Keywords

1 Introduction

Photochromic compounds that undergo a reversible transformation between two isomers having different absorption spectra upon photoirradiation have attracted much attention because various physicochemical properties, such as conductivity, fluorescence, and magnetism, can be modulated without any direct physical contact. Diarylethene is one of the most promising molecules exhibiting the excellent performance, with high thermal stabilities, high photocyclization quantum yields, and high fatigue-resistant properties compared to other photochromic compounds [1]. In addition, diarylethenes can undergo the photochromic reaction even in the solid states, such as polymer films and the crystalline phase. The reversible changes in the various physicochemical properties accompanying with the photochromic reaction of diarylethenes can be applied to optical memory media, various photoswitching devices, light-driven actuators, and so on.

Some diarylethenes exhibit fluorescence in the open-ring form and/or the closed-ring form. As shown in Fig. 15.1, the fluorescent diarylethenes are classified into three types: diarylethenes exhibiting fluorescence (i) in their open-ring isomers (turn-off mode) [2,3,4,5,6,7,8,9,10,11,12,13,14], (ii) in their closed-ring isomers (turn-on mode) [15,16,17,18,19,20], and (iii) in both open- and closed-ring isomers [21,22,23,24,25,26,27,28,29]. Their fluorescence intensities or spectra change upon alternating irradiation with ultraviolet (UV) and visible light because the fluorescence properties between their open- and closed-ring forms are significantly different. In most cases, the fluorescent diarylethenes have quite low fluorescence quantum yield (Φf). It is not easy to design the fluorescent diarylethene with high Φf in addition to the high reactivities in photocyclization and photocycloreversion. To overcome this point, molecular systems combining diarylethenes and fluorophores by chemical bonding or mixing were proposed. In the systems, the fluorescence on/off switching accompanying with the photochromic reaction was accomplished. When diarylethene is in the open-ring form, the fluorophore exhibits fluorescence. On the other hand, when diarylethene is converted to the closed-ring form, the fluorescence is quenched. The processes are based on an energy transfer or intramolecular electron transfer as described later. The fluorescence photoswitchable materials are one of the most promising systems for ultra-high density optical memories and display devices because the fluorescence signal can be detected even at a single-molecule level and the change can be visually observed by naked-eye.

Fig. 15.1
figure 1

Typical examples of diarylethenes exhibiting fluorescence (i) in the open-ring isomer, (ii) in the closed-ring isomer, and (iii) in both open- and closed-ring isomers

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Although various types of the fluorescent photoswitchable systems based on the photochromism of the diarylethene derivatives have been reported so far, most of the researchers have focused on their fluorescence switching behaviors only in solution. However, it is important to evaluate the fluorescence switching properties not only in the solution but also in the solid state for practical applications such as optical memories and display devices. Here, we have focused on the solid-state fluorescence photoswitching behavior using the diarylethenes and reviewed the progress and the development in this chapter. The researches on the fluorescence switching in solution are not described in detail here because many other excellent reviews reported previously will cover the results reported so far [1, 30,31,32,33,34].

2 Ultra-High Density Optical Memory

2.1 Fluorescence Photoswitching at a Single-Molecule Level

One of the advantages for fluorescence among various physicochemical properties is that the signal can be detected even at the single-molecule level. If a single molecule of diarylethene would work as one-bit memory, ultra-high density optical memory (1 Pbit/inch2) could be realized. In this part, various researches for the ultra-high density optical memory are presented.

Irie and coworkers made an effort to realize the ultra-high density optical memory based on a photochromic reaction of the diarylethenes. First, they have tried to observe the fluorescence photoswitching at the single-molecule level using a diarylethene–fluorophore dyad 4a that connects a fluorescent anthracene derivative to a photochromic diarylethene via a rigid adamantyl spacer (Fig. 15.2a) [35, 36]. The fluorescence intensity of 4a reversibly changed upon irradiation with UV and visible light in toluene. When diarylethene is in the open-ring form, the fluorophore exhibits fluorescence. On the other hand, when diarylethene is converted to the closed-ring form, the fluorescence is quenched because of an energy transfer from the excited-state fluorophore to the diarylethene closed-ring form (Fig. 15.2b). The fluorescence photoswitching behavior at the single-molecule level was investigated using confocal microscopy in a Zeonex polymer film doped with the closed-ring form 4b. Initially, the polymer film was non-fluorescent. Upon irradiation with visible light, the four fluorescent signals of 4a could be detected as shown in Fig. 15.2c. The signals disappeared by irradiation with UV light. After that, the visible light irradiation recovered the fluorescence signals. Therefore, they accomplished that the fluorescence photoswitching can be controlled by alternating irradiation with UV and visible light at the single-molecule level and provided the molecular design for the ultra-high density optical data storage.

