Review

Photochromism of diarylethene molecules and crystals

By Masahiro IRIE*1,†

(Communicated by Kenichi HONDA, M.J.A.)

Abstract: Photochromism is defined as a reversible transformation of a chemical speciesbetween two isomers upon photoirradiation. Although vast numbers of photochromic moleculeshave been so far reported, photochromic molecules which exhibit thermally irreversible photo-chromic reactivity are limited to a few examples. The thermal irreversibility is an indispensableproperty for the application of photochromic molecules to optical memories and switches. We havedeveloped a new class of photochromic molecules named “diarylethenes”, which show the thermallyirreversible photochromic reactivity. The well designed diarylethene derivatives provide out-standing photochromic performance: both isomers are thermally stable for more than 470,000 years,photoinduced coloration/decoloration can be repeated more than 105 cycles, the quantum yield ofcyclization reaction is close to 1 (100%), and the response times of both coloration and decolorationare less than 10 ps. This review describes theoretical background of the photochromic reactions,color changes of the derivatives in solution as well as in the single crystalline phase, and applicationof the crystals to light-driven actuators.

Keywords: photochromism, diarylethene, single crystal, optical memory, optical switch,actuator

Introduction

Photochromic molecules are characterized bytheir reversible color changes upon photoirradia-tion.1)–3) Typical examples of photochromic mole-cules, which show such color changes, are shown inFig. 1. The upper two molecules, azobenzene andspiropyran, change the color from colorless to orangeand blue, respectively. These traditional moleculesare classified into T-type (thermally reversible)photochromic ones. The photogenerated right-sideisomers are thermally unstable and the colors slowlydisappear in the dark. On the other hand, recentlyinvented lower two molecules, furylfulgide and di-arylethene, both of which change the color fromcolorless to red, are classified into P-type (thermallyirreversible, but photochemically reversible) photo-chromic ones. In these P-type photochromic mole-

N N N N

N

CH3 CH3

O NO2R

N

CH3 CH3

NO2

OR

-+

CH3

OO

CH3

CH3

O

O O

OCH3

OO

CH3

CH3CH3 CH3 CH3

H3C

F2 F2

S

CH3

SH3C

F2F2

H3C CH3 S

CH3

SH3C

F2F2

H3C CH3

Fig. 1. Typical examples of photochromic molecules.

*1 Department of Chemistry and Research Center for SmartMolecules, Rikkyo University, Tokyo, Japan.

† Correspondence should be addressed: M. Irie, Departmentof Chemistry and Research Center for Smart Molecules, RikkyoUniversity, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501,Japan.

Proc. Jpn. Acad., Ser. B 86 (2010) [Vol. 86,472

doi: 10.2183/pjab.86.472©2010 The Japan Academy

cules, photogenerated right-side isomers are ther-mally stable and scarcely return to the left-sideisomers in the dark at room temperature. Thethermally irreversible property is indispensable forthe application to optical memories and switches.The thermal irreversibility of diarylethene deriva-tives will be discussed in detail later.

The color changes are ascribed to photoinducedelectronic structural changes of the molecules fromthe left- to the right-side isomers. During theelectronic structural changes the molecules alsochange the geometrical structures. Azobenzene iso-merizes from the trans-isomer to the cis-isomer.Spiropyran converts from the closed-ring form tothe open-ring form. Photochromic molecules changeboth their electronic as well as geometrical structuresupon photoirradiation.

A typical example of the electronic and geo-metrical structural changes of a diarylethene deriv-ative is shown in Fig. 2. Figure 2a and b show thechemical structures of the open- and the closed-ringisomers of 1,2-bis(2,5-dimethyl-3-theinyl)perfluoro-cyclopentene (1a) and their absorption spectral

changes (color changes).4) In the left-side open-ringisomer two thiophene rings have no interaction andthe spectrum is similar to a substituted thiophene.On the other hand, in the right-side closed-ringisomer :-electrons are delocalized throughout themolecule and the HOMO–LUMO gap becomessmall. As a result, the spectrum shifts to longerwavelengths. The red-color is ascribed to thedelocalization of :-electrons. At the same time, themolecule changes the geometrical structure. Thetop- and side-views of the molecule are shown inFig. 2c. As can be seen from the side-view, thicknessof the molecule becomes thinner when the moleculeundergoes the photocyclization. The top-view of themolecule indicates that the base width of the triangleshape (blue broken line) decreases from 1.01 nm to0.90 nm and the height increases from 0.49 nmto 0.56 nm. Inferring from the shape change it isconcluded that the diarylethene molecule shrinksupon photocyclization and expands upon photo-cycloreversion.5),6)

