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Cite this:Phys.Chem.Chem.Phys.,

2014, 16, 13483

Photoconversion of 6,13-a-diketopentacene in thecrystalline phase†

Sadahiro Masuo,*ab Katsuki Tanaka,a Mariko Oea and Hiroko Yamadabc

The photoconversion of a crystalline-phase precursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione (PDK),

into pentacene was examined. Upon irradiation with 470 nm light, the photoconversion proceeded even in the

crystalline phase. Although the rate of the crystalline phase photoconversion was much slower than that in

solution, quantitative photoconversion was achieved upon lengthy photoirradiation. The photoconversion

of PDK was also confirmed at the single mm-sized crystal level by observing fluorescence images and

spectra of the PDK crystal using confocal microscopy. Destruction of the crystal was observed when

some of the PDK molecules in the crystal were photoconverted into pentacene. These results indicate

that the crystalline-phase photoconversion of PDK proceeded through the destruction of the crystal.

Quantitative photoconversion was achieved as a result.

Introduction

Organic semiconducting molecules are very attractive for thefabrication of organic thin-film transistors (OTFTs) for use invarious molecular electronics applications, such as electronicpaper and flexible displays.1–3 Because of its high carrier mobility aseither a single crystal4 or a thin film,5–8 pentacene is one of the mostpromising candidates for use as a p-type semiconductor and iscurrently used in organic electronics applications such as OTFTs5–8

and organic thin-film photovoltaic cells.9–12 Unfortunately, thelow solubility of pentacene in common organic solvents limitsthe preparation of pentacene devices to the use of the expensivevacuum deposition method, which is a significant obstacleto its widespread use. Thus, soluble precursors with thermallyor photochemically removable leaving groups have receivedincreasing attention.13–22 One such reaction is the photochemicalconversion of an a-diketone precursor into anthracene with theproduction of two CO molecules.23 Photoirradiation of diketonecompounds at the n–p* absorption band leads to the release of twomolecules of CO, with quantitative production of the resultingacenes. This reaction has recently been applied to the photo-chemical synthesis of pentacene and larger acenes.24–37 A photocon-vertible precursor, 6,13-dihydro-6,13-ethanopentacene-15,16-dione(PDK, Fig. 1), with a bicyclo[2.2.2]octa-2,3-dione frameworkwas recently proposed as a soluble acene photoprecursor.24–32

PDK is soluble in common organic solvents and can be convertedinto pentacene upon photoirradiation, with the release of two COmolecules (Fig. 1).24,25 This reaction is induced only by photo-irradiation, in contrast to the reactions of monoketone pentaceneprecursors.21,38 PDK was successfully converted to pentacene ina 1.4% quantum yield by photoirradiation in toluene under anAr atmosphere; nearly 100% of the PDK was converted uponsufficiently lengthy photoirradiation.25,28 Recently, the mechanismof photoconversion (i.e., the excited-state dynamics) of diketoneprecursors of acenes, including PDK, in solutions was investi-gated using time-resolved spectroscopy techniques and theore-tical calculations.39,40

The details of the photoconversion of diketone precursorsof acenes in the crystalline phase remain to be elucidated.A photoreaction involving large changes in geometry, structure, ormolecular volume shows a substantially different efficiency in thecrystalline phase than in solution due to the restriction of volumechanges in the crystalline phase. Among photoreactions in thecrystalline phase, pioneering works include the photodimerization

Fig. 1 Photoconversion of PDK and molecular structures of PDK andpentacene, based on single-crystal structural data.26 The dihedral anglebetween the two naphthalene planes is 1081 in PDK.

a Department of Chemistry, Kwansei Gakuin University, 2-1 Gakuen, Sanda,

Hyogo 669-1337, Japan. E-mail: masuo@kwansei.ac.jpb CREST, Japan Science and Technology Agency (JST), Chiyoda-ku 102-0075, Japanc Graduate School of Materials Science, Nara Institute of Science and Technology,

Ikoma 630-0192, Japan

† Electronic supplementary information (ESI) available: Movies of Fig. 4 and 6.See DOI: 10.1039/c4cp01607f