Fig. 15.2
figure 2

Reprinted by permission from Ref. [35]. Copyright 2002 Springer Nature

a Molecular structure and b schematic illustration of fluorescence photoswitching by an energy transfer of diarylethene–fluorophore dyad 4a, and c images of the single-molecular fluorescence photoswitching of four-individual diarylethenes.

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However, the anthracene derivative as used above was decomposed after a few cycles of the fluorescence photoswitching. To realize the ultra-high density optical memory, the excellent fatigue resistance of the fluorophore is required. Here, the fluorophore moiety was replaced from the anthracene derivative to the perylenebisimide derivative having high photochemical stability, high Φf, and large molar extinction coefficient. The improved diarylethene–fluorophore dyad 5a (Fig. 15.3a) exhibited the fluorescence photoswitching behavior even in the polymer film as well as 4a. Dyad 5a exhibited excellent photostability compared with 4a. It was estimated that the photochromic performance of 5a was kept after being excited around 106 times [37].

Fig. 15.3
figure 3

Reprinted with the permission from Ref. [38]. Copyright 2007 American Chemical Society

a Molecular structures of diarylethene–fluorophore dyads 5a and 6a, b–e histograms of (b, d) on-time and (c, e) off-time in PnBMA (b, c) and PMMA (d, e) containing 5, and (f, g) a schematic diagram of the potential energy surfaces of a diarylethene f in the gas phase and g in the polymer matrix. The histograms were constructed from the time trace of fluorescence intensity of 5 in the polymer matrixes irradiated with both 488 and 325 nm light.

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As a result of the improvement of the durability, a unique environmental effect on the photochromic reaction of dyads 5a and 6a at the single-molecule level was found [38]. Figure 15.3b–e shows on and off histograms observed for a single molecule 5a in two kinds of polymer films. The histogram has an exponential shape in poly(n-butyl methacrylate) (PnBMA) with a low glass transition temperature (Tg) near room temperature, which indicates that the photocyclization/cycloreversion quantum yields are constant. On the other hand, the histograms of the exponential shape were not observed when Tg of the polymer is higher than room temperature like poly(methyl methacrylate) (PMMA). The result suggests that the quantum yields of the photochromic reaction are not constant and increase with an increase in the number of absorbed photons. The abnormal histograms can be explained by a multilocal minima model (Fig. 15.3f, g). The diarylethene molecule simply undergoes the photochromic reaction in the soft environment with low Tg. The rigid matrix with high Tg provides multilocal potential surfaces in the ground and excited states, which prevent the one-step photochromic reaction and require multistep photoexcitations to reach the final reaction process. The rigid matrix with high Tg leads to unique photochromic reaction behavior. However, it may complicate the fluorescence photoswitching behavior at the single-molecule level. The result revealed the importance of selecting a suitable matrix around the molecules to realize the ultra-high-density optical memory.

2.2 Fluorescence Photoswitching in Film Loaded with a Large Amount of Molecules

The fluorescence photoswitching at a single-molecule level in the previous part has shown the feasibility of ultra-high density optical memory. The photoswitching was performed in the polymer films loaded with a very small amount of the molecules. To realize the ultra-high density optical memory, it is necessary to demonstrate the fluorescence photoswitching in the solid states loaded with a large amount of the molecules. However, most of the organic fluorophores become non- or very weak fluorescent with increasing concentration of the fluorophores due to concentration quenching, which is a serious problem for the application. To overcome this problem, Park and coworkers proposed to use cyanostilbene derivative as the fluorophore moiety [39]. Cyanostilbene is one of the molecules exhibiting aggregation-induced emission (AIE) [40,41,42]. The molecules having AIE characteristics exhibit weak or no emission in dilute solution but exhibit strong emission in the aggregated state. A diarylethene derivative bearing the cyanostilbene moiety (7a) (Fig. 15.4) also exhibited strong fluorescence with increasing concentration. The Φf values for 7a increased from 0.00002 in dilute solution to 0.051 in nanoparticles fabricated by a reprecipitation method. The increased Φf value is due to the formation of J-aggregate. The fluorescence intensity of 7a in the nanoparticles decreased accompanying with photocyclization of the diarylethene and almost quenched at the photostationary state (PSS) (fluorescence on/off contrast > 10). On the other hand, the photocyclization conversion at PSS was 35%, which suggests that the fluorescence photoswitching of the nanoparticles was affected by not only intramolecular energy transfer but also intermolecular energy transfer. The strong fluorescence intensity at on state and the reversible fluorescence photoswitching with high on/off contrast (>19) were observed even in the polymer film loaded with a large amount of 7a (20 wt%).