The electronic as well as geometrical structuralchanges at the molecular level can be adopted invarious macro-scale applications. The electronicstructural changes can be applied to optical memorymedia and photo-switching devices.7)–10) On the otherhand, the geometrical structural changes can bepotentially applied to light-driven actuators.11)–15)

For such applications, photochromic moleculesshould fulfill the following requirements.1. Thermal stability of both isomers2. Fatigue-resistant property3. High sensitivity4. Rapid response5. Reactivity in solid state

We have developed a new class of photochromicmolecules named “diarylethenes”, which fulfill theabove all requirements simultaneously.16)–19) Diaryl-ethenes are derivatives of stilbene. When phenyl ringsof stilbene are replaced with five-membered hetero-cyclic rings, such as thiophene or furan rings, bothisomers become thermally stable and coloration/decoloration cycles can be repeated many times.Characteristic properties of diarylethene derivativesare summarized as follows.1. Both isomers are thermally stable: half-life

times at room temperature are as long as470,000 years

2. Coloration/decoloration can be repeated morethan 105 cycles

3. The quantum yield of coloration is close to 1(100%).

Abs

orba

nce

1.0

0.5

0

Wavelength / nm

300 400 500 600 700

Top-view

Side-view

0.49 nm 0.56 nm

1.01 nm 0.90 nm

(c)

(b)

(a)

S

CH3

SH3C

F2F2

F2

H3C CH3 S

CH3

SH3C

F2F2

F2

H3C CH3

1a 1b

Fig. 2. (a) Chemical structures of the open- and the closed-ringisomers of 1,2-bis(2,5-dimethyl-3-theinyl)perfluorocyclopentene(1a) (b) absorption spectra of the open- (solid line) and theclosed-ring (red line) isomers and (c) top- and side-views of thegeometrical structures of the open- and closed-ring isomers.

Photochromism of diarylethene molecules and crystalsNo. 5] 473

4. Response times of both coloration and decolor-ation are less than 10 ps

5. Many of diarylethene derivatives undergo pho-tochromism even in the single crystalline phaseIn this review, theoretical background of the

photochromic reactions, color changes of the deriv-atives in solution as well as in the single crystallinephase, and application of the geometrical structuralchanges to light-driven actuators will be described.

Theoretical study

Thermal irreversibility is an indispensable prop-erty for the application of photochromic moleculesto memories and switches. Although tremendousefforts were carried out in the 70s–80s to provide thethermal irreversibility to photochromic systems, allattempts to modify existing photochromic moleculesfailed.7) We had to wait until fortunate finding

of thermally irreversible photochromic molecules.Finally in the beginning and the middle of the 80sfurylfulgide and diarylethene were found to undergothermally irreversible photochromic reactions, re-spectively.20)–23) Although the photogenerated colorsof these molecules remained stable at room tempera-ture, the reason why the molecules show the thermalirreversibility was not understood. It was stronglydesired to reveal the basic principle of the thermallyirreversible photochromic reactivity. We tried tosolve the problem based on MO theory.

A theoretical study on 1,3,5-hexatriene to cyclo-hexadiene-type photochromic reactions was carriedout to reveal the thermally irreversible reactivity.24)

Typical molecules of the hexatriene molecular frame-work are diarylethene derivatives having phenyl orheterocyclic rings. Semiempirical MNDO calculationwas carried out for diarylethene derivatives 2–5(Scheme 1).

Figure 3 shows state correlation diagrams forthe electrocyclic reactions from 2a to 2b and from 3ato 3b in a conrotatory mode. Full lines of the figureshow that the interconnecting states belong to thesame symmetry groups. According to the Woodward-Hoffmann rule based on :-orbital symmetries for1,3,5-hexatriene, the cyclization reaction is allowedin the conrotatory mode in the photoexcited state.The cyclization reactions of both molecules can notproceed in the ground state because of large energybarriers. The open-ring isomers excited to B statesconvert to the closed-ring isomers in the groundstate.

H

H

h ν

h ν', ∆

2a 2b

h ν

h ν', ∆X X X X

H

H

3a: X = O4a: X = NH5a: X = S

3b: X = O4b: X = NH5b: X = S

Scheme 1.

3a2b

3b

2aFig. 3. State correlation diagrams in conrotatory mode for the reaction from 2a to 2b and from 3a to 3b.

