 4d). Furthermore, the transient absorption signal at 770 nm has the clear quadratic dependence on the excitation intensity (Figure 4e). Because the excitation intensity is still low for the simultaneous two-photon absorption process, this result shows that the imidazolyl radical is formed by the stepwise two-photon absorption of the ZnTPP moiety. The full mechanism for the stepwise two-photon induced photochromic reaction of ZnTPP-ImD is described as follows (Figure 5).


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Figure 5. Stepwise two-photon induced photochromic reaction scheme of ZnTPP-ImD.72

Firstly, the visible light is absorbed by the ZnTPP moiety and the lowest excited state is formed. Secondly, at high excitation intensity, the lowest excited state of the ZnTPP moiety absorbs another photon to produce the higher excited state. Thirdly, the electron of the higher excited state of the ZnTPP moiety transfers to the ImD moiety, which produces the charge separated state. Since the previous electrochemical studies showed that the electron injection to the [2.2]PC bridged imidazole dimer induces the bond cleavage between the imidazole rings,63 the charge separated states also induces the bond cleavage and the biradical is formed. The back electron transfer from the ImD moiety to the ZnTPP moiety occurs within 30 ps. Finally, the biradical form is generated and it decays with the half-life of 38 ms. This stepwise two-photon induced photochromic reaction can be initiated by a weak nanosecond laser pulse (8.8 MW cm−2) in spite that this stepwise two-photon absorption process uses the S1 state as the intermediate state. In most of the molecular systems, the Sn state quickly relaxes to the S1 state nonradiatively (Kasha’s


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rule). There are two plausible reasons why the efficient electron transfer occurs from the Sn state of ZnTPP-ImD. First reason is that the wave function of the Sn states is delocalized over the ZnTPP and imidazole ring due to the direct connection by a covalent bond. The second reason is that the energy level of the charge separated state is lower than the Sn states. Another important fact to realize the fast photochromic reaction via the electron transfer from the higher excited states is that the C–N bond between the imidazole rings cleaves once the electron is injected to the lowest unoccupied molecular orbital (LUMO) of the imidazole ring.63 This study suggests that this type of stepwise two-photon induced fast photochromic materials can be extended to other combinations of molecular units by carefully designing the energy level of each molecular units.

Fast photochromism involving stepwise two-photon absorption process.14 The first example demonstrated the novel fast photochromic reaction induced by the stepwise two-photon absorption process via the higher excited state. However, the power threshold of the stepwise two-photon absorption is still high because the S1 state of the ZnTPP moiety, whose lifetime is ~nanoseconds, is used for the intermediate state of the two-photon process. Next example introduces the stepwise two-photon absorption using the photogenerated transient species of fast photochromic compounds. This stepwise two-photon process is achieved by the combination of fast photochromism with the valence isomerization between biradical and quinoid. The biradical and quinoid are usually described as a resonance hybrid or thermal equilibrium in biradical systems.85–94 On the other hand, the distinct biradical and quinoid forms are observed in some flexible molecules due to the large activation energy barrier between these forms.14,95–98 The optical properties of the distinct biradical and quinoid forms are completely


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different. Therefore, if the spin-spin interaction can be controlled by the incident light intensity, the fast photochromic compounds will be an efficient platform for the stepwise two-photon absorption process. Among several synthesized fast photochromic compounds which have the though-bond spin-spin interaction,14,71,97–100 bis(imidazole dimer) (bis(ImD), Figure 6a) shows an unique photochromic behavior involving the stepwise two-photon absorption process.14

Figure 6 (a) Photochromic reaction scheme of bis(ImD). (b) Transient absorption spectra of bis(ImD) in degassed benzene (interval: 50 ms). (c) UV−vis absorption spectra for the degassed benzene solution of bis(ImD) (298 K, 2.1 × 10−5 M) measured after repeated irradiation with 355 nm laser pulses. (d) The ∆absorbance at 590 nm of bis(ImD) in benzene (1.5 × 10−5 M) as a function of excitation intensity. The excitation light was a 355-nm nanosecond laser pulse (pulse width = 5 ns).14

Bis(ImD) has two photochromic units in a molecule. Under low excitation intensity, a photochromic reaction of a photochromic unit of bis(ImD) occurs by the one-photon absorption.


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On the other hand, under high excitation intensity, additional photon is absorbed by the photogenerated biradical form, and further photochromic reaction occurs to produce the tetraradical form, which quickly isomerizes to the quinoid form. Figure 6b shows the transient absorption spectra of bis(ImD) excited at relatively low excitation intensity (355nm of nanosecond laser pulse, 3 mJ). The photogenerated transient species under low excitation intensity can be assigned to the biradical form. The absorption spectrum is similar to that of the biradical form of [2.2]PC imidazole dimer, although the spectrum of the biradical form of bis(ImD) slightly shifts to the longer wavelength and the color looks bluish green. At relatively low excitation intensity, the decay of the biradical form follows the first-order decay kinetics and the half-life is 88 ms. While the decay follows the first-order decay kinetics in almost whole visible regions, a small residual component is observed at ~600 nm. The residual absorption component becomes larger by repeated irradiations of the UV pulses (Figure 6c). This component is assigned to the quinoid form generated by the stepwise two-photon absorption process, which was characterized by UV-vis and infrared absorption spectroscopies. As expected, the formation of the quinoid form has the quadratic dependence on the excitation intensity as shown in Figure 6d, and therefore, this result shows that the quinoid form is generated by the stepwise two-photon absorption process. The quinoid form is long-lived and 4 days are necessary to revert to the initial colorless solution. As explained before, the photon flux density of the conventional CW UV LED (365 nm, 100 mW, the diameter of the irradiation spot = 1 cm) is calculated to be 2.3 photons nm−2 ms−1. Since the lifetime of the photogenerated biradical form is 88 ms, this brief calculation suggests that the stepwise two-photon absorption of bis(ImD) can be induced by a CW LED. Actually, the stepwise photochromic reaction of bis(ImD) can be induced even by a very weak UV LED (5 mW). Interestingly the combination of the weak UV