Fig. 15.4
figure 4

Molecular structure of fluorescent photoswitchable diarylethene 7a

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Métivier, Nakatani, and coworkers have investigated the amplification effect of the fluorescence switching in polymer films doped with a diarylethene derivative and a BODIPY derivative that is one of the typical organic fluorophores [43]. They prepared two polymer films with different concentrations of the fluorophore, PF-1 and PF-2. The concentration of the fluorophore in PF-2 is 10 times higher than that in PF-1. Single molecules of the diarylethene closed-ring form could quench eight fluorophore molecules in PF-2 and two fluorophore molecules in PF-1. The amplification effect of fluorescence quenching by the intermolecular energy transfer from the multiple fluorophores to the single diarylethene closed-ring form in polymers and nanoparticles was also investigated [44,45,46]. Thus, the increase of the concentration of the diarylethenes and the fluorophores results in more efficient fluorescence photoswitching when strong fluorescence of the fluorophore is observed even in high concentrations. However, a further increase in the number of doped molecules to the polymer film may cause problems such as a phase separation. Herein, a polymer bearing diarylethene and cyanostilbene moieties in the main chain were prepared and the fluorescence photoswitching behavior was investigated [47]. The neat polymer film exhibited strong fluorescence and the reversible fluorescence photoswitching with high on/off contrast (>10).

To achieve the high fluorescence on/off contrast and to suppress the undesired photoreaction on the readout, the absorption spectrum of the fluorophore is required to be separated from absorption peaks of both isomers of the diarylethene. A large overlap between the fluorescence spectrum of the fluorophore and the absorption spectrum of the diarylethene closed-ring form is also required. Park and coworkers succeeded in solving these points using a fluorophore (DHBO) that undergoes excited-state intramolecular proton transfer (ESIPT). As shown in Fig. 15.5a, DHBO enol form isomerizes to keto form through ESIPT process. As a result, DHBO keto form exhibited large Stokes-shifted fluorescence. Moreover, DHBO exhibits stronger fluorescence in the solid state (Φf = 0.1) in comparison with that in chloroform (Φf = 0.02). A pair of diarylethene 8a and DHBO satisfies the essential spectral points for the efficient fluorescence switching (Fig. 15.5b). High on/off contrast fluorescence switching (fluorescence on/off contrast > 290), non-destructive readout, and the reversibility were accomplished as shown in Fig. 15.5c [48].

Fig. 15.5
figure 5

Reprinted with the permission from Ref. [48]. Copyright 2006 American Chemical Society

a Molecular structures of 8a and DHBO, and four-level excited-state intramolecular proton transfer (ESIPT) process of DHBO. b Absorption spectra of PMMA film loaded with 8a in the open-ring form (black line) and in the PSS upon irradiation at 365 nm (blue line) and absorption and fluorescence spectra of PMMA film loaded with DHBO (red lines). c Non-destructive readout capabilities of 8a/DHBO-loaded PMMA film in the open-form state (black lines, λex = 415 nm, 200 μW cm−2) and in the PSS upon irradiation at 365 nm (blue lines, λex = 415 nm, 200 μW cm−2). Inset shows the fluorescence photoswitching of the ESIPT fluorescence in 8a/DHBO-loaded PMMA film.

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2.3 Three-Dimensional Fluorescence Recording

Previous studies have dealt with ultra-high-density optical memory in two-dimensional planes. If three-dimensional (3D) reading becomes possible, the capacity of the recording medium will be dramatically improved. Irie and coworkers demonstrated 3D erasable optical recording in a single crystal of a fluorescent diarylethene, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene (9a) [2]. Figure 15.6a–h shows eight confocal laser scanning microscopic images of the recorded spots in the single crystal of 9a. Diarylethene 9a shows reversible turn-off mode fluorescence photoswitching even in the single crystal using an Ar ion laser (336–363 nm) and an Ar-Kr ion laser (488 nm). The recorded spots were detected as dark spots, and the size was approximately 200 nm in plane and 1.5 μm in depth, which corresponds to the recording density higher than 5 Tbit/cm3. On the other hand, Belfield and coworkers accomplished 3D two-photon recording and two-photon fluorescence readout using polymer film containing a diarylethene and a fluorene derivative [49, 50]. Two layers separated by 50 μm in depth direction were written in the polymer film by two-photon absorption of 800 nm light. The fluorescence of the fluorene derivative was monitored for the readout.

Fig. 15.6
figure 6

Reprinted with the permission from Ref. [2]. Copyright © 2001 The Japan Academy

a–h Sequence of eight confocal laser scanning microscopic images of the recorded spots in a single crystal of 9a: image interval = 0.4 μm, i reflection mode confocal image of the crystal surface with 538 nm light, and j the molecular structure of diarylethene 9a.