Received 14th April 2014,Accepted 19th May 2014

DOI: 10.1039/c4cp01607f

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of trans-cinnamic acid,41–43 photodimerization of single crystal–single crystal,44–47 and photopolymerization of poly(diacetyl-enes).48,49 One of the interesting phenomena induced byphotoreaction in the crystalline phase is the change in themorphology or the shape of the crystal.50,51 For example,reversible surface morphology changes were observed uponthe photoirradiation of single crystals of photochromic diaryl-ethenes.50 Rapid and reversible changes in the shape of singlecrystals upon photoirradiation have also been reported.51 Thesemacroscopic changes in the crystal shape are induced bymicroscopic structural changes in the molecules in the crystals.In this work, the crystalline phase photoconversion of PDK intopentacene was examined to gain a fundamental understandingof the process. The photoconversion of PDK also requires largestructural and molecular volume changes for the transformationof the bent PDK molecule, which has a dihedral angle of 1081,into the planar pentacene molecule (Fig. 1). These changes likelyinfluence the efficiency of the photoconversion. In this study, acrystalline film was prepared to compare the photoconversionefficiency in the crystalline phase and in solution. To investigatephotoconversion in the crystalline phase in more detail, thephotoconversion of single mm-sized crystals was also investigatedusing optical microscopy.

ExperimentalEfficiency of PDK photoconversion in solution and in thecrystalline phase

Photoconversion of PDK in solution was observed as follows.PDK was dissolved in toluene at a concentration of 1.5 � 10�4

M, and the solution was degassed by bubbling Ar for 20 min.The solution was then irradiated using a blue light-emittingdiode (LED) lamp (wavelength: 470 nm, intensity: 6.5 mW cm�2,Edmund Optics). To monitor the photoconversion, UV-Visabsorption spectra were measured at intervals of 1 min through-out the photoirradiation.

Photoconversion of PDK in the crystalline phase was observedas follows. PDK was first dissolved in chloroform, and the concen-tration of the solution was carefully determined from the absor-bance at 465 nm (molar extinction coefficient of PDK at 465 nm:1220 M�1 cm�1).25 A crystalline film was then prepared from thechloroform solution by drop-casting 1.0 mL of solution onto a cleancoverslip. The prepared crystals were ground until the individualcrystal size was less than 2 mm because the penetration depth(depth at which the intensity falls to 1/e of its initial intensity) oflight at 470 nm was estimated to be approximately 2 mm. Subse-quently, the ground crystalline film was irradiated for a given timeperiod with a blue LED (intensity: 6.5 mW cm�2 or 150 mW cm�2)under a N2 atmosphere. After each photoirradiation, the crystal-line film was dissolved in 3.0 mL of chloroform, and the absorp-tion spectrum was measured to estimate the concentration ofunconverted PDK. By comparing the concentrations of PDKbefore and after photoirradiation, the amount of photoconvertedPDK was determined for each irradiation time. The aboveprocedure was performed at room temperature in a dark room.

Photoconversion of PDK at the single lm-sized crystal level

Photoconversion of a single mm-sized PDK crystal was observedby monitoring the fluorescence images and spectra by confocaloptical microscopy during photoirradiation. The mm-sized PDKcrystals were placed on a clean coverslip, which was set on thestage of an inverted confocal microscope (IX71, Olympus). As alight source for both photoconversion and excitation for thefluorescence images and spectra, a 488 nm continuous-wavelaser (Cyan-MA5, Spectra-Physics) was focused onto the surfaceof the single mm-sized PDK crystal through an objective lens(100�, Olympus) under a N2 atmosphere. The fluorescence spectraand images were obtained using a spectrometer (SpectraPro2358,Acton Research Corporation) with a cooled CCD camera (PIXIS400B,Princeton Instruments) and video camera (Handycam, Sony),respectively. The fluorescence spectra were measured with anaccumulation time of 3 s. All measurements were performed atroom temperature in a dark room.

Results and discussionPhotoconversion efficiency of PDK in solution and in thecrystalline phase

Before investigating the photoconversion of PDK in the crystal-line phase, the photoconversion of PDK dissolved in toluenewas confirmed. The change in the UV-Vis absorption spectra ofPDK in toluene (1.5 � 10�4 M) during photoirradiation underan Ar atmosphere is shown in Fig. 2a. A blue LED (470 nm,6.5 mW cm�2) was used as the light source for photoconversion.