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LED (5 mW) with white light of a halogen lamp at the visible region (420-700 nm, 500 mW) accelerates the stepwise two-photon absorption process although the initial form of bis(ImD) does not have any absorption at the visible region. Figure 7 shows the color changes of benzene solution of bis(ImD) after visible (420-700 nm, 500 mW), UV (365 nm, 5 mW), and both CW light irradiation for 25 seconds each.

Figure 7 Color changes of a benzene solution of bis(ImD) (293 K, 9.4×10−5 M) after CW light irradiation for 25 s (Vis: λex = 420−700 nm, 500 mW; UV: λex = 365 nm, 5 mW). Also see movie S2 in the reference 14.

The visible light does not induce any stepwise photochromic reaction and the weak UV light very slightly induces the stepwise photochromic reaction. On the other hand, irradiation of both UV and visible light greatly accelerates the photochromic reaction. The detailed analyses of the dependence of the excitation wavelength shows that the stepwise two-photon absorption process can be induced by the combination of UV and the visible light shorter than 420 nm, while the combination of UV and visible light at 550 nm does not induce the stepwise reaction.

Summary and future outlook. As was shown in Figure 2a, the time scales from tens of microseconds to hundreds of milliseconds are one of the most important time scales for human daily lives because these time


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scales cover the time resolutions of human visions and conventional photodetectors. The reaction speeds of the fast photochromic compounds cover these time scales, and therefore, the fast photochromic compounds are promising for ophthalmic lenses, updatable holographic materials,64,65,101 and fluorescence switching materials.66 In addition to these potential applications, the studies introduced in this perspective demonstrated potentials of fast photochromic compounds for the novel advanced photofunctional materials. The efficient stepwise photochromic reactions were achieved by the combination of the fast photochromism with other optical properties such as the biradical-quinoid valence isomerization. If the lifetime of the photogenerated transient species is longer than microseconds but shorter than seconds, the power threshold of the stepwise two-photon absorption process can be clearly observed. Thus, these bi-chromophore systems of fast T-type photochromic molecules realize the photosynergetic effect: the cooperative interaction involving two photons and two photochromic units. The fast photochromic reactions involving the stepwise two-photon absorption processes have several potential impacts as below.

Quotes: The extremely low power threshold of the two-photon absorption process opens up novel potential applications which cannot be achieved by conventional photofunctional materials.

1) Material sciences The stepwise two-photon absorption process had been considered to be less important in industry as compared to the simultaneous two-photon absorption process because the simultaneous process can excite the system with light which cannot be absorbed by the onephoton process while the stepwise process cannot. In the meanwhile, the stepwise two-photon


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absorption process reported in this perspective demonstrated the extremely low power threshold, which is even initiated by very weak CW LED. The extremely low power threshold of the twophoton process affords many opportunities for novel photofunctional materials. One of the potential applications is an intelligent optical filter which blocks the incident light to protect human eyes and detectors only when the light intensity is over the threshold (optical power limiting filter whose power threshold is extremely low). Moreover, the stepwise two-photon process also has a potential for up conversion systems with weak CW LEDs. One of the drawbacks of these systems may be that the total energy level after the up conversion by using photochromic compounds is lower than that by using excited states. This is because the energy level of the transient species of the photochromic compounds is the thermodynamically-unstable ground state while the S1 and T1 states are excited states. However, the up conversion is still possible by using a thermodynamically-unstable transient species as an intermediate state of the stepwise two-photon absorption process. 2) Photochemistry The application of the fast photochromic reaction to the stepwise two-photon absorption process extended the concept of the stepwise two-photon absorption process. That is, the study demonstrates that the intermediate state of the stepwise two-photon absorption process can be replaced from the “long-lived” excited states to the “short-lived” transient species of the photochromic reactions. The process greatly reduces the power threshold and increases the range of choices of molecular units for efficient two-photon processes. Recently, we developed novel radical dissociation-type photochromic molecules called pentaarylbiimidazole (PABI)71 and phenoxyl-imidazolyl radical complex (PIC).99,100 In these systems, the photogenerated radical pair is substituted at the ortho-position of the phenylene ring, and therefore, PABI and PIC are


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expected to have the strong through-bond spin-spin interactions. These molecular frameworks would give further opportunities to develop novel advanced photofunctional materials. Furthermore, another notable point is that the stepwise two-photon reaction can be achieved by the combination of fast photochromic reactions with other reversible reactions, which are not restricted by biradical-quinoid valence isomerization. If the transient species of the fast photochromic molecule has other photophysical properties such as emissions,34–36 triplet-related reactions, proton transfers, and energy transfers, these systems could be also efficient stepwise two-photon reaction systems. Furthermore, the strategy of the fast photochromic reactions demonstrated in bi-chromophore systems can be extended to multi-chromophore systems such as nanoparticles, nanofibers, crystals. Therefore, the fast photochromic reactions can be a novel platform for photofunctional systems realizing the photosynergetic effects.