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2.4 Non-destructive Readout by Intramolecular Electron Transfer

The fluorescence quenching process as described above is based on the energy transfer from the excited fluorophore to the diarylethene closed-ring form or the photochromic reaction of the fluorescent diarylethene itself. However, such a readout process destroys the recording simultaneously with readout in the optical memory. Although the influence was minimized using very weak light for the readout and adopting the diarylethene derivatives with a quite low photocycloreversion quantum yield, they cannot lead to an essential solution. Here, the intramolecular electron transfer (IET) process was adopted to accomplish the complete non-destructive readout for the ultra-high density optical memory. The reduction and/or oxidation potential of the diarylethene can be altered upon the photochromic reaction. The changes in the redox potentials of the diarylethene activate or deactivate the pathways of the IET process between the diarylethene and fluorophore to the charge separation state. The radiative process of the excited fluorophore can be suppressed when the IET process occurs. The IET process makes it possible to separate the absorption spectra of both open- and closed-ring forms of the diarylethene and the fluorescence spectrum of the fluorophore. Several researchers have tried to design and synthesize various types of diarylethene–fluorophore dyads to achieve non-destructive readout in fluorescence photoswitching based on the IET process [51,52,53,54,55].

In 2011, Fukaminato and coworkers successfully demonstrated non-destructive fluorescence readout of a diarylethene–fluorophore dyad 10a (Fig. 15.7a) in solution and at the single-molecule level based on the IET mechanism [56]. As shown in Fig. 15.7b, the fluorescence spectrum of the perylenebisimide derivative as the fluorophore moiety and the absorption spectra of the open- and closed-ring forms of the diarylethene moiety were completely separated. In addition, energy gaps for the charge separation in dichloromethane were calculated to be 1.23 and −8.55 kcal/mol for 10a and 10b by Rehm–Weller equation. It indicates that it is probable that the IET process takes place only for 10b in the solution. As shown in Fig. 15.7c, only the Φf of the closed-ring form decreases with increasing the dielectric constant of the solvent. As predicted from this result, the fluorescence of dyad 10 can be reversibly switched by alternating irradiation with UV and visible light in polar solvents. In addition, the fluorescence intensities of the open- and closed-ring forms did not change even when irradiated with 532 nm light (2.5 mW/cm2) for 2 h (Fig. 15.7d). These results suggest that non-destructive fluorescence readout can be realized using dyad 10a. They have investigated the fluorescence photoswitching and non-destructive fluorescence readout at the single-molecule level. Poly(methyl acrylate) (PMA), which was selected as a polar and soft polymer, loaded with 10b was prepared. As shown in Fig. 15.7e, a non-destructive fluorescence readout based on the IET mechanism was successfully demonstrated even at the single-molecule level. To summarize the results so far, various attempts on the fluorescence photoswitching using the diarylethenes have been proposed and demonstrated. Their results will advance the realization of the ultra-high density optical memory.

Fig. 15.7
figure 7

Reprinted with the permission from Ref. [56]. Copyright 2011 American Chemical Society

a Molecular structure of the diarylethene–fluorophore dyad 10a, b absorption and fluorescence spectra of each component of 10 in 1,4-dioxane: absorption spectra of the open-ring form and closed-ring form of the diarylethene moiety, and absorption and fluorescence spectra of the fluorophore moiety, c relative fluorescence quantum yields versus the dielectric constant for 10a (open circle) and 10b (closed circle), d change of fluorescence intensities at 568 nm under excitation at 532 nm of 10a (open circle) and 10b (closed circle) against irradiation time, and e single-molecule fluorescence photoswitching in a poly(methyl acrylate) (PMA) film containing 10b: sequential wide-field fluorescence images of 10 embedded in the PMA film (upper) and the corresponding fluorescence intensity trajectory of a single molecule of 10, represented as a white circle in the upper images.

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3 Display Materials

3.1 Fluorescence Photomodulation Between Dual Colors

In the previous part, we reviewed various researches on the fluorescence on/off photoswitching in the solid states for the ultra-high density optical memory. On the other hand, multicolor fluorescent photomodulation materials that exhibit multiple distinguishable fluorescence signals are especially attractive due to their potential applications in flexible full-color displays and in next-generation lighting sources.

In 2012, efficient photoresponsive fluorescence color tuning has been demonstrated by combining two kinds of cyanostilbene derivatives having AIE characteristics, CN-MBE and TPA-2CNMBE, and diarylethene 8a (Fig. 15.8a) [57]. A polymer (PMMA) film containing these three molecules was prepared. CN-MBE and TPA-CNMBE exhibited the violet-blue and orange-red fluorescence in PMMA film, respectively. When 8 was in the open-ring form, the orange-red fluorescence was observed because of the partial energy transfer from CN-MBE to TPA-2CNMBE. On the other hand, the fluorescence color significantly changed to violet-blue via white when 8a was converted to 8b. The result can be explained by the difference in the energy transfer efficiency from two kinds of the fluorophores to 8b. Figure 15.8b shows the energy level of CN-MBE, TPA-2CNMBE, and 8a/8b calculated by time-dependent density functional theory (TD-DFT). The orange-red fluorescence of TPA-2CNMBE was largely quenched by 8b in comparison with that of CN-BME because the energy level of 8b was closer to that of TPA-2CNMBE than that of CN-MBE. Based on such a mechanism, the photoresponsive fluorescence color tuning between orange-red and violet-blue colors was achieved as shown in Fig. 15.8c.