Fig. 2 Photoconversion of PDK in toluene under an Ar atmosphere upon6.5 mW cm�2 LED irradiation. (a) UV-Vis absorption spectra measured at0, 2, 4, 6, 8, 10, 20, and 30 min during photoirradiation. (b) Time profile ofthe photoconversion of PDK, as estimated from absorption spectra.

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Before photoirradiation, a broad peak at approximately 465 nmwas observed, which was assigned to the n–p* transition of thediketone moiety. As the absorbance of this peak decreasedgradually with irradiation, new absorption peaks at 577, 534,and 498 nm, which were attributed to pentacene, increased.The appearance of isosbestic points at 484, 432, and 365 nmindicated the quantitative photoconversion of PDK to pentacene.25

The amount of photoconverted PDK was estimated from thedecrease in absorbance at 465 nm (molar extinction coefficient:1220 M�1 cm�1).25 It was difficult to estimate the amount ofphotoconverted PDK from the absorbance of pentacenebecause pentacene precipitated out of the toluene as the photo-conversion proceeded. Although the absorption spectrum ofpentacene overlaps that of PDK at 465 nm, the concentration of PDKwas estimated by compensating for the contribution of pentacene at465 nm using eqn (1):25

Abs (PDK:465) = Abs (465) � Abs (577) � 0.128, (1)

in which Abs (PDK:465) is the absorbance of PDK at 465 nm, Abs(465) is the observed absorbance at 465 nm, Abs (577) is theobserved absorbance at 577 nm (attributed to pentacene), and0.128 is the ratio of the absorbance of pentacene at 465 nm and577 nm. A time profile of the amount of the photoconverted PDKis shown in Fig. 2b. The amount of photoconverted PDK reachednearly 100% upon sufficiently lengthy photoirradiation.

Photoconversion of PDK in the crystalline phase was alsoinvestigated by monitoring the change in the UV-Vis absorptionspectra. A thin, crystalline film of PDK was irradiated with150 mW cm�2 of LED light. The absorption spectra afterphotoirradiation of the chloroform solution in which the PDKcrystals were dissolved are shown in Fig. 3a. As the irradiationtime increased, an increase in the absorption of pentacene anda simultaneous decrease in the broad absorption of the diketoneat approximately 465 nm were observed. The absorption spectrameasured at 60 and 120 min during the photoirradiation showeda broad absorption band at approximately 678 nm. This broadband is characterized as the absorption of the aggregatedpentacene. Although pentacene is insoluble in chloroform, somepentacene molecules and aggregates generated in the PDKcrystals were most likely dispersed in the chloroform by dissolu-tion of the PDK. These absorption spectra confirm the photo-conversion of PDK into pentacene, even in the crystalline phase.To investigate the efficiency of the photoconversion in thecrystalline phase, a time profile of the amount of photoconvertedPDK estimated from the absorption spectra is shown in Fig. 3b.The time profile was measured by irradiation with 150 mW cm�2

of LED light. After 2 h of photoirradiation, the amount ofphotoconverted PDK became constant at 98 � 2.3%. This valuewas almost equivalent to that of the PDK in solution, indicatingthat nearly all PDK was photoconverted into pentacene, even inthe crystalline phase, upon a sufficient length of photoirradia-tion. The time profiles of the PDK conversion estimated both insolution and the crystalline phase are shown in Fig. 3c. The timeprofile of the photoconversion varied with light intensity. Tocompare the photoconversion rates, the time profiles for bothcases were measured using a light intensity of 6.5 mW cm�2,

which revealed that the photoconversion rate in the crystallinephase was much slower than that in solution. The estimatedphotoconversion rate in the crystalline phase was 65 times slowerthan that in solution, assuming that the amount of photo-converted PDK is linearly proportional to the photoirradiationtime. The reactivity of some one-photon absorption processes inthe crystalline phase is known to increase nonlinearly with lightintensity. For example, Masuhara et al. reported that the reactivityof the photochromic reaction of spirooxazine microcrystalsincreased nonlinearly with the excitation intensity.52,53 We didnot address such excitation power-dependent behavior in thepresent study. As shown in Fig. 1, the photoconversion of PDKis accompanied by a large structural change; PDK molecule, whichpossesses a dihedral angle of 1081 between two naphthaleneplanes, must adopt a planar structure to form pentacene. In thePDK crystal, PDK molecules stack with strong p–p overlappingbetween the naphthalenes of two PDK molecules. The distance

