fabrication of patterned images in photochromic organic microfibers

Communication

1010

Fabrication of Patterned Images inPhotochromic Organic Microfibers

Jung Lee, Chan-Woo Lee, Jong-Man Kim*

Photochromic spiropyran molecules were embedded in electrospun polymer microfibers.Electrospinning of a clear viscous chloroform solution containing a spiropyran and a matrixpolymer, such as polystyrene and polyethylene oxide, affords polymer microfibers that arephotoswitchable. Photomasked, 365nm UV irradiation ofthe microfibers results in the generation of patterned colorimages owing to the selective transformation of the spir-opyran molecules from their ring-closed SP to ring-openedMC form. The UV-irradiated areas display brilliant redfluorescence, which changes to green fluorescence uponprolonged irradiation.

Introduction

Owing to the light-driven molecular switching phenom-

enon, photochromic compounds have been extensively

investigated as key materials in optical memory, switching,

and sensor devices.[1–18] Photochromic materials, such as

azobenzene, spiropyran, phenoxyquinone, and bisthieny-

lethene derivatives, undergo reversible structural changes

when irradiated with UV light, leading to interconversions

of two distinct isomeric forms. The reactions involved in the

photochromic processes include trans-cis isomerization

(azobenzenes), ring-opening (spiropyrans), phenyl migra-

tion (phenoxyquinones), and ring-closure (bisthieny-

lethenes). In each case, the reverse process takes place

upon irradiation with visible light.

Among photochromic compounds, spiropyran deriva-

tives have gained great attention as a result of interesting

properties that include facile synthesis, distinct chromic

transition, and fluorescence of one isomeric form in certain

J. Lee, J.-M. KimDepartment of Chemical Engineering, Institute of Nanoscienceand Technology, Hanyang University, Seoul 133-791, KoreaE-mail: [email protected]. LeeDepartment of Chemistry, University of Ulsan, Ulsan, 680-749,Korea

Macromol. Rapid Commun. 2010, 31, 1010–1014

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

environments.[19–23] In general, spiropyrans can exist in

two isomeric states, including a ring-closed, less-polar spiro

form (SP) and a ring-opened, more polar merocyanine form

(MC) (Scheme 1). A number of investigations have explored

the photochromic properties of spiropyran derivatives in

various environments, such as organic solvents,[24] aqueous

solutions,[25] ionic liquids,[26] as well as polymer[27] and

organogel matrices.[28] Very recently, spiropyran-

embedded organic nanofibers have been described.[29]

Patterned functional images have great utility in

applications of displays, optical memory devices, molecular

switches, and sensors. Over the past decade, our efforts have

focused on the development of techniques to produce

patterned images in polymer matrices based on the so

called ‘‘precursor approach’’.[30] In this approach,

precursor molecules undergo photoinduced structural

transformations to produce products that have different

absorption and emission properties than the original

DOI: 10.1002/marc.201000019

Fabrication of Patterned Images in Photochromic . . .

Scheme 1. Photochromism of a spiropyran.

Figure 1. A schematic outlining of the strategy employed for the

substrates. In this way, patterned functional images are

generated by selective irradiation through a photomask. By

using this method, we have been able to fabricate a wide

variety of patterned color and/or fluorescence images in

polymer films.[31–33] More recently, we have shown that

patterned color/fluorescence images can be readily formed

in electrospun polymer fibers.[34] Accordingly, photo-

masked UV irradiation of precursor molecule-embedded

electrospun nano/microfibers leads to formation of pat-

terned images. In our continuing studies in this area, we

questioned whether it would be possible to produce

patterned images by UV irradiation of a spiropyran,

encapsulated in electrospun fibers. If successful, polymer

fibers containing spiropyran derivatives would participate

in a write-and-erase role through sequential UV and visible

irradiation.
fabrication of patterned color images in electrospun polymerfibers.
Experimental Part

Materials and Instruments

The photochromic spiropyran 10 ,30-dihydro-10,30,30-trimethyl-6-

nitrospiro[2H-1-benzopyran-2,20-(2H)-indole], poly(ethylene

oxide) (PEO) (Mw ¼ 300 000 g�mol�1) and polystyrene (PS)

(Mw ¼280 000 g �mol�1) were purchased from Aldrich. SEM images

of the spiropyran-embedded fibers were obtained by using a JEOL

(JSM-6330F) FE-SEM instrument. The UV-irradiated PDA-contain-

ing fibers were coated with Pt for 30 s. SEM images were obtained at

an accelerating voltage of 15 kV. Optical images were observed

with an Olympus microscope (IX71).