Fig. 15.8
figure 8

Reprinted with the permission from Ref. [57]. Copyright 2012 American Chemical Society

a Molecular structures of CN-MBE, TPA-2CNMBE, and 8a, b schematic illustration of the proposed FRET processes among CN-MBE, TPA-2CNMBE, and 8 after UV and visible light irradiation, and c photoresponsive reversible fluorescence images of a CN-MBE/TPA-2CNMBE/8-doped polymer film system.

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However, this fluorescence color photomodulation involves a significant decrease of intensity on the overall fluorescence spectrum. To realize ideal color-specific photoswitching between multiple fluorescence colors, it is needed that fluorescence of one fluorophore can be selectively switched upon the photochromic reaction while other fluorophores are not affected by light irradiation. In addition, the energy transfer from one fluorophore to the others must be prohibited. The problems were solved using two types of ESIPT fluorophores: (2-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (HPI) and 3-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalene-2-ol (HPNIC) (Fig. 15.9) [58]. The large Stokes-shifted fluorescence of HPI and HPNIC was observed because of the ESIPT process from enol form to keto form. The fluorescence maximum wavelength (λf) of HPI and HPNIC was 462 and 590 nm, respectively, while they have similar absorption maximum wavelength (λabs) (at 318 nm and 365 nm for HPI and HPNIC, respectively). Thus, there is almost no overlap between the absorption spectrum of HPNIC and the fluorescence spectrum of HPI, meaning that the energy transfer from excited HPI to HPNIC is negligible. As expected, a polymer film containing HPI and HPNIC (HPI:HPNIC = 1 wt%:1wt%) exhibited a dual peak fluorescence spectrum, which is almost the same as the fluorescence spectrum calculated by the simple addition of HPI and HPNIC. A diarylethene derivative, 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (11a), whose color changes from colorless to yellow upon irradiation with UV light, was selected as the photochromic compound. The blue ESIPT fluorescence of HPI can be selectively switched by the photochromic reaction of 11a because the absorption spectrum of 11b overlaps with only the fluorescence spectrum of HPI. Three-component polymer film containing HPI, HPNIC, and 11a shows white fluorescence at the initial state. Upon irradiation with UV light, the white fluorescence color was modulated to the orange by the selective quench of the blue fluorescence of HPI. The fluorescence color returned to white upon irradiation with visible light. Therefore, color-specific photoswitching between white and orange fluorescence colors was successfully demonstrated.

Fig. 15.9
figure 9

Molecular structures of HPI, HPNIC, and 11a

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On the other hand, Akagi and coworkers reported another approach to construct the multicolor fluorescent photomodulation system [59]. Their concept is to mix several kinds of nanoparticles consisting of the fluorescent photoswitchable polymers. They synthesized the fluorescent photoswitchable aromatic conjugated polymers (P1, P2, and P3) having a diarylethene moiety at the side chains and the fluorescent aromatic conjugated polymers (P1′, P2′, and P3′), as shown in Fig. 15.10a, b. The nanoparticles consisting of the polymers fabricated by a reprecipitation method exhibited the blue (for P1 and P1′), green (for P2 and P2′), or red fluorescence (for P3 and P3′) in water, respectively. The fluorescence of the nanoparticles consisting of P1, P2, or P3 was quenched upon irradiation with UV light, and almost completely quenched at PSS (Fluorescence on/off contrast = 473.2, 850.7, and 98.9 for P1, P2, and P3). The high on/off contrast is ascribed to the large contribution of intermolecular energy transfer in the nanoparticles. A white fluorescence photoswitchable film was obtained by casting the solution mixing the three kinds of the nanoparticle solution at a molar ratio. The white fluorescence can be switched between the fluorescent and quenched states upon alternating irradiation with UV and visible light. In addition, the three components cast films containing two polymer nanoparticles having a diarylethene moiety and the other polymer nanoparticle without a diarylethene were fabricated. In these films, the energy transfer between individual nanoparticles was prohibited due to a long distance between the nanoparticles. Therefore, as shown in Fig. 15.10c, only two fluorescence colors were quenched upon irradiation with UV light, while the other fluorescence was not altered. As a result, the color of the cast films was converted from white to blue, white to green, and white to red, respectively.