M. IRIE [Vol. 86,474

Although full lines of Fig. 3 suggests that theclosed-ring isomers, 2b and 3b, can convert to theopen-ring isomers, 2a and 3a, in the ground state,the cycloreversion reactions have to overcome someenergy barriers. The barriers correlate with theground state energy difference between the open-and the closed-ring isomers. The calculated values ofthe difference for 2, 3, 1,2-di(3-pyrrolyl)ethene (4),and 1,2-di(3-thienyl)ethene (5) are shown in Table 1.When the energy difference is large, as in the caseof 2, the energy barrier becomes small and thereaction is expected to take place readily. On theother hand, the reaction barrier becomes large whenthe energy difference is small as shown for 3. In thiscase, the cycloreversion reaction hardly takes place.The energy difference between the open- and theclosed-ring isomers controls the stability of thephotogenerated closed-ring isomers.

During the cyclization reaction phenyl andheterocyclic rings change the structures as shownin Scheme 2. The structural changes indicate thataromaticity of the groups change during the reac-tions. The energy difference between the right andleft side groups was calculated, as shown in Table 2.The aromatic stabilization energy of the aryl groupscorrelates well with the ground state energy differ-ence. The highest energy difference was calculated forthe phenyl group and the lowest for the thienylgroup. The aromaticity is the key molecular propertywhich controls the energy barrier or the thermalstability of the closed-ring isomers.

The theoretical prediction was confirmed by thesynthesis of diarylethene derivatives with varioustypes of side groups, as shown in Fig. 4.16) When theside groups are furan, thiophene, benzothiophene,benzofuran, thiazole, oxazole rings, which have lowaromatic stabilization energy, the closed-ring isomersare thermally stable and hardly return to the open-ring isomers. On the other hands, photogeneratedclose-ring isomers of diarylethenes with phenyl orindole rings, which have rather high aromaticstabilization energy, are thermally unstable. They

return to the open-ring isomers even in the dark. Thebasic principle that diarylethene derivatives havingside groups with low aromatic stabilization energyexhibit thermal irreversibility has been establishedbased on the theoretical consideration as well as thesupporting experimental results.

To reveal the details of the photoisomerizationreactions ab initio MO study was carried out fora model system.25)–27) The energy profile of thepotential surface is shown in Fig. 5 as a function ofthe internuclear distance (q) between two reactivecarbons. The photocyclization reaction of the open-ring isomer proceeds as follows. Upon excitation from1A to 1B state, the 1B state will relax to theminimum having C–C distance of 1.779Å or cross-over into 2A state and then relax to 2Ao. The 2Aominimum is located between the closed- and theopen-ring isomers. From this 2Ao point the systemdecays to the ground state open- or closed-ringisomers. The photocyclization reaction proceedswithout any reaction barrier.

The cycloreversion reaction of the closed-ringisomers to the open-ring isomer proceeds as follows.Upon excitation to the 1B state, the system reaches2A state and then 2Ac state will be populated. Going

Table 1. Relative ground-state energy differences between theopen- and closed-ring isomers

CompdConrotatory

(kcal/mol)

1,2-diphenylethene (2) 27.3

1,2-di(3-pyrrolyl)ethene (4) 15.5

1,2-di(3-furyl)ethene (3) 9.2

1,2-di(3-thienyl)ethene (5) −3.3

C C

C C

X X

Scheme 2.

Table 2. Aromatic stabilization energy difference

Group Energy (kcal/mol)

Phenyl 27.7

Pyrrol 13.8

Furyl 9.1

Thienyl 4.7

Photochromism of diarylethene molecules and crystalsNo. 5] 475

from the 2Ac closed-ring excited state to 2Ao, there isa barrier caused by C–C bond fission. When thesystem arrives at the 2Ao point, both cycloreversionas well as cyclization reactions can take place. Therate of population of the 2Ao state by the cyclo-

reversion process is less than the population from thecyclization process, because of the presence of anenergy barrier. This is consistent with the exper-imental results that the cyclization quantum yield islarger than that of cycloreversion in dithienylethenederivatives.

The energy barrier from 2Ac to 2Ao controlsthe quantum yield of cycloreversion reaction. Whenthe barrier is large, the cycloreversion quantumyield is expected to decrease.28) The experimentalquantum yields were plotted against the calcu-lated energy differences between 2Ao and 2Acstates, E(2Ao) − E(2Ac), for five diarylethene de-rivatives. The quantum yields were found todecrease linearly with increasing energy differences.The good correlation indicates that the energysurface in the excited state well explains thecycloreversion reaction process. The theoreticalcalculations successfully predict the photochemicalquantum yields.