Preparation of Spiropyran-Embedded Electrospun

Fibers

A typical procedure for fabrication of the spiropyran-encapsulated

electrospun polymer fiber is as follows: A chloroform solution

(4 mL), containing spiropyran (20 mg) and PS (1.48 g,

(Mw ¼280 000 g �mol�1), was pumped through a 25 G metal

syringe needle at a constant rate of 0.5 mL �h�1 by using a syringe

pump (KD Scientific model 200 series). Application of a high voltage

(15 kV) to the metal syringe needle allowed generation of

microfibers, which were collected on the surface of a grounded

aluminium plate (the working distance between the tip of needle

and the collector was 15 cm). Electrospinning was performed under

ambient conditions.



Generation of Color Patterns

A photomask was placed on a spiropyran-embedded electrospun

fibermat. Photoirradiation was performed using a typical handheld

laboratory 365 nm UV lamp.

Results and Discussion

The key strategy and procedure employed for generating

patterned images is schematically presented in Figure 1. A

viscous organic solution containing the spiropyran and the

matrix polymer is placed in a syringe. A high voltage (15 kV)

is then applied to the syringe needle causing ejection of a

charged polymer jet containing microfibers, which are

collected on the surface of a grounded aluminum plate

placed 15 cm from the needle tip. Photomasked UV

irradiation of the electrospun fiber generates the MC form

of the spiropyran in exposed areas, providing patterned

color images in the polymer fibers.

Figure 2 displays photographs of scanning electron

microscope (SEM) and optical microscope images of typical

electrospun fibers. Inspection of the SEM images (Figure 2A)

shows that the spiropyran-embedded fiber mat, obtained

by electrospinning of a chloroform solution containing

polystyrene (PS) (20 wt.-%) and spiropyran (0.3 wt.-%), is

composed of microfibers. Optical microscope images

recorded after electrospinning also demonstrate that the

www.mrc-journal.de 1011

J. Lee, C.-W. Lee, J.-M. Kim

Figure 3. Photographs of spiropyran-embedded electrospun poly-ethylene oxide (PEO) and polystyrene (PS) fiber mats before (A),after (B) photomasked UV irradiation, and after heat treatment ofthe UV-irradiated fiber mats (C).

Figure 2. SEM images of electrospun polystyrene fibers containingthe spiropyran compound (A). Optical (B,C) and fluorescence(D) microscopic images of the fibers obtained before (B) andafter (C,D) UV irradiation.

1012

process produces microfibers (Figure 2B). Interestingly, UV

irradiation of the spiropyran-embedded polymer fibers

yields blue-colored microfibers (Figure 2C), which are

indicative of successful generation of the ring-opened MC

form of the spiropyran molecules. Additional evidence for

the SP-to-MC transformation comes from the observation

that the UV-irradiated fibers display strong red fluorescence

(Figure 2D). It is well known that the fluorescence quantum

yield of the SP form is extremely low while in certain

environments the MC form emits relatively strong

fluorescence.[35] In general, the fluorescence quantum

yields of the ring-opened spiropyrans are low in solution

but are significantly enhanced when the MC forms are part

of solid aggregates or in matricies.[36] Consequently, the

fluorescence emission observed from the UV-irradiated

microfibers is not surprising. Importantly, no significant

morphological difference is seen between the SEM and

optical microscopic images of fiber mats recorded before

and after UV-irradiation (data not shown).

The feasibility of generating color patterns using the

photochromic spiropyran-embedded electrospun fiber

mats was explored next. The polymer fiber mats, obtained

in the manner described above, were irradiated with

365 nm UV light (1 mW � cm�2) through a photomask for

30 sec. Interestingly, colored images were produced only in

the UV-exposed areas, indicating that selective transforma-

tion of the SP form of the photochrmic molecule to the MC

form takes place in the polymer fibers (Figure 3). Different

color patterned images are obtained depending on which

matrix polymer is used. For example, purple-colored images

are generated when PEO serves as the matrix polymer. In

contrast, UV-irradiation of the spiropyran-embedded PS

fiber mat produces a blue-color pattern. In general, UV



irradiation of spiropyrans tends to yield purple-colored

materials in a polar media and blue-colored compounds in a

nonpolar environment.[37] Thus, the fact that PEO is more

polar than PS appears to be the cause of the differences in

color of the materials formed by UV irradiation. Impor-

tantly, the patterned color images are erasable by heat

treatment. Thus, placing the patterned fiber mats on a hot

plate at 120 8C causes immediate disappearance of the

images. Moreover, the patterned images can be regenerated

by photomasked UV irradiation. The write-erase-write

property of the PS fiber mats is found to be superior to that of

the PEO fiber mats. The spiropyran-embedded PS fiber mats

could be subjected to the write-erase cycles for more than

20 times without difficulty (Figure 4).

