multicolor photochromism of polymolybdate-citric acid composite films

Chinese Journal of Chemistry, 2005, 23, 1037—1041 Full Paper

* E-mail: [email protected]; Tel.: 0086-10-82616517; Fax: 0086-10-82616517; † Present Address: College of Chemistry, Jilin University, Changchun

130023, China Received October 25, 2004; revised March 25, 2005; accepted April 5, 2005. Project supported by the National Natural Science Foundation of China (Nos. 50221201, 90301010, 20373077 and 20471062), the Chinese Academy

of Sciences and the National Research Fund for Fundamental Key Projects No. 973.

© 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Multicolor Photochromism of Polymolybdate-Citric Acid Composite Films

WANG, Jing(王静) ZHANG, Guang-Jin(张光晋) YANG, Wen-Sheng†(杨文胜) YAO, Jian-Nian*(姚建年)

Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

Photochromic polymolybdate-citric acid composite films were fabricated. It was found that after UV irradiation the composite films with different molar ratios of organic/inorganic components exhibited different colors. The UV-irradiated films showed dark blue, dark khaki and light sea green colors when the ratios were 1.0, 0.3 and 0.2, respectively. It was identified by Raman spectra that the polymolybdate species formed in the composite films after UV irradiation were sensitive to the ratios of the organic/inorganic components, thus resulting in the different colors of irradiated films. Citric acid played an important role during the photochromic process. Under UV light irradiation, it served as hole scavenger that suppresses the recombination of photogenerated electrons and holes to make the polymolybdates show UV light photochromism.

Keywords polymolybdate, composite, photochromism

Introduction

Photochromic materials, which exhibit variation in color upon irradiation with light, have attracted increas-ing attention due to their potential applications to optical memories and optical switches.1 Polyoxometalates, which can trap electrons at appropriate metal sites (usu-ally octahedral MoO6 sites) upon photoexcitation of O→Mo (LMCT, ligand-to-metal charge transfer) band, have been considered as one of the most promising can-didates for practical applications.2,3 The colors of the UV irradiated polyoxometalates are related with the extent of delocalization of d1 electrons in polyoxometa-late lattices.3 For example, in [NH3Pr-i]6[Mo8O26- (OH)2]•2H2O the d1 electron is localized within almost a single edge-shared MoO6 octahedral lattice, which re-sults in the reddish brown color. While in the Keggin framework of polyoxometalate, the electron is delocal-ized over corner-shared MO6 octahedra, which leads to a blue coloration.4-6 Usually these polyoxometalate photochromic materials suffer from the shortcoming of single UV induced colorization, i.e., from colorless (or white) to blue (or brown) upon irradiation. Here, we report the preparation of polyoxomolybdate composite films by a simple approach from a colloidal solution containing ammonium molybdate and citric acid. The composite films show multicolor photochromism being dependent on the molar ratios of the citric acid to mo-lybdenum in the colloidal solution. Such a multicolor

photochromism of polyoxometalate will be meaningful for the development of multifrequency optical memories and displays.

Experimental

All the chemicals were purchased from Aldrich and used without further purification. The water used was purified by Milli-Q gradient system (Millipore, MA, USA) to reach an electrical resistivity of 18.2 MΩ•cm－1. The poly(vinyl alcohol) (PVA) was used as matrix for the preparation of polymolybdate/citric acid films. The PVA (average polymerization degree 1750±50) was dissolved in water by heating for 3 h at 80 . Ammo-nium molybdate (0.054 mol•L－1) was mixed with ap-propriate amount of citric acid (0.38 mol•L－1) to obtain the composite solution with defined molar ratio [C]/[Mo]＝[citric acid]/[molybdenum]. Then the composite solu-tion was mixed with equal volume of PVA solution (5.6 wt%) to form a homogeneous viscous solution. The composite films were prepared by casting the solution onto quartz or glass substrates. All of the films annealed at 50 were colorless and exhibited good homogene-ity and transparency with excellent adherence to the substrates. The thickness of the composite films was approximately 1 µm. In our experiment, three films were prepared from the composite solution with differ-ent molar ratios (1.0, 0.3 and 0.2), which were denoted as film A, B and C, respectively. XRD experiments