Color changes

As noted in the introduction, photoinduced colorchanges are the most pronounced phenomenonobserved in photochromic molecules. Figure 6 showstypical color changes of several diarylethene deriva-

Thermally stable molecules

CH3

CH3HC3

F2

F2 F2

CH3

F2

F2 F2

CH3

F2

F2 F2

SS H3C

6b

SS H3C

7b

OS H3C

8b

CH3

F2

F2 F2

CH3H3CNN CH3

F2

F2 F2

NN CH3

F2

F2 F2

SS H3C

9b

SS H3C

10b

OO H3C

11b

Thermally unstable molecules

NN

CH3

H3C

F2F2 F2

CH3 CH3

CNNC

NN

CH3

H3CCH3 CH3

CH3H3C

CH3

H3C

CH3

CH3

H3C

H3C14b13b12b

Fig. 4. Thermal stability of photogenerated closed-ring isomers of diarylethene derivatives having various types of side groups.

q (Å)

Fig. 5. Potential energy profile of the model system.

M. IRIE [Vol. 86,476

tives. When hexane solutions of the derivativesare irradiated with UV light, the colorless solutionsturn to yellow, red, blue and green. The chemicalstructures of the derivatives, 15b–18b, are alsoshown in the figure. The color change is ascribed tothe electronic structure change of the diarylethenesfrom the open- to the closed-ring isomers. The coloris controlled by the length of :-conjugation of theclosed-ring isomers. In 15b the :-conjugation islocalized in the central part, while in 16b and 17bthe :-conjugation delocalizes throughout the mole-cules and the length of 17b is longer than that of16b. Therefore, the absorption maximum of 17b(blue) is longer than that of 16b (red). Nowadays, itis possible to estimate the colors using theoreticalcalculations.

In normal photochromic systems they intercon-vert between only two states: colorless and colored. Ifmulti-color photochromic molecules are prepared,they are useful for multifrequency optical memoriesand displays. We took an approach to incorporatethree photochromic units in one molecule.29) Theadvantage of the one-molecule system over mixturesystems is high image resolution, constant colorbalance in a large area and possible application tomultifrequency single-molecule memory. Figure 7ashows the fused trimer (19a) along with anticipatedphotochromic reactions. The fused trimer has threedithienylethene moieties, bis(2-thienyl)ethene, (2-thienyl)(3-thienyl)ethene and bis(3-thienyl)ethene,with two thiophene rings in common. Upon irradi-ation with UV (313 nm) light the hexane solutioncontaining 19a turned black. When UV (313 nm)

and blue (460 nm) light was simultaneously irradi-ated, the solution turned sky blue. When the solutionwas irradiated with UV (313 nm) and red (633 nm)light, the solution turned to red. The solution turnedto yellow when the solution was irradiated UV(313 nm) and yellow (578 nm) light. The solutioncontaining 19a can develop blue, red, and yellowcolors upon irradiation with appropriate wavelengthof light, suggesting that all of the three diarylethenemoieties can cyclize upon irradiation, as shown inFigure 7b.

Most of diarylethene derivatives undergo photo-chromism even in the single crystalline phase, becausethe geometrical structural change during the photo-chromic reaction is very small. Figure 8 shows typicalcolor changes of several diarylethene single crystals.30)

Upon irradiation with UV light, colorless crystalsturn yellow, red, blue or green, depending on themolecular structure of the diarylethenes. The coloredforms are stable even at 100 °C and never return to

F2F2 F2

F2F2 F2

UV Vis.

SS

H3C CH3

CH3

H3C SSH3C CH3

CH3

H3C

15bYellow

(425 nm)

16bRed

(505 nm)

SS

CH3

H3C

F2F2 F2

SN

CH3

H3C

F2F2 F2

CH3CH3O

CN

CH3

17bBlue

(575 nm)

18bGreen

(665 nm)

Fig. 6. Typical color changes of several diarylethene derivativesin hexane solution upon photoirradiation.

F2 F2S

SF2

F2 F2S

SF2

UV

(a)

SSF2

19a

S

F2 F2 F2F2

F2 SSF2

19c

S

F2 F2 F2F2

F2

Red

Vis.

UV Vis. UV Vis.

SSF2

F2 F2S

S

F2 F2 F2F2

F2

F2

BlueSSF2

F2 F2S

S

F2 F2 F2F2

F2

F2

Yellow

UV

Vis.