AUTHOR INFORMATION Notes The authors declare no competing financial interests. Biography Yoichi Kobayashi is an assistant professor at Aoyama Gakuin University. He received his Ph.D. degree from Kwansei Gakuin University in 2011. He worked as a postdoctoral fellow for Research Abroad of Japan Society for the Promotion of Science (JSPS) at University of Toronto from 2011 to 2013. His current research focuses on development and spectroscopy of novel photofunctional materials.


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Katsuya Mutoh is a postdoctoral fellow at Aoyama Gakuin University. He received his Ph.D. degree from Aoyama Gakuin University in 2015. His current research interests are focused on the design and development of the photosynergetic materials based on the fast switchable photochromic compounds. Jiro Abe received his B.S. (1986), M.S. (1988), and Ph.D. (1991) degrees from Waseda University. In 1991, he joined the Faculty of Engineering, Seikei University, as a research assistant. In 1992, he moved to Tokyo Institute of Polytechnics and was promoted to associate professor in 1997. In 2000, he moved to the Graduate School of Engineering, Tokyo Metropolitan University. He moved to Aoyama Gakuin University in 2004 and was appointed as full professor in 2010 at the same university. Concurrently, he was a project leader of CREST, Japan Science and Technology (JST) from 2010 to 2016. His current research interests are the development of novel fast-switchable photochromic compounds and the photofunctional molecular assemblies.

ACKNOWLEDGMENT This work was supported partly by the Core Research for Evolutionary Science and Technology (CREST) program of the Japan Science and Technology Agency (JST), JSPS KAKENHI Grant Number JP26107010 in Scientific Research on Innovative Areas “Photosynergetics”, and JP15K17846 in Grant-in-Aid for Young Scientists (B) for Y.K. from MEXT, Japan. 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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REFERENCES (1)

Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.; McCord-Maughon, D.; et al. Two-Photon Polymerization Initiators for Three- Dimensional Optical Data Storage and Microfabrication. Nature 1999, 398, 51–54.

(2)

Kawata, S.; Kawata, Y. Three-Dimensional Optical Data Storage Using Photochromic Materials. Chem. Rev. 2000, 100, 1777–1788.

(3)

Kawata, S.; Sun, H. B.; Tanaka, T.; Takada, K. Finer Features for Functional Microdevices. Nature 2001, 412, 697–698.

(4)

König, K. Multiphoton Microscopy in Life Sciences. J. Microsc. 2000, 200, 83–104.

(5)

Helmchen, F.; Denk, W. Deep Tissue Two-Photon Microscopy. Nat. Methods 2005, 2, 932–940.

(6)

Belfield, K. D.; Bondar, M. V.; Hernandez, F. E.; Przhonska, O. V.; Yao, S. Two-Photon Absorption of a Supramolecular Pseudoisocyanine J-Aggregate Assembly. Chem. Phys. 2006, 320, 118–124.

(7)

He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Multiphoton Absorbing Materials: Molecular Designs, Characterizations, and Applications. Chem. Rev. 2008, 108, 1245– 1330.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8)

Page 26 of 47

Chung, S.-J.; Kim, K.-S.; Lin, T.-C.; He, G. S.; Swiatkiewicz, J.; Prasad, P. N. Cooperative Enhancement of Two-Photon Absorption in Multi-Branched Structures. J. Phys. Chem. B 1999, 103, 10741–10745.

(9)

Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Two-Photon Absorption and the Design of Two-Photon Dyes. Angew. Chem. Int. Ed. 2009, 48, 3244–3266.

(10)

Hänninen, P. E.; Soini, E.; Hell, S. W. Continuous Wave Excitation Two-Photon Fluorescence Microscopy. J. Microsc. 1994, 176, 222–225.

(11)

Booth, M. J.; Hell, S. W. Continuous Wave Excitation Two-Photon Fluorescence Microscopy Exemplified with the 647-nm ArKr Laser Line. J. Microsc. 1998, 190, 298– 304.

(12)

Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source. J. Am. Chem. Soc. 2008, 130, 6928–6929.

(13)

Tsuboi, Y.; Shimizu, R.; Shoji, T.; Kitamura, N. Near-Infrared Continuous-Wave Light Driving a Two-Photon Photochromic Reaction with the Assistance of Localized Surface Plasmon. J. Am. Chem. Soc. 2009, 131, 12623–12627.

(14)

Mutoh, K.; Nakagawa, Y.; Sakamoto, A.; Kobayashi, Y.; Abe, J. Stepwise Two-PhotonGated Photochemical Reaction in Photochromic [2.2]Paracyclophane-Bridged Bis(imidazole Dimer). J. Am. Chem. Soc. 2015, 137, 5674–5677.