Intriguing changes in the fluorescence properties of the

spiropyran-embedded fibers take place upon prolonged UV-

irradiation. Figure 5 displays optical and fluorescence

microscope images before and after UV irradiation,

monitored with a green filter (excitation at 460–495 nm).

As described above, no fluorescence is seen from the fibers

before irradiation (Figure 5A, right). UV irradiation for 30 s

yields blue-colored fibers (Figure 5B, left), which emit

almost no fluorescence (Figure 5B, right). Of course, the

irradiated fibers shown in Figure 5B emit red-colored

fluorescence when observed with a red-filter (excitation

wavelength range of 530–550 nm, see Figure 2D). Surpris-

ingly, further UV-irradiation of the blue-colored mats for

15 min generates fibers that emit very bright green

DOI: 10.1002/marc.201000019

Fabrication of Patterned Images in Photochromic . . .

Figure 5. Optical and fluorescence microscope images of thespiropyran-embedded polystyrene fibers obtained before (A),after 30 s (B), and 15min (C) UV irradiation. The fluorescencemicroscopic images are obtained with a green filter (excitation at460–495nm).

Figure 4. Photographs of a spiropyran-embedded electrospunpolystyrene fiber mat after photomasking at 365 nm UV irradia-tion for 20 s (A, left) and after heat treatment for 10 s at 120 8C(A, right). The patterns displayed in B and C are obtained after 10(B) and 20 (C) writing and erasing cycles, respectively.



fluorescence (Figure 5C, right), along with simultaneous

bleaching of the blue color (Figure 5C, left).

We initially believed that the disappearance of the blue

color was associated with transformation of the ring-

opened MC form to the ring-closed SP form of the spiropyran

molecules in the fiber. However, this is not the case since the

SP form does not emit green fluorescence. In addition,

irradiation of the bleached fibers with 365 nm light does not

induce the MC-to-SP transition. Thus, it appears that the

spiropyran moieties in the polymer fiber undergo photo-

oxidation to produce green fluorescent species upon the

prolonged irradiation. Although the photooxidation pro-

duct(s) has not yet been identified, the observations made in

this study are interesting since they show that red (short

irradiation time) and green (long irradiation time) fluor-

escent microfibers can be readily generated by simply

controlling the time of UV-irradiation.

Conclusion

The effort described above has demonstrated that pat-

terned images can be generated by UV-irradiation of

photochromic spiropyran-embedded elctrospun microfi-

bers. Electrospinning of a chloroform solution containing a

spiropyran and a matrix polymer results in the generation

of microfibers that encapsulate the photochromic mole-

cules. UV irradiation of the spiropyran containing fibers

through a photomask affords patterned color images,

whose fluorescence properties are dependent on the matrix

polymer used. Finally, prolonged UV irradiation of the fibers

results in the production of green fluorescent microfibers,

which are likely the result of photooxidation of the

spiropyran moieties.

Acknowledgements: The authors gratefully thank the NationalResearch Foundation of Korea for financial support through theBasic Science Research Program (20090083161), the Center forNext Generation Dye-sensitized Solar Cells (2009-0063368),and the International Research & Development Program(K20901000006-09E0100-00610).

Received: January 7, 2010; Revised: January 29, 2010; Publishedonline: April 13, 2010; DOI: 10.1002/marc.201000019

Keywords: electrospinning; patterned Images; photochromism;spiropyran

[1] G. H. Brown, Photochromism, Wiley-Interscience, New York1971.

[2] Organo Photochromic and Thermochromic Compounds, J. C.Crano, R. J. Guglielmetti, Eds., Kluwer Academic/PlenumPublishers, New York 1999.

[3] Molecular Switches, B. I. Feringa, Ed., Wiley-VCH, Weinheim2001.

[4] G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 2000, 100,1741.

www.mrc-journal.de 1013

J. Lee, C.-W. Lee, J.-M. Kim

1014

[5] C. C. Corredor, Z.-L. Huang, K. D. Belfield, Adv. Mater. 2006, 18,2910.