UV +Blue Light

UV +Yellow Light

UV +Red Lightcolorless

19d 19b

(b)

19a 19c 19d 19b

Fig. 7. Photochromic reactions of fused trimer 19 and the colorchanges upon irradiation with the appropriate wavelength oflight.

Photochromism of diarylethene molecules and crystalsNo. 5] 477

the initial colorless form at room temperature in thedark. The color disappeared by irradiation withvisible light. The photoinduced coloration/decolora-tion cycles can be repeated more than 104–105 timeswithout any destruction of the crystals.

Multi-color single crystals can be prepared bymixing appropriate diarylethene derivatives withsimilar geometrical structures.31)–33) We preparedmixed crystals composed of above derivates, 11, 10and 20 (Scheme 3).33)

These three diarylethene derivatives have verysimilar geometrical structures, but the colors of theclosed-ring isomers are different. Colorless 11a, 10aand 20a turn yellow, red and blue upon UVirradiation. These three derivatives could be readilymixed in almost any ratios into single crystals. Thecolor change of the three component crystal uponirradiation with the appropriate wavelength of light

UV

Vis.

S SCH3

CH3

F2F2 F2

S S

CH3

CH3

F2 F2

F2

S S

CH3

CH3H3C CH3

F2

F2F2

S S

CH3

F2

F2F2

S S

CH3

CH3H3CF2F2

F2

CH3O2N NO2

F2F2

F2

S SCH3

CH3

F2

F2

F2

S S

CH3

CH3

F2F2

F2

S S

CH3

CH3H3C CH3

F2

F2

F2

CH3

F2F2

F2

S S

CH3

H3C CH3

F2F2

F2

CH3O2N NO2

F2F2

F2

UV

Vis.

CH3 CH3S SCH3

S SCH3 CH3 S SCH3

Fig. 8. Color changes of several diarylethene single crystals upon photoirradiation.

> 620 nm

370 nm > 690 nm

435 nm

435 nm

Fig. 9. Color changes of a three-component crystal containingdiarylethene 11a, 10a and 20a upon irradiation with theappropriate wavelength of light.

O

NN

O

CH3

F2

F2 F2

O

NN

O

CH3

F2

F2 F2UV

Vis.

F2

F2 F2

OO H3C

11a

F2

F2 F2

OO H3C

11b

UV

F2

F2 F2

S

NN

S

CH3

H3C

10a

F2

F2 F2

S

NN

S

CH3

H3C

10b

Vis.

UV

SS

CH3

H3C

20a

SS

CH3

H3C

20b

Vis.

Scheme 3.

M. IRIE [Vol. 86,478

is shown in Fig. 9. The crystal changes from colorlessto yellow, red, and blue upon photoirradiation.

The multicolor change was also observed for asingle-component crystal under polarized light.34)

Figure 10a–d show the color change of single crystal(010) surface of 1,2-bis(2-methyl-6-nitro-1-benzothio-phen-3-yl)perfluorocyclopentene (21a). Before pho-toirradiation the crystal was colorless (Fig. 10aand b). Upon irradiation with UV light, the crystalturned yellow at a certain angle (3 = 0). When thecrystal was rotated as much as 90° (3 = 90), the colorturned blue. The yellow color reappeared at 180°.The clear dichroism from yellow to blue is explainedby the regular orientation of the diarylethene in the

crystals. Figure 10e shows the chemical structure of21b and the molecular packing of the diarylethenesviewed from (010) surface. The diarylethene has twoelectronic transitions, long and short axes, and thelong and short axes transitions correspond to 600 nmand 465 nm absorption bands. When the polarizedlight coincides to the long axis, the crystal exhibitsblue color (600 nm), while the color changes to yellow(465 nm) when the polarized light coincides to theshort axis. Even single component diarylethenecrystal can show two colors under polarized light.

Geometrical structural changes

As described in the introductory part, photo-

(e)

F2

F2 F2

a

S S

CH3

H3C

O2N NO2

c

Fig. 10. Photographs of single crystal 21a under polarized light before (a, 3 = 0°; b, 3 = 90°) and after (c, 3 = 0°; d, 3 = 90°) irradiationwith UV light. 3 is the rotation angle of the crystal. (e) crystal packing viewed from the (010) surface and the chemical structure of21a.