Fig. 3 Photoconversion of crystalline-phase PDK under a N2 atmosphere.(a) The absorption spectra of chloroform solutions of dissolved PDK crystalsafter 150 mW cm�2 LED photoirradiation for 0, 30, 60, and 120 min. (b) Timeprofile of the photoconversion of crystalline-phase PDK, as estimated fromthe absorption spectra. 150 mW cm�2 LED light was used. (c) Comparison ofthe time profiles in solution (black squares) and in the crystalline phase (whitetriangles). The photoconversions were performed using 6.5 mW cm�2 LEDlight in both solution and the crystalline phase.

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between naphthalenes is 3.596 Å.26,28 The much slower photo-conversion rate of the crystalline phase relative to that insolution is therefore attributed to the restriction of molecularstructural changes in the crystal lattice.

Photoconversion of PDK at the single lm-sized crystal level

To obtain more detailed information about the photoconversion ofPDK in the crystalline phase, the photoconversion of a single mm-sized crystal was investigated using confocal optical microscopy. Inthis experiment, a 488 nm CW laser was used as the light sourcefor both photoconversion and the fluorescence measurements.The laser beam was focused onto the surface of the PDK crystalthrough an objective lens. Optical microscope images acquiredduring the photoirradiation are shown in Fig. 4. Immediately afterphotoirradiation, a green emission was observed at the focal point(Fig. 4b). As irradiation continued, the emission color graduallychanged from yellow-green to red (Fig. 4c–e). A color change at thefocal point was also observed in a transmission image afterphotoirradiation. The coloration at the focal point after irradiationis shown in Fig. 4f and was compared to the image acquiredbefore photoirradiation (Fig. 4a). These changes in the emis-sion color and crystal color signaled the photoconversion ofPDK into pentacene. The fluorescence spectra of the single PDKcrystal obtained during photoirradiation are shown in Fig. 5.

In this experiment, the PDK crystal was irradiated with an80 W cm�2 laser beam to slow down the photoconversion andobserve the changes in the fluorescence spectra in more detail.Note that below 500 nm, the fluorescence spectra were cut offby a dichroic mirror that was inserted into the microscope toreflect the laser beam. Immediately after photoirradiation, abroad peak with vibronic structures was observed in the range from500 to 600 nm and attributed to PDK. After 30 s of photoirradiation,the PDK signal disappeared with the simultaneous appearance of afluorescence spectrum with peaks at 605 and 647 nm. Thisspectrum was assigned to the isolated pentacene. The intensity ofthe spectrum increased with irradiation. After 25 min of photo-irradiation, a new fluorescence peak at 690 nm, assigned to thepentacene aggregate, appeared. With further irradiation, theintensity attributed to the isolated pentacene decreased and thatof the pentacene aggregate increased. The onset of fluorescenceat 690 nm indicated the coalescence of neighboring pentacenemolecules into an aggregate, that is, PDK was efficiently photo-converted into pentacene. The results indicated that PDK under-went partial conversion. Increasing the duration or intensity ofirradiation to increase the conversion of PDK resulted in thedestruction of the PDK crystal.

Fig. 6 shows microscope images of the destruction of thesingle crystal during photoirradiation. A CW laser was focusedonto the single crystal located in the center of the images. Afterphotoirradiation, yellow-green-colored fluorescence assigned toPDK was observed from the crystal (Fig. 6b). The color of thefluorescence changed to orange as irradiation continued (Fig. 6c).This color change was attributed to the photoconversion from PDKinto pentacene, as mentioned above. After further photoirradiation,destruction of the single crystal was observed (Fig. 6d). Observationof many single crystals consistently showed this fluorescence colorchange and the subsequent destruction of the crystal. The timerequired for crystal destruction depended on the laser intensity.Quantitative data were not obtained because of the time variationdepending on the size and shape of the crystals.