[6] A. J. Myles, N. R. Branda, Adv. Funct. Mater. 2002, 12, 167.[7] F. M. Raymo, M. Tomasulo, Chem. Soc. Rev. 2005, 34, 327.[8] C. J. Barrett, J.-I. Mamiya, K. G. Yager, T. Ikeda, Soft Matter

2007, 3, 1249.[9] M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature

2002, 420, 759.[10] S.-J. Lim, B.-K. An, S. Y. Park, Macromolecules 2005, 38, 6236.[11] H. Cho, E. Kim, Macromolecules 2002, 35, 8684.[12] M. Bossi, V. Belov, S. Polyakova, S. W. Hell, Angew. Chem. Int.

Ed. 2006, 45, 7462.[13] T. A. Golovkova, D. V. Kozlov, D. C. Neckers, J. Org. Chem. 2005,

70, 5545.[14] S. Wang, W. Shen, Y. Feng, H. Tian, Chem. Commun. 2006,

1497.[15] E. J. Harbron, D. A. Vincente, D. H. Hadley, M. R. Imm, J. Phys.

Chem. A 2005, 109, 10846.[16] G. Jiang, S. Wang, W. Yuan, L. Jiang, Y. Song, H. Tian, D. Zhu,

Chem. Mater. 2006, 18, 235.[17] J. L. Bahr, G. Kodis, L. D. L. Garza, S. Lin, A. L. Moore, T. A. Moore,

D. Gust, J. Am. Chem. Soc. 2001, 123, 7124.[18] D. V. Kozlov, F. N. Castellano, J. Phys. Chem. A 2004, 108,

10619.[19] V. I. Minkin, Chem. Rev. 2004, 104, 2751.[20] K. Fukushima, A. J. Vandenbos, T. Fujiwara, Chem. Mater.

2007, 19, 644.[21] C. W. Lee, Y. H. Song, Y. Lee, K. S. Ryu, K.-W. Chi, Chem.

Commun. 2009, 6282.



[22] F. M. Raymo, S. Giordani, Org. Lett. 2001, 3, 1833.[23] I. L. Medintz, S. A. Trammell, H. Mattoussi, J. M. Mauro, J. Am.

Chem. Soc. 2004, 126, 30.[24] X. Guo, D. Zhang, Y. Zhou, D. Zhu, J. Org. Chem. 2003, 68, 5681.[25] Y. Shiraishi, K. Adachi, M. Itoh, T. Hirai, Org. Lett. 2009, 11,

3482.[26] S. Zhang, Q. Zhang, B. Ye, X. Li, X. Zhang, Y. Deng, J. Phys.

Chem. B 2009, 113, 6012.[27] S. Wu, J. Lu, F. Zeng, Y. Chen, Z. Tong, Macromolecule 2007, 40,

5060.[28] A. Shumburo, M. C. Biewer, Chem. Mater. 2002, 14, 3745.[29] F. D. Benedetto, E. Mele, A. Camposeo, A. Athanassiou, R.

Cingolani, D. Pisignano, Adv. Mater. 2008, 20, 314.[30] For a recent review in this area, see J.-M. Kim, Macromol.

Rapid Commun. 2007, 28, 1191 and references therein.[31] J.-M. Kim, T.-E. Chang, J.-H. Kang, K. H. Park, D.-K. Han, K.-D.

Ahn, Angew. Chem. Int. Ed. 2000, 39, 1780.[32] J. M. Kim, T. E. Chang, J.-H. Kang, D. K. Han, K.-D. Ahn, Adv.

Mater. 1999, 11, 1499.[33] J.-M. Kim, Y. B. Lee, S. K. Chae, D. J. Ahn, Adv. Funct. Mater.

2006, 16, 2103.[34] S. K. Chae, H. Park, J. Yoon, C. H. Lee, D. J. Ahn, J.-M. Kim, Adv.

Mater. 2007, 19, 521.[35] M.-Q. Zhu, L. Zhu, J. J. Han, W. Wu, J. K. Hurst, A. D. Q. Li, J. Am.

Chem. Soc. 2006, 128, 4303.[36] M. Matsumoto, T. Nakazawa, R. Azumi, H. Tachibana, Y.

Yamanaka, H. Sakai, M. Abe, J. Phys. Chem. B 2002, 106, 11487.[37] X. Song, J. Zhou, Y. Li, Y. Tang, J. Photochem. Photobiol. A:

Chem. 1995, 92, 99.

DOI: 10.1002/marc.201000019

fabrication of patterned images in photochromic organic microfibers

Documents