Photochromism of diarylethene molecules and crystalsNo. 5] 479

chromic molecules change electronic as wellas geometrical structures upon photoirradiation.Figure 2c shows the typical geometrical structurechange of 1 along with the photoisomerization. Thediarylethene molecule shrinks when it converts fromthe open- to the closed-ring isomers, as evidenced byin situ X-ray crystallographic analysis.5),6) If suchstructural change takes place in a crystal, the crystalbulk shape is anticipated to follow the shape changesof the component diarylethene molecules. The mostconvenient way to detect the crystal shape changeis to measure the surface morphology by using anatomic force microscopy (AFM).

We carried out AFM measurement of the singlecrystal of 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-perfluorocycopentene (22a).35) The surface mor-phology change was followed before and after UV(365 nm) irradiation and after visible (>500 nm)irradiation. Figure 11 shows the AFM images. Uponirradiation for more than 10 s with UV light, stepsappeared on the (100) surface. No step formationwas discerned during the irradiation for the initial

10 s but appeared after the induction period. Thestep height is 1.0 nm. The step disappeared byirradiation with visible light. When the irradiationtime was prolonged, the number of steps increasedand steps with height of 2.0 and 3.0 nm appeared.The height was always multiples of the minimumstep height of 1.0 nm, which corresponds to onemolecular layer. The morphological change wasreversible and correlated with the color change.This result indicates that the change of molecularshape is linked to macro-scale shape of materialsin the densely packed crystal. In densely packedcrystalline systems the strain energy generated bythe molecular shape change directly influences theshape of the bulk crystal. Based on this view weprepared small-size single crystals (10–100 µm) ofphotochromic diarylethenes and demonstrated thatthe bulk shape of crystals changes upon photo-irradiation.11)

Figure 12 shows the color and shape changes of1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopen-tene (23a) and 1,2-bis(5-methyl-2-phenyl-4-thia-

(a)

UV

(b)

Vis.

(c)

CH3

F2F2 F2

CH3H3C

SSH3C

22a

Fig. 11. AFM images of the (100) surface of single crystal 22a (a) before and (b) after irradiation with 365nm light and (c) afterirradiation with visible (6 > 500nm) light.

M. IRIE [Vol. 86,480

zolyl)perfluorocyclopentene (24a) crystals upon irra-diation with UV (365 nm) and visible (>500 nm)light. The color of the two crystals turned blueand violet, respectively. The colors are stable inthe dark, but disappeared on irradiation withvisible light. UV irradiation changed the cornerangle of the single crystal 23 from 88° and 92° to82° and 98° and hence the shape from a square to alozenge. On the other hand, the crystal 24 exhibitedreversible contraction/expansion in length as muchas 5–7% upon alternate irradiation with UV andvisible light.

A rod-like crystal was also prepared from 24a bysublimation. The rod-like crystal mounted at one endon a glass surface bent upon irradiation with UVlight. The rod bent towards the direction of theincident light. This effect is due to the gradient in theextent of photoisomerization caused by the highabsorbance of the crystal, so that the shrinkage of theirradiated part of the crystal causes bending just as ina bimetal. The bent rod-like crystal became straightupon irradiation with visible light. Such reversible

bending could be repeated over 80 cycles, as shown inFig. 13.

The crystal rod (about 200 micrometers longand 5 micrometers diameter) was used to launch atiny silica-particle (diameter 80 micrometers), asshown in Fig. 14. The rapid response and relativelylarge elastic modulus of the crystal rod enabled to hitthe silica-particle as if it were a tennis ball.

It is a long-standing ambition for chemists toconstruct molecular systems which exhibit mechan-ical movement based on geometrical structuralchanges of individual molecules and have the sys-tems perform mechanical work.36),37) The presentresult indicates that densely packed crystals can beused to link the molecular scale events to macro-scalephenomena. The shape change of component mole-cules in crystal upon photoirradiation is directlylinked to the macro-scale shape change of the crystal.The photoinduced change of molecular shape at themolecular level is successfully linked to the macro-scale mechanical motion of the crystal and resulted ina net task to launch a tiny silica-particle.

Acknowledgments

The author is grateful for the support from theMinistry of Education, Culture, Sports, Science andTechnology (Grant-in-Aid for Scientific Research inPriority Area “New Frontiers in Photochromism(471)” No. 19050008). The author also wishes toexpress his sincere thanks to colleagues and studentsfor their extraordinary efforts to accomplish theresearch.

(a)

Et

F2

F2 F2

Et

F2

F2 F2λ = 365 nm

λ > 500 nm

23a 23b

S SEt S SEt

(b)

N

S S

NCH3

H3C

F2

F2 F2

N

S S

NCH3

H3C

F2

F2 F2λ = 365 nm

λ > 500 nm

24a 24b

Fig. 12. Photoinduced color and shape changes of single crystalsof (a) 23a and (b) 24a upon irradiation with UV and visiblelight.