26

Page 27 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15)

Cai, X.; Sakamoto, M.; Yamaji, M.; Fujitsuka, M.; Majima, T. C-O Bond Cleavage of Benzophenone Substituted by 4-CH2OR (R = C6H5 and CH3) with Stepwise Two-Photon Excitation. J. Phys. Chem. A 2005, 109, 5989–5994.

(16)

Cai, X.; Sakamoto, M.; Hara, M.; Inomata, S.; Yamaji, M.; Tojo, S.; Kawai, K.; Endo, M.; Fujitsuka, M.; Majima, T. Homolytic Cleavage of C–Si Bond of pTrimethylsilylmethylacetophenone upon Stepwise Two-Photon Excitation Using TwoColor Two-Laser Flash Photolysis. Chem. Phys. Lett. 2005, 407, 402–406.

(17)

Ishii, K.; Hoshino, S.; Kobayashi, N. Photodecarbonylation of Ruthenium Carbonyl Octaethylporphyrin via Stepwise Two-Photon Absorption of Visible Light. Inorg. Chem. 2004, 43, 7969–7971.

(18)

Ishii, K.; Shiine, M.; Shimizu, Y.; Hoshino, S.; Abe, H.; Sogawa, K.; Kobayashi, N. Control of Photobleaching in Photodynamic Therapy Using the Photodecarbonylation Reaction of Ruthenium Phthalocyanine Complexes via Stepwise Two-Photon Excitation. J. Phys. Chem. B 2008, 112, 3138–3143.

(19)

Boca, S. C.; Four, M.; Bonne, A.; van der Sanden, B.; Astilean, S.; Baldeck, P. L.; Lemercier, G. An Ethylene-Glycol Decorated ruthenium(II) Complex for Two-Photon Photodynamic Therapy. Chem. Commun. 2009, 4590–4592.

(20)

Hirata, S.; Totani, K.; Yamashita, T.; Adachi, C.; Vacha, M. Large Reverse Saturable Absorption under Weak Continuous Incoherent Light. Nat. Mater. 2014, 13, 938–946.

(21)

Hirata, S.; Vacha, M. Large Transmittance Change Induced by Exciton Accumulation under Weak Continuous Photoexcitation. Adv. Opt. Mater. 2015, 4, 297–305.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22)

Page 28 of 47

O’Connor, N. A.; Berro, A. J.; Lancaster, J. R.; Xinyu, G.; Jockusch, S.; Nagai, T.; Ogata, T.; Lee, S.; Zimmerman, P.; Willson, C. G.; et al. Toward the Design of a Sequential Two Photon Photoacid Generator for Double Exposure Photolithography. Chem. Mater. 2008, 20, 7374–7376.

(23)

Tasdelen, M. A.; Kumbaraci, V.; Jockusch, S.; Turro, N. J.; Talinli, N.; Yagci, Y. Photoacid Generation by Stepwise Two-Photon Absorption: Photoinitiated Cationic Polymerization of Cyclohexene Oxide by Using Benzodioxinone in the Presence of Iodonium Salt. Macromolecules 2008, 41, 295–297.

(24)

Sundaresan, A. K.; Jockusch, S.; Li, Y.; Lancaster, J. R.; Banik, S.; Zimmerman, P.; Blackwell, J. M.; Bristol, R.; Turro, N. J. Adiabatic Ring Opening in Tethered Naphthalene and Anthracene Cycloadducts. Photochem. Photobiol. Sci. 2010, 9, 1082– 1084.

(25)

Yagci, Y.; Jockusch, S.; Turro, N. J. Photoinitiated Polymerization: Advances, Challenges, and Opportunities. Macromolecules 2010, 43, 6245–6260.

(26)

Yanai, N.; Kimizuka, N. Recent Emergence of Photon Upconversion Based on Triplet Energy Migration in Molecular Assemblies. Chem. Commun. 2016, 52, 5354–5370.

(27)

Carling, C.-J.; Nourmohammadian, F.; Boyer, J.-C.; Branda, N. R. Remote-Control Photorelease of Caged Compounds Using Near-Infrared Light and Upconverting Nanoparticles. Angew. Chem. Int. Ed. 2010, 122, 3870–3873.


28

Page 29 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28)

Boyer, J.-C.; Carling, C.-J.; Gates, B. D.; Branda, N. R. Two-Way Photoswitching Using One Type of Near-Infrared Light, Upconverting Nanoparticles, and Changing Only the Light Intensity. J. Am. Chem. Soc. 2010, 132, 15766–15772.

(29)

Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Simultaneous Phase and Size Control of Upconversion Nanocrystals through Lanthanide Doping. Nature 2010, 463, 1061–1065.

(30)

Kouno, H.; Ogawa, T.; Amemori, S.; Mahato, P.; Yanai, N.; Kimizuka, N. Triplet Energy Migration-Based Photon Upconversion by Amphiphilic Molecular Assemblies in Aerated Water. Chem. Sci. 2016, 7, 5224–5229.

(31)

Amemori, S.; Sasaki, Y.; Yanai, N.; Kimizuka, N. Near Infrared-to-Visible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0-T1 Absorption. J. Am. Chem. Soc. 2016, 138, 8702–8705.