1038 Chin. J. Chem., 2005, Vol. 23, No. 8 WANG et al.


found that the films were amorphous. The thicknesses of the films were measured with a

surface profile measuring system (Detake 3 Model JGP-560). UV-vis absorption spectra were recorded on a Shimadzu UV-1601 PC spectrophotometer. Raman spectra were obtained by a Ranishaw-2000 microprobe spectrometer (514 nm, Ar＋ ion laser). Low laser power (5 mW) was applied to avoid the decomposition of samples due to local heating effect. FT-infrared meas-urements were taken on an FTS1665 FT-infrared spec-trometer. The X-ray photoelectron spectra (XPS) were recorded on a USW HA150 photoelectron spectrometer at BSRF using monochromated Al-k (1486.6 eV) radia-tion. EPR spectra were recorded at room temperature with a Bruker ESP 300E X-band spectrometer. Photo-chromic experiments were performed in ambient condi-tions with relative humidity 50%—60% using a 500- watt high pressure Hg lamp as the light source.

Results and discussion

As shown in Figure 1 (Figure 1O) and Figure 2 (Figure 2, curve o), all of the prepared original films are colorless and show no absorbance in visible region. Under UV irradiation, the prepared transparent films with different molar ratios turn different colors. For film A with molar ratio of 1.0, after UV irradiation the film exhibits dark blue color (Figure 1A). The absorption spectrum of the colored film A displays a strong broad symmetric band in the region of 500—900 nm (λmax＝670 nm) (Figure 2, curve a). Actually, when the molar ratio is above 1.0, the films exhibit similar char-acteristics. For film B with molar ratio of 0.3, dark khaki color was achieved as shown in Figure 1B. Cor-respondingly, an absorption band in the region of 400—550 nm was observed (Figure 2, curve b) for the colored film B. When the molar ratio was between those of Film A and Film B, a transition color (cornflower blue) was found for the colored film. As for film C with molar ratio of 0.2, the irradiated film shows light sea green color (Figure 1C) with an absorption band cen-tered at 738 nm, accompanied by a shoulder at 614 nm, and also a band in the region of 400—550 nm (Figure 2, curve c). It is well known that the absorptions in the visible region are induced by intervalence charge trans-fer (IVCT, Mo5+

↔Mo6+) and d-d transition of the re-duced polymolybdate species.7,8 So the different shapes and positions of the absorption bands suggest that the polymolybdate species formed under UV irradiation should be different in the films with different colors.

Raman analyses were performed to determine the formed polymolybdate species in the colored films. Raman spectra of all the original films show similar vibrational bands at 941 and 885 cm－1 (Figure 3A),9

which has been attributed to the terminal Mo＝O stretching mode and bridged Mo-O-Mo vibrational mode of 6

7 24Mo O － , respectively.10 This indicates that before UV light irradiation the polymolybdates of the

Figure 1 Color changes of films before and after UV light irra-diation: the original film before UV light irradiation (O) and films with molar ratio [C]/[Mo] of 1.0 (A), 0.3 (B) and 0.2 (C) after UV light irradiation for 30 s.

Figure 2 Spectral changes of films before and after UV light irradiation: the original film before UV light irradiation (o), and films with molar ratio [C]/[Mo] of 1.0 (a), 0.3 (b) and 0.2 (c) after UV light irradiation for 30 s.

three films are all 67 24Mo O － species. After UV light

irradiation the Raman spectra of the colored films be-came different from each other. For film A with [C]/ [Mo]＝1.0, after UV light irradiation the terminal Mo＝O stretching band shifted from 941 to 971 cm－1 (Figure 3B, curve a), indicating that new species was formed after light irradiation in film A. The Raman spectra of the film B with [C]/[Mo]＝0.3 remained unchanged after UV light irradiation (Figure 3B, curve b). A new band at 968 cm－1 became visible for film C with [C]/[Mo]＝0.2 after UV light irradiation (Figure 3B,

curve c), indicating that parts of the 67 24Mo O － speci-

es were converted into other species in film C after light irradiation. It is known that the frequency of the Mo＝O stretching vibration band shifts to higher wavenumber as the degree of polymerization of polymolybdate is