Fig. 13. Reversible bending a crystal-rod of 24a upon irradiationwith UV and visible light.

Photochromism of diarylethene molecules and crystalsNo. 5] 481

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(2000) X-ray crystallographic study on single-crystalline photochromism of bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene. J. Am. Chem. Soc.122, 1589–1592.

6) Yamada, T., Kobatake, S. and Irie, M. (2000) X-RayCrystallographic Study on Single-Crystalline Pho-tochromism of 1,2-Bis(2,5-dimethyl-3-thienyl)per-fluorocyclopentene. Bull. Chem. Soc. Jpn. 73,2179–2184.

7) Irie, M. (1994) Photo-reactive Materials for Ultra-high Density Optical Memory. Elsevier, Amster-dam.

8) Irie, M. (2000) Photochromism: memories andswitches-introduction. Chem. Rev. 100, 1683–1890.

9) Irie, M. and Matsuda, K. (2001) Memories. InElectron Transfer in Chemistry (ed. Balzani, V.).Springer, Berlin. pp. 215–242.

10) Irie, M. (2001) Photoswitchable molecular systemsbased on diarylethenes. In Molecular Switches (ed.Feringa, B. L.). Wiley, Weinheim, pp. 37–62.

11) Kobatake, S., Takami, S., Muto, H., Ishikawa, T.and Irie, M. (2007) Rapid and reversible shapechanges of molecular crystals on photoirradiation.Nature 466, 778–781.

12) Al-Kaysi, R.O., Müller, A.M. and Bardeen, C.J.(2006) Photochemically driven shape changes ofcrystalline organic nanorods. J. Am. Chem. Soc.128, 15938–15939.

13) Al-Kaysi, R.O. and Bardeen, C.J. (2007) Reversiblephotoinduced shape changes of crystallineorganic nanorods. Adv. Mater. 19, 1276–1280.

14) Uchida, K., Sukata, S., Matsuzawa, Y., Akazawa,M., de Jong, J.J.D., Katsonis, N. et al. (2008)Photoresponsive rolling and bending of thincrystals of chiral diarylethenes. Chem. Commun.(Camb.), 326–328.

15) Koshima, H., Ojima, N. and Uchimoto, H. (2009)Mechanical motion of azobenzene crystals uponphotoirradiation. J. Am. Chem. Soc. 131, 6890–6891.

16) Irie, M. and Uchida, K. (1998) Synthesis andproperties of photochromic diarylethenes withheterocyclic aryl groups. Bull. Chem. Soc. Jpn.71, 985–996.

17) Irie, M. (2000) Diarylethenes for memories andswitches. Chem. Rev. 100, 1685–1716.

18) Kobatake, S. and Irie, M. (2004) Single-crystallinephotochromism of diarylethenes. Bull. Chem. Soc.Jpn. 77, 195–210.

19) Morimoto, M. and Irie, M. (2005) Photochromism ofdiarylethene single crystals: crystal structures andphotochromic performance. Chem. Commun.(Camb.), 3895–3905.

20) Darcy, P.J., Heller, H.G., Strydom, P.J. andWhittall, J. (1981) Photochromic heterocyclicfulgides. Part 2. Electrocyclic reactions of (E)-,-2,5-dimethyl-3-furylethylidene (alkyl-substitutedmethylene) succinic anhydrides. J. Chem. Soc.,Perkin Trans. 1, 202–205.

21) Heller, H.G. and Langan, J.R. (1981) Photochromic

Fig. 14. A shot of a silica micro-particle by crystal-rod of 24aupon irradiation with 365nm light pulse.

M. IRIE [Vol. 86,482

heterocyclic fulgides. Part 3. The use of (E)-,-(2,5-dimethyl-3-furylethylidene)(isopropylidene) suc-cinic anhydride as a simple convenient chemicalactinometer. J. Chem. Soc., Perkin Trans. 2, 341–343.

22) Irie, M. and Mohri, M. (1988) Thermally irreversiblephotochromic systems. Reversible photocyclizationof diarylethene derivatives. J. Org. Chem. 53, 803–805.

23) Hanazawa, M., Sumiya, R., Horikawa, Y. and Irie,M. (1992) Thermally irreversible photochromicsystems. Reversible photocyclization of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocycloalkenederivatives. Chem. Commun. (Camb.), 206–207.