(32)

Mahato, P.; Monguzzi, A.; Yanai, N.; Yamada, T.; Kimizuka, N. Fast and Long-Range Triplet Exciton Diffusion in Metal-Organic Frameworks for Photon Upconversion at Ultralow Excitation Power. Nat Mater 2015, 14, 924–930.

(33)

Valeur, B.; Berberan-Santos, M. N. Molecular Fluorescence: Principle and Applications; 2nd ed.; Wiley-VCH: Weinheim, 2012.

(34)

Tomasulo, M.; Sortino, S.; Raymo, F. M. A Fast and Stable Photochromic Switch Based on the Opening and Closing of an Oxazine Ring. Org. Lett. 2005, 7, 1109–1112.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35)

Page 30 of 47

Deniz, E.; Tomasulo, M.; Cusido, J.; Yildiz, I.; Petriella, M.; Bossi, M. L.; Sortino, S.; Raymo, F. M. Photoactivatable Fluorophores for Super-Resolution Imaging Based on Oxazine Auxochromes. J. Phys. Chem. C 2012, 116, 6058–6068.

(36)

Garcia-Amorós, J.; Swaminathan, S.; Sortino, S.; Raymo, F. M. Plasmonic Activation of a Fluorescent Carbazole-Oxazine Switch. Chem. Eur. J. 2014, 20, 10276–10284.

(37)

Thapaliya, E. R.; Swaminathan, S.; Captain, B.; Raymo, F. M. Autocatalytic Fluorescence Photoactivation. J. Am. Chem. Soc. 2014, 136, 13798–13804.

(38)

Zhang, Y.; Swaminathan, S.; Tang, S.; Garcia-Amorós, J.; Boulina, M.; Captain, B.; Baker, J. D.; Raymo, F. M. Photoactivatable BODIPYs Designed to Monitor the Dynamics of Supramolecular Nanocarriers. J. Am. Chem. Soc. 2015, 137, 4709–4719.

(39)

Tian, H.; Wang, S. Photochromic Bisthienylethene as Multi-Function Switches. Chem. Commun. 2007, 781–792.

(40)

Gust, D.; Andréasson, J.; Pischel, U.; Moore, T. A.; Moore, A. L. Data and Signal Processing Using Photochromic Molecules. Chem. Commun. 2012, 48, 1947–1957.

(41)

Zhang, J.; Zou, Q.; Tian, H. Photochromic Materials: More than Meets the Eye. Adv. Mater. 2013, 25, 378–399.

(42)

Pischel, U.; Andreasson, J.; Gust, D.; Pais, V. F. Information Processing with Molecules Quo Vadis? ChemPhysChem, 2013, 14, 28–46.


30

Page 31 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43)

Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 12174–12277.

(44)

Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Multiphotochromic Molecular Systems. Chem. Soc. Rev. 2015, 44, 3719–3759.

(45)

Organic Photochromic and Thermochromic Compounds Volume 1 Main Photochromic Families; Crano, J. C.; Guglielmetti, R. J., Eds.; Plenum: New York, 1999.

(46)

Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. 2000, 100, 1685–1716.

(47)

García-Amorós, J.; Velasco, D. Recent Advances towards Azobenzene-Based LightDriven Real-Time Information-Transmitting Materials. Beilstein J. Org. Chem. 2012, 8, 1003–1017.

(48)

Sousa, C. M.; Berthet, J.; Delbaere, S.; Polonia, A.; Coelho, P. J. Fast Color Change with Photochromic Fused Naphthopyrans. J. Org. Chem. 2015, 80, 12177–12181.

(49)

Hayashi, T.; Maeda, K. Preparation of a New Phototropic Substance. Bull. Chem. Soc. Jpn. 1960, 33, 565–566.

(50)

Hayashi, T.; Maeda, K.; Takeuchi, M. A Kinetic Study of the Photochromism of 2,2′,4,4′,5,5′-Hexaphenyl-1,1′-Biimidazolyl with Electron Spin Resonance. Bull. Chem. Soc. Jpn. 1964, 37, 1717–1718.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(51)

Page 32 of 47

Hayashi, T.; Maeda, K.; Morinaga, M. The Mechanism of the Photochromism and Thermochromism of 2,2′,4,4′,5,5′-Hexaphenyl-1,1′-Biimidazolyl. Bull. Chem. Soc. Jpn. 1964, 37, 1563–1564.

(52)

Kishimoto, Y.; Abe, J. A Fast Photochromic Molecule That Colors Only under UV Light. J. Am. Chem. Soc. 2009, 131, 4227–4229.

(53)

Fujita, K.; Hatano, S.; Kato, D.; Abe, J. Photochromism of a Radical Derivative with Intense Coloration and Fast Decoloration Performance. Org. Lett. 2008, 10, 3105–3108.

(54)

Hatano, S.; Abe, J. Activation Parameters for the Recombination Reaction of Intramolecular Radical Pairs Generated from the Radical Diffusion-Inhibited HABI Derivative. J. Phys. Chem. A 2008, 112, 6098–6103.

(55)

Mutoh, K.; Shima, K.; Yamaguchi, T.; Kobayashi, M.; Abe, J. Photochromism of a Naphthalene-Bridged Imidazole Dimer Constrained to the “Anti” Conformation. Org. Lett. 2013, 15, 2938–2941.