Photochromism Chin. J. Chem., 2005 Vol. 23 No. 8 1039


increased.11 Thus it is deduced that the formed species in the three colored films after UV irradiation have dif-ferent degree of polymerization. For film B, because of the similarity of the Raman spectrum with that of

67 24Mo O － (Figure 3B, curve b), the formed species in

film B after UV irradiation should be reduced Mo7 spe-cies. For film C, the Raman data of the new formed spe-cies (Figure 3B, curve c) are quite similar to those of pre-reported two-electron reduced species of

1014 46Mo O － ,3 indicating that parts of 6

7 24Mo O － species have been converted into 10

14 46Mo O － after UV light irradiation in film C. For film A, although the exact ag-gregation mode of the new formed species could not be made sure, it should be a larger polymolybdate cluster since the Mo＝O stretching band appeared at a larger wave number in the Raman spectrum (Figure 3B, curve a).

Figure 3 Raman spectra of the films before (A) and after (B) UV light irradiation films with molar ratio [C]/[Mo] of 1.0 (a), 0.3 (b) and 0.2 (c).

It is known that organic polyhydroxy molecules in-teract with polymolybdate ions through hydrogen bonding or coordination bonding.12 FTIR spectral analyses were carried out to illustrate the complex formed in the composite films. The frequency of C＝O stretching vibration peak is sensitive to bonding mode between citric acid molecules and polymolybdates. The peak appears at higher wavenumber for hydrogen

bonding, whereas at lower wavenumber for coordina-tion bonding. For film A with [C]/[Mo]＝1.0, the C＝O stretching vibration peaks attributed to νas(COOH) and νas(COO－) peaks,13-15 appeared at 1720 and 1633 cm－1 respectively (Figure 4, curve a), indicating that in film A the citric acid molecules bond to the polymolybdates through hydrogen bonding. For film B with [C]/[Mo]＝0.3, appearance of a new peak at 1586 cm－1 ascribed to νas(COOMo) peak (Figure 4, curve b) suggested that the citric acid molecules partly coordinate to the polymo-lybdates. For film C with [C]/[Mo]＝0.2, the peak at 1720 cm－1 almost disappeared, while peak at 1586 cm－1 became obvious (Figure 4, curve c), indicating that the citric acid molecules are coordinated with the polymo-lybdates at this time.

Figure 4 FTIR spectra of the films with molar ratio [C]/[Mo] of 1.0 (a), 0.3 (b) and 0.2 (c).

XPS analyses were carried out to determine the mo-lar ratios of Mo5＋ to Mo6＋ in the colored films. Figure 5 presents the Mo(3d) core level XPS spectra of the three films. After curve fitting, the two peaks at 232.3 and 235.4 eV could be assigned to the core levels of Mo6＋(3d5/2) and Mo6＋(3d3/2), respectively. While an-other two peaks located at 231.3 and 234.5 eV were as-cribed to Mo5＋(3d5/2) and Mo5＋(3d3/2), respectively.16 The molar ratios of Mo5＋ to Mo6＋ in the colored films were determined to be 0.63 for film A (Figure 5a), 0.35 for film B (Figure 5b) and 0.22 for film C (Figure 5c) by the peak area ratios of Mo5＋(3d5/2) to Mo6＋(3d5/2). It is seen that, with decreasing molar ratio of citric acid to molybdenum, the quantity of light reduced Mo5＋ in the colored film was decreased.

For such an organic/inorganic composite system, the organic component (for example, alkylammonium) usu-ally served as the electron donor during photochromic process.17,18 This could be proved by EPR analysis in our system. EPR spectrum of film A reveals two EPR signals at g＝1.9503 and 2.1314 (Figure 6, curve a). The line at g＝1.942 (A＝51 G) ascribed to paramagnetic Mo5＋ species19 indicates that the unpaired electron is delocalized over the polymolybdate cluster in the col-ored film A.20 The signal of g＝2.144 was attributed to

1040 Chin. J. Chem., 2005, Vol. 23, No. 8 WANG et al.


Figure 5 Deconvoluted XPS valence band spectra of Mo(3d) core levels after UV irradiation: films with molar ratio [C]/[Mo] of 1.0 (a), 0.3 (b) and 0.2 (c).

disappearing of citric acid radical in solution. This sug-gested that under UV irradiation a charge transfer proc-ess occur between polymolybdates and citric acid molecules, resulting in the reduction of polymolybdates and the oxidation of citric acid molecules. For film B and C, no signal of paramagnetic Mo5＋ has been ob-served (Figure 6, curve b and c), indicating that the electrons are spin-paired at this time.