24) Nakamura, S. and Irie, M. (1988) Thermallyirreversible photochromic systems. A theoreticalstudy. J. Org. Chem. 53, 6136–6138.

25) Guillaumont, D., Kobayashi, T., Kanda, K.,Miyasaka, H., Uchida, K., Kobatake, S. et al.(2002) An ab initio MO study of the photochromicreaction of dithienylethenes. J. Phys. Chem. A106, 7222–7227.

26) Asano, Y., Murakami, A., Kobayashi, T., Goldberg,A., Guillaumont, D., Yabushita, S. et al. (2004)Theoretical study on the photochromic cyclo-reversion reactions of dithienylethenes; on the Roleof the Conical Intersections. J. Am. Chem. Soc.126, 12112–12120.

27) Nakamura, S., Kobayashi, T., Takata, A., Uchida,K., Asano, A., Murakami, A. et al. (2007) Quan-tum yields and potential energy surfaces: a theo-retical study. J. Phys. Org. Chem. 20, 821–829.

28) Shibata, K., Kobatake, S. and Irie, M. (2001)Extraordinarily low cycloreversion quantum yieldsof photochromic diarylethenes with methoxy sub-stituents. Chem. Lett., 618–619.

29) Higashiguchi, K., Matsuda, K., Tanifuji, N. and Irie,M. (2005) Full-color photochromism of a fused

dithienylethene trimer. J. Am. Chem. Soc. 127,8922–8923.

30) Fukaminato, T., Kobatake, S., Kawai, T. and Irie,M. (2001) Three-dimensional erasable opticalmemory using a photochromic diarylethene singlecrystal as the recording medium. Proc. Jpn. Acad.,Ser. B 77, 30–35.

31) Morimoto, M., Kobatake, S. and Irie, M. (2003)Multicolor photochromism of two- and three-component diarylethene crystals. J. Am. Chem.Soc. 125, 11080–11087.

32) Kuroki, L., Takami, S., Shibata, K. and Irie, M.(2005) Photochromism of single crystals composedof dioxazolylethene and dithiazolylethene. Chem.Commun. (Camb.), 6005–6007.

33) Takami, S., Kuroki, L. and Irie, M. (2007) Photo-chromism of mixed crystals containing bisthienyl-,bisthiazolyl-, and bisoxazolylethene derivatives. J.Am. Chem. Soc. 129, 7319–7326.

34) Kobatake, S., Yamada, M., Yamada, T. and Irie, M.(1999) Photochromism of 1,2-bis(2-methyl-6-nitro-1-benzothiophen-3-yl)-perfluorocyclopentenein a single-crystalline phase: dichroism of theclosed-ring form isomer. J. Am. Chem. Soc. 121,8450–8456.

35) Irie, M., Kobatake, S. and Horichi, M. (2001)Reversible surface morphology changes of a photo-chromic diarylethene single crystal by photoirra-diation. Science 291, 1769–1772.

36) Merian, E. (1966) Steric factors influencing thedyeing of hydrophobic fibers. Text. Res. J. 36,612–618.

37) Irie, M. (2008) Photochromism and molecularmechanical devices. Bull. Chem. Soc. Jpn. 81,917–926.

(Received Dec. 18, 2009; accepted Feb. 24, 2010)

Profile

Masahiro Irie received his BS (1966) and MS (1968) degrees from Kyoto Universityand his PhD (1973) degree in radiation chemistry from Osaka University. In 1968, hejoined the Faculty of Engineering, Hokkaido University, as a research associate andstarted his research on photochemistry. In 1973, he moved to Osaka University and waspromoted to associate professor at the Institute of Scientific and Industrial Research in1978. In 1988, he was appointed professor at the Institute of Advanced Material Study,Kyushu University; in 1996, he was reappointed professor of chemistry at the Faculty ofEngineering. In 2007, he moved to Tokyo and is now professor at Rikkyo University. Hehas been conducting research on photochromic molecular systems for the last 30 years. Inthe middle of the eighties he invented a new class of photochromic molecules, thediarylethenes, which undergo thermally irreversible and fatigue resistant photochromic reactions. At present, thesederivatives are widely used in functional molecular materials as key elements of optical switches. He has receivedthe following awards: Award of the Society of Polymer Science, Japan (1988), Award of the JapanesePhotochemical Association (1993), Vinci d’Excellence (1995), Docteur Honoris Causa of University of Bordeaux I(2003), the Chemical Society of Japan Award (2004), Mukai Award (2007), The Purple Ribbon Medal (2007) andTheodor-Förster-Preis (2008).

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