The removal of materials by irradiation with a high intensitylaser beam is known as laser ablation. Laser ablation cangenerally be understood as the decomposition, vaporization,

Fig. 4 Optical microscope images of a single mm-sized PDK crystalduring photoirradiation. A 488 nm CW laser was used as the light sourceand focused on the surface of the crystal. The laser intensity at the focalpoint was 46 kW cm�2. (a) and (f) Transmission images of the PDK crystalbefore and after photoirradiation, respectively. (b)–(e) Fluorescenceimages acquired during photoirradiation; images (b–e) were recorded at0, 6, 13, and 24 s during photoirradiation, respectively. The scale bar inimage (a) represents 10 mm.

Fig. 5 Fluorescence spectra of a single PDK crystal during photoirradia-tion. The laser intensity at the focal point was 80 W cm�2. The spectrawere measured at 0, 0.5, 1, 25, and 150 min during photoirradiation. Eachfluorescence spectrum was acquired with a 5 s integration time.

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or shattering of materials by absorbed laser energy. However, theobserved destruction of the crystal in this work cannot beattributed to laser ablation because the photoconversion of PDKinto pentacene was observed before the destruction of the crystal,i.e., the absorbed laser energy was consumed for the photoconver-sion. The mechanism of destruction may involve strain on thecrystal structure due to photoconversion. As mentioned above, thephotoconversion of PDK is accompanied by a structural change inthe molecule, i.e., the transition of the bent structure of PDK tothe planar pentacene structure. This structural change reducesthe molecular volume. Therefore, the destruction of the crystalmay occur due to the strain arising from the change in themolecular volume of the crystal. Another possible mechanism ofdestruction is the generation of CO gas during photoconversion,which would increase the internal pressure of the crystal, resultingin crystal destruction at sufficiently high pressures. The mecha-nism of destruction might be further investigated by observingthe correlation between crystal destruction and crystal structureusing a variety of PDK derivatives with different crystal struc-tures. These results indicate that the photoconversion of PDK inthe crystalline phase is accompanied by crystal destruction.

Conclusions

Photoconversion of crystalline PDK into pentacene was inves-tigated using spectroscopy techniques. Photoconversion of athin crystalline film revealed that PDK could be quantitativelyphotoconverted into pentacene upon sufficiently lengthy photo-irradiation, even in the crystalline phase. However, the photo-conversion rate in the crystalline phase was much slower thanthat in solution. At the single mm-sized crystal level, the

photoconversion of PDK was also confirmed by the fluores-cence spectra. Importantly, crystal destruction was inducedwhen some of the PDK molecules in the single crystal werephotoconverted into pentacene upon lengthy photoirradiationor photoirradiation at high intensity. This crystal destruction islikely due to internal strain resulting from the change inmolecular volume during the photoconversion. Our results alsoindicate that the photoconversion of PDK in the crystallinephase is accompanied by a morphological change, i.e., nearly100% of the PDK in the crystalline phase is photoconverted intopentacene, accompanied by the destruction of the crystals. Thecrystal is likely broken down to the final size that can stand thestrain; thus, if a crystal could be prepared at this final size, itwould not be destroyed during photoirradiation. The resultsobtained in the present work provide insight into photoreactionsin the crystalline phase and the fabrication of organic devicesusing photoconvertible precursors.

Acknowledgements

This work was partly supported by CREST from the JapanScience and Technology Agency (JST) and by Individual SpecialResearch A from Kwansei Gakuin University.

Notes and references

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Fig. 6 Optical microscope images of the destruction of a single mm-sizedPDK crystal during photoirradiation. A 488 nm CW laser was focused onto thesurface of the crystal. The laser intensity at the focal point was 100 kW cm�2.(a) Transmission image of the PDK crystal before photoirradiation. (b)–(d)Fluorescence images acquired during the photoirradiation; images (b–d) wererecorded at 5, 30, and 60 s during photoirradiation, respectively. A scale bar inimage (a) represents 15 mm.

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