Fig. 15.10
figure 10

Reprinted by permission from Ref. [59]. copyright 2014 Springer Nature

(a, b) Chemical structures of a fluorescent photoswitchable polymers P1, P2, and P3, b fluorescent polymers (P1′, P2′, and P3′), and c photographs of fluorescent photoswitching between white and RGB in the mixture nanoparticle cast films: P1′ + P2 + P3, P1 + P2′ + P3, and P1 + P2 + P3′ are the white-to-blue system, white-to-green system, and white-to-red system, respectively.

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3.2 Photo-Patternable Electroluminescence on Organic Light-Emitting Diode

The rational designs of the fluorescent photoswitchable system enable not only the single fluorescence color on/off switching but also the fluorescence modulation between dual colors. Such materials have attracted much attention for the practical application such as organic light-emitting diodes (OLEDs), which are promising devices in flat-panel displays and in smart illumination technologies. The fabrication process of full-color OLED display consisting of RGB-emissive dyes is complicated and costly for fabricating large-area displays because the display must be selectively deposited precisely onto the substrates. Kawai and coworkers proposed the photo-patternable electroluminescence on OLEDs using diarylethene 12a (Fig. 15.11a) [60]. The open-ring isomer 12a is non-fluorescent, while the closed-ring isomer 12b exhibits the orange fluorescence in the amorphous film. The photochromic reaction upon irradiation with UV and visible light can be undergone even in the amorphous film; thus, the amorphous film 12a behaved as photo-patternable luminescence turn-on material. They fabricated a multilayer bottom-emission type OLED using 12b as an emissive dopant in the emitting layer, as shown in Fig. 15.11b. The fabricated device with 12b exhibited the yellow electroluminescence with an external quantum efficiency of 0.20% (Fig. 15.11c). In addition, patterned visible light irradiation on the device is area-selectively isomerized 12b to 12a, which results in the clear electroluminescence patterning as shown in Fig. 15.11d. The result shows the possibility of direct photo-patterning fabrication of full-color OLEDs using the fluorescent switchable system by the diarylethenes.

Fig. 15.11
figure 11

Reproduced from Ref. [60] by permission of The Royal Society of Chemistry

a Molecular structure of diarylethene 12a, b structure of bottom-emission type OLEDs using compound 12b as a dopant on an emitting layer in the wet process, and (c, d) photographs of c yellow EL from the device and d patterned EL from the device.

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4 Crystalline State Fluorescence Behavior of Diarylethenes

4.1 High Contrast Fluorescence Photoswitching in Crystal

Crystals can be promising for the development of further efficient fluorescent photoswitchable materials because the molecules are regularly and very densely aggregated in the crystal. In this part, we introduce two topics, crystalline state turn-off and turn-on mode fluorescence photoswitching with high contrast and characteristics fluorescence behavior of inverse-type diarylethene in solid states.

As described above, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene (9a) exhibits turn-off mode fluorescence switching accompanying with the photochromic reaction even in the single crystal (Fig. 15.6). However, the fluorescence on/off contrast is not high due to the low photoconversion yield in the single crystalline state. Fukaminato et al. designed a diarylethene derivative linked to the benzothiadiazole derivative (13a) (Fig. 15.12a) to obtain fluorescent diarylethene with efficient turn-off mode fluorescence photoswitching properties in the single crystal [61]. Each performance of the diarylethene and the fluorophore moieties in dyad 13a can be carried out at a high level because they were separated by an ether bond. Initially, the single crystal of 13a exhibited strong green fluorescence. Upon irradiation with area-selective UV light, the fluorescence intensity of the crystal was down to the background level. As shown in Fig. 15.12b, the size of the dark area gradually decreased toward the center with the recovery of the fluorescence upon irradiation with visible light. On the other hand, the fluorescence signal of quenched area in the PMMA film containing 13a is uniformly recovered under irradiation with 438 nm light (Fig. 15.12c). The photocyclization conversion becomes high as the position is close to the center because UV light intensity has Gaussian distribution. The difference in the photocyclization conversion can be negligible in the PMMA film due to the small contribution of the intermolecular energy transfer. On the other hand, the fluorescence intensity of the single crystal suddenly and largely changes upon irradiation with visible light, which is ascribed to the intermolecular energy transfer from a large number of the fluorophores to the small amount of diarylethene closed-ring forms in the crystal. The amplified intermolecular energy transfer process results in the high fluorescence on/off contrast and the fluorescence recovery behavior upon decreasing the size of the dark area.

Fig. 15.12
figure 12

Reproduced from Ref. [61] by permission of The Royal Society of Chemistry

a Molecular structure of diarylethene–fluorophore dyad 13a, b fluorescence images under excitation with 438 nm light to the single crystal of 13a, c a PMMA film containing 13a (4 wt%); (i) before UV irradiation, (ii) after irradiation with 390 nm light, (iii)–(v) fluorescence recovery under excitation with 438 nm light.