Therefore, when the molar ratio was decreased, the produced Mo5＋ was decreased, and vice versa. In film A, the citric acid molecule bonding to the polymolyb-dates through hydrogen bonding was proved by IR analyses (Figure 4, curve a). The amount of citric acid in film A with [C]/[Mo]＝1.0 is much more than those

Figure 6 EPR spectra (X band) of the UV-irradiated films with molar ratio [C]/[Mo] of 1.0 (a), 0.3 (b) and 0.2 (c).

of the other two films B and C, which will make the corresponding produced Mo5＋ by UV light irradiation the most. The XPS analysis showed that about 38% of Mo6＋ was reduced to Mo5+ for film A (Figure 5a). It is known that the reduced Mo5＋ is prone to aggregate into larger species.21,22 Therefore, due to the presence of great amount of reduced Mo5＋, the Mo7 clusters are readily condensed into species with higher degree of polymerization. In such a large polymolybdate species (the dark blue species, λmax＝670 nm), the electrons are highly delocalized as indicated by the EPR spectrum (Figure 6, curve a). This will lower the energies of the d-d and IVCT transitions.23 Thus a wide band can be observed at longer wavelength (500—900 nm) and film A presents a dark blue color. In film B, citric acid molecule interacted with the polymolybdates through hydrogen bond and coordination bond was proved by IR analyses (Figure 4, curve b). With the decrease of molar ratio in film B to [C]/[Mo]＝0.3, the amount of pro-duced Mo5＋ in the colored film by UV light irradiation decreased. The XPS analysis reveals that about 26% of Mo6＋ was reduced to Mo5＋ for film B (Figure 5b). That is, each Mo7 cluster can get about two electrons. The two electrons are paired as suggested by the EPR spectrum (Figure 6, curve b), and they are localized over the Mo7 species (the dark khaki species). As a re-sult, the energies of d-d and IVCT transitions became higher.24,25 Absorption in the region of 400—550 nm was observed and film B exhibited dark khaki color. In film C with [C]/[Mo]＝0.2, citric acid molecule coordi-nated with the polymolybdates was proved by IR analyses (Figure 4, curve c). The amount of citric acid in the film is the lowest and XPS analysis demonstrated that only about 18% of Mo6＋ has been reduced to Mo5＋ by UV light irradiation (Figure 5c). At this time, each Mo7 cluster can get about 1.4 electrons. Namely, in the colored film C some of Mo7 can get two electrons，but others only get one electron. A pair of one electron re-duced Mo7 clusters are inclined to be condensed into a diamagnetic species as suggested by the EPR results (Figure 6, curve c), 10

14 46Mo O － (the blue species, λmax

Photochromism Chin. J. Chem., 2005 Vol. 23 No. 8 1041


＝738, λsh≈614 nm). In [Mo14O46]10－ species, the electrons are delocalized over the four central MoO6 octahedra.3 Therefore, the absorption at 738 nm accom-panied by a shoulder at 614 nm assigned to

1014 46Mo O － and those in the region of 400—550 nm re-

lated to two electrons reduced Mo7 species have been observed and the film displayed a light sea green color.

Conclusions

The photochromic polymolybdate-citric acid com-posite films with different molar ratios have been pre-pared by sol-gel method. The molar ratios affected the species formed in the UV irradiated films, thus resulting in the multicolor of the composite films, i.e., dark blue ([C]/[Mo]＝1.0), dark khaki ([C]/[Mo]＝0.3) and light sea green colors ([C]/[Mo]＝0.2). This study would be helpful to the further application of such polymolyb-date/organic composite films to multifrequency opti-cal-electronic devices.

References and note

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(E0410255 LI, W. H.; FAN, Y. Y.)

multicolor photochromism of polymolybdate-citric acid composite films

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