(56)

Hatano, S.; Sakai, K.; Abe, J. Unprecedented Radical-Radical Reaction of a [2.2]Paracyclophane Derivative Containing an Imidazolyl Radical Moiety. Org. Lett. 2010, 12, 4152–4155.

(57)

Harada, Y.; Hatano, S.; Kimoto, A.; Abe, J. Remarkable Acceleration for Back-Reaction of a Fast Photochromic Molecule. J. Phys. Chem. Lett. 2010, 1, 1112–1115.


32

Page 33 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58)

Kimoto, A.; Tokita, A.; Horino, T.; Oshima, T.; Abe, J. Fast Photochromic Polymers Carrying [2.2]Paracyclophane-Bridged Imidazole Dimer. Macromolecules 2010, 43, 3764–3769.

(59)

Mutoh, K.; Abe, J. Photochromism of a Water-Soluble Vesicular [2.2]ParacyclophaneBridged Imidazole Dimer. Chem. Commun. 2011, 47, 8868–8870.

(60)

Takizawa, M.; Kimoto, A.; Abe, J. Photochromic Organogel Based on [2.2]Paracyclophane-Bridged Imidazole Dimer with Tetrapodal Urea Moieties. Dye. Pigment. 2011, 89, 254–259.

(61)

Yamashita, H.; Abe, J. Photochromic Properties of [2.2]Paracyclophane-Bridged Imidazole Dimer with Increased Photosensitivity by Introducing Pyrenyl Moiety. J. Phys. Chem. A 2011, 115, 13332–13337.

(62)

Mutoh, K.; Abe, J. Comprehensive Understanding of Structure- Photosensitivity Relationships of Photochromic [2.2]Paracyclophane-Bridged Imidazole Dimers. J. Phys. Chem. A 2011, 115, 4650–4656.

(63)

Mutoh, K.; Nakano, E.; Abe, J. Spectroelectrochemistry of a Photochromic [2.2]Paracyclophane- Bridged Imidazole Dimer: Clarification of the Electrochemical Behavior of HABI. J. Phys. Chem. A 2012, 116, 6792–6797.

(64)

Ishii, N.; Kato, T.; Abe, J. A Real-Time Dynamic Holographic Material Using a Fast Photochromic Molecule. Sci. Rep. 2012, 2, 819.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(65)

Page 34 of 47

Ishii, N.; Abe, J. Fast Photochromism in Polymer Matrix with Plasticizer and Real-Time Dynamic Holographic Properties. Appl. Phys. Lett. 2013, 102, 163301.

(66)

Mutoh, K.; Sliwa, M.; Abe, J. Rapid Fluorescence Switching by Using a Fast Photochromic [2.2]Paracyclophane-Bridged Imidazole Dimer. J. Phys. Chem. C 2013, 117, 4808–4814.

(67)

Shima, K.; Mutoh, K.; Kobayashi, Y.; Abe, J. Enhancing the Versatility and Functionality of Fast Photochromic Bridged Imidazole Dimers by Flipping Imidazole Rings. J. Am. Chem. Soc. 2014, 136, 3796–3799.

(68)

Nakano, E.; Mutoh, K.; Kobayashi, Y.; Abe, J. Electrochemistry of Photochromic [2.2]Paracyclophane-Bridged Imidazole Dimers: Rational Understanding of the Electronic Structures. J. Phys. Chem. A 2014, 118, 2288–2297.

(69)

Iwasaki, T.; Kato, T.; Kobayashi, Y.; Abe, J. Chiral BINOL-Bridged Imidazole Dimer Possessing Sub-Millisecond Fast Photochromism. Chem. Commun. 2014, 50, 7481–7484.

(70)

Yamaguchi, T.; Hilbers, M. F.; Reinders, P. P.; Kobayashi, Y.; Brouwer, A. M.; Abe, J. Nanosecond Photochromic Molecular Switching of a Biphenyl-Bridged Imidazole Dimer Revealed by Wide Range Transient Absorption Spectroscopy. Chem. Commun. 2015, 51, 1375–1378.

(71)

Yamashita, H.; Abe, J. Pentaarylbiimidazole, PABI: An Easily Synthesized Fast Photochromic Molecule with Superior Durability. Chem. Commun. 2014, 50, 8468–8471.


34

Page 35 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(72)

Kobayashi, Y.; Katayama, T.; Yamane, T.; Setoura, K.; Ito, S.; Miyasaka, H.; Abe, J. Stepwise Two-Photon Induced Fast Photoswitching via Electron Transfer in Higher Excited States of Photochromic Imidazole Dimer. J. Am. Chem. Soc. 2016, 138, 5930– 5938.

(73)

Murakami, M.; Miyasaka, H.; Okada, T.; Kobatake, S.; Irie, M. Dynamics and Mechanisms of the Multiphoton Gated Photochromic Reaction of Diarylethene Derivatives. J. Am. Chem. Soc. 2004, 126, 14764–14772.

(74)

Miyasaka, H.; Murakami, M.; Itaya, A.; Guillaumont, D.; Nakamura, S.; Irie, M. Multiphoton Gated Photochromic Reaction in a Diarylethene Derivative. J. Am. Chem. Soc. 2001, 123, 753–754.