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On the other hand, Morimoto, Irie, and coworkers demonstrated high contrast fluorescence photoswitching in single crystals of diarylethene derivatives having benzothiophene S,S-dioxide at the aryl moieties (14a, 15a) (Fig. 15.13) [62]. Diarylethenes 14a and 15a exhibit the reversible photochromic reaction and turn-on mode fluorescence photoswitching upon irradiation with UV and visible light in solution and in the single crystal. The single crystals emit no fluorescence when the diarylethenes are in their open-ring isomers. Upon irradiation with UV light, the crystals become fluorescent due to the generation of the fluorescent closed-ring forms. Thus, the single crystals underwent turn-on mode fluorescence photoswitching with high fluorescence on/off contrast.

Fig. 15.13
figure 13

Molecular structures of diarylethenes 14a and 15a

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4.2 Characteristic Fluorescence Properties of Inverse-Type Diarylethenes in Crystal

Diarylethene derivatives can be categorized into two groups, a normal type and an inverse type, with respect to the orientation of the two aryl rings as shown in Fig. 15.14 [63]. The inverse-type diarylethenes are one of the typical fluorescent diarylethenes and show characteristic fluorescence and photochromic reaction behavior in the solid states. They have slightly red-shifted absorption spectra due to long π-conjugation compared with the normal-type diarylethenes and emit blue fluorescence in their open-ring forms [2, 3, 11,12,13,14]. The fluorescence intensity of the open-ring forms decreases upon irradiation with UV light. Upon irradiation with visible light, the fluorescence intensity returns to its initial one. As mentioned above, 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene (9a) exhibits turn-off mode fluorescence switching accompanying with the photochromic reaction even in the single crystal [2]. The fluorescence in the crystal is composed of short-lifetime component (λf = 435 nm, τ < 1.0 ns) and the long lifetime component (λf = 495 nm, 1.0 ns < τ  < 20 ns). Detailed analysis by time-resolved fluorescence measurement, X-ray crystallographic analysis, and polarized fluorescence measurements revealed that the short-lifetime component is ascribed to the monomer, and the aggregate formed by the intermolecular interaction between two thiophene rings of neighboring molecules in the crystal have a longer lifetime and a longer λf compared with the monomer [3].

Fig. 15.14
figure 14

Chemical structures of normal and inverse-type diarylethene derivatives

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In 2005, Yu and coworkers found that an inverse-type diarylethene, 1,2-bis(4-methyl-2-(2-pyridyl)thiazolyl)perfluorocyclopentene (16a) (Fig. 15.15), exhibited stronger fluorescence around 500 nm (Φf = 0.20) in the crystalline state compared with the solution (Φf = 0.005) [11]. Diarylethene 16a exists in the antiparallel conformation in crystal. The distance between the reactive carbons in the crystalline phase was 3.56 Å, which is sufficiently short for the photocyclization to take place in the crystalline phase [64]. However, the crystal did not show any photocyclization. Métivier and Nakatani et al. reported that the nanoparticles consisting of 16a show the intermediate photocyclization reactivity and the Φf compared with the acetonitrile and in the crystal [12]. Based on the result, they assume that the conformational changes that are necessary for the photocyclization may be hindered when the environment around the diarylethene becomes rigid, such as in the crystal. Similar observation has been made for the other inverse-type diarylethene, 1,2-bis(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene (1a) as described in Fig. 15.16a [65]. Two polymorphic crystals of 1a, crystals 1a-α and 1a-β, can be obtained by recrystallization from acetone and n-hexane solutions, respectively. Although all the diarylethene molecules in the crystals existed in the antiparallel conformation with the distances between the reactive carbons shorter than 4.2 Å, 1a cannot undergo the photocyclization in the crystalline phase. However, the photocyclization of the diarylethene having the methyl groups (11a) instead of the phenyl groups can be observed even in crystal [66, 67]. On the other hand, the substitution of the methyl group to the methoxy group at the 3-position of thiophene rings (17a) also caused loss of the photocyclization reactivity in the crystal [68]. Miyasaka and coworkers revealed that the photocyclization quantum yield of 1a decreases with an increase in the solvent viscosity in solution [63]. It can be concluded that specific substituents such as the phenyl and methoxy groups on the inverse-type diarylethenes may have some geometric or electronic effect prohibiting the photocyclization as the environment around the molecule becomes rigid although the reason and the mechanism are not clear yet.

Fig. 15.15
figure 15

Molecular structures of diarylethenes 16a and 17a

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Fig. 15.16
figure 16

Reprinted from Ref. [65]. Copyright 2017, with permission from Elsevier. Reprinted from Ref. [69]. Copyright 2018, with permission from The Chemical Society of Japan

a Molecular structure of inverse-type diarylethene 1a, b optical microscopic photographs of crystal 1a-β before and after heating at 50 °C for 80 min, and c optical microscopic photographs of crystal 1b before and after irradiation with visible light. The photographs were observed under excitation with 365 nm light.

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In the viewpoint on the solid-state fluorescence properties, the inverse-type diarylethenes having the phenyl groups to R2 positions have attractive solid-state fluorescence properties and can be expected as a new AIE molecular skeleton. As mentioned above, the crystal of 16a exhibited stronger fluorescence compared with the solution [11, 12]. Additionally, both crystals 1a-α and 1a-β exhibited the largely red-shifted and stronger orange and yellow fluorescence (Φf = 0.52 and 0.50 for crystals 1a-α and 1a-β) compared with the n-hexane (λf = 480 nm and Φf = 0.017). Furthermore, the polymorphic phase transition from crystal 1a-β that has one hexane molecule in the unit cell to crystal 1a-α upon heating was found (Fig. 15.16b). In the phase transition process, the fluorescence color changed from yellow to orange via dark state because the phase transition includes the collapse of the β-crystalline phase accompanying the exclusion of the n-hexane molecules and the crystallization of the α-crystalline phase [65]. Crystals consisting of the closed-ring form (crystal 1b) underwent the photocycloreversion upon irradiation with visible light. As shown in Fig. 15.16c, the open-ring form crystal (crystal 1a′) produced by the photocycloreversion exhibited green fluorescence that is a significant difference from those of crystals 1a-α and 1a-β [69]. Although the molecular geometry and intermolecular interactions in crystal 1a′ cannot be revealed due to the lack of crystallinity after the photocycloreversion, weaker intermolecular interactions in crystal 1a′ compared with crystals 1a-α and 1a-β may lead to the green fluorescence because the orange and yellow fluorescence of crystals 1a-α and 1a-β is ascribed to the intermolecular π-π interactions between the phenyl rings. Therefore, the introduction of steric substituents to the phenyl rings may modulate the intermolecular π-π interactions to result in a dramatic change in the solid-state fluorescence properties. As shown in Fig. 15.17a, b, inverse-type diarylethenes 18a-21a having various alkyl chains at the p-position of phenyl rings exhibit the green fluorescence that is a similar color to that of crystal 1a′. As expected, the molecules in crystals 18a-21a have no π-π intermolecular interaction and only van der Waals interaction, which indicates that the alkyl chains prevent the formation of the strong π-π interaction between the phenyl rings to result in only slight red-shifted fluorescence [70]. Therefore, the inverse-type diarylethene 1a and the derivatives exhibit different fluorescence colors depending on the intermolecular interaction in different states. In addition, 18a-21a have crystallization-induced emission (CIE) characteristics to exhibit strong fluorescence in the crystalline phase compared in n-hexane and in the amorphous phase (Fig. 15.17b). The amorphous solid of 18a which has the most remarkable CIE characteristics among 18a-21a was crystallized after mechanical scratching followed by heating at 90 °C because the small crystal nuclei were fabricated by scratching and the growth of the crystal nuclei was performed by heating. As shown in Fig. 15.17c, reversible fluorescence recording based on CIE characteristics and mechanical scratching and heating induced crystallization was successfully demonstrated. By partly scratching and heating at 90 °C for 3 min for the amorphous solid that was prepared by heating the crystals at 150 °C, green fluorescent letters of “D” or “E” were clearly written. The letter was completely erased by heating at 150 °C for 30 s. As introduced up to this point, inverse-type diarylethenes having phenyl groups show unique behavior in the solid states, such as multicolor fluorescence depending on intermolecular interactions and responsiveness for external stimuli such as heat and scratching, which may be useful for potential applications such as in sensors and security materials.

Fig. 15.17
figure 17

Reprinted from Ref. [70]. Copyright 2019, with permission from Elsevier

a Molecular structures of inverse-type diarylethenes 18a-21a, b fluorescence spectra of 18a in the crystalline phase (green line) and in the amorphous phase (red line); inset shows the fluorescence quantum yields of 18a-21a in the crystalline phase and in the amorphous phase, and c reversible fluorescence recording of 18a by partly scratching and heating observed at room temperature upon excitation at 365 nm.

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5 Summary

In this chapter, we have introduced the solid-state fluorescence switching using diarylethene. The fluorescence switching can be achieved by combining a diarylethene with a fluorophore or using a fluorescent diarylethene in solution. On the other hand, in the solid states, it is difficult to obtain similar properties as in solution due to problems such as concentration quenching and environmental effect of the matrix. However, various types of solid-state fluorescence-switchable systems using the diarylethenes based on rational molecular design and smart experimental approach have been proposed and successfully demonstrated for practical applications, such as optical memory, display devices, and sensor materials. A series of the researches will advance the realization of the applications based on the fluorescence switching of the diarylethene.