(75)

Miyasaka, H.; Murakami, M.; Okada, T.; Nagata, Y.; Itaya, A.; Kobatake, S.; Irie, M. Picosecond and Femtosecond Laser Photolysis Studies of a Photochronic Diarylethene Derivative: Multiphoton Gated Reaction. Chem. Phys. Lett. 2003, 371, 40–48.

(76)

Ishibashi, Y.; Murakami, M.; Miyasaka, H.; Kobatake, S.; Irie, M.; Yokoyama, Y. Laser Multiphoton-Gated Photochromic Reaction of a Fulgide Derivative. J. Phys. Chem. C 2007, 111, 2730–2737.

(77)

Ishibashi, Y.; Tani, K.; Miyasaka, H.; Kobatake, S.; Irie, M. Picosecond Laser Photolysis Study of Cycloreversion Reaction of a Diarylethene Derivative in Polycrystals: Multiphoton-Gated Reaction. Chem. Phys. Lett. 2007, 437, 243–247.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(78)

Page 36 of 47

Tani, K.; Ishibashi, Y.; Miyasaka, H.; Kobatake, S.; Irie, M. Dynamics of Cyclization, Cycloreversion, and Multiphoton-Gated Reaction of a Photochromic Diarylethene Derivative in Crystalline Phase. J. Phys. Chem. C 2008, 112, 11150–11157.

(79)

Ishibashi, Y.; Okuno, K.; Ota, C.; Umesato, T.; Katayama, T.; Murakami, M.; Kobatake, S.; Irie, M.; Miyasaka, H. Multiphoton-Gated Cycloreversion Reactions of Photochromic Diarylethene Derivatives with Low Reaction Yields upon One-Photon Visible Excitation. Photochem. Photobiol. Sci. 2010, 9, 172–180.

(80)

Mori, K.; Ishibashi, Y.; Matsuda, H.; Ito, S.; Nagasawa, Y.; Nakagawa, H.; Uchida, K.; Yokojima, S.; Nakamura, S.; Irie, M.; et al. One-Color Reversible Control of Photochromic Reactions in a Diarylethene Derivative: Three-Photon Cyclization and Two-Photon Cycloreversion by a Near-Infrared Femtosecond Laser Pulse at 1.28 µm. J. Am. Chem. Soc. 2011, 133, 2621–2625.

(81)

Suzuki, M.; Asahi, T.; Masuhara, H. Photochromic Reactions of Crystalline Spiropyrans and Spirooxazines Induced by Intense Femtosecond Laser Excitation. Phys. Chem. Chem. Phys. 2002, 4, 185–192.

(82)

Suzuki, M.; Asahi, T.; Masuhara, H. Cooperative Photochemical Reaction Mechanism of Femtosecond Laser-Induced Photocoloration in Spirooxazine Microcrystals. ChemPhysChem 2005, 6, 2396–2403.

(83)

Uchida, K.; Yamaguchi, S.; Yamada, H.; Akazawa, M.; Katayama, T.; Ishibashi, Y.; Miyasaka, H. Photoisomerization of an Azobenzene Gel by Pulsed Laser Irradiation. Chem. Commun. 2009, 4420–4422.


36

Page 37 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(84)

Okuhata, T.; Kobayashi, Y.; Nonoguchi, Y.; Kawai, T.; Tamai, N. Ultrafast Carrier Transfer and Hot Carrier Dynamics in PbS−Au Hybrid Nanostructures. J. Phys. Chem. C 2015, 119, 2113–2120.

(85)

Chichibabin, A. E. Über Einige Phenylierte Derivate Des p, p-Ditolyls. Chem. Ber. 1907, 40, 1810.

(86)

Mayer, U.; Baumgartel, H.; Zimmermann, H. Über Biradikale, Chinone Und Semichinone Der Imidazolyl-Reihe. Angew. Chem. 1966, 78, 303.

(87)

Montgomery, L. K.; Huffman, J. C.; Jurczak, E. A.; Grendze, M. P. The Molecular Structures of Thiele’s and Chichibabin's Hydrocarbons. J. Am. Chem. Soc. 1986, 108, 6004–6011.

(88)

Kikuchi, A.; Iwahori, F.; Abe, J. Definitive Evidence for the Contribution of Biradical Character in a Closed-Shell Molecule , Derivative of. J. Am. Chem. Soc. 2004, 126, 6526– 6527.

(89)

Kikuchi, A.; Ito, H.; Abe, J. A New Family of Pi-Conjugated Delocalized Biradicals: Electronic Structures of 1,4-bis(2,5-Diphenylimidazol-4-Ylidene)cyclohexa-2,5-Diene. J. Phys. Chem. B 2005, 109, 19448–19453.

(90)

Kubo, T.; Shimizu, A.; Uruichi, M.; Yakushi, K.; Nakano, M.; Shiomi, D.; Sato, K.; Takui, T.; Morita, Y.; Nakasuji, K. Singlet Biradical Character of Phenalenyl-Based Kekule Hydrocarbon with Naphthoquinoid Structure. Org. Lett. 2007, 9, 81–84.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(91)

Page 38 of 47

Stable Radicals: Fundamentals and Applied Aspectd of Odd-Electron Compounds; Hicks, G. R., Ed.; John Wiley&Sons Ltd.: West Sussex, UK, 2010.

(92)

Sun, Z.; Wu, J. Open-Shell Polycyclic Aromatic Hydrocarbons. J. Mater. Chem. 2012, 22, 4151–4160.

(93)

Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011–7088.

(94)

Kubo, T. Recent Progress in Quinoidal Singlet Biradical Molecules. Chem. Lett. 2015, 44, 111–122.

(95)

Zeng, Z.; Sung, Y. M.; Bao, N.; Tan, D.; Lee, R.; Zafra, J. L.; Lee, B. S.; Ishida, M.; Ding, J.; Lopez Navarrete, J. T.; et al. Stable Tetrabenzo-Chichibabin’s Hydrocarbons: Tunable Ground State and Unusual Transition between Their Closed-Shell and Open-Shell Resonance Forms. J. Am. Chem. Soc. 2012, 134, 14513–14525.

(96)

Sun, Z.; Zeng, Z.; Wu, J. Zethrenes, Extended p Quinodimethanes, and Periacenes with a Singlet Biradical Ground State. Acc. Chem. Res. 2014, 47, 2582–2591.

(97)

Mutoh, K.; Nakagawa, Y.; Hatano, S.; Kobayashi, Y.; Abe, J. Entropy-Controlled Biradical–quinoid Isomerization of a P-Conjugated Delocalized Biradical. Phys. Chem. Chem. Phys. 2015, 17, 1151–1155.

(98)

Kobayashi, Y.; Shima, K.; Mutoh, K.; Abe, J. Fast Photochromism Involving ThermallyActivated Valence Isomerization of Phenoxyl-Imidazolyl Radical Complex Derivatives. J. Phys. Chem. Lett. 2016, 7, 3067–3072.


38

Page 39 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(99)

Yamashita, H.; Ikezawa, T.; Kobayashi, Y.; Abe, J. Photochromic Phenoxyl-Imidazolyl Radical Complexes with Decoloration Rates from Tens of Nanoseconds to Seconds. J. Am. Chem. Soc. 2015, 137, 4952–4955.

(100) Ikezawa, T.; Mutoh, K.; Kobayashi, Y.; Abe, J. Thiophene-Substituted PhenoxylImidazolyl Radical Complexes with High Photosensitivity. Chem. Commun. 2016, 52, 2465–2468. (101) Kobayashi, Y.; Abe, J. Real-Time Dynamic Hologram of a 3D Object with Fast Photochromic Molecules. Adv. Opt. Mater. 2016, DOI: 10.1002/adom.201600218.


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Figure 1. Energy diagrams for (a) simultaneous and (b) stepwise two-photon absorption processes. 51x35mm (300 x 300 DPI)


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Figure 2. (a) Typical time scales of lifetimes of photogenerated transient states. (b) Schematics of stepwise two-photon absorption processes via the S1 state, T1 state, and the colored isomer of T-type photochromic reactions. 113x157mm (300 x 300 DPI)



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Figure 3. Photochromic reaction schemes of (a) HABI, (b) [2.2]PC bridged imidazole dimer, and (c) pictures representing the time profiles of the color change of [2.2]PC bridged imidazole dimer upon CW UV light irradiation (365 nm).52 94x107mm (300 x 300 DPI)


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Figure 4. (a) Photochromic reaction scheme of ZnTPP-ImD. (b, c) Transient absorption spectra of ZnTPPImD in degassed benzene (4.6 × 10−5 M) excited with a 532 nm picosecond laser pulse at (b) low (0.06 mJ mm−2) and (c) high (1.2 mJ mm−2) pump power. (d) Difference spectrum obtained from the transient absorption spectra excited at low and high pump intensities at 300 ps. The transient spectrum of ZnTPP-ImD at 2 ms time delay excited with a nanosecond laser pulse is also shown as a red line. (e) Pump power dependence of the logarithmic transient absorption signal of ZnTPP-ImD in degassed benzene (4.6 × 10−5 M) excited at 532 nm and probed at 770 nm. The slope of the straight line is 2.72 143x250mm (300 x 300 DPI)



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Figure 5. Stepwise two-photon induced photochromic reaction scheme of ZnTPP-ImD.72 82x52mm (300 x 300 DPI)


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Figure 6. (a) Photochromic reaction scheme of bis(ImD). (b) Transient absorption spectra of bis(ImD) in degassed benzene (interval: 50 ms). (c) UV−vis absorption spectra for the degassed benzene solution of bis(ImD) (298 K, 2.1 × 10−5 M) measured after repeated irradiation with 355 nm laser pulses. (d) The ∆absorbance at 590 nm of bis(ImD) in benzene (1.5 × 10−5 M) as a function of excitation intensity. The excitation light was a 355-nm nanosecond laser pulse (pulse width = 5 ns).14 88x43mm (300 x 300 DPI)



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Figure 7. Color changes of a benzene solution of bis(ImD) (293 K, 9.4×10−5 M) after CW light irradiation for 25 s (Vis: λex = 420−700 nm, 500 mW; UV: λex = 365 nm, 5 mW). Also see movie S2 in the reference 14. 39x18mm (300 x 300 DPI)


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TOC graphic 49x49mm (300 x 300 DPI)


Fast Photochromic Molecules toward Realization of Photosynergetic

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